Laser beam welding results in the smallest heat affected zone compared to the other welding processes mentioned.

Heat Affected Zone (HAZ):
The heat affected zone is the area surrounding the weld where the material's microstructure and mechanical properties have been altered due to the heat generated during welding. It is important to minimize the size of the HAZ as it can affect the integrity and strength of the welded joint.

Explanation:

1. Shielded Metal Arc Welding (SMAW):
- SMAW, also known as stick welding, involves the use of a consumable electrode coated in flux.
- The heat generated during welding is relatively high, resulting in a larger HAZ.
- The flux coating helps to protect the weld pool and reduce the formation of defects, but it does not significantly minimize the HAZ size.

2. Gas Welding:
- Gas welding utilizes a fuel gas and oxygen to produce a flame for welding.
- The heat generated in gas welding is relatively low compared to other processes.
- As a result, the HAZ size is smaller compared to SMAW, but it is still larger compared to laser beam welding.

3. Laser Beam Welding:
- Laser beam welding is a high-energy density welding process that uses a focused laser beam to join materials.
- The laser beam generates a concentrated heat source, resulting in a very small and localized HAZ.
- The high energy density allows for precise control of the heat input, minimizing the thermal effects on the surrounding material.
- This process is particularly useful for welding materials with high melting points or for applications where minimal distortion and small HAZ are critical.

4. Thermit Welding:
- Thermit welding is a fusion welding process that uses a chemical reaction between a metal oxide and a reducing agent to generate heat.
- The heat generated in thermit welding is high, resulting in a relatively large HAZ.
- This process is commonly used for joining railway tracks, pipes, and other large metal structures where the size of the HAZ is not a major concern.

In summary, laser beam welding results in the smallest heat affected zone among the mentioned welding processes due to its high energy density and precise control of heat input.