Deep Drawing Tests Research Articles

Formability and Failure Mechanisms of Continuous Glass Fiber-Reinforced Polypropylene Composite Laminates in Thermoforming Below the Melting Temperature.

In this study, the thermoforming formability of continuous glass fiber-reinforced polypropylene (CGFRPP) laminates below the melting temperature were investigated. The forming limits of CGFRPP laminates were explored using flexural tests, Erichsen tests and deep drawing tests. The failure mechanism of CGFRPP in thermoforming was investigated by observing typical failure specimens using a microscope. The results show that the flexural performance and Erichsen performance are optimal at 130 °C and 2 mm/min. At 160 °C and 100 mm/min, the deep drawing performance is optimal. The restriction of fibers by the matrix is affected by the deformation temperature, and the creation of defects is affected by the deformation rate. During forming, the CGFRPP laminates undergo shear and extrusion deformations, resulting in wrinkles, delamination, and fiber aggregation.

Tribological investigations of water-based lubricants for application in the deep drawing process

This paper describes the tribological investigation of water-based lubricants in the context of a deep drawing process. Therefore, different methods were conducted in order to determinate the suitability of these lubricants for a deep drawing process. Three lubricant manufacturers each provided their lubricants as part of this work. The sheet materials investigated are an aluminum material (AA6014) and two steel materials (DC04, DP800), each with a thickness of 1.5 mm. All sheet materials were examined in the as-delivered condition as part of this work. The hypothesis for this research proposal is that it is possible to achieve comparable tribological properties in sheet metal forming using water-based lubricants as when using mineral oil-based lubricants. The aim is to optimize a tribological system for water-based lubricants when used as additional lubrication in order to replace mineral oil-based lubricants and, as a result, to shorten the representative process chain by one process step (cleaning). In the course of this work, topographical measurements of the sheet materials were carried out in order to investigate the lubricant holding capacity of the sheet materials. Furthermore, strip drawing tests were performed to determine the friction coefficients of the different lubricant-sheet combinations. The final step was to conduct deep-drawing tests to determine the limits of use of the water-based lubricants by continuously increasing the holding-down force.

Study on the formability of copper foils during multi-step micro deep drawing

In this work, the formability of copper foils was analyzed based on a series of multi-step micro deep drawing (MMDD) tests. The copper foils were annealed at temperatures ranging from 300 to 700 °C for 5 min, followed by tensile tests and electron backscatter diffraction (EBSD) tests for the exploration of mechanical behavior and microstructural characterization of annealed specimens. The results show that the as-received copper foils exhibit poor formability and cannot form micro cups during MMDD. When the annealing temperature attains 500 °C, a homogeneous microstructure was formed inside copper foils, enhancing the formability of materials and improving the surface quality of formed products. Nevertheless, both the elongation and tensile strength decrease dramatically due to the easy occurrence of incoordination deformation in coarse grain bands with further increase in annealing temperature to 700 °C. Consequently, marked wrinkling is generated on the surface of drawn cups and obvious thinning occurs at the nose radius region due to metal flow. In addition, the results show that the E texture component within the copper foils is favorable for MMDD. Overall, an optimal annealing temperature of 500 °C is obtained with improved formability of copper foils and forming quality of drawn cups.

Calibration of Yld2000-2D Anisotropy Yield Criterion with Traditional Testing and Inverse Identification Strategies.

In order to improve predictive capabilities of numerical simulations, Yld2000-2D yield criterion is used to model the plastic anisotropic behaviors of AA5086 sheets. The parameters of Yld2000-2D yield criterion are identified based on the traditional testing strategy and the inverse identification strategy, respectively. The traditional testing strategy considers uniaxial and equi-biaxial tensile tests. The inverse identification strategy relies on the finite element model update (FEMU) method that couples with a biaxial tensile test using a dedicated cruciform specimen or the Pottier bulging test. The identified parameters are preliminarily evaluated by comparing predicted and experimental yield stresses, r-values, and yield loci. Then, the deep drawing test and simulations are performed. The identified parameter sets of Yld2000-2D yield criterion are further evaluated in terms of practical forming by comparing the predicted earing profile height with the experimental results. The results show that the inverse identification strategy can be an effective alternative to identify the parameters of Yld2000-2D yield criterion, and a well-designed heterogeneous test could lead to a better identification result.

Study on the forming fracture of the crash box with extruded Al6060-T6 profile

This study was performed to analyze the forming fracture of crash box with extruded Al6060-T6 profile. Deep drawing test was performed to evaluate the formability of the extruded profile. For the various specimens, deep drawing tests were repeated to measure the maximum stroke at which fracture did not occur. The finite element analysis for the deep drawing tests was performed to calculate the principal strain through the obtained maximum stroke. The forming limit diagram for the design of the concave forming shape was calculated through the obtained principal strains with the FEA. The forming shape was optimized by using the design variables and FLD by using the FEA. The design variables are the central curvature, the lateral curvature, and the forming depth. The tools were made with the optimizing model and the crash box has been formed without fracture.

Modeling of constitutive behavior and ductile fracture of AA5A06 aluminum alloy sheet in warm hydroforming process

In order to predict the formability of lightweight materials, the flow stress and fracture behavior of AA5A06 aluminum alloy which is very sensitive to temperature and strain-rate were investigated in this study. First, a modified Misiolek constitutive equation with a work-soften term is proposed to calculate the flow stress of the sheet metal. To predict ductile fracture in sheet forming operations, Cockcroft-Latham damage criterion is applied here and ductile fracture threshold under each condition is estimated using Response Surface Method. Finally, warm hydroforming deep drawing tests were conducted at various conditions and the experiment results are compared with the numerical simulation result to evaluate the new model’s feasibility and accuracy in forecasting the mechanical response of the material in warm forming conditions. Results show that the proposed model has a good accuracy compared with the traditional power law models and the FEM simulation conforms well to the tests.

Application of an Oleophobic Coating to Improve Formability in the Deep-Drawing Process

The competition among sheet-metal-forming manufacturers in recent years has become more severe. Many manufacturers have survived by cutting their production costs. Increasing the formability, which could reduce the production costs, is the focus of many manufacturers and engineers. In the present research, to increase the formability over the limiting drawing ratio (LDR) in the cylindrical deep-drawing process, the application of oleophobic coating is proposed. An SUS304 (JIS standard)-stainless-steel cylindrical deep-drawn component was used as the investigated model. First, we applied the oleophobic coating in the sheet-metal-forming process, and tribology tests were carried out to examine the friction coefficients, which were reduced by approximately 60% compared with those of standard lubricant use (Iloform TDN81). Next, deep-drawing tests were performed to investigate the drawing ratio (DR). The LDR recommended in the past could be overcome, and it increased by approximately 12% with the oleophobic coating use. Finally, the deep-drawing mechanism using an extremely low friction coefficient was clarified as well. Based on these results, an oleophobic coating could be applied in the cylindrical deep-drawing process to increase the LDR. The results also clearly expose the multidisciplinary approach that combines an oleophobic coating application and the sheet-metal-forming process.

Multi-scale modelling of evolving plastic anisotropy during Al-alloy sheet forming

Accurate modelling of local evolving texture and yield surface evolution in complex strain path are of crucial importance to the accurate sheet metal forming simulations. This work proposed a novel multi-scale computational scheme, in which the full-field crystal plasticity (CP) modelling is mapped and real-time updated into each integration point following the local strain path in the finite element (FE) model. By the real-time close-loop feedback and update between the FE model and CP model, the evolving texture and its induced anisotropic plasticity evolution are considered utilizing on-the-fly virtual tests in the full deformation field. To carefully verify the developed computational framework, a comprehensive validation including plastic anisotropy characterization, cyclic hardening tests and deep drawing tests is conducted in an AA6016-T4 aluminium sheet with strong cube texture using ESAFORM Benchmark 2021. By comparison with experimental results and the conventional approaches with constant yield surface for the forming case, the newly developed method is thoroughly validated, showing that the significant texture evolution is observed in large plastic deformation and various strain paths lead to different texture evolution. The local evolving texture induced plastic anisotropy evolution has a considerable effect on the prediction accuracy of the deep drawing simulations, such as earing profile, wall thickness and loading history. Additionally, this method does not increase too much computation cost, enabling cost-effectively simulation of complex forming processes.

Deep drawability of electron beam welded tailored blanks of commercially pure titanium thin sheets

In this work, commercially pure titanium tailor-welded blanks (TWBs) were fabricated from sheets of thicknesses 1.0 mm (Ti1.0) and 1.2 mm (Ti1.2) using electron beam welding (EBW). It was observed that the titanium base materials had an equiaxed microstructure, whereas the FZ of the weld consisted of coarsened prior β grains with internal α substructures. Transverse and longitudinal tensile tests were conducted to evaluate weld integrity and weld properties, respectively. The deep drawing behaviour of both the base materials and TWB had been investigated by conducting laboratory-scale deep drawing tests. The limiting drawing ratios (LDR) of the Ti1.0 and Ti1.2 base materials were 2.14 and 2.21, respectively. The LDR of TWB was 2.14, which was same as that of the Ti1.0 base material. The CPB06 yield criterion and weld properties were incorporated in the FE modelling to study the deep drawing behaviour of the TWB for the first time. The FE model successfully predicted the non-uniform material flow and thickness distribution in the deep drawn TWB. Moreover, the weld line movement was found to be towards the Ti1.0 side at the cup top region and towards the Ti1.2 side at the cup bottom of the TWB deep-drawn cup.

Material Modeling of Perforated Sheet Metals with Different Hole Arrangements by Homogenization Method

The homogenization method is effective for analyzing macroscopic mechanical properties from the substructure of a material. In perforated sheet metal, macroscopic mechanical properties depend on the pattern of hole arrangement. In this study, the macro–plastic properties of perforated sheets with 60° standard staggered and 90° square arrangements are modeled. Biaxial stress is applied to the unit cells given the periodic boundary condition, and the contours of equal plastic work are obtained. The yield surfaces of the perforated sheets on the tension–compression combined stress state are strongly affected by the hole arrangement. To model the yield surface, a yield function for the rotational symmetry peculiar to the perforated sheet is proposed. The yield surfaces are modeled using the CPB2006 yield function that takes into consideration tension–compression asymmetry. Also, a surface–interpolated differential hardening model is applied to model the changes of yield surfaces. The analysis results obtained using the modeled yield surfaces are compared with the experimental ones obtained on the uniaxial tensile and deep drawing tests. The results of both tests show good agreement.

Estimating Forming Conditions for Deep Drawn SUS304 Square Cups Having no Delayed Cracks Using FE Simulation

Forming conditions for deep drawn SUS304 square cups having no delayed cracks are determined using finite element (FE) simulations based on the threshold % of wall thickening for delay cracking obtained from deep drawing tests of SUS304 cylindrical cups. The maximum drawing ratio and the initial blank thickness are fixed at 1.87 and 0.8 mm, respectively in this study taken into the considerations of the capacity of the press machine i.e., 100 kN. The blank holding forces (BHF), punch corner radius, rp and die corner radius, rd are varied to obtain 75 mm x 75 mm square cups having % of local thickening less than the threshold value i.e., 47% in the cracking zones located adjacent to the top corners of the cups. A quarter 3D FE simulation model of the deep drawing process is constructed, the wall thickness distributions of the cups are calculated. The simulation results showed that the forming conditions for the crack-free cups could be obtained with BHF ≥ 47 kN, rp ≥ 8.5 mm and 6.8 mm ≤ rd ≤ 9.5 mm. A method for designing the forming conditions of deep drawn SUS304 square cups is proposed in this study.

Anisotropic mechanical behavior prediction of aluminum alloy sheet based on an anisotropic GTN model: Modeling, simulation and experimental investigation

The rolled aluminum alloy sheets usually have anisotropic properties that affect its ductile fracture behavior. In order to study the plastic anisotropy and ductile fracture behavior of aluminum alloy 6016 sheet, the Hill48 anisotropic yield criterion is introduced into the Gurson–Tvergaard–Needleman (GTN) model forming an anisotropic GTN model. In this model, two groups of Hill48 anisotropic yield constants, which are respectively calculated by Lankford's coefficient in different directions and the combination of the Lankford's coefficient and yield stress in different directions, are separately used to describe anisotropic mechanical behaviors. The failure void volume fraction is identified through microstructure analysis and the other damage parameters are calibrated through a finite element (FE) inverse calibration method. The abilities of these two groups of Hill48 anisotropic yield constants in predicting anisotropic mechanical behaviors of aluminum alloy 6016 sheet are examined by performing FE simulations of 0°, 45°, and 90° plate tensile tests, deep-drawing tests and Erichsen cupping tests. Results show that the Hill48 anisotropic yield constants calculated by the Lankford's coefficient in different directions have a better ability to predict local anisotropic plastic deformation especially the earing effect in the deep-drawing test, but the Hill48 anisotropic yield constants calculated by the combination of the Lankford's coefficient and yield stress in different directions have a better prediction ability in predicting global anisotropic mechanical behaviors of the aluminum alloy 6016 sheet.

Establishment of the unified critical wrinkling limit curve of thin-walled parts forming

Wrinkling is a common defect in sheet metal forming. Different forming conditions lead to complex stress loading paths, which is the main factor that makes wrinkling difficult to control. It is of great significance to accurately predict the wrinkling phenomenon of sheet metal under different forming loads. Three kinds of typical wrinkling tests are selected in this paper, which are Yoshida buckling test (YBT), wedge tensile test and deep-drawing test for conical parts. The numerical analysis models of three typical working conditions are established by using “Buckle + Dynamic-Explicit” algorithm in ABAQUS software. The accuracy of the simulation algorithm is verified by comparison experiments. The law of the critical wrinkling stress and strain data under three working conditions is studied. The unified critical wrinkling limit curve was established by using stress ratio and strain ratio based on MATLAB software. Finally, the accuracy of the curve is verified.

Investigation on granular medium forming formability of TA1 titanium alloy cylinder-shaped parts

In order to investigate the formability of the granular medium forming (GMF) based on the Mohr–Coulomb constitutive model with the tri-axial compression test of granular medium and the true stress–strain curves of TA1 titanium alloy from uniaxial tensile tests, the numerical simulation of TA1 titanium alloy sheet deep drawing with finite element method was performed, and the deep drawing tests were also carried out. Simulation analysis and test results show that the GMF process is suitable for titanium alloy sheets and can effectively improve the uniformity of the wall thickness of the formed parts, reduce the tendency of wrinkles, and improve the forming quality.

Determination of Fracture Limit Line in Principal Strain Space by Shear Tests

The main goal of this paper was to define a new limit line, which expands Forming Limit Curve (FLC) more precisely Fracture Forming Limit Curve (FFLC). The new limit line was defined by damage models transformed from triaxiality-fracture strain space to the major/minor strain space and we focused on the shear area. Because FLC and FFLC have some limitations, for example, some Advanced High Strength Steels (AHSS) fractures before thinning. Another limitation can be seen on the left side of the diagrams in the shear region, where the part can break in the process without crossing the FLC or FFLC. The different shapes of samples were used for finding a new line of the FFLC and Deep Drawing Test (DDT) was used for verification. The fracture points from measurement were used for calibration of simple uncoupled damage models. The simple model means model, which is calibrated based on one or two measurements (tensile test or tensile test + shear test). This paper was used an Advanced High Strength Steel (AHSS) DP1000 sheet with a thickness of 0.8 mm for the experiments.

A new Device for Determination of Forming-Limit-Curves under Hot-Forming Conditions

Forming-Limit-Curves (FLCs) are widely used to characterize the forming behaviour of materials. Nevertheless, the determination of FLCs for hot forming processes is a challenging and time-consuming procedure. At the Chair of Automotive Lightweight Design, a new device for the determination of FLCs under hot forming conditions was developed. In a sealed isothermal oven atmosphere, blank holder, die and punch are used for the FLC determination relying on the DIN EN ISO 12004-2. External hydraulic forces actuate the punch and blank holder. To ensure insulation from the thermal load set inside the hot oven atmosphere an innovative cooling system with extensive use of additively manufactured components with integrated cooling channels is used. For the feed of the different deep drawing tests geometries an airlock is installed, which allows keeping the temperatures constant during the testing procedure. Afterward the forming limits for the different geometries are evaluated with optical measurement systems. The developed test rig proved the ability to determine FLCs for isothermal temperatures up to 800° C.

Effects of Variable Punch Speed and Blank Holder Force in Warm Superplastic Deep Drawing Process

A cylindrical deep drawing test was conducted for the purpose of improving the drawability, product accuracy, and quality in warm deep drawing using a superplastic material with large strain rate dependence. Then, the effects of blank holding force (BHF) and punch speed (SPD) on the flange wrinkle behavior and wall thickness distribution were investigated by experiments and theoretical analysis. A Zn-22Al-0.5Cu-0.01Mg alloy superplastic material SPZ2 with a sheet thickness of 1 mm was employed as the experimental material, and a cylindrical deep drawing experiment with the drawing ratio (DR) of 3.1 and 5 was performed at 250 °C. A good agreement was qualitatively obtained between the elementary theory on the flange wrinkle limit, the fracture limit, and the experimental results. In addition, the authors examined each variable BHF and SPD method obtained from the theory and experimentally demonstrated that the variable BHF method has a great effect on uniform wall thickness distribution and that variable SPD has a great effect on shortening the processing time for superplastic materials. Furthermore, the authors demonstrated the effectiveness of the variable BHF/SPD deep drawing method that varies both BHF and SPD simultaneously.

A simple and computationally efficient stress integration scheme based on numerical approximation of the yield function gradients: Application to advanced yield criteria

In this paper, we have modified the stress integration scheme proposed by Choi and Yoon [1]; which is based on the numerical approximation of the yield function gradients, to implement in the finite element code ABAQUS three elastic isotropic, plastic anisotropic constitutive models with yielding described by Yld2004-18p [2], CPB06ex2 [3] and Yld2011-27p [4] criteria, respectively. We have developed both VUMAT and UMAT subroutines for the three constitutive models, and have carried out cylindrical cup deep drawing test simulations and calculations of dynamic necking localization under plane strain tension, using explicit and implicit analyses. An original feature of this paper is that these finite element simulations are systematically compared with additional calculations performed using (i) the numerical approximation scheme developed by Choi and Yoon [1]; and (ii) the analytical computation of the first and second order yield functions gradients. This comparison has shown that the numerical approximation of the yield function gradients proposed in this paper facilitates the implementation of the constitutive models, and in the case of the implicit analyses, it leads to a significant decrease of the computational time without impairing the accuracy of the finite element results. In addition, we have demonstrated that there is a critical loading rate below which the dynamic implicit analyses are computationally more efficient than the explicit calculations.

Influence of heat treatments on the formability of the 6061 Al alloy sheets: experiments and GTN damage model

The research mainly focused on the forming limit curves (FLC) of the 6061 Al alloy subjected to heat treatments with various (1 mm, 1.6 mm, 2 mm, and 2.5 mm) sheet metal thicknesses. Five peculiar heat treatments, including natural aging temper (T4) and highest-strength (T6) temper, were devised to study the influence of heat treatments on the formability of 6061 Al alloy sheets. Tensile tests were executed to determine the plastic behavior of the alloy under specific heat treatments. Laser marked FLC specimens were deep-drawn with the proposed spring-attached Nakajima deep drawing test setup. A finite element model (FEM) of the deep drawing experiment was constructed to investigate the nucleation and growth mechanism of the voids. The developed model was compared and verified with the experimental FLC, fracture locations, and dome height. The FEM results were differed from the experimental FLC by less than 5% considering all heat treatments and sheet thicknesses. The results declared that sheet metal thickness has a positive effect on the effective strain up to failure. On the other hand, the duration of artificial aging time reduced effective strain to failure controlled by nucleation and growth of voids.

Investigations of hot-dip galvanized dual-phase steel (DP600+Z) sheet metal on selectively oxidized tool steel surfaces under dry deep-drawing conditions

As reported in previous studies conducted within the priority program 1676 “Dry forming - Sustainable production through dry machining in metal forming”, selectively oxidized tool surfaces represent a promising approach for the realization of a dry forming process. For the transfer of the findings to an actual industrial deep drawing process, the choice of sheet material is of great importance. The present study reports investigations on hot-dip galvanized dual-phase steel that was deep-drawn with selectively oxidized tools made from hardened 1.2379 tool steel. The data are compared to the deep-drawing behavior of electrolytically galvanized deep-drawing steel, which was used as a reference. The applied oxide system consisted of α-Fe2O3 and was generated in a tailored atmosphere with controlled partial pressure of oxygen. A specially developed modular deep-drawing tool was used for the deep-drawing tests, which allowed carrying out cup-drawing tests in which parts of the drawing ring featured different oxidation conditions. The differently set load cases on the drawing ring were then evaluated and examined with respect to the resulting surface properties of the component produced. The changes in the surface morphology prior to and after deep drawing were analyzed using high resolution analysis and the interactions between tool coating and sheet metal were examined. The results obtained provided information about the deep-drawing properties of the sheet metal materials and confirmed the advantages of α-Fe2O3 tool coating used as a low-friction separating layer for use in dry deep drawing.