Nakazima Test Research Articles

Forming limit analysis of AA5052 sheet under bilinear strain path by designing a novel large-scale gripping fixture

The combined strain paths have significant effects on the formability on the sheet metal. In this work, aiming at a widely-used AA5052-O sheet metal, the uniaxial tension and the Nakazima tests were conducted on the large-scale specimens subjected to the pre-tension deformation. First, a novel gripping fixture was designed and manufactured to achieve the uniform pre-strains of the large specimen. Then, two types of strain paths of the tension - tension and tension - bulging were implemented to reveal the evolutions of the mechanical properties and forming limit curves (FLCs), respectively. Moreover, the forming limit stress curves (FLSCs), which are more insensitive to the changes of strain paths, are calculated from the experimental FLCs to improve the prediction accuracy of finite element (FE) simulations. Compared to the conventional necking criterion of path transition and the FLC criterion, the FLSC criterion can effectively enhance the simulation precision in the biaxial tension region.

Predicting the forming limit diagram of the fine-grained AA 1050 sheet using GTN damage model with experimental verifications

In this research, forming limit diagram (FLD) of aluminum alloy 1050 (AA 1050) sheet produced by Accumulative Roll Bonding (ARB) is investigated numerically and experimentally. The Gurson-Tvergaard-Needleman (GTN) ductile damage model is used to predict sheet failure and obtain its FLD using numerical simulation in Abaqus/Explicit. Nucleation and growth of voids in the material during the deformation is the basic concept of the GTN damage model. This damage model has nine basic parameters that obtaining through experimental tests is time-consuming and costly, and in some cases, impossible. Thus, the present study tries to obtain the above parameters for fine-grained aluminum 1050 fabricated by ARB using the finite element method. Therefore, after considering each parameter’s interval, numerical simulation and the anti-inference method are used in the uniaxial tensile test to identify GTN parameters for the AA 1050 sheet using FEM. The optimum parameters of the GTN model are used in the FEM of the Nakazima test for FLD prediction. Also, The FLD of the fine-grained aluminum sheet is obtained experimentally using the Nakazima test. Finally, the numerical and experimental FLDs are compared for validation.

Influence of cold rolling process on the Lankford coefficient, drawability and texture of AISI 430 ferritic stainless steel

The following paper addresses the large demand for further comprehension of the ferritic AISI 430 steels owing to deep drawing processes and accompanying forming technologies, such as rolling. Their good corrosion resistance and relative low cost, make them reasonably well competitors in a great deal of applications. There has been thus evaluated in a pilot mill the influence of three different cold rolling passes with 76% thickness reduction to the drawability and mechanical properties of stainless steel plates. Erichsen, Nakazima and tensile tests were complemented with ODF, pole figure, SEM-FEG observations. It was believed that changes on the cold rolling passes, which alters the anisotropy of materials, would favorably influence deep drawing applications. Experiments revealed, however, that changes defining the deep drawing parameters from 15, 20 and 24 passes, were not statistically significant after rolling. Notwithstanding, texture gradients are reported.

The Effect of Preheating Temperature on the Forming Limit Diagram of AA1050/AA7050 Al Multilayered Sheets Produced by Accumulative Roll Bonding (ARB)

Multilayered sheets of AA1050 and AA7050 Al alloys produced by hot accumulative roll bonding (ARB) are tested to estimate their forming limit diagrams (FLDs). Sheets processed with preheating at 450 and 500 °C for up to six ARB cycles are submitted to Nakazima tests with an in situ digital image correlation system and tensile tests to analyze the mechanical behavior. X‐ray measurements and electron backscatter diffraction are performed to obtain the crystallographic texture and the mesotexture, respectively, and thus characterize the heterogeneous microstructure. A bimodal grain size distribution is observed in these sheets since AA7050 layers present elongated and fine grain size with nanometric precipitates and AA1050 layers present coarser and equiaxed grains. In addition, texture analysis indicates both rolling and shear components in the sheets. The FLDs show that the forming properties did not follow the expected rule of mixtures when compared with monolithic alloy sheets. Overall, the multilayered AA1050/AA7050 processed by means of ARB at 500 °C presents better formability than that at 450 °C, due to the combination of lower anisotropy measured by the Lankford coefficients values close to 1 and low normal anisotropy Δr values.

Microstructure-Based Modelling of Flow and Fracture Behavior of Tailored Microstructures of Ductibor® 1000-AS Steel

Emerging grades of press-hardening steels such as Ductibor® 1000-AS are now commercially available for use within tailor-welded blanks (TWBs) to enhance ductility and energy absorption in hot-stamped automotive structural components. This study examines the constitutive (hardening) response and fracture limits of Ductibor® 1000-AS as functions of the as-quenched microstructure after hot stamping. Three different microstructures consisting of bainite and martensite were obtained by hot stamping with die temperatures of 25 °C, 350 °C, and 450 °C. Mechanical characterization was performed to determine the hardening curves and plane-stress fracture loci for the different quench conditions (cooling rates). Uniaxial-tension and shear tests were conducted to experimentally capture the hardening response to large strain levels. Shear, conical hole-expansion, plane-strain notch tension, and Nakazima tests were carried out to evaluate the stress-state dependence of fracture. A mean-field homogenization (MFH) scheme was applied to model the constitutive and fracture behavior of the mixed-phase microstructures. A dislocation-based hardening model was adopted for the individual phases, which accounts for material chemistry, inter-phase carbon partitioning, and dislocation evolution. The per-phase fracture modelling was executed using a phenomenological damage index based upon the stress state within each phase. The results revealed that the 25 °C hot-stamped material condition with a fully martensite microstructure exhibited the highest level of strength and the lowest degree of ductility. As bainite was formed in the final microstructure by quenching at higher die temperatures, the strength decreased, while the ductility increased. The predicted constitutive and fracture responses in the hot-stamped microstructures were in line with the measured data. Accordingly, the established numerical strategy was extended to predict the mechanical behavior of Ductibor® 1000-AS for a broad range of intermediate as-quenched microstructures.

Forming limit diagrams and mechanical properties of AA1050/Ti CP2 multilayered composite produced by the accumulative roll bonding technique

In this study, the AA1050/Ti CP2 metallic multilayered composite was produced by accumulative roll bonding (ARB) using AA 1050 and commercially pure titanium Ti (CP-Ti ASTM grade 2) for three cycles at ambient temperature. The microstructure evolution was investigated by scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). Also, the tensile test and Vickers microhardness measurement were conducted to evaluate mechanical properties. The forming limit diagrams (FLD) were obtained for a metallic multilayered composite for the first time by the Nakazima test. It was observed that a continuous and well-bonded composite was fabricated after the first cycle. For the subsequent two cycles, necking and fracture of Ti reinforcement occurred in the Al matrix. The maximum tensile strength was reached after two cycles with almost a 98% increase compared to the annealed aluminum sample. However, the tensile strength was reduced after the third cycle. The uniform elongation considerably decreased after the first cycle and was maintained for the subsequent two cycles. The microhardness improved by 124% and 47% for Al and Ti, respectively, in the whole process of ARB in comparison with annealed Al and Ti. As a formability criterion, the level of forming limit curve (FLC) was reduced sharply after the first ARB cycle. For subsequent cycles, this reduction was neglectable. The Al/Ti composite fracture surfaces were a combination of shear ductile and brittle after the third pass.

Deformation behaviour and formability analysis of thin brass sheet: experiments and modelling

ABSTRACT In this study, the hot deformation and formability behaviour of commercially Brass sheet under uniaxial and multiaxial loading conditions at a wide range of temperatures (room temperature −773 K) have been investigated by experiments and theoretical modelling. Uniaxial tensile test and Nakazima test at different test temperatures have been conducted. Different constitutive models, namely, Johnson-Cook (JC) model, modified Zerilli–Armstrong (m-ZA) model, modified JC model, integrated Johnson Cook and Zerilli–Armstrong (JC-ZA) model, have been used to understand the prediction behaviour of flow stress of Brass. Further anisotropic yield criteria, namely, Hill’48 and Barlat’89 yield functions have been utilised for yield loci prediction. From Nakazima test, the experimental forming limit diagram has been plotted. Other formability parameters, namely, limit dome height, thickness distribution, and strain distribution have been analysed. The theoretical FLD by Marciniak Kuczynski (MK) model. By employing the uniaxial constitutive models, anisotropic yield criteria have been investigated. The predicted MK-FLD coupled with the JC-ZA model and Barlat’89 yield criteria shows good agreement with measured experimental FLD.

Effect of Initial Temper on the Warm Forming Characteristics of a High Strength 7000-series Al-Zn-Mg-Cu Alloy

In this work, the formability of a developmental 7000-series copper containing aluminium alloy was assessed at room temperature (RT), 150°C, 175°C and 200°C in pre-aged (PA), peak-aged (T6) and overaged (T76) tempers using Nakazima tests with stereoscopic digital image correlation (DIC) strain measurement. The limit strains were identified using a novel curvature-based approach to detect the formation of an acute neck. The tensile mechanical properties in these warm forming processing routes were characterized with and without a paint bake cycle. Finally, a thermo-mechanical tensile simulator was used to evaluate the constitutive response of the PA and T76 tempers as a function of strain-rate and time at 175°C. Formability results found the selected PA temper to have a good room temperature formability and a mild positive response to the selected warm-forming cycles. The T6 and T76 tempers both exhibited increases in formability in response to warm forming. The PA temper had a significant positive response to short-duration warm forming and subsequent paint baking, with the yield strength increasing from 420 MPa to 512 MPa following this thermal cycle. For the T6 temper, the warm-forming cycle showed a trend characteristic of retrogression and re-aging, with the warm-forming cycle dropping the yield strength from 566 MPa to 534 MPa and the subsequent paint-bake re-aging to 554 MPa. The effect of aging during pre-heating prior to warm forming on the warm constitutive response of the PA and T76 tempers was also investigated. Both tempers exhibited rather different aging responses to short-duration thermal cycles. In the PA temper, this manifested as an increase in at-temperature yield strength and loss of hardening rate. In contrast, the T76 temper exhibited a drop in strength since this temper is already over-aged prior to warm forming. Both the PA and T76 tempers showed comparable at-temperature strain-rate sensitivity.

Material characterisation for strength and formability limits of DP 1180 sheet

ABSTRACT Dual phase steels were specifically developed to address the weight and safety-related problems of the automotive industry. Nevertheless, these steels behave quite differently from conventional high strength steels (single phase) during the stamping processes. The unusual material behaviour limits the application of these steels. This study focuses on the material characterisation of cold rolled and hot dip galvanised dual phase 1180 sheet material. Microstructure and elemental analyses were performed using an optical microscope and a scanning electron microscope with energy dispersive X-ray spectroscopy. Mechanical properties were determined through Rockwell hardness, Vickers hardness, tensile, Nakazima, and hole flanging tests. A digital image correlation system was utilised for Nakazima and hole flanging tests. The average micro hardnesses of the steel and the coating were 341 HV0.5 and 51 HV0.01 respectively. Uniform elongation was ∼6.6%. The lowest point on the forming limit diagram was ∼0.1. The average hole expansion ratio was ∼22%.

High Temperatures Deformation and Formability Evaluation Of α+β Brass Sheet by Experimental and Theoretical Approaches

ABSTRACT The present work proposes a systematic procedure for the evaluation of high temperatures deformation and formability of α + β brass undergoing the stretch forming process. Firstly, flow stress has been predicted by the most popular Johnson-Cook (JC) uniaxial constitutive model at a wide range of temperatures (773 K, 873 K and 973 K) and strain rates (0.001 s−1, 0.01 s−1 and 0.1 s−1). For yielding performance evaluation, FLC integrated with Barlat’89 function has been substituted by the traditional Hill’48 function. The necking curves have been constructed with the uniaxial constitutive model and eminent the Barlat’89 yield criterion. The fracture locus is mathematically derived from the forming limit curves (FLCs). These FLC have been experimentally deceived through a series of standard Nakazima tests. Further, the predicted FLCs by Marciniak–Kuczynski (M–K) model have been compared for different loading conditions. The predicted FLCs with Barlat’89 yield function performed outstanding compared to Hill’48 function. Furthermore, formality studies by means of forming limit diagram, limit dome height and surface strain distribution have been analysed.

Experimental determination of forming limit diagram for α + β brass at hot forming conditions

Brass is an alloy of copper and zinc; it has a composition of 60% and 40% respectively. It plays a major role in manufacturing of musical instruments, electrical applicants like electrical sockets and household articles like kitchen utensils, keys, door knobs, etc. It has unique properties like malleability, corrosion resistance, low melting point and formability characteristics. Forming limit diagrams is significant performance indexes for evaluating the formability of a material. In the present study comparative experimental investigation of forming behaviour of brass has been performed at two different temperatures (500 °C and 600 °C) and strain rates (0.001,0.01 & 0.1 s−1). Subsequently, experimental forming limit curve, thickness distribution curves and limiting dome height curves have been plotted at different temperatures and strain rates using Nakazima test samples. Comparison between these set of results shows significant variations in formability of material. Furthermore, scanning electron microscope is used to observe the fractured specimens and SEM images are taken. The scanning electron microscope, however, it is much more substantial in failure analysis of over load fractures.

Insight into the Influence of Punch Velocity and Thickness on Forming Limit Diagrams of AA 6061 Sheets—Numerical and Experimental Analyses

In this article, the forming limit diagram (FLD) for aluminum 6061 sheets of thicknesses of 1 mm and 3 mm was determined numerically and experimentally, considering different punch velocities. The punch velocity was adjusted in the range of 20 mm/min to 200 mm/min during the Nakazima test. A finite element (FE) simulation was carried out by applying the Johnson–Cook material model into the ABAQUSTM FE software. In addition, a comparison between the simulation and the experimental results was made. It was observed that by increasing the punch velocity, the FLD also increased for both thicknesses, but the degree of the improvement was different. Based on these results, we found a good agreement between numerical and experimental analyses (about 10% error). Moreover, by increasing the punch velocity from 20 mm/min to 100 mm/min in 1 mm-thick specimens, the corresponding FLD increased by 3.8%, while for 3 mm-thick specimens, this increase was 5.2%; by increasing the punch velocity from 20 mm/min to 200 mm/min in the 3 mm-thick sheets, the corresponding FLD increased by 9.3%.

Mechanical and micro-structural characterization of hot rolled brass foil

To assist the large-scale production of micro parts by micro sheet metal forming processes, mechanical and microstructural behaviors of metal foils are required. In the present study, an attempt has been made to characterize these properties of 100-μm thickness as-received brass foil. Mechanical characterization of foil has been done with tensile tests, bending tests and Nakazima tests. From the tensile test analysis, it has been observed that change in strain rate has a negligible impact on flow stress. However, serrations observed in flow stress are dependent on strain rate. The Electron backscatter diffraction (EBSD) analysis of as-received and post-deformed specimens shows the concentrated rise in Kernel average misorientation, high angle grain boundaries and low angle grain boundaries in deformed specimens. The fracture morphology study shows the presence of ductile fracture in the middle region of the fracture cross-section with shear lips in the edge region. Bending experiments show that the spring-back angle increases with the increase of bending angle up to 120°. After that, with an increase in bending angle, the spring-back angle decreases. The forming limit curve (FLC) obtained from the Nakazima test was compared to FLC obtained from different prediction criteria. From this study, it was observed that the FLC obtained from Marciniak-Kuczynski (M-K) criterion showed a good prediction of FLC for brass foil. However, FLCs obtained from Swift-Hill criterion, Abspoel empirical criterion and Pamstamp's Matwizard module over estimated foil's formability.

Study on the Formability and Texture evolution of AA6061 Alloy Processed by Repetitive Corrugation and Straightening.

The enhanced mechanical properties obtained by refining the grain size down to the ultrafine-grained (UFG) regime have attracted considerable attention in recent years. The severe plastic deformation (SPD) techniques allow obtaining ultrafine-grained materials. Different SPD techniques permit processing sheet shape materials such as repetitive corrugation and straightening (RCS) and accumulative roll bonding (ARB). In this study, the formability of an AA 6061-T6 processed by RCS was evaluated. The forming limit diagrams (FLD) were obtained by Nakazima tests of samples in initial condition (T6 state) and after one and two RCS cycles. The FLD curves showed that the forming capacity decreased from the first RCS cycle. Likewise, uniaxial tensile tests at different temperatures and strain rates were conducted to analyze the effect of the RCS process on the strain rate sensitivity. They showed a relatively high strain rate sensitivity coefficient in the samples after one and two RCS cycles, which indicates an improvement of i) the capacity of the material to delay the onset of the necking and ii) the formability at increasing temperatures. Finally, texture analysis was carried out employing X-ray diffraction, calculating the orientation distribution functions (ODFs). The initial texture showed a predominant cube texture component, whereas, for further RCS cycles, a weakening of the cube texture and an increment of the S texture component were observed.

Development of a Nakazima formability test methodology for high temperature applications on titanium alloys sheet metal

Forming Limit Curves (FLCs) in applications of hard-to-form materials results essential to optimise the complex forming processes involved. The need of applying high temperature in most of these applications increases the importance of FLCs for cost-effective procedure designs. Nevertheless, either a lack of knowledge is detected relative to this field in literature, or the experimental methods proposed to obtain FLCs result complex to be executed. In the present work, an easy-to-use methodology based on the traditional Nakazima test is proposed and the research has been applied to different titanium alloys. For that, a heated Nakazima device was developed and some new geometry for test samples was proposed. The results agree with those obtained by other authors in similar conditions, what validates the approach developed herein.

Research on High Temperature Stamping Forming Performance and Process Parameters Optimization of 7075 Aluminum Alloy.

The stress strain curve of 7075 aluminum alloy in the temperature range of 310 °C to 410 °C was obtained by Gleeble-3800. By Nakazima test, the isothermal thermoforming limit diagrams of 7075 aluminum alloy at different deformation temperatures and stamping speeds were acquired. Moreover, the parameters of automotive S-rail hot stamping process were optimized by GA-BP neural network. The results show that the forming limit curve of 7075 aluminum alloy increases as the deformation temperature and stamping speed increase. The predicted optimal parameters for hot stamping of automotive S-rails by GA-BP neural network are: stamping speed is 50 mm/s, friction coefficient between die and blank is 0.1, and blank holder force is 5 kN. The maximum thinning rate at this process parameter is 9.37%, which provided a reference for 7075 aluminum alloy automotive S-rail hot stamping.

An innovation in finite element simulation via crystal plasticity assessment of grain morphology effect on sheet metal formability

Experimental and numerical study regarding the uniaxial tensile test and the forming limit diagram are addressed in this paper for AL2024 with the face-centered cube structure. First, representation of a grain structure can be obtained directly by mapping metallographic observations via scanning electron microscopy approach. Artificial grain microstructures produced by Voronoi Tessellation method are employed in the model using VGRAIN software. By resorting to the finite element software (ABAQUS) capabilities, the constitutive equations of the crystal plasticity were utilized and implemented as a user subroutine material UMAT code. The hardening parameters were calibrated by a trial and error approach in order to fit experimental tensile results with the simulation. Then the effect of the changing grain size, the heterogeneity factor, and the grain aspect ratio were studied for a uniaxial tensile test to emphasize the importance of the microstudy behavior of grains in material behavior. Furthermore, the polycrystal plasticity grain distribution was employed in the Nakazima test in order to obtain the forming limit diagram. The crystal plasticity-driven forming limit diagram reveals more accurate strains, taking into account the involving the micromechanical features of the grains. An innovative approach is pursued in this study to discover the necking angle, both in tensile test or Nakazima samples, showing a good agreement with the experiment results.

Prediction of the bending and out-of-plane loading effects on formability response of the steel sheets

Failure in sheet metal forming can occur by necking, fracture or wrinkling. By using a forming limit diagram (FLD) as a powerful tool to prevent sheets metal failures in the forming process, provides parameters controlling throughout forming. There are different developed methods for predicting FLDs, which estimate sheet metal forming strains limits. Assessment of FLD estimation reveals that there is a dependency between the effect of several factors containing normal stress, shear stress, sheet thickness, mechanical properties, metallurgical properties, yield function, strain path, and bending with formability. In this research, the effects of bending via two finite element models are investigated. In the first method, the out-of-plane deformation is applied by increasing punch displacement to study the effects of bending. In the second method, the effect of bending is investigated via changing punch diameter (25, 50, 70 and, 100 mm). The Marciniak–Kuczynski (M–K) theory is used to predict the time of localized necking in finite element simulations. Furthermore, a novel method for the determination of the inhomogeneity coefficient is presented in M–K model to simulate the groove width for M–K model. To verify finite element simulation results, Nakazima tests with 50 and 100 mm punch diameters were done as experimental studies. The comparison of experimental results and finite element analysis illustrates that the increasing bending or the out-of-plane loading can improve formability. At the end, the effect of bending on FLD is reported as an equation based on minor and major strains.

Evaluation of Temperature Effects on Forming Limit Diagrams of AA6061-T6 Considering the Marciniak and Kuczynski Model

Abstract In this study, forming limit diagrams (FLDs) of AA6061-T6 have been determined numerically and experimentally at room and elevated temperatures. The computational method has been based on the Marciniak and Kuczynski (M-K) model in conjunction with the Johnson-Cook (J-C) constitutive model. In the present numerical approach, four nonlinear equations have been solved simultaneously via the Quasi-Newton Technique. Moreover, the effects of temperature on formability have been discussed. To verify the theoretical results, out-of-plane Nakazima tests have been performed at the same temperatures, and a good match between the present numerical approach and experimental data has been seen. Both the experimental and numerical results illustrated that as the temperature increased, the FLD moved upward.

Evaluation of forming limit diagrams using Nakazima out-of-plane test and incremental forming process for two-phase magnesium–lithium alloy sheet

The focus of this paper is to investigate the formability of two-phase Mg–Li sheets through drawing forming limit diagrams (FLDs) obtained from Nakazima out-of-plane test and incremental forming process. For this goal, Mg LZ71 manufactured by casting and then as-cast ingot converted to plate by hot rolling, and finally, the sheets were annealed. Then, the microstructure of the LZ71 alloy was assessed by an optical microscope. Also, the mechanical properties of the LZ71 samples were investigated using uniaxial tensile tests. Furthermore, to achieve isotropic samples, anisotropy parameters of as-cast, as-rolled, and annealed samples were determined. Afterward, the formability of the produced LZ71 sheet was studied with the help of FLD. In the present investigation, the FLDs of the LZ71 sheet were obtained using two methods, including the Nakazima out-of-plane test and the incremental forming process. The results showed that Mg LZ71 has a dual-phase microstructure, which causes considerable formability. Also, the results showed that FLD of the Mg LZ71 sheet in incremental forming is higher than that of the conventional Nakazima test.