Severe Plastic Deformation of Magnesium Alloys by Equal Channel Angular Pressing

Abstract

Magnesium and its alloys possess good mechanical properties like low density, good castability, high specific strength and low cost. Requirement of magnesium alloys is more towards industrial applications, particularly in aerospace and automotive industries. The main limitation is the poor room temperature formability, with limited number of slip systems, due to their hexagonal closed packed (HCP) structure. Formability at room temperature in Mg alloys can be improved by grain refinement, as fine-grained structure improves ductility. Severe plastic deformation (SPD) is a technique where high strains are induced into the material to produce fine grained structural material. They also have few limitations, like high wear rate and low corrosion resistance, which hinders the usage of magnesium in many applications. Wear rate and corrosion resistance can be improved by subjecting the material to SPD, thereby, increase the applications of magnesium alloys. Equal channel angular pressing (ECAP) is one of the simple and most effective methods of SPD to improve the material properties. ECAP is a process in which the workpiece is subjected to shear deformation and thus, severe plastic strain is induced into the material without any change in the cross-sectional dimension of the sample. Microstructure has major effect on mechanical properties. ECAP process leads to ultrafine-grained microstructure in the material which may show superplastic deformation at low temperature and high strain rate. In HCP structured metals, grain refinement also leads to textural changes i.e. high strengthening in some particular directions. Magnesium alloys are available in various systems – (a) Mg-Al system where aluminum addition improves the mechanical property as well as the castability. Addition of aluminum up to 6% and more makes the alloy age-hardenable. Manganese (Mn) addition plays a vital role in grain refinement for magnesium alloys. (b) Aluminum-manganese (AM) series magnesium alloys are widely used in manufacturing of various automobile components such as seat frames, instrument panels etc., due to their better damping, better toughness, impact absorption and elongation properties compared to aluminum-zinc (AZ) series alloys.In the present study, ECAP was performed on AM70, AM80 and AM90 alloy with varying percentage of aluminum and manganese. In spite of various applications of AM series magnesium alloy, limited work has been done to improve the physical properties of AM series magnesium cast alloys. We may further improve these properties by using ECAP, so that its application can be expanded in different areas of engineering. ECAP was carried out using hot die steel (HDS) die with channel angle (Φ) as 110° and outer arc of curvature (Ψ) as 20° using route BC. Unprocessed and ECAP processed samples were subjected to microstructural studies and tested for mechanical properties. Strength and hardness values showed increasing trend for the initial 2 passes of ECAP processing and then started decreasing with further increase in the number of ECAP passes, even though the grain size continued to decrease in all the successive ECAP passes. However, the strength and hardness values still remained quite high when compared to the initial condition (as-cast and homogenized). This behavior was found to be correlated with texture modification in the material as a result of ECAP processing. Wear and corrosion tests were conducted to study tribological and corrosion behavior of ECAP processed samples. Results showed reduction in wear mass loss for the ECAP processed samples. Coefficient of friction (COF) was studied for different loads and improvement in COF values was observed for ECAP processed samples compared to initial condition. Potentiodynamic polarization and electrochemical impedance spectroscopy test showed improvement in corrosion resistance of ECAP processed samples. Immersion test showed similar trend with increased corrosion resistance of ECAP processed samples with low hydrogen evolution.