Workpiece Positioning during Magnetic Pulse Welding of AluminumSteel Joints A study was conducted of the axial positioning (working length) and Introduction Magnetic pulse welding (MPW) is a solid-state impact welding process that enables the joining of tubular or flat partners. Through the fast discharge of capacitor banks, a magnetic field is generated in a coil, which leads to eddy currents in the electrically conductive outer (flyer) piece positioned in close vicinity. Repelling Lorentz forces between the coil and flyer cause the flyer to accelerate toward the inner (parent) workpiece. Under adequate conditions, a metallurgical joint between flyer and parent part is formed. The process enables joint creation of dissimilar materials, such as steel/aluminum and copper/ aluminum, which are not easily welded by standard techniques due to their differences in melting points, thermal conductivity, and specific heat (Ref. 1). During MPW, the only heat generated is due to the deformation of the parts during collision and the eddy currents in the flyer part. There is no input of external heat, thus the formation of brittle intermetallic phases that weaken the joint is largely avoided. The ability to join similar and dissimilar metals and alloys, as well as its large repeatability and production capability, has fostered the application of MPW in various commercial sectors; this includes automobile, aerospace, nuclear, packaging, consumer products, and electrical industries in recent years. Comprehensive reviews of the process principles and current research activities in the field of MPW are given in Refs. 1, 2, and 3. WELDING RESEARCH The properties of the welding front, which are influenced greatly by the evolution of the flyer deformation and the collision process, are ultimately the determining factors in welding success. Under the correct conditions, a mass flux containing oxides and debris (the “jet”) is formed, which cleans the surfaces and allows for metallurgical bonding (Ref. 4). The three main factors typically considered in joint formation are the collision angle , the collision velocity vc, and the radial impact velocity vim. These parameters are geometrically interrelated. This criterion is based on the similarity of MPW with the explosion welding process, which has been under investigation for several decades (Ref. 2). Limiting conditions for weld formation need to be investigated for every specific combination of machines, tools, and workpieces, but, in general, radial impact velocities slower than 250 m/s and impact angles lower than 5 deg are regarded as unfavorable for MPW (Ref. 2). Figure 1 shows a schematic of the welding front as well as a micrograph of a welding interface. Stern et al. provide a more in-depth analysis of the jet material for Al-Al and Al-Mg samples. Here it was reported that the composition and mechanical behavior of the jet was primarily dependent on the density and melting point of the respective materials (Ref. 5). The welding front contour is determined by many factors, including the radial coil/flyer and flyer/parent spacings, as well as the axial arrangement of the coil and flyer (the “work- coil parameters for cylindrical workpieces BY A. LORENZ, J. LUEG-ALTHOFF, J. BELLMANN, G. GÖBEL, S. GIES, C. WEDDELING, E. BEYER, AND A. E. TEKKAYA ABSTRACT Magnetic pulse welding (MPW) enables the fabrication of joints via the harnessing of Lorentz forces, which result from discharging a current pulse through a coil. In the process, an outer piece (flyer) is accelerated onto an inner piece (parent), and welding is achieved using propagating impact fronts. The working length of the experimental setup allows for various shapes of the deformation front, and each configuration has its own advantages and drawbacks. The objective of this work is to show how the working length of tubular MPW specimens affects the front propagation as well as to indicate ways to optimize the front propagations, which are vital to the welding result. It is shown that for steelaluminum joints, three different front regimes exist, which are related to geometrical factors. These results may be used to avoid seemingly favorable but nevertheless suboptimal conditions for flyer movement, which reduce the weld quality and energy efficiency of the process. Additionally, the methodology presented here may allow for faster process optimization without the need for timeconsuming metallographic analyses. KEYWORDS • Magnetic Pulse Welding (MPW) • Working Length • Impact Welding • Aluminum • Steel A. LORENZ, J. BELLMANN, and E. BEYER are with the Institute of Manufacturing Technology, Technische Universität, Dresden, Germany, and the Fraunhofer Institute for Material and Beam Technology (IWS), Dresden, Germany. J. LUEGALTHOFF (joern.luegalthoff@ iul.tudortmund. de), S. GIES, C. WEDDELING, and A. E. TEKKAYA are with the Institute of Forming Technology and Lightweight Construction, Technische Universität, Dortmund, Germany. G. GÖBEL is with the Fraunhofer Institute for Material and Beam Technology (IWS), Dresden, Germany, and the University of Applied Sciences, Dresden, Germany. MARCH 2016 / WELDING JOURNAL 101-s
Welding Journal | March 2016
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