Experimental Research On Ultrasonic Assisted Friction Stir Welding Of Dissimilar Aluminum To Copper Alloys

Dissimilar aluminum-copper alloys composite joints can make full use of the excellent properties of the two materials but reduce costs,and have broad application prospects in power,refrigeration and other industries.Due to the great difference of physical properties between aluminium and copper and the poor metallurgical compatibility,it is difficult to obtain Al-Cu joints with good performance by fusion welding.When conventional friction stir welding(FSW)is used to weld aluminium and copper,there are still some problems,such as different material fluidity of aluminium/copper alloys and the formation of hard and brittle intermetallic compounds(IMCs).Ultrasound vibration assisted friction stir welding(UVaFSW)of aluminum alloys has been proved to promote the plastic flow of welded materials and improve the micro structure and properties of joints.In this study,FSW and UVaFSW experiments were carried out for the butt joining of 2 mm thickness 6061-T6 aluminium alloy and copper alloy(T2 copper)sheets.The experimental results show that the application of ultrasonic vibration in UVaFSW can effectively broaden the process parameters window,promote the mixing of Al and Cu,and strengthen the mechanical interlocking of heterogeneous materials.With ultrasonic vibration,the distribution of microhardness in the nugget zone is more uniform,the tensile strength of the joint is increased,and the fracture position is shifted to the nugget zone.When UVaFSW was made at 600 rpm rotation speed and 60 mm/min welding speed,the tensile strength and elongation of joints are 78.32%and 56.96%higher than those of FSW joints.Because of the large potential difference between aluminium and copper,the joint is prone to galvanic corrosion,and there are large corrosion pits on the aluminium side.In the area where mixing is intense,the corrosion pit area is small,which can improve the corrosion resistance of the joint to a certain extent.The effect of ultrasonic vibration on friction stir welding process of AI-Cu dissimilar alloys was investigated.Thermocouples were used to measure the thermal cycle in the welding process,and the results show that the effect of ultrasonic vibration on the thermal cycle is not obvious.By sampling the axial pressure and torque of tools during welding process by force-torque sensor,it is found that applying ultrasonic vibration can effectively reduce the welding load.This indicates that the acoustic softening effect plays a dominant role when the ultrasonic vibration acts on the welded materials,which promotes the plastic deformation of the materials and reduces the flow stress.The flow situations of heterogeneous materials at the cross-section of FSW and UVaFSW welded joints and the horizontal cross-section at different depths around the exit hole were indirectly observed and analyzed.It was found that the flow and mixing degree of Al-Cu heterogeneous materials increased with the application of ultrasonic vibration,and the mechanical interlocking condition of joints was better.At different rotational speeds,the IMCs layer at the interface between nugget zone and copper side of FSW and UVaFSW joints is mainly Al2Cu.When ultrasonic vibration is applied,the composition of IMCs layer remains unchanged,but the thickness of IMCs layer decreases.Especially at 500 rpm rotation speed and 60 mm/min welding speed,not only the thickness of intermetallic compound layer decreases by 61%,but also the number of layers decreases from two layers to one layer.