Welding Magnesium Alloys: Processes, Filler Metals, Defects and Fire Safety

What Is Magnesium Welding?

Magnesium welding is the joining of magnesium alloys by fusion or solid-state processes under shielding that excludes oxygen, nitrogen and moisture.

Magnesium alloys are chosen where weight matters: aerospace housings, automotive castings, portable equipment and sporting goods. A part welded from magnesium must also tolerate the metal’s low stiffness and its tendency to corrode galvanically against other metals. The figure compares density with other structural metals.

Why Is Magnesium Difficult to Weld?

Several properties combine to make magnesium demanding. None is unusual alone, but together they remove much of the tolerance that steel welding allows.

The oxide is a particular trap. Its volume is smaller than the metal it forms from (a Pilling-Bedworth ratio of about 0.8), so it cracks and does not seal the surface, a behaviour discussed in the guide to high-temperature oxidation of metals. Oxide left in the joint becomes inclusions, and unprotected hot metal oxidises quickly.

Which Magnesium Alloys Can Be Welded?

Most commercial magnesium alloys can be fusion welded, but weldability varies with composition. Wrought alloys with moderate aluminium are the easiest; high-aluminium castings and some rare-earth alloys need extra care.

Alloy names follow the ASTM system: the first two letters give the main alloying elements (A for aluminium, Z for zinc, E for rare earths, W for yttrium, M for manganese), the numbers give nominal percentages, and the final letter marks a version. Alloys with more than about 1.5% aluminium are the ones most prone to stress corrosion cracking after welding, which is the reason for stress relief below.

Which Welding Process Should You Choose?

Gas tungsten arc welding is the standard choice for sheet and repair, with gas metal arc welding for thicker work and solid-state or beam processes for high-volume or critical joints. Oxy-fuel welding is not used because flux and flame contamination are difficult to remove.

Whichever process is used, a documented procedure and trained welders are expected under a quality system such as the ISO 3834 welding quality system, and critical joints are often verified through third-party inspection in welding.

How Do You Prepare Magnesium for Welding?

Clean the joint immediately before welding, because the oxide re-forms within hours and a dull grey or white film is already present on stock. The sequence is:

1. Degrease with a suitable solvent or alkaline cleaner to remove oil, cutting fluid and marking inks.
2. Remove the oxide by chemical pickling, which is traditional for critical work, or by mechanical means such as scraping or brushing with a clean stainless steel brush kept only for magnesium.
3. Clean filler rod or wire in the same way, and keep it in a sealed container.
4. Weld soon, commonly within hours of cleaning, and re-clean if the surface dulls.

Chemical pickling often uses chromic acid-based solutions, which contain hexavalent chromium and are tightly regulated. Use a compliant alternative where the specification permits and manage waste to local rules. Do not use steel wool or brushes that have touched steel, because embedded iron accelerates corrosion later. Machine shavings and grinding dust must be collected and handled as combustible waste.

How Do You Shield and Set Up the Weld?

Pure argon is the usual shielding gas, supplied from clean equipment with dry hoses. Helium or argon-helium blends can be used for deeper penetration on thick sections. Gas purity matters because moisture is the main source of hydrogen porosity.

• Backing and trailing shields keep the hot metal covered as it cools; the root side needs an argon purge or a grooved backing bar.
• Tungsten for alternating current is pure or zirconiated, ground to a balled tip. Check any tungsten and shield gas recommendations in the filler specification.
• Fixtures must clamp firmly because high thermal expansion drives distortion. Use backing bars that conduct heat evenly.
• Heat input should be as low as the joint allows to limit grain growth and cracking. Fast travel and tight fit-up help.
• Preheat is generally avoided on thin wrought sheet, but a modest preheat, often in the range of 100 to 150 °C, is used for thick or restrained castings. Confirm with the procedure.

How Do You Select Filler Metal?

Filler metal is chosen from the AWS A5.19 range of magnesium electrodes and rods, matched to the base alloy and to cracking resistance. In many cases the filler has a slightly higher aluminium content than the base metal, which narrows the freezing range and reduces hot cracking.

Do not mix zirconium-bearing and aluminium-bearing alloys in the same weld, because aluminium combines with zirconium and removes its grain refining effect.

How Do You Weld AZ31B Sheet With GTAW?

This sequence shows a typical alternating-current GTAW procedure for AZ31B sheet. It is a guide only; settings come from a qualified welding procedure.

1. Degrease the AZ31B sheet, then remove the oxide with a dedicated stainless steel brush or approved chemical cleaning, and clean the filler rod.
2. Fit the joint tightly in a clamped fixture with a grooved backing bar, and start the argon purge on the root side.
3. Select AC GTAW with a pure or zirconiated tungsten, balled tip, and pure argon shielding at the flow rate in the procedure.
4. Set low current for the sheet thickness, check on scrap, and fit a trailing shield if the procedure calls for one.
5. Tack weld at close spacing, using the same filler and shielding as the final weld.
6. Weld at steady, fast travel with ER AZ61A filler, keeping the rod end inside the gas shield at all times.
7. Hold the shield over the weld until it has cooled and the bead has lost its glow, then inspect for bright, uniform metal with no black oxide.
8. Stress relieve as the procedure requires, then clean, coat and inspect the weld.

A trial weld on scrap of the same thickness is worth doing before the part. Visual inspection should show a bright, consistent bead with no black or grey oxide, no cracks and no visible porosity. Heat tint colours are a cue, not a measurement.

What Defects Occur and How Are They Prevented?

For critical parts, inspection includes visual testing, penetrant testing, radiography or other methods set by the specification. Welded magnesium used under cyclic load needs attention to fatigue, because weld toes and porosity act as initiation points, as estimated in the fatigue life calculator. Toughness checks follow the principles in the Charpy impact testing guide, although magnesium alloys have low impact energy compared with steel.

What Happens in the Weld Microstructure?

Magnesium has a hexagonal close-packed crystal structure with limited slip systems, which is one reason it is less ductile than aluminium at room temperature. In the weld, the fusion zone solidifies as fine dendrites, and the heat-affected zone coarsens or partially melts.

• Eutectic at boundaries. In aluminium-bearing alloys, a low-melting eutectic of magnesium and Mg₁₇Al₁₂ (the beta phase) forms near 437 °C and can melt in the heat-affected zone, which lowers the cracking threshold. The behaviour of such boundary films is part of the topic covered in grain boundaries in metals.
• Grain refinement. Zirconium in ZE and WE alloys nucleates fine grains, which is why those alloys resist hot cracking.
• Heat input and grain size. Low heat input and fast cooling give finer grains and higher strength, while slow cooling coarsens the structure.
• Friction stir welds recrystallise at below melting temperature, giving a fine-grained stir zone without solidification defects.

What Post-Weld Treatment Is Needed?

Welds in aluminium-bearing magnesium alloys are normally stress relieved, because residual tensile stress combined with a humid environment can cause stress corrosion cracking. Typical practice for wrought AZ31B is a low-temperature treatment, often quoted around 260 °C for about 15 minutes, but the exact temperature and time depend on the alloy, the thickness and the specification, so verify them.

• Stress relieve in a controlled furnace and avoid temperatures that cause grain growth or incipient melting.
• Cast alloys may also be re-solution treated or aged, but this is specialist heat treatment.
• Remove spatter and any flux residue, and inspect the weld before coating.
• Apply conversion coating and paint after cleaning to protect the surface.

How Do You Manage Fire and Fume Risk?

Solid sheet and plate are hard to ignite and burn only when heated near melting for a sustained period. Chips, turnings, fines and dust ignite far more easily. The main controls are:

• Housekeeping. Remove chips and dust promptly into dedicated, labelled metal containers kept dry.
• Dust control. Avoid dry grinding in the weld area, and follow NFPA 484 for combustible metals.
• Segregation. Keep magnesium work away from other metal dust and from oil-soaked rags.
• Fire plan. Provide Class D extinguishers, train welders on their use and tell the fire service that magnesium is on site.
• Fume. Welding generates magnesium oxide fume. Use local extraction and respiratory protection as the risk assessment requires.

The wider set of weld shop hazards, including fume and fire, is described in the guide to welding hazards linked earlier, and gas supply safety applies to the argon cylinders used for shielding.

How Do You Protect Welded Magnesium From Corrosion?

Magnesium is the most anodic of the common structural metals, so it corrodes preferentially when coupled to steel, copper or stainless steel in a wet environment. Welded joints add residual stress and a changed microstructure, which can raise susceptibility.

• Isolate dissimilar metals. Use insulating washers, sealants and coated fasteners, and screen material pairs with the galvanic corrosion risk estimator.
• Coat the part. Conversion coatings and paint systems protect the surface; avoid scratching the coating at the weld.
• Keep out iron and copper contamination. Traces of iron, nickel or copper greatly accelerate corrosion, so use dedicated tools.
• Use the anodic behaviour deliberately. Magnesium is also used as a sacrificial anode material, compared in the sacrificial anode materials comparison and applied in cathodic protection in offshore structures.

What Are Common Mistakes When Welding Magnesium?

• Welding over oxide. The film must be cleaned immediately before welding.
• Using the wrong filler. Mixing aluminium-bearing and zirconium-bearing alloys harms grain structure.
• Using a shared steel brush. Iron contamination causes rapid corrosion later.
• Leaving out the backing purge. The root oxidises and weakens.
• Skipping stress relief on aluminium-bearing alloys.
• Using water on a fire. This can make the fire worse.
• Letting chips and dust accumulate. They are the main ignition hazard.
• Welding damp or porous castings without repair preparation.

Quick Reference: Welding Magnesium

Key Terms

Magnesium alloy

A metal based on magnesium with additions such as aluminium, zinc, rare earths or zirconium.

AZ31B

A wrought magnesium alloy with about 3% aluminium and 1% zinc, widely used for welded sheet.

Oxide film

The thin, non-protective magnesium oxide layer that forms on exposed magnesium.

Hot cracking

Cracking that occurs during solidification in the weld or just after it.

Backing purge

An inert gas flow under the joint that protects the root from oxidation.

Stress corrosion cracking

Cracking from the combined action of tensile stress and a corrosive environment.

Class D extinguisher

A fire extinguisher designed for combustible metal fires such as magnesium.

Friction stir welding

A solid-state process that joins metal by stirring it with a rotating tool below its melting point.

Frequently Asked Questions

Conclusion

Welding magnesium succeeds when four things are controlled: a clean, oxide-free surface, effective argon shielding including the root, the right filler and low heat input, and a plan for stress relief and corrosion protection. The fire risk from chips and dust needs its own discipline, with Class D protection and no water. Process and parameter details should always come from a qualified procedure and the alloy supplier. To see how metallurgical principles apply across other non-ferrous alloys, the next step is the Non-Ferrous Metals category, and the Welding Metallurgy category covers related joining topics.

About This Guide

This guide was prepared by the MetallurgyZone technical team from the sources listed under Standards and References. Verify temperatures, filler selection and safety requirements against the current standard, the alloy supplier and your qualified welding procedure.

Standards and References

• AWS A5.19/A5.19M, Specification for Magnesium Alloy Welding Electrodes and Rods, American Welding Society (aws.org).
• NFPA 484, Standard for Combustible Metals, National Fire Protection Association (nfpa.org).
• ASTM B94, Standard Specification for Magnesium-Alloy Die Castings, and ASTM B80, Standard Specification for Magnesium-Alloy Sand Castings, ASTM International (astm.org).
• ISO 3834, Quality requirements for fusion welding of metallic materials, International Organization for Standardization.
• ANSI/AWS Z49.1, Safety in Welding, Cutting, and Allied Processes, American Welding Society.

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