An exploratory study into the feasibility of magnetic pulse welding Kevin Loncke Promotor: prof. dr. ir. Wim De Waele Begeleiders: Koen Faes (BIL) Masterproef ingediend tot het behalen van de academische graad van Master in de ingenieurswetenschappen: bouwkunde Vakgroep Mechanische constructie en productie Voorzitter: prof. dr. ir. Patrick De Baets Faculteit Ingenieurswetenschappen Academiejaar 2008-2009 De auteur en promotor geven de toelating deze scriptie voor consultatie beschikbaar te stellen en delen ervan te kopi¨eren voor persoonlijk gebruik. Elk ander gebruik valt onder de beperkin- gen van het auteursrecht, in het bijzonder met betrekking tot de verplichting uitdrukkelijk de bron te vermelden bij het aanhalen van resultaten uit deze scriptie. The author and promoter give the permission to use this thesis for consultation and to copy parts of it for personal use. Every other use is subject to the copyright laws, more specifically the source must be extensively specified when using from this thesis. Gent, Juni 2009 De promotor De begeleider De auteur Prof. dr. ir. W. De Waele ir. K. Faes Kevin Loncke Acknowledgments With the finishing of this master thesis, my time at Ghent University has almost come to an end. I am truly grateful to a large number of people who have helped me throughout my study and the fulfilling of this master thesis. First and foremost, I would like to express my appreciation to my promotor Prof. Dr. Ir. Wim De Waele, and my mentor Ir. Koen Faes. They were always prepared to give advice and feedback during the experimental research. I very much appreciate the time they took to read and re-read the numerous drafts of this thesis. Most importantly, I would like to thank them for the understanding of my sometimes over-booked schedule, and the freedom they gave me to plan the experiments as it suited my agenda best. I would also like to thank the people of the Belgian Welding Institute that helped me find my way around in the laboratory: Michel De Waele for explaining me the tools I needed to use; Anja Buyse and Gert Oost for taking their time to etch the samples; the people of the technical staff to make the workpieces; and all the others that were prepared to answer my countless questions throughout the semester. During the hundreds of hours that I spent behind this desk while writing this work and pondering about all possible aspects of magnetic pulse welding, there was one special person always ready for me. I would like to thank my girlfriend and future wife Jessica for encouraging me during the times when I felt the deadline was coming too close, for keeping me company on skype during my breaks, and for showing her sincere interest in the work I have done. I would also like to thank my fellow students of the VTK and IAESTE for sharing all those fun moments the past academic year, and more importantly making me realize that I was far from the only student combining a thesis with working at the student union. Last but not least, I would like to thank my parents for offering me the opportunity to complete these studies, for encouraging me during the hard first years of my studies, for appreciating my efforts done throughout this time, for understanding when I was cranky due to too big work - time ratios, and for buying me a coffee machine for my student home. Kevin Loncke Ghent, 1 June 2009 iii iv An exploratory study into the feasibility of magnetic pulse welding by Kevin Loncke Master thesis presented in fulfillment of the requirements for the degree of Master of Civil Engineering Academic year 2008{2009 Promotor: Prof. Dr. Ir. Wim De Waele Mentor: Ir. Koen Faes (BIL) Faculty of engineering Ghent University Department of Mechanical construction and production Chairman: Prof. Dr. Ir. Patrick De Baets Summary In this work a study was done of the magnetic pulse welding process. The influence of the stand-off distance, thickness of the flyer tube, position of the field shaper, the shape of the inner workpiece and the material were discussed, based on the results of a series of experiments. A series of formulas was set up to help understand the influence of the parameters. A literature study on the explosive welding process was used to get a better understanding of the bonding and the deformation behaviour. Keywords magnetic pulse welding, principle, process parameters, weld characteristics An exploratory study into the feasibility of magnetic pulse welding Kevin Loncke Supervisor(s): Wim De Waele, Koen Faes Abstract— This article displays the principle of the MPW process. The operations required, and the high quality of the weld. Limita- influence of the process parameters and the deformation behaviour is dis- tions of the process lay in the geometries that are possible to cussed. These are based on a series of experiments, a set of formulas and a comparison with explosive welding. combine, while the sensitivity to process parameters and the in- Keywords—magnetic pulse welding, principle, process parameters, weld accessibility of the welding zone are disadvantages. The process characteristics is very similar to the explosive welding process, apart from the energy source that is used to move the flyer material. I. INTRODUCTION III. EXPLOSIVE WELDING OINING of dissimilar materials offers great opportunities Jfor e.g. the automotive industry and heat-exchanging sys- A. Welding windows tems. Solid state welding processes like magnetic pulse welding Except for the source of energy, explosive welding (EXW) (MPW) offer this possibility. But because research in MPW is is very similar to MPW. The EXW theory learns that the weld fairly new, the knowledge about and experience with the process holds the best strength properties when a wavy interface is cre- is concise. In order to use the process in industrial applications ated without an intermediate layer [2]. The quality of the weld on a wide scale, a good understanding of the bonding and the depends on the impact velocity v and impact angle α. Graphs influence of the process parameters is required. c show welding windows with the required range of these two pa- II. PRINCIPLE rameters. The angle α should be within a certain range, and vc should be in the subsonic range in order to allow jet formation. The magnetic pulse welding process starts by charging a bank The impact velocity vc should also be high enough to create a of capacitors upto a desired energy level. This energy level is wavy interface. The welding windows can serve as a tool to characterized by the voltage level in the capacitors. When the compare the required parameters of different material combina- capacitors are charged, the current is instantaneously released tions. through a coil (figure 1[1]), this way inducing a magnetic field. A field shaper is placed inside the coil to concentrate the mag- B. Wavy interface netic field in the desired area. The field will be blocked by the flyer tube, depending on the skin depth of the material. Through The waves at the interface show an increase in the wavelength simple calculations, it was shown that for the experiments in this and amplitude for an increasing vc. For an increasing α the work, more than 80% of the field is blocked. The difference in amplitude will first increase and immediately decrease, while magnetic field causes a radial pressure on the flyer tube, due to the wavelength will only show an increase. These relations are which it will impact to the inner workpiece. used to guess the change in vc and α in the MPW experiments. IV. SET OF FORMULAS The voltage level V could be calculated starting from the re- quired impact velocity vc with a set of equations. The chosen value of the impact velocity gives the total pressure that is re- quired to deform and accelerate the flyer tube. This pressure leads to a value of the magnetic field, taken into account the dif- fusion of the magnetic field through the tube. From the magnetic field density, the required current peak in the coil is calculated. This leads to the required voltage level, by using the equipment’s Fig. 1. Magnetic Pulse system layout. characteristics. The formulas were used to get an idea of the impact veloci- So far the research has been limited to tubular workpieces and ties that were achieved in the experiments. Due to the simplifi- few experiments with sheets. A great variety of material combi- cations, the pressure required for the deformation of the work- nations has already been successfully welded which illustrates pieces is underestimated. The formulas are therefore not accu- the main advantage of the process: the possibility to combine rate for workpieces with higher thicknesses t and higher stand- dissimilar materials. Other advantages are a reduction of the off distances s, because these require more deformation energy. manufacturing costs because there are no pre or post welding Moreover, the effect of the field shaper overlap is not taken into account. The formulas are therefore not accurate, but give a rea- residual stresses in the flyer tube. These porous welded areas sonable estimation of the impact velocity and show the influence should be avoided, as they clearly degrade the quality of the of the parameters for experiments with lower deformation. weld. V. EXPERIMENTAL PROCEDURE D. Influence of the parameters The experiments showed that a A. Workpiece layout • Shape of the field shaper: straight surface of the inner workpiece, as shown in figure 2, The experiments were done using the general layout shown in should be preferred over a slant surface. figure 2. The varying parameters were the materials, stand-off The collar on the inner workpiece only influences the deforma- distance s [mm], thickness of the flyer tube t [mm], shape of the tion behaviour of the flyer tube if l exceeds a certain value.