Study On Flow Behavior Of Cu/Al Dissimilar Materials Friction Stir Welding And Microstructure Of The Joint

Friction stir welding is a solid-phase connection technology,which has many advantages including low heat input,small welding distortion and high-quality joints.It is applied to the welding of non-ferrous metals and dissimilar metals.Copper(Cu)and aluminum(Al)are non-ferrous metals commonly used in the industry,and Cu/Al composite structures are widely used in power,energy,etc.Cu/Al dissimilar metal structure with excellent performance can be obtained through the application of friction stir welding,but the current research on Cu/Al dissimilar metal friction stir welding focuses on certain aspects and is not systematic.Therefore,the writer conducted a systematic study on Cu/Al dissimilar metal friction stir welding in this paper,and thoroughly analyzed the characteristics of temperature field,welding,flow,fracture and metallurgical bonding.After the comparative analysis of the limitations of the cone-shaped stirring tool in the welding process of dissimilar metals,a cylindrical stirring tool,which is more conducive to the connection of Cu/Al dissimilar metals,is developed.Additionally,a comparative analysis is conducted on their welding quality.Results show that the cylindrical stirring tool has more advantages in metal crushing and refining.Test results of process characteristics show that Cu is placed on the AS side,the average strength of the joint is 148.5mpa,which is higher than that of Cu on the RS side,indicating that Cu placed on the AS side can improve the joint strength.Fracture J-integral is used to study the fracture tendency of the joint.Results show that the performance of the joint will be affected by various factors such as the crack location,size and angle.Severe stress concentration will appear in the root segment under tension,weakening the performance of the joint,and the concentration of stress at the tip increases sharply with the increase of the unwelded area.Considering the effect of dissimilar metals mixing on heat production during welding process,Finite Element Model of thermal force direct coupling is built for friction stir welding of dissimilar metals.Calculating results show that asymmetry of FSW causes the obvious differences in stress of retreating side and advancing side and the deformation heat generation with the result that the temperature of the RS side is about 15?higher than that of the AS side.The temperature field of Cu/Al dissimilar metal FSW is affected by the material properties and the process of FSW,showing obvious asymmetry.Under the same parameters,when Cu is placed on the RS side,the maximum temperature difference between Cu and Al is 54°C and when Cu is placed on the AS side,the maximum temperature difference between Cu and Al is67°C with 19.4%of temperature difference fluctuation.Besides,the offset of the stirring tool will affect the temperature field of Cu/Al dissimilar metals.When the stirring head tilts to the Al side,it can reduce the heat loss and improve the efficiency of the heat source.The study of the metal flow behavior during friction stir welding shows that during FSW of the same material,the strain rates of Al and Cu are 2s^-1and 1.8s^-1 respectively while that of Cu/Al dissimilar metal is nearly 0s^-1.Therefore,the effects of shaft shoulder and stirring pin on the flow of dissimilar metals are studied respectively,and the flow model of FSW of Cu/Al dissimilar metals is established.From the result,we can notice that the transfer of Cu is mainly driven by Al and the flow molding of dissimilar metals is formed.When researching the Cu/Al dissimilar metal metallurgy area,it is found that the metallurgical bonding area mainly appears in the dissimilar metal bonding interface and the dissimilar metal mixing area.More specifically,a layered metallurgical bonding area of Cu Al₂+Cu₉Al₄ is formed in the bonding interface of dissimilar metals,and three different kinds of metallurgical composite structures are formed in the mixing area whose structures are affected by the diffusion process and the size of Cu particles and expand along with the Cu particle,Al atoms cannot diffuse completely into Cu,the structural changes of the metallurgical zone are Cu Al₂,Cu Al₂+Cu₉Al₄,Cu Al₂+Cu₉Al₄+Cu.The study on the diffusion behavior of Cu and Al shows that the actual inter-diffusion depth(4.92?m)of the two elements is higher than the theoretical value(2.47?m).The diffusion is affected not only by thermally-induced diffusion,but also by the strong plastic flow process of friction stir welding,which ultimately results in the actual diffusion depth being higher than the theoretical value.Based on the self-filling,stirring and crushing of friction stir welding,the dislocation reset process is used to repair the internal defects of Cu/Al dissimilar metal joints,and the refining of dissimilar metal mixed zone is completed to strengthen joints.Results show that internal defects of joints can be completely repaired and its tensile strength increases from 60.5MPa to155.6MPa,reaching the optimal strength of the joint in the process test.However,the dislocation reset process promotes the diffusion of Cu and Al atoms again,and the thickness of the IMCs in the metallurgical bonding area has increased.the structure of the area has evolved from the initial Cu Al₂+Cu₉Al₄ to the Cu Al₂+Cu Al+Cu₉Al₄.