Defect size and fracture strength in solid-state additive manufacturing

• Applicants: Dr. Frank Gärtner; Professor Dr.-Ing. Thomas Klassen
• Subject Area: Coating and Surface Technology ; Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 448318292

Cold spraying is attracting serious attention for additive manufacturing and repair of structural components based on high-strength materials. However, for lifetime prediction and to guarantee safe use in automotive or aerospace applications, deeper insights regarding failure mechanisms and process resilience are needed. Deposit formation in cold spraying is based on high strain deformation by high velocity impacts of solid powder particles. Thus, deposits can be strain hardened to a high extent, with microscopically inhomogeneous strain distribution and microstructure. In addition, deposits regularly contain a certain degree of non-bonded internal interfaces as defects. As key influence on potential mechanical strength, the overall area of these non-bonded interfaces can be minimized by appropriate selection of process parameters. However, up to now, none of the existing descriptions includes the role of defect sizes and local ductility/toughness for the fracture strength of such solid-state manufactured parts. To elucidate critical mechanisms and to develop new strategies for tailoring properties, the joint research approach combines critical experimental investigations and concomitant multi-scale modelling for describing influences of local toughness, stresses, and possible crack growth. To cover materials with different strengths, pure Al, Ni and Ti are chosen as model system. Defect types are deliberately introduced by controlled oxidation and annealing of part of the feedstock powder. Parameters are designed to build up deposits with the same ratio of bonded to non-bonded interfaces, however, with different absolute defect size by using different powder size cuts. Experiments and modeling will also deal with properties after post-spray heat treatments, used to adjust local ductility and toughness, and to improve particle interfaces bonding. Experimentally determined strength, toughness, detailed microstructure and fracture analyses serve to study the relationships between powder sizes, crack sizes, and remnants of surface oxides, in order to distinguish their contribution. Multi-scale simulations are applied to model the fracture behavior based on experimental data for single impacts and bulk deposits. The simulations allow insights into local strain and toughness distribution, to develop tensile failure models for elemental single splats, and to define transfer functions for complete deposits. The combination of both, experiments and simulation, will identify the key influences for a comprehensive understanding of coating strength.The findings of this project will enrich the understanding of cold sprayed deposit failure, and help to develop strategies for powder design and kinetic spray parameter windows for reaching mechanical properties comparable to bulk material. Moreover, these strategies and models might be transferable also to other additive manufacturing techniques.

• DFG Programme: Research Grants
• International Connection: China
• Partner Organisation: National Natural Science Foundation of China
• Cooperation Partner: Professor Dr.-Ing. Wenya Li