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Issu de : 24th International Conference on Wear of Materials, 16-20 avril 2023, Banff, Alberta, Canada; International audience; The chemical composition and microstructure of metallic parts are parameters that affect plastic deformation accommodation mechanisms under friction stresses. The content of alloying elements impacts the stacking fault energy and the plastic deformation, and thus affects the tribological behavior. Depending on this content and the levels of mechanical stresses, plastic deformation of some alloys obtained under non-equilibrium conditions can occur. It is caused by different mechanisms, such as perfect slip and/or partial dislocation slip, as well as by phase transformation. The purpose of this study is to determine the influence of the plastic deformation accommodation mechanisms on tribological behavior by studying the effect of the iron content in cobalt-based alloys. For manufacturing purposes, cobalt-based coatings are produced on steel substrate using a additive manufacturing process (SLM). With this process, the microstructures of the cobalt-based coatings are essentially metastable FCC phase at room temperature with different contents of diluted iron. Tribological tests were carried out with a ball-to-disc contact. Iron contents were estimated by EDS-SEM. Analytical techniques such as XRD and EBSD were used to identify the microstructural changes observed in TTS due to tribological loading. The friction coefficient is linked to the evolution of the plastic deformation mechanisms activated to accommodate the contact. In particular, in addition to work-hardening phenomena, phase transformations are possible, namely a metastable FCC phase gives an HCP phase and a metastable FCC phase gives an α′-BCC phase under tribological loading. Both types of transformations can occur, individually or simultaneously, depending on the iron content in the coating.

International audience; The Selective Laser Melting process was used to perform cobalt-based alloy coatings on a C35 steel substrate. The relationships between interlayer times, iron dilution, crystalline structures, and micro-hardness were studied for different numbers of layers with an initial lased powder layer of 50μm thickness. Reducing the interlayer time increased the temperature reached in the melting bed and promoted matter transport from the substrate. The coating thickness consisted of a Co-Cr-Fe mixture, divided into two zones: a transition zone near the interface and a stabilized zone towards the substrate. The real coating thickness was found to be always greater when the interlayer time was reduced. For an interlayer time equal to 11 s (series 2), the iron dilution was always higher than for an interlayer time ranging between 42 s and 16 s (series 1), leading to a higher coating thickness. The microstructural state was also dependent on the interval time between successive layers. The coating microstructure was always cellular because of the high cooling rate. XRD analysis of the surface showed that this microstructure is essentially composed of two non-equilibrium phases: FCC and α’ BCC. The high hardness is due to the high content of iron which induces a martensite phase (series 2). Starting from 5 layers for series 1 and from 6 layers for series 2, the α’ BCC phase disappeared if the iron content on the coating surface was reduced by more than 45% content in weight. The mean coating hardness decreased with an increasing number of layers because of the decrease in the iron content. Finally, the micro-hardness of the FCC phase, for its part, was found to be dependent on the iron content in solution in the Co matrix.

International audience; During friction stir lap joint welding, hooks are systematically formed at the interface of dissimilar Al2024/Ti-6Al-4V plates. Various hooks dimensions and shapes are observed according to the welding parameters (welding speed, rotational speed and double-pass). Based on digital image correlation (DIC) during tensile shear tests, it is found that hooks features govern the fracture behaviours and, hence, the mechanical properties of the overall welds. In particular, three fracture behaviours are obtained: at the interface, in the Al2024 and across the Ti-6Al-4V plate, which was never reported in dissimilar Al/Ti studies. The aim of this work is (i) to study the influence of FSW parameters on the mechanisms of hooks formation and (ii) to understand the role of both AS and RS hooks on the mechanical behaviour of the welds, by the means of two new approaches. First, hooks are investigated by X-ray tomography which provided some innovative insight into the material flow (3D viewing). Based on these qualitative and quantitative results, hooks are used as tracers and two new scenarios of AS and RS generation are proposed. Then, the contribution of both hooks on the stress distribution is clearly revealed by some calculations of the stresses reached around AS and RS hooks. Contrary to RS hook, AS hook is sharp and, hence, the stress reached at the top of AS hook is characterized by a stress concentration. The results showed that the fracture behaviours are greatly influenced by hooks size and the formation of microstructural defects inside the Ti-6Al-4V, which depend on the welding parameters. In conclusions, this work revealed the relationship between the achievable parameters, the resulting hooks dimensions and the fracture behaviours. In addition, based on the understanding of the material flow, some ways of optimization of the mechanical properties are proposed.

WOM 2019 - 22nd International Conference on Wear of Materials, 14-18 April 2019, Hyatt Regency Miami, Miami, FL, USA; International audience; This paper examines plastic mechanisms that lead to damage to the sub-surfaces of ductile metallic contact areas and, by extension, to the creation of wear particles. The word “damage” is used here as a generic term to designate all topological, morphological and/or microstructural modifications to the surfaces that are the consequence of interfacial shear stress under friction. During friction, the accommodation mechanisms of plastic deformation by the M3 mode in the contact area can be different, depending on the microstructures of the alloy and the first sites (bulk materials).For example, for tempered martensitic steels under frictional stresses, plasticity strain occurs in Tribologically Transformed Surfaces (TTS) under dislocation gliding. The creation of new dislocations and the rearrangement of all these dislocations into a “bamboo”-type structure leads to a decrease in hardness, or softening of the steel.Otherwise, depending on the stacking fault energy in alloy microstructures, plasticity may occur through mechanisms involving either perfect dislocation gliding and/or partial dislocation gliding. In addition to hardening, the mobility and morphology of dislocations are also related to stacking fault energy, which can promote phase transformation leading to shear strain in TTS. These shear strain mechanisms have a great influence on the internal flow of wear particles and on the formation of the “Third-body layers”. These findings are given in a review of tribological results of measurements carried out on a wrought X38CrMoV5 grade steel and on cobalt-base thick coatings, using tribometers, under various loadings, test temperatures and sliding speeds. The cobalt-base thick coatings are deposited on steel by several processes which modify the nominal chemical composition of the cobalt alloy and make it possible to study the influence of the iron content on shear strain under friction stresses.

WOM 2019 - 22nd International Conference on Wear of Materials, 14-18 April 2019, Hyatt Regency Miami, Miami, FL, USA; International audience; This paper deals with the influence of the mechanisms of plasticity on the evolution of the friction coefficient in cobalt-based hardfacings. Particularly depending on the alloying element content and so on the stacking fault energy, plasticity in Co-based alloys may occur through mechanisms involving either perfect dislocations gliding and/or partial dislocation gliding. As the coalescence of stacking faults by partial dislocations promotes the phase transformation of the Co, this study focus on the impact of this phase transformation on the evolution of the friction coefficient.Stellite 21 (Co-27Cr-5Mo-0.25C) hardfacings deposited on a steel substrate by two different processes, namely metal inert gas (MIG) and laser, are studied. The tribological properties are evaluated with a ring on disc tribometer under high load (800 daN) and continuous sliding (5 mm/s) at room and high temperatures (450 °C and 600 °C). The wear volume is characterized by confocal microscope. From micro-hardness measurements and SEM observations the work-hardening and the plastic strain of the Tribological Transformations of Surfaces are identified in relationship with chemical analysis (by EDS). Moreover X-rays diffraction and EBSD reveal the crystal structure evolution.As-deposited Co-based hardfacings have a face-centered cubic (FCC) structure. Meanwhile depending on the process of deposition on the steel substrate, the nominal composition of the Stellite 21 can be modified due to dilution effect. So, the iron content is found to be higher in the MIG hardfacing than in the laser one, leading to different mechanisms of plasticity, respectively by perfect dislocations gliding and by phase transformation (FCC to hexagonal closed-packed (HCP)). Moreover, a significant influence of the phase transformation on the friction coefficient has been evidenced: without phase transformation the cobalt remains in the FCC structure and the friction coefficient is stable during the test, while a decrease of its value occurs during the FCC to HCP phase transformation.

International audience; The trend in the automotive field is to lower the weight and to enhance safety in vehicles. Thecompetitive environment in the sheet metal forming industry leads the manufacturer in an effort to reduce costs. One of the solutions proposed by the automotive industry is the reduction of sheet thickness by using high strength steels (HSS) for structural parts. However because of their mechanical properties, it is easier to form this steel grade by hot stamping and to shear by hot blanking. The blade tools are submitted to high thermo- mechanical stresseswhich induce blade surface damages. The increase of tool lifetimes contributes to cost saving. To assess the wear damage mechanisms of tool grades in the hot blanking process, we propose to study their tribological behaviour with an experimental test based on a hot blanking device simulator. This device is designed to reproduce the shearing of a blank heated at high temperature. After an observation of a part of industrial blade we show that our test device leads to the same wear mechanisms. Blades present adhesive wear from blank coating(Al-Si), plastic deformation and a microstructural evolution.

International audience; Hot blanking is a promising process to produce cars’ body parts. The high thermo-mechanical stresses in blanking operations may damage blade tools. To get better insight into the damage mechanisms and their evolutions, a finite element numerical model to simulate the process and achieve the thermo-mechanical loadings is developed. The effect of blank’s initial temperature, the cutting clearance and thermo-mechanical behaviour of tools are investigated. Experimental hot blanking tests are performed in order to examine the relevance of the simulations.

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20th International Wear of Materials Conference (WOM), Toronto, CANADA, APR 12-16, 2015; International audience; Aeronautic forging dies are subjected to very high loads and temperatures for a long contact time between the pre-heated parts and dies. Cobalt-based hardfacings are commonly deposited on dies and their main wear mechanism is large plastic deformation of the die radii. This paper deals with the wear damage mechanisms of three different cobalt-based hardfacings: Stellite 21 deposited by a MIG process, Stellite 21 and Stellite 6 deposited by a LASER process. The tribological tests are carried out on a high load Ring on Disc tribometer at room temperature. The postmortem investigations are undertaken by SEM observations, micro-hardness measurements as well as by X-ray diffraction analyses. Results show that the increase of the hardness, in order to improve the wear behaviour, can be achieved by a higher carbon content and by a lesser iron dilution that depends on the deposition process. A very important work-hardening, up to 90%, is also observed under sliding conditions and a relationship is established between the increase of the micro-hardness and the plastic strain level. Two different plastic strain mechanisms are observed. For high (MIG) or low (LASER) iron dilution levels, the plastic strain causes respectively a reorientation of grains or a FCC to HCP phase transformation; the latter being associated with a lower friction coefficient.

In machining, it is clearly noticed that the cutting tool wear influences the cutting process. However, it is difficult with experimental methods to study the effects of tool wear on several machining variables. Thus, in the literature, some earlier studies are performed separately on the effect of tool flank wear and crater wear on cutting process variables (such as cutting forces and temperature). Furthermore when the workpiece material adheres in cutting tool, it affects considerably the heat transfer phenomena. Accordingly, in this work the finite element analysis (FEA) is performed to investigate the influence of combination of tool flank and crater wear on the local or global variables such as cutting forces, tool temperature, chip formation on the one hand and the effects of the oxidized adhesion layer considered as oxide (Fe 2 O 3 /Fe 3 O 4 /FeO) on the heat transfer in cutting insert on the other hand. In this investigation, an uncoated cutting insert WC–6Co and medium carbon steel grade AISI 1045 are used. The factorial experimental design technique with three parameters (cutting speed Vc, flank wear land VB, crater wear depth KT) is used for the first investigation without adhesion layer. Then, only linear investigation is performed. The analysis has shown the influence of the different configurations of the tool wear geometry on the local or global cutting process variables, mainly on temperature and cutting. The simulation’s results show also, the highly influence of the oxidized adhesion layer (oxide Fe 2 O 3 /Fe 3 O 4 /FeO) on the heat transfer.

Issu de : ICTMP 2014 - 6th International Conference on Tribology in Manufacturing Processes & joining by plastic deformation, Darmstadt, GERMANY, June 22-24, 2014; International audience; High temperature forging tools are highly damaged by wear shearing under cyclic loading which reduces the life of tools. In real conditions, depending upon the tool areas, the level of wear can change. The surfaces of tools can be treated by cobalt-based hardfacing using different welding processes. This study focuses on tribological damages of Stellite 21 deposited by MIG process. Wear tests are carried out at room and high temperature on a ring on disc tribometer under high normal load. Different testing conditions are examined. The surface plastic strain due to the friction shear stresses is demonstrated by different methods like SEM observations, micro-hardness measurements and XRD analyses. More particularly, it is shown that the initial (200) crystallographic preferred orientation due to the welding process is modified into (111) crystallographic preferred orientation due to shear stresses regardless the loading. Moreover, a relationship between the gradient of the plastic shear strain at the friction subsurface and the level of the micro-hardness measurement has been established. In regard of the results, when the shear strain exceeds a threshold, the micro-hardness measurement is stabilised. The ultimate value could be induced by the stacking fault energy that is considered as weak in Stellites.

SCIE Article le 06/02/14; International audience; This original study investigates the damages of WC-6Co uncoated carbide tools during dry turning of AISI 1045 steel at mean and high speeds. The different wear micromechanisms are explained on the basis of different microstructural observations and analyses made by different techniques: (i) optical microscopy (OM) at macro-scale, (ii) scanning electron microscopy (SEM), with back-scattered electron imaging (BSE) at micro-scale, (iii) energy dispersive spectroscopy (EDS), X ray mapping with wavelength dispersive spectroscopy (WDS) for the chemical analyses and (iv) temperature evolution during machining. We noted that at conventional cutting speed Vc ≤ 250 m/min, normal cutting tool wear types (adhesion, abrasion and built up edge) are clearly observed. However, for cutting speed Vc > 250 m/min a severe wear is observed because the behavior of the WC-6Co grade completely changes due to a severe thermomechanical loading. Through all SEM micrographs, it is observed that this severe wear consists of several steps as: excessive deformation of WC-6Co bulk material and binder phase (Co), deformation and intragranular microcracking of WC, WC grain fragmentation and production of WC fragments in the tool/chip contact. Thus, the WC fragments accumulated at the tool/chip interface cause abrasion phenomena and pullout WC from tool surface. WC fragments contribute also to the microcutting and microploughing of chips, which lead to form a transferred layer at the tool rake face. Finally, based on the observations of the different wear micromechanisms, a scenario of WC-6Co damages is proposed through to a phenomenological model

International audience; High-chromium white cast irons are commonly used in applications requiring excellent abrasion resistance, as in the mining and mineral ore processing industry. Their excellent abrasion resistance is mainly due to their solidification microstructures. In this study, 15 experimental high-chromium white cast alloys containing different chromium andmolybdenumcontents and a ratio Cr/C of 10 are examined. The wear experiments were carried out on a pin-on-disc tribometer at 700 degrees C. Microstructure has the most important role in dictating friction. It was observed that the wear loss was insignificant compared with that obtained at room temperature. The mean friction coefficient was also lower than that observed at room temperature sliding wear.

International audience; A methodology is developed in this article to assess the wear damage occurring on a die radius in a deep-drawing process. Profilometric and topographic measurements were performed with a white-light-source confocal microscope on the rubbed part of the die radius, using a repeatable procedure. Four new quantitative damage criteria were defined from 2D profiles, to quantify and determine the degree of regularity of both adhesive and abrasive damage. This method allows different tool steels to be classified in terms of their adhesive and abrasive wear behavior, with fairly good repeatability and reliability.

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17th International Conference on Wear of Materials, Las Vegas, NV, APR 19‐23, 2009Extrait de : WOM 09 / sous la dir. de K.C. LUDEMA et S.J. SHAFFER; International audience; High‐chromium white cast irons are commonly used in applications requiring excellent abrasion resistance, as in the mining and mineral ore processing industry. Their excellent abrasion resistance is mainly due to their solidification microstructures. During their solidification, high levels of chromium (16‐32%) lead to the formation of a high‐volume fraction of eutectic M(7)C(3)‐carbides, which may or may not be associated with primary carbides in a heterogeneous austenitic/martensitic dendritic structure. Generally, in common white high‐chromium cast iron, the molybdenum content is less than 3 wt.% (by weight) so as to avoid a perlitic transformation. It is reported that by addition of molybdenum in quantities of more than 3 wt.%, new carbides (M(2)C, M(6)C) are formed which greatly increase the high‐temperature wear resistance. In this paper, 15 high‐chromium white cast alloys containing different chromium contents (16 wt.%Cr to 32 wt.%Cr) and molybdenum (Mo free to 9 wt.%Mo) are examined. For each alloy, the chemical composition and volume fraction of carbides and matrix have previously been determined. The matrix microstructure and the type of carbides depend on the relative contents of molybdenum and chromium. The wear experiments are carried out on a pin‐on‐disc tribometer at room temperature. The pin is made of cast iron. The wear mechanisms are observed by scanning electron microscopy (SEM). It is observed that the pin height loss, the evolution of friction versus time curves and the mean friction coefficient are largely dictated by the matrix microstructure. The carbides volume fraction and the macroscopic hardness both play only a minor role. The pin height loss is significant for a single‐phased matrix and the mean friction coefficient is high. When the matrix is multiphased, the pin height loss tends towards zero and the coefficient of friction is lower. Detailed SEM observations and analysis of the evolution of the friction versus time curves indicate the substantial contribution of the large carbides in friction contact.

17th International Conference on Wear of Materials, Las Vegas, NV, APR 19-23, 2009; International audience; In machining processes, surface roughness and dimensional accuracy of machined parts depend on tool wear. Cemented carbide based cutting tools remain widely used in machining processes for their wear resistance. The aim of this paper is to study wear mechanisms of a WC-Co cutting tool grade under tribological conditions. Dry friction experiments are carried out on a high-speed pin-on-disc tribometer considering WC-Co pins against steel discs made of an AISI 1045 grade. Furthermore, two type-K thermocouples are embedded in WC-Co pins in order to estimate the contact temperature during sliding tests. A large sliding speed range is considered: from 60 up to 600 m/min. Results deal with wear rate versus tribological conditions, evolution of friction coefficient and temperature versus sliding speed and evolution of wear mechanisms. WC-Co tribological pins exhibit different wear mechanisms: abrasion, adhesion, transgranular WC micro-cracking and WC/WC debonding. Circulation of debris in the friction contact depends on sliding speed and on test duration. The evolution of surface temperature versus friction coefficient is studied too. Furthermore, a particular attention is paid on the relationship between the thermal energy in the pin and the mechanical energy in the contact. The thermal energy is calculated from the temperature values of K-thermocouples and the mechanical energy dissipated in the pin is calculated from friction coefficient evolutions. Relationships are established through modelling approaches.

Issu de : WOM 09 - 17th International Conference on Wear of Materials 19-23 April 2009 • Las Vegas, USA; International audience; Given the specific micro‐structure of some steel grades, under tribological conditions the sub‐surface material or sub‐layers of sliding bodies are prone to cumulative cyclic plastic deformation, leading to the formation and emission of wear debris. In the present paper, a new wear model based on a cyclic ratchetting‐type plastic deformation of subsurface material is proposed. It is considered that the debris is formed and the wear‐loss occurs when the accumulated plastic deformation at sub‐surface exceeds ``a critical strain'' or ``rupture limit''. The model takes into account the number of cycles or test duration, a characteristic thickness of the sub‐layer dependent on tribological conditions and material properties, the shear rupture ductility and an average plastic strain increment. The average plastic strain increment is estimated by numerical simulation of pin‐on‐disc friction. A very close correlation is found between the predicted and experimental wear heights versus time and/or versus the number of cycles. The wear investigations were carried out on a high‐temperature pin‐on‐disc tribometer under dry friction conditions. Experiments were performed under constant load, speed and disc temperature for different durations. The steel grade involved was a tempered martensitic tool steel X38CrMoV5 (AISI H11). Wear mechanisms were investigated by Scanning Electron Microscopy (SEM) observations in surface and cross‐section.

International audience; Cemented carbide based cutting tools remain widely used in machining processes for their wear resistance. Knowledge and modelling of wear processes are of prior importance to get models to predict and increase the cutting tool service life. The present paper deals with some results of a research work that study relationships between wear and temperature in the case of pin-on-disc tribological experiments. Topic of the paper is focussed on the estimation of the pin/disc contact temperature by coupling experimental measurements and computational methods. Friction experiments are conducted with WC-6Co pins against steel discs made of an AISI 1045 grade. Furthermore, WC-6Co pins are instrumented with two type-K thermocouples. A large sliding velocity range is considered in the study: from 100m/min up to 600m/min. The present paper is focussed on the 600m/min velocity. During these tribological tests, tangential forces and thermocouple temperatures are measured and monitored. From these values, the heat flux in the WC-6Co pin is estimated by two different ways. On one hand by considering the unidirectional Fourier law, on the other hand by estimating the heat partition coefficient [1] between the disc and the pin. The heat partition coefficient is determined from a physical approach based on the consideration of the mechanical power dissipated in the contact. In both cases, the heat transfer in the pin is then modelled by finite element methods. It is necessary to perform numerical analysis to estimate the pin/disc interface temperature because of the impossibility to measure it directly during friction tests. Results of experiments and of numerical simulations are compared.

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16th International Conference on Wear of Materials 15-17 April 2007 • Montreal, Canada; International audience; The wear rate of sliding materials seems impossible to predict. It is clearly a function of substrate material, but we are learning that many surface events are also important. In particular, studies on the last 20 years have shown that substances develop between two sliding bodies have a strong influence on both friction and wear. These substances had become known as "third bodies". Whereas third bodies have been proven to be important, they have not been completely characterized. The third bodies have been studied, but it appears that the geometries of the sliding pairs are also important. This paper describes some experiments in which the composition of third-body materials are controlled by temperatures and in which the durability of third-body films are influenced by the geometry of contact between sliding bodies.

International audience

International audience; Injection molded parts often show several types of surface defects. Different authors mentioned that those defects are due to the interaction between the mold and the melt. This contact has a strong influence on the flow front behavior, and depends on processing parameters like melt and mould temperatures, ram speed during injection, mould surface properties and roughness, mould thickness and polymer melt rheological properties … After a literature review of "flow marks" defect, it appears that the "alternate flow marks" are caused by an instability of the Fountain flow front. An assumption proposed by some researchers is that the slip is the formation mechanism of this instability. We assume that this mechanism is the start of the flow front instability during the filling stage. We use numerical simulations performed with Forge2  commercial software, in order to generate non-symmetrical contact boundaries conditions. We introduce some sliding conditions at the wall during a flow between two parallel plates. Our preliminary computations confirm that the instability is created in this way.

Issu de : WOM 05 - 15th International Conference on Wear of Materials; International audience; The tribology behaviour of the die radius, in the deep-drawing process, results from surface interactions between the metal sheet and the tool under contact pressure. Friction, degradation of the sheet, surface quality and tool wear are a result of this interaction. This article aims to study degradations of the radius portion of a die in the deep-drawing process. The study on an industrial press is complex, so we developed our own test facility, an experimental device, which represents a deep-drawing process-simulator. It allows friction between the metal sheet and a die radius. It can dissociate the diverse mechanical effects exerted on the die radius by enabling a study of the component conditions found in deep-drawing (effect of the blankholder, of the pulling forces, the sliding rate, etc.). An experimental study was conducted on a low-carbon steel sheet and a X160CrMoV12 steel die radius. It revealed that the die radius surface was degraded by ploughing and transfer mechanisms. The distribution of these degradations on the die radius was localised in two areas but varied in intensity depending on the exit angle between the sheet and the die radius. The die radius friction coefficient has been modelled with respect to the mechanical characteristics of the deep-drawing process-simulator. This model leads to a correlation between the friction coefficient and the degradation evolutions on the die radius.

International audience

International audience; A significant part of the energy in forging is used to break the interfacial junctions due to friction between the tool and the workpiece. The life of hot‐forging tools is usually limited by complex interactive mechanisms under cyclic loading such as abrasive, adhesive and scaling wear, thermal and mechanical fatigue, and plastic deformation. This contribution deals with the wear mechanisms of the tempered martensitic X38CrMoV5 steel (AISI HI 1) under high‐temperature and dry‐sliding wear. The investigations are carried out with high‐temperature pin‐on‐disc tests. The pin is cut from bars of X38CrMoV5 steel treated at 42 and 47 HRC. The disc is made of common steel (AISI 1018, XC18). Temperature of the disc ranges from 20 to 950 degreesC. Before the test starts, the disc is first pre‐heated for 1 h. The experiments are performed under constant load and velocity. The friction coefficient decreases quasi‐linearly with the rising disc temperature up to 800 degreesC. Over this temperature, it decreases drastically for the 42 HRC steel but remains linear For the 47 HRC steel. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) investigations have revealed that wear is essentially due to abrasion, plastic deformation and fatigue. Set of cracks due to contact rolling fatigue is observed on the pin and the disc. Those cracks are located on the transferred scale on the pin and on the oxide scale of the disc wear track. The cross‐section observations of the pin have revealed a plastically deformed zone beneath the surface. In this sub‐surface layer, the tempered martensitic microstructure seems to be more aligned due to friction and the plastic deformation.

13th International Conference on Wear of Materials, VANCOUVER, CANADA, APR 22-26, 2001; International audience; In this study an experimental methodology to study surface wear of injection moulds on a full scale polymer injection moulding machine is developed. A comparative study of four different tool materials for mould cavities is carried out in situ. The polymer injection conditions (geometry cavities, machine settings, polymer) are chosen in order to accelerate the degradation of the mould cavity surface. The reinforced polycarbonate resin Lexan 341R-739, containing 40% of weight of short glass fibres, well known for their abrasive character, was injected with jetting and normal injection conditions. Jetting conditions implies high pressure and velocity of the injected resin. The die wear is discussed in relation to the polymer flow conditions and the die materials.

13th International Conference on Wear of Materials, VANCOUVER, CANADA, APR 22-26, 2001; International audience; in hot forming processes, the interface tool/product is important for the quality of the finished product. In hot rolling, the scale formed on the roll material plays an essential role. As soon as the contact oxide-oxide is established between the roll and the slab, friction allows the process to start. However, the oxide scale will continue to grow under the cyclic action of hot contacts and water cooling, and will be subjected to thermo-mechanical stresses. Beyond a certain critical thickness, the oxidized surface layer of the cylinders has not sufficient mechanical strength to withstand the shear stresses. The wear and friction behavior of the oxides appearing on the surface of the hot working rolls is not well known. The influence of these oxides on the friction and, consequently, the quality of the products of finishing mills, seems very significant. So, in this study, we investigate the evolution of the friction coefficient and the wear, according to the growth, the nature and the thickness of the formed scale. We use a high temperature pin on disc tribometer. The pin consists of material ``rolls'' while the disc consists of the slab. The pin is instrumented with thermocouples in order to couple the friction coefficient measurements with the thermal gradient in the pin and the surface temperature and the formed oxides, Then, the characterization of the surfaces is done by scanning electronic microscopy (SEM) and EDS analyses. We use the method of sin(2) Psi to evaluate residual stresses of oxide and correlate these data with shear stress behavior.

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International audience; Pack cementation chromizing is a process of chromium deposit by activated cementation. When the substrate is a high carbon content steel, a layer of carbides M(7)C(3) and M(23)C(6)(M=Cr,Fe) is formed. Consequently the substrate surface composition evolves significantly during the treatment.The thermodynamic modelling carried out aimed at studying the influence of this evolution on the deposit mechanisms when using ferrochromium as source of chromium.Both the condensed solutions properties of the C-Cr-Fe system (with the aid of the sub-lattices model) and the stoichiometrics substances (condensed and gazeous) properties of the C-Cr-Fe-N-H-Cl system (with the aid of COACH databank) have been modelled.The two equilibriums which take place have also been modelled (with the aid of the GEMINI2 computation code, by minimizing the Gibbs energy). Firstly the equilibrium between the ferrochromium and the halide compound which are present in the cement. Secondly, the equilibrium between the gazeous phase of this first equilibrium and the substrate surface. For this last equilibrium we have carried out some computations which describe the evolution of the substrate surface composition.Our computations show that in our experimentals conditions (source of chromium and halide compound: ferrochromium with 30% of iron and ammonium chloride; substrate:XC65 steel; temperature:980 degrees C):In the beginning of the treatment there is only a chromium deposit according to the reactions of exchange ((CrCl2)+(substrat)-->(depose)+(FeCl2) and reduction ((CrCl2)+(H-2)-->(depose)+2(HCl)) which occur in similar proportionsthen, when the M(23)C(6) carbide is present in the substrate surface, the reaction of reduction becomes strongly predominant until it occurs a co-deposit of chromium and iron, according to the reaction of reduction of those two elements.Comparison of chromium and iron activities in the ferrochromium and M(23)C(6), carbide explains this evolution : the activities of chromium and iron in the ferrochromium can be simultaneously higher than the activities of those two elements in the M(23)C(6) carbide, that is to say in the substrate surface.Quantitative analysis carried out by electron microprobe has shown that our thermodynamic modelling was realistic.

The aim of this experimental work is to identify the levels of damage necessary to activate different plastic strain mechanisms under volume fatigue and surface fatigue stresses (obtained indirectly by friction tests) of CoCrMo type hardfacing obtained by the SLM additive manufacturing process. While the surfacing processes on aeronautical mechanical parts are preferably performed by powder projection, the Selective Laser Melting (SLM) process is based on "powder bed" technology within the ICA-Albi laboratory. The interactions between powders and laser beams by powder bed technology can be considered as exploratory compared to powder projection processes and thus lead to coatings with microstructural particularities. The study of the plastic deformation behaviour via specific material parameters is undertaken by defining a mass percentage of additive elements necessary to activate the plastic deformation mechanism by phase transformation. This thesis initiates a study on the damage behaviour under multi-stress loading. Four-point plane bending fatigue tests are performed for different levels of mechanical loading to stress the volume. Tribological tests are carried out to engage different physical mechanisms of strain accommodation and thus study the synergy between frictional loading and surface degradation. In combination with tribological and fatigue tests, microstructural investigations and physical-chemical analyses have allowed the identification of damage mechanisms.; L'objectif de ce travail expérimental est d'identifier les niveaux d'endommagements nécessaires pour activer différents mécanismes de déformation plastique sous sollicitations en fatigue volumique et en fatigue de surface (obtenue indirectement par des essais de frottement) de rechargements de type CoCrMo obtenus par le procédé de fabrication additive SLM. Si le procédé de rechargement sur des pièces mécaniques aéronautiques se fait préférentiellement par projection de poudre, au sein du laboratoire ICA-Albi, le procédé de Selective Laser Melting (SLM) est basé sur la technologie "lit de poudres". Les interactions "poudres/faisceau laser" par technologie en lit de poudres peuvent être considérées comme exploratoires par rapport aux procédés par projection de poudre et ainsi conduire à des revêtements présentant des particularités microstructurales. L'étude du comportement sur la déformation plastique via des paramètres spécifiques matériaux est engagée en définissant un pourcentage en masse d'éléments d'addition nécessaire pour activer le mécanisme de déformation plastique par transformation de phase. Cette thèse initie une étude sur le comportement à l'endommagement sous sollicitations multi-contraintes. Des essais en fatigue flexion plane 4 points sont effectués pour différents niveaux de chargements mécaniques permettant de solliciter le volume. Des essais tribologiques sont réalisés pour engager des mécanismes physiques d'accommodation de la déformation différents et ainsi étudier la synergie entre la sollicitation de frottement et la dégradation de surface. Associées aux essais tribologiques et aux essais en fatigue, des investigations microstructurales et des analyses physico chimiques ont permis d'identifier les mécanismes d'endommagement.

Dissimilar Al2024/Ti-6Al-4V welding presents opportunities of light-weighting in the aircraft industry, large consumer of titanium alloys, through the replacement of titanium monolithic structures with “hybrid” Al2024/Ti-6Al-4V structures. However, due to their large differences in physical properties and the formation of brittle intermetallic compounds, Al2024/Ti-6Al-4V welding by conventional fusion joining processes is very difficult. Friction stir welding (FSW) emerges as a promising candidate for Al2024/Ti-6Al-4V welding, since it is a solid state joining process.The aim of this thesis is to understand the relationship between the process parameters, the metallurgical evolutions induced at different scales (from the material flow to the grain and the atomic scales) and the mechanical behaviour of the welds, in order to identify optimized welding conditions regarding to the mechanical properties. In particular, since welds are governed by thermal, material flow and metallurgical phenomena, research studies are organized around these three points.The use of infrared thermography, coupled with numerical results, allow to establish a relationship between FSW parameters and a heat index. Then, the characterization of the welds reveals the formation of “macroscopic” discontinuities (hooks, continuous Alclad film) whose features depend on the material flow. On the scale of the microstructure, metallurgical heterogeneities, mainly related to the evolutions of the precipitation state, are observed in the Al2024. Finally, at the interface, chemical heterogeneities, which can lead to the formation of intermetallic compounds, are identified. The contribution of all these evolutions on the mechanical behaviour of the welds is assessed by digital image correlation. In particular, it is shown that the evolutions related to the material flow are predominant for the studied welding conditions. All the results obtained in this work offer some interesting perspectives for the optimization of the mechanical properties.; Le soudage hétérogène Al2024/Ti-6Al-4V présente des possibilités d’allègement dans le secteur aéronautique, grand consommateur d’alliages de titane, à travers le remplacement des structures monolithiques en titane par des structures dites « hybrides » Al2024/Ti-6Al-4V. En raison de leurs grandes différences de propriétés physiques et de la formation de phases fragiles à l’interface, leur soudage par les procédés classiques par fusion n’est pas maitrisé. Le procédé de soudage par friction malaxage (FSW), s’opérant à l’état solide, se positionne comme un candidat prometteur au soudage Al2024/Ti-6Al-4V. Cette thèse vise à appréhender les relations existantes entre les paramètres du procédé, les évolutions métallurgiques induites à différentes échelles (du flux de matière aux échelles du grain et atomique) et le comportement mécanique des soudures, en vue de dégager des conditions de soudage optimisées au regard des propriétés mécaniques. En particulier, les soudures étant régies par des phénomènes thermique, d’écoulement de matière et métallurgique, les travaux de recherche sont organisés autour de ces trois aspects. Tout d’abord, l’instrumentation des essais FSW par thermographie infrarouge, couplée à des résultats numériques, ont permis de relier les conditions de soudage étudiées à un indice de chaleur. La caractérisation des soudures a ensuite révélé la formation de discontinuités « macroscopiques » (crochets, film continu de clad) dont les caractéristiques sont tributaires du flux de matière. A une échelle plus fine, des hétérogénéités métallurgiques, principalement attribuées à des évolutions de l’état de précipitation, sont observées dans l’Al2024. Enfin, à l’interface des hétérogénéités chimiques, pouvant donner lieu à la formation de composés intermétalliques, ont été identifiées. La contribution de toutes ces évolutions sur le comportement mécanique des joints soudés a été évaluée par corrélation d’images. En particulier, il est montré que les évolutions liées au flux de matière prédominent pour les conditions de soudage étudiées. L’ensemble des résultats obtenus dans ce travail ouvrent des perspectives intéressantes pour l’optimisation des propriétés mécaniques.

Car manufacturers are building using a self-hardening high strength boron steel (22MnB5) for safety and environmental concerns. But the shaping of sheets made of these kind of steel imposes on tools severe mechanical stresses and strong thermal transfer. The integration of a blanking function in transfer presses is a response to reach high profitability, but blades are subjected to the same durability problems. The aim of this thesis is to understand how hot blanking tools are wearing in order to give to manufacturers tool material guidelines (X38CrMoV5-3 or X70CrMoV5-2) and process parameters guidelines. To do so, an estimation of mechanical stresses and thermal solicitation is required. The methodology is based on four steps: literature search, hot blanking trials on a specific module of ICA laboratory industrial pilot MEFISTO, finite element simulations of the operation, and microstructural analysis. The state of art is focused on the description of the operation, the physical and mechanical properties of the blank material and tools materials, the blank/tool interface behavior, and the techniques used simulate the hot blanking process. Trials on the hot blanking module provided blanking force and observations of worn blades. These informations are needed to validate the finite element model. This model shows that mechanical stresses are local, intense and slides in the blade cutting edge. Moreover, the blade heating generates a surface temperature close to tool steel tempering temperature. Experimental results compared to numerical ones allow to understand the link between thermo-mechanical stresses and how the damage occurs.; Les constructeurs automobiles utilisent des aciers auto-trempant à haute résistance au bore manganèse (22MnB5) pour des questions de sécurité et environnementale. La mise en forme à chaud de tôles constituées de cet acier impose aux outillages des contraintes mécaniques sévères et un échauffement intense. L'intégration d'une fonction de cisaillage sous presse à suivre permet de répondre à des problématiques rentabilité de production, mais les lames sont confrontées à des problèmes similaires de durabilité. L'objectif de cette thèse est de comprendre la dégradation des outils de cisaillage à chaud afin de proposer des guides de choix de matériaux d'outil (X38CrMoV5-3 ou X70CrMoV5-2) et de paramètres du procédé. Cela requiert l'estimation des sollicitations mécaniques et thermiques dans la partie active de l'outil. La méthodologie développée passe par quatre étapes : une recherche bibliographique, des essais de cisaillage à chaud sur un module spécifique du pilote MEFISTO de l'ICA, le développement d'un modèle thermomécanique éléments finis de l'opération et des analyses des microstructures. Un état de l'art portant sur la définition de l'opération de découpage de tôles, sur les propriétés physiques et mécaniques du matériau de tôle et des lames, le comportement à l'interface tôle/outil et sur les techniques permettant de simuler le découpage a été proposé. Des essais sur le module de découpage ont permis d'accéder à des résultats d'effort et des observations des endommagements des lames de cisaillage. Ces informations sont utilisées pour valider le modèle de calcul éléments finis. Ce modèle montre que les contraintes mécaniques sont locales, intenses et se déplacent dans l'arête de coupe. De plus, l'échauffement de l'outil engendre une température de surface proche des températures de revenu des matériaux d'outil. La comparaison des résultats de simulation numérique et de l'étude expérimentale permettent de comprendre le lien entre les dégradations observées et les sollicitations thermomécaniques.

In the field of hot forging of aeronautical parts, the steel dies are commonly hardfaced, on few millimeters thick, by a cobalt-based alloy (Stellite 21) deposited by arc welding (MIG). As part of this thesis, this "classic" hardfacing is compared to Stellite 21 and Stellite 6 hardfacings deposited by two emerging processes in this area, the PTA and the LASER one. The objective is to assess surface damage mechanisms, especially induced by plastic strain, of these various hardfacings. Tribological tests (laboratory and semi-industrial) were used to create surface damage comparable to those observed in industrial dies. Associated with these tests, multiscale microstructural, structural and mechanical investigations have been performed (tensile, bending, microhardness, OM, SEM, STEM, XRD, EBSD). According to the « material/process » couple, plastic strain mechanisms by perfect dislocation glide and by FCC to HCP phase transformation have been identified. The activation of the latter has been connected to the temperature of the allotropic phase transformation (FCC/HCP) in cobalt. This temperature depends on (i) the alloying elements, varying according to the deposited grade (Cr, C, ...), (ii) the dilution (Fe content evolution) connected to the welding parameters and (iii) the number of deposited layer. Moreover, a significant influence of the phase transformation on the evolution of the friction coefficient has been evidenced. Indeed, if the phase transformation is not observed, the friction coefficient is stable during the test, while a drop of the friction coefficient curve is connected with the FCC to HCP phase transformation. Moreover, the plastic flow of dendrites is observed at the extreme surface, on a few tens of micrometres in thickness, in the direction of sliding. It is associated with a high morphologic and crystallographic texturing of the identified phase (FCC or HCP), with the highest atomic density planes mostly oriented parallel to the sliding surface. The results also show that, under tribological laodings, a significant hardening is observed on the surface (up to 90%) and a correlation has been established between the increase in the microhardness and the plastic deformation ratio.; Dans le domaine du forgeage à chaud de pièces aéronautiques, les matrices en acier sont couramment rechargées, sur quelques millimètres d’épaisseur, par un alliage base cobalt (Stellite 21) déposé par procédé de soudage à l’arc (MIG). Dans le cadre de ce travail de thèse, ce rechargement « classique» est comparé à des rechargements Stellite 21 et Stellite 6 déposés par deux procédés émergents dans ce domaine, le PTA et le LASER. L’objectif est d’apporter des éléments de compréhension aux mécanismes d’endommagement de surface, notamment par écoulement plastique, de ces différents rechargements afin de dégager des voies d’amélioration pour augmenter la durée de vie des matrices. Pour cela, des essais tribologiques (semi-industriels et laboratoire) ont été mis en œuvre pour créer des endommagements de surface comparables à ceux observés sur matrices industrielles. Associées à ces essais, des investigations microstructurales, structurales et mécaniques multi-échelles ont été réalisées (traction, flexion, microdureté, MO, MEB, MEB-STEM, DRX, EBSD). Selon les couples « nuance/procédé » de rechargement, des mécanismes de déformation plastique par glissement des dislocations parfaites et par transformation de phase CFC en HC ont été identifiés. L’activation de ce dernier a pu être reliée à la température de transformation allotropique CFC/HC du cobalt. Cette température dépend à la fois (i) des éléments d’addition, variant en fonction de la nuance déposée (Cr, C,...), (ii) de la dilution (variation de la teneur en Fe) liée aux paramètres de soudage et (iii) du nombre de couches déposées. De plus, une influence significative de la transformation de phase sur l’évolution du coefficient de frottement a été mise en évidence. En effet, dans le cas où la transformation de phase n’est pas observée, le coefficient de frottement est stable durant l'essai alors qu'une chute de la courbe de coefficient de frottement a été reliée avec la transformation de phase CFC en HC. Parallèlement, l'écoulement plastique des dendrites est observé en extrême surface sur quelques dizaines de micromètre d'épaisseur dans la direction de glissement. Cet écoulement est associé à une forte texturation morphologique et cristallographique de la phase identifiée (CFC ou HC), avec une orientation des plans de plus grande densité atomique parallèlement à la surface de glissement. Les résultats montrent également que sous sollicitations tribologiques, un important durcissement est observé en surface (jusqu'à 90%) et une corrélation a pu être établie entre l'augmentation de la microdureté et le taux de déformation plastique.