Influence of friction on roughening of the matt surface in aluminium pack rolling

A model has recently been presented to simulate inhomogeneous deformation of polycrystalline materials. This model was based on a finite element analysis using an isotropic plasticity model for the material with a distribution of strengths to simulate the crystallographic texture. By considering plane-strain compression of monolithic material, the displacements generated at the midplane successfully provided a prediction of the roughness at the interface of pack-rolled aluminium foil. In the current study, the predictions of the model are explored further. First, the influence of the friction coefficient μ at the interface between the sheets is assessed using a plane-strain compression model of two sheets. It is found that roughness does not depend greatly on μ when μ>0.1, being close to that predicted by the monolithic model. However, the roughness does increases significantly as μ falls below 0.1. Secondly, a plane-strain rolling model is used to clarify the effects of friction between the roll and the strip. It is found that neither the friction at the roll surface nor the roll diameter has a strong effect on the roughening of the internal matt surface. It is concluded that the model using plane-strain compression of monolithic material is appropriate to predict the roughness on the matt surface of pack-rolled materials.     

A testing method of cold forging performance of steel wires

The flow stress of wire material, the friction coefficient and the sticking capability of the lubrication coating on the wire surface are evaluated with a single test. In this test, the specimen cut off from the end of a wire coil is set on a V-groove die and then is compressed into the V-groove by a punch. Friction coefficient is determined by the ratio of the punch load and the die load. The flow stress curve is calculated using the punch load-punch stroke curve. The proposed method is suitable for the use in the wire manufacturing shops for cold forging.     

Friction Factor Evaluation Using Experimental and Finite Element Methods for Al-4%Cu Preforms

In this study, ring compression tests and finite element (FE) simulations have been utilized to evaluate the friction factor, m, under different lubricating conditions for powder metallurgical (P/M) Al-4%Cu preforms. A series of ring compression tests were carried out to obtain friction factor (m) for a number of lubricating conditions, including zinc stearate, graphite, molybdenum disulfide powder, and unlubricated condition. FE simulations were used to analyze materials deformation, densification, and geometric changes, and to derive the friction calibration curves. The friction factor has been determined for various initial relative densities and different lubricating conditions, and a proper lubricant for cold forging of P/M Al-4%Cu preforms is found. Studies show that the use of lubricants has reduced the friction. However, increase in the number of pores in the preforms leads to excessive friction. The FE simulation results demonstrate a shift in the neutral plane distance from the axis of ring specimen, which occurred due to variations in the frictional conditions and initial relative densities. The load requirement for deformation, effective stress, and effective strain induced, and bulging phenomena obtained by FE simulations have a good agreement with the experimental data.     

基于有限元分析的摩擦系数测定

在塑性体积成形有限元模拟中,摩擦模型和摩擦系数的确定是一个关键问题.本文采用圆环镦粗法研究体积成形中摩擦系数的确定.利用有限元分析确定一组摩擦系数标定曲线,用圆环镦粗试验确定圆环内径变化率百分比与圆环高度压缩百分比关系曲线.通过曲线对比得到模具材料和成形材料之间的摩擦系数.针对6A02CZ材料在钢模具中体积成形时6A02CZ材料与钢之间的摩擦系数进行确定,6A02CS材料在无润滑剂和润滑油做润滑剂两种情况下的摩擦系数分别为0.325和0.3.研究表明,采用有限元方法和圆环镦粗试验相结合的方法更适合塑性体积成形中的有限元分析.     

T-shape upsetting–extruding test for evaluating friction conditions during rib–web part forming

Since the deformation features of existing friction testing methods are not as similar as those of the closed die forging process of rib–web parts, the friction conditions evaluated by these methods are not consistent with the actual situation. In this paper, a new friction testing method, the T-shape upsetting–extruding process employing a rectangular blank, is proposed for evaluating the friction conditions during rib–web part forming process. It was found that rib height and the web width are both sensitive to friction conditions. Therefore, the ratio of rib height to web width was chosen as a criterion for friction evaluation. With the utilization of a commercial FEM program, DEFORM, the effects of the geometrical parameters and testing variables on the friction sensitivity were investigated, and the friction conditions between different interfaces were analyzed. In order to verify the proposed method, friction factors of a water-based graphite lubricant measured by three different methods (the proposed one, the ring compression test and the double-cup extrusion test) at 723 K were used to simulate the isothermal deformation process of a cross-rib part. The experimental results suggest that the proposed method is more accurate than the other two methods for rib–web part forming process.     

Correlation between Poisson ratio and Mohr–Coulomb coefficient in metallic glasses

Yielding in metallic glasses is often described in terms of the Mohr–Coulomb criterion, τ + μσ ≥ τ₀. It suggests that a material yields when a combination of shear (τ) and normal stresses (σ), linked through the friction coefficient (μ), reaches the critical shear stress (τ₀). In this paper, an increase of the friction coefficient for increasing Poisson ratio is foreseen, if the elastic limit observed under uniaxial (tensile or compressive) and shear stresses remain constant. Experimental values of Young and shear moduli and Poisson ratio of metallic glasses have been collected from the literature. Compressive and tensile yield stresses have been also collected and elastic limits have been calculated. The elastic limit observed under compressive stress decreases with increasing Poisson ratio and it appears similar for metallic glasses based on the same metals. The values of the friction coefficient for metallic glasses have been obtained from the fracture angle observed under uniaxial stress (compressive or tensile) and from the ratio between compressive and tensile strength. Experimental data of the friction coefficient appear rather scattered and a clear trend as a function of Poisson ratio cannot be outlined. The increase of toughness of metallic glasses with increasing Poisson ratio has been ascribed to the corresponding reduction of the compressive elastic limit.     

Evaluation of environmentally friendly lubricants for cold forging

The authors proposed double-layer-type environmentally friendly lubricants, which were composed of an undercoat, superior in adhering to a material, and an overcoat, superior in reducing the friction between the material and the die. The performance of these lubricants for cold forging was evaluated by the ring compression test, the combined forward rod-backward can extrusion-type friction test and the combined forward conical can-backward straight can extrusion-type friction test. The double-layer-type lubricants showed comparable friction characteristics and anti-pick-up properties to a conversion coating lubricant, when the film thickness and surface treatment before coating were improved. In a practical application by cold multistage forging, the double-layer-type lubricants showed a similar performance to a conversion coating lubricant.     

Lubrication and Friction of Magnesium Alloys in Warm Forging

In order to realize precision forging of magnesium alloys, the frictional behavior of ZK60 (Mg-6%Zn-0.5%Zr) is studied. At room temperature, the friction test of ZK60 sliding over the surfaces of cemented carbide tool with and without TiC+TiCN+TiN and DLC (diamond like carbon) coating is carried out. It is found that the DLC coated film is effective to reduce the coefficient of friction and the oxide film of magnesium increases friction. For warm forging, ring compression tests are carried out at temperatures from 200 °C to 300 °C. Several liquid lubricants are applied onto the tool surfaces by controlling the film thickness and the effect of lubrication on friction is examined. Some methods for heating and lubrication for warm forging of magnesium alloys are suggested.     

Application of numerical simulation for wear analysis of warm forging die

Warm forging process has better forming precision than hot forging process and has better formability than cold forging. But warm forging die sustains higher temperature and working pressure, the die wear is faster than those of hot forging and cold forging. The purpose of this research is to combine the experimental techniques, wear model and numerical simulation method to predict the wear of warm forging die. The non-isothermal ring compression test was adopted to estimate the friction coefficient in different temperatures and the on-line temperature recording system was setup to correct the heat transfer coefficient of the interface. The wear coefficients in different temperatures were acquired from high temperature wear experiment. Finally, the Archard wear theory and DEFORM, a FEM code, were used to analyze the warm forging of automotive transmission outer-race and predict the die wear condition.     

Improvement of the liquid segregation phenomena of semisolid aluminum alloys by the multistage strain rate control in the compression test

The deformation behavior of semisolid aluminum alloys in the range of solid fraction (f _s ) between 55 and 90% was investigated through compression tests for the variation of strain rate. In order to obtain the optimal conditions of the net shape forging process, the rheological behavior of aluminum alloy in the semisolid state has been examined by using parallel plate compression. The strain rate is the value of the strain rate corresponding to the stress to avoid the liquid segregation phenomena during compression tests. The material constants of semisolid material in the stress-strain curve are proposed to perform the numerical analysis for a die design of semisolid forging. The starting materials used in this experiment are A357, 86S (similar to A319), and A390 alloys, which are fabricated by the electromagnetic stirring process. The intelligent compression test with a controlled strain rate was performed. The liquid segregation in the overall cross-sectional areas is controlled as the multistage variation of the pressing velocity and variation of the solid fraction during the compression process. The characteristics of materials flow between solid and liquid phases, considering the liquid segregation, are discussed for various solid fractions and pressing velocities.     

铝合金和不锈钢摩擦焊接参数的预测和优化(英文)

搅拌摩擦焊是一种固态连接工艺,具有低的热量输入、高的生产效率、容易操作和环境友好等优点而被广泛使用。搅拌摩擦焊能够焊接不同类型的黑色金属和有色金属,而传统的熔焊则不能。焊接工艺参数如摩擦压力、顶锻压力、摩擦时间和施压时间对接头的强度起主要作用。本研究旨在建立一个经验方程去预测搅拌摩擦焊AA6082和AISI304不锈钢接头的拉伸强度。采用响应面法去优化搅拌摩擦焊工艺参数,从而得到接头的最大拉伸强度。     

Tribology of thermal sprayed WC–Co coatings

This paper looks at the tribology of thermal sprayed WC–Co based coatings and covers the high energy air–sand erosion resistance and slurry jet impingement erosion performance, dry and wet sliding tribology of thermal spray WC–Co based coatings as well as the abrasion and abrasion–corrosion of these coatings. The tribological and tribo-corrosion performance of the coatings will be related to their mechanical and corrosion properties as well as deposition parameters, microstructure and actual composition. For example, the anisotropic microstructure of thermally sprayed WC–Co–Cr coatings, in particular the low fracture toughness in a direction parallel to the substrate, has been observed to affect the nature of crack formation under 200 μJ air–solid particle erosion conditions. Voids and occasionally other microstructural features (i.e., cobalt lakes, splat boundaries, interfacial inclusions) in the coating act as crack initiation sites. The erosion rate was dominated by cracks within 5 μm of the surface and was relatively insensitive to total length of cracks, showing a near-surface damage front controls the erosion rate and this region is coincident with the region of maximum shear stress induced by erodent impacts. Optimisation of the deposition parameters of HVOF 86WC–10Co–4Cr coatings show an improvement in erosion resistance of more than 50% over the conventional D-gun applied coating of identical nominal composition. The variation in the slurry erosion performance of the thermally sprayed coatings is also linked to directional fracture toughness and crack propagation paths which are influenced by the presence of pores, inhomogeneous carbide distributions and substrate grit blast remnants. The influence of slurry jet angle is more pronounced under 0.4 μJ energy conditions where maximum erosion occurred at 90° and the minimum at 30° in contrast to 7 μJ slurry erosion rates which were independent of jet angle. This reflects the lower levels of fluctuating stresses imparted to the coating during low energy slurry impacts leading to the impact angle having a greater effect on sub critical crack growth rate than for higher energy conditions.The abrasion resistance of these coatings was found comparable to sintered cermets of the same composition. The synergistic effects between micro and macro abrasion and corrosion for detonation gun (D-gun) sprayed WC–10Co–4Cr coatings are shown to be significant and depend on the environment. The size effect of the abradant relative to the microstructure and splat size is important as well as the propensity for the various phases to passivate to control corrosion levels. Comparisons between exposed and freshly polished coating surfaces in strong NaOH solutions (pH 11) show that significantly lower wear rates were seen for the exposed surface due to a negative wear–corrosion synergy due to selective phase removal and the effects of localised passivation.Dry and wet sliding wear resistance of these coatings is shown to be high (wear rates of 10⁻¹⁶–10⁻¹⁸ m³/Nm) with modest coefficient of friction levels between 0.2 and 0.5. The presence of oxides on the binder phases appears to influence the friction and wear levels. Wear appears to be by carbide ejection and/or by tribo-chemical processes.     

Effect of coating characteristics on friction and formability of Zn-Fe alloy electroplated sheet steel

The effects of coating composition and coating weight on friction characteristics and formability under different deformation modes, as well as their mechanisms, were studied on zinc-iron electroplated drawing-quality special-killed (DQSK) sheet steels. Friction tests, simulative formability tests, and coating characterization were performed. The experiments revealed that the iron concentration of the coating has a tremendous effect on friction in the deep-drawing operation, but only a slight effect in the stretching operation. Uniaxial tension tests indicated that both coating weight and coating composition affectn-value andr-value. The dependence of friction characteristics,n-value, andr-value on iron concentration is related to the variation of coating hardness caused by the variation of iron concentration in the coating.     

FE simulation of the development of flaws during injection forging

Several types of flaws develop during the injection forging of components; among these a prominent form results from the instability of the free length of the billet. The material in the die cavity buckles or slides laterally along the anvil; consequently, die filling is effected by asymmetrical deformation of the billet. This FE simulation considers the influence of several parameters which influence the developments of flaws during injection forging. By considering friction conditions at the anvil, the aspect ratio of the primary deformation zone, the exit geometry and the inhomogeneity of the material, the types of flaws which are initiated and the subsequent die filling are simulated using ABAQUS code. Marginal changes in the friction conditions influence the stability of the billet; billets which were unstable when μ = 0.01 were, generally, sufficiently stable when μ = 0.03 to effect flawless die filling. Simulation confirms the experimentally proven, limiting aspect ratio of the primary deformation zone to be between T = 1.6 and 1.8. The deformation of the billet graduates from non-symmetrical deformation at aspect ratios greater than 1.7 to bending at ratios greater than 2.0. larger exit radii improve the flow characteristics and had been shown to reduce the energy requirements; however, the simulation shows that instability would occur at a lower aspect ratio when a large exit radius was incorporated in the injection chamber. Lack of inhomogeneity in the material will also result in instability and asymmetrical die filling at low aspect ratios.     

The curling characteristics of static inside-out inversion of metal tubes

The curling behavior of quasi-static inside-out inversion of tubes was analyzed using an energy method technique based on the critical condition for more precise design. (1) The first part is to derive the critical bending radius as a curling criterion to distinguish between curling and flaring on a conical die, in which the strain hardening exponent n and half-apex angle of die α have very marked effects on theoretically; however, the friction coefficient μ-dependence is not as great as the n. As a result, a much more satisfactory correlation between prediction and experiment is obtained and Kitazawa's study is a special case of this work. (2) The second part is to theoretically find the range of semicircular curling ( ) and the compressing load P of inversion in tube inversion with a quarter circle die. It appears that the material properties of the aluminum tubes used have little effect on the curling behavior of inversion. It is noted that factors μ and n obviously affect the position of semicircular curling of inversion but not its range size. On the other hand, n in contrast to μ affects significantly the compressing load. Good agreement between the theoretical prediction and experimental results is obtained for the compressing load, except in the case of the sharp fillet die radius for the strictly bending effect to be ignored in the prediction.     

压铸模乳液涂料的润滑机理

压铸模乳液涂料的润滑机理在于它的润滑剂大部分受热分解而释出气体。在整个压铸过程中，在模具型腔和金属间形成持久而又适度的气隙，润滑层由释放出的气体和残留的润滑剂所构成，所以润滑剂的发气特性和高温摩擦因数是控制乳液涂料的关键性能。在压铸过程开始时同温摩擦因数起主导作用。接着气体润滑的作用控制着整个过程。对于大型铝合金和镁合金铸件来说，必须在配方中加入高耐热性的成分，在整个压铸过程中才能够持久而且稳定地释放出气体。     

Preparation of Al2O3/DLC/Au/MoS2 chameleon coatings for space and ambient environments

We have investigated the tribological properties of nanocomposite “chameleon” coatings, which adapt their low friction behavior with the surrounding environmental humidity and temperature. The material system of interest included alumina (Al₂O₃) in an Au matrix with diamond-like carbon (DLC) and MoS₂ nanoparticle inclusions. The coating design included formation of nanocrystalline hard oxide particles for wear resistance, embedding them into an amorphous matrix for toughness enhancement, and inclusion of nanocrystalline and/or amorphous solid lubricants for friction adaptation to different environments.Chemical analysis was used to ascertain a correlation between chemical bonding of species and frictional properties. Friction measurements were studied in cycling between humid air and dry nitrogen conditions at room temperature and during heating in air to 500 °C. It was observed that both graphitic carbon and MoS₂ worked together to give low friction in variable humidity environment, while Au was valuable for a low friction at elevated temperatures. Friction coefficients were found to be 0.02-0.03 in dry nitrogen, 0.1-0.15 in humid air, and 0.1 in air at 500 °C. Thus the tribological property results have shown that the system provides “chameleon” type adaptation behavior in different environments relevant for aerospace systems.     

T形锻件挤压模设计

T形锻件因形状所致,其挤压成形无法采用传统的整体凹模结构。现给出了T形锻件无飞边挤压模具结构。该模具采用斜面自锁纵向对合凹模结构,侧面设有凹模顶出装置,锻件在两半凹模和凸模形成的封闭模腔中挤压成形,生产的锻件无飞边。     

铝合金板温成形过程中凸凹模圆角处摩擦的测量

在温成形过程中铝合金板与模具表面的接触和摩擦行为十分复杂,不同的接触区域摩擦状况不尽相同。本文分析了板料成形中摩擦测量的国内外发展状况,采用自行设计的新型摩擦测量装置完成了铝合金板温成形过程中凸凹模圆角处摩擦系数的测量。该测量装置的特点是可以模拟板料的真实变形过程,因而可以获得更为准确的测量结果。     

WC粉末电镀Cu预处理后热喷涂层的磨损特性

采用电镀技术,在WC粉末表面镀一定量Cu,用常规氧-乙炔焰喷涂制备WC涂层.在干摩擦条件下,对WC涂层/GCr15、45钢/GCr15摩擦副摩擦性能进行测试,SEM分析摩擦面形貌,EDS分析摩擦面元素分布.结果表明,WC涂层/GCr15摩擦副较45钢/GCr15摩擦副摩擦系数高,耐磨性远优于45钢/GCr15摩擦副;WC涂层磨损机理为脆性剥落和磨粒磨损;WC涂层/GCr15摩擦副摩擦面局部粘接,部分GCr15基材粘于WC涂层.