材料成型与控制工程毕业论文 .doc

材料成型与控制工程毕业论文镁合金铸轧区温度场仿真及组织研究单 位 名 称：材料与冶金学院专 业 名 称：材料成型与控制工程镁合金铸轧区温度场仿真及组织研究镁合金是最轻的金属结构材料，其比强度和比刚度高，阻尼性及机加工性好，具有防震、屏蔽电磁波等优异性能，近年来得到极大重视，在国防、航空航天以及3C、汽车等民用工业部门得到了广泛地应用。镁合金的双辊薄带连续铸轧技术是当今有色行业主要研究的课题之一，具有短流程、低能耗及节省投资等优点。应用模拟软件，进行镁合金双辊薄带连续铸轧过程的数值模拟，寻求最佳工艺参数，为镁合金的连续铸轧提供理论基础。本文通过大型模拟软件ANSYS研究了不同的浇注温度、不同的铸轧速度以及不同的辊带间对流换热系数对铸轧区内镁熔体温度场和流场的影响；通过轧卡实验得到一定条件下的铸轧区凝固样品，并观察其凝固组织；论文得到了如下结果：（1）随着浇注温度的升高，铸轧区内的整体流动趋势差别不大，各处温度均有所升高，液穴长度增大，液固两相区增大，轧辊咬入端附近两相区凝固壳变薄，凝固终点位置靠近轧制出口端，出口板带温度也升高。（2）随着铸轧速度的提高，铸轧区内的液穴长度增大，液固两相区增大，铸带表面温度升高，凝固壳变薄，凝固终点位置向轧制出口端靠近。铸轧区中部的两个漩涡略向轧制出口端偏移。（3）随着辊带间对流换热系数的增大，铸轧区内的液穴的长度减小，液固两相区变小，凝固壳略变厚，凝固终点位置向咬入端偏移。铸轧区中部的两个漩涡也向咬入端偏移。（4）铸轧区的凝固组织，靠近轧辊边部的晶粒比中间部位的细小且等轴化程度更高；边部的晶粒则是越靠近轧制出口端越细小。关键词：数值模拟，镁合金，铸轧，温度场，流场 Numerical Simulation on Temperature Field and the Study on Microstructure of Cast-rolling Zone of Magnesium AlloyMagnesium alloys are the lightest constructional metal materials. Due to their excellent properties such as high specific strength and stiffness, good damping and machinability, shock resistance, electromagnetic shielding, magnesium alloys are deemed as one of the most potential materials, and have received more and more attention. Magnesium alloys are rapidly applied to national defence, aeroplane, 3C, automobile and so on. The technique of twin-roll strip continuous cast-rolling of magnesium alloys is one of the main research tasks in nonferrous industry now, it possesses the advantages such as short procedure, low energy consumption, less investment. Simulative software can simulate the process of twin-roll strip continuous cast-rolling of magnesium alloys to get the proper processing parameters, and provide theoretical basis for continuous cast-rolling of magnesium alloys.In this paper, the effect of point of pouring, cast-rolling speed and heat convection coefficient was studied on the temperature and flow field of magnesium melt in the cast-rolling zone by ANSYS; also solid sample was obtained by experiment to study the solid microstructure in the cast-rolling zone, and the conclusions were obtained as follow:（1）With the increment of point of pouring, the flow tendency in the cast-rolling zone is hardly changed, the temperature everywhere in the cast-rolling zone increases more or less, the length of the liquid cave increases, the semi-solid region enlarges, the semi-solid shell around the nip point thins, freezing point is near to outlet and the temperature of strip in outlet also increases.（2）With the increment of cast-rolling speed, the length of the liquid cave increases, the semi-solid region enlarges, the surface temperature of strip increases, the semi-solid shell thins, freezing point is near to outlet and the two eddies in the middle of the cast-rolling zone shift to outlet appreciably.（3）with the increment of heat convection coefficient, the length of the liquid cave reduces, the semi-solid region diminishes and the semi-solid shell thickens appreciably. The freezing point and the two eddies in the middle of the cast-rolling zone are near to the nip point.（4）For the solid microstructure of the cast-rolling zone, the grains around the rolls are finer and more equiaxial than those in the middle of the cast-rolling zone. For those grains around the rolls, the nearer they are to outlet , the finer they are. Key words: numerical simulation，magnesium alloy，cast-rolling，temperature field，flow field 目 录任务书··································································································································i中文摘要·······························································································································iiABSTRACT······················································································································iii第1章 绪论·························································································································11.1 金属镁及其合金···········································································································11.1.1 镁的基本性质及特点································································································11.1.2镁合金的合金成分、牌号标记及其分类·································································31.1.3镁合金的应用及国内外发展现状·············································································51.2 板带铸轧技术的提出与发展现状20·····································································71.2.1 国外简况····················································································································71.2.2 国内简况····················································································································81.3 铸轧技术的数值模拟现状························································································81.4 本文研究的意义和主要内容···················································································91.4.1本文研究的意义·······································································································101.4.2本文研究的主要内容·······························································································11第2章 铸轧过程数值模拟的基本理论····························································122.1流场计算的基本理论·································································································122.1.1流场的基本控制方程·······························································································122.1.2流场湍流模型···········································································································142.1.3通用微分方程的离散化···························································································182.2温度场计算的基本理论····························································································202.2.1热量传递的基本方式·······························································································202.2.2传热中的能量守恒···································································································222.2.3传热中的微分方程···································································································232.2.4传热中的边界条件···································································································23第3章 镁合金铸轧区温度场的数值计算·······················································253.1模型的假设条件··········································································································253.2物理模型及网格划分································································································.253.3计算的工艺参数··········································································································263.4计算的物理性能参数·································································································273.5初始条件和边界条件·································································································283.6求解策略························································································································28第4章 数值模拟的计算结果与分析·································································304.1浇注温度对温度场和流场的影响··········································································304.2铸轧速度对温度场和流场的影响··········································································344.3辊带间对流换热系数对温度场和流场的影响··················································394.4上下辊径的不同对流场的影响··············································································42第5章 铸轧区凝固组织研究·················································································43第6章 结论·······················································································································45参考文献·····························································································································46结束语···································································································································47第1章 绪论1.1 金属镁及其合金1.1.1 镁的基本性质及特点镁在地壳中是继铝、铁、钙和钾元素之后分布最广泛的元素，占地壳重量2.5%。在海水中，镁的含量仅次于氯元素和钠元素，约占0.13%。镁及其合金是常用金属结构材料中最轻的一种。纯镁的熔点651，比重1.74g/cm3 (是钢的1/4，铝的2/3)，常见镁合金密度从1.31.9g/cm3不等。镁的原子序数为12，相对原子质量为24.32，电子结构为1S22S22P63S2，位于周期表中第3周期第2族。镁的晶体结构为密排六方，在25时的晶格常数为：a=0.3202nm，c=0.5199nm；晶胞的轴比为c/a=1.6237，配位数等于12，原子半径为0.612nm。这种晶体结构滑移系少，有脆化倾向，使得普通商业镁合金的力学性能较差，但镁的比重小，合金化能力强，可与其他金属构成力学性能优异，化学稳定性高，抗腐蚀能力强的轻合金。镁的其他一些重要的物理及化学参数见表1.11,2和表1.23,4。表1.1 镁的物理性质和化学性质Table 1.1 Physical and chemical properties of pure magnesium性质量 纲量 值原子性质原子序数12轨道电子状态1s22s22p63s2原子量24.305原子体积cm3×mol-114.0质量性质密度kg×m-31738(20°C)；650°C 时：1650(固态)，1580(液态)体积收缩%4.2(液体凝固)，5(固体冷却：65020°C)热性质熔点°C650±1沸点°C1090开始再结晶温度°C150固态线膨胀系数mm×m-1°C-129.9(20500°C)液态体膨胀系数°C-1380´10-6(651800°C)热导率W×m-1×K-1156(27)W×m-1×K-1146(527)比热kJ×kg-1×K-11.025(20°C)熔化潜热kJ×kg-1360377升华热kJ×kg-161136238(25°C)汽化潜热kJ×kg-151505400燃点°C632635自扩散系数10-10cm2×s-14.4(468°C);36(551°C);210(627°C)热力学比热容J×mol-1×K-1298923K: 26.29-1.01´10-3T-1.60´105/T2+8.41´10-6T29231600K: 212.74-205.66´10-3T-350.15´105/T2+61.56´10-6T2固体熔化焓J×mol-132.7±1熵值J×mol-1×K-132.7±0.1(25°C)电性质泊松系数0.33电导率IACS38.6%电阻率W×cm2×m-10.0453(20°C)接触电位MV+44电化学当量mg×°C-1126标准电极电位V-2.40电离电位eV7.65(1+)15.05(2+)磁性质磁化率Mks0.006270.00632导磁率-1.000012霍尔常数W×m×A-1×m-1-1.06´10-6镁在金属中是电化学顺序最后的一个，因此还具有很高的化学活泼性。镁的室温塑性很差。纯镁单晶体的临界切应力只有(4849)×105Pa，纯镁多晶体的强度和硬度也很低，因此都不能直接用做结构材料。纯镁的主要用途是配制镁合金及其他合金。镁合金是镁在工业中应用的主要形式，与其它金属和工程塑料相比，镁合金具有无可比拟的优点，主要表现在以下几个方面5-7：(1) 镁合金的密度很小，只及钢铁材料的1/4，铝合金的2/3，是最轻的结构金属，能有效降低汽车，航天部件的重量，节省能源。(2) 镁合金的比强度很大，略低于比强度最高的纤维增强材料。(3) 镁合金的比刚度和铝合金、钢铁材料差不多，但远高于工程塑料。(4) 镁合金阻尼性很好，吸收能量的能力很强，具有极佳的防震性能，可用于振动剧烈的场合，用在汽车上可增强汽车的安全性和舒适性。(5) 镁合金热传导性好，是工程塑料的300倍，可用于制造要求散热性能好的电子产品。(6) 镁合金是非磁性材料，电磁屏蔽性能很好，抗电磁波干扰能力强，可用于手机等通讯产品。(7) 镁合金机加工性很好，外观美丽，质感好，可做笔记本电脑，照相机等部件的外壳。(8) 镁合金尺寸稳定，收缩率很小，不易因环境的改变而改变(相对于工程材料)。表1.2 纯镁的一些物理参数随温度的变化Table1.2 Variation of some physical parameters of magnesium with temperatures温度/ºC密度/g.cm-3比热/J(Kg.K)-1热导率/W(m.K)-1热膨胀率/10-6K-1粘度/mPa.s201.7381.02515525.01001.7241.03426.920028.84001.69232.56001.6221.32736.3650（固）1.6101.360650（液）1.5801.3221.251.1.2 镁合金的合金成分、牌号标记及其分类(1)镁合金的合金成分与牌号标记8镁合金的标记方法有多种，各国的标准也不一样，其中美国ASTM标准的标记规则应用最为广泛。按ASTM标准的标记规则，化学元素用12个字母标记，其后的数字表示该元素在合金中的名义成分，用质量分数表示，四舍五入到最接近的整数。字母的顺序按在实际合金中含量的多少排列，含量高的化学元素在前，如果两种元素的含量相同，则按英文字母的先后顺序排列。例如，AZ91表示合金Mg-9Al-1Zn，但该合金的实际化学成分是w(Al)8.3%9.7%和w(Zn)0.4%1.0%。紧接着表示化学成分的英文字母和表示元素的质量分数，有时还用A、B、C、D等后缀表示同一牌号合金在某一特定范围内的改变。铝、镁合金中各字母所代表的化学元素如表1.3所示。由于ASTM标准中的标记既适用于镁合金也适用于铝合金，因此既使像铁那样在镁合金中仅以杂质形式出项的元素，但考虑到是铝合金中的元素，故也列于表1.3中。可见ASTM标准的方法是按合金中所含有的主要化学成分标记镁合金之间的区别。表1.3 ASTM标准中镁合金的英文字母代号所表示的化学元素Table1.3 Chemical element of magnesium alloy indicated by English letter in ASTMEnglish letterSymbol of elementChinese nameEnglish letterSymbol of elementChinese nameAAl铝MMn锰BBi铋NNi镍CCu铜PPb铅DCd镉QAg银ERE混合稀土RCr铬FFe铁SSi硅GMg镁TSn锡HTh钍WY钇KZr锆YSb锑LLi锂ZZn锌我国对镁合金的标记方法比较简单，用两个汉语拼音字母和其后的合金顺序号（阿拉伯数字）组成。依据前两个汉语拼音字母将镁合金分为4类：变形镁合金、铸造镁合金、压铸镁合金和航空镁合金。合金的顺序号表示合金之间的化学成分差异。变形镁合金用MB两个汉语拼音字母表示，M表示镁合金，B表示变形；铸造镁合金用ZM两个汉语拼音字母表示，Z表示铸造，M表示镁合金；压铸镁合金虽然也属于铸造镁合金，但还是专用两个汉语拼音字母YM表示。用于航空的铸造镁合金与其他铸造镁合金在牌号上略有区别，即ZM两个字母与代号的连接加一个横杠。例如5号航空铸造镁合金用ZM-5表示。可见，我国对镁合金标记的特点是按成形工艺划分镁合金的。(2)镁合金的分类根据不同标准，镁合金有几种不同的分类方法：根据化学成分的不同，工业镁合金按主添加元素为Mn，Al，Zn，Zr和稀土分为5个基本合金系：Mg-Mn，Mg-Al-Mn，Mg-Al-Zn-Mn，Mg-Zr，Mg-Zn-Zr，Mg-稀土-Zr，Mg-Ag-稀土-Zr和Mg-Y-稀土-Zr等。此外，在某些镁合金中Th也是添加元素之一。尽管含Th镁合金可具有优良的性能，但因Th具有放射性，现已基本不用。镁合金也可分为含Al和不含Al镁合金，又由于不含Al镁合金一般都用Zr作为晶粒细化剂(Mg-Mn除外)，故也可分为含Zr和不含Zr镁合金。按产品形态，可分为铸造和变形合金，后者又可分为锻压合金、挤压合金和轧制合金。除以上常用镁合金外，还有其它一些新系镁合金，如Mg-Zn-Cu系，典型合金有砂铸合金ZC63和变形合金ZC71；Mg-Li系合金，其中LA141A和LS141A已在航空航天工业得以应用。近来，结合新工艺方法，一些新型镁合金体系得以开发和应用，如快凝(RSP)合金，如EA55RS；非晶镁合金，如著名的三元合金Mg-M-Ln，其中M为Cu或Ni，Ln为La系元素，如Y；金属基复合材料(MMC)，如以SiC，玻璃，Al2O3和石墨等作为纤维强化添加剂的AZ91，AZ31及Mg-Li系合金等。这些合金的强度比一般镁合金高得多，甚至高于一般铝合金的强度。1.1.3 镁合金的应用及国内外发展现状1755年人类发现了镁的化合物，镁作为有使用价值的材料始于1808年，直到1886年镁合金才在德国开始工业化生产9。19世纪初全世界原镁的产量只有10t，几乎都在德国。直至第一次世界大战，镁的生产厂家还仅限于德国，而且镁的产量很小。在两次世界大战，特别是第二次世界大战中镁的用量急剧增加，到1943年仅美国镁的产量就达到了184 000t，比1939年的5倍还多。战争中镁主要用于军事目的，大多用在飞机上，典型的应用是发动机部件、机体和着陆轮。战后镁的主要用途由军事转为民用。镁合金压铸件最早出现在20世纪20年代中期的德国，到现在用镁压铸件来减轻汽车重量的努力至少已有80年的历史。20世纪70年代末，随着国际性能源危机的临近，汽车工业再次将投资的焦点转向了轻质的镁合金材料。美国和德国政府也参与进来，组织了一系列的攻关项目9-11，目标是减轻汽车重量，节省能源。20世纪80年代，镁合金材料开发取得了长足进展。近年镁合金在汽车和3C产品上的应用格外引人注目。镁合金具有高的导热性、抗磁干扰能力、可压铸薄壁件和易于回收等优点，广泛应用于3C电子产品。目前全世界汽车尾气排放CO2所造成的污染占大气污染的60%70%。当今举世瞩目的温室效应和臭氧保护层破坏等都与汽车排放的污染物有关，汽车排放的污染被认为是世界重大公害之一，已严重地威胁到人类的生存和发展，因而人们期待着用镁合金作为轻质材料应用于汽车，以减轻汽车重量、节约能源、降低污染、改善环境。发达国家现在正在大力度地开发镁基材料，镁基材料被认为是最具开发和应用潜力的“绿色材料”8,12,13,14。20世纪70年代以来，各国尤其是发达国家对汽车的节能和尾气排放提出了越来越严格的限制，19931994年欧洲汽车制造商提出“3L汽油轿车”的概念。美国制定了“PNGV”（新一代交通工具伙伴）的合作计划，其目标是生产出消费者可承受的每100Km耗油3L的轿车，且整车至少80%以上的零部件可以回收。这些要求使汽车制造商采用新材料、新工艺和新技术，生产重量轻、耗油少、符合环境要求的新一代汽车。据测算，汽车自重减轻10%，其耗油效率可提高5.5%。如果每辆汽车能使用70Kg的镁合金，CO2的年排放量就能减少30%以上。镁合金作为实际应用中最轻的结构金属材料，在汽车的减重和性能改善中的重要作用日益受到人们的重视。世界各大汽车公司已将镁合金制造汽车零部件作为重要的发展方向15-19。在欧美国家中，各国的汽车厂商正极力争取采用镁合金零件的多少作为汽车技术领先的标志。在未来的七八年中，欧洲汽车制造业使用镁合金将占总消耗量的14%，预计今后将以10%20%的速度递增，2005年将达到20万t。我国是一个摩托车的生产、消费和出口大国，也是一个潜在的汽车生产和消费大国。我国已经在长春、上海、十堰、重庆等城市形成了以汽车、1000万辆各型摩托车的能力。按2001年汽车产量237万辆、摩托车产量1000万辆，每辆汽车镁合金用量20Kg，每辆摩托车用量5Kg计算，该行业将使用镁合金9.7万t8。除压铸镁合金被大量应用在交通工具、3C产品上外，变形镁合金应用也开始受到重视，并随着镁合金连接和表面处理等技术的不断完善，使之在交通工具、3C产品上被应用诸如摩托车、自行车、残疾人用车、手提行李车，以及一些体育和生活工具中的应用也越来越显示出美好的前景。可见，随着镁合金生产和应用技术的不断完善，材料性能/价格比的进一步提高，镁合金不仅在汽车、3C产品上的用量会逐步地增加，而且其应用也将会进一步扩大。表1.4 1997年世界镁合金用量最大的10个汽车生产企业Table1.4 Top ten magnesium alloy usage of automobile manufacturing enterprisesin the world in 1997镁 合 金 用 量 最 大 的10 个 汽 车 企 业镁 合 金 用 量 最 大 的 10 种 车 型汽车企业镁合金用量/t镁合金车型镁合金用量/t福特17 500GM Full Sized Vans- Savana & Express26.3通用9 400Daimler Benz SL17.020.3克莱斯勒7 050GM Minivans-Safari & Astro16.7丰田4 200Ford F-150 Truck14.9奔驰2 700VW Passat Audi A4 & A613.614.5奥迪1 600Porsche Boxster Roadster9.9大众1 250Buick Park Avenue9.5宝马700Alfa Romeo1569.3菲亚特500Daimler Benz Slk Roaster7.7保时捷250Chrysler Minivans5.8总计45 1501.2 板带铸轧技术的提出与发展现状201.2.1 国外简况板带铸轧技术的发展至今也有150多年的历史，早在1846年，英国的贝塞麦(Bessemer)就提出，从两个旋转辊上方浇铸金属液，通过一对内部具有循环冷却作用的铸轧辊辊缝间隙结晶、凝固、变形后从下边引出铸轧带坯。但限于当时的技术水平和工艺技术条件而未获成功。在这以后的近100年里该项技术一直不为人们所重视，直到1930年，德国的容汉斯(Junghans)等首次报道广立式连续铸轧成功的消息。1951年美国的亨特、道格拉斯(Hunter、Douglas)两家公司联合，将失败的贝塞麦(Bessemer)方法重新进行了研究，将生产方式上注式改为下注式，并改善厂两辊的冷却方式，采用了可控制金属液静压力炉前箱，终于在1955年顺利地铸轧出宽型薄铝带还、创立了双辊式铸轧机，所以亦称Hunter式铸轧机。因这种向下向上的铸轧法其供料嘴的安装调整十分不便，1962年亨特公司提出铸轧辊中心线连线与地平固成75度夹角的倾斜式双辊式铸轧机。继而法国的斯卡尔公司研制出称之为3C(Continuous Caster Between Cylinders)法的双辊水平式铸轧机。这种铸轧机与上述两种的不向之处在于两铸轧辊中