INFLUENCE OF FILLER MATERIAL ON MICRO AND MACRO-MECHANICAL BEHAVIOUR OF LB-WELDED T-JOINT FOR AEROSPACE APPLICATIONS

3.2 mm thick laminated skins of AA7475 alloy and 2.5 thick extruded stringers of the alloy PA765 were T-jointed by laser beam welding. Two different joints were fabricated using identical welding parameters but two different alloys as welding wires, AA5356 and AA4047. The microstructure, the tensile and the failure behaviours of the two welded joints are studied by means of optical and electronic microscope, microindentation and tensile testing. Weld beads obtained using AA5356 result larger than those obtained using AA4047. Microstructure of the welded joints is quite similar: at the centre of the fusion zone, cellular dendrites are present; near the fusion line in the fusion region, a chill zone and parallel dendrite arms are found; an evident heat-affected zone is observed between the fusion line and the parent material. Microhardness tests reveal that the partially-melted zone is a weak band in the welded specimens. Indeed, for both joints, failure took place at the partially-melted zone upon tensile loading. Hoop testing displays that AA4047 joints present a lower yield stress than AA5356 ones, although both joints show similar tensile strength and elongation at break. AA4047 joints result more strength than AA5356 ones upon T-pull testing.