摘要
汽车工业及航天航空工业的迅猛发展催生了市场对带有复杂内腔及弯曲孔道铝合金铸件的需求,致使具有高强度的树脂砂芯被大量应用于此类铸件的生产,造成了严重的环境污染；且树脂砂芯的溃散性较差、发气量偏大。水溶性型芯能够很好地解决这类问题,但其低强度和高吸湿率制约了其在生产中的应用。为此,本文在综述现有铝合金水溶性型芯优缺点的基础上,提出了制备铝合金用无机盐粘结剂耐火材料芯,系统研究了影响该水溶性型芯强度的因素和提高该型芯的强度及抗吸湿性能的方法及机理,获得了高强低吸湿的铝合金用型芯砂配方及制备工艺。
本文的主要内容如下：
1)研究了无机盐种类、原砂种类以及加热温度、加热时间等对型芯抗拉强度的影响；通过正交试验获得了优化的型芯砂基础配方及制芯工艺,其抗拉强度约0.5MPa。
2)对比了不同增强物质对1型芯的增强效果,获得了高强型芯配方。实验结果表明：高岭土、晶须及硅藻土均能显著提高型芯的强度,增强后型芯的抗拉强度度超过1.5MPa,相对于基础配方型芯的抗拉强度提高了2倍；采用复合增强后,型芯的抗拉强度可达2.5MPa,相对于基础配方型芯的抗拉强度提高了4倍。
3)分析了基础配方的粘结机理及增强配方的增强机理。在制芯过程中,包覆在砂粒表面的无机盐溶液会在砂粒表面逐渐结晶析出形成粘结桥,从而使型芯砂建立起强度；增强物质加入后改变了无机盐的结晶形态,由未增强型芯砂中的片状、块状或针状结构变为致密平滑的结构,从而使型芯砂的强度得到了大幅度的提高。
4)确定了以吸湿率为主以表面硬度及抗拉强度为辅的评价型芯抗吸湿能力的指标,并获得了型芯在一定湿度及温度下的吸湿特性方程。
5)探讨了沉淀法在提高型芯抗吸湿性能中的效果。其结果表明：以溶有氯化钙的酒精溶液处理型芯的抗吸湿性能得到显著提高。这是因为：一方面沉淀反应中反应产物的抗吸湿性能优于反应物的抗吸湿性能,另外一方面沉淀法处理型芯能屏蔽原有型芯砂粒表面无机盐粘结机的多孔表面。在RH=75～85%、T=20～25℃的环境下,沉淀法处理的型芯砂在4h内的强度损失率为15%,是未处理型芯砂强度损失率的30%。
6)获得了高强低吸湿铝合金用水溶性型芯砂配方,并分析了其水溶溃散机理。实验结果表明：该高强低吸湿配方制得的型芯砂抗拉强度达2.5Mpa,发气量仅6.2mL/g,且水溶溃散性能良好,抗吸湿性能提高了一倍；尽管碳酸钾与硅砂在600～800℃下会发生烧结,但由于烧结反应发生在硅砂与碳酸钾的界面上且烧结反应进行得不充分,因此未添加增强物质的型芯因型芯的多孔结构和碳酸钾极易溶于水的特性能够快速溃散；添加增强剂的型芯由于增强剂不会与碳酸钾反应,不会产生新的烧结,因此也具有优良的水溶溃散性能。
The rapid development of automobile and aerospace industies gives birth to the demand of alluminum castings with hollow configurations or internal channels, and results in resin sand core with high tensile strength being wildly used in manufacturing these castings, which causing serious enviromental pollution. Resin sand core also has bad colapsibility and high gas evolution. These can be settled by water soluble core, but its low strength and high hydroscopicity limits its application in practice. In the present dissertation, advantages and disadvantages of present water soluble core used in alluminum casting was summarized to develop inorganic salt refractory core. Afterward, methods and mechanisms of improving the strength and the humidity resistance of water soluble core were systematically researched. And then the process and the fomula of water soluble core with high strength and low hydroscopicity were obtained.
The main contents of this paper were as follows:
1) Influence of inorganic salts, base sands, heating temperature and heating time on tensile strength was investigated; the process and the basic fomula with the tensile strength 0.5MPa were obtained through orthogonal test optimization.
2) The strengthening effect of the cores using different strengthening material was compared and the formula of the core with high strength was obtained. The results showed that:kaolin, magnesium borate whisker and diatomite could prominently improve the strength of the core and the tensile strength of the core could exceed 1.5MPa, which was two times higher than that of the basic fomula; the tensile strength of the core achieved 2.5MPa using composite strengthening method, which was 4 times higher than that of the basic fomula.
3) Bonding mechanism and strengthening mechanism were analized. During the process of the core making, inorganic salt crystallized out to form bond briges from its solution which coated on the surface of sand and then the strength established; strengthening meterals could change the crystal morphology of the inorganic salt, turned flaky, massive and acicular structure to compact and smooth structure, which obviously improved the tensile strength.
4) Hygroscopicity was established as the main index to evaluate the humidity resistance of the cores while surface hardness and tensle strength were the supplimentary, and the moisture absorption characteristic equation of the core was obtained.
5) Humidity resistance improved by precipitation method was investigated. The results showed that:the humidity resistance of the water soluble core treated by alcoholic solution dissolved with calcium chloride was improved prominently on the account of the porous surface coated by calcium chloride and the high humidity resistance products generated in the reaction between calcium chloride and potassium carbonate; under the condition of RH=75～85%, T=20～25℃, the strength loss of the water soluble core treated by preipitation method was 15%, which was 30% of that without any treated.
6) The fomular of the water soluble core used in alluminum casting with high strength and low hygroscopicity was obtained and the mechanism of the colapsibility was investigated. The result showed that:the core made of the fomular with high strength and low hyproscopicity had high strength (2.5 Mpa), low gas evolution (6.2 mL/g), and good colapsibility and humidity resistance; potassium carbonate and silica sand could sinter at 600～800℃, but the sintering reaction was insufficiency and takes place at the solid-solid contact between potassium carbonate and silica, so cores without strengthening had good colapsibility on account of its porous structure and the property of potassium carbonate soluble in water easily; strengthening materials could not react with potassium carbonate, so there was no new sinter reaction in strengthening cores, which guaranteed that strengthening cores also had good colapsibilities.
引文
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