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 The shape changes once the punch is withdrawn, because of the materials elasticity.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.     

Formabilities of steel/aluminium alloy laminated composite sheets

The tensile properties and press formabilities of laminates experimentally produced from mild steel and various aluminium alloy sheets are examined. The tensile properties of the laminates are approximately predictable by the mixture rule of the properties of the individual sheets. The forming limits in deep drawing, as well as stretch forming due to various types of fractures of the laminated composite sheets, cannot be predicted without considering the stress and strain histories of the individual sheets during forming. Furthermore, it is found that the drawability, as well as the stretch formability, is improved by setting the mild steel sheet on the punch side for the case of aluminium alloy sheet with comparatively high ductility, and by sandwiching the aluminium alloy sheet with the mild steel sheets for the case of low ductility.     

铝/钢薄板无铆连接接头成形过程及形貌控制

目的 研究铝/钢薄板无铆连接过程中接头形貌的影响因素。方法 采用有限元模拟方法分析了铝/钢薄板在平底模具下接头形貌的成形过程。结合变形区域的金属流动情况,解释了互锁结构的形成机理,并分析了不同工艺参数对接头形貌的影响。结果 接头互锁结构主要是依靠铝板填充钢板的凹陷部位而形成的,抑制钢板与模具接触一侧的金属流动有助于接头底部和侧壁的钢板拉薄,进而形成内部凹陷,促进互锁结构的形成。摩擦因数对接头形貌参数影响较大,增大摩擦因数可显著提高接头的互锁量。结论 冲头半径、冲头圆角、凹模深度以及摩擦因数对颈厚值和互锁量均有显著影响。通过控制这些影响因素,可以得到良好的接头。此外,接头的失效形式以铝板颈厚较薄处的剪断失效为主,因此对于采用铝上钢下的无铆连接,保证颈厚值相对于互锁量更加重要。     

渐进成形A3003铝板减薄带分析及数值模拟研究

研究渐进成形过程中板料减薄带的变化,可以提供合理的加工参数,提高板料的成形性能和加工利用率,减少零件破裂失效.基于渐进成形过程中金属板料轮廓的变化与理想情况下轮廓的区别,对渐进成形初始成形阶段A3003铝板减薄带的产生原因和剪切力的变化过程进行了理论分析,并通过有限元模拟分别从未变形区金属板料的长度和强度两个角度对板料渐进成形过程中未变形区下沉的影响,以及成形角度和杨氏模量对变形区回弹的影响两个方面,对减薄带的产生原因进行研究.结果表明:板料未变形区的下沉和变形区的回弹使板料在初始加工阶段形成一段平缓区域,工具头在平缓区域的变形性质发生了变化,平缓区域发生剪切变形导致了板料在初始加工阶段形成了减薄带;渐进成形时减小板料未变形区的长度,增大板料与垂直方向的角度可以一定程度上阻碍减薄带的产生,模拟结果与理论分析相符合.     

OPTIMUM JOINING CONDITIONS IN A MECHANICAL PRESS JOINT

Mechanical press joining has been used in sheet metal work because it is a simple process and offers the possibility of joining dissimilar sheet metals, such as steel and aluminum alloy sheets. The mechanical press joint strength was found to vary with joining conditions, such as sheet thickness and punch diameter. The optimum joining conditions of the mechanical press joint under complex loading can be determined by correlating strength ratio with diameter ratio and sheet thickness ratio. The failure mode was considered during estimation of the joining strength. Under this experimental condition, the optimum strength ratio was acquired at a sheet thickness ratio of 1.0 and a diameter ratio of 1.683.     

Prediction of non-uniform thinning in superplastically formed spherical domes

Abstract

Superplastic forming is an attractive manufacturing process, which allows the production of complex sheet metal components. The gas pressure bulging of metal sheets has become an important forming method. As the bulging process progresses, significant thinning in the sheet material becomes obvious. A prior knowledge about non-uniform thinning in the product after forming helps the designer in the selection of initial blank thickness. This paper suggests a simple procedure to obtain the variation in thickness of a gas pressure formed spherical dome at any instant of time during the bulging process. This simple procedure is validated by comparing predicted and measured thicknesses of a formed titanium hemispherical dome.     

Effect of continuous strain path changes on forming limit strains of DP600

The strain path may change in actual sheet metal‐forming processes, so the determination of formability of sheet metal should consider the nonlinear strain path. For identifying the forming limit (FL) strains under nonlinear strain path, a conventional two‐step procedure with unloading is classically used to produce the strain path change, which results in no continuous measure of strain. The in‐plane biaxial tensile test with a cruciform specimen is an interesting alternative to overcome the drawbacks of conventional method. The strain path change can be made without unloading during a single test. In this work, the experimental FL strains of DP600 sheets under two types of nonlinear strain path are investigated and then compared with those under linear strain paths. The Oyane ductile fracture criterion is used in the finite element simulation to predict the experimental results.     

On the numerical implementation of a shear modified GTN damage model and its application to small punch test

Gurson-type models have been widely used to predict failure during sheet metal forming process. However, a significant limitation of the original GTN model is that it is unable to capture fracture under relatively low stress triaxiality. This paper focused on the fracture prediction under this circumstance, which means shear-dominated stress state. Recently, a phenomenological modification to the Gurson model that incorporates damage accumulation under shearing has been proposed by Nahshon and Hutchinson. We further calibrated new parameters based on this model in 22MnB5 tensile process and developed the corresponding numerical implementation method. Lower stress triaxiality were realized by new-designed specimens. Subsequently, the related shear parameters were calibrated by means of reverse finite element method and the influences of new introduced parameters were also discussed. Finally, this shear modified model was utilized to model the small punch test (SPT) on 22MnB5 high strength steel. It is shown that the shear modification of GTN model is able to predict failure of sheet metal forming under wide range of stress state.     

Thinning analysis for hot forming of single-piece hemispherical dish end using finite element simulation and its empirical modeling

Hot forming is an important forming method for production of small and medium sized single-piece pressure vessel dish ends. Thinning of the blank material with increased degree of forming is quite obvious. Present work shows finite element (FE) analysis of hot forming of single-piece hemispherical dish end (SPHDE) for predicting maximum thinning of blank during forming. Thinning analysis is performed for SPHDE of two different ferrous material grades SA-387 Gr22 and SA-516 as per ASME, Section II, Part A (2004). Percentage thinning results of FE analysis are verified with the practical material thinning data. Based on the thinning analysis, empirical relation for calculation of percentage thinning as a function of different dish ends′ geometry parameters is proposed. Empirically predicted percentage thinning is observed to be in good agreement with practical and FE simulation results. Thinning results obtained based on proposed FE simulation and empirical model can be helpful to design engineer in selecting thickness of developed blank considering thinning allowance for hot forming of SPHDE. Thinning analysis data can be used for optimizing the dish end geometrical parameters for minimum thinning during forming.     

板料成形中韧性断裂准则应用研究进展

对板料成形中的成形极限应力图、最大变薄率、成形极限图以及韧性断裂准则等预测成形极限的方法,进行了综述和分析。指出利用韧性断裂准则,不但能够较好地预测塑性差的板料成形极限,而且还能考虑应变路径的变化。利用有限元方法模拟时,韧性断裂准则既可以应用到完全耦合的弹塑性损伤模型的增量方法中,也可以应用到一步有限元逆算法中。为了准确地预测成形极限,除了提高有限元模拟精度外,应找到一种本质地反映材料性能的韧性断裂准则。     

A study on sheet formability by a stretch-forming process using assumed strain FEM

The effects of sheet thickness and frictional condition between the punch and sheet on formability is predicted and compared with the experimental results of the Erichsen test as a stretch-forming process. The material and geometrical nonlinearity are considered. A hypoelastic–plastic model is used and strain-field stabilization is taken into account using the Assumed Strain Finite-Element Method. By considering the contact problem and applying the nonlinear finite-element method, the force and dome height for aluminum and steel sheets are computed and compared with the experimental results. The Oyane criterion is used to access the formability of the sheet. A good agreement was found between the experimental and predicted formability results of aluminum and steel sheets. The results show that for decreasing friction coefficient the position of tearing is moved from the rim to the pole. Also, by increasing the work-hardening exponent, formability is increased.     

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.     

Numerical and experimental analysis of wrinkling during the cup drawing of an AA5042 aluminium alloy

The recent trend to reduce the thickness of metallic sheets used in forming processes strongly increases the likelihood of the occurrence of wrinkling. Thus, in order to obtain defect-free components, the prediction of this kind of defect becomes extremely important in the tool design and selection of process parameters. In this study, the sheet metal forming process proposed as a benchmark in the Numisheet 2014 conference is selected to analyse the influence of the tool geometry on wrinkling behaviour, as well as the reliability of the developed numerical model. The side-wall wrinkling during the deep drawing process of a cylindrical cup in AA5042 aluminium alloy is investigated through finite element simulation and experimental measurements. The material plastic anisotropy is modelled with an advanced yield criterion beyond the isotropic (von Mises) material behaviour. The results show that the shape of the wrinkles predicted by the numerical model is strongly affected by the finite element mesh used in the blank discretization. The accurate modelling of the plastic anisotropy of the aluminium alloy yields numerical results that are in good agreement with the experiments, particularly the shape and location of the wrinkles. The predicted punch force evolution is strongly influenced by the friction coefficient used in the model. Moreover, the two punch geometries provide drawn cups with different wrinkle waves, mainly differing in amplitude.     

Evaluation of bending limit curves of aluminium alloy AA6014-T4 and dual phase steel DP600 at ambient temperature

Bending/hemming operations are extensively used in automotive industries for assembling the car body panels. Besides the process mechanics, bending operation differs from biaxial sheet metal forming operations in failure mechanism also. Hence, the limit strains that material can sustain are different for both the operations. Thus, the conventional FLC proposed by Keeler and Goodwin fails to predict formability in bending/hemming operations. This necessitates the development of bending limit curves. In this work, bending limit curves are determined experimentally for AA6014-T4 and DP600. Effect of punch radius, nature and level of pre-strain on the bending limits is also studied. Thus, BLC can be used as a post-processing criterion in finite element simulations to assess formability during bending/hemming operations.     

金属板材屈服行为与塑性失稳力学模型在微尺度下的应用

板材屈服准则与塑性失稳模型是精准描述高性能构件成形或服役过程的基础与前提。在板材塑性成形过程中,试样几何尺寸、材料晶粒大小、自由表面粗化和织构分布等都会对材料的塑性变形行为产生不可忽略的影响,导致单一尺度下的本构模型和断裂准则不能有效预测微观尺度下的材料变形行为和各种缺陷,大大限制了合金板材在航空、航天、汽车、医疗等工业上的应用。对现有屈服准则的研究进展进行了较为全面的回顾,从Hill、Hershey-Hosford和Drucker这3个系列出发,分别进行了对比分析,并总结了目前国内外用于验证屈服准则的金属板材双向拉伸实验机发展状况。基于不同的破裂失稳机理,将失稳模型分为宏观失稳准则、韧性断裂准则和耦合材料损伤演化的韧性断裂准则,并分别进行了归纳和阐述。此外,随着微成形技术的逐步推广,也对宏观塑性成形理论在微尺度下的应用进展进行了说明,指出了宏观屈服准则和失稳模型在微尺度下的不足和缺陷。最后讨论了宏观屈服准则和失稳模型今后的发展趋势以及宏观塑性成形理论在微尺度下的应用前景。     

Effect of different yield criteria on prediction of wrinkling initiation of interstitial-free galvanized steel sheet

Thin interstitial-free (IF) galvanized steel sheets have great usage in auto industry due to their high formability and corrosion resistance. However presence of wrinkle waves in deep drawn parts of thin IF steel sheets is a common challenging issue. Achieving a reliable method to predict the wrinkling onset enables the manufacturers to avoid wrinkling by modification of forming parameters. In the present study, sidewall wrinkling of an IF-galvanized steel sheet was both analytically and experimentally examined during a conical cup wrinkling test. Using an energy method approach together with a newly developed deflection function, Hosford and Hill-1948 yield criteria were employed to predict the critical values of stress and cup height at the onset of wrinkling. According to the obtained results, applying Hill-1948 yield criterion resulted in prediction of wrinkling onset closer to the experimental value than Hosford yield criterion. It was also found that the newly presented deflection function has a great suitability for prediction of the real status of wrinkling phenomenon.     

Rotation friction pressing riveting of AZ31 magnesium alloy sheet

A kind of joining method for magnesium alloys, rotation friction pressing riveting (RFPR), is proposed in this paper. In RFPR operation, a rivet with a plug rotating at high speed is brought to contact with the riveted sheets, generating frictional heat between the rivet and riveted sheets, which softens the sheet materials and enables the rivet to be drilled into the sheets under reduced force. When fully inserted, the rivet is stopped rotating, and the plug is immediately pressed into the shank of the rivet by a punch. The expansive deformation of the rivet shank occurs under the action of the plug, thereby forming a mechanical interlock between the rivet and the sheets to fasten the sheets together. The studies show that RFPR of AZ31 magnesium alloy sheet can be carried out at ambient temperature, and provides the joints with superior shear strength and fatigue property when compared with self-piercing riveting (SPR). The effects of the operating parameters of RFPR process on the quality of the joints were investigated in the study. The results shows that while the rivet rotation speed little affects the shear strength of RFPR joints, the punch pressure has a significant influence on the mechanical properties of the RFPR joints. A numerical analysis was also performed to understand the effect of the punch pressure on the interlock between the rivet and the sheets, and the stress and strain distribution inside the sheet materials around the rivet. The results show that the interlock increased with the punch pressure and there is residual compressive stress inside the sheet materials, which seems to explain the good fatigue property of RFPR joints observed.     

基于韧性准则的金属板料冲压成形断裂模拟

该文基于Rice-Tracey韧性失效准则,探索金属板料冲压成形的断裂数值模拟方法。首先设计两种试验,并结合有限元分析结果确定双相钢板料的材料失效参数;然后,基于Abaqus软件的显式模块Explicit,编写采用Rice-Tracey韧性失效准则的用户自定义材料子程序VUMAT;最后对双相钢薄板深拉成形过程中的断裂行为进行数值模拟,并进行了试验验证。结果表明,数值模拟结果与试验结果吻合较好。可见,Rice-Tracey韧性准则可用于金属板料冲压成形的断裂预测。     

脉冲电流对超薄316L板微流道成形精度的影响

目的 研究在金属双极板脉冲电流辅助冲压工艺中,将脉冲电流单独作用于保压阶段、冲压阶段以及作用于整个冲压过程(冲压阶段+保压阶段)3种通电方式对极板流道深度及壁厚分布的影响。方法 以316L不锈钢薄板作为实验材料,调节流经样品的电流参数,分别为0、10、20、30 A,分析脉冲电流参数和通电方式对极板流道深度及壁厚分布的影响规律。结果 在高电参数下,当采用在冲压阶段通电、在保压阶段断电的方式2时,在保压阶段可促进应力松弛,具有最佳的回弹抑制效果,其成形深度回弹可减小73.88%。在不同电参数下,3种成形方式对双极板流道壁厚分布具有相似的影响。槽顶和槽底的壁厚减薄相对较小,减薄量为原板材厚度的2%~6%;流道直壁壁厚减薄为原板厚度的9%~15%;而外圆角和内圆角区域的壁厚减薄较大,达到27%~30%。结论 在金属双极板脉冲电流辅助冲压工艺中,以不同方式施加脉冲电流,均可以显著提升流道深度,对壁厚影响并无明显差别。特别地,采用在冲压阶段通电、在保压阶段断电的方式2可以显著提升对流道回弹的抑制效果。     

汽车用烘烤硬化板 BH340 及深冲板 St 15 成形性能分析

进行了汽车用烘烤硬化钢板 BH340 及深冲钢板 St15 的力学性能、时效特性和烘烤硬化性能实验,并利用有限元方法模拟了 2 种钢板的成形极限和破裂点应变路径。 分析后指出, BH340 烘烤硬化效应较显著,时效效应不明显,对于汽车外覆盖件成形、涂漆烘烤工艺具有较好的适应性; St15 经形变强化后仍具有良好的塑性,适用于多道次成形工艺。 另外,2 种钢板均具有较为明显的面内异向性。