7050铝合金蠕变时效成形本构模型研究

为研究7050T451铝合金蠕变时效本构模型,在160℃、不同应力条件下进行单轴拉伸蠕变试验,分析了蠕变应变、屈服强度和微观组织随时间的变化规律.基于高强铝合金析出强化理论,建立了能描述蠕变时效成形宏观及微观变化的本构方程,并运用遗传算法对材料常数进行拟合优化.研究表明,该模型在不同应力水平下与试验结果吻合良好,能够用来模拟分析蠕变时效成形过程.     

铝合金时效成形及时效成形铝合金

介绍了铝合金时效成形的方法、基本原理、工艺特点及应用情况,概括了国外研究铝合金时效成形工艺及相关可时效成形铝合金的现状,重点阐述了时效成形对铝合金微观组织结构的影响.     

回归再时效中预时效温度对7050铝合金应力腐蚀性能的影响

采用硬度测试、电导率测试、慢应变速率拉伸、透射电镜和扫描电镜等方法,研究了回归再时效热处理工艺中预时效温度对7050铝合金微观组织和应力腐蚀性能的影响。结果表明:随着预时效温度升高,回归再时效后7050铝合金晶内析出相从以GP区为主转变为以η′相为主,晶界析出相逐渐粗化,晶界变得不连续分布,合金应力腐蚀敏感性降低;但晶界无沉淀析出带宽度增加,120℃时达到140nm,易导致应力集中和阳极溶解,合金抗应力腐蚀性能下降。预时效温度为80℃,即稍微欠时效时,7050铝合金抗应力腐蚀性能较好,在缓慢应变速率(10-6s-1)和3.5%NaCl溶液腐蚀介质下,合金抗拉强度为473.5MPa,伸长率为10.67%,应力腐蚀指数为0.05824。     

7050-T7451铝合金低周疲劳平均应力松弛规律

为了研究平均应变对7050-T7451铝合金低周疲劳力学行为的影响,开展了不同应变比(R=-1、-0.06、0.06和0.5)下的室温恒幅低周疲劳试验。结果表明:在对称循环应变下,材料总体表现为循环软化特征;而在非对称循环应变下,材料表现为初始硬化后的循环稳定行为。非对称循环应变导致了材料出现与应变幅相关的平均应力松弛现象。采用Landgraf模型和非线性Maxwell模型分别研究了7050-T7451铝合金的平均应力松弛规律。结果表明:Maxwell模型能够较准确地描述材料的平均应力循环松弛特征,而Landgraf模型更适用于低应变幅下的平均应力松弛描述。     

工艺参数对7075铝合金带筋壁板时效成形回弹的影响

目的 研究7075航空高强度铝合金带筋壁板时效成形过程中工艺参数对零件回弹的影响,以提高筋板类零件的产品质量。方法 利用正交试验开展不同工艺参数组合下的带筋壁板时效成形试验,并对带筋壁板时效成形后的回弹率进行极差分析和规律曲线分析。结果 带筋壁板时效成形后的回弹同时受到时效参数和筋板结构参数的影响,各因素按对回弹的影响程度由大到小的顺序依次为时效时间、筋条厚度、筋条高度和时效温度,筋条结构参数对调节构件回弹有重要作用,并且回弹率实测值基本处于回弹率随工艺参数变化的拟合曲线附近。结论 回弹率随时效时间和筋条高度的增加表现为非线性下降趋势,随筋条厚度的增加呈现线性上升趋势。7075铝合金带筋壁板时效成形后的回弹行为可以用回弹率回归方程进行较为合理的描述。     

7050铝合金热压缩变形的流变应力本构方程

对7050铝合金在应变速率为0.01～10s-1、变形温度为250～450℃条件下的流变应力行为进行了实验研究.结果表明:7050铝合金热压缩变形中发生了明显的动态回复与动态再结晶,流变应力随应变速率的增加而增加,随温度的增加而降低;通过线性回归分析计算出7050材料的应变硬化指数n以及变形激活能Q,获得了7050铝合金高温条件下的流变应力本构方程.     

7050铝合金结构件热处理与冷成形过程残余应力演化规律的数值模拟

基于Hansel-Spittel黏塑性流变应力模型以及应力-应变曲线构建7050铝合金在不同温度区间的材料本构方程.采用有限元仿真技术,首先分析试块级试样淬火与冷压缩过程的残余应力变化,模拟的分布规律与试块的超声测试分析结果一致.在此基础上,研究带筋条结构的铝合金结构件淬火热处理与冷变形工艺残余应力演化规律,并对结构件开展了超声残余应力测试和机加工变形验证.结果表明:淬火后残余应力呈外压内拉分布.不同的冷变形工艺对结构件淬火残余应力的消减程度有较大差异.对于冷压缩工艺,当变形量超过2％后,辐板压缩对辐板心部的应力状态改善较好,而筋条压缩只改善筋条局部位置的应力状态.3％压下量的冷拉伸工艺对整体的应力状态改变较大,可同时有效改善筋条部位和辐板部位残余应力的均匀性.经冷拉后,铝合金结构件机加工变形程度可得到明显改善.     

时效成形对2324铝合金组织及性能的影响

研究了时效成形对2324铝合金力学性能的影响,利用透射电镜观察了时效成形过程中的微观组织.研究了时效成形产生的"应力位向效应"及其对力学性能的影响,对"应力位向效应"的特征和产生原因进行了分析.结果表明:由于外加应力造成了平行应力方向和垂直应力方向扩散系数的差异,导致析出相择优生长.无应力时效时析出相沿{201}晶面族均匀析出,时效成形过程中析出相沿平行(111)方向择优生长,择优生长方向主要取决于外加应力的方向.     

pH值对7050铝合金膜致应力和应力腐蚀敏感性的影响

采用慢应变速率拉伸法和流变应力差值法研究了7050铝合金在3.5%(质量分数)NaCl水溶液中膜致应力和应力腐蚀敏感性随pH值的变化规律。结果表明:当pH≤7时,随着pH值的增大,膜致应力和应力腐蚀敏感性均下降,当pH7时,膜致应力和应力腐蚀敏感性随着pH值的增大而提高;而当pH=1,14时,腐蚀的类型为剥蚀,合金基体发生剥落,表面没有钝化膜产生。pH值在6~9之间时,膜致应力随pH的变化比较平缓,而pH在2~5和10~13之间时,膜致应力变化则较为剧烈,整体的变化曲线呈山谷形。膜致应力和应力腐蚀敏感性具有很强的相关性。XPS研究表明,膜致应力值与钝化膜的成分有关。     

Analysis and prediction of microstructural effects on long-term fatigue performance of an aluminum aerospace alloy

A program of experimental and analytical tasks has been conducted to define the linkage(s) between microstructural characteristics and fatigue performance in an aluminum alloy typically used for airframe structural applications. The first goal was to develop data for quantitatively linking measurable characteristics of material microstructure with long-term fatigue performance. The second goal was to develop models to predict fatigue performance based on the microstructural characteristics. The work focused on several process variants of aluminum alloy 7050-T7451 plate. This material was chosen because of its widespread use for flight-critical airframe structural components, and the particular characteristics associated with the manufacturing, service and maintenance of thick section components. Within the framework of this objective, life-limiting microstructural features have been identified and ranked by severity, and models to quantitatively describe the evolution and growth of macrostructural cracks from those features have been developed.The modeling framework has been applied to predict the cyclic lifetime of the 7050 alloy process variants based on the populations of life-limiting microstructural features. In addition, the models have been used to show how changes in the material characteristics may affect the fatigue performance. This includes predictions of the effect of changing the life-limiting microfeature size and shape distributions, and the effect of changing material strength properties.The use of this modeling approach to probabilistically describe the implications of changes in the microstructure has been demonstrated, thereby allowing the effects of material pedigree to be predictively linked with the structural integrity of end components. The modeling framework has potential applications in airframe design support processes, and as a tool for use in material and product form selection processes.     

Influence of anodization on the fatigue strength of 7050-T7451 aluminium alloy

In recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050-T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material. A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.     

The effect of laser power density on the fatigue life of laser-shock-peened 7050 aluminium alloy

Laser shock peening (LSP) is an innovative surface treatment method that can result in significant improvement in the fatigue life of many metallic components. The process produces very little or no surface profile modification while producing a considerably deeper compressive residual stress layer than traditional shot peening operations. The work discussed here was designed to: (a) quantify the fatigue life improvement achieved by LSP in a typical high strength aircraft aluminium alloy and (b) identify any technological risks associated with its use. It is shown that when LSP conditions are optimal for the material and specimen configuration, a —three to four times increase in fatigue life over the as-machined specimens could be achieved for a representative fighter aircraft loading spectrum when applied at a representative load level. However, if the process parameters are not optimal for the material investigated here, fatigue lives of LSP treated specimens may be reduced instead of increased due to the occurrence of internal cracking. This paper details the effect of laser power density on fatigue life of 7050-T7451 aluminium alloy by experimental and numerical analysis.     

Fatigue life recovery in aluminium alloy aircraft structure

Abstract This paper presents the results of several DSTO research programmes which investigated the effectiveness of the fatigue life enhancement method used on RAAF F/A‐18 aircraft – glass bead peening. The research identified ways in which process improvement could enhance this effectiveness, and developed a procedure for mid‐life reworking of critical airframe parts to effectively restore the original fatigue life. The procedure included removing a very thin layer of material (and with it, any undesirable manufacturing features and accumulated fatigue cracking) from the surface. Further life recovery can be achieved, if required, by applying an optimized peening procedure. This process has been developed to allow restoration of fatigue life to critical airframe components which are thought to be accumulating fatigue crack damage faster than desired.     

工艺因素对TC4合金应力松弛行为的影响

研究了TC4合金应力松弛行为的特征和工艺因素的影响.结果表明,TC4合金应力松弛行为可以分为两个阶段:第一阶段,应力松弛非常快,第二阶段应力松弛缓慢,但长时间应力松弛后剩余应力趋向于一极限值;应力松弛行为可以用应力松弛极限和应力松弛速率两个参量进行表征;温度升高时,松弛速率加快,应力松弛极限降低;同一温度不同初应力作用的应力松弛,应力松弛极限相同.     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

CrMoWV钢的应力松弛行为及其预测

以12Cr-1Mo-1W-0.25V耐热钢550,600℃的8760h松弛实验数据作为对象,研究短时间松弛数据准确而有效预测长时间松弛应力的方法。在采用松弛模型对长时间松弛应力进行直接拟合外推时,发现模型参数与所采用拟合数据的时间长度呈规律性的变化。提出考虑模型参数规律变化的时序参数法,以高精度预测长时间松弛应力。通过对比时序参数法与直接拟合外推法的预测结果,认为时序参数法在用短时间松弛数据预测长时间松弛应力上具有明显优势,预测结果的准确性较直接外推法高。