Diffusion assisted hardness recovering and related microstructural characteristics in fusion welded Al-Mg-Si alloy butt

The mechanical properties of the precipitation strengthened Al-Mg-Si alloys are largely influenced by their long term and thermo related processing in practical application, which still desires further investigation. In this work, the hardness and precipitate microstructure variations of the fusion welded 6082-T6 butts during post-weld natural aging (NA) and paint baking (PB) processes were systematically investigated, employing the methods of hardness testing, scanning electron microscope (SEM), transmission electron microscope (TEM). The results show that the hardness loss of the butt during welding can be recovered in both processes of NA and PB, especially in the subzones of the fusion line. Mechanisms are closely related to precipitation evolution. The origination of this improvement is attributed to re-precipitation of GP zones during NA and beta '' phase during PB, by treating the welding, cooling and NA/PB as heat treatment processes of solid solution, quenching and aging respectively. Solid-state diffusion of the alloying elements between the seam and base material was observed and played an important role in the above processes. It causes enrichment of Mg and Si elements around the fusion line, resulting in higher supper-saturation level that further assists in the post-weld re-precipitation processes in this location. Therefore, the fusion line shows higher hardness values in any state than the fusion zone and heat affected zone, which provides new acknowledgements for reacquainting the performance of the fusion line in such welded butt. The data obtained in this work provides important guidance for application and design of the Al-Mg-Si alloys in light-weighting fields.