【作者】 谢丰广；

【导师】 刘相华； 管仁国；

【作者基本信息】 东北大学 ， 材料加工工程， 2009， 博士

【摘要】 针对半固态成形技术研究现状及发展趋势,本文提出一种半固态合金的制备新方法(Vibrating Wavelike Sloping Plate Process—VWSP法),开发了相应的波浪型倾斜板振动技术。结合国家自然科学基金项目任务书,以Al-6Si-2Mg合金为研究对象,对合金在波浪型倾斜板振动作用下的动态凝固行为及其组织形成机理进行了研究,研究了工艺条件对半固态合金组织的影响规律,优化了工艺参数,对波浪型倾斜板振动法制备的半固态坯料的二次加热与触变模锻成形进行了实验研究。获得的主要工作进展如下：(1)利用自行设计的波浪型倾斜板振动装置,成功制备出半固态合金材料。将熔化的合金浇注到具有波浪型表面的倾斜板上,在金属流动过程中同时对倾斜板施加振动,通过流动与振动的混合作用,使金属发生动态形核长大,最终得到了球化率较高、组织均匀细小的半固态材料。(2)通过数值模拟与实验,得到了倾斜板长度为400mm,倾角为45°条件下,合金在倾斜板表面的温度场分布。在不同浇注温度条件下熔体从入口到出口的温降差在66℃～77℃之间,合金熔体在底部温降最快,在侧面温降次之,在上表面温降较慢,在宽度方向对称面上温降最慢。振动的波浪型倾斜板比非振动的平直的倾斜板更容易使熔体温度场保持均匀分布。(3)分析了波浪型倾斜板表面合金凝固过程中的传热与溶质分布规律。合金在斜板表面的温度从浇注入口到出口逐渐降低。由于流动与振动作用,熔体中溶质的扩散能力加强,使得熔体在斜板表面各个部位的成分均匀分布,这为球形晶的形成创造了条件。(4)揭示了波浪型倾斜板振动作用下合金熔体的动态凝固过程组织形成机理。存在三种形核机制：倾斜板大面积表面上的异质形核、均匀成分场内部熔体整体爆发形核流动与振动共同作用下晶核从倾斜板表面的游离,即晶核增殖。晶粒长大存在两种机制：直接球状长大；界面失稳后的枝晶长大,并不断破碎和球化。(5)发现了工艺参数对合金凝固组织的影响规律和制备Al-6Si-2Mg合金半固态坯料最佳工艺参数范围,即倾斜板长度为400mm,倾角为45°左右,浇注温度为660-690℃,振动频率为50～60Hz,采用正弦波形的倾斜板表面。(6)揭示了半固态合金二次加热组织演化规律,优化了二次加热工艺条件。为制备理想的半固态金属浆料,二次加热最优工艺条件为：保温温度为610℃,保温时间为60min。(7)对波浪型倾斜板振动法制备的半固态合金锭坯的变形力学特性和变形机制进行研究,获得了Al-6Si-2Mg合金半固态本构模型。半固态合金锭坯的压缩过程可分为弹性变形阶段和稳定流动变形阶段。其本构关系方程为：(8)对Al-6Si-2Mg合金半固态坯料实施触变锻造成形,触变性能优良,所得制品外观形状完整,无表面缺陷,制品尺寸符合要求,T6热处理后制品的维氏硬度达到100HV,在同等热处理条件下,比传统的A356压铸件硬度提高30%以上。更多还原

【Abstract】 According to the research and development of semisolid process at present, in this paper, a novel process, Vibrating Wavelike Sloping Plate process-VWSP, for preparing semi-solid alloy was proposed, according to the Natural Science Foundation of China, Al-6Si-2Mg alloy was selected as study object, heat and solute diffusion, solidification thermodynamics and kinetics during dynamical solidification process under action of vibrating wavelike sloping plate were studied. Fine non-dendrite microstructure formation mechanism and effects of process conditions on the microstructure of semi-solid alloy were studied, and the process conditions were optimized. The reheating and thixoforming of semisolid billet prepared by VWSP process were studied. Main work and results were described as follows:(1) Semi-solid alloy material was successfully prepared by using a self-designed vibrating wavelike sloping plate process device. When alloy melt was cast on the wavelike sloping plate, under actions of vibration and flow, alloy nucleated and growed, which finally led to fine and spherical microstructure formation.(2) Through numerical simulation and experiment, temperature distributions of alloy on the sloping plate with the length of 400mm and the sloping angle of 45°were obtained. The decreasing value of temperature from the casting mouth to the exit was in the scope of 66℃～77℃. The decreasing velocity on the melt bottom was fastest, that on the side wall of melt was secondly faster, that on the melt surface was relatively slowly, and that on the symmetrical plane of the width was most slowly. Vibrating wavelike sloping plate could more easily kept even temperature distribution of the alloy than flat sloping plate.(3) Heat and solute diffusions during solidification of the alloy were obtained. The temperature along the central line of the sloping plate decreased linearly from the casting mouth to the exit. Due to flowing and vibrating, solute diffusion in the melt was strengthened, thus it led to the uniform distributions of solute, which created good conditions for spherical grain formation.(4) The microstructure formation mechanism during dynamical solidification of semisolid alloy was revealed. Three nucleation mechanisms occured:Heterogeneous nucleation on the surface of sloping plate; Eruptive nucleation in the melt with homogenous solute distribution; Nucleus departed from the sloping plate surface under flushing and vibrating, which would cause nucleus multiplication. The growth of non-dendrite microstructure had two patterns:the first was direct globular growth; the second was dendrite growth after the interface was unstable, which was followed by dendrite breaking and spheroidization.(5) The effects of process parameters on solidification microstructures and optimal parameters for preparing semisolid Al-6Si-2Mg alloy were revealed, the sloping plate angle was 45°, the plate length was 400mm, casting temperature was between 660℃and 690℃, the vibrating frequency was between 50Hz and 60Hz, and the sloping plate was designed with sinusoid shape.(6) Microstructure evolution mechanism during remelting process of semisolid alloy was investigated, and remelting process parameters were optimized. the optimal reheating conditions was that the reheating temperature of 610℃and holding time of 60min.(7) The deformation mechanism and mechanical properties of semi-solid alloy were studied. Deformation constitutive equations were established. The deformation process of semi-solid alloy could be divided into two stages:elasticity deformation stage and stable flow deformation stage. the deformation constitutive equations were as follows:(8) Thixo-forging of semi-solid alloy was successfully implemented, and the shape of the product was good, and the surface has no defects, the sizes of the wheel meet the demand of the object. The Vickers-hardness of the product was 100HV after T6 treatment and was improved by 30% compared with conventional casting product of A356 alloy at the same heat treatment.更多还原