| The current development and application of magnesium and magnesium alloys are restricted by the problems in its processing and corrosion resistance. Magnesium alloy components are mainly die castings produced on the basis of its high plasticity. Given the application of proper welding technology, components with more complicated structure and larger size can be produced, thus expanding the application of magnesium alloy. In this paper, low-melting filler metals for AZ31B magnesium alloy were developed, which can be the foundation of magnesium alloy joining at a lower temperature.In this paper, Mg-Al-Zn filler metals were prepared through crucible melting and flux protecting. Their melting characteristics, metallographic structure, spread abilities, wetting abilities, shear strength of the lap joints and tensile strength of the butt joints were measured or analyzed through spreading and wetting capacity test, shear test, tensile test in terms of differential thermal analysis, optical microscopic structure inspection, SEM and energy spectrum analysis, X-ray diffraction analysis and fluorescence analysis. The research results are as follows:1) The melting temperatures of the filler metals ranged from 336℃to 464℃. The brazing temperatures were comparatively low, the highest of which was 510℃. The solid-liquid temperature range was narrow, which was conducive to the brazing. The self-designed argon protection device had received good effects, and the brazing process was stable when the equipment was used in conjunction with the self-designed brazing joint adjust jig. 2) A large number of oversize primary precipitated phases of Al and Mg in as-cast filler metals were found through the analysis of the metallographic structure of the filler metals. Due to the high solidification rate, there were many non-equilibrium metallographic structures in the filler metals. The solidification of the brittle eutectic phases mainly depended on the primary precipitated phases. The as-cast filler metals show great brittleness.3) The wettabilities and spreadabilities of the filler metals were comparatively good, with the minimum wetting angle being 13.2°. The filler metals brazed well, with smooth brazing interface and no obvious brazing defects. There was a great difference between the metallographic structure of the joints and the as-cast filler metals. The bases were allα-Mg, around which were eutectic structures of Mg-Al and Mg-Zn. The eutectic structures of #1 and #2 filler metals were oversize, bone-shaped and in a continuous netlike distribution, while the eutectic structures of #3 filler metal were cellular and lamellar. The brittle eutectic structures in the filler metals showed unfavorable impact on properties of the joints. The corrosion resistance of the filler metals was better than that of the base metal. The self-corrosion potential of the #3 filler metal was close to that of the base metal, so its joint with the base metal is better than the others.4) The lap joint of the #3 joint had the highest shear strength, which was 60.44MPa, up to 60% of that of the base metal. The highest tensile strength was 77.29MPa, up to 30% of that of the base metal. The fractures of the joints were all brittle fractures which extended along the brittle eutectic structures in the joints. |
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