Grades 6–8 · Materials & Matter
How can welding make two metal pieces become one joint?
A welded frame may be built from separate metal pieces, yet its joints do not need bolts to hold the pieces together. In fusion welding, a heat source melts a small region of the original metal at the joining edges. The rest of each piece remains solid.
In the diagramFusion welding joins metal at a local melt poolOne welding family shown in cutawayBefore joiningSolidified weldTwo parent-metal edgesA continuous solid jointLocal melting and solidification unite the original edges.
The liquid forms a local weld pool. Some processes add filler metal to the pool; others can join suitable edges without it. As the heat source moves away, the pool cools and solidifies. Metal now extends continuously across the region that used to separate the pieces.
The surrounding solid metal also experiences heat, which can change its structure. A reliable weld therefore depends on more than seeing a bead on the surface: material choice, preparation, protection from unwanted reactions and inspection all matter. The drawing explains fusion welding only. Other welding families can join materials through pressure and deformation without melting the original pieces.
About this explanation
Starting knowledge: Understand melting and solidification and distinguish a local change from a whole-object change.
Sources
- TWI — What is Fusion Welding?
- TWI — What is Solid State Welding?