Welding Magnesium

Magnesium welds with the same AC TIG approach as aluminum, but it brings two problems aluminum does not: castings crack easily under heat, and the chips, dust, and grinding swarf are flammable. Get the technique right and magnesium produces light, strong welds on motorsport wheels, gearbox and transmission cases, and aerospace brackets. Ignore the fire-safety side and a pile of fines or a water-charged extinguisher turns a routine repair into a hazard.

AC TIG (GTAW) is the primary process. Run AC polarity with 100% argon at 15-20 CFH. The AC cycle does the same double duty it does on aluminum: the EN (electrode negative) half drives penetration, and the EP (electrode positive) half breaks up the surface oxide through cathodic cleaning. Set AC balance toward the EN side for most work and increase EP cleaning only as much as a dirty surface demands, since extra EP overheats the tungsten. Magnesium conducts heat fast and melts at a somewhat lower temperature than aluminum (AZ31 runs roughly 630C at the liquidus versus about 660C for aluminum) and over a range rather than a single point, so use a light touch and move deliberately.

Tungsten selection follows the same logic as other AC work. Zirconiated or lanthanated tungsten holds a stable ball and resists spitting on AC, sized one step up from steel because AC loads the electrode harder. Our tungsten electrode guide covers the trade-offs between types and how to prep the tip for AC versus DC.

The common wrought alloys are AZ31B (roughly 3% aluminum, 1% zinc) in sheet and plate, and AZ61A (roughly 6% aluminum, 1% zinc) in extrusions and stronger structural shapes. Both are readily weldable. Cast alloys, including AZ91 die castings and the rare-earth grades like EZ33A used in high-temperature parts, are more crack sensitive and need careful heat management. Identify the alloy before you weld, since filler choice and preheat both depend on it.

Filler selection matches the base alloy and comes from AWS A5.19, the specification for magnesium alloy welding electrodes and rods. AZ61A and AZ92A rods weld the AZ-series wrought grades, with AZ92A preferred where crack resistance matters on castings and restrained joints. EZ33A rod is used on EZ33A and other rare-earth castings and high-temperature components. Never reach for an aluminum filler rod, the chemistry will not match and the weld will crack. Confirm the current rod designation and parameters against the manufacturer datasheet and SDS for the specific filler you buy.

Preheat is the lever that keeps castings and thick sections from cracking. Light wrought sheet usually needs none, but thick plate, restrained joints, and most castings benefit from preheat to slow the cooling rate through the heat-affected zone. Heat the part uniformly and verify with a temperature crayon or infrared thermometer rather than guessing. Follow the alloy producer or ASM Handbook preheat and interpass guidance for the specific alloy and thickness, do not invent a number, and keep interpass temperature controlled so you are not stacking heat into a part that is already hot.

Now the part that separates magnesium from every other base metal: fire safety. Magnesium chips, dust, and fine grinding swarf are flammable, and a solid magnesium part can ignite if it is heated enough or if fines accumulate. The rules are not optional.

• Never use water, CO2, foam, or a standard ABC extinguisher on burning magnesium. Water reacts with hot magnesium to release hydrogen and can cause a steam or hydrogen explosion. CO2 and ABC media will not smother a metal fire.
• Keep a Class D dry powder extinguisher rated for magnesium within reach, or dry sand to smother small fires. Class D media is the only correct choice for a combustible-metal fire. Our fire-prevention guide lists which extinguisher class covers which hazard.
• Control the swarf. Do not let grinding fines, chips, or dust pile up. Clean up between operations, store fines in a covered metal container, and keep magnesium dust away from water and from any source of sparks. Grind magnesium and steel on separate setups so a steel spark does not land in magnesium dust.

These practices line up with NFPA 484, the standard for combustible metals, and OSHA guidance on magnesium and Class D extinguishing media. Verify the current edition and clause numbers against the published standard before you build a written procedure around them.

Surface prep follows the aluminum playbook. Remove the oxide with a dedicated stainless brush that has never touched steel, degrease with acetone, and weld promptly before the oxide reforms. The cleaner the surface, the less EP cleaning you need and the cooler you can keep the tungsten. For an overview of how magnesium fits alongside the other base metals and where to start, see the welding materials hub.

This guide is general welding reference, not a procedure qualification or a substitute for the manufacturer datasheet, the governing code, or a qualified welding engineer. Magnesium fire hazards are serious. Confirm filler designations against AWS A5.19, follow the alloy producer’s preheat data, and follow NFPA 484 and your local fire code for storage, handling, and extinguishing before you cut, grind, or weld magnesium.