This research investigated the influences of some key factors in the prestressed laser peen forming (PLPF) process, namely, the plate thickness, the coverage ratio, and the prestress, on the deformation of 2024-T351 rectangular plates through experiments and numerical simulations. In the experiments, laser parameters, such as the laser energy and spot size, were kept unchanged, and prestress was applied through a piece of self-developed, four-point-bending equipment. The curvature radius of the samples was measured through a digital radius gauge. A corresponding finite element analysis (FEA) model of PLPF was also established to simulate the full procedure of the PLPF, including prebending, laser shock peening, and spring back. Based on the PLPF experimental results, an artificial neural network (ANN) was trained to help to design the process parameters, including the coverage ratio and the amount of prebending, according to the plate thickness and the target curvature radius. By adding a penalty term to the loss function, the amount of prebending (AOP) can be reduced as much as possible. The validation of the ANN was confirmed by three other PLPF experiments.

Laser peen forming (LPF) is an appealing technique for forming metal sheets using high-energy, short-duration laser pulses. The deformation of the target metal plate is closely related to the magnitude and distribution of laser-induced residual stress. Consequently, the relationship between process parameters and residual stress is worth researching. In this research, two process parameters in LPF, laser energy and coverage ratio (spot distance essentially), and one workpiece parameter, plate thickness, were examined through an element method (FEM) of multiple square-spot laser shock peening (SSLSP). Corresponding experiments of SSLSP on aluminum alloy 2024-T351 test blocks were conducted, together with an X-ray diffraction (XRD) residual stress measurement and a surface morphology observation. The FEM simulation and experimental results show that congested laser spots had a significant influence on the magnitude of compressive residual stress; higher laser energy was beneficial to the depth of the compressive stress layer but could decrease its magnitude. Therefore, for better forming ability, higher laser energy and a higher coverage ratio are beneficial; for surface strengthening, laser energy should not be too large, and the coverage ratio should be larger than 100% to ensure that the residual stress on the treated surface is compressive, resulting in better surface integrity.

Laser peen forming (LPF) is an advanced plate forming process using high-energy, short-duration laser pulses. Compared with traditional shot peen forming (SPF), the location of every single laser shock can be determined precisely, which makes it possible to control the deformation of the plate to meet the target deformation by designing a proper spot array. In this study, based on theoretical calculation and finite element analysis (FEA), we first discussed the variation of laser-induced residual stress and bending moment with the distance between laser shock spots. After that, based on the mechanical analysis, FEA and LPF experiment, we studied the influence of non-uniform spot array and different coverage ratio on the deformation of square 2024-T351 aluminum alloy test plates. Through research, it was first proved that spot distance has a significant effect on bending moment, and as the distance decreases, the interaction between the residual stress fields becomes stronger. The non-uniform spot array can overcome the inherent spherical tendency of shot peening to a certain extent.

为研究拉伸速率和温度对TC4钛合金性能的影响,采用电-热-力完全耦合方法,运用林建国统一粘塑性本构模型,使用有限元分析软件ABAQUS,对不同温度和拉伸速率下的TC4钛合金电热拉伸过程进行模拟研究,并选取目标温度为750℃、拉伸速率为1 mm·min-1的这一组模拟结果与试验结果进行对比.对比结果显示:有限元模拟中通电加热后得到的温度场与试验中同一阶段采用热像仪测得的温度场分布相似,标距段中间部位水平线上的平均温度相差仅为2.1℃;此外,有限元模拟得到的拉伸力-位移曲线与试验曲线的变化趋势相近,最大拉伸力相差0.014 kN,误差约为2.41％,最大拉伸力出现时的伸长量相差0.148 mm.可见有限元模拟结果与试验结果较为吻合,证明了有限元模拟方法的可靠性.有限元模拟得到的拉伸力-位移曲线和试验曲线均表明:相同应变速率下,温度越高,钛合金塑性越好,在应变速率较高时,温度的影响作用减弱;在相同温度下,应变速率越低,钛合金的塑性越好,在温度较低时,应变速率的影响作用减弱.

为了对物体表面形貌进行精准的无损测量,设计了一种基于激光位移传感器的物体表面形貌测量系统,该系统由硬件部分和软件部分组成,硬件部分包括激光位移传感器、光栅位移平台、PCL833三轴正交编码计数器卡,软件部分包括数据采集软件和MATLAB数据处理程序.被测物体在光栅位移平台上按一定的轨迹移动,数据采集软件将X、Y、Z方向的离散点坐标数据进行存储,最后采用高斯滤波,由MATLAB程序对数据进行处理,并绘制被测物体的三维形貌图.

为了分析激光喷丸作用后,以2024-T351铝合金为代表的高强航空铝合金材料表面和内部残余应力的分布情况,并为后续板料激光喷丸变形分析打下基础,在ABAQUS软件平台上,构建了一种方形光斑激光冲击的有限元计算模型,对方形光斑激光冲击诱导的残余应力场进行了数值模拟,分析了在无搭接和搭接率为25％的两种情况下,激光冲击产生的残余应力场在空间上的分布特征,发现冲击坑中心处残余压应力值较大且均匀性较好,边缘处出现了较大的拉应力且应力梯度较大.通过进行无搭接的激光喷丸实验和X射线衍射应力测试实验,验证了有限元模型的有效性.试件表面形貌通过激光位移传感器进行了测量观察.

The true stress-strain curves of 2024 aluminum alloy plate are investigated under different deformation conditions through the cyclic bending and uniaxial tensile tests. Under this condition, the equivalent strain limit is improved with the increase of the times of circle bending, which is linear accumulated along the line that is perpendicular to the neutral layer on which the bending strain is zero. Furthermore, the tensile test with the cyclic bended plate samples were carried out. The equivalent strain of layer is calculated and compared with original tensile test. It is found that the ultimate strain limit of average equivalent strain of the layers on section is extended with a significant value.

使用CSS-44100电子万能试验机对通直流电加热的TC4钛合金试件进行单向拉伸试验,得到了TC4钛合金试件的载荷-位移曲线.在变形过程中,通过减小电流以控制材料截面积减小而导致的电阻升高,从而调整试件的温度.采用切块法,将试件均匀分段,每段视为等温,从而在变形试件上建立了温度场计算模型,进一步运用林建国的粘塑性本构模型建立了试件应力-应变的计算方程.对TC4钛合金的通电拉伸试验数据进行分析计算.结果 表明:所建立的计算方法有效,可用于钛合金非绝热状态下电热单向拉伸本构模型模型参数的辨识.

带筋整体壁板因其质量轻、承载效率高等优点在现代飞机中被大量使用,其基本组成单元为单筋结构,喷丸成形是该类复杂型面零件成形的首选方法.为研究单筋件的喷丸成形规律,建立多弹丸撞击有限元模型,得到不同喷丸覆盖率下的残余应力分布;基于直接应力法建立单筋件的喷丸成形有限元模型,得到了单筋件不同区域喷丸处理对其成形的影响规律,建立了具有"波浪形"外形面单筋件的喷丸成形模拟方法,为带筋整体壁板喷丸成形实际应用提供了参考.

针对激光喷丸成形模拟中涉及的诱导应力的确定问题及整体壁板的成形模拟,建立了用于分析激光喷丸成形过程中局部区域受力情况的激光冲击有限元模型,并通过试验验证了其有效性.分析了激光冲击中心处的诱导应力随模型深度及激光冲击时间的变化规律,提出了激光冲击过程中以合力最大为依据的动态诱导应力的提取方法,得到了激光喷丸参数对稳态及动态时沿深度方向的平均诱导应力的影响规律.基于直接应力法建立了典型单筋结构的激光喷丸成形有限元模型,并赋予以合力最大为依据的动态诱导应力进行成形模拟,将模拟结果与已有的试验结果进行对比,结果表明以合力最大为依据的动态诱导应力可用于进行激光喷丸成形的有效模拟.

激光喷丸技术是一种先进的金属塑性成形和表面强化技术,相比于弹丸喷丸,激光喷丸能量密度更大,因而成形能力更强,可以用于成形刚度更大的钣金件,如飞机整体壁板,在航空航天领域有广泛的应用前景.构建一种多尺度激光喷丸成形模拟方法,包括激光喷丸诱导应力场的计算方法和基于直接应力法的工件成形曲率的预测方法.预测结果通过2024–T351铝合金块状试件和典型截面单筋件激光喷丸试验得到了验证,试验结果与模拟结果吻合较好,表明此模拟方法有效可行.

Incremental sheet metal forming (ISF) is well known as a die less and flexible forming method and is widely used for prototype part fabrication. It can extremely improve the forming limit of sheet metal. But how to get the stress-strain curve of material with improved strain limit to be used for analysis of incremental forming process? Comparing with classic global forming method, sheet metal incremental forming is local forming method. It means that any time only a small zone of entire component is deforming and moving correspondingly with the forming tool head and is undergoing a complex loading history during the forming process. Based on these characteristics, in the present study, a constitutive model is proposed to describe the stress-strain curve of material and to research the mechanism of high forming limit phenomena in ISF experimentally and theoretically. Three aspects are focused on: firstly, a constitutive model is proposed to describe the stress-strain curve of material and to adapt to the complex loading history and to analysis the mechanism of high forming limit. Secondly, an incremental forming paradigm with truncated conical shape is conducted with geometrical relationship between tool and sheet metal to calculate the strain path of sheet metal around the tool head. Thirdly, the mechanical properties are gained by tension test before and after truncated conical component made by incremental forming. The forming limit diagram is obtained by measuring and calculating the grids printed on the surface of truncate conical component. The calculation algorithm with the proposed constitutive model is programmed to calculate the plastic forming strain and stress of truncated conical component during incremental forming process. The proposed constitutive model is compared with experimental results and can be used to analysis forming limit of incremental sheet metal forming. (C) 2020 The Authors. Published by Elsevier B.V.

提出了带筋整体壁板激光喷丸成形工艺参数优化设计方法,建立了平板试件激光喷丸成形等效诱导应力的计算模型以及带筋整体壁板的激光喷丸成形曲率半径的预测模型.利用带筋整体壁板成形曲率半径计算成形弯矩,根据成形弯矩计算得到壁板激光喷丸成形的等效诱导应力,运用诱导应力计算反算激光喷丸参数,并根据曲率半径的预测误差,通过迭代优化激光喷丸成形工艺参数.采用铝合金平板试件进行正交试验和带筋整体壁板激光喷丸成形试验,结果表明,带筋整体壁板激光喷丸成形工艺参数优化设计方法可行.

大型机翼整体壁板是现代大型飞机重要的大型承力整体结构件并且通常直接构成飞机的气动外形.喷丸成形是现代大型轻质高强铝合金整体壁板件成形制造的首选技术方法,但如何实现大型机翼整体壁板的精确喷丸成形一直是现代航空制造技术领域的一个难点问题.针对这一工程问题,本文采用系统化的方法,将影响大型机翼整体壁板喷丸成形精度的因素分解为壁板平面板坯误差、成形参数设计准确度、成形参数控制精度、环境因素.针对这些因素,采用基于变形位能最小的板坯优化设计来减小由板坯导致的成形误差;采用数据拟合、人工神经网络以及解析模型计算相结合的喷丸成形参数综合设计方法来提高喷丸参数设计的精度和效率;建立了板坯修正模型以修正环境温度、喷丸设备参数波动等因素对成形件形状和尺寸的影响;对于从喷丸设备上下线后仍存在的外形贴模误差,则采用手提喷丸机进行局部的渐进式校形喷丸至外形贴模.壁板喷丸成形的工程实践表明,本文所提出的系统化方法能够有效提高大型机翼整体壁板喷丸成形的精度和效率,并可满足工业生产的需求.

钛合金型材弯曲构件由于性能优异而逐渐成为先进民用复合材料机身的主要承力构件,其成形质量直接关系到飞机的装配精度.本文针对OT4M钛合金L型材的拉弯成形工艺进行了研究,建立了拉弯过程的解析模型,并在不同的试验条件下进行了多组热单轴拉伸试验来探究型材的热变形行为,利用绝缘模具开展了钛合金型材的电热拉弯成形试验,结果表明,材料的塑性变形能力受变形温度和速度影响显著,拉弯成形时绝缘模具的应用可使钛合金型材保持在相对较高的温度490℃下成形,进而降低了回弹.

当采用位移法进行型材拉弯成形过程有限元模拟时,需要设计夹头的运动轨迹.针对于拉弯件引导线是复杂的二维变曲率问题难以处理,提出了一种根据型材拉弯零件数模截面形心引导线离散点数据,得出不同预拉量和在拉弯过程中截面中性层内移量与夹头在拉弯过程中的位移轨迹坐标的关系的计算模型,并根据有限元边界条件的定义要求,通过变换和归一处理,建立了拉弯成形模拟过程中夹头的位移边界条件定义数据的计算方法.通过有限元模拟实例验证,表明所建立的拉弯成形夹头轨迹设计算法准确可靠.

采用标准试样在材料试验机上按照规定的试验方法和程序,通过准静态单向拉伸试验得到航空领域常用2024、6061、7075三种规格的铝合金板料不同状态、不同厚度和取向(轧制方向、横向、45°方向)的拉伸性能参数及应力应变数据.选用Hockett-Sherby双Voce模型,通过最小二乘法拟合未经淬火处理的合金板料的真实应力—真实应变曲线,选取Swift模型拟合经淬火处理的合金板料真实应力—应变曲线,得到相应的模型参数.

In this investigation, the attention is focused on the minimum bending radii of 2196-T8511 and 2099-T83 Al-Li alloy extrusions. To predict the failure of Al-Li alloys, sheet and extrusion stretch bending tests are developed, carried out and simulated using finite element model. The theoretical minimum bending radius is introduced to derive a safe lower limit for the bending radius which can serve as a guideline for tool and product design. Stretch bending tests of Al-Li alloys are performed using the three-point bending test and displacement-controlled stretch bending test at room temperature. The finite element model incorporates three-dimensional solid elements and ductile damage modeling. The experimental results show that Al-Li alloy extrusions in stretch bending show three types of failures, occurring at the unbent region near the entrance of the jaws, at the region below the exit of the die and within the region in contact with the die, respectively. Comparison between predicted values and experimental results has been made, a consistent agreement being achieved, reflecting the reliability of the present model. The three types of failure mechanisms which compete with each other are tensile localization failure, die-corner failure and shear failure, respectively. Based on the analytical models, experiments and simulations, it appears that the three distinct failures need to be applied to predict the minimum bending radius and range of failures that can occur with 2196-T8511 and 2099-T83 Al-Li alloy extrusions in stretch bending.

Residual stresses occur in composite structures during curing process which play an important role in the deformation and mechanical properties of composite, especially for thick laminates. However, the experimental measurement of curing residual stresses is often costly and complicated. Alternatively, computational tools are used to predict the curing residual stresses. Considering the effect of multi-scale in composites, this paper proposes a multi-scale model to predict the residual stresses of composites during the curing process. At the part level, a macro-scale three-dimensional model, which incorporated the thermo-chemical model and residual stress model, is developed by considering the time-dependent properties of material performances during curing process. The two sub models are mathematically coupled to solve for the process with variables interactively to obtain part-level temperature, degree of cure gradients and macro curing residual stresses. At the reinforcement level, a representative volume elements (RVE) is employed to calculate the micro-scale residual stresses by using the results of macro-scale simulations. The results show there is a significant difference in the calculation of micro residual stresses by introducing the effect of multi-scale model. Subsequently, the effect of different boundary conditions and fiber arrangement are discussed.

For the electric-thermal stretch bending of titanium alloy, a numerical simulation method of sequential coupling of electro-thermal stretch bending process was established, and a multi-process and multi-field coupling numerical simulation process of electro-thermal stretch bending with turntable was realized.Then, the dynamic explicit thermal-mechanical coupling analysis algorithm was selected based on J-C fracture criteria, and a 3D model predicting the forming limit and fracture of electro-thermal stretch bending was built.Comparing the experimental result with simulation result of extrusion T-type profile for Ti-6Al-4V titanium alloy, it is found that the failure of material in the electric-thermal stretch bending process is mainly because of the excessive tensile force on the pre-stretching and post-stretching stages.Therefore, the main factors influencing on the forming limit of electric-thermal stretch bending are the heating temperature or current density, the pre-stretching force, the temperature or cooling time of the post-stretching process and the post-stretching force.Furthermore, the predicted relative errors of limit stress for the pre-stretching and post-tensioning are 19.7% and 19.1% respectively, and the effectiveness of model is verified.