Investigating evolution of microstructures and mechanical properties in metal injection molding Ti6Al4V with sintering processing by quasi in-situ strategy

Abstract

Manipulation of the pore defects, adjustments of α/β phases fractions, optimization of the mechanical properties are helpful for sintered Ti6Al4V alloys fabricated Metal injection molding (MIM) powders. A pseudo quasi in-situ method with vacuum encapsulation and quenching has been proposed to control dynamical characteristics of pores, grain sizes and phase fractions of MIMed Ti6Al4V powders during sintering process. The dependency of pores annihilation and grains growth on sintering time and temperature has been established. Higher sintering temperature shortens the transition period of pore morphology from connected irregular to enclosed spherical characteristics. The annihilation of pores satisfies the exponential function as well as the power law with temperature and time, respectively. The activation energy has been calculated to be 252.2 kJ/mol comparable to self-diffusion activation energy of Ti according to Arrhenius fitting of the changes of grain size with sintering temperature. Additionally, the diffusional kinetic coefficient was 28.57 at 1200 °C, about five times larger than that at 1100 °C. The fraction of α/α′ phase decreases with increasing of sintering temperature and time. The existence of impurity elements (O, C) may result increasing the stability of α phases and morphology transforming of α phases from lamellar to equiaxed. Finally, sintered MIM-ed Ti6Al4V alloys with lowest porosity sintered at 1200 °C for 4 h have the tensile strength of 922 ± 5 MPa, the yield stress of 833 ± 4 MPa and the elongation of 12.8 ± 0.5%.