Study on superplastic blow-forming in a rectangular closed-die

The objective of this paper is to explore the plastic deformation behavior of the sheet during blow-forming a superplastic sheet into a rectangular closed-die by the finite element method. A finite element commercial code “DEFORM” is used to carry out the simulations and calculate the pressurization profile and the sheet thickness distribution during the blow-forming process. A pressure control algorithm is proposed to keep the maximum strain rate in the deformation zone of the sheet equal to the target value, which corresponds to the highest m-value of the material being SPFed. The thickness distributions of the formed product using constant pressure control and keeping target strain rate control are compared. Experiments using 8090 Al–Li sheets on superplastic blow-forming in a rectangular closed-die are also carried out. The theoretical predictions of thickness distribution of the product are compared with experimental results.     

On the optimization of superplastic blow-forming processes

The superplastic blow-forming process of thin sheets is analyzed, and an optimal stable deformation path that reduces production time is obtained. The analysis is based on an analytical model for the superplastic forming (SPF) of a long rectangular box made of Ti-6Al-4V alloy at 900 °C use of a microstructure-based constitutive equation for the strain rate and grain growth, a stability criterion, and a variable strain rate control. It is shown that by imposing a variable strain rate control scheme derived from the stability analysis, an optimal forming time can be developed while maintaining a stable deformation path. Some other control schemes also show effectiveness in either reducing the localized thinning in the formed sheet or reducing the required forming time. Effects of friction and initial grain sizes on the forming pressure profile and the thickness distribution of the formed sheet are also investigated.     

Deformation characteristics and cavitation during multiaxial blow forming in superplastic 8090 alloy

This work examined the effect of multiaxial stress on deformation characteristics of a superplastic aluminum alloy 8090 by deforming the sheet into a die with a cylindrical cavity. Several interrupted tests were performed to bulge the sheets to various depths for different strain rates, the formed parts were utilized to evaluate the deformation status, thickness distribution, local strain states, and cavitation. It was found that evolution of cavity volume fraction with forming time could be related to the thinning behavior of the deformed sheet during forming. Decrease in cavity volume fraction at the central region was observed in the later stage of forming as the thickness of the deformed sheet remained constant for all test forming rates.     

细晶AZ31薄板超塑气胀成形有限元分析

为研究细晶AZ31镁合金薄板超塑气胀成形的最佳工艺参数，利用有限元软件对超塑胀形过程进行了有限元数值模拟分析。通过选取不同的应变速率敏感性指数和成形压力，来分析应变速率敏感性指数和成形压力对厚度分布均匀性的影响，同时对成形过程中的应变速率变化进行了模拟分析。模拟结果表明．应变速率敏感性指数和成形压力对厚度分布均匀性影响很大；应变速率分布情况良好，在所要求的应变速率范围之内，说明应变速率敏感性指数，压力等参数的设计合理。     

Superplastic process modeling of plane strain components with complex shapes

Computational process models using membrane element method are developed in this paper for the superplastic forming of plane strain boxes with complex cross-sectional details. Many practical superplastic components manufactured in industry have sloping sidewalk with die bottoms either corrugated and/or at angles to the sides. The new method is used to develop process models for such configurations and the resulting software can be used interactively in a computer. The method is useful to a designer in the parametric study of die geometry, die wall friction, initial thickness, and material property, or to determine if a specific geometry is suitable for superplastic forming. The kinematics of deformation are illustrated, and the numerical results of the model are compared with continuum finite element solutions and also with experimental data.     

Design and experimental validation of a two-stage superplastic forming die

The conventional superplastic forming (SPF) of complex and deep geometries can result in excessive thinning and necking. To address these issues, a two-stage SPF process has been developed and demonstrated in forming trials using a superplastic aluminum sheet alloy. Within a single die, gas pressure is used to form the blank into a preform die cavity prior to the pressure being reversed to form the sheet into the final component cavity. The preforming of the blank creates length of line, while preserving metal thickness in certain regions to improve the thickness profile of the final part. In this work, a preform has been designed to improve the forming of a complex component by providing a superior thickness profile as compared to a conventional single stage forming cycle. Finite element analysis was used to guide the design of the preform cavity since the preforming surface was not intuitive and could cause wrinkling in the final part.     

凸凹模受力模型与有限元分析

通过对板料成形时的受力分析,建立了凸凹模在拉深过程中的受力模型,完成了对凸凹模的有限元数值分析,得到了凸凹模的应力和变形分布情况。对板料进行强度、刚度校核,得到了随板料厚度的变化凸凹模孔口所受的压力、凸凹模的最大应力和位移的变化规律。可为模具结构的设计、模具材料的选择提供依据。     

MODELING OF SUPERPLASTIC FORMING PROCESS FOR ALUMINUM ALLOYS WITH STRAIN HARDENING EFFECT

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变摩擦正反向超塑成形壁厚均匀的TC4钛合金深筒形件

1C4钛合金深筒形件厚度精度要求为(1.6±0.2)mm,据此设计了多种预成形模形状,采用MARC有限元模拟研究不同形状预成形模对最终深筒形件侧壁厚度分布的影响规律及不同预减薄区域的作用.分析预成形模和终成形模的表面摩擦系数分别对成形件壁厚分布的影响,并提出模具型面变摩擦控制厚度分布的方法.结果表明:预成形模对于板料压边部分环形带区域和筒形件底部区域的局部预减薄,对最终侧壁的厚度分布有非常大的改善.同时,合理地增大预成形模的表面摩擦能显著增加预成形的局部减薄作用,对于提高工件最终壁厚分布的均匀性有利.减小终成形模的摩擦,可以使板料趋于整体变形,壁厚分布趋于均匀.根据模拟结果,采用机械加工方法增加预成形模表面摩擦系数,在终成形模表面喷涂BN润滑剂降低摩擦系数.最后,通过正反向超塑成形试验制得厚度分布在1.50～1.78mm范围内的TC4钛合金深筒形件.     

Fracture types and thickness distribution in superplastic sheets formed with plastic injection molding

This paper investigates the fracture types and thickness ratio distribution in superplastic Zn/Al sheets formed during a hybrid process combining superplastic sheet forming with plastic injection molding. Three types of sheet fractures, edge cracking, central cracking and combined cracking, are observed. The success of forming superplastic sheets with rib features using injection molding with various parameters, including melt temperature, injection pressure and mold temperature, are investigated. They are presented in the form of a molding area with areas of various fracture types. Central cracks occur when superplastic sheets are formed with injection molding with higher melt temperature, while edge cracks occur with higher injection pressure. When the melt flow is parallel to the sheet rolling direction, the areas of edge crack are enlarged. The sheet thickness ratio distribution is obtained for various injection parameters and rib depths. Observation of the sheet thickness distribution for variation parameters, the tendency of the sheet fracture type can be generalized.     

Loading path optimization of tube hydroforming process

Optimization methods along with finite element simulations were utilized to determine the optimum loading paths for closed-die and T-joint tube hydroforming processes. The objective was to produce a part with minimum thickness variation while keeping the maximum effective stress below the material ultimate stress during the forming process. In the closed-die hydroforming, the intent was also to conform the tube to the die shape whereas in the T-joint design, maximum T-branch height was sought. It is shown that utilization of optimized loading paths yields a better conformance of the part to the die shape or leads to a higher bulge height. Finite element simulations also revealed that, in an optimized loading path, the majority of the axial feed needs to be provided after the tube material yields under the applied internal pressure. These results were validated by conducting experiments on aluminum tubes where a good correlation between the experimental results and simulations were obtained.     

On the evaluation of superplastic characteristics using the finite element method

In recent years, there has been a considerable interest in the application of superplastic forming in the aircraft and automotive industries to realize complex parts. This requires a detailed design of the technological process in order to better exploit its peculiar potentialities. Nowadays, the finite element method represents the mainly used technique to plan the sheet metal forming processes whose simulation requires determination of material constants for superplastic materials. The aim of the present work is aimed to show a simple method to characterize superplastic materials. In this study, constant pressure free bulging for superplastic materials is analysed by finite element method and expressions to determine the constants of the superplastic materials have been proposed. In order to support the proposed method, numerical and experimental activities have been carried out which show the good reliability of values obtained from proposed expressions.     

铝合金圆锥形零件粘性介质温成形研究

采用热力耦合有限元数值模拟方法对铝合金圆锥形零件粘性介质温成形过程进行了模拟分析,研究了成形过程粘性介质和板材的温度分布、不同温度条件下成形零件壁厚分布、成形载荷等.结果表明,圆锥形零件的底部圆角区域为成形危险区域.非等温粘性介质温成形过程中,在粘性介质内部形成的非均匀温度场影响了板材的温度分布.当粘性介质温度略低于板材温度时,坯料中心区域温度较低,有利于延迟底部圆角成形时的破裂,提高了零件壁厚的均匀性.分别进行了室温和加热时铝合金圆锥形零件粘性介质压力成形试验,试验结果与数值模拟具有相同的规律.     

切角板坯对纯铜薄板矩形盒拉深影响的试验及数值分析

在大量纯铜薄板矩形盒拉深试验的基础上,结合有限元计算,分析了矩形盒的拉深特性。指出,矩形切角板坯使法兰曲边变形分布得以改善,非拉深变形抵抗弱化,提高了拉深成形性。有限元模拟结果显示矩形板坯拉深断裂点产生在凸模肩部转角附近,且始终承受两向不等拉伸,应变计算值与实际拉深测定值相符;切角板坯的拉深断裂点则转移至接近凹模口的侧壁处,且始终处于压剪组合变形状态,断裂时板厚应变相当小,认为倾向于剪切断裂。切角板坯断裂点的拉、压应力组合效应使该点板厚几乎不变,是提高拉深极限的重要因素之一。     

Modeling of Transformation Superplastic Forming of Ti Alloys

Transformation superplastic forming is an attractive alternative forming technique to microstructural superplastic forming, since it requires no special microstructures and, therefore, eliminates the limitation of superplastic forming capability to only expensive materials with stable high-temperature fine grains. Transformation superplasticity occurs through biasing the internal stress produced from an allotropic phase transformation by a small external stress. In this work, finite element modeling was implemented to study the transformation superplastic forming of domes from flat circular thin plate samples. The evolution and distribution of stress, strain, and dome thickness was analyzed in detail. The thickness distributions in the formed domes were compared with the theoretical predictions of two models, which assume different stress states in the domes. The appropriate stress state was identified through this comparison. Different gas pressure amplitudes were applied during forming to investigate the effect on the formed-dome apex height, when the forming time was fixed. This article was presented at Materials Science & Technology 2007, Automotive and Ground Vehicles symposium held September 16-20, 2007, in Detroit, MI.     

Failure analysis in superplastic materials

Superplastic alloys and metals possess the ability to undergo large uniform strains prior to failure. Isothermal superplasticity of sheet metal is a phenomenon due to both peculiar process condition and material intrinsic characteristics. The material must have a grain size of less than 10 μm, the forming temperature of around half the absolute melting point and a very low strain rate (in the order of 10⁻⁵–10⁻³ s⁻¹).The instability of superplastic flow under uniaxial stress state has been the subject of different studies. In this paper, under biaxial stress conditions, instability analysis of superplastic PbSn60 alloy using the finite element method is investigated. An original model has been implemented successfully in commercial finite element code in order to predict the imminent failure of material during superplastic forming processes.     

SPF model applications for aluminum

A closed-form, plane strain model (PS2) is compared with a finite element model (SUPFORM3) in analyzing the forming of rectangular pans from 2090-OE16 aluminum sheet. The errors introduced by improperly using a plane strain model are quantified. Some preliminary verification of SUPFORM3 is shown, and the effect of friction on the distribution of thickness in the superplastic forming (SPF) of a rectangular pan is presented. SUPFORM3 is used to predict thickness distribution of male formed access doors, selecting proper starting gauges, assessing producibility, estimating forming time, and developing pressure time cycles. SUPFORM3 will be used to form complex parts and flat-bottomed pans such that post-SPF properties samples can be cut from the flat areas of the flat-bottomed pans, yet have the same strain and strain rate history as the critical areas in the complex part. It is concluded that SUPFORM3 is a useful tool for predicting thickness distributions resulting from and pressurization schedules for superplastic forming.     

工业铝合金汽车覆盖件的超塑成形研究

采用工业铝合金 5 1 82就一款汽车上的前挡泥板零件进行了超塑成形的试验研究。根据零件的形状特点确定使用超塑气压胀形工艺 ,并进行了模具型腔曲面设计和模具结构设计。应用数值模拟的方法对型腔曲面设计进行了优化 ,使预测出成形零件所需的变形量处在材料的变形能力之内。最终的成形试验结果表明 ,成形工艺和模具设计合理可行。     

Superplastic Forming of Triangular Channels with Sharp Radii

The superplastic forming process is used to form light weight components with complex features in one manufacturing step. However, the non-uniformity of the produced part thickness and the possibility of severe thinning are among its major disadvantages. The goal of this parametric study was to investigate feasible geometries for triangular channels to be manufactured by superplastic forming. The channels considered had sharp radii and served as secondary features extending along a circular path at the base of a shallow cup. An axisymmetric finite element model in ABAQUS? was used to simulate the forming process. Effects of the aspect ratios of both the cup and the triangular channel on the thickness distribution and the pressure profiles were investigated. An experimental setup was used for validating the simulation results for AA5083 at 500 °C.     

Two-Stage Superplastic Forming of a V-Shaped Aluminum Sheet into a Trough with Deep and Irregular Contour

A sheet metal trough of aluminum alloy is manufactured by a two-stage gas forming process at 500 °C. The product with sloped side walls is of ~1.2 m length and ~260 mm opening width, comprising two near-conical-shaped sinks at two ends. The depth of one sink apex is ~350 mm, which results in the depth/width ratio reaching 1.4. To form such a deep and irregular trough, a superplastic aluminum alloy 5083 initially bent into a V-shaped groove was prepared prior to the gas forming work. Within a single die, gas pressure is used to form the V-shaped blank into a preform die cavity prior to the pressure being reversed to form the sheet into the final component cavity. The preforming of the V blank creates a uniform length of line in order to improve the thickness profile of the final part. In this work, a preform has been designed to improve the forming of a complex component by providing a superior thickness profile as compared to a conventional single-stage forming cycle. However, a serious wrinkling situation was encountered, the cause of which has been analyzed using basic mechanics as well as numerical simulation.