Wu Guohua

Effect of cooling condition on microstructure of semi-solid AZ91 slurry produced via ultrasonic vibration process

The effects of cooling conditions on the microstructure of semi-solid AZ91 slurry produced via ultrasonic vibration process were investigated. AZ91 melts were subjected to ultrasonic vibration in different temperature ranges under different cooling rates. The results show that fine and spherical α-Mg particles are obtained under ultrasonic vibration at the nucleation stage, which is mainly attributed to the cavitation and acoustic streaming induced by the ultrasonic vibration. The reduction of lower limit of ultrasonic vibration temperature between the liquidus and solidus increases the solid volume fraction and average particle size. Increasing cooling rate increases the solid volume fraction and reduces the average shape factor of particles. The appropriate temperature range for ultrasonic vibration is from 605 °C to 595 °C or 590 °C, and the suitable cooling rate is 2-3 °C/min.

Effect of heat treatment on microstructures and mechanical properties of sand-cast Mg-10Gd-3Y-0.5Zr magnesium alloy

Abstract Influence of heat treatment on the microstructures and mechanical properties of sand-cast Mg–4Y–2Nd–1Gd–0.4Zr magnesium alloy was investigated, and the tensile fracture mechanisms of the studied alloys under different conditions were also discussed. The results show that the optimum T4 and T6 heat treatment conditions for the as-cast Mg–4Y–2Nd–1Gd–0.4Zr alloy are 525 °C, 8 h and (525 °C, 8 h)+(225 °C, 16 h), respectively, with regard to the microstructure observation, DSC heating curve and mechanical properties. The hardness, yield strength, ultimate tensile strength and elongation of the Mg–4Y–2Nd–1Gd–0.4Zr alloy treated by optimum T6 heat treatment are HV91, 180 MPa, 297 MPa and 7.4%, respectively. Moreover, the Mg–4Y–2Nd–1Gd–0.4Zr alloys under different heat treatment conditions exhibit different tensile fracture modes.

Non-flux purification behavior of AZ91 magnesium alloy

Abstract The effects of non-flux purification techniques on the mechanical properties and microstructure of AZ91 magnesium alloy were investigated by ICP, OM, XRD and SEM. The results show that Ar spraying with high flow rate could remove non-metallic inclusions and improve the mechanical properties of AZ91. The alloy obtains the best properties after argon spraying for 30 min at the melt temperature of 740 °C. The ceramic foam filter (CFF) could effectively improve the ultimate tensile strength and elongation of AZ91 alloy, especially the elongation, which increase with increasing pores per inch (ppi) and the thickness of CFF. Non-flux purification does not change the microstructure of AZ91 alloy. However, filtration has a certain effect on the fracture pattern of AZ91 alloy. To improve the mechanical properties effectively, both filtration and gas spraying should be utilized together.

Influence of heat treatment on microstructures and mechanical properties of gravity cast Mg–4.2Zn–1.5RE–0.7Zr magnesium alloy

Abstract The microstructures and mechanical properties of Mg–4.2Zn–1.5RE–0.7Zr alloy were investigated under different heat treatment conditions. The as-cast alloy consisted of α -Mg phase, T -phase and Mg 51 Zn 20 phase. After aging treatment (single-step (325 °C, 10 h) and two-step (325 °C, 4 h)+(175 °C, 14 h)), neither T -phase nor Mg 51 Zn 20 phase dissolved into the matrix and the coarsening of α -Mg phase was not significant. When peak-aged at 325 °C for 10 h, dense short rod-like β′ 1 phase precipitated in the matrix. Further ageing at 325 °C led to coarsening of β′ 1 phase and a decrease in number density. Alloy aged at 325 °C for 10 h achieved the highest yield strength (YS) and ultimate tensile strength (UTS) of 153.9 MPa and 247.0 MPa, which were increased by 48 MPa and 23 MPa from as-cast condition, respectively. While the elongation slightly decreased to 15.6%. Comparatively, the YS and UTS of alloy two-step aged by (325 °C, 4 h)+(175 °C, 14 h) showed little difference from those of single-step aged alloy, but with a lower elongation of 13.4%. In addition, the fracture surfaces of Mg–4.2Zn–1.5RE–0.7Zr alloy under different thermal conditions were mainly characterized by quasi-cleavage feature, but with differences in the details.

Effect of inclusion on service properties of GW103K magnesium alloy

The effects of inclusions on microstructure, mechanical property, corrosion behavior of Mg-10Gd-3Y (GW103K) alloys by unrefining, MgO ceramic filtering and JDMJ flux refining were investigated, respectively. The results indicate that with decreasing significantly the number and size grade of inclusions for the alloy refined with JDMJ flux, tensile strength and elongation increase; however, the yield strength is less than that of the alloy refined with MgO ceramic filter and unrefined alloy. With the decrease of the inclusions contents, the corrosion rate of the alloys quickly vary from 2.419 mg/(cm^2•d) to 1.265 mg/(cm^2•d). After inclusion content is reduced to 0. 385%, the corrosion rate has almost no changes. Finally, the relationship between the volume fraction of inclusions and service properties of GW103K alloy under different conditions are established quantitatively.

Effect of complex melt-refining treatment on microstructure and mechanical properties of sand-cast Mg–10Gd–3Y–0.5Zr alloy

Abstract The effect of complex melt-refining treatment (melt flux incorporating with rotating gas bubble stirring) on microstructure and mechanical behavior of the sand-cast Mg–10Gd–3Y–0.5Zr alloy was investigated. In addition, the melt purifying mechanism of the complex melt-refining treatment for the sand-cast alloy was discussed systematically. The results show that the new melt-refining method can significantly improve melt quality and mechanical behavior of the tested alloy, i.e., compared to the reference unpurified alloy, the volume fraction of inclusions decreased from 0.47% to 0.28%, the ultimate tensile strength and elongation for T6-treated alloy increased from 245 MPa and 0.7% to 312 MPa and 4.5%, respectively. Especially, combining 1% flux with rotating gas bubble stirring can get even better purifying effectiveness than conventional sole 2% flux purification; the use of melt flux decreased by 50% and significantly reduced environmental pollution.

Influence of cooling rate on solidification behavior of sand-cast Mg−10Gd−3Y−0.4Zr alloy

Abstract The effect of the cooling rate ranging from 1.4 °C/s to 3.5 °C/s on the solidification behavior of the sand-cast Mg–10Gd–3Y–0.4Zr alloy was studied by computer aided cooling curve analysis (CA-CCA). With the increase in cooling rate, the nucleation temperature (Tα,N) increases from 634.8 °C to 636.3 °C, the minimum temperature (Tα,Min) decreases from 631.9 °C to 630.7 °C, the nucleation undercooling (ΔTN) increases from 2.9 °C to 5.6 °C, the beginning temperature of the eutectic reaction (Teut,N) increases, the time of the eutectic reaction shortens, solidus temperature decreases from 546.0 °C to 541.4 °C, and solidification temperature range (ΔTS) increases by 6.1 °C. The increased nucleation rate () is supposed to be the main reason for the increased ΔTN. Increased value (Teut,N–Teut,G) and shortened time of the eutectic reaction cause the change in the volume fraction and morphology of the second phase.

Effects of processing parameters on microstructure of semi-solid slurry of AZ91D magnesium alloy prepared by gas bubbling

Abstract The semi-solid slurry of AZ91D magnesium alloy was prepared by gas bubbling process. The effect of processing parameters, including gas flow rate, cooling rate and stirring end temperature, on microstructure of AZ91D semi-solid slurry was investigated. With increasing the gas flow rate from 0 to 5 L/min, the average size of primary α-Mg particles decreases from 119.1 to 77.2 μm and the average shape factor increases continuously from 0.1 to 0.596. The formation of non-dendritic primary α-Mg particles during gas bubbling is the result of combined effects of dendrite fragmentation and copious nucleation. With increasing the cooling rate from 3.6 to 14.6 °C/min, the average particle size of primary α-Mg phase decreases from 105.0 to 68.1 μm while the average shape factor peaks at 9.1 °C/min. Both high and low cooling rates can induce dendritic growth of primary α-Mg particles. Changing the stirring end temperature from 590 to 595 °C has little effect on the average size and shape factor of primary α-Mg particles in AZ91D semi-solid slurry. The insensitivity of semi-solid microstructures to the stirring end temperature is attributed to the sufficient quantity of primary particles formed in the melt.

Influence of pouring temperature on solidification behavior, microstructure and mechanical properties of sand-cast Mg-10Gd-3Y-0.4Zr alloy

Abstract Influence of the pouring temperature ranging from 680 to 780 °C on the solidification behavior, the microstructure and mechanical properties of the sand-cast Mg−10Gd−3Y−0.4Zr alloy was investigated. It was found that the nucleation undercooling of the α-Mg phase increased from 2.3 to 6.3 °C. The average α-Mg grain size increased from 44 to 71 μm, but then decreased to 46 μm. The morphology of the eutectic compound transformed from a continuous network into a discontinuous state and then subsequently into an island-like block. The volume fraction of β-Mg 24 RE 5 phase increased and its morphology transformed from particle into rod-like. The increase in pouring temperature increased the solute concentration. YS increased from 138 to 151 MPa, and UTS increased from 186 to 197 MPa. The alloy poured at 750 °C had optimal combining strength and ductility. The fracture surface mode transformed from quasi-cleavage crack into transgranular fracture, all plus the dimple-like fracture, with the micro-porosity and the re-oxidation inclusion as major defects. The average α-Mg grain size played a main role in the YS of sand-cast Mg−10Gd−3Y−0.4Zr alloy, besides other factors, i.e. micro-porosity, morphology of eutectic compounds, re-oxidation inclusion and solute concentration.

Low temperature mechanical properties of as-extruded Mg–10Gd–3Y–0.5Zr magnesium alloy

Abstract Influence of multi-cycle cryogenic treatment and tensile temperature on microstructure, mechanical properties and fracture mechanism of as-extruded Mg–10Gd–3Y–0.5Zr magnesium alloy was investigated. The results show that there have no significant changes in tensile properties of the tested alloy after 10 d in liquid nitrogen immersion or 10 cycles of high-low temperature treatment at all test temperatures. The room temperature ultimate tensile strength increases from 398 MPa to 417 MPa after 20 cycles of high-low temperature treatments. Compared with the room temperature, the tested alloys exhibit higher tensile properties at low temperatures. At −196 °C, the yield strength and ultimate tensile strength of the as-extruded-T5 Mg–10Gd–3Y–0.5Zr alloy are 349 MPa and 506 MPa, respectively, increasing by about 18% and 27%, respectively. The transgranular cleavage fracture mechanism is observed at room temperature, while at low temperatures both ductile fracture and cleavage fracture behaviors coexist.