Page 30 - MetalForming March 2020

FABRICATION
Projection Welding —Facts and Fundamentals
A primer to familiarize yourself with this popular process, provided by Bill Brafford, site content expert, www.howtoresistanceweld.info.
What is a Projection Weld?
Projection welds developed via the resistance welding (RW) process use the design or shape of the part to make discreet individual-point contacts to concentrate the current during weld- ing. In most applications, one of the workpiece surfaces features multiple, small, formed projections—round dim- ples, elongated ridges or rings, or the extended corners of weld nuts. When the mating parts come together, the projections concentrate the current flow and generate heat in these loca- tions. The hot projections then collapse as the weld nugget forms.
Note: In the case of cross-wire weld- ing (two round wires placed together at 90 deg. to form a point of contact), a weld forms at the crossing location.
Projections (shown here on a nut) concentrate current flow and generate heat in these locations. The hot projections then collapse as the weld nuggets form.
Where Can I Find Weld Schedules/Parameters for Projection Welding (PW)?
Several sources exist for PW sched- ules, some included in industrial stan- dards and literature. Considering that nut-and-bolt welding comprises a large
percentage of PW, it would seem that standards might be available. However, I am not aware of a standard for nut- and-bolt welding. Due to the multitude of nut-and-bolt designs, users and stan- dard-writing groups have not been able to collect the necessary data to form a consensus and develop a standard.
Can a Projection Weld Cause Spatter or Expulsion?
PW, like any RW process, if not per- formed properly can generate weld spatter or expulsion. The key variables to control: alignment, projection size and shape, force follow-up, and weld schedule.
Alignment: Weld pressure must force the projections into contact simultaneously and at the same pres- sure, otherwise one or more projections will carry most of the current and over- heat. An overheated projection or pro-
Another example of projection welding—threaded spuds welded to a tank head.
28 MetalForming/March 2020
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