Microstructure and kinetics of intermetallic phase growth of three-layered A1050/AZ31/A1050 clads prepared by explosive welding combined with subsequent annealing

Abstract

The effect of annealing has been investigated with respect to interface microstructure and evolution of intermetallic phases in three-layered explosively welded A1050/AZ31/A1050 specimens. Both interfaces in the state after welding were characterized by wavy shape morphology with intermediate phases, which formed segmented structures. Two different morphologies consisted of equiaxed and columnar grains were observed within the Mg2Al3 phase, while the Mg17Al12 phase was built of only columnar grains. Furthermore, a small amount of Mg23Al30 was detected by X-ray synchrotron diffraction. Annealing at 350 °C strongly induced the Mg23Al30 phase development in the form of discontinuous layer between Mg2Al3 and Mg17Al12 phases after 10 h of annealing. Kinetics calculations indicated that, Mg2Al3 phase growth at 300 °C was controlled by different mechanisms according to the location: volume diffusion and chemical reaction at the upper interface and solely by volume diffusion at lower one. The growth of Mg17Al12 was governed only by volume diffusion. Furthermore, the same mechanisms were observed during annealing at 400 °C, however this heat treatment significantly changed the microstructure i.e. the grain size and shape. It was also established that the nanohardness of both Mg2Al3 and Mg17Al12 was about 350 HV.