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MPW is said to have been first used in Russia for welding the end closures onto nuclear fuel rod holders in the early 60's. MPW has huge potential as a mass joining process but today remains at an embryonic stage limited by equipment inadequacies due to problems associated with power handling. Today MPW is used mainly for joining small parts often in exotic materials or relatively soft dissimilar materials. Larger machines are becoming available that partly address the power handling limitations. Trib treatment offers a potential solution to this dilemma by improving two aspects of MPW first - by reducing overall energy to weld between round co-axially overlapping parts, and second - by reducing local energy to weld at some part of an overlap where welding energy is marginal, at for example the start/finish split of a single turn induction coil. The practical limitations of MPW relate to handling very high electrical currents. These currents are generated by discharging a bank of charged capacitors rapidly through a coil that surrounds one of the parts to be welded. The current passing through the coil induces an opposing current within the surrounded part, the reaction force between opposing magnetic fields forcing the parts being welded together at sufficient velocity (energy) to cause welding. This method of accelerating a body is theoretically capable of very high velocity of the order of 15,000m/s compared with approximately 5,000m/s for explosives, but in practice is limited to lower velocities of less than 1,000m/s by the problems associated with power management, for example coils tend to vaporise! Lower power magnetic pulse machines are used extensively for forming metals - known as Magnetic Pulse Forming (MPF) where the energy levels required are typically less than half that for welding. A typical use for these machines is the folding over and crimping of flanged edges on canisters for example on automotive oil filter cartridges. MPF machines do not develop sufficient velocity for welding. However by reducing the welding threshold velocity with Trib, some MPF machines may be tooled of making low energy impact welds. Conventionally for successful MPW the first difficulty to overcome is the limitation of the switch, this must be able deliver a rate of rise of current that challenges available technology. For very high energy discharges, spark gaps are used - which are difficult to control and are prone to erratic behaviour. Ignitrons, employing an arc struck between electrodes within mercury vapour and controlled externally by a trigger mechanism known as an igniter are more stable. Several ignitrons arranged in parallel boost current capacity but the stray inductance effects of connecting cables limit di/dt . Ignitrons are successfully employed in low and medium power MP forming and welding equipment's. Ignitrons were used as resistance or spot welding switches. Solid state Silicon Controlled Rectifiers (SCR's) have replaced ignitrons for resistance welding and are used in MPF machines. High power SCR's were developed principally for motor speed controls, being semiconductors they are prone to over-load damage, either voltage or current transients. Semiconductor switches will probably provide the best engineering solution eventually when combined with Trib Treatments for producing reliable high volume production MPW units. Another major limitation of MPW equipment concerns the electrical connections between capacitors, switch and coil. The coil in particular has electrical interfaces that suffer severe erosion, which limits coil life, due to uneven surface contact and subsequent variations of local resistivity. The coils themselves are single turn inductors made of high strength steel, thus they are not particularly good conductors of electricity. They need to be made of steel to withstand the operating stresses. Thus significant advantages are gained by reducing stress on equipment by reducing the energy to weld. Patents are pending on Trib methods of reducing the impact velocity threshold. More information available upon request.
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for more information email: info8@tribtech.com
The TribTech name derives from "tribos" - Greek for 'rubbing'. 'TribTech' is a trade name used by Ball Burnishing Machine Tools Ltd. of 12 Brookmans Av. Hatfield, Herts. AL9 7QJ. United Kingdom; Company Reg. No. 1408807, VAT Reg. No. 421 6210 04; a knowledge based company that develops, patents and licenses technology. All rights reserved by Ball Burnishing Machine Tools Ltd. Last modified: May 01, 2008 copyright © 1999/2008. The information and data provided herein should be considered generally representative for the tools and technologies described. In all cases users should carefully evaluate the tools and technologies to determine their suitability for a particular purpose. |
