纯钛及钛合金强力旋压损伤演化规律及韧性断裂预测

发布时间：2018-08-16 12:03

【摘要】：旋压成形技术作为制造大型薄壁回转类零件最有效的工艺方法,已越来越多的应用于纯钛及钛合金的塑性成形过程。但是,纯钛及钛合金属于难变形金属,在旋压过程中容易产生韧性断裂,严重制约了纯钛及钛合金旋压技术的应用。本文针对纯钛及钛合金强力旋压成形过程中的韧性断裂问题,研究了TA2纯钛在不同应力状态下的韧性断裂行为,利用韧性断裂准则预测了TA2纯钛筒形件可旋性试验和板材可旋性试验的旋压极限及损伤演化过程,揭示了筒形件和板材可旋性试验中旋压破裂的韧性断裂机制。针对Ti-15-3钛合金筒形件强旋过程中的表面开裂现象,研究了减薄率对筒形件内外层损伤演化的影响,揭示了低塑性材料筒形件强旋的韧性断裂机制。开展了TA2纯钛不同应力状态下的韧性断裂实验,分析了不同应力状态下塑性变形的特点,研究了应力状态改变对TA2纯钛韧性断裂行为的影响。在高应力三轴度下,随着应力三轴度的增加,材料的断裂应变逐渐减小。负应力三轴度下压缩试验的断裂应变最大。低应力三轴度下纯剪切试验的断裂应变较小,仅高于缺口半径R2的圆棒拉伸试验的断裂应变。将韧性断裂准则耦合到有限元软件ABAQUS中,预测了TA2纯钛不同应力状态下韧性断裂试验的损伤分布,确定了各个韧性断裂准则的适用范围,并获得了各个韧性断裂准则在不同应力状态下的损伤极限。开展了TA2纯钛筒形件的可旋性试验研究,确定了裂纹的萌生位置及其筒形件强力旋压的可旋性。利用韧性断裂准则预测了TA2纯钛可旋性试验的损伤分布。结果表明,除了Ayada准则和Freudenthal准则,其他准则都可以预测TA2纯钛管材的外表面裂纹萌生。研究了应变、应力状态等因素对TA2纯钛可旋性试验韧性断裂的影响。根据旋压变形应力应变呈周期性变化的特点,提出了周期应力三轴度和周期罗德系数概念,分析了断裂位置内外层金属的应力状态随旋压过程的演化规律。研究结果表明,断裂位置的最外层金属由于高应力三轴度和大塑性变形的影响在旋压过程中最有可能首先开裂。研究了网格高宽比对韧性断裂准则预测精度和损伤累积的影响。在所有选用的准则中,C-L准则具有最高的预测精度并且受网格高宽比改变的影响较小,而网格高宽比对R-T准则和MC准则的预测精度有很大影响。此外,相比于压缩试验,拉伸试验预测的筒形件可旋性数值更接近实验值,相应的预测精度也高于压缩试验。针对Ti-15-3钛合金筒形件强旋中的破裂情况,进行了不同减薄率下的损伤演化预测,研究了低塑性材料筒形件强旋过程中表面裂纹的产生机制。对于本文使用的韧性断裂准则,只有McClintock准则的预测规律与试验结果相符。由于同时考虑了第一主应力和第二主应力对损伤累积的影响,在中间减薄率下,McClintock准则捕捉到了最内层金属第二主应力在隆起变形阶段和减薄变形阶段的符号变化,从而预测了一个可以消除Ti-15-3表面裂纹的减薄率安全区域。进行了TA2纯钛板材可旋性试验研究和损伤预测研究。分析了应力、应力状态等因素对TA2纯钛板材可旋性试验韧性断裂行为的影响。结果表明,最大剪切应力对内层金属的韧性断裂影响较小,但内层金属在整个成形过程中具有较高的应力三轴度。虽然外层金属的应力三轴度低于内层,但是外层金属具有高于内层的等效塑性应变,并且最大剪切应力对外层金属的韧性断裂影响显著。因此,对于本文所选用的韧性断裂准则,主要考虑最大主应力或应力三轴度对韧性断裂影响的模型,其预测的最大损伤值出现在旋压板材的内层,与实验不符。只有Freudenthal,LeRoy以及Lou和Yoon提出的韧性断裂准则,可以预测TA2纯钛板材可旋性试验中外层金属的裂纹萌生。
[Abstract]:As the most effective technology for manufacturing large thin-walled rotating parts, spinning has been widely used in the plastic forming process of pure titanium and titanium alloys. However, pure titanium and titanium alloys are difficult to deform and are prone to ductile fracture during spinning, which seriously restricts the application of pure titanium and titanium alloy spinning technology. Aiming at the problem of ductile fracture of pure titanium and titanium alloy during high-strength spinning, the ductile fracture behavior of pure titanium TA2 under different stress states is studied. The spinning limit and damage evolution of TA2 barrel-shaped parts and sheet-metal spinnability tests are predicted by the ductile fracture criterion. The barrel-shaped parts and sheet-metal spinnability tests are revealed. Ductile fracture mechanism of spinning fracture in spinning test. Aiming at the surface cracking phenomenon of Ti-15-3 titanium alloy cylindrical parts during strong spinning, the effect of thinning rate on damage evolution of inner and outer layers of cylindrical parts was studied. The ductile fracture mechanism of low plastic material cylindrical parts under strong spinning was revealed. The characteristics of plastic deformation under different stress states are analyzed, and the effect of stress state change on the ductile fracture behavior of pure titanium TA2 is studied. At high stress triaxiality, the fracture strain decreases with the increase of stress triaxiality. The fracture strain of compression test under negative stress triaxiality is the largest. The ductile fracture criterion is coupled to the finite element software ABAQUS to predict the damage distribution in the ductile fracture test of pure titanium TA2 under different stress states. The applicable ranges of the ductile fracture criteria are determined and the ductile fracture criteria are obtained. Damage limits of TA2 titanium cylindrical parts under different stress states were studied. The location of crack initiation and the spinnability of TA2 titanium cylindrical parts under strong spinning were determined. The damage distribution of TA2 titanium cylindrical parts under spinnability test was predicted by using ductile fracture criterion. The results show that there are other criteria besides Ayada criterion and Freudenthal criterion. The effect of strain, stress state and other factors on the ductile fracture of TA2 titanium in spinning test was studied. According to the periodic variation of stress and strain during spinning, the concepts of periodic stress triaxiality and periodic Rhode coefficient were put forward, and the stress of inner and outer layers of metal at the fracture position was analyzed. The results show that the outermost metal in the fracture position is most likely to crack first in the spinning process due to the influence of high stress triaxiality and large plastic deformation. In addition, compared with the compression test, the predicted value of cylinder spinnability in tensile test is closer to the experimental value and the corresponding prediction accuracy is higher than that in compression test. The damage evolution of titanium alloy cylindrical parts under different thinning rates was predicted and the mechanism of surface crack formation during strong spinning was studied. For the ductile fracture criterion used in this paper, only McClintock criterion was found to be consistent with the experimental results. The McClintock criterion captures the symbolic changes of the second principal stress of the innermost metal during the uplift and the thinning stages at the intermediate thinning rate, thus predicting a safe thinning rate region for eliminating surface cracks on Ti-15-3. The results show that the maximum shear stress has little effect on the ductile fracture of the inner layer metal, but the inner layer metal has higher stress triaxiality in the whole forming process. The stress triaxiality is lower than the inner layer, but the outer layer metal has higher equivalent plastic strain than the inner layer, and the maximum shear stress has a significant effect on the ductile fracture of the outer layer metal. Only the ductile fracture criterion proposed by Freudenthal, LeRoy and Lou and Yoon can predict the crack initiation of the outer metal in the spinnability test of pure titanium TA2 plate.
【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG306

【参考文献】