Explanation:
Laser beam welding (LBW) is the welding process that consists of minimum heat affected zone compared to other welding processes listed.

Heat Affected Zone (HAZ):
The heat affected zone is the region surrounding the weld where the base material experiences a significant change in microstructure and mechanical properties due to the heat input during welding. The size of the HAZ is influenced by several factors, including the welding process, heat input, material properties, and cooling rate.

Comparison of Welding Processes:

1. Shielded Metal Arc Welding (SMAW):
- SMAW, also known as stick welding, involves the use of a consumable electrode coated with flux.
- It produces a relatively larger heat affected zone due to the slower cooling rate and higher heat input.
- The HAZ can be prone to higher distortion and residual stresses.

2. Laser Beam Welding (LBW):
- LBW uses a highly concentrated laser beam to melt and join the base materials.
- It offers precise control over the heat input, resulting in a smaller heat affected zone.
- The laser beam provides a concentrated energy source, allowing for faster heating and cooling rates.
- LBW is particularly suitable for welding thin materials and intricate parts.
- The small HAZ reduces the chances of distortion and preserves the mechanical properties of the base material.

3. Ultrasonic Welding (USW):
- USW is a solid-state welding process that uses high-frequency mechanical vibrations to generate heat at the joint interface.
- It is commonly used for joining thermoplastics and some metals.
- USW produces a localized heat affected zone, but the overall size is typically smaller compared to other welding processes.
- The heat input and cooling rate depend on the specific application and material being welded.

4. Metal Inert Gas Welding (MIG):
- MIG welding, also known as Gas Metal Arc Welding (GMAW), uses a consumable electrode and a shielding gas.
- It produces a moderate heat affected zone, depending on the welding parameters and material being welded.
- The cooling rate and heat input can vary depending on the welding conditions.

Conclusion:
Among the listed welding processes, laser beam welding (LBW) provides the smallest heat affected zone. The concentrated laser beam allows for precise control over the heat input, resulting in faster heating and cooling rates. This smaller HAZ minimizes the chances of distortion and preserves the mechanical properties of the base material.