Convection and joint characteristics in aluminum alloy melting zone during resistance spot welding of dissimilar Fe-Al material in external magnetic field

In resistance spot welding (RSW) of Fe and Al alloy, an intermetallic compound (IMC) is formed at the joining interface, and it is known that the strength of the joint decreases as the IMC becomes thicker. One of the factors that determines the thickness of the IMC is the temperature field during welding, which is considered to vary greatly depending on convection within the Al alloy melting zone. Therefore, changes in convection are considered to possibly affect the state of IMC formation, but there have been few detailed studies of this phenomenon. In this study, convection in the Al alloy melting zone of an Fe-Al alloy RSW was varied and the effect on the joint characteristics was investigated. For this study, to focus on the electromagnetic force generated in the Al alloy melting zone, convection in the Al alloy melting zone was changed by applying a magnetic field using neodymium magnets. In-situ evaluation of the convection using synchrotron radiation and cross-sectional images of the joint revealed that the presence of a magnetic field during welding causes a non-axisymmetric change in convection in the Al alloy melting zone, which deflects this zone. The magnetic field increases the driving force for convection and homogenizes the temperature field at the joining interface, leading to the formation of a thin uniform IMC near the joint center. Furthermore, from a cross tension test and observation of the fracture surface of the joint after the test, it was clarified that the cross tension strength of the joint welded under a magnetic field was improved by the propagation of cracks into the Al alloy melting zone during the test.