Development of a wrought Mg‒7Sn‒3Zn alloy with improved strength and ductility

Abstract

The current focus of research in Mg alloys is to develop wrought alloys suitable for the automotive industry. The challenges to be overcome are low strength, poor ductility, and high cost associated with wrought Mg‒based alloys. With this objective, wrought Mg‒7Sn‒3Zn (TZ73) alloy was developed in the present work, which does not contain expensive rare‒earth (RE) elements and has much higher strength than the most commonly used commercial AZ31 alloy, with a reasonable ductility. The alloy was prepared by squeeze casting, followed by homogenization at 300°C for 24 h to dissolve the low‒temperature eutectic and achieve uniform composition in the as‒cast microstructure. The processing map was generated by conducting hot compression tests in the temperature range 200 ‒ 450°C and the strain rate range 10‒3 – 101 s‒1. Based on the results from the processing map, the homogenized TZ73 alloy was rolled at 350°C. It was also subsequently annealed at 215°C. A detailed investigation has been carried out at each stage of the thermo‒mechanical processes. The hot‒rolled sheet exhibited a high strength (0.2% PS: 315 MPa, UTS: 362 MPa) with reasonable elongation‒to‒failure (9%). Strengthening is contributed to the alloy by the combined effect of grain refinement, solid solution strengthening by Zn and Sn atoms, fine Mg2Sn particles, and crystallographic texture. After annealing, the strength was reduced but % elongation‒to‒failure was increased, the alloy exhibiting tensile properties ‒ 0.2% PS: 218 MPa, UTS: 311, and elongation‒to‒failure: 18%.

The properties of the Mg‒based alloys are highly dependent on the crystallographic texture. A strong basal texture develops on rolling, which is detrimental to formability. The crystallographic texture depends on the strain path during rolling. Therefore, cross rolling (CR)

and reverse rolling (RR) were also carried out at 350°C. The RR process with refined microstructure and weaker texture exhibited better tensile properties (0.2% PS: 325 MPa, UTS: 386, and elongation‒to‒failure (9%) with minimum anisotropy) than CR. The basal texture is also weakened by introducing high shear stresses by having different circumferential velocities of the upper and lower rollers, known as asymmetric rolling (ASR). ASR was also employed for this alloy and the ASR sample exhibited the same strength levels as in symmetric rolling but higher elongation‒to‒failure. The present work shows that the wrought TZ73 alloy can be a promising candidate for the automotive industry.