Fusion welding joins parts by creating a local molten pool that contains base material and sometimes added filler. As the heat source moves away, the pool solidifies into a weld.

A fusion weld therefore combines heat transfer, fluid flow and solidification in a strongly localized thermal cycle.

The fused region is the weld metal. Adjacent base material that did not melt but experienced a thermal cycle is the heat-affected zone (HAZ).

The energy delivered per unit travel length is often approximated by a heat input

$$H=\eta\frac{P}{v},$$

where $P$ is source power, $v$ is travel speed and $\eta$ represents the fraction effectively delivered to the workpiece.

For $P=4\ \mathrm{kW}$, $v=5\ \mathrm{mm/s}$ and $\eta=0.75$,

$$H=0.75\frac{4000}{5}=600\ \mathrm{J/mm}.$$

Higher heat input generally creates a larger, slower-cooling thermal field, but penetration and pool shape also depend on source concentration, joint geometry and material transport.

Because the weld pool solidifies from surrounding solid material, grains can grow directionally into the pool. Composition can also vary through dilution between filler and base material.

Fusion welding is consequently not only a way to attach geometry. It creates a new local material whose microstructure and defects depend on the thermal history of the joining process.