The use of ultra-high-strength 7000 aluminum alloys has an important role in future lightweight structures in the field of mobility due to their low density combined with high strength. However, 7000 aluminum alloys can only be fusion welded to a limited extent, since welding defects can rarely be prevented.
Friction Stir Welding (FSW) offers the possibility to join these materials; however, as for fusion welds, the strength is still reduced compared to the base metal. The reason for this is to a high extent the local heat input, which changes the local material properties. In addition, geometric and metallurgical notches are introduced at the transition from the weld metal to the heat-affected zone, which have a significant influence on the fatigue strength. The goal of this investigation is to enable a reliable fatigue strength assessment of the welded joints at 7000 aluminum alloys, taking into account the manufacturing induced properties.
For the characterization of the butt joint specimens, cross-sections were prepared and examined metallographically, which determined the local weld geometry as well as the microstructure in the joining area. In addition, hardness measurements were carried out to identify the hardness differences in the base metal, the weld and the heat-affected zone. Furthermore, the angular distortion was determined with a contour measurement of all specimens.
The fatigue strength of Friction Stir Welded (FSW) and Laser Beam Welded (LBW) butt joint specimens was determined using force-controlled fatigue tests. An evaluation is made using linear-elastic and elastic-plastic approaches. In a first step, the reference radius concept is applied in an attempt to map the influence of geometric notches on the fatigue strength, assuming linear-elastic stress-strain behavior. In a second step, the stress averaging approach according to Neuber is used. With this approach, it is possible to consider additionally the influence of the material properties, such as the hardness, on the local endurable stresses with an empirically derived microstructural length. Finally, the elastic-plastic strain-life approach is applied that takes directly the elastic-plastic material behavior into account. Particular challenge of integrating the complex material state (gradients in the material properties, geometric and metallurgical notches) into the evaluation are shown.