International Conference – Innovative technologies for joining advanced materials – tima09

Magnetic Pulse Welding – A New and Innovative Joining Process F. Zech, H. Cramer, L. Appel Schweißtechnische Lehr- und Versuchsanstalt SLV München, NL der GSI mbH, Germany E-mail: [email protected]

Abstract Magnetic Pulse Welding is one of various application possibilities of electromagnetic pulse technology. It works to the principle of fast capacitor discharge in a magnetic coil. A power pulse is generated by electromagnetic induction. It causes a contact-free and fast deformation of the working piece. Thus overlap and push-in joints on tubelike geometries can be produced. Depending on deforming speed and contact conditions of the workpiece surface, the joining process can be used for crimping and welding

In the case of explosion welding, a propagating pressure wave is generated by a chemical reaction (figure 2, left side). The pressure wave deforms and accelerates a plate onto another plate. A cold welding is performed along a propagating contact line which is caused by high impact speed.

The lecture points out the principles of electromagnetic pulse technology, explains the components and functions of the system. It shows specific joining geometries and examples of applications as well as metallurgical characteristics of the produced joints.

Introduction Figure 2: Features by magnetic pulse systems

Figure 1: Activities performed by magnetic pulse systems Magnetic pulse technology is based on the principle of a sudden plastic deformation caused by an electromagnetic force pulse. In addition to welding, the magnetic pulse technology can be used for crimping, deforming, cutting and punching (figure 1).

Comparison: Explosion Welding - Magnetic Pulse Welding Formation of the joint and joining mechanisms of magnetic pulse welding and explosion welding are comparable – only power sources are different.

Figure 3: Explosion welding of steel/aluminium The fusion line shows a wave-type structure which is typical for the procedure caused by local material plastification and intermixture at the contact face (figure 3). To obtain a proper collision speed, a defined gap is required between the plates which acts as an acceleration distance. The required gap distance is obtained by inserting spacers (angle plates) between the plates to be joined. During welding, the spacers are also compressed and welded. Explosion welding is preferably used for production of large-scale platings and claddings, also using materials of different type.

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why aluminium or copper are often selected as material for the outer joining piece. If materials of lower electrical conductivity are used, an additional sleeve made of Al or Cu may be used as an enhancer. The contact-free and cold joining procedure is especially suitable for joints of the same kind as well as for joining different materials. Another feature is the extremely short process time. Figure 4: Principle and geometries of magnetic pulse technology Using a capacitor discharge process, a high current pulse of approx. 100 – 1,000 kA (10 kV, 100 kJ) is generated (figure 4) in the framework of electromagnetic pulse technology. This high current pulse flows through a coil and generates a strong and pulse-type magnetic field. A metallic workpiece is positioned inside the coil. The induced current flow in the workpiece generates a magnetic field of opposite direction. In this way, an electromagnetic force pulse is caused between coil and workpiece.

Magnetic Pulse Welding System of the SLV Munich The magnetic pulse system (figure 6) consists of 4 components: a high voltage cabinet, an energy storage cabinet, a work table with magnetic coil and an operating panel.

Figure 6: 35 kJ magnetic pulse welder at SLV München The displayed magnetic coil is a multi-turn coil system with an internal slotted copper core acting as field former for a sleeve diameter of 25 mm. Work piece feeding and positioning of the laboratory operation is carried out using a manual feeding unit with insulated plastic elements (PE1000, green). Figure 5: Magnetic pulse waveform Provided that the coil is stable and free from deformation or damage, the magnetic force generates a deformation of the workpiece inside the coil as a radial compression. Deformation is carried out extremely quick within the first pulse half-wave (figure 6) during a period of time of around 50 μs (0.05 ms). Depending on deformation speed and contact conditions of workpiece surfaces, the joining process can be used for crimping (force transmission by force and shape) or welding (metallic joint) for overlap joints according to the examples in figure 4. If 2 workpieces are positioned inside the coil, e.g. overlapping ends of tubes, the outer tube will be pressed onto the inner tube. The very short pulse time limits the induction depth, comparable to a high frequency induction. The magnetic force only compresses the outer tube radially, but not the inner tube. However, it may be required to prevent the inner piece from mechanical deformation caused by the outer piece. A sufficiently high wall thickness or a support may be suitable to stabilize the inner tube. This is

The only parameter to be adjusted is capacitor charging voltage (kV). All other conditions are defined by geometric arrangement of the coil system (field former), work piece geometry (wall thickness, radial gap, axial overlap) and positioning inside the magnetic coil.

Examples of Magnetic Pulse Welded Joints Trial pieces of magnetic pulse welded Al/Al joints are displayed in Figure 7. Charging voltage was increased from 4 to 8 kV. An increasing charging voltage and energy (E = ½ CU² = ½ LI²) causes a faster and/or stronger deformation of the aluminium sleeve. A metallic joint is formed in the overlap area (figure 8). Fusion line shows non-fused areas at the beginning and the end. However, fused area is 2 - 3 times longer than the wall thickness of the sleeve. In other words, the joint shows a higher strength than the tube sleeve. In addition, the joint is liquid and gas tight (pressure tight). A protection of the open edge areas is only required in the case of corrosional stress.

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be observed. As a cold welding procedure, the magnetic pulse welding is especially suitable for joining different material. Figure 10 shows an aluminium tube ∅25 x 1,5 mm which was welded to a copper piece ∅17 mm. The radial gap between the joining faces was approximately 2.5 mm before deformation. The gap-free end of the copper piece is only used for fixation and/or centering of the components. Caused by the extremely quick deformation and the collision of the joining areas, a wave-like structured joining zone is created which has a similar structure compared with explosive welding. Figure 7: Al/Al magnetic pulse welds

Figure 8: Welding zone of Al/Al MP welds Magnetic pulse welding is regarded as a cold welding procedure. Especially with aluminium materials, a softening caused by heat influence is avoided and the strength of the base material is also preserved in the area of the joining zone. A possible softening of the aluminium material can be detected when observing a hardness drop.

Figure 10: Different materials - aluminium/copper MP weld

Applications of Serial Production During the past years, the magnetic pulse welding has obtained a wide area of applications mainly in the range of automotive components. However, there are many other manufacture jobs: the process will be used if thin walled tubular structures and boxes made of light weight materials like aluminium are to be welded, especially if a high quality style and high economic efficiency are required, provided that the joint is an overlap joint. Typical examples are aluminium casings for filter or cooling media (air conditioning systems).

Figure 9: No heat effect on Al MP welds Figure 9 shows the hardness of both aluminium components. One can see that the hardness of the joining area (FZ) is virtually identical compared with the base material (GW). A thermal influence to the material cannot

Figure 11 displays a thin walled aluminium box which is produced by magnetic pulse welding in serial production. It is faster and more economical compared with conventional MIG welding. In addition, the workpiece shows a higher strength without thermal softening. An aluminium tube 6061 ∅96 x 1,7 mm is welded to a circular disk made of Al 6061 (figure 12). The width of the circumferential joining area is approx. 8 mm with an overlap of approx. 13 mm. The welding time is < 1 ms, independent from size of

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worlpiece and length of circumferential joint.

Figure 14: Air drying canister /PSTproducts/

Figure 11: Al/Al AC accumulator /PULSAR/

Another field of application can be found in the field of electrical conducts and connections of copper and aluminium cables. When it comes to mechanical crimping of cable lug with relatively slow deformation speed, a disadvantage is backspringing of material with reduced clamping force of the cable. In addition, resistances generated by surface oxides, may cause power loss and dangerous heating under high current. Partially metallic joints are obtained using magnetic pulse welding for cable lugs (figure 15). Such contact areas are located between sleeve and outer wires as well as between the wires themselves. Such a direct metallic connection avoids almost any transition resistance – an ideal connection suitable for a low loss transmission of high electrical power.

Figure 12: Welding Zone of AC-Accumulator Various magnetic pulse welded parts of serial production are shown in figure 13 and 14.

Figure 15: MP-Welding of power cables /PULSAR/

The air drying canister is a prototype, MP welded with a PSTproducts machine PS60 (60 kJ). It is made of Al/Al, atomic welded and helium tight, with no heat effects, scheduled for 750,000 units per year.

Figure 16 presents the differences in welding or crimping using the magnetic pulse system.

Figure 13: MPW receiver drier /TI Automotive/

A metallic connection is generated during welding. A radial gap is required between the two pieces (acceleration distance) and a high deformation speed in connection with suitable surfaces and contact conditions (impact speed and impact angle). The harder the material can be deformed – depending on strength of the alloy and wall thickness – the higher is the required pulse power. Crimping can be carried out with lower deformation speed and does not require any radial gap between the faces and/or any acceleration distance to achieve a sudden contacting. Crimping is functionally carried out together with a defined shaping of the inner workpiece.. When deforming the outer piece, a deformation of the inner piece forms fractional connection and form closure.

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Conclusion A joining process using an electromagnetic force pulse is suitable to weld and crimp thin walled tubular overlap joints. The procedure is not very much known or spread due to different designations (MPW, MPU, EMPT, …). An increasing demand on fast and economic joining procedures, on low heat affection of materials, on production of joints with different materials and advantages of a touch-free and fast deformation open up a fast growing field of application for magnetic pulse technology in serial production.

Figure 16: Drive shafts, Al/St mp welded and StSt mp crimped, ∅50 mm

SLV München supports users to introduce this new technology.

References Further examples on MP crimping are presented in figure 17.

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Figure 17: Examples of electromagnetic pulse crimping /PST products/ Commercial systems for deforming and welding using the MP technology are available in modular performance stages from 5 to more than 100 kJ. Commonly used magnetic pulse machines are equipped with a compression coil, i.e. a coil located at the outside, which compresses a piece in the inside. There are some other variants available like expansion coils positioned in the inside of a workpiece as well as flat coils to generate axial force. The coil system must bear the magnetic counter force and is exposed to high stress during operation. When it comes to production lots in serial production from 100,000 to 2,000,000 units per year, a regular replacement of the coil system is to be scheduled depending on dimension and stress, e.g. after 50,000 100,000 welding cycles. In serial production, a contribution of e.g. EUR 0.05 - 0.20 per welding cycle is calculated as "Coil Cost per Weld". Compared with other welding processes, there are no costs for welding commodities, gases, tools etc..

Zech F., Cramer H. and Appel L., "Metallografic Investigation of MPW interfaces", First technical conference on industrialized magnetic Pulse welding and Forming, SLV Munich, July 3rd 2008 Kallee S.W., "Magnetic Pulse Welding as an Enabler of LightWeighting in the Automotive Industry", First technical conference on industrialized magnetic Pulse welding and Forming, SLV Munich, July 3rd 2008 Shribman, V.,"Magnetic Pulse Joining of Light Metal Castings", First technical conference on industrialized magnetic Pulse welding and Forming, SLV Munich, July 3rd 2008 Mussi, P., "Magnetic Pulse Welding on Receiver Drier for heat Ventilation Air Conditioning", First technical conference on industrialized magnetic Pulse welding and Forming, SLV Munich, July 3rd 2008