Effect of the parameters in spot welding
The passage of more current than what is required causes the following defects.
I. Weld expulsion
ii. Cavitations
iii. Weld cracking
iv. Reduced mechanical properties
v. Electrode embedment in the surface
On the other hand, less current results in:
i. Requirement of oversized electrodes.
ii. Reduction in the weld properties.
iii. Unfused interface.
Higher pressure causes
A. Lowering of contact resistance and hence less heat generated.
B. High distortion as against the following defects due to less pressure:
- Pitting
- Reduction in electrode life.
C. Expulsion of metal from within the weld along with more weld time results in:
- boiling
- porosity
- plastic contact surfaces
- growth of nugget up to electrode faces.
The uniformity of the weld depends on the cleanliness of the surface.
Rough estimates of parameters are given by hand books for different materials and thicknesses. Based on these values trials can be conducted on the welding machine available in order to fix the exact parameters, as the parameters vary slightly from machine to machine in the case of resistance welding.
When two plates of two different thicknesses are to be welded, the weld nugget grows towards the thicker side. When two plates of different electrical conductivities are resistance welded, the weld nugget grows towards the higher resistivity side. In the above two cases, the upper and lower electrodes of different diameters, are to be chosen.