INFLUENCE OF MILLING PARAMETERS ON PARTICLE SIZE AND MICROHARDNESS OF CRYOMILLED AND SPARK PLASMA SINTERED CP Ti
Abstrakt
Ultra-fine grained (UFG) materials are in the spotlight because of their enhanced mechanical properties. UFG material can be prepared by bottom-up process consisting of agglomeration of nano-particles or by top-down process of severe plastic deformation (SPD) In this study, Ti Grade 2 powder underwent attritor milling in liquid nitrogen (LN) and argon (LAr) and was consequently consolidated via spark plasma sintering (SPS).
The influence of milling liquid (LN vs. LAr), of milling balls material (stainless steel vs. tungsten carbide) and of other milling parameters (e.g. speed, duration) on size and shape of powder particles, chemical contamination and microhardness was investigated.
Particle size reduction was generally not observed, while their morphology changed significantly due to simultaneous disintegration and intensive cold welding during the milling process. Using LN as a cooling medium resulted in very high nitrogen content (up to 3 wt.%), and consequent embrittlement and hardening with microhardness values attaining 800 HV.
LAr seems to be more suitable for milling of titanium, but a process control agent (stearic acid) must be added to prevent excessive cold welding. Microhardness of material without nitrogen contamination rose from original 178 HV to 200-300 HV range, depending on milling efficiency.