Effect of laser powder bed fusion parameters on the microstructural evolution and hardness of 316L stainless steel

Abstract

In this study, parts were fabricated using variations of laser power, scanning speed, and hatch spacing (volumetric energy density (VED)) to understand the effect of processing parameters on the structure and properties of 316L stainless steel. It was observed that parts with good microstructural integrity and properties (hardness, porosity, and density) were obtained when using VEDs between 40 and 100 J/mm3. Also, VED was valuable when comparing the extent of consolidation and unfused/unmelted powders (porosity). The individual printing parameters offered a better understanding of the microstructure evolution, part density, and hardness of the material when compared with the VED. Also, it was shown that the relative beam spot size to the hatch spacing creates unique conditions that result in either a melt track offset or overlap which dictates the energy requirement for the creation of a part with good qualities. Melt track overlaps require low power while melt track offsets require high power to create parts with good qualities (density, porosity, and hardness). In addition, it was observed that the hatch spacing dominates the scanning speed in determining part porosity while the scanning speed dominates the hatch spacing in determining part density. The individual printing parameters had a significant effect on the morphology, size, and spatial distribution of columnar and cellular subgrain structures. Higher laser power and high scanning speed resulted in coarser, well-defined cellular and columnar subgrains with relatively low hardness. Also, increasingly hatch spacing resulted in finer subgrain structures with dense columnar structures and sparsely distributed cellular structures.