Microstructure evolution and mechanical properties of large-size AZ31 magnesium alloy block fabricated by wire arc additive manufacturing

With the urgent demand for lightweight and integrated structural parts in the aerospace field, the use of cold metal transfer wire arc additive manufacturing (CMT-WAAM) to form large-size magnesium alloy structural parts has become a research hotspot. In this paper, a large AZ31 magnesium alloy multilayer block with a lower porosity was successfully prepared by CMT-WAAM. The AZ31 block exhibits a fully equiaxed grain morphology, the average grain diameters of the top, middle, and bottom are 16.1 µm, 12.4 µm, and 10.4 µm, respectively. The submicron spherical particles measuring 0.09 to 0.4 µm are Mg17Al12 phases, irregular particles are Al8Mn5 + Mg17Al12 composite phases and large particles measuring 5 to 10 µm are Al8Mn5 in AZ31 block. The UTS of CMT-WAAM AZ31 block from the bottom of 239.9 ± 5.0 MPa to the top of 237.3 ± 3.1 MPa, and the EL from the bottom of 22.7 ± 0.6 % to the top of 30.0 ± 1.1 %, exceeding the casting standards. In the CMT-WAAM processed AZ31 alloy, the pre-existing coarse second phase particles in the AZ31 filaments acted as nucleating agents, and the electromagnetic stirring effect in the WAAM process broke up the dendrites and increased the number density of nucleation sites to improve the rate of nucleation, and the complex thermal conditions in the WAAM process restricted the growth of the grains and the precipitated phases. The mechanical properties of AZ31 alloy were improved by effectively refining its grain size and precipitated phase size.