| The semisolid forming (SSF) has become one of the hotspots in the field of metal processing in recent years. Comparing with traditional die-casting processes, SSF has special technical advantages and wide application prospect. Therefore, the theory and technique about SSF have been received high concerns of the researchers in many countries and SSF has come into the industrial application stage in some countries. Near net-shape forming (NNF) is one of the outstanding advantages of SSF, and it can enhance the utilization efficiency of materials and save resource. In addition, SSF can reduce macro-segregation, avoid internal defects and ultimately improve the performance of the formed components. And it also can reduce the detrimental heating effect in the used molds and prolong their period of validity. Thus, SSF has been regarded as a new alternation of metal forming technology in the 21st century.Magnesium alloys have many excellent properties, such as light weight, high strength/weight ratio, and super noisy and oscillatory damping, which were preferable for the reduction of automobile weight, energy saving and environment protecting. However, they have some unacceptable disadvantages. Magnesiumalloys were very flammable, which bring the practical production into many problems. On the other hand, magnesium alloy components were usually produced by the die-casting method and molten metal fills the mold room at high velocity and high pressure, which lead the metal to entrap air and produce porosity inevitably in the casting. The inferior performance of the formed components not only reduces the finished product ratio, but also fairly restricts the application of magnesium alloys.Strain induced melt activation (SMA), as a producing method of the semisolid globular grains, is not only widely studied to attain the semisolid aluminum and magnesium materials but also is regarded as the most promising method to attain the semisolid microstructure. The combination of SMA for producing the semisolid starting materials and thixocasting for forming the magnesium casting can effectively solve the technical problems in the producing process, which exhibits an excellent prospect in the producing process of magnesium components.So far, the researches of fabricating the semisolid magnesium alloys were mainly focused on the different influence of the technical factors and the analysis of the globular grain evolution process. The large difference in the recognition lies in the globular grain forming process and mechanism. The viewpoints about the effect of predeformation on the semisolid microstructral character were not unanimous in the learning field. Although the internal and external researchers have made plentiful studies in the material field, some technical problems were still required to be thoroughly studied, such as the effect of predeformation on the globular grain size, the effect of semisolid isothermal heat treatment process on the evolution of the semisolid microstructure and the effect of primary as-cast microstructure on the formation of the semisolid globular grains, in addition with the influence elements of mechanical properties.The forming mechanism and model of fabricating the semisolid globular grains by SIMA method have been studied in this paper. The influencing factors, such as kinetics, predeformation and isothermal heat treatment, were studied to establish the processing criteria. In addition, the effect of the original as-cast microstmcture and the mechanical properties of the semisolid microstmcture fabricated by SIMA method were also studied in detail. The main researches were following: (1) Mechanism of globular grain formation in Mg alloy fabricated by SIMA The main evolutional mechanism of the semisolid globular grains was the recrystallization.dr DL 2aAVJ 1 1dt dC[{l-ko)m A// r0 rThe systemic internal energy of the AZ91D alloy increased because of the predeformation. The recrystallization occurred during the sequent heat treatment. The recrystal grains became individual grains with the increase of the liquid phase. The individual grains had different interface curves and the interface energy increased with the increase of the interface curves. The interface with high energy easily melted and increasingly resulted in the spheroidization of the solid phases, which ultimately made them become semisolid globular grains.^eutectic jtm% subgrain ^^subgrain ^-?s.liquid -^liquid islandboundary ￡*7^"fv| boundary J solute eutectic(a) (b) (c) (d) (d)Fig.l Evolutional process of semisolid microstmcture produced by SIMA method (a) Original dendritic crystal (b) Occurrence of recrystallization (c) Grain growth ofrecrystallization (d) Occurrence of Liquid phase (e) Formation of globular grains The evolutional model of the globular grains was established as illustrated in Fig. 1. The evolutional process of the semisolid globular grains were as follows: predeformation-*recovery and recrystallization-*recrystal grain combining andgrowth-* eutectic phases melting—secondary dendrite combining—solution atom diffusion along the grain boundary—numerous liquid production — subgrains separated into globular grains.The analysis provides the evolutional rules in size of the semisolid grains during the semisolid isothermal heat treatment. The grains with the size below the average value would increasingly melt and ultimately vanish. On the contrary, the grains with the size above the average value would increasingly grow. All above ultimately resulted the reduction of the semisolid globular grains.(2) Influencing factors of kinetics of globular grain formation in Mg alloy fabricated by SIMAThe influencing factors of kinetics of globular grain formation in Mg alloy were mainly comprised of predefromation and semisolid isothermal heat treatment, which not only affect the formation, morphology and distribution of the globular grains but also influence the liquid/solid fraction in the semisolid materials.Magnesium with a compacted hexagonal microstructure was not easy to deform, so the predeformation was required to be performed in the heating condition. The globular gain size associated with the heating temperature and time, and low temperature and short time accorded to fine grains under the effect of the plentiful strain energy stored in the predeformed alloys. However, low temperature and short time resulted in the fracture of the processed specimens, which was unfavorable for the sequent processing.The predeformation is one of the most important factors of kinetics of globular grain formation in Mg alloy fabricated by SIMA. When the compressive ratio was below 10%, the globular grain size is obviously reduced with the increase of the compressive ratio. The descending trend above will be weakened between 10% and 20%. When the compressive ratio surpassed 20%, the predeformation wouldhave no obvious effects on the globular grain size. Therefore, the compressive ratio should be chosen as 14%-20% in the fabrication of the semisolid microstructure by SIMA method.The spheroidization degree and size of the attained globular grains, in addition with the liquid/solid ratio in the semisolid microstructure, associated with the temperature and time of the semisolid isothermal heat treatment. The increase of the semisolid heating temperature would accelerate the evolution of the globular grains, but the excessively high temperature would result in the reduction of the solid-liquid fraction in the semisolid materials and affect the superiority of SSF. The short heating time was unfavorable for the spheroidization of the semisolid grains, but the excessively long heating time would make the semisolid grains relatively coarse and was also unfavorable for the semisolid thixocasting. According to our experimental researches, during the fabrication of the semisolid globular AZ91D microstructure, the relative optimum semisolid heating temperature and time should be respectively choosen 570 °C -575 °C and 10min-20min.(3) The effect of the original as-cast microstructure on the semisolid globularmicrostructure.The effect of the original as-cast microstructure was not negligible in the SSF process. With the increase of Al content in the original as-cast microstructure, the amount of the eutectic MgnAl^ would increase in the predeformed alloys and accelerate the evolution of the semisolid globular grains because of the large liquid/solid fraction. So the processing of the magnesium alloys with different Al content should choose different technical processes. The attained semisolid globular grain size also associated with the original as-cast grain size, and the finer former according to the finer latter. But the above effect was not obvious in SSF,and the main reason was that the evolution mechanism of fabricating the semisolid AZ91D alloys was the recrystallization mechanism. The addition of the lanthanon to the original as-cast microstructure would be favorable for the fabrication of the fine semisolid microstructure. The above function was different with the type and amount of lanthanon. The lanthanon namiparticules discretely distributed in the magnesium alloy so as to enhance the propagation of dislocations, to resist the climb and slip of dislocations and ultimately restrain the combination and growth of the recrystallization grains. In the same way, the lanthanon namiparticles also have a resistant effect on the combination and growth of the semisolid grains during the semisolid isothermal heat treatment. (4) The mechanical properties of the thixoformed magnesium alloys The mechanical properties of the thixoformed magnesium components were largely improved, such as the tensile strength improved by 38.7% and the elongation ratio improved by 140%. After T4 heat treatment, the tensile strength and elongation of the semisolid magnesium alloy respectively reached 284.5MPa and 13.8% and were improved by 51.6% and 248.11% compared with the average die-cast with metal mold. However, the thixoforming had no obvious effect on the hardness degree of the semisolid AZ9 ID alloys.The size of the semisolid globular grains had an inherited effect on the tensile strength and elongation of the thixoformed semisolid AZ91D alloys. The corresponding formula was as follows: 0b =421.21—2.527d and 6=22.947— 0.183d.
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