Sn对Mg-Al-Ca系合金热裂敏感性的影响

发布时间：2018-11-04 11:06

【摘要】：镁合金如今被公认为是轻型的环保金属材料,是因为其具有较低的密度、较高的比刚度及良好的铸造性能。但由于镁合金的凝固区间较宽,在浇注的过程中容易有热裂纹生成从而降低了合金本身的性能,限制了它在重工业方面的发展,因此对镁合金热裂敏感性的研究有着重要的意义。为了研究Sn对Mg-Al-Ca系合金热裂敏感性的影响,通过Clyne-Davies模型对MgAl_7Ca_1Snx(x=0,0.5,1,2)系合金的热裂倾向性进行预测。采用“T”型热裂浇注模具,通过A/D转换,用计算机对MgAl_7Ca_1Snx系合金凝固过程中铸件的温度、收缩应力信号等数据进行采集,研究其断口处裂纹萌生和扩展的相关信息。利用X射线衍射(XRD)和扫描电子显微镜(SEM)对MgAl_7Ca_1Snx系合金的显微组织和热裂断口区域的形貌进行观察。揭示合金凝固末期的残余液相分数、脆弱区域的凝固温度变化、第二相种类等因素对MgAl_7Ca_1Snx系合金热裂倾向的影响。将MgAl_7Ca_1合金的研究结果与常用的铸造合金AZ91的热裂行为进行了对比。实验结果表明:合金热裂倾向从大到小顺序为MgAl_7Ca_1Sn1MgAl_7Ca_1Sn2MgAl_7Ca_1Sn0.5MgAl_7Ca_1AZ91。由于MgAl_7Ca_1Sn1合金在所有合金中其凝固温度区间最宽、脆弱区域的凝固温度变化最大、凝固末期形成的液膜最少、枝晶干涉点后析出的第二相阻碍枝晶间的补缩等多种原因使之热裂倾向性最大。对于MgAl_7Ca_1Snx合金来说,当Sn的含量为1 wt.%时,此时生成的热裂纹最大,当Sn含量为0 wt.%时,产生的热裂纹最小。在Sn含量较少时,铸件发生热裂的机理主要是晶间搭桥理论。由于晶间桥接的强度难以抵抗铸件的凝固收缩应力,凝固收缩应力会破坏枝晶间的桥接从而使铸件产生裂纹;随着Sn含量的继续添加,铸件发生热裂的机理主要是液膜理论和凝固收缩补偿理论。此时铸件在凝固过程中生成的液膜会变厚、分离的枝晶间也将得到了充分的补缩,因此铸件没有热裂纹产生。
[Abstract]:Magnesium alloys are now recognized as light, environmentally friendly metal materials because of their low density, high specific stiffness and good casting properties. However, due to the wide solidification range of magnesium alloy, hot cracks are easy to occur in the casting process, which reduces the properties of the alloy itself and limits its development in heavy industry. Therefore, it is of great significance to study the hot cracking sensitivity of magnesium alloys. In order to study the effect of Sn on the hot cracking susceptibility of Mg-Al-Ca series alloys, the hot cracking tendency of MgAl_7Ca_1Snx (x0 0 0. 5 1 / 2) series alloys was predicted by Clyne-Davies model. By means of "T" hot cracking casting mould and A / D conversion, the data of temperature and shrinkage stress of MgAl_7Ca_1Snx alloy during solidification were collected by computer. The information of crack initiation and propagation at the fracture surface is studied. The microstructure of MgAl_7Ca_1Snx alloy and the morphology of hot crack fracture region were observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of residual liquid fraction at the end of solidification, the change of solidification temperature in the fragile region and the type of the second phase on the hot cracking tendency of MgAl_7Ca_1Snx alloy were revealed. The hot cracking behavior of MgAl_7Ca_1 alloy was compared with that of common cast alloy AZ91. The experimental results show that the hot cracking tendency of the alloy is in the order of MgAl_7Ca_1Sn1MgAl_7Ca_1Sn2MgAl_7Ca_1Sn0.5MgAl_7Ca_1AZ91. from large to small. Because MgAl_7Ca_1Sn1 alloy has the widest range of solidification temperature in all alloys, the solidification temperature of fragile region is the biggest, and the liquid film formed at the end of solidification is the least. The second phase precipitated after the dendrite interference point hinders the interdendritic shrinkage and so on, which results in the greatest hot cracking tendency. For MgAl_7Ca_1Snx alloy, when the content of Sn is 1 wt.%, the thermal crack is the largest, and when the content of Sn is 0 wt.%, the thermal crack is the smallest. When the content of Sn is low, the mechanism of hot cracking of castings is mainly the theory of intergranular bypass. Because the strength of intergranular bridging is difficult to resist the solidification shrinkage stress of the casting, the solidification shrinkage stress will destroy the bridging between the dendrites and cause the casting to produce cracks. With the addition of Sn, the hot cracking mechanism of castings is mainly liquid film theory and solidification shrinkage compensation theory. In this case, the liquid film formed during the solidification process of the casting will become thicker, and the separated dendrite will be fully condensed, so there are no hot cracks in the casting.
【学位授予单位】：沈阳工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.22;TG292

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