Cold weld (false weld) means no proper weld nugget formed during the resistance welding process. It is one of the most common weldability problems in the automotive industry in multi-sheet spot welding with dissimilar materials. Increasing false welds can harm the safety of the cars drastically or the production line must stop and rerun.

Why no weld nugget?
The reasons can be complicated, but they can be classified into two categories.
1. Bad contact
The electrical current must flow effectively through the intended welding area to form a weld nugget. When the electrode doesn’t contact properly with the workpieces during resistance welding, it can lead to cold weld. Similarly, the gap between the workpieces also results in a false weld.
2. Bad parameters
If you don’t get a good weld though the electrodes make good contact, insufficient heating might be a problem. In this case, you need to check your process parameters. Low current and short welding time can be a reason for insufficient heat. The wrong electrode tip with a large contact area can cause the current to spread thus not enough heat to melt the metal.
Another reason is the design of the material stack-up. Spot welding of multi-sheets with a high thickness ratio makes it tricky to have a well-positioned nugget. The heat generation starts between sheets when the current flows through the metal sheets. If the sheets have equal thickness and material, it is easy to have balanced heat in every sheet. If the sheets have different thicknesses, the thicker sheet has higher resistance than the thin sheet. Thus heat generation is towards a thicker sheet. In spot welding, a thickness ratio under 3:1 is generally acceptable range.

Now let’s change the material of the thicker sheet to high-strength steel. High-strength steel has a higher resistivity than low-carbon steel. The difference in the resistivity disrupts the heat balance, causing more heat to be generated in the thicker higher strength sheets. You might get a cold weld in the thinner low-carbon steel sheet.
How to minimize cold weld (false weld)?
Secure the contact
Ensure the consistent electrode-to-workpiece contact and the no gap between the workpieces. If there’s an unavoidable gap due to alignment errors or stamping tolerances, make sure the electrodes can close the gap before the welding current flows. You can read our case study about welding sheets with a gap.
Find optimized parameters
If the contact is secured, optimize your parameters for your sheet combinations by adjusting the current/time/force. If you have a more challenging combination with dissimilar material, you might want to try the below methods:
- Use electrodes with different diameter
- Pulse welding
- Pre-heating
- Multiple steps in the current
The key to achieving the best weld nugget is to generate balanced heat. The physical test is crucial, but simulation can help you by visualizing heat generation inside the weld and the final nugget form. You might get insight out of simulations and can reach a better solution faster.
Try our unique solution, SORPAS® weld planning to find the optimal parameters which is the most beloved feature by our customers in industries. You can easily customize the welding program with multi-pulses, add pre-pulse, change the electrode tip, and anything you can imagine.