Microstructure of butt laser joints of aluminium alloy 6056 sheets with an AS12 filter

Butt joints of 1.6 mm thick sheets of Al-Mg-Si-Cu alloy 6056 in the T4 temper have been produced using a 3 kW Nd:YAG laser with an Al-1 2 wt-%Si filler wire. The fusion zone consists of a very fine dendritic structure (DAS=5 mu m) with a grain size of 100 gm. Its composition is controlled by the ratio between the welding speed and filler wire speed. The interdendritic space is rich in all alloying elements and contains several complex phases. The dendrite interior still contains sufficient solute to enable precipitation hardening during a post-welding heat treatment. The grain boundaries of the base material close to the weld nugget appear to be affected by liquation during the welding process, with spatial extension controlled by the welding speed. The microstructure of the heat affected zone is characterised by GP zone dissolution. The resulting hardness decrease can be fully recovered by a T6 or T78 post-welding heat treatment. The comparatively lower hardness of the fusion zone can also be improved by precipitation hardening during a post-welding heat treatment.