The objective of this this study is to investigate the distribution of crack initiation sites in the unsteady solidification conditions of pulsed laser welding for an AA2024 aluminum alloy, using both analytical and experimental methods. The employed analytical model considers the competition between volume change rate and liquid flow rate during the final stages of solidification. The former has a direct relationship with solidification rate (r), the latter has an inverse relationship with the length of the crack-vulnerable zone (l), and the risk of cracking decreases with lower values of r × l. According to this model, the distribution of crack initiation sites is more strongly influenced by the solidification rate profile, specifically the solidification rate at the fusion line (rFL) and at the weld center (rWC). The crack initiation site will move toward the weld center as (rWC-rFL)/rFL increases. The model states that in square-wave pulse welded samples, a decrease in rFL caused by preheating causes crack initiation sites to move from the FL toward the WC and crack severity to decrease. Moreover, it was shown that ramp down pulse shaping was more effective at reducing cracks, as both r and l can be controlled and, in turn, r × l can be reduced enough to prevent crack initiation. The model’s reliability was assessed using experimental pulsed laser welding tests that considered the influence of base metal preheating and temporal ramp down pulse shaping.
