Introduction:
Spot welding is a type of resistance welding process in which two or more metal sheets are joined together by applying heat and pressure at specific points. The composition and thickness of the base metal play a crucial role in determining various parameters of the spot welding process.

Explanation:
The composition and thickness of the base metal have a significant impact on the spot welding process. Let's discuss how they determine the holding times, the amount of weld current, and the amount of squeeze pressure.

1. Holding Times:
- Holding time refers to the duration for which the welding current is applied to the joint.
- The composition and thickness of the base metal affect the heat transfer characteristics during spot welding.
- Different metals have different thermal conductivity and heat absorption capacity, which determines the time required to achieve proper fusion and solidification of the weld.
- Thicker base metals generally require longer holding times to ensure sufficient heat penetration and proper bonding.
- Similarly, different compositions of base metals may have different melting points, which can influence the holding times required for successful welding.

2. Amount of Weld Current:
- Weld current refers to the electrical current that passes through the metal sheets during spot welding.
- The composition and thickness of the base metal determine the electrical resistance offered by the joint.
- Thicker base metals generally have higher electrical resistance, requiring higher weld currents to generate sufficient heat for welding.
- Different compositions of base metals may also have different electrical conductivities, leading to variations in the amount of weld current required.

3. Amount of Squeeze Pressure:
- Squeeze pressure refers to the force applied to hold the metal sheets together during spot welding.
- The composition and thickness of the base metal influence the deformability and ductility of the joint.
- Thicker base metals require higher squeeze pressures to ensure proper contact and intimate contact between the metal sheets.
- The composition of the base metal can affect the mechanical properties, such as hardness and strength, which may require specific squeeze pressures to achieve desired weld quality.

Conclusion:
In conclusion, the composition and thickness of the base metal have a significant impact on spot welding. They determine the holding times, the amount of weld current, and the amount of squeeze pressure required for successful welding. Understanding these factors is crucial for achieving high-quality spot welds and ensuring the durability and strength of the joint.