Preparation and characterization of sintered Ti-6A1-4V powder compacts

Abstract

Sintered Ti6Al4V powder compacts were prepared using atomized spherical and angular powders in the porosity range of 29-63%. Cylindrical green powder compacts cold compacted at various compaction pressures and then sintered at 1200 C for 2 h. The final porosities and average pore sizes were determined as functions of the applied compaction pressure and powder type. The compression deformation behavior of Ti6Al4V powder compacts was also investigated at quasi-static (1.6x10-3-1.6x10-1s-1) and high strain rate (300 and 900 s-1) conditions using conventional mechanical testing and Split Hopkinson Pressure Bar techniques, respectively. The mean pore size of the compacts varied between 29 and 171 Um depending on the particle size range of the powders used and the compaction pressure applied. Microscopic studies of as-received powders and sintered powder compacts showed that sintering at high temperature (1200oC) and subsequent relatively slow-rate cooling in the furnace transformed the microstructure of spherical powder from the acicular alpha to the Widmanstatten microstructure and angular powder from bimodal to equiaxed+ Widmanstatten microstructure.In compression testing, at both quasi-static and high strain rate conditions, the compacts failed primarily by shear band formation along the diagonal axis 45 C to the loading direction. Increasing strain rate was found to increase both the flow stress and the compressive strength of spherical powder compacts while it did not affect the critical strain for shear localization. The mechanical properties of angular powder compacts were further shown to be a function of powder size; larger the particle size higher the percentage of equiaxed structure while in compacts of particles <100 um relatively large voids resulted in reduced strength and ductility. Microscopic analyses of deformed but not failed and failed spherical powder compact samples further showed that fracture occurred in a ductile (dimpled) mode consisting of void initiation and growth in alpha phase and/or at the alpha/beta interface and macrocraking by void coalescence in the interparticle bond region. In angular powder compacts, the failure was granular brittle type at the interparticle bond region while the compact samples of particles <100 um fractured transgranularly through the voids. The strength of the sintered compacts was further shown to satisfy the strength requirements for cancellous bone replacement. The strength of the compacts having porosity level of 40% and/or lower was comparable with that of human cortical bone.Compared to Ti powder compacts of previous studies, Ti6Al4V powder compacts provided higher strength and increased porosity level of the compacts suitable for cortical bone replacement.