Al-Si合金半固态浆料制备技术及应用研究

发布时间：2018-07-16 11:38

【摘要】：近些年能源消耗和环境污染问题的日益突出,为轻量化材料铝合金在汽车方面的使用提供了广阔的空间。半固态压铸由于充型平稳,凝固收缩小,生产的铸件致密,可热处理强化等优点,被认为是突破铝合金在汽车轻量化应用方面限制的主要途径。然而这项技术目前还没有得到广泛的推广应用,主要原因在于目前的浆料制备技术难以满足高质量和高效率的生产要求。本文以常用的铝硅铸造合金A357和319s为研究对象,采用基于控制凝固技术的热平衡法浆料制备技术,以实现半固态流变压铸成形工程化应用为目的,开展了铝合金半固态浆料制备、球状晶组织演化形成机理、浆料充填行为和半固态压铸件组织与性能方面系统的研究。通过本文研究对进一步完善控制凝固技术在半固态浆料制备和推动半固态流变成形工程应用化方面具有一定的指导意义。本文主要研究的内容和取得的研究成果如下:分析了金属液在热平衡法制浆过程的冷却行为,理论推导了热平衡法过程的传热模型,推导出了浆料温度场的表达式。结果表明,在给定合金和坩埚材料的条件下,浆料温度场的分布是浇注温度和坩埚初始温度的函数,共同决定半固态浆料的微观组织。以A357合金为研究对象,研究了工艺参数对浆料组织的影响规律。研究结果表明:(1)浇注温度对浆料组织影响最大,随着浇注温度的降低,金属液在液相线附近获得较大的过冷度,随着熔体的内生流动在坩埚中形成大量的自由晶并保留下来,使浆料最终凝固组织为细小、均匀分布的球状晶组织。采用截线法测量晶粒平均尺寸,当浇注温度从690℃降到630℃时,边部晶粒平均尺寸由820μm降到248μm,中部晶粒平均尺寸由706μm降到223μm,心部晶粒平均尺寸由659μm降到186μm。(2)旋转转速对浆料组织的影响根据浇注温度而变化,当高于660℃浇注时,坩埚中的熔体具有较低的表观黏度,随着转速的增加,熔体的对流效果加强,晶粒组织明显细化;当低于645℃浇注时,坩埚中熔体的表观黏度增加,旋转过程难以产生明显的对流效果,转速的增加对浆料组织影响不明显。(3)随着旋转时间的延长,小晶粒溶解消失,大晶粒逐渐长大,以Ostwald熟化机制长大,通过LSW模型计算颗粒尺寸变化,得长大速率为22.47μm~3/s。(4)相同制浆条件下,浆料重量主要影响熔体的过冷度,浆料重量越小,浇注过程中受到的激冷效果越明显,熔体中晶核的数目增多,晶粒越细小。通过对制浆过程的分析,建立了描述热平衡法制浆过程中形核和长大的理论模型。在研究形核机制时,提出了一次形核和二次形核的概念。基于晶粒游离论和瞬态形核理论,对一次形核过程进行了分析,浇注过程的激冷作用,使熔体过冷形核,产生大量的自由晶并保存下来,这些大量晶核的存在是获得细小、均匀球状晶组织的主要原因;在二次形核过程中提出了边部冷却层和有效对流区的概念,二次形核过程中,旋转对流促进了有效对流区内枝晶臂的根部熔断,实现二次形核过程的晶粒增殖。基于过冷熔体中成分过冷对晶粒长大形貌的影响,研究了初生固相颗粒在高密度晶核、缓慢冷却和旋转对流条件下晶粒的长大机制。高密度晶核的存在减小了晶粒的长大空间和长大速率,决定了晶粒的最终尺寸,而高密度晶核温度场和成分场的叠加以及缓慢冷却和旋转对流都促进了凝固界面前沿温度场和成分场的均匀化,提高了界面的稳定性,促进晶粒沿各方向均匀生长,最终形成球状晶组织。在工程化应用方面,以319s合金为研究对象,对热平衡法半固态流变成形浆料流动机制和成形件组织与力学性能进行系统性研究:(1)通过压铸过程浆料流动稳定性的研究,建立了热平衡法制浆工艺与浆料稳定性的变化规律:浇注温度一定时,浆料的稳定性主要由固液相配比决定,表现为随着浆料温度的降低,稳定性先增强后降低;浆料温度一定时,浆料稳定性主要由初生相形貌决定,表现为随着浇注温度的降低,稳定性先增强后降低。实验中获得的319s合金浆料流动最稳定时的制浆工艺为:浇注温度630℃-640℃,浆料固相百分数35%-45%。(2)以制备高端复杂精密汽车零部件为例,对压铸件充型过程、组织和力学性能进行分析。研究结果表明:在优化后的半固态浆料制备工艺下,压铸充填过程浆料流动平稳,铸件内部可获得细小均匀分布的初生固相,铸件致密无缺陷,经热处理后铸件性能大幅提高,可满足高端复杂精密零部件的技术要求。
[Abstract]:The problems of energy consumption and environmental pollution have become increasingly prominent in recent years. It provides a broad space for the use of lightweight material aluminum alloy in automobile. Semi solid die casting is considered to be a breakthrough in the lightweight application of automobile because of its smooth filling, small solidification shrinkage, compact production of castings and heat treatment. However, this technology has not been widely used at present. The main reason is that the current slurry preparation technology is difficult to meet the requirements of high quality and high efficiency. This paper uses the commonly used aluminum silicon cast alloy A357 and 319s as the research object, and uses the heat balance method based on controlled solidification technology to prepare the slurry. In order to realize the engineering application of semi solid rheological die casting, the preparation of semi solid slurry of aluminum alloy, the formation mechanism of spheroidal crystal structure, filling behavior of the slurry and the structure and properties of semi solid die casting are studied. Through this study, the preparation and promotion of semi-solid slurry in the control and solidification technology are further perfected. The application of semi solid rheological forming engineering has some guiding significance. The main contents and achievements of this paper are as follows: the cooling behavior of the metal liquid in the process of heat balance pulping is analyzed. The heat transfer model of the heat balance process is deduced and the expression of the temperature field of the slurry is derived. Under the condition of fixed alloy and crucible material, the distribution of the temperature field of the slurry is the function of the pouring temperature and the initial temperature of the crucible. The microstructure of the semisolid slurry is determined jointly. The effect of the process parameters on the size of the slurry is studied with A357 alloy as the research object. The results show that: (1) the pouring temperature has the greatest influence on the size of the slurry. When the pouring temperature is reduced, the metal liquid obtains a large supercooling degree near the liquidus. With the melt endogenetic flow in the crucible, a large number of free crystals are formed and retained. The size of the slurry is finally solidified into fine and uniformly distributed spherulites. The average size of the grain is measured by the intercept method, and the pouring temperature is reduced from 690 to 630. The average grain size of the edge is reduced from 820 u m to 248 m, the average size of the central grain is reduced from 706 mu m to 223 m. The average grain size of the heart is reduced from 659 Mu to 186 M. (2). The influence of rotation speed on the slurry structure varies according to the pouring temperature. The melt in the crucible has a lower apparent viscosity when it is higher than 660 C. Adding, the convection effect of the melt is strengthened and the grain structure is obviously refined. When the casting is below 645 C, the apparent viscosity of the melt is increased and the rotating process is difficult to produce obvious convection effect. The effect of the rotational speed on the size of the slurry is not obvious. (3) the small grain dissolves with the prolongation of the rotation time, and the large grains grow up gradually, with Ostwald ripening. When the mechanism grew up, the size change of the particles was calculated by LSW model. Under the same pulping condition of 22.47 m~3/s. (4), the weight of the slurry mainly affected the supercooling of the melt. The smaller the weight of the slurry, the more obvious the effect of the chilling effect in the casting process, the increase of the number of the molten MICROTEK core and the finer grain. A theoretical model describing nucleation and growth in the process of heat balance pulping is established. In the study of the nucleation mechanism, the concept of the first nucleation and two nucleation is proposed. Based on the theory of grain free and transient nucleation, the process of the first nucleation is analyzed and the chilling of the casting process is used to make the melt supercooled and produce a large number of free crystals. The existence of these large nuclei is the main reason for obtaining fine and uniform spheroidal crystals. In the process of two nucleation, the concept of side cooling layer and effective convection zone is proposed. In the two nucleation process, rotating convection promotes the fusion of the root of the dendrite arm in the effective convection zone and realizes the grain proliferation in the two nucleation process. Based on the effect of component supercooling in supercooled melt on grain growth, the grain growth mechanism of primary solid particles in high density nucleation, slow cooling and rotating convection is studied. The existence of high density nucleation reduces the growth space and growth rate of grain, determines the final size of grain, and the temperature field of high density nucleation and nucleation. The superposition of the composition field, the slow cooling and the rotating convection both promote the homogenization of the temperature field and the composition field at the front of the solidification interface, improve the stability of the interface, promote the uniform growth of the grain along all directions, and eventually form the spherical crystal structure. In the engineering application, the 319s alloy is used as the research object, and the semi solid rheological forming of the heat balance method is used. The mechanism of slurry flow and the microstructure and mechanical properties of the forming parts are systematically studied. (1) through the study of the stability of the slurry flow in the process of die-casting, the change law of the stability of the slurry technology and slurry is established. When the pouring temperature is fixed, the stability of the slurry is mainly determined by the ratio of the solid to liquid phase, which shows the decrease of the slurry temperature. Stability first increases and then decreases; when the slurry temperature is fixed, the stability of the slurry is mainly determined by the primary phase appearance. With the lowering of the pouring temperature, the stability first increases and then decreases. The pulping process of the most stable flow of 319s alloy slurry in the experiment is: the pouring temperature is 630 C -640 C, and the percentage of the slurry is 35%-45%. (2). The mold filling process, microstructure and mechanical properties of the high end complex precision automobile parts are analyzed. The results show that the slurry flow in the die casting filling process is stable under the optimized semi solid slurry preparation process, and the small and uniform distribution of the primary solid can be obtained inside the casting. The casting is compact and without defect, and the casting is cast after heat treatment. The performance is greatly improved, which can meet the technical requirements of high-end complex precision parts.
【学位授予单位】：北京有色金属研究总院
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG292

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