Due to the great potential for resource conservation, the direct recycling of aluminum chips using solid-state-recycling processes is a promising alternative to energy-intensive melting with a subsequent forming process. Processes used to date achieve the break-up of the oxide layers of the individual chips, which is necessary for optimum welding, by means of high degrees of deformation by severe plastic deformation (SPD). However, the process window of these methods is relatively restricted and consequently not established industrially. A new, innovative approach of solid-state-recycling is spark plasma sintering (SPS). The application of the SPS process enables complete welding of the chips and thus allows significantly optimized material characteristics to be expected. By mixing different chip types, hybrid semi-finished products can be produced, which enable a wide range of applications due to combinations of properties.
As a basis, EN AW-6060 aluminum chips were milled from cast bars. Subsequently, the chips were cleaned in an ultrasonic bath to remove impurities and pre-compacted into briquettes using a manual hydraulic press. After cold compaction, the three stacked briquettes were sintered by direct current (DC) to heat the chip material by Joule heating. Temperature, pressure, and duration of the sintering process were varied to determine the process parameter-dependent performance of the semi-finished products. Light and scanning electron micrographs were used to gain information about microstructural details. Temperature has been shown to be the most important influencing factor for good bond strength and has to exceed 450 °C for sufficient bonding, while minimum pressure of about 40 MPa is necessary to prevent delaminations. Together with the defect structure of the specimens taken from the semi-finished products, determined by computed tomography, these could be correlated well with the fatigue properties. The results were used to establish a microstructure-based model to evaluate the capability based on process parameters.