The conventional cold-compaction-and-sinter powder metallurgy (PM) approach offers an efficient solution to the near-net shape or preform fabrication of titanium and its alloys for cost reduction and improved chemical homogeneity and refined microstructures. The methods for compacting titanium powder are similar to those used for other ductile powders. The high-purity titanium in the most ductile state is similar to annealed copper in terms of elastic modulus, hardness, elongation, and ultimate tensile strength. The properties of titanium are sensitive to the impurity level, in particular to nitrogen, oxygen, carbon, and iron. Hardness is a convenient measure of the quality of a titanium sponge product. Dilatometric studies of the sintering of titanium-nickel alloys show similar observations and confirm that oxide films on titanium powder surfaces do not need to be reduced by the atmosphere or disrupted by a chemical additive.

Cold compaction and sintering of titanium and its alloys for near-net-shape or preform fabrication

Attempts have been made to ascertain the mechanism of mass transport through the evaluation of the activation energies for different temperature ranges and through the sintering diagram approach of Ashby during sintering of microcrystalline titanium powders (average particle size of ∼3 μm). Present studies reveal that the sintering in the alpha range is controlled by lattice diffusion with source of matter at grain boundaries and the sintering in beta region appears to be controlled by lattice diffusion along with some kind of grain boundary rotation mechanism.

Sintering kinetics of micrometric titanium powder

Powder metallurgy (P/M) is a particularly attractive manufacturing process for titanium (Ti) components as conventional shape casting routes are not practical due to the reactivity of molten Ti with most gas atmospheres and with most materials which might serve as crucibles. This chapter discusses the conventional press-and-sinter Ti P/M process. It begins by providing a historical overview of the methods of production for Ti powder and the characteristics of the powders produced. It then reviews key issues in the cold pressing and sintering processes of Ti and its alloys.

Sintering of titanium and its alloys

Sintering behaviour of Ti–2Ni and Ti–5Ni elemental powders

The sintering densification trajectory for titanium powder is identified in terms of the interaction between mass transport processes and microstructure evolution. During initial heating, as surface oxides dissolve, surface diffusion forms bonds between contacting particles without densification. Grain boundaries form in the bonds due to random crystal orientations at the contacts. Except for mixed powder Kirkendall swelling, subsequent diffusion in these interparticle grain boundaries leads to densification. Most importantly, the alpha-beta transformation provides strain, defects, and interfaces that accelerate densification in the 800–1100 °C temperature range. This is below a typical peak sintering temperature. Final densification involves beta phase volume diffusion and grain boundary diffusion. Densification slows due to grain growth and the loss of grain boundary area. Pores close near 92% density to trap impurities and reaction products inside the closed pores, often limiting sintered density to about 95% of theoretical. High final density requires slow heating or long holds at intermediate temperatures to evaporate impurities prior to pore closure. The master sintering curve is a means to link densification to process parameters without concern over detailing this cascade of transport mechanisms and microstructure changes.

Titanium sintering science: A review of atomic events during densification

The influence of phase transformation on the sintering kinetics of Ti powder has been studied comparing the behaviour of Ti compacts under the influence of cyclic heating. The results show that the effect of a~f3 transformation is to promote sintering. The reverse transformation f3~a has no effect on sintering rate. Total shrinkage of Ti compacts sintered for 5 h by cyclic heating with transformation was about twice that of compacts sintered for the same time without transformation. In the former compacts, twinlike structures were observed. PM/0163

Z. Hara

Papers

Increase of Sintering Rate of Titanium Powder During Cyclic Phase Transformation