Materials Science Forum Vols. 488-489

Abstract: Magnesium alloys are becoming more and more attractive because of their low density, high specific stiffness and strength, good heat conductivity, high electromagnetic interference shielding and damping capabilities, etc. Magnesium alloy components produced by semi-solid processing (SSP) has the merits of high dimension stability and quality. Moreover, it is desirable to apply the SSP technology to produce components free of gas holes and porosity which are usually found in those produced by the die casting. SSP of magnesium alloys has been utilized in Japan and Taiwan, China. In this paper, preparing methods of semi solid slurry appropriate for magnesium alloys are introduced, including stirring, SSIT-semi solid isothermal heat treatment, pouring temperature controlling, grain refinement and so on. The effects of process parameters on the microstructure of semi-solid slurry are also discussed. In addition, thixomolding technology practically utilized in the semi-solid processing of magnesium alloys is also introduced. At last the prospect of Semi-solid processing technologies for magnesium alloys are discussed.

Abstract: Magnesium alloy slurry was prepared using Strain-Induced Melt Activation(SIMA) technique. The samples were quenched into water so as to fix the high temperature instantaneous microstructures. The microstructure evolution of compressed deformation Mg alloy is studied in the process of continuous heating and iso-temperature of semi-solid state. The results indicate that deformed Mg alloy (AZ91) has first occurred to have the conversion of dendrite crystal-oriented isometric crystals in the continuous heating process. When the temperature rises to the range of semisolid state, the region with high energy at the pressed stripes begins to melt, showing that the cellular structures emerge in the crystal boundary and melting micro-pool phenomena appear within the crystals. With the iso-temperature time in semisolid state prolongs, the isometric crystals can be gradually converted into spherical crystal grains.

Abstract: The effects of vacuum degree, pattern density, pouring temperature and metallic static head on the mold-filling velocity of AZ91D alloy in the expendable pattern casting (EPC) process have been investigated in the paper, and the influencing law of these technological factors on the mold-filling velocity are obtained. The results indicate that the influence of vacuum degree on the mold-filling velocity of AZ91D alloy in the EPC process is greatest, pattern density and pouring temperature are next respectively, and metallic static head is small relatively.

Abstract: Compositional variations of magnesium alloys AZ91D and AM60B in a crucible during hot-chamber die casting process have been investigated. The sampling of molten metal for chemical composition analysis was carried out at three positions (A: bottom, B and C: 100 and 300 mm upside from the bottom, respectively) within a crucible of 600 mm in depth. The picked molten metal was poured into a steel mold to cast into a rod 40mm diameter and 120mm length. Results show that heavier elements such as Al, Zn, Mn, Fe, etc. were concentrated at the bottom of crucible, which is resulted from long-time, repeated casting operations without bailing out. The macro segregation of alloying elements and impurities are partly caused by the reaction between the alloying elements. Short-periods of holding time up to 250 minutes, however, did not significantly change the concentration of Al and Zn at the entrance of gooseneck.

Abstract: We developed the new technology for Thixomolding® of magnesium alloys to meet latest market requirements as the following. (1) Flood feed screw to stabilize chips feeding and its melting. (2) High speed and quick response new series machine to improve injection performance. (3) Long nozzle to increase the yield ratio about parts per shot.

Abstract: The as-cast microstructures of two dilute Mg-Zr and three dilute Mg-Mn alloys were studied using optical microscopy and scanning electron microscopy, including EDX analysis. The results show that the as-cast microstructure of the Mg-Zr alloys was composed of non-dendritic, equiaxed Mg grains, with a few Zr particles within the Mg grains and along grain boundaries. The grain size of the Mg-Zr alloy was significantly reduced by the Zr addition and a fine grain structure was achieved when the zirconium concentration was above 0.4wt. %. The as-cast structure of the Mg- Mn alloys contained columnar, dendritic grains. Two types of Mn particles (equiaxed large particles and rod-like or plate-like small particles) were observed in the as-cast Mg-Mn alloys. The volume fraction of particles and the size differences between the large and small particles increased with an increase of Mn concentration.

Abstract: In this paper a rheological model is presented which describes the rheological behaviors of liquid-like semisolid magnesium alloy under a simple shearing flow. On the basis of Chen and Fan’s mono-dispersion microstructure model of semisolid metal slurry, the particle size distribution is considered in this model. It is believed that it is the state of agglomeration which determines the rheological behaviors of the slurry, whereas the external flow conditions such as shear rate and shearing time, affect the rheological properties by changing the state of agglomeration. The expressions of collision rate between two agglomerates, effective solid fraction and the formula of apparent viscosity of Chen and Fan’s model are corrected according to the experimental results and statistical mechanics. Finally calculated apparent viscosity and the average number of the particles of AZ91D by the developed model as functions of shear rate are presented. These results show that there is a one to one coupling between the rheological properties of the magnesium alloy slurries and the state of the agglomeration.

Abstract: AZ31 magnesium plates with 1mm in thickness were fabricated by casting and subsequent hot rolling process. Both wheel-band continuous casting (modified Properzi) and horizontal continuous casting (HCC) machine were designed and employed to produce AZ31 alloy plates. The plates with 5x50mm in size produced by modified Properzi process exhibited good surface qualities that were sustained during following rolling process. Surface qualities of HCC plates with 120x30mm in size were very sensitive depending on the processing parameters. Homogenization treatment or surface machining before rolling was effective to get good thin plate. By means of rolling process, microstructures were refined effectively to the size of 7~10µm.

Abstract: The influence of various homogenizing parameters on the microstructure of ZK60 magnesium alloy ingot has been investigated. By means of metalography observation and micro-hardness measurement, the effect of annealing temperature and time on the homogenization processes were studied, which indicated that the homogenized microstructure was more sensitive to annealing temperature than to annealing time. An optimized homogenizing process 470°Cx14h was suggested to improve the deformability of ZK60 alloy.

Abstract: Direct chill (DC) casting is a dominant way of producing aluminum and copper alloy billets. In the past we developed a new technique, namely Low Frequency Electromagnetic Casting (LFEC), in which low-frequency electromagnetic field (LFEF) is incorporated to DC casting processing. In this work, LFEF was introduced to the magnesium DC casting processing. Experimental investigations and numerical simulation indicate that application of LFEF to DC casting magnesium alloy can significantly improve the surface quality of the AZ91 billets, refine microstructure and reduce both macro and micro segregations. It was also found that high casting velocity become possible for magnesium alloys through LFEC processing.