Mechanism-based characterization and evaluation of the performance of resistively sintered semi-finished products based on recycled aluminum chips

• Applicants: Professor Dr.-Ing. Bernd-Arno Behrens; Professor Dr.-Ing. Frank Walther
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 454199925

Due to the increasing scarcity of resources, demands for energy-efficient recycling processes for aluminum (Al) alloys are becoming ever greater. The direct Solid-State-Recycling of Al scrap promises significant energy savings compared to conventional remelting recycling processes, while reducing material loss due to combustion. Previously used processes realize the break-up of the chips’ oxide layers, which is necessary for an optimal welding, by severe plastic deformation (SPD). However, the process window of SPD processes is severely restricted and therefore not established in industry. Furthermore, due to the insufficient break-up of the oxide layers in SPD processes, the cyclic properties of profiles produced by conventional remelting-recycling are far from being achieved. A new, innovative approach of Solid-State-Recycling is Spark-Plasma-Sintering (SPS). It has been shown that the process is basically suitable for the successful consolidation of Al chips. In contrast to the SPD processes, the mechanism of SPS is not based on the break-up of the oxide layers by high degrees of deformation, but on the recrystallization and the associated activation of pressure-superimposed diffusion processes. The application of the SPS process enables a complete welding of the chips and thus allows the expectation of significantly optimized properties.The aim of this research project is to investigate the influence of oxide layers on the resulting properties of products based on Solid-State-recycled chips by SPS. The envisaged process chain, consisting of a cold-forge process, SPS and cold extrusion process, should guarantee the production of homogeneous and pore-free components. In addition to tensile and load increase tests, single step tests are carried out on selected specimens in order to be able to carry out a process-oriented microstructure-correlated characterization. Additionally, investigations with a FIB-SEM as well as intermittent tensile and fatigue tests in combination with XRD- and CT-technology are used. The aim of the investigations is to obtain information about the welding quality of the chips and the failure mechanisms with a relatively small number of specimens. Selected process-relevant parameters are varied, and compared by means of characterization methods. The process-related microstructural properties of the chip-based profiles will be correlated with the mechanical properties, which results in a structure-based analysis of the damage mechanisms, the understanding of which is absolutely necessary for the design of optimal process conditions on the basis of a fundamental understanding of the process-structure-property relationship. The feedback of the results of the mechanical investigations to the manufacturing process should enable resource-saving and efficient machining. In addition, the results are compared with conventionally produced semi-finished products in order to classify the results.

• DFG Programme: Research Grants