Papers by Author: Qi Chi Le

Abstract: A new type of AZ31-1.3Ca-1.0Sm-0.3La alloy was obtained in this study by adding Ca, Sm and La to AZ31 alloy. Detailed analysis results on second phases showed that Al₂Ca phases, Al₂Sm phases with two kinds of morphologies formed in as-cast AZ31-1.3Ca-1.0Sm-0.3La alloy besides Mg₁₇Al₁₂ phases, and La atoms mainly dissolved in Al₂Ca/Sm phases. The average grain size of as-cast AZ31-1.3Ca-1.0Sm-0.3La alloys was 212 μm and the grain sizes distributions were uniform. After the hot extrusion, the average grain size decreased to 5.4 μm and the grain sizes distributions were uneven. The base texture of as-extruded AZ31-1.3Ca-1.0Sm-0.3La alloy was strong, and the maximum density value was 3.25. The yield strength, ultimate tensile strength and elongation of as-extruded AZ31-1.3Ca-1.0Sm-0.3La alloy was 216 MPa, 280 MPa and 16% at RT, and 145 MPa, 188 MPa and 42% at 150°C, respectively, which are much higher than those of the common MB2 alloy both at the room temperature and 150 °C.

Abstract: The effect of extrusion temperature on the mechanical properties of low-Zn containing wrought Mg alloys manufactured by indirect extrusion was studied. The experiments were performed on Mg-Zn-Mn based alloy with addition of LaMM (La rich misch metal). The alloys presented typical bimodal grain structure consisted of large elongated un-DRXed grains and fine recrystallized grains when extruded at relatively low temperature. The alloy showed excellent balance of strength and elongation. The distinct grain structure generated in this study allowed the influence of un-DRXed grains on yield strength to be investigated.

Abstract: The traditional fixed-frequency ultrasonic technology (FUT) and the variable-frequency ultrasonic technology (VUT) were used in direct-chill (DC) casting of an AZ80 magnesium alloy with the diameter of ∅255mm. The microstructure evolution, mechanical properties and macro-segment of Al element were investigated. The results show that both FUT and VUT can refine the α-Mg grain while the VUT has better efficiency which can reduce the grain size from 679-1454 μm (DC) to 150-241 μm. The homogeneity of grain size was also improved. These two technologies can also change the morphology and distribution of Mg₁₇Al₁₂ phase and tiny and refined morphology of Mg₁₇Al₁₂ phase and a homogeneous distribution can be obtained after VUT treatment. The YS and UTS were increased following the microstructure evolution, displaying the YS of 88.9-95.1 MPa and UTS of 164.3-173.1 MPa after VUT treatment. The segregation rate of Al element were improved with VUT treatment. Keywords: magnesium alloy; variable frequency ultrasonic technology; microstructure; mechanical properties;

Abstract: The effect of Ca and RE metal additions on the precipitation and microstructure of as-cast AZ91 alloy was systematically investigated. It was found that Ca and RE additions could result in phase and microstructure changes. The XRD pattern showed the crystallite phase of as-cast AZ91 alloys consists of α-Mg matrix and β-Mg₁₇Al₁₂, however, after adding 1.5wt. % Ca and 0.8wt. % RE (0.5wt. % Sm and 0.3wt. % La), peaks coincident with Al₂Ca, Al₂Sm and Al₁₁La₃ intermetallic compounds were found, suggesting the generation of relative precipitates. The SEM images indicated that in as-cast alloys, the Al₂Ca intermetallic compound was located at grain boundaries with a lamellar structure, and the Al₂Sm intermetallic compound was homogeneously distributed in the α-Mg matrix or near the grain boundaries with a polygonal structure, and the Al₁₁La₃ intermetallic compound was located at grain boundaries with a needlelike structure. These intermetallic compounds could reduce the amount of β-Mg₁₇Al₁₂ and refine the microstructure of as-cast AZ91 alloy.

Abstract: In recent years, the development and utilization of renewable generation have attracted more and more attention, and the grid puts forward higher requirements to the energy storage technology, especially for security, stability and reliability. The liquid metal battery (LMB) consists of two liquid metal electrodes and a molten salt electrolyte, which will be segregated into three liquid layers naturally. Being low-cost and long-life, it is regarded as the best choice for grid-level large-scale energy storage. This paper describes the main structure and working principle of the LMB, analyzes the advantages and disadvantages of the LMB when compared with the traditional batteries, and explores the feasibility and economy when it is used as a kind of large-scale energy storage applied in the power grid. The paper also makes a comprehensive comparison on the performance of several LMBs, and points out the LMB’s research and development in the future.

Abstract: Effect of different melt temperatures, holding time, cooling rate and C₂Cl₆ degasser treatment on the density of Mg-6Zn-0.5Y alloy was studied. The experimental results indicated that the density of RPT samples decreased with increasing of melt temperature, which indicating that the hydrogen content increased with the increment of temperature. And the results also showed that the density of RPT samples rapidly decreased at first and then decreased slowly with increasing of holding time, suggesting that the hydrogen content rapidly increased at first and then reached a saturated state with increasing of holding time. When cooling rate increased, the density of ingots gradually increased at the same melt temperature. When the melt was treated by 1%, 3% and 5% C₂Cl₆ degasser at 720°C, the density of ingots gradually increased compared with the ingot without treatment by C₂Cl₆ degasser. The density of ingot reached the maximum value when C₂Cl₆ degasser content was 3%, i.e, the ingot density value increased from 1.8649g.cm^-3 of the untreated sample to 1.8774g.cm^-3 of the sample treated by 3% C₂Cl₆ degasser, indicating that 3% C₂Cl₆ degasser treatment has an optimum degassing effect.

Abstract: Low-frequency electromagnetic casting (LFEC) processing could improve not only the metallurgical quality of magnesium billet including refining grain size, reducing regional microstructural difference and lightening segregation, but also its surface quality due to the effect of applied electromagnetic field according to the results by microstructure observation and the numerical simulation. In this research in-situ temperature measurement was carried out in LFEC processing in order to investigate heat transfer behavior of billet during solidification. The effects of the electromagnetic conditions (frequency and the intensity) together with the casting temperature on the sump and the mushy zone were investigated in detail. The results indicate that all the casting conditions affect the temperature field of magnesium billet markedly during solidification. Electromagnetic field could decrease not only the sump depth but also the difference of regional temperature field along the solidification direction leading to much more uniform cooling rate.

Abstract: Microstructures and Mechanical Properties of as-cast and extruded Mg–2wt.%Zn alloys containing dilute misch metal were investigated. RE additions were added in the form of relatively inexpensive cerium-rich misch metal (MM). The results indicate that the dilute MM addition has evidently effect the microstructure and mechanical properties of Mg-2wt.% Zn alloy. The grain refinement was improved and the amount of second phase precipitation was increased as increasing MM content in Mg-2wt.%Zn alloy from 0.2% to 1.5%, and the hardness determination and tensile test results show that the hardness of its ingot and the tensile strength of its extrusion rod were also enhanced, however, excessive MM addition would promote second phases forming network precipitating in grain boundary, which would result in the marked reduction of the plasticity of the alloy. Therefore, it is recommended that the MM content not exceed 0.6% in order to obtain high strength companying with relative higher elongation. The tensile properties of as-extruded Mg-2Zn-0.6wt.%MM alloy rod were UTS=266.7MPa, YS=213MPa, EL=14.5% which is similar to the corresponding properties of AZ31.

Abstract: Effects of low frequency electromagnetic field on surface quality, microstructure and hot-tearing tendency of direct chill casting of Φ500mm ZK60 magnesium alloy billets were investigated. The results show that with the application of the low frequency electromagnetic field, the surface quality of Φ500mm ZK60 magnesium alloy billets has been markedly improved, and the depth of cold fold is decreased. In the conventional direct chill casting, the microstructures of the billet, especially at the center, are coarse. The distribution of the grain size is non-uniform throughout the billet. From the edge to the center, the microstructure gradually changes from fine to coarse in all billets. However, under the low frequency electromagnetic casting, the microstructures of the billet is significantly refined, the distribution of the grains size is relatively uniform from the billets edge to the billets center. And it also shows that the hot-tearing tendency of direct chill casting Φ500mm ZK60 magnesium alloy billets under low frequency electromagnetic field is significantly reduced.

Abstract: Microstructures and macrosegregation of AZ80 magnesium alloy billets cast with and without the electromagnetic vibration were investigated. Compared with the conventional DC casting, microstructures of the billet have been greatly refined and the macrosegregation had been effectively reduced in electromagnetic vibration casting conditions. Increasing the electromagnetic vibration intensity, including both the stationary magnetic field and alternating magnetic field, leads to finer grains and more uniform distribution of solute elements in the billet.