Moulage par injection de pièces poreuses en NiTi pour des applications biomédicales

Objectif du projet: Fabriquer des pièces poreuses en NiTi de module élastique proche de celui de l’os. La porosité doit être ouverte et la taille des pores comprise entre 100 et 500 μm. Méthodes | Expériences | Résultats: Des pièces en NiTi ont été fabriquées par moulage par injection de poudres (PIM) à partir de poudres de Ni et de TiH2. Deux géométries ont été testées : Des pastilles pour des essais de compression et des barrettes pour des essais de flexion 3 points. Afin de caractériser les pièces frittées, des observations au microscope optique et électronique, des essais de compression cyclique, des essais de flexion cyclique et calorimétriques (DSC) ont été réalisés. L’essai de flexion met en évidence un module élastique proche de celui de l’os (20GPa). La DSC indique des températures de transformations austénitiques et martensitiques voisines de la température ambiante. Le taux de porosité se situe entre 31 et 43 % en fonction des températures de frittage. La taille des pores se situe entre 5 et 30 μm au lieu des 100 à 500 μm souhaités. L’utilisation de space - holders semble inéluctable pour obtenir des pores d’une si grande taille. Trois températures de frittage ont été testées : 800, 1000 et 1200°C. Les caractéristiques mécaniques obtenues pour les deux dernières températures de frittage sont très bonnes. Dans la perspective d’une application industrielle, un frittage à 1000°C semble le meilleur compromis coût – caractéristiques mécaniques

Réalisation des pièces en alliage de titane par technologie MIM

Objectif du projet : réalisation par technologie MIM (Metal Moulding Injection) de pièces à partir de’hydrure de titane ayant les caractéristiques du titane pu

Enhanced powder sintering of near-equiatomic NiTi shape-memory alloys using Ca reductant vapor

An alternative powder metallurgical process (called VPCR process) for the fabrication of single-phase NiTi shape-memory alloys is presented. It is based on a vapor phase metallothermic reduction operating during the NiTi compound formation. This process allows to achieve high chemical homogeneity. It is effective not only for obtaining homogeneous near-equiatomic NiTi without undesirable oxides (Ti4Ni2Ox) and carbonitrides (TiC1−xNx), but also avoids the formation of other Ni–Ti intermetallics such as Ti2Ni, Ni4Ti3 and Ni3Ti. Furthermore, the produced near-equiatomic NiTi (Ti-50.2 at.% Ni) exhibits improved shape-memory effects associated with the reversible B2 ↔ B19′ transformation

Fabrication process and shape-memory effect of NiTi alloys synthesized from Ni/TiH2 powder mixture

An alternative powder metallurgical fabrication route for the production of NiTi shapememory alloys is presented. It uses TiH2 instead of Ti as raw powder. TiH2 powder was chosen because it has firstly the potential to decrease the oxide content of the final product due to the presence, in-situ, of hydrogen and secondly it is commercially available in very small sizes, thanks to its brittleness. NiTi alloys of two different chemical compositions were processed from elemental Ni, Ti and TiH2. The effect of TiH2 on the microstructure was investigated using X-ray diffraction, scanning electron microscopy, differential scanning calorimetry and shape-memory measurements. The alloys of near-equiatomic composition are single-phase NiTi. Ti-rich alloys show, in addition to the NiTi phase, a small quantity of Ti2Ni/Ti4Ni2Ox phases. The phase transformation enthalpies of the Ti-rich NiTi alloys are very close to those of reference melt-cast alloys. The samples exhibit substantial shapememory effects

Alternative powder metallurgical processing of Ti-rich NiTi shape-memory alloys

Ti-rich NiTi shape-memory alloys have been fabricated from elemental powders using an alternative powder metallurgical method based on the use of a reducing metal vapor during sintering. The produced alloys show high chemical homogeneity, low porosity and large shape-memory effects with recovery strains up to 3.9% for an applied stress of 80 MPa

Elastic behavior ::superelasticity

Superelasticity refers to the unusual ability of certain metallic alloys to undergo very large recoverable deformations. This unique property is linked to the existence of a crystallographically reversible stressinduced martensitic transformation (Otsuka and Wayman 1998). Superelasticity was first observed in a Cu–Zn alloy by Reynolds and Beyer (1952), and was then found in a number of other alloys, in particular Cu–Al–Ni, Cu–Zn–Al, and Ni–Ti. This last alloy system has given rise to a number of important practical applications

Processing of titanium-based materials from low cost titanium hydride powders ::pim and tape casting

This paper summarizes recent developments on titanium and titanium-nickel materials processed from titanium hydride powders. The feasibility of using this route to process performant, complex parts is assessed. Feedstocks for PIM were prepared with binders composed of polyethylene, paraffin wax and stearic acid. Solvent debinding in heptane was followed by thermal debinding and dehydrogenation at 500°C under argon. Titanium parts were sintered at 1200°C under argon. The ultimate tensile strength (666 MPa) and elongation to fracture (15%) meet the requirements for titanium grade 4. Watch bracelet segments were injection molded, showing good shape preservation and reproducibility. In addition, shape memory titanium-nickel parts with controlled porosity from 31 to 43% were sintered under argon at temperatures from 800°C to 1200°C. Slurries for tape casting composed of powder, solvent, plasticizer and binder were used to produce green tapes, which were debinded and sintered to obtain porous titanium thin sheets with 25% porosity

Net-shape manufacturing of NiTi shape-memory parts

NiTi parts have been produced by two net-shape manufacturing techniques: metal injection moulding (MIM) and three-dimensional printing (3DP). MIM parts were processed by using pre-alloyed powders, and two layer-manufacturing techniques: dropping of a binder on powder beds, and dropping of a solvent on powder-polymer granule beds. Both MIM and 3DP sintered parts are porous, and exhibit shape memory behaviour. MIM parts processed from Ni and TiH2 based feedstocks show a shape memory effect of more than 4%

Powder sintering and shape-memory behaviour of NiTi compacts synthesized from Ni and TiH2

The use of TiH2 to produce NiTi shape-memory alloys from elemental powders is investigated. Mixtures of Ni and TiH2 powders are cold-pressed and sintered in vacuum or under protective atmosphere. Some of the sintered NiTi compacts are subjected to an additional post-HIP treatment to further increase their density. Post-HIPped NiTi compacts with approximately 7% porosity are obtained. A detailed characterization of the microstructure of the compacts is carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The phase transformation behaviour is studied by differential scanning calorimetry (DSC). The specimens present significant shape-memory effects. A recovery strain of up to 1.6% is reached under a stress of 120 MPa