Low-cycle fatigue behavior and local deformation of friction welded and heat-treated steel-aluminum hybrid components

In this study, the local deformation and fatigue behavior of steel-aluminum hybrid components produced by friction welding and subsequent heat treatment are analyzed. During heat treatment the mean thickness of the intermetallic compound layer (IMC-layer) increased from 0.17 to 0.32 μm. The ultimate tensile strength of the friction-welded hybrid specimens increased from 211 MPa to 255 MPa in the T6 heat-treatment condition. In-situ tensile tests, combined with digital volume correlation of X-ray microscopy tomography data, were carried out to investigate local strain distributions. It was found that the maximum strains did not occur directly at the interface but rather at a distance of 1.0–1.3 mm in the aluminum alloy. Cyclic fatigue tests were employed to investigate the low-cycle fatigue behavior. Crack initiation in the heat-treated specimens occurred in the joining zone followed by crack propagation through the aluminum alloy and forced fracture formed at the interface between the IMC-layer and the steel. Strain amplitudes > 0.4% resulted in small proportions (< 20%), strain amplitudes < 0.4% resulted in larger proportions (> 40%) of aluminum remaining on the fatigue fracture surfaces. The T4-treated and T7-treated hybrid specimens withstood strain amplitudes > 0.45% without immediate failure in the IMC-layer at the joining zone. T6-treadted and non-heat-treated hybrid specimens failed in the first cycle. At strain amplitudes < 0.3% T6-treated and non-heat-treated hybrid specimens achieved higher number of cycles to failure then the T4-treated and T7-treated hybrid specimens.