Which current ensures a good spot welding?
Author: Prof. Dr.-Ing. Peter Puschner,
Welding expert and consultant of ELMA-Tech GmbH
True to the motto "higher, faster, further" a race was started several years ago, with one believed to be able to fix the quality and suitability of spot welding machines for use in the car repair at a single number: The maximum welding current producible by the machine. Or: The higher the better!
Meanwhile 15 kA are laying on the table, that are waiting to be surpassed.
This idea was closely linked to the now completely outdated knowledge that those high and ultra high strength steels, at that time just in the launch phase, can’t be welded otherwise.
The state of the art
Currents above 10 kA are counterproductive and generate strong spatter with all the negative side issues and a not reproducible joining result. The ancient formula "a good point has to splatter" has lost its raison d'être a long while ago.
In fact, the specific resistances of the high- and ultra high-strength steels are significantly higher than that of standard steels. This means that for a given current a higher energy input results in any case, according to the formula E = i² * R * t.
In countless attempts thousands of plate combinations were welded for the release of repair machines in institutions commissioned by the automobilists. All with currents well below 12 kA, typically below 10 kA.
Limits of mains power supplies in repair shops
Machines with maximum current up to 12 kA can be operated with fuse protections of 3 x 32 A operate in almost all European repair shops with 400 V three-phase supplies, although the present phase currents briefly make a multiple. Everyone who thinks one can or needs to provide 15 kA for a spot, requires a 63 A fuse protection and the shortest power lines to the distribution box. A condition that is rarely realized in car repair shops. Abroad, often with low voltages in three-phase systems, the fuses must be increased to 126 A, the power cables for this purpose are as thick as an arm … A hardly enforceable or true scenario.
How important therefore is a value of 15 kA with respect to a high-end weld?
Not important! The significance of 15 kA for a repair spot welding machine for workshops has approximately the same quality as "red cars go faster". Even in the production such high currents in all steel connections are off-limits. Here someone just followed the idea during the specification that a transformer, which is not being driven to its limits, is all in all more stable for 25,000 points per day in three shifts.
But, which repair service places more than 2000 points a day? And also here a rethinking has begun: Stability is not a question of oversizing, but solely a question of technical design.
Nowadays, a good, high-quality spot weld is only a question of the quality of the process control. Regarding this point, today's machines are in competition. And for that you don’t need 13, 14 or 15 kA, but a deep understanding of the joining. This is precisely what we have been working on successfully since 2004.