Spot welding on galvanized sheetgreenspun.com : LUSENET : Resistance Welding : One Thread
we generally faced weak spot welding on galvanized sheets. What parameters should be changed to carry out quality spot welding on galvanized sheet as compared with on normal sheet? What is the difference in tip dressing standards between galvanized and normal sheets? We normally tip dressing manually.
-- Ghulam Farooq (email@example.com), September 23, 2002
Galvanized welding is completely different than bare because of the thin layer of zinc in the center of the molten nugget. Bare metal can just be heated to the plastic state and forged together, because there is no zinc in between the pieces. If the zinc is not removed or dispersed it will fracture along that layer, like gluing two pieces of wood, with waxed paper stuck in between. Two ways deal with this: 1. Use upslope to gently move the zinc aside. This must be setup by experimenting with JUST the upslope heat, you don't want it to weld, you just want to uncover a 1/16" dia spot on your coupon when you peel it apart. 2. Use impulse heat to allow you to pump a lot of heat into the weld, but still maintain containment by using the 2-3 cycles cool between pulses to cool the surface, so it doesn't melt through. Generally two to five heat pulses are used, and they will total up to about 25% more energy than the equivalent schedule for bare metal. Tip cooling is EXTREMELY important. Most auto plants use just impulse, but some use upslope and impulse both. Upslope is considered controversial, because the earlier versions were not "balanced firing" and (I have no proof) I suspect they would sometimes saturate the transformer, which would make intermittent cold welds. Impulse firing also can saturate the transformer, even with carefully balanced firing, so make sure your weld controller has "antisaturation protection" This is standard on Medar, Square D, WTC, Robotron, ADC, ATek, but in some cases you may have to select this feature in the setup section of the controller. -Dave Bacon
-- David Bacon (firstname.lastname@example.org), October 03, 2002.