Analysis and comparative study of factors affecting quality in the hemming of 6016T4AA performed by means of electromagnetic forming and process characterization

Hemming is commonly one of the last operations for stamped parts. For this reason it is of critical importance on the performance and perceived quality of assembled vehicles. However, designing the hemmed union is a complicated task and is deeply influenced by the mechanical properties of the material of the bent part. Significant problems can arise in this operation when bending aluminum alloys, because cracks can appear due to the localized strain during hemming as a result of the low ductility of automotive aluminum alloys. This paper presents the development of the electromagnetic forming (EMF) technology for auto body-in-white parts hemming. A relatively simple experimental procedure to perform a hemming operation based on the principle of EMF is presented in order to compare the variation in the quality parameters of a hemmed joint. The achieved results are compared with the corresponding geometry hemmed utilizing the conventional process. At the same time, the study is completed with the development of a new simulation method for the EMF technology. The results obtained during this study prove the capability of the EMF to obtain quality hem unions simplifying the complicated conventional hemming operation. In this study a loose coupling EMF hemming simulation method has been developed using Maxwell^® 3D to solve the electromagnetic field computation and Abaqus^® to solve the mechanical computation. This simulation method shows good agreement with the physical experiments. Finally, the EMF hemming process is characterized by analyzing the influence of main input parameters on the quality output parameters.     

Experimental Observations of Quasi-Static-Dynamic Formability in Biaxially Strained AA5052-O

To establish the efficacy of electromagnetically assisted sheet metal stamping (EMAS), a series of combined hydraulic bulging and electromagnetic forming (EMF) experiments are presented to evaluate the biaxial quasi-static-dynamic formability of an aluminum alloy (AA5052-O) sheet material. Data on formability are plotted in principal strain space and show an enhanced biaxial formability beyond the corresponding experimental results from conventional forming limit diagram. The plastic strains produced by the combined process are a little larger than or at least similar with those obtained in the fully dynamic EMF process. In addition, the biaxial forming limits of aluminum sheets undergoing both very low and high quasi-static prestraining are almost similar in quasi-static-dynamic bulging process. Limit formability seems to depend largely on the high-velocity loading condition as dictated by EMF. It appears that in quasi-static-dynamic forming, quasi-static loading is not of primary importance to the material’s formability. Based on these observations, one may be able to develop forming operations that take advantage of this formability improvement of quasi-static-dynamic deformation. Also, this could enable the use of a quasi-static preform fairly close to the quasi-static material limits for the design of an EMAS process.     

管件电磁胀形极限的实验研究

电磁成形可明显提高铝合金的的成形性,因此在汽车工业中有广泛的应用前景。本文根据电磁胀形特点对管件电磁胀形的成形极限进行实验研究,建立了1060纯铝和3A21铝合金的电磁成形极限线,并且研究了尺寸对3A21铝环的极限成形性能的影响。     

Improvement of the drawability based on the surface friction stir process of AA5052-H32 automotive sheets

Based on the imperative social demand for lighter vehicles, lightweight materials such as aluminum alloys are expected to replace conventional steels in many automotive applications. In automotive parts manufacturing, most of the components produced in conventional stamping operations are geometrically complex as the blanks are subjected to both stretching and drawing deformations. However, aluminum alloys have intrinsic drawbacks, such as the inferior formability of these materials, although the effects of the weight reduction in terms of performance are highly promising. In an effort to improve the formability of aluminum alloy sheets, the surface friction stir process is proposed in this study. This process locally modifies the surface of automotive aluminum alloy sheets via stirring and advancing on the surface of the sheet, similar to the Friction Stir Welding (FSW) process that utilizes a probe without a pin. When the surface of the sheet is modified locally by stirring, dynamic recrystallization due to the severe shear deformation along with heat resulting from the friction occur due to changes in the micro-structure and mechanical properties in the stirred zone, while the dislocation density and grain size refinement are curtailed. In this work, the drawability performance of AA5052-H32 sheets (thickness 1.5 mm) that were welded using the surface friction stir process was experimentally and numerically investigated in cylindrical cup drawing tests. When applied to AA5052-H32 automotive sheets, the surface friction stir process improved the drawability of the entire aluminum alloy sheet. For numerical simulations, the non-quadratic anisotropic yield function Yld2000-2d was employed along with isotropic hardening, while the formability was evaluated by utilizing theoretical forming limit diagrams (FLD) based on Hill's bifurcation and M-K theories.     

Material Formability and Coil Design in Electromagnetic Forming

Pulsed electromagnetic forming is based on high-voltage discharge of capacitors through a coil. An intense transient magnetic field is generated in the coil and through interaction with the metal work-piece; pressure in the form of a magnetic pulse is built up to do the work. Data on formability of two aluminum alloys employed for exterior (6111-T4) and interior (5754) automotive body panels will be shown. Comparison of traditional Forming Limit Diagrams obtained by stretching of aluminum sheet with hemispherical punch to the results on formability, where hemispherical punch is replaced by a coil will be provided. It will be shown that material formability in high-rate forming conditions can significantly depend upon interaction with the forming die: electromagnetic forming into an open round window provides only slight improvement in formability, while forming in a V-shape die or into a conical die indicates a significant improvement. An important part of the electromagnetic forming technology is the design of the coil. The coil failure modes and measures preventing them are discussed. This article was presented at Materials Science & Technology 2006, Innovations in Metal Forming symposium held October 15-19, 2006 in Cincinnati, OH.     

汽车轻量化用铝合金板冲压成形性研究

在实现汽车轻量化的进程中,越来越多的铝合金应用于汽车工业中。在总结汽车用铝合金板特点及其力学性能的基础上,提出了铝合金板在冲压成形过程中的开裂、回弹和表面损伤等问题及相应解决措施,并得出了材料特性、润滑条件和压边力是影响铝合金板冲压成形性能的主要影响因素。     

Sharp flanging and flat hemming of aluminum exterior body panels

Limitations of conventional flanging and hemming technologies require increased radii of flanges and roped hems when aluminum alloys are used for production of closure panels. A new process of flanging has been developed based upon the idea of redistributing plastic strains through the larger area, delivering additional metal into the bending zone, and creating an additional axial compression. Comparison of the newly developed and conventional flanging process indicated that the new process expands the bending ability of aluminum alloy 6111-T4 by allowing an additional 10% of prestrain to the panel compared with previous forming operations. The advantages of the new flanging operation can be transferred to the hemming operation by allowing an additional 10% of prestrain through the whole sequence of forming and assembly operations. Employment of suggested flanging technology makes possible the flat hemming operation of panels stamped from aluminum alloy 6111-T4 if the thickness of the interior panel is 1 mm or more.     

Deep drawing of square cups with magnesium alloy AZ31 sheets

The square cup drawing of magnesium alloy AZ31 (aluminum 3%, zinc 1%) sheets was studied by both the experimental approach and the finite element analysis. The mechanical properties of AZ31 sheets at various forming temperatures were first obtained from the tensile tests and the forming limit tests. The test results indicate that AZ31 sheets exhibit poor formability at room temperature, but the formability could be improved significantly at elevated temperatures up to 200 °C. The test results were then employed in the finite element simulations to investigate the effects of process parameters, such as punch and die corner radii, and forming temperature, on the formability of square cup drawing with AZ31 sheets. In order to validate the finite element analysis, the deep drawing of square cups of AZ31 sheets at elevated temperatures was also performed. The experimental data show a good agreement with the simulation results, and the optimal forming temperature, punch radius and die corner radius were then determined for the square cup drawing of AZ31 sheets.     

Experimental Observations of 5A02 Aluminum Alloy in Electromagnetically Assisted Tube Hydroforming

To establish the efficiency of electromagnetically assisted tube hydroforming, a typical experimental test for hydroforming, i.e., hydrobulging, was carried out on a 5A02 tube blank by using a combined quasi-static axial feeding and pulsed electromagnetic hydrobulging method. Data on the formability of an aluminum alloy 5A02 tube employing this combined loading method is compared with data for traditional quasi-static tests. The results show that the formability of aluminum alloy undergoing a quasi-static–dynamic process is dramatically increased beyond that exhibited in quasi-static or fully dynamic tests. The ultimate expansion ratio of an aluminum alloy tube undergoing a pulsed electromagnetic hydrobulging process is greatly increased beyond that exhibited in quasi-static hydrobulging tests. Both the expansion ratio and the effective strain exhibited in electromagnetically assisted tube hydroforming tests are about four and two times of that in quasi-static and fully dynamic hydrobulging tests, respectively. The forming limits of aluminum samples with both low and high prestrain levels are almost similar in the electromagnetically assisted tube hydroforming process, which makes it possible to stretch the aluminum alloy to a higher quasi-static prestrain level without weakening its total quasi-static–dynamic formability.     

Numerical modeling and deformation analysis for electromagnetically assisted deep drawing of AA5052 sheet

Electromagnetic forming(EMF) is a high-velocity manufacturing technique which uses electromagnetic (Lorentz) body forces to shape sheet metal parts. One of the several advantages of EMF is the considerable ductility increase observed in several metals, with aluminum featuring prominently among them. Electromagnetically assisted sheet metal stamping(EMAS) is an innovative hybrid sheet metal processing technique that combines EMF into traditional stamping. To evaluate the efficiency of this technique, an experimental scheme of EMAS was established according to the conventional stamping of cylindrical parts from aluminum and the formability encountered was discussed. Furthermore, a “multi-step, loose coupling” numerical scheme was proposed to investigate the deformation behaviors based on the ANSYS Multiphysics/LS-DYNA platform through establishing user-defined subroutines. The results show that electromagnetically assisted deep drawing can remarkably improve the formability of aluminum cylindrical parts. The proposed numerical scheme can successfully simulate the related Stamping-EMF process, and the deformation characteristics of sheet metal reflect experimental results. The predicted results are also validated with the profiles of the deformed sheets in experiments.     

Improvements in finite-element simulation for stamping and application to the forming of laser-welded blanks

During stamping-die design, the formability in sheet-metal forming process has been evaluated by the geometrical functions in ‘Die-Face CAD’, which has been developed and improved by Toyota Motor Corporation. When evaluation by these functions is difficult, formability has been estimated by performing experiments using test dies in which the forming defects are similar to those in the actual process.

A numerical method has been developed in order to substitute numerical analysis for experiments using test dies for the accurate prediction of defects in sheet-metal forming. The elastic-plastic FEM with the commercial code ‘JNIKE3D’ has been improved in the areas of: (1) the material constitutive equation; (2) the consideration of the pressure distribution on the blank-holder; and (3) the evaluation of breakage initiation. Using the improved method, the square-cup drawing process and the hemming process have been analyzed. Numerical results for strain, breakage initiation, and hemming deflection were in good agreement with experimental results. The formability of laser-welded blanks and the most efficient process to form them were evaluated also using the improved method.     

铝板弯曲电磁校形实验研究

回弹是弯曲成形的主要缺陷,传统的弯曲工艺消除回弹的效果并不理想.电磁成形是一种高速成形技术,能提高成形性能,改善应力分布,有效地控制回弹.以1060铝板为研究对象,提出一种用于V形件弯曲校正匀压力线圈,以利于提高成形效率,对不同厚度的铝板毛坯进行电磁弯曲校形实验.实验结果表明:随着放电能量的增加,V形件回弹逐渐减小直至消除;坯料越厚,消除回弹所需的能量越大;坯料宽度对工件回弹没有影响;在较低的能量下对工件进行多次放电,随着放电次数的增加,回弹逐渐减小,最终被消除;离折弯线区域越近,工件塑性变形功越大.     

5052铝合金板材磁脉冲动态拉伸塑性失稳分析

为揭示磁脉冲成形的增塑机制,采用理论分析与微观组织观察相结合的方法对5052铝合金板材磁脉冲动态拉伸过程中动态成形行为和塑性失稳机制进行了系统研究.结果表明,惯性力在动态成形中起主要作用,惯性力对试样的结构失稳具有抑制作用,从而使试样的塑性提高并产生分散失稳;5052铝合金动态成形和准静态成形的成形性质相似,不会产生特殊的组织结构,塑性变形机制均为位错滑移机制;准静态成形过程以均匀单系位错滑移为主,断裂伴随着位错的缠结和攀移;而动态成形过程中,位错滑移趋于多系开动,在大面积区域出现明显的交滑移现象,且滑移带较准静态成形时窄且密,位错组态更均匀;动态成形的多系滑移和位错均化作用可在比准静态成形高的多的塑性应变水平下形成,从而使材料表现出较高的塑性和强度.     

Experimental and numerical analysis of multilayered steel sheets upon bending

In this study, the bending formability of multilayered steel sheets is evaluated by tensile tests, V-bending tests, and hemming tests. Enhanced formability was observed in these experiments, namely, the constituent high-strength materials were elongated beyond the original fracture strain limit. As a result of this effect, multilayered steel sheets were successfully formed in V-bending tests and even in hemming tests. Observations using a scanning electron microscope verified that no delamination occurred at interfaces. To represent the geometrical features of a multilayered steel sheet, a solid-element model under an isostrain condition was utilized in finite element modeling, where the rule of mixtures was adopted to obtain the flow curve of the constituent high-strength material, and a good agreement with experimental results was observed. Analyses using this finite element model were conducted to investigate the effect of the geometry on the springback of multilayered steel sheets undergoing V-bending.     

航空铝锂合金热成形研究进展

与传统铝合金相比,铝锂合金拥有更低的密度、更高的比强度、更好的耐腐蚀性,在航空、航天和航海领域得到了广泛应用。铝锂合金的优异性能归因于在铝基体中添加元素锂。但铝锂合金存在常温延伸率低、回弹大和各向异性强等问题,这些问题严重限制了铝锂合金的应用。为解决铝锂合金常温成形性差的问题,国内外学者针对铝锂合金热成形工艺开展了大量研究。本文首先介绍铝锂合金的发展,随后基于基础实验、失稳理论以及损伤理论三个方面介绍铝锂合金热成形研究进展,了解铝锂合金高温宏微观变形机理以及损伤演化规律,从而实现铝锂合金零件在高温条件下成形成性一体化控制;最后对航空铝锂合金热成形的发展趋势进行了展望。本文可为航空铝锂合金材料热成形生产工艺的制定提供一定的理论参考。     

Research on formability of 5052 aluminum alloy sheet in a quasi-static–dynamic tensile process

In order to establish the efficacy of electromagnetically assisted sheet metal stamping (EMAS), the formability of 5052 aluminum alloy sheet in a quasi-static–dynamic tensile process is experimentally investigated using a combined quasi-static tension and the pulsed electromagnetic forming (EMF) method. Data on the formability of aluminum alloy 5052-O employing this combined loading method is compared with data for traditional quasi-static tensile tests. Results show that the formability of aluminum alloy sheet undergoing a quasi-static–dynamic tensile process is dramatically increased beyond that exhibited in quasi-static tensile tests, and a little higher than or at least similar with that obtained in the fully dynamic EMF process. The forming limits of aluminum samples with both low and high pre-strain levels are almost similar in quasi-static–dynamic tensile process, which makes it possible stretching the sheet to a higher quasi-static pre-strain level without weakening its total quasi-static–dynamic formability. This would enable the use of a quasi-static pre-form fairly close to the quasi-static material limits for design of an EMAS process in manufacturing large aluminum alloy shell parts.     

Application of electromagnetic-assisted stamping (EMAS) technique in incremental sheet metal forming

The application of high velocity electromagnetic-assisted stamping (EMAS) technique in incremental sheet metal forming has been proposed. EMAS whose principle is based on Lorenz force is a hybrid forming process that uses both quasi-static conventional stamping technique and electromagnetic forming actuators built into sharp corners and other difficult-to-form contours to form metals. The recent push to use more artificial intelligent (AI) aluminum alloys in automobile and aircraft industries as a result of increasing demand for fuel efficient cars and aircrafts, large size vehicle panels, improved formability limit of materials and weight reduction have placed EMAS as one of the best high velocity forming technique.It is believed that the end result of this vision will lead to more cost effective land and aerospace vehicles.     

Enhancement of formability of aluminum alloys in multi-stage forming operations by a local intermediate heat treatment

Within this paper a new approach to enhance the formability of aluminum alloys in multi-stage forming processes will be presented. The technology??s key idea is the local adaption of the mechanical properties after a first forming step and their optimization for the subsequent forming operation. The partial change of the mechanical properties is obtained by a short term heat treatment between two forming steps. Based on the new property distribution the material flow during the second forming is improved and the formability of the material can be enhanced. The presented work covers all necessary steps for a successful application of the technology. After a material characterization in dependency of the pre-straining and the heat treatment, the heat affected zone, which is a result of the high heat conductivity of aluminum alloys was analyzed. In the end appropriate heat treatment layouts were designed via numerical simulation and the enhancement of formability was demonstrated with a real multi-stage forming process.     

Tailored heat treated accumulative roll bonded aluminum blanks: failure under bending stresses

Ultrafine-grained accumulative roll bonded (ARB) sheet metals of aluminum alloys have a high potential for lightweight construction. The mechanical properties can be enhanced regarding strength and ductility by the combination of ARB and a local heat treatment according to the Tailor Heat Treated Blanks technology. The present investigation focuses on the failure behavior of ultrafine-grained ARB blanks. Due to the low formability of these high-strength ARB metals, a detailed understanding of the failure mechanisms is essential. For this purpose, an established approach to determine the different stages of damage of the material for conventional materials is now applied to multilayered aluminum in the as-received and heat-treated condition. In this context, air bending tests are used to qualify and quantify the bending and forming behavior of ARB sheets of AA1050A and AA6016 aluminum alloys.     

Enhancement of bending formability of brittle sheet metal in multilayer metallic sheets

The formability of multilayer metallic sheets is evaluated by tensile, V-bending, hat bending and hemming tests. A monolithic type-420J2 stainless steel sheet cannot be formed because of poor elongation as small as 1.7%. Marked enhancement of the bending formability was observed in the bending of type-420J2 stainless steel sheets when they are layered by type-304 stainless steel sheets and composed into a multilayer metallic sheet. The mechanism of the enhancement of the formability of type-420J2 stainless steel in a multilayer metallic sheet is investigated analytically by focusing on the delay of the initiation of necking, and by performing stress analysis by finite element method (FEM).