Electromagnetic Ring Expansion Research Articles

Effect of strain rate on ductility of Cu TU1 in electromagnetic ring expansion

It is still a controversial issue that whether ductility of metals under high strain rates is varying or not. This paper presents results of a series of electromagnetic ring expansion experiments to study on this problem. Three types of probes were used to measure experimental signals. As a result, the strain at the fracture time can be acquired through in-situ diagnosis. It is found that for a typical metal material, i.e. Cu TU1 under strain rates between 1×103 and 9×103 s−1, the ductility is varying and the fracture strain increases monotonically. These results are new solid evidence of rate-dependent ductility of metals.

Mechanical properties and microstructure evolution of 2219 aluminum alloy via electromagnetic ring expansion & electromagnetic treatment

As an emerging technology to solve the bottleneck of plastic processing of light alloys, electromagnetic forming usually causes a complex mechanical response and microstructure evolution of materials. In this work, based on the electromagnetic ring expansion (EMRE) experiment, the relationship between microstructure evolution and mechanical properties of AA2219-T6 under EMRE with different discharge energies is established. In particular, to reveal the influence of strain and temperature parameters on the output response during the electromagnetic pulse process, a Q235 steel sleeve and an epoxy resin sleeve with different thermal conductivities are applied to constrain the deformation of the ring, respectively, thus the performance under electromagnetic treatment (EMT) are investigated. It is found that, under the threshold discharge energy of 4.23 kJ, the microhardness of the ring via EMT with the steel sleeve is similar to that of AA2219-T6, but increases by 20.9% compared with the ring via EMT with the epoxy sleeve and 17.1% compared with the ring via EMRE. The electromagnetic pulse has no significant effect on the evolution of precipitates in AA2219-T6-EMT (Steel), but the adiabatic temperature rise in AA2219-T6-EMT (Epoxy) caused redissolution of G.P.Ⅱ zones. With the gradual increase of discharge energy under EMRE, the microhardness of AA2219-T6 increases initially and followed by a decrease, while the fracture strain increases continuously. When discharge energy reaches 8.63 kJ, 2219 aluminum alloy has the optimal mechanical properties that, compared with AA2219-T6, the microhardness increased by 14.2% and the fracture strain increased by 18.0%. The dislocation density shows a trend of first increasing and then decreasing with the increase of discharge energy. Combined with the observed phase transformation, it is shown that dislocations generated by EMRE will promote G.P.Ⅱ zones to transform into θ′′ phases, and affected by dislocation density, the transformation effect is most obvious at 8.63 kJ. In addition, the transformation of precipitates greatly increases the probability of hindering the motion of dislocations, which promotes the precipitates to be dispersed, and AA2219-T6 is further strengthened after EMRE. As discharge energy continues to increase, the dislocations begin to annihilate and G.P.Ⅱ zones are redissolved due to the adiabatic temperature rise.

Rate-related study on mechanical properties and fracture characteristics in aluminium alloy via electromagnetic ring expansion test

The uniform one-dimensional tension can be easily realized by the electromagnetic ring expansion (EMRE) test at high strain rates of 103-104 s−1, which utilizes strong pulsed electromagnetic force to deform materials. However, the accurate and rapid acquisition of the time-dependent stress, strain and strain rate in EMRE test has been a bottleneck that hinders its wide application. Based on the electromagnetism and the principle of virtual velocity in plastic dynamics, a stress analytical model considering both the electromagnetic and inertial components is established. The whole EMRE process of deformation, strain localization and fracture for 5052 aluminium alloy (AA5052) which is widely used in the automotive and aerospace industries is collected by a measurement system with microsecond time-resolved imaging, and then the true stress-strain-strain rate relationship is calculated based on the above analytical model. In addition, the dynamic mechanical properties of AA5052 under EMRE test are contrastively analyzed with those under the split Hopkinson tension bar (SHTB) and quasi-static tests. The ductility and formability of AA5052 under EMRE are improved due to the inertial and eddy current effects which is demonstrated by fracture characteristics observed using Scanning Electron Microscopy (SEM).

High strain rate effect on tensile ductility and fracture of AM fabricated Inconel 718 with voided microstructures

The paper describes Electromagnetic Ring Expansion Tests (ERET) performed on Laser Melting Powder Bed Fusion (LPBF) Inconel 718 stress relieved test pieces, to establish the effect of a randomly dispersed spherically voided microstructure on tensile ductility, fracture, and fragmentation at high strain rate (10−3 < ε < 104 s−1). An empirical model to predict porosity type and growth rates as a function of laser energy density was established, to select the LPBF process parameters to fabricate test pieces under stable conduction and keyhole melting. The size, shape, distribution of macro and keyhole pores in the test pieces obtained for ERET testing were characterised. At high strain rate the number of ring fragments for the highest porosity doubled, accompanied by a reduction in true strain at maximum uniform elongation and fracture strain. The trend for reducing fracture strain with increasing porosity at high strain rate was described by a decaying power law. Overall, there was a significant positive strain rate effect on tensile ductility at lower porosities attributed strain rate hardening (Hart, 1967) [1]. Fracture surfaces containing the highest porosity identified four different void coalescence mechanisms that helped explain the influence of larger pores on the stress state in the alloy.

Mechanism of high velocity electromagnetic deformation of Al–Mg alloy

Electromagnetic ring expansion experiments, as a one-dimensional simplification of the electromagnetic forming process, were performed on AA5083 aluminium alloy at room and cryogenic temperatures. The ductility of the AA5083 alloy is considerably improved at a high strain-rate under electromagnetic loading conditions, compared with that under quasi-static tensile test conditions. The average strain to failure of the specimen obtained at cryogenic temperature is lower than that at room temperature, despite the specimen being expanded at a higher deformation velocity. Characterisation of deformation texture and microstructure evolution was carried out using electron backscatter diffraction and transmission electron microscopy techniques. The results indicate that the significant enhancement in ductility of AA5083 during the electromagnetic ring expansion test at room temperature is a consequence of dynamic recovery, due to the temperature rise in the metal ring caused by the eddy current heat, in addition to an inertial effect at high strain rates.

Effect of Induced Current on the Flow Stress in the Electromagnetic Ring Expansion

The electromagnetic forming (EMF) is a promising metal-forming process for the complex geometric shape and lightweight construction. Material behavior in EMF is one of the important researches. Herein, an experiment system of the electromagnetic ring expansion is employed to study the flow stress in the EMF with the consideration of the induced current, high strain rate, and temperature rise. Compared with the simulation results, the experiment results are analyzed to investigate the effect of induced current on the flow stress of the aluminum alloy during the EMF.

Suitability of the electromagnetic ring expansion test to characterize materials under high strain rate deformation

Characterisation of materials under high strain rate is always challenging, and requires sophisticated apparatus. Magnetic pulse forming techniques can involve high strain rate deformation and the electromagnetic ring expansion test (ERET) is considered here to determine its suitability to characterise materials at high strain rate. The multi-physics and high speed nature of the process requires advanced numerical modelling techniques to gain insight and understanding of stress development when used as a material characterisation technique. In order to evaluate the suitability of the ERET to fully characterise the material behaviour at high strain rate, stress states were investigated using 3D coupled mechanical-electromagnetic simulations on a validated numerical model. The numerical simulations were performed in LS-Dyna ® with a modified Johnson-Cook (MJC) material model in this study. Albeit the preliminary results show the development of biaxial stress during the test, a shielding mechanism is proposed to eliminate the axial component of the stress on the rings. This study indicates that the ERET has a potential to be used for characterisation of the material parameters under the influence of high strain rate.

Dynamic Ductility and Fragmentation for Aluminum Alloy Using Electromagnetic Ring Expansion

In order to investigate the effects of ductility enhancement for aluminum alloy under the high-velocity EMF condition, the electromagnetic ring expansion test is carried out. Through a use of Photron digital high-speed video camera, the sequence of deformation and failure in the expanding ring are determined. Subsequently, the variation of expansion velocity with time is measured. In particular, this work reveals the effects of expansion velocity on the ductility, necking characteristic, fracture behavior of Al 5052-O and 6061-T4. In addition, the dynamic ductility of Al 5052-O under electromagnetic ring expansion experiment is compared with that in split Hopkinson tension bar (SHTB) test and quasi-static tensile case. It is shown that the number of necks and fragments increase as the maximum expansion velocity increases. Furthermore, the uniform strain and fracture strain augment with increasing expansion velocity, which is more evident for Al 5052-O. The fracture surfaces of ring fragments are characterized by the larger and deeper dimple-like structure at higher expansion velocity. Compared with the SHTB tests at the same strain rate, the ductility of Al 5052-O under electromagnetic ring expansion is better. The ductility enhancement results from not only the inertia effect but also the changes in material constitutive behavior.

Effects of Deformation Rate on Ductility of Ti-6Al-4V Material

To determine the velocity field in which the Ti-6Al-4V titanium alloy sheet was sensitive to the strain rate, the formability at different strain rates were tested using a ring sample in the electromagnetic ring expansion experiment. For the test of titanium alloy, an aluminum alloy loop was used as a driver ring due to improve energy efficiency. The expansion velocity and strain rate of titanium ring were predicted by a numerical simulation method which had been verified by experimental data obtained with a high speed camera. The uniform strain was defined as the ratio of the change and the one after expansion of the cross sectional area in this study. Since the fracture strain of a material was related to the aspect ratio (ratio of length to diameter) of samples, the uniform strain was chosen to characterize the ductility of the material. The results indicate that when the tensile speed of Ti-6Al-4V titanium alloy sample is 2mm/min (quasi-static), the strain rate of deformation is 6.67×10-4 1/s and its uniform strain (ɛu) reaches 0.102. Instead, the uniform strain is only 0.032 when the expanding speed of 46.7 m/s is faster than the quasi-static speed. However, the uniform strain increases proportionally with the increment of the strain rate, and exceeds the quasi-static uniform strain reaching 0.11 or more when the deformation speed is faster than 286 m/s, in which the strain rate exceeded 6935.6/s. Therefore, the deformation speed of 286 m/s or the strain rate of 6935.6 1/s could be considered as the threshold to improve the ductility of Ti-6Al-4V titanium alloy.

Study on the homogeneity of deformation under electromagnetic expansion of metal tube

In this paper, a 3D numerical simulation method was put forward by coupling magnetic-structural fields, and its reliability was verified through comparing with the experimental data offered by Suzuki. According to the characteristics of electromagnetic expansion of metal tube, this simulation method was used to analyze the uniformity of tube expansion when the length of tube was less than that of the coil. Meanwhile, a curve of homogeneity of electromagnetic tube expansion with different aspect ratios (ratio of length to diameter) of tube was given based on the criterion of uniformity on deformation at the condition of specific parameters presented in this paper. By analyzing the mechanical characteristics of tube expansion, a length range of tube was found where the tube deformed uniformly. The results derived from this paper provided a good guidance for optimization design of electromagnetic ring expansion experiments which will be conducted on and this could ultimately promote the application of electromagnetic forming.

Identification of optimized constitutive model parameters at a high strain rate using electromagnetic ring expansion test results

In this article, parameters of the material constitutive model have been identified at high strain rate electromagnetic ring expansion test using experimental data taken from literature for oxygen-free electronic copper (OFE Cu). The experimental data were processed using a finite-element optimization procedure in which the measured deformation has been applied to a specimen. An optimal set of material constants for JohnsonCook constitutive model have been computed by minimizing the standard deviation of differences between experimental and calculated stressstrain curves. The applicability of identified parameters has been validated successfully by using simulation of reverse Taylor impact test results. The presented method in this article was found to be an effective method for identifying the material model parameters at high strain rates.

Development of Electromagnetic Ring Expansion Apparatus for High-Strain-Rate Test

The structure of the experimental set-up for electromagnetic ring expansion and its electric properties are presented in the paper. While earlier expanding ring apparatuses were usually equipped with shaped-charge, thyratron or ignitron switches, we propose to use the impulse high-voltage thyritor as a switch to discharge capacitor bank. Furthermore, the preliminary results of the copper ring fragmentation properties are presented for different initial charge of capacitor bank (0,5 ÷ 2 kV). On the basis of our calculated and preliminary experimental data of the ring motion for expansion at 2 kV, we estimated the radial ring velocity, which was close to 250 m/s.

Increased ductility in high velocity electromagnetic ring expansion

Thin rings have been rapidly expanded using large, transient magnetic fields to study the effect of deformation velocity on strains to failure of ductile metals. A classical electrodynamics analysis similar to one developed previously by Gourdin was employed to estimate sample velocities. Within expansion velocities studied (50 to 300 m/s), the experimental results show that ductility of Al 6061 and OFHC Cu increases monotonically with increasing velocity. In each case, sample strain at failure is almost twice as great at 300 m/s as in the static condition. Comparison to a one-dimensional rigid-viscoplastic dynamic finite element method analysis suggests that inertial effects are mainly responsible for enhanced ductility over a wide range of velocity.

Effect of sample size on ductility in electromagnetic ring expansion

Effect of sample size on ductility in electromagnetic ring expansion

Analysis and assessment of electromagnetic ring expansion as a high-strain-rate test

The rapid expansion of electromagnetically launched thin metal rings is analyzed in detail, with particular emphasis on the determination of the expansion speed, currents, and specimen temperature as a function of launch voltage. Electrodynamic calculations for copper, tin, and lead rings are presented and compared with typical experimental results. Currents derived from Rogowski-probe records and expansion speeds measured with a velocity interferometer (VISAR) are found to be in excellent agreement with these calculations. Currents are large, of the order of tens of kiloamperes, and heating of the specimen can be significant. It is found that, for good conductors such as copper, the maximum useful strain rate will be limited by the Joule heating of the specimen. For materials with resistivities and strengths substantially larger than those of copper, direct expansion of a specimen ring is difficult. Use of a composite ring, in which a high-conductivity driver is used to launch a low-conductivity specimen, is discussed as a means of overcoming this limitation.