Line distributed pores in laser powder bed fusion manufactured dies: A critical factor in die failure for high pressure die casting

Abstract

Additive manufacturing technologies, such as laser powder bed fusion (LPBF), have attracted a significant amount of attention for their capability in fabricating components of complex geometries with improved efficiency and design flexibility. However, the presence of pores in additively manufactured high pressure die casting dies has emerged as a critical issue affecting the performance and reliability of the dies. A numerical model for the formation of pores due to the instability of the keyhole and molten pool in a 18Ni300 steel is created. Computational fluid dynamic (CFD) simulation revealed that the keyhole in the molten pool undergoes inward ebbing and is wrapped by the liquid interface behind the laser beam, forming gas bubbles along the laser tracks. Some of the bubbles will either coalesce or breakup in remelting by the laser beam above in building the next layer forming a string of pores along the LPBF build direction, while others remain unchanged. The line distributed pores are the primary sites for cracking, and the cracks grow along the LPBF build direction. As a result, these longitudinal cracks cause the premature failure of dies with conformal cooling channels (CCC).