Originally Posted by onehundred80
If the parts are both cold then they will maintain the same size difference hot or cold. To achieve any useful change in the sizes the part with the hole is heated and the stud cooled.
Time is money so it is quicker to spot weld the large head of the headed stud. The head is not flat, so it would indicate to me that the small contact area of the head resistance welds to its mating part.
To use a PRW method on any application you have to account for the entire weld cycle. This involves bringing the parts into tooling, closing the gun, applying a set force, waiting for a set hold time, possibly applying a post-weld heat treat (if it's a hardened part, ie. group 3 or 4 steels), waiting for a 2nd hold time, opening the gun, and then finally removing the part. Compare that to an automated line where many parts are cooled at the same time through a moving helical line and are brought together with their female companion part (not cooled) and placed together on a rotating table where the parts never stop moving. BAW, Shuanghuan, Quingling, and Hafei all have been doing this for many years.
Originally Posted by onehundred80
Heating and cooling parts take time, time to heat and time to cool, each of these processes take power which also costs money. If the parts are handled manually special precautions have to be taken.
Are you aware of the power requirements for a typical 150KVA MFDC transformer.
Originally Posted by onehundred80
The female part is quite thin, .06 - .08 and a band of an interference fit in this takes no effort to turn the bolt, as the steel will crush under the load and mushroom effectively reducing the grip.
Typical Projection welds are not a weld at all. If you look closely at a prepared metallurgical sample of a projection weld you'd see no mixing of parent metals. Just about every projection weld is basically a solid-state bond. Projection welds are NEVER meant to be anything more than a holder for a fastener to be used in compression. Projection welds in the automotive industry are strictly used to aid in the assembly process. If that bracket is a hot-stamped AHSS group 3 or 4 material it can have a substantial amount of strength. Even if it's a relatively mild DP980 it's over 900MPa Tensile. If it's Martensitic it could be up to or more than 1600MPa tensile. I suppose I could find out, but I don't want to remove my lower control arm just to put it in my microhardness machine.
Originally Posted by onehundred80
On thick parts the interference can be minimal, and heat can be applied to reduce any dimensional interference at room temperature to a negative number when heated. On thin parts it just does not work the same as noted above, even more so on small diameter parts.
I fail to follow this. On thicker parts the surface area is much larger, allowing a substantial amount greater force to be applied.
Originally Posted by onehundred80
This part has to take the force of turning a nut on and off. The torque of turning a rusted nut is often greater than the torque specs to install it. In this case the stud broke as it was probably over tightened some time.
Actually, I'm more than positive the engineers that designed this part only intended for it to be fastened one time. I've dealt with thousands of automotive engineers over the years in this industry and have yet to find one who takes special precautions to make it easy to work or repair anything on a vehicle. Moreover, if ease of repair was what was on their mind then there's even more reason not to weld it. Once the weld is broken there's a substantial weakening of the structure with the loss of parent metal, and heat-cracking and metal embrittlement through the heat-affected zone. In this case I'm fairly sure the stud broke from exactly what the OP stated...... hitting a huge pothole.....twice.
Originally Posted by onehundred80
Heating the part 250°F. (to 320°F.) would result in an expansion of, .00000633 x .316 x 250 = .0005.
Originally Posted by onehundred80
This is hardly a good tight fit and to get that the part would have to be exactly the same size as the hole to start and it would be a light press fit at that - size to size. Cooling the stud to achieve that shrinkage would involve cooling it down to room temperature 70°F - 250°F = -180°F which is hard to do and would gain a poor interference fit anyway - .001.
You can take that up with the Asian and European automakers. I've only seen it in practice about 30 times on everything from M2 to M16 fasteners.