Effect of transverse magnetic field on microstructure and mechanical properties of Ti-6Al-4 V manufactured by Laser-MIG hybrid welding

In this study, an external transverse magnetic field was used to assist laser-metal inert gas (MIG) hybrid welding of Ti-6Al-4 V (TC4). The microstructure and mechanical properties, such as microhardness and tensile properties of the weld joints, under an external magnetic field (EMF) and a reference weld joint without the EMF were investigated. The results showed that the laser-arc interaction zone (LAIZ) and heat-affected zone (HAZ) expanded, and the laser fusion zone (LFZ) and fusion zone (FZ) changed with the application of the EMF, resulting in a change in the distribution of microhardness. The microstructure of the FZ was comprised of Widmanstatten α′-grains and primary boundary β-grains without the EMF. After applying the EMF, the diameter of the primary globular β-grains in the FZ decreased by 38.6%, from 78.4 to 48.1 μm, and a small amount of residual dot-like β-grains appeared in the FZ of the joint. The mechanical properties of the weld seam improved significantly under the EMF; the average microhardness in the FZ increased by 9.7%, and the failure strain and stress of the tensile specimens improved by 45.6% and 6.6%, respectively, owing to the solid solution strengthening of the residual dot-like β-grains and finer primary β-grain boundary. In addition, a mixed fracture mode was observed on the fracture surfaces. This study revealed that the elementary microstructure of laser-MIG hybrid welded TC4 was correlated with the welding heat input of the EMF.