Final answer:
Fusion welding involves heating the material beyond its melting point, which is the temperature at which a solid begins the transition to a liquid state. Option B is correct.
Step-by-step explanation:
Fusion welding involves heating the material beyond its melting point. The correct answer to the question is B. Melting point. When a crystalline solid is heated, the energy of its atoms, molecules, or ions increases, eventually reaching a point where the solid begins to transition into a liquid state.
This happens at the melting point, and the temperature of the solid will not rise further until all of it has melted. Only after all of the solid has melted will continued heating result in an increase in the temperature of the liquid.
The melting point is distinct from the elastic limit, where a material can return to its original shape after the stress is removed, and the plastic deformation point, where the material undergoes permanent deformation. The glass transition temperature is another concept where thermoplastics become more flexible and deformable, but this is not the same as melting.
This process is essential for welding because only in a molten state can materials be fused together.