WJS London branch

Report of meeting held on 17th April 2008 at Central Sussex College, Crawley

I am indebted to Charlie Barraclough, the Chairman of the London Branches for compiling this report on the April meeting of the London Branches WJS which was held at Central Sussex College, Crawley on the 17th April.

The speaker, Geoff Melton of TWI Arc Welding Department, was introduced by Bert Buckingham. Bert noted that many years ago welding consumables were developed to make up for some of the short-comings of welding power sources, but equipment was now available which was able to make up for the idiosyncrasies of the welding processes and even to some extent of the lack of experience of welders. Geoff introduced the aims of power source development as improvement in control, quality and productivity. This had enabled improvement in fit-up tolerance and extending the capability of equipment to deal with a wider range of parameters.

About 50% of the market still uses old-fashioned transformer-rectifiers. Large primary transformers were required to step down to welding voltage, and the voltage was crudely controlled by the saturation of the transformer. With transistor control the voltage was smoothed by analog control, but a large amount of heat was generated which needed considerable cooling. Because of current limitations a large number of transistors were needed. These were later replaced with switching thyristors which gave a square wave and much higher power rating and lower heat loss.

With switching much higher frequencies are possible, and inverter power sources use this feature to directly switch the incoming rectified mains voltage to over 100kHz, doing away with the primary transformer altogether. Since high frequency transformers are an order of magnitude smaller than low frequency transformers, the secondary circuit transformer which is used to step the voltage down to welding voltage is very small. Furthermore the voltage and current can be digitally controlled.

The advantages of the current generation of digitally controlled inverter power sources are not only size and power rating: much better control of transfer is possible, giving MIG (metal inert gas) welds a quality which is 'near-TIG' (tungsten inert gas). Welding noise is reduced as is stray EMR (electro-magnetic radiation) (no interference with Radio 1!). Dip, spray and pulsed MIG have been extended to a point where one-knob 'synergic' control is possible from very low to very high current. Feedback of arc voltage and waveform to the control processor allows voltage to be reduced at the moment of detachment to prevent explosive transfer. Such systems are now being introduced with 'self-tuning' ability, so that corrections are made to allow for any increase in wire feed speed or nozzle to workpiece variation.

AC (alternating current) MIG is now also possible, as arc ignition can be maintained during the wire-negative stage, and this offers very high productivity. Other processes have also benefited; higher frequencies with TIG allow arc constriction and welding with very low currents; variation of frequency with SAW (submerged arc welding) have increased both penetration and deposition rates possible. Computer control has been made more user-friendly, and computer or even mobile phone control of wave-form and other characteristics is now possible.

Paul Bryant of Central Sussex College gave the vote of thanks, and a party of the attendees visited the welding facilities of the college.