Micro-scale model based study of solidification cracking formation mechanism in Al fiber laser welds

Effect of welding speed on solidification cracking susceptibility in fiber laser welding of 6013 aluminum alloy was investigated by considering both mechanical and metallurgical factors. A finite element model was developed at columnar grain scale to calculate the strain localization in the mushy zone. Based on the Rappaz-Drezet-Gremaud (RDG) criterion and study from the developed micro-scale model, the pressure drop in the inter-dendritic liquid film was used as cracking index to investigate the formation of solidification cracking. Cooling rate, solid fraction, local strain rate and microstructure characteristic resulted from different welding speeds were examined. Cracking sensitivity was shown to decrease with the increase of welding speed in the range between 2.5 m/min and 3.5 m/min in fiber laser welding of 6013 aluminum alloy. Numerical calculations showed that mechanical tensile strain in the solid grain was in the order of magnitude of 10−4while the strain in the liquid film is in the order of magnitude of 10−2. The total pressure drop at the root of columnar grains was more than 150 kPa, which was deemed as the critical pressure drop to form a crack in this study. The transverse solidification cracking was prone to initiate in the second half of mushy zone where the solid fraction was between 90% and 94%.