Lifetime prediction of hybrid friction stir welded aluminum joints - process and fatigue modeling taking microstructure into account

• Applicants: Professor Dr. Alexander Hartmaier; Professorin Dr.-Ing. Ghazal Moeini; Professor Dr.-Ing. Thomas Niendorf
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 450156756

Additive manufacturing (AM) offers an excellent opportunity for the implementation of novel lightweight construction concepts through the combination of unprecedented design freedom as well as unique material properties that cannot be achieved using conventional processing routes. Realization of cost-efficiently manufactured hybrid joints with excellent mechanical properties is a critical step towards the application in industries such as automotive and aerospace. To achieve this goal the understanding of suitable welding processes must be deepened further. During the first funding period a combined approach of experimental and model-based investigations was used to demonstrate that the build chamber size limitations of the AM process in focus can be overcome by using friction stir welded joints of additively manufactured and cast aluminum. However, the comparatively weak cast zones governed the fatigue behavior of these joints. The investigations also showed that there are still significant challenges in the field of friction stir welding of dissimilar material combinations with regard to material behavior predictions of the weld zone itself. Thus, in order to meet the requirements of modern lightweight construction it is necessary to advance the design of hybrid welded joints based on both experiments and modeling. Here, it is important to thoroughly understand the complex interrelation of temperature, material flow, microstructure, residual stresses and their effect on the final fatigue properties. In order to close prevalent research gaps the project will focus on the friction stir welding process itself. In addition to the experimental determination of the material properties, both the welding process and the fatigue life will be modeled and validated based on experimental results using a three-way approach. Material models for fatigue life simulation will be built based on the existing material models elaborated in the first funding period. In order to focus on more homogeneous hybrid joints being characterized by superior properties, eventually shifting the failure-determining area into the welding zone, the conventionally processed high-strength alloy AA7075 is used as a welding partner for the additively manufactured AlSi10Mg. Mechanical and microstructural investigations are carried out on the components made from conventional AA7075 and their friction stir welded joints. Combined with experimental data of the friction stir welding process, the results serve as the basis for the realization of welding process and fatigue models. Design of these models is carried out on the basis of real microstructures and welding conditions. The aim is to predict the resulting microstructural zones and their individual fatigue life, and thus precisely forecast crack initiation and fatigue life.

• DFG Programme: Research Grants