A numerical analysis of sheet metal formability for automotive stamping applications

A theoretical failure model is presented for the numerical prediction of the forming limit strains of automotive sheets. The model uses the Swift’s diffuse necking and Hill’s localized necking concepts in describing tearing-type sheet metal failures and a computational scheme is proposed in which the failure conditions are expressed in incremental forms. The Bauschinger effect is included properly in the deformation modeling using an additive backstress form of the nonlinear-kinematic hardening rule. The necking conditions and plasticity model are transformed into a set of algebraic equations that may be applied both for proportional and non-proportional strain-controlled loadings. An iterative approach is employed in the incremental solution of algebraic equations. The formability analyses are conducted using the proposed theoretical model and the forming limit strains of two new generation auto sheets (Trip600 1.4 mm, DP980 1.15 mm) are estimated. The numerically generated FLC are compared with the experimental data and the FLC calculated with the Keeler–Brazier equation. For both steels, the model produced conservative plain–strain intercept values, FLC₀, when compared with the predictions of Keeler–Brazier equation. Also the negative minor strain part of the experimental FLD’s is estimated with sufficient accuracy. For the positive minor strain side, however, the predictions are lower than both the experimental fit and the standard curve.     

Application of a feasible formability diagram for the effective design in stamping processes of automotive panels

The objective of this study is to propose a method of process design that uses a feasible formability diagram, which denotes the safe region without fracture and wrinkle, for the effective and rapid design of stamping processes. To determine the feasible formability diagram, FE-analyses have been performed for combinations of process variables that correspond to the orthogonal array of design of experiments. Subsequently, the characteristic values for fracture and wrinkle have been estimated from the results of FE-analyses on the basis of the forming limit diagram. The characteristic values for all combinations within a whole range of process variables have been predicted through the training of an artificial neural network. The feasible formability diagram has been finally determined for all combinations of process variables. The stamping processes of automotive panels to support suspension module, such as the turret suspension and the wheel house, have been taken as examples to verify the effectiveness of process design through feasible formability diagram. A comparison of the FE-simulation results with the experimental results reveals that the design of stamping processes through feasible formability diagram is efficient and suitable for actual processes.     

汽车用铝合金板拉伸性能与成形能力的相关性

为了找到适合于评估汽车用铝合金板成形性能的参数,本文在模拟试验获得的样本数据基础上,采用回归分析方法,分析了汽车用铝合金板单向拉伸性能参数与其拉深性能、胀形性能之间的对应关系.研究表明,在单向拉伸性能参数中,硬化指数n对汽车用铝合金的拉深极限凸模行程的相关性最为显著;而总延伸率δ对胀形极限凸模行程的相关性最为显著.上述结果表明,n值和δ值分别是评估汽车用铝合金板拉深性能和胀形性能的最佳参数.     

Modified texture and room temperature formability of magnesium alloy sheet by Li addition

The effect of Li addition on the mechanical responses of magnesium alloy sheets was examined in correlation with the concurrent microstructure and texture evolution. The basal texture was drastically weakened and the basal pole rotated to the transverse direction (TD) owing to the Li addition. The Mg alloy sheet containing lithium element exhibited larger uniform elongation and its Erichsen value (IE) increased by?~?59 %. Enhanced the ambient formability of the extruded Mg alloy sheet fabricated by Li addition was accomplished by the tilted weak basal texture. The microstructure evolution and properties were characterized and compared.     

Microstructure and press formability of a cross-rolled magnesium alloy sheet

Cross-rolling, in which the roll axis is tilted by 7.5° towards the TD-direction, was carried out on a commercial magnesium alloy. The (0002) texture intensity of the cross-rolled specimen was lower than that of the unidirectionally rolled specimen, and the (0002) texture of the cross-rolled specimen was inclined about 10° towards the TD-direction. Also, the grain size of the cross-rolled specimen was smaller than that of the unidirectionally rolled specimen. As a result of the Erichsen tests at 433-493 K, the press formability of the cross-rolled specimen was higher than that of the unidirectionally rolled specimen. The high formability of the cross-rolled specimen is attributed to both the modification of (0002) texture and the enhancement of grain refinement by the cross-rolling.     

Measurement of texture and formability parameters with a fully automated,ultrasonic instrument

A fully automatic, ultrasonic instrument to measure texture and formability parameters on metal sheet is described. Arrays of EMAT transducers are used to transmit and receiveS _o Lamb waves propagating at 0°, 45°, and 90° with respect to the rolling direction. By analyzing the frequency dependence of the phase of the received signals, the long wavelength limit of the velocities is obtained. Included is a discussion of this algorithm, and subsequent processing steps to predict the ODC'sW ₄₀₀,W ₄₂₀, andW ₄₄₀. On steel, the prediction of drawability parametersr and r based on a correlation developed previously by Mould and Johnson is also discussed. Results of blind field trials at facilities of three suppliers/users of steel sheet for automotive applications and one supplier of aluminum sheet for beverage can production are reported. The former confirmed the Mould-Johnson correlation for lowr material but indicated that refinements are needed for modern steels with highr. The aluminum data suggest a correlation between W₄₄₀ and the degree of four-fold earing.     

Design of texture for improved formability of high-strength steel

The effect of crystallographic textures on the formability of BCC steel sheets has been studied by using crystalline plasticity finite element analysis (FEA) and experiments. It was confirmed that one of the important reasons why the conventional high-strength steel sheet has poor formability was due to lack of {111} fiber texture components —γ-fiber texture—. In this paper, a texture adjusted design method is proposed to improve the formability of conventional high-strength steel sheets. First, an artificial γ-fiber texture is defined in terms of a rotationally symmetric Gaussian distribution of deviation angles, which has a certain scatter width along the given γ-fiber skeleton line in Euler space. The analytic textures are designed by introducing the artificial γ-fiber texture into the conventional high-strength steel model. The blending coefficient corresponding to the {111}/{001} volume fraction ratio is selected as the design parameter. Then, an optimum crystallographic texture of steel sheet is found through the limit dome height (LDH) formability tests by employing as objective function, which is evaluated by a maximum thinning ratio of the deformed sheet. Further, it is demonstrated that the sheet with the optimum texture shows the best straining in VDI benchmark stamping tests.     

The stamping behavior of an early-aged 6061 aluminum alloy

The stamping behavior of 6061 aluminum alloy with various conditions of early-aging is investigated in the present study. The relationship between the stamping performance, microstructure and mechanical property for this alloy is also discussed. Experimental results show that the 6061 aluminum alloy with a 10–30 min. early-aging at 160 °C will exhibit excellent stamping performance. The burnished surface of these treated alloys can reach a quite high value of 47%. Meanwhile, the mechanical strength and impact toughness have important effects on the stamping behavior of 6061 aluminum alloy. The moderate values of mechanical strength and toughness will exhibit an optimal stamping performance.     

Post-welding formability of AZ31 magnesium alloy

The plastic flow behaviour and formability of friction stir welded AZ31 magnesium alloys were widely investigated. Flow curves were obtained in extended ranges of temperature (250–350 °C) and strain rate (0.001–0.1 s⁻¹) by means of uniaxial tensile tests; furthermore, forming limit curves were determined using the hemispherical punch method in the same range of temperature but with a constant crosshead speed of 0.1 mm/s. The results were compared with those obtained, under the same experimental conditions, on the base material. The flow stress levels of joint and base material are very similar up the peak of the flow curve although the equivalent strains at the peak and to failure are usually lower than those of base material. However, at the highest temperature and lowest strain rate investigated (350 °C and 0.001 s⁻¹), the flow behaviour of the welded joint tends to be similar to the one of the base alloy. Finally, formability of the friction stir welded material, evaluated in terms of forming limit curves, is usually lower than the one of the base material.     

Effect of annealing on formability of aluminium grade 19000

Forming limit diagrams are used by the stampers to solve sheet metal forming problems. In practice, sheet metals have been subjected to various combinations of strain. Necking during sheet metal forming, sets the limit to which the sheet metal can be formed. Forming limit diagram is an effective tool to evaluate the formability of sheet metal in various strain conditions. The information upon the formability of the sheet metal is important for both sheet metal manufacturers and users. In this work, a study has been made on the formability of aluminium 19000 grades annealed at three different temperatures namely 160 °C, 200 °C and 300 °C for sheet thickness of 2.00 mm. The tensile properties and formability parameters were experimentally evaluated and they are related to forming limit diagram. Strain distribution profiles obtained from the forming experiment have been analyzed. The fractured surface of the formed samples were viewed using scanning electron microscope (SEM) and the SEM images were correlated with fracture behaviour and formability of sheet metal. The sheet which is annealed at 300 °C has been found to possess good drawability and stretchability compared to other two annealed sheets.     

基于广义成形理论的冲压翘曲缺陷的消除方法

针对覆盖件冲压成形中的翘曲缺陷,阐述了翘曲产生的原因,在有限元环境下,从调节金属流动的角度出发,基于广义成形理论,提出一种通过调整金属流动使得应变分布均匀或对称,从而消除翘曲缺陷的方法,并编写针对性的有限元分析软件模块SFT Shape Change,通过算例验证了该方法的有效性。     

Explaining texture weakening and improved formability in magnesium rare earth alloys

Wrought magnesium alloys are rarely used due to their poor formability which is caused by strong textures created during processing. Addition of rare earth (RE) elements including Y, Ce, La, Gd and Nd weakens these strong basal textures and significantly improves formability. Developing a mechanistic understanding of this effect is critical in leading alloy design towards a new class of highly formable magnesium alloys. This fall in texture intensity occurs during recrystallisation and only requires very low solute RE additions, 0·01 at.-% in the magnesium–Ce case. These additions retard dynamic recrystallisation and increase non-basal slip; however, a full understanding of the RE effect has yet to be obtained, with a variety of mechanisms proposed. Recent research in these areas is critically reviewed.     

Challenges in the formability of the next generation of automotive steel sheets

Improving strength may lightweight car bodies by thinning the structure gauge while bearing the same crash loads. Development of the third generation advanced high-strength steels aiming to boost material ductility to over 30% total elongation has become the common sense of potential solutions for stamping. Not only total elongation but also several other parameters, involving presswork hardening, anisotropy ratio, strain rate sensitivity, uniform elongation, edge sensitivity and internal stress between phases, are critical in determining the formability under the complex stress and strain distributions during stamping. We clarify here the logic of formability and difficulties in this context of three types of steel: quenching and partitioning, δ-transformation-induced plasticity and medium Mn-TRIP.     

Effect of stamping deformation on microstructure and properties evolution of an Al–Mg–Si–Cu alloy for automotive panels

The effect of 5 % tensile deformation, which simulates the stamping process of Al–Mg–Si–Cu automotive outer panels, on the microstructural evolution during age strengthening, has been investigated. In addition, its benefit on key mechanical properties including hardness, yield strength, ductility, and corrosion resistance has been linked to the microstructural features. It was found that the aging precipitation sequence, SSSS → clusters and G.P. zones → β″ → β′ + Q′ → Q, was not influenced by the dislocations introduced through the stamping deformation prior to aging. On the other hand, stamping deformation could promote the formation of precipitates and refine the precipitates because of the enhanced heterogeneous nucleation and the accelerated precipitation kinetics, leading to superior strength of the alloy at the early stage. Meanwhile, the larger amount of Cu incorporated into nanoprecipitates leads to better intergranular corrosion resistance of the stamped alloy compared with the unstamped one. Due to the reduction in free Si amount at grain boundaries, the formation of fine subgrain structures and the increase of dislocation accumulation, the ductility of the stamped alloy was increased.     

Investigation of a surface defect and its elimination in automotive grade galvannealed steels

The galvannealed coating have gained importance as a potential solution for auto body panels owing to their superior performance like weldability, paintability and after painting corrosion resistance. Efficient production of galvannealed strip through hot dip galvanising and subsequent annealing root is a challenge as occurrence of surface defects results in material downgrading and rejection. The present work deals with one type of surface defect present at the bottom surface of the galvannealed strip with high defect severity. The defect appearance was like a shining spot by naked eye. It was characterised using Optical Microscope, Scanning Electron Microscope coupled with Energy Dispersive Spectroscopy to analyse shape, size, morphology and composition. The defect was further analysed by Confocal Laser Scanning Microscope (CLSM). The features of CLSM like high resolution, high depth of focus, 3D surface topography were useful in determining the relative depth of the defect at different regions. The results indicated that the defect occurrence was due to mechanical abrasion of coating with deposited dust of Fe–Zn intermetallic phases at the galvannealing tower top roll. The production line parameters were modified to maintain the temperature and cleanliness of the top roll and a reduction in defect severity was achieved.     

Evolution of rotated Brass texture by cross rolling: implications on formability

In the present work, the possibilities of tailoring crystallographic texture via cross rolling are presented. It is shown that a strong rotated Brass texture develops upon cross rolling in aluminium alloys which also remains intact during the subsequent recrystallisation annealing treatment. The governing mechanisms behind the evolution of deformation and recrystallisation texture are discussed in terms of effect of strain path on stability of deformation texture components and strain-induced boundary migration mechanism, respectively. In addition, the likelihood of rotated Brass texture having a positive effect on formability is discussed in terms of sluggish cross-slip criteria as the rotated Brass texture presents a unique scenario where cross-slip is inhibited along all the three principal directions.     

600 MPa级元宝梁用钢冲压开裂原因分析

为解决元宝梁冲压开裂问题,用OM和SEM等仪器与ABAQUS有限元仿真软件对600 MPa级元宝梁用钢冲压开裂原因进行了分析.结果表明:600 MPa级元宝梁用钢化学成分、力学性能与冷弯性能均满足标准要求,金相组织为准多边形铁素体与少量珠光体的混合组织,不存在带状组织;开裂件断口形貌呈现出明显鱼骨状形貌,钢板中心分层,两侧分布起源于中心分层并沿同一方向扩展的三角形台阶状断口,不同形貌断口均属于韧性断裂,而断口整体形貌显示材料在断裂前没有发生缩颈过程.利用ABAQUS软件对"先冲压再压边工艺"与"先压边再冲压工艺"两种工况进行有限元分析,结果显示:先冲压再压边工艺边部应力集中更严重,边部节点应力超过了材料抗拉强度从而导致撕裂现象的发生;由"先冲压再压边工艺"改为"先压边再冲压工艺"后元宝梁冲压开裂率由50%降至0,材料边部变形均匀,未见缩颈与开裂现象发生.     

三种强度级别的双相钢成形性能研究

对三种不同强度级别、不同厚度的双相钢进行了系统的成形性试验,综合分析了双向钢的力学性能、杯突性能、冷弯性能、成形极限。研究表明:双向钢的延伸率A及加工硬化指数n值随强度级别上升而下降;杯突试验结果表明,杯突的高度随双相钢强度级别上升而下降,随材料厚度升高而升高;双相钢冷弯性能随材料强度级别上升而下降;双轴应变下的胀形试验表明双向钢具有良好的成形性;基于软钢的FLD0(是指成形极限曲线FLC与纵坐标e1的交点的纵坐标值,它是FLD(成形极限图)上的重要的特征点)经验公式并不能直接套用在双向钢上。     

The influence of texture on the mechanical properties of a superplastic magnesium alloy

Superplastic (SP) characteristics of a MA15 magnesium alloy subjected to tensile deformation in both textureless and textured states with identical fine grain structures have been studied. The existence of crystallographic texture is found to be the cause of the anisotropy of flow stress and plasticity. The shift of SP optimum strain rate towards higher values due to the strengthening of texture is observed. A special experiment has indicated that the difference of the alloy behaviour in textureless and textured states results from the operation of grain boundary sliding stimulated by the intragrain dislocation slip.