Study of size effect in micro-extrusion process of pure copper

The size effect on material deformation behaviors are characterized by grain size, part feature size, forming material size and interfacial condition. These factors have a close relationship with material flow behavior, which in turn affects the geometry accuracy of micro-formed parts. In this study, a general-purpose tooling set for realization of micro-forward, backward, combined forward rod-backward can and double cup extrusions is developed and the micro-extrusions of pure copper with different grain sizes are conducted. The size effect phenomena are analyzed based on the deformation load, interfacial friction behavior and microstructure evolution. It is found that the interfacial friction is high in micro-extrusion processes and the grain size effect on deformation load is sensitive to the friction force at the tooling–workpiece interface. The microstructures of the extruded parts show the occurrence of inhomogenous deformation and a large number of slip bands passing through the grain boundaries to accomplish the strain continuity in the cases with coarse grains. In addition, the flow stress curves obtained from micro-compression are used to model the micro-extrusion processes using finite element (FE) simulation based on the conventional material model. It is found that the conventional material model is not applicable in simulation of the material deformation behavior and evaluation of the interfacial friction in micro-extrusion processes due to the size effect. This research therefore provides an in-depth understanding of size effect in micro-extrusion processes, which is critical to further formulate design rules to facilitate the development of micro-parts.     

The size effect on micro deformation behaviour in micro-scale plastic deformation

When the geometry of metal deformed part is scaled down to micro-scale, the understanding and prediction of micro deformation behaviour becomes difficult. This is because the conventional material deformation models are no longer valid in micro-scale due to the size effect, which affects the deformation behaviour in micro plastic deformation, and thus leveraging the traditional knowledge of plastic deformation from macro-scale to micro-scale is not meaningful. In this paper, the size effect on micro-scale plastic deformation and frictional phenomenon are investigated via micro-cylindrical compression test, micro-ring compression test and Finite Element (FE) simulation. The experimental results show the occurrence of various size-effect related deformation phenomena, including the decrease of flow stress and the increases of: (a) irrational local deformation, (b) the amount of springback, and (c) the interfacial friction stress with the decreasing specimen size. The research further verifies that the established surface layer models, with the identified surface grain, the internal grain properties and the measured friction coefficients, are able to predict micro deformation behaviour. The research thus provides an in-depth understanding of size effect on deformation and frictional behaviours in micro-scale plastic deformation.     

An ALE hydrodynamic lubrication finite element method with application to strip rolling

A method that incorporates the hydrodynamic lubrication analysis into the arbitrary Lagrangian Eulerian (ALE) finite element analysis is developed for steady-state strip rolling simulation. By employing the ALE formulation, only part of the workpiece, which is subjected to large plastic deformation within the roll-bite region, is modelled, so that the computational cost is substantially reduced. In the hydrodynamic lubrication formulation, the effect of surface roughness on the lubricant flow is taken into consideration by the use of an average flow model. The friction stress is expressed in terms of forming variables such as surface roughness, lubricant and workpiece properties, film thickness, forming speed and process geometry. Furthermore, the elastic deformation of rolls is also analysed by the boundary element method to avoid the finite element discretization inside the rolls. Two numerical examples, aluminium and steel strip rolling processes, are presented to demonstrate the merits of the proposed method.     

超声振动辅助塑性成形及形性预测研究进展

随着微机电系统等领域的快速发展,对零件成形精度与性能的要求日益增加。超声振动辅助塑性成形是一种典型的能场辅助塑性成形工艺,相比于传统塑性成形工艺,具有流动应力低、材料成形能力高、界面摩擦少、成形质量较好等优势,被广泛应用于难成形材料加工、微成形、复杂构件成形等塑性成形过程。然而,由于不同塑性成形工艺中金属的变形行为特性存在较大差异,对塑性成形质量与成形性能进行预测有利于实现成形过程的形性协同控制。介绍了超声振动辅助塑性成形在体积成形工艺（镦粗、挤压、拉拔等）与板料成形工艺（拉伸、拉深、渐进成形、冲压等）中的应用及发展概况,讨论了超声振动对材料塑性变形过程中宏观表现与微观演化的影响。在已有研究基础上,重点分析了超声振动辅助塑性成形过程中成形能力预测（流动应力、成形极限等方面）和成形性能预测（表面性能、力学性能、微观组织等方面）的研究进展,为金属零部件成形高质量形性调控提供理论参考,并展望了超声辅助塑性成形工艺的发展趋势。     

Modelling flow stress behaviour of aluminium alloys during hot rolling

Abstract

A mathematical model is proposed to predict the flow stress behaviour of aluminium alloys under hot rolling conditions. To do so, a dislocation model for evaluating flow stress during deformation is coupled with a finite element analysis to access metal behaviour under non-isothermal and variable strain rate conditions. Then, with the aid of the proposed model, a hot strip rolling process was simulated. In order to verify modelling results, flow stress behaviour of an aluminium alloy is studied employing hot compression tests in various temperatures and strain rates and the model was examined on this material. Non-isothermal hot rolling experiments were carried out and good agreement was found between predictions and experiments.     

Kaltgewalztes Flachzeug aus unlegierten weichen Stählen — Nach DIN 1623 Blatt 1

Cold-Rolled Flat Products of Plain Carbon Mild Steel According to DIN 1623 Sheet 1 . Cold-rolled flat products of plain carbon mild steel and among the most important finished products made by iron and steel works and rolling mills as well as being among the most important semi-products for industrial manufacture. Metallurgical and rolling developments during the course of the last twenty years have resulted in considerable improvements in the main cold forming properties of sheet and strip. The chief demand made of manufacturers of cold-rolled flat products is for sheet and strip which has been fully annealed and rerolled for good cold formability. Because of the various stresses on materials in the different forming processes, it is difficult to make a reliable prediction from test results about the behaviour of the material during actual forming operations. The new edition of sheet 1 of DIN 1623 prescribes the conventional test methods and their results as regards ensuring cold formability. The Standard does not include the additional means of using numerical values for vertical anisotropy r, work hardening exponent n and the maximum drawing ratio (ßo)_max.     

Inferences on plastic properties and coefficient of friction during simultaneous compression deformation of dissimilar sintered powder metallurgical preforms

Abstract

Plastic working of powder metallurgical (PM) material necessitates the development of fundamental data such as flow stress, densification behaviour, coefficient of friction, apparent strength coefficient, apparent strain hardening exponent, plastic Poisson's ratio, etc. In the present work compression and standard ring compression tests have been carried out to generate the fundamental data for simultaneous deformation of sintered steel and copper powder metallurgical preforms. The results reveal that the behaviour of individual materials during simultaneous deformation is strongly influenced by local micromechanical interactions at the metal - metal interface. In addition to this, the test conditions (iso-stress and iso-strain) strongly influence the severity of interaction. The interfacial friction coefficients are less than that of the same material when tested between hard tools. The optimal process parameters with higher interfacial friction, which can enhance the solid state joining of dissimilar materials, have been identified. The flow stress of the composite (steel - copper combination) during simultaneous deformation can be estimated if the flow stress of the individual materials comprising the combination/composite are known. With these studies, it should be possible to extend the inferences to the major deformation processes.     

特种能场辅助微塑性成形技术研究及应用

特种能场辅助微塑性成形技术是利用声、光、电、磁等特殊能量源对微型零件变形过程进行调控的先进制造技术。特种能场已被证明在宏观尺度下对于降低零件加工难度、提高尺寸精度、改善材料微观组织、提升构件力学性能、提高表面质量等存在促进作用。然而,在微塑性成形过程中,材料的变形特性在尺寸效应的影响下与宏观情况存在一定差异。梳理了特种能场辅助微塑性成形技术的研究进展,总结了微型零件在特种能场辅助下的成形特点。其中,着重综述了超声场辅助微成形中体积效应和表面效应的宏观表现及微观机理,展示了多种微成形工艺中超声场对微型零件成形质量的提升效果。同时,重点概述了电场辅助微成形时材料力学性能及微观组织演变规律,剖析了电致塑性效应产生的本质原因。此外,列举了激光、电磁、高压流体等其他特种场辅助微成形的原理及作用效果。最后,对特种能场辅助微成形的发展趋势进行了展望。     

Material behavior modelling in micro/meso-scale forming process with considering size/scale effects

Size effects make most know-how of traditional macro forming inappropriate for the micro forming process. Material behavior greatly varies in micro forming process with the decreasing of the scale. The purposes of this paper are to analyze the influence of material size effects and establish a martial model in micro/meso-scale. By combining surface model with theories of single crystal and polycrystal, a mixed material model, which contains a size dependent term and a size independent term, is proposed in this paper. It finds that the flow stress of material in micro/meso-scale is between that of single crystal model (lower bound) and polycrystal model (upper bound). Based on a mixed material model, the influence of size effects is discussed in both micro bulk forming process and micro sheet forming process. At the end, the validity of the mixed model is approved by comparing the simulations with the experimental results.     

Deformation behaviour in superplastic 5083 Al alloy during biaxial gas pressure forming with lubrication

Abstract

Effect of lubrication on deformation behaviour of a superplastic material has been given little attention, although it is important for industrial application. In this paper, a superplastic 5083 Al alloy under biaxial deformation was investigated by deforming the sheet into a cylindrical die cavity with and without lubrication. Several interrupted tests were performed to bulge the sheets to various depths for two different strain rates, the formed parts were then utilised to evaluate the effect of lubrication on metal flow, thickness distribution and cavitation. It was found that reducing the interfacial friction by use of a lubricant improved the metal flow after the deformed sheet had made contact with the bottom surface of die. Changes of the metal flow during forming not only developed a better thickness distribution of the formed part, but also reduced cavitation levels.     

高温合金薄壁W截面密封环滚压成形壁厚变化研究

目的研究滚压成形过程中环件壁厚的变化规律。方法基于ABAQUS/Explicit平台建立了GH4169薄壁W截面密封环多道次滚压成形三维弹塑性有限元模型,分析了滚压成形过程中环件壁厚沿周向及轴向的变化情况以及工艺参数对环件壁厚变化的影响规律。在此基础上,进行与模拟条件一致的试验研究,并与模拟结果进行了对比分析。结果滚压成形过程,环件壁厚沿周向分布均匀而沿轴向分布不均匀;随着变形量增大,减薄带宽度增大;进给辊进给速度增大,环件壁厚不均匀性加剧;随着驱动辊转速、进给辊与环件间摩擦因数的增大,环件壁厚减薄呈减小趋势。结论环件滚压成形过程中,应合理分配各道次变形量,适宜的工艺参数为:驱动辊转速为2 rad/s,进给速度为0.2 mm/s,摩擦因数为0.1。     

Parameter identification of hardening laws for bulk metal forming using experimental and numerical approach

The simulative prediction of material behaviour in forming processes necessitates a precise determination of the material parameters. The present work focusses on the modelling of the isostatic part of the flow stress using a flow curve with an analytical suppression of the influence of friction and an adequate analytical law. The experimental data are obtained from isothermal upsetting tests with various upsetting ratios. The different ratios are based on a variation of the height of the sample, remaining the diameter constant. For the proposed flow stress law five parameters are identified. In order to decrease the number of function evaluations, a new reduction model method based on both analytical and sequential quadratic programming (SQP) algorithms is developed and applied to identify flow stress law parameters. A comparison with traditional SQP algorithm is also done. A 3D finite element model is built in order to simulate a side pressing test and an experimental validation is done. As numerical results fit very well experimental data, the proposed model achieves a precise prediction of the flow behaviour. The identification of the other parts of the model (i.e. dependencies on strain-rate and temperature) are conducted in further works.     

Modelling of microstructure evolution during hot rolling of AA5083 using an internal state variable approach integrated into an FE model

Hot rolling, a critical process in the manufacturing of aluminum sheet products, can significantly impact the final properties of the cold rolled sheet. In this research, a mathematical model was developed to predict the through-thickness thermal and deformation history of a sheet undergoing single stand hot rolling using the commercial finite element (FE) package, ABAQUS^™. A physically based internal state variable microstructure model has been incorporated into the FE simulation for an AA5083 aluminum alloy to predict the evolution of the material stored energy and the subsequent recrystallization after deformation is complete. The microstructure predictions were validated against experimental measurements conducted using the Corus pilot scale rolling facility in IJmuiden, the Netherlands for an AA5083 aluminum alloy. The model was able to predict the fraction recrystallized as well as the recrystallized grain size reasonably well under a range of industrially relevant hot deformation conditions. A sensitivity analysis was carried out to determine the influence of changing the material constants in the microstructure model and deformation conditions on the predicted recrystallization behaviour. The analysis showed that the entry temperature was the most sensitive process parameter causing significant changes in the predicted driving force for recrystallization, nucleation density, fraction recrystallized, and recrystallized grain size.     

Numerical analysis of flow behaviour,grain size prediction and experimental verification of hot rolled ultralow carbon niobium microalloyed steel

Niobium bearing ultralow carbon micro alloyed dual phase steel grade steel with chemical composition of 0.045 wt.% carbon, 0.04 wt.% niobium, 0.59 wt.% chromium, 0.9 wt.% manganese, 0.28 wt.% silicon is investigated. The design of thermomechanical treatment needs knowledge about the flow behaviour at high deformation temperatures, to be used for this steel in industrial plants. Therefore, this work aims to predict, for the investigated steel, the flow behaviour and mean flow stress. A phenomenological constitutive model is established to derive the flow stress using the hyperbolic sinusoidal Arrhenius mathematical model. The relation between stress and strain with Zener-Hollomon parameter is studied. Mean flow stress has been figured out by measurements taken from compact slab production plant log data using a constitutive model. The constitutive model is further verified by multiple hits of flat compression setup mode at BährTTS820 physical simulator. The tests were performed under specific thermochemical schedule simulating compact slab production plant. The results show good agreement between the calculated flow stress, Non-recrystallization temperature and experimentally measured values obtained from the physical simulation. Ferrite grain size is modelled and predicted then validated by experimental rolled specimens in a practical hot strip mill.     

PC轧机轧制过程耦合数值模拟研究

开发了一种分析三维板带轧制过程轧件与轧辊的耦合变形的计算机模拟系统。它耦合了三维刚塑性有限元法,弹性有限元法和计算辊系变形的影响函数方法。采用该系统成功地对PC轧机(Paircrossedrollingmill)轧制过程进行了分析,得到了轧制压力横向分布,前后张力横向分布,金属横向流动,以及轧后板带的横向板厚分布,包括板凸度和边部减薄大量信息等。     

纳米材料微阵列超塑微成形机理与尺度效应

微成形技术是未来批量制造高精密微小零件的关键技术,但是,微小尺度下材料的塑性变形行为不仅表现出明显的尺度效应,而且零件尺度已经接近常规材料的晶粒尺寸,每个晶粒的形状、取向、变形特征对整体变形产生复杂的影响,难以保证微成形的工艺稳定性。本项目采用纳米材料进行微成形,制造微阵列,零件内部包含大量的晶粒,可以排除晶粒复杂性的影响,而且纳米材料具有超塑性,在超塑状态下,变形抗力和摩擦力都明显降低,从而显著降低微成形工艺对模具性能的苛刻要求,提高工艺稳定性和成形精度。目前,纳米材料超塑性微成形技术方面的研究极少,变形时纳米材料的力学行为、变形机理、尺度效应、位错演化、力学模型等关键问题还有待研究。采用电沉积技术制备晶粒尺寸可控的纳米材料,将工艺实验研究、性能测试、组织分析、力学性能表征、数值模拟相结合,深入探究了纳米材料微阵列超塑性微成形机理和成形规律,以促进该技术的广泛应用。     

Strain behaviour of aluminum 99.5 in micro-forming conditions of the coining process

The coining process can be identified as a shallow die forging. By using this process, it is possible to produce very fine surface geometries. This leads to dimension scaling into micro dimensions and causes the specific material behavior. In the micro area, continuum laws are not strictly followed anymore. The leading role in determining the deformation and strain level is taken by the fine surface geometry dimension and the size of the crystalline grain of the workpiece material. This is known as the size effect and defines all micro-forming processes. In order to provide monitoring of the filling of the smallest die dimensions, additional supporting procedures should be used. In this case, radiography testing procedures for observing the specific points of the workpiece geometry are applied, characterized with very small dimensions (less than 0.5 mm). Testing aluminum samples are formed using two techniques – open and closed die coining, with identical surface geometry. Their crystalline structures are in two-grain sizes of 34 μm and 80 μm. Scanning results show different material behaviour and different surface deformation for the same level of forming force in all four testing cases.     

Numerical investigation of the influence of frictional conditions in thread rolling operations with flat dies

Numerical simulation technology has become an important part of the process design stage in bulk metal forming operations. With increasing computing performance, three dimensional simulations within the product design process are thus becoming increasingly feasible. However, the modeling of friction within numerical simulations is still posing a challenge, especially in very friction sensitive processes, such as rolling of axisymmetric parts. Within the presented work, the process of rolling with flat dies with consideration of friction (according to Amontons-Coulomb) is simulated and analyzed. With the developed model, tribological loads as well as the influence of friction is investigated. These numerical findings are contrasted with experimental results obtained with an industrial forming machine. It is shown that the numerically obtained results are highly sensitive regarding numerical and physical contact modeling parameters. This is due in part to the highly varying frictional conditions (relative sliding velocity, contact normal stress) within the contact zone. Additionally, it is shown that the contact zone exhibits properties that favor static friction rather than sliding friction. This observation is especially important for an adequate empirical characterization of friction for thread and profile rolling processes.     

Mechanical behaviour of nitrogen-alloyed austenitic stainless steel hardened by warm rolling

This investigation deals with the hardening effect, by warm rolling, of the alloy Uranus B66^®, a nitrogen-alloyed austenitic stainless steel produced by Creusot-Loire Industrie. The warm rolling process leads to substructural changes from the appearance of planar slips at low deformation, the micro-twins formation followed by sequences of their bending, breaking and disappearance at intermediate deformation, and finally to the formation of heavily deformed domains at the highest warm rolling reduction. The mechanical behaviour of the warm rolled Uranus B66, under quasi-static tensile and quasi-static and dynamic compression tests, has been analysed. Warm rolling increases the mechanical resistance to a saturation level and decreases the ductility when compared to that of the as-received material. The dynamic flow stress after warm rolling up to 85% increases to such a level that brittle fracture occurs after small plastic deformation. The origin of the strength saturation is related to the terminal microstructure derived from the warm rolling deformation.     

Influences of temperature and grain size on the material deformability in microforming process

This paper investigated the influences of temperature and grain size on the deformability of pure copper in micro compression process. Based on the dislocation theory, a constitutive model was proposed taking into account the influences of forming temperature, Hall-Petch relationship and surface layer model. Vacuum heat treatment was employed to obtain various grain sizes of cylindrical workpieces, and then laser heating method was applied to heat workpieces during microforming process. Finite element (FE) simulation was also performed, with simulated values agreed well with the experimental results in terms of metal flow stress. Both the FE simulated and experimental results indicate that forming temperature and grain size have a significant influence on the accuracy of the produced product shape and metal flow behaviour in microforming due to the inhomogeneity within the deformed material. The mechanical behaviour of the material is found to be more sensitive to forming temperature when the workpieces are constituted of fine grains.