Papers by Keyword: Non-Dendritic Structure

Abstract: Semisolid casting and non-dendritic solidification of commercially pure tin (about 1 °C freezing range) and Zamak 3 alloy (about 10 °C freezing range) by a modified serpentine channel method were studied. It was shown that semisolid casting of very small freezing range metals with a non-dendritic structure was possible using this method. The results showed that the wall of the copper serpentine channel mold acted as a substrate for heterogeneous copious nucleation of primary solid particles and the channel provided sufficient self-steering action to disperse the nuclei in the melt. The average diameter and shape factor of the primary particles in the semisolid cast CP-Sn sample was measured to be about 107 μm and 0.75, respectively. The average diameter and shape factor of the primary particles in the semisolid cast Zamak 3 alloy was measured to be about 16 μm and 0.8, respectively. Hardness of semisolid samples was slightly higher than those of conventional gravity cast samples.

Abstract: AZ91D magnesium alloy is one of the most widely used magnesium alloys in the production of metal forming, which use the characteristics from liquid state to solid state of metal to form. The present status of the research and application of the semi-solid forming for AZ91D magnesium alloys at present was reviewed in this paper, including the microstructural characteristics, the thixotropic and rheological behavior, the forming process of semi-solid for AZ91D magnesium alloys and the mechanical properties of the parts made of semi-solid magnesium alloys. The developing prospects and the key points of the semi-solid forming for AZ91D magnesium alloys were forecasted, and the industrial application of the alloy were also discussed.

Abstract: The cooling slope technique has been developed in recent years, which controls the nucleation and growth of the primary grains during solidification to achieve fine and non-dendritic microstructures. In this study, A356 Al alloys were processed through a modified cooling slope technique to obtain fine, non-dendritic microstructures, in which the cooling rate of the cast crucible was controlled. Three process parameters, namely pouring temperature, inclined slope angle, and the cooling rate of the cast crucible, were varied during the processing. The cooling slope was water-cooled with a constant water flow rate. The solid fraction and the size distributions of the primary grains along the vertical and horizontal positions of the cast ingots were measured individually. The macro-segregation was examined in terms of the distribution of the solid fraction. The yields of the ingots were calculated for studying the efficiency of the cooling slope technique. The effects of the three process parameters on the microstructures, macro-segregation, and yields were studied by the Taguchi method.

Abstract: The purpose of this study was to develop a new method for formation of non-dendritic microstructure during sand casting of aluminum alloys without any previous special processing. For this purpose, sand molds with three different types of running systems were designed. The first design included a traditional reverse tapered vertical sprue. The second design was similar to the first one except for a conical sand core placed inside the lower half portion of the sprue creating a narrow gap for the flow of the melt into the mold. In the third design, the core was fitted with metal chills. Molten A356 alloy was then cast from fully liquid state under gravity and centrifugal casting conditions in the molds and the microstructures of the castings were studied. The results showed that non-dendritic microstructure could be achieved by using the last design under low superheat temperature and centrifugal casting conditions. The findings can pave the way for small quantity production of semisolid castings in expendable molds.

Abstract: Continual improvement of product quality has been a long challenge to Semi-Solid Metal (SSM) technology. By conventional semi-solid processes, this might be attained at the expense of economical production. The advent of Inclined Cooling Plate (ICP) process has already realized the development of non-dendritic SSM while satisfying qualitative, quantitative and economical requirements collectively. In spite of its potential advantages, functional mechanisms of this process are not yet clearly understood that makes its optimal utilization obscured. Basically, such understanding needs a picture of the process. As the first step, this picture is pursued through physical modeling of the ICP process i.e. direct observation of an analog system by virtue of transparent character of a model alloy (succinonitrile-acetone). Based on this phenomenological model, a picture of the process is presented as follows: flowing molten alloy down ICP, multiple regions form typically on the plate i.e. a chilled layer at the vicinity of the plate surface, a two-phase mushy zone on the chilled layer and ambient liquid far from the plate surface. In this process, interaction of the liquid forced-flow with mushy zone separates solid particles from the stationary mush on the plate resulting in a two-phase mixture which is responsible for the formation of slurry i.e. SSM.

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.