Effects of heat accumulation strategies on defects and microstructure of pure chromium fabricated by laser powder bed fusion: An experimental and numerical study

The process of employing laser powder bed fusion (L-PBF) process to refractory materials, such as chromium (Cr), remains challenging because of its high ductile-brittle transition temperature. Therefore, a strategy is required to increase the processing temperature to prevent defects. The focus of this study is to clarify the effect of the preheat temperature and scan length variations on defects during the L-PBF process with an experimental and numerical study. Applying a high preheat temperature and short scan length was effective in mitigating the defects. By tuning the heat accumulation strategies, the determined relative density measured by the Archimedes principle and optical measurement of pure Cr parts increased from 97.2% to 99.8% and 97.4% to 99.2%, respectively. The numerical study indicated that deepened and elongated melt pool geometry owing to heat accumulation promoted epitaxial growth. Strong crystallographic texture formation with epitaxial growth led to higher grain size, lower high-angle grain boundary misorientation, Kernel average misorientation, and Taylor factor, which resulted in densification by preventing defects. The hardness of L-PBFed Cr samples gradually decreased with the stronger crystallographic texture formation and compressive yield strength exhibited the same phenomenon. However, the high densification sample with strong crystallographic texture promised the highest compressive strength and strain because it allows single and multi-slip operation during the compressive deformation without premature fracture. This study is a pivotal moment in heat accumulation strategies to achieve high densification for brittle materials fabricated by the L-PBF process while proposing an approach to ensure the reliability of structural applications.