【作者】 夏明许；

【导师】 袁森；

【作者基本信息】 西安理工大学 ， 材料加工工程， 2003， 硕士

【摘要】 镁合金作为21世纪的绿色产业，已经引起国内外的广泛关注。有关镁合金半固态成形技术的研究方兴未艾。而SIMA法制备半固态镁合金具有独特的优势。 本文主要研究SIMA法制备镁合金半固态坯料工艺过程中二次加热熔化激活的工艺参数及组织球化机制。首先对经过挤压形变的AZ91D镁合金分别进行连续加热及等温保温，然后水淬快速激冷固定其高温瞬时组织。定量测试晶粒的等积圆直径和球化度等形貌参数，比较形变组织的半固态球化效果，采用金相分析、XRD、TEM、EDAX等方法观测组织演变过程特点，并考察第二相及微区成分的变化。通过对半固态再结晶和半固态熔化过程中的组织转变、晶界蜂巢及晶内液池现象的分析，结合相变热力学和动力学原理，提出SIMA法制备镁合金半固态球状化组织的形成机制及工艺参数。研究结果表明： 1、等温温度和保温时间是形变镁合金组织球状化的两个重要控制因素。随着保温温度的升高，获得半固态球状组织所需的等温时间逐步缩短。在相同的加热速度条件下，560℃保温5min的工艺效果与570℃保温4min、580℃保温2min基本相当。 2、相同等温温度条件下，改变等温时间，等积圆直径的分布随时间的延长趋向均匀，晶粒球化度接近于正态分布。平均等积圆直径由大变小再变大再变小，呈波浪状变化；而晶粒平均球化度则由小变大再变小，呈抛物线状。 3、相同保温时间条件下，改变等温温度，等积圆直径大的晶粒随温度的升高而增加，球状化程度高的晶粒数增多，平均等积圆直径和平均球化度同时增大。而等温温度对最终球化组织的平均等积圆直径和平均球化度的影响呈抛物线关系。 4、连续升温和等温保温过程中再结晶形成等轴晶，为半固态晶粒球状化提供了必 西安理工大学硕士学位论文要的组织条件．．保温温度升高和连续加热速度加诀都会使半固态再结晶的速度提高。5、形变组织半固态保温过程中伴随着局部高能区的熔化，表现为晶界蜂巢状结构和晶内液池现象，这是半固态合金保持固相特征并具有触变性的基本条件。6、半固态晶粒球状化的基本原因是晶界熔化，其控制因素是因液界面处原子扩散迁移的速度和界面曲率。过程机制为：挤压形变组织在加热过程中首先发生再结晶长大变为颗粒状等轴晶；随温度升高和保温时间延长，等轴晶发生局部熔化，液固界面的曲率过热使晶粒外形向球状转变。更多还原

【Abstract】 Magnesium alloy, as one of the green industry in the 21st century, had caught more and more international researchers’ attention. The technology of the magnesium alloy’s semi-solid processing was studying. Moreover the fabrication of the semi-solid magnesium alloy by SIMA had the unique advantage.The reheating parameters of the fabrication and the mechanism of the globalization were investigated systematically in this article. The extruded AZ91D alloy was heated continuously and soaked at constant temperature .Then it was quenched in the water and the instant texture at high temperature was acquired. The equal-grain-area-circle diameter and the roundness were defined to scale the form factor and quantified the degree of the grain’s globalization. Metallographic analysis, XRD, TEM and EDAX were expired to observe the evolution of the microstructure, the changing of the phase and the changing of the component. Through the analysis of the evolution of the microstructure, the honey comb at the grain boundary and the pool in the grain, furthermore according to the phase change thermodynamic and kinetics, the conclusions of the globalization mechanism and the reheating parameters were drawn as follows.1. When the preformed Magnesium alloy’s texture was globalizing, there were two important control factors, which were holding temperature and holding time, affecting the globalization processing. With the holding temperature going up, the globalization velocity accelerated gradually. At the same heating velocity, the result of holding 5min at 560 X? was the same as holding 4min at 570 癈 and as holding 2.5min at 580"C.Ill2,XI’AN UNIVERSITY OF TECHNOLOGY2. At the same holding temperature, when the holding time increased, the equal-area-circle diameter trended to uniform, and the distribution of the roundness approached to Gaussian distribution. The average of the equal-area-circle diameter was changed from long to short and then to long, to short at last, as a wave. The average of the roundness was changed from low to high and to low again, as para-curve.3. At the same holding time, when the holding temperature changed, the number of the long equal-area-circle diameter grain increased, and the average of the equal-area-circle diameter lengthened, the average of the roundness heightened. The relationship between the holding temperature and the average of the equal-area-circle diameter or the average of the roundness was parabolic.4. After continuously heated and isothermally soaked, the equiaxed grain, as the result of the recrystallization, was the necessary texture condition of the semi-solid grain’s globalization processing. The semi-solid recrystallization velocity was run-up when the holding temperature risen and the heating velocity accelerated.5. When it was soaked in the semi-solid temperature, the preformed texture was accompanied with the part high-energy-area melting. Its presentations were the honey combs at the grain boundary and the pools in the grain. It was the essence condition that the semi-solid metal maintained the solid character and had the thixotropy.6. The melting of the grain boundary is the reason of the semi-solid grain globalization, and its control factors were the atomic diffusion velocity and the liquid-solid interface curvature. The globalization course was that the preformed texture recrystallized and became the granular equiaxed grain, when the temperature rose and the holding time prolonged, the granular grain melted at some area and at the liquid-solid interface the grain globalized at the role of the curvature overheated.更多还原