EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

A major concern with laser based powder bed fusion (LB-PBF) is the limited number of qualified alloys. This is especially true for low-alloy steels, where the carbon content can adversely affect processability by increasing the number of defects (e.g. cracks). This study examines the effect of layer thickness (20, 40 and 60 µm) on the microstructure and processability of 4130 and 4140 low-alloy steels. Microstructure and defect analysis indicate a strong effect of the layer thickness on defect formation, where an increase in layer thickness increases the propensity for lack of fusion defects. Melt pool depth measurements revealed that they were larger than any tested layer thickness, indicating that this lack of fusion was likely the result of instabilities within the powder bed. It was also found that the chosen layer thickness influenced the intrinsic heat treatment of LB-PBF, where a smaller layer thickness improved martensite tempering. This was an important factor that influenced cold cracking defects within the 4140 alloy. From these results, processing windows were established that allows to produce defect-free high-density (>99.8%) 4130 and 4140 low-alloy steels at each tested layer thickness The increase in layer thickness to 40 µm and 60 µm allowed for build rate improvements of up to ~106% and ~165%, respectively.