挤压铸造Mg-12Zn-4Al-0.5Ca合金组织与性能研究

发布时间：2018-06-30 08:11

本文选题：Mg-Zn-Al-Ca + 挤压铸造　；参考：《上海交通大学》2015年硕士论文

【摘要】：目前,工业上应用较多的镁合金主要是Mg-Al和Mg-RE系合金。Mg-Al系镁合金成本较低,同时具有良好的室温性能,但其组织中含有低熔点共晶相Mg17Al12,因此耐热性能较差。Mg-RE系镁合金由于高强耐热,在航空、航天等国防领域得到了一定应用,但其成本高昂,很难在汽车、电子等民用领域大规模推广。Mg-Zn-Al系合金是上世纪70年代开发的一种新型镁合金,其高温性能优于Mg-Al系合金,成本又远低于Mg-RE系合金,是极具潜力的低成本耐热镁合金。但是,该系合金铸造成形过程中热裂倾向严重,极大限制了其工业应用。液态挤压铸造、半固态流变挤压铸造既能细化合金组织,提高力学性能,又可以显著降低合金热裂倾向,其在Mg-Zn-Al系合金中的应用,可以极大推动这种低成本耐热镁合金的发展。本文以自主开发的Mg-12Zn-4Al-0.5Ca(ZAX12405)合金为研究对象,采用液态挤压铸造和半固态流变挤压铸造两种方法对该合金进行成形。首先,系统研究了液态挤压铸造工艺参数对ZAX12405合金微观组织和力学性能的影响规律,结果发现:增加压力会带来合金组织致密度的显著提升以及一定程度上的晶粒细化;浇注温度的降低使合金微观组织得到细化,但同时合金的流动性也会下降,进而导致凝固组织中的缩孔率增加、致密度下降;在合金凝固结束前延长保压时间对提升组织致密度、实现晶粒细化均有利,一旦凝固结束,继续延长保压时间无明显作用。在此基础上,获得了优化的液态挤压铸造工艺参数:压力120MPa,浇注温度650°C,保压时间30s。该工艺下合金的室温抗拉强度、屈服强度和延伸率分别达到211MPa、113MPa和5.2%,相比于重力铸造ZAX12405合金,其室温抗拉强度和延伸率分别提升了40%和300%。其次,通过气体搅拌法制备出高质量的ZAX12405合金半固态浆料,得到了优化的制浆工艺参数(浇注温度为580°C,吹气流量为8L/min),并揭示了ZAX12405合金制浆过程中非枝晶组织的形成机制。最后,对气体搅拌法制备的ZAX12405合金浆料进行了流变挤压铸造成形。同时,对比分析了重力铸造、液态挤压铸造和流变挤压铸造ZAX12405合金的室温、高温力学性能。研究发现:相比于液态挤压铸造,半固态流变挤压铸造ZAX12405合金的室温和高温力学性能都有进一步提高,室温抗拉强度、屈服强度和延伸率分别达到223MPa、113MPa和7.2%,200°C时抗拉强度和延伸率达到134MPa和38.1%。相较于其他两种成形方式,半固态流变成形合金力学性能提升的主要原因是:α-Mg晶粒细化和圆整化,以及第二相组织细化与分布弥散化。
[Abstract]:At present, Mg-Al and Mg-RE series magnesium alloys have low cost and good room temperature properties. But its microstructure contains low melting point eutectic phase Mg17Al12, so the heat resistance of mg RE magnesium alloy is poor, because of its high strength and heat resistance, it has been used in aviation, aerospace and other national defense fields, but its cost is too high to be used in automobiles. Mg-Zn-Al alloy is a new type of magnesium alloy developed in 1970s. Its high temperature property is better than that of Mg-Al alloy, and the cost is much lower than that of Mg-RE alloy. It is a potential low cost heat-resistant magnesium alloy. However, the hot cracking tendency of the alloy during casting is very serious, which greatly limits its industrial application. Liquid squeeze casting and semi-solid rheological squeeze casting can not only refine the microstructure of the alloy, improve the mechanical properties, but also significantly reduce the hot cracking tendency of the alloy. Its application in Mg-Zn-Al alloy can greatly promote the development of this kind of low cost heat-resistant magnesium alloy. In this paper, the Mg-12Zn-4Al-0.5Ca (ZAX12405) alloy, which was developed by ourselves, was formed by liquid squeeze casting and semi-solid rheological squeeze casting. Firstly, the effect of the process parameters of liquid squeeze casting on the microstructure and mechanical properties of ZAX12405 alloy was systematically studied. With the decrease of pouring temperature, the microstructure of the alloy will be refined, but at the same time, the fluidity of the alloy will also decrease, which will lead to the increase of shrinkage porosity and the decrease of density in solidified microstructure. Prolonging the holding time before solidification is beneficial to increase the density of microstructure and realize grain refinement, but once solidification is over, it has no obvious effect on prolonging the holding time. On this basis, the optimized technological parameters of liquid squeeze casting are obtained: pressure 120 MPA, pouring temperature 650 掳C, holding pressure time 30 s. The tensile strength, yield strength and elongation of the alloy at room temperature are up to 211 MPA and 5.2 MPA, respectively. Compared with the gravity cast ZAX12405 alloy, the tensile strength and elongation at room temperature are increased by 40% and 300%, respectively. Secondly, the high quality ZAX12405 alloy semi-solid slurry was prepared by gas stirring method, and the optimized pulping process parameters (pouring temperature 580 掳C, blowing flow rate 8 L / min) were obtained, and the formation mechanism of non-dendritic structure in ZAX12405 alloy pulping process was revealed. Finally, the Zax 12405 alloy slurry prepared by gas stirring method was formed by rheological squeeze casting. At the same time, the mechanical properties of ZAX12405 alloy in gravity casting, liquid squeeze casting and rheological squeeze casting at room temperature and high temperature were compared and analyzed. It is found that compared with liquid squeeze casting, the mechanical properties of ZAX12405 alloy in semi-solid rheological squeeze casting have been further improved at room temperature and high temperature, and the tensile strength at room temperature has been improved. When the yield strength and elongation reached 223 MPA and 7.2 掳C, respectively, the tensile strength and elongation reached 134 MPA and 38.1 MPA, respectively. Compared with the other two kinds of forming methods, the main reasons for the improvement of mechanical properties of the semi-solid rheological alloy are: 伪 -Mg grain refinement and rounding, and the second phase microstructure refinement and distribution dispersion.
【学位授予单位】：上海交通大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG249.2;TG146.22

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