Experimental and Numerical Investigations into Magnetic Pulse Welding of Aluminum Alloy 6016 to Hardened Steel 22MnB5

Abstract

By means of magnetic pulse welding (MPW), high-quality joints can be produced without some of the disadvantages of conventional welding, such as thermal softening, distortion, and other undesired temperature-induced effects. However, the range of materials that have successfully been joined by MPW is mainly limited to comparatively soft materials such as copper or aluminum. This paper presents an extensive experimental study leading to a process window for the successful MPW of aluminum alloy 6016 (AA6016) to hardened 22MnB5 steel sheets. This window is defined by the impact velocity and impact angle of the AA6016 flyer. These parameters, which are significantly dependent on the initial gap between flyer and target, the charging energy of the pulse power generator, and the lateral position of the flyer in relation to the inductor, were determined by a macroscopic coupled multiphysics simulation in LS-DYNA. The welded samples were mechanically characterized by lap shear tests. Furthermore, the bonding zone was analyzed by optical and scanning electron microscopy including energy-dispersive X-ray spectroscopy as well as nanoindentation. It was found that the samples exhibited a wavy interface and a transition zone consisting of Al-rich intermetallic phases. Samples with comparatively thin and therefore crack-free transition zones showed a 45% higher shear tensile strength resulting in failure in the aluminum base material.

Analytical Estimation of Electromagnetic Pressure, Flyer Impact Velocity, and Welded Joint Length in Magnetic Pulse Welding

“…A direct analytical solution of the governing equation (Equation ( 1)) along with the necessary boundary conditions (Equations ( 2) and ( 3)) is very difficult, if not impossible [32]. Efforts to numerically calculate the transient EM field by solving the governing equation along with the necessary boundary conditions using finite element methods are reported for MPW of tubes [5,33] and sheets [3,34]. These efforts are a testimony of several inherent computational challenges, such as the requirement of very fine element sizes to discretize the solution domain as well as the use of very small time-steps to model the steep gradient of the EM field and its evolution [3,33].…”

“…Efforts to numerically calculate the transient EM field by solving the governing equation along with the necessary boundary conditions using finite element methods are reported for MPW of tubes [5,33] and sheets [3,34]. These efforts are a testimony of several inherent computational challenges, such as the requirement of very fine element sizes to discretize the solution domain as well as the use of very small time-steps to model the steep gradient of the EM field and its evolution [3,33]. To keep the modelling calculations manageable, many simplifications are considered.…”

“…To keep the modelling calculations manageable, many simplifications are considered. For example, the elements in the solution domain are considered isotropic and the EM field, "little away" from the coil and flyer-target assembly, is assumed to be negligible [3,35]. Likewise, the dynamic change of the flyertarget geometry is also neglected for the calculation of the EM field [3,36].…”

“…The joint length is referred to the intimate contact along the flyer-target interface as a result of the EM pressure of the flyer, its progressive impact on the target and the consequent visco-plastic deformation of the flyer-target assembly [3,5]. The joint length is often observed using microscopy and measured by means of peel tests [2,7,33].…”

“…An advance estimate of the EM pressure and the flyer impact velocity is required to design a suitable process schedule and to assess the joint quality in MPW [2]. The use of comprehensive numerical models [3] and wide-ranging experiments [4] is growing, to improve the insightful understanding of MPW, but these are computationally intensive [5] and time-consuming [6]. A practical framework is needed for a fast and reliable estimate of the EM pressure, flyer impact velocity, and the joint dimensions for a given process schedule to enhance the commercial application of MPW [7].…”