welding
Welding joins two pieces of metal by melting them together so they solidify as one continuous piece, the way you can melt the ends of two candles and press them into a single stick. Unlike bolting or gluing, a good weld makes the joint effectively part of the parent metal, which is why welding builds ships, skyscrapers, pipelines, pressure vessels, and car bodies. Most structural welding is fusion welding, where an intense heat source melts a small pool of metal at the joint.
In fusion welding an arc, flame, laser, or electron beam melts the edges of the two parts into a shared weld pool, usually with a filler metal added to fill the gap. The pool solidifies almost instantly, so the weld metal is really a tiny casting: it freezes with a cast grain structure, columnar grains, and the same risks a casting has, such as gas porosity, shrinkage, and solidification cracks. Right beside the melted zone sits the heat-affected zone, where metal that never melted was still cooked hot enough to change its microstructure.
The honest reality is that a weld is a metallurgically complex, often weakest, part of a structure. It combines a cast weld metal, an altered heat-affected zone, residual stresses and distortion from uneven heating and cooling, and the chance of hydrogen-induced cracking. Welds are therefore common origins of fatigue and fracture failures, and are heavily inspected by nondestructive testing. Good welding is as much about controlling heat input, cleanliness, and cooling rate as about filling the joint.
A steel pipeline is arc welded with a filler rod: the arc melts a pool that solidifies as cast weld metal, and just beside it a heat-affected zone can form hard, brittle martensite if the steel cools too fast, so preheating slows the cooling.
A weld is a tiny casting plus a heat-affected zone, so it inherits both casting flaws and unwanted phase changes.
A weld is not just glued metal: it is a cast weld pool plus an altered heat-affected zone with locked-in residual stress, which is why welds are frequent origins of fatigue cracks and must be inspected.