Temporal pulse shaping and solidification cracking in laser welded Al-Cu alloys

Solidification cracking in pulsed laser welds is a result of complex interactions between the alloy composition, the welding process conditions and the resulting solidification process, and the thermomechanical strains that are generated during welding. In pulsed laser welding, temporal shaping of the laser beam pulse has been reported to reduce or eliminate solidification cracking; however, a fundamental understanding of the influence of the temporal pulse shape on the cracking event has yet to be established. In this study, the cracking sensitivity of overlapped Nd:YAG laser spot welds was determined for binary aluminum-copper alloys. A pulse shape was then developed which reduced cracking in individual spot welds in Al-3.73 wt.% Cu. Using a numerical model, the thermal conditions during welding were predicted and were shown to be significantly different for two different pulse shapes. The resulting microstructures of the welds were examined and the primary dendrite spacing was shown to be in good agreement with the numerical predictions. Overlapped welds made using the temporally-shaped pulses also exhibited significantly less cracking. The results show that temporal pulse shaping can be effective for reducing cracks in both individual and overlapping spot welds.