Manufacturing of fine-grained titanium by cryogenic milling and spark plasma sintering

Abstract

Commercially pure (CP) gas-atomized titanium powder was processed by wet cryogenic milling in liquid argon and compacted by spark plasma sintering. The time-dependent sintering evolution at different temperatures was evaluated by using the master sintering curve (MSC) approach with the aim of achieving a material with maximum relative density and minimum grain size. Carrier-gas hot extraction (CGHE) confirmed a purity consistent with ASTM standard of Grade 4 CP Ti. Grain size and texture were determined by EBSD. An apparent activation energy of sintering of 115 kJ/mol was found based on the MSC approach. It is significantly lower than the activation energy of self-diffusion in Ti. This is attributed to an enhanced diffusion rate due to high concentration of defects in the powder after milling. The relative density was correlated with the resulting grain size and a general trade-off relationship between achieving high relative density and maintaining small grain size was found. The distribution of oxygen after milling and subsequent sintering at low temperatures is heterogeneous as determined by complementary XRD, CGHE and microhardness measurements. The distribution of oxygen becomes homogeneous with increasing sintering temperature. The microhardness of the material was shown to depend on residual porosity, the content and distribution of oxygen and also on texture. Processing parameters of milling and sintering were optimized to achieve fully dense, fine grained material with a low contamination by nitrogen and oxygen.