Xingwei Zheng

Directional Solidification and Microsegregation in a Magnesium-Aluminum-Calcium Alloy

Creep-resistant Mg-4Al-4Ca (AX44) alloy was solidified under different growth/cooling rates using a directional solidification (DS) technique. The solidification behavior and microsegregation of the alloying elements in the castings was investigated. Computational thermodynamics calculations of phase equilibria and experimental observations confirm that the solidification microstructure in this alloy consists of α-Mg, C36-(Mg,Al)2Ca, and C14-Mg2Ca phases. A relationship was established between the primary dendrite arm spacing and the cooling rate, which can be used to predict mechanical properties such as yield strength and creep. The microsegregation of alloying elements (Al and Ca) predicted by the Scheil model in AX44 agrees well with the electron probe microanalysis (EPMA) measurements, suggesting that the Scheil model can be used in microstructure simulation of magnesium castings.

Effect of process parameters on structure and macrosegregation upon direct chill casting of Mg–Nd–Zn–Zr alloy

Abstract Direct chill (DC) semicontinuous casting process has been successfully used to produce sound Mg–3·0Nd–0·4Zn–0·4Zr (NZ30K) billets. The influence of process parameters such as casting speed, casting temperature on the microstructure and macrosegregation was studied. The results show that the casting speed affects the macrosegregation greatly while it has a slight influence on the grain size of the billet; the casting temperature has a slight influence on macrosegregation of the billet while the grain size of the billet increases as the casting temperature increases. The optimal process parameters have been experimentally determined as follows: casting temperature 700°C and casting speed 90 mm min−1. The ultimate tensile strength, yield strength and elongation of billets cast at the optimal casting parameters are 196 MPa, 125 MPa and 16·5% respectively.

Influence of shot peening on notched fatigue properties of magnesium alloy ZK60

Abstract The influence of shot peening on high cycle fatigue performance of notched specimen was investigated for ZK60 and ZK60-T5 magnesium alloys. The results show that the notched fatigue strengths (at 107 cycles) for ZK60 and ZK60-T5 alloys increase from 150 and 155 MPa to 220 and 240 MPa at the optimum Almen intensity of 0·30 and 0·40 mmN respectively. In comparison to ZK60 alloy in extruded condition, higher notched fatigue performances of both unpeened and peened specimens were observed for ZK60-T5 alloy.

Smooth and notched fatigue performance of aging treated and shot peened ZK60 magnesium alloy

The influence of shot peening (SP) on high cycle fatigue (HCF) performance of smooth and notched specimens of hot-extruded ZK60 magnesium alloy was investigated and compared to that of hot-extruded and T5 aging-treated ZK60 magnesium alloy referred to as ZK60-T5. The increases in fatigue properties at the optimum Almen intensities were found to depend on the material states. In contrast to ZK60 alloy, higher smooth and notched fatigue properties for both unpeened and peened specimens were observed for ZK60-T5 alloy. Meanwhile, the improvement of fatigue life for notched specimen by SP was much more than that for the smooth specimen. The mechanism by which the compressive residual stress induced by SP resulted in the improvement of fatigue performance of smooth and notched specimens for ZK60 and ZK60-T5 alloys was discussed.