【作者】 程素玲；

【导师】 周尧和；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2005， 硕士

【摘要】 镁合金是航空、航天及汽车工业中应用较多的一种有色金属,其比重（约1.8）比铝还小,具有比强度和比刚度高,减震能力好以及良好的电磁屏蔽性等优点。镁合金作为一种轻质材料,近年来越来越受到汽车和电子行业的重视。但是普通镁合金在熔炼和浇铸过程中不可避免地要发生剧烈的氧化,甚至燃烧。目前工业生产中通常采用熔剂或气体保护法进行熔炼,也有的同时加入微量的Be减缓其氧化。但是,熔剂保护法和气体保护法致使熔炼工艺及设备复杂化、易产生央杂,并且污染环境;微量的Be亦不能完全阻止镁熔体的氧化燃烧,且Be具有剧毒。 本论文针对以上问题,采用合金化阻燃的方法,研究了Ca对镁合余阻燃和力学性能的影响。AZ91合金是目前工业上应用最为广泛的镁合金,因此以AZ91合金为基体,并添加Y优化表面膜结构及基体组织。主要研究内容及结果如下: （1） 采用热分析法详细研究了Ca、Y对AZ91合金阻燃性能的影响。Ca对AZ91合金的阻燃性能有明显的提高作用,随着Ca含量的增加,AZ91合金的燃点随之提高,通过燃点测试实验得出Ca的百分含量x与相应合金的燃点T之间的关系如下: T=179x+524 由于Y在Mg-Al合金中极易与A1反应,生成Al₂Y,使得该合金中存在的单质Y很少（<0.25wt%）,因此,AZ91合金加Y后燃点只能提高10～30℃左右。然而,加入少量的Y（0.15～1wt%）后,AZ9l-xCa合金的燃点达750℃（熔炼温度）所需的最低Ca含量可由1.26wt%降低为1.18wt%左右。 （2） 利用X-射线衍射分析（XRD）、扫描电镜分析（SEM&EDS）和热重分析仪（TGA）研究了AZ91、AZ91-xCa、AZ91-xCa-yY合金的氧化膜组织、结构、阻燃元素的分布及高温氧化动力学。AZ91合金加Ca后表面生成了由MgO、CaO、Al₂Ca、MgAl₂O₄和Mg3_N2组成的复合氧化膜,提高了氧化膜的致密度,但是该表面膜较厚且具有脆性,在合金受热变形过程中易发生开裂;在此基础上加入少量Y后,相于对于只加Ca的合金,由于Y₂O₃的生成使氧化膜的致密度大大提高,合金在高温下生成的氧化膜较薄,并且氧化膜的塑性得到了提高。 （3） 采用光学显微镜、X-射线衍射及扫描电镜研究了Ca和Y对AZ91合西北工业大学工学硕卜学位论文金组织的影响,并通过室温拉伸性能测试考察了合金元素Ca、Y对AZgl合金铸态拉伸性能的影响。AZgl合金中加入1 .0一1 .swt%的Ca后,一方面使p相的数量减小,另一方面使合金的晶粒尺寸减小。因此Ca的加入提高了AZ91合金的强度。在此基础上加入微量的Y,可使合金组织进一步细化。Y与Al反应生成AbY,在晶内和晶界上无规则地弥散分布,强化了合金组织,使合金的抗拉强度进一步提高。但是随着Y量的增加,A12Y相的尺寸增大,·割裂了合金基体从而使抗拉强度降低。本实验得到Y的最佳含量为0.llwt%。 （4）综合考虑合金元素Ca、Y对AZgl合金的阻燃和拉伸性能的影响,得出阻燃镁合金的最佳成分为AZgl一1 .18叭%Ca一0.llwt%Y。关键词AZ91镁合金;Ca;Y;阻燃性能;氧化膜;氧化动力学;力学性能更多还原

【Abstract】 Magnesium alloy as a kind of nonferrous metal was used in aeronautics, astronautics and automobile industry extensively. It is even lighter than aluminum alloy. It has the advantages of high special strength, high special rigidity, high damping property and electromagnetism shielding. Magnesium alloy is increasingly used in automobile and electric industry as a lightweight material in recent years. However, conventional magnesium alloy will be oxidized severely during melting and pouring process because of high activity of magnesium element. Fluxes and protective gases are usually used as burning prohibitors in commercial manufacture. Beryllium element is also used sometimes to slow down the oxidation reaction of magnesium and air. However, both flux and protective gas would lead to some problems such as inclusion, environmental pollution and complication of melting process and equipment. Beryllium has tremendous toxicity and it can’t restrain oxidation reaction absolutely.In the present dissertation, calcium was selected as alloying element and its effect on ignition-proof and tensile properties was investigated. AZ91 alloy was selected as master alloy since it is the most widely used magnesium alloy at the present time. Yttrium was also added into AZ91 alloy to optimize the microstructure of oxidation film and matrix of it. The research aspects and results of the paper are as follows:(1) Thermal analysis method was used to study the effect of calcium on ignition-proof property of AZ91 alloy. The ignition-proof of AZ91 was increased significantly after calcium was added. As the content of calcium was increased, the ignition point of AZ91 was raised. By the determination of ignition point, the relationship between the addition amount of calcium and ignition point of the alloy was obtained as following:T=179x+524Since yttrium is prone to reaction with aluminum to form Al2Y, so single-element yttrium is limited in Mg-Al alloy (<0.25wt%), the ignition point of AZ91 will onlyincrease 10~30℃ after yttrium is added. However, the content of calcium will decreased from 1.26wt% to 1.18wt% when the ignition point of AZ91 reaches 750℃ (melting temperature) after a little amount of yttrium is added.(2) X-ray diffraction (XRD), scanning electron microscope (SEM&EDS) and thermalgravitational analysis (TGA) was used to research the morphology and structure of oxide film, the distribution of ignition-proof element and oxidation kinetics at elevated temperature. The oxidation film of AZ91 composed of MgO, CaO, Al2Ca, MgAl2O4 and Mg3N2 was formed with the calcium addition. The density of it was enhanced. But it was thick and brittle and was prone to crack in the process of elevated temperature distortion. With calcium addition, the density of oxide film formed on AZ91 would be increased because of the formation of Y2O3 after yttrium was added. The oxide film was thin and the plasticity was higher than that of alloy with single calcium addition.(3) Optical microscope, XRD, SEM and tensile property test were employed to study the effect of calcium and yttrium on the structure and the tensile property of AZ91 alloy. The room-temperature tensile strength of AZ91 alloy would be enhanced due to the reduction of β phase and grain size in AZ91 alloy if the addition amount of calcium was 1.0~1.5wt%. With a little amount of yttrium addition, the tensile strength would further enhanced because of the disperse distribution of Al2Y formed in grain and grain boundary by the reaction of Al and Y. However, the tensile strength of AZ91 would reduce when the size of Al2Y phase was increased and the matrix was disserved along with the increment of yttrium content. The appropriate amount of yttrium addition is 0.11 wt% in this investigation.(4) According to above studies, the composition of ignition-proof magnesium alloy with excellent ignition-proof property and tensile strength was AZ91-1.18wt%Ca-0.11wt%Y.更多还原