When the material is thick and the projection starts to collapse the metal sometimes needs a place to flow to in order to get out of the way of the two parts coming together. This can be done by projection design. Frequently a small cavity or depression is formed into the part around the base of the projection. Excess material can flow into this area and let the two parts come into more intimate contact.
Reference: AWS C1.1 Recommended Practices for Resistance Welding
RWMA Manual Section 1, Chapter 3