38MnB5热成形钢高温变形行为及本构方程

利用Gleeble-3500热模拟试验机对38MnB5热成形钢的高温变形行为进行研究, 分别在650~950℃温度区间内, 以0.01、0.1、1和10 s-1的应变速率对其进行等温单向拉伸测试, 并得到相应条件下的真应力-应变曲线.结果表明: 38MnB5热成形钢流变应力随着变形温度的升高而减小, 随着应变速率的增大而增大.当应变速率逐渐增加时, 热变形时发生的动态回复和动态再结晶效果并不显著, 而当温度逐渐升高时, 二者作用逐渐加强.考虑了温度、应变速率和应变的综合复杂影响, 建立38MnB5热成形钢高温下的本构方程.此本构方程通过对流变应力、应变、应变速率等实验数据的回归分析, 得到与变形温度、应变速率和应变相关的材料参数多项式.计算结果与实验结果对比发现, 通过本构方程所获得的计算值与试验值吻合良好.      

热压铍材静-动态变形行为及其本构方程

采用改进的PTW方程和LM参数方程构建了一个新的金属材料本构方程模型;应用Hopkinson压力杆法获得热压铍材不同温度和应变速率下静-动态应力-应变曲线数据,确定了本构方程中材料参数。结果表明,计算曲线与实验曲线吻合的较好,该本构方程可应用于预测热压铍材不同温度和应变速率下静-动态流变应力。     

基于单向拉伸仿真对各向异性薄板本构修正的研究

金属板料拉伸直至断裂的全程本构关系或真应力—塑性应变曲线对成形仿真具有重要的意义,由于传统单向拉伸试验获取的真应力—塑性应变曲线在颈缩后失真,所以对成形仿真具有不真实性.本文利用试验—有限元辅助方法,在ABAQUS中合理简化构建模型,基于DC04单向拉伸试验的真应力—塑性应变本构,引入Hill48各向异性屈服准则,采用...     

Al-Cu-Mg-Ag合金热压缩变形行为的预测

采用了热模拟实验机研究了Al-Cu-Mg-Ag耐热铝合金的热压缩变形行为。实验的温度和应变速率分别为340～500℃,0.001～10 s-1。分别用了本构方程和人工神经网络来对Al-Cu-Mg-Ag合金的流变行为进行了分析和模拟。神经网络的结构是3-20-1;输入参数是温度,应变速率和应变;输出参数是流变应力。结果表明该合金的流变曲线出现加工硬化、过渡、软化和稳态流变这4个阶段,流变应力随着应变速率的增加而增大,随着变形温度的下降而减少。用所建立的神经网络模型预测了变形温度和应变速率对流变应力的影响,预测的结果与热压缩变形的基础理论吻合得很好,而且该模型可以很好地描述Al-Cu-Mg-Ag合金的流变应力,在应变速率为0.001～10 s-1的条件下,其平均相对误差分别为3.68%,3.98%,1.53%,3.53%和2.04%。这表明神经网络的预测性能优良,具有很强的推广能力。同时通过本构方程和神经网络的预测结果比较看出神经网络模型的相关系数比较高,而且神经网络比本构方程有更好的预测性能。神经网络可以预测不同应变下的相应的流变应力,但是本构方程只可以根据不同的应变速率和温度来预测峰值应力。     

成形极限图的测试、应用和可信度分析

成形极限图通常是通过钢模胀形试验测得，实际测量的成形极限图与ASAME自动应变测试分析系统模拟计算的成形极限曲线吻合较好。应用成形极限图分析冲压零件成形的安全裕度和进行选材预测时，对以平面应变和胀形为主的成形零件具有较高的可信度，而对以深拉延变形为主的成形零件可信度不高。     

热轧低碳钢高温变形行为与显微组织

采用 Gleeble-1500试验机对低碳钢进行热变形试验,获得了真应力-真应变曲线,进而研究了变形温度为900~1200℃,应变速率为0·1~10 s－1对材料热变形行为的影响。通过非线性回归获得了材料在不同变形条件下的材料常数,建立材料的热变形本构方程,进而分析了热变形低碳钢的微观组织演变及极限压缩率的变化规律。结果表明：基于热变形方程真应变为0·5时的热变形激活能Q为216·95 kJ/mol,利用该本构方程计算的峰值应力与试验得到的应力-应变曲线的峰值应力吻合较好;应变速率1 s－1,变形温度1100℃下的显微组织较其他温度相比都要细小、均匀,此时其极限压缩率最大可以达69％,可在此工艺条件下实现较大的塑性变形,且变形后具有较好的综合力学性能。     

耦合位错密度的6111铝合金热变形本构模型

利用Gleeble-1500热模拟试验机对6111铝合金进行高温拉伸试验,研究了其在变形温度为350、450和550℃以及应变速率为0.1、1和10 s-1时的热变形行为.6111铝合金的流变应力随温度升高而减小,随应变速率增大而增大,其热变形从应变硬化阶段过渡到稳态变形阶段.建立了综合考虑应变、温度和应变速率对流变应力的影响以及耦合位错密度的统一黏塑性本构模型,并通过遗传优化算法求解出本构模型中的材料常数.模型计算得到的真应力-真应变曲线与试验数据吻合较好.      

6111铝合金热变形流变行为及本构模型

试验材料为厚2mm的6111铝合金,利用ZWIKE100KN高温材料试验机对该材料在350~550℃,0.1~10s-1应变速率下进行热拉伸试验.结果表明:受位错密度的影响,6111铝合金的流变应力随温度的升高而降低,随应变速率的增大而增大;可以分为应变硬化和饱和稳态流变两个阶段.基于Voce饱和外推模型(H-S模型)构建以温度、应变、应变速率为变量因素的6111铝合金流变应力本构模型,通过回归拟合试验数据求解模型中的参数.试验数据与计算该模型得到的预测曲线吻合较好,验证了该模型的可行性.     

高铌微合金钢高温变形流变应力预测模型

 以流变应力曲线的唯象特征和本构方程的传统理论为基础,针对高铌微合金钢开发出一种新的描述高温变形应力 应变曲线的数学模型。该模型包括两个基本方程,确立了应力与应变、温度、变形速率的数学关系,预测了加工硬化和动态再结晶各阶段流变应力的变化,预测结果与实验结果吻合良好。     

新型微合金化C-Mn-Al高强度钢的热变形行为

通过Gleeble-1500热模拟单轴压缩试验,研究了一种含1.79% Al （质量分数）的以Al替代Si微合金化高强度钢在温度为900-1100℃、应变速率为0.01-30 s-1条件下的热变形行为.建立了考虑应变量对材料常数影响的双曲正弦本构方程,利用建立的本构方程预测的应力-应变曲线与实验值吻合良好,表明建立的本构方程可以对实验钢的流变应力给出相对准确的预测.建立了实验钢的加工图,根据加工图分析确定了实验钢的动态再结晶区为1000-1100℃和0.01-1 s-1.组织观察表明在动态再结晶区实验钢发生了动态再结晶,而失稳区对应的组织出现了变形集中带或“项链”组织.最后将建立的本构方程和加工图联合运用,为更全面地研究实验钢在不同变形条件下的热变形行为提供了方法.      

Effect of matrix constitutive behavior and inclusions on forming limits of Fe-42 pct Ni alloy sheet

The effects of matrix constitutive behavior and nonmetallic inclusions on forming limit strains have been examined with five laboratory heats of Fe-42 pct Ni alloy. The inclusion volume fraction was varied between approximately 0.01 and 1.59 pct by proper selection of Mn, S, and O contents. Each ingot was processed into 0.38-mm-thick sheets and heat treated to the recovered or the recrystallized condition. Forming limit strains were obtained from uniaxial tensile, plane-strain tensile, and hydraulic bulge tests by circle grid analysis. In any strain path, the limit strain increased with increasing the strain hardening exponent, n. The forming limit strains on the right-hand side of the forming limit diagram (FLD) decreased with inclusion volume fraction, while no effects of the inclusion volume fraction were observed on the left-hand side. A decrease in the slope of the FLD on the right-hand side due to an increase in the inclusion volume fraction qualitatively agrees with the theoretical calculation by Graf and Hosford, which was conducted on the basis of the Marciniak-Kuczynski (M-K) theory.     

Effects of texture gradients on yield loci and forming limit diagrams in various aluminum-lithium sheet alloys

Marked through-thickness variations of preferred crystallographic orientations in aluminum-lithium (Al-Li) sheet alloys have been observed and documented. These metallurgical features could have an effect on the way in which these materials distribute strain during plastic deformation. From a theoretical or a practical point of view, it is important to investigate these texture effects on plastic-deformation properties and particularly on forming limit strains. In this work, quantitative texture data, which were determined by X-ray and neutron diffraction techniques, were used with a polycrystal model to predict the yield locus of recrystallized and unrecrystallized AA8090 and AA2090 Al-Li sheets. The conventional AA2024 alloy in the annealed condition was also investigated as a reference material. Subsequently, these yield loci were used to calculate forming limit diagrams (FLDs) in the stretching range, using the Marciniak-Kuczynski (M-K) approach with strain rate potentials to describe the constitutive properties of the sheets. A simple critical-thickness-strain criterion was used to predict the FLD in the drawing range. The predicted FLDs were found to be in fair agreement with experimental curves obtained under punch-stretching conditions. In general, experimental trends were accounted for by the results predicted using the average texture data. However, the texture gradients do not completely explain the large scatter observed in the experimental forming limits and the high average limit strain of the recrystallized AA8090.     

7085铝合金的高温压缩流变应力及软化行为

对均匀化炉冷态7085铝合金进行高温压缩实验,研究该合金在变形温度为350~450℃、变形速率为0.001~0.1 s 1和应变量为0~0.6条件下的流变应力及软化行为。结果表明:流变应力在变形初期随着应变的增加而迅速增大,出现峰值后逐渐软化进入稳态流变;随着变形温度的升高和应变速率的降低,峰值流变应力降低。采用包含Zener-Hollomon参数的Arrhenius双曲正弦关系描述合金的流变行为。分析和建立了应变量与本构方程参数(激活能、应力指数和结构因子)的关系,研究发现本构方程参数随应变量的增加而减少。合金的流变行为差异与动态回复再结晶和第二相粒子相关。     

Numerical analysis of the formability of an aluminum 2024 alloy sheet and its laminates with steel sheets

A criterion for ductile fracture is applied to the formability prediction of an aluminum 2024 alloy sheet and its laminated composite sheets. Axisymmetric deep-drawing processes of the 2024 sheet and the laminates clad by mild steel sheets are simulated by the finite-element method. From the calculated distributions and histories of stress and strain, the fracture initiation site and the forming limit are predicted by means of the ductile fracture criterion. The predictions so obtained are compared with experimental observations. The results show that the fracture initiation in the 2024 sheet with no appearance of necking is successfully predicted by the present numerical approach. Furthermore, it is found that the formability of the 2024 sheet is improved by sandwiching it with the mild steel sheets.     

Crystal plasticity forming limit diagram analysis of rolled aluminum sheets

Numerical simulations of forming limit diagrams (FLDs) are performed based on a rate-sensitive polycrystal plasticity model together with the Marciniak-Kuczynski (M-K) approach. Sheet necking is initiated from an initial imperfection in terms of a narrow band. The deformations inside and outside the band are assumed to be homogeneous, and conditions of compatibility and equilibrium are enforced across the band interfaces. Thus, the polycrystal model needs only to be applied to two polycrystalline aggregates, one inside and one outside the band. Each grain is modeled as an fcc crystal with 12 distinct slip systems. The response of an aggregate comprised of many grains is based on an elastic-viscoplastic Taylor-type polycrystal model. With this formulation, the effects of initial imperfection intensity and orientation, initial distribution of grain orientations, crystal elasticity, strain-rate sensitivity, single slip hardening, and latent hardening on the FLD can be assessed. The predicted FLDs are compared with experimental data for the following rolled aluminum alloy sheets: AA5754-0-A, AA5754-0-B, AA6111-T4-A, AA6111-T4-C, and AA6111-T4-D.     

初始晶粒尺寸对2024铝合金热变形行为和组织的影响

利用永磁搅拌近液相线铸造和普通铸造方法制备不同晶粒尺寸的2024铝合金铸锭,利用Gleeble-1500热模拟试验机研究初始晶粒尺寸对不同压缩变形条件下2024铝合金的热变形行为和变形后显微组织的影响。研究表明:2024铝合金的热变形行为依赖于变形条件和初始组织。初始晶粒尺寸对流变应力的影响是:当应变速率小于0.1 s~(-1)时,流变应力随晶粒尺寸减小而减少;当应变速率为10 s~(-1)时,流变应力随晶粒尺寸减小而增大。降低变形温度会弱化晶粒尺寸对流变应力的影响。热压缩流变应力随应变速率增大而增大,随变形温度升高而减小。应变速率为10 s~(-1)时,热压缩应力应变曲线呈现周期性波动;只在粗晶2024铝合金中发现变形剪切带。     

Simulation of the effect of texture on limit strain in biaxially stretched steel sheet

Limit strains for biaxial stretching are predicted by the Marciniak and Kuczynski (M-K) method, suitably modified to take into account the textures present in sheet steels. The series expansion method is used for this purpose in the framework of the Taylor/Bishop and Hill theory. The effect on the limit strain of yield surface shape is allowed for by the use of an improvedP parameter. The experimental limit strains of an AKDQ steel are compared with the forming limit curves (FLCs) predicted on the assumption of two sets of slip systems with different critical resolved shear stresses (CRSSs). The limit strains in equibiaxial stretching with assumed groove orientations are also predicted for some idealized materials containing specific preferred orientations.     

Calculations of forming limit

The method of calculating the shape of forming limit diagrams (FLDs) using a high-exponent yield criterion with the Marciniak and Kuczynski (M-K) analysis has been extended to include the effects of changing the strain paths and applied to aluminum alloy 2008 T4. Calculations incorporating abrupt path changes agreed with the general trends found experimentally. If the first stage of strain is under biaxial tension, the subsequent FLD shifts to the right and down with respect to the original FLD, whereas it shifts to the left and up when the first stage of strain is in uniaxial tension. Calculations introducing gradual strain-path changes, characteristic of stretching over a hemispherical dome, predict that the minimum of the FLD shifts to the right.     

Study of Forming Limit for Rotational Incremental Sheet Forming of Magnesium Alloy Sheet

As a lightweight material, magnesium is being increasingly used for automotive parts. However, due to a hexagonal-closed-packed (hcp) crystal structure, in which only the basal plane can move, magnesium alloy sheets exhibit a low ductility and formability at room temperature. Press forming of magnesium alloy sheets is conventionally performed at elevated temperatures of 200 °C to 250 °C and thus is known as energy consumed forming. Therefore, in view of an energy saving forming technology, we study magnesium alloy sheet forming by a rotational incremental sheet forming (RISF) at room temperature, where the rotational tool generates local heat of specimen enough to accelerate plastic deformation. The flow curves of the magnesium alloy sheet are obtained and calculated at elevated temperatures, while the yield loci of the magnesium alloy sheet are measured at room temperature. Using RISF, a square cup of 80-mm width, 80-mm length, and 25-mm height is then formed from a magnesium alloy sheet at room temperature. In addition, the strain distribution is obtained and compared with the forming limit curve (FLC) by considering the effect of the tool radius and is found to effectively predict the forming limit of a magnesium alloy sheet in RISF.     

Effects of Recovery Behavior and Strain-Rate Dependence of Stress–Strain Curve on Prediction Accuracy of Thermal Stress Analysis During Casting

Recovery behavior (recovery) and strain-rate dependence of the stress–strain curve (strain-rate dependence) are incorporated into constitutive equations of alloys to predict residual stress and thermal stress during casting. Nevertheless, few studies have systematically investigated the effects of these metallurgical phenomena on the prediction accuracy of thermal stress in a casting. This study compares the thermal stress analysis results with in situ thermal stress measurement results of an Al-Si-Cu specimen during casting. The results underscore the importance for the alloy constitutive equation of incorporating strain-rate dependence to predict thermal stress that develops at high temperatures where the alloy shows strong strain-rate dependence of the stress–strain curve. However, the prediction accuracy of the thermal stress developed at low temperatures did not improve by considering the strain-rate dependence. Incorporating recovery into the constitutive equation improved the accuracy of the simulated thermal stress at low temperatures. Results of comparison implied that the constitutive equation should include strain-rate dependence to simulate defects that develop from thermal stress at high temperatures, such as hot tearing and hot cracking. Recovery should be incorporated into the alloy constitutive equation to predict the casting residual stress and deformation caused by the thermal stress developed mainly in the low temperature range.