Contact Conditions in Skin-pass Rolling

The special contact conditions in skin-pass rolling of steel strip is analysed by studying plane strain upsetting of thin sheet with low reduction applying long narrow tools and dry friction conditions. An extended sticking region is estimated by an elasto-plastic FEM analysis of the plane strain upsetting. This sticking region causes a highly inhomogeneous elasto-plastic deformation with large influence of work-hardening and friction. A numerical analysis of skin-pass rolling shows the same contact conditions, i.e. an extended sticking region around the center of the contact zone. The calculated size of the sticking region with varying contact length and pressure/reduction is experimentally verified by plane strain upsetting tests measuring the local surface deformation of the work pieces after unloading.     

接触表面性质对圆柱滚子轴承混合润滑的影响

目的为了提高圆柱滚子轴承的润滑性能,研究滚子与外圈表面纹理及硬弹比对圆柱滚子轴承混合润滑的影响。方法基于平均流量模型、非牛顿效应、热效应和粗糙峰的弹塑性变形,建立了圆柱滚子轴承有限长线接触热混合润滑模型。研究了表面纹理及硬弹比对膜厚、膜厚比、载荷比、平均摩擦系数及最高温度的影响。结果随着表面纹理参数的增大,油膜厚度逐渐减小,粗糙峰接触压力、平均摩擦系数和载荷比逐渐增,最高温度先减小后增大,最小膜厚先增大后减小。但表面纹理参数小于1/3时,对最小膜厚的影响非常小。表面硬度引起的完全塑性变形、弹塑性和塑性变形、完全弹塑性变形对润滑状态的影响不同。在硬弹比处于0.01～0.03时,粗糙峰同时发生弹塑性和塑性变形,油膜厚度、最小膜厚、载荷比、平均摩擦系数、粗糙峰接触压力及最高温度不随表面硬度而变化。当硬弹比小于0.01时,粗糙峰产生完全塑性变形;当硬弹比大于0.03时,粗糙峰发生完全弹塑性变形。这两种情况的载荷比、平均摩擦系数、粗糙峰接触压力及最高温度均随着表面硬弹比的增大而增大。在不同工况下,表面硬度与表面纹理参数对圆柱滚子轴承润滑状态的影响存在差异。在表面纹理参数小于1/3时,表面硬度的影响占主导地位;在表面纹理参数大于1.0时,表面纹理参数的影响占据主导地位。结论表面纹理参数等于1.0时,润滑状态最好;硬弹比处于0.01～0.03时,综合润滑性能最好。在不同条件下,表面纹理参数与表面硬度对润滑影响的程度不同。因此,圆柱滚子轴承混合润滑中,存在最佳的表面纹理参数和表面硬弹比。     

Basic investigations on boundary lubrication in metal forming processes by in situ observation of the real contact area

In metal forming processes tribology plays a significant role for process feasibility and tool lifetime. Boundary and mixed lubrication conditions are predominant in metal forming processes. Hence the appearing real contact area which is mainly influenced by the topography of tool and workpiece has a major impact on friction conditions. For an improved understanding of friction at the interface between tool and workpiece, the flattening behavior of idealized asperities under boundary lubrication conditions is investigated by applying a translucent tool. Various liquid lubricants have been used. It has been revealed that the usage of certain lubricants leads to higher resistance of the asperity against flattening which would be in favor of reduced friction. Topography analysis and finite element simulations have been performed in order to get more insight into the mechanisms leading to this effect. The findings have been transferred to real topographies by micro upsetting tests with ground surfaces.     

Finite element analysis of the effect of friction in high pressure torsion

High pressure torsion (HPT) is one of the most important techniques among various methods that create severe plastic deformation in the production of bulk materials with nano/ultrafine grained microstructures. Since the driving force in deforming the workpiece in HPT is surface friction, understanding of the friction effect is critical for successful application of HPT. In this study, the friction effect in HPT was analyzed using the finite element method. The distribution of effective strain on the contact surface of the HPT samples under different friction conditions was investigated. The friction force influenced the effective strain more in the middle and edge regions than in the central region. The condition for the minimum friction factor that could achieve a sticking condition between the surfaces of the dies, and the samples in the middle and edge regions, was investigated. There was a critical friction coefficient in which the effective strain varies sharply with an increasing friction coefficient.     

Study of workability limits of porous materials under different upsetting conditions by compressible rigid plastic finite element method

Workability limits must be considered when designing powder metallurgy (PM) forging processes. This research successfully applied the general upsetting experiment method to the deformation of porous materials. Based on the plastic theory of porous materials, the compressible rigid plastic finite element method is used to simulate the deformation processes of cold upsetting of disks and rings for porous metal materials with a full account of contact friction boundary conditions, the height-to-diameter ratio, the initial relative density, and the die and workpiece geometry. Furthermore, a successful analysis of the cold forging process results in the prediction of the stress, the strain, and the density field. By coupling with the ductile fracture criterion, which is a strain-based criterion obtained by Lee and Kuhn, possible defects leading to material failure have been checked. This research reveals that larger height to diameter and a lesser friction factor can delay the local strain locus to intersect with the Lee and Kuhn’s fracture line and restrain formation of the surface crack. Meanwhile, it reveals that the initial relative density has only a very small influence on the strain to fracture in compression, and it shows the forming behavior of the ring and disk with the curved die. According to Lee and Kuhn’s results, the calculated results agree well with the experimental results.     

Determination of workpiece flow stress and friction at the chip–tool contact for high-speed cutting

This paper presents a methodology to determine simultaneously (a) the flow stress at high deformation rates and temperatures that are encountered in the cutting zone, and (b) the friction at the chip–tool interface. This information is necessary to simulate high-speed machining using FEM based programs. A flow stress model based on process dependent parameters such as strain, strain-rate and temperature was used together with a friction model based on shear flow stress of the workpiece at the chip–tool interface. High-speed cutting experiments and process simulations were utilized to determine the unknown parameters in flow stress and friction models. This technique was applied to obtain flow stress for P20 mold steel at hardness of 30 HRC and friction data when using uncoated carbide tooling at high-speed cutting conditions. The average strain, strain-rates and temperatures were computed both in primary (shear plane) and secondary (chip–tool contact) deformation zones. The friction conditions in sticking and sliding regions at the chip–tool interface are estimated using Zorev's stress distribution model. The shear flow stress (k_chip) was also determined using computed average strain, strain-rate, and temperatures in secondary deformation zone, while the friction coefficient (μ) was estimated by minimizing the difference between predicted and measured thrust forces. By matching the measured values of the cutting forces with the predicted results from FEM simulations, an expression for workpiece flow stress and the unknown friction parameters at the chip–tool contact were determined.     

旋转工作台静压导轨摩擦因数测试实验研究

旋转工作台之间的摩擦对机床整机动态性能的影响很大,是可动结合部间存在阻尼的主要原因。实际摩擦因数的大小受到接触面材料、表面粗糙度、表面平整度、接触面间的润滑状况、受载状况以及接触面间相对速度的大小等诸多因素的影响,常常与参考值有较大出入,因此有必要准确测出接触面间的摩擦因数。提出一种测试摩擦因数的思路,不需要任何辅助设备,仅仅依靠机床数控系统本身的反馈数据,即可方便地计算出旋转工作台静压导轨的摩擦因数。测试了4种不同工况下的速度-摩擦因数曲线,分析了不同油压下旋转工作台静压导轨的转速与摩擦因数的关系,为整机动力学仿真提供了基础数据。     

利用圆环镦粗法测定7050铝合金高温变形摩擦系数

采用圆环镦粗法进行了7050铝合金高温变形摩擦系数测试实验,研究了7050铝合金在不同变形速度、不同变形温度、不同变形程度、不同润滑剂条件下的高温变形摩擦。实验结果表明,润滑状态对其高温变形摩擦影响最大,变形温度次之。     

Slip-line field model of micro-cutting process with round tool edge effect

This paper presents a slip-line field model which considers the stress variation in the material deformation region due to the tool edge radius effect. The Johnson-Cook constitutive model is applied to obtain the shear flow stress and hydrostatic pressure as functions of strain, strain-rate, and temperature in the primary shear zone. The friction parameters between the rake face and chip are identified from cutting tests. The sticking and sliding contact zones between the tool and chip are considered in the secondary shear zone. The total cutting forces are evaluated by integrating the forces along the entire chip-rake face contact zone and the ploughing force caused by the round edge. The proposed model is experimentally verified by a series of cutting force measurements conducted during micro-turning tests. Micro-cutting process is analyzed from a series of slip-line field simulations.     

基于热混合润滑的钢/橡胶接触表面水润滑增强调控

目的 增强钢/橡胶摩擦副的润滑性能,为提高混合润滑状态下水润滑轴承的性能提供参考。方法 建立水润滑条件下钢/橡胶摩擦副的热混合润滑模型,讨论热效应对润滑性能的影响,并在此基础上进一步研究表面粗糙度、水基润滑剂黏度和供水压力对水润滑增强调控的作用。结果 与等温解相比,热效应使Stribeck曲线发生了右移,摩擦因数和载荷比增大,膜厚比降低。最高水膜温度随着转速的增加而升高,热效应对混合润滑性能的影响显著。减小摩擦副表面粗糙度,Stribeck曲线向左移动。在相同转速下,载荷比随着表面粗糙度的减小而降低,膜厚比反之。表面粗糙度越大,水膜温度越高,最高温度位于出口区,且钢的表面温度低于水膜和橡胶的表面温度。当水基润滑剂的黏度增大时,膜厚比增大,载荷比和最高水膜温度降低,Stribeck曲线发生左移。增加供水压力可以改善水膜压力分布,使水膜承载区增大、压力减小,粗糙峰接触压力和承载区减小,导致载荷比减小、膜厚比增加,Stribeck曲线向左偏移,水膜最高温度降低。当接触区由边界润滑向混合润滑过渡时,水膜最高温度出现拐点,且水膜最高温度拐点随着供水压力的增加而左移。结论 热效应会降低摩擦副的混合润滑性能,因此在混合润滑中不能忽略。考虑热效应时,通过减小表面粗糙度,或增加水基润滑剂黏度和供水压力,均有利于增强钢/橡胶接触表面水润滑的混合润滑性能。     

Determination of flow factors for the semi-analytical prediction of friction coefficients

Friction conditions in sheet metal forming are mainly influenced by the surface topography of blank and tool, its mechanical properties and the properties of the intermediate medium, the lubricant. Aiming towards an analytical determination of friction coefficients for sheet metal forming, such factors should be included in suitable models and be weighted accordingly. In addition to the elastic–plastic deformation of the surface topography of the blank as a result of increasing nominal surface pressure, the influence of the lubricant can be considered using the Reynolds equation. In the present investigation, the flow factors of the elasto-hydrodynamic lubrication and mixed friction, which take into account the effects of surface topography and orientation, were calculated as a function of nominal surface pressure and nominal contact area in terms of plastically deformed surfaces. Asperity deformation is evaluated with a numerical contact model using the flow curve of base metal to calculate local contact forces.     

考虑表面形貌的接触应力分析

目的 研究规则和实际表面形貌对二维平面线接触模型的影响.方法 基于线接触几何特性,考虑表面受载后的弹性变形特性,结合规则和实际粗糙表面的形貌特征,并考虑磨合对表面的影响,采用移动平均滤波方法对实际粗糙表面进行光滑处理,利用共轭梯度法,求解表面接触压力和摩擦力,计算二维平面内接触近场应力分布,同时采用修正的离散卷积快速傅里叶变换方法提高计算效率.对比验证二维接触模型的准确性,并对比规则和实际粗糙形貌表面受法向载荷和切向载荷时平面应力分布云图中的应力分布和大小.结果 圆面与圆形微凸体接触时,圆形凸体半径越小,最大Mises应力值越大,半径增大,最大Mises应力减小.多个圆形微凸体及正弦微凸体对应力分布影响类似.当考虑切向力时,会对应力分布云图中的应力分布形状和大小产生巨大影响.在相同平面表面形貌时,摩擦系数越大,Mises应力越大,并且应力沿摩擦力方向发生偏移.结论 数值模型能准确计算出粗糙平面的应力.当圆面与规则表面轮廓平面接触时,与光滑表面接触时相比,Mises应力分布呈现很大不同,微凸体越小,应力越集中;摩擦力会使接触压力和近场Mises应力产生偏移,摩擦系数越大,偏移越明显.当考虑实际粗糙表面时,在粗糙界面尖端接触区域产生应力集中.经过磨合表面采用平均滤波光滑处理后,粗糙界面尖端接触区域应力集中将大幅减小.     

Investigation of Influencing Parameters for Tribological Conditions in Dry Forming Processes

Environmental awareness and a growing demand for efficient resource utilization encourage the realization of lubricant-free forming processes. A first step in accomplishing dry sheet metal forming is to gain knowledge about the changing tribological conditions and to identify the relevant influencing parameters. The commonly used flat strip drawing test was selected to investigate the tribological conditions in the flange area of deep drawing processes. The influencing factors of contact pressure and varying drawing velocities were analyzed under dry and lubricated conditions. Additionally,the tool and workpiece surfaces were characterized. Besides lubrication, the contact pressure mainly determines the tribological conditions. In lubricated tests higher normal pressure reduces friction, whereas without lubrication higher pressure results in slightly increasing friction. A changing drawing velocity affects the friction when lubricant is applied. In dry experiments, no influence of velocity was found. Results of surface characterization reveal adhesion as main wear mechanism under dry conditions. Based on the investigated influence of the process parameters, an increase in process understanding for dry forming operations is derived.     

基于接触摩擦量化作用的镁合金轧制力预报研究

本研究针对镁合金热轧制工艺,通过协调有限元分析结果及物理试验数据,提出一种关于热轧制接触摩擦因子的确定方法,耦合多轧制工艺参数进一步拟合出其求解关系式,工艺参数包括：轧制温度,轧制速度以及压下率。在之前对AZ31B镁合金板材轧制过程轧制力预报模型及温度场数学模型建立研究的基础上,综合考虑接触单位压力及摩擦应力两方面因素的作用,对前期轧制力预报模型进行了优化重构,经确定接触摩擦作用到轧辊的压力约占整体轧制力的4.36%。     

粉末润滑界面边界层状态微观形态观察和分析

目的粉末润滑边界层状态预示着润滑条件的优劣。粉末颗粒以其良好的润滑性能和承载能力以及对极端工况的适应性,被广泛应用于诸多领域,因此需对粉末润滑界面的摩擦润滑性能展开深入研究。方法利用UMT-2多功能摩擦磨损测试仪,结合形状测量激光显微系统对粉末润滑界面边界层的微观形态进行观察和机理分析。重点研究载荷(1、2、4、6 MPa)、速度(2、4、8、16 r/min)、粉末层厚度(1.5、3.5、5.5、7.5μm)以及表面粗糙度(0.389、0.745、1.751、3.112μm)等工况条件对粉末润滑的影响机制。结果较大的接触载荷有利于形成致密的边界层,提高润滑性能。在速度较低时,不利于形成完整、致密的边界层,润滑较差,甚至产生爬行现象,随着速度的增加,润滑效果改善,但过大的速度又会加快粉末耗散。适量的粉末层厚度能够使摩擦界面间形成完整、厚实的边界层,提供良好的润滑能力。粗糙度适当的表面有利于粉末润滑剂存储、边界层的形成以及润滑作用的维持。结论合理选择接触载荷、速度、粉末层厚度以及试样表面粗糙度有助于提高粉末颗粒的润滑性能。     

The strain path and forming limit analysis of the lubricated sheet metal forming process

Few previous attempts have been made to analyze numerically the strain path and the forming limit in complex lubricated sheet metal forming. Since usual approaches of solving the lubrication model are limited to axisymmetric and plane strain cases only, this paper developed a unified procedure for combining the finite element code of sheet metal forming, the current lubrication/friction model and forming limit theory, to predict the strain path and fracture strains for either a steady or an unsteady three-dimensional process including both axisymmetric and plane strain cases. The availability of the method must be proved by a published problem, and an axisymmetric stretch forming process was therefore adopted as a benchmark. Numerical results showed that the present analysis provides good agreement with the experimental data of the strain path and the fracture strain for various tribological parameters such as lubricant viscosity and composite roughness of tooling and workpiece, and the advantage of the developed model is that it can be applied to solve the complicated 3D geometric problems.     

表面织构及供油量对润滑性能影响的建模分析

目的研究不同供油条件下织构表面的润滑性能。方法首先,建立考虑表面织构的乏油润滑模型,求解修正雷诺方程获得乏油工况下考虑织构表面的润滑油膜厚度以及压力分布。然后,依据求得的润滑油膜厚度判断计算域内各点润滑状态,通过接触压力及油膜厚度分别计算边界润滑、混合润滑以及流体润滑状态下的切应力,并积分求得摩擦力进而得到摩擦系数。结果模拟了供油层厚度为50~500 nm以及充分供油条件下三种织构的润滑行为,获得了不同润滑状态下表面织构的摩擦系数。速度为0.1 m/s时,供油量对接触区油膜厚度的影响较小,不同润滑状态下织构表现出不同的润滑性能。速度为0.2 m/s时,供油层厚度对油膜厚度的影响较大,随着供油层厚度的增大,膜厚明显增加,摩擦系数在供油层厚度为200 nm时最小。结论接触副处于流体润滑状态时,织构表面不具有减摩效果。接触副处于边界润滑状态时,织构表面具有减摩效果,并且织构较密时,摩擦系数较小。接触副处于混合润滑状态时,织构过于稀疏或密集时均不具有减摩效果,但是合理分布的织构具有减摩效果。     

不同润滑条件下带钢冷轧后表面形貌演变规律研究

在3种润滑条件下对酸洗后的热轧带钢试样进行了冷轧实验,采用扫描电镜跟踪观察冷轧前后试样的表面形貌,采用TR200型粗糙度测量仪测试轧制前后试样的表面粗糙度轮廓,研究了润滑条件对冷轧带钢表面形貌演变规律的影响。结果表明：轧件表面形貌主要取决于表面凹坑和辊痕的尺寸,随着轧制道次的增加,轧件与轧辊表面的实际接触面积增加,轧件表面凹坑和辊痕的尺寸均减小,轧件表面光洁度提高;轧制5道次后,干摩擦条件下轧制的试样表面显微凹坑最少,但表面存在明显的辊痕;油润滑条件下轧制的试样表面辊痕较少,但存在少量未被轧合的显微凹坑;乳化液润滑条件下轧制的试样表面残留的显微凹坑和辊痕都很少,表面质量最好。     

铝板冷轧润滑模型及应用

提出一种建立于流体动力学及塑性变形原理基础上的铝板冷轧润滑模型。可用于预测分析轧制变形区内油膜厚度、前滑、轧制压力及摩擦力分布等。该模型首先求解板材入口速度，然后得到油膜厚度，进行了高粘度矿物油铝板轧制试验，实测了不同压下率下入口油膜厚度，结果表明：实测值与计算值相当吻合，该模型为进一步研究板材冷轧及其润滑过程，优化生产工艺提供了依据。     

Textured surfaces for deep drawing tools by rolling

In this paper, macroscopic textured tool surfaces manufactured by rolling are investigated. Focus is on selective adjustment of friction by local texturing of tool areas to influence the material flow during deep drawing operations. Flat strip drawing tests were performed using friction elements with open textures. The texturing influences the friction conditions and affects the material properties of the stripes. The use of these surfaces results in a significant increase in friction, which allows an additional control of the material flow during sheet drawing operations. The main mechanisms for increased drawing forces are elastic deformation near the area of the texture and local plastic deformation on the sheet surface. Using strips made of mild steel, the texturing leads to an increased roughness of the sheet metal surface and, in the case of high surface pressure, to plastic deformations of the strips. Compared to conventional measures like draw beads, rolled-textured surfaces allow to retard the material flow during sheet drawing operation without excessive strain hardening in the sheet material.