Multiaxial cyclic behaviour of extruded and forged AZ80 Mg alloy
Multiaxial cyclic behaviour of extruded and forged AZ80 Mg alloy
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DOI：
10.1016/j.ijfatigue.2019.06.015
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发表时间：
2019-10-01
影响因子：
6
通讯作者：
Su, X.
中科院分区：
文献类型：
--
作者：
Gryguc, A.;Behravesh, S. B.;Su, X.
研究了挤压态和闭式挤压锻造态AZ 80镁合金在250 ℃、20 mm/s锻造条件下的准静态和循环响应。伺服液压，完全反向，应变控制的单轴“推拉”，纯扭转，和双轴疲劳试验也进行了闭模锻造合金。研究了两种单轴加载路径（纯轴向和纯剪切）和三种不同的双轴加载路径（比例加载和相位角为45 °和90 °的非比例加载）。挤压态坯料经锻造后，轴向屈服应力提高了46%，断裂伸长率提高了35%，极限强度提高了13%。对锻件横截面上局部织构、显微组织和表面硬度的空间变化进行了研究和讨论。锻造坯料中心的腹板区域表现出几乎完全再结晶和细化的显微组织，具有强烈的基底织构，而两侧的肋区域表现出部分再结晶和较粗糙的显微组织，具有较不强烈的织构。锻造材料在轴向方向上的疲劳寿命相对于在应变幅小于0.35%的挤压材料显示出显著的改善。然而，在更高的应变幅下，锻造材料（相对于挤压态）中存在显著更高的循环应力（和相关的疲劳损伤），导致在完全反向应变控制的疲劳载荷下的寿命降低。对于纯轴向应变路径，在稳定的响应中观察到的应变能密度（SED）的总量仅是循环次数的函数，并且对材料条件（挤压或锻造）不敏感。然而，对于锻造材料，在研究的所有应变路径中，只有纯剪切路径表现出显著不同的总SED与寿命的关系。此外，对于一个给定的周期数的总SED的量减少时，双轴加载时，虽然遵循相同的一般关系作为纯轴向应变路径。减少总SED的影响，对于一个给定的周期数发生在所有的双轴应变路径，与比例是最不突出的，然后45度和90度的相位分别逐渐更加明显。利用Smith沃森Topper（SWT）和Jahed-Varvani（JV）模型预测单轴和双轴循环试验的疲劳寿命。分别使用两种单轴加载路径对每个模型的内参数进行校准，然后将这些相同的参数相互结合使用，以预测双轴情况下的寿命。SWT和JV模型都能够给出可接受的预测，然而JV模型的预测更可靠，特别是当循环响应具有明显的不对称性时。双轴疲劳响应在一定程度上由轴向分量主导，原因有两个：首先，剪切方向上的滞后形状根据相位角和轴向应变振幅发生显著变化，而轴向滞后回线对其他加载轴的存在或调制保持一定不变。第二，在除纯剪切外的所有应变路径中，初始裂纹扩展模式主要是横向于轴向主导双轴失效模式的拉伸裂纹提供了对现有的基于能量的双轴疲劳寿命预测模型的修正因子的定性证明，使其能够可靠地预测各种不同双轴应变路径下挤压和锻造AZ 80镁合金的寿命。
The quasi-static and cyclic response of as-extruded and closed-die extruded-forged AZ80 Mg alloy forged at 250 degrees C and 20 mm/sec was investigated. Servo hydraulic, fully reversed, strain controlled uniaxial "push-pull", pure torsion, and biaxial fatigue testing were also conducted on the closed-die forged alloy. Two uniaxial loading paths (pure axial and pure shear) and three different biaxial loading paths were investigated (proportional loading, and non-proportional loading at phase angles of 45 and 90). After forging of the as-extruded billet, the yield stress in the axial direction increased by 46%, the failure elongation by 35% and the ultimate strength by 13%. The spatial variation in both the local texture, microstructure and superficial hardness throughout the cross section of the forging was investigated and discussed. The web region in the center of the forged billet exhibited a virtually fully recrystallized and refined microstructure with intense basal texture, whereas the rib regions on each side exhibited a partially recrystallized and coarser microstructure with less intense texture. The fatigue life of the forged material in the axial direction showed dramatic improvement relative to the as-extruded material at strain amplitudes which were less than 0.35%. However, at higher strain amplitudes, there are significantly higher cyclic stresses (and associated fatigue damage) in the forged material (relative to the as-extruded), resulting in a decrease in life under fully reversed strain controlled fatigue loading. For the pure axial strain path, the total amount of strain energy density (SED) observed in the stabilized response was a function of number of cycles only, and was insensitive to material condition (as-extruded or forged). However, for the forged material, of all the strain paths investigated, only the pure shear path demonstrated a significantly different relationship of total SED vs. life. Furthermore, the amount of total SED for a given number of cycles decreases when biaxial loading is introduced, though following the same general relationship as the pure axial strain path. The effect of a decrease in total SED for a given number of cycles occurred in all biaxial strain paths, with proportional being the least prominent, then 45 degrees and 90 degrees out of phase being progressively more pronounced respectively. Both the Smith Watson Topper (SWT) and Jahed-Varvani (JV) models were utilized to predict the fatigue life of the uniaxial and biaxial cyclic tests. Calibration of each models intrinsic parameters were done using both of the uniaxial loading paths individually, then these same parameters were used in combination with one another to predict the life in the biaxial cases. Both the SWT and JV models are capable of giving an acceptable prediction, however the JV model's prediction is more reliable, especially when the cyclic response has pronounced asymmetry. The biaxial fatigue response is somewhat dominated by the axial component for two significant reasons; firstly, the shape of the hysteresis in the shear direction dramatically changes based on phase angle and axial strain amplitude, whereas the axial hysteresis loop remains somewhat invariant to the presence or modulation of the other loading axis. Secondly, the initial crack propagation mode is predominantly transverse to the axial direction in all strain paths except for pure shear (which exhibits longitudinal cracking).Tensile cracking dominating the biaxial failure mode provides qualitative justification of a modification factor to an existing energy based biaxial fatigue life prediction model, enabling it to predict the life of extruded and forged AZ80 Mg reliably for a variety of different biaxial strain paths.