Study On Microstructures And Properties Of MIG Welding Of Al-Cu Alloy With Medium-thick Plate By Electromagnetic Pulse

Aluminum alloy has become a widely used metal material in recent decades because of its low cost,good corrosion resistance,thermal and electrical conductivity,and low density.The demand for the use of medium-thick plate aluminum alloy in the fields of marine,military and aerospace is also increasing year by year.However,the medium-thick plate aluminum alloy in multi-layer multi-pass welding is prone to high porosity,large grain size,unfused between layers,poor weld formation and other problems.In order to solve the above problem,this thesis takes8 mm thick 2219 aluminum alloy as the experimental research object.It uses a Fronius MIG welder,utilizing a monolithic pulse welding with a built-in dedicated aluminum welding expert system.To study the effect of external AC longitudinal magnetic field on the quality of weld formation,microstructure and mechanical properties of welded joints.Through macroscopic analysis,microstructure and mechanical properties testing,we investigate the effect of the introduction of AC longitudinal magnetic field on weld forming quality,microstructure and mechanical properties and summarize the mechanism of action.The magnetic field distribution near the magnetic head and test plate was simulated using COMSOL Multiphysics software.Based on this,the magnet head structure was designed and built to be used with the excitation power supply to excite the magnetic field.It ensures that the device excites a magnetic field of 10 m T or more to act on the welding arc,molten drop transition and molten pool flow.The experiments were conducted using a two-layer,three-pass butt weld without a pad on the back of the weld.The effect of the excitation parameters on the weld seam was studied by means of an orthogonal test when the welding parameters were determined without the external magnetic field.The shape and dimensions of the weld seam after the magnetic field has been applied are better than those of a weld without a magnetic field.The AC longitudinal magnetic field causes the arc to rotate alternately,and the heat of the arc is lost to the shielding gas in the form of thermal convection,reducing the heat input to the weld.It leads to an increase in melt width and sidewall depth and a decrease in residual height.From the microstructure analysis,it is found that the introduction of AC longitudinal magnetic field induces the transformation of coarse columnar crystals to equiaxed crystals.The hardness and tensile strength are increased after the application of the magnetic field,and the porosity and slag defects between the weld channels are significantly reduced.At 5 A/60 Hz magnetic field,the average grain size in the central area of the weld is increased by 24.18%compared to that without magnetic field,and the grain size is decreased.The tensile strength was increased by 30.27%,the elongation after break was increased from 5% to 10.05%,and the number of weld porosity was significantly reduced.This is due to the impact of the AC longitudinal magnetic field,which changes the arc pattern,droplet transition and drives the stirring and flushing action between fluids inside the liquid melt pool.It promotes the escape of gas,refines the weld grain structure and obtains good weld properties.