Shear Assisted Processing and Extrusion (ShAPEÔ) of AZ91E Flake: A Study of Tooling Features and Processing Effects Jens T. Darsell, Nicole R. Overman, Vineet V. Joshi, Scott A. Whalen, and Suveen N. Mathaudhu (Submitted May 14, 2018) Charges of melt-spun AZ91E flake were indirectly extruded into tubes using Shear Assisted Processing and Extrusion (ShAPEÔ). The effect of instrument parameters and tool features on densification and microstructural evolution was studied. At a constant extrusion ratio, the tool rotational velocity varied from 75 to 300 rpm and was demonstrated to reduce the forge force by a factor of four. Modification of the extrusion die face with successively more aggressive scrolled features was found to enhance material flow into the extrusion orifice which led to a 30% reduction in spindle torque. Microstructure, texture and hardness are reported for the range of rpm and scroll geometries investigated. It was observed that the ShAPEÔ process is able to retain the average grain size of the as-spun flake (2.5-4 lm) while simultane- ously imparting strong textural alignment in the resultant tube. Keywords extrusion, friction extrusion, magnesium, SEM, ShAPE, Shear Assisted Processing and Extrusion 1. Introduction Magnesium alloys have been widely investigated due to their high strength-to-weight ratios, making them promising for applications in the automotive, consumer electronic and biomedical industries (Ref 1-3). It has been shown that the use of magnesium alloys in automotive bumper beams, crush tips and intrusion beams would result in significant vehicle weight savings (Ref 4, 5); however, the use of magnesium alloys in structural applications is primarily influenced by their limited ductility and the yield anisotropy observed in compres- sion and tension. The ratio of compressive yield stress (CYS) to tensile yield strength (TYS) varies from 0.5 to 0.7 in conventional magnesium alloys and is primarily associated with the activation of different deformation mechanisms (Ref 6, 7). Efforts to overcome yield asymmetry have primarily focused on precipitation heat treatments, solid solution strengthening, grain refinement and texture alignment with respect to the extrusion axis (Ref 7-10). Several researchers have suggested that smaller grains encourage yield symmetry, with an optimum (CYS/TYS = 1) grain size estimated to be 0.8-1.0 lm for AZ31 (Ref 11, 12). It has also been found that the preferred basal textural alignment for yield symmetry is 45° from the extrusion axis (Ref 10). Jain et al. (Ref 8) found that aged AZ80 exhibited less yield asymmetry than in a solution heat-treated condition due to reduced rates of twinning in the presence of precipitates. Several researchers have also found that the addition of solid solution elements such as Y (Ref 9, 13, 14), Sr (Ref 15), La, Gd (Ref 16) or Li (Ref 17) can improve yield anisotropy. Recent high-performance magnesium alloys typically under- go solid solution strengthening via the addition of rare earth elements and have exhibited CYS/TYS ratios equal to or greater than 1 (Ref 9). Grain refinement and texture alignment usually involves a combination of conventional processing techniques and other severe plastic deformation (SPD) pro- cesses (Ref 18, 19). Despite improved material performance, these multi-step techniques are projected to add significant cost to the end product. A superior approach would achieve the desired strength, textural alignment and grain refinement in a single-step process. In this work, Shear Assisted Processing and Extrusion (ShAPEÔ) is used to extrude tubing from AZ91E flake, in a single-step process, while simultaneously influencing texture, grain size and hardness through variation in processing parameters. ShAPEÔ is similar to indirect/direct extrusion, with the key difference being that the die rotates at high rpm as it displaces axially into a charge (Ref 20-23). The die face is grooved with a helical pattern (herein referred to as scrolls) to assist the flow of the material into the extrusion die orifice. The scrolls generate friction at the interface, such that the shear deformation generates enough heat to plastically deform the material without the need for additional process heat. The design of the scroll profile, rpm of the ram and axial feed rate are expected to influence the grain size, texture, hardness and dispersion of the second phases within the extrusion. Recent work using ShAPEÔ to extrude 7.5-mm-diameter tubing with 0.75 mm wall thickness from ZK60 and Mg- 2wt.%Si alloys showed that as the rpm increased, the grain size decreased and the basal planes tilted with respect to the extrusion axis (Ref 22). The combination of grain refinement and off-axis basal plane alignment resulted in a CYS/TYS ratio near unity which enabled the energy absorption of ZK60 and Mg-2wt.%Si to be similar to that of AA6061. It was also Jens T. Darsell, Nicole R. Overman, Vineet V. Joshi, and Scott A. Whalen, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352; and Suveen N. Mathaudhu, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352; and Department of Mechanical Engineering, University of California, Riverside, Riverside, CA. Contact e-mail: jens.darsell@pnnl.gov. JMEPEG ÓASM International https://doi.org/10.1007/s11665-018-3509-1 1059-9495/$19.00 Journal of Materials Engineering and Performance