The main purpose of heat treatment in metals is to improve their mechanical properties. Heat treatment involves controlled heating and cooling processes that alter the microstructure of the material, leading to various enhancements in material properties. Let's analyze the options to understand why this is the correct choice:

1. To reduce the cost of metal: This is incorrect. Heat treatment involves energy costs and processes that do not inherently reduce the cost of the metal. Its primary role is to change material properties rather than cost.
2. To improve mechanical properties: This is the correct answer. Heat treatment processes such as annealing, quenching, and tempering can significantly enhance mechanical properties such as strength, hardness, ductility, and toughness.
3. To change the color of the metal: Incorrect. Although some heat treatment processes might lead to color changes due to surface oxidation effects, changing color is not the main purpose.
4. To add weight to the metal: Incorrect. Heat treatment does not add weight to the metal; its mass remains largely unchanged after the process.

Conclusion: Thus, the main aim of heat treatment is to enhance the mechanical properties of metals, which facilitates improved performance in various applications, from automotive parts to industrial machinery components.