Study of effect of load on springback in sheet metal bending

Springback remains a major concern in sheet metal bending in fabricating any final product within the permissible tolerance. Apart from the geometrical and material parameters, springback is significantly affected by the forming load also and the present study is focused on it. Sheet metal bending process involves large rotation and strain as well as large springback due to elastic recovery of the material. Therefore, a large deformation algorithm based Finite Element software was used to model a typical sheet metal bending process employed in manufacturing cylindrical structures. A Total-Elastic-Incremental-Plastic (TEIP) algorithm has been incorporated in an in-house software to handle large deformation and the elastic recovery during the unloading process. In addition, experiments have been performed on aluminum, brass, copper and mild steel sheets and substantiated with the FEM analysis.     

Microstructural effects on the springback of advanced high-strength steel

The application of advanced high-strength steels (AHSS) has been growing rapidly in the automotive industry. Because of their high-strength, thinner sheet metals can be used for body components to achieve both weight savings and increased safety. However, this will lead to greater springback deviation from design after the forming operation. Fundamental understanding and prediction of springback are required for springback compensation and tooling design. While various types of continuum mechanics based models have been proposed to simulate the mechanical behavior of advanced high-strength steels, few of them consider microstructural effects such as material heterogeneity. In this study, through sheet thickness strength variation has been observed in DP 780 and TRIP 780 steels. Finite-element simulation indicates that the through thickness effect (TTE) can have a significant impact on the springback behavior of these sheet metals. This is verified through our experimental work using draw-bend testing. The results suggest that microstructural effects should be considered to accurately simulate springback of AHSS. Based on these results, implications of different microstructural designs will be discussed.     

Experimental Analysis on the Frictional Behaviour of Drawbeads in Sheet Metal Forming

In sheet metal forming, drawbeads are commonly used to control uneven material flow, which may cause defects such as wrinkles, fractures, surface distortion and springback. Although friction may not directly change the limiting strain of steel sheets, the tribological conditions in the contact zone between the sheet surface and the tool surface play an important role in determining the limits of the forming process. Friction in the drawbead contact zones affects the flow of the material in the tool and is used deliberately to control the stamping process. Therefore in this study, the frictional behaviour of drawbeads is experimentally investigated by the drawbead friction test. To characterize the effect of processing variables on the friction coefficients, tests are performed for various sheets, lubricants and bead materials suffering different surface treatments. The results obtained from the drawbead friction test show that the friction and drawing characteristics of deforming sheets were strongly influenced by the strength of sheet, viscosity of lubricant and hardness of bead surface.     

预应变对TRIP型双相不锈钢拉伸变形行为的影响

预变形通常作为工程构件服役前的关键预处理工艺,用以提高金属构件的力学性能.为了明确预应变对拉伸变形过程中力学行为和相变行为的影响,在INSTRON 8801试验机上进行了一系列准静态拉伸试验,研究了一种TRIP型双相不锈钢的拉伸变形行为及其预应变敏感性,同时结合TEM分析、EBSD分析和原位磁性测量讨论了微观机理.结果...     

先进镀锌高强钢中液态金属脆化的研究进展及挑战

先进高强钢广泛用于汽车车身结构,能够在保证车辆安全性的同时使整体车身减重,满足节能减排的要求,在汽车轻量化方面已凸显出巨大的应用潜力.通常汽车用高强钢板需要进行表面镀锌处理以提高车身结构的抗腐蚀能力.但镀锌高强钢板在点焊过程中容易产生液态金属脆化,降低焊接接头的承载性能.镀锌热成形高强钢在热成形过程中也会产生液态金属脆...     

Material Characterisation for Reliable and Efficient Springback Prediction in Sheet Metal Forming

In this paper, a novel experimental‐numerical methodology for an accurate prediction of springback after sheet forming is presented. An advanced phenomenological material model is implemented in the FE‐code ABAQUS. It includes the Bauschinger effect, the apparent reduction of the elasticity modulus at load reversal after plastic deformation, the strain rate dependence and the elastic‐plastic anisotropy and its evolution during the forming process. The required material parameters are determined from stress‐strain curves measured in tension‐compression tests. These tests are carried out with a special test rig designed to avoid buckling of the specimens during compression. The benefits of this procedure for springback prediction are demonstrated. Additionally, parameters for the phenomenological models are determined from texture simulations.     

金属板材成形性能的力学分析

根据金属材料拉伸试验所获得的强度、塑性指标、成形性能如Rp0.2、Rm、A、n、r等,并结合成形极限图(FLD)试验实测的金属板材拉胀成形性能,介绍了一整套用于评价金属板材塑性成形性能的实验室分析技术.文章以两种厚薄规格的冷轧板材为例,根据拉伸应变硬化指数动态跟踪技术实测的n值随应变量变化的趋势,揭示了上述两种材料拉伸性能与成形性能异同特点的力学本质.     

大口径直缝焊管自由弯曲回弹试验与仿真分析

主要介绍方大特钢轧钢厂生产线的设备概况,以及在进行直角扁钢品种开发过程中碰到的问题和解决办法。     

Experimental and Numerical Study of Electromagnetic Forming of AZ31B Magnesium Alloy Sheet

Wrought magnesium alloys are interesting materials for automotive and aeronautical industries due to their low density in comparison to steel and aluminium alloys, making them ideal candidates when designing a lower weight vehicle. However, due to their hexagonal close‐packed (hcp) crystal structure, magnesium alloys exhibit low formability at room temperature. For that reason, in this study a high velocity forming process, electromagnetic forming (EMF), was used to study the formability of AZ31B magnesium alloy sheet at high strain rates. In the first stage of this work, specimens of AZ31B magnesium alloy sheet have been characterised by uniaxial tensile tests at quasi‐static and dynamic strain rates at room temperature. The influence of the strain rate is outlined and the parameters of Johnson‐Cook constitutive material model were fit to experimental results. In the second stage, sheets of AZ31B magnesium alloy have been biaxially deformed by electromagnetic forming process using different coil and die configurations. Deformation values measured from electromagnetically formed parts are compared to the ones achieved by conventional forming technologies. Finally, numerical study using an alternative method for computing the electromagnetic fields in the EMF process simulation, a combination of Finite Element Method (FEM) for conductor parts and Boundary Element Method (BEM) for insulators, is shown.     

Experimental Study on Springback of Sheet Metal Single Point Incremental Forming

Sheet metal single point incremental forming (SPIF) is a new technology for flexible process.The spring- back phenomenon in single point incremental forming has been discussed.Effects of forming angle and shape of the part are analysed using simple experimental method.Tool diameter, sheet thickness, step size, material parameters and the interaction of them are also analysed by using orthogonal test.The results show that the primary factor af- fecting springback is forming angle.In addition, springback is decreased when the specimen has a larger forming angle.The order of the four factors that influence springback is tool diameter, sheet thickness, step size and materi- al parameters.The forming precision will increase if springabck is decreased by optimizing the forming parameters.     

A Review of the Physical Metallurgy related to the Hot Press Forming of Advanced High Strength Steel

The automotive industry requirements for vehicle weight reduction, weight containment, improved part functionality and passenger safety have resulted in the increased use of steel grades with a fully martensitic microstructure. These steel grades are essential to improve the anti‐intrusion resistance of automotive body parts and the related passenger safety during car collisions. Standard advanced high strength steel (AHSS) grades are notoriously difficult to be press formed; they are characterized by elastic springback, poor stretch flangeability and low hole expansion ratios. Hot press forming has therefore received much attention recently as an alternative technology to produce AHSS automotive parts. In this contribution, the physical metallurgy principles of the hot press forming process are reviewed. The effect of composition on CCT curves of standard CMnB hot press forming steels is discussed taking the deformation during press forming into account. Furthermore,the effect of the static strain ageing processes occurring during the paint baking cycle on the in‐service mechanical properties of press hardened steel will be presented. The influence of temperate and strain rate on the flow stress during press forming and the final room temperature mechanical properties will be discuss ed. Moreover, the issues related to coatings on B‐alloyed CMn hot press forming steel will be critically reviewed. In particular the combined effects of thermal cycle and deformation on the degradation of the Al‐10%Si coating will be discussed in detail. Finally, the properties of both Al‐based and Zn‐based coating systems are compared, and the possibility of the formation of a diffusion barrier during press forming is discussed.     

Robustness of the sheet metal springback cup test

The robustness of a proposed test for elastic springback characterization of sheet metal has been examined using a matrix of defined experimental errors. A series of flat bottom deep drawn cups made from AISI 1010 steel sheet were examined. It was found that misalignment of the blank over the forming tool and error in the vertical location where the springback ring was cut from the cup sidewall had the largest effect on the resulting springback opening. Other experimental errors involving cup height and ring width were found to be less important. The effect of in-plane anisotropy of mechanical properties on springback was negligible. The results are examined in terms of measured through thickness residual stresses and elastic bending of beams with circumferential thickness gradients.     

High Temperature Tribological Studies on Surface Engineered Tool Steel and High Strength Boron Steel

The popularity of hot sheet metal forming processes in the recent years has necessitated research efforts to improve tool life and control the friction level during hot forming operations. In this work, the tribological properties of tool steel and ultra high strength boron steel (UHSS) pairs at elevated temperatures have been studied by using a special hot sheet metal forming test rig that closely simulates the conditions prevalent in the real process. This test involves linear unidirectional sliding of a preheated UHSS sheet between two tool steel specimens where new workpiece material is continuously in contact with the tool surface. The study is aimed at investigating different surface treatments/coatings applied on either the tool or sheet surface or on both. The results have shown that it is possible to control the coefficients of friction through surface treatments and coatings of the tool and workpiece materials. The application of a coating onto the sheet material has a greater influence on the friction compared to changing the tool steel surface. After running‐in, the investigated tool steel variants show almost similar frictional behaviour when sliding against the same sheet material. Although coating the UHSS sheet reduces friction, it abrades the tool surface and also results in transfer of the sheet coating material to the tool surface.     

Forming properties and springback evaluation of copper beryllium sheets

Copper beryllium (CuBe) alloys possess excellent strength and conductivity. They have become the most important materials used for producing high reliability connectors and interconnections for electrical and electronic applications. As demand for high connection density in electrical and electronic products grows, springback behaviors become increasingly critical in fabricating these miniaturized contact components from sheet base materials. In the present article, a study of the springback behavior of CuBe sheets under different heat treatments is presented, with the goal of providing reliable information needed for fabricating more intricate connection parts. Both experimental and analytical techniques were adopted. The tensile tester was first used to study the springback related tensile properties. The governing tensile parameters on springback were identified, and their variations for sheets with different heat treatments were studied. It was found that a bilinear constitutive relationship can best characterize the stress strain behavior of the CuBe alloy. A closed form solution based on this bilinear relationship was formulated to predict the springback for the CuBe sheets at bending conditions. A V-shaped bend tester having an interchangeable punch to accommodate multiple radii was designed and built to evaluate the springback properties of CuBe sheets. A good correlation was found between the analytical predictions and experimental data. A parametric study, as an example, was also performed to provide the springback information needed for designing complicated connectors. formerly Director of R&D with NGK Metals Corp., Reading, PA 19612     

Formability of sandwich sheet materials in plane strain compression and rolling

The stress states developed during room temperature, plane strain compression modes of deformation of stainless steel clad aluminum and aluminum clad strainless steel sheets have been investigated in order to gain insight into the formability of bonded ductile sandwich sheet materials in primary metalworking processes. Assuming uniform, isostrain deformation in the component layers, sandwich compression stress-strain curves were predicted to be rule of mixtures averages of component compression stress-strain curves. These predictions showed good agreement with experimental data when friction and in-homogeneous deformation were taken into account. Since the through-thickness applied pressure can be assumed to be the same in both components of thin sandwich sheet materials, in-plane stresses which are tensile in the harder component and compressive in the softer component of a clad sheet are developed in order to satisfy the yield conditions. The nature of these in-plane stresses was confirmed by measurements of residual stress distributions in rolled clad sheet specimens, and it was shown how the tensile stress in the harder component may lead to unstable flow and failure of this component during forming. The observed failures were similar in both plane-strain indentation and rolling tests. Although the initiation of instability in symmetric clad sheet metals appears to be independent of the arrangement of the component layers, the process of final localization leading to fracture was observed to depend heavily on the layer arrangement. S. L. SEMIATIN, formerly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University.     

Process Modeling for Precision Forming of Discrete Parts – State of the Technology and Critical Issues

This paper presents an overview on the application of FE simulation as a virtual manufacturing tool in designing manufacturing processes for precision parts. The processes discussed include forging, sheet metal forming and hydroforming. Determination of reliable input parameters to simulate a process is a key element in successful application of process simulation for process design in all the mentioned areas. These issues are discussed in detail. Practical examples of application of FE simulation are presented for improvement of the existing metal forming process and/or designing new metal forming process for manufacturing discrete precision parts in forging, sheet metal forming and hydroforming.     

Simulation of the forming of mash seam welded sheets

The paper presents results on the optimization of the process of mash seam welding for improving the formability of the joined sheet metal semi-products. An integrated concept including the welding, smoothing and heat treatment of mash welded sheets has been developed. As an example, fatigue tests and the simulation of the forming behaviour of a mash seam welded tensile specimen simulated by the finite element method (FEM) is discussed. The determination of the material properties of the seam which, so far, have been determined experimentally is thus considerably simplified.     

Mechanical Properties Optimization Metal Bars and Sheets Made of 34CrNiMo6 Steel

Currently available and well known materials treated by a special procedures can provide superior properties in the comparison to presently obtained ones.Special treatment procedures are efficiently developed with the use of physical simulators nowadays.Physical simulators allow treatment optimization on small scale laboratory samples which can be subsequently transferred to real production processes later on.Nevertheless,there is always need of successful transfer from small scale laboratory experiments to real production of for example metal sheets.In the current paper thermo-mechanical procedure developed for 34CrNiMo6 steel under laboratory conditions is applied to metal sheets production.There were obtained very promising properties on the bulk material samples processed in the physical simulator,the challenge was to obtain similar properties on a real metal sheets by a transfer of laboratory procedure to metal sheet production process.In the considered case the combination of tensile strength and elongation were especially required for considered application in a train body.In order to achieve excellent elongation for high strength hardened steel,very fine microstructure has to be attained through the thermo-mechanical process optimization.Very good results were achieved on produced metal sheets.6% elongation was attained allowing the possibility of further material processing,e.g.bending of sheets,with tensile strength exceeding 1500 MPa.The study presented here confirms usefulness of thermo-mechanical simulators for new procedures development.With the use of the simulator of thermaldeformation cycles a materials with optimized properties for certain application can be efficiently developed.     

Process Modeling and Optimization in Sheet Metal Forming – Selected Applications and Challenges

This paper presents an overview on the application of FE simulation as a virtual manufacturing tool in designing process for manufacturing sheet metal parts. Input parameters to simulate a process are key elements in successful simulation for process design. In this paper several methods to determine input parameters for process simulation are discussed. Practical examples of application of FE simulation are presented for improvement of existing and/or designing new forming processes for manufacturing sheet metal parts.     

Springback in simple axisymmetric stampings

A series of experiments was conducted to measure springback (calculated from dome height measurements) in a simple, stretched sheet metal part formed using a 50.8-mm-radius hemispherical punch. Parts were formed from three materials [5182-0 aluminum, 2036-T4 aluminum, and aluminum-killed (AK) steel] using three different binder geometries (lockbead, stinger, flat), and punch penetration was varied between 1.27 and 21.6 mm, limited by failure of the aluminum sheet. The steel and 5182-0 aluminum were chosen to possess similar gage and tensile properties to highlight the effect of elastic modulus, while the 5182-0 and 2036-T4 aluminum were chosen to possess similar gage and modulus but different tensile behavior to highlight the effect of strength differences. Springback increased with increasing strength and decreasing modulus. A major finding was that, for a specific material, a primary factor influencing spring-back was the binder geometry, with the lockbead showing the least springback and least variation with punch penetration: by contrast, the stinger and flat binders exhibited much greater springback, and the magnitude of the springback was strongly influenced by the extent of punch penetration. In the worst case, the springback was more than 30 pct of punch penetration. The effect of binder region restraint on springback was evaluated by comparing the part geometry both before and after strain-free removal of the binder region by electrodischarge machining. The magnitude and sign of the binder region restraint depended on binder geometry and punch penetration but was always less than the springback observed in removing the part from the die initially.