An analysis of the isothermal hot compression test

The nominally isothermal, uniaxial hot compression test has been analyzed with special reference to the effects of temperature nonuniformities and friction on sample deformation and flow stress estimates. A simple one-dimensional analysis was used to establish the influence of initial temperature nonuniformities, strain rate, and the temperature dependence of the flow stress on flow localization tendencies. Noticeable strain concentrations were predicted to occur only at high strain rates (∼10 s⁻¹) in materials such as titanium alloys, but not in steels, for typical values of the initial temperature nonuniformity. More extensive numerical (finite element method) simulations of the compression test with various values of the friction shear factor corroborated the conclusions of the flow localization analysis. In addition, it was established that initial temperature nonuniformities, as well as friction, have an almost negligible effect on flow stress data deduced from measurements of average pressurevs true height strain, at least for reductions of the order of 50 pct. The analysis results were supported by observations of the deformation behavior of a near-gamma titanium aluminide and a low-alloy steel. S.I. OH, formerly Research Leader, Net Shape Manufacturing Group, Battelle Memorial Institute, Columbus, OH, 43201. S.L. SEMIATIN, formerly Senior Research Scientist, Metalworking Group, Battelle Memorial Institute, Columbus, OH 43201.     

Measurement of friction under sheet forming conditions

Among the tests used to evaluate friction in sheet forming processes, bending-under-tension (BUT) tests have received considerable attention. A new test was developed incorporating a smooth increase of wrap angle during deformation, even at high deformation rates, thus replicating typical conditions at die radii. A corresponding data analysis procedure was introduced to separate the effects of bending and friction and to account for the strain-rate sensitivity of the test material. The new test and analytical procedure were used to investigate the frictional behavior for four sheet alloys: interstitialfree (IF) steel, high-strength galvanized (HSG) steel, 2008-T4 aluminum, and 70/30 brass. A range of testing rates, pin radii, and lubrication conditions were employed, and the variation of friction coefficient (μ) as a function of contact radius, contact angle, and punch speed was measured. Complex variations ofμ with respect to tool radius and wrap angle were observed, consistent with previous studies. No significant, systematic variation ofμ was found for punch speeds ranging from 1.7 to 170 mm/s, contrary to reports in the literature. Failures in 2008-T4 aluminum shifted from tensile localization to bending as the tool radius was reduced. The other materials consistently failed by tensile localization.     

An inverse analysis of the hot uniaxial compression test by means of the finite element method

A computer simulation using the finite element method is carried out in the present work to model a hot compression test. Real constitutive equations of the hot flow behaviour of a medium carbon microalloyed steel, experimentally determined, are implemented in a commercial computer code and an inverse analysis is performed to determine under which conditions the experimental test can be considered as valid. In other words, the degree of strain and strain rate heterogeneity as well as the isothermatility of the test, for the given material, are verified. The effect of the friction coefficient is also included in the current analysis.     

刹车速度对铜基粉末冶金摩擦材料性能的影响

以粉末冶金法制备铜基粉末冶金摩擦材料, 采用洛氏硬度计和夏比冲击试验机对摩擦材料的力学性能进行表征, 利用MM-3000型摩擦磨损性能试验台研究了刹车速度对材料摩擦磨损性能的影响, 并借助电子扫描显微镜(scanning electron microscope, SEM)观察了摩擦材料的微观形貌。研究表明:铜基粉末冶金摩擦材料的摩擦磨损性能与刹车速度密切相关, 随着刹车速度的增大, 摩擦吸收功率近似线性增长, 而摩擦系数呈先增大后减小的趋势; 在高速刹车条件下, 铜基体自身发生软化会破坏摩擦材料表面形成的氧化膜, 降低了分子键的抗剪切强度, 从而增大了磨损量。     

Effect of friction,strain hardening,and strain rate hardening on the metal flow in rod extrusion

The metal flow in a rod extrusion is theoretically discussed in relation to a frictional condition, strain hardening and strain rate hardening. An existing rigid plastic FEM-program was modified in order to take a rate sensitivity of a material into consideration through a constitutive equation of the form. An interfatial friction was incorporated by a constant Coulomb-frictional coefficient. When the frictional coefficient is smaller than 0.1, a large effect of the friction does not appear. The larger the n-value is, the stronger the distortion of the grid in a billet becomes even under small frictional resistance. The effect of the m-value is similar qualitatively to that of the n-value. When the rate sensitive material is extruded at very low punch speed under large frictional resistance, the material is strained very irregularly all over the billet at an early stage.     

Features of formation and the structure,composition, and properties of electrospark coatings on the ZhS6U nickel alloy with the use of the KhTN-61 SHS-Ts alloy

Features of the mass transfer of a KhTN-61 SHS-Ts electrode material onto a substrate of a ZhS6U nickel alloy with a variation in frequency (1000–1400 Hz) and time (25–70 μs) of pulsed discharges was studied. A complex of studies of the structure, composition, and properties of the formed coatings was carried out. The optimum frequency-energetic mode of the treatment (E = 0.30 J, f = 1000 Hz, and τ = 50 μs), which was characterized by a high rate of coating at a satisfactory degree of the roughness of the surface layer, is found. As a result of electrospark alloying (ESA) at the optimum energetic parameters, 40-μm-thick coatings with a uniformity of 95% and a microhardness of 5.2 GPa form on the surface of the nickel alloy. The ESA treatment of a ZhS6U nickel alloy with the use of KhTN-61 SHS-Ts electrode material allows us to enhance its wear resistance (by more than a factor of 10), hardness (by a factor of 2), and heat resistance, as well as to decrease (by a factor of 5) the friction coefficient.     

SiO2/ZrO2复合陶瓷组元对铜基摩擦材料摩擦磨损性能的影响

在粉末冶金铜基摩擦材料中添加6%(质量分数)的SiO2/ZrO2复合陶瓷组元,研究SiO₂和ZrO₂的质量分数对摩擦材料摩擦磨损性能的影响,并分析其机理。结果表明:随w(SiO₂)/w(ZrO₂)比值减小,铜基摩擦材料的密度和硬度增大。高速制动时,摩擦材料的摩擦因数和摩擦稳定因数较小。SiO₂可有效提高摩擦因数,ZrO₂可降低摩擦副的磨损率。当w(SiO₂)/w(ZrO₂)为2/4时,摩擦材料具有较好的摩擦磨损性能,高速制动下平均摩擦因数为0.326,摩擦稳定因素处于较高水平,为0.71,对偶数材料损伤在可接受范围内。SiO₂较易脱落而形成磨粒,ZrO₂与基体界面结合状态较好,所以随SiO₂含量减少,主要磨损机制从磨粒磨损转变为黏着磨损和磨粒磨损,最后转变为剥层磨损。     

粉末冶金摩擦材料综述

本文简要地综述了为提高并稳定粉末冶金摩擦材料摩擦系数,提高耐磨性、耐热性、抗腐蚀能力和改进导磁性、降低成本而作的配方试验与所得到的性能;并介绍了粉末冶金摩擦材料的各种制造方法,包括现行的与正在试验的制造方法;导后综述了关于粉末冶金摩擦材料发展前景的若干评价。     

A theoretical sensitivity analysis for full- dome formability tests: parameter study forn,m, r,and μ

Sheet material formability has been assessed by many kinds of tests and analyses presented in the literature; the most popular ones are based on hemispherical punch stretching geometry. An axisymmetric version of such a test was analyzed using SHEET-3 [a three-dimensional (3-D) finite element method (FEM) program] to determine the sensitivity of the test to parametersn,m, r, and μ. A realistic and self-consistent failure criterion was established to predict the limiting cup height at failure (LDH _f ) in sheet specimens from calculated strain distribution data. A 2^k factorial experiment was conducted for variations in the four variables (k = 4). The sensitivity of LDH _f to variations inn andm is constant, while its sensitivity to variations in μ. is decreased as μ, is increased. The significance of the influence of normal anisotropy on LDH_f is unclear, being dependent upon the choice of yield theory. Direct interactions between material properties and friction were, for the most part, not found to strongly influence test results at the parameter levels examined. Materials with different properties will typically not fail under the same strain conditions in full-dome cup tests. Cup test results are influenced by a number of parameters, including some not considered here,e.g., sheet thickness and test temperature. The FEM model for the full-dome cup test, including a prediction of forming limits, provides a useful technique for sorting out important effects that individual parameters have on the strain state in the sheet at failure and the predicted final punch height at failure. D.A. Burford, formerly Senior Research Associate with the Department of Materials Science and Engineering, The Ohio State University     

Simulation of the hot-tension test under cavitating conditions

A theoretical analysis of the isothermal hot-tension test under cavitating conditions for sheet samples was performed using a “direct-equilibrium” approach. The effects of cavity growth rateη, initial cavity volume fractionC _v0, strain-rate sensitivity exponentm, and specimen taper on engineering stress-strain curves, strain profiles, and failure modes were established. For a given value ofm, it was predicted that the engineering stress-strain curves of cavitating and noncavitating samples are almost coincident except near failure. In fact, during quasistable deformation, the required load for a cavitating material is slightly higher than that for a noncavitating material because of strain rate and effective area effects. Model results also delineated the competition between failure controlled by localized neckingvs fracture, the latter being defined by a critical-volume fraction of cavities. Specifically, at low strain-rate sensitivitiesm and cavity growth ratesη, failure was predicted to be controlled by necking. By contrast, at high values ofm andη, fracture prior to localized necking was predicted to predominate; in these cases, the cross-sectional area at the failure site was appreciable, thus resembling a form of brittle fracture. The validity of the modeling approach was confirmed through the analysis of data in the literature. However, model results did suggest that caution should be taken in the interpretation of experimental data because various combinations ofC ^v0 and η can result in the same total elongation. Formerly Visiting Scientist, Metals and Ceramics Division, Materials Directorate, Wright Laboratory, WL/MLLN     

Experimental investigation and finite element modeling of hemispherically stretched steel sheet

Hemispherical stretching experiments and corresponding finite element modeling (FEM) were performed on three aluminum-killed (AK) steels. Strain distributions measured from photogrids on the specimen surface were compared to those predicted by a three-dimensional (3-D), membrane, rigid-viscoplastic FEM program. The material model uses Hill's nonquadratic theory for normal anisotropy and Coulomb friction. The new anisotropy coefficient,M, and friction coefficient, μ, have opposite effects on the strain distribution. Balanced, biaxial simulations of highM materials required unrealistically high friction coefficients to produce agreement with measured strains. The discrepancies call into question the validity of Hill's nonquadratic yield surface and the method of measuringM. J.R. KNIBLOE, formerly Graduate Student, Department of Materials Science and Engineering, The Ohio State University     

Anelastic behavior of barium-titanate-based ceramic materials

The internal friction (Qsu−1) and Young’s modulus (E) of BaTiO₃-based ceramics were measured vs temperature from −100 °C to 150 °C. Rectangular bars of high-density (96 to 99 pct) ma-terials were driven electrostatically in flexural vibration at a resonance frequency of about 3 kHz, at maximum strain levels of about 10⁻⁶. The curves ofQ ⁻¹(T) andE(T) allow the study of the following three phase transformations: tetragonal to cubic (about 130 °C in pure material), orthorhombic to tetragonal (about 0 °C in pure material), and rhombohedral to orthorhombic (about −80 °C in pure material). Internal friction and modulus data were obtained on pure material and on materials doped with niobium and cobalt to give semiconducting and insulating X7R behavior. Permittivity, dielectric loss, and microstructure data are given and used to aid interpretation of the mechanical measurement data. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee     

Experimental Analysis on the Frictional Behaviour of Drawbeads in Sheet Metal Forming

In sheet metal forming, drawbeads are commonly used to control uneven material flow, which may cause defects such as wrinkles, fractures, surface distortion and springback. Although friction may not directly change the limiting strain of steel sheets, the tribological conditions in the contact zone between the sheet surface and the tool surface play an important role in determining the limits of the forming process. Friction in the drawbead contact zones affects the flow of the material in the tool and is used deliberately to control the stamping process. Therefore in this study, the frictional behaviour of drawbeads is experimentally investigated by the drawbead friction test. To characterize the effect of processing variables on the friction coefficients, tests are performed for various sheets, lubricants and bead materials suffering different surface treatments. The results obtained from the drawbead friction test show that the friction and drawing characteristics of deforming sheets were strongly influenced by the strength of sheet, viscosity of lubricant and hardness of bead surface.     

板成形中摩擦因数测定方法

采用新型摩擦因数测定装置进行了摩擦因数测定的实验研究，为进一步开展摩擦因数测定及摩擦边界的研究打下了基础，为有限元模拟的真实性提供了参考依据。     

温度分布对铜基摩擦材料点蚀损伤的影响

通过离合器惯性实验台耐热实验,结合对非均质粉末冶金摩擦层结构模型和温度场的分析结果,研究了特定工况下的铜基粉末冶金摩擦副点蚀损伤现象,分析了温度分布对摩擦材料点蚀损伤的影响。结果表明:湿式铜基摩擦材料在长时间过载或热负荷集中时,由于摩擦表面产生局部高温,摩擦层内部产生较大的温度梯度和热应力,在铜基体与石墨接触区域会产生裂纹并出现铜基体的脱落与转移,发生点蚀;摩擦层上的点蚀程度由外侧向内侧逐渐加重后再减轻,中部点蚀现象最严重;在同一道摩擦层上,距离径向油槽较远区域的点蚀现象严重。     

Effects of work-hardening and rate sensitivity on the sheet tensile test

The importance of strain hardening and strain-rate hardening in sheet tensile tests has been investigated using finite-element modeling (FEM). A Hollomon-type material law was examined with n-values (logarithmic work hardening rate) between 0.0 and 0.5 andm- values (logarithmic strainrate sensitivity) between -0.002 and 0.3. A von Mises yield condition with isotropic hardening rule completed the material model. The results of these simulations showed that, as expected, the uniform elongation(e _u ) depends mainly onn, with a small dependence onm. The post-uniform elongation (e _f —e _u ;e _f = total elongation) depends on bothn andm, and is highly nonlinear in either variable for large parametric values. That is, combined increases ofn andm extend ductility more than the additive increase expected from separate effects. Earlier presumptions thatn is unimportant in the post-uniform region appear incorrect. Approximate relations between (e _f ,e _u ) and (n,m) were developed quantitatively. These relations have been compared with superplastic data in the literature and appear to agree well, given uncertainties in specimen geometry and test procedure. K. CHUNG, formerly Postdoctoral Research Associate, Department of Metallurgical Engineering, The Ohio State University     

热连轧粗轧辊磨损的有限元数值模拟和工艺优化

针对轧钢厂GCr15轴承钢240 mm × 240 mm方坯粗轧阶段轧辊磨损较严重的情况,采用Archard磨损数学模型模拟分析了轧件压下量、轧辊硬度、热传导系数及摩擦因子在一道次成形后对轧辊磨损规律的影响。模拟结果表明,轧辊硬度越高,轧辊抗磨损能力越强;热传导系数对轧辊磨损的影响较小;当摩擦因子f＞0.25时,其摩擦因子对轧辊磨损量变化明显;当轧件压下量在△h＜50 mm 时,轧件压下量对轧辊的磨损量影响显著。根据所得结果,结合现场轧制工艺和轧辊材质,将使用的球墨铸铁Ⅰ轧辊[抗拉强度≥400 MPa,硬度HRC值40,热传导系数18 kW/（m²·℃）,摩擦因子0.3]改成球墨铸铁Ⅱ轧辊[抗拉强度≥500 MPa,硬度HRC值45,热传导系数17kW,/（m²·℃）,摩擦因子0.2],并将压下量由70 mm降至50 mm,使轧辊单槽过钢量由优化前10000 t提高至优化后的18000~20000 t。     

耐磨合金材料在烧结生产中的应用

本文针对烧结生产中各种磨损机制的作用，以及部分耐磨钢种的物化特性作了有益的探讨，针对烧结生产使用的耐磨钢性能做了比较，提供了部分选材方案和对比实验结果，对耐磨金属材料的应用具有一定的参考价值。