Microstructures And Properties Of Dissimilar Aluminium Alloy And Pure Copper Joints Improved By Ultrasonic Vibration-assisted Friction Stir Welding

The necessity of joining dissimilar materials like aluminium(Al)and copper(Cu)is for the two materials to complement each other on the required performance for an outstanding potential application.Copper and its alloys are largely used in power generation,transportation and electronic sectors due to their exceptional electrical and thermal conductivity properties.But copper is cost and heavy.Nonetheless,Aluminium alloys have also excellent electrical and thermal conductivity properties,most especially,it has a lower density than that of copper metal.The advantage of lower density aluminium alloys possesses over copper can make aluminium a potential replacement of copper due to its weight and cost issues.The pursuit of Al/Cu joint is mainly directed to their potentials in the electric vehicle battery,electric vehicle wiring and battery charging station in the transportation sectors and so on.Hence,achieving high-quality Al/Cu joint is vital to the sustainability of electric vehicles and other products.However,joining Al and Cu encounter many challenges.It is very challenging to join Al and Cu by fusion welding technologies due to the greater mismatch of thermal conductivity and melting point between them.There are also complete differences in all their chemical and thermal properties.Although friction stir welding(FSW)can be used to join Al and Cu.It is also a very difficult task to achieve sound Al/Cu FSW joint.This thesis focuses on the dissimilar butt joining aluminium alloy(AA6061-T6)and pure copper(C10100)plates of thickness 2 mm by ultrasonic vibration-assisted friction stir welding(UVaFSW).Comparative investigations were conducted between the welds made by FSW and UVaFSW processes.Different test conditions(Test 1,Test 2,Test 3 and Test 4)were used to systematically investigate the effects of ultrasonic vibration on the weld surface finish,microstructure,mechanical properties and electrical properties on the FSW joint.It was found that the ultrasonic vibration influenced various changes on the weld surfaces of FSW joint by reducing the roughness and thus,promotes smoothness,which is an indication of improving surface quality.Also,a higher degree of mixing was observed with strong intercalation of the layered structures due to the improved materials flow.The joint strengths were found to improve in the UVaFSW joints especially at the optimum tool rotation rate of 600 rpm in Test 1.This welding condition also produces the best weld surface quality among other weld surfaces.Besides,the thickness of the(intermetallic compounds)IMC phase layers was found to reduce to about 50.76%under the influence of ultrasonic vibration,which eventually enables the production of good quality joints.The fracture position of the joint was found to shift from the fracture susceptible region(Al/Cu boundary)to the less fracture susceptible region(Al side),and there are dimple features on the fracture surface under the presence of ultrasonic vibration,because the morphology of the IMC phase layers in that region reduces.Different base materials positions demonstrate that when copper base material was positioned at the advancing side(type 1 joint),the acoustoplastic effects were different on the entire joint formations from those when the copper was positioned at the retreating side(type 2 joint)in Test 3.The acoustoplastic effect was found to comparatively reduce the weld surface roughness amounts in both types of joints.Moreover,the(ultimate tensile strength)UTS and(yield strength)YS of the joints in type 2 joints have been respectively increased by about 28.00 MPa and 12 MPa in the UVaFSW joints,while in the type 1 joints,the UTS increment was very little,only by about 7.00 MPa.The IMC phase layer thickness in type 1 joint was reduced by about 80.23%with ultrasonic vibration addition,whereas in type 2 joint a reduction of the IMC phase layer thickness under the presence of ultrasonic vibration was not observed,but the drastic reduction of the amount and compactness of the copper fragments in the weld nugget zone,which is due to the features of the content of the mixing regions(i.e,composite-like structure).Besides,the joint properties for each type of the joint were found to be determined by different factors,as type 1 joint was determined by the thickness of the IMC phase layer at the joint interface,while type 2 joint was determined by the amounts of copper fragments in the weld nugget zone.However,the nugget zone strengths(UTS and YS)in type 1 joint were also enhanced by about 30.00 MPa and 11 MPa,respectively with ultrasonic vibration addition,while a minor improvement of about 3.00 MPa for the UTS in type 2 joint was established with the addition of ultrasonic vibration,which demonstrated the quality of both welds.In short,favourable joint features and quality joint are observed in type 2 joint than in type 1 joint with ultrasonic vibration addition.When different tool offsets were used in Test 4,the sizes of the copper fragments varied depending on the levels of the offset and the processes employed.The copper fragments areas and minimum Feret diameters were found to be significantly reduced for up to about 73.30%and 42.60%,respectively with ultrasonic vibration addition when the tool was offset towards Al side(Case ? joint).On the other hand,the reduction of the fragment areas is up to about 60.60%and for the minimum Feret diameters is up to about 24.60%with the ultrasonic vibration addition under zero tool offset(Case ? joint).Thus,it resulted in the complete changes in the joint mechanical performance by improving the joint strength and elongation of the joint fracture in Case ? and Case ? joint.Beside,defective weld surfaces in both FSW and UVaFSW joints were achieved when the tool was offset towards the Cu side(Case ? joint).The compactness of the fragments was reduced by dispersing the fragments all-over the width of the weld nugget zone with ultrasonic vibration addition.It was also found that the concerns for the tool offset in Al/Cu joint can be avoided with ultrasonic vibration addition since sound and efficient joint properties and performance have been established in both Case ? and Case ? joints.Moreover,both microstructures of Case ? and Case ? joints were completely governed by the tool offset plans and the welding processes.