基于数值仿真的冲压成形界面接触压力

为探究冲压成形过程中板料-凹模圆角区界面接触压力,采用有限元静力算法建立了U形件小圆角半径弯曲成形过程的数值仿真模型,完成了板料和模具界面接触压力数值模拟,并参数分析了钢板强度、相对圆角半径、压边力和摩擦系数对板料界面接触压力分布的影响.研究表明:与压边力和摩擦系数相比,钢板强度和相对圆角半径更明显地影响着板料界面接触压力,并随着材料强度增加和相对模具圆角半径减小界面接触压力明显增加;随着压边力和摩擦系数的增加,界面接触压力宽度也随之增大.     

Investigation of Surface Damage in Forming of High Strength and Galvanized Steel Sheets

Powdering/exfoliating of coatings and scratching are the main forms of surface damage in the forming of galvanized steels and high strength steels (HSS), which result in increased die maintenance cost and scrap rate.In this study, a special rectangular box was developed to investigate the behavior and characteristics of surface damage in sheet metal forming (SMF) processes.U-channel forming tests were conducted to study the effect of tool hardness on surface damage in the forming of high strength steels and galvanized steels (hot-dip galvanized and galvannealed steels).Experimental results indicate that sheet deformation mode influences the severity of surface damage in SMF and surface damage occurs easily at the regions where sheet specimen deforms under the action of compressive stress.Die corner is the position where surface damage initiates.For HSS sheet, surface damage is of major interest due to high forming pressure.The HSS and hot-dip galvanized steels show improved ability of damage-resistance with increased hardness of the forming tool.However, for galvannealed steel it is not the forming tool with the highest hardness value that performs best.     

The limit drawing ratio and formability prediction of advanced high strength dual-phase steels

Dual phase (DP) steels, being among advanced high-strength steels (AHSS), have been successfully used in the sheet metal stamping of automotive components for its great benefit in reducing vehicle weight while improving car safety. In their practical application, one of the major challenges is related to formability prediction of onset crack. This paper first conducts limiting drawing ratio (LDR) experiments to identify the maximum blank diameter of SPFC340, DP600, DP800 and DP1000 with onset crack. The fracture modes of these four types of blanks are then compared and classified into two categories: necking crack and shear crack. Further for DP1000, appropriate hardening formula is determined to fit the flow curve derived by the tensile test. Three yielding models (Hill-48, Batlat-89 and Banabic-2005) are compared with each other in the numerical simulation of DP1000 LDR onset crack. The investigation shows that a Swift and Hockett–Sherby combined formula is in good agreement with the flow curve of the tensile test and Batlat-89 yield model successfully predicts the onset shear crack of DP AHSS.     

Characterisation of formability behaviour of multiphase steels by micromechanical modelling

Multiphase steels are new Advanced High Strength Steels (AHSS), which have been developed for the automobile industry for the purpose of reducing of car body weight. These steels offer an excellent combination of high strength and large elongation due to the coexistence of harder and softer phases in their microstructure. The advantageous properties of multiphase steels can be utilised by regulating the type, amount, formation and spatial distribution of the different constituent phases. To describe the influences of the heterogeneous microstructure on the mechanical properties and the complex fracture mechanisms, an approach is presented using Representative Volume Elements (RVEs) on a micro level. Three dimensional RVE simulations were conducted under considerations of metallographic analysis for a Dual Phase (DP) steel and a Transformation-Induced Plasticity (TRIP) steel. The Gurson–Tvergaard–Needleman (GTN) damage model was applied to investigate the local crack initiation in steel sheets during various forming processes. In conclusion, a failure prediction based on microstructure is proposed for the material characterisation in sheet metal forming of multiphase steels.     

On twist springback in advanced high-strength steels

The sheet metal components made of advanced high-strength steel (AHSS) become fairly attractive in reducing weight and enhancing operational performance of products. However the corresponding forming process often generates more severe springback. This paper aims to investigate the behavior of twist springback in advanced high strength sheet components, where a twist rail was considered and the corresponding die and measurement tool were developed. Finite element model of the twist rail was first validated through a try-out test and then is used to carry out a parametric study on the twist springback. The results lead to an in-depth understanding of how the design and process parameters, such as transition ratio of cross-section width, corner angle, drawbead depth and material strength grade, affect the twist springback, thereby providing some insights into sensitivity analysis for the design of products and corresponding processes.     

An assessment of various process strategies for improving precision in single point incremental forming

Single point incremental forming (SPIF) is a process with the capability to form complex geometries using a tool of very simple geometry, without the need for a matching die. However, large elastic springback resulting from the die-less nature of the process can cause problems if high levels of accuracy are required. The aim of this investigation is to use numerical modelling to investigate different strategies to improve the process precision. A finite-element (FE) model has been used to investigate the effects of adding a backing plate, a supporting kinematic tool and modifying the final stage of the tool path. The results show that the backing plate will minimise the sheet bending near to the initial tool contact location; the additional kinematic tool will reduce springback; and the extension of the tool path across the base of the sheet will eliminate the pillow effect. The cumulative effect of introducing these features to the process shows an improvement in the overall accuracy of the profile and in the thickness distributions of the final product. The results contribute to a better understanding of springback in SPIF.     

高强钢板热冲压成形热力耦合数值模拟

为研究高强钢板的热冲压成形性,采用ABAQUS软件对高温下22MnB5高强钢板沟槽形件冲压成形进行了数值模拟研究.建立了基于热力耦合的弹塑性有限元模型和热成形下的材料模型,通过对沟槽形件热成形进行数值模拟,考察了压边力、模具间隙和凹模圆角半径等工艺参数对热成形时温度分布和回弹的影响,给出了热成形中产生回弹的机理,确定了合适的工艺参数,通过热成形试验验证了数值结果的可靠性.     

Numerical study of contact conditions in press hardening for tool wear simulation

In the press hardening industry, industrial and academic efforts are being directed toward predicting tool wear to realize an economical manufacturing process. Tool wear in press hardening is a tribological response to contact conditions such as pressure and sliding motion. However, these contact conditions are difficult to measure in-situ. Furthermore, press hardening involves high temperatures, and this increases the complexity of the tribo system. The present work investigated the contact conditions of press hardening with a commercial FE code (LS-DYNA) as a base for tool wear simulation. A press hardening experiment was established in industrial environments and evaluated through FE simulations. The numerical model was set up so as to approximate the manufacturing conditions as closely as possible, and the sensitivity with respect to the friction coefficients was examined. The influence of numerical factors such as the penalty value and mesh size on the contact conditions is discussed. The implementation of a modified Archard’s wear model in the FE simulation proved the possibility of tool wear simulation in press hardening. Finally, a comparison between the tool wear simulation and the measured wear depth is presented.     

Investigation on failure process and structural optimization of a high pressure letdown valve

High pressure letdown valve in direct coal liquefaction is used to adjust the flow rate of coal–oil slurry that enters into the downstream separator. Severe erosion–cavitation wear is found on the valve spool, seriously affecting the safety and reliability of unit. The majority of this paper investigates the failure process of valve spool and proposes a corresponding structural optimization via computational fluid dynamics (CFD) methodology. Three geometries of different failure states are selected as the computational domains in the numerical simulation. The Schneer–Sauer model, particle rebound-velocity model and erosion model are employed to calculate the cavitation phenomenon and erosion rates distribution. Experiments of flow rates and cavitation on valve model under different pressure drops are conducted to validate the accuracy of numerical approach. Results showed that the damage development of valve spool aggravates the erosion–cavitation wear. The maximum erosion rates are located on the top of spool head in all the three states. The erosion rates on spool arc surface are two orders of magnitude higher than that on parabolic surface. The decrease in radius of spool head reduces the intensities of erosion–cavitation wear. The numerical results are in agreement with actual failure morphologies of valve spool in different states.     

Wear-related failures of nitrocarburized steels: Some microstructural and morphological observations

This paper describes some characteristic microstructural and morphological features of nitrocarburized tool and die steels after wear testing (standard pin-on-disk technique). The steels under study are used for the fabrication of dies and molds for metal working (cold- and hot-work) and plastic injection molding. Surface spalling by interface failure and/or intralayer fracture was observed for the two classes of tool steels (AISI D6 and a CrMoV steel), and uniform surface layer removal by micropolishing and microcutting was observed on the precipitation-hardened (PH) stainless steel.     

Experimental determination of spring back and thinning effect of aluminum sheet metal during L-bending operation

In automotive industry, significant efforts are being put forth to replace steel sheets with aluminum sheets for various applications. Besides its higher cost, there are several technical hurdles for wide usage of aluminum sheets in forming. Major problems in aluminum sheet metal forming operations are deformation errors and spring back effect. These problems are dependent on the number of parameters such as die and tool geometry, friction condition, loading condition and anisotropic properties of the metal.To predict the exact shape, the geometry based punch contact program must be used. The shape changes once the punch is withdrawn, because of the materials elasticity. Prediction of such a spring back effect is a major challenging problem in industry involving sheet metal forming operations. It also needs applying appropriate back tension during the forming complex shapes. Slight deformation of the metal leads to non-axisymmetric loading. One can predict the residual stress by determining plastic and elastic deformation. Thus appropriate spring back effect can be investigated.The present investigation was carried out to determine the spring back and thinning effect of aluminum sheet metal during L-bending operation. Number of specimens with thickness varying from 0.5 mm to 3.5 mm were prepared. The experiments were conducted for different clearances between punch and die. It is observed that, beyond a particular clearance for each thickness of the sheet metal, the spring back and thinning effects were linearly increasing. However, below the critical clearance, scratches on the surface of the sheet metal were seen due to wear. The scratches were analyzed through Scanning Electron micrographs. As the clearance between punch and die reduces further, more wear on the punching surface was observed. And, as the clearance increases it leads to increase the spring back effect and fracture propagation.     

Wear particles deposit formation on the polishing tool working surface

Based on the quantum-mechanical theory of scattering, the paper describes interaction between workpiece debris particles and tool wear particles in the contact zone in polishing. It is shown that the forward scattering of debris particles occurs mostly on the debris particles, and the probability of their scattering on wear particles is insignificant. The distribution function of the total cross-section of debris particles scattering along the radius of circular zones of the tool working surface has been determined together with coordinates of the deposit localization zone as well as approximate number and dimensions of the deposit fragments. The results of experimental verification of the theoretical calculations are given.     

Experiences from experimental and numerical springback studies of a semi-industrial forming tool

The work reported in the present paper constitutes a part of a project on simulation of springback in sheet metal forming. Previous work in this project has been concentrated on material modeling and characterization with focus on springback applications. It has been demonstrated that, with proper considerations of all aspects of the material model and the material properties, excellent springback results can be obtained for simple problems. At the simulation of real, industrial parts, a number of additional problems are encountered. Many of these problems are associated with deviations from nominal geometries and other properties. These are examples of factors that influence the outcome of the forming process, but are unknown to the analyst, and can therefore not be considered in the simulation of the forming process. Other phenomena are known to exist, but due to their complexity, they are practically impossible to consider in industrial simulations. Examples of such phenomena are the true frictional behavior in contacts between the blank and the tools, and the flexibility of the press and the forming tool. The influence of these kinds of effects is discussed in the present paper. In the current study, a semi-industrial tool, specially designed to catch those springback problems that are encountered in the forming of real industrial, parts, is used. The tool includes several characteristics that can be found in typical forming tools, such as several draw radius steps and change-over in section geometries. Effects like flange/wall angle changes, sidewall curl and twist are obtained at springback. The sensitivity of the predicted springback is evaluated with respect to various numerical factors, such as the friction coefficient, the material model, and the mesh density. Finally, the quality of the predicted springback behavior for four different materials, commonly used in the automotive industry, is evaluated.     

Grundlagen,Herstellung und Anwendungsaspekte des Supersolidus Flüssigphasensinterns von hochlegierten Werkzeugstählen und Metallmatrix‐Verbundwerkstoffen

Principles, manufacturing and application aspects of super solidus liquid phase sintering of high‐alloyed tool steels and metal matrix composites Iron‐based metal matrix composites (MMC) are applied for abrasive wear resistant applications. A common production route uses hot isostatic pressing (HIP) of metal and carbide powders, a comparatively cost intensive process. Using high‐alloyed tool steels as matrix materials it is possible to obtain dense materials by liquid phase sintering with an internally formed liquid phase. This contribution describes the basic principles of densification of the matrix materials taking thermodynamic calculations into consideration. It points out a production route for processing particulate reinforced, high wear resistant composite materials by sintering. Beside the sintering behaviour concepts for heat treatment as well as the abrasive wear resistance are discussed.     

基于FEM-Archard模型的高强钢冲压成形粘模行为评估

为探明高强钢冲压成形过程中的粘模机理,利用数值模拟与工艺试验相结合的方法系统评估了其宏观粘模行为.以高强钢SPFC590为研究对象,采用方盒拉深成形工艺,基于FEM-Archard磨损模型研究了弯曲-拉深复合变形模式下界面压力与滑移距离对粘模行为的影响规律.研究结果表明,方盒拉深过程中模具的宏观粘模行为主要集中在界面压力较大的拉深圆角与弯曲直边相接处附近,并随着拉深次数的增加(即滑移距离的增大),粘模量不断增大且不断向拉深方向扩展.同时,基于FEM-Archard磨损模型得到的粘模评估结果与方盒拉深试验结果相一致.     

Forming sheet metals by means of multi-point deep drawing method

Multi-point deep drawing (MPDD) is an advanced manufacturing technology for 3D sheet metal parts and it can form a variety of part shapes without the need for solid dies. In this study, a test set has been prepared for multi-point deep drawing process utilizing the multi-point forming technology. Drawability attributes of gradually rectangular shaped container have been observed using a sheet, which has the quality of Erdemir 7114 and is suitable for deep drawing process, and also using multi-pointed punch with a given tool geometry and a draw velocity. The blank shape to be drawn without wrinkling and tearing has been determined. Wrinkles and dimples are the major forming defects in the MPDD process. In conventional deep drawing, the method to form sheet metal with a blank holder is an effective way to suppress wrinkling; and the same is true in MPDD. The process of multi-point forming technology decreases production cost of die, provides flexible usage, and it is convenient to achieve the most even deformation distribution.     

高强度钢板U形件热冲压凹模结构拓扑优化

为降低热冲压凹模的生产成本和使用成本,基于板料热冲压数值模拟对凹模结构进行了拓扑优化设计.运用有限元软件ABAQUS建立热力耦合有限元模型,对高强钢板U形件的热冲压成形和淬火过程进行了数值仿真.提取凹模与板料间关键工况下接触应力作为凹模拓扑优化的外在载荷,建立约束凹模结构关键区域节点位移的体积最小化拓扑模型,对热冲压凹模结构进行拓扑优化设计,最终实现结构减重20%,且优化后凹模的变形和应力与优化前的结果相差甚微.研究内容对热冲压过程数值模拟和模具结构拓扑优化研究具有一定参考价值.     

An analytical solution for the unloading in spherical indentation of elastic-plastic solids

This paper presents a contact modelling with emphases on the unloading and subsequent reloading during the contact between a rigid sphere and an elastic-perfectly plastic half-space. By using the superposition of Steuermann solutions of elastic contact bodies of profiles having the form A_nr²ⁿ, analytical solutions of unloading or subsequent reloading for the contact force, contact displacement and contact pressure distribution are derived, all of which are expressed in terms of the contact radius. To demonstrate the present analytical solutions, curves between contact force and contact radius, contact pressure at the contact centre and contact radius, contact force and contact displacement, and contact pressure distribution are compared with those obtained using the nonlinear finite element analysis method.     

The influence of tool wear particles scattering in the contact zone on the workpiece surface microprofile formation in polishing quartz

The paper describes the interaction and scattering of wear particles in the tool-workpiece contact zone in the course of polishing and explains the oscillatory structure of scatter of the tool wear particles on debris particles and on wear particles. The differential cross-section of scattering of the wear particles on the debris particles as well as on the wear particles is shown to be maximum in the case of forward scattering on the central segments of the contact zone. We have calculated the coordinate dependence of the differential cross-section of scattering of tool wear particles and the dependence of the polished surface microprofile height on the radius of circular zones. The theoretical curve of polished surface profile microirregularities is shown to fit well the experimental one for a quartz workpiece.     

Finite element modelling and experimental investigation of single point incremental forming process of aluminum sheets: influence of process parameters on punch force monitoring and on mechanical and geometrical quality of parts

New trends in sheet metal forming are rapidly developing and several new forming processes have been proposed to accomplish the goals of flexibility and cost reduction. Among them, Incremental CNC sheet forming operations (ISF) are a relatively new sheet metal forming processes for small batch production and prototyping. In single point incremental forming (SPIF), the final shape of the component is obtained by the CNC relative movements of a simple and small punch which deform a clamped blank into the desired shape and which appear quite promising. No other dies are required than the ones used in any conventional sheet metal forming processes. As it is well known, the design of a mechanical component requires some decisions about the mechanical resistance and geometrical quality of the parts and the product has to be manufactured with a careful definition of the process set up. The use of computers in manufacturing has enabled the development of several new sheet metal forming processes, which are based upon older technologies. Although standard sheet metal forming processes are strongly controlled, new processes like single point incremental sheet forming can be improved. The SPIF concept allows to increase flexibility and to reduce set up costs. Such a process has a negative effect on the shape accuracy by initiating undesired rigid movement and sheet thinning. In the paper, the applicability of the numerical technique and the experimental test program to incremental forming of sheet metal is examined. Concerning the numerical simulation, a static implicit finite element code ABAQUS/Standard is used. These two techniques emphasize the necessity to control some process parameters to improve the final product quality. The reported approaches were mainly focused on the influence of four process parameters on the punch force trends generated in this forming process, the thickness and the equivalent plastic deformation distribution within the whole volume of the workpiece: the initial sheet thickness, the wall angle, the workpiece geometry and the nature of tool path contours controlled through CNC programming. The tool forces required to deform plastically the sheet around the contact area are discussed. The effect of the blank thickness and the tool path on the punch load and the deformation behaviour is also examined with respect to several tool paths. Furthermore, the force acting on the traveling tool is also evaluated. Similar to the sheet thickness, the effect of wall angle and part geometry on the load evolution, the distribution of calculated equivalent plastic strain and the variation of sheet thickness strain are also discussed. Experimental and numerical results obtained allow having a better knowledge of mechanical and geometrical responses from different parts manufactured by SPIF with the aim to improve their accuracy. It is also concluded that the numerical simulation might be exploited for optimization of the incremental forming process of sheet metal.