Prediction of forming limit curve for pure titanium sheet

Commercially pure titanium (CP Ti) has been actively used in the plate heat exchanger due to its light weight, high specific strength, and excellent corrosion resistance. However, researches for the plastic deformation characteristics and press formability of the CP Ti sheet are not much in comparison with automotive steels and aluminum alloys. The mechanical properties and hardening behavior evaluated in stress–strain relation of the CP Ti sheet are clarified in relation with press formability. The flow curve denoting true stress–true strain relation for CP Ti sheet is fitted well by the Kim–Tuan hardening equation rather than Voce and Swift models. The forming limit curve (FLC) of CP Ti sheet as a criterion for press formability was experimentally evaluated by punch stretching test and analytically predicted via Hora's modified maximum force criterion. The predicted FLC by adopting Kim–Tuan hardening model and appropriate yield function shows good correlation with the experimental results of punch stretching test.     

Influence of Vanadium Content on Hot Deformation Behavior of Low-Carbon Boron Microalloyed Steel

Single-pass compression tests were performed to investigate the hot deformation behavior of low-carbon boron microalloyed steel containing three various vanadium contents at 900-1100℃ and strain rate of 0.01-10 s~(-1) using the MMS-300 thermal mechanical simulator.The flow stress curves of investigated steels were obtained under the different deformation conditions,and the effects of the deformation temperature and strain rate on the flow stress were discussed.The characteristic points of flow stress were obtained from the stress dependence of strain hardening rate;the activation energy of investigated steels was determined by the regression analysis;the flow stress constitutive equations were developed;the effect of vanadium content on the flow stress and dynamic recrystallization(DRX) was investigated.The result showed that the flow stress and activation energy(3-6.5 kJ mol~(-1)) of the steel containing 0.18 wt% V were significantly higher than those of the steels with0.042 wt% and 0.086 wt% V,which was related to the increase in solute drag and precipitation effects for higher vanadium content.DRX analysis showed that the addition of vanadium can delay the initiation and the rate of DRX.     

流动应力-应变关系对5754O铝合金板成形极限的影响(英文)

为了合理描述单向拉伸试验曲线,给出了一种修正的Swift型流动应力—应变关系。基于两种流动应力—应变关系,采用Yld2000-2d屈服准则计算5754O铝合金板的成形极限应变图(FLD-strain)。通过对比理论和实验结果,发现基于修正的Swift型的应力—应变关系所计算的FLD-strain能够合理地描述实验结果。虽然常用的Voce型应力—应变关系能够精确地描述均匀变形阶段的变形行为,但基于该应力—应变关系计算的FLD-strain明显低于实验结果。结果表明,板料的强化率越高则相应的成形极限也越高。为了描述板料在非均匀变形阶段的变形行为和成形极限,建议了一种用于确定合理的流动应力—应变关系的方法。     

Effect of electric current heating on hot deformation resistance and microstructure of steels

The effect of electric current (EC) heating on the high temperature mechanical behavior and microstructure of plain carbon and austenitic stainless steels was examined using a Gleeble thermomechanical simulator. In stainless steel, EC heating is shown to reduce the flow stress, strain hardening rate, the apparent activation energy for deformation and to increase the strain rate sensitivity of the flow stress. These are accompanied by the acceleration of dynamic recrystallization and by an increase in recrystallized grain size. The EC effect is more pronounced at lower Zener-Hollomon parameters. In plain carbon steel, EC has almost no influence on flow stress. The EC heating effect on mechanical behavior and microstructure is related to deformation resistance, electric resistivity and heat conductivity of the material being tested.     

Surface roughness changes on a hot-dipped galvanized sheet steel during deformation at low strain levels

During a study on surface roughening as a function of deformation, an anomalous behavior of the surface roughening for hot-dipped galvanized sheet steel was observed during low levels of imposed strain. A mechanistic model, which explains this surface roughening behavior, is presented. Surface topography parameters, including surface roughness, and relative zinc crystal orientation data, were measured after different amounts of deformation were imposed on the sheet. The proposed model is consistent with both sets of data. The model indicates that the zinc crystals on the surface of the coated sheet steel will initially deform by twinning, which causes a slight increase in surface roughness. The twin regions of the zinc crystals are then favorably oriented for dislocation glide, and the roughness during further deformation will slightly decrease. Once the zinc crystal orientations have homogenized, standard surface roughening due to adjacent grain constraint occurs. Further evidence to support the model include: (1) Twinning in a single crystal of zinc causes a roughness increase of the same magnitude as that found in the zinc coated steel after low strain deformation, (2) twinning of zinc has been observed in other studies of zinc-coated sheet steels, and (3) discontinuous yielding of the steel and recrystallization of the zinc during the sheet deformation did not occur and are not the cause for both the surface roughness and crystal orientation results.     

AZ31B镁合金热拉伸流变应力研究

针对不同加工方法制备的AZ31B镁合金薄板,利用热拉伸试验机和金相显微镜对其在不同温度和变形速率下的流变应力进行了实验研究。结果表明,变形温度和变形速率对热拉伸时镁合金的流变应力有显著影响,峰值流变应力随应变速率的降低和变形温度的升高而降低。峰值流变应力随板材的厚度增加而发生变化,低温时厚度效应较为明显。退火处理对冷轧板的峰值流变应力影响较小,冷轧板可直接用于热加工成形。峰值流变应力变化规律:挤压板>热轧板>冷轧板。     

An experimental study on the formability of intermetallic nickel aluminide

In this paper, the formability of ductile intermetallic Ni₃Al and limiting strains at room temperature are presented. Uniaxial tensile tests are carried out to identify uniform elongation, work hardening behavior, and fracture character. Forming limit diagrams are determined and the effect of biaxial deformation on limiting strains and failure modes are shown. The bulk formability is investigated by cold upset testing on continuous cast and extruded bars. Forming limit diagrams, both for bars and sheets, do not show a significant effect of thermomechanical processing and sheet thickness at the range of 0.30-0.46 mm. These observations are in agreement with common ductile metals where larger deformations are obtained in biaxial modes. Microcrack initiation at grain boundaries is shown to contribute to lower limiting strains in the plane strain region during sheet metal stretching and compression of intermetallic nickel aluminide.     

Failure During Sheared Edge Stretching

Failure during sheared edge stretching of sheet steels is a serious concern, especially in advanced high-strength steel (AHSS) grades. The shearing process produces a shear face and a zone of deformation behind the shear face, which is the shear-affected zone (SAZ). A failure during sheared edge stretching depends on prior deformation in the sheet, the shearing process, and the subsequent strain path in the SAZ during stretching. Data from laboratory hole expansion tests and hole extrusion tests for multiple lots of fourteen grades of steel were analyzed. The forming limit curve (FLC), regression equations, measurement uncertainty calculations, and difference calculations were used in the analyses. From these analyses, an assessment of the primary factors that contribute to the fracture during sheared edge stretching was made. It was found that the forming limit strain with consideration of strain path in the SAZ is a major factor that contributes to the failure of a sheared edge during stretching. Although metallurgical factors are important, they appear to play a somewhat lesser role.     

Constitutive relations for determining the critical conditions for dynamic recrystallization behavior

A series mathematical model has been developed for the prediction of flow stress and microstructure evolution during the hot deformation of metals such as copper or austenitic steels with low stacking fault energies, involving features of both diffusional flow and dislocation motion. As the strain rate increases, multiple peaks on the stress-strain curve decrease. At a high strain rate, the stress rises to a single peak, while dynamic recrystallization causes an oscillatory behavior. At a low strain rate (when there is sufficient time for the recrystallizing grains to grow before they become saturated with high dislocation density with an increase in strain rate), the difference in stored stress between recrystallizing and old grains diminishes, resulting in reduced driving force for grain growth and rendering smaller grains in the alloy. The final average grain size at the steady stage (large strain) increases with a decrease in the strain rate. During large strain deformation, grain size reduction accompanying dislocation creep might be balanced by the grain growth at the border delimiting the ranges of realization (field boundary) of the dislocation-creep and diffusion-creep mechanisms.     

Deformation behavior of thin copper films on deformable substrates

The role of strain hardening for the deformation of thin Cu films was investigated quantitatively by conducting specialized tensile testing allowing the simultaneous characterization of the film stress and the dislocation density as a function of plastic strain. The stress–strain behavior was studied as a function of microstructural parameters of the films, such as film thickness (0.4–3.2 μm), grain size and texture. It was found that the stress–strain behavior can be divided into three regimes, i.e. elastic, plastic with strong strain hardening and plastic with weak hardening. The flow stresses and the hardening rate increase with decreasing film thickness and/or grain size, and are about two times higher in (111)-grains compared to the (100)-grains. These effects will be discussed in the light of existing models for plastic deformation of thin films or fine grained metals.     

正方形薄板激光三维成形球冠面的应力场分析

激光成形是利用激光束的能量修整板材曲率的一种新型柔性无模成形加工方法。为了描述激光三维成形中的弹塑性变形过程,以进一步揭示其三维成形的机理,文章采用显式动力学方法分析了交叉线扫描激光三维成形正方形薄板为球冠面的物理过程,揭示了激光扫描过程中正方形薄板的温度场、应力场、应变场的变化情况,虽然激光引起薄板温度变化产生的热应力仍是三维成形的主要因素,但是激光三维成形与激光直线扫描成形不同,激光三维成形扫描过程中在薄板内部形成的应力波是影响激光三维成形的主要因素之一,薄板的应变、变形等均与应力波有关,本文揭示的物理行为为下一步的机理研究打下了良好的基础。     

2A12硬铝合金热拉伸流变行为及本构建模

在变形温度350~500℃、应变速率0.01~1 s~(-1)条件下,利用Gleeble-3500热模拟实验机对2A12硬铝合金板进行热拉伸实验。结果表明:峰值应力随温度升高而减小,随应变速率提高而增大;随着应变速率减小,断裂总伸长率升高,而均匀伸长率降低;应变速率较低时,其断裂总伸长率在350~450℃时较高,升高到500℃时迅速降低,均匀伸长率则对温度变化不敏感;应变速率较高时,试样断裂总伸长率对温度变化不敏感,均匀伸长率随温度升高而降低。根据实验结果,采用Z参数建立的流变应力本构模型,能较好地描述2A12铝合金板材热拉伸变形下的流变行为。     

Experimental and numerical determination of texture evolution during deep drawing tests

This paper deals with the study of texture evolution during deep drawing tests. The aim of these tests is to obtain the whole range of deformation paths encountered in real industrial processes. Nakazima tests are carried out in order to obtain deformation states ranging from simple tension to equibiaxial stretching. Shear tests are also achieved in a simple shear setup developed for this work. The experimental strain fields are determined accurately with an optical measurement system based on the correlation of digital images. Texture measurements were carried out for the undeformed material and after each test. The texture evolution is compared to the texture obtained from an elastic–plastic rate-independent polycrystalline material model recently developed. Boundary conditions applied in the model reproduce the in-plane stretching of the material with an out-of-plane stress relaxation procedure in order to reach plane stress conditions.     

汽车覆盖件弹塑性失稳机理实验模拟研究

利用YBT模型,结合密栅云纹试验,模拟了汽车用钢板的拉伸过程。通过记录、测量、计算和分析,获得试样全部变形过程中的整场连续、瞬时位移、应变和应力信息。结果表明,汽车覆盖件回弹类缺陷是卸载不均匀压缩诱发的局部弹性失稳;起皱类缺陷是加载不均匀导致的塑性失稳。     

Investigation on deformation behavior of sheet metals in viscous pressure bulging based on ESPI

In order to analyze the effect of viscous medium on the deformation behavior of sheet metals in viscous pressure bulging (VPB), the entire deformation process including instability and fracture was investigated real-timely by the aid of electronic speckle pattern interferometry (ESPI). Images of speckle patterns were captured continuously to obtain fringe patterns representing the full field strain rate. Values of strain rates were calculated based on the fringe patterns. The evolution of the weak region from the initial defect to the groove until crack was also observed through the fringe patterns. The onset of diffuse and localized necking were determined qualitatively and quantitatively. Experimental results show that the deformation of sheet metals in VPB passed through five states, namely, uniform deformation, strain localization, diffuse necking, localized necking and fracture. A defect emerged in strain localization. The growth of the defect caused the diffuse necking and generated a groove. The groove expanded mainly in length direction until the localized necking occurred. Finally the specimen fractured as a result of groove deepening. The tangential adhesive stress provided by viscous medium in VPB restricted the locally larger strain of the specimen. The diffuse necking was postponed greatly. Theoretical prediction of the limit strains of sheet metals in VPB would be made based on the experimental results in further work.     

Multiscale mechanics of TRIP-assisted multiphase steels: I. Characterization and mechanical testing

The mechanical behaviour of transformation-induced plasticity (TRIP)-assisted multiphase steels is addressed based on three different microstructures generated from the same steel grade. The mechanisms responsible for the work-hardening capacity and the resulting balance between strength and resistance to plastic localization are investigated at different length scales. The macroscopic mechanical response is determined by simple shear, uniaxial tension, Marciniak and equibiaxial tension supplemented by earlier tensile tests on notched and cracked specimens. It is shown that the transformation rate reaches a maximum for stress states intermediate between uniaxial tension and equibiaxial tension. At an intermediate length scale, the true in situ flow properties of the individual ferrite–bainite and retained austenite phases are determined by combining neutron diffraction and digital image correlation. This combined analysis elucidates the partitioning of stress and strain between the different constitutive phases. Based on these results, supplemented by transmission electron microscopy and electron backscattered diffraction observations, a general overview of the hardening behaviour of TRIP-assisted multiphase steels is depicted.     

Ductile fracture behavior of 5052 aluminum alloy sheet under cyclic plastic deformation at room temperature

Ductile fracture behavior of a 5052 aluminum alloy sheet undergoing cyclic plastic deformation is investigated in order to clarify the effect of cyclic plastic deformation on formability enhancement in incremental stretch sheet forming at room temperature. In the incremental forming, formability markedly increases owing to strain distribution and accumulation effects. The former effect is activated when the deformation region expands along tool paths. Thus, localization of deformation, which leads to necking or fracturing, can be prevented. On the other hand, local strain is accumulated without fracturing when a blank sheet is repeatedly subjected to out-of-plane deformation at the same position. In this paper, the effect of the strain accumulation due to cyclic deformation generated by bending and unbending is primarily focused on to discuss the effect on deformability. To apply cyclic plastic straining to the specimen, a cyclic stretch-bending test was adopted. A cyclic tensile test was also conducted for larger bending curvature. The experimental results show that cyclic bending–unbending affects the ductility of sheet metals. The fractography obtained by scanning electric microscopy also indicates that fewer and smaller voids are observed particularly on bending the inner side than on the outer side.     

FE-simulation of machining processes with a new material model

In the field of materials mechanics the influence of the state of stress on the plastic deformation behavior of metals is known since decades. However, the state-of-stress influences are usually not considered in structural or processing simulations. Nevertheless, its application in the numerical investigation of manufacturing processes seems very promising since, for example, machining processes are characterized by complex states of stress. Consequently, its incorporation in the computation of the workmaterial's flow stress may increase the physical conformity and accuracy of cutting FE-analysis.This paper presents the creation and experimental validation of a 3D-FEM model of the longitudinal turning process with an extended modified Bai–Wierzbicki material model (extended MBW model). This newly developed material model evaluates the influence of state of stress as well as damage on the strain hardening behavior. In addition, it takes temperature and strain rate effects into consideration, whose influences are both typically higher in cutting processes than in structural–mechanical problems.For the validation of the proposed material model, longitudinal turning experiments were conducted on AISI 1045 steel. Four different cutting tools and process conditions were investigated, which cover a broad range from finishing to roughing. A high speed camera was used to film the chip formation and chip flow in order to compare it to the simulation results. The three cutting forces components were also collected. Measured chip temperatures were taken from the literature. The validation showed that the implementation of the selected material model results in a close agreement between experimentally obtained and predicted chip geometries, cutting forces and chip temperatures.     

RELATIONSHIP BETWEEN TENSILE DEFORMATION BEHAVIOUR AND MICROSTRUCTURAL PARAMETERS OF A DUAL PHASE STEEL

Based on the deformation model of dual phase steels,an expression for the stress of martensile in dual phase steels is derived,it predictes that the onset of plastic deformationof martensite (transition strain) depends on the strain hardening of ferrite and on thestrength of martensite.The relationship between the flow stress and microstructural parameters of a 0.12C-0.9Mn dual phase steel was investigated using the expression forthe flow stress of dual phase steel.By calculating the stress ratio and the stress-strainpartition coeffictient,the load transition and the stress-strain partition between two phases are studied.It shows that the deformation of dual phase steel lies between the isostress and isostrain states and the stress-strain pratition changes continuously during the deforamtion.