Influence of grain refinement on hot cracking in laser welding of aluminum

Hot cracking is a major problem in casting and laser welding of important technical aluminum alloys. A well-known method to reduce this problem is grain refinement. The mechanism of hot cracking prevention by grain refinement is not fully understood yet. In this study, different amounts of titanium and boron were added into weld in laser welding of AA 6082, which lead to different grain structures. A hot cracking test (DELTA test) was carried out to investigate the influence of grain refinement on hot cracking susceptibility of the welds. In order to understand the mechanism of hot cracking prevention by grain refinement, an analytical model was developed, which describes the influences of the grain structure and solidifications parameter on the three following factors, the duration of the mush zone, the capillary pressure for hot cracking, as well as the permeability of the dendritic network in the molten pool. It reveals that the hot cracking susceptibility is determined by the combination effect of these three factors. There is an optimum grain size for a minimal hot cracking susceptibility. It agrees with the experimental results in DELTA test.