Effect of substrate preheating by induction heater on direct energy deposition of AISI M4 powder

Abstract

This study discusses the effect of substrate preheating by an induction heater on the deposition characteristics of high-speed tool steel M4 powder. The powder is highly susceptible to cracking because of the very rapid heating and cooling rates induced by a high-energy laser during the deposition process and the residual stress evolution after deposition. Experimental investigations showed that a slower cooling rate can be induced by substrate preheating with the assistance of induction heating, which results in a coarser microstructure and a significantly longer spacing between cellular and dendrite arms. A high micro-hardness level may be obtained despite the microstructure evolutions occurring during the induction heating-assisted laser metal deposition because of the higher volume fraction of hard and stable carbides resulting from the increased holding time for precipitation. The tensile strength of the specimen prepared with substrate preheating increases. This effect may be attributed to the fine-grained martensite structure exhibited in the inter-diffusion and heat-affected zones in the sample that was not preheated. On the contrary, no distinct martensite structure was observed in the induction-heated sample. The fracture surface showed a typical brittle fracture, in which the crack propagated along the inter-dendritic boundaries. The results presented in this paper can be considered as guidelines for practical hardfacing applications that use high-alloy tool steel powders to achieve highly improved die and mold performances in terms of wear resistance and toughness.