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In this work, the onset of failure induced by severe strain at elevated temperature was numerically estimated with cross-formed empirical hardening law describing material softening. The hardening law can replicate the rate-sensitive behavior of aluminum alloy 7075 sheets (thickness of 2.0 mm) with initial hardening and progressive material deterioration caused by dynamic recrystallization, dynamic recovery, and micro-void development. The characterized material was applied to the two-step hybrid forming process consisting of a drawing at 400 °C followed by a pneumatic forming at 470 °C to produce a shock absorber housing with an extremely complex shape. The user-defined subroutine codes, VUMAT (ABAQUS/Explicit) and UMAT (ABAQUS/Standard), were sequentially utilized for the drawing and the pneumatic forming, respectively. The identified hardening parameters based on uniaxial tensile tests were validated by simulating the two-step hybrid forming process and compared with the conventional Voce type law (converging function) and the combined Swift-Voce type law (ever-increasing function) since they play a key role in accurately predicting the onset of failure induced by severe strain localization. Finally, simulation results are reasonably well matched with experiments in terms of the moment of failure occurrence, failure location, final blank shape, and thickness distribution.

Metal–plastic composite tubular structures that combine the high strength and stiffness of metallic tubes with the lightweight and flexible properties of plastic tubes, exhibit considerable potential to provide lightweight structures with improved performance compared to conventional monolithic tubular parts. However, the lack of well-confirmed technologies for connecting metal–plastic composite tubes that can ensure good structural stability and strength has proven problematic in actual applications. In this study, an innovative mechanical joining process via a hydroformed threaded coupling is proposed to achieve the successful joining of a metal–plastic composite layered tube (CLT). For the threaded connection, the CLT with an internal thread to act as a sleeve and the coupling tube with an external thread to act as a screw were joined together. A three-layer CLT composed of AISI 304 stainless steel/polyvinyl chloride (PVC)/AISI 304 stainless steel was investigated by free bulging and thread hydroforming. For successful forming into a threaded CLT, the optimal loading path was designed using analytical forming pressure and numerical contracting stroke. The mechanical tests based on compression and lateral three-point bending tests provided several key indicators for the quantitative performance of the thread-coupled parts. The reliability and applicability of the threaded coupling confirmed that the proposed joining process has strong potential for successful utilization in connecting metal–plastic composite tubes for structural applications.

In this study, the formability behavior of Ti-6Al-4V alloy sheets under uniaxial and multi-axial loading conditions at 923 K (650 °C), which has received less attention till now, has been extensively investigated by experiments and numerical modeling. Uniaxial tension tests at various strain rates and limit dome height (LDH) tests at the elevated temperature, covering the uniaxial stretch, plane strain, and biaxial stretch deformation modes, were conducted, and the corresponding constitutive models were constructed. Based on the experimentally characterized mechanical properties and associated constitutive laws, finite element simulations of the LDH tests were conducted, and the formability of the investigated material was analyzed using the Marciniak-Kuczyinski theory by employing the von-Mises isotropic yield function and Voce hardening law, including strain-rate sensitivity. At the elevated temperature, a strong positive rate sensitivity effect was observed. Finally, the predicted load-displacement responses of the LDH tests were in good agreement with the measured data. The corresponding formability analysis can contribute to the design of optimal hot forming processes for Ti-6Al-4V sheets.

Weld zones in tailor welded blanks have different mechanical properties than the base metals and thus have significant effects on formability. In this work, flow stresses of weld zones in laser-welded dissimilar dual-phase steels were measured. Strains at the weld zones of notched specimens were measured using the digital image correlation method and the flow stress data were corrected considering the anisotropy. For validation, finite element simulations were performed for the cup drawing process by considering the measured weld zone properties, and the weld line movements were compared with the measurements.

Electromagnetic forming (EMF) is an emerging nonconventional technology that relies on the usage of electromagnetic forces to deform a workpiece at high speed. Recently, studies on the application of EMF have been underway, and evaluating the characteristics of an EMF system, that is, the system circuit parameters (resistance, inductance, and capacitance), is necessary to understand the effect of circuit configuration on the performance and to design the coil. In this study, two practical methods were utilized for evaluating the system circuit parameters of a high-voltage EMF system connected with a flat spiral coil involving wires and connectors: the semi-analytical method and inverse optimization method. The semi-analytical method simply obtains the system circuit parameters by subtracting the coil circuit parameters from the equivalent circuit parameters. The coil circuit parameters are calculated analytically from the coil geometry, while the equivalent circuit parameters are extracted from the measured equivalent electric current profile. In the inverse optimization method, an optimization process is performed to obtain the system circuit parameters that achieve the best match between the predicted and measured equivalent electric current profiles. As examples of EMF application, two types of bulge tests, with circular and elliptical shapes, were performed, and they were successfully predicted by the EMF simulation with the obtained system circuit parameters. As a result, the feasibility of the two utilized methods as practical approaches to understand the effect of circuit configuration on the performance and to design the coil was verified.

Electromagnetic pulse (EMP) technology is an emerging non-conventional technology that relies on the usage of electromagnetic force to deform a workpiece at high speed. Recently, studies on the application of EMP technology have been underway, and identifying the characteristics of an EMP system, that is, the system circuit parameters (resistance, inductance, and capacitance), is necessary to understand the effect of resonance circuit configuration on the performance and to design the coil. An EMP equipment commonly consists of a power supply and capacitor bank for charging and discharging high voltages, and it is connected to a conductive coil actuator. The capacitor bank, wires, and connectors are collectively referred to as the EMP system, and the system circuit parameters are represented by only the capacitance, but the internal resistance and inductance caused by the wire and connectors are also included. Although the two types of practical methods for identifying the system circuit parameters were developed in our previous work, only limited verifications with flat spiral coil were performed. This study provides the prediction of the equivalent electric-current profile, based on the identified system circuit parameters, flowing in the resonance circuit connected with two types of helix coil actuators. As a result, it is verified whether the identified system circuit parameters work uniquely when the type of coil actuator changes.

Recently, as the demands to improve automotive fuel efficiency grow, several attempts are being conducted on the development of lightweight automotive components using magnesium (Mg) alloy sheets having the highest specific strength among metal materials. In a window regulator module which is a switchgear that raises and lowers the automotive side window pane, the weight of the rail accounts for about 40 % of the total weight, while the production cost of the rail is about 10 % of the total cost, that is the weight reduction effect is higher than the cost burden. Therefore, the application of Mg alloy sheets to the window regulator rails is more likely to be commercialized. The aim of this study is to investigate the warm forming behavior of Mg alloy sheets in the manufacturing of window regulator rails at the temperature range from 150 to 250 °C. AZ31B Mg alloy sheets with a thickness of 1.4 mm were used, and their mechanical properties were evaluated at the warm temperature range. The warm forming behavior was predicted by the finite element simulation. Based on the simulation results, the Mg alloy window regulator rail was successfully manufactured without fracture using the warm forming.

In this study, a room temperature forming process for high-strength aluminum alloys was investigated. To overcome the low formability of the peak-aged 7075 aluminum alloy (7075-T6) at room temperature, the sheet metal was subjected to solution heat treatment followed by rapid quenching. Mechanical tests, including uniaxial and balanced biaxial tension tests, were performed, and the Nakajima test was conducted to evaluate the formability of the sheet metal. On the basis of the plastic deformation behavior, finite element models including the plastic yielding and strain hardening laws were constructed. The proposed forming process incorporating the W-temper heat treatment was successfully applied to a realistic automotive part, which could not be manufactured by conventional room temperature forming without prior heat treatment. The numerical accuracies among various plasticity models were evaluated from a comparison of the thickness profiles. In addition, the paint-bake process was applied to the automotive component to evaluate the strength recovery.

The feasibility of room-temperature (cold) forming of peak-aged 7075 high-strength aluminum alloy (7075-T6) sheets after solution heat treatment followed by water quenching was investigated. The anisotropic plastic properties of the heat-treated aluminum sheet (7075-WT) were experimentally characterized by performing uniaxial tensile tests (along different loading directions) and a balanced biaxial tension test. Limit dome height (LDH) and cylindrical cup drawing experiments were conducted to evaluate the formability at room temperature. With the obtained anisotropic mechanical properties, different yield functions, including Yld2000-2d, Hill 1948, and von-Mises, were determined and implemented in the finite element simulations to numerically predict the formability at room temperature. The formability of the 7075-WT sheet at room temperature could be significantly enhanced owing to the prior heat treatment, and the finite element simulations performed using the anisotropic yield functions and Voce hardening law were in good agreement with the measured LDH. The LDH of the 7075-WT sheet in the plane strain and biaxial stress modes and the earing in the circular cup drawing tests could be better predicted non-quadratic Yld2000-2d function than those by isotropic and Hill 1948 quadratic yield functions.

In this study, the mechanical properties of a high-strength laminated vibration damping steel sheet (LVDSS) with stretch-dominated deformation behavior were characterized and successfully applied to formability prediction. The high-strength LVDSS was fabricated with dual-phase 590 steel sheets as outer skins and polymeric adhesive layer as a core via roll bonding. The uniaxial and biaxial mechanical behaviors were evaluated by performing uniaxial tension and in-plane biaxial tension tests, respectively, while the forming limit was investigated by conducting punched dome tests. Then, based on the experimental data, the mechanical properties of the LVDSS were characterized, viz., the hardening curve, yield function, and forming limit curve. As an example of LVDSS application, a square cup drawing test was conducted and a dashboard prototype was fabricated using an LVDSS, and the deformation and failure behaviors were successfully predicted via a finite element simulation performed by inputting the characterized mechanical properties.

A crystal-plasticity approach based on three-component dislocation density model is proposed, as a virtual experimental model, to accurately predict non-proportional anisotropic hardening behavior of ultra-thin sheet metals, a potential candidate for PEMFC (proton-exchange membrane fuel cell) bipolar plate. The model introduces three different populations of dislocations named forward, reverse and latent, which are selectively activated depending on load path changes. To validate the predictive capability as a tool for the virtual experiment, the model is validated by predicting flow behaviors of 1.2 mm thick extra-deep drawing quality steel sheet under compression–tension and two-step tension test. The model is then applied to the 0.1 mm thick ultra-thin ferritic stainless steel sheet. The model parameters are identified by the two-step tension test. The proposed model with the identified parameters can predict the tension–compression stress–strain curves, which are difficult to measure experimentally due to premature buckling for such thin sheet metals. The potential application of the crystal plasticity based virtual modeling to the springback prediction in the thin sheet metal parts is discussed in this study.