A MODIFIED ALGORITHM FOR DEALING WITH HOLDER FRICTION BOUNDARY CONDITION IN SIMULATION OF SHEET METAL FORMING

This paper investigates the equivalent model for the friction boundary condition between blank and holder during sheet metal forming simulation, and proposes an equivalent algorithm, the thickness threshold value algorithm, for the first time based on thickness weighting. This algorithm assumes that the holder clearance is uniform during sheet metal forming; and the main reason for the unevenness of friction force distribution under the holder is that the uneven deformation of the blank leads to its uneven thickness distribution, which makes the local pressure on the blank distributed unevenly. The algorithm proposed in this paper can effectively simulate the influence of the unevenness on the forming process. Validity of this algorithm is verified by a comparison between the simulation results and the experimental ones for the drawing process of a car spring base.

A first order friction model for lubricated sheet metal forming

This paper presents the derivation of a first order friction model for lubricated sheet metal forming.Assuming purely plastic real contacts, Newton's law of viscosity, and a square root behavior of the hydrodynamic coefficient of friction with respect to the hydrodynamic Hersey parameter an analytic model is found. The model predicts the coefficient of friction as a function of the relative pressure, the relative Hersey parameter and the real contact coefficient of friction. Questions about local and global friction are raised in the validation of the model against flat tool sheet experiments. For some flat tool sheet experiments reasonable agreements are obtained assuming a rigid punch pressure distribution. The restricted number of user inputs makes the model useful in early tool design simulations.

MESHFREE METHOD BASED ON POINT COLLOCATION FOR METAL FORMING SIMULATION

A meshfree method based on reproducing kernel approximation and point collocation is presented for analysis of metal ring compression. The point collocation method is a true meshfree method without the employment of a background mesh. It is shown that, in a point collocation approach, the remesh problem because of the mesh distortion in FEM （finite element method） and the low efficiency in Galerkin-based meshfree method are avoided. The corrected kernel functions are introduced to the stabilization of free-surface boundary conditions. The solution of symmetric ring compression problem is compared with a conventional finite element solution, and reasonable results have been obtained.

Interactive mechanism and friction modelling of transient tribological phenomena in metal forming processes: A review

The accurate representation of tribological boundary conditions at the tool–workpiece interface is crucial for analysis and optimization of formability,material flow,and surface quality of components during metal forming processes.It has been found that these tribological conditions vary spatially and historically with process parameters and contact conditions.These time-dependent tribological behaviours are also known as transient tribological phenomena,which are widely observed during forming processes and many other manufacturing application scenarios.However,constant friction values are usually assigned to represent complex and dynamic interfacial conditions,which would introduce deviations in the relevant predictions.In this paper,transient tribological phenomena and the contemporary understanding of the interaction between friction and wear are reviewed,and it has been found that these phenomena are induced by the transitions of friction mechanisms and highly dependent on complex loading conditions at the interface.Friction modelling techniques for transient behaviours for metal forming applications are also reviewed.To accurately describe the evolutionary friction values and corresponding wear during forming,the advanced interactive friction modelling has been established for different application scenarios,including lubricated condition,dry sliding condition(metal-on-metal contact),and coated system.

Test of Friction Parameters in Bulk Metal Forming Based on Forward Extrusion Processes

Existing methods for examining the friction parameters in metal forming all have advantages and disadvantages.Based on the theory of plasticity,the current study established quantitative correlations among friction coefficient/factor,yield stress of the workpiece material,load and die geometry in the forward extrusion with a conical die,and then designed a procedure for testing the friction parameters in forming processes using the correlations.A series of extrusion experiments along with the numerical simulations,using AA 7050 specimens under various lubricating conditions,were carried out.The results proved that the method can obtain the friction coefficient/factor with an acceptable precision.Theoretically,since the effects of material properties,forming velocity,temperature and surficial condition,etc.,on the deformation can be directly considered in the operation,this method is applicable to a wide range of material types and forming conditions.To avoid the occurrence of "barreling phenomenon" under large load which may lead to failure of the operation,it is recommended that half angle of the conical die ranges from 5 to 10 degrees.

Galling phenomena in metal forming

Galling phenomena in metal forming not only affect the quality of the engineered surfaces but also the success or failure of the manufacturing operation itself.This paper reviews the different galling conditions in sheet and bulk metal forming processes along with their evolution and the effects of temperature on galling.A group of anti-galling methods employed to prevent galling defects are also presented in detail.The techniques for quantitatively measuring galling are introduced,and the related prediction models,including friction,wear,and galling growth models,are presented to better understand the underlying phenomena.Galling phenomena in other processes similar to those occurring in metal forming are also examined to suggest different ways of further studying galling in metal forming.Finally,future research directions for the study of galling in metal forming are suggested.

Using Neural Network to Predict the Interfacial Friction and Flow Stress of Materials via Ring Compression

In this study a neural network approach is proposed to realize an automatic numerical prediction of the interfacial friction factor and the flow stress of materials. Decrease in the inner diameter and reduction in the height of the ring are taken as input

Determination of the Friction Coefficient in the Flat Strip Drawing Test

Current work is focused on the influence of friction in deep drawing process. Friction measurements were also conducted using a modified tribotester based on strip sliding between tools. Four different tool surfaces were tested under similar contact conditions regarding contact area, normal pressure, sliding speed, lubricant and surface characteristics to calculate the friction coefficient between the tool surface and a high strength low alloy steel sheet HSLA 380. The results showed that friction coefficient varies over a wide range with different lubricating conditions and different sliding velocities. For some sliding velocities, the coefficient of friction is stable and low, while for others it is unstable and higher. Results of the experiments reveal that this novel tribotester is a very useful tool to evaluate and compare the friction between steel sheet and tool surfaces in alloyed steel for cold working applications. The outcomes have only small dispersion within the different test series, which indicates a stable process with good repeatability. The test method enables comparison of different surface finishes and treatments, lubricants and coatings in terms of friction and galling under conditions similar to those found in sheet metal forming processes. The four different types of surfaces considered for this study were grinded, polished, nitrided and quenched/tempered. The main difference among the tested tools in this work was the surface roughness, which was found to have a strong influence on friction.

Pressurized environments directly influence friction and wear of dry steel contacts—Investigations in a novel high fluid pressure tribometer

To reduce the usage of classical lubricants in deep drawing,a new tribological system based on volatile lubricants was developed.Therefore,a volatile medium is injected under high pressure into the interstice between drawing tool and sheet metal.Depending on temperature and pressure,the temporary lubricant may exist in its gaseous or liquid phase.In this study,a novel high fluid pressure tribometer was designed to investigate the friction and wear of dry steel contacts under comparable conditions like in dry deep drawing.Therefore,a new ball-on-disc tribometer was designed and integrated into a high-pressure vessel.To specifically investigate the effects of different environments(technical air,liquid and gaseous carbon dioxide,nitrogen,argon)at atmospheric and high pressure(0.1 MPa,6 MPa)on tribology,the specimens and all components were operating unlubricated.During the experiments,the friction was measured continuously.Results show that the highest friction occurs in air and the lowest in carbon dioxide environment.Subsequent to the experiments,the wear of the specimens was assessed along with changes in surface chemistry related to tribochemical reactions.Therefore,the tribology of the dry sliding contacts is correlated to changes of the surface chemistry.Also differences as well as similarities regarding the different fluid environments are shown.As the results show,the differences between the media used are most pronounced at elevated pressure.Concluding,this work gives clear indications on the suitability of volatile lubricants in dry friction or rather gas lubrication,especially for dry deep drawing.

U形件拉深成形回弹影响因素的计算机仿真(英文)

回弹是板料冲压成形中的主要缺陷之一 ,控制回弹涉及到回弹量的准确预算。利用DANAFORM软件 ,对U形件拉深成形回弹影响因素进行了计算机仿真 ,得到了压边力、凸凹模间隙、摩擦系数、材料特性等参数对回弹影响的数值结果 ,并对模拟结果进行了分析。

板料成形数值模拟研究

介绍了板料成形动态显式有限元模拟的基本理论 ,讨论了成形模拟中接触摩擦算法 ,起皱、破裂的判断及回弹模拟等关键技术。最后 ,通过介绍NUMISHEET’ 99三组标准考题及主要模拟研究结果 ,讨论了有限元模拟技术在板料成形中的应用。

板成形中摩擦系数测定的研究进展

本文阐述了摩擦系数在板成形中的作用及板成形中摩擦系数测定的国内外的发展状况 ,采用作者设计的新型摩擦系数测定装置进行了摩擦系数测定的实验研究 ,为进一步开展摩擦系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重要的参考依据。

板料成形接触摩擦过程的有限元模拟技术研究

本文在分析了板料成形过程接触摩擦问题特点的基础上 ,提出了接触搜索、界面滑动约束、法向接触力以及摩擦力计算等问题的有限元模拟算法 ,建立了板料成形过程接触摩擦问题的有限元模拟模型 ,最后通过数值算例与飞机蒙皮拉形模拟试验结果的比较 ,验证了此模型计算精度及数值稳定性良好。

板材冲压成形有限元数值模拟界面摩擦约束处理方法

基于弹塑性大变形板材冲压成形增量有限元法,采用线性弹簧单元提出一种空间三维板材冲压成形等效界面摩擦力的处理方法.这种方法的优点是可以反映界面摩擦力的同步、被动产生效果,同时它还可以保证约束后刚度矩阵的对称性.将提出的摩擦约束处理算法引入自主开发的板材成形模拟软件FASTAMP,计算了汽车油底壳的成形过程,计算结果的厚度分布与实际冲压件吻合比较好,验证了等效界面摩擦力约束处理方法的有效性.

改善摩擦条件以实现省力成形

摩擦是金属成形中一个关键影响因素,它产生于存在相对运动的金属和模具接触面上。恶劣的摩擦条件将阻碍金属的流动,使成形载荷显著增加,成形能耗增加。因此,采用合适的途径来改善金属成形中的摩擦条件,对于降低成形力,促进金属流动,增加工件的成形质量和精度、提高模具的使用寿命等方面都具有明显的实际应用价值。目前,有限元数值模拟技术已经成为预测金属流动、成形载荷和模具受力的重要手段,因此,金属成形计算中的摩擦作为重要的边界条件获得了广泛的重视及深入的研究。在对不同摩擦试验方法研究的基础上,可知摩擦因子及因数随着材料新表面生成率、接触压强的增加而增大。对改善摩擦条件的金属表面处理方法、包套挤压、振动辅助成形等考虑摩擦的省力成形途径进行简要介绍。

金属成形数值模拟中的接触单元法

给出了一种基于罚函数方法及库仑摩擦规律的点—面接触形式的接触单元法。根据等向滑动函数的概念 ,推导了粘着接触与滑动接触的接触单元刚度矩阵。由于这种方法在单元积分点引入接触约束 ,扩大了接触搜索面积 ,因而接触计算更加稳定。数值计算表明该方法能得到较好的结果。

用新型摩擦装置测定薄板成形的摩擦系数

采用一种新型的测量板成形中工具与工件之间摩擦系数的模拟装置，试验测定了不同厚度及成分的钢板在不同条件下的摩擦系数。被测钢板有０８Ａｌ，ＩＦ钢，ＩＦ钢镀锋板，测试中改变润滑条件，辊距、辊径和板厚，板表面取向。结果表明摩擦系数与润滑和冲头的几何形状密切相关。

薄板深冲成形过程中摩擦系数的测定

采用作者设计的一种新型测量薄板深冲成形过程中工具与工件之间摩擦系数的模拟装置 ,测定了相同材质的带材在不同条件下的摩擦系数。测试过程中改变润滑条件、辊距及带材的表面取向 ,测试结果表明摩擦系数与润滑和冲头的几何形状有很大关系。为计算机有限元模拟提供真实摩擦边界条件 。

板料成形过程模具圆角摩擦测试实验装置的研究

板料成形研究中的一个难点是如何采用实验方法定量测定板料成形过程中板料与模具之间的摩擦系数。通过对板料成形时的变形特点进行分析,提出一种新的板料成形过程模具圆角摩擦测试实验方法,并研制出相应的摩擦测试实验装置。该实验装置考虑了板料成形时周向收缩、厚向增厚的变形特点,能够在线测试出室温和加热状态下板料成形过程模具圆角处的摩擦系数。摩擦测试结果表明,所研制的摩擦测试装置具有灵敏度高、可重复性好、使用方便的特点,为深入研究板料成形过程中摩擦系数随工艺参数的变化规律提供了实验手段。

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。