基于累积塑性能的热变形开裂准则及应用研究

发布时间：2018-03-26 14:40

本文选题：铸态Ti60合金　切入点：热变形行为　出处：《西北工业大学》2015年博士论文

【摘要】：热塑性加工技术具有生产效率高、材料利用率高、产品质量稳定等优点，而且还能有效改善工件的力学性能，在金属零件制造过程中占据了重要的地位。在热塑性加工成形领域内，金属材料成形性能的研究一直是人们关注的重点之一，而金属的韧性断裂是影响成形性能的重要因素。随着计算机硬件和软件技术的发展以及损伤断裂力学理论的日臻完善，利用数值模拟技术，将损伤断裂力学引入有限元计算成为先进热塑性加工领域的前沿研究方向之一。然而，现有国内外的韧性开裂准则均基于室温变形而建立，对于热变形开裂的预测有一定的局限性，而且高温高应变速率条件下临界开裂参数难以测定。因此，建立热变形开裂准则以迅速、准确地预测可能出现的开裂行为，成为了热塑性加工技术发展迫切需要解决的瓶颈问题之一。为此，本文以铸态Ti60钛合金为研究对象，研究了合金的热变形行为，揭示了合金的开裂行为及其机制，建立了热变形开裂准则，并结合有限元模拟分析技术，系统分析了合金大规格棒材开坯工艺过程，优化了开坯工艺参数，并进行试验验证。主要研究内容和结果如下：基于流动应力应变数据，揭示了不连续屈服程度与晶粒尺寸成反比关系，构建了考虑应变因素的本构方程，采用动态材料模型，绘制了Ti60合金的热加工图，优化了Ti60钛合金的加工窗口，理想加工参数为变形温度1120℃，应变速率为0.01~0.1s-1和变形量为45-60%。基于热压缩试验样本，研究了Ti60合金的开裂行为及机制，其表面主要表现为45°剪切开裂和纵向自由表面开裂两种宏观断裂形貌，其微观形貌分别表现为拉长型的韧窝和等轴韧窝，且合金开裂程度随着变形温度的降低、应变速率和变形程度的增加而增加。裂纹易于在α/β相界、β晶界或氧化层中形核，并且倾向于沿着β晶界或者沿着β晶粒内的α/β丛域扩展。提出了一种采用双镜头、强补光以及专业摄影架等方式进行修正了的高速摄影新方法，有效解决了裂纹观测时裂纹产生位置随机、观测视场亮度较暗以及拍摄画面不稳定而存在临界裂纹捕获困难的问题，准确测量了合金热压缩过程的临界开裂变形量，提高了观测裂纹出现位置及时间的精度。通过对现有典型开裂准则的理论分析以及所预测的开裂位置和临界损伤变化规律可知，Frudenthal准则可以预测热压缩过程发生的心部开裂，而CL、OK、Brozzo、McClintock、Oyane、RT准则均可预测赤道处自由表面开裂，这与实际完全吻合。其中，OK准则能较好综合考虑建立开裂准则时所考虑的应力因素，且在一定程度上等价于Kuhn经验准则和McClintock空洞合并模型。基于引起Ti60合金韧性开裂的诱发应力为环向拉应力，本文选用累积塑性能模型的OK准则进行修正以期建立Ti60合金的热变形开裂准则。综合累积塑性能模型OK准则力能函数形式和温度补偿应变速率Zener-Hollomon因子，最终建立了用于预测Ti60合金高温热变形的韧性开裂准则。采用FORTRAN语言二次开发子程序将热变形开裂准则嵌入商用有限元软件DEFORM-3D中，实现了合金变形-传热-损伤耦合分析，预测结果与试验结果吻合较好，验证了模型的有效性。最后，借助已嵌入开裂模型子程序的DEFORM-3D有限元软件，以实现充分破碎铸态组织和防止开裂为目标，，对大尺寸Ti60合金铸锭进行不同砧型(平砧、120oV型砧和U型砧)拔长、单道次下不同压下量、压下速度和进给量以及多道次下拔长开坯、镦拔开坯和翻转角度等不同工艺参数下的有限元模拟。平砧拔长坯料端面开裂损伤严重及产生“端面窝心”缺陷，但实际开坯过程中无需更换模具，开坯效率较高。U型砧和120oV型砧具有较高的心部应变和较好的防开裂的效果，但型砧尺寸随试样截面尺寸变化而变化，开坯效率较低。单道次变形条件较优的变形量、压下速度以及进给量分别为30%、10-15mm/s和130mm。基于单道次优化的工艺参数，拔长开坯或镦拔开坯时，四个道次后不仅可实现铸锭组织被充分打碎，且表面未出现明显的开裂。且增加45°翻转而进行的六方拔长比90°翻转进行的四方拔长具有更好的截面应变积累和更加均匀的应变分布。实验证实，选择优化的开坯工艺参数，大尺寸Ti60钛合金铸锭变形均不会产生明显开裂。
[Abstract]:Thermoplastic processing technology has the advantages of high production efficiency, high material utilization rate, the advantages of stable product quality, but also can effectively improve the mechanical properties of the workpiece, occupy an important position in the metal parts manufacturing process. In the hot plastic forming field, research on the forming properties of metal materials has been one of the focus of attention however, fracture toughness of metal is an important factor affecting the forming performance. With the development of computer hardware and software technology development and fracture mechanics theory, numerical simulation technology, the damage and fracture mechanics into the finite element method has become one of the frontier research field of advanced thermoplastic processing. However, the current domestic and overseas toughness fracture criterion is based on room temperature deformation and the establishment, has some limitations for prediction of thermal deformation and cracking, high temperature and high strain rate under the condition of critical cracking The parameters are difficult to measure. Therefore, the establishment of thermal deformation and cracking criterion to quickly and accurately predict the cracking behavior may occur, become one of the urgent need to solve the bottleneck problem of the development of thermoplastic processing technology. Therefore, the as cast Ti60 alloy as the research object, studied alloy hot deformation behavior, reveals the cracking the behavior and mechanism of the alloy, hot deformation and fracture criterion is established, combined with the finite element simulation analysis, system analysis of large size bar alloy billet process, optimized process parameters, and test. The main research contents and results are as follows:
Based on the flow stress and strain data, reveals the discontinuous yielding degree and grain size is inversely proportional to the relationship, construct constitutive equation considering strain factors, using the dynamic materials model, drawing the hot processing map of Ti60 alloy, the optimization of the processing window of Ti60 titanium alloy, the ideal processing parameters for the deformation temperature of 1120 DEG C, should be the transmission rate of 0.01~0.1s-1 and 45-60%. deformation
Hot compression test sample based on the cracking behavior and mechanism of Ti60 alloy was studied, the surface is mainly 45 degrees and the longitudinal shear crack free surface cracking of two macroscopic fracture morphology, the microstructure were elongated type of dimples and dimple, and alloy cracking degree with the temperature decreasing, strain rate and deformation degree increases. Cracks in the alpha / beta phase boundary, nuclear beta grain boundaries or oxide, and tend to along grain boundaries or along the grain in the beta beta alpha / beta bundle domain extension.
Put forward a new method using dual lens, high-speed photography was fixed and fill strong professional photography frame etc., effectively solve the crack observation when crack location is random, observation field brightness is dark and image instability and critical crack capture difficult problem, accurate measurement of the critical cracking alloy hot compression process the amount of deformation, improve the crack observation time and the accuracy.
Through the theoretical analysis of the existing typical fracture criterion and crack position prediction and critical damage variation shows that the Frudenthal criterion can predict the heart of cracking, hot compression process of CL, OK, Brozzo, McClintock, Oyane, RT criterion can predict the equator free surface cracking, which are consistent with the real one., the OK criteria can better integrated into considering crack criterion of the stress factor, and equivalent to a certain extent in the experience of Kuhn criterion and the McClintock cavity with model induced by Ti60. The toughness of the alloy cracking stress for tension based on the cumulative plastic OK criterion performance model was modified to establish Ti60 alloy thermal deformation Cracking Criterion.
Comprehensive cumulative plastic performance model OK criterion can function form and temperature compensated strain rate factor Zener-Hollomon, finally established for predicting ductile fracture criterion of thermal deformation of Ti60 alloy at high temperature. Using FORTRAN language to develop two times the thermal deformation and fracture criterion for embedded subroutine of the commercial finite element software DEFORM-3D, the alloy thermoviscoplastic damage coupling the analysis and prediction results are in good agreement with the test results, verify the validity of the model.
Finally, with the help of finite element software DEFORM-3D has been embedded in cracking model subroutine, to achieve full of broken cast structure and prevent cracking as the goal, different type of anvil of large size ingot of Ti60 alloy (flat anvil, anvil type 120oV and type U anvil) pull long, single pass under different pressure, and multi pass pull long billet under speed and feed pressure, finite element simulation of upsetting cogging and flip angle under different process parameters.
Flat anvil and blank end cracking damage serious "face." defects, but the actual opening without the need to replace the mold blank in the process of high efficiency billet type.U and type 120oV anvil high heart strain anvil and good anti cracking effect, but the anvil with the size of cross section size of samples changes billet, low efficiency.
Single pass deformation deformation conditions. The optimum pressure speed and feed rate were 30%, 10-15mm/s and 130mm. single optimization based on process parameters, drawing billet or upsetting billet, four times after not only can realize the ingot microstructure was fully broken, and surface cracking of Ming Dynasty the section strain and increase the pull. Four 45 degree turn and six party pulled longer than the 90 degree turn for long has better accumulation and more uniform strain distribution. Experiments proved that the optimized blooming process parameters, large size Ti60 titanium alloy ingot deformation are not obvious cracking.

【学位授予单位】：西北工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.23

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