In-situ high-speed X-ray imaging of crack formation and elimination mechanisms in AA7075 alloy during laser remelting

Abstract

The inherently high susceptibility to hot cracking of AA7075 alloy poses significant challenges in its Laser Powder Bed Fusion (LPBF) manufacturing process, thereby impeding its widespread adoption in aerospace and automotive industries. However, the laser-matter interactions and melt pool dynamics in laser Additive Manufacturing (AM) remain obscure, particularly in how cracks initiate and propagate during the process. In the present study, in-situ high-speed X-ray imaging technique was employed to characterize the crack formation and elimination during the laser remelting process on LPBF fabricated AA7075 substrates. The remelting process resembles scanning on preceding layers during LPBF. The microstructure of substrates and the processing parameters were investigated. Two modes of crack formation were unveiled: one initiating from the inherent crack defects, propagating upwards through vulnerable areas, and the other originating within the final depression zone of the melt pool, extending downwards across the vulnerable areas. On a substrate with rich defects, cracks tend to initiate from the inherent cracks. Another crucial finding is that the rupture of gas pores can induce fluctuations in the melt pool, leading to the elimination of cracks. The efficacy of crack elimination within the melt pool is highly influenced by variations in the inherent microstructure of the substrates and the applied processing parameters. These results will provide enlightening insight into crack reduction and elimination in LPBF.