Effects of base and filler chemistry and weld techniques on equiaxed zone formation in Al-Zn-Mg alloy welds

Abstract

Equiaxed zone (EQZ) formation in Al-Zn-Mg alloy welds as affected by base metal, filler metal chemistry and weld techniques is studied. Filler metal chemistry and welding techniques have great influence on the formation of EQZ microstructure as base metal composition has. In an effort to characterise the equiaxed grain zone formation in Al-Zn-Mg alloy welds two commercial Al alloys AA7018 and RDE40 were selected. Gas tungsten arc welding in continuous current, pulsed current and arc oscillation mode were applied to weld the base materials. The influence of Sc containing fillers have been studied and compared with the commercial filler material. Mechanical and metallurgical characterisation were carried out in the EQZ. Intergranular corrosion in EQZ was studied according to ASTM G 110-92. Results reveals that RDE40 with low solute contents showed wider EQZ but relatively better corrosion and mechanical properties compared to AA7018 EQZ. Gas tungsten arc welding in pulsed and arc oscillation mode fusion boundary region exhibits better corrosion and mechanical properties compared to continuous current mode welds. Addition of Sc to the AA5556 filler combined with pulsed mode resulted in elimination of EQZ, better corrosion and mechanical properties compared to welds made with conventional AA5556 filler and also the presence of Sc within the EQZ so called unmixed zone has been observed.