| The development of the railway transportation is an important indicator of a country’s modernization,and it is an inherent requirement of the 14th Five-Year Plan to improve the intelligentialize of China’s railway transportation.In order to improve the comfort and running speed of high-speed rail,aluminum alloys are widely used in the structure of the train body.However,the welding speed of aluminum alloy MIG arc welding is slow speed and some welding defects will occur with the increase of the welding speed,which greatly restricts the further development of the railway transportation industry.Recent studies have shown that the external magnetic field(EMF)-assisted MIG welding has the characteristics of low cost,low energy consumption and high efficiency.Our group developed a pulsed compound EMF assisted high-speed arc welding process of mild steel.By combining a self-designed magnetic field generator with the welding torch,a stable transverse magnetic field and an alternating parallel magnetic field in the arc space were generated simultaneously to control the molten metal flow in the weld pool.The maximum welding speed can reach 2.0m/min while no weld bead defect occurred for the welding of mild steel.However,aluminum alloy and mild steel differ greatly in physical properties such as magnetic properties,melting point,and thermal conductivity,which caused significant difference in the weld ability of the two materials.Whether the compound magnetic field can be applied to the MIG welding of aluminum alloy to improve the welding speed and quality is still lack of researchThis thesis carried out the magnetic field assisted high-speed MIG welding of aluminum alloy in bead-on-plate and butt welding ways by selecting 4mm thick 6N01 aluminum alloy as the base metal.The influence of the excitation parameters on the weld bead formation was investigated and the feasibility of high-speed MIG welding of aluminum alloy with the assistance of the compound EMF was demonstrated.The effect of the EMF on the arc behavior was analyzed by visual sensing to clarify the suppression mechanism of the weld bead defects by the EMF.Through parameter optimization,it was found that the welding seam quality of butt welding and bead-on-plate welding was the best when the excitation current was 5A and the excitation frequency was 100Hz.Under this condition,the EMF can adjust the arc behavior effectively to enhance the lateral spreading ability of the liquid metal in the weld pool,so the weld bead defects such as undercuts and humping bead can be suppressed.The bead-on-plate welding speed can increase to 2.5m/min and the butt welding speed can increase to 2.1 m/min with the assistance of EMF.The performance of the high-speed MIG butt-welded joint of aluminum alloy was analyzed.The experimental results show that the compound EMF can suppress the weld bead defects and the second phase coarsening in the weld seam zone in high-speed welding,so the mechanical property of the weld joint was improved.With the application of EMF,the width of the softened zone was reduced by 26.6%and the tensile strength of the joint was 71.9%of the base metal,which was close to strength of the joint obtained in low-welding speed.Besides,the impact toughness of the joint in high-speed welding with EMF reached 98.5%of the joint in low-welding speed.Finally,the MIG welding speed of aluminum alloy increased from 0.75m/min to 2.1m/min,and the welding efficiency increased by 280%without the sacrifice of joint quality so that the high-quality and efficiency welding of aluminum alloy was realized. |
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