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Forming limit curves (FLC) at high temperatures under plane-stress states are beneficial for guiding the process parameter design of hot forming superalloy sheets. Constrained by the high deformation temperatures, the establishment of FLCs of superalloy sheets at high temperatures is extremely difficult. This paper conducts an experimental and modelling research to establish the physical-based model with the accurate prediction of hot flow behavior as well as micro grain sizes, using which, the forming limit curves can be predicted. First, the uniaxial tensile tests under different temperatures and strain rates and electron backscattered scattering detection (EBSD) observations of typical conditions were conducted to obtain the hot flow behavior and grain evolution. Second, the dome test at room temperature was performed, and the corresponding simulation was further carried out to obtain the forming limit at room temperature under plane-stress states. Using the above results, a plane-stress constitutive model for GH3128 superalloy was established enabling plane-stress test results under typical hot conditions to be accurately predicted. The prediction accuracies of hot uniaxial and biaxial stretching results are 94.2% and 95.4%, respectively. [ABSTRACT FROM AUTHOR]

Knowledge of the deformation behaviour of Ti-6Al-4V using single-point micro incremental forming (SPMIF) is very important to understand the physics behind the microstructural changes, and forming limit. In SPIF, shape changes in sheet metals up to ultra-thin sizes can be performed without using a die and punch (does not require any specific tooling as in the conventional forming process) and hence, this process is recommended for the fabrication of parts in the aerospace, automobile, and bio-medical industries. Furthermore, in SPIF, the components are manufactured using a hemispherical end tool moving along a predefined path with an enhanced forming limit. The present research work has focused on studying the formability, microstructure, mechanical properties and fracture mechanics of Ti–6Al–4V alloy foils during SPMIF. The importance of spindle speed on the forming limits of the Ti–6Al–4V alloy foil was studied and it was found that the maximum forming limits were achieved at higher spindle speeds (200 rpm) due to strengthening of basal texture and weakening of prismatic texture components. A forming limit strain (FLS) was drawn at different spindle speeds (100, 150, and 200 rpm). XRD, EBSD and TEM analyses were performed for the phase analysis, orientation and dislocation density respectively. The fracture behaviour was investigated and the void coalescence parameters were compared with respect to spindle speed. [ABSTRACT FROM AUTHOR]

Electromagnetic forming (EMF) is a high-speed forming process with significant potential for manufacturing bipolar plates. However, titanium's low conductivity requires the use of a driver sheet in traditional EMF, limiting its formability improvement. This study proposed a novel electromagnetic forming method with independent loading of the magnetic field and current (EMF-ILMFC), enabling separate application of electromagnetic force and Joule heating. The dynamic deformation behavior of ultra-thin titanium was investigated using three different loading methods: quasi-static (QS), electromagnetic bulging with driver sheets (EMB-DS), and electromagnetic bulging with the new method (EMB-ILMFC). The study revealed that the forming limit under EMB-ILMFC was significantly higher than under EMB-DS and QS, with a maximum improvement of 139.1%. Numerical simulations and experimental analyses were conducted to elucidate the thermal effects on formability during dynamic deformation. The results indicate that thermal effects reduce flow stress, extend the duration of inertia and high strain rates, and thereby improve the formability of titanium.

Aluminum alloy possesses poor plastic deformation ability under quasi-static stamping (QS). Hence, the plastic deformation behavior and microstructure evolution of 5052 aluminum alloy in electromagnetic driven stamping (EMDS) and QS under different heat treatment conditions were investigated in this paper. Compared with QS, EMDS can respectively improve the strength and plasticity of 5052-O aluminum alloy by 0.9% and 12%, as well as 4.6% and 30% in 5052-H32 state. The enhancement effect of EMDS on 5052-H32 aluminum alloy is more obvious. This is due to the 5052-H32 aluminum alloy has more initial dislocations, which will promotes multi-system slip and cross-slip of dislocation at high strain rates. Microscopic analysis indicates that the 5052-H32 aluminum alloy in EMDS possessed a more uniform dislocation distribution and a higher grain refinement degree. For a higher forming height, the grain boundary, dislocation and texture exhibit more obvious effects on the strength of the materials. It provides a theoretical basis for the forming mechanism of 5052 aluminum alloy during EMDS under different heat treatment conditions.

Manufacturing of thin-walled components with protrusions holds importance for the advancement of extrusion technology, expanding the application scope of extrusion processes and achieving lightweight equipment. To accomplish these objectives, a novel two-step combined extrusion process is proposed, drawing on numerous research achievements and theories in flange forming. Detailed analysis of metal flow characteristics during extrusion is conducted using finite element simulation, and the strain heterogeneity index is introduced to quantitatively evaluate the influence of different process parameters and die cavity dimensions on the forming results. The results indicate that the forming quality of the protrusions depends on lower extrusion speeds, appropriate friction coefficients, and rounded die inlet corners. Furthermore, the generation mechanism of defects during the forming process is investigated through the analysis of billet and die cavity dimensions, leading to the establishment of a 3D forming limit diagram. Finally, multiple sets of physical experiments are designed and conducted to validate the feasibility of the forming process and simulation results. [ABSTRACT FROM AUTHOR]

To address the difficulty in forming ultra-thin stainless steel strips, this study focuses on 304 stainless steel ultra-thin strips. By conducting tension and forming limit experiments, the basic mechanical properties and FLC (Forming Limit Curve) of the material are determined, and its formability is systematically investigated. Additionally, to improve testing efficiency and reduce resource consumption, this paper predicts the FLC forming curve of the ultra-thin strip based on the M–K ductile damage model, which is then validated against experimental results, establishing a reliable FLC prediction model. Moreover, to relate it to practical industrial production applications, this study simulates the stamping process of box-shaped components made from the ultra-thin strip based on the theoretical model, exploring the influencing factors of stamping processes on the formability of the ultra-thin strip. The research findings indicate that among the hard, semi-hard, and soft stainless steel ultra-thin strips, the soft one exhibits the best formability, and the 0.05 mm thickness is less formable compared to the 0.1 mm strip. The simulation results demonstrate that the M–K ductile damage theory can reasonably predict the formability of the ultra-thin strip. Furthermore, optimizing the chamfer size in the stamping process, reducing the friction coefficient between the die and the ultra-thin strip, and lowering the stamping speed effectively improve the formability of the ultra-thin strip. [ABSTRACT FROM AUTHOR]

There are various methods to evaluate the forming limit of a sheet, and these criteria can be classified as position-dependent, time-dependent, and position-time dependent according to the basis of judgment. However, these criteria have a single function and can only find the forming limit of the sheet and cannot determine the strain distribution, strain change, or fracture location during the sheet forming process. This paper introduces a time–location-dependent method, i.e., the spatial strain rate method, which is used to detect the onset of necking of a sheet. The spatial strain rate is directly based on the strain and can not only find the forming limit of the sheet but also depict the strain distribution and strain variation during the two phases of the experimental process—distributed instability and concentrated instability—as well as predict the location of sheet fracture. The spatial strain rate of AA5083 aluminum alloy of different widths was analyzed and verified in detail via Nakazima experiments using digital image correlation techniques and compared with the guidelines published in the literature in recent years. [ABSTRACT FROM AUTHOR]

Low formability, low productivity and forming quality are the main factors restricting the industrial application of single point incremental forming technology. Ultrasonic vibration was introduced into the single point incremental forming process to improve the forming limit of sheet metal by changing the deformation mechanism of material. A device combining ultrasonic vibration with plastic deformation processing technology and single point incremental forming was developed in this work. Taking cone parts of 1060 aluminum as the research object, the effects of ultrasonic vibration amplitude and frequency on equivalent stress and equivalent plastic strain in forming process were analyzed by combining numerical simulation and experimental research. Then, the forming angle of cone parts was used as the standard to study the influence of ultrasonic vibration parameters on formability. The results show that the equivalent stress decreased and equivalent plastic strain increased by superimposing ultrasonic vibrations, ultrasonic vibration can significantly change the formability and improve the forming limit of 1060 aluminum, the forming limit of 1060 aluminum can be maximized by selecting ultrasonic vibration parameters: amplitude 0.03 mm and frequency 30 kHz. [ABSTRACT FROM AUTHOR]

In addressing the difficulty of forming Fiber Metal Laminates (FMLs), a laminate composed of a 0.1 mm-thick 304 Stainless Steel Foil (SSF) and Carbon Fiber Reinforced Plastics (CFRP) as the base material is prepared using a specific process. Subsequently, forming limit diagrams are obtained through hemispherical bulge forming experiments to investigate the forming performance of FMLs laminates. The damage phenomena of various forming specimens during the thermal stamping process are analyzed. Based on different damage forms, corresponding theoretical models are established to simulate and predict the forming damage modes of FMLs through simulation. The research results indicate that the stamping formability of FMLs is influenced by the damage tolerance of the thin stainless steel foil. During the thermal stamping process, FMLs may experience delamination, fracture/crack of the thin stainless steel foil, and rupture/tearing/wrinkling of the prepreg. To address these issues, cohesive elements, GTN-M-K damage, and Hashin damage models are established, providing a reasonable simulation prediction of the stamping damage phenomena of FMLs. [ABSTRACT FROM AUTHOR]

This work focuses on the limit strains of 5754-O aluminum alloy sheets with consideration of the hardening law effect. Based on uniaxial tension test data, the hardening laws of Swift, Voce, LSV and Hockett–Sherby were applied to determine the mechanical properties. The fitted parameters and the Yld2000-2d yield function were introduced into the Marciniak–Kuczynski (M–K) theory to predict the forming limit curve (FLC). This prediction was not consistent with the Nakajima test results. Assessment of the effect of the hardening law on the predicted FLC indicated that the hardening law affected the yield surface evolution through the hardening rate. Afterward, an improved LSV hardening law was proposed to depict the plastic stress–strain relationship, and both the theoretical prediction and the numerical simulation verified the validity of the improved model. The results were compared with the test data, and good agreement was shown. [ABSTRACT FROM AUTHOR]

Accurately predicting the forming limit (FL) of sheet metals between equi-biaxial tension (EBT) and uniaxial tension (UT) is a research focus in sheet metal forming field. However, up to now, there is still a lack of an uncoupled ductile fracture criterion (DFC) which not only can predict the FL of sheet metals between EBT and UT accurately but also can be easily extended to other DFCs for bulk and sheet metals, preventing embedding only one uncoupled DFC into finite element analysis (FEA) software to satisfy the application requirements of fracture initiation prediction within different stress state ranges. The aim of this paper is to address the issue. The problems encountered in recently developed DFCs are firstly revealed and then a new uncoupled DFC is presented according to the analyses of void evolution of sheet metals. To fully understand the proposed DFC, parametric studies are conducted. In addition, the proposed DFC and four recently developed DFCs are employed to forecast the FL strains of three different sheet metals and the extensibility of the proposed DFC is studied. Research results show that the proposed DFC can not only forecast the FL of various sheet metals between EBT and UT reasonably but can also be easily extended to a unified uncoupled DFC for bulk and sheet metals, enabling the implantation of one DFC into FEA software to meet the application requirements within different stress state ranges. Moreover, the capability of the presented DFC to forecast FL in actual forming process is also verified. [ABSTRACT FROM AUTHOR]

With the development of product miniaturization in aerospace and other fields, nickel-based superalloy thin sheets with excellent comprehensive properties have become crucial materials for micro-components in these fields. Due to the complexity of the initial microstructure and strengthening mechanism, there is a lack of in-depth research on the mesoscale formability of nickel-based superalloys under the coupled effects of multiple factors such as feature size (i.e., the ratio of specimen thickness to grain size, t/d), particle size and volume content of the precipitated phase. In this paper, a test platform was constructed for the application of high-precision strain measurements to the analysis of the mesoscale forming limit, and the mechanisms for the influences of feature size and precipitation on the mesoscale forming limit of a nickel-based superalloy were studied. The results showed that feature size influenced the forming limit of the nickel-based superalloy thin sheet at the mesoscopic scale, and the formability decreased with decreasing t/d and increasing γ′′/γ′ particle size and volume content. Based on surface layer theory and ductile fracture theory, the influences of t/d and γ′′/γ′ phases on the forming limit of nickel-based superalloy thin sheets at the mesoscopic scale were revealed. The coupled effects of surface roughening caused by free surface grains and hindered dislocation movement induced by precipitated phases resulted in strain localization and reduced the forming properties of nickel-based superalloy thin sheets at the mesoscopic scale.

In this paper, a novel method was proposed to improve the production efficiency and formability of titanium alloy, which was named triple-layer sheet hot stamping with a cold die cooling (THF). The titanium alloy sheet was clamped by two steel sheets with no adhesive added, then heated, transferred and formed together. In order to verify the ability in enhancing formability of this process, spherical part forming tests were carried out to study the formability under the single-layer sheet hot stamping (SHF) and THF. Finally, a plug socket part was formed to verify the practicability of the novel forming process. The finite element (FE) model was established to analyze the forming process finding that the formability of the parts were significantly improved after using THF process. Comparing with SHF process, the stamping depth increases by 135.7% at 900 °C in the THF process. The forming limit strain increases from 0.18 of the single-layer sheet to 0.51 of the triple-layer sheet. The forming limit strain obtained by the THF process is close to that of isothermal forming. The friction state is one of the main reason for improving formability. Moreover, for plug socket parts, there is almost no thinning of the part in the THF process at 900 °C. In conclusion, it has great advantages in improving the formability of titanium alloy using the THF process.

Blades are important parts of aero-engines, and their manufacturing technology directly affects the preparation level and development speed of aero-engines. In view of the many problems existing in the current blade preforming process, this paper proposes a potential way to prepare blade preforms by using cross wedge rolling (CWR). Furthermore, considering the shape characteristics of the blade, a blade preform with an elliptical section formed by CWR was thoroughly investigated. The roll's profile curve is of significant importance to forming quality, and its equation was deduced based on the plane meshing principle between the roll and the rolling piece and verified by finite element (FE) analysis and CWR experiments on a homemade platform. Meanwhile, the accuracy of the established FE model was quantitatively tested and used to study the forming limit expressed by eccentricity e, and the forming limit was approximately 0.8 (i.e., e ≤ 0.8). Based on this, a blade preform for an aero-engine compressor was designed with two elliptical sections, and it was well formed by the FE model, which verified the feasibility; these research results can also be extended and applied to the preform of other nonrotating parts and other materials. [ABSTRACT FROM AUTHOR]

Accurate prediction of the forming limit at necking of strongly anisotropic thin-walled tubes is an urgent requirement in tube hydroforming. M-K model is one of the most widely used prediction models. However, the failure criterion in M-K model is based on the ratio of major true strain increments in the groove to uniform regions, which sometimes results in overestimation of the limit strains. To address this issue, a combined model of M-K model and ductile fracture criterion (DFC) for tube was proposed in this paper, in which the failure condition of the groove region is determined by the DFC. The characteristics of the M-K + DFC model were analyzed by combining with the DF2012 phenomenological DFC. To validate the M-K + DF2012 model, the limit strains at necking of an AA6061 tube with strong in-plane anisotropy were tested through tube-controllable biaxial loading experiments. The results show that the forming limit curve (FLC) predicted by the M-K + DFC model does not exceed the FLC calculated by the M-K model and the fracture forming limit curve (FFLC) determined by the corresponding DFC. The M-K + DF2012 model provides a reasonable forming limit prediction of the AA6061 tube, and the shortcoming of the M-K model mentioned earlier is overcome. Meanwhile, a reasonable constitutive model considering the in-plane anisotropy is very important in the forming limit prediction of a strongly anisotropic thin-walled tube. [ABSTRACT FROM AUTHOR]

The forming limit of aluminum alloy (AA) 7075 sheets depending on the deformation mode, the strain-rate, and the temperature was evaluated under the pneumatic stretching technique at elevated temperatures. In order to perform the pneumatic stretching test and to prevent the unfavorable fracture on the edge with the targeted strain-rate, the previously introduced die shape designs were utilized with the pressure-time profile. The two different kinds of strain-rate and temperature-sensitive forming limits were determined, where one is the necking-based forming limit (NFL) and the other is the fracture-based forming limit (FFL). The results show that FFL is highly deviated from NFL at 400 °C than those at 470 °C due to abrupt strain localization, which leads to high dome height at the pneumatic forming during the relatively lower elevated temperature. The microscopical analysis (optical and scanning electron) reveals that higher temperature condition, 470 °C, has early fracture due to the high diffusion of micro-voids and faster coalescence of the voids. The well-known three parameters fracture criteria e.g. Johnson-Cook (J-C), Modified Mohr-Coulomb (MMC3), Lou, and Hu fracture model were multiplicatively extended to strain-rate relationships based on the suggested general equation and independently characterized for each elevated temperature with the error analysis.

The conventional theoretical forming limit diagram (FLD) based on the plane stress state has defects due to the neglect of the stress in the thickness direction of the sheet. It is urgent to study the FLD which considers the stress in the thickness direction. Through the finite element simulation, it is further confirmed that the stress in the thickness direction will be induced in the deformation process. At the same time, the effect of the induced stress in the thickness direction on FLD is not the same as that always applied stress in the thickness direction during the deformation process. Based on the Modified Maximum Force Criterion (MMFC) instability theory, C-H instability theory, Mises and Hill48 yield criterion, the effects of the two stresses on FLD are compared. The induced stress in the thickness direction also affects the sensitivity of the coefficient of normal anisotropy r , but does not significantly change the effect of the stress-strain index n on Forming Limit Curves (FLC). The theoretical calculations of the three materials TRIP780, AA5182, and 5754O are compared with the experimental data, which proves that the FLD which considers induced stress in the thickness direction is more accurate than the FLD of the plane stress. [ABSTRACT FROM AUTHOR]

To meet the requirement of lighter weight and better performance in tube hydroforming, one of the most important tasks is to accurately predict the forming limit of thin-walled tubes under nonlinear loading paths. This work established the M-K+DF2012 model, a combination of the M-K model and the DF2012 ductile fracture criterion, for the forming limit prediction of thin-walled tubes under nonlinear loading paths. In this model, the failure of the groove is determined by the DF2012 criterion, and the corresponding strains in the uniform region are the limit strains. The limit strains of an AA6061 aluminum alloy tube under a set of linear loading paths and two typical nonlinear loading paths were tested. Parameter values of the M-K+DF2012 model for the tube were determined based on the experimental limit strains under linear loading paths, and the limit strains under the two nonlinear loading paths were predicted. Then the strain-based forming limit diagram (ε-FLD) and the polar effective plastic strain FLD (PEPS-FLD) of the tube under different pre-strains were predicted and discussed. The results show that the limit strains of the tube are obviously path-dependent, and the M-K+DF2012 model can reasonably capture the limit strains of the tube under both linear and nonlinear loading paths. The predicted ε-FLD shows a strong dependence on the pre-strain, while the predicted PEPS-FLD is weakly strain path-dependent and almost path-independent on the right-hand side for the AA6061 tube. [ABSTRACT FROM AUTHOR]

The profile with rectangular ribbed (日-shaped) section is applied to the anti-collision beam of commercial vehicles, which not only improves the anti-collision performance but also improves the lightweight of the car. With the gradual decrease of the bending radius of the profile, the profile is prone to defects such as wrinkling of the inner wall or fracture of the outer wall. In this paper, the ABAQUS, a finite element simulation software, is used to establish the simulation model of the profile with rectangular ribbed section during the process of rotary draw bending, and the stress and strain laws are analyzed. The bending forming limit of the profile with rectangular ribbed section is studied by choosing the fracture of the outer wall as the evaluation standard. And the influences of bending speed, friction coefficient, and thickness on the forming limit are obtained. The minimum bending radius decreases as the bending speed increases and increases with increasing of friction coefficient and thickness. [ABSTRACT FROM AUTHOR]

It has been reported that the forming limit of materials can be improved during electromagnetic forming (EMF). However, the detailed mechanism of this phenomenon remains unclear. The effect of initial state on the EMF process of AA1060 is systematically investigated. AA1060-O and AA1060-H24 alloys are subjected to quasi-static (Q-S) stamping and EMF, respectively. Compared to the Q-S stamping, the forming limit and hardness of AA1060-H24 sheet are increased by 68.3% and 9.4% under EMF. However, the forming limit of AA1060-O sheet is not improved. It is found that both groups of aluminum alloys exhibit high dislocation density after Q-S stamping and the dislocation movement occurs via the plane slip mode. Moreover, dislocation cells and wavy slips are observed in AA1060-H24 sample after EMF. In the case of AA1060-O sample, the intragranular dislocation density is low without wavy slip. Therefore, the wavy slip mode may be the reason for improved forming limit under the influence of EMF. In addition, the textural strength of EMF sample is different from the Q-S deformation sample and EMF sample exhibits a higher Taylor factor value. The changes in grain size, dislocation density and texture strength alter the overall strength. The current study provides the theoretical basis for the mechanism of improved forming limit under the action of EMF.

The experimental acquisition of electromagnetic forming limit is significantly confined due to the non-uniform pulse pressure of electromagnetic actuator and the complex deformation behavior of sheet metal. Hence, it was proposed to determine the electromagnetic forming limit diagram of aluminum alloy sheet by combining theoretical model, numerical simulation and experimental verification. The Marciniak–Kuczynski model embedded with Cowper-Symonds rate-dependent hardening model was derived in order to consider the strain rate effect on the electromagnetic formability of aluminum alloy sheet. The self-loop samples were newly designed with long oval coil for electromagnetic forming experiments to accomplish the typical deformation paths of uniaxial tension and plane strain. The strain rates and strain paths were investigated by numerical simulations. With the electromagnetic forming limits under uniaxial tension and plane strain conditions, the Marciniak–Kuczynski model was calibrated to fully determine the electromagnetic forming limit diagram. The electromagnetic bulging experiment was carried out to validate the electromagnetic forming limit diagram. Compared with the quasi-static forming limit diagram, the electromagnetic forming limit diagram of AA5182-O aluminum alloy is much higher. The electromagnetic forming limits under uniaxial tension, plane strain and equal-biaxial tension conditions are improved by 43%, 53% and 59%, respectively. [ABSTRACT FROM AUTHOR]

• Empirical failure point in tight-radius three-point bending through direct crack geometry observation. • In-situ profile scanning method for the bending test, enabling quantification of slow crack propagation. • Analysis of crack growth behaviour in diverse high-strength steels and rolling directions. • Phenomenological model for estimating crack depth in tight-radius three-point bending of high strength steels. Determining the bending-fracture limit, also referred to as local formability, is crucial for sheet metal forming. However, this is limited by existing methods such as the VDA-238-100 standard due to limitations in its failure criterion based on the punch load. This study addresses this limitation by proposing a quantitative crack-based failure criterion and investigating its relationship to the load-drop point. Along with surface and cross-sectional optical analyses, an integrated line-scanning system for the VDA-238 test is developed, and advanced high-strength steel (AHSS) grades are analysed with yield strengths ranging from 980 to 1500 MPa in both the rolling and transverse directions. The results showed that, while the load-drop point marks a rapid shift in the crack-propagation direction, the true onset of crack initiation occurs earlier, with different patterns for each AHSS and direction. For all the AHSS and directions tested, a crack depth exceeding 1 % of the sheet thickness occurs at 90 % of the switch-off stroke in the VDA-238 test, suggesting an empirical tendency toward microcrack growth for a more accurate evaluation of the local formability based on early cracks. [ABSTRACT FROM AUTHOR]

The characterization of forming limit utilizing Cu-Cr-Zr-Ti alloy sheets during the multistage forming process is imperative for successfully fabricating thrust chamber liners used in satellite launch vehicles. Therefore, the present work studies the effect of prestrain on the forming limit and failure behaviour of this alloy under different deformation paths. A stretch-forming setup was utilized to induce 8% equi-biaxial prestrain (EBP), and subsequently, these prestrained specimens were deformed till the onset of failure using the same setup. After prestraining, a shift in the failure strains was observed in the principal strain space, whereas it was found to be absent in the polar effective plastic strain, principal stress and effective plastic strain-stress triaxiality loci. The cup height along different deformation paths was marginally decreased in the prestrained specimens because of the reduction in failure strains, and the fracture occurred in the prestrained region. Further, failure analysis revealed intergranular cracks in the specimens deformed along plane strain and uniaxial deformation paths due to the presence of micron-size Cr-rich precipitates along grain boundaries. However, intergranular and transgranular cracks were present in the prestrained specimens, which were deformed equi-biaxially similar to the as-received specimens. The presence of nano-size Cr-rich precipitates inside the grains acted as nucleation sites for voids during equi-biaxial stretching, leading to transgranular cracks. It was also observed that failure in the prestrained and as-received specimens occurred at a similar effective plastic strain value due to the combined effect of void density and void size. Based on experimental observations, it was inferred that the forming limit of Cu-Cr-Zr-Ti sheets under a two-stage forming process was primarily affected by the presence of micro and nano-size Cr-rich precipitates. • Effect of biaxial prestrain on failure behaviour of Cu-Cr-Zr-Ti was characterized. • Prestrained and as-received specimens failed at similar effective plastic strain. • Cr-rich precipitate acted void nucleation site which affected the failure modes. • Intergranular and transgranular cracks were observed in prestrained specimens. [ABSTRACT FROM AUTHOR]

A technology named as electrically assisted electromagnetic forming (EAEMF) was used to improve the plastic deformation ability of 5052 aluminum alloy at room temperature. The ANSYS software is used to study the force and deformation of workpiece based on sequential coupling method. The simulation results show that the pulse current can increase the current density, improve the uniformity of current distribution, and increase the forming height of the sample. The primary strain and secondary strain are increased by 53.8% and 27.4% respectively. Under the same deformation condition, the elongation of EAEMF sample increases by 160%, while the strength decreases slightly. From the perspective of microstructure evolution, the dislocation density of EAEMF sample is lower than that of EMF sample, and the pinning effect of the second phase particles on the dislocation is weakened. Under the action of the pulse current, the dislocations continue to move around the second phase particles with higher energy, and the dislocations appear directional arrangement. [ABSTRACT FROM AUTHOR]

Sheet metal forming limit diagram (FLD) is a comprehensive index used to evaluate sheet metal formability. In this paper, the forming limit curves of GH605 sheet materials with 0.2 mm and 2.5 mm thickness under three different heat treatment conditions were obtained by numerical simulation of bulging. Then, the simulated forming limit results of the three materials with 0.2 mm thickness were compared with the experimental results. The results show that the simulated FLDs can be in good agreement with the experimental FLDs. The influences of work hardening index (n value) and anisotropy index (r value) on the forming limit are analysed from the view of material property difference. The comprehensive simulated and experimental results show that the formability of the sheets after the two kinds of solution treatment is significantly better than that of the annealed sheets. The forming limit curve increases with the increase of n value and r value, and the influence of n value on the forming limit curve is more significant than that of r value. The forming limit diagram established by numerical simulation can provide a theoretical basis for material development and process selection.

Accurately forecasting the forming limit of metal foils is essential in their micro-forming due to the appearance of necking failure easily. In order to accurately and easily forecast the forming limit of metal foils influenced by size effect, an idea for failure modeling of metal foils using the mechanism that different fracture modes of metal foils are related to shear bands is presented in this work. Based on the idea, a simple ductile failure criterion that considers the maximum shear stress and grain size effect but excludes the stress triaxiality is proposed, motivated by the demand to decrease the complexity of parameter calculation tests in micro-forming failure prediction. The proposed criterion and another criterion called as the μ-DF2022 model are applied to capture the forming limit curves of various thick copper and 304 stainless steel foils with different grain sizes to verify its advantage and performance, and the corresponding mechanisms of their prediction capabilities are also elucidated. Furthermore, the proposed and μ-DF2022 models are applied to forecast the limit drawing ratio of a 304 stainless steel foil to further show its performance and advantage in real micro-forming. The applications show that the proposed failure criterion effectively forecasts the forming limit of metal foils between equi-biaxial tension and uniaxial tension influenced by size effect. Moreover, the prediction accuracies using the proposed model and the μ-DF2022 model are comparable, whereas the proposed model does not require complex micro equi-biaxial tension tests, which are typically unavailable to many engineers and researchers, to calibrate its material parameters. Therefore, it is recommended to utilize the proposed criterion to capture the forming limit of metal foils in micro-forming. This work advances the forming limit modeling method of metal foils and provides insight into the application of uncoupled ductile failure criteria in the micro-forming of metal foils, which contributes to the development of their optimal micro-forming processes. [Display omitted] • An idea for failure modeling adopting the mechanism that distinct fracture modes of metal foils with various grain sizes are related to shear bands is presented. • A simple failure criterion for the forming limit prediction of metal foils is established based on this idea. • The criterion reduces the complexity of parameter calibration process by eliminating the need for micro equi-biaxial tension tests. • Stress triaxiality has a small effect on forming limit of metal foils with fewer than about eleven grains in thickness direction. [ABSTRACT FROM AUTHOR]

• A theoretical framework for constructing Meso/Micro scale forming limit criteria of composite metal foils is provided. • A new ductile failure criterion for micro forming of composite metal foils is developed. • Interfacial layer, grain size and free surface roughening effects of heterogeneous materials are considered in the new criterion. • The proposed criterion is able to effectively predict the forming limit of composite metal foils between equi-biaxial tension and uniaxial tension. • The validity of the proposed failure criterion is demonstrated via forming limit strains and a practical micro-forming process. In order to design robust and reliable micro forming processes for composite metal foils, it is crucial to accurately forecast their forming limit under various strain paths. To date, however, there are few criteria that can effectively predict the forming limit of composite metal foils influenced by size effect. To solve this problem, a new uncoupled failure criterion is developed in this study, which considers the effects of interfacial layer, free surface roughening, grain size and strain path of heterogeneous materials, based on the deformation characteristic and failure mechanism of composite metal foils. Subsequently, it is presented the methodology for calculating the material constants of the proposed failure model. Additionally, the experimental forming limit strains of Ni/Cu clad foils with various grain sizes are utilized to show the prediction capability of the proposed model. To further confirm the application capability of the new failure criterion in practical micro forming, the finite element method incorporating the proposed criterion is employed to predict the failure behavior of Ni/Cu clad foil micro-channels in rubber pad forming. Comparisons indicate that the proposed model can accurately capture the forming limit of the composite metal foils influenced by size effect. This work provides a new scientific basis for determining the forming limit of composite metal foils. [ABSTRACT FROM AUTHOR]

The conventional theoretical forming limit diagram (FLD) based on the plane stress state has defects due to the neglect of the stress in the thickness direction of the sheet. It is urgent to study the FLD which considers the stress in the thickness direction. Through the finite element simulation, it is further confirmed that the stress in the thickness direction will be induced in the deformation process. At the same time, the effect of the induced stress in the thickness direction on FLD is not the same as that always applied stress in the thickness direction during the deformation process. Based on the Modified Maximum Force Criterion (MMFC) instability theory, C-H instability theory, Mises and Hill48 yield criterion, the effects of the two stresses on FLD are compared. The induced stress in the thickness direction also affects the sensitivity of the coefficient of normal anisotropy r, but does not significantly change the effect of the stress-strain index n on Forming Limit Curves (FLC). The theoretical calculations of the three materials TRIP780, AA5182, and 5754O are compared with the experimental data, which proves that the FLD which considers induced stress in the thickness direction is more accurate than the FLD of the plane stress.

To meet the requirement of lighter weight and better performance in tube hydroforming, one of the most important tasks is to accurately predict the forming limit of thin-walled tubes under nonlinear loading paths. This work established the M-K+DF2012 model, a combination of the M-K model and the DF2012 ductile fracture criterion, for the forming limit prediction of thin-walled tubes under nonlinear loading paths. In this model, the failure of the groove is determined by the DF2012 criterion, and the corresponding strains in the uniform region are the limit strains. The limit strains of an AA6061 aluminum alloy tube under a set of linear loading paths and two typical nonlinear loading paths were tested. Parameter values of the M-K+DF2012 model for the tube were determined based on the experimental limit strains under linear loading paths, and the limit strains under the two nonlinear loading paths were predicted. Then the strain-based forming limit diagram (ε-FLD) and the polar effective plastic strain FLD (PEPS-FLD) of the tube under different pre-strains were predicted and discussed. The results show that the limit strains of the tube are obviously path-dependent, and the M-K+DF2012 model can reasonably capture the limit strains of the tube under both linear and nonlinear loading paths. The predicted ε-FLD shows a strong dependence on the pre-strain, while the predicted PEPS-FLD is weakly strain path-dependent and almost path-independent on the right-hand side for the AA6061 tube.

In order to accurately predict the forming limit curve of DP780 steel sheet under the condition of warm forming,the ductile fracture prediction model was introduced into the numerical simulation. Based on the unidirectional tensile test results analysis of DP780 steel sheet in the temperature range of 573 K ～ 873 K and strain rate range of 6. 67×10-4 s-1～ 6. 67×10-3 s-1,the warm forming performance of DP780 steel sheet is better under the condition of temperature of 673 K and strain rate of3. 33×10-3 s-1 determined by the product of strength and ductility. And through the simple tensile test and simulation under this deformation condition,determined the main factors affecting the ductile fracture of DP780 steel sheet. Thus a model for predicting tensile fracture and shear fracture of DP780 steel under different strain paths was developed,by combining it with the warm forming constitutive model and the Hill ’48 yield criterion,the finite element simulation of the rigid mold bulging test was realized,and the forming limit curve of DP780 steel sheet at the temperature of 673 K was predicted,The mathematical model of forming limit was established. The results show that the forming limit results predicted by this method have better precision.

Advanced manufacture technologies, oriented to implants featured with high reliability, low cost and high biocompatibility, are key approaches to advance the rehabilitation efficacy and shorten the recovery period of patients. Considering the shortage of current fabrication processes, the incremental sheet forming (ISF) which is featured as a flexible forming technique is applied to manufacture implants. This review begins with the overview of ISF for different kinds of materials and implants. Then, the forming quality of implants formed by ISF was comprehensively reviewed and discussed in various aspects including the geometric accuracy, surface quality, and the forming limit. Moreover, various micro manufacturing processes and the size effect were summarized for the purpose of surface modification of medical implants. Based on the investigation of micro manufacture techniques, a novel macro-micro hybrid ISF technique was proposed to form the macro-profile and the micro-structures of implants synergistically through a single manufacturing process with different scales. Finally, future scope and potential research directions were addressed.

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Roll forming can improve the material utilization rate and production efficiency of cups with a curved rotary profile, but there is no basis for the determination of forming limit. The DEFORM-3D software was used to simulate the roll forming of cups. The influence of the billet wall thickness and bottom thickness, coefficient of friction, radius of roller, and the fillet radius of the punch on the forming limit was studied, and the damage value and velocity vector were analyzed. The results showed that the forming limit of the billet’s wall thickness in roll forming for a cup is about 62%. With the increase of the ratio of the formed cup’s wall thickness to the billet’s bottom thickness, the forming limit of wall thickness will be slightly reduced. A larger radius of roller, fillet radius of punch, and friction coefficient between punch and billet and a smaller friction coefficient between roller and billet are good for decreasing the damage value and improving the roll-forming limit. According to the numerical simulation results, the roll-forming limit diagram of cups is established, and the accuracy of the forming limit diagram is verified by experiments.

With the development of the shipbuilding, chemical engineering, and military industry, the demand for bending the metal plates with large curvature is increasing dramatically. Take the shipbuilding as an example, the line heating method is widely applied. However, this traditional method has significant disadvantages. In order to improve the manufacturing efficiency and precision, a novel method called incremental bending has been presented by our team, which is based on the minimum energy principle and model-less control method. However, due to the limited formability of the metal plates at room temperature, the bending curvature of the plates during the incremental bending process cannot meet the requirements of the shipbuilding industry. Researches show that heating can reduce springback and improve the metal formability effectively. So here in this paper, a new method called heat-assisted incremental bending is presented. In this method, the metal plate is supported by several supporting pillars and the punch moves according to the loading trajectory, which is calculated by minimum energy method. In addition, the induction heating system is applied to heat the plates at the punching positions. The bending process continues step by step until the metal plate achieves the designed shape finally. During this study, the springback behavior of the metal plates during the heat-assisted incremental bending process was investigated based on theory, numerical simulation, and experiment. Then, the metal plates with large single curvature and variable curvature were deformed based on the novel thermal-mechanical coupled metal forming process. The experimental results show this new forming process can better control the springback behavior of the metal plates and achieve the objective metal plates with lager curvature with higher processing efficiency and accuracy. [ABSTRACT FROM AUTHOR]

The precise determination of the forming limit strains of a sheet metal is necessary in order to accurately predict the onset of failure in sheet metal forming processes. The method used to detect the onset of necking (i.e. the necking criterion) is a most important factor in formability analysis as it significantly affects the forming limit strains. Many different necking criteria have been developed, which take advantage of the flexibility and high resolution of digital image correlation (DIC) strain measurements. Several time-dependent and time/geometry-dependent necking criteria were carefully investigated in order to evaluate their ability to reliably and consistently detect the onset of necking of TRIP780 sheet specimens that were stretch-formed over a hemispherical punch. It was found that the geometry-dependent, surface slope criterion was the most robust and consistent criterion of those evaluated. [ABSTRACT FROM AUTHOR]

In this study, an investigation on the densification behavior and forming limit of powder metallurgy composites containing hard carbide particles were carried out. The selected composite materials are Al4TiC, Al4WC, Al4Fe3C and Al4Mo2C. The compacts of 0.82 and 0.86 relative density and 0.4 and 0.6 aspect ratio were prepared on a 1-MN capacity hydraulic press using suitable die-set assembly. Sintering was carried out at 594 ℃ for 60 minutes in an electrical muffle furnace. Hot upsetting was carried out at the sintering temperature immediately after the sintering process and the forming process was stopped once visible cracks were seen on the free surface. Flat dies on the upper and lower surface were employed under dry friction conditions during hot upsetting. Finally, the densification behavior and forming limit of sintered-forged aluminium composite preforms is presented in this research work. It was concretely noted that Al4TiC produced the best density amongst all the above mentioned aluminium composites. Further, it was seen that Al4Mo2C and Al4WC produced good final height and diameter strain at fracture.

Forming limit is an important performance index and process parameter in sheet metal processing. In this paper,A method for the thermal incremental forming of liquid while supporting and heating was proposed. An inverted cone was studied as a research object. The influence of process parameters such as the temperature,the distance between the layer and the diameter of the tool head on the thermal incremental forming was studied by means of orthogonal test,and the theoretical analysis is carried out. The results show that the forming limit angle increases obviously with the increase of the forming temperature,and the smaller the distance between the layers increases with the increase of the layer spacing. The limit angle of the tool head diameter is not significant.

Copyright of China Mechanical Engineering is the property of Editorial Board of China Mechanical Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper reviews the most recent models for description of the anisotropic plastic behavior and formability of sheet metals. After a brief review of classic isotropic yield functions, recent advanced anisotropic criteria for polycrystalline materials of various crystal structures and their applications to cup drawing are presented. Next, the discussion focuses on novel formulations of anisotropic hardening. A brief review of the experimental methods used for characterizing and modeling the anisotropic plastic behavior of metallic sheets and tubes under biaxial loading is presented. The experimental methods and theoretical models used for measuring and predicting the limit strains, development of new tests for determining the Forming Limit Curves (FLC), as well as on studying the influence of various material or process parameters on the limit strains are presented. [ABSTRACT FROM AUTHOR]

This work was aimed to experimentally and theoretically investigate the formability of a new magnesium alloy sheet at room temperature. The fracture forming limit diagram was predicted by MMC3 and DF2014 models, where the non-linear strain path effect was taken into account by means of damage accumulation law. In order to obtain the instantaneous values of the stress triaxiality and the Lode parameter during the deformation process, strains tracked by digital image correlation technique were transformed into stresses based on the constitutive equations. The fracture forming limit diagram predicted by the fracture models was compared with the forming limits obtained by ball punch deformation tests. The prediction errors were evaluated by the accumulative damage values, which verified the advantages of ductile fracture models in predicting the forming limits of the magnesium alloy sheets. [ABSTRACT FROM AUTHOR]

In the present work, the effect of hardener to resin ratio of epoxy adhesive on the formability of adhesive bonded steel sheets has been studied. Forming limit curve has been evaluated by hemispherical dome tests at predefined strain‐paths. They are predicted by finite element simulations using strain mapping method. Cohesive zone model has been used to model the interface between skin and adhesive core. An improvement in the forming limit of sandwich steel sheets has been observed when the hardener to resin ratio is changed from 0.6 : 1 to 1 : 1 due to the improved plasticity of adhesive core layer at 1 : 1 ratio as compared to others. The forming limit of sandwich sheet made at 1 : 1 ratio is equivalent to base steel sheet of same thickness and grade. This shows the potential use of sandwich sheet in place of base steel sheet. The forming limit curve predictions agree well with experimental data for base sheet, while reasonable agreement is observed in case of sandwich sheet. Numerical prediction of interface delamination show insignificant influence of hardener to resin ratio on the onset of delamination and significant effect of strain‐paths. [ABSTRACT FROM AUTHOR]

In this paper, a new approach is proposed for stress state - dependent flow localization in bifurcation failure model bounded through ductile damage in dynamically loaded sheets. Onset of localized necking is considered in phenomenological way for different strain rates to draw the forming limit diagram (FLD). Using a strain metal hardening exponent in the Vertex theory related to the strain rate helps investigate ratedependent metal forming limits. Besides, the paper utilizes the model of ductile damage as a function of strain condition, stress states (triaxiality and Lode parameters), and the symbols of stiffness strain to predict the onset of the necking. It is worth noting that updated level of elasticity modulus in the plastic deforming is attributed as an essential index for the ductile damage measuring. According to original formulations, a UMAT subroutine is developed in the finite element simulation by ABAQUS code to analyze and connect the related constitutive models. Results reveal that the FLD levels increase for St 13 material through enhancing the strain rate. [ABSTRACT FROM AUTHOR]

The superplastic formability of the as-rolled 2A97 Al-Li alloy was studied under various stress states with the use of both tensile tests and bulge tests deformed at 350-430 °C. The flow stress curves of the 2A97 alloy at the test temperature with an initial strain rate ranging from 1x10-3 s-1 to 3x10-2 s-1 were characterized by the modified Hooke equation and the Grosman equation. The bulge tests were performed under four different strain paths, ranging from equi-biaxial to approaching plane strain. The effects of temperature and bulge-tool aspect ratio on the dome height, forming pressure, thickness distribution, and cavitation were investigated. Dome height and forming pressure appear to be more sensitive to tool ratio than temperature. Increasing the temperature in bulge forming was found to improve the thickness distribution in the formed parts, but it may lead to excessive cavitation. The results of the forming-limit diagram show significant scatter in terms of failure strain. It indicates that a determination of the forming limits of superplastic 2A97 cannot be represented with a simple forming-limit diagram and additional metrics for internal cavitation need to be considered. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Various approaches have been attempted by researchers to break through the limitation of axisymmetric shape in metal spinning. In this study, a synchronous multipass spinning method for offset blanks is proposed. The motion of the roller and the rotation of the mandrel are synchronized to achieve asymmetric shapes. The roller trajectory is represented by a set of scattered data which contains the axial position of the roller, the radial position of the roller, and the rotational angle of the mandrel. Conventional spinning for offset blanks is also conducted in comparison with synchronous spinning. The geometries of the products with different offset values λ are examined. The axial distribution of wall thickness varies at different circumferential positions of the product due to the oblique shape. The thickness increases from the short side to the long side of the product. A larger offset value λ results in a larger product diameter due to the springback of the material. Forming limits in terms of roller feed ratio f, offset value λ, blank thickness t0, blank diameter D, and the number of passes are investigated. Both fracture and wrinkling occur in the experiments. Due to the oblique shape, wrinkling tends to occur during the first several passes on the long side of the flange, and fracture tends to occur during the last several passes on the long side of the product. The experiments are more likely to succeed when a small offset ratio λ, a small blank diameter D, a small feed ratio f, or a large blank thickness t0 is used. Compared with synchronous spinning, the difference between the axial thickness distributions on both sides of the product is smaller in conventional spinning due to the uniform feed ratio. In terms of forming limits, conventional spinning has a slightly better performance. [ABSTRACT FROM AUTHOR]

For the free-bending forming of complex bending components, the forming limit significantly influences the complexity of the forming materials. In this study, the wrinkle factor Iw, which comprehensively affects the forming limit of free-bending, was obtained by theoretical analysis. The process parameters affecting the forming limit of free-bending were systematically studied by using the ABAQUS finite element simulation platform. Moreover, the effects of series of parameters on the relationship curve and wrinkling factor under different eccentricity conditions were also analyzed. The free-bending forming limit under the optimal conditions was obtained, which provides a process optimization method to improve the forming limit for practical test and production. Furthermore, the forming experiments were conducted on tubes with different wall thickness, and it was verified that the wall thickness factors significantly influenced the forming limit of the tubes. Finally, the forming experiments were carried out on 6061-T6 aluminum alloy, H2 copper, and H62 brass tubes. The results proved that the yield strength was the key material parameter affecting the forming limit of metal tubes. [ABSTRACT FROM AUTHOR]