复杂钛合金铸件凝固和热等静压过程缩孔形成与湮灭机理及数值模拟研究

In the casting process of complex titanium alloy products, shrinkage defect is easily formed during its solidification. These casting defects are typically eliminated by hot isostatic pressing (HIP). However, the developing precision casting processing design and well-designed post-processing by HIP still unreachable to effectively remove the shrinkage defect to guarantee the high quality of casting products, particularly for castings with complex structures and shapes. The main reason of these difficulties is due to the blank of researching on the mechanism of the shrinkage formation and elimination during the solidification in the centrifugal or gravity casting processes and post-processing by HIP for the complex titanium alloys casting. In this study, the combination of experimental and numerical simulation methods is used to explore the mechanism of entire life cycle of shrinkage defect evolution from the 'forming' during solidification process and 'elimination' in HIP process. Firstly, on the basis of consideration of gravity, centrifugal force, surface tension, dynamic fluid flow inside the melt pool, dendrite growth on the solidification front and other influential factors, mathematical models of formation of shrinkage defect are built. Secondly, based on diffusion mechanism of material creep, the models of elimination of shrinkage defect are established under coupling effect of thermodynamics and mechanical in HIP. Thirdly, the dynamical process of formation and elimination of shrinkage defect is simulated by quantitative characterization methods of shrinkage morphology to achieve quantitative prediction of shrinkage evolution and to guide optimization of casting and HIP process design. The research achievements will provide strong theoretical and technical supports for quantitative control of shrinkage defect of complex titanium casting products.

复杂钛合金铸件在凝固过程容易产生缩孔类缺陷,一般通过采用热等静压工艺来消除，但是现阶段的精密铸造+热等静压的工艺尚不能完全消除缩孔类缺陷而获得高品质复杂钛合金铸件，究其原因主要是由于复杂钛合金铸件在离心/静止铸造条件下凝固过程以及后续的热等静压过程中缩孔形成与湮灭机理尚不清楚。本研究拟采用实验研究与数值模拟相结合的方法来探索复杂钛合金铸件缩孔缺陷从凝固过程"形成"至热等静压过程"湮灭"整个生命周期的演变机理。首先，在考虑重力、离心力、表面张力、动态熔池内部液体流动、凝固前沿枝晶生长等因素的基础上，建立缩孔形成的数学模型；其次，基于材料蠕变扩散机理建立热等静压热力耦合作用下缩孔湮灭的数学模型；再次，通过缩孔形貌定量化表征方法，进行缩孔从形成到湮灭全过程的数值模拟，实现缩孔演变过程的定量研究，并指导铸造和热等静压工艺的优化设计。本研究成果将为复杂钛合金铸件缩孔缺陷定量化控制提供理论和技术支撑。

复杂钛合金铸件在凝固过程容易产生缩孔类缺陷，一般采用热等静压工艺来消除，但是现阶段的精密铸造+热等静压的工艺尚不能完全消此类缺陷，究其原因主要是由于复杂钛合金铸件在离心/静止铸造条件下凝固过程以及后续的热等静压过程中缩孔形成与湮灭机理尚不清楚。本研究采用了实验研究与数值模拟相结合的方法来探索复杂钛合金铸件缩孔缺陷从凝固过程“形成”至热等静压过程“湮灭”整个生命周期的演变机理。在缩孔的形成方面，首先，在考虑重力、离心力、表面张力等因素的基础上，建立缩孔形成的数学模型；其次对模型进行求解，采用C++语言开发出铸件凝固过程缩孔预测模块并与华铸CAE软件对接；再次，设计了U形铸钢铸件的重力铸造实验和U形钛合金铸件的立式离心铸造实验，验证模型在大气压下重力铸造工艺和真空中不同转速的立式离心浇铸工艺下均有很高的缩孔预测精度；最后从实际大批量生产的结构较为复杂的重力铸造铸钢铸件和立式离心铸造钛合金铸件中各选取两组铸件，采用缩孔预测模型对其铸造工艺进行凝固过程模拟，模拟结果与实验结果吻合。在缩孔湮灭方面，首先，通过实验与数值模拟相结合研究了热等静压温度对钛合金铸件缩孔湮灭的影响，得到了适合钛合金铸件进行热等静压的温度并验证了有限元方法的可行性；其次，对真实缩孔进行形貌和体积等效，解决含有真实缩孔的钛合金铸件在热等静压数值模拟过程中难以收敛的问题；再次，基于材料弹塑性理论，蠕变扩散机理建立热等静压热力耦合作用下缩孔湮灭的数学模型；最后将此数学模型分别应用于含有预制缩孔和自然缩孔钛合金铸件热等静压过程的数值模拟中，模拟结果与实验结果吻合较好，此外还观察了缺陷弥合区的微观组织形貌。将以上获得的两个模型应用于发动机机匣件的铸造及后续的热等静压的数值模拟中，实现了缩孔演变过程的定量化表征。本研究成果将为复杂钛合金铸件缩孔缺陷定量化控制提供理论和技术支撑。