Development of novel process parameter for laser-powder bed fusion based additive manufacturing of crack prone AISI M2 high speed steel

AISI M2 is a W-Mo based High-speed steel (HSS) that contains high carbon (0.95-1.05 wt. %) along with tungsten (6.0-6.7 wt. %), molybdenum (4.7-5.2 wt. %), vanadium (1.7-2.0 wt. %), and chromium (3.8-4.5 wt. %) as major alloying elements. It is known for its high hardness (700-800 HV) and wear resistance, even at elevated temperatures (500-600 °C). The presence of martensite, along with eutectic carbides in the microstructure, is responsible for these properties. Laser powder bed fusion (L-PBF) is characterized by the rapid solidification of a micrometre-sized molten pool. However, the use of L-PBF to manufacture M2 HSS components possesses significant challenges. Due to rapid solidification and the complex thermal history prevail in this process, along with the formation of hard and brittle martensite and eutectic carbides during cooling, significant residual stresses develop. These stresses invariably lead to cracking and delamination in the L-PBF printed parts. This study aims to develop a robust set of process parameters for L-PBF to print crack-free, dense M2 HSS parts. Base plate preheating and an optimized scanning strategy were explored to reduce the thermal gradient and minimize residual stresses. The investigation began with single-scan track deposition using 72 different combinations of laser power and scan speed, analysing melt pool dimensions, track stability, and bead modes. A processing window for thin, stable beads in the conduction mode was identified to print cubes. The optimal combination of process parameters was established to successfully print crack-free cubes with 400 °C base plate preheating and a novel scanning strategy. The relative density of the sample reached 99.5%. Microstructural and mechanical characterisation was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), microhardness testing, and compression testing. Based on the developed strategy, industrial adaptation of the M2 HSS forging tooling are explored.