Abstract: The addition of a heteronucleating metamorphic agent is the main way to obtain fine-grained tissue. In order to study the effect of TiB₂ particles on the microstructure properties of 5356 aluminum alloy welds, Al-Mg welded joints were fabricated by powder-cold metal transfer (CMT) process, and the effects of TiB₂ particle addition amount on weld structure and mechanical properties were analyzed. The results show that the addition of TiB₂ particles can significantly refine the grain of the weld structure, weaken the grain growth and texture characteristics, increase the proportion of large-angle grain boundaries, reduce the size of the second phase, and promote its diffusion distribution. As the heterogeneous nucleation core, TiB₂ particles promote the precipitation of the second phase of Al₃Mg₂, reduce its size, make its distribution more uniform, and significantly reduce the size of the weld structure grain. Compared with the weld specimen without particle addition, the addition of 0.73% TiB₂ reduces the average grain size from 96.7 μm to 64.2 μm, increases the tensile strength from 202.4 MPa to 216.7 MPa, and increases the elongation after fracture from 19.8% to 25.5%. The average hardness of the weld increases with the increase of TiB₂ content, and the hardness with 0.89% TiB₂ increases from 65.18 HV to 73.97 HV. The fracture mechanism of the weld metal is ductile fracture.