Effect of Longitudinal Magnetic Field on the Microstructure and Properties of A-TIG Welding with Different TiO2 Coating Amounts

Abstract

In order to improve the poor weldability and low quality of welded joints during the welding of magnesium alloys, a longitudinal magnetic field is introduced in the welding process of A-TIG welding with a fixed magnetic field frequency (30 Hz) and magnetic field current (1.5 A). Experimental analysis is performed on the effect of the magnetic field on the microstructure and mechanical properties of welded joints under different TiO2 active agent coating amounts. The results show that the grain size tends to decrease and then increase with the increase in the active agent coating under the magnetic field. This is mainly because the active agent changes the arc morphology, which in turn affects the melt pool motion. The Lorentz force generated by the longitudinal magnetic field acts on the molten pool and will have an agitating effect on the pool. Both the magnetic field and the active agent are convective to the melt pool, and when the magnetic field and the active agent act together will further enhance the convective effect. However, when the active agent is too thick, it will affect the fluidity of the molten pool during welding and reduce the quality of the welded joint. Under the action of magnetic field, when the active agent coating amount is 3 mg/cm2, the grain size is the finest and the mechanical properties are the best. At this time, the tensile strength was 292 MPa, elongation was 11.2%, and hardness was 78.9 HV in the weld zone and 77.8 HV in the heat-affected zone. Further analysis of the melt pool change and grain refinement mechanism under the combined effect of the magnetic field and active agent revealed that the magnetic field promotes the solidification of the second phase in the weld tissue, but the effect on the heat-affected zone is not obvious. The addition of the magnetic field was found to refine the grains by EBSD testing, reducing the average grain size by 1.43 μm. This indicates that the introduction of the magnetic field in the A-TIG welding process improves the mechanical properties and microstructure of the welded joint, which is conducive to solving the problem of poor weldability in the welding process of magnesium alloys.