Papers by Author: Yuan Sheng Yang

Abstract: The microstructure and mechanical properties of the high-purity magnesium (99.99wt.% Mg) extruded by single direct extrusion experiment were investigated. For the extrusion speed of 0.2mm/s, the microstructure of extruded Mg rods was composed of equiaxed fine dynamical recrystallized (DRXed) grains and some elongated coarse un-DRXed grains. The yield strength (YS) and the elongation of the extruded bars were 105.3MPa and 46.7% respectively. In the case of extrusion speed of 4.0mm/s, the DRXed grains were remarkably coarsened and the elongated coarse un-DRXed grains vanished, meanwhile lots of twins occurred and the intensity of basal-plane texture increased a little. With the extrusion speed being raised from 0.2mm/s to 4.0mm/s, the YS and the elongation decreased to 60.5MPa and 22.1% respectively, but the ultimate tensile strength (UTS) was improved from 154.7MPa to 178.8MPa.

Abstract: As a novel material, Mg-based foam material not only has fantastic physical characteristics, such as low density, high specific surface area, high specific strength and stiffness, and good biocompatibility, but also has special functional properties, for example, electromagnetic wave shielding, vibration reduction, sound absorption, and so on. It can be widely applied to aeronautical and aerospace, military, shipbuilding, transportation, automotive and medical industries. Mg-based foam material was prepared by direct foaming in magnesium alloy liquid in this study, and the effect of technological parameters on the products, such as addition amount of and granularity of SiC particles and MgCO₃ particles, stirring temperature, stirring time, stirring velocity, foaming temperature and foaming time, was investigated. The aim of this research was to develop a new technology which could fabricate large scale Mg-based foam material in air condition. The results showed that variations of technological parameters may affect preparation of the foam materials in some extent and resulted in the changes of the products in apparent density, porosity and structural uniformity. The light weight Mg-based foam with homogeneous pores could be obtained by suitable combination of the technological parameters.

Abstract: Microarc oxidation coatings were prepared on the surface of AZ31 magnesium alloy profiles. Oxidation time of the coatings was between 5min-10min. The phase structure, surface morphology and corrosion resistance of the coatings were investigated using SEM, XRD, copper sulfate spot test and polarization curve test. The results indicate: the main phase compositions of the microarc oxidation coatings are MgO, Mg₂SiO₄ and MgSiO₃; with increasing pulse voltage, the micropore diameter of the coating surface becomes larger, the micropore number reduces and the coating surface roughness increases; the corrosion current density of magnesium alloy reduces significantly after microarc oxidation treatment. The pulse voltage of microarc oxidation should be controlled between 240V-360V to obtain the best corrosion resistance.

Abstract: The effect of Sr addition on the as-cast microstructure of AS31 alloy was studied. The experimental results show that the grain size decreased obviously from 1116μm to 221μm for AS31 alloy with addition of 0.05%Sr, and promote the formation of globular grains. The morphology of eutectic Mg₂Si phase is changed from Chinese script to polyhedral shape or fine fibers shape. The grain refinement mechanism is attributed to the enrichment of Sr element at solidification interface front, which restricts the growth of α-Mg primary dendrite. The modification of the eutectic Mg₂Si phases results from the dissociative Sr changing the preferred growth manner of them.

Abstract: Long route and high inclusion content are two common problems existing in the present process for magnesium alloys. To solve these problems, a new process technology, in which the crystalline magnesium is used as the raw material, is developed. The process is melting flux→ adding raw materials in batches → stirring→ holding→ pouring (MASHP). The mechanisms of protection and purification mechanisms are also analyzed. The results show that the MASHP process has high protection and purification effects on magnesium alloys. By the in-batches addition mode of raw materials, no oxidation and evaporation loss of magnesium produce during heating and melting, and the inclusions that self-contained in the crystalline magnesium and that produced by oxidation during heating and melting can be minimized. High quality magnesium alloys can be produced by the new process.

Abstract: The microstructure and tensile properties at room temperature of as-cast, solution-treated and aged samples of a Mg-5Sn-5Zn (wt%) alloy are investigated. The microstructure of the as-cast sample consists of -Mg primary, Mg-MgZn eutectic, divorced eutectic Mg2Sn and secondary precipitation Mg2Sn. After solution treatment, all the MgZn particles and the majority of Mg2Sn phases are dissolved into the matrix and the tensile strength and the elongation of the alloy increase obviously. During aging process many fine particles precipitate which result in the improvement of the yield strength.

Abstract: Low Voltage Pulsed Magnetic Casting (LVPMC) is developed for grain refinement castings in recent years. This paper investigates the grain refinement effect of LVPMC on superalloy K417 and deals with the effects of cooling rate and superheating on grain refinement, as well as grain refinement mechanism. The experimental results show that the grains in the alloy are equiaxed and refined to 60 m averagely. The melt flow and Joule heat during solidification are modeled and simulated to reveals the grain refinement mechanism. It is considered that the melt vibration and convection caused by the pulsed magnetic field, as well as cooling rate and superheating contribute to the refinement of solidified grains.

Abstract: The microstructure evolution of magnesium alloy AZ91D solidified with different electric current pulses and cooling rates was investigated and a new method, Low-voltage Electric Current Pulses (LVECP), to produce semi-slurry magnesium alloy was developed in this paper. The experimental results showed that the electric current pulses during solidification changed morphology of dendrites and the equiaxed, non-dendritic grains formed. The size of the primary a-Mg particles in semi-solid AZ91D alloy and the sphericity of the particles decreased with increase of discharging the voltage and treating time of LVECP. The increase of the cooling rate during the solidification of AZ91D alloy with LVECP promoted the formation of finer a-Mg particles, but the value of the sphericity of the particles rised. The formation of equiaxed, nondendritic structure by LVECP might be attributed to the electric current pulses increase the nucleation rate, restrained growth of the dendrites, and made dendrite arms remelted during the solidification of AZ91D alloy.