Unravelling the cracking mechanism in wire-based laser-directed energy deposition processing high-strength aluminum alloy

Abstract

High-strength aluminum alloys exhibit high cracking susceptibility in laser-based additive manufacturing. Understanding the mechanisms behind cracking and identifying the primary factors are crucial to preventing cracking, especially when dealing with difficult-to-process materials. Therefore, it is necessary to uncover the cracking mechanisms during successive deposition. In this study, the cracking mechanism is investigated in laser-directed energy deposition processing AA7075 alloy in terms of solidification conditions, microstructure, and residual stress. Based on the results, the cracking phenomenon observed during successive deposition is induced by insufficient backfilling to solidification shrinkage leading to solidification cracking. The melt-pool lifetime and the maximum melt-pool temperature are the primary factors determining the solidification cracking susceptibility. After the initiation, the cracks grow in two directions parallel to the building direction. The growth downwards is attributed to the liquation cracking mechanism, while the growth upwards results from the solidification cracking mechanism. The delayed cracking (cracking after a certain number of layers have been deposited) is identified as the consequence of the competitive growth between grains with preferential growth direction and highly misaligned grains.