Publications by Michel Rappaz | EPFL Graph Search

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In 1952, Frank (Proc R Soc Lond Ser-Math Phys Sci 215:43-46, 1952) already postulated that Icosahedral Short Range Order (ISRO) of atoms in the liquid could possibly explain the large nucleation undercoolings measured in metallic alloys by Turnbull and Fis ...

A new approach to the modelling of grain structure formation in solidification processes is proposed. Based upon a two-dimensional cellular automata technique, the model includes the mechanisms of heterogeneous nucleation and of grain growth. Nucleation occurring at the mould wall as well as in the liquid metal are treated by using two distributions of nucleation sites. The location and the crystallographic orientation of the grains are chosen randomly among a large number of cells and a certain number of o

Nickel-Titanium (NiTi) shape memory alloys are often used in medical component devices, for instance as guide wires for neurological surgery applications. Manufacturing of such devices becomes more and more challenging, especially considering the need to join them with other metals, like Stainless Steel (SS). Laser welding is a promising technique to realize and to guaranty the mechanical stability of dissimilar metal welds. However, inherent differences in chemical compositions may lead to fracture of the

In this paper we present briefly two possible approaches for modelling the laser cladding process. The first approach is 2D and consists in finding the shape of the molten pool given the fraction of the laser power which is available at the surface of the workpiece and given the amount of powder sticking to the molten pool. The powder reaching the workpiece is assumed to melt instantaneously at the surface of the molten pool, it is then mixed into the molten pool according to the computed velocity field. Be

The gradual transformation of a mushy zone during alloy solidification, from a continuous liquid film network to a fully coherent solid, has been simulated using a granular model. Based on a Voronoi tessellation of a random set of nucleation centers, solid ...

The appearance of hot tears, a very severe defect in castings, is a major limitation to the production of foundry cast parts and to the productivity of continuous casting processes such as the direct chill casting of aluminum alloys. As an example, the casting speed of the direct chill casting of billets is limited for some aluminum alloys because of their high propensity to develop hot tears, which initiate at non-zero liquid fraction at the bottom of the sump. In order to predict the occurrence of hot tea

The equiaxed solidification of Al-20 wt.% Zn alloys revealed an unexpectedly large number of fine grains which are in a twin, or near-twin, relationship with their nearest neighbors when minute amounts of Cr (1000 ppm) are added to the melt. Several occurr ...

Directional solidification experiments on hypoperitectic Cu-Sn alloys have been performed at very low velocity in a high thermal gradient to ensure planar front growth of both phases. The diameter of the sample has been reduced to 500 mu m to strongly redu ...

Matter removal in the electron discharge machining (EDM) is essentially due to a thermal mechanism. The control of the thermal process (i.e. the time dependent energy flow given by the discharge and the local temperature evolution) is therefore of crucial importance for efficiency and surface quality improvement. We report the first results of our research aimed at determining these quantities. Two types of measurements have been performed at the global and single discharge scale. 1) In-depth temperature me

Rapid advances in atomistic and phase-field modeling techniques as well as new experiments have led to major progress in solidification science during the first years of this century. Here we review the most important findings in this technologically impor ...

Single pan thermal analyses (SPTA) have been performed on Cu–14.5 wt.% Sn, Cu–21.3 wt.% Sn and Cu–26.8 wt.% Sn peritectic alloys. For this purpose, a SPTA assembly has been built and calibrated. As the latent heat is a function of temperature and compositi ...

As a necessary step toward the quantitative prediction of hot tearing defects, a three-dimensional stress–strain simulation based on a combined finite element (FE)/discrete element method (DEM) has been developed that is capable of predicting the mechanica ...

The paper by Karma and Tourret (this volume) in this special issue focuses on multiscale modeling approaches ranging from atoms to microstructure. In the present one, the most recent and significant modeling contributions dealing with the scale of solidifi ...

Micro-macrosegregation calculations have been performed for a rectangular cavity containing either a Pb-48 wt pct Sn alloy or a Sn-5 wt pct Pb alloy. The numerical results calculated with a finite volume method (FVM) and a finite element method (FEM) are compared with experimental results previously obtained by Hebditch and Hunt.([1]) The two methods are based on the same average conservation equations governing heat and mass transfer and the same assumptions: lever rule, equal and constant density of the s

Hot tearing, a severe defect occurring during solidification, is the conjunction of tensile stresses which are transmitted to the mushy zone by the coherent solid underneath and of an insufficient liquid feeding to compensate for the volumetric change. In most recent hot tearing criteria, one of the critical issues is the definition of a coherency point which, in low-concentration alloys, corresponds to the bridging or coalescence of the primary phase. A coalescence model has been developed recently using t

Upon solidification, most metallic alloys form dendritic structures that grow along directions corresponding to low index crystal axes, e.g., directions in fcc aluminum. However, recent findings[1,2] have shown that an increase in the zinc content in Al-Zn ...

Hot tear formation has been observed during the solidification of a succinonitrile–acetone (SCN-acetone) alloy by pulling the columnar dendrites in the transverse direction with a stick. It is observed that the cracking of the mushy zone (hot tears) always occurs at grain boundaries. At low volume fraction of solid, the opening can be compensated for by leanersolute interdendritic liquid (i.e., “healed” hot tears). At higher volume fraction of solid, hot tears directly nucleate in the interdendritic liquid

The effect of minute Cr additions on dendritic growth directions in Al-20 wt pct Zn alloy has been investigated. As first evidenced by Gonzales and Rappaz (Metall Mater Trans A 37:2797-2806, 2006) Al-Zn alloys exhibit a Dendrite Orientation Transition (DOT ...

It has been shown recently that minute additions of Cr in Al-Zn alloys refine the solidified grain structure and induce multiply-twinned grains which can only be explained if the icosahedral symmetry is considered (Kurtuldu et al., 2013). It has been sugge ...

We have recently shown that minute solute element additions to liquid metallic alloys can strongly influence the nucleation of the fcc phase and act as a grain refinement method. Electron back-scattered diffraction observations revealed a concomitant incre ...

Michel Rappaz

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Solidification of Metallic Alloys: Does the Structure of the Liquid Matter?

Probabilistic modelling of microstructure formation in solidification processes

Characterisation of Laser Welds, between Nitinol and Stainless Steel Wires

Laser-Powder-Material Interactions in the Laser Cladding Process

A Granular Model of Equiaxed Mushy Zones: Formation of a Coherent Solid and Localization of Feeding

Prediction of Hot Tears in DC-Cast Aluminum Billets

Influence of Cr on the nucleation of primary Al and formation of twinned dendrites in Al-Zn-Cr alloys: Can icosahedral solid clusters play a role?

Competition of the primary and peritectic phases in hypoperitectic Cu-Sn alloys solidified at low speed in a diffusive regime

Temperature measurements and numerical analysis of the heat transfer in samples submitted to electron discharge machining (EDM)

Solidification microstructures and solid-state parallels: Recent developments, future directions

Application of single pan thermal analysis to Cu-Sn peritectic alloys

Simulation of Semi-Solid Material Mechanical Behavior Using a Combined Discrete/Finite Element Method

Modeling and characterization of grain structures and defects in solidification

Numerical Simulation of Macrosegregation : a Comparison between FVM and FEM Predictions and a Confrontation with Experiments

HOW DOES COALESCENCE OF DENDRITE ARMS OR GRAINS INFLUENCE HOT TEARING ?

Dendritic Growth Morphologies in Al-Zn Alloys-Part I: X-ray Tomographic Microscopy

Hot Tear formation and coalescence observations in organic alloys

Influence of Minor Cr-Additions to the Growth of Columnar Dendrites in Al-Zn Alloys: Influence of Icosahedral Short Range Order in the Liquid

Influence of icosahedral short range order on diffusion in liquids: A study on Al-Zn-Cr alloys

Thermodynamic Aspects of Homogeneous Nucleation Enhanced by Icosahedral Short Range Order in Liquid Fcc-Type Alloys