Investigation of process parameters of laser powder bed fusion to manufacture crack-free M2 high-speed steel

This study detailed the optimisation of process parameters for the laser powder bed fusion (LPBF) additive manufacturing process to produce crack-free, high-density M2 high-speed steel alloy. The high carbon content of this steel increases its susceptibility to cracking under the steep thermal gradients of the LPBF process. The work identifies optimised processing parameters for manufacturing defect-free M2 high-speed steel with minimal residual stress. A matrix of samples was built by varying laser power and scan speed, under 180°C platform heating. It was found that there are three distinct sample behaviours, categorised by volumetric energy density (VED): build failure at high energy density, macro-cracking at intermediate energy density and sound but porous structures at low energy density. X-ray diffraction analysis of cracked samples revealed phase transformations with varying VED, suggesting that the thermal gradient played a significant role in crack formation. While analysing the set of samples without visible macro-cracking, a critical VED of 27 J/mm 3 was identified, resulting in a crack-free sample when using a relatively low power (225 W) and low speed (1000 mm/s) combination, which reduces thermal stress. Using this optimal baseline, the process was further refined by adjusting the laser hatch distance. Decreasing the hatch distance from 140 µm to 120 µm successfully minimised internal porosity by promoting ideal melt pool overlap. The optimised process yielded fully dense, crack-free parts with a relative density of 99.38% (8.11 g.cm −3 ) and an as-built hardness of 643 HV0.5.