Unequal shape or size projections can cause issues in projection welding. If one or more or the projections are not the same shape or size, current will flow through the part differently. The current will flow through all projections making contact at the end of the squeeze portion of the weld cycle. The larger cross section projections will not heat as fast as smaller ones. The smaller mass projections in contact will heat faster and start to collapse sooner than the larger ones in contact. If a projection height is short it may not even be in contact at current initiation. It won’t make contact until the other projections start to collapse. When contact does occur arcing or expulsion is likely from this short projection. Quality could be compromised. Before it makes contact the other projections are carry its current so they are overheated. Again the process is not in control.
Poor electrode alignment or being out of parallel can be a serious problem with projection welding. Generally projection welding involves making several small projections welds with one electrode delivering the force and current to the part. Therefor if the electrode contacts one side of the part first those projections will be in the most intimate contact and carry most of the current. Projections on the other side of the part will have little if any force and could be subject to expulsion when the current initiates. This is both dangerous and bad for weld quality since the projection may be expelled when the current initiates.
Projection welding is a resistance weld where the design or shape of the part is used to make discreet individual point contacts to concentrate the current during the welding process. In most applications multiple small projections are formed on one surface of the parts to be welded. These projections can be round dimples, elongated ridges, circular, or the extended corners of weld nuts. Two round wires placed together at 90 degrees form a point contact. This would also be a projection weld. When the mating parts are brought together these projections concentrate the current flow and generate the heat in these locations. When the projections get hot they collapse as the weld nugget forms. After cooling the result is several weld nuggets holding the part together. Of course in the case of cross wire welding there is one weld per crossing location but normally many are welded at the same time as in fencing.
Nut welding is used to describe the projection welding process used to attach threaded nuts to a metal substrate. This is a projection weld because the nuts either have a partial or full ring projection or multiple corner edges are formed into projections. This is a very common form of welding in the automotive and appliance industry. These weld nuts provide the assembly points for all of the screws and bolts used in assembly of these products. It is a reliable fast repetitive process.
Yes, cross wire welding is a projection weld. There is no formed projection present on the wire or rod. When you bring the wire together at 90 degrees to each other they make a point contact. This then becomes the projection for projection welding. Many products are projection welded using this process including fencing, grating and rebar.
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