Comparison of microstructure and mechanical properties of TIG and laser welding joints of a new Al–Zn–Mg–Cu alloy

Abstract

Sheets of a new Al–Zn–Mg–Cu alloy with T6 temper were welded using TIG and laser welding method. Microstructure characteristics and mechanical properties of joints were contrastively analyzed. Results show that the width of the fusion zone (FZ) and heat affected zone (HAZ) in laser joint is obviously smaller. The FZs in TIG and laser joints are both made up of dendritic equiaxed grains and lots of coarse particles, the mean grain size is 33.9μm and 6.1μm respectively, and the coarse particles are identified as the T (AlZnMgCu) phase. The two factors of grain size and alloying elements distribution are combined to result in almost the same microhardness of the two FZs. Moreover, the grain morphology, grain size and degree of recrystallization of the two HAZs are almost the same with base metal (BM). Due to the higher heating and cooling rate of welding thermal cycles in HAZ of laser joint, the microhardness is obviously higher. The minimum microhardness of the two joints is obtained in FZ where the fractures always occur in the tensile tests, the ultimate tensile strength of TIG and laser joints is 436.2MPa and 471.1MPa, the elongation of them is about 7.5% and 5.1% respectively.