Enhancing equiaxed grain formation in a high-alloy tool steel using dual laser powder bed fusion

This study is the first to show how dual laser (DL) powder bed fusion (PBF) combined with in situ heat treatment (HT) via substrate plate heating promotes the formation of equiaxed grains in the high-carbon high speed steel PM HS 10-2-5-8 containing 1.64 wt% C. The solidification microstructure is significantly affected by overlapping the melting laser beam with a defocused high-power heating laser beam. This heating concept results in a reduced temperature gradient G at the solidification front leading to a columnar to equiaxed transition (CET) and a fine-grained equiaxed microstructure with exceptional mechanical properties. Dense and crack-free DL specimens could be obtained by using a scan speed of 800 mm/s, a melting laser beam with a power of 130 W and a spot size diameter of 80 µm, followed by the heating laser beam with an offset of 160 µm, a power of 600 W and with a defocused spot size diameter of 400 µm. The study also found that by using DL-PBF, it is possible to reduce the temperature of the substrate plate for in situ HT from 800°C for single laser (SL) PBF to 600°C for DL-PBF.