Microstructure and Mechanical Properties of ZK60 Mg Alloy Processed by Cyclic Expansion-Extrusion (CEE) at Different Temperatures

Abstract

The cyclic expansion extrusion (CEE) process was applied to the ZK60 Mg alloy, and the influence of processing temperature was investigated.  The CEE process at a processing temperature of 190 ºC showed significant refining of α-Mg grains to a size of approximately 9 μm.  The mechanical properties of ZK60 Mg alloy were greatly affected by the CEE processing temperature.  The tensile test fracture morphologies of unprocessed and CEEed ZK60 Mg alloy revealed the combination of ductile and brittle fracture mechanisms.In this work, the cyclic expansion extrusion (CEE) process was applied to ZK60 Mg alloy. The correlation between the evolved microstructure and mechanical properties was investigated. The CEE process was performed at a constant ram speed (15 mm/min) and at different processing temperatures (190, 270, and 350 °C). Optical and scanning electron microscopes, X-ray diffraction instruments, Vickers hardness tester, and tensile testing machine were utilized to examine the influence of CEE processing temperature on the characteristics of ZK60 Mg alloy. The XRD analysis showed that two phases were presented in the matrix of ZK60 Mg alloy, namely α-Mg and MgZn 2, in small amounts. The CEE process reduced the size of α-Mg grains due to dynamic recrystallization, especially at the processing temperature of 190 °C. A slight coarsening of the α-Mg grains was observed with increasing processing temperature to 270 and 350 °C. The hardness value of ZK60 Mg alloy was enhanced by about 11 to 19% using the CEE process compared to the as-extruded sample. The processing temperature greatly affected the mechanical properties, where a significant improvement of about 24% yield strength, 9% ultimate tensile strength, and 38% elongation was observed using a processing temperature of 190 °C. The characterization of the tensile fracture surface of the tested samples indicated that the ductile-brittle fracture mode was responsible for the failure.