熔模铸造条件下Ti6Al4V合金铸件与陶瓷型壳间界面换热系数研究

RESEARCH ON THE INTERFACIAL HEAT TRANSFER COEFFECIENT BETWEEN CASTING AND CERAMIC SHELL IN INVESTMENT CASTING PROCESS OF Ti6Al4V ALLOY

建立了Ti6Al4V合金铸件/铸型界面换热系数(h)的一维反算模型,从数学及数值模拟的角度研究了型壳热物性参数和热电偶定位等参数对h计算的影响,分析了不同参数影响的不同特点,据此对型壳热物性参数和热电偶定位位置等进行了修正,提高了h反算精度.修正计算参数后的反算结果表明,Ti6Al4V合金熔模铸造条件下,h的变化可分为4个阶段:(1)铸件为液态,h维持约440 W/(m2·K);(2)铸件表面生成完整凝固层,此阶段h下降近60%;(3)凝固层不断增厚至铸件凝固,此阶段h下降接近峰值的20%;(4)铸件凝固后,h随温度缓慢下降.在三维模型中对反算得到的h进行了验证,得到的模拟温度与实测温度基本吻合,表明反算得到的h较为准确,可以应用于Ti6Al4V合金熔模铸造过程的数值模拟中.

Investment casting process is an important way to get complex parts of titanium alloy. However there are few research on the interfacial heat transfer coefficient （h） between casting and shell thus the temperature simulation of investment casting process of titanium alloy is often inaccurate. In order to get a relatively accurate h,a one-dimensional mathematical model for the reverse calculation of h between casting and shell in investment cast- ing process of Ti6A14V alloy was built and the analytic relationship between temperature and time/heat flux was es- tablished. Considering the calculated h is significantly affected by the error of parameters such as the specific heat capacity and thermal diffusivity of shell and position of thermocouples, research on the error of these parameters is essential. The relationship between the error of these parameters and the temperatures in the casting and shell was studied and it was found that the effect of different kind of error on the temperature field was obviously different. An experiment based on the one-dimensional mathematical model was done and temperatures of different positions were measured. Based on the effect of different kind of error and the difference between the calculate temperature field and the measured temperatures, the proportion of effect of each kind of error was assessed. These errors were revised on the basis of the assessment, thus a relatively accurate h between the casting and shell was obtained. The relationships between h and thickness of the solidified layer on the casting/temperature at the surface of casting can be divided into 4 stages： （1） Metal was liquid and h kept about 440 W/（m^2·K）; （2） Solid layer appeared on the sur- face, and h declined nearly 60%; （3） Solid layer grew up before metal became completely solid and h declined near- ly 20% of its maximum; （4） After metal solidified, h declined slowly as temperature on the surface of casting dropped. These relationships were applied in a three-dimensional model for numerical simulation of the tempera- ture field. Temperatures of different positions in casting and shell were calculated and calculated temperatures agreed with measured ones well. Thus the accuracy of h was identified and it can help solve problems in the pro- duction in investment casting process of Ti6A14V alloy.