What process involves the fusion of metal under the application of heat?

Welding is the process that involves the fusion of metal under the application of heat, making it a critical technique in metalworking and fabrication. In welding, a significant amount of heat is applied to the base metals, causing them to melt and fuse together as they cool and solidify, creating a strong bond between the two pieces. This process can involve various methods, including arc welding, MIG (Metal Inert Gas) welding, and TIG (Tungsten Inert Gas) welding, each utilizing different heat sources and techniques to achieve the fusion of metals.

The other methods listed—brazing, soldering, and hardfacing—serve different purposes. Brazing and soldering involve joining materials using a filler metal that has a lower melting point than the base metals, without melting the base metals themselves. Hardfacing, on the other hand, is a technique used to strengthen and provide abrasion resistance to the surface of a material, typically involving welding as a way to deposit hard material onto a softer base. However, the essential characteristic that defines welding is the actual melting and fusion of the base materials, which distinguishes it from the other processes mentioned.