| Copper has good electrical and thermal conductivity,but it is a scarce resource and expensive.Aluminium is second only to silver and copper in terms of electrical conductivity,and it is less dense,more resourceful and less expensive.If using aluminium/copper composite joints,it can give full play to the various advantages of aluminium and copper,reduce product costs and broaden the range of applications.The current aluminium/copper dissimilar mediumthick plate composite structure in the field of power engineering has a wide range of application needs.However,if conventional single side friction stir welding for aluminium/copper dissimilar plate welding,in order to ensure the welding heat generation,it is generally necessary to use a large size tool,resulting in the effect of the large shoulder,the upper part of the workpiece will generate more heat,which will produce excessively thick layer of intermetallic compounds;The pin causes insufficient heat production at the bottom of the workpiece,which will lead to insufficient flow of materials at the bottom of the workpiece and prone to defects such as a cavity,unwelded,and it is difficult to obtain high-quality joints.In order to achieve high-quality welding of medium-thick aluminum/copper dissimilar plates,we have carried out double side friction stir welding(DS-FSW),double side friction stir Z shape lap-butt welding(DS-FSZW),Splat cooling assisted double side friction stir welding(SDS-FSW)and Ultrasonic assisted Double side friction stir Z shape lap-butt welding(UDSFSZW)on 12 mm thick 6061-T6 aluminium alloy and pure copper.Comparing and analyzing the influence of process modification on weld formation,joint microstructure and mechanical properties patterns.All process tests used a small tool for double side welding of the workpiece to reduce the welding heat input and inhibit the thickening of the intermetallic compound layer,the results showed that during the conventional double side friction stir welding of aluminium/copper dissimilar medium-thick plates,the pure copper was placed on the advancing side,the pin was offset to the aluminium side by 2.7 mm,the plunge depth of the shoulder was kept at 0.2 mm,when the First-pass weld on the front of the workpiece was made at 600 rpm400 mm/min and the Second-pass weld on the back of the workpiece was made at 800 rpm-400 mm/min,the maximum tensile strength of the joint was 151.15 MPa,reaching 61.69%of that of the pure copper base metal.In order to increase the contact area between aluminium and copper,optimise the thermal distribution of the joint and stagger the weak position of the weld on both sides,a Z-shaped joint modification was carried out and double side friction stir Z shape lap-butt welding(DSFSZW)process was proposed,and the influence of welding speed and rotational speed on the mechanical properties of the joint was systematically studied.The results showed that the maximum tensile strength of the joint was 174.17 MPa,reaching 71.09%of that of the pure copper base metal,when the lap width of the plate was 4 mm and the welding process parameters of 700 rpm-400 mm/min were used on both sides.Compared to conventional double side friction stir welding,the tensile strength of the DS-FSZW joint was increased by 24%.In order to reduce the thickness of the intermetallic compound layer of the DS-FSW joint in aluminium/copper dissimilar medium-thick plates,an experiment was carried out with the Splat cooling assisted double side friction stir welding(SDS-FSW)process.It was found that when the welding heat input was low,the additional cooling did not have a significant effect on the improvement of the mechanical properties of the joint.When both sides of the workpiece were welded by using the welding process parameters of 850 rpm-300 mm/min of medium welding heat input,the joint performance was optimal,with a maximum tensile strength of 159 MPa,reaching 65%of the pure copper base metal.The tensile strength of the SDS-FSW joint was increased by 42%compared to a conventional double side friction stir welding joint with the same parameters.Analyzing the microstructure of the aluminium/copper interface of the joint,it was found that the average thickness of the IMCs layer at the aluminium/copper interface of both DS-FSW and SDS-FSW joints generally tended to increase and then decrease as the distance from the upper surface of the First-pass weld or the Second-pass weld increased.By comparing the DS-FSW and SDS-FSW processes,it was found that SDS-FSW not only effectively suppressed the thickening of the IMCs layer,but also resulted in a more uniform thickness of the IMCs layer at the aluminium/copper interface of the joint along the thickness direction.EBSD analysis revealed that the grain size at the top and middle of the weld nugget zone of the First-pass weld in the SDS-FSW joint was significantly finer compared to the conventional DS-FSW process,due to the application of a rapid cooling external field which reduced the peak welding temperature and the high-temperature dwell time,thereby inhibiting the grain growth in the joint.The bottom of the weld nugget zone was farther away from the workpiece surface,which was less affected by the rapid cooling field,so the difference in grain size between the bottom of the welding nugget zone is smaller.In order to further improve the mechanical properties of the joints,the ultrasonic assisted double-sided Z-friction stir welding(UDS-FSZW)process was carried out.When both sides of the workpiece were welded with process parameters of 850 rpm-400 mm/min,the maximum tensile strength of the joint was 184.88 MPa,reaching 75.46%of that of the pure copper base metal.The tensile strength of the UDS-FSZW joint was increased by 23.62%compared to the DS-FS W joint with the same welding process parameters.Through the DS-FSW,DS-FSZW,SDS-FSW and UDS-FSZW joints fracture location and fracture morphology analysis,it found that even under the optimal welding process parameters,these four processes to obtain the aluminium/copper joints were fractured in the aluminium/copper interface,indicating that the aluminium/copper interface was the weakest part of the aluminium/copper joint.The fracture analyses revealed that the aluminium side of the fracture was distributed with dimples,tearing ridges and cleavage planes,which belong to a mixed ductile-brittle fracture. |
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