Investigation of the bonding mechanisms of aluminum chips during SPD-and FAST-based solid-state recycling processes

Considering the reduction of resources and environmental concerns, the recycling of aluminum alloys is gaining more importance day by day. In this context, especially in light of the recycling of aluminum chips, which constitute a significant part of aluminum scrap, the solid-state or meltless recycling process stands out with its advantages, such as lower energy consumption, less metal loss due to the chemical reactivity and larger surface area of metal chips, minimally harmful gas and solid waste emissions compared to the conventional remelting method. Solid-state recycling techniques are basically based on two main categories: severe plastic deformation (SPD) and powder metallurgy (PM). The common purpose of both techniques in terms of chip solid-state recycling is to improve the mechanical properties while reducing energy requirements. The two main factors that serve this purpose are the quality of the chip bonding and the microstructure of the resulting bulk material. However, an important obstacle to the establishment of strong chip bonding is the oxide layers that aluminum alloys form on their surfaces when exposed to air. In the face of bonding requirements, the hot extrusion method, which is one of the SPD methods that provides a very high level of plastic strain to the material with additional shear force, has become a prominent method for obtaining bulk or semi-products from aluminum chips. However, this method carries the risk of peripheral coarse grain (PCG) zone formation due to excessive heat generation and suffers from insufficient fragmentation of oxide layers, large thermal energy requirements, and processing costs.