Research On The Microstructure And Mechanical Properties Of Al-12.7Si-0.7Mg Alloy Joints Welded By AC TIG Welding

Recently, a high-Si Mg-containing wrought aluminum alloy has been developed. Compared with6063aluminum alloy, this new developed wrought Al-Si alloy has excellent mechanical properties and very good application prospect. In this study, the base material used was a4mm thick Al-12.7Si-0.7Mg alloy plate cut from hot extruded Al-12.7Si-0.7Mg alloy profiles. The welding samples were butt-welded using automatic TIG straight seam welding machine. After welding, the joints were solution treated at500～540℃for0.5hour and the artificial aged at180℃for1-3hours. After heat treatment, the microstructures and mechanical properties of as-treated and as-welded joints were analyzed by microstructrue observation, microhardness measurement and tensile test. In addition, electrochemical corrosion behavior was preliminary evaluated by electrochemical measurement technique. The following main results are obtained:1. When Φ1.2mm ER4047was used as filler metal and AC square-wave was applied, the joint with best tensile properties could be produced using TIG welding parameters as70Hz AC frequency,160A welding current,73%half wave balance,210cm/min wire feeding speed and20cm welding speed. Under the above welding parameters, the ultimate strength of the welded joint is183MPa (reaching98.4%that of the base material), the yield strength is102MPa and elongation is8.84%.2. Microhardness of WZ was markedly higher than FZ and HAZ in the weld joint. After heat treatment, the microhardness of each zone (WZ, FZ and HAZ) increased and the difference of microhardness among each zone decreased.3. The strength of weld joints was enhanced greatly after T6heat treatment. The strength of joints increase with increasing solid-solution temperature or aging time.4. The best tensile property of joint appeared after solid-solution at520℃for0.5hour and aging at180℃for3hours. Under this optimized T6heat treatment, the yield strength and ultimate strength of the joint are243MPa and323MPa, respectively (equivalent to82.9%and90.2%those of the BM in T6state, i.e., solid-solution at540℃for0.5hour and aging at180℃for3hours). The enhancement in the values of yield strength and ultimate strength are respectively138.2%and76.5%as compared with the as-welded joint.5. The results of electrochemical corrosion measurement showed that, although the corrosion trend of the weld joint is relative high, the corrosion rate of the weld joint is lower than that of the BM, which means the electrochemical corrosion reaction is hard to proceed in the joint. Therefore, the corrosion resistance of TIG weld joint is generally better than that of the BM.