Studies on Mechanical Properties and Corrosion Behavior of Ze41 Magnesium Alloy Subjected to Equal Channel Angular Pressing

Abstract

An exceptional combination of properties such as high specific strength, good damping capacity and abundant availability confirm magnesium alloys as a potential candidate for applications in automobile, biomedical, aerospace as well as electronics industries. However, the limited ductility of Magnesium and its alloys as well as their relatively lower corrosion resistance still remains a hindrance to extend their applications. Equal channel angular pressing (ECAP) has immense potential to attain remarkable grain refinement thereby improving mechanical properties such as yield strength, ultimate tensile strength and percentage of elongation of Mg alloys. In the present work, ZE41 Mg alloy is subjected to two step ECAP. The microstructure and mechanical properties of ZE41 Mg alloy before and after two step ECAP are investigated. Further, the corrosion and galvanic corrosion behaviour of ZE41 samples are evaluated in 0 M, 0.1 M and 1 M NaCl solutions to mimic conditions encountered in automobile applications. Two step equal channel angular pressing carried out on as received ZE41 Mg alloy resulted in a remarkable grain refinement. As compared to grain size of 46μm in as received sample, refinement upto 2.5μm is achieved after 8th pass equal channel angular pressing (ECAP). The combined effect of crystallographic orientation and grain refinement is investigated by analysing the mechanical properties and corrosion behaviour of ZE41 Mg alloy using electron back scattered diffraction (EBSD). The first stage comprises of 1st, 2nd, 3rd and 4th passes at a processing temperature of 300 °C while the 5th, 6th, 7th and 8th passes are ECAPed at 275 °C in second stage. The mechanical properties of ZE41 Mg alloy 158 MPa yield tensile strength (YTS), 230 MPa ultimate tensile strength (UTS) and 7 % elongation in as received condition is enhanced to 236 MPa YTS, 295 MPa UTS and 19.76% respectively after first stage ECAP. The yield tensile strength deteriorated due to the effect of texture predominating grain refinement during the second stage ECAP. The corrosion resistance of ZE41 Mg alloy is significantly enhanced by ECAP and is inferred from electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation results. The role of microstructure is minimal on corrosion behaviour of ZE41 Mg alloy due to extra resistance when tested in 0M NaCl. However, the influence of grain refinement greatly influenced the improvement in corrosion resistance of ZE41 Mg alloy rather than crystallographic orientation observed from EBSD analysis. In contrast, the crystallographic orientation predominated the effect of grain refinement during ZE41 Mg alloy corrosion in chloride containing 0.1M and 1M NaCl solutions. From the observation of results it is found that equal channel angular pressing has the dual advantage of improving mechanical properties and corrosion resistance of ZE41 Mg alloy. Further, the galvanic corrosion behaviour of as received and ECAPed ZE41 Mg alloy coupled with Al7075 alloy is investigated using zero resistance ammeter (ZRA) in three different corrosive environments 0 M, 0.1 M and 1 M NaCl to mimic the conditions experienced in engineering applications. The mechanism of galvanic corrosion for ZE41 Mg alloy –Al7075 Aluminium alloy is explained. It is observed that a robust surface film containing composite layer of oxide/hydroxide of magnesium and aluminium is established in 0 M NaCl solution. However, only a single layer of magnesium oxide/ hydroxide is detected in chloride containing environments. ECAP improved the resistance to galvanic corrosion by 58% and 54% when compared with as received ZE41 Mg alloy in 0 M and 1 M NaCl solution respectively. In contrast, galvanic corrosion resistance decreased by 26% in 0.1 M NaCl after equal channel angular pressing while the as received samples evinced pits unfavourable to be used in engineering applications. ECAP is a promising method to combat galvanic corrosion encountered by ZE41 Magnesium alloy used in automobiles and components of military vehicles.