Limiting Dome Height Research Articles

Comparison of electromagnetic forming of friction stir-welded blanks of dissimilar material AA 5052-AA 6061 with conventional forming process

In this work, studies on formability of Tailor Welded Blanks (TWB) were carried out by means of both quasi-static and high speed forming process. TWBs were manufactured as welded blanks of dissimilar material combination. Welding of AA 5052-H32 to AA 6061-T6 aluminium alloy of 1 mm thickness was done with the help a friction stir welding (FSW) process. Limit dome height (LDH) test was performed by both conventional hydraulic press forming (HPF) and by a high-speed forming process called electromagnetic forming (EMF) process, to investigate the forming behaviour of the TWB. Subsequently forming limit curves (FLCs) were plotted to quantify forming behaviour. Dome heights and FLCs of TWB and base materials are compared. Similar comparison has been made between conventional forming and EM forming process. It is found that the formability of TWB increases considerably with EMF process with base AA 5052 material showed largest increase in formability. Effect of weld line offset on the formability was also investigated. When the weld line was offset by 25 mm towards AA 6061 side of the TWB, the welded blanks showed maximum forming behaviour by EMF process in terms of both dome height and FLC.

Hot stamping of AA6082 tailor welded blanks: Experiments and knowledge-based cloud – finite element (KBC-FE) simulation

A novel hot stamping technique known as ‘Solution Heat treatment, Forming and in-die Quenching (HFQ®)’ was employed to manufacture lightweight structural components from AA6082 tailor-welded blanks (TWBs) of different thickness combinations: 1.5–1.5 and 2.0–1.0mm. A finite element (FE) model was built to study the deformation characteristics during the hot stamping process. The FE model was successfully validated by comparing simulation results with experimental ones. Subsequently, the verified simulation results were analysed through a novel multi-objective FE platform known as ‘Knowledge-Based Cloud – Finite Element (KBC-FE)’. KBC-FE operates in a cloud environment and offers various advanced unique functions via functional modules. The ‘formability’ module was implemented in the current study to predict the limiting dome height and failure mode during the hot stamping process. Good agreements were achieved between the predicted and experimental results, from which studies were extended to predict the forming features of 2.0–1.5mm TWBs. The ‘formability’ module has successfully captured the complex nature of a hot stamping process, featuring a non-isothermal and non-linear loading path. The formability of TWBs was found to be dependent on forming speed and blank thickness, out of which the latter has a dominant effect.

Forming Limit Differences in Hemispherical Dome and Biaxial Test During Equibiaxial Tension on Cruciform

Traditionally, the mechanical properties of materials have been characterized using the uniaxial tension test. This test is considered adequate for simple forming operations where single axis loading is dominant. Previous studies, however, have noted that the data acquired from this type of testing are not enough and additional details in other axes under simultaneous deformation conditions are important. To analyze the biaxial strain, some studies have suggested using the limiting dome height test and bulge test. However, these tests limit the extent of using multi-axial loading and the resulting stress pattern due to contact surfaces. Therefore, researchers devised the biaxial machine which is designed specifically to provide biaxial stress components using multiple and varying loading conditions. The idea of this work is to evaluate the relationship between the dome test data and the biaxial test data. For this comparison, cruciform specimens with a diamond shaped thinner gage in the center were deformed with biaxial stretching on the biaxial testing machine. In addition, the cruciform specimens were biaxially stretched with a hemispherical punch in a conventional die-punch setting. Furthermore, in each case, the process was simulated using a three-dimensional (3D) model generated on abaqus. These models were then compared with the experimental results. The forces on each arm, strain path, forming, and formability were analyzed. The differences between the processes were detailed. It was found that biaxial tests eliminated the pressurization effect which could be found in hemispherical dome tests.

Implementation of Yld96 anisotropy plasticity theory for estimation of polar effective plastic strain based failure limit of pre-strained thin steels

In present work, tensile and stack compression tests were performed along different orientations to evaluate the yield behaviour and strength directionalities of several equi-biaxial, plane strain and uniaxial pre-strained extra deep drawing (EDD) thin steel sheets. Strain based forming limit diagrams (ε-FLD) were evaluated by deforming along six different paths with strain ratio ranging from 0.91 to −0.34, and the dynamic shift of failure strains was established with the cusp moved equal to the amount and direction of minor strain component of the pre-strained materials. Hence, polar effective plastic strain based FLD (PEPS-FLD) was estimated using the Yld96 anisotropy plasticity theory to restrict the dynamic shift of failure strains irrespective of the amount and type of pre-strain. Unlike ε-FLD, the consideration of PEPS-FLD along with Yld96 anisotropy coefficients of as-received EDD steels in FE model was encouraging to predict the limiting dome height (LDH) close to experimental data within 6% error. Also, insights into the role of effective plastic strain consumed and strain path on decrease in LDH of pre-strained steels were analysed.

Great room temperature stretch formability of fine-grained Mg-Mn alloy

The effect of grain size and forming speed on room temperature formability was examined using Mg-0.3at%Mn binary alloy with different average grain sizes (2.5 and 18.5µm). Erichsen tests revealed that the limited dome height (LDH) was clearly influenced by these factors. LDHs increased with finer grain sizes and/or lower forming speeds. The highest LDH was obtained to be 8.2, which is superior to that in the conventional aluminum (A5083) alloy (LDH=4.3), in the present Erichsen testing conditions. The results obtained from the tensile tests under strain rate ranges similar to those of the Erichsen tests showed high strain rate sensitivity (m-value, where m=0.06–0.13 for the meso-grained alloy and 0.11–0.22 for the fine-grained alloy, respectively), irrespective of the extrusion direction vs. the tensile direction. Deformed microstructural observations after tensile and Erichsen tests revealed the existence of deformation twinning; however, the area/volume fraction of its microstructural feature was too small to affect the deformation mechanism. Instead of the deformation twinning-related fracture, cavitation due to grain boundary sliding was the origin of fracture during the present Erichsen tests. The further activation of grain boundary sliding was an effective method to improve the room temperature formability of magnesium alloys.

Cryorolling and warm forming of AA6061 aluminum alloy sheets

ABSTRACTCryorolling is a severe plastic deformation (SPD) process used to obtain ultrafine-grained aluminum alloy sheets along with higher strength and hardness than in conventional cold rolling, but it results in poor formability. An alternative method to improve both strength and formability of cryorolled sheets by warm forming after cryorolling without any post-heat treatment is proposed in this work. The formability of cryorolled AA6061 Al alloy sheets in the warm working temperature range is characterized in terms of forming limit diagrams (FLDs) and limiting dome height (LDH). Strain distributions and thinning in biaxially stretched samples are studied. Hardness of the formed samples is correlated with ultimate tensile strength to estimate post-forming mechanical properties. The limit strains and LDH have been found to be higher than in the case of the conventional processing route (cold rolled, annealed and formed at room temperature), making this hybrid route capable of producing sheet metal parts of aluminum alloys with high strength and formability. In order to combine the advantages of enhanced formability and better post-forming strength than the conventional cold rolled and annealed sheets, warm forming at 250°C has been found to be suitable for this alloy in the temperature range that has been studied.

Microstructures, Forming Limit and Failure Analyses of Inconel 718 Sheets for Fabrication of Aerospace Components

Recently, aerospace industries have shown increasing interest in forming limits of Inconel 718 sheet metals, which can be utilised in designing tools and selection of process parameters for successful fabrication of components. In the present work, stress-strain response with failure strains was evaluated by uniaxial tensile tests in different orientations, and two-stage work-hardening behavior was observed. In spite of highly preferred texture, tensile properties showed minor variations in different orientations due to the random distribution of nanoprecipitates. The forming limit strains were evaluated by deforming specimens in seven different strain paths using limiting dome height (LDH) test facility. Mostly, the specimens failed without prior indication of localized necking. Thus, fracture forming limit diagram (FFLD) was evaluated, and bending correction was imposed due to the use of sub-size hemispherical punch. The failure strains of FFLD were converted into major-minor stress space (σ-FFLD) and effective plastic strain-stress triaxiality space (ηEPS-FFLD) as failure criteria to avoid the strain path dependence. Moreover, FE model was developed, and the LDH, strain distribution and failure location were predicted successfully using above-mentioned failure criteria with two stages of work hardening. Fractographs were correlated with the fracture behavior and formability of sheet metal.

Effect of Spot-Weld Line Orientation on Formability for Patchwork Blank

In recent years, in order to achieve weight reduction and crashworthiness, automotive part was manufactured using patchwork blanks. The smaller patches were welded on a main sheet to achieve a local reinforcement, which is called the patchwork. In this study, the effect of the spot-weld line orientation on the formability of patchwork blanks for high-strength low alloy (HSLA) steel was investigated in uni-axial tension mode by limiting dome height (LDH) test. A FE-analysis and experiment was performed to gain a better understanding of the formability for limiting dome height and strain distribution. As a result of the LDH test and FE-analysis, differences in the dome heights and forming limits were observed patchwork blanks with spot-weld line orientation of 0°, 45°, and 90°, respectively. For the 0° and 45° patchwork blanks, the dome heights was lower than those of 90° patchwork blanks because of the stress concentration in the pole side.

Finite Element and Experimental Analyses on the Formability of Steel Sheets Produced by Compact Endless Cast and Rolling

Although the compact endless cast and rolling mill (CEM) is a promising candidate as a next-generation energy-saving steel process, due to its short history, the formability of the steel sheet produced by the CEM process are not known yet. Herein, drawability and stretchability of low-carbon steel sheets produced by the CEM process are investigated and compared with those of conventional hot-rolled low-carbon steel sheets, to estimate its applicability to industrial parts. Finite element analyses using the Gurson-Tvergaard-Needleman damage model were conducted and compared with the experimental results. Homogeneous microstructure and relatively strong textures of {111}||ND γ-fibers and 〈110〉||RD α-fibers were developed in the CEM-processed steel in comparison with the conventional hot-rolled specimen. The drawn cup of the CEM specimen showed weak earing phenomena, while having higher limiting drawing ratios (2.0 and 1.95 in the experimental and numerical simulation, respectively). Furthermore, a difference in limit dome height between the two specimens is negligible. Therefore, it is confirmed that CEM-processed steels have comparable properties of strength and formability, provide an effective manufacturing process, and exhibit good potential as a next-generation energy-saving process.

Mechanical Properties and Formability of Cold Rolled Friction Stir Welded Sheets in AA5754 for Automotive Applications

The effect of cold rolling on mechanical properties and formability of friction stir welded blanks in AA5754 aluminium alloy was investigated. To this purpose, FSW experiments were performed on 2.5mm thick sheets with rotational and welding speeds equal to 1200rpm and 100mm/min, respectively, and tool sinking of 0.1mm. Part of the FSWed sheets was cold rolled, with the rolling direction perpendicular to the welding line, until the obtaining of a constant sheet thickness equal to 2.2mm. Finally, the mechanical properties and formability of joints, both before and after cold rolling, were evaluated by means of uniaxial tensile and hemispherical punch tests; the experimental results, in terms of stress vs. strain curve, microstructure, and limiting dome height, were analysed and discussed.

Implementation of YLD-96 Plasticity Theory in Formability Analysis of Bi-axial Pre-strained Steel Sheets

In the present work, 15% bi-axial pre-strain with 0.98 strain ratio was induced in extra deep drawing (EDD) steel sheets using an in-plane stretch forming set-up. Subsequently the quasi-static uni-axial tensile responses were evaluated experimentally, and the YLD-96 plasticity theory was implemented to estimate the anisotropic coefficients of both as-received and pre-strained materials. The strain based forming limit diagrams (ɛ-FLD) were evaluated by deforming sheet materials under six different strain paths ranging from 0.94 to -0.31 in a limiting dome height (LDH) test set-up. It was found that the failure strains of the pre-strained material decreased significantly with a shift towards tension-tension region, and hence, the polar effective plastic strain-FLD (PEPS-FLD) was estimated using the formulated YLD-96 plasticity theory to restrict the dynamic shift of failure strains. The FE models of LDH testing of both as-received and pre-strained materials were developed, and the formability was predicted successfully by incorporating PEPS-FLD as a damage model within an error of approximately 6%.

Study of Limiting Dome Height and Temperature Distribution in Warm Forming of ASS304 Using Finite Element Analysis

Stretch forming is an important sheet metal forming process in which thin sheets are plastically deformed into desired shapes. All structural steels undergo changes in their properties when subjected to cryogenic temperatures and may become brittle at lower temperatures, but Austenitic stainless steel (ASS 304 grade) having FCC structure is found to be more durable and strong due to less interstitial defects. Even when subjected to lower temperatures, it remains tough. In the present work, the stretch forming of ASS 304 with varying width specimens is modelled using Finite Element Methods. These models are simulated using Barlat 3-parameter anisotropy material model to study the formability of material using Limiting Dome Height (LDH) technique at various temperatures like 25°C, 150O C and 300°C.

Experimental and Finite Element Studies of Stretch Forming Process for Ti-6Al-4V Alloy at Elevated Temperature

In this study, experimental and Finite Element (FE) analysis has been studied of stretch forming process for Ti-6Al-4V alloy at 400°C. Initially, uniaxial tensile tests experiments have been conducted to study the material properties at 400°C. Furthermore, stretch forming experiments have been performed using Nakazima test. The various quantitative parameters such as Forming Limit Diagram (FLD), Limiting Dome height (LDH) and thickness distribution are evaluated at 400°C. Additionally, Finite Element (FE) analysis of stretch forming process has been done using commercial available DYNAFORM software with LSDYNA server. Two different anisotropic yield criteria namely; Hill 1948 and Barlat 1989 with Hollomon Power Law (HPL) are consider for FE simulations. The results of FE simulations are compared with the experimental results. It has been observed that Barlat 1989 yield criterion prediction is more accurate than Hill 1948.

Development of Electrohydraulic Forming Process for Aluminum Sheet with Sharp Edge

Electrohydraulic forming (EHF), high-velocity forming technology, can improve the formability of a workpiece. Accordingly, this process can help engineers create products with sharper edges, allowing a product’s radius of curvature to be less than 2 mm radius of curvature. As a forming process with a high-strain rate, the EHF process produces a shockwave and pressure during the discharge of an electrical spark between electrodes, leading to high-velocity impact between the workpiece and die. Therefore, the objective of this research is to develop an EHF process for forming a lightweight materials case with sharp edges. In order to do so, we employed A5052-H32, which has been widely used in the electric appliance industry. After drawing an A5052-H32 Forming Limit Diagram (FLD) via a standard limiting dome height (LDH) test, improvements to the formability via the EHF process were evaluated by comparing the strain between the LDH test and the EHF process. From results of the combined formability, it is confirmed that the formability was improved nearly twofold, and a sharp edge with less than 2 mm radius of curvature was created using the EHF process.

Determination of forming limit diagrams of AA6013-T6 aluminum alloy sheet using a time and position dependent localized necking criterion

The forming limit behaviour of AA6013-T6 aluminium alloy sheet was characterized under isothermal conditions at room temperature (RT) and 250°C using limiting dome height (LDH) tests. Full field strain measurements were acquired throughout testing using in situ stereoscopic digital image correlation (DIC) techniques. Limit strain data was generated from the resulting full field strain measurements using two localized necking criteria: ISO12004- 2:2008 and a time and position dependent criterion, termed the “Necking Zone” (NZ) approach in this paper, introduced by Martinez-Donaire et al. (2014). The limit strains resulting from the two localization detection schemes were compared. It was found that the ISO and NZ limit strains at RT are similar on the draw-side of the FLD, while the NZ approach yields a biaxial major limit strain 14.8% greater than the ISO generated major limit strain. At 250°C, the NZ generated major limit strains are 31-34% greater than the ISO generated major limit strains for near uniaxial, plane strain and biaxial loading conditions, respectively. The significant variance in limit strains between the two methodologies at 250°C highlights the need for a validation study regarding warm FLC determination.

Failure prediction of AZ31B magnesium alloy sheet based on a micro-mechanical void model incorporating the asymmetric plasticity constitutive law

In this paper, a fracture criterion based on a micro-mechanics void model was applied to ductile fracture model of AZ31B magnesium alloy sheet. In particular, the fracture model was coupled with an advanced plasticity model in which the asymmetric cyclic response of magnesium alloy due to combined slip and twin deformation can be reproduced accurately. The extended McClintock fracture model adopted in this study as a fracture criterion for the magnesium alloy sheet takes into account the effect of shear deformation by adding Lode parameter dependency to the original model. Both the fracture model and the plasticity model were implemented in the explicit finite element code, LS-DYNA, via a user-defined material subroutine. The parameters of the proposed fracture model were identified using multi-directional mechanical tests, such as uniaxial tension, simple shear, and notched tension tests. As validation, the proposed model was compared with existing models for predicting the fracture patterns in test specimens and limit dome heights.

Effect of microstructure and texture on forming behaviour of AA-6061 aluminium alloy sheet

AA-6061 aluminium alloys are used extensively in automobile and aerospace industries owing to their excellent combination of mechanical and physical properties. This alloy exhibits prominent anisotropy in mechanical properties when produced as cold rolled sheets. In manufacturing, sheet metal anisotropy may have severe consequences for downstream processes such as stamping and deep drawing operations. In the present study, correlations among in-plane anisotropy, strain path and formability of AA-6061 sheet metal are investigated. Using limit dome height tests, forming limit diagrams (FLDs) were constructed for three different sheet directions by applying appropriate strain localization and fracture criteria. For each sheet direction and strain path, microstructure and texture evolution were also observed to identify the origin of in-plane anisotropy and formability of the AA-6061 sheet metal alloy. A detailed analysis of microstructure under different strain paths suggests the direction and texture dependent yield locus to be a significant factor for in-plane anisotropy. Intra-granular crack propagation in a particular sheet direction (transverse direction: TD) may lead to reduced formability of AA-6061. Formability, on the other hand, appears to be highly correlated to the relative fraction of specific texture components. Certain critical texture components such as Cube {001} <100> and Brass {011} <211> influence the forming behaviour of AA-6061 aluminium alloy significantly.

Formability analysis of pre-strained AA5754-O sheet metal using Yld96 plasticity theory: Role of amount and direction of uni-axial pre-strain

Automotive industries are very much interested in formability of different pre-strained aluminum alloy sheets in the context of multistage stamping to fabricate complex components. In the present work, different uni-axial pre-strains of 6.4% and 12.2% were induced in AA5754-O aluminum alloy both along rolling direction (RD) and transverse direction (TD). The true stress-strain response, limiting dome height (LDH) and strain based forming limit diagram (ε-FLD) of as received and all pre-strained materials were evaluated experimentally. The anisotropy constitutive material model was developed using the Yld96 plasticity theory in-conjunction with the Hollomon isotropic hardening law to predict the yield strength evolution of the pre-strained materials. Also, it was found that the limiting strains in ε-FLD shifted significantly depending on the amount and direction of uni-axial pre-strain. Hence, the limiting strains of the as-received materials were transposed into stress space to estimate the stress based forming limit diagram (σ-FLD) using the anisotropy constitutive material model. Further, the dynamic shifts of ε-FLDs of four different pre-strained materials were predicted by successfully decoupling the σ-FLD of as-received materials within root mean square error of 0.008. Finite element models of both uni-axial pre-straining and subsequent LDH tests were developed, and the forming behavior of the pre-strained materials were predicted implementing the Yld96 plasticity model and estimated σ-FLD. It was found that LDH was significantly influenced by the amount of pre-strain, and the maximum thinning location shifted close to pole in the case of 12.2% pre-strained materials. However, the effect of uni-axial pre-strain direction on both LDH and maximum thinning location in AA5754-O material was very negligible.

The effects of strength ratio on the forming limit diagram of tailor-welded blanks

In the present study, the influences of strength ratio (SR) are studied on the formability and forming limit diagram (FLD) of tailor-welded blanks (TWBs). AISI 340, St 12 and St 14 steel sheets with equal thickness of 1 mm were used as different kinds of steel to make TWB with different SR. TWBs were obtained by CO2 laser welding of different steel sheets. Limit strength ratio (LSR) is introduced as a new useful factor to predict the FLD of TWB with different SR. Results of this research show that with increasing of difference of TWB’s SR and LSR, formability and the level of FLD will decrease. By SR increasing, limit dome height decreases and some defects such as weld line movement increase. The experimental findings show that the SR of TWB can effect on the position of fracture in the TWB products.

Prediction of Thickness Distribution in Sheet Metal Forming by Modified Limiting Dome Height Test with and without Blank-Holding Force

Prediction of Thickness Distribution in Sheet Metal Forming by Modified Limiting Dome Height Test with and without Blank-Holding Force