Microstructural characteristics and material removal mechanisms in nano-scale finished surface of mirror-like injection mould fabricated by laser metal deposition and post treatments

Abstract

The manufacture of injection moulds with mirror-like surfaces by laser metal deposition is a state-of-the-artdevelopment trend in manufacturing industries. The high surface quality and long-term service performance are two critical demands for laser-metal-deposited injection moulds. In this paper, the microstructure and material removal mechanisms of laser-metal-deposited moulds after nano-scale surface finishing and post treatment were analyzed. Our results confirmed that the differences in temperature gradient and melting composition during solidification would result in both columnar and equiaxial dendritic αʹ-Fe grains distributed in the mould surfaces, and the surface defects of the moulds after nano-scale finishing were directly related to the aggregation of brittle M23(C,B)6 networks stripped out from inhomogeneous αʹ-Fe matrices. Both laser-surface-heat and vacuum-furnace-heat post treatments were beneficial to the microstructural homogenization. The former could achieve directional solidification microstructures with columnar αʹ-Fe dendrites and well-distributed M23(C,B)6 networks, effectively relieve stress concentration during finishing and successfully enhance surface quality and performance. The latter would produce the formation of retained γ-Fe and M2(C,B) particles, weakening surface quality of moulds. Therefore, the laser-surface-heat post treatment seemed to be one of the most promising ways to address microstructural homogenization of laser metal deposited moulds.