Aluminum Alloy Sheet Metal Research Articles

Fast-cooling of aluminium alloy sheet metal for hot-stamping applications

The paper reports an investigation into two rapid sheet metal cooling designs that are suitable for applications in hot-stamping production -air cooling and contact cooling, aiming at improving formability of the high-strength aluminium sheets for the forming of components with complex geometries with enhanced overall process efficiency. The investigation involved both numerical modelling and experimental trials using an in-house developed prototype cooling tools. A series of contact cooling experiments were conducted, considering process variables such as initial blank temperatures, cooling die temperatures, die materials, surface conditions, etc. The results demonstrated that the cooling rate of the aluminium blank could be accurately controlled by both methods with appropriate pre-setting of the equipment and control of the process parameters. The integration of the cooling system into a polit hot-stamping production was also tried, considering industry-scale applications related factors. The results can serve as a design guidance for an industry to introduce such cooling methods and manufacturing facilities.

Identification of thermo-viscoplastic behavior for AA6061 under in-plane biaxial loadings

This work aims to investigate the thermo-visco-plastic behavior of an aluminum alloy (AA6061) sheet metal submitted to in-plane biaxial loadings under warm conditions. Biaxial tensile tests are performed in a temperature range from room temperature to 160 °C, and in a strain rate range from quasi-static to the so-called “intermediate” strain rate (up to few s−1). The specimen shape used in this study has been previously defined and validated by the authors to identify the viscoplastic hardening models of metallic alloys at large strains. A specific device leading to a uniform temperature in the sample is associated with a dynamic biaxial traction bench to carry out the temperature and strain rate dependent characterizations. From these experiments, both the experimental forces measured on each axis of the in-plane biaxial specimen and the principal strains at the center of the specimen are obtained. These experimental data are then used in an inverse analysis loop, based on a finite element model of the biaxial test, to calibrate the parameters of a thermo-viscoplastic strain hardening model by minimizing the difference between the experimental and numerical principal strains at the center point of the specimen. Finally, it is shown that the identified hardening model well predicts the flow stress of AA6061 under different temperatures and strain rates for a strain level of up to 40%.

Prediction and Verification of Forming Limit Diagrams Based on a Modified Shear Failure Criterion

The forming limit diagram plays an important role in predicting the forming limit of sheet metals. Previous studies have shown that, the method to construct the forming limit diagram based on instability theory of the original shear failure criterion is effective and simple. The original shear instability criterion can accurately predict the left area of the forming limit diagram but not the right area. In this study, in order to improve the accuracy of the original shear failure criterion, a modified shear failure criterion was proposed based on in-depth analysis of the original shear failure criterion. The detailed improvement strategies of the shear failure criterion and the complete calculation process are given. Based on the modified shear failure criterion and different constitutive equations, the theoretical forming limit of TRIP780 steel and 5754O aluminum alloy sheet metals are calculated. By comparing the theoretical and experimental results, it is shown that proposed modified shear failure criterion can predict the right area of forming limit more reasonably than the original shear failure criterion. The effect of the pre-strain and constitutive equation on the forming limits are also analyzed in depth. The modified shear failure criterion proposed in this study provides an alternative and reliable method to predict forming limit of sheet metals.

Study on formability and microstructure evolution of hot deep drawing manufactured 7005 aluminum alloy sheet metal

The hot deep drawing process with an in-die heating system was proposed on the forming of 7005 aluminum alloy to improve the formability due to the limitation of ductility during severe plastic deformation at room temperature. The feasibility of hot deep drawing AA7005 sheet (only 0.8 mm) was investigated and evaluated by Swift cup tests from RT to 450 °C. The priority of main process parameters, namely temperature, blank holder force, and stamping speed, on sheet metal forming limit were discussed by analysis of range and variance. Experimental results indicated that elevating the temperature could fundamentally improve the deep drawing performance of the alloy, which is manifested by the revolutionary change in sheet metal behavior — from the failure of deformation at lower temperatures (RT-200 °C) to achieving a limiting drawing ratio of 2.19 at 450 °C. The optimal process parameters under HDD conditions are given as drawing temperature of 400 °C∼450 °C, BHF of 3 kN, and punch speed of 15–20 %. The thickness variation increases with rising temperature corresponding to the formability enhancement during the process within acceptable limits. DRV and DRX are the flow softening mechanism under the high temperature and large strain in HDD process obtained from the analysis of microstructure and texture evolution. The elongation and rotation of grains further substantiated the interrelationship between drawing performance and microstructure evolution. Improving the deep drawing performance through HDD process can provide practical guidance for the application of AA7005 sheet metal.

Prediction of forming limit curve for AA6061-T6 at room and elevated temperatures

Forming Limit Curve (FLC) has been widely adopted as a practical criterion for evaluating the formability of sheet metals. Predicting a reliable FLC by a virtual methodology could lead to robust process optimization before expensive tool manufacturing. In order to increase the predictive capabilities of the virtual forming tools, an accurate modeling of the forming limit curve should be considered at room and elevated temperatures. In this work, the isothermal forming limit curves of 6000 series aluminum alloy sheet metal are predicted by performing numerical simulations of Nakajima test. A stress triaxiality and Lode angle based ductile fracture criterion is used to determine the forming limit curve. Also, the ductile fracture criterion is extended to add the impact of temperature on ductile fracture prediction. The hybrid experimental-numerical approach is used to calibrate the ductile fracture criterion. The forming limit curve of AA6061-T6 sheet metal, with a thickness of 1 mm, is predicted using the calibrated ductile fracture criterion at room and elevated temperatures. Numerical simulations are performed in 3D with the finite element code Abaqus. The limit strains are determined for specimens undergoing deformation under different strain paths. Influence of temperature on the predicted forming limit curve is discussed.

Experimental and numerical investigation of ductile fracture for AA6061-T6 sheets at room and elevated temperatures

Warm forming is largely employed to enhance the poor formability of aluminum alloys at room temperature. The knowledge of the effect of stress state and temperature on the forming limits is an important aspect for the control of warm forming operations. Therefore, the objective of this work is to investigate the influence of temperature on the ductile fracture of AA6061-T6 aluminum alloy sheet metal under different stress states, ranging from shear to biaxial tension. For this purpose, experiments and numerical simulations of uniaxial tensile tests on dog-bone shaped specimen, notched specimens with different radius, specimens with a central hole and shear specimens are conducted at room and elevated temperatures. The hybrid experimental–numerical approach is used to identify the fracture strain and the corresponding stress state parameters (i.e. stress triaxiality and Lode parameter) to derive the fracture loci at room temperature, 150∘C and 200∘C. To accurately model the material behavior, the positive strain rate sensitivity in the flow stress response at elevated temperature is considered and attention is paid to constrain the finite element model by using the real experimental boundary conditions. The strain rate dependence of fracture strain is not considered in this work. A strong dependency of the ductile fracture on the temperature and stress state is evidenced. Also, an extension of Lou’s ductile fracture criterion which includes the impact of temperature on ductile fracture prediction is proposed. The prediction accuracy of the original and extended criteria is evaluated. It is found that Lou’s ductile fracture criterion is well suited to predict accurately ductile fracture at elevated temperatures under isothermal conditions whereas the extended form is very useful to predict ductile fracture initiation under non-isothermal conditions.

Numerical and experimental verification of an iterative coupling method for analyzing the Lorentz-force-driven sheet metal stamping process

The Lorentz-force-driven stamping process is a newly developed forming process, which combines the advantages of the quasi-static stamping and electromagnetic forming to a certain extent. It involves complex physical problems such as strong electro-magneto-mechanical interactions and considerable deformation. Therefore, it is still a challenge to accurately predict the electromagnetic parameters (coil current and Lorentz force, etc.) and workpiece deformation (forming height and material flow) during the stamping process for different forming conditions. To solve this problem, taking the cylindrical cup forming of 5052-O aluminum alloy sheet metal as an example, a numerical method based on the iterative calculation of a circuit-electromagnetic model and an electromagnetic-mechanical model is developed and systematically validated. Numerical tests show that, compared with the non-iterative model, the proposed iterative mode can significantly improve the simulation accuracy for different forming conditions (such as varied discharge voltages and initial gaps between the coil and the punch). It has been confirmed that the iterative procedure can well consider the effect of the increased distance between the coil and the punch on the calculations of electromagnetic and mechanical parameters. Furthermore, numerical and experimental investigations for a Lorentz-force-driven stamping system using a discharge circuit with an additional crowbar branch are carried out, showing the simulation data of discharge current and workpiece deformation agree well with that of experiment measurement. This confirms the applicability of the proposed method to different discharge circuits.

Influence of temperature on the formability of an aluminum alloy

Warm forming is widely used as increasing the temperature is a solution to improve the formability of aluminum alloys. The stress (or strain) state is one of the most important factors affecting the formability of metals. In warm forming, the temperature and strain rate also play an important role on the deformation and fracture behavior. Figuring out the relationship between formability, temperature, strain rate and stress state is of great importance for providing more understanding of ductile fracture in warm forming conditions. Therefore, the objective of this work is to investigate the influence of temperature on the ductile fracture of a 6000 series aluminum alloy sheet metal under different stress states. Dogbone specimens, notched tensile specimens with different radius, tensile specimens with a central hole and shear specimens are used to cover a wide range of stress states. The hybrid experimental-numerical approach is used to identify the fracture strain and the corresponding stress state parameters (i.e. stress triaxiality and Lode parameter). To this end, fracture tests are carried out at 200°C using a tensile machine to determine the instant of fracture. Numerical simulations of the tensile tests are performed in 3D with the finite element code Abaqus to predict the strain field and calculate the evolution of the stress state. To accurately model the material behavior the positive strain rate sensitivity in the flow stress response at elevated temperature is considered. The results show a strong dependency of the ductile fracture on the temperature, strain rate and stress state.

Optimization of Hot Rolling Parameters of Aluminum Alloy Sheet Metal based on Multiscale Crystal Plasticity Finite Element Method with Dynamic Recrystallization

Optimization of Hot Rolling Parameters of Aluminum Alloy Sheet Metal based on Multiscale Crystal Plasticity Finite Element Method with Dynamic Recrystallization

Electromagnetic forming of aluminum alloy sheet metal utilizing a low-frequency discharge: A new method for attractive forming

In existing electromagnetic sheet metal forming processes, the sheet metal is typically deformed away from the coil under repulsive Lorentz forces. This deformation mode makes it difficult to apply the forming technology to dent repair without disassembly of the automobile body and aircraft fuselage. To solve this problem, a new electromagnetic attractive forming process with a low-frequency discharge current was developed in this work. Different from previous attractive forming methods, only a single coil and a single power supply are required that is low cost and easy to implement. Both numerical and experimental studies have demonstrated that an AA1060-H24 aluminum alloy sheet with a thickness of 1 mm can be effectively attracted towards the coil with a maximum deformation of more than 8 mm after a single discharge. More importantly, the mechanism of the attractive forming process has been clarified in detail through the analysis of the Lorentz force characteristics and deformation process of sheet metal. It has been pointed out that the current waveform after the first peak plays a vital role in improving the attractive force and the attractive forming process, which can be optimized by introducing a crowbar circuit. Meanwhile, it is found that the deformed shape of sheet metal caused by a repulsive force helps to facilitate the subsequent reverse deformation under an attractive force. This is also an important reason why an obvious attractive forming can be achieved, although the attractive force is much smaller than the repulsive force during the forming process. The obtained results have the significance for fundamental understanding of electromagnetic forming process, and further applications can be expected using the developed forming method.

Experimental and Simulation Investigation on the Ultrasonic-Assisted Bending of Annealed and Aged AA6061 Aluminum Alloy Sheets

This paper aims to investigate the influence of ultrasonic vibration on bending parameters of AA 6061 aluminum alloy sheet. Required setup involves generator, horn, load cell, and die. Four amplitudes of 0, 3.68, 9.2, and 14.72 µm with three different horn speeds of 20, 50, and 80 mm/min were considered. Moreover, two processes of heat treatments, namely annealing and aging, have been applied. It was found that application of ultrasonic vibration generally improved bending parameters. Ultrasonic vibration reduced required bending force and also spring-back angle; with increasing amplitude of vibration, more reduction in force and spring-back angle was seen. Furthermore, at lower horn speed, the required force to bend the sheet metal was less than the case with higher horn speed. In addition, due to the low strength of the annealed aluminum alloy sheet metals (AA6061-O) in comparison with aged ones (AA6061-T6), less spring-back was observed in annealed sheet metals.

Springback Prediction of Aluminum Alloy Sheet under Changing Loading Paths with Consideration of the Influence of Kinematic Hardening and Ductile Damage

Springback prediction of sheet metal forming is always an important issue in the industry, because it greatly affects the final shape of the product. The accuracy of simulation prediction depends on not only the forming condition but also the chosen material model, which determines the stress and strain redistributions in the formed parts. In this paper, a newly proposed elastoplastic constitutive model is used, in which the initial and induced anisotropies, combined nonlinear isotropic and kinematic hardenings, as well as isotropic ductile damage, are taken into account. The aluminum alloy sheet metal AA7055 was chosen as the studied material. In order to investigate springback under non-proportional strain paths, three-point bending tests were conducted with pre-strained specimens, and five different pre-straining states were considered. The comparisons between numerical and experimental results highlighted the hard effect of both kinematic hardening and ductile damage on the springback prediction, especially for a changed loading path case.

Formability enhancement of EN AW-5182 H18 aluminum alloy sheet metal parts in a flash forming process: testing, calibration and evaluation of fracture models

Currently, it is state of the art to use precipitation hardening 6000-series aluminum alloys to manufacture high-strength aluminum automotive parts by extrusion or in a cold forming process. Alternatively, it is also possible to produce such parts by the use of non-precipitation hardening 5000-series aluminum alloys in a work-hardened condition. Therefore, BENTELER Automobiltechnik GmbH developed a special sheet forming process, henceforth referred to as “flash forming process”. The application of the flash forming process, consisting of a rapid heat treatment and a subsequent cold die stamping, increases the forming capability of the work-hardened 5000-series aluminum sheets and results in high-strength parts with a very good ductility and weldability. In addition, this thermal assisted forming process allows a cost-saving production of such high-strength aluminum parts due to lower material costs of 5000-series aluminum alloys than those of a 6000-series material. Furthermore, the weight-saving effects of “flash formed” parts can be higher compared to extruded or cold formed 6000-series aluminum alloys. The suitability of the process is evaluated by forming a commercial AW-5182 H18 aluminum sheet to a crash-relevant automotive part. However, to accurately simulate the flash forming process itself, a temperature dependent fracture model is necessary. Investigations on a coupon basis also showed that the effect of adiabatic heating due to plastic work cannot be neglected. In cooperation with Paderborn University, a detailed mechanical testing, aided by digital image correlation (DIC) and thermal imaging, is carried out to characterize the yield, hardening and fracture behavior at elevated temperatures. The experimental tests are followed by the calibration of a FLD and an incremental stress state dependent fracture model in LS-DYNA. Finally, the simulation models are validated on a cross die deep drawn cup.

A numerical scheme of convex yield function with continuous anisotropic hardening based on non-associated flow rule in FE analysis of sheet metal

A non-associated flow rule (NAFR) model is developed by adopting the convex function YLD-2004 as the yield stress function and the plastic potential function. The yield stress function coefficients are continuously updated which are associated with the change of directional uniaxial yield stress and biaxial yield stress to simulate the anisotropic hardening behaviour. That was achieved by implementing the numerical identification procedure of coefficients into stress integration procedure. The coefficients of plastic potential function are constant and identified by the uniaxial and biaxial r-value. This constitutive model is capable of describing anisotropic hardening and yield behaviour of strongly textured aluminium alloy sheet metal. In this paper, the model was implemented into the FE code via ABAQUS subroutine to predict the deep drawn cup earing and directional flow stresses of the AA5042-H2 aluminium alloy. The new anisotropic hardening model shows better agreement with experiments compared with the isotropic hardening model.

A new temperature-dependent anisotropic constitutive model for predicting deformation and spring-back in warm deep drawing of automotive AA5086-H111 aluminium alloy sheet

A new temperature-dependent anisotropic hardening and yield function based on non-associated flow rule (NAFR) which accurately describes the anisotropic properties of aluminium alloy sheet metal is presented in this paper. This new NAFR coupling yield function has been successfully implemented in commercial FEM software Abaqus via user material subroutine UMAT to model warm forming of AA5086-H111 aluminium alloy and good agreement was found in the predicted earing profiles and spring-back compared with experiments. The new NAFR coupling yield function has been shown be superior to the existing Hill48 (associated flow rule) yield function making it suitable for FEA of warm deep drawing and design of tooling for various automotive panels and components.

Study on Thickness Distribution and Spring Back Phenomena of Sheet-Bulk Forming for Aluminum Alloy

With the advance of technology, aluminum alloy has been widely used in industry. In this study, the formability and spring back phenomena of thick aluminum alloy sheet metal were investigated. Deform-3D and Simufact.forming software were used to simulate the sheet-bulk forming. To find the optimal element size, the convergence analysis method for mesh of simulation process was utilized. In addition, thickness distribution and spring back phenomena of the rectangular blank with round feature of corners were also evaluated. Simulation results were compared to scanning file that obtained via ATOS based on spring back phenomena from experiments. Study results show that the optimal element size is 2 mm. The cross section that is near binder area in x-direction and near quarter area of laptop keyboard in y-direction has minimum spring back. Solid-shell element is better than tetrahedral element for analyzing sheet bulk metal forming process.

Research on transition region in sectional multi-point forming of saddle-shaped parts

The design of the transition region is significant for effective sectional multi-point forming (SMPF) of sheet metal. In this work, the characteristics of the SMPF and design of the transition region in SMPF were systematically studied, and the reasonable design scheme of transition region is summarized. Numerical simulations of a saddle-shaped workpiece formed with different transition region design schemes were performed with finite element analysis software and compared with the design without the transition region. The results show that the reasonable design of the transition region can effectively inhibit indentation and wrinkle defects in the sheet metal during the SMPF process. The values of stress and strain were found to be smaller with uniform distribution compared to the design without the transition region. The flow of sheet metal was also more uniform and the distribution of thickness more reasonable. Finally, a series of experiments on 2024 aluminum alloy sheet metal were tested on the multi-point forming press (MPF) to verify the accuracy of simulation results. The results conclusively prove that the reasonable transition region design can greatly improve the forming effect of sectional forming.

Welding deformation analysis of aluminum alloy and steel sheet metal parts based on resistance spot welding

Auto-body lightweight is becoming an important trend of energy saving and emission reduction. Various materials assembled together would be used to fabricate the auto-body to satisfy this demand. However, the assembly dimensional quality is difficult to be controlled in the real processing due to the huge differences in material properties. Therefore, it is necessary to control and analyze the welding deformation of aluminum alloy and steel. In this work, the orthogonal design experiment has been adopted to analyze the welding deformation of steel–aluminum sheet parts. Subsequently, the response surface model is proposed to establish the relationship between welding deformation and welding parameters by finite element analysis, which is verified by physical experiments again. Meanwhile, the anti-deformation method is also used to decrease the assembly deformation successfully. And the case of Z-shaped planes is applied to further illustrate the proposed method of this work. Finally, the results show that both the methods have a good simulation effect and a high prediction accuracy for the assembly dimensional quality.

Sheet metal anisotropy and optimal non-round blank design in high-speed multi-step forming of AA3104-H19 aluminium alloy can body

A new anisotropic yield function which accurately describes the complex anisotropy of aluminium alloy sheet metal is proposed in this paper. A non-linear least square method is used to determine the coefficients of this yield function based on the experimental results. By employing the back-Euler stress integration algorithm, this anisotropic constitutive model has been successfully implemented in commercial FEM software ABAQUS via user material subroutine UMAT to predict earing profile of AA3104-H19 deep-drawn and redrawn cups. Good agreement was found in the predicted earing profiles and those obtained in experimental deep-drawn and redrawn cups. The FE model was used in an iteration optimisation process to determine the optimal shape of non-round blanks to obtain the ear-free deep-redrawn cups, and non-round tooling was designed accordingly. Results showed the height of the ears is significantly reduced in the cup formed using this non-round blank compared with that of a conventional round blank which is important for smooth running of high-speed multi-step forming of aluminium can body.

Influence of ductile damage on springback prediction of aluminum alloy sheet under changing loading paths

Springback prediction during the metal forming, especially under complex loading paths, is always one big issue in industry, which is also attracting attentions of many researchers. The accuracy of simulation prediction depends not only the forming conditions, but also the chosen material model. In the classical model, the yield function and Young’s modulus both play important roles in the springback prediction. Normally, the yield function represents initial anisotropy, and ignores the subsequent anisotropy. Since the yield surface shape, especially its evolution are difficult to measure. At the same time, the average Young’s modulus decreases with the plastic deformation, which will highly affect the final springback values. In this study, a recently proposed damage model concerning Lode angle parameter, stress triaxiality and also micro-cracks is employed to simulate the fracture phenomena of chosen AA7055. A mixed hardening anisotropic finite plasticity has also been added to the fully coupled isotropic ductile damage in the given constitutive equations. The aluminium alloy sheet metal AA7055 is chosen as the test material. In order to investigate its springback phenomena under variable strain paths, the three point bending tests are conducted with three different pre-strains. Through the comparisons between numerical results and experimental observations, it can be found that the great influence of ductile damage on the springback.