Projection welding of nuts on zinc coated surface

[nlazic]

During our projection welding of nuts on zinc coated surface we often have a problem. Nut supposed to be welded in three points, but often happened that nut is correctly welded only on two points , sometimes just on one, and rarely poor welded at all. After welding, part with nut is going on painting, and when part destroyed for analyses, because of paint rest, we can clearly see what?s happened.
When this happened, our client have problem during assembling, because coaxiality is being lost, and screw cannot pass.
It will be great if any of you have any suggestion how to solve this problem. Thanks

 

[tonefordays]

Hello,

Here are a few things to consider,

1) Be sure all the equipment is clean and free of any loose debris, there should be no scale, rust, oil, moisture, paint, etc. I would suspect the zinc plating could be contributing the problem.

2) Check that machine settings are actually within proper range for material being welded. If you are following a welding procedure, be sure ALL variables are within limits. Changes in thickness, material type could affect the result.

3) Set up, you said that the coaxial position of the nut was off, this could be a result of the part moving during welding. If the part is moving, this could also affect the weld fusion and also present issues like are having. Check all contact areas for proper fit up.

4) Welding equipment, make sure there is good contact and that fixtures aren't worn out or otherwise preventing electrical contact. Fixtures and welding contacts also need to be clean and free of any of the debris mentioned above. Over the course of production, these contact areas could encounter build up from dirty parts, thus affecting the electrical characteristics.

As with most welding processes, good fit up, and cleanliness, are essential for successful welding.

Hope this helps.

 

[hogheavenfarm]

I would also suspect the coating being probable cause. You need a real clean surface for the welder, and if one of the projections does not 'take', the nut will twist (up of down) when a bolt is threaded in, binding it up. Do a quick impact test if you can on every so many pieces to get an idea of how prevalent it is, then work on your variables, one at a time.

 

[Jen Kirley]

Welcome to the Cove!

I did some research and found a short article on projection welding of nuts on zinc coated surfaces, in this case sheet metal. TheFabricator(dot)com (I have no affiliation) titled Projection welding for nut and bolt attachment: Competing or complementary joining methods? that gives some insight:

The current for projection welding is generally less than that required to produce correspondent spot welds. The projection will heat rapidly, and excessive current will melt it and result in expulsion. However, the current must be at least high enough to create fusion before the projection has completely collapsed.

A short welding time might be desirable from a production standpoint, but it will require correspondingly higher amperage. Therefore, it is important to optimize welding parameters to prevent overheating and metal expulsion. In some cases, such as when welding fasteners to high-strength steels (HSS), impulse welding may be advantageous to control the heating rate. This also is helpful for thick-sheet projection welding and when welding metals with low thermal conductivity.

A second source, TWI (again, I am not affiliated) gives more specific advice you may want to experiment with on their page titled How do I resistance spot weld coated steels?:

Guideline welding conditions are available in standards such as BS1140 (British Standard specification for uncoated and coated low carbon steel sheet). Electrode force is usually increased by 10 to 20% compared to the equivalent setting for uncoated steel of a given thickness, otherwise a poorer process tolerance (narrower welding range) is likely. Longer weld times are also beneficial, allowing 2 to 3 cycles for the coating to be displaced before weld growth starts. Some users prefer a double pulse condition, in an effort to control the coating removal by using an initial low current pulse. However, a single pulse is normally quite satisfactory. Substantially higher welding currents are needed compared with uncoated steel to compensate for the low interface resistance caused by the coating. The actual increase depends on the coating type and material thickness combination.

As already recommended by my fellow Cover, ensure your weld-area surfaces are free of surface contamination of all types.

I hope this helps!