基于晶界液化和再凝固行为数值模拟的高温合金焊接热影响区微裂纹形成机理与控制因素研究

The numerical model of constitutional liquation for precipitates such as NbC carbides, Laves phase and gamma prime γ' will be constructed in order to investigate the occurrence of weld heat-effected zone (HAZ) microfissures in superalloys. Combined with isothermal liquation, Gleeble physical simulation experiments and microstructural investigation the mechanism and controlling factors of grain boundary liquation will be manifested through numerical simulation. On the basis of experimental observation of the solidifying characteristics of the liquid caused by liquation and theoretical analysis an initiating criterion for liquid film migration (LFM) will be put forward. In succession the numerical model of LFM and normal solidification of the liquid film on grain boundary will be established and used in simulation to reveal the law and influencing factor that operates during the liquid resolidification. Finally a whole course simulation including the liquation of precipitate phases, the resolidification of the liquid film and the welding stress-strain evolution will be conducted in combination with varestraint cracking tests to investigate the influences of various processing parameters on the susceptibility to liquation cracking, where the parameters include material compositions, microstructures and welding parameters..The proposal aims at building a thorough numerical simulation technology for dynamic phase transformation concerning with liquation cracks ranging from precipitate solution, liquation to the liquid resolidification either by LFM mode or by normal solidification mode during welding of superalloys. The simulation technology established will make it possible to predict the susceptibility of superalloys to weld HAZ liquation cracking by the whole course simulation of the thermo-mechanic-microstructural evolution. Through the deepening of the understanding about the laws that governs the grain boundary liquation, liquid resolidification and even the liquation cracking, the modification of chemical composition, the pre-weld heat treatments, the selection of welding parameters and the utilization of auxiliary measures will obtain appropriate theoretical guidance and the effectiveness of liquation cracking prevention will be improved.

针对高温合金焊接热影响区微裂纹的形成，建立析出相（NbC、Laves相和γ'）组分液化数值分析模型，并结合等温液化试验、Gleeble试验和显微组织分析，通过模拟揭示晶界液化形成机理和控制因素；针对晶界液化形成的液膜，通过试验观测其重新凝固的特点，并结合理论分析，提出液膜迁移（LFM）的启动判据，建立LFM和正常凝固的理论模型，通过模拟计算揭示液膜再凝固的规律和控制因素。最后通过对晶界液化、液膜凝固及焊接应力应变的全程模拟，并结合可变拘束裂纹试验，揭示材料成分、组织因素和工艺规范对液化裂纹倾向的影响规律。.本项目将建立起从析出相液化到晶界液膜LFM和/或正常凝固的相变模拟技术，使采用热-力-组织演变全程模拟预测高温合金焊接液化裂纹倾向成为可能。通过深化对晶界液化和再凝固乃至液化裂纹规律的认识，可为合金成分改进、焊前热处理和焊接规范的选取以及辅助工艺的使用提供理论指导，提高液化裂纹防治效果。

高温合金具有较大的焊接热影响区（HAZ）液化裂纹倾向。为了获得对液化裂纹形成机理和控制因素的规律性认识，围绕晶界液化及由此形成的晶界液膜的再凝固行为开展了数值模拟研究。.首先通过焊接性试验、Gleeble热-力物理模拟试验及微观分析，获得了对高温合金焊缝凝固及HAZ晶界液化和再凝固现象的直观认识，为数值模拟研究提供了实验基础。.揭示了Gleeble热-力模拟测试热塑性用于低塑性铸造合金焊接性评价的局限性，并发现拉断力和拉断冲程能够间接反映K438铸造合金的组织或冶金变化，可与断面收缩率参量基本等效，首次提出通过Gleeble热-力模拟试验测试拉断力参量来评价低塑性铸造合金焊接性的研究方法。拉断力参量测试表明，与固溶处理比，均匀化可明显降低K438的液化裂纹倾向。.构建了焊接HAZ晶界析出相（NbC、Laves相、γ'）的组分液化模型。基于传热和物质扩散方程有限差分，编制了Visual FORTRAN 计算程序，完成了焊接热影响区NbC组分液化过程的数值模拟，揭示了组分液化的一般过程及工艺因素的影响。.基于晶界液膜附近固液相界面自由能平衡和摩尔自由能公切线构图法，建立了包含固相协调应变能和曲率作用的溶质浓度差驱动液膜迁移的数值模型，并提出了液膜迁移（LFM）启动的初步判据。.基于改进的MF模型建立了晶界液膜的溶质再分布正常凝固数值模型，首次完成了组分液化形成的γ-Nb-C伪三元晶界液膜的凝固路径模拟，成功地预测了低熔共晶的组成和数量。.完成了与高温合金焊接热影响区液化裂纹形成有关的晶界组分液化、液膜迁移、溶质再分布正常凝固和焊接热-力过程各环节的数值模型的建立，使从晶界析出相溶解、组分液化到晶界液膜LFM和/或正常凝固的全程相变模拟技术基本得以确立。.本项目研究，把对焊接HAZ晶界液化裂纹的认识提高到新的高度，并为材料改进、工艺选取以及辅助工艺使用提供了理论指导。研究结果非常有助于提高航空发动机和地面燃气轮机热端部件焊接液化裂纹的防治效果，对确保关键结构的安全性和可靠性将发挥重要的作用。