Effects of back-diffusion on solidification cracking susceptibility of Al-Mg alloys during welding: A phase-field study

The effects of Mg back-diffusion on liquid channel segregation and morphology, and hence on the solidification cracking susceptibility (SCS) of Al-4.0 wt% Mg alloy, were investigated using phase-field simulations using a multistep computational strategy. We demonstrated that Mg back-diffusion can strongly reduce liquid channel segregation and facilitate dendrite coalescence, promoting the dendrites to bond extensively to resist cracking. When back-diffusion is considered, the predicted SCS index decreases significantly. On the other hand, we found that increasing either solid-state diffusion coefficient (D-s) or partition coefficient (k) can enhance Mg back-diffusion and hence promote dendrite coalescence; however, the underlying mechanisms are essentially different. Increasing either D-s or k can significantly flatten the T-(f(s))(1/2) curves, and reduce the index and SCS. Our simulation results explained why Al-4.0 wt% Mg alloy can have low SCS despite its wide freezing temperature range (because of Mg back-diffusion) and why the drop in SCS of Al-Mg alloys is much more significant than that of Al-Cu alloys when considering back-diffusion (owing to the larger k) from the perspective of liquid channel segregation and morphology as well as the SCS index. Results from this work are consistent with analytical predictions and previous experiments. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.