Answer:

In TIG (Tungsten Inert Gas) welding, an electric arc is formed between a non-consumable tungsten electrode and the workpiece. The electrode is typically made of tungsten because of its high melting point and low rate of vaporization. To protect the weld pool and the tungsten electrode from oxidation and contamination, an inert gas is used.

The correct answer is option C, which states that the gases used in TIG welding are argon and helium. Let's discuss why these gases are used in detail.

1. Argon:
Argon is the most commonly used gas in TIG welding. It is an inert gas, which means it does not react with other elements. Here are the reasons why argon is used:

- Inertness: Argon does not react with the molten metal or tungsten electrode, preventing oxidation and contamination of the weld pool. This helps in producing high-quality welds with excellent mechanical properties.

- Stability: Argon gas provides a stable and consistent arc during welding. It helps in maintaining a steady heat input, which is crucial for achieving precise control over the welding process.

- Thermal conductivity: Argon has a high thermal conductivity, which means it can quickly dissipate heat from the weld area. This helps in reducing the heat-affected zone and minimizing distortion in the workpiece.

2. Helium:
Helium is another gas used in TIG welding, often in combination with argon. Here's why helium is used:

- Increased heat: Helium has a higher thermal conductivity and ionization potential than argon. When mixed with argon, helium increases the heat input during welding, allowing for higher welding speeds and better penetration.

- Deeper penetration: Helium gas produces a more focused and narrower arc, resulting in deeper penetration into the workpiece. This is beneficial for welding thicker materials or when deeper weld penetration is required.

- Hotter arc: Helium also increases the temperature of the arc, which can be advantageous for welding materials with high thermal conductivity, such as aluminum. The hotter arc helps in overcoming the heat dissipation of these materials and ensures proper fusion.

Conclusion:
In TIG welding, argon and helium gases are used. Argon provides inertness, stability, and good thermal conductivity, while helium increases heat input, penetration, and arc temperature. The combination of these gases allows for precise control over the welding process, high-quality welds, and the ability to weld a variety of materials.