微细C5210磷青铜板料的尺寸效应研究

为研究常温下晶粒尺寸和厚度对C5210磷青铜力学性能的影响,引入尺寸效应影响因子φ=厚度/晶粒尺寸。结果表明,厚度不同时,随着φ从14.7减小到6.3,屈服强度减小了48%,延伸率由25.5%减小到18.2%;晶粒尺寸不同时,随着φ的减小,屈服强度先快速下降,当φ减小到3.5后便缓慢下降,延伸率则先增大再减小,φ为3.5时延伸率达到最大值29.2%。通过扫描电镜观察拉伸试样断口,均为韧性断裂。厚度不同时,随着φ的增大,韧窝数量增多且尺寸增大,材料塑性较好;晶粒尺寸不同时,φ从10.6减小至3.5,断面收缩率增加,韧窝尺寸增大,材料塑性较好。而当φ值继续减小到0.8时,韧窝数量减少,且尺寸变小,材料的塑性变差。     

C5210磷青铜薄板成形行为的尺度效应（英文）

为研究厚度、晶粒尺寸对C5210磷青铜薄板力学性能和成形性能的影响,通过不同温度退火热处理得到不同晶粒尺寸的试样,然后在常温下对具有不同厚度和晶粒尺寸的试样进行单向拉伸试验。结果表明:当厚度在50~800μm范围内,材料的屈服强度随着厚度的减小而增大,而加工硬化指数和伸长率随着厚度的减小而减小,提出了描述屈服强度随厚度减小而增大关系的修正模型;材料的屈服强度随着晶粒尺寸的增大而减小,但加工硬化指数随着晶粒尺寸的增大而增大,伸长率则随着晶粒尺寸的增大先增大到一个峰值后再减小。通过扫描电镜观察拉伸试样的断口形貌发现所有试样断裂方式均为韧性断裂,并且随着厚度的增大断口韧窝的密集度增大,而晶粒尺寸越大的试样断口韧窝密集度越小。     

黄铜薄板单向拉伸第Ⅱ类尺寸效应

以黄铜薄板为研究对象,通过对不同厚度、不同宽度试样的单向拉伸,研究厚度和宽度变化对薄板单向拉伸过程应力和塑性性能的影响。实验结果表明,厚度变化对应力的影响十分显著。当厚度小于10倍左右内禀尺寸,薄板变形不均匀性加剧,不断增大的几何必需位错加强了薄板的硬化,流动应力随厚度的减小而增大,表现出明显的第Ⅱ类尺寸效应。但是当应变增大到0.013以后,几何必需位错所占比例不断减小,其对材料的硬化作用明显减弱。此外,随着厚度的减小,薄板的断裂机制由韧窝微孔聚集断裂转变为沿晶断裂,板料的塑性性能持续下降。拉伸试样宽度减小使流动应力略微下降,但对塑性性能影响不大。     

5Cr油管电化学充氢后的力学行为

通过拉伸实验研究电化学充氢对5Cr油管钢拉伸性能的影响表明：随充氢时间和充氢电流密度的增加,5Cr油管的强度和塑性有所减小。用扫描电镜（SEM）观察断口发现其形貌以韧窝为主要特征,与未充氢试件相比,充氢试样拉伸断裂后韧窝尺寸变小、变浅,数量增多,这说明材料的塑性损失增大。充氢到一定值后材料由韧性断裂转变为脆性断裂。     

循环载荷幅值对7075-T7451铝合金疲劳裂纹扩展行为的影响

在不同幅值循环载荷条件下对7075-T7451铝合金紧凑拉伸(CT)试样进行拉伸疲劳试验,对其疲劳裂纹扩展速率和应力强度因子幅值ΔK进行了研究,并用扫描电子显微镜观测试样的断口形貌。结果表明：随着循环载荷幅值的增大,试样的疲劳寿命缩短,裂纹的扩展速率增大;试样宏观断口形貌的裂纹稳态扩展区域减小,而瞬时断裂区域增大。稳态扩展区主要以疲劳条带扩展机制为主,且疲劳条带间距随循环载荷幅值的增大而增大;瞬断区的断口形貌以韧窝断裂为主,韧窝尺寸随循环载荷幅值的增大而减小。     

不锈钢超薄板力学性能的尺度效应

为研究304不锈钢超薄板力学性能与试样尺寸、晶粒尺寸的相关性,应用自制的超薄板专用夹具进行拉伸试验,分析超薄板的厚度、晶粒尺寸对材料力学性能的影响。试验结果表明,随着试样厚度减小,材料的屈服强度先减小而后明显增大。这是因为,当试样厚度大于200μm时,表面层效应起主导作用;当试样厚度小于200μm时,应变梯度效应和表面层钝化膜共同作用。同一厚度下,改变试样的晶粒尺寸结果表明,当厚度尺寸和晶粒尺寸的比值小于2时,板料在厚度方向已没有内部晶粒,位错很容易滑移出自由表面,应力-应变曲线显著降低。     

静态及动态充氢对SM490B纯净钢拉伸性能的作用

用拉伸的方法研究了静态充氢和动态充氢对SM490B纯净钢力学性能的作用,结果表明:随着充氢电流密度的增加,试样的延伸率连续降低。动态充氢试样的氢致塑性损失明显大于静态充氢拉伸试样的氢致塑性损失。氢原子提高了试样的屈服强度,而试样的抗拉强度随电流密度增大而减小。随着充氢电流密度的增加,静态拉伸试样的断口均为韧窝状,而动态拉伸试样的断口形貌由韧窝状向准解理状变化。     

Q&P钢微观组织及力学性能试验研究

以先进高强钢QP钢为对象,研究其微观组织结构及力学性能。对试样进行了单向拉伸试验,对不同材料的拉伸断口进行了SEM分析,并对不同变形程度的试样进行了XRD分析。结果表明,QP钢的硬化指数n值较高,硬化行为明显,相比DP钢、MS钢具有较好的塑性成形能力;QP钢属于韧性断裂,韧窝较大较深,塑性性能优于DP钢和MS钢;QP钢中的残余奥氏体体积转化率与应变大小和应变率相关。     

镀锌层对高强度镀锌薄板力学性能的影响

通过宏观单向拉伸实验和断口微观组织观察,研究分析了镀锌层对G550高强度镀锌薄板力学性能的影响,实验中所使用的试样厚度t分别为0.28,0.39,0.45和0.58mm,镀层厚度均为50μm左右。结果表明:这种镀锌板在去除镀锌层前后力学性能变化较为明显,尤其是屈服极限、抗拉极限、塑性应变比r、硬化指数n和伸长率δ变化较为明显,而杨氏模量E和泊松比μ几乎无变化;这种镀锌板的屈服极限和抗拉极限均高于裸板,而塑性应变比r、硬化指数n和伸长率δ均小于裸板;相比较于裸板,这种镀锌板的成形性能有一定的下降,后续成形工艺的设计需要考虑镀锌层的存在。     

薄壁纯钛管室温力学性能的研究

通过单向拉伸试验研究了四种规格薄壁纯钛管在三种拉伸速率下的室温基本力学性能。结果表明:随着管壁晶粒尺寸的减小,材料的抗拉强度、硬化指数增大;随拉伸速率的增大,抗拉强度逐渐增大,伸长率、硬化指数逐渐减小;薄壁纯钛管的应变速率敏感性指数总体较小,且随着应变的增大而逐渐减小。     

Constitutive model based on dislocation density and ductile fracture of Monel 400 thin sheet under tension

In micro-scaled plastic deformation, material strength and ductile fracture behaviors of thin sheet in tension are quite different from those in macro-scale. In this study, uniaxial tensile tests of Monel 400 thin sheets with different microstructures were carried out to investigate the plastic deformation size effect in micro-scale. The experimental results indicate that the flow stress and fracture strain departure from the traditional empirical formula when there are only fewer grains across the thickness. And the number of dimples on the fracture surface is getting smaller with the decreasing ratio of specimen thickness to grain size. Then, a constitutive model based on dislocation density considering the free surface effect in micro-scale is proposed to reveal the mechanism of the flow stress size effect. In addition, a model is proposed considering the surface roughening inducing the thickness nonuniform and the decrease of micro-voids resulting from the reduction of grain boundary density with the decreasing ratio of specimen thickness to grain size. The interactive effects of the surface roughening and the decrease of micro-voids result in the earlier fracture in micro tension of the specimen with fewer grains across the thickness.     

Numerical simulation of two-stage deep drawing of complex-shaped parts

1　INTRODUCTIONInrecentyears ,withtherapiddevelopmentofcomputersoftwareandhardware ,andtheintersectionandcombinationofcomputertechnology ,graphics ,mechanicsandprocessengineering ,thecomputeraid edengineering(CAE)technologybasedonnumericalsimulationhasbeenwidelyappliedinthesheetmetalformingfield[1,2 ] .Nevertheless ,untilnow ,thenu mericalsimulationisconfinedtosimulatesomebasicmodesofdeformationsuchasbending ,deepdraw ing ,bulgingandflanging .Forthedeformationofcomplexwork pieces ,forexa…     

AZ31镁合金铸轧和常规轧制板的变形组织及形变特征

在变形温度为150～400 ℃、应变速率为0.3～0.000 3 s~(-1)条件下,在Gleeble1500热模拟机上采用等温拉伸试验对AZ31镁合金铸轧和常规轧制板的高温塑性及组织演变进行研究.结果表明:两种AZ31镁合金板的峰值应力和峰值应变均随着变形温度的降低和应变速率的增加而逐渐增大.铸轧板的应变硬化指数和应变速率敏感系数均大于常规轧制板的.在高温低应变速率变形条件下,铸轧板的晶界滑移引起的空洞尺寸、体积分数和密度均大于常规轧制板的.低应变速率下拉伸变形后的动态再结晶晶粒尺寸随温度的升高逐渐增加;不同变形条件下铸轧板的晶粒尺寸均小于常规轧制板的;再结晶晶粒尺寸和Z参数呈幂律关系.     

微细薄板尺度依赖的材料本构模型的构建(英文)

通过表面层理论和金属晶体塑性变形原理解释微细薄板材料在塑性变形中产生尺寸效应的内在机理。引入尺度参数,对经典的Hall-Petch公式进行修正,建立基于表面层模型理论的尺度依赖材料模型。利用所建立的材料模型分析微细薄板厚度及其晶粒尺寸对材料成形流动性能的影响。在晶粒尺寸一定的情况下,随着微细薄板厚度的减小,材料流动应力逐渐降低;晶粒尺寸越大的微细薄板,其流动变形的尺寸效应现象越明显。利用不同厚度的不锈钢和纯铜箔材的微细薄板拉伸真应力-应变曲线对所建立的材料模型进行验证,计算结果与实验结果比较吻合,验证了所建立的材料模型的合理性。     

热机械加工对EW75镁合金组织与性能的影响

采用金相显微镜、扫描电镜和拉伸试验机等手段,研究了恒温多向锻造工艺对EW75镁合金显微组织和力学性能的影响,并分析了动态再结晶机制。结果表明,500和470℃锻造态EW75镁合金薄板都发生了动态再结晶,而440和410℃锻造态EW75镁合金薄板中可见大量细小颗粒状析出相,未见明显动态再结晶组织,且在平行于锻压方向上,可见原始晶粒形貌以及颗粒状析出相,而在垂直于锻压方向上,可见明显加工变形流线。470℃锻造态EW75镁合金薄板发生了完全动态再结晶,平均晶粒尺寸约为15μm,且合金中动态再结晶晶粒大部分为大角度晶界。经过锻造处理后的EW75镁合金薄板的强塑性相较于固溶态均有明显提升,随着锻造温度的降低,EW75镁合金薄板的抗拉强度和规定塑性延伸强度都呈现逐渐上升的趋势,而断后伸长率则表现为先增加后减小的特征。在470℃锻造时,EW75镁合金薄板具有最好的塑性,这主要与完全再结晶协调塑性变形以及较大的晶粒间取向差有助于晶面滑移有关。     

Investigation of size effects on material behavior of thin sheet metals using hydraulic bulge testing at micro/meso-scales

Reliable material models are necessary for accurate analysis of micro-forming and micro-manufacturing processes. The grain-to-feature size ratio (d/D_c) in micro-forming processes is predicted to have a critical impact on the material behavior in addition to the well-known effect of the grain size (d) itself as manifested by the Hall–Petch relation. In this study, we investigated the “size effects” on the material flow curve of thin sheet metals under hydraulic bulge testing conditions. The ratio of the sheet thickness to the material grain size (N=t₀/d) was used as a parameter to characterize the interactive effects between the specimen and the grain sizes at the micro-scales, while the ratio of the bulge die diameter to the sheet thickness (M=D_c/t₀) was used to represent the effect of the feature size in the bulge test. Thin sheets of stainless steel 304 (SS304) with an initial thickness (t₀) of 51 μm and three different grain sizes (d) of 9.3, 10.6, and 17 μm were tested using five bulge diameters (D_c) of 2.5, 5, 10, 20, and 100 mm. A systematic approach for determining the flow curve of thin sheet metals in bulge testing was discussed and presented. The results of the bulge tests at different scales showed a decrease in the material flow curve with decreasing N value from 5.5 to 3.0, and with decreasing M value from 1961 to 191. However, as M value was decreased further from 191 to 49, an inversed relation between the flow curve and M value was observed; that is, the flow curve was found to increase with decreasing M value from 191 to 49, a new observed phenomenon that has never been reported in any open literature. New material models, both qualitatively and quantitatively, were developed to explain the size effects on the material flow curve by using the N and M as the characteristic parameters of relative size between the grain, the specimen (i.e., sheet thickness), and the part feature (i.e., bulge diameter). The explanation and prediction of the flow curve behavior based on these models were shown to be in good agreement with the bulge test results in this study and in the literature.     

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.     

板料n值测量方法的研究

在板料冲压成形过程中 ,板料的硬化指数n值具有十分重要的意义 ,因而n值的测量无论对理论研究还是对工程实践都非常必要。因此 ,涌现出众多的硬化指数n值的测量方法。到目前为止 ,常用的测量方法主要有一点法、两点法、阶梯形试件拉伸法等。这些方法各有其优缺点 ,其共同不足就是用这些方法测定的n值反映的只是均匀塑性变形阶段的硬化性能。为此 ,本文基于常规单向拉伸试件在拉断后出现的锥形现象 ,以拉伸过程中的3个特征点 (屈服之后靠近屈服的一点、最大载荷时刻的点和断裂时刻的点 )为基础 ,提出了一种测量n值的新方案———三点法。在此方案中 ,用 3个特征点表征的两个n值分别描述了板料在均匀塑性变形阶段和随后大塑性变形阶段的硬化性能 ,而大塑性变形在板料成形过程中是必不可少的。本文通过对软钢板、紫铜板等进行的试验研究 ,证明了此方案是可行的。而且 ,作为板材成形性重要参数的r值也可以在试验中同时获得     

Effects of plastic strain and strain path on youngs modulus of sheet metals

The effects of plastic strain and strain path on Young’s modulus of sheet metals are experimentally investigated using low carbon steel, stainless steel, aluminium, copper and brass sheets of 1 mm thickness. These sheets are firstly deformed to different plastic strains under a few strain paths from balanced biaxial stretching to uniaxial tension. Then, a small uniaxial tension test specimen is cut from each deformed sheet and Young’s modulus is measured using electrical-resistance strain gauges glued to both surfaces of the specimen. The experimental results show that Young’s moduli of the low carbon steel and stainless steel sheets decrease with increasing plastic strain, while those of aluminium, copper and brass sheets hardly change with the plastic strain. In all materials, however, the effect of the strain path on Young’s modulus is not necessarily evident. It is confirmed that Young’s modulus of the low carbon steel sheet can be recovered to the initial value of undeformed sheet by a subsequent annealing. In addition to Young’s modulus, the effects of the plastic strain and the strain paths on Poisson’s ratio of these materials are also shown.     

Improvement of the drawability based on the surface friction stir process of AA5052-H32 automotive sheets

Based on the imperative social demand for lighter vehicles, lightweight materials such as aluminum alloys are expected to replace conventional steels in many automotive applications. In automotive parts manufacturing, most of the components produced in conventional stamping operations are geometrically complex as the blanks are subjected to both stretching and drawing deformations. However, aluminum alloys have intrinsic drawbacks, such as the inferior formability of these materials, although the effects of the weight reduction in terms of performance are highly promising. In an effort to improve the formability of aluminum alloy sheets, the surface friction stir process is proposed in this study. This process locally modifies the surface of automotive aluminum alloy sheets via stirring and advancing on the surface of the sheet, similar to the Friction Stir Welding (FSW) process that utilizes a probe without a pin. When the surface of the sheet is modified locally by stirring, dynamic recrystallization due to the severe shear deformation along with heat resulting from the friction occur due to changes in the micro-structure and mechanical properties in the stirred zone, while the dislocation density and grain size refinement are curtailed. In this work, the drawability performance of AA5052-H32 sheets (thickness 1.5 mm) that were welded using the surface friction stir process was experimentally and numerically investigated in cylindrical cup drawing tests. When applied to AA5052-H32 automotive sheets, the surface friction stir process improved the drawability of the entire aluminum alloy sheet. For numerical simulations, the non-quadratic anisotropic yield function Yld2000-2d was employed along with isotropic hardening, while the formability was evaluated by utilizing theoretical forming limit diagrams (FLD) based on Hill's bifurcation and M-K theories.