SPOT WELDING (RESISTANCE)
Sometimes we have to join, permanently, metal parts (iron sheet or stainless steel) that have common flat areas. In In these cases, spot or resistance welding is highly recommended. choice.
The method consists of passing through a very small (dots with diameters between 2mm and 6mm approx.) an electric current of high current and low voltage. The process would be:
- Two electrodes, usually flat tip round, imprison the components to be joined, facing each other, exerting a controllable pressure (usually pneumatic).
- It is passed from one electrode to the other, passing through the sheets to be joined, a low-voltage, high-voltage electrical current intensity over a short period of time, usually measured in cycles in the case of alternating current machines (we remember that in Spain 1s=50 cycles. This current passes through the contact surface electrode-sheet1+sheet thickness1+sheet1-sheet2 contact surface+thickness of sheet2+electrode-sheet2 contact surface. Taking into account that the electrodes will be made of high conductivity materials, the area of greatest resistance (lower conductivity) will be the joining surface between the plates to unite, so it will be there where a very high temperature is generated.
- The high temperature generated melts the sheets. The molten zone will be a zone (called a bob) from the center the center of the section of the electrodes and increasing size with time, since the pressure of the electrodes prevents liquid metal from flowing into the gap between plates.
- Time must be controlled to try to avoid that the molten metal reaches beyond the compressed zone, since in this case would be projected by the accumulated internal pressure. To these metal losses cast is called "projections" and, while undesirable (especially for operator safety and assembly cleanliness), does not imply a joint defective.
- Once the current stops flowing, it remains pressure until the material resolidifies (period of cooldown) and sometimes even increased to achieve a of "forging".
The solidified metal “binds” the two plates and would have a intermediate metallic composition, since the two metals are mixed during the liquid phase.
A correctly given point should hold up to one attempt at separation beyond the strength of the base metal, breaking the plate more thin and sticking to the other.
In these welds all the parameters are quite critical, from the shape and parallelism between the tips of the electrodes to the Cleaning all surfaces that will come into contact.
Another widely used resistance welding is welding. bumps, which we will talk about in another specific entry.
Sheet metal welding will also deserve a separate chapter galvanized.