流态床冷却定向凝固技术研究进展

Research Progress of Directional Solidification Technology with Fluidized Bed Quenching

镍基单晶高温合金具有良好的高温力学性能,是航空发动机和燃气轮机热端部件的首选材料。高速凝固法是一种通过抽拉系统将充满熔融金属的模壳移出加热区进行冷却的Bridgman定向凝固技术。其由于具有工艺成熟、设备结构简单及凝固组织相对稳定等优点,已成为制备航空发动机涡轮叶片最常用的定向凝固技术。然而在制备燃气轮机用大尺寸单晶高温合金叶片时,由于高速凝固法的温度梯度较低限制了抽拉速率的提高,制备单晶叶片的过程中容易产生缺陷,使得叶片合格率较低。此外,采用较低的抽拉速率还会导致生产周期延长和能耗增加。而后续发展的气冷法和液态金属冷却法也存在很多问题,如气冷法的温度梯度仍然较低;液态金属冷却法虽然温度梯度较高,但是存在液态金属锡(Sn)会污染铸件、设备结构复杂难以维护和成本较高等问题。因此需要发展一种高温度梯度、不污染铸件、设备结构简单且成本较低的定向凝固技术。流态床冷却法是一种采用固体颗粒和惰性气体两相复合冷却的Bridgman定向凝固技术。流态床具有较强的冷却能力,并且可以利用浮力设置一种浮在流态床表面的动态挡板,从而有效阻止热量从热区向冷区传递,因此流态床冷却法的温度梯度较高。同时,采用流态床冷却不会对铸件造成污染,并且流态床冷却定向凝固设备结构简单、易于维护、成本较低、生产周期较短,因此流态床冷却法在制备大尺寸单晶高温合金叶片方面有着良好的应用前景。本文阐述了流态床冷却法的原理及特点,介绍了流态床冷却定向凝固设备的结构,总结了流态床冷却定向凝固设备中动态挡板的研究进展,分析了影响流态床冷却能力的因素,并对流态床冷却定向凝固技术的发展进行了展望。

Due to the excellent high-temperature mechanical performance, nickel-based single-crystal superalloys have developed indispensable mate-rials in the preparation of advanced aero-engines. The high rate solidification(HRS) method is a Bridgman directional solidification technology, which moves the mold filled with molten metal out of the heating zone for cooling. Because of its simple equipment and stable solidification structure, it has been the most common technology for the preparation of aero-engine blades. However, in the process of preparing large-sized single-crystal superalloy turbine blades, the low thermal gradient of HRS method restricts the increasing of withdrawal rate, which leads to increase the tendency of defects formation, resulting in low yield of the blades. The lower withdrawal rate would also increase the production cycle and energy consumption. The gas cooling casting(GCC) method and liquid metal cooling(LMC)method developed later also have many problems, such as high cost, pollution of Sn, and the difficulties in equipment maintenance. Therefore,it is necessary to develop advanced directional solidification technology with high thermal gradient, clean cooling medium, simple equipment structure, and low cost. The fluidized bed quenching(FBQ) method is a Bridgman directional solidification technology that uses solid particles and inert gas for cooling.The FBQ method can provide a higher thermal gradient due to the excellent cooling capacity and the existence of a dynamic baffle. The dynamic baffle effectively prevents heat transfer from the hot zone to the cold zone. Additionally, the fluidized bed does not contaminate the castings and it has the advantages of simple structure, low cost, short production cycle, and easy maintenance. Therefore, the FBQ method has a good application prospect in the process of large-sized nickel-based single-crystal superalloy turbine blades. In this paper, the principles and characteristics of fluidized bed quenching are clarified, the structure of directional solidification technology with fluidized bed quenching are introduced, the research progress of dynamic baffles in directional solidification equipment with fluidized bed quenching are summarized, the influencing factors of the fluidized bed quenching capacity are analyzed, such as gas velocity and particle size, and the future research aspects of directional solidification technology with fluidized bed quenching are also prospected.