【摘要】：鎂合金如今被公認(rèn)為是輕型的環(huán)保金屬材料,是因?yàn)槠渚哂休^低的密度、較高的比剛度及良好的鑄造性能。但由于鎂合金的凝固區(qū)間較寬,在澆注的過程中容易有熱裂紋生成從而降低了合金本身的性能,限制了它在重工業(yè)方面的發(fā)展,因此對(duì)鎂合金熱裂敏感性的研究有著重要的意義。為了研究Sn對(duì)Mg-Al-Ca系合金熱裂敏感性的影響,通過Clyne-Davies模型對(duì)MgAl_7Ca_1Snx(x=0,0.5,1,2)系合金的熱裂傾向性進(jìn)行預(yù)測(cè)。采用“T”型熱裂澆注模具,通過A/D轉(zhuǎn)換,用計(jì)算機(jī)對(duì)MgAl_7Ca_1Snx系合金凝固過程中鑄件的溫度、收縮應(yīng)力信號(hào)等數(shù)據(jù)進(jìn)行采集,研究其斷口處裂紋萌生和擴(kuò)展的相關(guān)信息。利用X射線衍射(XRD)和掃描電子顯微鏡(SEM)對(duì)MgAl_7Ca_1Snx系合金的顯微組織和熱裂斷口區(qū)域的形貌進(jìn)行觀察。揭示合金凝固末期的殘余液相分?jǐn)?shù)、脆弱區(qū)域的凝固溫度變化、第二相種類等因素對(duì)MgAl_7Ca_1Snx系合金熱裂傾向的影響。將MgAl_7Ca_1合金的研究結(jié)果與常用的鑄造合金AZ91的熱裂行為進(jìn)行了對(duì)比。實(shí)驗(yàn)結(jié)果表明:合金熱裂傾向從大到小順序?yàn)镸gAl_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溫度區(qū)間最寬、脆弱區(qū)域的凝固溫度變化最大、凝固末期形成的液膜最少、枝晶干涉點(diǎn)后析出的第二相阻礙枝晶間的補(bǔ)縮等多種原因使之熱裂傾向性最大。對(duì)于MgAl_7Ca_1Snx合金來說,當(dāng)Sn的含量為1 wt.%時(shí),此時(shí)生成的熱裂紋最大,當(dāng)Sn含量為0 wt.%時(shí),產(chǎn)生的熱裂紋最小。在Sn含量較少時(shí),鑄件發(fā)生熱裂的機(jī)理主要是晶間搭橋理論。由于晶間橋接的強(qiáng)度難以抵抗鑄件的凝固收縮應(yīng)力,凝固收縮應(yīng)力會(huì)破壞枝晶間的橋接從而使鑄件產(chǎn)生裂紋;隨著Sn含量的繼續(xù)添加,鑄件發(fā)生熱裂的機(jī)理主要是液膜理論和凝固收縮補(bǔ)償理論。此時(shí)鑄件在凝固過程中生成的液膜會(huì)變厚、分離的枝晶間也將得到了充分的補(bǔ)縮,因此鑄件沒有熱裂紋產(chǎn)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.
【學(xué)位授予單位】：沈陽(yáng)工業(yè)大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TG146.22;TG292

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