Research On Slurry Evolution And Microstructure And Properties Of Thixomolding Magnesium Alloy

Research on Slurry Evolution and Microstructure and Properties of Thixomolding Magnesium AlloyThixomolding is the unique commercial SSF used to manufacture magnesium products, the products of which have near net shape, low shrinkage, high mechanical properties and high dimensional stability. With these advantages, this technology has great applied potential. Thixomolding is used widely in developed countries but just startes in China due to foreign technology protection. In order to improve national thixomolding technology, highly sophisticated thixomolding machine with independent intellectual property rights has been designed and built by Changchun Limei Technology Development Company and Jilin University. Based on the new machine, basic theory and process of thixomolding are researched on in this paper.Solid fraction which is determined by slurry temperature is an important factor to product performance. The slurry temperature is controlled by thermocouples which are located on the outer barrel wall in this thixomolding, so the slurry temperature can not be measured directly and the solid fraction of slurry can not be calculated accurately. In order to solve the problem, the law of slurry movement and evolution in screw shear system, the heat transfer process of screw shear system, and the effect of process parameters such as barrel temperature, screw rotation speed on the microstructure and mechanical properties are researched on. Conclusions of present study as following:First of all, loose particulates model, solid plug model, molten pool model, and semisolid slurry forming model are proposed to explain the law of slurry movement and evolution in screw shear system. The process starts when particulates leave the hopper and enter the screw section through the feed throw. The particulates are not compacted, there is relative motion between particulates, and the screw channels are not full filled. During the compaction stage, the screw channels become increasingly filled with feedstock particulates. The solid plug conveyance relies on a difference in friction between the solid plug and the barrel on one side and the screw on another side. The higher friction with the barrel generates a breaking force, which contributes to a sliding of the solid plug on the screw. With the forward movement of solid plug, barrel temperature gradually rises above the melting point of AZ91D, The solid plug starts to melt. The molten film, with the thickness exceedingδ, is then scraped off and collected in the molten pool by the advancing screw side. With the reduction in the melt's viscosity, the feedstock melting is controlled by external heat, and the contribution of shear should be neglected. Since heat is supplied from outside, and the barrel has a higher temperature than the screw, melting from the barrel direction dominates. With the forward movement of feedstock, screw temperature also rises above the melting point of AZ91D, and the feedstock contacting with screw channels begin to melt. The solid plug will be broken, and then gradually formed the semi-solid slurry with a certain amount of solid particulates suspended in the liquid alloy.Solid fraction, which is an important factor to the product, is controlled by screw shear system majorly. The slurry temperature is controlled by thermocouples which are located on the outer barrel wall, so the slurry temperature can not be measured directly and the solid fraction of slurry can not be calculated accurately. The heat transfer of screw shear system is simulated with finite element software ABAQUS, and the difference of temperature between outer barrel wall and slurry is calculated when screw shear system reached steady state. When the barrel temperatures are 580°C, 590°C, 600°C, 610°C respectively, the slurry temperature calculated are 577.67°C, 586.97°C, 590.70°C, 596.59°C respectively. The experiment proved that the difference between calculated and simulated solid fraction is no more than 15%.The process parameters including barrel temperature and screw rotating speed are closely related to screw shear system. And the effect of these parameters on the microstructure and mechanical properties are discussed. The results show that typical thixomolding AZ91D microstructure includes unmelted solid phase and liquid phase. There are three kinds of unmelted solid phase with different shape, such as spherical solid particle, irregular solid particle, and solid particle with liquid pool inside. The phases of thixomolding AZ91D are a-Mg and intermetallic Mg17Al12. With the rising barrel temperature, the solid fraction decreases, and the strength and elongation of thixomolding AZ91D increase. The fracture mode is quasi-cleavage fracture, for structures with higher solid fractions, the crack penetrates preferentially between the primary solid and the matrix, for low-solid fractions, the crack penetrates along the interface between the Mg17Al12 phase and the a-Mg. With the increasing screw rotating speed, more protective gas is involved in semi-solid slurry, resulted in the increasing porosity of sample, so the strength and elongation of thixomolding AZ91D decrease. The optimized barrel temperature and screw rotation speed are 600°C, 143r/min respectively.The results of this study make great contribution to the basic theory of thixomolding, and as a theoretical guidance it could be used to optimize the screw structure of thixomolding. In practical application, through predicting the solid fraction of thixomolding magnesium alloys, the time of adjusting process parameters is reduced and the production costs are saved.