Solidification characteristics of aluminium alloys

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The main goal of this project is to understand the role of alloy composition in the formation of defects in welds, taking into account the fraction liquid evolution and the morphology of the microstructure during solidification. The research will focus on the development of an algorithm predicting the temperature where new phases start to crystallize and calculating their specific growth rates. Since cooling rates are very fast in welding, the algorithm will have to take into account phenomena associated to liquid-solid interface moving at high velocities. Experimentations will be conducted in controlled conditions by using a differential scanning calorimeter (DSC), which measures the heat released during solidification. The cooling curves obtained with this instrument will be analysed using an original mathematical treatment so that it will be possible to compare theoretical and experimental heat rate extraction. This work will be used to investigate hot cracking in welds, which is caused by residual stresses exceeding the strength of the not completely solidified butt weld. Experimentations of hot cracking in welding will be conducted to know the critical stress and critical temperature where the alloy is prone to crack. The role of iron in the fusion zone will be particularly investigated in two alloys of the 6XXX series. It seems that intermetallic phases containing iron could play an important role in the initiation of hot cracking.