Abstract: The addition of heteronucleating metamorphic agent is the main way to obtain fine grain 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 transition (CMT) process, and the effects of TiB₂ particles on weld structure and mechanical properties were analyzed. The results showed that the addition of TiB₂ particles could significantly refine the grain structure of the weld, 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 distributionAs a 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. Compared with the weld specimen without particle addition, the addition of TiB₂ with 0.73% mass fraction reduced the average grain size from 96.7 μm to 64.2 μm, increased the tensile strength from 202.4 MPa to 216.7 MPa, and increased the elongation from 19.8% to 25.5%. The average hardness of the weld increased with the increase of TiB₂ content, and the hardness of TiB₂ increased from 65.18 HV to 73.97 HV.The fracture mechanism of weld metal is ductile fracture.