Influence of layer thicknes on the processability and productivity of low alloys steel

Rasmus Gunnerek and William Hearn

Chalmers University of Technology, Department of Industrial and Materials Science, CAM2

A major concern with laser powder bed fusion (L-PBF) is the limited number of qualified alloys. This is especially true for low alloyed steels, where the carbon content can adversely affect processability by increasing the number of defects (e.g. cracks and/or pores). Previous work has demonstrated that high density components of two low alloy steels (4130 and 4140) can be produced by optimizing the process parameters at 20 µm layer thickness. However, at this layer thickness the build rate is relatively low and hence the usability of these alloys is limited. Therefore, the aim of this study is to increase the build rate by varying the layer thickness from 40 µm to 60 µm and optimizing the process parameters. Studies on defect formation changes when varying layer thickness, is emphasized and therefore a minimum of process parameters is altered i.e. scan speed and laser power. This investigation demonstrates that a 40 μm layer thickness yielded a wider processing window than 60 μm layer thickness. Examinations of the microstructure revealed a greater number of lack-of-fusion defects at a larger layer thickness, which were the cause of this narrower process window. The melt pool depth at a 60 μm layer thickness was estimated to 100-240 μm, indicating that this lack of fusion porosity stemmed from instabilities within the powder bed. It was also found that an increased layer thickness yielded a lower hardness. This was connected to a more pronounced in-situ tempering that occurred at a 60 μm layer thickness. As a result, processing windows for 4130 and 4140 alloys, at layer thicknesses of 40 and 60 μm, were established, allowing to produce defect free, high density (>99.8%) components. Increasing layer thickness allowed to improve the build rate of these materials by up to ~165% in comparison to the previously developed parameters.