Development of a new Mg-25Al-5Zn filler metal containing trace Y for brazing AZ91D magnesium alloy

In this work, the investigation focused on the effects of trace Y on the microstructure and characteristics of the Mg-25Al-5Zn filler metal, and then the optimal Mg-25Al-5Zn-0.5Y was used to braze the AZ91D magnesium alloy. Five kinds of filler metals showed good wettability on AZ91D magnesium alloy. Compared with Mg-25Al-5Zn filler metal, the wetting area of Mg-25Al-5Zn-0.5Y filler metal was increased by 134 % and the wetting angle was decreased by 58 %. This was mainly due to the spontaneous segregation of the active element Y at the interface. Thus, the surface tension was reduced and the wettability of the solder was improved. Different from the precipitation of alpha-Mg near the interface at different brazing temperatures, the distribution of alpha-Mg in brazing seam was more homogenous as the holding time increased. The microhardness of the brazing seam gradually increased from AZ91D side to the center of the brazing seam due to a lot of beta-Mg17Al12 with high microhardness in joint. However, with the increase of brazing temperature and holding time, a large number of alpha-Mg solid solutions with low microhardness were precipitated in the brazing seam. As a result, the microhardness of the brazed joint gradually decreased. The shear strength of the brazed joint reached the maximum value (42 MPa) at 520 degrees C for 60 s. After calculation, the contribution of grain boundary strengthening and solid solution strengthening to shear strength was 67.98 % and 28.96 %, respectively.