The Research On Sim Solidification Process And Semisolid Rheo-forming Of Magnesium Alloys

Magnesium alloys present a great potential in automobile and electronic industries with the human’s increasingly strict demand on energy conservation and emission reduction and the product quality. The grain in microstructure of Mg alloys under conventional solidification is coarse and the precipitated phase is nonuniform, thus the mechanical properties and corrosion resistance of Mg alloy cannot meet the requirement of structural components; die-casting is the main shaping technology for Mg products, but the castings always involve in voids and cannot be strengthened by heat treatment. Semisolid forming uses semisolid slurry contains fine and spherical solid grains, thus the bottleneck of widely application of Mg alloy is able to solve with the development of SSF. This research focuses on the microstructure control of Mg alloy and with the objective of realizing SSF of Mg alloy. The research content contains solidification microstructure refining, non-dendritic slurry preparation, rheo-diecasting and the heat treatment of semisolid die-castings. The results of this research have guiding significance on further improvement and innovation of Mg alloy refining technology, and promoting the further application of SSF in Mg production. The main results are as follows:A novel solidification controlling method called as self-inoculation method (SIM) is developed on the basis of microstructure control theory and the features of current refining technologies, the equipment of SIM was manufactured and a series of cast experiments were carried out. Results indicate that compared to conventional solidification, SIM has favorable refining effect on grain size and the morphology of secondary phase of both AM60and AZ31alloy, thus the microstructure and composition uniformity of alloy were enhanced. The essential effect of SIM parameters on solidification process was verified:the melt treatment temperature, addition amount of self-inoculants and slope angle of fluid director collectively effect on the melt temperature at the exit of fluid director, the fine refining effect can be achieved due to the grain breeding if the above parameters were better matched. The model which describes the melting conditions of self-inoculants and the mathematic relation between different parameters of SIM were developed, which proposes a rule for the further optimizing of SIM parameters for other alloys.The research on formation mechanism and evolution process of semisolid microstructure under SIM indicate that:SIM promotes the grain multiplication in initial solidification of alloy melt, the temperature guidance was uniform in the continuous cooling stage, and the resulted microstructure is fine and spherical, thus the high grain density and uniform temperature field are the basic conditions for directly formation of spherical microstructure from alloy melt; the result from slurry preparation experiments indicate that the main parameters of SIM have similar effect on morphology of primary a-Mg, there is a wide operable parameter range of SIM thus the flexibility and operability of slurry preparation were enhanced; the analysis of nucleation and growth feature of a-Mg considered that self-inoculants create the appropriate thermodynamics dynamics conditions for heterogeneous nucleation, and the fluid director promote undercooling of bulk melt and created the free crystals; the grain grow in a uniform temperature field due to the liquid convection, the actual undercooling in front of interface was weakened thus the preferred growth was suppressed. In continuous cooling and isothermal holding of slurry, the particle separation occurred under the function of interface curvature and surface tension, the undercooling was decreased because of concentration field overlying, mergence and Ostwald ripening is the main coarsening mechanism of primary during prolonged isothermal holding.Results of rheo-diecasting experiments indicate that:the solidification behavior of semisolid slurry can be divided into three stages:dependent growth of secondary a-Mg, independent nucleation of secondary a-Mg and the final eutectic reaction. The size and shape factor of a-Mg decreases with the increase of injection speed, and the liquid segregation at the edge of castings decreases with the reducing of slurry solid fraction and the increase of injection speed. The solid fraction and particle size increases with the prolonging holding time, and the particle size and roundness reduces with the increase of holding temperature. Voids is the main defect of AM60alloy produced by RDC, reducing injection speed and increasing solid fraction are helpful for improving the density of castings; the fracture occurred along the β phase in the secondary solidification area when the defects is less, and in this condition the fracture of AZ31occurred both in and on the surface of primary phase. When the defects are frequent, the crack generated in voids and then extended along the secondary a-Mg.Results of heat treatment indicate that, the microstructure evolution of AM60during solid solution can be divided into three stages:rapidly dissolution of β phase combined with rapidly a-Mg coarsening and the normal coarsening of a-Mg particles. In the first stage, the β phase dissolve in a-Mg rapidly results in a rapid coarsening of secondary a-Mg, and at the same time the primary a-Mg grow rapidly by combining secondary particles. In the secondary stage the a-Mg particles grow with a low rate under the combination mechanism. In ageing process, the β phase precipitates from a-Mg in the form of discontinuous precipitation cell, the amount of β phase increases with the increase of ageing temperature and the morphology presents a coarsening tendency. The tensile strength and elongation increase with the prolonging of solid solution and then decreased at the excessive treatment time, the peak value of237MPa and13.9%was achieved when solid solution treated operated at430℃with16hours. The ageing strength effect of AM60was not obvious. The rheo-diecasted AZ31alloy has certain solid solution strength ability, and the peak value of mechanical properties was obtained during a shorter solid solution time compared to AM60alloy, the peak value of220MPa and14.7%was obtained when the solid solution operated at400℃withlh.