Mechanism for the formation of ductile and high strength material based bonding between steel and aluminum during hybrid casting by using an PVD applied adhesion promoter on steel

• Applicants: Professor Dr.-Ing. Xiangfan Fang; Professor Dr. Andreas Leineweber
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Coating and Surface Technology ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 535878895

During steel-Al hybrid casting (HC), steel inserts are recast with Al melt in order to produce low-cost lightweight components in very restricted package space. Until now, only force- and form-locking bonding can be produced in HC so far, which are insufficient for sever (crash) loading conditions. It was not possible up to now to produce material based joints via interdiffusion by using commercial Zn, ZnFe and AlSi coatings. A new PVD coating of the steel sheet, developed by the applicant Fang, involving the elements Al-Fe-Si-(Mn) enables a ductile material based bonding between steel and cast Al with high-strength. Optimized PVD parameters and target compositions result in a layer exhibiting two sub-layers. The first 2-3 µm thin layer consists of binary (η, θ) and ternary (2, 3, 5c, 6, 10) Fe-Al-Si intermetallic phases (IMPs). The second columnar sublayer, ~20 µm thick, consists of an Al solid solution with dissolved Si and traces of Fe. Failure of this material based bonding occurs predominantly in the second sublayer and less frequently in the first sublayer or even in the Al casting, and is characterized by honeycomb-like fracture surfaces. The fracture behavior of these HC specimens can thus be considered ductile. The mechanical properties of the individual sublayers have not been investigated so far. Several questions arise: a) Does the first sublayer remain intact due to good adhesion and sufficient strength or due to inherent ductility? Is the thin first sublayer actually ductile? b) Why does the second sublayer predominantly fail? c) How can the second sublayer be improved? d) How does the phase composition and the resulting mechanical behaviour of the intermediate layer change with changing manufacturing parameters (T-t; temperature-time)? To clarify these questions and to understand the formation mechanisms of the ductile and high-strength material based HC joint between steel and Al, five research hypotheses are formulated, which should be studied, and verified/adapted in 6 work packages. The T-t curves of the entire coating process (PVD and CC) are measured experimentally, simulated via FEM, and the resulting IMPs are related to the phase diagram using SEM-EBSD and XRD characterisation. Plasticity and failure of the coating are investigated by incremental stepwise loading. For this purpose, the entire PVD coating process is examined and the previous weaknesses of the coating are resolved. In addition, synthetic bulk samples of the individual IMPs of the first sub-layer are used to evaluate their mechanical properties. Based on the obtained knowledge, the layer and the HC process can be optimized and an deeper understanding of the IMP layer formation leads to dedicated design of the PVD adhesion promoting layer.

• DFG Programme: Research Grants