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See detailOn the Laser Cladding of Ti Alloy Ti-6Al-4V with Low Laser Power
Mertens, Anne ULg; Paydas, Hakan ULg; Tchuindjang, Jérôme Tchoufack ULg et al

in Pilchak, Adam (Ed.) Proc. of Ti 2015: 13th World Conference on Titanium, Vol. 6: Additive and Near Net Shape Manufacturing (in press)

Laser cladding is an economic layer-by-layer near-net-shape process for the production and the repair of metallic parts. In this process, a metallic powder is projected onto a substrate while being molten ... [more ▼]

Laser cladding is an economic layer-by-layer near-net-shape process for the production and the repair of metallic parts. In this process, a metallic powder is projected onto a substrate while being molten by a laser beam. Laser sources with fairly high power – i.e. typically 2kW − are often used to ensure short building times and high productivity. However, this approach has limitations. Indeed, it is very difficult to produce thin walls at high laser power. Moreover, an increase of the incident energy may give rise to a relatively coarser microstructure, and this will in turn affect the mechanical properties of the component. In order to address these issues, this paper aims at assessing the potential of a laser source with a lower maximum power of 300W to enhance the flexibility of the process. Two types of samples – i.e. thin walls or bulk deposits − were produced at low laser power from alloy Ti-6Al-4V. Their geometry, microstructures and local hardness are characterised and correlated with the thermal history experienced during fabrication. [less ▲]

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See detailLocal thermal history during the laser additive manufacturing of steel
Mertens, Anne ULg; Hashemi, Seyedeh Neda ULg; Tchuindjang, Jérôme Tchoufack ULg et al

Conference (2016, February 09)

Laser additive manufacturing techniques – such as laser cladding − were developed in the late 1990s as economic layer-by-layer near-net-shape processes allowing for the production or restoration of ... [more ▼]

Laser additive manufacturing techniques – such as laser cladding − were developed in the late 1990s as economic layer-by-layer near-net-shape processes allowing for the production or restoration of complex metallic parts. These techniques are characterised by ultra-fast solidification and cooling rates, thus giving rise to ultra-fine out-of-equilibrium microstructures with potentially enhanced mechanical properties. However, the general quality of a component, as well as its dimensional accuracy, microstructure and final usage properties are strongly influenced by the local thermal history experienced during fabrication. As a consequence, a deeper understanding of the influence of the various processing parameters on the local thermal history during fabrication is of paramount importance, and it can only be reached by combining detailed experimental studies and modeling. Keeping this in mind, the present paper reviews on-going investigations carried out in the Metallic Materials Science Unit of the University of Liege in the laser additive manufacturing of HSS and of stainless steel 316L. [less ▲]

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See detailLaser cladding as repair technology for Ti6Al4V alloy: Influence of building strategy on microstructure and hardness
Paydas, Hakan ULg; Mertens, Anne ULg; Carrus, Raoul et al

in Materials & Design (2015), 85

Laser cladding is a metal deposition technique used to fabricate or repair components made from high value metallic alloys. In the present work Ti-6Al-4V deposits with variable thickness are made to ... [more ▼]

Laser cladding is a metal deposition technique used to fabricate or repair components made from high value metallic alloys. In the present work Ti-6Al-4V deposits with variable thickness are made to assess the use of laser cladding as a repair technology. Both the effect of the building strategy (BS) and the incident energy (IE) on the metallurgical characteristics of the deposits in relation to their complex thermal history have been studying. It is shown that for the configuration consisting in a decreasing track length (DTL) under high IE, a gradient of cooling rate exists that leads to the presence of different phases within the microstructure. Conversely homogeneous microstructures are present either for the configuration with a constant track length (CTL) under high IE, and for the strategy obtained from a DTL under low IE. Depending on the possible heat accumulation the nature of the phases are determined together with hardness maps within the deposits. Some qualification criteria are set prior to tensile tests to selected the adequate candidate-deposit that does not weaken the cladded material when it is stressed. A thermo-metallurgical scheme is proposed that helps understanding the effect of both the BS and the IE on the microstructure. [less ▲]

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See detailProduction of alumina foundry moulds by additive manufacturing for low and high melting points alloys prototypes
Bister, Geoffroy; Deschuyteneer, Dorian; Hautcoeur, Dominique et al

Poster (2015, October 26)

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See detailStudy of the Crystallization Behavior and the Subsequent Martensitic Transformation in a High Chromium Cast Steel submitted to Different Austenitization Temperatures
Tchuindjang, Jérôme Tchoufack ULg; Habraken, Anne ULg; Lecomte-Beckers, Jacqueline ULg

Conference (2015, September)

The crystallization behavior a High Chromium Cast Steel (HCCS) has been determined under Equilibrium conditions with ThermoCalc® software, and under non-Equilibrium conditions through Differential Thermal ... [more ▼]

The crystallization behavior a High Chromium Cast Steel (HCCS) has been determined under Equilibrium conditions with ThermoCalc® software, and under non-Equilibrium conditions through Differential Thermal Analyses (DTA). The results are compared to each other and the differences between the predicted solidification path and the actual one obtained from DTA (Figure 1) are highlighted. The microstructure in the as-cast conditions is characterized while using both scanning electron microscopy (SEM) and hardness measurements. The SEM analysis involves a combination of methods such as backscattered electron observation, energy dispersive spectrometry and backscattered-electron diffraction with X rays, which ensure proper identification of complex solidification carbides and the matrix made of martensite and retained austenite (Figure 2a). Solid state transformations occurring during the heating stage up to the austenitization temperature are studied towards both DTA and dilatometry, especially the dissolution/precipitation reactions. Different austenitization temperatures are considered prior to cooling down to room temperature during dilatometry tests to determine the starting points (Ms) for martensite transformation. In addition a cryogenic quenching within liquid nitrogen is performed which allows the completion of the martensitic transformation (Figure 2b) and help determining the critical point for the end of the martensitic reaction (Mf points). An enhanced discussion based on the influence of the cooling rate and the undercooling is developed to explain the differences that are observed between equilibrium and non-equilibrium conditions, which are mainly related to the solidification sequence and the carbides formed. The influence of the austenitization temperature and the matrix composition/supersaturation on both Ms and Mf points is also discussed. [less ▲]

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See detailComparison of the wear behavior of high speed steel grades obtained from conventional casting and laser cladding
Hashemi, Seyedeh Neda ULg; Lecomte-Beckers, Jacqueline ULg; Montrieux, Henri-Michel ULg et al

Conference (2015, September)

Tools Steels are alloys which withstand severe mechanical and physicochemical stresses in service. Therefore their alloying design that involved both the original chemical composition and the casting ... [more ▼]

Tools Steels are alloys which withstand severe mechanical and physicochemical stresses in service. Therefore their alloying design that involved both the original chemical composition and the casting route is crucial in order to achieve a tailored microstructure exhibiting enhanced wear performances. Tools steels obtained from conventional casting processes had received lot of attention so far as they yield typical microstructure composed of a quasi-continuous network of coarse grain boundary carbides with grain size ranging between 20 to 200 µm. Direct energy deposition applied to Tools Steels represents a new emerging technique that may allow ultrafine grained microstructures due to the higher cooling rates achieved especially in the solidification range. In this paper, four tool steels grades were studied, one of them being obtained from a conventional casting process and the other ones originated from the direct energy deposition. Differential Thermal Analysis helps enhancing the solidification sequence of the studied alloys, while their microstructure after subsequent heat treatment was characterized by the means of both optical and electron microscopes together with hardness measurements. Tribological tests carried out at room temperature and at high temperature were performed while using a “pin-on-disc” device. Based on the friction coefficient and the wear rate, the wear performances of the tool steel were determined and compared with one another. The influence of metallurgical features (the grain size, or the nature, the size and the distribution of carbides) on the wear behavior was also enhanced. The setting of the wear test parameters together with the microstructure of the studied materials seems to strongly influence the subsequent abrasion and wear mechanisms. [less ▲]

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See detailCarbides and grain size quantification methods in high speed steel alloys
Tchuindjang, Jérôme Tchoufack ULg; Paydas, Hakan ULg; Hashemi, Seyedeh Neda ULg et al

Conference (2015, July)

High speed steel (HSS) grades are widely used as in highly stressed applications where carbides content promote enhanced mechanical properties together with oxidation and wear resistances. Such a ... [more ▼]

High speed steel (HSS) grades are widely used as in highly stressed applications where carbides content promote enhanced mechanical properties together with oxidation and wear resistances. Such a tribological behaviour is mainly due to the presence of primary carbides. It is thus important to quantify the carbide volume fraction in HSS to correlate this result with the mechanical and tribological properties of the materials. Depending of the steel composition several types of carbides are precipitated in HSS: MC (Nb and V-rich), M2C (Mo and W-rich), Cr-rich M7C3 and some others, depending of the chemical composition. Two groups of HSS were analyzed in this study: cast HSS manufactured by centrifugal casting and cladded HSS manufactured by laser cladding. Depending on the chemical composition and the manufacturing process of the studied material, several methods have been used to quantify the carbide volume fraction and the grain size. [less ▲]

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See detailDevelopment of quantification methods applied to HSS alloys for carbides volume fraction and grain size assessments
Tchuindjang, Jérôme Tchoufack ULg; Paydas, Hakan ULg; Hashemi, Seyedeh Neda ULg et al

Poster (2015, July)

High speed steels (HSS) are used in applications where enhanced mechanical properties together with hot oxidation and wear resistances are required. Such an improved physico-chemical behaviour is mainly ... [more ▼]

High speed steels (HSS) are used in applications where enhanced mechanical properties together with hot oxidation and wear resistances are required. Such an improved physico-chemical behaviour is mainly due to the presence of primary carbides within the microstructure. Depending of the initial chemical composition several types of carbides can precipitate in HSS such as MC (Nb and V-rich), M2C (Mo and W-rich), Cr-rich M7C3, etc. Two groups of HSS were analysed in this study, which are the so-called cast HSS grades that are manufactured by a centrifugal casting process and the cladded HSS manufactured by laser cladding. Purpose: Quantify the carbide volume fractions and the grain size within HSS grades in order to set correlations between the microstructure and both the mechanical and the tribological properties of the materials. Findings: The metallurgical features such as carbides nature and shape have been determined that are related to the manufacturing process of the studied material, and several methods have been used to quantify the carbide volume fractions and to assess the grain size. [less ▲]

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See detailTipTopLam - Rapid manufacturing laser de pièces en titane (2008-2015), volet 1: Caractérisation microstructurale
Carton, Marc; Contrepois, Quentin; Mertens, Anne ULg et al

Report (2015)

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See detailDetermination of Stress and Strain Fields in Cast and Heat Treated Bimetallic Rolling Mill Rolls
Neira Torres, Ingrid; Tchuindjang, Jérôme Tchoufack ULg; Sinnaeve, Mario et al

in Proceedings of the Iron & Steel Technology Conference and Exposition (AISTech 2015 & ICSTI) (2015, May)

Efficient work rolls for Hot Strip mill roughing stands are usually produced as bimetallic spun cast rolls with a core material made of Spheroidal Graphite Iron (SGI) and a shell material made of High ... [more ▼]

Efficient work rolls for Hot Strip mill roughing stands are usually produced as bimetallic spun cast rolls with a core material made of Spheroidal Graphite Iron (SGI) and a shell material made of High Chromium Steel alloy, semi-High speed steel or adapted High Speed steel chemistry. In this paper, the evolution of stress fields and microstructure during post casting cooling and subsequent heat treatment of a standard high chromium steel is targeted. This knowledge is a first step to analyze the different events that could happen during these production stages and provides the residual stress fields as well as the final roll microstructure which are interesting data for roll service life. As far as roughing mill work rolls for roughing mills are concerned, current trends in terms of work roll design are forcing the roll producer to go towards higher usable shell thicknesses or increased residual shell thickness after scrap diameter in order to delay potential fatigue phenomena at the shell-core interface. One important parameter could not be evaluated in this study, it concerns the pollutions of the core material by alloying elements of the shell during production, which could affect to some extent the mechanical properties and microstructure of the core. The macroscopic thermo-mechanical-metallurgical Finite Element model used to perform simulations, takes into account coupled effects. Input data parameters are identified by experimental tests such as compression tests at constant strain rate and numerical procedures such as inverse method. After the model presentation, the material data set is commented. Whole experimental campaign could not be presented, however interested readers are referred to [1] where more details on the material study of High Chromium Steel alloy can be found. The section of simulation results show the phase transformation histories and stress profiles of a typical roll of diameter 1.2 m and shell thickness of 0.08 m. An additional sensitivity analysis of the results to some material data such as induced plasticity transformation coefficient and shift of the martensitic transformation start temperature parameters is reported. Finally, some conclusions analyze the interest and drawbacks of the described model and its results. [less ▲]

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See detailModelling of stress field evolution in bimetallic rolling mill rolls during the manufacturing process
Gilles, G.; Neira Torres, I.; Tchuindjang, Jérôme Tchoufack ULg et al

in Proceedings of the 5th Conference on rolling mill rolls for metals processing Rolls 5 2015 (2015, April 22)

This paper presents a study of the phase transformation histories and stress profiles in a typical bimetallic roll subjected to a post-casting cooling followed by a tempering heat treatment. The ... [more ▼]

This paper presents a study of the phase transformation histories and stress profiles in a typical bimetallic roll subjected to a post-casting cooling followed by a tempering heat treatment. The simulations are performed using the non-linear finite element code "Lagamine" and a thermo-mechanical-metallurgical model able to take into account coupled effects. The sensitivity of the finite element results to some parameters which are not accurately identified is investigated. [less ▲]

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See detailPhase Transformations and Crack Initiation in a High-Chromium Cast Steel Under Hot Compression Tests
Tchuindjang, Jérôme Tchoufack ULg; Neira Torres, Ingrid; Fores, Paulo et al

in Journal of Materials Engineering and Performance (2015), 24(5), 2025-2041

The mechanical behavior of the fully austenitic matrix of high-chromium cast steel (HCCS) alloy is determined by external compression stress applied at 300 and 700 C. The microstructure is roughly ... [more ▼]

The mechanical behavior of the fully austenitic matrix of high-chromium cast steel (HCCS) alloy is determined by external compression stress applied at 300 and 700 C. The microstructure is roughly characterized toward both optical and scanning electron microscopy analyses. Dilatometry is used during heating from room temperature up to austenitization to study the solid-state phase transformations, precipitation, and dissolution reactions. Two various strengthening phenomena (precipitation hardening and stress-induced bainite transformation) and one softening mechanism (dynamic recovery) are highlighted from compression tests. The influence of the temperature and the carbide type on the mechanical behavior of the HCCS material is also enhanced. Cracks observed on grain boundary primary carbides allow establishing a rough damage model. The crack initiation within the HCCS alloy is strongly dependent on the temperature, the externally applied stress, and the matrix strength and composition. [less ▲]

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See detailMetal matrix nanocomposite coatings for improving the usage properties of metallic materials
Mertens, Anne ULg; Lecomte-Beckers, Jacqueline ULg

Conference (2015, January)

The use of metal matrix nanocomposite is at the present time not as widespread as for their polymer-based counterparts. Yet, the addition of nanoparticles in a metallic matrix also allows to significantly ... [more ▼]

The use of metal matrix nanocomposite is at the present time not as widespread as for their polymer-based counterparts. Yet, the addition of nanoparticles in a metallic matrix also allows to significantly improve its usage properties, thus opening new and interesting prospects in terms of applications. This presentation aims at providing a broad overview of the various usage properties that can be enhanced by the addition of nanoparticles as illustrated by a number of examples taken from the scientific literature as well as from the researches carried out by the Metallic Materials Science Unit at the University of Liege. [less ▲]

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See detailRAPPORT SCIENTIFIQUE & TECHNIQUE FINAL - Projet BLUM – RW 6800
Lecomte-Beckers, Jacqueline ULg; Tchuindjang, Jérôme Tchoufack ULg; Paydas, Hakan ULg

Report (2015)

Le projet BluM® s’inscrit dans la continuité du projet Blisk. En effet les trade-off architecturaux du projet Blisk plan Marshall ont conduit Techspace Aero à se tourner vers le développement d’un concept ... [more ▼]

Le projet BluM® s’inscrit dans la continuité du projet Blisk. En effet les trade-off architecturaux du projet Blisk plan Marshall ont conduit Techspace Aero à se tourner vers le développement d’un concept encore plus porteur en terme de masse et plus ambitieux en terme de défi technologique que les blisk « classiques ». Il s’agit du développement de composant rotorique multi étage monobloc, d’où l’acronyme BluM® pour Bladed Drum. Sur les turboréacteurs actuels, les compresseurs basse pression comportent un tambour, dont le fût se compose de glissières circulaires dans lesquelles on introduit les aubes. Le projet apporte des solutions nouvelles à la conception du compresseur basse pression. Avec le BluM®, le principe du tambour est conservé sur lequel les aubes seront soudés par friction, une technique qui commence à se développer dans le domaine de la propulsion aéronautique. Elle offre un soudage à la fois précis et très résistant car les propriétés mécaniques du titane sont conservées. En intégrant l’aube au tambour, on supprime l’encoche et le pied d’aube et l’on réalise ainsi des gains de masse importants. Le concept BluM® permet d’envisager une réduction de masse de 20% sur l’ensemble rotorique. En outre le projet vise à mener une série d’études, d’évaluation et de modélisations très avancées de l’impact du concept sur la performance aéromécanique. L’objectif du projet est de développer cette technologie pour lui permettre d’atteindre un niveau de maturité suffisant pour être éligible sur un programme de développement à partir de 2014. Pour atteindre ce niveau de maturité les activités comprendront le développement de nouvelles méthodologies de dimensionnement et de modélisation, des essais partiels (matériaux, procédés, banc partiel) et la conception et la production d’un prototype pour validation en essai moteur. [less ▲]

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See detailThermal Treatments of AlSi10Mg Processed by Laser Beam Melting
Mertens, Anne ULg; Dedry, Olivier ULg; Reuter, David et al

in Bourell, Dave (Ed.) Proceedings of the 26th International Solid Freeform Fabrication Symposium (2015)

Recent studies have shown that AlSi10Mg processed by Laser Beam Melting (LBM) exhibits a much finer microstructure when compared to its cast counterpart as a consequence of the much faster cooling rates ... [more ▼]

Recent studies have shown that AlSi10Mg processed by Laser Beam Melting (LBM) exhibits a much finer microstructure when compared to its cast counterpart as a consequence of the much faster cooling rates imposed in the LBM process. Such microstructural refinement causes a significant increase in strength and hardness, to such an extent that as-fabricated LBM AlSi10Mg was reported to present hardness value of 127 ± 3 Hv0.5, similar to the hardness of high pressure die cast AlSi10Mg in the aged condition (i.e. 130-133 Hv). Yet, little attention has been given so far to the influence of thermal treatments on the microstructure and mechanical behavior of LBM AlSi10Mg. The present work hence aims to investigate the effect of two different types of heat treatments – i.e. (i) stress relief and (ii) solutionizing and ageing − on the microstructure, hardness and tensile properties of LBM AlSi10Mg. [less ▲]

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See detailInfluence of Fibre Distribution and Grain Size on the Mechanical Behaviour of Friction Stir Processed Mg-C Composites
Mertens, Anne ULg; Simar, Aude; Adrien, Jérôme et al

in Materials Characterization (2015), 107

Short C fibres-Mg matrix composites have been produced by friction stir processing sandwiches made of a layer of C fabric stacked between two sheets of Mg alloy AZ31B or AZ91D. This novel processing ... [more ▼]

Short C fibres-Mg matrix composites have been produced by friction stir processing sandwiches made of a layer of C fabric stacked between two sheets of Mg alloy AZ31B or AZ91D. This novel processing technique can allow the easy production of large-scale metal matrix composites. The paper investigates the microstructure of FSPed C fibre-Mg composites in relation with the fragmentation of the C fibres during FSP and their influence on the tensile properties. 3D X-ray tomography reveals that the fibres orient like onion rings and are more or less fragmented depending on the local shear stress during the process. The fibre volume fraction can be increased from 2.3% to 7.1% by reducing the nugget volume, i.e. by using a higher advancing speed in AZ31B alloy or a stronger matrix alloy, like AZ91D alloy. A higher fibre volume fraction leads to a smaller grain size which brings about an increase of the composite yield strength by 15 to 25%. However, a higher fibre volume fraction also leads to a lower fracture strain. Fracture surface observations reveal that damage occurs by fibre/matrix decohesion along fibres oriented perpendicularly to the loading direction. [less ▲]

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