Page 45 - MetalForming May 2014

the machine. The frame of the welding machine and the electrode assembly must be rigid to avoid flexure and mis- alignment under load. The electrode force must be able to hold each pro- jection firmly against the stamping during the initial period of the weld time, and must have good follow-up characteristics when the projections collapse during the welding process. Slow follow-up will result in metal expulsion before the parts come together.
Also ensure that the surfaces of the electrodes are flat and well-aligned. They must have clean contact points and be free from oil, dirt or film. The upper electrodes also must be on the same centerline. While most RPW setups employ Class 2 copper-alloy electrodes, harder tungsten-copper inserts can be used to minimize elec- trode wear. The weld pin of the lower electrode should be of a properly insu- lated material, either ceramic or a material with a nonconductive coating.
Finally, the weld pin should be spring- or air-loaded. Incorporating an air-supported pin not only will help cool the weld, it also will expel any weld spatter that might occur.
Follow the Prescribed Welding Procedure
To determine the optimum weld schedule—electrode force, weld time in cycles and welding current—first consider the type of nut or stud to be welded, along with the size and loca- tion of the projections. Then consider the thickness and material grade of the stamped part. This information also allows a fabricator to establish a typical minimum torque-to-failure test for the settings selected.
To verify that a minimum specified torque is achieved with the parameters selected, destructive testing most likely is required. Fabricators typically use either a push-off or peel test to check that a piece of the material can be pulled from the sheet at each projec-
tion point. If this cannot be accom- plished, the welding parameters (elec- trode force, weld cycle time and cur- rent) can be adjusted to reach the desired torque requirements. Typically, a short weld time (in cycles) requires more current, and a longer weld time requires less current.
For most applications, the current required for RPW is less than what is required for corresponding spot weld- ing. RPW current must be sufficient to create fusion before the projections completely collapse. While shorter welding time is desirable from a pro- duction standpoint, higher current is necessary to create quality welds.
To prevent overheating and metal expulsion, take care to optimize the weld schedule. To weld fasteners to high-strength steels, consider using impulse welding to better control heating rate. Ensure that electrode force is sufficient to flatten the pro- jections completely when they reach welding temperature, and that the fas-
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MetalForming/May 2014 43