Qudong Wang

Effects of rare earths on the microstructure, properties and fracture behavior of Mg–Al alloys

Abstract AZ91–xRE and Mg–6Al–xRE magnesium alloys were studied, where x is 0, 1, 2 and 3% (in weight percent, wt.%), respectively. Influence of rear earths (RE) on the microstructure was investigated. Fine morphology could be achieved by high cooling rate. By casting fluidity spiral specimens, fluidities of the alloys were achieved. The hardness and microhardness of the alloys was tested. RE improved fluidity and hardness. By casting specimens in permanent mold, tensile properties of the alloys with different RE additions at ambient and elevated temperatures were studied. RE had little effect on ambient temperature tensile strength of AZ91 alloy but greatly improved that of Mg–6Al alloy and high temperature tensile properties of both alloys. The fracture behavior of the alloys, which was changed by RE and high temperature, was examined by scanning electron microscopy (SEM) and optical microscopy. Fracture of the alloys is predominantly brittle cleavage or/and quasi-cleavage failure.

Behavior of surface oxidation on molten Mg–9Al–0.5Zn–0.3Be alloy

Abstract Excellent ignition proof performance was obtained during the melting of Mg–9Al–0.5Zn–0.3Be alloy directly in the atmosphere. XRD and AES analysis indicated that the oxide film on the surface of the molten Mg–9Al–0.5Zn–0.3Be alloy exhibited a duplex structure, which was in agreement with the result of thermodynamic analysis. The outer layer of the oxide film mainly consists of MgO, which grows according to the parabolic law. The inner layer is a mixture of MgO and BeO, and its growth follows a linear law approximately. This inner layer acts as a barrier to reduce the outward diffusion of Mg 2+ , which leads to the excellent ignition proof performance. An oxide model was established to describe the oxidation procedure on the surface of the molten Mg–9Al–0.5Zn–0.3Be alloy.

An understanding of the hot tearing mechanism in AZ91 magnesium alloy

A new method of investigating the hot tearing behavior of magnesium alloys is developed. The results of measurement, by EDAX and optical microcopy revealed that the end-solidifying temperature was around the eutectic temperature under practical solidification conditions, because of a quantity of eutectic caused by dendritic segregation. The temperature at which hot tearing occurred most probably in the AZ91 alloy was the practical end-solidifying temperature, which was 424.2 °C in the present solidification conditions. The appearance of eutectic was the inducement for hot tearing of AZ91 alloy.

Effects of Zn and RE additions on the solidification behavior of Mg–9Al magnesium alloy

Abstract Effects of Zn and RE additions on the solidification behavior of Mg–9Al alloy were studied. Zinc (Zn) and rare earths (RE) elements were added to a maximum quantity of 1.2 and 1.6wt.%, respectively. Cooling curves were obtained under commercial production conditions. Hot tearing susceptibility was investigated using crack-ring molds. Microstructure was analyzed by metallographic examination and electron probe micro-analysis. Zinc additions decreased the end-solidifying temperature, promoted the precipitation of Mg 17 Al 12 in grain boundaries and increased the hot-tearing susceptibility coefficient (HSC). Al 4 RE phase precipitation slowed down the temperature decrease at the initial stage of solidification. RE additions had little effect on HSC of Mg–9Al with low or no Zn content, while when Zn content was exceeded 0.8 wt.%, RE additions decreased HSC of Mg–9Al alloys distinctly.

Evaluation of the effect of vacuum on mold filling in the magnesium EPC process

The combination of magnesium alloys with the expendable pattern casting (EPC) process will bring a bright future for the application of magnesium alloys. Vacuum is a pre-requisite parameter in the EPC process of magnesium alloys, because without vacuum, the fluidity of the magnesium alloy in the EPC process is too poor to fill the mold completely, especially for the thin-section castings. In this investigation, the effect of vacuum on the fluidity of AZ91 magnesium alloy has been explored. A modified model has been presented to explain the effect of vacuum on mold filling, which was verified by optical microscopy. The results obtained indicate that vacuum is the most effective parameter in improving the fluidity, the effect of vacuum on the fluidity interacting strongly with the pouring temperature and coating. Vacuum greatly changes the mass and heat transfer in the EPC process. Vacuum may not only control the profile of the metal–foam interface, which will influence the mass transfer process, but may also greatly speed up the removal rate of pattern decomposition products at the metal–coating interface. It also changes the primary heat-transfer mode to heat convention, which has a great influence on the distribution of the casting temperature field and solidification process. The microstructures of castings cast with vacuum exhibit a fine grain size and a small amount of precipitated Mg17Al12, but vary insignificantly with the location in the castings.

Effects of heat treatments on microstructure and mechanical properties of Mg-15Gd-5Y-0.5Zr alloy

Abstract Microstructure and mechanical properties of Mg-15wt.%Gd-5 wt.%Y-0.5wt.% Zr alloy were investigated in a series of conditions. The eutectic was dissolved into the matrix and there was no evident grain growth after solutionized at 525 °C for 12 h. The evolution of the phase constituents from as-cast to cast-T4 was as follows: α-Mg solid solution+Mg 5 (Gd,Y) eutectic compound → α-Mg solid solution+ spheroidized Mg 5 (Gd, Y) phase → α-Mg supersaturated solid solution+cuboid-shaped compound (Mg 2 Y 3 Gd 2 ). And the precipitation sequences of Mg-15Gd-5Y-0.5Zr alloy were observed, according to the hardness response to isothermal ageing at 225–300 °C for 0–128 h.

Hot-tearing susceptibility of Mg–9Al–xZn alloy

Abstract Hot-tearing susceptibility of Mg–9Al– x Zn alloys is studied. Crack-ring molds were used to test the hot-tearing susceptibility. In situ thermal analysis, optical microscopy, scanning electron microscopy (SEM) and line scan were used to examine the hot-tearing behavior. Hot-tearing originates along the grains at the end of solidification. With Zn additions, the quantity of phase with low melting point in grain boundaries is increased, its melting point is decreased and the hot-tearing susceptibility is increased. The segregation of Zn element in grain boundaries is the main contribution to the high hot-tearing susceptibility of Mg–9Al– x Zn alloys.

Microstructure and texture characteristics of ZK60 Mg alloy processed by cyclic extrusion and compression

Abstract The microstructure and crystallographic texture characteristics of an extruded ZK60 Mg alloy subjected to cyclic extrusion and compression (CEC) up to 8 passes at 503 K were investigated. The local crystallographic texture, grain size and distribution, and grain boundary character distributions were analyzed using high-resolution electron backscatter diffraction (EBSD). The results indicate that the microstructure is refined significantly by the CEC processing and the distributions of grain size tend to be more uniform with increasing CEC pass number. The fraction of low angle grain boundaries (LAGBs) decreases after CEC deformation, and a high fraction of high angle grain boundaries (HAGBs) is revealed after 8 passes of CEC. Moreover, the initial fiber texture becomes random during CEC processing and develops a new texture.

Grain refinement of magnesium alloys processed by severe plastic deformation

Abstract Grain refinement of AZ31 Mg alloy during cyclic extrusion compression (CEC) at 225–400 °C was investigated quantitatively by electron backscattering diffraction (EBSD). Results show that an ultrafine grained microstructure of AZ31 alloy is obtained only after 3 passes of CEC at 225 °C. The mean misorientation and the fraction of high angle grain boundaries (HAGBs) increase gradually by lowering extrusion temperature. Only a small fraction of twinning is observed by EBSD in AZ31 Mg alloys after 3 passes of CEC. Schmid factors calculation shows that the most active slip system is pyramidal slip and basal slip {0001} at 225–350 °C and 400 °C, respectively. Direct evidences at subgrain boundaries support the occurrence of continuous dynamic recrystallization (CDRX) mechanism in grain refinement of AZ31 Mg alloy processed by CEC.

Dry sliding wear behavior of cast Mg–11Y–5Gd–2Zn magnesium alloy

Abstract Dry sliding wear tests on as-cast and cast+T6 Mg–11Y–5Gd–2Zn magnesium alloys were performed using a ball-on-plate configuration. The wear rates were measured within a load range of 3-15 N, sliding speed range of 0.03-0.24 m/s, test temperature range of 25–200 °C and at a constant sliding distance of 400 m. The wear tracks, worn surfaces and wear debris of the alloys were analyzed using scanning electron microscope (SEM). The results show that the wear rate of the alloys increases almost linearly with increasing applied load and decreases with increasing sliding speed. The wear rate of the as-cast alloy is higher than that of the cast+T6 alloy. The amount of Mg 12 Y 1 Zn 1 phase, surface oxidation and retained wear debris affect the wear rate. The dominant wear mechanisms under the test condition are abrasion and plastic deformation.