Welding is the process of joining two pieces of metal by creating atomic bonds between them, typically through intense heat, pressure, or both. At its core, welding works by bringing metal surfaces close enough together that their atoms intermix and lock into a shared crystal structure, forming a joint that can be as strong as the original material itself.
Two Fundamental Approaches
Every welding method falls into one of two categories: fusion welding or solid-state welding. The difference comes down to whether the metal actually melts.
Fusion welding melts the base metals at the joint. The molten material from both pieces flows together, sometimes mixed with additional filler metal fed into the joint from a wire or rod. As this pool of liquid metal cools, it solidifies into a single, continuous structure called the fusion zone. This is how most common welding works, including stick welding, MIG, and TIG.
Solid-state welding never melts the metal. Instead, it uses pressure, heat below the melting point, or both to force the surfaces into intimate contact until their atoms bond. Friction stir welding is a well-known example: a rotating tool plunges into the joint line and generates intense friction heat while physically stirring the softened (but still solid) metal together. This approach is especially useful for metals that lose important properties when melted. Brass alloys, for instance, can lose zinc through evaporation during fusion welding, which degrades their corrosion resistance. Keeping the temperature below the melting point avoids that problem entirely.
How an Electric Arc Creates the Heat
Most fusion welding uses an electric arc as its heat source. The arc forms when electricity jumps across a small gap between an electrode (the welding torch or rod) and the workpiece. This sustained electrical discharge superheats the surrounding gas into plasma, reaching temperatures well above what’s needed to melt steel.
The direction of current flow matters. When the electrode is negative and the workpiece is positive (called straight polarity), roughly two-thirds of the arc’s heat concentrates at the electrode and one-third at the workpiece. This melts the electrode faster, which is useful when you want to deposit a lot of filler metal quickly. Reverse the polarity so the electrode is positive, and more heat shifts to the workpiece, driving deeper penetration into the base metal. Welders choose polarity based on whether they need to fill a gap or burn deep into thick material.
The Role of Shielding Gas
Molten metal reacts aggressively with oxygen and nitrogen in the air. If exposed, the weld pool absorbs these gases and forms brittle oxides and nitrides that weaken the joint. Shielding gas solves this by flooding the weld area with an inert or semi-inert blanket, usually argon, helium, carbon dioxide, or a blend of these.
Shielding gas does more than just protect the puddle, though. It also influences arc stability, how smoothly molten droplets transfer from the electrode to the workpiece, and how deeply the arc penetrates. Research into ionized gas metal arc processing has shown that optimizing the shielding gas can produce welds with virtually no spatter or fume, because the droplets transfer smoothly from the wire tip to the base metal rather than exploding outward. So while “shielding” is the primary job, gas selection is really a tuning parameter for the entire process.
Preparing the Joint
Before welding begins, the edges of the metal pieces need to be shaped so the weld can fully penetrate the joint. On thin material, two square-cut edges butted together may be enough. On thicker plate or pipe, the welder (or a machinist) cuts a groove into the edges to create space for the weld metal to fill. Common groove shapes include V-grooves, J-grooves, and U-grooves, each named for their cross-sectional profile.
The choice depends on thickness, access, and the strength the joint needs to carry. A single-V groove, for example, is beveled on one side and works well when the welder can only access the joint from one direction. A double-V is beveled from both sides and distributes the weld metal more evenly, reducing distortion. J-grooves and U-grooves require more machining but waste less filler metal on very thick sections. Without some type of groove on thick material, it’s essentially impossible to achieve full penetration through the joint.
Five basic joint configurations cover nearly every welding scenario: butt joints (two pieces edge to edge), tee joints (one piece perpendicular to another), corner joints, lap joints (overlapping pieces), and edge joints. Each can be welded with different groove preparations and weld styles depending on the load the finished joint must support.
What Happens to the Metal Around the Weld
The weld itself, where the metal melted and resolidified, is only part of the story. The surrounding metal that got hot but never actually melted undergoes its own transformation. This region is called the heat-affected zone, or HAZ, and it’s often where weld failures originate.
The HAZ isn’t uniform. Closest to the weld, where temperatures reached roughly 1,000 to 1,300°C, the metal’s grain structure completely recrystallizes and the grains grow large, sometimes doubling in size compared to the original material. This coarse-grained zone tends to be softer and weaker in tension, though it can actually maintain decent toughness. Farther from the weld, where peak temperatures stayed between 700 and 900°C, only partial recrystallization occurs. This partially transformed zone can be harder and stronger at room temperature but more brittle under impact loading, with impact energy dropping by around 22% compared to the unaffected base metal.
These changes happen because heat rearranges the internal crystal structure of the metal. The size, shape, and orientation of the grains shift depending on how hot the metal got and how quickly it cooled. Managing this, through preheat, controlled cooling rates, and post-weld heat treatment, is a major part of producing a reliable weld.
What Determines Whether a Metal Welds Well
Not all metals are equally cooperative. Steel’s weldability depends heavily on its chemical makeup, particularly its carbon content and alloying elements. Engineers use carbon equivalent formulas to estimate how a given steel will behave during welding. These formulas assign weighted values to elements like manganese, chromium, nickel, and molybdenum, then combine them into a single number that predicts hardenability and cracking risk. The International Institute of Welding adopted a standardized version of this formula in 1967, and variations of it appear in welding codes worldwide.
A higher carbon equivalent means the steel is more likely to form hard, brittle microstructures in the heat-affected zone as it cools, which makes it prone to hydrogen cracking. Steels with a high carbon equivalent typically require preheating before welding and careful control of hydrogen sources (moisture in electrodes, for example) to avoid cracking after the weld cools.
Health Hazards of Welding Fumes
Welding produces a visible plume of fume that contains a complex mix of metal particles and gases. The metals in that fume read like a periodic table sampler: iron, manganese, chromium, nickel, cadmium, lead, zinc, and many others, depending on the base metal and filler being used. The gases include carbon monoxide, nitrogen dioxide, ozone, and phosgene.
Two components deserve special attention. Hexavalent chromium, produced when welding stainless steel or chromium-containing alloys, is a known human carcinogen. OSHA sets its permissible exposure limit at just 5 micrograms per cubic meter over an eight-hour shift, an extremely small amount that’s easy to exceed without proper ventilation. Manganese in welding fumes is linked to neurological effects resembling Parkinson’s disease with chronic overexposure. Adequate ventilation, local exhaust at the arc, and respiratory protection are the primary defenses against these exposures.