Materials Science Forum Vols. 488-489

Abstract: The demand for magnesium alloys has increased significantly during the past decade in the automotive and electronic industries where weight reduction becomes increasingly an important issue. At present, high-pressure die casting (HPDC) is a dominant process in production of magnesium alloy components. However, magnesium alloy components produced by HPDC suffer from porosity problem and this limits the enhancement of mechanical properties through subsequent heat treatments. The semi-solid processing (SSP) is an emerging new technology for near-net shape production of engineering components, in which the alloys are processed in the temperature range where the liquid and solid phases coexist. The SSP has various advantages over the conventional casting processes. It offers the castings with high integrity and less porosity and allows subsequent heat treatments for enhancement of mechanical properties. For these advantages, the SSP of magnesium alloys has received increasing attention in recent years. In the present study, the continuous casting process was developed for the production of magnesium billets for the subsequent SSP. The process utilizes an electromagnetic stirring system in order to obtain desired microstructure with an excellent degree of homogeneity in both microstructure and composition. Prototypes of an air conditioner cover and a telescope housing were produced using the SSP of the continuously cast magnesium alloy billets.

Abstract: Magnesium has the lowest density among the commercially available structural metals and its superior physical and mechanical properties make magnesium alloys extremely attractive for applications requiring lightweight, good castability and damping capacity. In recent years, due to the demand for lightweight magnesium components in the automobile and electronic industries, the world market for magnesium alloys has increased rapidly. At the same time, the research activities on manufacturing process and characterization of magnesium alloys have also increased significantly. The research activities have been concentrated on many different fields, including the development of high pressure die casting process and semi-solid processing, development of high strength wrought alloys and components, and characterization of microstructure, mechanical properties as well as corrosion properties. In this article, the recent progresses achieved in these research areas at Seoul National University will be introduced.

Abstract: Research on rheology of semisolid slurries suggests that an ideal semisolid slurry for semisolid metal (SSM) processing is one in which a suitable volume fraction of fine and spherical particles dispersed uniformly in a liquid matrix. Such ideal semisolid slurry can be obtained by enhancing the effective nucleation and promoting spherical growth during solidification. Experimental investigation and theoretical analysis of solidification under forced convection allowed us to identify the conditions for achieving enhanced effective nucleation and promotion of spherical growth. Such conditions can be summarised as uniform temperature and chemistry throughout the whole volume of liquid alloy, high shear rate and high intensity of turbulence during the solidification process. Based on such understandings, a new SSM processing technology, rheodiecasting (RDC), has been developed for the production of components with high integrity. AZ91D Mg-alloy was used to optimise the rheo-diecasting process and component production trials. The experimental results indicate that the rheo-diecast samples have close-to-zero porosity, fine and uniform microstructure and much improved strength and ductility. Rheo-diecasting process is particularly suitable for production of high-safety, airtight and highly stressed components in the automotive industry.

Abstract: Recent research and development status of wrought magnesium alloys in China is reviewed, and more attentions are paid to structure controlling, plastic processing, welding, surface treatment and product application. The control of alloy phases, grain refinement, deforming behavior and twin-roll strip casting process are discussed. The existing problems on the research and development of wrought magnesium alloys in China are analyzed.

Abstract: Twin roll casting has been used to produce AZ31 magnesium alloy strip. The size of the cast strip is 5mm thick and 250mm wide. The microstructure of the as-cast AZ31 alloy strip consists of columnar zones from the surface to mid-thickness region and equiaxed zones in the mid-thickness region. The secondary dendrite arm spacing varies from 5.0 to 6.5 μm through the thickness and width of the strip and is much smaller than 34μm of an AZ31 alloy DC ingot used for comparison. In the temperature ranging from room temperature to 400°C, the tensile strength and elongation of the as-cast strip are higher and smaller than those of DC ingot, respectively. Elongation is evidently improved by homogenization treatment and becomes more than that of the DC ingot. Such an improvement of elongation is though to be caused by homogenization of segregation of Al and Zn atoms in the dendrite cell boundaries. The non-homogenized and homogenized strips were subsequently hot rolled. Their hot rolled sheets have much higher value of elongation with equivalent strength, compared to the hot rolled sheet of flat-bar extruded from DC ingot in the temperature ranging from 200°C to 300°C.

Abstract: Magnesium has a hexagonal close-packed crystal structure which makes it more difficult to deform than aluminum. Conventional strip production usually requires several process steps to reach the final strip thickness. Strip casting can reduce some of the process steps and make strip processing simpler and easier, especially for alloys with poor deformability. The twin-roll casting process can directly obtain strips with thickness less than one or two millimeters. In this paper, the metallurgical characteristics of the twin-roll strip casting process were analyzed and discussed. A laboratory scale vertical twin roll caster in prepared AZ31 magnesium alloy strips, with 1.0 to 2.0 mm thick and 150mm wide. Process stability in the thin strip casting process of the alloy has been studied, the casting temperature (superheat) was proven to be a key factor influencing process stability and casting strip quality. The as-cast microstructure of the alloy was analyzed and evaluated by optical microscopy, which showed that the as-cast microstructure was composed of developed dendrites when the superheat was high, and of a rosebush-like structure when the superheat was low. When the casting temperature was close to the liquid phase temperature，the as-cast microstructure became global or grainy, and the grain size of the cast strip was very small. This remarkably improved the deformability of the as-cast strip.

Abstract: AZ31 and experimental ZMA611 alloys were strip cast into 2 mm thick strips. The as-cast AZ31 alloy strip consists of columnar dendrites. On the other hand, as-cast ZMA611 alloy strip shows equiaxed dendritic structure through the thickness of strip. Strip cast AZ31 alloy in H24 condition has equivalent yield and tensile strengths with similar ductility compared to commercial ingot cast AZ31-H24 alloy, indicating that strip casting is a viable process for the fabrication of Mg alloy strips. The ZMA611 alloy has a large volume fraction of fine dispersoid particles in the microstructure, resulting from the beneficial effect of strip casting on microstructural refinement. It has been shown that the ZMA611 alloy has superior tensile properties compared to commercial ingot cast AZ31-H24 alloy, suggesting the possibility of the development of new wrought Mg alloy sheets by strip casting.

Abstract: This paper analyzed the effect of deformation process of wrought magnesium alloys on the mechanical properties. The rolling technology and process key technology for Mg sheet metal are discussed. The technical flows of rolling processes are confirmed.

Abstract: Strip continuous casting by twin roll method is a rapidly solidifying process, which can directly produce thin strips,reduce energy consumption and product cost of various metal materials. However, the casting of magnesium alloys with large solidifying ranges lead to some problems such as surface and internal defect, variation of solidification finish point for Mg sheet. In this research, the process conditions for a twin roll caster are analyzed by thermal flow simulation. The effect of type and dimension of delivery nozzles on production of Mg alloy are studied. A proper match of the setback distance of thin nozzle and casting speed is studied by thermal flow simulation, based on the characteristics of Mg melt handling, flow dynamics control and solidification.

Abstract: Conventional manufacturing processes of metal and alloys sheets are rolling. The poor formability of magnesium alloys at room temperature makes rolling difficult. In the present paper, a manufacturing technique of sheets by powder rolling was employed to fabricate magnesium alloy sheets. The technique consisted of roll compaction, sintering, re-rolling and annealing. Powders of Mg-3 wt% Al mixed using a global mill were roll compacted into green sheets using two counter rotating rolls to the thickness of approximately 0.60 mm. Roll compacted green sheets were sintered at 823 K in argon atmosphere. Sintered green sheets were then re-rolled at ambient temperature to approximately 0.22 mm to obtain fully dense sheets. There was no significant edge cracking observed in the process of cold rolling. It is reasonable to believe that powder rolling is a promising technique for manufacturing magnesium alloys sheets.