Formability Enhancement of AA5182-O During Electro-Hydraulic Forming

In this research, formability improvement of AA5182-O aluminium sheet during electrohydraulic forming (EHF) was investigated by means of experimental and finite element analysis. Free and conical die formed EHF was carried out on grid sheet blanks and formability improvement was measured by comparing grid analysis results at each EHF condition with different forming limit curves (FLCs). It is found that AA5182-O shows minor improvement in formability when formed freely into EHF. But a significant rise in effective strain in safe grids is observed when EHF into 34, 40 and 45-degree conical dies provided a critical threshold of input energy has been used. In order to understand the mechanical aspects of formability improvement, related factors such as strain rate, stress triaxiality, and compressive through-thickness stress were studied using finite element simulation with an accurate description of the hardening behaviour of AA5182-O. Another advantage of the numerical simulation carried out in this work is that unlike previously published works, the driving force for EHF deformation was not simplified as uniform pressure and it resembles the actual process of EHF. Ignition and growth model was used in conjunction with Coupled Eulerian-Lagrangian (CEL) approach to simulate the EHF pulse formation. Moreover, 3D solid elements were used instead of shell elements and this facilitated measurement of stress in elements located in the bulk of sheet material. The tensile flow behaviour of AA5182-O sheet was investigated in the strain rate range of 0.001 to 1000s−1 and at different material directions (RD, DD, and TD) by means of both phenomenological models and neural networks (NNs). Genetic algorithm (GA) and linear regression analysis methods were used to calculate the constants in Johnson-Cook (JC), Khan-Huang-Liang (KHL) and modified Voce hardening functions, and user-material subroutines were developed and used in FE software. Moreover, in order to predict the rheological behaviour of AA5182-O without the limitations of a mathematical function, two types of feed-forward back-propagation neural networks were trained and used in the FE model. Simulation results were compared with experimental tensile flow curves and the most accurate method is used to predict the mechanical response of AA5182-O in FE simulations of the EHF process. FE results suggested that a combination of EHF process related parameters including compressive through-thickness stress (negative stress triaxiality) generated during the deformation could postpone the failure, when specimens are formed into a die cavity (EHDF). Also, the increased velocity and significant impact pressure at the final stage of deformation not only prevent strain localization but also help in further suppressing the damage. It is found that very high peak strain rates develop in the sheet as it contacts the die surface which further postpone the failure since AA5182-O exhibits positive strain rate sensitivity at such high-strain rates. Moreover, damage mechanisms of AA5182-O sheets were investigated during EHF tests and are compared with those occur during quasi-static (QS) deformation. The results confirm that void nucleation, growth and coalescence are the main damage mechanisms of AA5182-O at both high and low strain rates. It is found that Mg2Si particles do not significantly influence void formation and the main source of void nucleation is cracking of Al3(Fe-Mn) intermetallic particles. More importantly, it is found that specimens deformed under QS conditions contained more voids in areas away from the sub-fracture surface but EHF specimens exhibit higher rate of void growth close to sub-fracture areas. Optical microscopy results confirmed that void formation is suppressed by increasing the applied energy in EHF. And more importantly, the growth of voids is suppressed due to the high-velocity impact of the sheet against the die which plays an important role in increasing formability of AA5182-O aluminium sheet in EHF. Optical microscopy showed that AA5182-O grains experience significant shearing strain during the EHF deformation in the apex area of conical EHDF specimens. The results of transmission electron microscopy showed that dislocation density increases when specimens are formed using EHF process but the magnitude of this increase is not significantly greater than in quasi-static deformations. Finally, it is concluded that the combination of high strain rate deformation and compressive through-thickness stress during the deformation, leads to formability improvement of AA5182-O in EHDF

Impulse-Based Manufacturing Technologies

In impulse-based manufacturing technologies, the energy required to form, join or cut components acts on the workpiece in a very short time and suddenly accelerates workpiece areas to very high velocities. The correspondingly high strain rates, together with inertia effects, affect the behavior of many materials, resulting in technological benefits such as improved formability, reduced localizing and springback, extended possibilities to produce high-quality multi material joints and burr-free cutting. This Special Issue of JMMP presents the current research findings, which focus on exploiting the full potential of these processes by providing a deeper understanding of the technology and the material behavior and detailed knowledge about the sophisticated process and equipment design. The range of processes that are considered covers electromagnetic forming, electrohydraulic forming, adiabatic cutting, forming by vaporizing foil actuators and other impulse-based manufacturing technologies. Papers show significant improvements in the aforementioned processes with regard to: Processes analysis; Measurement technique; Technology development; Materials and modelling; Tools and equipment; Industrial implementation

Investigation of the Formability of TRIP780 Steel Sheets

The formability of a metal sheet is dependent on its work hardening behaviour and its forming limits; and both aspects must be carefully determined in order to accurately simulate a particular forming process. This research aims to characterize the formability of a TRIP780 sheet steel using advanced experimental testing and analysis techniques. A series of flat rolling and tensile tests, as well as shear tests were conducted to determine the large deformation work hardening behaviour of this TRIP780 steel. Nakazima tests were carried out up to fracture to determine the forming limits of this sheet material. A highly-automated method for generating a robust FLC for sheet materials from DIC strain measurements was created with the help of finite element simulations, and evaluated against the conventional method. A correction algorithm that aims to compensate for the process dependent effects in the Nakazima test was implemented and tested with some success

Experimental determination of the forming limits of DP600 and AA5182 sheets in electrohydraulic free forming

Electrohydraulic forming is a pulsed metal forming process that uses the discharge of electrical energy across a pair of electrodes submerged in fluid to form sheet metal. Pulsed metal forming processes, including electrohydraulic forming, have been shown to increase the formability of sheet metals, which is of significant industrial interest. An experimental procedure was developed to quantify the formability in electrohydraulic free forming (EHFF) that is consistent with the quasi-static formability assessment convention. Novel sheet metal specimen geometries were created to quantify the formability across the entire minor strain spectrum. The experimental EHFF forming limit curve (FLC) was determined for both AA5182-O and DP600 sheets. Compared to their respective quasi static FLCs, DP600 shows no formability improvement in EHFF while AA5182-O shows formability improvement over the entire range of minor strains including an 11% engineering strain improvement at the plane strain intercept. Numerical modeling indicated that peak strain rates reach approximately 2500 to10,000 s-1

Multi-scale Characterization of Hyperplasticity and Failure in Dual Phase Steels Subject to Electrohydraulic Forming

In this research, three commercial dual phase steel sheets, i.e. DP500, DP780 and DP980, were formed under quasi-static and high strain rate conditions using the Nakazima test and Electrohydraulic Forming (EHF), respectively. In EHF, as a result of a high-voltage electrical discharge between two electrodes in a water chamber, a shock wave was produced which travelled through the water and formed the sheet into the final shape. When a 34° conical die was used in EHF, significant formability improvement, known as hyperplasticity, was achieved in the specimens compared to the specimens formed in the Nakazima test. In this research, hyperplasticity as well as failure in the specimens were characterized at different scales of observation. Quantitative metallography showed relative deformation improvement of around 20% in ferrite and 100% in martensite when formed under EHF. Dislocations in ferrite and deformation twinning in martensite were found to be responsible for the significant improvements of deformation in the constituents under EHF. As a mechanism of failure, voids were found to nucleate in the ferrite/martensite interface due to decohesion. However, under EHF, the significant deformation improvement of martensite enhanced the plastic compatibility between ferrite and martensite. Consequently, the strain gradient across the ferrite/martensite interface, i.e. decohesion, was reduced and nucleation and growth of the interfacial voids was suppressed. Furthermore, quantitative analysis of the voids showed that void growth in the specimens formed under EHF was slower than in the specimens formed in the Nakazima test. The reason was attributed to impact of the sheet against the die that generates significant compressive and shear stresses which act against void growth. Therefore, under EHF, coalescence of the voids to form micro-cracks was postponed to higher levels of strains which resulted in suppression of failure. Fractography of the specimens showed ductile fracture as the dominant type of fracture under both quasi-static and high strain rate forming conditions. In addition, limited quasi-cleavage fracture was observed in DP780 and DP980 specimens. Shear fracture was also observed in the specimens formed under EHF

Investigation of the Formability Enhancement of DP600 Steel Sheets in Electrohydraulic Die Forming

The objectives of this thesis are to quantify the increase in formability of DP600 steel sheets in electrohydraulic die forming (EHDF) and describe the mechanisms that lead to a formability enhancement. Marciniak tests and EHDF tests were conducted to obtain the conventional and EHDF forming limit curves of this sheet material, respectively. EHDF tests with a V-shaped die indicated that, globally, there was no formability improvement; however a 100% improvement was achieved locally near the apex of the specimen. A formability enhancement of over 75% can be achieved in EHDF with a conical die, provided the discharge energy is sufficient (≥12.5 kV). Numerical simulations of these tests showed that the combination of high strain rate and inertial effects helps to delay the onset of necking prior to the sheet contacting the die. But contact phenomena play an even more significant role to improve sheet formability by decreasing the stress triaxiality

Simulation of Electrohydraulic Forming Using Anisotropic, Rate-dependent Plasticity Models

Electrohydraulic forming (EHF) is a pulsed forming process in which two or more electrodes are positioned in a chamber filled with a liquid and a high-voltage discharge between the electrodes generates a high-pressure to form the sheet. Deformation history of a sheet material in EHF process shows substantial changes in the strain rate of the material during the forming process. In this research, the mechanical properties of DP600, TRIP780, and AA5182-O were obtained at different strain rates. Uniaxial tensile tests showed significant strain-rate sensitivity in all three material orientations (RD, DD, and TD) for DP600 and TRIP780. In contrast, AA5182-O exhibits almost near-zero strain-rate sensitivity. Several anisotropic yield functions were calibrated at various strain rates to evaluate the effect of strain rate on the flow surface shape. By comparing the quasistatic and updated flow surfaces of DP600 and TRIP780 predicted by Yld2000-2d, results show a relatively considerable effect of updating anisotropy coefficients for higher strain rates. Several rate-dependent anisotropic material models (plane stress and general) were developed, by combining updated anisotropic yield functions and a rate-dependent hardening model (KHL). The developed models were implemented as user-defined material subroutines (VUMATs) based on implicit stress integration algorithm for ABAQUS/Explicit code to simulate electrohydraulic free-forming (EHFF) and die-forming (EHDF) processes. EHF simulations were completed, using Eulerian elements and ignition-and-growth model. The EHFF process was simulated for four different geometries (representing four different strain paths). Also, the EHDF process was simulated using a conical die The EHFF simulation results for the DP600 biaxial specimen showed that von Mises predicts a maximum effective plastic strain around 11% greater than Yld2000-2d for the same amount of applied energy. The EHDF simulation result for DP600 showed that with the same applied energy magnitude, von Mises overpredicts major, minor and through-thickness shear strains and consequently effective plastic strain (14% higher) compared to Yld2004-18p. Results showed that 82% of the effective plastic strain occurs under a proportional biaxial strain path before contacting the die. Also, results showed that von Mises overpredicts maximum absolute compressive through-thickness stress and shear strain compared to the values predicted by Yld2004-18p

Impulsive sheet metal forming based on standoff charge for conical geometry

Recently, explosive forming has gained much attention from researchers to overcome problems of conventional methods in manufacturing complex geometries such as cone. Despite these developments, analytical studies especially on cone with sharp apex angle are rarely reported. Past analytical studies in explosive forming on cone ignored the effects of friction between the blank and the die, redundant work in the work sheet blank and strain rate on blank material behaviour. Likewise, in finite element (FE) method, Arbitrary Lagrangian Eulerian (ALE) approach, most frequently method in the past is very time consuming and costly especially for large number of simulation tests. An alternative to ALE, Coupled Acoustic-Structural Analysis (CASA) approach has been seen gradually applied to model damage on the marine structure subjected to under water explosion but reports on its applications in modelling of explosive forming is somehow very limited. Moreover, in the past reported works, estimation of explosive mass, deformation history and damage accumulation models were analysed independently which creates difficulties to predict all aspects of the blank behaviour simultaneously. An integrated model that addresses these three issues concurrently is however, not available. The main aim of this research is to establish a satisfactory explosive mass estimation equation for modelling cone forming behaviours under integrated conditions with reasonable number of trials, i.e. simulation and experimental. Analytical model based on the impulse method was adopted to estimate the explosive mass by considering the effects of deformation efficiency and strain rate during cone forming process. This was done prior to establishment of FE model. ABAQUS software was used to develop a FE model based on CASA approach. Both models were validated via a series of experimental tests. Three different circular blank materials were tested, i.e. AISI 1006, Cu-ETP and Al 6061-O subjected to C-4 explosive forming under water. Four geometrical parameters were varied in the experiments. They were blank diameter (100 and 110mm), blank thickness (0.8, 1 and 1.2 mm), standoff distance (130, 150 and 170 mm) and half apex angle of cone (45 and 60 degree). Height of deformed cone was measured after each test and these results was used an indicator for the right explosive mass determination. An analytical equation was established by taking into consideration the effects of strain rate, friction and redundant work during forming process. Verification via experimental tests showed that the error of explosive mass required for forming all blank materials into a complete cone is about 20% ± 2.91. The developed FE model was also able to predict concurrently the deformation history, thickness distribution and damage accumulation in a good agreement with experiments. In conclusion, this study provides very encouraging evidences that both impulse method and CASA approach can be used together for predicting material behaviours during explosive forming process