Natalie Kübler

Working with different corpora in translation teaching

Introduction

Studies on corpus use for translation, translation teaching and translation studies started during the nineties. [START_REF] Baker | The role of corpora in investigating the linguistic behaviour of professional translators[END_REF] investigated translators' behaviour by studying parallel corpora in order to assess the distance between the language of translation (what she called third code) and the language of native speaker texts. Around the same time, [START_REF] Aston | Corpus use and learning to translate[END_REF] outlined the uses of corpora in translation and translation teaching, and the first Corpus Use and Learning to Translate (CULT) conferences took place (Beeby et al. 2009;Bernardini and Zanettin (eds.) 2000;Zanettin et al. 2003). Much work has since been put into creating and enhancing corpora and corpus tools in order to study translation, and to showing how appropriate corpora can help translators find the information they need to translate.

Despite this interest in academic circles, professional translators are still not very keen on using corpora for translation. A survey carried out in 2005 by the European project MeLLANGE 1 (also reported in [START_REF] Bernardini | Corpora for Translator Education and translation Practice: Achievements and Challenges[END_REF], obtained 623 completed questionnaires, the majority returned by UK professional translators, but also by professionals in France, Italy, and Germany. This survey showed that of the 40.5% who collected domain-specific-texts, most (69.4%) saved these in electronic form and used computer software (mainly word processing packages), to explore them.

In contrast, 41.9% said they had never heard of corpora, even if they would be interested in learning more about them (see table 1). Table 1. Results from the MeLLANGE questionnaire Why do so many professional translators not use corpora? First, not all training syllabuses include corpus use as a skill to be taught. And where it is included, students also learn to use other tools, such as translation memories and Google, which may seem easier to approach. Second, corpus resources are not equally available in all languages and domains, and although query tools have improved considerably over the last decade, they still require specific competences and may not seem user-friendly. Third, corpus skills are never mentioned in job advertisements on the translation market [START_REF] Bowker | Computer-aided Translation Technology[END_REF]). Fourth, the impact of cultural studies seems to have reduced interest in linguistics in the translation community. All of these factors may negatively influence attitudes towards the use of corpora, not just of professional and trainee translators, but also of translator trainers. There is still limited use of corpora in pedagogic settings. As [START_REF] Frankenberg-Garcia | Raising teachers' awareness to corpora[END_REF] notes as far as language teaching is concerned, insufficient attention is paid to training teachers to use corpora. This is true of translation teaching as well.

So how can we render the use of corpora more attractive? A great deal of work is being put into providing better tools for term extraction and corpus querying. We will concentrate here on the teaching situation, and ways in which trainee translators can be taught how to obtain relevant information from corpora. Beeby et al. (2009) propose that the conjunction of corpora and translation in teaching can be seen from two perspectives:

-Learning to use corpora to translate, i.e. using corpora as tools and corpus linguistics as a method to find linguistic information useful in the translation process; -Learning to translate using corpora, i.e. studying the process of translating using corpora, as in [START_REF] Castagnoli | Designing a learner translator corpus for training purposes[END_REF] who show how using a learner translator corpus in the classroom can lead to raising students understanding of different translation strategies. This paper illustrates how different types of corpora can be used from the first of these perspectives.

The activity of translation

We are concerned here with pragmatic specialised translation. Pragmatic translation, defined by Newmark (1988: 133) as a practice taking into account the reader's or the readership's reception of the translation, is the type of translation in which corpus use is at its best. A pragmatic translator should know that the source text may not be perfect, and that the most important thing to take into account is its communicative intent [START_REF] Froeliger | Felix culpa : congruence et neutralité dans la traduction des textes de réalité[END_REF]. This means that the translator must take the real world into account, and have a global general knowledge of the source and target cultures. Specialized translation will be viewed as the translation of texts written in languages for specific purposes (LSPs), not just as conventionally defined, such as science, medicine, or law, but also in the broader senses of general academic language or general business language. Pragmatically translating LSPs requires not only knowledge of the source and target cultures in general, but also knowledge of very specific areas. Even a very well-educated translator may not know the terminology, phraseology, or even grammar of a particular specialist domain. So what is the solution? Calling on the specialists for help? Becoming specialized in a specific domain? Experts often disagree about interpretations in their field, and for a translator to become specialized can be a frustrating experience, as they will never be as specialized as the true specialists. Using corpora can help the translator to acquire specialized knowledge in a subject area, to discover a specific domain, and to find linguistic information, which enables them to convey the intent of the source text.

The issues of intent, genre and register are central to the translation task, but common linguistic difficulties should not be put aside, or cultural gaps. Students must be trained to think 'bilingually', which means being able to understand the source text (meaning and intent) in the context of a specialised domain, and to formulate what they have understood in the target language. This means finding out about terminology, phraseology, and more delicate questions such as semantic prosody. Training future translators means raising their linguistic awareness, both in the source and target language. They must be taught to avoid literal translations, distortions of meaning, use of the wrong register, and so on. These are all areas where corpora can help, as one of the tools available in the translation process. As [START_REF] Bernardini | Corpora for Translator Education and translation Practice: Achievements and Challenges[END_REF] notes, translation is in many ways an ideal field for corpus applications. This paper focuses on specialized translation, trying to show in what ways corpus use is ideal in this case. Examples from practical situations in the translation classroom will show some advantages and drawbacks of corpus use for the translation process.

Corpus Types and Corpus Tools

Depending on the type of task to be performed, different types of corpora may be called for.

However, not all may be equally available. What seems the ideal tool for the translator is a parallel corpus, in which source texts are aligned with their translations in the target language. An appropriate parallel corpus can provide the terminology and phraseology necessary for the translation, as well as examples of alternative translation strategies. However, parallel corpora do not exist for many language pairs and domains, and to compile one requires specific competences and is very time-consuming. Comparable corpora, i.e. collections of texts dealing with the same subject in the source language and the target language, can help overcome problems of 'artificiality' in parallel corpora (Kübler 2003: 41) and can largely make up for the lack of those [START_REF] Frankenberg-Garcia | Compiling and using a parallel corpus for research in translation[END_REF]. These are much easier to compile, even if the task of extracting bilingual terminology and phraseology from them is more complex. Varantola (2002) calls small comparable corpora compiled from the Web disposable corpora. They may never be re-used, but are nonetheless well-suited for a specific task. Such corpora are collections of texts in a specific subject area, which may belong to different genres. For example, a specialized corpus in physics can consist of papers from specialized journals, textbook materials, and popular science articles. These can be backed up by reference monolingual corpora, such as the BNC for British English, the COCA for American English, or Kosmas for German, to provide information about more general linguistic features.

While all these types of corpora can be used in the translation classroom (and also by translation professionals), their limited availability represents one of the reasons why professional translators use them so little. English is well served as far as reference corpora are concerned. But French does not have a reference corpus like the BNC. The Frantext corpus, which contains 4000 texts from the end of the 16 th century up to the 21 st in arts, literature, sciences and technologies, is hardly a reference corpus for contemporary French. While currently being completed with more contemporary texts, it is not a balanced corpus, even if it contains 210 million running words, and at most may be useful to literary translators. Some newspaper-based tools are available: Thus Glossanet 2 allows users to create and query their own on-line specialized corpora from French and Belgian newspapers; concordances are sent to the user by e-mail. Les Voisins de le Monde 3 allows users to search ten years of Le Monde and find collocates for arguments which are governed by verbs, nouns, or adjectives, or predicates, i.e. PoS which govern argument collocates, but also find the distributional neighbours of arguments and predicates (i.e. arguments or predicates which share the same collocates).

When working in specialized translation with French as a target language, parallel or comparable specialized corpora are also necessary. All information concerning LSPs, such as terminology and different text types, has to be found in specialized corpora. However, it is not always easy to build a comparable French/English corpus in a specialized domain, as French documents belonging to the same genres as the English ones may be rare. For instance, scientific research articles are rarely written or translated into French, as most French scientists publish directly in English. Textbook materials and popular science articles in French are easier to find. So working with comparable corpora in these areas raises issues of how comparable the corpora really are. In technical domains, comparable French texts may be easier to find, but they are not always devoid of mistakes. There may also be confidentiality issues, such as proprietary information of a company. This is especially a problem for terminology and specialized phraseology. [START_REF] Zanettin | DIY corpora: The www and the translator[END_REF] reported the advantages of compiling do-it-yourself corpora to translate specific documents. His hope that corpora would 'find their place in the translator workstation together with other corpus resources and computer-assisted tools ' (2002: 8) does not yet seem to have been fulfilled, however. [START_REF] Bernardini | Corpora for Translator Education and translation Practice: Achievements and Challenges[END_REF] called for work in three different areas: the role of corpus work for awareness-raising, the construction of translator-oriented (e-)learning material, and the fact that corpus construction and corpus searching tools should be more user-friendly. But [START_REF] Aston | Corpora in translators training[END_REF] note that the situation has still not changed much. So part of translator education for corpus use must still deal with corpus creation and corpus query tools. Various tools can be used in the classroom, which are equally applicable in professional situations, such as Wordsmith 4 , AntConc 5 , ParaConc 6 , Xaira 7 . Let us see how these issues can be faced in the classroom.

Translation tasks and basic exercises

Corpora can play different roles at different stages of the translation process: (a) during the documentation phase, in which translators look for initial information on content, terminology and phraseology in the source and target language; (b) during the translation phase, in which translators look for solutions to specific terminology and phraseology problems; (c) during the revision phase, in which they investigate other alternative strategies. Each of these phases can be seen as involving a series of tasks. For instance, the first phase requires:

identifying the genre and register of the document to be translated; -collecting a corpus; -exploring the domain and understanding difficult or unknown concepts; -acquiring useful information on linguistic points, particularly terminology and phraseology, in both source and target languages.

To perform these tasks, the translator or learner translator must have already acquired a number of concepts in linguistics, and have attained a certain level of linguistic awareness. This is one reason why corpus linguistics should form part of translator training curricula, as a precondition to using corpora in the translation process. Students must first understand what a corpus is, what types of corpora exist, and what is in them. Frankenberg-Garcia (2010) proposes tasks to raise student awareness of these issues, such as understanding different corpora, formulating corpus queries, and interpreting corpus output. These can all be adapted for translator training, and need not necessarily be linked to translation tasks as such.

Translation tasks

In the two tasks which are described below have been carried out by French-speaking students at the University Paris Diderot in the frame of a Master's in specialised translation for several years l Working together to agree on terminology and phraseology; and

l Revising the translation.

Task 2: Individual translation of a specialized text in any domain

This is a year-long individual project, achieved by second-year master's students, and that deals with a larger text, usually about 5000 words. It must be in a specialized domain, but may be of any genre. The translation process is again divided into a series of sub-tasks.

Identifying genre and register

Understanding genre is vital to the translation process, because the same genre can present very different linguistic features according to the language. For example, the genre of user manuals uses a much more formal register in French than in English. In English, giving instructions is done using the second person (you), whereas French tends to use the infinitive or a third person form:

Example 1: extracts from a comparable EN/FR corpus on digital cameras EN You must put these file back after the firmware update has been completed. make sure you put time aside to learn it properly.

The SSFDCs are keyed, so you can't insert them backwards.

You can adjust the brightness of the LCD at any time by holding down the DISP button.

FR

Les utilisateurs expérimentés peuvent contrôler et ajuster la sensibilité de l'appareil.

Nikon Capture permet d'ajuster la taille de sortie.

Mettre le cordon série sur l ordinateur et brancher la fiche sur l appareil photo.

As the translator's first task is to define the genre of the text to be translated, emphasis is placed on raising students' awareness of what genre is and how to distinguish different genres. The main questions to be asked are:

-What is the speaker's purpose and topic? -Who is the intended audience?

In answering these questions, the student should relate them to linguistic and rhetorical features. Is it a didactic text, in which definitions and explanations are given? If it is a scientific article, then there will probably be few definitions and explanations, as the author will take it for granted that the reader already knows a lot about the subject. If it is a manual for a washing-machine, then the reader will not be presumed to know anything technical about the field, and the translation should be kept as clear and simple as possible. Genre and domain are also important from a terminological point of view: a highly specialized scientific text will require a lot of research on terminology, while an article in a newspaper may require less such work, but more on other points, such as general and cultural knowledge. Once the genre of the text has been determined, the internal characteristics of the text can be studied. Simple exercises with corpora can give students insights into these characteristics and lead them to understand more about internal linguistic characteristics, which are associated with the genre, such as the difference in addressing the reader in French English user's manuals, or the use of nous for the author in French scientific articles, whereas English tends to use the passive.

Take the word hypothesis, which is supposed to occur quite often in scientific articles. Even before collecting a comparable corpus for the specific domain, a general corpus can provide important information. The examples below come from the BYU BNC and the BYU COCA 10 .

Asking students to search for the collocates of hypothesis in the fiction and the academic subcorpora of BNC and COCA very easily led them to see differences between the two genres.

There are far more significant collocates for hypothesis in the academic subcorpus than in the fiction one (100 against 24 in COCA, 100 against 12 in BNC). In the fiction subcorpus of COCA, only one of these collocates is not a grammatical word, namely test. In the academic subcorpus, almost 50% of the top 24 significant collocates are non-grammatical words, as shown in table 2. The collocates for hypothesis in the two subcorpora show a number of differences in register. The academic subcorpus has no pronouns among the top collocates, whereas four pronouns are near the top of the list for the fiction corpus. The degree of formality seems lower in fiction, witness also the presence of contracted forms. These simple results can then lead to a discussion of other linguistic differences.

Compiling a comparable specialised corpus

DIY corpora represent a useful documentation resource in specialised translation. Over the years, our students have compiled comparable corpora in a number of specialised domains [START_REF] Kübler | Corpora and LSP translation[END_REF], using the keywords found in the research article on which they are working. As mentioned above, for some domains, especially scientific ones, this poses problems insofar as there are very few research articles written in French. (A parallel corpus is also impossible because research articles are almost never translated into French.) 11 In such cases students are guided to look for PhDs 12 didactic texts, and websites on popular science.

Students are made aware that their corpora may not be fully comparable, and that this can present drawbacks, particularly as far as terminology is concerned. Young French researchers in hard sciences read scientific literature in English but write their PhDs in French. Even though there has been a French scientific terminology for a long time, most new concepts are coined in English and then translated into French almost literally. As PhDs are not produced for publication, this terminology may not be fully reliable. Some didactic texts can be found on university websites, but most of the French texts that staff use in their university teaching are very difficult to get hold of.

There is, on the other hand, plenty of popular science in French available on the web, but again this may not be a reliable source of terminology.

Learning more about the domain

In understanding the source text, not only are language problems at stake, but also cultural ones. In texts that deal with specialized subjects it is necessary to get acquainted with the domain. An expert in the domain can help the translator understand it, but it is not always possible to have an expert at hand, and here corpora can play an important role (see, for example, [START_REF] Maia | Training Translators in Terminology and Information Retrieval using Comparable and Parallel Corpora[END_REF]). In the Earth Science translation task described above, students are required to write definitions for a series of terms before they start to translate, in order to improve their understanding of the domain. They are then encouraged to search for definitions in their corpora, using linguistic markers to find these [START_REF] Pearson | Terms in Context[END_REF]).

In the examples below, the markers is a and i.e. in English, and est un(e) and c'està-dire in French, were used to find term definitions.

Example 2: results of corpus searches for definitions, using linguistic markers is a

Gelifluction is a thaw-related solifluction (Matsuoka, 2001) that is controlled by elasto-plastic soil deformation.

est un

Un bassin avant arc est un bassin océanique situé entre la subduction et la terre, au contraire d'une fosse océanique qui est le bassin provoqué par la subduction elle-même.

i.e.

an extrusive (i.e. mud-volcanic) rather than an intrusive (i.e. diapiric) mechanism builds up the mud domes… c'est-à-dire Les roches sédimentaires, c'est-à-dire ces roches qui proviennent de la transformation de sédiments comme les sables et les boues…

Terminology and phraseology

The use of corpora to find term equivalents has been abundantly described in the literature (e.g. [START_REF] Bowker | Working with Specialised Language: a Guide to Using Corpora[END_REF][START_REF] Kübler | Corpora and LSP translation[END_REF][START_REF] L'homme | La terminologie: principes et techniques[END_REF][START_REF] Maia | Training Translators in Terminology and Information Retrieval using Comparable and Parallel Corpora[END_REF]. While much current research is focused on the automatic retrieval of term equivalents (see Zweigenbaum 2008, for example), there are as yet no available tools for this. So translators must learn how to query comparable corpora to locate term equivalents. This methodology is nowadays well-documented and can also be applied to look for phraseological equivalents. Showing students that one of the prepositional collocates for hypothesis is with led to an exercise aimed at raising their awareness of the different definitions of a collocation (two words with or without a grammatical relationship, which co-occur in a statistically significant manner), and of the existence of phraseological units that appear more often in academic than in fictional texts. Example 3 shows concordances from the BNC academic and fiction subcorpora. A simple gap-filling exercise, in which students have to fill in the blanks using the words in boldface, shows that there are a number of phraseological units associated with hypothesis (help with, compatible with, etc.), and that hypothesis + with collocation reported by the tool does not necessarily imply a syntactic relationship between node and collocate, since, in the last example from the BNC academic subcorpus, with occurs in a different sentence from hypothesis.

Example 3: phraseological units associated with hypothesis

BNC academic subcorpus

only that can NAEP not help with research hypothesis formation or, evidence which was inconsistent with the working hypothesis fasten onto those that agree with his hypothesis and overlook those behaviour which is inconsistent with the hypothesis of budget.

This is compatible with the hypothesis that prenatal nutrition affects

Our results support this hypothesis. Reactions with carcinoembryonic formation in these patients is also consistent with this hypothesis proposed hypothesis that Mozart suffered with Tourette's syndrome.

BNC fiction subcorpus

it dishonesty; comparing facts with a hypothesis. However, even if The expression avec virulence can only be used with speech verbs, whereas de manière offensive, is found with verbs describing a physical or virtual attack, but not in the discourse.

In formulating the target language text so that it is adapted to its culture, a general monolingual corpus is the student's best friend, as it helps them to find correct collocations, colligations, semantic preferences and prosodies.

Discovering the importance of semantic prosody in the translation process

Specialized texts in scientific domains are usually thought to have very limited semantic prosodies.

It is commonly held that connotations, be they positive or negative, should be banned from scientific articles, which should be purely factual. Semantic prosody is however an important feature of scientific writing, which is linked to evaluation. Students often do not realise the importance of semantic prosody in their own language, and this leads them to awkward translations. [START_REF] Stubbs | Words and Phrases. Corpus studies of lexical semantics[END_REF] describes the English verb cause as having a negative semantic prosody: the things that are caused are generally undesirable. This is also the case of the French causer. Pinpointing the differences between the general and the specialized corpus helps students become aware of the problem, and avoid using causer as the translation equivalent of those English verbs of causation that do not have a negative semantic prosody.

Conclusions

I hope to have shown how available and do-it-yourself corpora can be used in specialized translation training for all phases of the translation process. The examples illustrate the need to raise students' awareness in using corpora, rather than merely looking for translation equivalents. A complete translator education cannot avoid linguistic concepts. This can be done simultaneously with discussion of how to find translation equivalents, which is made much easier by using corpora.

If corpus use is to become more popular among professional translators, it is essential not only to provide them with appropriate user-friendly, integrated tools, but also to teach them the rudiments of corpus linguistics as part of their training. In the future it should be an obligatory part of all translation schools' curricula. Choosing very recent scientific articles in which students have almost no prior knowledge helps them grasp very quickly how useful corpora are and how important linguistic analysis is. This choice allows translation students to appropriate the tools and methods corpora and corpus linguistics provide. They can then apply those to any domain and genre of text.

Notes

  (since 2004). Therefore all the examples mentioned here come from real-life classroom situations. , Students have thus access to a variety of corpora and tools that are listed here: the corpora and tools used by French speaking students in translating from English into French are: -a ten-million word corpus of Le Monde 8 (which represents one year) and a homemade concordancer using Perl regular expressions; -the Les Voisins de Le Monde web interface to the 200-million-word Le Monde corpus ; -the English/French Europarl 9 corpus (Koehn 2005) with ParaConc; and a series of English/French and DIY comparable corpora in Earth Science 8 and in digital camera technology, which have been compiled by students over the years, and queried either with the home-made online concordancer or with AntConc. Task 1: Group translation of a research article in an imposed specific domain: Earth Science First-year master's students are assigned specialised research articles in two or three sub-domains of Earth Science, such as volcanoes, the birth and evolution of mountains, plate tectonics, hydrology, ice, climatology, and mud volcanoes. Each sub-domain and each article are assigned to a group of students. The articles are then divided into sections of about one thousand words, and each student is assigned one section. The aim of the project is to achieve a complete translation of the articles, with a consistent terminology for the sub-domain. The pedagogic objective is to lead students to discover the use of corpora in the process of translating a specialized text in a group translation context. The task is divided into a series of subtasks, some of which also relate to other courses. These are: l Defining the genre; l Collecting a corpus; l Exploring the domain and understanding difficult or unknown concepts; l Acquiring bilingual information on domain-specific terminology and phraseology, and on the phraseology of scientific argumentation; l Conveying information appropriately in the target (native) language;

Table 2 .

 2 The significant 24 collocates of hypothesis in the fiction subcorpus of COCA, and the top 24 collocates in the academic subcorpus. The collocates are ordered by significance.

		COCA fiction subcorpus		COCA academic subcorpus
		78,752,154 tokens		79,292,295 tokens
		WORD
	1	N'T	1	SUPPORTED
	2	YOUR	2	SUPPORT
	3	YOU	3	THIS
	4	HE	4	TESTING

  The next step consists in finding equivalents for these phraseological units, using in this case the corpus in Earth Science. Looking for hypothèse(s) preceded by avec (as a translation equivalent of with) gives the following results, which provide insights as to French phraseological equivalents:His translation hypothesis was to use agressivement. However, during the revision phase, this was felt to sound incorrect in French, so it was checked against Le Monde:In this newspaper corpus, agressivement seems to modify adjectives rather than verbs. As its contexts differ substantially from those of aggressively in the source text, there is no support for agressivement being an appropriate translation. Looking for aggressively in the Europarl parallel corpus, the interesting equivalent de manière offensive was noted, which comes from the domain of war. Offensif collocates with missile, armée, guerre, stratégie and appears in many metaphors. Les Among these neighbours, two have a very close meaning to agressif, namely offensif (already noticed in the Europarl corpus) and virulent. Looking for their collocates in Les Voisins de le Monde showed that the first is used in war metaphors, the second in other metaphors in the discourse. This finally led to the following French translations of the source text sentences

	containing aggressively:
	Example 6: agressivement in Le Monde Example 8a: text extract of avec virulence
	Example 4: avec....hypothèse in a hydrology corpus L'industrie du copyright (qui regroupe l'industrie du film et celle du jeu vidéo, les télévisions ainsi que les
	Mussolini a rompu avec la ligne agressivement antibourgeoise des premiers faisceaux, maisons d'édition musicales et littéraires) les attaque avec virulence.
	compatible avec cette hypothèse Chapitre III. Etude géochimique au milieu de cette foule agressivement banalisée, on trouve quelques
	en corrélation avec cette hypothèse. Ils permettent de mettre en par la recherche d un son cru, agressivement dépouillé, son travail de producteur 8b : text extract of de manière offensive
	toluène est compatible avec cette hypothèse. Les thiols à courte à l'énoncé ce qu'il pourrait avoir d agressivement masochiste. N'empêche : même Les attaquants ont découvert cette faille et s'en prennent de manière offensive aux index des réseaux P2P les
	avec les Mammifères. Cette hypothèse essaye d'expliquer plus populaires afin de les empoisonner.
	est en accord avec une hypothèse de réajustement tectonique du
	en accord avec cette hypothèse. sur ce volcan est également en
	accord avec cette hypothèse qui, si elle s'avère applicable
	a été réalisée avec une hypothèse de calcul plus réaliste que
	Where equivalents are not found in specialized corpora, general corpora can often provide answers
	for specialized translations. The following example relates to the translation of aggressively in a Voisins de Le Monde gave the following results:
	computer science article entitled 'Index poisoning attacks in peer-to-peer file-sharing systems'. In
	his comment on translation problems, the student, who had not been able to compile an Example 7: neighbours of agressivement in Les voisins de le Monde
	English/French comparable corpus in this domain because of the lack of research articles in French, conforme autoritaire xénophobe virulent radical irresponsable brutal inacceptable spontané opposer neutre
	efficace cibler intelligent muscler inadmissible arbitraire contraire généreux provocateur souple violent
	noted that aggressively was used in two different contexts in the source text:
	concerter semblable volontariste préventif juger unilatéral pervers provocant dangereux digne contradictoire
	inhabituel consensuel énergique défensif civiliser confus ferme similaire scandaleux courageux mesurer
	Example 5: extracts of aggressively in two different contexts in a computer science article prudent identique répressif archaïque transparent paradoxal habile maladroit cohérent suicidaire offensif
	the "copyright industry" (including the music, film, television, gaming, and book publishing industries) is discriminatoire polémique destructeur menaçant injuste pragmatique délibérer rationnel coordonner guerrier
	aggressively attempting to curtail the unauthorized distribution of content in P2P file sharing systems nuancer inhumain indigne répréhensible conciliant revendicatif
	Attackers have discovered this vulnerability and are now aggressively index poisoning popular file-sharing
	systems.

  In the 200million-word Le Monde corpus, nearly all of the objects of forms of causer have negative connotations: Example 9: direct objects of the verb causer in French dommage irréversible dégât irréparable dommage irréparable dégats tracas important dégât frayeur dégât matériel r dégât tort ravage dommage désagrément préjudice traumatisme gêne émoi perturbation déception ennui embarras lésion remous trouble embouteillage désastre nuisance chagrin souffrance déboire souci blessure perte infection trou choc désordre décès mort pollution plaie sorbonne destruction inondation scandale douleur émotion mal ruine incendie fracture accident cancer pénurie université surprise chute inquiétude disparition bruit maladie catastrophe préoccupation maximum danger malaise peur retard commune drame problème difficulté tension victime effet risque crise millionIn scientific English, on the other hand, to cause does not always have a negative semantic prosody. This is probably the reason why more and more cases of causer in scientific French have unconnotated objects, which at times seems awkward: Example 10: causer followed by unconnotated objects

migration de joints de grains, ?causant une évolution progressive (better: générant) partie distale peut ?causer la superposition de différents régimes (better: provoquant) surface du sédiment. Le flux ?causé par l'action des marées a été (better: engendré)


A Baazaoui
email: ahlem.baazaoui@enit.fr

T Fourcade

O Dalverny

J Alexis
email: djoel.alexis@enit.fr

M Karama

Experimental and Numerical Study of Interfacial Fracture Parameters of a Brazed Joints

Keywords: AuGe solder, shear test, fractography, thermomechanical, interfacial fracture, cohesive zone model, finite element analysis

This paper deals with an identification methodology of the interfacial fracture parameters to predict the lifetime of a metallic brazed joint. The methodology is based on an experimentalnumerical study whereby the optimal parameters are obtained. The experimental data, using the scanning electron microscope analysis, allowed approving that failure of the assembly based AuGe solder seems first to appear near the interfaces. These results were confirmed by micrographs analysis of the solder/insert and solder/substrate interfaces. Then, using shear test results and parametric identification coupled with a finite elements model (FEM) simulation, the damage constitutive law of the interfacial fracture based on a bilinear cohesive zone model are identified. The agreement between the numerical results and the experimental data shows the applicability of the cohesive zone model to fatigue crack growth analysis and life estimation of brazed joints.

Introduction

Solder joint failure, either due to thermal loads or mechanical loads, is a significant reliability concern in power electronic packaging. The majority of the research show that failure seems first to appear near the solder/die and/or solder/ceramic substrate interfaces [START_REF] Ren | Fracture analysis on die attach adhesives for stacked packages based on in-situ testing and cohesive zone model[END_REF] (Baazaoui et al., SF2M2013) (Baazaoui et al, 2013) [START_REF] Müge | Thermomechanical fatigue failure of interfaces in lead-free solders[END_REF]. These failure modes are mainly due to the presence of a high porosity ratio [START_REF] Msolli | Viscoplastic behavior of diamond die attach subjected to high temperature conditions[END_REF] and/or brittle intermetallic compounds [START_REF] Müge | Thermomechanical fatigue failure of interfaces in lead-free solders[END_REF] (Baazaoui et al., CFM2013) [START_REF] Yao | Energy based micromechanics analysis on fatigue crack propagation behavior in Sn-Ag eutectic solder[END_REF]. To understand the interfacial cracking phenomena in adhesive joints and thus to optimize the design of high temperature packaging, cohesive zone models (CZM) (Crisfield & Alfano , 2001) (Davila & [START_REF] Camanho | Mixed-Mode Decohesion Finite Elements for the Simulation of Delamination in Composite Materials[END_REF] [START_REF] Li | Use of mode-I cohesivezone models to describe the fracture of an adhesively-bonded polymer-matrix composite[END_REF] have been widely used. The CZM can be considered as an improvement of the Griffith theory (linear fracture mechanics). Indeed, the CZM is able to describe the entire fracture process including crack initiation and propagation and taking into account the presence of a process zone upstream developing the crack tip. Since the developing works by Barenblatt [START_REF] Barenblatt | The mathematical theory of equilibrium cracks in brittle fracture[END_REF] and Dugdale [START_REF] Dugdale | Yielding of steel sheets containing slits[END_REF], many cohesive zone models have been proposed in the literature [START_REF] Tvergaard | The relation between crack growth resistance and fracture process parameters in elastic-plastic solids[END_REF]) [START_REF] Needleman | Numerical simulations of fast crack growth in brittle solids[END_REF] [START_REF] Ortiz | Computational modelling of impact damage in brittle materials[END_REF] (Zavattieri & Espinosab, 2001). All of them start from the assumption that one or more interfaces can be defined, where crack growth is allowed by the introduction of a possible discontinuity in the displacement field. This interfacial fracture has been modeled successfully in many applications, including the power electronic packaging, using the cohesive zone approach based on a traction-separation law [START_REF] Needleman | A continuum model for void nucleation by inclusion debonding[END_REF]. This law is defined the relation between the traction load exerted by the joining layer as a function of the separation between the fracture surfaces or across the entire layer. For pure-mode, three important parameters governing a traction-separation law are the cohesive element stiffness , the maximum traction stress and the energy dissipation at failure . The governing law for a specific interface is often found by assuming a relation of a certain form and then matching a finite element simulation with experimental observations. In this work, we propose to study and characterize the interfacial fracture behaviour of a metallic brazed joint. The first part of the study deals with the experimental characterisation performing the connection process of copper inserts using Au88Ge12 eutectic solder alloy, then the shear test based on the image correlation followed by a fractography analysis. In the second part, the constitutive law of the bilinear cohesive zone model and the finite element modelling of the assembly are presented. Finally, the estimation of the bilinear CZM parameters were performed based on an experimental-numerical methodology.

Experimental Characterizations of Brazed Joints

Experimental Procedure. The assembly is constituted by copper inserts of dimensions 4.7×4.7×2 mm which are bonded on copper substrates of dimensions 30×10×2 mm at a temperature of 420°C using the eutectic solder alloy Au88Ge12. The multilayer structure are presented in the Fig 1 .a. More details in specimen elaboration has been demonstrated in elsewhere (Baazaoui et al., SF2M2013) (Baazaoui et al., CFM2013). Notably, reference mark with dark color, which represented by the points A and B an shown in Fig. 1.b, is bonded to measure the displacement of the assembly and the displacement here is defined as the variation of non-contact measurement reference marks along the top surface of the inserts under shear loads. Compared with this latter result, the obtained shear strength of 47.5 MPa in our study is very low. Indeed, the failure seems first to appear in the Cu/substrate interfaces. To verify these assumptions, the failure zones for the AuGe specimen were analyzed using the scanning electron microscope (Fig. 3). The EDX analysis of the specimen failure shows the presence of the AuGe and Ni. The micrographs and EDX analysis of AuGe broken specimen illustrate a mixed fracture: adhesive and brittle failure in the two Cu/solder interfaces and cohesive and ductile failure in the joint.

Numerical simulations of the interfacial fracture

To simulate the behaviour and failure mechanism of solder interconnects under the shear loading, a fracture mechanics criteria based cohesive zone model was employed. The behaviour laws of cohesive elements are usually described in terms of traction versus separation which related the interface's relative displacement to the traction vector (Fig. 4). Generally, the cohesive elements behave elastically until damage initiation and obey a softening behaviour afterwards. initiation and propagation at the interfaces of electronic assemblies. They also approved the convergence between the numerical simulation and experimental results.

Constitutive law of the cohesive zone model. For pure-mode I and pure-mode II or mode III loading, the shape of the constitutive bilinear CZM law is shown in Fig. 4.b. This available tractionseparation law (TSL) is to assume the linear elastic behaviour followed by the initiation and evolution of damage. A high initial stiffness is used to hold the top and bottom faces of the decohesion element together in the linear elastic range. Once the peak values of the normal or shear nominal stress are attained, the stiffnesses are gradually reduced to zero. The analytical expression of the bilinear cohesive model law is as follows (Camanho & Davila, 2002) [START_REF] Alfano | On the influence of the shape of the interface law on the application of cohesive-zone models[END_REF]):

= ≤ 1 - < < 0 ≥ (Eq. 1) = - - (Eq. 2)
Where and represent the relative displacement at damage initiation and complete failure as shown in Fig. 5, respectively.

refers to maximum value of the relative displacement attained during the loading history. and are given by [START_REF] Alfano | On the influence of the shape of the interface law on the application of cohesive-zone models[END_REF]): = and = (Eq. 3)

represents the fracture energy of the cohesive elements. A fundamental aspect in the formulation of the cohesive constitutive model is the requirement that the energy dissipated at a fracture propagation must be equal to the fracture energy, the following relation (Eq. 3) must be satisfied: In this paper, only pure-mode II debonding problem is considered so that the analysis is simplified by focusing the attention on scalar interface relationships relating one relative-displacement component to the dual stress component . In this case, the CZM characteristic parameters are the cohesive element stiffness or the cohesive layer modulus , the fracture energy and the strength .

Cohesive zone modeling of the interfacial fracture. To simulate the behaviour and failure mechanism of AuGe solder interconnects under shear loading, a 2D finite element model has been developed incorporating the cohesive zone model which defined in the previous section and using the commercial finite element software ABAQUS®. The geometrical and boundary conditions are shown in Fig. 5. This model is composed of copper insert, copper substrate which are bonded together with the eutectic solder alloy Au88Ge12, two cohesive layers and a rigid shear tool. The two cohesive zone layers are placed in the solder/insert and solder/substrate interfaces, where fatigue failure is typically observed. The thickness of the cohesive and joint layers, measured experimentally, is about 4 and 50 µm, respectively. The two cohesive layers were modeled using a 4-nodes bilinear quadrilateral element of type 2D cohesive (COH2D4). The length of the cohesive elements was 100 µm. In the thickness, the cohesive zone must be discretized with a single layer of cohesive elements (Needleman & Xu, Numerical simulations of fast crack growth in brittle solids, 1994). In this case, the path of the cracking is naturally determined by the overall field and the cohesive response. The solder, the insert and the substrate layers were meshed with a 4-node bilinear 2D plane stress quadrilateral elements (CPS4R). The size of the elements in the solder is the same as the cohesive elements (100 µm) in the length and 10 µm in the thickness. 14670 8850 Specific heat capacity (J.Kg -1 .K -1 ) 134 380 Thermal expansion coefficient (µm.m -1 .K -1 ) 12. [START_REF] Alfano | Finite Element Interface Models for the delamination analysis of laminated composites : mechanical and computational issues[END_REF] 17.3 In the current model, we supposed that the two cohesive layers have the same interfacial properties and an isotropic mechanical behaviour. The procedure used to determine the CZM law's parameters is explained in the next section. The AuGe solder and the copper insert and substrate were considered to have elastoplastic behaviour. All mechanical proporties for AuGe and copper materials were respectively listed in the Table 1. The plastic behaviour of the AuGe solder and the copper substrates was taken from the experimental results of Hosking et al. and S. Msolli, respectively [START_REF] Hosking | Intermediate Temperature Joining of Dissimilar Metals[END_REF] [START_REF] Msolli | Modélisation thermomécanique de l'assemblage d'un composant diamant pour l'électronique de puissance haute température[END_REF]. This assembly is subjected to shear loading with a displacement rate of 1 mm/min and at a temperature room of 20°C. The displacement rate is applied in a reference point located in the rigid shear tool. Identification procedure method of the interfacial constitutive law Identification algorithm. One of the biggest challenges faced by researchers who work with cohesive zone models is to determine the model parameters because standard experimental tests do not exist [START_REF] Bhate | Fatigue crack growth and life descriptions of Sn3.8Ag0.7Cu solder joints: a computational and experimental study[END_REF]. As a matter of fact, the CZM links the microstructural failure mechanisms to the displacement fields governing bulk deformations. Thus and as mentioned before, a cohesive zone is characterized by the properties of the material, the damage initiation condition and the damage evolution function. As defined in the previous section, the considered constitutive law is the bilinear cohesive zone model. In this case, the properties of the interfacial behaviour required to be identified are the cohesive element stiffness or the cohesive layer modulus for the elastic behaviour and the strength and the fracture thoughness for the damage constitutive law. In this study, we considered the modulus of elasticity as known during the identification procedure and it is supposed to be the same as the joint layer (69.2 GPa). Thus, the number of the CZM parameters to identify is 2; the fracture energy and the strength of the interfaces layer. The identification of this two interfacial fracture parameters was achieved using the results from the shear test and corresponds to the resolution of an inverse identification problem. A usual methods of resolving such a problem is summarized in Fig. 6 [START_REF] Fourcade | Parametric identification of elastic-plastic constitutive laws using spherical indentation[END_REF]. The input data from which identification is done are the experimental shear test results, the behavior law of the solder/insert and solder/substrate interfaces associated with a set of initial parameter and the numerical model of the test. For a set of parameters , a difference vector is then calculated as:

= , … , , (Eq. 5)
Where = .

is the mean experimental shear test curve, is the simulated one and is the weight and have a value of 1 in order to maximize the weight of the value at maximum depth, which is the one with the smallest uncertainty. The cost function is then defined as the Euclidean norm of the difference vector (Eq. 6). = (Eq. 6) The identification is led by minimizing the cost function using the Levenberg-Marquardt algorithm from the scipy.optimize python library. The unknown parameters and were initially set to 47 MPa and 4.8 N/mm, respectively, and the domain of these two parameters is given in Table . 2. The initial values were chosen over the experimental range. The maximal value of the cohesive elements strength is set to the value of the AuGe joint strength at failure. Estimation of the interfacial fracture parameters and discussion. The values of the strength and the fracture energy was determined using an iterative procedure to obtain the best match between model predictions and the experimental measurements of the load-displacement behaviour…..

Summary

To predict the lifetime of the AuGe brazed joint, a bilinear CZM parameters of Cu/AuGe interfaces were identified using inverse method with experimental shear test and finite element modelling. The experimental data allowed approving the load-displacement behaviour of the joint and the two Cu/solder interfaces. The SEM and EDX fractography analysis show a mixed fracture: adhesive in the two Cu/solder interfaces and cohesive in the joint. The interfacial fracture of these assembly was then simulated using 2D finite element modelling. The parameters of the bilinear CZM for the case of mode II loading were obtained based on inverse methodology and the results seem to be in good agreement with the experimental results……..

Fig. 1 .

 1 Fig. 1. Sample preparation, a) multilayer structure of the bonding process, b) testing configuration of the AuGe specimen. To determine the load-displacement behaviour of the assembly, displacement-controlled shear tests with a constant rate of 1 mm/min were operated at room temperature by INSTRON apparatus dedicated to perform shear tests on electronic devices. Meanwhile, an innovative non-contact detecting system using the correlation camera was introduced to measure displacement of the assembly. The database of the experimental results obtained is used for identification of the interfacial fracture parameters of the brazed joint. Experimental result analysis. Fig.2 shows the shear test curves obtained by INSTRON apparatus (Fig. 2.a) and video displacement measurement (Fig. 2.b). For both load-displacement results, the maximal applied load exceeds 1050 N. The crosshead and real displacement at failure reaches 1.27 mm and 0.136 mm, respectively. The latter result (Fig. 2.b) defines the accumulation displacements of the AuGe brazed joint and the two Cu/solder interfaces and the extra displacement in the first curve (Fig. 2.a) is induced by the load frame, load cell and load clips.

Fig. 2 .

 2 Fig. 2. Shear test results for 1 mm/min of a) effective apparatus results and b) correlation camera results.The load-real displacement behaviour obtained by the camera correlation show an important and non-linear plastic deformation. Knowing the maximal load and the insert surface, the shear strength

Fig. 3 .

 3 Fig. 3. SEM Micrographs of AuGe specimen failure and EDX analysis of the same zone.

  The specific energy dissipated by the cohesive element can be calculated from the area under thecurve. Different constitutive laws of the cohesive zone model were proposed in the literature (Needleman, 1987) (Tvergaard & Hutchinson, 1992) (Needleman & Xu, 1994) (Ortiz & Camacho , 1996) (Zavattieri & Espinosab, 2001) (Hutchinson & Tvergaard, 2002). Since the shape of the function may influence the results of the simulation (Alfano, 2006)[START_REF] Scheider | Micromechanical based derivation of traction-separation laws for cohesive model simulations[END_REF], it is crucial to identify a law that is suitable for capturing the interfacial fracture extension behaviour of the cohesive layers. According to the nature of the material (ductile, brittle,…), the type of loading considered (monotonic or cyclic loadings), the cohesive zone model can be defined. The fractography results of the AuGe solder based assembly carried out in the first section have been shown a brittle damage in the Cu/solder interfaces. To predict the interfacial and brittle damage behaviour observed experimentally, the irreversible and bilinear cohesive zone model(Alfano & Criesfield, 2001) (Camanho & Davila, 2002) may apply. This bilinear cohesive zone model is currently the most widely used for interfacial fracture behaviour simulation due to its availability in ABAQUS® software. Thus, it has been widely used to characterize the interfacial damage behaviour in power electronic packaging[START_REF] Ren | Fracture analysis on die attach adhesives for stacked packages based on in-situ testing and cohesive zone model[END_REF] [START_REF] Yao | Cohesive fracture mechanics based numerical analysis to BGA packaging[END_REF]. These studies have shown the performance of the bilinear cohesive zone methods for describing and predicting fracture

Fig. 4 .

 4 Fig. 4. a) Configuration of the cohesive fracture, b) TSL for bilinear cohesive zone model.

Fig. 5 .

 5 Fig. 5. Finite element model of sandwiched assembly joined by AuGe solder.

Fig. 6 .

 6 Fig. 6. Block diagram of the identification of CZM constitutive law from simple shear tests (Fourcade et al., 2014).

Table 1 .

 1 Properties of the AuGe and copper materials used for assembly[START_REF] Hosking | Intermediate Temperature Joining of Dissimilar Metals[END_REF])[START_REF] Msolli | Modélisation thermomécanique de l'assemblage d'un composant diamant pour l'électronique de puissance haute température[END_REF].

	Properties	AuGe Copper
	Elastic modulus (GPa)	69.2	128
	Poisson's ratio	0.32	0.36
	Yield strength (MPa)	160	233
	Tensile strength at failure (MPa)	200	268
	Thermal conductivity (W.m -1 .K -1 )	44.4	398
	Mass density (Kg.m -3 )

Table 2 .

 2 Domain for cohesive zone model parameters during identification process.

	Min	Max


Jeremie Graneix
email: jeremie.graneix@enit.fr

Jean-Denis Beguin
email: bjean-denis.beguin@enit.fr

Joel Alexis
email: cjoel.alexis@enit.fr

Talal Masri
email: dtalal.masri@enit.fr

Weldability of superalloys Hastelloy X by Yb : YAG laser

Keywords: laser welding, Hastelloy X, microstructure, mechanical properties

Requirements of aircraft parts welded becoming increasingly severe especially in terms of reproducibility of geometry and metallurgical grade of weld bead; laser welding is a viable method of assembly to meet these new demands by its automation to replace longer term the manual TIG welding process. The purpose of this study is to determine the weldability of Hastelloy X alloy by the butt welding process Nd: YAG laser. To identify influential parameters of the welding process (laser power, feed rate, focal diameter and flow of gas) while streamlining testing, an experimental design was established with the CORICO software that uses the graphic correlation method. The position of the focal point was fixed at -1/3 of the thickness of the sheet. The gas flow rate and the power of the beam seem to have a major role on the mechanical properties and geometry of the weld. The strength of the weld is comparable to that of base metal. However, there is a significant drop in the elongation at break of about 30 %. The first observations of the cross section of the weld by scanning electron microscopy coupled with EBSD analysis show generally a molten zone presenting dendritic large grain compared to the equiaxed grains of the base metals without a heat affected zone.

Introduction

Hastelloy X is a commercially available nickel-chromium-molybdenum superalloy with a good oxidation resistance, a good mechanical properties at high temperature and a significant formability; sine qua criteria for the choice of materials for the production of chambers turbojet combustion which is part of this study [START_REF] Hastelloy | Haynes international report H-3009C[END_REF]. Arc welding technique is commonly used for the manufacturing of parts but the aeronautical requirements becoming increasingly severe especially in terms of reproducibility of geometry and metallurgical grade fillet weld. Laser welding is a viable method of assembly to meet these new demands by its automation to replace longer term the manual TIG welding. The high power CO 2 laser is extensively used for practical applications such as cutting and welding laser welding. The CO 2 laser is very used in the industry with regard to Yb:YAG laser which until now was not rather powerful but this changes. The aim of this study was to evaluate the effect of Yb:YAG laser beam parameters on the microstructure and mechanical properties of the laser beam welded superalloys Hastelloy X to define a field of weldability. The implementation of an experimental design approach is required due to the multitude of input parameters and the complexity of the phenomena involved [START_REF] Benyounis | [END_REF][3].

Experimental procedure

Alloy studied is the nickel-chromium-molybdenum alloy Hastelloy X which composition is given in Table 1. The sheet metal is in the annealed condition: it was heat treated between 1150°C and 1175°C and cooled rapidly. Treatment and cooling are carried out under a protective atmosphere [START_REF] Benyounis | [END_REF]. Their thickness is 1.2 mm. The samples were cut and butt welded by TRUMPH TruLaser Cell 3000 machine. The laser beam is a Yb: YAG TruDisk maximum power 3.3 kilowatts continuous, equipped with a special optical fiber comprising a core fiber 100 µm and a coaxial fiber 400 µm. This fiber thus allows to weld sheet metals with a distributed power density following a classic Gaussian surface (small fiber, SF) either another a torus (large fiber, LF). Two laser parameters are particularly important for welding: the power density of the laser beam and the interaction time. They determine how much energy per area (W.m -2 ) and per unit of time (J.m -2 ) is introduced into the workpiece process [START_REF] Vora | Dilution of molybdenum on aluminum during laser surface alloying[END_REF]. The power density can be controlled by changing the laser power, the size of the focus spot and the configuration of fiber. The interaction time can be adjusted for spot work by changing the irradiation time or by altering the feed rate. To identify influential parameters of the welding process while streamlining testing, an experimental design was established by the CORICO software to understand the new possibilities offered by this new technology [7]. Five parameters were selected for this study: the laser power (500 -2500 W), the speed welding (1 -8 m.min -1 ), the focal diameter (110 -750 µm) which depends on the fiber configuration and the gas flow. The fiber configurations are: the laser beam can be carried by the core of the fiber (SF) or by the periphery of the fiber (LF). The Table 2 presents 30 configurations of welding of the experiment design defined with the software CORICO. The position of the focal point relative to the surface of the sheet has not been considered as a variable parameter. Indeed, much research in this area showed a penetration at a maximum focal length of -1/3 of the thickness of the sheet relative to the surface and this whatever the focal diameter and the feed speed [3].

During the laser welding operation, the specimens were clamped rigidly by a fixture which was designed and manufactured for this study, to align the laser beam with the butt joint and avoid any thermal distortion. To limit the oxidation of the lower face of samples, an argon gaseous protection is realized (20 l.min -1 ). The microstructures of the base metal and the welds have been studied from observations by optical microscopy (Leica wild M420 stereo microscope) and scanning electron microscopy (FEG-SEM JEOL 7000F). Dimensional limits for specific common imperfections and shape dimensions in laser beam fusion welding are also provided in the referred specifications, in particular, square joints in butt configurations need to be checked in terms of their reinforcement R, low weld LW, drop-thru DT, underfill UF, undercut UC, and shrinkage groove SG. To validate the weldability of these alloys, dimensions of cord section are measured and compared to standard specifications described in EN ISO 6947 (Fig. 1). A more precise metallurgical study was led for the optimized cords. EBSD analyses were performed using a JEOL 7000F field emission scanning electron microscope at an accelerating voltage of 20 kV equipped with a Nordlys II F+ camera. EBSD analyses were helpful in both analyzing the microstructure of the alloy and the welds. EBSD data were then post-processed with the commercial orientation imaging software package Oxford Channel 5. To minimize measurement errors, all grains comprising less than 3 pixels were automatically removed from the maps before data analysis. A 15• criterion was used to differentiate low angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs). The grain size was quantified by measurement of grain area (ignoring annealing twin boundaries) and calculation of the equivalent grain diameter assuming each grain as a circle. EDX analysis (oxford Silicon Drift Detector X-Max) were realized to specify the physico-chemical transformations in the welded cords. The mechanical properties of the welds were determined by means of micro-tensile to be compared with those of the parent material.

Results and discussion

The microstructure of the base metal consists of equiaxed grains of average size between 100 and 150 microns for Hastelloy X whatever plane plate studied (Fig. 2a). An EBSD analysis was realized in the plan RD-LT of the sheet of Hastelloy X. The figure 1.b present an IPF ZO mapping. Both plats show no texture. Several grains present macles. M 6 C's carbides are present in grains and in boundaries grains (Fig. 1c,d) The tensile mechanical properties in different directions in the plane RD-LT have been determined for the base metal (Table 3). No anisotropy of mechanical properties for the two sheets metal is demonstrated confirming previous microstructural observations.

0° 208±10 446±3 799±2 55±2 45° 211±15 446±11 793±11 53±2 90° 190±20 456±5 806±5 58±8
This study was conducted in several stages with CORICO software: a preliminary study was conducted to determine the limits of weldability defined from geometric criteria fillet weld. A second study clarified the weldability taking into account the mechanical properties of fillet weld and highlights the influence of each parameter on the welding quality of the welded joint. Corico method is based on analysis of partial correlations between all the variables of the experimental design. The domain of weldability was determined from 30 tries defined by the experimental design for the homogeneous assembly. The figure 3 presents the domains of weldability; that is the domains where the geometrical criteria of cords are verified. The welding speed of 2.75 m.min -1 seems the most interesting because the welded cords meet the criteria of the standard about is the diameter of the focal point and for a power superior to 1000 W. For speeds of welding more important, the power must be superior in 1500 W and the focal diameter between 200 and 370 microns. The figure 4a presents the evolution of the mechanical strength of the weld according to the variables: the focal diameter, the speed of welding and the power. It exists a correspondence enters domain of weldability and the maximal strength of cords. The speed of 2.75 mm.min -1 seems to be also the most relevant to obtain the most resistant cords. The break of tensile samples for the same speed is localized in the parent material and not in the welds ( Fig. 4b). The response surface methodology was used to explore the relationships between several explanatory variables (speed, power and focus diameter) and one or more response variables (UTS and elongation).

The figure 4 presents the response surfaces of the ultimate tensile strength according to the experimental variables. The mechanical strength is maximal when the speed of the head is average (of the order of 4.5 m.min -1 ) and the power superior to 1000 W (Fig. 5a). The focal diameter seems to be a factor of the first order because the strength is maximal for diameters lower than 350 microns whatever the speed and the power (Fig. 5b,c). The response surfaces of the elongation at rupture according to the same variables are presented to the figure 6. The same trends are a b highlighted. The maximal elongation is obtained for focal diameters lower than 110 µm, values of power lower than 1500 W and average speeds of welding of the order 4.5 mm.min -1 .

Figure 5.

Response Surfaces presenting the evolution of the elongation at rupture according to the welding speed, the power and the focal diameter.

Two optimizations were calculated by the software CORICO by fixing several criteria of desirability. The first optimization was calculated by setting as the standard IN ISO 6947 for the geometrical criteria references and by maximizing the mechanical properties. The second modelling was determined by taking as criteria the maximum of the strength of the welds while trying to minimize the geometrical defects (hollow, gutter) cords. The optimum parameters are summarized in table 4 for the small and large focal diameter for both homogeneous welding. Microstructure and mechanical properties of these optimized fillet welds were characterized to better understand the influence of welding parameters. Fusion zone shape and final solidification structure of these two alloys were evaluated as a function of laser parameters. The shapes of the weld beads optimized in cross section are shown in Figure 6. A typical form of a Keyhole welding mode is observed for butt weld obtained with small focal, and an X-shaped weld is more representative of a conduction welding method obtained with a large focal. Conduction mode is characterized by a low penetration welds. The weld depth is determined by a combination of heat conduction and convection of the liquid in the solder bath. As against, the mode called keyhole, wherein evaporation and ejection of metal occurs, achieves substantial penetration depths. The dimensions of optimized welds, defined in the figure 1 Fields of deformation of the sample welded with the small focal of the present first optimization locally a more important rate of deformation due to a concentration of stress in the weld which presents a low weld defect. The sample welded with the large focal of the present first optimization has a less important rate of deformation in the melted zone and uniformly distributed. This can be led by the geometry of the weld which is exempt from defects The premature break is however localized in the melted zone, at the level of the median interdendritic plan. Samples welded with the parameters of the second optimization, whatever is the focal diameter present a rate of homogeneous deformation in the weld and in the parent material. The differences of mechanical behavior of the various welds cannot thus give some explanation only by the geometry of the welded cords; the microstructure in a more local scale thus has to play a very important role.

To verify this hypothesis, EBSD analyses were realized on all the weld in the plan LT-ST. They are presented in the figure 9. These analyses were made in the same conditions as those made for the parent material (FEG-SEM JEOL 7000F coupled with OXFORD Nordlys Fast camera). The observed microstructure presents a growth epitaxic from the parent material of the denditric colonar grains in the melted zone. Differences of microstructure appear between cords realized with the parameters of the first optimization and the second optimization. Colonnars grains are less disorientated with regard to the axis of tensile tests for the samples of the second optimization. Furthermore, équiaxes grains appear in the center of these melted zones. These differences can explain the best mechanical behavior of these welds.

Conclusion

The weldability of Hastelloy X alloy was defined based on welding parameters such as the laser power, the speed, the focal diameter and the gas flow. The most influential parameters seem to be the laser power and the focal diameter. The implementation of experimental design allowed to define two types of optimization. The second optimization based on mechanical criteria seems more interesting than the first optimization based on geometrical criteria of cords. Indeed, the second optimization allows to obtain welds which have similar properties in that of the alloy.

Further analyzes of the carbon and more specifically of the state of precipitation in the weld are planned in order to precise the mechanical behavior of the weld.

Figure 1 .

 1 Figure 1. Standart geometrical specifications described in EN ISO 6947.

Figure 2 .

 2 Figure 2. Base metal Hastelloys X (a) Optical microscopy observation. (b) Inverse Pole Figure orientation map (IPF Z0). (c) EDX analyses of M 6 C carbide types.

Figure 3 .

 3 Figure 3. Weldability field of the Hastelloy X.

Figure 4 .

 4 Figure 4. (a) Mechanical propreties field (UTS) of the welds of Hastelloy X. (b) Fracture field of the Hastelloy X welded samples.

Figure 6 .

 6 Figure 6. Metallographic observations of welds obtained with the welding optimal parameters.The strength of the optimized welds was determined from transverse tensile tests. The obtained curves are presented to the figure 7. The curve of the hastelloy X, determined in the same direction, is also given for comparison.

Figure 7 .

 7 Figure 7. Evolution of the strength of cords according to the strain determined from tensile transverse tests realized on welded test samples.

Figure 8 .

 8 Figure 8. Evolution of the local deformation across the weld for an global elongation of 21% determined by DIC.

Figure 9 .

 9 Figure 9. EBSD maps (IPF Z0) of the welds in cross section .

Table 1 .

 1 Chemical composition of the Hastellloy X (wt %).

	Ni Cr Co Mo W	Al	Fe Mn Cu	Si	C
	Bal 22.1 1.38 9.0 0.45 0.13 18.7 0.66 0.08 0.35 0.07

Table 2 .

 2 Design matrix defined by the software CORICO.

		Power (w)	Speed (m.min -1 )	Ø SF (µm)	Ø LF (µm)	Gas flow (l.mn -1 )
	S1	1000	6.25	305 375	25
	S2	1500	1.00	110 450	10
	S3	2500	8.00	175 525	40
	S4	2500	4.50	240 600	32
	S5	1500	8.00	240 600	32
	S6	1000	6.25	175 525	25
	S7	1500	1.00	305 675	32
	S8	2000	6.25	175 525	17
	S9	2000	8.00	305 675	17
	S10	2000	2.75	305 675	40
	S11	1000	6.25	240 600	32
	S12	2000	4.50	305 675	10
	S13	2000	6.25	370 750	40
	S14	1500	4.50	370 750	17
	S15	1000	2.75	240 600	32
	S16	2500	2.75	270 750	17
	S17	1500	6.25	110 450	40
	S18	500	2.75	175 525	25
	S19	2500	1.00	240 600	25
	S20	500	4.50	205 675	10
	S21	1000	4.50	370 750	17
	S22	500	8.00	110 450	25
	S23	2000	2.75	175 525	32
	S24	1500	4.50	240 600	25
	S25	500	8.00	110 450	10
	S26	500	1.00	370 750	10
	S27	500	1.00	110 450	40
	S28	2500	1.00	110 450	10
	S29	2500	8.00	370 750	40
	S30	500	1.00	110 450	10

Table 3 .

 3 Mechanical properties of Hastelloy X.

	Angle of orientation with	Rigidity E [GPa]	Yield Strength [MPa]	Ultimate Tensile Strength [MPa]	Elongation [%]
	regard to rolling direction

Table 4 .

 4 Optimized parameters determined by the software CORICO for homogeneous welding Hastelloy X.

		1 st optimization 2 nd optimization
		SF	LF	SF	LF
	Power (W)	1571	2430	1679	2269
	Speed (m.min -1 )	7.65	3.17	4.67	2.63
	Ø focal (µm) Gas flow (l.min -1 )	350 40	518 39	349 39	610 39