Plastic deformation behavior of bi-metal tubes during magnetic pulse cladding: FE analysis and experiments
Introduction
Metal tubes are utilized in a wide range of industrial applications. Bi-metal tubes, specifically, possess the distinct advantages of high mechanical strength and effective protection against corrosion due to their layered structure, where different metals comprise the inner and outer layer. This type of design must bond mechanically or metallurgically one metal to a cylindrical tube; bonding along the interface is the most important feature of bi-metal tubes.
Many processing technologies have been developed to manufacture bi-metal tubes over the past few decades. The magnetic pulse cladding process is a relatively novel approach. Magnetic pulse cladding (MPC) is based on sequentially impact welding portions of long tubes to fabricate bi-metal tubes (Yu et al., 2014). The main principle at work during MPC is the magnetic force that makes the flyer tube produce plastic deformation, welding the base tube though high-intensity impact that is first released by a capacitor suddenly discharging (Fig. 1). Bonding occurs over a small overlapping fraction of the two surfaces. The process thus requires only a small amount of energy stored on the capacitor.
The MPC process consists of a series of progressive impact welding operations, which may be regarded as a sequential electromagnetic compression process of an outer tube onto an inner tube. Due to the complex and transient nature of the process, researchers often attempt to model the electromagnetic process numerically (Psyk et al., 2011). Problems inherent to magnetic pulse welding are of particular interest to many researchers and developers. Zhang et al. (2009) analyzed oblique magnetic pulse welding (MPW) of pre-flanged AA6061-T6 and Cu101 metal sheets using the Electromagnetism (EM) module in LS-DYNA. In their study, impact velocity was confirmed through experimental measurement by Photon Doppler Velocimetry (PDV). The reasons for the presence of a no-weld zone at the center of the Al-to-Al sheets were given by simulation of electromagnetic (EM) welding of flat sheets in Kore et al. (2010). In Serizawa et al. (2009), the collision behavior of the Fe and Al plates during the magnetic pressure seam welding were numerically investigated via the commercial Euler-Lagrange coupling software MSC Dytran, where the relation of the plastic strain pattern to successful joint was proved. Numerical investigations performed by Shim et al. (2011) lead to the optimum parameters to maintain the quality of joints during magnetic pulse welding of aluminum/steel pipes. Considering the simulation strategy, the magnetic pulse welding process was simulated by loose coupling of electromagnetic and mechanical FEM software with the commercial code ANSYS in Uhlmann1 and Ziefle, (2010). In their work, an empirical approach was also presented to help determine the location of welds, which gives the possibility of modelling the welding process by parameter-controlled bonding at the welding interface. Although much efforts have been made to the simulation of the magnetic pulse welding process, the approach mentioned above only deals with a single operation and is therefore not appropriate for the magnetic pulse cladding process. Magnetic pulse cladding is similar to magnetic pulse welding but does differ, because its configuration varies based on the discharge sequence of the multi-step cladding process (see Fig. 1c). Up to now, yet, there is no knowledge about magnetic pulse cladding from the numerical point of view.
The so-called “bamboo-like” shape (or ripple), an undesired outer diameter change over the tube length at consistent intervals, is liable to form in a bi-metal tube under any inappropriate processing parameters (Yu et al., 2014). The bi-metal tube is unacceptable once this diameter fluctuation exceeds a critical level. Yu et al. have experimentally shown that the generation of ripples in the outer diameter is irrespective of feeding length. However, how the plastic deformation behavior relates to the formation of bamboo-like shapes has not been included in the previous work. Plastic deformation behavior determines collision behavior, which is directly associated with fundamental process parameters (impact velocity and angle.) The collision condition is critical to bonding strength during the impact welding process (Psyk et al., 2012, Groche et al., 2014). Until now, however, geometric change in clad tubes related to the spatial distribution of magnetic pressure in the multi-step cladding process has not been researched. Additionally, there is a general lack of research regarding the influence of the field shaper geometry on plastic deformation behavior; although it has been shown that modification on the work zone of the field shaper has a positive effect on the cladding quality of bi-metal tubes (Yu et al., 2014).
In the present paper, the primary goal is a better understanding of the MPC process, particularly the dynamic plastic deformation response of bi-metal tubes subjected to progressively-loaded magnetic force. A FE model of the multi-step cladding process was developed in the first step. Subsequently, the dynamic response was evaluated based on the established approach. In addition, the effect of the field shaper geometry on the plastic deformation behavior was analyzed, the objective of which is the identification of the geometrical characteristics of the field shaper affecting the collision patterns to facilitate the field shaper design. Finally, the characteristics of the electromagnetic field and the distribution of the magnetic force were discussed.
Section snippets
Finite element model
The multi-step cladding process was simulated sequentially. Fig. 2 shows the numerical scheme on the basis of the ANSYS/LS-DYNA software platform. In the first step of MPC, an electromagnetic (EM) model in the ANSYS/EMAG module was used to calculate the transient magnetic field. Next, the resultant magnetic forces were extracted as the loading condition in the mechanical model (LS-DYNA module) to predict dynamic plastic deformation. For EM field simulation in the next step, the coil (including
Changes in morphology
Fig. 8 shows the experimental results for the three kinds of field shapers in the following process parameters: the radial gap of 1.8 mm and discharging voltage of 15 kV (the discharge energy of 11.25 kJ). Sample formed with C-field shaper at feeding length of 12 mm (Case A), can be seen in Fig. 8(a). Samples formed with P1-field shaper (Case B) and P2-field shaper (Case C), can be seen in Fig. 8(b) and (c), respectively. As exhibited in Fig. 8, the periodical raised rings (or ripples) appears in
Conclusions
The most notable conclusions of this study can be summarized as follows:
- (1)
A numerical scheme for the multi-step cladding process was proposed here, and comparison between simulations and experimental data show that the proposed numerical scheme reliably predicts system performance. This provides a basis for better understanding of deformation mechanisms.
- (2)
Inharmonious plastic deformation behavior resulted in ripples on the surface of the clad tube. To achieve a sound contour quality, a profiled
Acknowledgment
The authors gratefully acknowledge both the National Natural Science Foundation of China (Grant No. 51475122) and National Basic Research Program of China (973 Program) [2011CB012805] for their kindly financial supports of this work.
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