Development of new high strength Al – Sc filler wires for fusion welding 7000 series aluminium aerospace alloys

Abstract

It has been reported that the transition metal scandium (Sc) can improve the weldability and mechanical properties of aluminium aerospace alloys that are normally considered to be 'unweldable'. However, little is currently known about the mechanisms by which Sc leads to such improvements. Here, the effect of the Sc concentration in the fusion zone of metal inert gas (MIG) welds in a typical 7000 series Al aerospace alloy has been investigated in detail. It was found that at a critical Sc level (~ 0.4 wt-%) a dramatic level of grain refinement occurs, leading to a highly uniform, ultrafine (~ 10 μ m) grain structure across the entire the fusion zone. Grain refinement was accompanied by an increase in the concentration of solute that was retained in solid solution after solidification, which led to a reduction in the volume fraction of eutectic per unit grain boundary area, and a significant age hardening response in the fusion zone. The tensile properties of single pass MIG welds, produced with an Al–Sc filler wire, were compared to welds made using commercial Al filler wires recommended for welding 7000 series alloys (5087, 5180, and 5039). The Sc containing filler wire greatly outperformed all the commercial filler wires, both in terms of weld strength and ductility. Further improvements in the tensile properties of the welds were achieved by exploiting the enhanced aging response of the Al–Sc filler wires, through welding in a solution heat treated condition and using a post-weld aging treatment. The underlying metallurgical processes by which Sc brings about these improvements are discussed.