【作者】 吴纪红；

【导师】 曹占义；

【作者基本信息】 吉林大学 ， 材料学， 2007， 硕士

【摘要】 采用应变诱发熔化激活法(SIMA)制备AZ91D半固态坯料。对半固态制备过程中的主要工艺参数进行了研究,依次探讨了浇铸模具材料、预变形量及等温处理温度和时间对半固态球状晶尺寸及固-液相比等的影响规律,并分别优化给出了适合AZ91D镁合金半固态坯料制备的等温处理工艺参数,在此基础上研究了各工艺参数对半固态AZ91D镁合金组织中各相显微硬度的影响规律。并且从SIMA法制备半固态工艺过程出发,研究了半固态球状晶形成的机理。更多还原

【Abstract】 Magnesium alloys are the lightest structure materials at present, which have a lot of excellent properties. The utilization of magnesium has penetrated the fields of engineering components in the automotive, spaceflight, computer industry, etc. However, they have some unacceptable disadvantages because magnesium alloy components are 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.The semisolid forming (SSF) has become one of the hotspots in the field of metal processing in recent years. There are a lot of methods to get the semisolid billets, such as mechanical stirring, Magnetohydrodynamic stirring(MHD),strain-induced melt activation process(SIMA)and Pray Deposition(PD). SIMA method omits the procedure of molten metal treatment and the metal is partial remelted so that the inflammation of the magnesium is avoided. SIMA is considered as a net-shape process in which the basic extruded or rolled bars are subjected to additional deformation to accumulate a large quantity of deformation energy to induce sufficient strain, then heated to the semisolid state to transform the dendritic structure to develop a fine, uniform and spherical microstructure.The forming mechanism of the semisolid globular grains by SIMA method has been studied in this paper. The technological parameters, such as the material of the die, predeformation and isothermal heat treatment, were studied to establish the processing criteria, and affirmed the importance of the recovery and recrystallization during the forming of the globular grains. In addition the micro-hardness was tested. The main researches were following:(1) There are a lot of influencing factors in SIMA process. The die materials can influence the as-cast structure and the semisolid structure of the AZ91D magnesium. Predeformation and isothermal heat treatments are the main kinetic forces of the globular grains’forming. In addition they can effect on the solid fraction, as well as the shape , size and size distribution of the particles.It is discovered that the effects of the die materials on the as-cast structures were more obviously than the semisolid structure. When using the copper die, the cooling rate was higher so that the dendrites were fine, comparing with the as-cast structure gained by iron die. The semisolid structures had few differences between them after SIMA process. It is evident that the recrystallization mechanism was more important than the dendrite breakage mechanism in process of the forming of the globular grains.The spheroidization degree and size of the attained globular particles, 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 chosen 560℃~570℃and 25min-35min.(2) The effects of the technological parameters on the micro-hardness of the AZ91D alloys fabricated by SIMA.The micro-hardness value of AZ91D magnesium was increased after predeformation. And the effects of the predeformation were larger on the eutectic phase than theα-Mg phase so that the micro-hardness of the eutectic phase increased more obviously. And most of the energy produced by the predeformation exited here. The micro-hardness varied with the various isothermal heat treatment temperature and time. With the increase of the temperature, the values of the micro-hardness decreased at first, and then increased. When prolonging the time, the micro-hardness value of the field became more and more up to a maximal value at the initial stage and then decreased. If the time was long enough, the value of the micro-hardness would decrease in the final. These changes mainly depend on the evolution mechanism of the microstructure.(3) Mechanism of globular grains formation in AZ91D alloy fabricated by SIMA.The recrystallization is the main mechanism for the formation of the globular grains in the AZ91D alloy. The systemic internal energy of the AZ91D alloy increased because of the predeformation. The recrystallization occurred during the sequent heat treatment. The recrystal grains became isolated and surrounded by the increasing liquid phase. The isolated 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. The size of the particles changes according to the following formula at the range of the solid-liquid temperature:The analysis provides the evolutional rules in size of the semisolid grains during the semisolid isothermal heat treatment. The particles with the size below the average value would increasingly melt and ultimately vanish. On the contrary, the particles with the size above the average value would increasingly grow. All above ultimately resulted in the reduction of the semisolid globular particles.更多还原