Research for microstructure and mechanical properties of AZ91 magnesium alloy welded joint with magnetic field and activated flux

Abstract

Although many experimental researches have been carried out on the effect of different fluxes and the mechanism responsible for the higher penetration in activated TIG welding of magnesium alloy, few works as reported in literatures are available concering the grain refinement and the improvement of mechanical properties of welding joints. This is because the activated flux has very limited or even negative effects on improving the mechanical properties of welded joints. In order to find a method that can improve welding efficiency and mechanical properties of welded joints, the longitudinal alternating magnetic field and NiCl2 activated flux were used during TIG welding of AZ91 magnesium alloy. The formation, mechanical properties, phase composition and crystal growth pattern of the weld seam were tested and analyzed to study the mechanism. The experimental results reveal that with proper parameter matching (magnetic field and activated flux), larger weld penetration and smaller form factor can be obtained, welding efficiency is improved accordingly, but the form factor with the magnetic field is bigger than that without magnetic field. When the activated flux amount is 3 mg cm−2 with the magnetic field, the optimal value of mechanical properties of welded joint is obtained, tensile strength is 385 MPa, elongation is 13.3%, micohardness is 67 HV, respectively. All of these are better than those without the magnetic field, the optimal activated flux amount is 2 mg cm−2. The application of magnetic field and activated flux has no noticeable effect on the phase composition of weld seam. Under the combined action of magnetic field and activated flux, the crystallization nucleation condition of molten pool was changed, the grain size was refined, the formation of twins was promoted, and the crystals selectively grew within the basal (0001) plane.