Friction stir welding of aluminium alloy AA5754 to steel DX54: Lap joints with conventional and new solution

Abstract

The demand for joining of aluminum to steel is increasing in the automotive industry. There are solutions based on Friction Stir Welding (FSW) implemented to join these two dissimilar metals but these have not yet resulted in a reliable joint for the automotive industrial applications. The main reason is the brittle intermetallic compounds (IMCs) that are prone to form in the weld region. The objective of this thesis was to develop and test an innovative overlap joint concept, which may improve the quality of the FSW between aluminum alloy AA5754-H22 (2 mm) and steel DX54 (1.5 mm) for automotive applications.

The innovative overlap joint concept consists of an interface with a wave shape produced on the steel side. The protrusion part of the shape will be directly processed by the tip of the probe with the intention of improving the mechanical resistance of the joint due to localized heat generation, extensive chemically active surfaces and extra mechanical interlocking. The innovative overlap joint concept was tested with three different travel speeds and axial forces. The welds were evaluated by visual inspection, microstructure observation and mechanical tests. Conventional overlap joints concept were produced to enable the evaluation of the efficiency of the innovative concept. Furthermore, multi-pass welds, with 2 and 3 passes, for both overlap joint concepts were conducted to verify the improvement of microstructure and mechanical performance.

With a single-pass weld the conventional overlap joint concept presented higher strength than the innovative concept. The main mechanisms governing this result were the formation of IMCs and higher reduction of effective thickness in the aluminium alloy sheet for the innovative lap joint concept. The multi-pass welds with 2 passes have shown the best tensile shear strength for both overlap joint concepts, about 50 \% efficiency in relation to the aluminium alloy base material. One issue affecting the results is the fact that the method applied to produce the pre-weld deformation in the steel sheet, at the contact interface, did not yield the originally planned geometric shape for the innovative overlap joint concept.