With the legislation enforcing more rigid CO2 emission targets, automobile manufacturers are adopting lightweight structures without compromising the rigidity and strength. Constellium HSA6TM is a newly developed aluminium 6000 extrusion alloy (Al-Mg-Si-Cu) which is extremely attractive to the automotive sector due to its excellent mechanical properties (e.g. ultimate tensile strength of ~400 MPa) offered through optimised precipitation hardening. However, welding during component manufacture of any age hardened Al alloy results in a heat-affected zone (HAZ) where the strengthening precipitates coarsen and dissolve and/or transform, leading to a reduction in the strength. The strength loss in the 6000 series alloys with elevated Cu content compared to standard 6000 alloys is not well established in the literature.
This research aims to understand the effect of three different welding processes – CMT (Cold Metal Transfer), Laser and FSW (Friction Stir Welding) on the microstructure and mechanical properties of the HSA6. The weldments were evaluated by both hardness and tensile tests to understand the strength loss in the HAZ. To complement the mechanical properties, site-specific TEM investigations were undertaken to understand the microstructure in the HAZ for the CMT weldment.
The investigation revealed that the FSW weldments offered the best tensile performance in both the as-welded condition and after a post-weld heat treatment, when compared with Laser and CMT welding processes. Hardness mapping showed that the HAZ was the softest region of the FSW weldment, while the fusion zone was the softest region in the Laser and CMT weldments. However, fracture in the CMT weldment occurred in the HAZ and not in the fusion zone, due to the larger cross-section area of the weld-bead. The microstructure in the weakest area of the HAZ in the CMT weldments shows the growth/formation of needle-shaped β’ and/or plate-type C precipitates and coarsening of the strengthening Q-type precipitates which nucleated on the Fe-Mn-Si and Zr containing dispersoids.