【作者】 肖树龙；

【导师】 陈玉勇； 田竟；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 博士

【摘要】 本文对钛合金的低成本熔模精密铸造技术进行了研究,通过对粘结剂、面层涂料及型壳各种性能的研究、型壳干燥与焙烧工艺的研究,确定了低成本氧化物陶瓷型壳的制备工艺;研究了钛合金与低成本氧化物陶瓷型壳的界面反应、钛合金的显微组织与力学性能;对熔模精密铸造充型与凝固过程进行了数值模拟,设计了浇注系统,优化了熔炼与浇注工艺参数,为实际浇注打下基础;开展了典型薄壁复杂铸件的熔模精密铸造离心浇注实验,制备出合格的铸件,并对铸件的质量和性能进行了测试,结果表明采用低成本氧化物陶瓷型壳在离心力场下能够浇注出质量和性能优良的铸件。对低成本熔模精密铸造面层涂料中的耐火粉料进行了粒度级配,并对采用级配粉制备的涂料的流变性能进行了研究。研究结果表明:随着粉液比的增大,涂料粘度增大;相同粉液比情况下,当细粉含量为40%的时候,涂料具有较低的粘度;粉液比为2.4、细粉含量为40%的涂料具有较好的流变性能。型壳的干燥时间随着环境温度的升高而缩短,随着环境湿度的增加而增加,当相对湿度大于70%时,型壳干燥时间显著增加;在一定范围内,空气流速的增加能够明显缩短型壳的干燥时间,当空气流速大于5m/s时,对干燥速度影响不大。完全粗粉时型壳的强度最低,随着细粉含量的增加,型壳的湿强度是增加的,当细粉含量达到40%时型壳具有最大的强度,随后型壳的强度又有所降低,并且型壳断裂时的位移也变小了;型壳焙烧后的强度随着焙烧时间的延长而增大,随着焙烧温度的升高,面层型壳的抗弯强度逐渐增加。根据实验结果,确定了低成本氧化物陶瓷型壳的制备工艺为:在环境温度为25℃-30℃,湿度40%-50%,空气流速2.5m/s-3m/s,干燥时间为8-10小时。型壳干燥干燥后进行焙烧,焙烧工艺为:升温至150℃后保温30分钟后迅速升温至600℃保温1小时,然后升至950℃保温2-3小时。通过Miedema模型和Kohler公式,推导了三元系钛合金中各元素活度的计算公式;计算了合金元素的含量对钛合金中各元素活度的影响,发现随着Al含量的增加,Al的活度增大,而Ti和Zr的活度是减小的。而Zr含量的增加则使Ti的活度增大,使Al的活度减小;计算了熔体温度对钛合金中合金元素活度的影响规律,发现随着温度的升高,Al元素的活度是减小的,Zr元素和Ti元素的活度是增加的。研究了钛合金与低成本氧化物陶瓷型壳的界面反应,结果表明,粒度级配和粉液比优化对减轻型壳与钛合金的界面反应、获得规则界面是有贡献的,特别是当粉液比为2.4、涂料中细粉含量为40%时,效果最为明显。采用XRD和XPS手段对钛合金表面相组成与生成物进行了研究,结果表明,钛合金的表面主要由型壳中的Al₂O₃和SiO₂组成,并且存在少量的Ti、Si化合物。从界面反应结果和XRD、XPS结果都显示出由于合金元素的添加降低了Ti的活度,Ti-Al-Zr合金与型壳的界面反应和扩散程度都比纯钛合金的要轻。针对大型薄壁复杂钛合金铸件,设计了三种浇注系统;分别对三种浇注系统熔模精密铸造充型过程的充型时间、温度场和速度场进行了数值模拟与分析,优选出合理的浇注系统;特别针对优选出的浇注系统的熔模精密铸造过程进行了凝固过程的数值模拟,分析了凝固过程中的温度场。对蜡模的制备工艺进行了研究,制备出了高表面光洁度的蜡模,表面粗糙度平均值达到Ra=1.04μm;采用低成本氧化物陶瓷型壳的制备技术,进行了钛合金铸件的型壳的制备,并在离心力场下采用水冷铜坩埚真空感应凝壳熔炼炉成功地浇注出了大型薄壁复杂钛合金铸件;对浇注出的铸件进行了显微组织、力学性能与质量检测。结果表明,采用本文研究的钛合金熔模精密铸造用低成本氧化物陶瓷型壳的制备技术及相应的离心浇注工艺,能够浇注出质量优良的大型薄壁复杂钛合金铸件。更多还原

【Abstract】 A novel low cost investment cast technology was developed for titanium alloy and was investigated in this thesis. The properties of binder, face coat slurry and mould shell were studied in detail, which help determine the technology of preparing mould shell including drying and baking technologies. The interface reaction between titanium alloys and the developed low cost oxide ceramic mould shell were studied. Microstructures and mechanical properties of the cast titanium alloys were studied as well. Numerical simulations of melt filling and solidification processes were carried out for the investment cast of titanium alloy, including designing pouring system, optimizing process parameters of melting and pouring, establishing theoretical basis for practical casting process. The large-size thin-wall complex-shape components were cast successfully by centrifugal investment cast. The quality and property tests of the casts verified that the newly developed low cost oxide ceramic mould shell is good enough to produce high quality titanium alloy cast in centrifugal force field.Size-grade distribution was investigated on the refractory powder of the low cost investment face coat slurry, and the rheological properties of slurry were studied. The results showed that the viscosity of slurry increased with increasing S/L ratio. A lowest slurry viscosity was found when the fine powder was 40% at the same S/L ratio. Better rheological properties of slurry were reached when S/L ratio was 2.4 and the content of fine powder was 40%.The drying time of mould shell was shortened with rising ambient temperature, but prolonged with increasing ambient humidity, particularly when ambient humidity was more than 70% the drying time was prolonged remarkably. Increasing air flow velocity could evidently shorten drying time, but an air flow velocity more than 5m/s did not change drying time much.A smallest bending strength of mould shell was obtained when the powder of slurry was composed of full coarse powder, and increasing the content of fine powder increased the bending strength. The bending strength of mould shell increased with increasing baking time and rising baking temperature as well.Based on the above results, the processing technology of preparing novel low cost oxide ceramic mould shell was determined to be: ambient temperature, 25℃-30℃; ambient humidity, 40%-50%; air flow velocity, 2.5m/s-3m/s; and drying time, 8-10 hours. The mould shell baking is with the following steps: holding for 30min at 150℃, holding for 1hour at 600℃followed by holding for 2-3hours at 950℃.The chemical activities of various elements in ternary titanium alloy system were computed on the basis of Miedema model and Kohler formula, and their activity coefficients were calculated. The simulation results showed that the increasing Al content results in an increase of activity coefficient of Al and a decrease of activity coefficients of Ti and Zr. Increasing Zr content increased the activity coefficient of Ti and reduced the activity coefficient of Al. It was also found that the activity coefficient of Al decreased with rising temperature and that the activity coefficients of Zr and Ti increased with rising temperature.The interface reaction between titanium alloys and the low cost oxide ceramic mould shell was studied. The results showed that the powder size grade distribution and optimized S/L contributed to weakening the interface reaction to obtain regular interface with an optimum state at an S/L of 2.4 and the content of fine powder of 40%. XRD and XPS analysis on the surface of cast titanium alloys showed that the surface of cast titanium alloy was composed of Al₂O₃,SiO₂ and little amount of Ti-Si compound. The results of interface reaction investigation and analysis of XRD and XPS confirmed that adding alloy elements into Ti alloy decreased the activity coefficient of Ti.To understand the characteristics of investment casting of large-size thin-wall complex-shape component, different runner systems were design and the cast filling time, temperature field and viscosity field were simulated during the processes of cast filling and solidification of titanium alloy.The processing technology of preparing wax patterns was investigated. The high surface finish wax patterns could be prepared using current technology, with an average roughness of about 1.04μm. The large-size thin-wall complex-shape titanium alloys casts were successfully cast in centrifugal force field using a water-cooling crucible vacuum induction skull melt （ISM） furnace. The experimental results showed that with the currently developed technology of preparing low cost oxide ceramic mould shell, ISM and centrifugal pouring technology, the good quality large thin-wall complex titanium alloy cast could be cast successfully.更多还原