Welding is a joining process that fuses two pieces of metal (or other materials) at their contacting surfaces using heat, pressure, or both. Unlike bolting or riveting, welding creates a permanent bond where the materials themselves merge together. The global welding machinery market hit roughly $35 billion in 2026, reflecting how central the process is to construction, manufacturing, automotive, and aerospace industries.
There are dozens of specific welding methods, but they all fall into two broad categories: fusion welding, which melts the base metals to join them, and solid-state welding, which uses heat and pressure without ever melting the material. Most welding you’ll encounter in everyday life, from car frames to building steel, is fusion welding.
How Fusion Welding Works
In fusion welding, an energy source heats the joint area until the base metals liquefy and flow together. As the molten pool cools and solidifies, it forms a single continuous piece of metal. Most fusion methods also add filler metal from an electrode or wire to strengthen and fill the joint. The critical challenge is protecting that molten pool from the surrounding air. Oxygen and nitrogen will react with liquid metal and weaken the finished weld, so every fusion process includes some form of shielding, whether that’s a gas, a layer of powdered flux, or a vacuum chamber.
Stick Welding (SMAW)
Stick welding, formally called Shielded Metal Arc Welding, is one of the oldest and most portable arc welding methods. It uses a consumable electrode, a metal rod coated in a dry chemical mixture called flux. When you strike the electrode against the workpiece, an electric arc forms between the two, generating enough heat to melt both the rod and the base metal into a shared pool.
As the flux coating vaporizes, it does two things at once: it releases a shielding gas that surrounds the arc and it deposits a layer of slag, a glassy crust, over the cooling weld. Together, these barriers keep atmospheric contamination out. Once the weld cools, you chip the slag away with a hammer and wire-brush the surface to reveal the finished joint underneath.
Stick welding’s big advantage is simplicity. The equipment is relatively inexpensive, and because shielding comes from the electrode itself, you don’t need external gas cylinders. That makes it practical for field repairs, construction sites, and remote locations. The tradeoff is speed: it’s a slower, more manual process than wire-fed alternatives.
MIG Welding (GMAW)
MIG welding, or Gas Metal Arc Welding, feeds a continuous solid wire electrode through a handheld gun while simultaneously flowing shielding gas, typically carbon dioxide or a carbon dioxide and argon mix, through the same nozzle. The arc forms between the wire tip and the workpiece, melting both together. Because the wire feeds automatically, you can weld longer seams without stopping to replace an electrode.
This continuous feed makes MIG welding faster and easier to learn than stick welding. It’s the most common choice for automotive bodywork, light fabrication, and hobbyist projects. The limitation is that the external shielding gas can be blown away by wind, making MIG welding best suited for indoor or sheltered environments.
TIG Welding (GTAW)
TIG welding uses a tungsten electrode that does not melt during the process. The arc forms between the tungsten tip and the workpiece, while an inert shielding gas (usually argon) flows from the torch to protect the weld zone. If extra material is needed to fill the joint, the welder feeds a separate filler rod into the pool by hand.
Because the electrode stays intact, the welder has independent control over heat input and filler addition. This produces a small, precise arc that’s ideal for thin sheet metal, pipe root passes, and materials like aluminum and magnesium that are difficult to weld cleanly with other methods. TIG became popular in the 1940s specifically for joining aluminum and magnesium in aerospace applications. When welding aluminum, alternating current is used because it helps break up the stubborn oxide layer on the metal’s surface. TIG welding delivers the highest quality and cleanest appearance of any common arc process, but it’s the slowest and demands the most hand skill.
Flux-Cored Arc Welding (FCAW)
Flux-cored welding looks similar to MIG welding on the surface: a continuously fed wire electrode, a handheld gun, an electric arc. The difference is the wire itself. Instead of a solid wire, FCAW uses a tubular wire with flux packed inside its core. As the wire melts, the flux generates shielding gases and protective slag from within the weld.
The self-shielded version of this process (FCAW-S) needs no external gas cylinder at all, which makes it especially effective for outdoor work, windy job sites, and structural steel erection where other processes would struggle with atmospheric exposure. FCAW also deposits metal faster than stick or MIG welding, so it’s a preferred method for heavy fabrication and shipbuilding where large volumes of weld metal are needed.
Solid-State Welding
Solid-state welding joins metals without melting them. Instead, a combination of heat, pressure, and sometimes motion forces the surfaces together until their atoms bond at the interface. The temperatures involved are high but stay below the material’s melting point.
Friction stir welding is the most widely known example. A rotating tool plunges into the joint line between two workpieces, generating intense frictional heat that softens the metal into a plasticized state. The tool then travels along the seam, mechanically stirring the softened material from both sides together. Research shows the bonding happens primarily through a creep-driven process where tiny cavities at the interface are squeezed shut by the surrounding deformed metal, rather than through simple diffusion. The result is a joint with minimal distortion, no solidification defects, and mechanical properties often superior to fusion welds. Friction stir welding is used extensively for aluminum structures in aerospace, rail, and marine construction.
Other solid-state methods include ultrasonic welding, which vibrates parts together at high frequency (common for plastics and thin metal foils), and diffusion welding, which holds parts in contact at elevated temperature and pressure for an extended period until atomic migration bonds the surfaces.
High-Energy Beam Welding
Laser beam and electron beam welding concentrate energy into an extremely small spot, reaching power densities above 100,000 watts per square centimeter. At that intensity, the metal doesn’t just melt; it vaporizes, forming a narrow vapor channel called a keyhole that allows the beam to penetrate deep into the material. Weld depth-to-width ratios can exceed 10 to 1, meaning you can make a very deep weld that’s barely wider than a pencil line.
Electron beam welding typically operates in a vacuum chamber, which makes it ideal for reactive metals like titanium that would contaminate in open air, and for applications requiring hermetic vacuum seals. It became the method of choice in nuclear and aerospace manufacturing during the 1970s and remains preferred for penetrations greater than about 12 mm. Laser welding can operate in open air and is more easily automated, making it dominant in high-volume automotive production lines where speed and precision matter.
The Five Basic Joint Types
Regardless of which welding process you use, the pieces being joined fit together in one of five standard configurations:
- Butt joint: Two pieces placed edge to edge in the same plane, welded along the seam. This is the most common joint in plate and pipe work. The edges may be left square or beveled into V, J, or U shapes to allow full penetration on thicker material.
- Tee joint: One piece meets another at a 90-degree angle, forming a T shape. Structural steel connections frequently use this configuration.
- Corner joint: Two pieces meet at their edges to form an L shape, either open or closed. Sheet metal enclosures and frames rely heavily on corner joints.
- Lap joint: Two pieces overlap each other, and the weld is placed along the edge of the overlapping section. This is common in sheet metal and automotive panels.
- Edge joint: Two pieces are set side by side with their edges aligned and welded along the top. This is typically used for joining thin sheet metal or reinforcing edges.
Shielding Gases and Their Roles
Shielding gases prevent oxygen, nitrogen, and moisture in the air from reacting with molten metal. The gas you choose affects arc behavior, penetration, and weld appearance. Argon is inert, meaning it won’t react with any metal, which makes it the standard choice for TIG welding and for aluminum MIG welding. Helium, also inert, produces a hotter arc than argon and is sometimes blended with argon for deeper penetration on thick aluminum. Carbon dioxide is the only reactive shielding gas that can be used in pure form. It’s inexpensive and provides good penetration on steel, but it creates a less stable arc and more spatter than argon-based mixtures. Most MIG welding on steel uses a blend of argon and carbon dioxide to balance cost, penetration, and weld quality.
Essential Safety Practices
Welding produces ultraviolet radiation intense enough to burn skin and eyes in seconds, along with metal fumes that can cause serious respiratory damage over time. Federal workplace safety standards require arc welding helmets with proper filter plates for all arc welding operations. Auto-darkening helmets, which switch from a clear viewing state to a dark filter the instant an arc strikes, have become the practical standard because they let you position your torch accurately before welding begins.
Protective clothing varies with the job but generally includes flame-resistant gloves, jackets, and long pants that shield against sparks and spatter. Ventilation is equally critical. In spaces smaller than 10,000 cubic feet per welder or with ceilings below 16 feet, mechanical ventilation is required at a minimum rate of 2,000 cubic feet per minute per welder. Confined spaces demand additional precautions to prevent toxic gas buildup and oxygen depletion. Proper fume extraction, either through local exhaust hoods positioned near the arc or general shop ventilation, protects against the long-term lung and neurological effects of chronic weld fume exposure.