Microstructural study of laser welds Al6056-AS12 in relation with hot tearing

T shape welds of 2.5 mm thick sheets of the Al-Mg-Si-Cu alloy 6056 were produced using two 3kW Nd:YAG lasers. The filler wire used was Al-12%wt Si. The welds were realised in the T4 temper. Optical observations revealed a very fine dendritic structure (D.A.S.=3mum) in the fusion zone with a grain size of about 100mum. The composition of the interdendritic space is rich in all alloying elements and multiphased. The dendrites interior still contain sufficient solute to enable precipitation hardening during a post-welding heat treatment. This precipitation has been characterised by transmission electron microscopy. The grain boundaries of the base material close to the weld nugget appear to be affected by the welding process. By a combination of optical and TEM observations, we have demonstrated that grain boundary liquation occurred. Microhardness tests reveal a large difference between the fusion zone and the base metal. After a post welding heat treatment, hardness increases not only in the base metal but also in the fusion zone due to precipitation hardening. The narrow heat affected zone which can be observed just after welding disappears after a post welding heat treatment.