Behaviour of aluminium/steel hybrid RSW joints under high cycle fatigue loading

Abstract

The lightweight construction of automotive car bodies is more and more important to reduce the fuel consumption and costs. It can be reached by use of high strength steels and aluminium alloy base materials. Recent car bodies contain both steel and aluminium alloys, therefore necessary to make reliable joints between them. Several joining methods were investigated like mechanical joining and welding. These joints typically were tested by quasi-static testing methods, rarely high cycle fatigue (HCF) test results can be found. The HCF loading basically determines the lifetime of these joints, so important to know the joint behaviour in this case. The resistance spot welding (RSW) is typically used for joining of car bodies and it is applicable for aluminium/steel hybrid joints too. During the welding very brittle intermetallic compound (IMC) formed which basically determines the joint properties, especially the IMC thickness and phases are influential. In this investigation 5754-H22 / DP600 and 6082-T6 / DP600 joints were made by RSW and HCF tests were performed. In case of HCF useful to compare the results of aluminium/aluminium and steel/steel joints too. Therefore, 5754-H22 / 5754-H22, 6082-T6 / 6082-T6 and DP600 / DP600 RSW joints were made and tested too. The joints were made by optimized RSW joining technologies and showed different behaviour during HCF loading. The failure modes showed differences, the shearing is occurred with a combination of base material fracture and plug failure mode also happens. After HCF tests the IMC thicknesses were measured in case of typical failure modes and show differences too. The resistance against HCF of hybrid joints are better than aluminium/aluminium joints and showed worse results compared with steel/steel joints.