Welding

Welding joins materials by producing local coalescence, using heat, pressure or both, with or without added filler. Fusion welding melts a joint region; solid-state methods can join without bulk melting. The surrounding heat-affected zone can change microstructure and properties even where material did not melt. Shrinkage and restraint can create residual stresses, while pores or incomplete bonding can weaken a joint. A welded structure must therefore be evaluated through the joint geometry and altered material regions, not just the parent material's nominal strength.

The heat-affected zone is the region of a material whose microstructure or properties are changed by nearby welding or thermal cutting without itself melting. It lies outside the fusion region and can contain different subregions corresponding to different thermal histories.^[ref]

In a steel, heating and cooling can change phases, grain size and hardness; in another alloy the thermal cycle may soften a previously strengthened region. Heat input, travel speed, geometry and thermal transport determine the affected region together. Measuring a weld therefore requires examining both the joint and adjacent material. Surface heat-tint colours alone do not uniquely identify the extent of the metallurgically altered region.^[ref]

Localized optical heating at a nanoscale metal junction can fuse a contact. In the demonstrated silver-wire process, the initial gap concentrates light; once electrical/physical contact forms, the optical hotspot changes and limits further heating. Comparing local morphology and electrical resistance with unwelded or uniformly heated controls tests whether contacts improve without broad substrate damage.^[ref]