L Cano-Salinas

X Sourd

K Moussaoui

S Le Roux

M Salem

A Hor

R Zitoune

Effect of process parameters of Plain Water Jet on the cleaning quality, surface and material integrity of Inconel 718 milled by Abrasive Water Jet

Keywords: EDS Energy-dispersive spectroscopy, f Traverse speed, IN718 Inconel 718, P Pressure, PWJ Plain, SE Secondary electron, SEM Scanning electron microscopy, SOD Stand-off distance, STD Step-over distance, Str Texture-aspect ratio, XRD X-ray diffraction

Abrasive Water Jet (AWJ) is considered as a promising milling method for difficult-to-machine aeronautical materials as Inconel alloy 718 (IN718). However, grit embedment during AWJ is known as a detrimental effect on certain applications as aircraft repair, where surface condition may play an important role on additive material technologies. To overcome this problem, Plain Water Jet (PWJ) has been used in the present study as cleaning process and demonstrated to be an effective method to remove grit particles from the surface with marginal alterations of surface state. In this paper, firstly the influence of AWJ process parameters on abrasive embedment and surface texture on IN718 specimens milled by AWJ were addressed. Then these surfaces were subjected to PWJ cleaning process and were extensively characterized in terms of grit embedment, surface texture and roughness, erosion depth, microhardness and residual stresses. Before cleaning, the milled surfaces presented a grit embedment level varying between 7 % and 14 %. One setting condition was selected for performing PWJ cleaning which reduced the grit level up to a quarter of the initial total surface area (less than 4 % in all cleaned surfaces) without relatively modifying neither the surface texture nor the erosion depth. Comparable microhardness gradients were observed before and after PWJ cleaning which corresponded to ~ 30 % higher than the bulk values at surface and then decreased beneath the surface up to 200 µm before to reach bulk value (~ 245 HV). Compressive residual stress state at surface initially induced by AWJ milling in some surfaces remained unchanged after PWJ process but in other ones was slightly relieved (~ 50 MPa). Residual stresses after PWJ process resulted from -630 MPa to -315 MPa depending on the milling process parameters.

Nomenclature and abbreviations

AWJ

Abrasive Water Jet. BSE Back-scattered electron

Introduction

Abrasive Water Jet (AWJ) technology for milling has found application in a wide range of sectors as the challenging aerospace industry. AWJ belongs to non-conventional machining process in which abrasive particles are mixed into a water jet entrained by high-pressures converted into high-velocities using a narrow nozzle (often Ø 0.76-1.50 mm) to erode the material when impacting onto the surface [START_REF] Momber | Principles of abrasive water jet machining[END_REF]. AWJ milling has been gaining use over conventional machining processes (e. g. milling and grinding) because it offers several advantages. AWJ has been shown to be suitable for removing material of complex geometry components as thin-curved blades because it keeps a constant thickness of material removal among the surface where conventional milling may not immune to it [START_REF] Donachie | Cleaning and finishing[END_REF]. Another popular cleaning method is the ultrasonic. Ultrasonic cleaning can remove strongly embedded contaminants from parts immersed in aqueous medium by using high-frequency sound waves in the range between 40 and 200 kHz. The ultrasonic energy is applied to the cleaning solution (water, various forms of alcohol, degreasing, acids, etc.) causing cavitation to scrub the surface free of pollutants [START_REF] Awad | Ultrasonic cleaning[END_REF]. However, this method requires a rigorous selection of frequency and amplitude of sound waves due to the tendency of the ultrasonic energy to damage parts. Furthermore, this cleaning method involves a rise in cost due to the tank filled with cleaning agents.

On the other hand, abrasive-less water jet, commonly called Plain Water Jet (PWJ), is recently reported in the literature as a surface treatmentfor improving fatigue strength by introducing compressive residual stresses, for medical applications where an increase in surface roughness is required [START_REF] Arola | Waterjet and abrasive waterjet surface treatment of titanium: a comparison of surface texture and residual stress[END_REF][START_REF] Lieblich | On the fatigue behavior of medical Ti6Al4V roughened by grit blasting and abrasiveless waterjet peening[END_REF][START_REF] Huang | Parametric effects on grit embedment and surface morphology in an innovative hybrid waterjet cleaning process for alpha case removal from titanium alloys[END_REF][START_REF] Arola | Improving fatigue strength of metals using abrasive waterjet peening[END_REF][START_REF] Azhari | Effect of multiple passes treatment in waterjet peening on fatigue performance[END_REF] or cleaning method, -e.g. for removing oxygen layer in titanium alloys surfaces usually formed after service at elevated temperatures [START_REF] Huang | Removal of heat-formed coating from a titanium alloy using high pressure waterjet: influence of machining parameters on surface texture and residual stress[END_REF].

However, the use of Plain Water Jet method for addressing the problem of grit embedment induced by AWJ milling is until today scarcely studied in the literature [START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF][START_REF] Sourd | Plain water jet cleaning of titanium alloy after abrasive water jet milling: surface contamination and quality analysis in the context of maintenance[END_REF][START_REF] Huang | Parametric effects on grit embedment and surface morphology in an innovative hybrid waterjet cleaning process for alpha case removal from titanium alloys[END_REF]. Moreover, there is a knowledge gap in the literature concerning surface integrity aspects after PWJ cleaning method (surface quality, material thickness removed, residual stresses, hardness, etc.). Rivero et al. [START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF] applied PWJ process to release the surface of embedded abrasives due to AWJ milling from alloy 718 surfaces. They demonstrated that all the embedded abrasive particles were removed under pressures between 160 and 360 MPa, traverse speeds ranging 0.1-0.9 m/min, stand-off distances varying 10-90 mm and from one to four jet passes. Further, they observed that, in several PWJ conditions, the roughness produced by AWJ was reduced by the cleaning operation between 2.5 % and 40 % because of water droplets eroded the peaks and valleys produced by abrasive particles impact. Nevertheless, the material removal induced by PWJ was not quantified in this study, which is the utmost importance for aeroengine components as thin-curved blades. Huang et al. [START_REF] Huang | Parametric effects on grit embedment and surface morphology in an innovative hybrid waterjet cleaning process for alpha case removal from titanium alloys[END_REF] studied AWJ milling for the removal of alpha case from titanium alloys followed by PWJ to remove the grit embedment. They have tested several experimental conditions for PWJ to evaluate the efficiency of grit removal. In the conditions performed in their study, it was found that there was better performance to remove the grit embedment from 20 % to 6-10 % when using SOD between 30-50 mm and for jet impingement angle of 90 • . Besides, when using traverse speeds of 0.5 and 1.5 mm/min as well as when increasing the number of jet passes, the grit embedment reduced from 29.8 % to less of 10 %. Further, they observed higher grit removal effectiveness with a PWJ scanning path in a feed direction perpendicular to that used for the AWJ milling. Arola et al. [START_REF] Arola | Improving fatigue strength of metals using abrasive waterjet peening[END_REF] reported that in titanium alloy residual stresses resulting from PWJ increased with abrasive size and jet pressure. Lieblich et al. [START_REF] Lieblich | On the fatigue behavior of medical Ti6Al4V roughened by grit blasting and abrasiveless waterjet peening[END_REF] reported that higher waterjet pressures in titanium alloy did not induce higher residual stresses, but only increased material erosion. They explained that higher amount of removed material allows a partial relief of residual stress in spite of the higher impact energy. Azhari et al. [START_REF] Azhari | Effect of multiple passes treatment in waterjet peening on fatigue performance[END_REF] investigated the influence of PWJ in stainless steel and reported that compressive residual stresses, hardness and roughness increased with higher number of passes, however, they suggested that higher levels of roughness may affect the fatigue strength, due to the tendency of crack initiation. Holmberg et al. [START_REF] Holmberg | Surface integrity after post processing of EDM processed Inconel 718 shaft[END_REF] evaluated the performance of high-pressure Plain Water Jet (360 MPa) in an Inconel 718 shaft to remove electrical discharge machining (EDM) residue by using only one processing condition, which resulted in a removed layer of 39.1 µm. Further, the authors indicated a surface roughness of 3.5 µm and compressive residual stresses with a peak of 750 MPa at ~ 7 µm beneath the surface that decreased along a depth of ~ 150 µm.

There is a lack in the literature providing an extensive analysis of surface state (topographical and mechanical aspects) in Inconel 718 before and after PWJ process with prior AWJ milling. It could be very interesting to estimate the relevance and the effectiveness of the not be quite appropriated. Since material is removed by erosion, AWJ milling induces a marginal thermal damage on workpiece as well as it is suitable for processing hard-to-machine materials as nickel-based aerospace superalloys, e.g. Inconel 718 [START_REF] Kong | An innovative method to perform maskless plain waterjet milling for pocket generation: a case study in Ti-based superalloys[END_REF].

Beyond these advantages, an issue widely acknowledged in the AWJ process is the residual abrasive particles which embedded mechanically at surface during milling [START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF][START_REF] Boud | Abrasive waterjet cutting of a titanium alloy: the influence of abrasive morphology and mechanical properties on workpiece grit embedment and cut quality[END_REF][START_REF] Fowler | A technical note on grit embedment following abrasive water-jet milling of a titanium alloy[END_REF][START_REF] Shipway | Characteristics of the surface of a titanium alloy following milling with abrasive waterjets[END_REF][START_REF] Arola | Waterjet and abrasive waterjet surface treatment of titanium: a comparison of surface texture and residual stress[END_REF]. These abrasive particles may be act as fatigue crack initiators [START_REF] Kong | An innovative method to perform maskless plain waterjet milling for pocket generation: a case study in Ti-based superalloys[END_REF][START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF][START_REF] Singh | Mechanical issues in laser and abrasive water jet cutting[END_REF], detrimental condition for a finished surface or for subsequent processes in repair application of damaged aircraft engine components where 3D additive manufacturing (AM) technology is deposited [START_REF] Bremer | Automated repair and overhaul of aero-engine and industrial gas turbine components[END_REF]. In metallic materials, it has been reported up to 40 % of the total area of a milled surface consisted of grit embedment, and in the best cases, 5 %, depending on machining conditions [START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF][START_REF] Fowler | A technical note on grit embedment following abrasive water-jet milling of a titanium alloy[END_REF][START_REF] Shipway | Characteristics of the surface of a titanium alloy following milling with abrasive waterjets[END_REF][START_REF] Arola | Waterjet and abrasive waterjet surface treatment of titanium: a comparison of surface texture and residual stress[END_REF][START_REF] Sourd | Plain water jet cleaning of titanium alloy after abrasive water jet milling: surface contamination and quality analysis in the context of maintenance[END_REF].

Since AWJ process is characterized by a large number of machining parameters, researchers have attempted to reduce the levels of grit embedment by varying the process parameters. However, the scientific publications regarding metallic materials are limited. Traverse jet speed (m/min), jet impingement angle ( • ), distance between jet passes (stepover distance, mm) or grit size (mesh #) have been reported as adjustable parameters that can influence the level of grit embedment. Rivero et al. [START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF] studied abrasive embedment in alloy 718 surfaces milled by varying different AWJ process parameters. It was reported that grit embedment increased with increasing jet pressure, traverse speed and stand-off distance with step-over distance; whilst, an optimum higher level of grit embedment of 20 % was reported to occur at 0.3 grmin of abrasive flow rate, whereas for 0.15 and 0.45 gr/min the grit embedment was of ~ 14 %. Arola et al. [START_REF] Arola | Waterjet and abrasive waterjet surface treatment of titanium: a comparison of surface texture and residual stress[END_REF] reported that abrasive embedment increased with increasing pressure and smaller abrasive particles in titanium surfaces. Boud et al. [START_REF] Boud | Abrasive waterjet cutting of a titanium alloy: the influence of abrasive morphology and mechanical properties on workpiece grit embedment and cut quality[END_REF] examined #80 mesh garnet abrasive from five different sources when through-cutting titanium alloy by AWJ and concluded that, despite the different morphology, crushing strength and hardness of each abrasive type, there was no significant difference in the grit embedment level. Shipway et al. [START_REF] Shipway | Characteristics of the surface of a titanium alloy following milling with abrasive waterjets[END_REF] examined two sizes of garnet abrasives particles by varying the jet impingement angles when milling titanium alloy surfaces. They observed that, for both abrasive sizes (#80, #200), the grit embedment increased with increasing the impingement angle due to the higher impulse during impact as the impingement angle was raised. Fowler et al. [START_REF] Fowler | A technical note on grit embedment following abrasive water-jet milling of a titanium alloy[END_REF] investigated the effect of traverse speed, grit size, milling direction and jet impingement angle on the level of abrasive embedment in titanium alloy surfaces milled by AWJ and reported that for the two studied abrasive sizes (#80, #200) with a constant traverse speed of 0.18 m/min, the forward milling resulted in high levels of grit embedment whilst backward milling resulted in lower levels of grit embedment for all impingement angles of 30 • , 60 • and 90 • . However, for a higher traverse speed (9.96 m/min) they reported that the two modes of milling had no strong influence on grit embedment. Patel [START_REF] Patel | Quantitative evaluation of abrasive contamination in ductile material during abrasive water jet machining and minimising with a nozzle head oscillation technique[END_REF] compared straight and oscillation cutting techniques in AWJ with respect to grit embedment, and they found that for ductile materials, nozzle oscillation reduced by up to five times the abrasive embedment in surface compared with a straight cutting.

On the other side, in order to remove contaminants from the surfaces, several cleaning methods have been reported in the literature. Mechanical or chemical cleaning processes are often used for removing metallic contamination, tarnish and oxidation resulting from hot working or heat treatments of nickel-base superalloy components [START_REF] Donachie | Cleaning and finishing[END_REF]. Mechanical cleaning methods include dry or wet abrasive blasting, polishing (with ceramic materials) and wet tumbling. Chemical methods are more used than mechanical means, namely: picking and vapor degreasing or alkaline cleaning followed by either water rinsing or soaking. The applicability of these methods is determined by the shape of the component, the surface finish requirements and the allowable metal loss, however, a combination of two or more methods are often necessary. Such methods may require rigorous inspections to prevent over-etched parts or excessive metal surface roughening; even hydrogen embrittlement, which it is not commonly to occur in super-alloys but are speed (f) and step-over distance (STD), as stated in Table 2, were performed resulting in twenty-seven different experimental conditions similar to those used in previous work [START_REF] Cano-Salinas | Influence of abrasive water jet parameters on the surface integrity of Inconel 718[END_REF].

Each specimen was milled by creating four "blind" pockets of 20 mm of length spaced 20 mm apart. These four pockets in each specimen were machined with identical milling parameters in order to ensure a good repeatability of the AWJ process. All pockets were milled following a raster scan tool strategy by keeping jet direction changes outside of the workpiece (Fig. 2).

PWJ cleaning

Over the "blind" pockets created by AWJ milling, Plain Water Jet (PWJ) post-processing was performed without abrasive particles in an attempt to remove embedded abrasive particles. Similar tool path and feed direction to those used for AWJ milling were employed for PWJ cleaning process.

Before cleaning of pockets, preliminary tests were performed in order to identify a proper set of PWJ parameters allowing to preserve the topographies of the milled pockets (both in terms of depth and surface roughness and texture) as well as their mechanical state (hardness and residual stresses). Thus, the set of parameters selected to perform the PWJ cleaning process (Section 3.2.1) in AWJ milled surfaces was fixed at: P = 200 MPa, SOD = 50 mm, STD = 0.5 mm, and f = 1.0 m/min. Water jet pressure of 200 MPa was selected because showed a good effectiveness to remove embedded abrasive particles with a minimal thickness of removed material. Stand-off distance was fixed at 50 mm according to Rivero et al. [START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF] who efficiently performed PWJ cleaning tests using a SOD between 10-90 mm. Initially, a SOD = 100 mm as to this used for AWJ milling was attempted to be used for PWJ cleaning but such value showed almost not efficient to remove grit, thus, in order to keep a relatively large value SOD for machining components with significant curvature, a value of SOD = 50 mm was selected. STD = 0.5 mm was selected in an attempt to improve cleanliness by increasing the overlap of the water jet passes in surface.

Among the twenty-seven milled specimens, only seven samples were selected to perform the PWJ cleaning in order to simplify the analysis, and from these seven specimens only the pockets number 3 and 4 were cleaned. Thus, three distinct degrees of texture (low, medium and high) produced by AWJ milling were selected from the evaluation of the texture-aspect ratio (Str parameter defined in Section 2.4). These selected specimens are: two specimens milled at P = 200 MPa chosen as isotropic textures, three specimens milled at P = 250 MPa as middle surface texture, and two specimens milled at P = 300 MPa as anisotropic surfaces in order to study different specimens of each different pressure used for AWJ milling.

Surface characterization techniques and procedures

The 3D surface topographies of the milled and cleaned pockets were measured by means of Alicona Infinite Focus 3D optical profilometer. From these surface mappings, the surface topography over an area of 5.7 × 5.7 mm at the center of the pocket as well as the thickness of material removal along 40 mm-long and 2 mm-wide, were measured following the experimental procedure fully described in previous work [START_REF] Cano-Salinas | Influence of abrasive water jet parameters on the surface integrity of Inconel 718[END_REF]. The scanning locations of the pocket measurements are shown in Fig. 3a.

The topography data were treated by means of AltiMap software. Surface topography was characterized using height, spatial and hybrid parameters according to the ISO 25178-2 standard. Such mentioned parameters have been used in previous works [START_REF] Avegnon | Areal surface texture and tool wear analysis from machining during powder bed fusion[END_REF][START_REF] Pawlus | Parametric description of oneprocess surface texture[END_REF][START_REF] Fu | Surface topography measurement of mirrorfinished surfaces using fringe-patterned illumination[END_REF] to characterize topographical properties of surfaces. The parameters used in this study were Sa, Str, Sal and Sdr. The arithmetic mean height parameter (Sa), is commonly used to characterize surface roughness without regard of the direction by quantifying the average absolute height of peaks and valleys of interest [START_REF] Avegnon | Areal surface texture and tool wear analysis from machining during powder bed fusion[END_REF]. As a complement to height parameter Sa, combination of the AWJ milling followed by PWJ process in alloy 718, for a multi perspective application: as milling operation for difficult-tomachine materials as Inconel 718 where a determined surface quality is desired, and also as a precedent step to the repair application where removal of damaged zone is required; in both cases a state free of abrasive residues is well preferred.

The purpose of the present investigation is, in a first stage, to quantify grit embedment and to characterize the surface in terms of texture of Inconel 718 surfaces milled by AWJ process. To achieve this objective, AWJ milled surfaces at different process parameters (viz. jet pressure, traverse speed and step-over distance) were taken from a previous study [START_REF] Cano-Salinas | Influence of abrasive water jet parameters on the surface integrity of Inconel 718[END_REF] and then the surfaces were subjected to PWJ cleaning process in order to remove the grit embedment by using a set of processing parameters by considering a trade-off among the removed grit and material erosion depth. After cleaning, the influence of Plain Water Jet (PWJ) process on grit embedment removal, the modifications in surface texture and roughness (by using Sa, Str, Sal and Sdr parameters), the thickness of removed material, the microhardness and the residual stresses were examined.

Experimental procedure

Equipment and material

AWJ and PWJ processes were conducted on a Flow MACH4-C Waterjet Machine. The cutting head of the machine is composed of an orifice diameter of 0.33 mm, a focusing tube length of 76 mm and a nozzle diameter of 1.016 mm. All tests for AWJ and PWJ were performed at a jet impingement angle to 90 • (cutting nozzle perpendicular to the cutting direction) with the nozzle in air. The overall view of the experimental setup is shown in Fig. 1.

Annealed commercially Inconel alloy 718 was utilized in this investigation. Inconel 718 was provided in plate form with 3.71 mm thickness. Test samples were prepared in rectangular form of 180 mm × 20 mm by AWJ cutting. The chemical composition of this alloy is given in Table 1. The hardness of the bulk material is 245 ± 15 HV measured as indicated in 2.4.

AWJ milling

Abrasive garnet (Wuxi Ding Long Minerals Co., Ltd) mesh #120 was employed for AWJ milling operation. For economical reasons, the abrasive size as well as the abrasive flow rate were kept fixed in this study (Table 2). A STD value of 100 mm was used to make possible the milling of workpieces with complex shapes as large curvatures. Full factorial tests, corresponding to three levels of jet pressure (P), traverse information describing the texture is given by Str and Sal spatial parameters and Sdr hybrid parameter. Spatial parameters are used to further describe the texture and orientation of a surface with respect to a whole image [START_REF] Avegnon | Areal surface texture and tool wear analysis from machining during powder bed fusion[END_REF]. The texture aspect ratio parameter (Str), is an evaluation of the surface texture isotropy. Str is a unit-less parameter and is denoted from 0 to 1 (or expressed as percentage between 0% and 100%) [START_REF] Leach | Characterisation of areal surface texture[END_REF]. An isotropic texture has the same properties regardless of the direction, whilst an anisotropic surface has a dominant texture direction [START_REF] Leach | Characterisation of areal surface texture[END_REF]. Str < 0.3 represents an anisotropic texture, whilst Str > 0.5 describes an isotropic texture [START_REF] Avegnon | Areal surface texture and tool wear analysis from machining during powder bed fusion[END_REF]. The autocorrelation length (Sal), provides information about the finesses of a surface, the smaller the Sal value, the finer the features [START_REF] Avegnon | Areal surface texture and tool wear analysis from machining during powder bed fusion[END_REF]. Sal parameter is defined as the horizontal distance of the autocorrelation function (ACF) which has the fastest decay to a specific value (s = 0.2) [START_REF] Leach | Characterisation of areal surface texture[END_REF]. For an anisotropic surface, Sal is in the direction perpendicular to the surface lay [START_REF] Leach | Characterisation of areal surface texture[END_REF]. Further, the developed interfacial area ratio (Sdr), is used as a measure of the surface complexity, it is typically described in percentage. A perfectly flat and smooth surface would have Sdr = 0 % [START_REF] Leach | Characterisation of areal surface texture[END_REF]. Surfaces were also observed using FEI Nova NanoSEM 450 Scanning Electron Microscope (SEM) with Back-Scattered Electron (BSE), secondary electron (SE) and Energy-dispersive spectroscopy (EDS) detectors. Before and after cleaning, nine micrographs (850 × 734 µm each image field) were taken at different position locations (Fig. 3b) on the pocket number 3 of all specimens. These micrographs were then post-treated with image analysis by means of ImageJ software by using a gaussian filter to reduce image noise and to enable detection of abrasive particles boundaries. Then, a suitable threshold was applied to identify the darkest zones corresponding to abrasive particles (image binarization), as illustrated in Fig. 4. To quantify the level of grit embedment, the surface area corresponding to abrasive particles was determined from the total area per image field and converted into a percentage. The grit embedment percentage is the average of the nine different measurements per pocket with its standard deviation (pocket 3 of all specimens).

Granulometry and shape factor of abrasive grit were studied in order to better understand the effectiveness of grit removal on the AWJ milled surfaces. A MasterSizer 3000 instrument from Malvern Panalytical was used for the granulometric analyses. This apparatus is based on the theory of laser diffraction (Mie or Fraunhofer theory). This relates the angle of the laser beam with the particle size. The quantity of the sample for the analysis was 20 ml of abrasive powder. The powder was feed into the apparatus and dispersed in an air stream at 3.5 bar of pressure. The granulometry was analyzed from two different samples of abrasives: abrasive grit as provided and, abrasive debris collected of the machine's catch tank after usage for AWJ milling. Each abrasive powder sample was measured twice (Measure A, Measure B) in order to duplicate measurements of each sample. Normal distribution model was used to fit the data.

The initial shape of single abrasive particles was measured by using SEM. Fifteen micrographs were taken, then, from these micrographs, 230 grain particle shape were extracted with image analysis by using Aphelion software. Circularity and elongation shape factors (both range from 0 to 1), were obtained to characterize these 230 particles. For a perfect circular particle, the circularity is equal to 1; elongation index is zero for a circle and approaches 1 for a long and narrow ellipse.

Residual stress measurements were performed by X-ray diffraction (XRD) technique. Residual stresses were evaluated in the middle of the machined pocket across and along (σ xx , σ yy ) the feed direction (Fig. 3a).

The XRD measurements were performed using 6-axis XRayBot® apparatus equipped with a goniometer with a pure Si solid detector and Mn radiation source (Mn-Kα1, λ = 2.10 Å). Setting parameters and computation data used are similar to those described in previous work [START_REF] Cano-Salinas | Influence of abrasive water jet parameters on the surface integrity of Inconel 718[END_REF].

Once the surface characterizations were completed, only two specimens, of the seven selected to be cleaned, were chosen to analyze the effect of the PWJ cleaning operation on the sub-surface hardness and for observations of grit removal in cross section. It was selected two specimens milled at two different pressures, both with different traverse speed and STD. The two selected specimens were those milled at P = 250 MPa, f = 1 m/min, STD = 0.5 mm and P = 300 MPa, f = 0.5 m/ min, STD = 1.5 mm. The cross sections of the pockets number 1 (uncleaned) and number 4 (cleaned) of these two specimens were cut on the middle of the pocket in two orientations: parallel and perpendicular to the feed direction (Fig. 5). In addition, one cross-section was cut of wrought material as provided in order to measure its microhardness. All cross-sections were prepared by grinding with different grit sizes of SiC paper (from P400 to P4000) and then by polishing with diamond powder to mirror-finished. The sub-surface microhardness measurements were performed using a microhardness testing MICROMET 5104 (from Buehler GmbH) driven by Omnimet MHT software according to the ISO 6507-1 standard. Vickers indenter was utilized to penetrate the

Table 1

Chemical composition of the wrought Inconel 718 material.

polished surfaces with a load of 200 gf applied during 10 s. In each crosssectional, three columns of 20 indentations in zigzag pattern were made toward the sub-surface, and an average of the sixty measured values was considered as the microhardness for each direction. The first row of indentations was located around 50 µm below the milled surface. Indentations, separated from each other in 80 µm, were performed up to 1.6 mm in-depth (Fig. 5). For the cross-sectional of the bulk sample, the procedure was identical to the milled cross-sections except for the number of indentations per column, where each filiation of 46 indentations were performed throughout the thickness of the plate.

Results and discussion

AWJ milling

Due to the limited thickness of material plate (3.71 mm), two specimens were shattered when milling at 250 MPa and 300 MPa of pressure both with traverse speed of 0.5 m/min and step-over distance of 0.5 mm. Thus, only twenty-five testing conditions could be analyzed in the present work.

Qualitative observation of milled surfaces

Examination of micrographs of IN718 surfaces milled by AWJ reveals that all the surfaces present embedded abrasive particles (Fig. 6); most of these particles being less than 50 µm in size. Since this size is much smaller than the original size (180 µm) of the abrasive (i.e. as received state) employed to mill the surfaces (Fig. 7b), it may be assumed that only fragments of original particles are embedded. This is because upon the jet impact on the surface, the abrasive particles can break into smaller portions [START_REF] Fowler | A technical note on grit embedment following abrasive water-jet milling of a titanium alloy[END_REF]. This can be verified by comparing the micrographs of the abrasive (as supplied) with the debris (after milling) respectively in Fig. 7a,c which clearly show that the abrasives break into very small portions either by impact on the surface and/or by collisions among the particles with the accelerated water in the mixing tube of the machine [START_REF] Momber | Principles of abrasive water jet machining[END_REF].

In Fig. 7b it can be observed that the abrasive garnet has not an homogeneous grain size, showing a 15 % in volume weight of abrasives with a size of 180 µm, while debris of abrasives after milling (Fig. 7c) show a large dispersion in size, most of these particles are nanometric sized, corresponding only 7 % of debris to 87 µm in size (Fig. 7d).

Further, Fig. 6b reveals two types of embedded abrasive particles: oxides and organic particles. From a performed analysis by energy dispersive X-ray spectroscopy (EDS) as shows Fig. 8b, the main oxide particles embedded are silica (SiO 2 ), which are the mainly constituent of the abrasive medium, followed by aluminum oxide (Al 2 O 3 ), ferric oxide (Fe 2 O 3 ) and magnesium oxide (MgO). Such chemical compositions of embedded particles were compared with those of the abrasive grit as received, and presented identical constituents (Fig. 8a), which corroborates that the dark zones in SEM micrographs correspond well to embedded grit. The superficial darkest zones (Fig. 6b) correspond to organic particles which could be carbon, oxygen or hydrogen. These organics can be removed more easily (e.g. by rinsing with acetone). However, oxide pollutants need be removed by a cleaning process because the embedment behavior difficult the ability to be removed [START_REF] Huang | Parametric effects on grit embedment and surface morphology in an innovative hybrid waterjet cleaning process for alpha case removal from titanium alloys[END_REF].

On the other side, SEM micrographs analysis reveals that the material removal of Inconel 718 is mainly produced by micro-cutting mechanism observed in form of plastic deformation induced by the impact of the abrasive particles on the target material (Fig. 9a-b). Repeated impacts by high-velocity particles induce deep craters and lip formations. Further, abrasive particle shape has an important influence on the erosion mechanism. Generally, the more sharp-edges abrasive grains, the greater the erosion [START_REF] Momber | Principles of abrasive water jet machining[END_REF]. Thus, material removal mechanism due to cutting-deformation is dominated by angular abrasive particles, whereas, the plowing deformation is significant for spherical particles [START_REF] Momber | Principles of abrasive water jet machining[END_REF]. In the micrographs of Fig. 9 is evident that micro-cutting due to plowing deformation is the predominant removal mechanism in IN718, which relates ductile behavior of the material. Further, Fig. 9c shows two types of crater formation: one due to an angular abrasive particle and the other owing to a rounded particle. Results about shape factor of single abrasive particles of garnet (Table 3) confirm a high index of circularity of particles (0.71), which is consistent with the plowing deformation mechanism mostly observed in the micrographs.

The images of Fig. 10a show different grain shapes for the garnet abrasive used in this study, namely spherical and sharp-edges particles. Moreover, in the field of erosion, it was reported that shape factor as well as the particle-hardness may have an influence on material removal. Angular and hard abrasive grains improve the material- removal process in ductile materials [START_REF] Momber | Principles of abrasive water jet machining[END_REF][START_REF] Fowler | The effect of particle hardness and shape when abrasive water jet milling titanium alloy Ti6Al4V[END_REF]. However, these topics are not part of the present study.

Effect of AWJ milling parameters on surface texture

The effect of process parameters on surface texture was examined from the surface topographies performed on AWJ milled pockets. Different textures were produced depending on the process parameters (Fig. 11). Jet pressure could intensify surface texture by increasing the amount of kinetic energy of the jet on the target material. The effect of traverse speed on the surface texture can be observed in Fig. 11b,d,f. These textures correspond to the surfaces milled at 0.5, 1 and 1.5 m/min respectively at the same pressure (P) and step-over distance (STD) parameters. As traverse speed increases, a less textured surface is produced. This is because a short time of a jet pass could not quite erode the surface.

Further, the surface texture also appears to be highly dependent on the step-over distance (STD) parameter. In order to illustrate this phenomenon, it is assumed that the distribution of the jet energy fits a Gaussian distribution (Fig. 12). In this case, a low STD is more likely to produce a fine surface texture due to greater overlapping between jet passes (Fig. 12a). Conversely, a higher STD generates rougher surface topographies, allowing peak formations in the zones where the jet passes are not overlap enough (Fig. 12b). Similar results have been observed when milling of carbon fiber-reinforced polymer (FRP) by AWJ process [START_REF] Hejjaji | Influence of controlled depth abrasive water jet milling on the fatigue behavior of carbon/epoxy composites[END_REF]. However, STD and traverse speed parameters, both strongly influence the surface texture at the same time. High STD values combined with low traverse speeds, regardless of the pressure, present well-marked grooves (Fig. 11c,e,g) even observed at naked eye. These grooves follow the jet pass direction.

However, the topographies obtained after AWJ milling, as depicted in Fig. 11, showed that the preferential texture orientation expected from Fig. 12 (a "theoretical" process of a single process parameter: STD) does not correspond exactly to the experimentally obtained surfaces (which are more complex) because the texture and directionality of the surface could be influenced depending on the intensity of the three combined process parameters (P, f, and STD) and only areal parameters can integrate information in all directions. The texture aspect ratio (Str), is one of the most important parameters when characterizing a surface in an a real manner [START_REF] Leach | Characterisation of areal surface texture[END_REF]. This spatial parameter (Str), is a numerical representation of the strength of orientation [START_REF] Avegnon | Areal surface texture and tool wear analysis from machining during powder bed fusion[END_REF].

The texture-aspect ratio (Str) of the surfaces produced after AWJ milling ranges from 0.2 to 0.9 depending on the process parameters, meaning a variety of isotropic and anisotropic surfaces. The most textured surfaces, exhibiting the lower Str (< 0.3), were produced at high STD (1.5 mm) regardless of the jet pressure used (Fig. 11c,e,g). On the contrary, more isotropic textures (Str > 0.5) were generated when using low pressure (200 MPa) with STD = 1 mm for all traverse speeds (Fig. 11b,d,f). However, the analysis of texture needs the complement of other areal parameters to better describe the surface texture. Sal values are not smaller for reduced STD as expected because the interaction between the different AWJ process parameters, instead low Sal values described more isotropic textures surfaces where the grooves of STD were not significant (Fig. 11d,f). Sdr parameter indicates that the flattest or smoothest surface was that with Sdr = 6 % followed by the surface with Sdr = 12 % (Fig. 11a,b).

Effect of AWJ milling parameters on grit embedment

Fig. 13 shows the evolution of the surface fraction of grit embedment as a function of the jet pressure (P) and the step-over distance (STD) for the three different traverse speeds (f). Each point in these graphs represents an average value of nine measurements in the pocket number 3 where examination where performed for each specimen. The grit embedment ranges from 7 % to 14 % of the total surface area, depending on the AWJ process parameters. This is way lower than the grit embedment reported in [START_REF] Sourd | Plain water jet cleaning of titanium alloy after abrasive water jet milling: surface contamination and quality analysis in the context of maintenance[END_REF] when milling Ti6Al4V by AWJ, which was around 40 % of the total surface area. This can be due to the machinability of the two materials. As Inconel 718 has a lower machinability than Ti6Al4V (12 % and 20 % respectively), it is more difficult for the abrasive grit to embed in the material.

The grit embedment seems to decrease as P increases as well as when decreasing STD and no clear effect was observed in function of the f parameter (Fig. 13a-c). In fact, if we refer to the literature review, during air jet blasting, the contamination rate increases with the increasing of the air pressure [START_REF] Getu | Conditions leading to the embedding of angular and spherical particles during the solid particle erosion of polymers[END_REF][START_REF] Hadavi | Numerical and experimental investigations of particle embedment during the incubation period in the solid particle erosion of ductile materials[END_REF]. Indeed, in the work of Getu et al. [START_REF] Getu | Conditions leading to the embedding of angular and spherical particles during the solid particle erosion of polymers[END_REF], it was mentioned that, with an increase in the impact velocity of the grit, the grit penetration depth also increases [START_REF] Getu | Conditions leading to the embedding of angular and spherical particles during the solid particle erosion of polymers[END_REF]. However, when considering the Abrasive Water Jet process, the physics occurring for the particle embedment can be different compared to the air jet blasting. In fact, the impact velocity of the grit increases with an increase of the water jet pressure. In this case, it can be supposed that, with an increase in the water jet velocity (or pressure) some embedded grit can be removed from the machined surface due to the pressure forces locally applied by the water on the wall of the grit. In addition, in the work of Rivero et al. [START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF] on milling of IN718 material, it was observed that, the grit embedment remains stable when the pressure varies from 160 MPa to 260 MPa. Further, it was reported by the same authors the effect that traverse speed increased the grit embedment, in contrast, in the present study no significant effect for f parameter was observed. It is important to noticed that, in the work of Rivero et al. [START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF], all the experimental tests have been conducted with a stand-off distance inferior to the one used in the present work (which is of 100 mm). In fact, when increasing the stand-off distance, the grit energy is reduced. Therefore, the influence of the traverse speed on grit embedment may be minor. The trend of a greater STD increased the grit embedment as depicted in Fig. 13 could be explained by the fact that smaller step-over distances lead to a larger overlapped area between different jet passes over the surface when

Table 3

Mean values of shape factors indexes of garnet abrasive particles used for AWJ process.

milling, which can remove embedded particles by subsequent jet passes.

PWJ cleaning process

Effect of PWJ cleaning post-process on surfaces milled by AWJ

PWJ cleaning process produced a significant reduction in the level of embedded abrasive particles in the surface of the seven specimens selected to be cleaned, as illustrated in Fig. 14.

The residual grit after PWJ cleaning process varied between 3 % and 4 % of the total milled surface regardless of the initial level of grit embedment produced by the AWJ (Fig. 15). This implies a reduction of embedded abrasives before and after PWJ cleaning from 4% to 11 % of the total surface area. The fact that the embedded particles were not completely removed could be explained by the existence of two types of embedded abrasive particles, namely deposited particles and submerged particles [START_REF] Rivero | Surface properties and fatigue failure analysis of alloy 718 surfaces milled by abrasive and plain waterjet[END_REF][START_REF] Huang | Parametric effects on grit embedment and surface morphology in an innovative hybrid waterjet cleaning process for alpha case removal from titanium alloys[END_REF]. Thus, it is assumed that the water jet, at the cleaning parameters selected for the present study, could not remove some submerged particles, located in a crater and/or partially covered by material. In Fig. 16 is depicted in cross sectional the embedded abrasive particles before and after PWJ cleaning. Fig. 16a,c shows embedded grit after AWJ process where submerged abrasives partially covered by a layer of IN718 material were observed. After PWJ cleaning, Fig. 16b,d shows the residual grit in the narrow corners of the cavities, when the water jet was not able to completely dislodge the abrasive. Most of residual grit is less than 20 µm and up nanometric size. This residual grit was analyzed in section by EDS and confirms the same chemical composition as that shown in Fig. 8b.

Before and after PWJ cleaning, the texture aspect ratio (Str), the roughness (Sa), the residual stress and the hardness results, were examined from pockets 3 and 4 (as before mentioned in Section 2.3). The average values were calculated from the two measurements performed at the same of parameters with standard deviation.

As shown in Fig. 17, the Str parameter (indicator of the degree of surface isotropy) remained almost constant after cleaning of the milled surfaces. This means that the impact of the water droplets on the surface not significantly modify the surface texture produced by the abrasive particles during AWJ milling. This can be observed in 3D surface textures of the milled pockets against the cleaned pockets depicted in Figs. 19-21 . It can be seen that surface topography remains almost the same before and after PWJ cleaning. However, in some specimens, surface texture slightly decreases (Fig. 21a-b) or increases (Fig. 20a-b). At the same time, when comparing the surface roughness (obtained in previous work [START_REF] Cano-Salinas | Influence of abrasive water jet parameters on the surface integrity of Inconel 718[END_REF]) before and after PWJ process, it is revealed that the roughness values are slightly increased after PWJ cleaning (Fig. 18). The increase in roughness or in texture after PWJ cleaning is probably due to the fact that the dislodging of the abrasive particles leads to additional craters. The case of decrease in surface texture is due to water droplets could slightly smooth the peaks produced by AWJ milling. It could be also noteworthy that even the texture-aspect ratio (Str) was almost identical before and after PWJ cleaning, Sal parameter would be relevant for milled surfaces as it is related to the STD process parameter (Fig. 12); Sal values should decrease as STD decreases (Figs. [START_REF] Holmberg | Surface integrity after post processing of EDM processed Inconel 718 shaft[END_REF][START_REF] Cano-Salinas | Influence of abrasive water jet parameters on the surface integrity of Inconel 718[END_REF][START_REF] Avegnon | Areal surface texture and tool wear analysis from machining during powder bed fusion[END_REF]. As well as, the developed area Sdr, give additional information regarding the surface complexity, especially when comparing several stages of processing [START_REF] Leach | Characterisation of areal surface texture[END_REF] as in the case of AWJ milling followed by PWJ cleaning process.

As expected, features become more refined as the Sal values decreases, however, Sal values not necessarily resulted in higher values for larger STD, indicating greater complexity of surfaces, better described by the hybrid parameter Sdr. Smoother surfaces have smaller percentage of Sdr (e.g. Fig. 21a). In all experiments (Figs. [START_REF] Holmberg | Surface integrity after post processing of EDM processed Inconel 718 shaft[END_REF][START_REF] Cano-Salinas | Influence of abrasive water jet parameters on the surface integrity of Inconel 718[END_REF][START_REF] Avegnon | Areal surface texture and tool wear analysis from machining during powder bed fusion[END_REF], Sdr values resulted smaller before than after PWJ cleaning, suggesting the interaction of different stages of processing, between the features initially produced by milling at a certain set of process parameters on which other smoother features were induced by different PWJ cleaning process parameters. Fig. 22 represents the effect of the PWJ process on the erosion depth on specimens obtained by different conditions of machining of the AWJ (previously published [START_REF] Cano-Salinas | Influence of abrasive water jet parameters on the surface integrity of Inconel 718[END_REF]). It is clear form these graphs, the eroded depth remains constant after cleaning operation conducted by the PWJ.

Microhardness measurements were conducted on two specimens before and after PWJ cleaning. A hardness gradient of about 30 % with respect to the bulk material is observed at about 40 µm beneath the surface with a return to bulk values (~245 ± 15 HV) at about 200 µm in depth (Fig. 23). Comparable microhardness values and gradients were measured in uncleaned and cleaned surfaces. This confirms that the plastic deformation of the surface layer is due to the abrasive particle impacts during AWJ milling, and that PWJ cleaning operation has not relieve surface stresses, since water droplets have not almost removed material. This can be verified when comparing compressive residual stress results before and after cleaning (Fig. 24). It is consistent that an increase in hardening co-exists with high compressive residual stresses at surface. Further, it is also observed that the level/extent of plastic deformation is depending on the kinetic energy and the exposure time of the jet. This explains that higher plastic deformation was induced by AWJ milling in the specimen milled at P = 300 MPa (Fig. 23b) than that milled at P = 250 MPa (Fig. 23a) (i.e. 333 ± 29 HV and 310 ± 8 HV respectively near to the surface).

Residual stresses after AWJ milling and PWJ cleaning show a compressive residual stress state at surface (Fig. 24) which is beneficial for fatigue life components [START_REF] Lieblich | On the fatigue behavior of medical Ti6Al4V roughened by grit blasting and abrasiveless waterjet peening[END_REF]. These compressive residual stresses after PWJ cleaning seem to be slightly reduced in both xx and yy directions overall the tests. Such reduction ranges between 30 MPa and 100 MPa depending on the AWJ milling parameters. This is probably due to two facts: (1) the dislodging of embedded particles leads to a relief of residual stresses and (2) because of the removal from the upper surface layer material which retained stresses. Muruganandhan et al. [START_REF] Muruganandhan | Investigation of water jet peening process parameters on AL6061-T6[END_REF] reported no significant changes in compressive residual stresses values after PWJ in aluminum alloy, affirming that this is due to the effect of the surface erosion. For improving compressive residual stresses, the process parameters should generate significant deformation in surface and subsurface region.

Further, it can be noticed that when milling, the jet produced slightly higher residual stresses in feed direction (yy) compared with those initially measured in the bulk material. However, even when difference between σxx and σ yy of the wrought IN718 material was 200 MPa due to the effect of rolling manufacturing, this residual stress state becomes more isotropic after AWJ milling.

Conclusions

The impact of PWJ cleaning operation was experimentally analyzed on Inconel 718 surfaces prior milled by AWJ process. Firstly, the influence of AWJ parameters (viz. water pressure, traverse speed and stepover distance) were analyzed on the degree of abrasive embedment and the surface texture. Then, the effectiveness of PWJ to remove the embedded grit with the minimal thickness loss and the minimal modification in surface state after cleaning were evaluated. For the experimental conditions investigated in this study, the following conclusions can be highlighted:

• Before PWJ cleaning, all surfaces presented embedded abrasive particles which surface fraction varied from 7 % to 15 % depending on the AWJ milling conditions. The grit embedment decreases when increasing the pressure but decreasing the step-over distance. • PWJ cleaning considerably reduced the fraction of embedded grit on the AWJ milled surfaces by up to 80 % of the total fraction. Residual abrasives fraction of ~4 % remained in all cleaned surfaces regardless of the initial level of grit embedment obtained from the different process parameters used for AWJ milling, meaning that, from micrographs observations, the 4 % of remaining grit is mostly submerged particles that could not be dislodged by PWJ cleaning. the material. Hence, taking into account the standard deviations, similar values (~250 HV) and gradients of microhardness were observed before and after PWJ cleaning with a peak of hardening at surface corresponding to 30 % higher than that of the bulk.

• The hardening is associated with compressive residual stresses which were produced by AWJ milling and were not considerably affected by PWJ cleaning (between 30 and 100 MPa of reduction), ranging from -300 to -650 MPa after PWJ cleaning, depending on the AWJ process parameters.

Thus, the present research allowed a thorough analysis of the surface state condition induced by the PWJ post-process applied after AWJ milling, such combination of processes is an interesting alternative for milling difficult-to-machine materials as IN718, as well as, for a repair application where is necessary to remove the damage zone of aeroengine components. Nevertheless, the principal disadvantage of AWJ milling is the grit embedment. High levels of embedded grit may induce fatigue failure for finished surfaces or be a detrimental condition for an intermediate stage. In such cases, PWJ cleaning proved to be an effective and economical post-processing after AWJ milling because it uses the same system without abrasive.

Fig. 1 .

 1 Fig. 1. Experimental setup for AWJ and PWJ processes with a schematic showing the SOD process parameter and the impingement angle between the nozzle and the sample.

Fig. 2 .

 2 Fig. 2. Overview of the specimen machined by AWJ at P = 300 MPa, f = 1 m/ min and STD = 1 mm showing milled pockets and the tool path.

Fig. 3 .

 3 Fig. 3. Location of pocket measurements (a) for the surface topography over an area of 5.7 × 5.7 mm and the profile of material removed thickness of 40 mmlong × 2 mm-wide, and (b) for the grit embedment over on nine different zones of an area of 850 × 734 µm each.

Fig. 4 .

 4 Fig. 4. (a) SEM micrograph of abrasives embedded at surface after AWJ milling at P = 300 MPa, f = 0.5 m/min and STD = 1.5 mm with its (b) image binarization showing the grit in white color and black background corresponding to Inconel 718 material.

Fig. 5 .

 5 Fig. 5. Schematic showing micro-indentations in the cross-sections of a milled pocket for microhardness measurements.

Fig. 6 .

 6 Fig. 6. SEM micrographs of IN718 milled surfaces by AWJ showing (a) the embedded abrasive particles in × 150 magnification with a zoomed view of grit embedment in × 500 magnification (P = 300 MPa, f = 1.5 m/min and STD = 1 mm); (b) different types of embedded abrasive particles in × 150 magnification (P = 200 MPa, f =1 m/min and STD = 0.5 mm).

Fig. 7 .

 7 Fig. 7. SEM micrographs in × 130 magnification of the morphologies of (a) the garnet grit #120 before employ for AWJ milling and (c) the grit debris after AWJ milling; and graphs of abrasive size distribution measured by laser diffraction spectroscopy (b) before AWJ milling and (d) after AWJ milling.

Fig. 8 .

 8 Fig. 8. EDS spectrum showing the chemical analysis of (a) the abrasive garnet grit as received state and (b) the embedded grit in surfaces after AWJ milling process.

Fig. 9 .

 9 Fig. 9. SE morphologies of samples milled at (a) P = 300 MPa, f = 0.5 m/min and STD = 1 mm in × 25 magnification; (b) P = 300 MPa, f = 1.5 m/min and STD= 1 mm × 150 magnification; (c) P = 250 MPa, f = 1 m/min and STD = 1.5 mm in × 500 magnification.

Fig. 10 .

 10 Fig. 10. (a) SEM micrograph of garnet abrasive grains with its (b) binary image showing different grain shapes and the circularity shape factor for a single particle.

Fig. 11 .

 11 Fig. 11. Different IN718 surface textures of pocket 2 of specimens milled by AWJ at P = 200 MPa with constant f = 0.5 m/min and (a) STD = 0.5 mm, (b) STD = 1 mm, (c) STD = 1.5 mm; P = 200 MPa with constant STD = 1 mm and (d) f = 1 m/min, (f) f = 1.5 mm; and f = 0.5 m/min with constant STD = 1.5 mm and (e) 250 MPa, (g) 300 MPa. The little arrow indicates the feed direction. Each surface area is 5 mm × 5 mm in size.

Fig. 12 .

 12 Fig. 12. Graphs describing the effect of the step-over distance (STD) parameter on surface textures produced by AWJ milling for (a) low STD and (b) high STD.

Fig.

  Fig. Effect of the AWJ milling parameters on the grit embedment area in function of step-over distances for various pressures and for a traverse speed of (a) 0.5 m/min, (b) 1 m/min and (c) 1.5 m/min.

Fig. 14 .

 14 Fig. 14. SEM micrographs of surfaces showing grit embedment in black color (a) before PWJ cleaning (specimen milled by AWJ at P = 200 MPa, f = 1.5 m/min and STD = 1.0 mm) and (b) after PWJ cleaning of the same specimen (PWJ cleaning performed at P = 200 MPa, f = 1 m/min and STD = 0.5 mm).

Fig. 15 .

 15 Fig. 15. Surface fraction of grit embedment after AWJ milling and PWJ cleaning for different AWJ process parameters, (a) P = 200 MPa, (b) P = 250 MPa and (c) P = 300 MPa.

Fig. 16 .

 16 Fig. 16. SEM micrographs (a) in cross-section perpendicular to feed direction of the specimen milled by AWJ at P = 250 MPa, f = 1 m/min and STD = 0.5 mm, and (b) in cross-section parallel to feed direction of the same specimen after PWJ cleaning; (c) in-cross section parallel to feed direction of the specimen milled by AWJ at P = 300 MPa, f = 0.5 m/min and STD = 1.5 mm, (d) in cross-section perpendicular to feed direction of the same specimen after cleaned by PWJ.

Fig. 17 .

 17 Fig. 17. Surface texture after AWJ milling and PWJ cleaning for different AWJ process parameters, (a) P = 200 MPa, (b) P = 250 MPa and (c) P = 300 MPa.

Fig. 18 .

 18 Fig. 18. Surface roughness after AWJ milling [20] and PWJ cleaning for different AWJ process parameters, (a) P = 200 MPa, (b) P = 250 MPa and (c) P = 300 MPa.

Fig. 19 .

 19 Fig. 19. Different IN718 surface textures (a) after AWJ milling at P = 200 MPa, f = 1 m/min and STD= 1 mm and (b) after PWJ cleaning of the same specimen; (c) after AWJ milling at P = 200 MPa, f = 1.5 m/min and STD = 1 mm and (d) after PWJ cleaning of the same specimen. (PWJ cleaning at P = 200 MPa, f = 1 m/min and STD = 0.5 mm). The little arrow indicates the feed direction. Surfaces are 5 mm × 5 mm in size.

•Fig. 20 .

 20 Fig. 20. Different IN718 surface textures (a) after AWJ milling at P = 250 MPa, f = 0.5 m/ min and STD = 1 mm and (b) after PWJ cleaning of the same specimen; (c) after AWJ milling at P = 250 MPa, f = 0.5 m/min and STD = 1.5 mm and (d) after PWJ cleaning of the same specimen; (e) after AWJ milling at P = 250 MPa, f = 1 m/min and STD = 0.5 mm and (f) after PWJ cleaning of the same specimen. (PWJ cleaning at P = 200 MPa, f = 1 m/ min and STD = 0.5 mm). The little arrow indicates the feed direction. Surfaces are 5 mm × 5 mm in size.

Fig. 21 .

 21 Fig. 21. Different IN718 surface textures (a) after AWJ milling at P = 300 MPa, f = 0.5 m/min and STD = 1 mm and (b) after PWJ cleaning of the same specimen; (c) after AWJ milling at P = 300 MPa, f = 0.5 m/min and STD = 1.5 mm and (d) after PWJ cleaning of the same specimen. (PWJ cleaning at P = 200 MPa, f = 1 m/min and STD = 0.5 mm). The little arrow indicates the feed direction. Surfaces are 5 mm × 5 mm in size.

Fig. 22 .

 22 Fig. 22. Material erosion thickness after AWJ milling [20] followed by PWJ cleaning for specimens milled at different process parameters, (a) P = 200 MPa, (b) P = 250 MPa, (c) P = 300 MPa. (The thickness of bulk plate material was 3.71 mm before AWJ milling).

Fig. 23 .

 23 Fig. 23. Microhardness values as a function of the distance to the processed surfaces towards core material before and after PWJ cleaning of the specimens milled at (a) P = 250 MPa, f = 1 m/min, STD = 0.5 mm, and (b) P = 300 MPa, f= 0.5 m/min, STD = 1.5 mm.

Fig. 24 .

 24 Fig. 24. Residual stresses at surface after AWJ milling [20] and PWJ cleaning for different specimens. (The solid and dotted gray lines indicate the magnitude of residual stresses measured on IN718 surfaces in initial wrought state).

Table 2

 2 Parameter setting for AWJ process.

	Ni (%)	Cr (%)	Fe (%)	Nb (%)	Mo (%)	Ti (%)	Al (%)	Co (%)	C	Mn (%)	Si (%)	S	P	B	Cu (%)	Ta (%)
									(%)			(%)	(%)	(%)
	53.47	18.28	18.2	5.06	2.98	0.96	0.51	0.17	0.03	0.09	0.08	0.0003	0.007	0.001	0.06	0.01

Acknowledgments

The authors gratefully acknowledge the financial support of CON-ACYT Mexico. The technical support of Karine Vieillevigne b is also thank.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


S Douté

G Munaretto

L Tornabene

M Pajola

A Lucchetti

PHOTOMETRIC AND ATMOSPHERIC CORRECTION OF CASSIS IMAGES

Introduction:

The Colour and Stereo Surface Imaging System (CaSSIS) [1] is the surface imaging instrument on the European Space Agency/Roscosmos Exo-Mars Trace Gas Orbiter (TGO), which was launched to Mars in March 2016 and began nominal science operations in April 2018. CaSSIS offers advanced colour and stereo imaging capabilities with the ability to acquire multispectral images both in front and behind the TGO during a single orbit. The acquisition is performed in four spectral channels -BLU, NIR, RED and PAN -allowing an improved discrimination among surface materials, particularly minerals containing ferrous and ferric iron. However, the spectral radiance measured by remote sensing instrument orbiting Mars is significantly influenced by the gaseous absorptions and aerosol scattering by airborne ferric iron oxide dust and water ice. This influences the apparent photometry and spectral signatures coming from the surface and impacts the ability to derive higher-level data products related to surface properties such as band ratios, spectral indices, and albedo maps.

In this contribution, we present an experiment aimed at correcting the atmospheric and photometric effects affecting the CaSSIS image MY35 013826 196 acquired at L S = 339 • , MY 35 over the Columbia Hills of Gusev crater, i.e. where the Spirit rover made in situ exploration. The processing, which also leads to an estimation of the atmospheric opacity (AOD), relies on a Digital Terrain Model (DTM), a CRISM derived reflectance model of the scene, and a series of image simulations. The added value of the correction is assessed qualitatively by comparing the original and corrected images as well as the corresponding Colour Band Ratio Composites CBRC1 proposed by [2]. The corrected image is also being used to validate the dark subtraction method, which is a simple empirical correction that shows some promise for minimizing the scattering effects of the atmosphere and isolating the surface component [Tornabene et al. 2021 LPSC; this conference].

Data: A photogrammetrically generated CTX DTM has been densified at 6 m/pixel (comparable to CaSSIS 4.6 m/pixel) according to [3] and has been co-registered with the CaSSIS image. The simulation is based on this topographical scene that is covered uniformly with a reference material described by a BRDF parametric model (RTLS) expressed at 91 wavelengths from 0.4-1.1 µm (i.e., the spectral sensitivity range of CaSSIS). The model ρ model is extracted from the processing of the CRISM EPF observation FRT3192 of the Columbia Hills [4] and corresponds to the bright reddish end member (dust dominated terrains). A simulation tool [START_REF] Douté | Simulating hyperspectral images for Martian 3D scenes[END_REF] is used to compute image cubes of atmospheric radiative transfer quantities and a realistic Bottom-Of-Atmosphere (BOA) simulated image. The latter corresponds to the real CaS-SIS illumination and viewing conditions (SZA=60.48 • , VZA=10.5 • , phase=60.55 • ) and expresses the upward radiance reflected by the surface under different irradiance terms (direct, diffuse, and reflected by the neighbouring facets). The calculation is performed for all wavelengths and the resulting products (direct and diffuse transmission through the atmosphere G d , path radiance R d , and BOA image boa ref image) are convolved by the spectral response function for each channel of the CaSSIS sensor. Since we do not know the AOD of the atmosphere at the moment of the acquisition, we try to obtain an estimate from the image itself. For that purpose we will consider a series of 21 AOD values regularly distributed in [0.1-1.5], each value leading to a set of radiative transfer products and BOA images to seek the best match to the actual CaSSIS imgage, and thereby constrain the best estimate for the AOD. Finally, as an indication of the residual shading in the normalised image, we calculate the spatial variance for each channel. We perform the previous steps for each possible value of AOD. As a result, curves of variance as a function of AOD are plotted.

In Fig. 1 (top) we note a strong decrease of variance for each channel until AOD ≈ 0.9-1.0. Then the PAN, RED, and NIR variances level off whereas the blue variance starts to increase. We estimate that the minimum of the BLU variance curve that corresponds to minimum shading in the image for this wavelength is indicative of the real AOD that can then be roughly estimated : AOD=1.0 ± 0.2. Plotting the calibration factors as a function of the AOD value is also very informative. Indeed in Fig. 1 (bottom) we note a monotonous decrease of the factors regardless of the channel from ≈ 2.5 to 0.8. Interestingly the curves cross the level 1.0 for 0.9-1.2 which is close to the AOD range estimated from the variance. Consequently we propose a rough estimate AOD ≈ 0.9, which is compatible with the climatology of [START_REF] Montabone | [END_REF]. This value of AOD is used for the final photometric and atmospheric correction of the image according to the equations of this section.

Discussion: Fig. 2 demonstrates that, compared to the TOA image, shading/illumination effects are minimised in the corrected image and the colour variations of the scene are generally enhanced, especially in areas once heavily shadowed or illuminated.

The Colour Band Ratio Composite CBRC1 is also generated for the TOA and corrected CaSSIS images. In Fig. 3 the comparison shows that both products shows globally the same spatial distribution of the minerals for the Columbia hills. However, the corrected image has clearly less artefacts, like the reddish hues in the TOA CBRC1 image that correlate with shadows where the atmospheric contribution is the highest. Our method for correcting the atmospheric and photometric effects will be tested in the near future for a scene that shows a higher mineral diversity and will be pivotal for performing quantitative photometric analysis of the surface of Mars.

  Aerosol optical depth estimation and correction of the CaSSIS image: For each AOD, a model of the CaSSIS Top-Of-Atmosphere TOA image is built, toa ref image = boa ref image * G d + R d , that we can compare to the real image (toa real image). A vector of multiplicative calibration factors is then estimated by taking the mean of the image ratio β = mean (toa real image/toa ref image) for each CaSSIS channel. These factors should account for any global radiometric mismatch between the real image and the simulation. Once the calibration factors are known, we can subtract the path radiance multiplied by these factors from the real TOA image. In addition, we divide the result by the total atmospheric transmission to get an evaluation of the BOA image: boa real image = (toa real image -βR d )/G d A photometric normalisation is performed by (i) dividing the latter by the reference BOA CaSSIS image and (ii) multiplying by a spectrum calculated according to the reference BRDF model expressed for uniform geometrical conditions (SZA=60.48 • , VZA=10.5 • , phase=60.55 • ): normalized image = boa real image/boa ref image * ρ model

Figure 1 :

 1 Figure 1: Evolution of the characteristics of the normalised image as a function of the AOD value (blue: BLU filter, green: PAN, red: RED, dark RED: NIR). Top: the amount of residual shading in the image. Bottom: calibration factors β.

Figure 2 :

 2 Figure 2: Comparison of two false color versions of CaSSIS image MY35 013826 196 (restricted here to the Columbia hills). Left: original. Right: corrected for photometric and atmospheric effects.

Figure 3 :

 3 Figure 3: CBRC1 product. Left: from the original image. Right: from the corrected image