镁合金熔模铸造成型工艺与组织性能研究.pdf

上海交通大学硕士学位论文 I 镁合金熔模铸造成型工艺与组织性能研究 摘 要 摘 要 镁合金熔模铸造由于存在一些未能有效解决的技术难题，如镁合金的氧化燃烧、金属-铸型的界面反应等，大大限制了镁合金熔模铸件的广泛应用本文尝试在熔模陶瓷型壳中添加不同阻燃剂，研究它们对镁合金熔模铸造的防氧阻燃效果，并观察了不同工艺条件对铸件组织性能的影响，以此寻求镁合金复杂薄壁结构件的成型工艺，获得一套有效的镁合金熔模铸造技术 通过在型壳中添加碳粉、硫铁矿粉、硼酸阻燃剂的铸造研究发现：阻燃熔模陶瓷型壳浇注出的 AZ91D 镁合金铸件表面比较光洁，无明显的浮凸产物，铸件颜色呈银白色金属光泽前两种阻燃剂主要通过浇注时产生的 CO2、SO2气体与镁液反应分别生成致密的 MgO+C、2MgO+MgS 保护膜起到防氧阻燃的作用；而硼酸则是通过硼酐在高温下溶解 MgO 形成 3MgO· B2O3釉质膜发挥作用 通过改变模壳温度、铸件壁厚条件浇注出 AZ91D 阶梯形熔模铸件的结果得出：铸型冷却条件对熔模铸件的显微组织和力学性能产生显著影响当铸件的壁厚较薄时，铸型的激冷作用使得镁液迅速冷却凝固，此时铸态组织细小均匀，相应的力学性能越高；当壁厚增大时，镁液内部热量来不及从型壳中传递出去，致使第二相组织迅速长大并连成条状甚至网状结构，最终导致力学性能下降。

通过对镁合金薄壁复杂结构件的熔模铸造成型和组织性能研究得出：重力浇注的镁合金复杂结构件容易出现充不满、缩陷、冷隔、卷气、热裂等缺陷，而低压浇注则有效地消除了这些缺陷，并得到表面光洁、尺寸精确、复型效果良好的铸件镁合金铸件经过固溶时效处理后，铸态组织中的第二相先固溶到基体中而后又弥散析出，起到了合金强化的作用，提高了铸件的力学性能此外，与复杂结构件本体取样相比，镁合金熔模单铸试样在不同状态下的组织性能与复杂结构件的变化规律基本相似，故其性能可作为同炉次浇注成型零件的有效参考 上海交通大学硕士学位论文 II 综上所述，采用镁合金熔模铸造成型薄壁复杂结构件时，应采取硼酸作阻燃剂涂灌型壳、低压浇注辅助充型、固溶时效热处理等工艺措施，从而获得结构完整、组织细小、性能优异的熔模铸件 关键词：关键词：镁合金，熔模铸造，界面反应，阻燃剂，成型工艺，组织性能 上海交通大学硕士学位论文 III STUDY ON PROCESS, MICROSTRUCTURES AND PROPERTIES OF MAGNESIUM-ALLOY INVESTMENT CASTINGS ABSTRACT Investment casting has been restricted to apply in the field of magnesium alloys, because the key problems have not been settled, such as the oxidation and ignition phenomena and metal-mold interface reaction during magnesium alloys being melting and casting. In this paper, the oxidation- and ignition-proofed effects of different agents on magnesium-alloy investment castings were investigated. And the influences of various conditions on the microstructures and properties of investment castings were also studied. Finally, an effective forming technology of the complex thin-walled components were explored.goo It was found that the surface of AZ91D investment castings was smooth and silvery-white after the additions of carbon or pyrite or boracic acid into the investment ceramic shells. The effects of carbon and pyrite flame retardants were attributable to the compact protective coatings of MgO+C and 2MgO+MgS, which formed during the reactions between magnesium melts and CO2 or SO2 gas. The boracic acid transformed into boride, which could dissolved magnesia and form 3MgO· B2O3 enamel film, and protected the casting surface at the elevated temperature. When the shell mold temperature and the cast thickness were changing, the microstructures and mechanical properties of AZ91D step-like investment castings were also changing as a result of the different cooling conditions. Magnesium melt solidified quickly at the thin wall of the castings, and fine as-cast microstructures and high mechanical properties were achieved. As the cast thickness increased, the second phases coarsed and connected with each other in as-cast microstructure, and the castings obtained poor mechanical properties consequently. The investment casting process of the complex thin-walled components was studied as well as the microstructures and properties of different magnesium-alloy castings. The defects, such as misrun, shrinkage depression, cold shut, gas inclusion, heat cracking, etc., existed in the investment castings which were poured under gravity. But with low pressure casting, these defects were eliminated efficiently, and the castings had smooth surface, accurate dimensions and precise profiles. Moreover, in all of magnesium alloys, the second phases were mostly solutionized after solid solution, and then precipitated dispersedly in the castings during ageing heat treatment. This enhanced the strength of magnesium alloys and improved the mechanical properties of the castings. In addition, the microstructures and properties of investment casting samples, which could be the potent references, were similar with the components under different states. 上海交通大学硕士学位论文 IV Ultimately, the complex thin-walled magnesium alloy components could obtain intact configuration, fine microstructures and good properties using investment casting process. But those measures should be taken during the process, namely, shell mold being affused with boracic acid, magnesium melts being poured under low pressure, castings being solid solution and ageing heat treated. KEY WORDS: magnesium alloy, investment casting, interface reaction, flame retardant, forming process, microstructure and property 上海交通大学上海交通大学 学位论文原创性声明学位论文原创性声明 本人郑重声明：所呈交的学位论文，是本人在导师的指导下，独立进行研究工作所取得的成果。

除文中已经注明引用的内容外，本论文不包含任何其他个人或集体已经发表或撰写过的作品成果 对本文的研究做出重要贡献的个人和集体，均已在文中以明确方式标明本人完全意识到本声明的法律结果由本人承担 学位论文作者签名：林志埙林志埙 日期： 年 月 日 上海交通大学上海交通大学 学位论文版权使用授权书学位论文版权使用授权书 本学位论文作者完全了解学校有关保留、使用学位论文的规定，同意学校保留并向国家有关部门或机构送交论文的复印件和电子版，允许论文被查阅和借阅 本人授权上海交通大学可以将本学位论文的全部或部分内容编入有关数据库进行检索，可以采用影印、缩印或扫描等复制手段保存和汇编本学位论文 保密保密□，在 年解密后适用本授权书 本学位论文属于 不保密不保密□ （请在以上方框内打“√√” ） 学位论文作者签名：林志埙林志埙 指导教师签名：蒋海燕蒋海燕 日期： 年 月 日 日期： 年 月 日 上海交通大学硕士学位论文 1 第一章第一章 绪论绪论 1.1 镁合金的特点与应用镁合金的特点与应用 1.1.1 镁合金的特点镁合金的特点 纯镁的密度为 1.74g/cm3，是铝的 2/3，钢的 1/4。

镁合金的密度一般低于 2.0g/cm3，是目前 最轻的金属结构材料之一，在工程应用时可大大减轻结构件的重量[1,2]与其它金属材料相比， 镁合金具有许多优点，比如：比强。