An arc welder is a machine that joins metal pieces together by creating an electric arc between an electrode and the metal workpiece. That arc generates intense heat, enough to melt the edges of both metal pieces and fuse them into a single, continuous joint. Arc welding is the most widely used welding method in construction, manufacturing, automotive repair, and home fabrication projects.
How an Arc Welder Works
The basic principle is straightforward. The welder sends electrical current through an electrode (a metal rod or wire) and directs it at the workpiece. When the electrode gets close enough to the metal surface, the electricity jumps across the tiny gap, forming a sustained arc of superheated plasma. This arc melts the base metal and, in most processes, the electrode itself. The molten metal from both sources flows together into a small pool, then cools and solidifies into a permanent bond.
The weld pool is extremely vulnerable while it’s molten. Oxygen, nitrogen, and hydrogen in the surrounding air can react with the liquid metal and weaken the joint. Every arc welding process uses some form of shielding to keep the atmosphere away from the weld pool. Some methods use a coating on the electrode that burns off and creates a protective gas cloud. Others pipe shielding gas directly to the weld area from an external tank. The most common shielding gases are argon, which produces a very stable arc with excellent control, and carbon dioxide, which increases heat input and weld penetration.
The Four Main Types
Stick Welding (SMAW)
Stick welding is the oldest and simplest form of arc welding. You clamp a flux-coated metal rod into an electrode holder, strike an arc against the workpiece, and manually guide the rod along the joint. The flux coating melts during welding and releases shielding gas, so no external gas tank is needed. The rods are typically 2 to 3 feet long, which means you have to stop and swap in a new one fairly often. Stick welding produces more spatter than other methods and the resulting joints aren’t the prettiest, but the equipment is inexpensive, portable, and works well outdoors or in windy conditions where shielding gas would blow away.
MIG Welding (GMAW)
MIG welding feeds a thin, continuous wire from a spool through a welding gun. The wire acts as both the electrode and the filler metal, so you can weld for long stretches without stopping to reload. An external tank supplies shielding gas (usually argon, carbon dioxide, or a mix of both) through the same gun nozzle. MIG is the most productive arc welding process for general work. It’s easier to learn than stick or TIG because the wire feeds automatically and you only need to control the gun’s speed and angle. It handles a wide range of metals including carbon steel, stainless steel, and aluminum, though each requires different wire and gas combinations.
TIG Welding (GTAW)
TIG welding uses a non-consumable tungsten electrode that creates the arc but doesn’t melt into the joint. When filler metal is needed, the welder feeds a separate rod into the weld pool by hand. Argon shields the weld area. This two-handed technique is slower and harder to master, but TIG produces the highest quality joints of any arc welding process, with clean appearance and minimal defects. It’s the go-to method for thin materials, precision work, and metals like titanium and magnesium that are extremely sensitive to contamination. TIG can also weld without any filler metal at all, simply fusing two pieces together with the heat of the arc alone.
Flux-Cored Welding (FCAW)
Flux-cored welding looks and feels a lot like MIG. It uses a continuously fed wire electrode through a similar gun setup. The difference is that the wire is hollow and filled with flux compounds. As the wire melts, the flux creates its own shielding gas, which means some flux-cored processes don’t need an external gas tank. This makes it practical for outdoor work and heavy structural welding where wind is a factor. It deposits metal faster than MIG, making it popular in shipbuilding and steel construction.
AC, DC, and Polarity
Arc welders run on either alternating current (AC) or direct current (DC), and the choice directly affects how the weld behaves. DC welding offers two polarity options. With DC electrode positive, more heat concentrates at the workpiece, producing deeper penetration into thick materials. With DC electrode negative, more heat stays at the electrode, which increases the rate of metal deposition but reduces penetration. This makes it better suited for thin metals that would burn through with too much heat.
AC welding alternates between both polarities many times per second. It’s less common for general work but useful in specific situations, particularly when welding aluminum with TIG, where the alternating current helps break up the oxide layer on the metal’s surface.
What Metals Can You Weld?
Arc welding covers a broad range of metals, but not every process works with every material. Carbon steel and low-alloy steel are compatible with virtually all arc welding methods, which is one reason steel dominates welded construction. Stainless steel works well with stick, MIG, TIG, and flux-cored processes. Cast iron can be arc welded but requires special electrodes and techniques to avoid cracking.
Aluminum is weldable with MIG and TIG, though TIG is strongly preferred for thinner sections and cosmetic work. Nickel alloys, commonly found in chemical processing and aerospace, work with stick, MIG, and TIG. More exotic metals like titanium, zirconium, and magnesium are almost exclusively TIG welded because they demand the cleanest possible shielding environment. Copper alloys can be stick or TIG welded depending on the specific alloy and application.
Inverter vs. Transformer Machines
Traditional arc welders use a heavy transformer to convert household or industrial power into welding current. These machines are durable and simple, but they weigh 32 to 45 kg (roughly 70 to 100 pounds) and convert only about 55% to 65% of the electrical energy they draw into actual welding power. The rest is lost as heat.
Modern inverter welders use high-frequency electronics to do the same job in a much smaller package. A typical inverter welder weighs 9 to 14 kg (20 to 30 pounds), roughly a third of a comparable transformer model. They’re 90% to 95% energy efficient, meaning at 140 amps of output, an inverter draws about 300 watts compared to 500 watts for a transformer unit. That’s around a 20% reduction in energy consumption. Inverters also give you finer control over the arc characteristics, which makes them easier to use at different skill levels. The tradeoff is that their electronics are more sensitive to dust and moisture, and repairs can be more expensive.
Duty Cycle Ratings
Every welder has a duty cycle rating that tells you how long it can run before it needs to cool down. The rating is expressed as a percentage of a 10-minute window at a specific amperage. A machine rated at 60% duty cycle at 200 amps can weld continuously for 6 minutes out of every 10, then needs 4 minutes of rest to avoid overheating.
Light-duty machines designed for hobbyists and occasional home use typically have a 20% duty cycle. Medium-duty welders used in small shops run at 40% to 60%. Heavy-duty industrial machines operate at 60% to 80%, allowing for near-continuous production welding. Running a welder past its duty cycle won’t necessarily damage it immediately, as most machines have thermal overload protection that shuts them off, but repeatedly pushing past the rating shortens the machine’s lifespan.
Eye and Fume Protection
The arc produces intense ultraviolet and infrared radiation that can burn your corneas in seconds, a condition welders call “arc eye.” OSHA sets minimum lens shade numbers based on the welding process and amperage. For stick welding at 60 to 160 amps, the minimum shade is 8. For MIG and flux-cored welding above 60 amps, it’s shade 10. TIG welding at under 150 amps requires at least shade 8, increasing to shade 10 above 150 amps. Most welders use auto-darkening helmets that stay light enough to see through until the arc strikes, then instantly switch to the appropriate shade.
Welding fumes are the other major hazard. The arc vaporizes metals from the electrode and workpiece, creating a plume of fine particles that can include iron, manganese, chromium, nickel, lead, and zinc, depending on what you’re welding. The process also generates gases like carbon monoxide, nitrogen dioxide, and ozone. Short-term exposure causes irritation and nausea. Long-term exposure without proper protection is linked to serious respiratory and neurological damage. Local exhaust ventilation positioned near the arc is the most effective control, pulling fumes away from your breathing zone before you can inhale them. When welding indoors, general ventilation with fresh air circulation is the bare minimum.