Voids at Welding Points
How can we prevent voids from forming around weld areas in electrocoated parts?
Q. We stamp and weld two CRS brackets together then send them out to be e-coated. More often than not, brackets come back to us with voids in the electrocoat around the welds, approximately the same diameter as the electrode we used to weld the brackets together or a little smaller. The e-coater wants us to tumble the brackets prior to e-coating, but that leads to the parts being bent. Do you have any suggestions?—D.O.
A. The intense heat generated during the welding operation causes the silicon (Si) in the steel substrate to diffuse to the surface, cool and harden. Because of surface tension differences between the Si and the welding material, the hard deposits typically appear round, and dark brown or amber in color.
One characteristic of the Si spots is that they have poor electrical conductivity. When electrocoating is involved, this condition produces voids or partial coats in the weld beads.
The intense heat generated during the welding operation not only introduces the challenges of weld spots, but also challenges with the surface adjacent to the welding seam or bead. This area is typically called the heat-affected area, and it is characterized by its dark brown or black discoloration. Although this heat -affected area is conductive and will electrocoat, the appearance and adhesion of the electrocoat will be marginal. This marginal performance is due to the inability of the phosphate system to develop a good phosphate crystal over such a heavily heat-oxidized surface.
Fabrication shops typically pre-wash parts with alkaline cleaners prior to welding to prevent burning or baking residues onto the metal surface and leaving it contaminated with excess carbon and other inorganic residues. If there is no pre-washing prior to welding, then additional performance issues will appear.
The best process to make a welded part ready for electrocoat would involve cleaning the Si spots, as well as the oxides and residues on the heat-affected areas.
The solutions to addressing both conditions are those employing mechanical forces such as blasting, wire brushing, sanding or tumbling, or employing chemical forces such as etching.
Some cleaning methods are preferred over others, and the right choice heavily depends on conditions such as the substrate, size of parts, type of manufacturing employed, type of electrocoat machine employed, etc. In your case, tumbling would be excluded because it bends the brackets.
This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.
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