【作者】 叶喜葱；

【导师】 郭景杰； 苏彦庆；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 博士

【摘要】 由于TiAl合金有许多突出的特点,其发展受到世人的关注和重视。TiAl合金的密度低,高的比强度和比弹性模量,在高温时仍可以保持足够高的强度和刚度,同时它还具有良好的抗蠕变及抗氧化的能力。由于TiAl基合金室温延性差,难以加工成形,研究者将目光集中在TiAl基合金铸造近净成形技术上,目前TiAl基合金的铸造成形技术有熔模铸造、反重力低压铸造、离心铸造等,但是这些铸造方法在成形小型薄壁TiAl基合金件时效率较低。因此,TiAl基合金薄壁叶片一般采用精锻件,但是这类薄壁叶片加工存在较大的变形问题,精加工余量采用手工抛光来去除,并靠截面样板来保证叶片气动形状,影响发动机的气动性能。本文提出了一种新的TiAl基合金近净成形铸造方法-底注式真空吸铸,通过对底注式真空吸铸过程的数值模拟和实验验证,详细研究了金属型底注式真空吸铸的充型和凝固规律。在此基础上研究了底注式真空吸铸TiAl基合金小型薄壁件的组织特征。底注式真空吸铸技术是将TiAl基合金在水冷坩埚中熔化,熔化了的合金通过坩埚底部的吸口充入到真空室金属铸型中凝固的近净成形铸造技术。底注式真空吸铸过程中,TiAl合金液是在真空条件下充型凝固的,克服了氧化物的形成,合金液在上下气体压差和自身重力的同向耦合下充型,充型动力大,即使在较小的过热度下也可成形小型薄壁件,利用金属铸型还可以获得细小的组织、减弱界面反应。吸口直径对底注式真空吸铸过程TiAl基合金熔体的流动行为产生很大的影响(型腔宽20mm,厚2mm,长60mm):当吸口直径为2mm时,合金熔体到达铸型底部后进行反向充填,在铸型的底部出现反向充填区,随着吸口直径的增大,反向充填区从底部向上部移动,当吸口直径为4mm时,反向充填区消失。影响流动产生这种行为的主要原因是由于吸口直径改变了合金液的最大射流宽度,随着吸口直径的增大,合金液的最大射流宽度增大。研究了吸口直径、浇注温度、浇注速度、铸型温度、换热系数和合金元素对充填率的影响:吸口直径和充填率呈线性关系;浇注温度的提高有利于充填率的提高;当浇注速度小于2m/s时,浇注速度和充填率成线性关系;铸型温度的提高也有利于充填率的提高;大的换热系数铸型使充填率降低;添加使TiAl基合金熔点提高、粘度增加的合金元素,不利于熔体的充型。适当的电流强度和电极高度有利于减少铸件中气孔缺陷。研究了工艺参数对凝固时间和凝固分数的影响:浇注温度的提高使浇口部位的合金液凝固时间延长,金属液前端临界凝固分数减少,有利于叶片隼部的补缩;吸口直径的增大、铸型温度的提高,有利于叶片隼部合金液在浇口部位的合金液后凝固,使叶片隼部的凝固封闭区减少,有利于叶片隼部的补缩;浇注速度的提高使叶片隼部的凝固封闭区先变小后增大,其临界速度为1m/s,当浇注速度小于1m/s时,有利于叶片隼部的补缩。确定了底注式真空吸铸TiAl基合金的局部凝固收缩率1.55%为缩孔,实验结果和数值模拟结果吻合。吸口直径对底注式真空吸铸TiAl基合金构件的缩孔影响最大,浇注温度次之,随着吸口直径的增大,浇注温度的提高,叶片隼部的缩孔减少。浇注温度的提高有利于应力的消除;吸口直径的增大,使叶片叶身部位的应力减少;铸型温度和浇注速度的提高,有利于叶片整体应力的减少;大的换热系数的铸型,使叶片的应力集中加剧。利用综合优化工艺参数图获得了充型完整且无宏观缩孔的薄壁铸件。研究了TiAl基合金薄板铸件、叶片铸件的组织特征。Ti-47Al合金薄板铸件晶粒尺寸和距底部距离有关,铸件底部的晶粒最小,随着距底部距离的增大,晶粒增大,铸件整个横截面全是等轴晶。Ti-47Al合金叶片铸件内部的晶粒也随着距底部距离的增大而增大,在距底部距离20mm处,叶片铸件的横截面从等轴晶向柱状晶转变,柱状晶的尺寸也随距底部距离的增大而变大。无论是薄板铸件还是叶片铸件,层片间距随距底部距离的增大而增大。和离心铸造对比,底注式真空吸铸得到的TiAl基合金组织偏析少,晶粒尺寸小,且晶粒尺寸分布均匀,无界面反应。W元素的添加细化了Ti-47Al合金的一次、二次枝晶臂;B、Si元素的添加,在Ti-47Al合金中生成第二相,形成的第二相主要分布在晶界处,阻碍晶粒的长大,从而细化了Ti-47Al合金的组织。更多还原

【Abstract】 TiAl based alloys are considered as promising structural materials in aerospace, energy and automotive industries. Their advantages are mainly low density, high specific strength, high specific stiffness, good oxidation resistance, and good creep property up to high temperatures. TiAl based alloys have low ductility, low deformation limit, and bad cold-forming property, so these bad properties limit the cold processing technology of TiAl based alloys.Casting is the most economical way to produce TiAl based alloy components. Different casting methods such as low pressure casting, centrifugal casting, shell mould casting etc. are used. However, there are some problems in casting small thin walled component of TiAl based alloy. Therefore, the TiAl based alloy precise forging thin blades were generally used in small engine. However, this thin blade has deformation problem, and precision allowance was removed by hand polishing, and the air-actuated shape of the blade was assured by cross section pattern.In this article, a new net-shape casting method for TiAl based alloy was developed - bottom pouring vacuum suction casting using permanent mold. Through the numerical simulation and experimental verification, the detailed study on filling and solidification process of TiAl based alloy melt during bottom pouring vacuum suction casting has been carried on. On the basis of the numerial simulation an experimental results, the defects and structure characteristics of small thin sheet Ti-47Al alloy components prepared by bottom pouring vacuum suction casting were studied also. Bottom pouring vacuum suction casting technique is to melting TiAl based alloy in a water-cooled crucible, and molten alloy will fill into metal mould through the suction hole at the bottom of the crucible,and then solidify. In the process of bottom pouring vacuum suction casting, the melt fills and solidifies under vacuum so as to overcome the oxide formation, on the other hand, melt the mould cavity in the coupled effect of the pressure difference and gravity, and the smaller superheat can also form small thin walled component, at the same time,use of metal mold can refine TiAl based alloy structure and weaken interface reaction.The suction hole diameter will affect filling behavior of TiAl based melt. With the increase of suction hole diameter, the max jet width increases. In this article, the effects of suction hole diameter, pouring temperature, pouring velocity, mold temperature, alloy element and coefficient of heat transfer on filling ratio have been studied. The suction hole diameter and filling ratio have linear relationship. With the increase of pouring temperature, the filling ratio increases. When pouring velocity is less than 2m/s, there has a linear relationship between the pouring velocity and the filling ratio. With the increase of the mold temperature, the filling ratio increases. The addition of alloy element, which will increase melting point and viscosity, will decrease the filling ratio. The proper current density and electrode height are benefit to decrease gas hole of casting.The effect of technology parameter on solidification time and solidification fraction has been studied. With the increase of pouring temperature, the solidification time of front melt is extended, which will benefit to feeding; The increase of suction hole diameter and mold temperature are benefit to solidification of blade hawk after the front melt solidification, and the solidification fraction is reduced, which will benefit to improve feeding; With the increase of pouring velocity, the solidification closed area decreases first and then increases, and the critical value is 1m/s, when pouring velocity is less than 1m/s, the increase of pouring velocity will benefit to improve feeding. The local solidification shrinkage ratio 1.55% of TiAl based alloy is used for shrinkage criterion, the experimental results and numerical simulation results agree well. The suction hole diameter has the greatest effect on shrinkage cavity of TiAl based alloy component, followed by the pouring temperature, with the increase of suction hole diameter and pouring temperature, the shrinkage cavity is reduced. With the increase of pouring temperature, the stress is reduced; the increase of suction hole diameter will lead to decrease blade leaf stress; the increase of pouring velocity and mold temperature will benefit to decrease blade stressIn this article, the qualified TiAl based alloy thin walled components have been obtained. The structure character of the thin sheet casting and blade casting of TiAl based alloy has been studied. The grain size of Ti-47Al alloy thin sheet casting increases with increasing distance from the bottom, and the entire cross section has equiaxed grain. The inner grain size of Ti-47Al alloy blade increases with increasing distance from bottom. The inner grain shape of Ti-47Al alloy blade is changes from equiaxed to columnar crystal at the distance from the bottom 20mm, and the columnar crystal fraction increases with increasing distance from bottom. The lamellar spaces of blade and thin sheet casting increase with the distance increase from bottom. Compared with centrifugal casting, the bottom pouring vacuum suction casting has several advantages, that is, less segregation, no interface reaction, and smaller grain size. The effect of alloying element on Ti-47Al alloy structure has been studied also in this article. The first secondary dendrite arm was refined, because of the W element addition; Because of B and Si element additions, second phase will form and result in refinement of Ti-47Al alloy structure.更多还原