Effects and distribution of TiC on the nanoparticle strengthening A-TIG welded AZ31 magnesium alloy joints

AZ31 magnesium alloy sheets were A-TIG welded using mixed activating flux, which contained different ratio of TiC nanoparticles (TiCp) and TiO₂ powder, as surface coating. From mechanical test, the values of ultimate tensile strength and microhardness increased from 120.2 MPa to 181.7 MPa, 46.4 HV to 84.2 HV, respectively, which mainly resulted from the grain boundary strengthening of α-Mg and the dispersion strengthening of TiCp. The theoretical calculation suggested that the improvement of the yield strength was mainly the combination of the Hall-Petch and Orowan strengthening mechanisms, and the mismatch strain caused by different coefficient of thermal expansion between the matrix and the particles. Numerical simulation showed that the TiCp had a centralized tendency toward middle and bottom part of welding pool and was confirmed by the micro-zone microhardness test results in the fusion zoon. (paper)