Research On Process And Lap Interfacial Characteristic Of Pinless Friction Stir Spot Welding Lap Joints Of Ti/Al Alloys

The connection of Ti/Al composite structure has potential application value in the aerospace and automotive industries.The application of Ti/Al composite structure will combine the advantages of Ti alloy and Al alloy.However,there are many differences in physical and chemical properties between Ti and Al alloys,it is difficult to achieve better metallurgical bonding.At present,friction stir welding（FSW）is considered to be the most promising welding technology with low heat input for dissimilar alloy connection.In this paper,a new method of pinless friction stir spot welding（PFSSW）was used for lap spot welding of TC4 titanium alloy and 2A12 aluminum alloy.The pinless friction stir spot welding process and interface formation characteristics of Ti/Al dissimilar metals was conducted a systematic study.The influence rule of welding process parameters on the macro forming,microstructure and mechanical properties of joints was analyzed.The EDS,SEM and micro area XRD were used to analyze the interface element distribution,interface structure and phase distribution of the joint.Because aluminum alloy is more susceptible to welding heat input and stirring head extrusion than titanium alloy,the microstructure of aluminum side of the joint and the interface combination are prone to change significantly under different process parameters.When the welding parameters were small,only the heat-affected zone and the base metal zone were formed on the aluminum side,and the interface was poorly combined.There was no continuous Ti Al₃ compound layer at the interface.When the welding parameters were moderate,there were four areas of fusion zone,fusion line zone,heat affected zone and base material zone on the aluminum side,and there was a Ti Al₃compound layer distributed in a strip shape at the interface.When the welding parameters were large excessively,it caused obvious thermal cracks and holes on the aluminum side of the joint,and the interface compound layer would become thicker.The significant factor leading to defects was a large amount of eutectic structure Al₂Cu Mg and Al₂Cu precipitated on the aluminum-side grain boundary of the joint.The test results of the mechanical properties of the joints show that the microhardness values on the aluminum side of the joint are symmetrical about the center of the solder joint and present a typical“W”shape distribution.The slight increase in hardness about the fusion zone is closely related to the existence of the low-melting eutectic structure Al₂Cu Mg and Al₂Cu at the grain boundaries.As the welding time,the rotation speed and the plunge of the tool increase,the tensile shear load of the welded joint increases first and then decreases.When the rotation speed of the tool at 1180rpm,the welding time at 90s and the plunge of the tool at 0.3mm,the maximum tensile shear load of the joint reaches 19.20k N.Under different welding parameters,there are four types of joint macro-fractures:interface fracture,aluminum side peeling fracture,aluminum side bottom fracture and aluminum side edge fracture.Microscopic fracture analysis showed that the joints were mainly brittle fractures,and obvious brittle fracture characteristics such as tearing edges and rock sugar patterns were found at the fractures.The temperature of the joint interface determines the reaction form of the joint.When the temperature is lower than the melting point of the aluminum base material,it is a solid-solid reaction.But when it is higher than the melting point of the aluminum base material,it shows typical solid-liquid fusion brazing reaction.The structure of the aluminum side of the joint during the fusion brazing reaction undergoes three stages of growth,partial melting and complete melting,and finally forms regions with different structures.The middle region of the joint interface is mainly Ti Al₃ compound layer,and the compound layer on both sides of the interface gradually disappears.Elements such as Cu were squeezed into the edge area of the joint interface by the tool pressure and heat input,which provided a good environment for the metallurgical reaction of the Ti/Al two elements.In the end,elemental analysis showed that Ti atoms diffused and dissolved to the aluminum side at the interface.Combining the measurement results of the interface temperature and the thermodynamic theoretical basis for the formation of Ti/Al compounds,the analysis confirmed that the most easily formed Ti-Al compounds at the interface is TiAl₃.