Fracture Characteristics and Analysis in Dissimilar Cu-Al Alloy Joints Formed via Electromagnetic Pulse Welding

Abstract

The aim of this study was to identify and analyze the fatigue fracture characteristics of dissimilar Al 6061 to Cu (UNS C11000) lap joints made with ultrafast electromagnetic pulse welding (EMPW) via fractography, stress analysis and finite element simulation. It was observed that EMPW generated an annular (or ring-shaped) bonding area, with weld zones and a central non-weld zone when viewed from the cross section. Two types of failure modes occurred in relation to the cyclic loading levels: base metal fracture or transverse through-thickness (TTT) crack growth at a higher loading level, and joint interfacial failure at a lower loading level. In the interfacial failure, fatigue crack initiated from the outer edge of annular welding area, and propagated to form an approximate elliptical boundary. Fatigue crack propagation was characterized by fatigue striations existing in discrete areas on the fracture surface. This was attributed to a coupled role of shear and normal stresses present in a tensile lap shear sample due to the bending moment caused by the inherent misalignment. The final rapid fracture started from elliptical boundary with elongated shear dimples. Both theoretical stress analysis and finite element model revealed the maximum stress and stress concentration along the outer edge, where fatigue crack initiation occurred.

Fatigue Performance and Interfacial Strengthening Mechanism in Al/Cu Dissimilar Lap Joints via Electromagnetic Pulse Welding

“…Subsequently, fast cracks mainly featured dimples propagated toward the inner edge. This fatigue fracture phenomenon was discussed in detail in our previous work [17]. Figure 4c shows the fatigue and dimple features along the interface in zone E during the transition from fatigue fracture to fast fracture.…”

“…The morphological characteristics on the fatigue fracture surface further indicate that the 14 kV joint has higher ductility than 12 kV and can withstand longer plastic deformation. As seen from Figures 4 and 5, the fatigue fracture mode on both voltages tends to be consistent, i.e., the stress concentration and fatigue crack initiated from the outer edge of the annular welding area until joint failure, which was attributed to a coupled role of shear and normal stresses present in a tensile lap shear sample due to the bending moment caused by the inherent misalignment [17]. Patel et al [24] also reported that the nugget edge is a region of stress concentration in USWed joints with triaxial stresses, which can lead to transverse through-thickness crack growth failure.…”

The Electrical and Mechanical Aspects of Aluminum and Copper Resistance Spot Weld Joints

“…Specifically, T2 grade commercially pure copper sheet and aluminum AA1050, both possessing a 1 mm thickness, were subjected to these welding experiments, as shown in Figure 1. Due to their remarkable electrical conductivity, these materials hold extensive applications in various industries, particularly electric cars [28]. The chemical composition was investigated using an optical emission spectrometer (spectroscopy).…”

Hierarchical Morphology and Formation Mechanism of Collision Surface of Al/Steel Dissimilar Lap Joints via Electromagnetic Pulse Welding

“…Eventually, material contacts and interactions extend along the interface at a high velocity, forming a variety of characteristics such as wavy and vortex diffusion layer, local melting and porous structure, etc. [6,7], which significantly affect the interfacial bonding properties of Al/steel joints.…”