Abstract
Aiming to improve the serious cracks in laser welding of 7075 aluminum alloy, a laser welding method with filler strip is proposed, and crack-free welded joints are obtained. With the increase of strip height, the cracks in the weld are changed from Y-shape cracks to I-shape cracks, and no crack occurs when the strip height is equal to 1.9 mm. Microstructure shows that cracks with smooth fracture surfaces occur along grain boundaries, which are classified into solidification cracks. Eutectic phases are observed along grain boundaries, which prove the presence of liquid films. A non-steady state cracking model is developed to illustrate the cracking mechanism and calculate the cracking susceptibility threshold, considering the change of the liquid backfilling ability in both time and space dimensions. It is found that with the increase of strip height, the couple of cooling rate, grain size and backfilling distance enhances the ability of liquid backfilling to suppress cracks. This is because the couple effect greatly reduces the local strain rate but has less impact on the liquid backfilling rate, which leads the cracking susceptibility lower than the threshold. Therefore, the cracks in the weld are well suppressed, which is in good agreement with the experiment results. This study provides a feasible method for crack-free welding of 7075 aluminum alloy.
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