How Does Welding Work? Joining Metal by Making It One Piece

Fusion welding melts the parent metal at the joint, usually together with a filler added from a rod or wire, forming a pool that solidifies into a continuous structure across the joint. The result is a joint with no interface, in contrast to soldering and brazing, which melt only a filler of lower melting point and rely on it wetting and bonding to solid parent metal, and in contrast to adhesive or mechanical joining. The consequence is that a properly executed weld in a suitable material can be as strong as the parts it joins, so designers can treat a welded structure as monolithic. The immediate difficulty is that molten metal reacts eagerly with oxygen and nitrogen from the air, producing porosity, oxide inclusions and brittleness, so every fusion process needs a way to keep the atmosphere away from the pool, and the differences between processes are largely differences in how they achieve that.

Each dominates a niche defined by cost, position, material and how much skill is required:

• •Manual metal arc, also called stick welding, where a consumable electrode coated in flux strikes an arc, the flux vaporising to shield the pool and forming a slag that is chipped off afterwards; cheap, portable and tolerant of wind and rusty steel, which is why it dominates site work
• •Metal inert gas and metal active gas welding, feeding a continuous wire through a torch that blankets the pool with shielding gas, which is fast, easy to learn and poorly suited to draughty outdoor conditions
• •Tungsten inert gas welding, using a non-consumable tungsten electrode with filler added separately by hand, which gives the greatest control and the cleanest results on thin material, stainless steel and aluminium, and is slow and demanding
• •Submerged arc welding, where the arc burns beneath a blanket of granular flux, used for long heavy seams in shipbuilding and pipe mills
• •Resistance spot welding, passing a large current through overlapping sheets so the contact resistance melts a nugget between them, which is how car bodies are assembled by robots at enormous rates
• •Laser and electron beam welding, delivering very concentrated energy for deep narrow welds with little distortion, at a high equipment cost
• •Friction stir welding, which uses a rotating tool to plasticise metal without melting it, producing excellent joints in aluminium and used in aerospace and rail

Beside every weld is a band of parent metal that was not melted but was heated enough for its internal structure to change, and it is usually the weakest part of the assembly. In steel, rapid cooling of that zone can form martensite, a hard brittle structure prone to cracking, particularly in thicker sections and higher carbon or alloy steels where the surrounding cold metal draws heat away quickly. The standard remedies are controlling heat input, preheating the work so it cools more slowly, and post-weld heat treatment to relieve stress and temper any hard structure. Hydrogen makes this far worse: hydrogen absorbed into the weld from moisture, rust, oil or damp flux diffuses into the hardened zone and causes cracks that may appear hours or days after welding, which is why low hydrogen electrodes are stored in heated ovens and why cleanliness is a structural requirement rather than good practice. In work-hardened or heat-treated aluminium the heat affected zone is simply softened, and the joint is designed on the assumption that it will be.

Metal expands when heated and contracts when it cools, and because a weld cools while restrained by the cold metal around it, the finished assembly pulls itself out of shape and holds locked-in residual stresses that can approach the yield strength of the material. Fabricators manage this by welding in a planned sequence, by alternating sides, by using jigs, by tack welding and by deliberately pre-setting parts out of position so they pull into alignment. Because a weld's interior cannot be seen, inspection matters enormously: visual examination, dye penetrant and magnetic particle testing for surface cracks, and radiography or ultrasonic testing for internal flaws, with critical work governed by qualified welding procedures and certified welders whose qualifications are tested and periodically renewed. The hazards are serious and specific, including intense ultraviolet light that burns the cornea in seconds, fumes containing metal oxides and, with stainless steel, hexavalent chromium, along with hot metal, electric shock and confined space risks from shielding gas displacing air.

Welding melts the edges of two parts so they solidify as one continuous structure, unlike brazing or bolting where the parts stay distinct, which is why a good weld can match the strength of the parent metal. Every process is organised around keeping air away from the molten pool, using flux, shielding gas or a vacuum. The band of metal heated but not melted beside the weld is usually the weak point, prone to hardening and hydrogen cracking in steel. Cooling under restraint distorts the assembly and locks in residual stress.