ELKANITE 20W3 APPLICATIONS
Elkanite 20W3 is a 75% tungsten 25% copper material produced by powder metallurgy techniques. Combining tungsten and copper in this way results in a material with good hardness, a high softening temperature and moderate electrical conductivity. Whilst the material offers the advantage of good hardness which is maintained to quite high temperatures, its disadvantage is its moderate electrical and thermal conductivities.
As with all resistance welding electrodes their running temperature while in operation determines the life that will be obtained. The cooler the operating temperature, the longer the electrode will last. One of the most important design features of a resistance welding electrode is therefore its efficient cooling. This is even more important with Elkanite 20W3 as its inherent electrical and thermal properties tend towards the generation of higher temperatures than would be expected from the copper alloy electrode materials Mallory 328 or Mallory 100. It is therefore recommended that Elkanite 20W3 is used as a facing on an efficiently water cooled copper alloy backing.
More specifically it should be used as a facing with a maximum thickness of 6mm, the facing being attached to either a Mallory 328 or Mallory 100 backing, by low temperature silver brazing. The electrode should be designed to put as much of the facing in contact with the parent material as is practical within the limits of the application. This reduces the generation of heat at the contact resistance face by lowering the current concentration passing through the facing. The backing itself should be as large as possible both to reduce current concentration and to provide a thermal mass. It should of course be cooled by passing water through internal galleries, these galleries running as close to the facing as possible. Care should also be taken in designing the water galleries to ensure that steam pockets will not be formed. The backing should be made from a copper alloy like Mallory 328 or Mallory 100 and not ordinary copper. This is because ordinary copper will be severely annealed during the brazing process used to attach the facing and could lead to deformation of the backing under the high loads applied to the electrodes during the welding operation. This deformation could result in a number of problems, however from the point of view of the facing, it could result in cracking of the joint.
METHOD OF ATTACHING
The preferred method of attachment is by “Low temperature silver brazing”. The recommended products for this are Elkaloy 42 for the filler metal (Associated Data 1300 : 150) and Elkaflux (Associated Data 1300 : 153)
Since tungsten copper is not easily wetted by brazing alloy filler metals the back of the facing should be pre-flushed with filler metal. This is simply done by coating the back of the facing with flux and melting onto it a layer of brazing alloy. Having done this, allow the backing to cool. Once cool remove the flux residues and generally clean up. The operation is now completed by fluxing the back and fluxing the facing. Put the two together and heat until the brazing alloy re-melts and flows. When the brazing alloy is molten it is good practice to move the facing backwards and forwards as this helps to remove flux and gas trapped in the joint. The tip should then be finally seated into position and the joint cooled.
When heating the assembly to brazing temperature it is important that this is carried out as rapidly as possible for two reasons. Firstly the fluxes used for silver brazing will become exhausted after a given time period, which will result in a poor quality joint. Secondly the copper alloys Mallory 328 and 100 soften with time at temperature, thus the longer they spend above 500 deg C the greater the degree of softening. Once the joint has solidified, water quenching is recommended to reduce the softening effect further.
Associated Data: Elkanite 20W3 Data Sheet. 1300 : 107
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