Study On Micro Structure Property Of Magnesium Alloy Welded Joint Under The Longitudinal Magnetic Field Associated With Active Agents

Magnesium alloy,due to its low density,high specific strength and good rigidity,has been widely used in fields of automobile,aerospace and sports equipment.However,high-quality welded joints of Mg alloy are difficult to be obtained because of its specific physical and chemical characteristics,which may lead to the occurrence of defects such as hot crackings,pores and nonmetallic inclusions.Therefore,it is necessary to improve the weldability of magnesium alloy and obtain high quality welding joint.Magnetically controlled welding and reactive welding have the advantages of low cost,high efficiency and good quality compared with traditional welding.Therefore,the active welding of magnesium alloy under longitudinal magnetic field are of great practical significance to improve the welding method of magnesium alloy.AZ91 magnesium alloy plates were welded by A-TIG with AC variable frequency longitudinal magnetic.The orthogonal experiment was used to optimize the process parameters.The change of arc shape was observed by high speed camera during welding.After welding,the forming condition,mechanical properties and microstructure of welded joints were tested under different parameters.The law and mechanism of active and magnetic field were studied.The main results of the paper are as follows:(1)The results of orthogonal experiments show that under the combined action of longitudinal magnetic field and chloride activator,the influence of four factors on weld forming coefficient is from large to small: magnetic field frequency> magnetic field current> welding current> coating amount.When the magnetic field current is 3.5A,the magnetic field frequency is 30 Hz,the welding current is 85 A and the coating amount is 1mg / cm2,the AZ91 magnesium alloy weld Hardness reaches a maximum of 73.83 HV.The microstructure of welded joints are composed of ?-Mg matrix and secondary phase ?-Al12Mg17 distributed along the grain boundary compound,which refine the grains apparently and enhance the mechanical properties.(2)According to the orthogonal experiment,it is found that under the combined action of longitudinal magnetic field and oxide activator,the influence of four factors on weld forming coefficient is from large to small: coating amount> magnetic field frequency> magnetic field current> welding current.The optimum parameters are magnetic field current is 1.5A,magnetic field frequency is 30 Hz,welding current is 80 A,coating amount is 3mg / cm2,weld hardness reaches maximum 76.3HV.On this condition,the microstructure of welded joints are refined which results in the enhancement of mechanical properties.(3)Based on high-speed photography techniques,it can be found that the arcs will rotate due to application of longitudinal magnetic field.Magnetic field on the welding pool plays the role of electromagnetic stirring,so that the microstructure of welding joint will be refined.When the oxide active agent is coated,the oxygen element can increase the flow rate of the weld by changing the flow pattern of the bath.When the chloride activator is coated,the chloride activator acts on the arc to increase the penetration depth.