Study On Microstructure And Mechanical Properties Of AZ31B Magnesium Alloy Ultrasonic Welded Joint

With the rapid development of modern society,magnesium alloy,as a highperformance lightweight material,has been widely used in automotive,aerospace and other fields.As the manufacturing industry continues to promote the development of more energy efficient,traditional steel structural parts can no longer meet the current market demand.On the contrary,magnesium alloys are being applied more and more widely because of their light weight,good specific performance,good shock absorption and low environmental pollution.In order to expand the application of magnesium alloy in the industrial field,the development of magnesium alloy welding structural parts has attracted more and more attention.Ultrasonic welding as a solid phase welding technology,welding temperature is low,no intermetallic compounds produced,suitable for homogeneous and heterogeneous metal welding,has the advantages of high efficiency,solid,green,is one of the most efficient metal welding process.In addition,ultrasonic welding process is easy to operate,welding time is short,environmental pollution is small,is a more ideal welding method.Firstly,ultrasonic welding of AZ31 B magnesium alloy was proposed in this paper,and the effects of different welding parameters on interface temperature,joint morphology and mechanical properties of magnesium alloy welding joints were studied,so as to determine the optimal welding parameters.The results show that the optimal welding parameters of 0.5 mm thick AZ31 B magnesium alloy are as follows: the welding energy is2500 J,the welding static pressure is 0.4 MPa,and the welding amplitude is 60%.With the increase of welding energy,the peak temperature of the interface at the welding joint showed an upward trend,and the degree of plastic deformation at the welding interface increased monotonously and appeared wavy and vortex-like morphology.When the welding energy reached 3500 J,obvious cracks appeared at the joint.With the increase of welding energy,the microhardness values of the welded joint first increased and then decreased.When the welding energy was 500 J,1500 J,2500 J and 3500 J,the hardness values of the interface bonding zone were increased by 17%,25%,33% and 28%compared with that of the base metal,respectively.The joint shear force increases first and then decreases with the increase of welding energy,and shows the same change trend with the increase of welding static pressure and welding amplitude.When the process parameters are too high,the welded joint will produce fatigue cracks due to stress concentration,which will reduce the performance of the welded joint.The fracture pattern of joint changes from interface fracture to fracture along the edge of welding area with the increase of welding parameters.Secondly,in order to improve the performance of multi-layer metal ultrasonic welding joint,the multi-layer AZ31 B magnesium alloy ultrasonic welding research was carried out,and the multi-layer magnesium alloy ultrasonic welding was realized based on the optimal process parameters.The experimental results show that the plastic deformation degree of the interface decreases with the increase of the number of welding layers and the grain in the weld center area is obviously refined.The morphology of welded joint can be divided into three stages with the increase of the number of welded layers: the first stage is the vortex occlusion stage,the second stage is the effective connection stage,and the third stage is the microconnection stage.The designed gradient welding energy can effectively compensate the energy loss caused by the increase of the number of welding layers.Compared with the constant welding energy of 2500 J,the joint interface bonding quality is higher.Finally,in order to further improve the ultrasonic welding performance of multi-layer AZ31 B magnesium alloy,heat treatment was conducted on the welded joints,and the influence of different heat treatment process parameters on the microstructure and properties of the welded joints was studied.The heat treatment process was designed to hold heat at 150℃,250℃ and 350℃ for 1 h.By comparing the properties of welded joints under different heat treatment parameters,It is found that the effect of heat treatment on grain size is the main factor affecting the welding quality and mechanical properties of workpiece.Most of the grains were recrystallized and refined after heat treatment at 250℃,but secondary recrystallization may occur in the severe plastic deformation area.The corresponding highest tensile shear force and elongation are 2.35 k N and 8.6%,respectively,and a large number of dimnesses appear at the tensile shear fracture,with typical plastic fracture characteristics.