TiAl基合金汽车气阀真空吸铸工艺研究

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	TiAl基合金汽车气阀真空吸铸工艺研究
	熊超
学位类型	博士
导师	李依依 ; 刘奎
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料加工工程
关键词	Tial基合金汽车气阀真空吸铸充型凝固组织性能 Γ-tial Based Alloys Automotive Valves Suction Casting Mould Filling And Solidification Process Microstructure And Mechanical Property
其他摘要	" 为了提高汽车发动机的燃油效率和减少环境污染，迫切需要轻质化的汽车气阀制备材料；TiAl基合金因密度低、比强度和比刚度高、高温力学和抗氧化性能良好等优点，成为制备轻质化汽车气阀的最佳候选材料。相比于铸锭热机械加工工艺和粉末冶金工艺，熔模铸造是最经济的TiAl基合金汽车气阀成型方法。熔模吸铸不仅结合了熔模铸造的优点，而且充型能力强，特别适合薄壁铸件的铸造，具有充型速度可控，夹杂少，铸件晶粒组织细化，氧含量低，原料利用率高等一系列优点，有望解决TiAl基合金气阀普通重力铸造和离心铸造中出现的增氧和气孔等问题。本论文采用水模拟、数值模拟与真空吸铸实验相结合的方法，系统研究不同工艺参数下TiAl基合金汽车气阀真空吸铸充型流动和凝固规律，在此基础上研究了真空吸铸TiAl基合金的组织特征及力学性能。水模拟实验在自行设计与组装的三套不同充型压力控制方式的吸铸水模拟装置中进行，研究发现利用壳型顶部通气孔粗略控制充型压力的普通吸铸水模拟过程，充型流动卷气生成大量的气泡；精确控制充型压力的“下进气法”和“上排气法”，充型流动平稳，充气流量<1.7 m3·h-1或排气流量<1.5 m3·h-1时，充型流动无气泡生成。数值模拟建立TiAl气阀真空吸铸顺序流动-传热耦合模型，并首次将坩埚系统纳入三维实体模型中，结果表明通过改变充型压力控制方式从普通吸铸到“上排气法”吸铸并降低排气流量，不仅能有效消除卷气形成的气泡，而且能显著降低自由表面卷气的体积分数，借助于四方体横浇道壳型，能将表面卷气降低到最低水平。小幅度的降低浇注温度和升高壳型温度对充型流动形态、表面卷气的体积分数及卷气形成气泡的行为均无明显的影响。小幅度的降低浇注温度和升高壳型温度都不同程度的加重气阀铸件阀杆部分的疏松缺陷。实际吸铸实验结果证明了吸铸水模拟和数值模拟的正确性，结合吸铸充型水模拟、充型和凝固数值模拟及实际吸铸实验结果，确定采用四方体横浇道壳型的TiAl基合金汽车气阀吸铸最佳工艺参数为：浇注温度1680 °C，壳型预热温度600 °C(壳型温度381 °C)，充型压差80 kPa，排气流量6~7.50 m3·h-1和吸铸保压时间73 s。浇注温度分别为1650 °C和1600 °C吸铸制备的Ti-45Al-8Nb-1B圆棒铸件显微组织为典型的全片层组织，晶粒尺寸细小；铸件氧含量分别为1100和1000 ppm，与同成分离心熔模铸造的气阀铸件氧含量比较，吸铸TiAl圆棒的氧含量降低了300～400 ppm。吸铸Ti-45Al-8Nb-1B圆棒铸件热等静压后的显微组织转变为近全片层组织，晶粒大小保持不变；铸件热等静压后的氧含量有所提高；铸件热等静压后缩孔疏松缺陷被压合，断裂强度明显提高；热等静压前后铸件断裂方式均为穿晶脆性断裂，热等静压前其脆性断裂失效的主要原因是铸件存在缩孔疏松缺陷，而大量长条状硼化物是吸铸圆棒拉伸试样热等静压后脆性断裂的主要原因。"; " The increasing demand for improving energy efficiency and reducing pollutant emissions in automotive engines has led to serious consideration of light weight materials used for manufacturing exhaust valves. γ-TiAl based alloys are a strong candidate for such an application owing to their low density, high specific strength and stiffness, high temperature strength retention, and good oxidation resistance. In comparison with the various valve manufacturing technologies which include thermo-mechanical processing and powder metallurgy, investment casting is regard as the most economic processing technology with high precision. In addition to the merits of investment casting, the advantages of investment suction casting (ISC) encompass better filling of castings, finer as-cast grain size, reduced inclusions, lower oxygen content of castings and much-improved utilization efficiency of raw materials, so ISC make a chance to reduce the high oxygen content of castings and remove the gas porosities occurred in traditional gravity casting and centrifugal casting of TiAl valves. In this dissertation, the mould filling and solidification process of TiAl valves during suction casting have been systematically investigated by water simulation, numerical simulation and suction casting experiments. On the basis of the water simulation, numerical simulation and experimental results, the structure characteristics and mechanical properties of γ-TiAl based alloys prepared by suction casting are also studied. The water simulation experiments have been carried out with three sets of experimental installations which are designed and assembled by authors. The results indicate that, in the water simulation of mould filling during suction casting process of TiAl valves, a great amount of entrapped gas bubbles occur during the filling flow with an rough filling pressure control method by means of the vents at the top of moulds (called a general suction casting); Tranquil filling patterns are obtained under an accurate filling pressure control method (“gas charging” or “air leakage”), and if the gas charging flow rate is smaller than 1.7m3·h-1 or the air leakage flow rate is smaller than 1.5m3·h-1,the gas bubbles disappear. A sequentially coupled flow-thermal model has been developed to investigate the suction casting process of TiAl valves in the numerical simulation, and a crucible system is involved in the 3D geometry of the numerical simulation, which can rarely be seen in some previous publications. It is found that, in the numerical simulation of mould filling and solidification process for TiAl valves during suction casting, by changing the filling pressure control methods from the general suction casting to the “air leakage” suction casting and reducing air leakage flow rates, the gas bubbles are eliminated effectively, and surface air entrainment attenuate dramatically; with resort to a mould with a tetragonal runner, the surface air entrainment decrease to the lowest level. Altering pouring temperature and mould temperature within a narrow range play an insignificant role in influencing the filling patterns, the surface air entrainment and the entrapped gas bubble behavior, but they are responsible for exacerbating shrinkage porosities formed in the valve stems. The real casting results are agreement with that of the water simulation experiments and numerical simulation. According to those results, the optimized casting parameters for the “air leakage” suction casting of TiAl automotive valves employed the mould with the tetragonal runner are found to be pouring temperature of 1680 °C, mould preheated temperature of 600 °C (mould temperature of 381 °C), filling pressure difference of 80 kPa, air leakage flow rate of 6~7.50 m3·h-1 and pressure hold-up time of 73 s. Ti-45Al-8Nb-1B round bar castings fabricated by suction casting with the pouring temperature of 1650 °C and 1600 °C have full lamellar microstructures and fine as-cast grain sizes. The oxygen contents of those castings are 1100 and 1000 ppm, respectively. Compared with that of the same chemical composition valves manufactured by centrifugal casting, the oxygen contents of the TiAl round bars prepared by suction casting decrease 300~400 ppm approximately. After hot isostatic pressing (HIP) treatment, the microstructures of the Ti-45Al-8Nb-1B round bars turn into nearly full lamellar, and their grain sizes keep invariant. The oxygen contents of the as-HIP round bars increase to a limited extent. Owing to the shrinkage porosities are impacted to closure, the ultimate tensile strength of the as-HIP castings improves pronouncedly. The main fracture modes in the as-cast and as-HIP castings are both translamellar brittle fracture, and the shrinkage porosities formed in the castings give rise to the brittle fracture of the as-cast castings, but the large boride ribbons of the castings cause the brittle fracture of the as-HIP castings.
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64413
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	熊超. TiAl基合金汽车气阀真空吸铸工艺研究[D]. 北京. 中国科学院金属研究所,2012.