Evolution of cracks in laser powder bed fused AA2024 and its processing parameter dependence

Abstract

Hot cracking is one of the most hideous issues in the fabrication of high-strength aluminum alloys via laser powder bed fusion (LPBF). This study investigated the evolution of hot cracking in LPBFed AA2024 and explored its processing parameter dependence. Results showed that most cracks distribute along continuous eutectic phases residing at grain boundaries. At the last stage of solidification, the eutectic composition with low melting point exists as a liquid film, which can be easily ruptured under stress caused by thermal contraction. This leads to the formation of cracks along grain boundaries. This cracking process is also related to the misorientation angle of grains, where higher angle results in more stable liquid films. Furthermore, increasing the laser power transforms the grain structure from fine columnar grains to coarse columnar grains, which eventually transform to a mixture of columnar and equiaxed grains. This grain structure transition results in the decrease followed by increase of critical stress for the rupture of liquid film. On the other hand, decreasing the scanning speed increases the eutectic phase content, which could enhance the permeability of mushy zone and thus reduce the cracking susceptibility. These findings could provide guidance for the fabrication of crack-free high-strength aluminum alloys via LPBF.