铸造镁铝钙合金的凝固行为、力学性能及半固态成形性能

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	铸造镁铝钙合金的凝固行为、力学性能及半固态成形性能
	梁松茂
学位类型	博士
导师	韩恩厚 ; 陈荣石
	2010
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	Mg－al－ca合金凝固路径力学性能半固态成形热力学计算
摘要	"本论文系统地研究了几组Mg－Al－Ca合金的凝固行为，第二相形成规律；显微组织与力学性能关系以及半固态温度下的显微组织演变规律和变形行为，并利用热力学模型对适合半固态成形工艺的Mg－Al－Ca合金进行成分预测。研究结果将为发展新的Mg－Al－Ca合金体系及成形工艺提供基础。利用冷却曲线热分析的方法，结合SEM/EDAX和X射线分析，研究了两组Mg－Al－Ca系合金(AZ91＋Ca和AM50＋Ca合金，分别简称为AZX91x和AMX50x合金)的凝固路径和显微组织形成规律。热分析结果表明，随着Ca含量的增加，AZX91x合金和AMX50x合金的液相线温度逐渐降低，而固相线温度的影响则各不相同；随着Ca含量的增加和Ca/Al比的提高，含Ca相由Al2Ca相转变为(Mg,Al)2Ca相和Mg2Ca相。利用偏振光彩色蚀刻对Mg-5wt.%Al-xCa (x = 0~4wt.%)合金的晶粒尺寸表征发现，增加Ca含量使得合金晶粒尺寸粗化，但是枝晶臂间距变小。分析研究了金属型重力铸造和压力铸造条件下Mg－Al－Ca合金的显微组织和力学性能。在金属型重力铸造条件下，AMX50x合金中形成粗大的含Ca相，其伸长率和抗拉强度都随Ca含量的增加而显著降低，屈服强度则先降低，然后稍微升高。热处理能够球化含Ca相，从而明显提高合金的强度和延伸率。(Mg,Al)2Ca相的强化效果要优于Al2Ca。在压力铸造条件下，AZX912合金的强度和伸长率均高于AZX911合金。MC-HPDC是将先进剪切熔体处理技术(MCAST)与常规HPDC相结合的成形工艺。能够得到组织均匀，铸造缺陷少的铸件，因此力学性能要高于常规HPDC铸件。研究了应变诱发熔化激活法(SIMA) Mg－Al－Ca合金在固液两相区显微组织演变规律和变形行为。结果表明，挤压后AZX911合金在515°C形成了球形固相颗粒被液相包围的典型半固态浆料组织，而AZX912合金在530°C才能得到这样的组织。对两种合金的半固态压缩变形分析表明，液相在半固态压缩变形过程中能够润湿晶界，有助于晶粒之间的相对滑动。虽然AZX912合金530°C下的液相分数高，但是液相中的固相Al2Ca相使得润滑作用减弱，所以其压缩变形应力仍然高于515°C 下AZX911合金。利用等通道挤压(ECAE)与二次重熔结合是一种新的SIMA方法。经过2道次ECAE加工的Mg－9wt.%Al合金在加热至半固态温度保温后能够得到细小球形固相颗粒分布在液相中的半固态浆料组织。同时发现，ECAE前的固溶处理使得半固态浆料中固相颗粒粗化速率加快，并且其中存在大量液相孤岛，不利于半固态成形。最后利用热力学计算软件Pandat，结合近期发表的Mg－Al－Ca热力学数据，采用两步凝固热力学模型，选择液相分数的温度敏感性，凝固温度区间，最高拐点等准则作为评价Mg－Al－Ca合金的半固态成型性能的参考标准，在较宽成分范围内对合金的半固态成型性能进行了分析预测。得出了适合进行半固态成形的Mg－Al－Ca合金的成分范围，分析结果表明AX73和AX74合金的半固态成形性能可以和A356相媲美，远远优于AZ91合金。"
其他摘要	"In this dissertation, we systematically investigated the solidification behaviour, phase formation rules, mechanical properties of some Mg-Al-Ca alloys, and then studied some properties which are concerned with the semisolid forming of Mg-Al-Ca alloys based on experiments and thermodynamic calculation. Based on these studies, some guidance and instructions on the development of new Mg-Al-Ca alloy systems and forming processes are expected to be obtained. By using Computer-Aided Cooling Curve Analysis (CA-CCA) system, together with SEM and XRD analysis, the solidification pathways and the phase formation rules of two groups of Mg-Al-Ca alloys (AZ91+Ca alloys and AM50+Ca alloys, named as AZX91x and AMX50x alloys respectively) have been obtained. The results show Ca addition decreases the liquidus temperature of Mg-Al alloys, but influences the solidus temperature in a more complex way. With increasing the Ca content and Ca/Al ratio, the Ca containing phases transform from Al2Ca to (Mg,Al)2Ca and Mg2Ca. The investigation on the effect of Ca on the grain size of Mg-5wt.%Al alloys shows that an increase in calcium content refines the dendrite arm spacing but coarsen the grain size of the alloys. The microstructure and mechanical properties of Mg-Al-Ca alloys processed by permanent mould cast and high pressure die cast (HPDC) have been investigated. For permanent mould cast AMX50x alloys, the ultimate tensile strength (UTS) and elongation of alloys decrease dramatically with the increase of calcium content, but the effect on proof tensile strength is much less significant. Heat treatment can spheroidize Ca containing phases, thus increase the UTS and elongation of the alloys. The (Mg,Al)2Ca phase has better strengthening effect than Al2Ca phase. MC-HPDC, which combines the Melt Conditioning by Advanced Shearing Technology (MCAST) and traditional HPDC, has been employed to reduce casting defects and thus improve the mechanical properties of AZX91x alloys. Unlike permanent mould casting, in both HPDC and MC-HPDC, AZX912 alloy has slightly higher tensile strength and elongation than AZX911 alloy. The microstructure evolution and deformation behaviour of extruded AZX911 and AZX912 alloys in the semi-solid state have been investigated. Partial remelting of extruded AZX911 alloys leads to a globular structure of the solid phase at 515°C whereas a temperature of 530°C is required for AZX912 alloy to obtain this kind of structure. Compression tests allowed to characterize the stress-strain behaviors at various temperatures and strain rates and it is shown that even at 530°C, the AZX912 alloy requires larger stresses for deformation than the AZX911 alloy at 515°C. Two passes ECAE is sufficient to generate enough plastic deformation for the following remelting to obtain a semi-solid microstructure with spherical solid particles. The solution heat treatment before ECAE process homogenized the distribution of solute element as well as resolved the Mg17Al12 phases, thus the obtained semisolid microstructure with more entrapped liquid and faster solid particle growth rate, which are detrimental to SSM processability. A two-stage thermodynamic model has been used to investigate the effects of compositional variations in the Mg-rich corner of Mg-Al-Ca ternary system on the semi-solid formability parameters, such as temperature sensitivity of liquid fraction, solidification interval, and highest knee points on liquid fraction vs. temperature curves. The criteria for semi-solid formability are identified and a range of compositions for Mg-Al-Ca alloys are evaluated in relation to these criteria. By comparing with A356 alloy and AZ91 alloy, a typical Mg-Al-Ca alloy AX74, is predicted to exhibit good semi-solid formability comparable to A356 and much better than AZ91."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64154
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	梁松茂. 铸造镁铝钙合金的凝固行为、力学性能及半固态成形性能[D]. 北京. 中国科学院金属研究所,2010.