Research On Welding Mechanism Of Aluminum Alloy Foils During Ultrasonic Metal Welding

Ultrasonic metal welding (UMW) is a rapid solid state processing techniquewith brilliant prospects. Ultrasonic additive manufacturing (UAM), based onultrasonic metal welding, makes possible the rapid prototyping of the parts withcomplicated structures. But so far there are many debates about the weldingmechanism of UMW, which to some extent limits its widespread application.Therefore, this paper conducted a comprehensive and profound research on matrixstructure evolution and interfacial bonding mechanism of aluminum alloy foilsbuilts fabricated by UMW under serial ultrasonic welding parameters, and thematrix hardness and interfacial shear strength were measured and analysed.This paper in experiments carried out the researches on microstructureevolution and mechanical properties of the builts through controlling ultrasonicamplitude, welding time and deformation extents so as,to clearly identificate theireffects. The evolution of the microstructure of aluminum alloy foils and weldinginterfaces under different ultrasonic welding parameters were studied throughorientation imaging map, crystal boundaries distribution map and orientaiondistribution functions (ODF) and so on. Features of interfacial bonding and failurefractures were observed by scanning electron microscope (SEM). The matrixhardness and interfacial shear strength were measured by digital micro hardnesstester and universal testing machine, respectively.The results show that the microstructures of aluminum foils are refined to someextent during ultrasonic metal welding. It is found that the increase of ultrasonicamplitude improves the level of grain refinement, and the content ofrecrystallization texture is decreasing and deformation texture is increasing. Theincrease of welding time would worse the uniformity of microstructure, and both therecrystallization texture and deformation texture are strengthed. The greatest affectfactor on matrix microstructure is deformation extent. The increase of deformationextent can promote continuous dymanic recrystallization (CDRX), and the grain sizeis refined and uniform. The deformation texture weakens, and the recrystallizationtexture is strengthed, conversely.Based on the strength of the interfacial friction affects, the welded interfacesare mainly divided into non-bonding zone, weak-bonding zone and strong-bondingzone. The surfacel oxide layers are kept in integerity in non-bonding zone, andmechanical connnection is the main bonding mechanism which has poor reliability.The interfacial oxide layers are broken in weak-bonding and strong-bonding zones,and contiuous dynamic recrystallization (CDRX) is the bonding mechanism, but the toughness and plasticity of strong-bonding zone is better. Ductile fracture is themain failure mode. The exist of oxides in dimples demonstrates oxides is thecrack-nucleated sources which would reduce the welding quality.Differences of matrix structures and interface bonding features directlydetermine the mechanical properties of the welded joints. With the increase ofultrasonic amplitude, the hardness of aluminum alloy foils decreases, and the shearstrength first increases, then decreases indicating the existence of a critical value.With the increase of welding time, the hardness first decreases, then increases, andthe shear strength decreases. While, the hardness and shear strength are positivelyrelated to the deformation extent.