The Research On Ultrasonic Vibration Assisted AZ41/AZ31B Magnesium Alloy Welding

Magnesium?Mg?alloy is widely used in industry such as aerospace series,instrument and apparatus,vehicle,electric products and military project,because of the characteristics of low density,high specific stiffness and specific strength,excellent electromagnetic shielding,good shock absorbency and machining performance etc.As a high-efficiency and reliable joining process,welding technique is developed to expand the application of simple shaped material.However,the welding quality of Mg alloy is poor owing to the volatilization,burning loss,crack and pore defects.How to realize the high strength and reliable joining of Mg alloy is the key problem to be solved to expand the application of Mg alloy.In this paper,AZ41 and AZ31B Mg alloy were joined by tungsten inert gas?TIG?welding process and the optimal joint can be obtained by adjusting welding parameters.Ultrasonic vibration was introduced into the TIG welding process,thus improving the microstructure and mechanical properties.The main research conclusions are shown as follows.TIG welding process was developed to realize the joining of AZ41/AZ31B Mg alloy.The test results indicated that,the macro morphologies of weldments were affected by the welding current and welding speed.The incomplete penetration occurred with low heat input.With the increase of heat input,the top surface of joint is characterized by crow's feet feature.However,the pore was observed with the excessive heat input.Different levels of grain growth appeared in the heat affected zone and fusion zone,and a number of Mg₁₇Al₁₂ precipitated phases occurred in the fusion zone.The micro hardness of fusion zone is 51.3HV.And the micro hardness of heat affected zone of AZ41 side and AZ31B side is 48.0HV and 47.8HV,which are lower than that of Mg alloy base material.With welding speed of 0.20m/min and welding current of 90A,the maximum tensile strength of joint was 242MPa.Besides,fracture occurred at the fusion zone and fracture line changed from straight line to indented line with the increase of heat input.Ultrasonic vibration was introduced into the TIG welding process.Results reveal that,the number of pore declined with ultrasonic power under 1.0kW and the number of pore increased with ultrasonic power exceeded 1.0kW.The grain refinement effect of ultrasonic vibration increased with the increase of ultrasonic power and resulted in the increase of micro hardness.With ultrasonic power of 1.0kW,the coarse grains are refined to about 26?m.However,applying ultrasonic power to 1.5kW did not make an observably further change in the grain size of fusion zone.With ultrasonic power of1.0kW,welding speed of 0.20m/min and welding current of 90A,the maximum tensile strength increased 8.7%to 263MPa over the joints without ultrasonic treatment.And fracture location changed from fusion zone to Mg alloy parent material.The corrosion resistance of Mg alloy welding parts was studied.The results show that the corrosion rate of welding parts is far more than that of Mg alloy parent metal and the corrosion rate increased with the increase of chloride ion concentration.The hydrogen evolution rate is 33.8ml/cm²·day with the joint was immersed into 5.5 wt.%NaCl solution.Under ultrasonic vibration treatment,the hydrogen evolution rate of joint declined owing to grain refinement and defect elimination.As the Mg alloy joints were immersed into 1.5 wt.%,3.5 wt.%and 5.5 wt.%NaCl solution,the weight loss rates of Mg alloy base metal were 3.2,4.5,5.9mg/cm²?day and the weight loss rates of welding parts were 23.2,31.3 and 40.6mg/cm²?day.Under ultrasonic vibration treatment,the weight loss rate of welding parts declined,whereas higher than that of Mg alloy parent material.