Towards Closed-Loop Recycling of Ceramic Particle-Reinforced Aluminium Alloys: Comparative Study of Resistance-Heating Sintered Primary and Solid-State Recycled Secondary SiCp/AlSi7Mg Composites

Abstract

Particle-reinforced aluminium matrix composites (AMC) with a high-volume fraction of ceramic reinforcement (>30 vol.%) combine high specific strength and stiffness with good wear resistance and thermal stability, resulting in their increasing popularity in high-load applications, such as brake discs and bearings. It is hence assumed that AMC will accumulate as scrap in the near future. Appropriate recycling strategies must therefore be developed to maintain AMC’s inherent properties. Melt-metallurgical recycling routes bear the danger of dissolving the ceramic reinforcement in the liquid metallic matrix and contaminating primary melts or forming intermetallic phases in secondary melts. Here, a solid-state AMC recycling route with crushing and sintering is investigated, wherein all steps are carried out below the solidification temperature of the aluminium matrix. A sintered primary AMC is mechanically converted into a particulate/powdery secondary raw AMC in coarse, medium, and milled quality (i.e., with d ≈ 7–12 mm, d ≈ 3–7 mm, and d < 300 µm) and subsequently resistance heating sintered to a secondary AMC under a variation of the sintering pressure. The two AMC generations are analysed and discussed regarding their microstructure and mechanical properties. Since the secondary AMC show reduced the mechanical strength, the fracture surfaces are analysed, revealing iron contaminations from the mechanical processing.

Combined Effect of Particle Reinforcement and T6 Heat Treatment on the Compressive Deformation Behavior of an A357 Aluminum Alloy at Room Temperature and at 350 °C

“…During this sintering process, the powder was heated by its own electrical resistance or indirectly by the used graphite tool. Due to the high heating and cooling rates, sintering temperatures close to the solidification temperature as well as the high sintering pressures, the used powder can be consolidated within a very short time in the range of few minutes [36]. Based on expertise from previous investigations [2,37], the sintering process was carried out in a vacuum for 5 min (holding time) at 520 • C and at a sintering pressure of 50 MPa to achieve nearly fully dense bodies (porosity < 1%) within a very short time to avoid microstructural coarsening effects.…”

On the Possibility of Using Secondary Alloys in the Production of Aluminum-Based Metal Matrix Composite

“…Among several process conditions, the authors found an optimum set of parameters to achieve recovery rates of 84% for the matrix alloy and 98% for SiC particles. Hirsch et al [21] recently investigated a solid-state recycling route for aluminum matrix composites with a high-volume fraction of ceramic reinforcement. They concluded that the contamination level of the milled composite and consolidation method significantly influenced the properties of the recycled MMC.…”