I Tomashchuk
email: iryna.tomashchuk@u-bourgogne.fr

P Sallamand

A Méasson

E Cicala

M Duband

P Peyre

Aluminum to titanium laser welding-brazing in V-shaped groove

Keywords:

Laser assisted joining of AA5754 aluminum alloy to T40 titanium with use of Al-Si filler wires was carried out. Continuous Yb:YAG laser beam was shaped into double spot tandem and defocalized to cover larger interaction zone in V shaped groove. Experimental design method was applied to study the influence of operational parameters on the tensile properties of the joints. Microstructure examination and fractography study were carried out to understand the relation between local phase content and fracture mode.

Within defined window of operational parameters, statistically important factors that influenced the strength of T40 to AA5754 joints in V groove configuration were Si content in the filler metal and groove opening angle on T40 side. The best quality joint showed joint coefficient of 90% (or 200 MPa of apparent UTS). Tensile strength of the joints was found to be determined by the proportion between well-developed and under-developed reaction zones of T40/melted zone interface. The formation of 2-25 m thick Si-rich interlayers composed by Ti 5 Si 3 and 2 proved to enhance the strength of brazed interface. The creation of very thin (<0.5 m) Si-rich layers at the bottom of the groove was found not sufficient to establish mechanical continuity of the joint and thus should be avoided.

Introduction

Dissimilar joining of aluminum and titanium alloys in lightened hybrid structures becomes relevant for application in transport and aircraft industries. However, it remains a difficult task because of important mismatch in physical properties, limited mutual solubility and formation of brittle intermetallic phases in aluminum-titanium (Al-Ti) system, reported by [START_REF] Raghavan | Aluminum-titanium[END_REF]. Successful joining of aluminum to titanium demands prevention of local accumulation of intermetallic phases, especially of TiAl 3 .

Direct laser joining of titanium alloys to aluminum alloys in keyhole mode often results in cold cracking of the weld. [START_REF] Majumdar | Formation of crack-free joint between Ti alloy and Al alloy by using a high-power CO2 laser[END_REF] reported that shift of CO 2 laser beam to titanium side during welding of Ti6Al4V to aluminum alloy resulted in formation of numerous cracks in the zones rich in TiAl and Al 3 Ti. However, [START_REF] Kreimeyer | Laser processing of aluminum-titanium-tailored banks[END_REF] reported the successful joining of AA6016 to Ti6Al4V with CO 2 laser shifted at 0.3-0.5 mm to titanium side. In this case, aluminum side was only slightly melted, and, consequently, the formation of intermetallic phases was reduced. Later, same result was confirmed by [START_REF] Casalino | Yb-YAG laser offset welding of AA5754 and T40 butt joint[END_REF]: intermetal-lic zone thickness was reduced down to 1 m and was no more an initiator of fracture during tensile test. [START_REF] Song | Interfacial microstructure and mechanicalproperty of Ti6Al4 V/A6061 dissimilar joint by direct laser brazing without fillermetal and groove[END_REF] tried opposite approach by shifting the laser beam completely to aluminum side and obtained crack-free joints. However, thickness of zone rich in intermetallic phases remained important (260 m) and average tensile strength of the welds did not overpass 64% of aluminum alloy. [START_REF] Tomashchuk | Direct keyhole laser welding of aluminum alloy AA5754 to titanium alloy Ti6Al4V[END_REF] reported that high speed welding (>5 m/min) by Yb:YAG laser could prevent important mixing between two melted materials and confirmed that laser beam offset at aluminum side is beneficial for weld regularity and tensile strength. [START_REF] Gao | Microstructure and tensile behavior of laser arc hybrid welded dissimilar Al and Ti alloys[END_REF] carried out hybrid laser-CMT welding of AA6061 to Ti6Al4V with Al-12Si filler wire. Accurate control of total heat input and fluid flow in the melted zone allowed reducing zone of intermetallic phase down to 1 m and reach joint coefficient of 95.5% of AA6061. All authors highlight the necessity to reduce the interaction between Ti and Al to thin diffusive interface. That is why many studies concentrate on strong brazing approach, when only aluminum side of the joint undergoes fusion.

Very promising results on joining of aluminum alloys with titanium alloys were obtained by laser assisted welding-brazing. The main advantage of laser beam in this case consists in providing accurate energy supply with well-known distribution law, which allows control the thickness of intermetallic layers. [START_REF] Vaidya | Improving interfacial properties of a laser beam welded dissimilar joint of aluminum AA6056 and titanium Ti6Al4V for aeronautical applications[END_REF] reported the formation of 1.8 m thick intermetallic layer at the interface between U-shaped AA6065 plate and Ti6Al4V sheet welded by laser in conduction mode (without keyhole formation).

In this case, fracture occurred in Al-rich melted zone. [START_REF] Peyre | Generation and characterization of T40/A5754 interfaces with lasers[END_REF] performed laser welding-brazing of AA 6061 aluminum alloy to Ti6Al4V in lap-joint configuration with no filler material and with Al-5Si filler wire, but the effect of Si on mechanical properties of the joint was found unnoticeable. phase Ti 7 Al 5 Si 12 followed by columnar layer of Ti(Al,Si) 3 . Later, Chen et al. (2011b) elucidated more in detail the mechanism of these interfacial reactions and highlighted barrier role of Ti 7 Al 5 Si 12 that prevents diffusion of Ti in the melted zone and in this way reduces the quantity of brittle Al-Ti intermetallics. Chen et al. (2010b) demonstrated the influence of heat flow along solid Ti side on the morphology of resulting Si-containing microstructures. In this experiment, the groove was machined only at aluminum alloy side, while titanium interface remained perpendicular to the top surface of the plate. Laser beam was modulated into rectangular spot with Gaussian distribution in cross-direction and uniform distribution along the joint line. It was observed that while the top of the interface demonstrated well-advanced inter-facial reactions, the bottom part suffered from lack of energy and served a start-point for crack propagation.

The uniform morphology of interfacial microstructures on solid Ti side was obtained by Chen et al. (2011a) when 45 • grooves both from titanium and aluminum sides were associated with modulated rectangular spot of the same shape as proposed by Chen et al. (2010b). According to the numerical model of heat transfer proposed by Chen et al. (2011a), the use of rectangular spot allowed the reducing of the difference of temperatures attained in different parts of titanium groove. In this way, UTS of 278 MPa was attained, and fracture took place in the seam. On the other hand, the use of common Gaussian distribution of laser energy resulted in impor- The present study is focused on the problem of efficient brazing of AA5754 alloy to T40 titanium with use of Al-Si filler wires of different composition. Defocalized Yb:YAG laser shaped into double half-spot configuration is used to enlarge interaction zone. Experimental design method is used to study the influence of operational conditions on the tensile properties of the joints.

Experimental set-up

3 mm thick aluminum alloy AA5754 and titanium T40 plates and Al-Si based filler wires 4043 (Al-5Si) and 4047 (Al-12Si) of 1.6 mm in diameter were used. Chemical composition of raw materials is given in Table 1.

Grooves with opening angles of 45 • or 30 • were machined at T40 side, according to experimental design plan (Table 3), whereas for AA5754 opening angle was fixed to 45 • . Welding configuration and sketch of the grooves are provided at Fig. 1. Welding experiments were performed with continuous Yb:YAG laser having maximal beam power of 6 kW and optical fiber with diameter of 600 m. Double half-spot welding head with focus distance of 200 mm was used. Initial laser beam was divided by beam splitter and thus formed two inversed half-spots with Gaussian distribution separated by a distance of 1.2 mm. This beam configuration allows to enlarge the interaction zone and to heat instantaneously base material and filler wire. In the present study, it was used in attempt to approach uniform temperature distribution on the walls of V-shaped groove.

In order to define a new shape and energy distribution of resulting double half-spot tandem, the characterization of laser beam with diagnostic system Prometec Laserscope UFF100 was carried out, according to the method introduced by [START_REF] Liu | Laser beam diagnostics for kilowatt pulsed Nd:YAG laser[END_REF]. This method consists in direct sampling of discrete portion of laser beam and recording of spatial power intensity. The Prometec Laserscope uses a hollow needle with a pinhole aperture which is rapidly swept through the laser beam (unfocused or focused) to measure the intensity distribution [START_REF] Graham | A comparison of rotating-wire and rotating-pinhole-type laser beam analysers when used to measure pulsed Nd:YAG laser beams[END_REF].

Basing on the results of preliminary experimental study, important defocalization of double half-spot was adopted (+4 to +8 mm from the top surface of the plates). This allowed avoiding the melting of T40 side and noticeably enlarged the interaction zone. The analysis of acquired beam profiles showed rather symmetrical laser power distribution between two spots (Fig. 2): each spot contained 50% of initial laser power.

The plates were welded in V-grove configuration (Fig. 1a). As the distance between two laser spots was fixed to 1.2 mm and filler supply was coaxial to double half-spot tandem, the filler wire of 1.6 mm in diameter was melted by both spots and produced partial screening of groove surface from laser radiation (Fig. 1b).

Preliminary study showed that optimal position of double halfspot lies within +0.4 and +0.9 mm laser offset to T40 side. The angle of 20 • between the filler wire and top surface of the plates was applied. Top and bottom gas protection was implied (pure argon with total debit of 20 l/min).

After preliminary identification of the window of operational conditions, the factorial design plan for 2 levels without repetition was applied to quantify the influence of seven varying parameters on tensile strength of obtained joints. The influencing factors taken in consideration are given in Table 2. Following this experimental design, eight welding conditions were tested (Table 3).

Cross-sections of the welds were polished and etched with Keller's reagent. 20 mm large transversal cuts of the weld underwent tensile test. Three tensile tests per welding condition were made. Fracture surfaces from both sides of broken welds were examined.

Microstructure and chemical composition of samples were studied by scanning electron microscope (JEOL) with fast EDS analyzer. Two crosscuts per condition were observed.

UTS of the welds was evaluated at room temperature in tensile testing machine (MTS Insight 30 kN) at a crosshead speed of 8.3 × 10 -5 m/s with rectangular samples of 20 × 100 mm 2 . Mechanical stability of the welds was characterized by linear tensile force (N/mm) because of different areas of weld cross-sections and by apparent UTS (MPa) that neglects the variation of thickness between the weld and the plates.

The identification of phase composition on fractured surfaces was carried out by X-ray diffraction (PANalytical X'Pert PRO) using a cobalt target. Scanning range of 20-140 • with a step scan of 0.0167 • and counting time of 200 s per step were used.

Results and discussion

General observations

Under all tested operational conditions (Table 3) melting of T40 side was completely avoided, while slight melting of AA5754 took place. Melted zone (MZ) composition was found very close to that of filler material (10 or 4 wt.% Si for 4047 and 4043 filler metals respectively, «1 wt.% Ti). Typical aspect and cross-section of the weld are illustrated at Fig. 3. No porosities or hot cracks were visible in the weld. The increase of grain size in HAZ of T40 was systematically observed. Heat affected zone can be observed only on T40 side where the increase of grain size took place. Typical defects of the melted zone are related with filling of the groove by melted wire. The bottom of T40 groove sometimes presents the lack of brazing or wettening, especially for 30 • groove angle.

Important variation of tensile properties depending on applied combination of influencing factors was observed (Table 4). The best joint (sample 3) showed tensile force of 596 N/mm or 200 MPa of apparent UTS, whereas the worst one (sample 8) showed only 92 N/mm or 30 MPa. Joint coefficient calculated as UTS weld /UTS AA5754 varied from 14 to 90%. Usually, fracture occurred at the interface between T40 and melted zone (Fig. 4a). In samples 6 and 7 the crack was initiated at the bottom part of T40/MZ interface but then propagated in Al-Si filler wire (Fig. 4b). The reason of this behavior will be discussed in Section 3.3. In sample 4 fracture occurred at the interface between AA5754 and MZ (Fig. 4c). Close analysis of the weld showed that the origin of this behavior was a lack of fusion at this interface. For all samples, low elongation values were noticed.

The analysis of diagram of effects for chosen factors of influence (Fig. 5) revealed that the tensile properties of the welds are strongly influenced by groove opening angles (x6) and by the composition of filler material (x1). In concordance with results of Chen et al. (2010b), the use of filler wire with higher Si content (12 wt.%) is beneficial for tensile properties of the welds. The addition of Si changes phase selection at the interface between solid titanium and melted zone. The formation of Si-rich phases and mild thermal cycle at titanium/melted zone interface can limit the diffusion of Ti in the melted zone and in this way reduce the quantity of brittle Al-Ti intermetallics. The opening angle of groove should be big enough to allow good wettability by liquid filler material. Therefore, the worst results were obtained with grooves having 30 • angle from T40 side. The quantification of significance of influencing factors is provided by p-values obtained with variance analysis ANOVA (Table 5). In order to decide whether the effect is significant or not, threshold value of p = 0.05 (probability level of 95%) was imposed (associated effects on average apparent UTS are provided in form of upper and lower limits on Fig. 5). In this particular study, it can be concluded that factors x1 (filler material) and x6 (opening angle) produced an effect strong enough to be considered as significant. Other factors like laser power, wire supply rate, welding speed, defocalization and offset from joint line had no significant influence on response function within defined window of operational parameters.

Microstructure of T40/MZ interface

The following study was focused on composition and morphology of T40/MZ dissimilar interface. It was found that up to four interface morphologies might coexist in a single sample because of a difference in local thermal history. Typical interface morphologies with associated phases are described below.The morphology of type 1 composed by well-developed columnar structure (Fig. 6) was usually observed at the top 1/3-1/2 part of T40/MZ interface. Here, the thickness of the interface was comprised between 13 and 25 m. Starting from T40 side, thin diffusion layer (<0.5 m) of Al in Ti was observed. It was followed by a 3-6 m thick layer of Ti diffusion in the melted zone, where strong segregation of Si was observed. This interlayer differed in morphology and composition, depending In samples with filler wire 4043 (5 %at Si) (Fig. 7), the interface was uniform and had typical composition of 64 %at. Al, 32 %at. Ti and only 4 %at. Si, which corresponds, according to Al-Si-Ti phase diagram [START_REF] Liu | On the reaction scheme and liquidus surface in ternary system Al-Si-Ti[END_REF], to the mixture of TiAl 3 , TiAl 2 and Ti 5 Si 3 phases. For the samples with filler wire 4047 (12 %at Si) (Fig. 6), this interlayer was more pronounced and non-uniform. Inner part had a composition of 43 %at. Al, 43 %at. Ti and 12 %at. Si, which also corresponds to the mixture of TiAl 3 , TiAl 2 and Ti 5 Si 3 phases. In outer part that contains less Si (67 %at. Al, 22 %at. Ti, 9 %at. Si), the formation of ternary phase 2 (Al 21 Si 46 Ti 33 -Al 8 Si 59 Ti 33 ) along with TiAl 3 was assumed.

Finally, important columnar structures composed of Ti(Al,Si) 3 (78 %at. Al, 19 %at. Ti, 3 %at. Si) in case of 4043 wire and of TiAl 3 + 2 (72 %at. Al, 20 %at. Ti, 8 %at. Si) in case of 4047 wire were observed. Si also accumulated in form of a halo around columnar structures. Next followed Al-Si medium composed by eutectic structure in case of 4047 wire and hypotectic structure in case of 4043 wire.

Interface of type 2 (Fig. 8) was usually perceived in the middle of T40/MZ interface. It had morphology similar to type 1 but colum-nar structures poor in Si were much less developed. The thickness of the whole interface was comprised within 4-6 m. EDX analysis of the interface allowed supposing the formation of TiAl 3 and Ti 5 Si 3 phases in the inner part of interlayer, when underdeveloped columns were composed by Ti(Al,Si) 3 . In case of 4043 filler, the accumulation of Si at T40/MZ interface was much less pronounced.

Interface of type 3 was observed in the middle and bottom parts of the samples welded with 4047 filler wire (Fig. 9). In this case, eutectic structure of 4047 accumulated at the interface and formed 1.8-2 m Si-rich layer with petal-like structure. EDX analysis of this interface allowed supposing the formation of TiAl 3 + Ti 5 Si 3 mixture.

Interface of type 4 (Fig. 10) was usually observed in the bottom part of the groove and was associated with use of 4043 filler material. In this case, the only observed structure was a very thin (<0.5 m) Si-rich layer.

For all samples, the expanses of these four types of morphologies at T40/MZ interface were localized with SEM imaging (Fig. A1) and expressed in% of total interface length (Table A1). The diagram of effects of operational parameters on formation of interface morphologies (Fig. A2) revealed the following tendencies. The morphology of type 1 forms very rarely (only in two samples) and corresponds to the excessive local heating of T40/MZ interface. No statistical conclusion can be made within chosen window of operational parameters. The formation of morphology of type 2 was found to be strongly promoted by use of Si-rich filler wire 4047 (Al12Si), opening angle of 30 • at T40 side of the groove and by laser offset of 0.4 mm to T40 side. The morphology of type 3 was found in all samples except sample 4 and is strongly promoted by groove opening angle of 45 • at T40 side of the groove and by laser beam offset of 0.9 mm to T40. The morphology of type 4 was found in all samples except the sample 4 and is strongly promoted by use of filler wire 4043 poor in Si (Al 5 Si). p values of influencing factors are provided in Table A2. The disposition of these morphologies along the interface is a complex function of operational parameters and depends on local thermal history of the interface that may be predicted by numerical modeling. In all cases, the morphology of type 4 was situated at the bottom of T40/MZ interface that remains the coldest part of the melted zone.

Fractography

As shown in Table 4 andon fractured surface on T40 side (Fig. 11a) shows two types of zones: rich in Si and Al, where Ti is absent (for example, for sample 3: 82 %at. Al, 0 %at. Ti, 18 %at. Si) and rich in Ti (28 %at. Al, 58 %at. Ti, 13 %at. Si). The tooth-like character of the interface between these two zones corresponds to the periodical arrival of melted filler wire at the bottom of the groove. In Al-Si rich zone, transgranular fracture mode was observed (Fig. 11b). Here, the fracture took place in the proximity of T40 interface, as the concentration of Si was systematically superior to that of filler wire. Visible Ti-rich zone is in fact the surface of T40 groove that suffered from weak adherence of filler material. In this zone, brazing was inefficient due to insufficient energy supply. These zones were generally situated at the middle or bottom of the weld, where T40 obviously did not receive enough heat for starting efficient diffusion process between Ti and filler elements.

XRD analysis showed that all samples broken at T40/MZ interface have fracture surfaces with very close phase content. Two sides of each break also have very close peak intensities (Fig. 12). The dominating phases were (Al), (Si), (Al 5 Si) and Ti(Al,Si) 3 , which allowed supposing that the fracture occured in outer layer of T40/MZ interface. Presence of ␣-Ti corresponds to T40 zones with little brazing.

In two samples (6 and 7, both with 4043 wire) the fracture was initiated at T40/MZ interface in the zone where lack of brazing is observed (interface of type 4), propagated to the beginning of the zone of effective brazing (interface of type 1, 2 or 3) and then continued in the filler metal. This can be seen on fractography of sample 6, composed by two clearly distinguished zones (Fig. 13a): filler wire with mixed fracture mode (Fig. 13b) and braze free T40 groove. From comparison between fractography and observed structure of T40/MZ interface (Fig. A1), it can be concluded that the interface of type 2 stopped the propagation of the crack along T40/MZ interface. XRD data for these two samples revealed the presence of (Al) and (Si) solid solutions as well as of ␣-Ti, but no intermetallic phases proper to T40/MZ interface.

Only in sample 4, a fracture occurred at the interface between AA5754 and MZ, because of the important lack of fusion: it can be seen that zone 1 of unmelted AA5754 dominates over zone 2 corresponding to MZ (Fig. 14). XRD study revealed the presence of (Al) and (Si) solid solutions as well as low quantity of Mg 2 Si.

After excluding atypical sample 4, the measurements of fractured surfaces corresponding to the zones of effective brazing at T40 side were made. These results were confronted with normalized expanses of morphology of type 4 at T40/MZ interface, obtained from SEM observation of cross-sections (Fig. A1; Table A1). Strong inverse linear tendency between the expanse of morphologies of type 4 and% of efficiently brazed T40/MZ surface (%EBS) was observed (Fig. 15a). The position of zones of inefficient brazing at T40/MZ interface was found in good concordance with location of morphology of type 4 (Fig. 16). In the same time, strong inverse linear tendency between the expanse of morphologies of type 4 and apparent UTS was observed (Fig. 15b). All this allows concluding that morphology of type 4 coincides with the zones of insufficient brazing and thus should be avoided.

Highest tensile strength (sample 3:200 MPa of apparent UTS or 90% joint coefficient) was obtained when efficiently brazed T40/MZ interface neared 100%. The use of Si-rich filler metal allowed enhancing tensile strength of joints comparing to results of our previous study focused on direct joining between AA5754 and Ti alloy [START_REF] Tomashchuk | Direct keyhole laser welding of aluminum alloy AA5754 to titanium alloy Ti6Al4V[END_REF], where joint coefficient of only 60% was reached. Samples 2 and 3, having highest tensile strength and%EBS, also show quite uniform microstructure of T40/MZ interface (Fig. A1). It means that the shaping of laser beam in double half-spot tandem allowed approaching uniform thickness of brazed T40/MZ interface for groove opening angle of 45

Conclusions

• Within defined window of operational parameters, statistically important factors that influence the strength of T40 to AA5754 joints in V groove configuration are: the nature of the filler metal (high Si content should be privileged) and groove opening angle on T40 side (angle of 45 • facilitates the flow of melted wire in the groove). • The tensile strength of the welds is strongly determined by the proportion between zones of T40/MZ interface having welldeveloped and under-developed structures. • At top and middle parts of the joints, well developed Si-rich structures with thickness comprised between 25 and 2 m composed by in Ti 5 Si 3, 2 and Ti(Al,Si) 3 were observed. • The formation of very thin (<0.5 m) Si-rich interfaces at the bottom of the groove is not sufficient for establishing mechanical continuity of the joint and thus should be avoided. • Optimal welding condition for joining of 3 mm thick T40 with AA5754 in butt configuration was identified as: groove opening 45 • /45 • , filler wire 4047 (Al12Si), laser power of 3.6 kW, filler supply rate of 2.2 m/min, welding speed 0.45 m/min, defocusing of 4 mm and offset of laser from joint line of 0.9 mm to T40 side. The resulting weld showed joint coefficient of 90% (or 200 MPa of apparent UTS comparing with AA5754). • Laser beam configuration in shape of double half-spot tandem was found promising for obtaining uniform microstructure thickness during welding-brazing in V-shaped groove with filler supply.

Fig. 1 .

 1 Fig. 1. Welding configuration (a) and draft of the groove (b).

Fig. 2 .

 2 Fig. 2. Laser beam profiles (expressed as% of beam power contained in the area): defocalization of +4 mm (a) and of +8 mm (b).

Fig. 3 .

 3 Fig. 3. Typical aspect and morphology of T40 to AA5754 joint: top view (a) and cross-section (b) of sample 2.

Fig. 4 .

 4 Fig. 4. Cross-sections of fractured welds: sample 2 (a), sample 7 (b) and sample 4 (c).

Fig. 5 .

 5 Fig. 5. Diagram of effects (confidence intervals for p = 0.05 are indicated with dot lines).

Fig. 6 .

 6 Fig. 6. T40/MZ interface of type 1 (sample 4, 4047 filler wire): interface morphology (a) and compositional maps of elements Al (b), Si (c) and Ti (d) with illustration of EDS signal variation across the interface.

Fig. 7 .

 7 Fig. 7. T40/MZ interface of type 1 (sample 6, 4043 filler wire): interface morphology (a) and compositional maps of elements Al (b), Si (c) and Ti (d) with illustration of EDS signal variation across the interface.

Fig. 8 .

 8 Fig. 8. T40/MZ interface of type 2 (sample 1): interface morphology (a) and compositional maps of elements Al (b), Si (c) and Ti (d) with illustration of EDS signal variation across the interface.

Fig. 9 .

 9 Fig. 9. T40/MZ interface of type 3 (sample 3): interface morphology (a) and compositional maps of elements Al (b), Si (c) and Ti (d) with illustration of EDS signal variation across the interface.

Fig. 10 .

 10 Fig. 10. T40/MZ interface of type 4 and evolution of EDS signal of Si along analysis line (bottom of sample 6).

Fig. 11 .

 11 Fig. 11. Fracture surface in case of crack propagation at T40/MZ interface (sample 3, T40 side): X-map of global view (a); zoom on fracture surface in Al-Si rich zone (b).

Fig. 12 .

 12 Fig. 12. XRD of fracture surfaces in case of fracture propagation at T40/MZ interface (sample 3).

Fig. 13 .

 13 Fig. 13. Fracture surface in case of crack propagation across T40/MZ interface and filler wire (sample 6, T40 side): X-map of global view (a); zoom on fracture surface in MZ (b).

Fig. 14 .

 14 Fig. 14. Fracture surface in case of crack propagation across AA5754/MZ interface (sample 4).

Fig. 15 .

 15 Fig. 15. Linear regression between% of T40/MZ interface occupied by morphology of type 4 (%T4) and% of efficiently brazed T40/MZ surface (%EBS) obtained from fractography data (a); linear regression between% of T40/MZ interface occupied by morphology of type 4 (%T4) and apparent UTS (b).

Fig. 16 .

 16 Fig. 16. Fractured surfaces from T40 side with indicated length of zone of insufficient brazing (top) and length of morphology of type 4 on weld crosscut (bottom): sample 2 broken at T40/MZ interface (a), sample 7 broken at T40/MZ interface and filler wire (b) and sample 8 broken at T40/MZ interface (c).

  • . The combined effect of two statistically significant factors (x1 and x6) can also be noticed on Fig. 15b. The group of samples having maximal tensile strength (No 2 and 3) corresponds to the combination of favorable values (x1 = 4047, x6 = 45/45 • ), the weakest samples (No 5 and 8) have combination of unfavorable values (x1 = 4043, x6 = 30/45 • ), when samples with average strength have one of these factors fixed on unfavorable value.

Fig. A1 .

 A1 Fig. A1. Distribution of different types of interface morphologies at T40/MZ interface.

Fig. A2 .

 A2 Fig. A2. Diagrams of effects of operational parameters on formation of different types of morphologies at T40/MZ interface: type 1 (a), type 2 (b), type 3 (c), type 4 (d).

Table 1

 1 Chemical composition and melting temperature of raw materials.

Chemical composition (wt.%)

Melting range ( • C) AA5754 Al: balance; Mg: 6; Si: ≤1 560-629 T40 Ti: balance; O: 0.25; N: 0.03; C:0.08; H: 0.015; F: 0.3 1668 4043 Al: balance; Si: 5.2; Fe: ≤0.08; Zn: ≤0.1; Cu: ≤0.3; Mn: ≤0.05; Mg: ≤0.05; Ti: ≤0.1 574-632 4047 Al: balance; Si: 12.0; Fe: 0.8; Zn: 0.2; Cu: ≤0.3; Mn: 0.15; Mg: 0.1 577-582

Table 2

 2 Influencing factors included in experimental design.

	Influencing factors	Code	Level 1	Level 2
	Filler wire material	x1	4047	4043
	Laser power (W)	x2	3000	3600
	Filler supply rate (m/min)	x3	1.8	2.2
	Welding speed (m/min)	x4	0.45	0.55
	Defocalization from the top surface (mm)	x5	+8	+4
	Groove opening angle T40/AA5754	x6	30 • /45 •	45 • /45 •
	Laser offset from joint line to T40 (mm)	x7	0.4	0.9
	tant variations of interfacial microstructures across the interface
	and poor UTS values.

Table 3

 3 Experimental layout.

	No	Influencing factors
		x1	x2	x3	x4	x5	x6	x7
	1	4047	3000	1.8	0.45	4	30 • /45 •	0.4
	2	4047	3000	1.8	0.55	8	45 • /45 •	0.9
	3	4047	3600	2.2	0.45	4	45 • /45 •	0.9
	4	4047	3600	2.2	0.55	8	30 • /45 •	0.4
	5	4043	3000	1.8	0.45	8	30 • /45 •	0.9
	6	4043	3000	1.8	0.55	4	45 • /45 •	0.4
	7	4043	3600	2.2	0.45	8	45 • /45 •	0.4
	8	4043	3600	2.2	0.55	4	30 • /45 •	0.9

Table 4

 4 Tensile test results.

	No	Tensile force (N/mm)			Apparent UTS (MPa)	Average elongation (mm)	Fracture position
		F 1	F 2	F 3	Average
	1	344	358	366	356	119	1.734	T40/MZ interface
	2	563	598	580	580	193	1.960	T40/MZ interface
	3	576	592	620	596	200	2.614	T40/MZ interface
	4	239	249	265	251	84	0.738	AA5754/MZ interface (lack of
								fusion)
	5	93	107	85	95	32	0.096	T40/MZ interface
	6	419	442	429	430	143	1.479	Initiated at T40/MZ interface
								and propagated in Al-Si filler
								wire
	7	421	468	473	454	151	1.765	Initiated at T40/MZ interface
								and propagated in Al-Si filler
								wire
	8	89	75	112	92	30	0.211	T40/MZ interface

Table 5 p

 5 values of factors influencing tensile strength.

	IF	x1	x2 + x3	x4	x5	x6	x7
	p	0.0463	0.3149	0.1914	0.2693	0.0264	0.2175

Acknowledgement

This work was supported by the French program Institut CARNOT Arts in the frame of the project ATTILA.

Appendix A. a Normalized expanse for every type of interfacial morphology was calculated as

• 100, where l i -total length of morphology of type i and L-total length of T40/MZ interface.


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Urban dew and rain in Paris, France: Occurrence and physico-chemical characteristics

Keywords: Dew water, Urban dew water, Rainwater, Water chemistry, Water quality, Paris (France)

This paper summarizes one year (April 2011 to March 2012) measurements on planar condensing surfaces of dew and rain events and related physico-chemical characteristics in the urban environment of Paris (city center). Yearly collected water was 3.48 mm for dew (63 events) and 593 mm for rain (146 events). The latter value compares well with rain data (547 mm and 107 events) collected within 12 km at Paris-Orly airport. An estimation of dew yield based on meteo data gives 2.35 mm and 74 events, to be compared with 17.11 mm and 196 events at Paris-Orly. These differences highlight the large reduction in dew events and dew yields in an urban area as compared to a close rural-like area. This reduction is not due to a sky view reduction but to heat island that increases air temperature and decreases relative humidity.

Analysis of dew (34) and rain (77) samples were done concerning pH, electrical conductivity (EC), major anions and cations as well as selected trace metals and other minor ions. Mean pH values are found similar for both, dew (6.5) and rain (6.1), rain being slightly more acidic than dew. The mean dew total ionic content (TIC 1.8 meq/l) and EC value (124 µS.cm -1 ) are about four times that of rain (0.45 meq/l; 35 µS.cm -1 ), meaning that total dissolved solids in dew is nearly four times that in rain. Sulfate and nitrate are the most acidifying components, calcium the most neutralizing constituent with ratio of mean total acidity/total alkalinity comparable for dew and rain (∼ 0.9). Sulfate and nitrate have mainly anthropogenic sources, whereas chloride and magnesium are mostly connected with marine air masses. Dew is a considerable factor of wet deposition of pollutants; ion concentrations, however, meet the WHO requirements for drinking water.

Introduction

Dew is the result of passive condensation of atmospheric water vapor into liquid. The necessary cooling power is provided by the radiation deficit between atmosphere and the condensing support. Dew has not to be confused with fog or rain, constituted of already condensed liquid droplets. With typically less than 70 Wm -2 radiative power available, the maximum yield cannot exceed 0.7 -0.8 lm -2 [START_REF] Monteith | Principles of Environmental Physics[END_REF][START_REF] Beysens | The formation of dew[END_REF][START_REF] Beysens | Dew nucleation and growth[END_REF][START_REF] Berkowicz | Urban dew collection under semi-arid conditions[END_REF]. This low yield, however, can be a welcome and useful contribution to the biosphere [START_REF] Zangvil | Six years of dew observations in the Negev Desert, Israel[END_REF][START_REF] Agam | Dew formation and water vapor adsorption in semi-arid environments-a review[END_REF][START_REF] Jacobs | Contribution of dew to the water budget of a grassland area in The Netherlands[END_REF][START_REF] Kidron | An Investigation of the Role of Water (Rain and Dew) in Controlling the Growth Form of Lichens on Cobbles in the Negev Desert[END_REF][START_REF] Uclés | Role of dewfall in the water balance of a semiarid coastal steppe ecosystem[END_REF] and to potable water in arid or semi-arid climates [START_REF] Nilsson | Initial experiments on dew collection in Sweden and Tanzania[END_REF][START_REF] Beysens | Pour des ressources alternatives en eau[END_REF][START_REF] Kidron | Analysis of dew precipitation in three habitats within a small arid drainage basin, Negev Highlands, Israel[END_REF]Beysens et al., 2006a;[START_REF] Lekouch | Rooftop dew, fog and rain collection in southwest Morocco and predictive dew modelling using neural networks[END_REF][START_REF] Tomaszkiewicz | Dew as a Sustainable Non-Conventional Water Resource, A Critical Review[END_REF][START_REF] Khalil | A review, dew water collection from radiative passive collectors to recent developments of active collectors[END_REF][START_REF] Mylymuk-Melnytchouk | Puits aériens : mythes et réalités ou Travaux russes & soviétiques sur la production d'eau à partir de l'air[END_REF], provided that water chemical and biological quality is acceptable (Beysens et al., 2006b;[START_REF] Muselli | Is dew water potable? Chemical and biological analyses of dew water in Ajaccio (Corsica Island, France)[END_REF][START_REF] Lekouch | Dew, fog, and rain as supplementary sources of water in south-western Morocco[END_REF][START_REF] Tomaszkiewicz | Dew as a Sustainable Non-Conventional Water Resource, A Critical Review[END_REF]. It can be also the source of plant disease [START_REF] Luo | Dew formation on rice under varying durations of nocturnal radiative loss[END_REF] and, in urban areas, cause degradation to roofs, cars and generally to any inorganic matter that can be corroded [START_REF] Rubio | Remoción de contaminantes por lluvias y rocíos en la región metropolitana[END_REF][START_REF] Rubio | Nitrite in rain and dew in Santiago city, Chile. Its possible impact on the early morning start of the photochemical smog[END_REF][START_REF] Xu | Laboratory observation of dew formation at an early stage of atmospheric corrosion of metals[END_REF]. Dew formation is a local phenomenon. The steps governing dew chemical composition are formation on dry deposition solids, dissolution of the soluble portion of the dry deposition by dew water, and sorption of gases into the dew solution.

For any precipitation to fall there is a need for a cloud. Clouds are formed when the relative humidity reaches 100% and with enough hygroscopic nuclei in air so that condensation can take place. Cloud droplets initially grow quickly but to eventually produce precipitation collision / coalescence processes are needed. When drops become heavy enough to overcome air resistance they fall as rain. The chemical composition of precipitation is a result of three different processes: nucleation scavenging (chemical configuration of nuclei determines initial cloud composition), in-cloud scavenging (take up of non-activated particles and trace gases (major important) including chemical reactions within droplets), sub-cloud scavenging (absorption of gases and take up of particles). Not every cloud brings rain and on average a condensation nuclei may undergo many cloud cycling processes including chemical transformation before it comes back to earth surface, often far away from its primary source.

Carbon dioxide plays a special role in the formation of acidity in the atmospheric liquid phase because of its high and constant concentration. An important pathway in alkalinity (carbonate) formation goes via condensation nuclei (nucleation and droplet formation) as well as aerosol scavenging. The last process contributes significantly to sub-cloud scavenging into falling raindrops. The ability to capture particulates is very relevant for dew chemical composition and is strong at the beginning and weakened at the end of the condensation process. The acidity from dissolved CO 2 , SO 2 , and NO x is mostly neutralized by Mg 2+ , Ca 2+ and NH 4 + ; sometimes a slight alkaline character is observed in dew samples. Dew events with the higher ionic concentration occur following long periods without rain.

Uptake of high soluble gases on atmospheric water is very fast and not a question of time.

Cloud cycling processes, microphysical conditions and heterogeneous reactions will influence dew and rain chemistry. When in equilibrium with atmospheric CO 2 , the HCO 3 -concentration is an exponential function of the pH-value. When the pH solutions is higher than 6.35 (pKa1 of H 2 CO 3 ) [HCO 3 -] can become important. But samples of the atmospheric multiphase system are most probably not in equilibrium with atmospheric CO 2 due to complex chemical compositions, microphysical processes and heterogeneous interactions and [HCO 3 -] can only be obtained by analytical estimation and not deriving Henry's law. A possible high contribution of bicarbonate to the ionic balance implies "errors" in the quality check between measured conductivity (where HCO 3 -is included) and the analyzed ions (where HCO 3 -is excluded). An observed difference between measured and calculated conductivity can also be caused by other not analyzed ions (e.g. formate, acetate, phosphate and in our study also ammonium).

In urban environment dew water composition is thus a function of both long range convected atmosphere and locally produced gas and aerosols. Several studies during the last decades have been concerned with dew chemistry in urban areas, in Santiago, Chile [START_REF] Ortiz | Hydrogen peroxide deposition and decomposition in rain and dew waters[END_REF][START_REF] Rubio | Nitrite in rain and dew in Santiago city, Chile. Its possible impact on the early morning start of the photochemical smog[END_REF][START_REF] Rubio | Hydroperoxides in dew water in downtown Santiago, Chile. A comparison with gas-phase values[END_REF][START_REF] Rubio | Factors determining the concentration of nitrite in dew from Santiago, Chile[END_REF], in Japan (Yokohama, Osaka, Tokyo) [START_REF] Okochi | Effect of acid deposition on urban dew chemistry in Yokohama, Japan[END_REF][START_REF] Okochi | Effect of atmospheric humiclike substances on the enhanced dissolution of volatile organic compounds into dew water[END_REF][START_REF] Takeuchi | Dominant factors of major and minor components and formation of hydroxyl alkane sulfonate in urban dew water[END_REF][START_REF] Takenaka | Difference in amounts and composition of dew from different types of dew collectors[END_REF]. Research were also carried out in Poland (Gdansk, Krakow, Wroclaw) [START_REF] Polkowska | Chemical characterization of dew water collected in different geographic regions of Poland[END_REF], Jordan (Amman) [START_REF] Jiries | Chemical composition of dew in Amman, Jordan[END_REF], Israel (Jerusalem) [START_REF] Berkowicz | Urban dew collection under semi-arid conditions[END_REF], Bordeaux (France) (Beysens et al., 2006a), Zadar (Croatia) [START_REF] Lekouch | Comparative chemical analysis of dew and rain water[END_REF] and New Delhi (India) [START_REF] Yadav | Pollutant scavenging in dew water collected from an urban environment and related implications[END_REF]. The physico-chemical analysis of dew gives information about atmosphere from where it was condensed. The obvious sources of aerosols deposited on the collectors are (i) emissions of hydrocarbons and nitrogen oxides coming from the road traffic, (ii) greenhouse gases and sulfur dioxide coming from nearby industries and house heating in winter, (ii) solid particles coming from the close environment (limestone, ashes of open fires, etc.). Analyses of urban dew usually indicate higher concentration levels of Ca 2+ , SO 4 2-and NO 3 -than in rural areas.

The aim of this study is then to determine in the urban environment of the historical center of Paris the volume and occurrence of dew as compared to rain and characterize the main chemicals present in dew and rain water. Such studies have not been performed so far to our knowledge in Paris and appear to be a good starting point to compare with further evolution in relation with air quality improvement and urban heat island.

Measurements and methods

Measurement site

The measurement site is located on an open terrace of the Ecole Supérieure de Physique et Dispersion of atmospheric pollutants is thus favored. However, the density of population and activities is quite high. Paris and its neighboring agglomeration concentrate about 90% of the regional population on a little more than 20% of the region area. According to Air Quality in Europe (2016), the metropolitan area is responsible for more than 75% of the regional NO x emissions, 70% of the hydrocarbons (volatile organic compounds VOC) and 50% of the particles. The NOx emissions of the region represent 10% of the national emissions. The principal sources of pollution are transportation, building heating and industry. Road transportation is responsible for 53% of the nitrogen oxides (NO x ) emissions of the Paris agglomeration, 15% of the VOC and 25% of the particles produced by an important fleet of diesel vehicles. Sulfur dioxide seems to be no longer an issue, due to the emission decrease of industrial activities in the region.

Dew and rain evaluation

In the present study, dew is collected on three planar condensers with same yield at 1m above the floor of the terrace, facing approximatively SW (220° direction, see Fig. 1). 1° resolution. The measurements were performed close to the edge of the building, 7.5 m above the ground level and about 2 m above the terrace (Fig. 1).

The calibration of the rain gauge was performed to convert the data in lm -2 (mm) of collected water. The pluviometer was calibrated by gently pouring different volumes of water and measuring the response of the weather station. The conversion ratio mm-condenser/mmstation is 8.51×10 -3 . The resolution of water collection is 0.014 mm. Only little dew water remains pinned at the condenser surface (less than 5% according to [START_REF] Beysens | Collecting dew as a water source on small islands: the dew equipment for water project in Bisevo (Croatia)[END_REF], then the yield is close to a scraped surface.

Condenser A', dedicated to another study, is used as a spare condenser when technical problems occur with condenser A. It is made with 7.5 mm thick corrugated fibrocement, with surface area 1.52×0.92 m 2 . Dew water is collected by gravity in a gutter and the corresponding volume is measured by a pluviometer. The resolution of water collection is 0.023 lm -2 . The measurements are taken every 15 min. The pluviometer was calibrated by pouring water. The conversion ratio mm-condenser/mm-station is 2.24×10 -2 . Due to fibrocement porosity, 0.08 mm is lost at each event [START_REF] Doppelt | Better collection of dew and weak precipitations[END_REF] and has been added to the data for correction. The condenser can be considered as reasonably thermally isolated due to its low thermal conductivity (< 0.17 Wm -1 K -1 ).

Condenser B is dedicated to chemical studies. It is made of a plastic foil thermally insulated beneath by 22 mm thickness Styrofam. The foil is 0.35 mm thickness. It is made with low density polyethylene in which are incorporated a few % of TiO 2 and BaSO 4 microsphères and an insoluble surface-active food agent at its surface (similar to [START_REF] Nilsson | Initial experiments on dew collection in Sweden and Tanzania[END_REF]manufactured by OPUR, 2016). The interest of such foil lies in its enhanced dew collection ability and its chemical inert properties. Water runs out by gravity in a PVC gutter and is collected in a polyethylene bottle. The samples are daily collected in the morning at fixed time (about 08:00 local time). The surface of the condenser is scraped to collect the smallest drops pinned at its surface. Dew and rain samples are collected in 50 ml polyethylene bottles without filtering (turbidity of water was generally very low). Volume of water is also measured at the chemistry laboratory when the bottle is not full up (sample lesser than 50 ml). The minimum volume for performing complete chemical analyzes as mentioned in section 3 is in general 3-5ml. Condenser and scraper are rinsed every day with distilled water. The foil was changed after the first 6 months of utilization, polyethylene being sensitive to UV. The water samples are kept at 3°C. They are carried out for analysis every 60 days in a refrigerated box to the chemistry laboratory (Laboratory of Hydrology of Aquitaine, University of Bordeaux). The number of analyzed samples was 34 for dew and 77 for rain.

Dew yields were measured primarily from condenser A (34 data), then B from chemical sample volume if absent (16 data) and, if also missing, from A' (13 data).. The distinction between dew, fog and rain is based on morning observation and the evaluation of the water collected rate. Dew rate is much smaller than rain' and cannot be higher than about 10 -3 lm - 2 min. -1 . For the measurement period (01/04/2011 -31/03/2012) one reports 63 dew, 5 fog and 146 rain events. Hoarfrost is rare in Paris and such data were not considered. Some measurements were lacking due to technical problems. No angle correction is made for fog and rain when converted in mm; one assumes that the collection area is the condenser surface area. As dew forms during night, daily water data collected on the condenser was chosen to be the sum of hourly data collected between dd-12:00 and dd+1-12:00. Some small uncontrolled bias might then appear when estimating the rain volume for sample collected at 8:00.

Dew yield estimation

Dew yield can be estimated from meteo data according to an analytical model by [START_REF] Beysens | Estimating dew yield worldwide from a few meteo data[END_REF]. The model is based on the approximation of a condenser temperature near T d and the recognition that most of the cooling energy is used to maintain the condenser surface at this temperature. The formulation was tested in laboratory and several places in the world with different climates. Agreement within typically 30% is found with dew measurements. Dew data collected every 15 min. with a condenser of 1 m 2 (or similar), inclined at 30° and thermally insulated from below corresponds to the following amplitude h 15 (in mm per 15 min.):

( ) [ ] d T a T . h h - - = 06 0 0 12 25 . 0 15 if positive and V 10 < V 0 h= 0 if negative or V 10 > V 0 (1)
The formula under square bracket is dew yield for a 12 h. night. The prefactor 0.25/12 corresponds to rescaling with data taken every 15 min.

A A' B M

The 2 nd term in the square bracket represents a negative contribution due to the condenser heat losses with ambient air, which are proportional to the difference condenser-air temperature T a -T d . The conditions about V 10 (wind speed at 10 m elevation) correspond to heat losses which become increasingly large above a wind speed threshold V 0 ≈ 4.4 m.s -1 where dew cannot form. Wind speed V 10 can be extrapolated from the measured data according to the classical logarithmic variation (see e.g. Pal Arya, 1988):

V(z) = V 10 ln(z/z c )/ln(10/z c ) (2)
The length z c (in the present study taken to be 0.1 m) is the roughness length, leading to V 10 ≈ 1.1V.

The 1 st term of Eq. ( 1), h 0 , is related to the cooling energy divided by the latent heat. Cooling originates from the radiation deficit between condenser and sky due to their different

-N/ . d T d T - H . H + . - . - H . H - . + . h       + ×       ×       × = (3)
The cloud coverage has been estimated from interpolating at 15 min. frequency data taken every 3h. at the airport meteo station Paris-Orly (12km) from the Paris measurement site.

The daily dew yield h (mm/day) is the daily sum of the h 15 , taken every 15 min.: The data obtained during daylight are automatically discarded in the calculation because of the too large difference in T a -T d , making h 15 negative.

Results and discussion

The following analyses were carried out: pH, electric conductivity (EC), chemical analysis for major cations (Na + , K + , Ca 2+ and Mg 2+ ) and major anions (Cl -, SO 4 2and NO 3 -) and minor ions (Fe 2+ , Cu 2+ , Cd 2+ , Mn 2+ , Pb 2+ , Zn 2+ , NO 2 -, Br -, -F). Inductively coupled plasma mass spectrometry (ICP-MS) was available for the determination of cations while ion chromatography was used for analyzing the anions. Unfortunately no analyses of ammonium were performed (see the discussion in section 3.4).

Many distributions of trace species coming from natural processes are strongly asymmetric, but they could be symmetrized by a lognormal distribution. The approximated location of the center of the frequency distribution can be determined by different measures. The most common is the arithmetic mean, but very susceptible to high values, especially with small amounts of samples. The median, that is the value being on the middle digit, is more appropriate. The Paris dew and rain data show strong right-skewed (lognormal like) distributions. The median is thus mostly significant smaller than the arithmetic mean. Very similar to the median is the volume weighted mean (VWM) which takes into account the effect of dilution by the dew or rain water amount and is useful in comparative studies. It can be calculated from VWM=

∑ 𝑣 𝑖 [X i ] 𝑁 𝑖=1 ∑ 𝑣 𝑖 𝑁 𝑖=1 . ( 5
)
where [X i ] is the concentration of ion X and v i is the water sample volume.

The analysis of correlations between two or several random variables can be performed by the calculation of their coefficient of correlation. This coefficient, the Pearson product-moment correlation coefficient r, is equal to the ratio of their covariance with the non-null product of their standard deviations. The coefficient r is a measure of the linear correlation between two variables X and Y. According to Chok (2008), this coefficient can be successfully used for analysis of non-normally distributed data as frequently found in this study.

Dew and rain yields

Daily dew and rain data and their summation data are presented on Fig. 2 and the statistics are summarized in Table 1. Dew yield is the largest in fall, with however a remarkable peak in summer. Both periods experienced a quite sunny period, corresponding to clear nocturnal sky.

The yearly yield (3.48 mm) is low although the sky view factor is large, due presumably to The data collected on site and those calculated with Eq. 4 are reported in Table 1 and in Fig. 2c where are shown the evolution of measured and calculated cumulated dew values. The numbers of dew days are comparable (measured: 63; calculated: 74), the maximum yields also (measured: 0.19; calculated: 0.17). The yearly sums only differ by 30% (measured: 3.48 mm; calculated: 2.35 mm). However, the calculated data evolution does not fit very well the experimental evolution. The main differences are due to the November (and to lesser extent, August) events, which are not properly taken into account in the model. Cloud cover in downtown Paris is also often smaller than in its suburb. We thus imposed the arbitrary lower mean value (N-1), which indeed gives a better agreement (Fig. 2c).

It is interesting to compare the calculated dew yields in Paris center and Paris-Orly. The results are reported in Table 1. The number of dew events (196) is more than two times higher than in Paris (74), the maximum yield also (Paris-Orly: 0.37 mm; Paris: 0.17 mm) and the annual yield is 5 times higher (Paris-Orly: 17.11 mm; Paris: 2.35 mm). This difference cannot be attributed to cloud coverage deviations that lead to a 30% difference nor the sky view factor (however crucial in many urban locations as stressed by [START_REF] Richards | Hardware scale modeling of summertime patterns of urban dew and surface moisture in Vancouver, BC, Canada[END_REF][START_REF] Richards | Urban and rural dewfall, surface moisture, and associated canopy-level air temperature and humidity measurements for Vancouver, Canada[END_REF]. It is rather the result of warmer and dryer downtown air [START_REF] Ye | Dew amounts and its correlations with meteorological factors in urban landscapes of Guangzhou, China[END_REF][START_REF] Muskała | Pollutant deposition via dew in urban and rural environment[END_REF] thanks to the so-called urban heat island (see Glossary of Meteorology, 2009). In Paris, the urban/rural gap is found between 1 and 6 K and its average for the 10 years 1990-1999 is 3.3 K [START_REF] Cantat | Paris urban heat island according to weather categories[END_REF]. The contrast is particularly noticeable at night. As a matter of fact, the nightly (21:00 -06:00) yearly mean is

pH measurements

The analytical results for dew and rain are shown in Tables 2 and3, respectively. Beside arithmetic means, median, minimum and maximum values, volume weighted means (VWM) are also given.

To calculate arithmetic and volume weighted means of the pH, the pH values is firstly converted to H + concentrations ([H + ] =10 -pH+3 (in mg/l). The calculated values are then converted back to pH units (pH = -log[H+]+3). pH arithmetic averages and VWM differ weakly for both dew and rain samples.

Dew pH variations are large, with values ranging between 5.3 and 10.5. The volume weighted mean was calculated to be 6.5. The median is at 6.7. Similar large variations are observed for rain pH that shows values close to dew's, ranging between 5.1 and 10.6 with a VWM of 6.1 and a median of 6.5. No seasonal variations are observed within the scatter of data, see Fig. 3.

No visible [H + ] dependence on dew and rain volume can be observed either. The differences between dew and rain pH values are small (Tables 23, Fig. 3), Paris rain being slightly more acidic than dew. This corresponds to the results found in some other urban areas as well as at humid sites [START_REF] Xu | Chemical Compositions of Dew and Scavenging of Particles in Changchun, China[END_REF].

Near-neutral pH values in dew and rain are caused by the neutralization of acidity and buffering by the alkaline elements of soil, dust and sea salt origin. This neutralization is confirmed by a strong correlation between the acidic ions (SO 4 2-and NO 3 -) and the major cations (Ca 2+ and Mg 2+ ) (see Tables 2-3 and section 3.7). 23).

Conductivity measurements

Dew electric conductivity EC (Tables 23) takes values in the study period between 26 μS/cm and 478 μS/cm. Volume weighted means, calculated by Eq. 5, is 124 µS/cm. Rain EC values extend from 2.7 μS/cm to 810 μS/cm with a volume weighted mean of 35 µS/cm. One notes that the dew EC mean value is rather large when compared to other studied urban locations in the world, meaning large total dissolved solids TDS [START_REF] Lekouch | Dew, fog, and rain as supplementary sources of water in south-western Morocco[END_REF][START_REF] Tomaszkiewicz | Dew as a Sustainable Non-Conventional Water Resource, A Critical Review[END_REF].

Total dissolved solids can be approximated by the formula TDS = k e EC, with TDS in mg/l and EC in µS/cm at 25°C (Atekwana et al., 2004). The correlation factor k e varies between 0.55 and 0.8. Taking the mean value k e ≈ 0.7 it comes for dew about (volume weighted) 87 mg/l. For rain TDS is about (volume weighted) 24 mg/l, that is, 3.5 times less than dew TDS.

The mean EC and TDS in dew are both higher than those in rain samples, reflecting thus higher total dissolved ions in dew.

Ion concentrations

Data quality assurance was performed by evaluating (see Tables 23) the percentage difference of the ionic balance BAL%=100×[(Σcation-Σanion)/(Σcation+Σanion)],

with

∑cations=[Na + ]+[K + ]+[Ca 2+ ]+[Mg 2+ ]+[ [H + ] ∑anions= [Cl -]+[SO 4 2- ]+[NO 3 - ]. (7)
Comparison is also made with the measured and calculated conductivities,

∆EC = 100×(EC c -EC)/(EC). ( 8
) EC c is the calculated conductivity according to EC c = 50.1[Na + ]+73.5[K]+53[Mg 2+ ]+59.5[Ca 2+ ]+349.7[H + ]+76.4[Cl -]+80.8[SO 4 2- ]+71.4[NO 3 - ].
The acceptability criterion is set as ≤ ± 20 % to comply with the requirements of the Global Atmosphere Watch Program for precipitation chemistry (World meteorological organization global atmosphere watch, 2004). As seen in Fig. 6, linear fits are observed for dew and rain data between Σcations and Σanions. In addition, linear fits are found between EC and EC c :

EC=0+1.017×EC c with r=0.99 (dew) and EC = 3.14 + 1.039×EC c with r=0.99 (rain).

A nearly equalized balance is highly valuable but is not a strong evidence for a true or complete analysis. Errors may compensate each other in data quality and even more in missing ionic species. The ions which unfortunately were not measured during our study (for example: NH 4+ and HCO 3 -) should contribute in the physico-chemical analysis. Ammonium is a fragile ion and the period of time between sampling and analysis could possibly be not short enough to avoid any oxidation reactions.

Several studies in Central Europe (e.g., [START_REF] Buijsman | Wet deposition of ammonium in Europe[END_REF][START_REF] Duffy | Environmental chemistry: A global perspective[END_REF]Duffy (2011), Galek et al. 2015b) or in India [START_REF] Yadav | Pollutant scavenging in dew water collected from an urban environment and related implications[END_REF] suggest that such ions should not be neglected. In urban environment, on average NH 4 + concentrations of 0.010-0.080 meq/l were observed in rain water compared to 0.002-0.004 meq/l at stations influenced to negligible extent by human activity. The special and temporal variation of pollutants in precipitation at 37 more or less rural stations in France was investigated from three different monitoring networks within more than 10 years (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) by [START_REF] Pascaud | Contrasted spatial and long-term trends in precipitation chemistry and deposition fluxes at rural stations in France[END_REF]. At several stations located in western and northern areas a decrease of NH 4 + concentration in atmospheric deposition was observed while the estimates of NH 3 emissions remained fairly stable. When considering all the sampling sites, the volume-weighted mean concentration of NH 4 + was 0.028 meq/l (ranging from 0.013 to 0.046 meq/1).

For dew water mean values from 0.05 meq/l (Alebić-Juretić and Sojat, 1998) and even 0.25 meq/l [START_REF] Polkowska | Chemical characterization of dew water collected in different geographic regions of Poland[END_REF] were found. [START_REF] Yadav | Pollutant scavenging in dew water collected from an urban environment and related implications[END_REF] found that in Delhi ammonium plays a dominant role in the acidity neutralization of dew and rain.

In our study the difference in the total concentrations of measured positive and negative charge balance is very low. One could thus speculate that ammonium plays a minor role as neutralizing component in rain and dew water in Paris as compared to the non-sea-salt base cations concentration, which contributes within about 40 % to the TIC. Tables 2 and 3 also summarizes the mean, maximum and minimal chemical compositions of dew and rain events in the period of study. Mean concentrations are volume weighted as for [H + ], see Eq. 5. The detectability threshold is 0.001 or 0.005 mg/l, depending on ions. Only data above the detectability threshold have been analyzed.

The minor ions (iron, copper, cadmium, manganese, lead, zinc, lead, nitite, brominate, fluorine) are in small quantity in both dew and rain water. In rain water, nitrite is quite low, with only 11 data above the detection level. As usual, nitrite concentration is somewhat higher for dew samples, due to its formation mainly by heterogeneous gas reactions occurring on wetted surfaces [START_REF] Alicke | OH formation by HONO photolysis during the BERLIOZ experiment[END_REF][START_REF] Acker | Concentrations of nitrous acid, nitric acid, nitrite and nitrate in the gas and aerosol phase at a site in the emission zone during ESCOMPTE 2001 experiment[END_REF][START_REF] Acker | Nitrite in dew, fog, cloud and rain water: an indicator for heterogeneous processes on surfaces[END_REF]. Lead, usually caused by the road traffic, exhibits a small value, corresponding to the removal of lead in car fuels.

The distribution of major ions in both rain and dew are similar, with (dew and rain) Ca 2+ > Na + > K + > Mg 2+ and (dew) SO4 2-> NO3 -> Cl -and (rain) SO4 2-≈ NO3 -> Cl -. However, ion concentrations in dew are about 4 times higher than in rain, in agreement with the calculation of TDS from the EC's as performed above in section 3.3 where a factor 3.5 is found. The dominant cation in both dew and rain is Ca 2+ . Its probable source is dust of the ground suspended in the low layers of the atmosphere but also the particles of the surrounding medium brought to the surface of the condenser by the wind. The main source of Ca 2+ is the abrasion of streets and sidewalks, weathering of plasters as well as building activities [START_REF] Polkowska | Chemical characterization of dew water collected in different geographic regions of Poland[END_REF]. Ions Na + and Mg 2+ are from marine origin, due to the strong oceanic influence in Paris (180 km from the ocean), as is also discussed below in section 3.6. The origin of potassium can be found in the presence of large agricultural areas around Paris.

Ions SO 4 2-and NO 3 -reveals pollution in dew and rain, coming from the local urban anthropogenic contributions. Sulfate, beyond the marine contribution (see below section 3.6), is of anthropogenic origin (combustion of sulfated fossil fuels), alike nitrates (road traffic).

Chloride is from marine origin (see below section 3.6).

It is interesting to note in Tables 2-3 that the chemical composition of Paris dew and rain urban water generally meets the WHO requirements. Although there is no requirements for pH (WHO pH revised, 2007), we have retained a current recommendation pH<9.5. Only very rare data exceeded the WHO requirements. For dew, one notices two NO 3 -data and one pH data. For rain, two pH data are out of the requirements. Even the lead concentration keeps below the maximum recommended.

Correlation anions-cations

The correlation between ions in dew and rain are presented in Table 4. The analysis of these correlations highlights the possible relations between two or several random variables. A measurement of this correlation is obtained by the calculation of their coefficient of correlation, r (see section 3), which is a measure of the linear correlation between two variables X and Y. According to Chok (2008), this coefficient can be successfully used for analysis of non-normally distributed data. Then Table 4 data indeed represents a measurement of the goodness of linear correlation. The coefficient of correlation lies between -1 and 1:

𝑟 = ∑ (𝑥 𝑖 -𝑥̅ )•(𝑦 𝑖 -𝑦 �) 𝑁 𝑖=1 �∑ (𝑥 𝑖 -𝑥̅ ) 2 𝑁 𝑖=1 •�∑ (𝑦 𝑖 -𝑦 �) 2 𝑁 𝑖=1 (9)
The results are listed in Table 4.

Table 4. Correlation coefficients r for dew (clear portion) and rain (shaded portion). Boldunderlined figures correspond to correlation higher than 0.9.

Concerning rain, four important correlations (> 0.9) were found. The correlation between Na + and Cl -corresponds to marine origin. The correlation between Ca 2+ and SO 4 2-is of continental and anthropogenic origin. The high correlations of SO 4 2-in rain with Na + (0.91) and Cl -(0.90) are due to long range transport of air masses bringing rain often influenced by maritime regions. The correlation of SO 4 2-in dew with Na + (0.41) and Cl -(0.87) is significantly lower due to more local (anthropogenic) pollutant influence.

Three correlations are found important in dew (> 0.9). A strong correlation is between Na + and Cl -and Mg 2+ and Cl -, indicating a marine origin. A strong indirect correlation is also found between Na + and Mg 2+ , presumably of marine origin. As an example, one has plotted in

Marine contribution

In order to measure the marine contribution of dew and rain ions, the sea salt fraction (SSF) and non-sea-salt fraction (NSSF) of marine salts were calculated. The totality of Na+ considered of sea origin is the principal reference of the present calculation [START_REF] Keene | Sea salt corrections and interpretations of constituent ratios in marine precipitation[END_REF][START_REF] Kulshrestha | Investigation into atmospheric deposition through precipitation studies at New Delhi (India)[END_REF]Al Obaidy and Himanshu Joshi, 2006) as well as the ionic concentration of sea water given by [START_REF] Riley | Introduction to Marine Chemistry[END_REF]. The parameters SSF and NSSF are thus calculated from the following equations:

%(𝑆𝑆𝐹) 𝑋 = 100×(𝑁𝑎)� 𝑋 𝑠𝑒𝑎 𝑁𝑎 𝑠𝑒𝑎 � 𝑋 (10) %(𝑁𝑆𝑆𝐹) 𝑋 = 100 -%(𝑆𝑆𝐹)𝑋 (11)
Here X is the concentration of ions as measured in dew and rain water, (X sea ) is the concentration of the sea water ions, and (Na sea ) the concentration of the sodium ion taken as a Here X is the species responsible for neutralization (Table 6). The high concentration of Ca²+ ions compared to the other ions in dew and rainwater implies that it is the more neutralizing cation, showing the strongest neutralization factor, followed by Mg²+ and K + . The NF for Ca 2+ in dew and rain are similar. The physico-chemical analysis of 34 dew and 77 rain samples show that dew and rain pH are nearly neutral with however rain being slightly more acidic than dew. This is in agreement with its lower neutralization factor. The mean dew total ionic content (TIC 1.8 meq/l) and EC value (124 µS.cm -1 ) are about four times that of rain (0.45 meq/l; 35 µS.cm -1 ), meaning that total dissolved solids in dew is nearly four times that in rain. Chemical composition of dew and rain water keeps similar trends. It reveals the importance of Ca² + and Na + cations and SO 4 2-, NO 3 -et Cl -anions. Sulfate and nitrate are the most acidifying components, calcium the most neutralizing constituent with ratio of total acidity/ total alkalinity comparable for dew and rain (≈ 0.9). The calculation of the sea salt fraction shows that only Cl -and Mg 2+ are influenced by the distance from the sea. Ca² + , SO 4 2-, NO 3 -and NO 2 -are of anthropogenic origin (domestic and heating fuels, road traffic, road and building carbonate particles).

Ions

It is interesting to note that, although being collected in strongly urbanized environment, both dew and rain water quite generally respect the requirements of WHO for drinking water. This result is encouraging as it indicates that rain and dew water could be exploited at a certain level.

Fig. 1 .

 1 Fig. 1. Condensers (A, A', B) and meteo station (M) on the terrace.

  emissivities and varies as (T d +273.15) 4 according to the Stefan law. The condenser emissivity is assumed to be close to unity; the sky emissivity is estimated according to[START_REF] Berger | Clear sky radiation as a function of altitude[END_REF]; it is a function of site elevation H (km), dew point temperature T d (in °C) and cloud coverage (N, in oktas) leading to:

  higher temperatures than in Paris neighborhood. As there are no recorded dew data available, no comparison can be made with other years. However, a detailed discussion with comparison of calculated dew yields in Paris and nearby at Paris-Orly airport is given below at the end of the section.Concerning rain, important rain storms were observed in August. Rain data can be compared to data collected every 3h. at the airport meteo station Paris-Orly, located within 12km from the dew measurement site. The number of rainy days in Paris (146) is more important than in Paris-Orly (107) because the detection threshold in Paris-Orly (0.2 mm) is higher than in Paris (0.004 mm). However, the maximum daily rain amounts are close (Paris: 4.1 mm; Paris-Orly: 5.02 mm) as the yearly sum (Paris: 593 mm; Paris-Orly: 547 mm), the somewhat higher yield in Paris being presumably due to the recording of smaller event, corresponding to the smaller detection level. The values found are less but comparable to a 30 years average (1981-2010, see meteo-France, 2016): 637.4 mm annual rainfall, 111.1 rainy days. Note that fall 2011 was one of the hottest falls since meteorological measurements are carried out in Paris.

  in downtown Paris and 10.3 K in Paris-Orly, whereas the dew point temperature is only slightly depressed with K in downtown Paris and 3.15 K in Paris-Orly thus exhibits a large dissimilarity, which explains the difference in dew yields.

Fig. 2 .

 2 Fig. 2. One year evolution of (a) dew and (b) rain yield (in lm -2 or mm). (c): Summation of daily dew from experimental data and from calculated data using Eq. 4 and cloud coverage N Orly (in oktas) as measured in Paris-Orly. Another calculation where the cloud cover N Orly -1 is used instead of N is shown for sake of comparison (see text). (d): Summation of the rain daily data shown in (b).

Figure 4

 4 Figure 4 reports the evolution for both dew and rain EC. Seasonal variations (if any) are less than the scatter of data. In Fig. 5 is shown the sample volume dependence of EC. Despite the

Fig. 5 .

 5 Fig. 5. Variation of electrical conductivity EC with nightly condensed volume h (mm/day). The lines are fits to EC=αh -1 (see text). (a) dew; (b) rain (two data are out of axes). Insets: log-log plots showing dilution dependence (line with slope -1, see text).

Fig. 6 .

 6 Fig. 6. Electrical neutrality. Sum of major anions (Na + , K + , Ca + , Mg 2+ , H + ) versus sum of major cations (Cl -, SO 2-NO 3 -) on a log-log plot. Lines are fits to Σcations = β Σanions, with β a proportionality constant expected to be unity. (a): Dew. The interrupted line is a fit to all data (β =0.80±0.04, uncertainty is one standard deviation, r = 0.91). The dotted line corresponds to a fit where 5 data have been removed because of very low collected volumes. It gives β =0.88 ±0.02, r = 0.97. (b): rain. The interrupted line is a fit to all data (β =1.05±0.02, uncertainty is one standard deviation, r = 0.98).

Fig. 7

 7 Fig.7 the correlation between Cl -and Na + , for both dew and rain. A linear fit [Cl -]=γ [Na + ] give (dew) γ = 1.15 ±0.06, r=0.92 and (rain) γ = 0.87 ±0.01, r=0.99. (Uncertainty is one standard deviation).

Fig. 7 .

 7 Fig. 7. Correlation between Cl -and Na + .(a): Dew, (b): Rain. Lines are fits (see text).

  water. All concentrations are expressed in meq/l. The results are presented in

  It is thus interesting to plot (Fig.8) the sum of acidifying anions (to the most alkaline cations (Ca 2+ +Mg 2+ ) in meq/l. Linear fit of the data to [SO 4 +NO 3 ] = δ [Ca+Mg] (in meq/l) gives (dew) δ = (0.64 ± 0.05), r = 0.75 and (rain) δ = 0.73±0.02, r = 0.94. (Uncertainties: one standard deviation). The ratio of total alkalinity/total acidity (Ca 2 + + Mg 2+ ) (mean concentrations in meq/l) is lower than 1 for both dew and rain water, with nearly the same values (dew: 0.89; rain: 0.87), indicating that water alkalinity is comparable for dew and rain.

Fig. 8 .

 8 Fig. 8. Acidification process: Relationship between the sum of alkaline cations (Ca 2+ +Mg 2+ ) and acidifying anions (SO 4 2-+NO 3 -). Full circles: dew; open squares: rain. The full lines are linear fits (see text). Full line: dew; interrupted line: rain.

  one year measurement (01/04/2011 -31/03/2012) of dew-rain frequency and volume in the historical center of Paris shows relatively small number of dew events (63 measured; 74 calculated from meteo data) compared to the estimate at a close location outside the city (Paris-Orly: 146 events). The low dew yields in Paris-center (3.5 mm measured, 2.5 mm estimated) when compared to Paris-Orly (estimated 17 mm) is not due to a limited sky factor downtown Paris but rather to the presence of heat island that increases air temperature and lowers relative humidity. Rain statistics downtown Paris and Paris-Orly were found comparable (146 events for 590 mm downtown, 107 events for 547 mm in Paris-Orly).

Table 1 .

 1 Dew and rain characteristics at Paris (01/04/2011 -31/03/2012).

Table 2 :

 2 Dew ion concentration statistics. N is the number of analyzed samples. When minimum concentration is zero, detection threshold is indicated as a substitute. Summed (total) volume is 2185ml).

	Measurement	Dew	Dew	Dew calc.	Rain	Rain
	(mm)	meas.	calc.	Paris-Orly	meas.	meas.
		Paris	Paris		Paris	Paris-
						Orly
	Nb. daily events	63	74	196	146	107
	h min (mm/day)	0.003 1.6 ×10 -4	1.6 ×10 -4	0.0046	0.2
	h max (mm/day)	0.19	0.17	0.37	86	29
	h mean (mm/day)	0.055	0.032	0.087	4.1	5.02
	Yearly sum h (mm)	3.48	2.35	17.11	593	547

Table 3 .

 3 Rain ion concentration statistics. N is the number of analyzed samples. When minimum concentration is zero, detection threshold is indicated as a substitute. Summed (total) volume is 327185 ml.

					vol. weighted	arithm.				WHO
		Rain	N	mean (VWM)	mean	min. median max.	mean
	Vol (ml)	77	4249	4249	5	850	85600
	H + (mM/l×10 4 ) 77	8.42	7.76	0.00025 3.2	89
	Cations (mg/l)
		Na +		77	1.1	3.8	0.1	1.4	81.5	200
	Major	K + Ca 2+ Mg 2+	77 77 77	0.68 3.05 0.17	1.0 4.2 0.4	0.1 0.2 0.005	0.8 2 0.02	16 43 0.17
		Fe 2+		77	0.03	0.03	0.005	0.01	0.18
		Cu 2+	77	0.0024	0.01	0.002 0.005	0.04	2
	Minor	Cd 2+ Mn 2+	77 77	0.001 0.0008	0.001 0.01	0.001 0.001 0.001 0.003 0.005 0.03	3 0.4
		Pb 2+		77	0.00022	0.007	0.005 0.008 0.008	0.01
		Zn 2+	77	0.035	0.06	0.005	0.04	0.49
	Anions (mg/l)
	Major	Cl -SO 4 NO 3 2--	77 77 77	2.6 3.7 4.1	7 5.4 4.5	0 0.2 0.1	2.5 3 2	140 65 38	250 250 50
	Minor	NO 2 Br -F -	-	45 45 45	0.06 0.03 0.06	0.09 0.016 0.02	0.01 0.01 0.01	0.04 0.03 0.04	0.33 0.08 0.33	50 1.5
	pH			77	6,1	6.1	5.1	6.5 c	10.6	9.5
	EC (25°C,
	µS/cm)		77	35	59.6	2.7	40	810	2500
	EC calc		77	29	57	2	35	750
	∑cations (meq/l) 77	0.23	0.45	0.02	0.28	5.8
	∑anions (meq/l) 77	0.22	0.38	0.01	0.25	5.6
	TIC (meq/l)	77	0.45	0.86	0.03	0.5

Fig. 3. Dew pH (full circles) and rain (open squares) evolution. The lines are volume weighted mean values (VWM ) and median values (from Tables

Table 5 .

 5 For both dew and rain water, the marine contribution for Mg 2+ and Cl -is clear, corresponding to sea salt particles in the atmosphere. The outsized values of chloride contribution in dew

	water (SSF=120%) and, to a lesser extent, magnesium (SSF=104%) are presumably due to the
	uncertainties in concentrations. Large NSSF (nearly 100%) values for ions SO 4	2-, NO 3	-Ca² + ,
	K + in dew and rain water indicate anthropogenic sources for these ions.

Table 5 .

 5 Sea salt water contribution (SSF) and non sea salt water contribution (NSSF). In bold

		Ions		Dew	Rain
						SSF (%)	NSSF (%)	SSF (%)	NSSF (%)
		Mg 2+	104	-4	78	22
		K	+			13	87	6	94
		Ca 2+	2	98	1	99
		Cl -		120	-20	77	23
		SO 4	2-	6	94	6	94
		NO 3	-	0	100	0	100
						are indicated the largest contributions.
	3.7. Neutralisation factor
	The acid ions (SO 4	2-, NO 3	-) contained in dew and rain water react with the base components
	Ca² + , Mg² + , K + . During this reaction, acidity disappears and water becomes nearly neutral. In
	order to determine the part played by the cations to neutralize the sulfuric and nitric acids, a
	factor of neutralization can be calculated according to:
						𝑁𝐹 𝑥 =	[𝑋] -+𝑆𝑂 4 [𝑁𝑂 3 2-]	(12)

Table 6 .

 6 Neutralization factor (NF).

		Dew NF	Rain NF
	Ca 2+	1.0	1.1
	Mg 2+	0.09	0.1
	K +	0.07	0.1

Acknowledgments

We gratefully thank C. Ohayon from Laboratoire d'Hydrologie d'Aquitaine for chemical analyses and M. Amal for help in analyzing the chemistry data.


Hussein Zahr
email: hussein.zahr@ensam.eu

Franck Scuiller
email: franck.scuiller@ecole-navale.fr

Eric Semail
email: eric.semail@ensam.eu

Five-phase SPM machine with electronic pole changing effect for marine propulsion

Keywords: Multi-phase machine, permanent magnets, magnet losses, iron losses, fractional-slot winding I. NOMENCLATURE SPM Surface-mounted Permanent Magnet FEA Finite

In this paper, the possibility of designing a fivephase Surface-mounted Permanent Magnet (SPM) machine with 20 slots and 8 poles for a low power marine propulsion system is examined. Due to its particular winding and surface magnet design, the machine inherently offers an electronic pole changing effect from 3×4 pole pairs at low speed to 4 pole pairs at high speed. At high speed, in the constant power range, according to Finite Element Analysis, the Maximum Torque Per Ampere strategy appears not to be the right solution to minimize the whole machine losses (copper, iron and magnets). In particular, a strategy that favors the 4-pole rotating field at high speed allows to mitigate the magnet losses, thus limiting the risk of magnet overheating.

II. INTRODUCTION

Multi-phase motors are widely used in electrical marine propulsion for reasons such as reliability, smooth torque and distribution of power [START_REF] Levi | Multiphase electric machines for variable-speed applications[END_REF]. For low power propulsion system (less than 10kW), the power partition constraint results from the low DC voltage (less than 60V) that supplies the drive. Hence increasing the phase number allows to limit the rating of the power electronic components. In addition, compactness objective can be more easily achieved if the phase number is considered as a design parameter. For instance, with five-phase machine, third harmonic current injection can be performed to boost the torque [START_REF] Semail | Right harmonic spectrum for the back-electromotive force of an n-phase synchronous motor[END_REF], [START_REF] Wang | Torque improvement of fivephase surface-mounted permanent magnet machine using third-order harmonic[END_REF]. Regarding the rotor, Permanent Magnet (PM) structure contribute to enhance the power density. In case of Surface-mounted Permanent Magnet (SPM) rotor, the ripple torque mitigation is facilitated. Furthermore, with five-phase SPM machine, third harmonic current injection can be used to eliminate the pulsating torque [START_REF] Scuiller | Third harmonic current injection to reduce the pulsating torque of a five-phase spm machine[END_REF]. If fractional-slot windings facilitate the reduction of cogging torque for SPM machine [START_REF] Zhu | Influence of design parameters on cogging torque in permanent magnet machines[END_REF], they also generate magnetomotive force harmonics that could result in excessive magnet losses. Machine with 0.5 slots per phase and per pole (𝑠 𝑝𝑝 = 0.5) are known to limit this effect [START_REF] Aslan | General analytical model of magnet average eddy-current volume losses for comparison of multiphase pm machines with concentrated winding[END_REF]. In addition, the slot filling can also be improved with this solution [START_REF] El-Refaie | Fractional-slot concentrated-windings synchronous permanent magnet machines: Opportunities and challenges[END_REF]. Therefore the machine here considered is a five-phase machine with 20 slots and 8 poles (20-8-5 configuration) for a marine propeller.

The 3-phase counterpart of this 20-8-5 machine has 8 poles and 12 slots (12-8-3 configuration). With reference to this 12-8-3 machine, the benefits of the 20-8-5 machine are examined in [START_REF] Zahr | Five-phase version of 12slots/8poles three-phase synchronous machine for marinepropulsion[END_REF] for the same design specifications: rated torque, power and external diameters are identical. With numerical computations of the two machines, this study shows that the 5-phase configuration allows a significant reduction of the magnet losses. In addition, the 5-phase machine facilitates the reduction of the ripple torques (cogging and pulsating) that is of critical importance at low speed.

This paper focuses on another property of the 20-8-5 machine. Due to its particular winding distribution, this machine inherently owns 3×4 pairs of pole and 4 pairs of pole. Hence an electronic pole changing effect can be obtained if the machine is designed to operate at low speed with 3×4 pole pairs or at high speed with 4 pole pairs. More generally the appropriate polarity has to be selected regarding the load demand in transient or steady state, taking into account the inverter rating and the efficiency or torque quality requirements. Pole changing methods by winding switching are well known for induction machine [START_REF] Osama | A new inverter control scheme for induction motor drives requiring wide speed range[END_REF]. For multiphase induction machine, pole phase modulation strategy can be applied [START_REF] Gautam | Variable speed multiphase induction machine using pole phase modulation principle[END_REF]. In [START_REF] Sadeghi | Wide operational speed range of five-phase permanent magnet machines by using different stator winding configurations[END_REF], the speed range of a five-phase PM machine is extended by switching between different stator configurations. A similar procedure is achieved in [START_REF] Nguyen | Different virtual stator winding configurations of open-end winding five-phase pm machines for wide speed range without flux weakening operation[END_REF] but with electronic switching. In this paper, the pole changing effect is electronically ensured by the inverter, depending of the levels of first and third harmonics of current.

The paper is divided into two parts. In the first part, the 20-8-5 machine design is introduced. The magnet layer to obtain the double polarity property is introduced. To master the sizing, numerical field calculations are reported and the torque/speed characteristic is estimated. The second part focuses on the control strategy in the constant power range that corresponds to the steady state operation of the propeller. Two strategies are described and compared regarding the copper, magnet and iron losses (estimated with FEA).

III. MACHINE DESIGN

A. Five-phase machine modeling

If the magnetic saturation and the demagnetization issue are not considered, it can be shown that a star-connected five-phase SPM machine behaves as two two-phase virtual machines that are magnetically independent but electrically and mechanically coupled [START_REF] Semail | Vectorial formalism for analysis and design of polyphase synchronous machines[END_REF]. Furthermore, as the rotor saliency can be neglected with SPM machines, the space harmonics are distributed among the two virtual machines: the virtual machine sensitive to the fundamental is called Main Machine (MM) whereas the other sensitive to the third harmonic is called Secondary Machine (SM). Actually the virtual machine is a physical reading of the mathematical subspace build on the linear application that describes the phase-to-phase magnetic couplings: this two-dimension subspace is usually represented with 𝛼𝛽-axis circuit in stationary frame or with 𝑑𝑞-axis circuit in rotating frame. As there is no saliency effect, no distinction has to be made between d-axis and q-axis inductance.

B. Design specifications

The machine is intended to be integrated in a pod for an electrical outboard. The propeller is driven by the electrical machine with a mechanical gear that reduces the electrical rotating speed by five. The main machine parameters are listed in table I. The electromagnetic circuit is sketched out in Fig. 1 (over a pole pair): the five-phase winding distribution with the corresponding first and third harmonic winding factors (𝑘 𝑤,1 and 𝑘 𝑤,3 ) can be observed. The magnet layer shape is arranged to make the two virtual machines enable to produce the same torque level. To reach this goal, the rotor pole consists of two radially magnetized magnets, each magnet covering one third of the pole arc, as illustrated by Fig. 1.

Winding Factor k w,1 =0.5878 k w,3 =0.9811

C. Field analysis

In this subsection, the electromagnetic behaviour of the machine is estimated with FEA software FEMM [START_REF] Meeker | Finite element method magnetics, version 4.2, users manual[END_REF] under magnetostatic hypothesis. In addition, saturation effects are not taken into account (linear assumption for the materials).

Fig. 2 shows the no-load back-emf waveform and spectrum. The double polarity of the machine can be inferred. The spectrum confirms that first and third harmonic terms are of the same order. The inductance values 𝐿 1 for the MM and 𝐿 3 for the SM are calculated by loading the machine with the rated current:

• 𝐿 1 = 3.1𝑚𝐻 for the MM (4 pole pairs)

• 𝐿 3 = 4.0𝑚𝐻 for the SM (3 × 4 pole pairs).

Fig. 3 shows the flux lines when loading the machine with fundamental rated current (for a given rotor position). The field intensity values allow to control the right sizing of the electromagnetic circuit: in the yokes and in the stator teeth, the field intensity is lower than 1.4T. Fig. 4 focuses on the cogging torque estimation. As previous, the cogging torque is negligible: its amplitude is less than 0.25Nm, that is less than 1% the rated torque.

Density Plot: |B|, Tesla 1.900e+000 : >2.000e+000 1.800e+000 : 1.900e+000 1.700e+000 : 1.800e+000 1.600e+000 : 1.700e+000 1.500e+000 : 1.600e+000 1.400e+000 : 1.500e+000 1.300e+000 : 1.400e+000 1.200e+000 : 1.300e+000 1.100e+000 : 1.200e+000 1.000e+000 : 1.100e+000 9.000e-001 : 1.000e+000 8.000e-001 : 9.000e-001 7.000e-001 : 8.000e-001 6.000e-001 : 7.000e-001 5.000e-001 : 6.000e-001 4.000e-001 : 5.000e-001 3.000e-001 : 4.000e-001 2.000e-001 : 3.000e-001 1.000e-001 : 2.000e-001 <0.000e+000 : 1.000e-001

D. Maximum reachable torque for finite Volt-Ampere rating

In this subsection, the torque/speed characteristic of the machine is calculated: for a given mechanical speed Ω 𝑚 (i.e. for a given electrical speed 𝜔), taking into account the maximum DC voltage and the maximum copper losses (driven by base current 𝐼 𝑏 ), the goal consists in finding the MM and SM current distribution that maximizes the electromagnetic torque [START_REF] Scuiller | Maximum reachable torque, power and speed for five-phase spm machine with low armature reaction[END_REF]. To solve this problem is equivalent to find the optimal d-axis and q-axis references for each virtual machine. The Maximum Torque Per Ampere (MTPA) for a given speed is then obtained. The optimization variable is defined as follows:

𝑧 = [ 𝐼 1 𝜃 1 𝐼 3 𝜃 3 ] 𝑇 (1)
The optimization variable is lower and upper bounded according to the following relations:

𝑍 𝑙𝑜𝑤 = ⎡ ⎢ ⎢ ⎣ 0 -𝜋 0 -𝜋 ⎤ ⎥ ⎥ ⎦ ≤ 𝑧 ≤ ⎡ ⎢ ⎢ ⎣ 𝐼 𝑏 𝜋 𝐼 𝑏 𝜋 ⎤ ⎥ ⎥ ⎦ = 𝑍 𝑢𝑝 ( 2
)
The objective is to maximize the electromagnetic torque. This goal is expressed in the following relation where 𝑇 is the average electromagnetic torque:

𝑧 * = 𝑎𝑟𝑔𝑚𝑖𝑛(-𝑇 (𝑧)) (3)
The average electromagnetic torque is the sum of the torque of each virtual machine. The MM torque (due to the fundamental of current) is denoted 𝑇 1 and the SM torque (due to the third harmonic of current) is denoted 𝑇 3 :

𝑇 1 = 5𝜖 1 𝐼 1 cos 𝜃 1 ( 4
)
𝑇 3 = 5𝜖 3 𝐼 3 cos 𝜃 3 (5)
The five-phase machine torque is then expressed as follows:

𝑇 = 5𝜖 1 𝐼 1 cos 𝜃 1 + 5𝜖 3 𝐼 3 cos 𝜃 3 (6)
Equation ( 6) is used in relation (3) to track the optimal current repartition 𝑧 * . The non linear constraint regarding the peak phase voltage is written in the following relation:

𝑓 𝑉 (𝑧) = max {𝑣(𝑝Ω 𝑚 𝑡, 𝑧), 𝑝Ω 𝑚 𝑡 ∈ [0..2𝜋]} -𝑉 𝑝𝑒𝑎𝑘 (7)
In equation [START_REF] El-Refaie | Fractional-slot concentrated-windings synchronous permanent magnet machines: Opportunities and challenges[END_REF], 𝑣(𝑝Ω 𝑚 𝑡, 𝑧) is the machine phase-to-neutral voltage for the current determined by 𝑧 at Ω 𝑚 speed. It should be noted that the considered voltage contains all the harmonics (i.e. not only the first and the third harmonics). The maximum allowable peak voltage is chosen to be half the bus voltage (thus meaning that linear modulation operation is targeted):

𝑉 𝑝𝑒𝑎𝑘 = 𝑉 𝑑𝑐 2 (8)
The constraint relative to the maximum RMS current is defined by the following equation:

𝑓 𝐼 (𝑧) = 𝑧(1) 2 + 𝑧(3) 2 -𝐼 2 𝑏 ( 9
)
The following expression summarizes the optimization problem under consideration:

𝑧 * = 𝑎𝑟𝑔𝑚𝑖𝑛(-𝑇 (𝑧)) with ⎧ ⎨ ⎩ 𝑍 𝑙𝑜𝑤 ≤ 𝑧 ≤ 𝑍 𝑢𝑝 𝑓 𝑉 (𝑧) ≤ 0 𝑓 𝐼 (𝑧) ≤ 0 (10)
Fig. 5 shows the resulting torque/speed characteristic corresponding to the resolution of optimization problem [START_REF] Gautam | Variable speed multiphase induction machine using pole phase modulation principle[END_REF]. It can be observed that the rated torque (about 34Nm) is obtained by using the virtual MM and the virtual SM in the same time, thus meaning that the five-phase machine here considered is designed to operate with first and third harmonic of current. In addition, the electronic pole changing effect is obtained since, at low speed, the MM (3 × 4 poles) mainly contributes to the torque whereas, at high speed, the SM (4 poles) torque becomes higher. The isopower line (7.7kW) drawn in fig. 5 allows to determine the constant power range that is between 2250rpm and 4500rpm (as specified in table I).

IV. CONTROL STRATEGIES IN THE CONSTANT POWER RANGE

This section focuses on the machine operation in the CPR. At low speed (below 2250rpm), the machine is supposed being in transient states and the MTPA strategy previously introduced is performed. At high speeds (between 2250rpm and 4500rpm), the constant power control is used: in this mode, since the torque is not maximized, the constant power can be obtained with different current distributions, depending on the considered objective. Therefore, two objectives will be examined: one that aims to reduce the copper losses and another that aims to maximize the MM torque contribution. These two strategies will be compared regarding the machine losses with FEA.

A. Constant Power Control with minimizing copper losses

For the control here described, the goal consists in minimizing the copper losses for a given electromagnetic power accounting the maximum allowable peak voltage. For a given torque, as the copper losses are minimized, this strategy is actually a MTPA one and is called CPR-MTPA in the following. The optimization problem can be written as follows:

𝑧 * = 𝑎𝑟𝑔𝑚𝑖𝑛(𝑧(1) 2 + 𝑧(3) 2 ) with ⎧ ⎨ ⎩ 𝑍 𝑙𝑜𝑤 ≤ 𝑧 ≤ 𝑍 𝑢𝑝 𝑓 𝑉 (𝑧) ≤ 0 𝑓 𝑇 (𝑧) = 0 (11)
In relation [START_REF] Sadeghi | Wide operational speed range of five-phase permanent magnet machines by using different stator winding configurations[END_REF], 𝑓 𝑇 (𝑧) is the constraint relative to the torque, simply obtained by dividing the required constant electromagnetic power 𝑃 𝑒𝑚 by mechanical speed Ω 𝑚 :

𝑓 𝑇 (𝑧) = 𝑇 (𝑧) - 𝑃 𝑒𝑚 Ω 𝑚 (12)
The speeds where the CPR can be achieved are deduced from the maximum torque/speed characteristic estimated in the previous subsection (2250rpm -4500rpm). Fig. 6a shows the obtained torque/speed characteristics: at low speed (below 2250rpm), the reported characteristic is the one according to the MTPA strategy [START_REF] Gautam | Variable speed multiphase induction machine using pole phase modulation principle[END_REF] whereas, at high speed, the reported characteristic is the one corresponding to the resolution of the CPR problem [START_REF] Sadeghi | Wide operational speed range of five-phase permanent magnet machines by using different stator winding configurations[END_REF] introduced in this subsection. Again the torque repartition between the two virtual machines confirms the electronic pole changing effect: at low speed, the SM is mainly used whereas, at high speed, the MM becomes predominant. Fig. 6b gives the corresponding power/speed characteristics. As specified, the CPR is actually obtained between 2250rpm and 4500rpm and the electronic pole changing effect already observed when, looking at Fig. 6a, is confirmed.

B. Constant Power Control with favoring Main Machine

In order to reinforce the pole changing effect at high speed, the constant power control (between 2250rpm and 4500rpm) is calculated so that the Main Machine torque contribution is maximized (always accounting the maximum allowable peak voltage). With this strategy, a limitation of the magnet eddy current losses is expected because the Main Machine owns 0.5 slot per pole and per phase [START_REF] Aslan | General analytical model of magnet average eddy-current volume losses for comparison of multiphase pm machines with concentrated winding[END_REF]. This strategy is called CPR-h1 and is obtained by solving the following optimization problem:

𝑧 * = 𝑎𝑟𝑔𝑚𝑖𝑛(-𝑇 1 (𝑧)) with ⎧ ⎨ ⎩ 𝑍 𝑙𝑜𝑤 ≤ 𝑧 ≤ 𝑍 𝑢𝑝 𝑓 𝑉 (𝑧) ≤ 0 𝑓 𝑇 (𝑧) = 0 (13)
In [START_REF] Semail | Vectorial formalism for analysis and design of polyphase synchronous machines[END_REF], 𝑇 1 denotes the torque produced by the Main Machine defined by (4). Fig. 7a and Fig. 7b show respectively the resulting torque/speed and power/speed characteristics when solving optimization problem (13) at high speed (at low speed, below 2250rpm, the MTPA characteristic is given). As aimed for, the torque produced by the Main Machine is maximized in the Constant Power Range. One can observe that, between 3200 and 4200rpm, the Secondary Machine operates in generator mode to facilitate the Main Machine power increase.

C. Losses analysis

Copper, magnet and iron (stator and rotor) losses are now considered with FE software Ansys Maxwell. Magnet and iron losses are estimated from the time variation of the flux density. The method to calculate the hysteresis, eddy current and excess losses is close to the one detailed in [START_REF] Lin | A dynamic core loss model for soft ferromagnetic and power ferrite materials in transient finite element analysis[END_REF]. For speed belonging to the CPR, the five-phase machine is simulated for the optimized currents corresponding to the CPR-MTPA and CPR-h1 strategies.

Fig. 8a and Fig. 8b give the copper, magnet and iron losses according to the speed for the CPR-MTPA and the CPR-h1 strategies respectively. With the CPR-MTPA strategy, as aimed for, the copper losses are minimized but, in Fig. 8a, it can be observed that copper losses do not represent the major losses. This trend is all the truer as the speed increases. Referring to the CPR-MTPA strategy that allows the lowest copper losses, the CPR-h1 strategy allows to reduce the whole losses up to 3200rpm as shown by Fig. 8b. This advantage is based on the significant decrease of the magnet losses. This result complies with the analysis carried out in [START_REF] Zahr | Five-phase version of 12slots/8poles three-phase synchronous machine for marinepropulsion[END_REF] where it is shown that the asynchronous space harmonics due the third harmonic generate eddy currents in the magnet layer. For SPM machine, magnet losses mitigation is critical because the magnet layer can not be cooled as easily as the stator winding. In steady state, the magnet losses should be carefully controlled to prevent from demagnetization due to overheating.

Fig. 9 gives another insight of the possible losses reduction with the CPR-h1 strategy: the efficiency versus speed for the Finally, for the SPM 20-8-5 machine, the MTPA strategy is not the right solution to maximize the efficiency and to limit the heating of the magnets.

V. CONCLUSION

In this paper, the estimations of the 20-8-5 machine performances are obtained without considering magnetic saturation and demagnetization issue. With a particular but quite simple design of the magnet layer, the 20-8-5 machine inherently owns 3×4 pole pairs and 4 pole pairs. With a single stator and single rotor structure, a magnetic gear behavior is then obtained by controlling two rotating fields. The torque/speed characteristic calculated by considering the current and voltage limitations imposed by the inverter confirms the electronic pole changing effect: at low speed, the SM (3×4 poles) mainly contributes to the torque whereas, at high speed, the MM (4 poles) torque becomes higher. As the propeller is specified to operate in the constant power range at steady state (between 2250rpm and 4500rpm), the machine losses are numerically estimated for two CPR control strategies: the CPR-MTPA that minimizes the copper losses and the CPR-h1 that favors the MM torque contribution. A better efficiency is obtained with CPR-h1 control up to 3200rpm, with a significant reduction of the magnet losses for the whole speed range. Finally, for the 20-8-5 machine here considered, the existence of an optimal Maximum Torque Per Losses (MTPL) strategy is then demonstrated and have to be explored in further studies.

Fig. 1 .

 1 Fig. 1. Electromagnetic circuit of the 5-phase machine

Fig. 2 .

 2 Fig. 2. No load back-emf at 2250rpm mechanical speed (FEA)

Fig. 3 .Fig. 4 .

 34 Fig. 3. Flux density (for fundamental rated current)

Fig. 5 .

 5 Fig. 5. Maximum reachable torque for the five-phase machine

Fig. 6 .

 6 Fig. 6. Torque and power with CPR-MTPA strategy

Fig. 7 .

 7 Fig. 7. Torque and power with CPR-h1 strategy

Fig. 8 .Fig. 9 .

 89 Fig. 8. Losses according to the speed in the CPR