The effect of different scanning strategies on microstructural evolution to 24CrNiMo alloy steel during direct laser deposition

Abstract

24CrNiMo alloy steel is a low-alloy high strength steel and was used to fabricate engine components and brake discs. In the current study, Direct Laser Deposition (DLD) was used to additively manufacture 24CrNiMo alloy steel samples with high strength and ductility. The microstructural evolution, texture and mechanical properties of deposited samples can be affected by changing inter-layer time intervals and scanning strategy between deposited layers. The granular bainite (GB) and lath martensite (LM) inside deposited samples were obtained by inter-layer successive deposition and 2-min intervals deposition, respectively, because of the different cooling rates of different inter-layer time intervals. The occurrence of 2-min intervals between cladding layers leads to higher cooling rates, resulting in finer grain size. In addition, the relatively strong textures could be transformed into the weak ones since the columnar dendrites were fragmented by the different heat transfer directions resulting from a scanning strategy of 90°-rotation between cladding layers. Less inclusions and un-melted powders inside deposited samples were obtained when a scanning strategy of 90°-rotation between deposited layers was applied. The experimental results indicate that a deposited sample with excellent tensile properties and microhardness was fabricated by an optimized laser scanning strategy. The optimized scanning strategy mainly include inter-layer 2-min intervals and 90°-rotation between deposited layers. Deposited samples fabricated by the optimized scanning strategy have comparable tensile strength and ductility to those of other forged low-alloy high strength steel. Therefore, this optimized laser scanning strategy can be applied to manufacture low-alloy steel with high tensile properties.