Sumit Bahl

The use of recycled or scrap or secondary aluminum to manufacture high pressure die castings (HPDC) for structural applications is an opportunity to reduce cost, energy expenditure and emission of greenhouse gases. Technical challenges that must be overcome to achieve this on a large scale include scrap sorting and availability of alloys that can take such feedstock while maintaining desirable performance. This review focuses on structural alloy design with scrap aluminum as feedstock. An overview of HPDC processing and relevant processing-microstructure-property relationships is provided. Common HPDC structural alloy systems in commercial use that accept primary aluminum as feedstock are reviewed. Scrap streams that are relevant for producing HPDC aluminum alloys are discussed. An overview of state-of-the-art tools that can be applied to develop sustainable HPDC alloy compositions is provided. Current sustainable alloy design practices associated with HPDC are summarized and salient metallurgical challenges and opportunities associated with sustainable HPDC alloy design are identified based on our review. It is concluded that design of sustainable compositions of HPDC aluminum alloys for structural applications is now possible, in particular for well-defined scrap such as that coming from automotive wheels. Many opportunities exist to develop new alloy compositions that can effectively utilize current and emerging scrap streams. A grand challenge is the development of new alloys that can incorporate post-consumer mixed scrap as feedstock.

This study investigates the effect of mechanical cold work (0, 50, and 90% cross-sectional area reduction via swaging) on the creep properties of cast aluminum alloys with high electrical conductivity including high-purity Al (HP-Al), Al-0.15Zr (Al-Zr), and Al-0.27Zr-0.08Sn (Al-Zr-Sn, wt%). The Al-Zr alloy is strengthened by Zr in solid solution while the Al-Zr-Sn alloy is additionally strengthened by Al3Zr nanoprecipitates (8 nm diameter). After 90% cold swaging, the alloys exhibit grains elongated along the swaging direction with their widths ranging between 8 – 152 µm perpendicular to the swaging direction. Creep testing at 200 °C shows that the minimum creep rate of all 90% swaged alloys shows high sensitivity to stress, which can be modeled via a threshold stress σth. Increasing swaging magnitude from 0 to 50 to 90% in Al-Zr-Sn improves the creep resistance without reducing electrical conductivity. The formation of subgrains, increased dislocation density, and columnar grain structure in swaged alloys all enhance creep resistance. HP-Al swaged to 90% exhibits much lower creep resistance (σth = 18 MPa) due to subgrain coarsening and the absence of precipitates compared to Al-Zr (σth = 40 MPa) and Al-Zr-Sn (σth = 40 MPa), which maintain stable subgrain structures and benefit from solid solution strengthening and Al₃Zr nanoprecipitates, respectively. Although Al₃Zr precipitates are known to stabilize the deformed microstructure by pinning grain-boundaries, this work demonstrates that Zr in solid solution alone can effectively stabilize the deformed microstructure resulting in enhanced creep resistance. Despite similar creep performances of Al-Zr and Al-Zr-Sn, the latter shows 70% higher room-temperature microhardness and slightly improved electrical conductivity (56 %IACS (International Annealed Copper Standard) vs. 57 %IACS, respectively). This work provides new insights into creep mechanisms of cold-worked Al–Zr alloys that will guide the design of heat-resistant Al conductors for high-demand applications.