铝合金弹性预应力超声喷丸成形试验

超声喷丸作为一种新型的板料成形技术,具有易操作,能耗少及优良成形性能等优点,广泛应用于板料成形中。但自由状态下,超声喷丸成形也易发生球面变形,因而提出了弹性预应力超声喷丸成形方法,以2024-T351铝合金壁板为研究对象,分析预弯力矩大小、成形轨迹和壁板厚度对成形曲率半径及成形性能的影响。试验中通过控制壁板展向的弯曲曲率来实现弹性预弯。结果表明:预应力超声喷丸能够大幅度减小壁板展向的成形曲率半径,同时增大弦向成形曲率半径,且随着预弯曲率半径的减小,这种减小和增大的效应随之逐渐增大。此外,与自由状态相比,预应力超声喷丸能够产生幅度和深度更大的残余压应力场,且表面粗糙度并未随着预弯曲率的增大而增加。分析认为,弹性预应力超声波喷丸能够进一步提高受喷材料的成形曲率并在一定程度上减小球面变形趋势,对大变形大厚度的单曲率零件成形具有重要的意义。     

薄板的各向异性对等离子电弧弯曲成形过程的影响

为了研究各向异性的金属薄板对等离子电弧加热弯曲成形的影响规律,采用等离子电弧以直线分别沿1Cr18Ni9Ti薄板轧制方向(RD)和垂直于轧制的方向(TD)进行加热弯曲成形试验.结果表明随着电弧扫描速度的降低,电弧功率增大以及扫描次数的增加,薄板塑性变形的各向异性影响减弱;反之则增强.该研究对薄板等离子电弧加热弯曲成形的实际应用有重要意义.     

Evolution of the residual stress state in a duplex stainless steel during loading

The evolution of micro- and macrostresses in a duplex stainless steel during loading has been investigated in situ by X-ray diffraction. A 1.5 mm cold-rolled sheet of alloy SAF 2304 solution treated at 1050°C was studied. Owing to differences in the coefficient of thermal expansion between the two phases, compressive residual microstresses were found in the ferritic phase and balancing tensile microstresses in the austenitic phase. The initial microstresses were almost two times higher in the transverse direction compared to the rolling direction. During loading the microstresses increase in the macroscopic elastic regime but start to decrease slightly with increasing load in the macroscopic plastic regime. For instance, the microstresses along the rolling direction in the austenite increase from 60 MPa, at zero applied load, to 110 MPa, at an applied load of 530 MPa. At the applied load of 620 MPa a decrease of the microstress to 90 MPa was observed. During unloading from the plastic regime the microstresses increase by approximately 35 MPa in the direction of applied load but remain constant in the other directions. The initial stress state influences the stress evolution and even after 2.5% plastic strain the main contribution to the microstresses originates from the initial thermal stresses. Finite element simulations show stress variations within one phase and a strong influence of both the elastic and plastic anisotropy of the individual phases on the simulated stress state.     

Texture development and hardening characteristics of steel sheets under plane-strain compression

Crystallographic texture development and hardening characteristics of a hot-rolled, low-carbon steel sheet due to cold rolling were investigated by idealizing the cold rolling process as plane-strain compression. The starting anisotropy of the test material was characterized by examination of the grain structure by optical microscopy and the preferred crystal orientation distribution by x-ray diffraction. Various heat treatments were used in an effort to remove the initial deformation texture resulting from hot rolling. The plastic anisotropy of the starting material was investigated with tensile tests on samples with the tensile axis parallel, 45°, and perpendicular to the rolling direction. The grain structure after plane-strain compression was studied by optical microscopy, and the new deformation texture was characterized by x-ray diffraction pole figures. These figures are compared with the theoretical pole figures produced from a Taylor-like polycrystal model based on a pencil-glide slip system. The uniaxial tensile stress-strain curve and the plane-strain, compressive stress-strain curve of the sheet were used to calibrate the material parameters in the model. The experimental pole figures were consistent with the findings in the theoretical study. The experimental and theoretical results suggest that the initial texture due to hot rolling was insignificant as compared with the texture induced by large strains under plane-strain compression.     

Effect of prestressed ultrasonic peen forming parameters on bending curvature and spherical deformation of plate

The elastic prestressed ultrasonic peen forming (UPF) was adopted in order to solve problems of insufficient bending deformation and large spherical deformation of plate during free UPF. The theoretical analysis of prestressed UPF and the influence of elastic prebending moment on deformation were analyzed. Spherical deformation coefficient was defined to quantificationally describe the spherical deformation. Experiments were conducted to compare the differences between free UPF and prestressed UPF processes and the effects of processing parameters on bending curvature and spherical deformation coefficient were studied. The results show that peening trajectory in chordwise direction is beneficial to enlarging spanwise bending deformation and decreasing spherical deformation coefficient. Large prebending curvature is helpful to increase spanwise bending deformation and decrease chordwise deformation, thereby obviously decreasing spherical deformation coefficient. Large spanwise deformation can be obtained under large firing pin velocity, small plate thickness and small offset distance. Large firing pin velocity plays a positive role in decreasing spherical deformation, while plate thickness and offset distance have little effect on it. Above all, prebending curvature and peening trajectory are the most important factors during prestressed UPF process. This study provides guidance for parameters optimization of prestressed UPF for wing plate with large thickness.     

Flexible Sheet Forming Technology by Double-sided Simultaneous Shot Peen Forming

The exceptionally flexible forming technology shot peen forming is used primarily on large, three-dimensionally curved sheet metal in the aircraft and aerospace industries. Depending on the kinetic shot energy, both convex and concave curvatures will be generated. The most recent development, double-sided simultaneous shot peen forming, brings about a higher productivity of the entire process. FEM simulations of single and multiple impacts are presented in order to evaluate the characteristics of concave curvatures generated by both single and double-sided peen forming.     

基于ABAQUS的大尺寸弹丸喷丸过程数值模拟

利用ABAQUS软件建立了单弹丸和多弹丸的喷丸有限元模型,研究了大弹丸喷丸2024铝板的动力学过程,分析了弹丸直径和喷丸覆盖率对板材塑性应变和沿弹坑表面及厚度方向残余应力的影响。模拟结果表明,增加弹丸直径可以使板材塑性应变层及残余压应力层深度增大,弹坑尺寸及弹坑"凸边"附近的残余拉应力也随之增大;塑性应变层和压应力层深度随喷丸覆盖率的提高而增大,但覆盖率达到一定程度时增加不再明显。     

热轧TB2钛合金织构多晶板动态压缩性能的各向异性

利用Instron 电子拉伸机和分离式霍普金生压杠(SHPB)实验装置, 研究了准静态和动态压缩条件下热轧TB2钛合金织构多晶板的力学性能。对TB2钛合金板材的轧向、横向以及轧制平面内与轧向成45°等3个方向进行了压缩实验, 得到了不同应变速率下的应力-应变曲线。结果表明：热轧TB2钛合金板材不同方向上的流变应力均随应变速率的增加而增加, 表现出明显的应变速率强化效应。准静态和动态力学性能均表现出各向异性, 且准静态和动态压缩行为规律不一致。考虑准静态和动态变形时可能的塑性变形机制, 基于微观晶体塑性变形理论, 定性讨论了热轧织构多晶TB2钛合金板屈服强度的各向异性     

Magnetic characterization of cold rolled and aged AISI 304 stainless steel

Magnetic easy axis predicted by the orientation distribution of the maximum amplitude of magnetic Barkhausen emission (MBE), which is obtained by magnetization in radial directions from the center of the specimens has been applied to determine the magnetic anisotropy on cold rolled and aged 304 SS in two sets of specimen. The maximum of the MBE has been found to orient along the rolling direction (RD) compared to the transverse direction (TD), indicating the presence of magnetic easy axis along the rolling direction for both sets. The strain induced martensite phase transformation has been determined using X-ray diffraction technique. The orientation distribution function (ODF) analysis has been carried out to obtain the crystallographic texture with cold rolling. ODF analysis revealed the 110 texture as the major. The magnetic anisotropy factor has also been determined with cold deformation and noticed that the strength of magnetic anisotropy decreases above 50% deformation for both the sets. Results have been explained considering two competitive effects, formation of crystallographic texture in the martensite phase and presence of compressive residual stresses along RD during cold rolling.     

Experimental study of shot peening and stress peen forming

Shot peening is a cold working process widely used to improve fatigue life of aerospace and automobile components. Stress peen forming is widely used in the aeronautic industry to produce thin components with complex shapes, involving double curvatures, such as wing skins. In this paper, quantitative relationships between the saturation, surface coverage and roughness with respect to peening time have been established based on aluminum Al2024 test strips. The influences of peening velocity and peening time on the resulting residual stress profiles have been experimentally presented. The quantitative relationships between the prebending moment and the resulting arc heights of narrow strips and square strips have been experimentally investigated. Experimental results show that with the increases of the prebending moment, the resulting arc height following the prebending direction increases and the tendency is almost linear. Quantitative equations of the saturation, coverage and roughness as well as the relationship between the prebending moment and resulting arc height can be used for the optimization of shot peening and stress peen forming process.     

表面曲率对激光冲击曲面材料表面残余应力场分布的影响

目的针对曲面材料在激光冲击作用下,表面曲率对激光冲击波传播存在影响,使其残余应力场分布情况不同于平面,分析其形成机理。方法将研究对象设置为凸模型,借助有限元软件ABAQUS,模拟了1500MPa冲击压力下,激光冲击波分别加载1/5、1/10、1/15曲率的7050铝合金试样。设置相应的平面试样作为对照组,并采用相同参数条件进行实验验证。结果当曲率为1/5时,冲击后的材料表面残余应力场分布不均匀,在母线方向的光斑边缘处,残余压应力仅为-237.0 MPa,塑性应变层深为0.5878μm;在圆周方向的光斑边缘处残余压应力为-258.5 MPa,较母线方向增加9.07%,塑性应变层深达到1.235μm,较母线方向增加110.11%。这一现象随着曲率的减小而逐渐消失,当曲率小于1/15时,表面残余应力场分布近似平面。结论激光冲击凸模型时,表面残余压应力场分布存在偏向现象,即试样沿母线方向的残余压应力值小于圆周方向,其对应的塑性应变深度也呈相同的规律。     

Impact of mechanical anisotropy on the geometry of flat-rolled fully pearlitic steel wires

The aim of the paper is to emphasize the role of anisotropy on metal flow in a bulk forming process, namely on lateral spread in pearlitic steel flat wire cold rolling. A precise final geometry of flat-rolled drawn wires is required in view of their industrial applications. Numerical modelling is undertaken to optimize the process and product properties. However, in the first part of the present study, the comparison between experimental and computed cross-sections shows a 10% discrepancy. It is shown first that temperature and friction are not the reasons for this underestimation, then that anisotropy may explain it. Indeed, ovalization of radially cut compression samples is observed at the end of wire-drawing, highlighting the appearance of wire-drawing-induced plastic anisotropy. Coefficients of Hill's quadratic anisotropic constitutive model have been identified at several stages of the wire-drawing and rolling process, using compression and shear tests, in order to study the sensitivity of the process to anisotropy and its influence on lateral spread. Then, the Finite Element Method (FEM) has been used to simulate rolling passes. Taking into account plastic anisotropy significantly improves the estimation of the final width; yet the underestimation is still 5% (instead of 10%). On the one hand, these results unambiguously prove that plastic anisotropy has an impact on flow during cold metal forming processes, provided free surfaces are present. On the other hand, they show further improvement is needed; it is suggested to use more sophisticated anisotropy models, as it has been done in sheet metal forming.     

Effect of plastic deformations and heat treatment on the behavior of the coercive force under load

Based on the measurements of the dependences of the coercive force of ferromagnetic steels on elastic tensile stresses, its minimum values and the positions of these minima have been found. Using the method of thermal neutron scattering, it has been experimentally proved that the stresses at which the coercive force is minimum is equal to the average value of the residual body stresses (internal stresses of the first kind). The latter were varied over wide limits with the aid of either a preliminary tensile plastic deformation to different degrees (steel St3) or using different tempering regimes after quenching (steel U8). In the first case, large residual compressive stresses arise in a significant part of grains along the direction of loading, which are caused by the anisotropy of Young’s modulus of iron. As a result of compensating these stresses by external tensile stresses, there appears a minimum of the coercive force, whose value is determined by an increase in the dislocation density at the stage of plastic deformation.     

多曲率旋转壳体液压胀形实验研究

研究了多曲率旋转壳体胀形过程中各部分的应变分布及位移变化规律。实验结果表明,在曲率方向不同时,壳体上曲率为负(内凹)的部分有向曲率为正(外凸)变化的趋势;在曲率方向相同时,曲率大的部分有向曲率减小的部分变化的趋势。     

弹塑性大变形有限元法在直缝焊管成型中的应用

采用弹塑性大变形有限元法研究了直缝焊管成型过程,针对边界非线性问题,首次导出了位移边界条件的迭代方法,分析了33.5mm×2mm双半径焊管管坯表面的应变分布规律。该方法也适用于冷弯过程。     

Effect of cyclic deformation on Barkhausen noise in a mild steel

The rms value of the Barkhausen noise and residual stresses were followed as a function of loading cycles in alternating bending fatigue of mild steel specimens. The measurements were carried out in several directions so that the changes in the angular anisotropy of the Barkhausen noise and reorientation of the macroscopic easy axis of magnetisation could be detected. In the loading direction the Barkhausen noise amplitude decreased due to build-up of compressive residual stresses. This was followed by either an increase or a decrease in the Barkhausen noise amplitude depending on the direction of the initial magnetic easy axis with respect to the loading direction. After the commencement of macroscopic plastic deformation the angular anisotropy of Barkhausen noise augmented substantially. In the transverse direction to the loading axis Barkhausen noise decreased in the beginning of loading, reached thereafter a minimum followed by an increase in Barkhausen noise. When cycling under larger strain amplitudes the loading direction and the direction transverse to the loading axis exhibited least changes as a function of loading cycles compared to other directions. The results indicated that following Barkhausen noise only in one direction, usually in the loading direction, as a function of loading cycles, may lead to misevaluation of the fatigue damage development. Monitoring of several directions gives much more reliable results and therefore it is highly recommended.     

冷轧Cu板动态压缩力学性能各向异性的研究

利用Instron电子拉伸机和Split-Hopkinson压杆(SHPB)实验装置,研究了准静态和动态压缩条件下冷轧和退火Cu板法向、轧向、横向的力学性能.不同应变率下的应力-应变曲线表明:冷轧和退火Cu板的流变应力均随应变率的增加而增加,表现出明显的应变率强化效应.冷轧Cu板准静态和动态压缩力学性能均呈现明显的各向异性:横向屈服强度最大,轧向最小,且低应变程度下的流变应力也具有同样规律.退火Cu板呈现近似各向同性.考虑准静态和动态变形时可能的塑性变形机制,基于微观晶体塑性变形理论的Taylor模型可定性地解释冷轧Cu板压缩力学性能的各向异性.     

Influence of the thermomechanical treatment on the microplastic behaviour of a wrought Al–Zn–Mg–Cu alloy

For plastic deformations smaller than the conventional limit of 0.2% for the yield stress, the so-called microplastic behaviour of a rolled Al–Zn–Mg–Cu alloy is investigated experimentally. Tension and compression responses are compared along the rolling and the transverse directions. The alloy shows an asymmetric response in tension and compression (i.e., compressive stress minus tensile stress for a given absolute plastic strain is non-zero). Moreover, this asymmetry changes sign between the rolling and the transverse directions. The difference between tension to compression is observed to decrease as the conventional limit is approached.The influence of the heterogeneous microstructure of the alloy and the fabrication process on these asymmetries is discussed. Modelling of the material response based on a self-consistent scheme is used to estimate the internal stresses resulting from the thermomechanical treatment, and also to investigate the influence of the heterogeneous elastoplastic behaviour of two types of constitutive grains (recrystallised and unrecrystallised). For a plastic strain smaller than 0.02%, the microplastic behaviour is not well described with the adopted model since the underlying assumption of uniform stress and strain fields per phase is questionable in this initial level of plasticity. However, the model shows that the asymmetries observed at plastic strains ranging from 0.02% to 0.2% are consistent with the intragranular stresses developed during the stretching step.     

Special features on the fracture of ordered segregated excess cementite in eutectoid steels

Special features of plastic deformation and fracture of excess cementite segregated under the conditions of high-temperature carbon supersaturation of steel are studied by the methods of light and scanning electron microscopy. It is shown that regularly oriented flakes of excess cementite with a lamellar structure are formed within the former austenite grains. The cementite flakes oriented along the, rolling plane fracture by a brittle mechanism forming numerous fragments. The behavior of free cementite under the conditions of cold rolling depends on the orientation of the flakes in the specimen. The mutual shift of the layers allows coarse cementite formations to deform under the action of compressive stresses without cracking. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 11–13, March, 1999.     

初始织构对AZ31镁合金孪生和织构演化的影响

为研究AZ31镁合金变形孪晶和塑性各向异性,基于率相关晶体塑性本构理论,采用有限元方法建立了具有不同初始织构的镁合金模型（包含滑移和孪生变形机制）,并引入孪晶体积分数,研究其压缩过程中织构演变、孪生和力学性能之间的关系。结果表明：晶体的塑性行为在很大程度上取决于初始织构,初始织构的差异导致了压缩行为的明显各向异性,轴向屈服强度和抗拉伸强度高,径向屈服强度和抗拉伸强度低。压缩塑性变形过程中随着变形量的增加,激活孪晶体积分数增高,且径向压缩激活孪晶体积分数越高,轴向压缩激活孪晶体积分数越低。模拟中出现明显孪晶的点与应力突变的点相吻合,当孪晶体积分数达到一定值时,应力发生突变,此时晶体取向发生显著变化,新的滑移系启动,反映了滑移和孪晶机制耦合对AZ31镁合金力学性能的影响。