Hyperplasticity mechanism in DP600 sheets during electrohydraulic free forming

Abstract

This work discussed the multi-scale mechanisms of hyperplasticity during the high strain rate electrohydraulic forming (EHF) process by exploring the formability of DP600 sheets in a state of uniaxial tensile stress. The experimental results showed that the limit strains and limit dome heights of the deformed specimens obtained by EHF were improved by 15 %–27 % and 22.54 %, respectively, as compared with quasi-static specimens, showing a hyperplasticity characteristic. The inertial effects that occurred during the EHF process were responsible for the macro-scale enhancement in terms of formability, which could generate an additional principal stress along the direction of stretching that slowed the velocity gradient of the necked elements to restrain uneven deformation, resulting in a 60 % broadening of the action zone of maximum Y-displacement. The proportion of inertial effects that contributed to the plastic deformation of the deformed specimens was 87.1 %, indicating that the vast majority of the deformation in the EHF process occurred as a result of inertial effects after the electrical energy was completely discharged. A larger dislocation density and a more uniform dislocation distribution were observed in the EHF specimens, which were regarded as the micro-scale causes of the hyperplasticity in the EHF process. Multiplication and entanglement of dislocations caused by the significant shear stress, together with the extensive nucleation of new dislocations caused by the high strain rates, demonstrated the micro-scale mechanism of hyperplasticity during EHF.

Introduction

Mass producing vehicles with lower fuel consumption economically has become a research hotspot, and an effective way to accomplish this goal is to replace conventional steel with lightweight and high-strength dual-phase (DP) steel sheets. However, the poor formability of DP steel at room temperature severely limits its comprehensive application in industry and other fields. High strain rate electrohydraulic forming (EHF), can deform metal sheets at speeds of up to several hundred meters per second and enhance the formability of work pieces; such work pieces demonstrate a higher degree of hyperplasticity than conventional forming methods. Priem et al. (2007) compared EHF with other high strain rate forming technologies (such as electromagnetic forming) and found that EHF had no material constraints and could be used to process work pieces with a wide range of materials. Therefore, EHF has significant advantages in promoting the industrial application of DP steel sheets.

To exemplify the hyperplasticity, Ahmed et al. (2017) conducted a series of EHF experiments on AA5052 sheets. It was found that the limit major strains of deformed specimens were 45–50 % higher than those obtained by quasi-static forming. However, these results pertained only to this material and thus were not representative, which needed to be further demonstrated. Rohatgi et al. (2011) studied the dynamic deformation behavior of AA5182 sheets during the EHF process, and they confirmed that EHF can universally improve the formability of metal sheets. Their results confirmed the hyperplasticity improvements achieved by using EHF. In addition, Rohatgi et al. (2012) found that the "ironing effect" of the conical die could further promote the enhancement of the velocity, strain and strain-rate of AA5182 specimens during an electrohydraulic die forming process. Subsequently, Rohatgi et al. (2014) determined that the limit strains of AA5182 sheets by means of free forming and conical die forming were enhanced by 2 times and 2.5 times, respectively, as compared with the quasi-static forming limit curve (FLC). Jenab et al. (2018) explicitly showed that the limit major strains of AA5182 sheets were increased by 40 % and 70 %, respectively, when specimens were formed with 34° and 40° conical dies during the EHF process. Therefore, their results quantified the importance of die shape. In addition, Woo et al. (2019) quantified the dynamic deformation behavior of metal sheets through numerical simulation and discovered that the maximum strain rate of AA6061 specimens was more than 3000/s during an EHF process. These numerical results were meaningful and could provide a theoretical basis for the experimental process.

Other investigations on the formability of steel materials under high strain rate conditions have been carried out in recent years. Kim et al. (2011) studied the formability of CQ steel sheets under different strain rates. They observed that the total elongation of the deformed specimen obtained at a strain rate of 100/s was increased by 5 %, and the limit strain was enhanced by 5.7 %. These results indicated that the effect of hyperplasticity during a higher strain rate was also applicable for steel sheets. Golovashchenko et al. (2013) conducted EHF experiments on DP500, DP590, DP780 and DP980 sheets. It was found that the formability in a biaxial-tension strain path was improved by 27.9–84.7 % as compared with quasi-static forming. Therefore, these results indicated that DP sheets were strain rate sensitive materials, and that a high strain rate could improve their formability. More specifically, Samei et al. (2013) reported that the formability of ferrite and martensite in DP500 specimens obtained by EHF was increased by about 20 % and 100 %, respectively. In addition, Maris et al. (2016) compared the EHF FLC with that obtained by quasi-static forming, and found that the engineering major strains of DP600 sheets were improved by 5 %. Further, Cheng et al. (2017) analyzed the die effects on the formability of DP600 steel sheets during an EHF process and showed that the limit strains were increased by 60 % in a biaxial-tension strain path and by 120 % in a plane strain path. Therefore, these investigations indicated that the die was beneficial to the improvement in formability of steel sheets during an EHF process.

To analyze the essential causes of hyperplasticity in high strain rate conditions, Balanethiram and Daehn (1992) conducted multiple high-speed uniaxial tension tests on IF iron, but they did not find any changes in the constitutive behavior of the material. Therefore, it was suggested that inertia was the cause of the improved formability that was achieved during a high strain rate forming process. Further, Balanethiram et al. (1994) confirmed that the inertial forces could restrain the development of micro-cracks and improve the formability of metal material by analyzing the formability of AA6061 sheets under high strain rate forming conditions. In addition to the inertial effect, Lee et al. (2007) found that the micro-voids and necking of AA6061 specimens were closed and postponed, respectively, under the action of a compressive stress that was generated by the high-speed impact of a conical die. Jenab et al. (2017) obtained similar conclusions, finding that the die effects could suppress the growth of voids by analyzing the failure mechanisms in AA5182-O sheets during an EHF process. Hassannejadasl et al. (2014) concluded that the inertial and the die effects were mainly responsible for the enhancement of formability achieved during a high strain rate forming process.

EHF is a forming process based on the form of bulging, which involves a complicated radial impulse force caused by the shock waves and the interactions between the liquid and the external structure. The above-mentioned causes for the enhanced formability in a high-speed uniaxial tension process were not sufficient to reveal the mechanisms of hyperplasticity during an EHF process. The existence of a die can greatly promote the EHF to improve the formability of materials, but it is not conducive to reveal the mechanisms of hyperplasticity of EHF in essence. Therefore—by establishing a plastic instability model of the DP600 deformed microelement—this work qualitatively demonstrates the affect of inertial effects on the hyperplasticity during an electrohydraulic free forming process from the perspective of a structural response. LS-DYNA program is adopted to carry out the finite element dynamic simulations, so as to quantitatively observe the influence of inertial effects on the forming limit of the tested specimens and to identify the proportion that the inertial effects contributed to the plastic deformation. Finally, the role of dislocations on the enhanced formability is discussed to reveal the micro-scale mechanism of hyperplasticity during an EHF process.