Refinement of Grains and Second Phases in Aluminum Alloys by Compressive Torsion Processing

In the compressive torsion process, a cylindrical workpiece is subjected simultaneously and continuously to compressive and torsional loading at elevated temperatures without changing its shape. As the workpiece was deformed under hydrostatic pressure, the compressive torsion process causes severe plastic deformation even to poor ductile metals, such as hypereutectic Al-Si alloy which has many brittle second phase particles, without any difficulty. Therefore, the compressive torsion process is a very attractive process for the refinement of not only grains but also brittle second phase. In the present work, the compressive torsion process was applied to a pure aluminum and a hypereutectic Al-Si alloy. The grain refinement, the second phase refinement and subsequent tensile properties of these alloys were investigated. As a result, The grain size of annealed pure aluminum was around 100 mu m. After processing, fine grains below 10 mu m were observed homogeneously over the radial cross section. The grain size was decreased with decreasing process temperature. The tensile strength was increased due to the grain refinement. On the hypereutectic Al-Si alloy, initial particle sizes of primary and eutectic Si were around 30 and 10 mu m, respectively. After the processing, both Si particles were refined below 5 mu m in the severely deformed region of the specimen. The total elongation was remarkably increased due to the Si particle refinement.