Céline Pascal

Co-sintering and microstructural characterization of steel/cobalt base alloy bimaterials

The objective of this study is to process a bimaterial that combines the mechanical strength of a martensitic steel (X3CrNiMo13-4) and the wear and corrosion resistance of a cobalt base alloy (Stellite 6). The powder metallurgy route includes three steps: co-compaction, debinding, and pressureless co-sintering. The experimental approach consists in studying the compaction, the debinding and the sintering behavior of single materials (dimensional changes during sintering, microstructure, and hardness after sintering) before studying co-sintering. The co-sintering temperature range is defined from thermo-chemical calculations and single material sintering experiments especially for Stellite 6. Finally, the co-sintering ability is evaluated (green and final densities, shrinkage mismatch, coefficient of thermal expansion…) and the bimaterial sintering is studied. Despite the shrinkage mismatch of single materials, cohesion is achieved between the two materials through the infiltration of the supersolidus liquid from the Co base alloy to the steel and through the formation of an interdiffusion layer between the two materials characterized by a composition gradient.

Elaboration of (Steel/Cemented Carbide) Multimaterial by Powder Metallurgy

A steel/cemented carbide couple is selected to generate a tough/hard two layers material. The sintering temperature and composition are chosen according to phase equilibria data. The choice of optimal sintering conditions needs experimental studies. First results evidence liquid migration from the hard layer to the tough one, leading to porosity in the hard region. The study of microstructure evolution during sintering of the tough material (TEM, SEM, image analysis) evidences the coupled mechanisms of pore reduction and WC dissolution, and leads to temperature and time ranges suitable to limit liquid migration. The sintering of the two layer material is then shown to need further compromises to avoid interface crack formation due to differential densification.

Pressureless co-sintering behaviour of a steel/cemented carbide component: model bimaterial

Abstract The co-sintering of model bimaterial {steel/cemented carbide} is studied. The cemented carbide consists of WC grain cemented by an Fe-rich binder. The steel is prepared from a powder mixture (Fe + WC + Cg). The experimental approach involves two steps: (1) the study of the single materials sintering (dimensional changes and weight losses during sintering, microstructure and hardness after sintering), (2) the study of the co-sintering of a two layer material which differs from the average behaviour of single materials. This difference arises from chemical interaction between the two materials such as M6C formation in the interfacial region. M6C formation is closely related to liquid formation in steel and liquid migration into the WC base material. A mechanism is proposed; it is based on WC dissolution and asymmetric diffusion of Wand C in the steel layer.

Fabrication of (Cemented Carbides/Steel) Bilayered Materials by Powder Metallurgy

This paper analyses the effects of five parameters (composition, compaction pressure, heating rate, sintering temperature and duration) on the sintering of a bilayered (cemented carbide/steel) material. Design of experiments is used to reduce the number of experiments and to analyse the results. After sintering, each sample is characterized (difference of shrinkage, shrinkage anisotropy, density and microstructure). Composition, sintering temperature and duration are the three main parameters which control the sintering of bimaterials, their microstructure and the interface quality. The heating rate and the compaction pressure have no significant effect in the tested domain.

Tungsten/Copper Functionally Graded Materials: Possible Applications and Processing through the Powder Metallurgy Route

Processing of W-Cu graded materials from attritor-milled W-CuO mixtures is described. The powder reduction steps are investigated by TG and XRD analyses and by microstructural observations (SEM, TEM). Sintering of reduced powder with different compositions is analysed by dilatometry. Sintering behaviour of the graded component processed by co-compaction of a 10/20/30wt%Cu multi-layer material is briefly discussed. Liquid Cu migration is observed and smoothes the composition gradient. Perspectives to control this migration are discussed.