Graded microstructure and properties of TiCp/Ti6Al4V composites manufactured by laser melting deposition

Abstract

In this paper, 50 vol% TiCp/Ti6Al4V composites without pores, cracks or poor fusion were fabricated by laser melting deposition (LMD) technology using 500 W laser power (lower power, LP) and 1000 W laser power (higher power, HP). The microstructure of different deposited layers was analyzed and the corresponding microhardness was measured. The results show that the composites are composed of undissolved TiC, in-situ TiC (eutectic TiC and primary TiC), α Ti and β Ti. It is interesting that the microstructure from the bottom layer to the top layer presents a graded distribution when all layers have the same composition. The eutectic TiC is mainly formed in lower layers while the primary TiC is mainly in upper layers. With increasing layer number, the size and number of the primary TiC grains increase and the primary TiC grows gradually from granules into dendrites, however, the number of the undissolved TiC particles decreases along the depositing direction. The simulation results show that the peak temperature of deposited layer increases while the cooling rate decreases with increasing layer number. Different thermal history affects the diffusion of carbon and the evolution of TiC, and then affects the microstructure distribution of the composites. This graded microstructure leads to the graded distribution of microhardness. The microhardness of the LP composite rises from 432.5 HV to 488.1 HV, while the HP composite rises from 435.5 HV to 604.3 HV from the bottom layer to the top layer, respectively.