Question

Resistance welding generates heat due to

• A. Radiation
• B. Friction
• C. Electrical resistance
• D. Chemical reaction

Hint:

Remember Joule's Law: \( H = I^2 R t \).
To generate heat efficiently at the interface:
- Maintain high contact resistance at the joint interface.
- Keep low contact resistance at the electrode-sheet interfaces (using high-conductivity copper electrodes and clean surfaces) to prevent electrode damage.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This question asks for the physical mechanism responsible for generating the heat required to create a joint in the resistance welding process (such as spot, seam, or projection welding).

Step 2: Key Formula or Approach:
Resistance welding relies on Joule heating. The heat generated \( H \) is given by Joule's Law: \[ H = I^2 R t \] where:
\( I \) is the welding current (Amperes).
\( R \) is the total electrical resistance of the circuit (Ohms).
\( t \) is the duration of current flow (Seconds).

Step 3: Detailed Explanation:

• Components of Resistance: The total resistance \( R \) in resistance spot welding consists of:
1. Bulk resistance of the metal sheets.
2. Contact resistance between the water-cooled copper electrodes and the sheets.
3. Contact resistance at the interface between the two sheets.

• Localized Heat Generation:
- The sheet-to-sheet interface has the highest localized contact resistance due to surface microscopic roughness and oxides.
- Because heat generated is directly proportional to resistance, the maximum heat is generated exactly at this sheet-to-sheet interface.
- This localized heating melts the interface metal, creating a molten pool that solidifies under electrode pressure to form a solid "weld nugget."
- Friction (used in friction welding), radiation (used in laser/electron beam welding), and chemical reactions (used in thermite welding) are not the primary heat sources here.


Step 4: Final Answer:
By definition, resistance welding generates heat purely via the passage of electric current through the contact resistance of the joint.
Therefore, option (C) is the correct choice.