Study On Welding Process,microstructure And Mechanical Properties Of Al/Cu Dissimilar Joints In Ultrasonic Vibration Enhanced Friction Stir Welding

Al/Cu dissimilar alloys structural components can be able to complementary advantages by reducing costs and achieving lightweight,which could have a good application prospect in new energy and electrical industry.However,due to the difference of physical and chemical properties between Al and Cu,it is difficult to obtain excellent joints by traditional welding methods.Besides,there are still some problems such as hard and brittle intermetallic compounds(IMCs)and low joint strength when conventional friction stir welding(FSW)was used for the joining of Al/Cu dissimilar metals.Therefore,researchers at home and abroad have proposed a variety of auxiliary modification technologies,such as laser assisted FSW and ultrasonic vibration assisted FSW.In this paper,ultrasonic vibration was applied directly in front of the workpiece(UVeFSW)for the joining of Al/Cu dissimilar metals.Through detailed characterizations of the joint micro-structure and performance,the effect of welding parameters and ultrasonic vibration on the Al/Cu FSW process and the joint properties was analyzed.The butt joining experiments of 2 mm thick 6061-T6 Al alloy plate and T2 Cu alloy plate was carried out by using FSW to study the effect of workpiece position and pin offset on welding appearance and joint micro-structure.After summarizing the experimental results,the following conclusions were drawn.When welding speed was 110 mm/min and tool rotational speed was 700 rpm,the welding apparence of Cu-RS/Al-AS plate position was better,and the welding force is much lower than that of Cu-AS/Al-RS plate position.The material flow behavior during welding with Cu-AS/Al-RS plate position was more complex than that with Cu-RS/Al-AS plate position,and the material mixing was also more intense.With Cu-RS/AlAS plate position,the thickness of interfacial IMC layer was smaller than that with Cu-AS/AlRS plate position.The IMCs layer of Cu-RS/Al-AS plate position was composed of Al2Cu and Al4Cu9,while the IMCs layer of Cu-AS/Al-RS plate position was not only composed of Al2Cu and Al4Cu9,but also a three-layer structure composed of Al2Cu,AlCu and Al4Cu9.The maximum tensile strength of the joint with Cu-RS/AI-AS plate position was more than 20%higher than that with Cu-AS/Al-RS plate position.When the copper was at the retreating side and the aluminum was at the advancing side,the tool rotational speed was 700 rpm and the welding speed was 110 mm/min,the welding apparence was better and the interface temperature was lower with the pin 0.5 mm away from the copper side than that of other pin offset conditions.It was found that the mechanical locking and the material flow were both better and the IMCs layer was thinner and more uniform,the ultimate tensile strength of the joint was higher with the pin 0.5 mm away from the copper side than that of other pin offset conditions..The Al/Cu FSW and UVeFSW experiments were carried out under the following conditions:tool rotational speed of 700 rpm and shoulder diameter of 18 mm,tool rotational speed of 450 rpm and shoulder diameter of 18 mm,tool rotational speed of 700 rpm and shoulder diameter of 14 mm.The results show that the ultrasonic vibration applied directly in the front of the workpiece can significantly optimize the welding process and the microstructure of Al/Cu FSW joints under various welding parameters.With the ultrasonic vibration applied to the FSW,the welding apperance was improved,the welding force was reduced,the welding temperature on both sides of Al and Cu was slightly increased.The ultrasonic vibration enlarged the plastic deformation volume of materials around the exiting hole around the pin,promoted the dissimilar material flow,and enhanced the mechanical locking between Al and Cu for a wide range of welding speeds,smoothed the Al/Cu bonding interface,and reduced the number and size of Cu fragments near the interface.It was found that ultrasonic vibration had a significant thinning effect on IMC layer thickness of Al/Cu FSW joint interface,but the layer number and phase composition of IMCs were not affected by ultrasonic vibration with different welding parameters.Besides,ultrasonic vibration improved tensile strength of Al/Cu FSW joints at different welding speeds under all of the three parameter combinations.Finally,the maximum ultimate tensile strength of the joint was achieved,which was 247.5 MPa,with the Cu-RS/Al-AS plate position,pin offset of 0.5 mm to the Cu side,tool rotational speed of 700 rpm,welding speed of 110 mm/min,and shoulder diameter of 18 mm.