Magnesium TIG Welding Guide: Setup, Safety, Technique

A fabrication shop owner grabs the TIG machine, lays out a bent magnesium bracket, and asks the welder to fix a cracked corner. Ten minutes later the shop is full of white smoke, the bracket is trashed, and the welder is asking why a puddle that was fine on aluminum kept crumbling on magnesium. The problem was never the machine. It was that magnesium welds differently, burns differently, and fails differently, and it punishes shortcuts harder than almost any metal a hobby welder will touch.

Magnesium TIG Welding: What It Is and Why It Matters

A repair job shows up at a fabrication shop almost every week: a customer sets a cracked magnesium housing on the bench and asks for a weld instead of a replacement casting. The gearbox must run by Friday, and the part feels so light it resembles a plastic mock-up. It is not an afternoon job.

So what is magnesium TIG welding? It is gas tungsten arc welding (GTAW) of magnesium alloys using alternating current and an inert argon shield, and for low-volume work where joint quality matters, it is the process you want. The Edison Welding Institute notes that GTAW and gas metal arc welding are the most common arc-based processes for aluminum and magnesium alloys, with GTAW generally the choice when low volume and superior weld quality matter.

In practice you strike an arc between a non-consumable tungsten electrode and the workpiece, hand-feed magnesium filler rod into the puddle, and blanket everything in argon so nothing in the air reaches the molten metal. That shield is not a nicety. Magnesium was one of the first metals welded commercially by TIG because welding in an inert gas atmosphere is distinctly advantageous for such a fast-oxidizing metal, as the CK Worldwide technical guide explains.

How magnesium TIG differs from aluminum TIG

From across the shop, the two processes look identical: AC power, a ceramic cup, a bottle of argon. The differences separate a decent aluminum weld from a magnesium mess.

• No flux crutch. Aluminum oxide can be broken chemically with flux, which is why aluminum flux welding procedures exist. Magnesium has no equivalent; its oxide must be broken mechanically by brushing or cathodically by the electrode-positive half of the AC cycle.
• A much narrower heat window. Magnesium alloys melt around 630 degrees C and begin to vaporize before the pool looks dangerously hot. Push the amperage and the metal boils instead of wetting out.
• Extreme contamination sensitivity. Steel dust, copper from a grinder, silicon from a cut-off wheel, a dirty steel brush: all of it ends up in the puddle. Surface prep decides the weld, which is why prepping metal properly before welding deserves its own treatment.

Then there is housekeeping. Magnesium chips, dust, and grinding fines are flammable, and burning magnesium is a dangerous thing to stand near. That fact changes how the job gets planned, from where you grind to where you rest the torch.

Where magnesium shows up in real products

Magnesium is a working structural metal, not an exotic curiosity. Common wrought magnesium TIG applications include cast and machined housings, transmission and gearbox covers, brackets and structural fittings, wheels, and aerospace and electronics components.

If you have repaired a mag wheel or handled a die-cast instrument housing, you have been near a magnesium weld. Our alloy wheel welding guide covers one of the most visible examples of the craft.

Three alloy families your procedure has to respect

Not every magnesium behaves the same. Welders encounter three practical families, and each one changes filler selection, crack risk, and preheat decisions.

Family	Typical grades	Where you meet it
Aluminum-zinc-magnesium (AZ)	AZ31B, AZ61A, AZ80A, AZ91D, AZ92A	Sheet, plate, extrusions, and the great majority of die castings
Aluminum-magnesium (AM)	AM60, AM80	Die castings where corrosion resistance and casting quality lead
Rare-earth or manganese-magnesium (EZ, WE, HM)	EZ33A, WE43, HM21A, HK31A	Elevated-temperature and aerospace castings

The family dictates the filler rod, and the filler rod dictates how much crack resistance you get. Get that wrong and the joint fails for reasons that have nothing to do with your technique.

The payoff: weight

Magnesium is one of the lightest structural metals, at roughly 1.74 g/cm3, about two-thirds the density of aluminum and less than a quarter that of carbon steel. That is why housings, covers, brackets, and wheels are made from it, and why a welded magnesium bracket can cut part weight dramatically compared with the aluminum version it replaced.

Material	Density (g/cm3)
Magnesium	~1.74
Aluminum	~2.70
Carbon steel	~7.85

Two pillars, and what this guide covers

Magnesium TIG is learnable, but it is a low-tolerance, high-consequence craft built on two non-negotiable pillars: clean metal and fire-safe housekeeping. Get either wrong and you do not get a weak weld, you get a scrapped part or a flash fire.

The guide follows the work in order: magnesium physics, alloys and filler rods, machine setup, joint preparation, a full job from setup to inspection, then the safety and defect troubleshooting chapters. It ends by comparing magnesium TIG against MIG, laser, brazing, adhesives, and friction stir welding, so you know whether TIG is the right process before you buy the rod.

One more thing worth saying early: magnesium TIG is not a scaled-up version of aluminum TIG. Treat magnesium as its own material with its own rules, and read the safety and troubleshooting chapters carefully. If you are comparing processes first, our aluminum MIG welding guide is a useful contrast, since MIG is often the production answer TIG replaces.

Why Magnesium Welds Differently: The Physics You Need to Know

With magnesium TIG welding, failures are rarely about skill. They’re about the metal itself. Four physical properties dictate nearly every decision, from cleaning to balance, and ignoring them will work against you no matter how steady your hand is.

1. A Tenacious Oxide Film

The first obstacle is magnesium oxide. While AZ31B, the most common wrought alloy, has a liquidus near 630 C, its surface oxide melts at roughly 5072 F (2800 C). That means your arc cannot dissolve it away. As the Edison Welding Institute notes for both aluminum and magnesium, this oxide must be removed through mechanical means or cathodic cleaning, or incomplete fusion is almost guaranteed.

2. A Narrow Solidification Window

Magnesium alloys freeze across a much narrower range than steel. Late in solidification, only a thin liquid film remains between grains. As the weld contracts, that film is pulled apart, leading to hot cracking, the most common failure in cast magnesium and heavily restrained AZ welds. Alongside this, alloying elements can segregate to the bead center, making filler selection and interpass control far more important than they would be with many ferrous metals.

3. Heat Sinks Away

AZ31B conducts heat at roughly 100 W/m-K, about double that of carbon steel. This rapid dissipation pulls heat sideways into the plate, causing the puddle to shrink quickly if travel speed isn’t matched to your amperage. The result is a narrow, cold-looking bead, increased risk of burn-through on thin stock, and a large amount of distortion from heat leaving through the base metal rather than the weld itself.

4. Hydrogen Porosity

Molten magnesium holds hydrogen in high amounts, while solid magnesium holds very little. Once the puddle freezes, that dissolved hydrogen has nowhere to go and forms porosity. The main culprits are moisture on the base metal, moisture on the filler rod, or condensation in gas lines. For this reason, shielding gas purity and rod storage deserve just as much attention as your arc settings.

Expansion, HAZ Stress, and AC Cathodic Cleaning

Magnesium’s coefficient of thermal expansion sits around 26 µm/m/K, roughly twice that of steel. On restrained joints, the heat-affected zone (HAZ) tries to expand while the weld metal tries to shrink. This tension can cause cracking hours after the arc is out.

These stresses are exactly why AC TIG is the standard. The electrode-positive half of the cycle drives ions into the oxide film, sputtering it off through cathodic cleaning. The electrode-negative half provides penetration. The balance control allows you to adjust that ratio. This trade-off between cleaning and penetration is the same balancing act covered on the MIG side in our aluminum MIG welding guide. For high-spec work, such as aerospace TIG welding, this balance must be repeatable and documented.

Physical property	Effect in the arc	Symptom when ignored
MgO film (5072 F / 2800 C)	Oxide survives weld heat, needs mechanical or cathodic removal	Incomplete fusion, cold lap, stubborn dross
Narrow solidification range	Thin liquid film between grains at freeze	Hot cracking, segregation, brittle bead centers
High thermal conductivity (~100 W/m-K)	Heat drains into plate, puddle shrinks fast	Narrow cold bead, burn-through, distortion
High molten / low solid hydrogen solubility	Hydrogen expelled on solidification	Porosity, pinholes, crater voids
CTE (~26 µm/m/K)	HAZ expansion against restrained weld metal	Distortion, HAZ cracking, delayed failure

These benefits come with a hazard. Breaking up that protective oxide creates fume, which can cause metal fume fever. For more details on exposure, see OSHA’s welding health and safety guidance.

Choosing Magnesium Alloys and the Right Filler Rod

Pick the wrong filler rod and everything else becomes secondary. Machine settings, cleaning, shielding gas, all of it can be perfect and the joint will still crack. Magnesium alloys look nearly identical on the bench, but they are not interchangeable, and the consumable you choose changes how the pool behaves as it solidifies.

The alloy families you will actually meet

Three groups cover almost all magnesium TIG work:

• Wrought AZ-series. AZ31B is the one you will see most, a rolled sheet and plate alloy with roughly 3 percent aluminum and 1 percent zinc, used for everything from enclosures to aircraft and automotive brackets. AZ61A and AZ80A are the extrusion and structural grades, so tube, angle and channel stock arrive in these.
• Cast AZ-series. AZ91D and AZ91E dominate high-pressure die castings, which is why they show up in housings, covers and brackets. AZ92A is the classic sand and permanent-mold casting alloy.
• Rare-earth and elevated-temperature grades. EZ33A, HM21A, HK31A and WE43 add rare earths to improve corrosion resistance and high-temperature strength. They appear in aerospace work and in parts that run hot, and they get their own filler. The aerospace TIG welding standards built around these grades are a separate universe from shop-floor AZ31B repairs.

Matching filler to base metal

The governing specification is AWS A5.19, the standard for magnesium alloy welding electrodes and rods. One piece of paperwork worth knowing: AZ92A filler is also listed as AMS 4395 and MSRR 9500/51. When an aerospace or government print calls up MSRR 9500/51 or AMS 4395, you are looking at AZ92A rod under a different name, so a supplier’s “milspec magnesium rod” may be exactly what your procedure requires.

The matching logic is simple enough to memorize:

• Wrought to wrought: AZ61A or AZ92A rod, with AZ92A preferred. NAVAIR 01-1A-34, WP 007 05 directs welders to use AZ92A or AZ61A rod for wrought-to-wrought joints and prefers AZ92A, because AZ61A is cheaper but more subject to weld cracking.
• Cast to cast: AZ92A.
• Rare-earth castings and elevated-temperature service: matching EZ33A rod, which is specified for joining rare-earth magnesium alloys such as HK31A and HM21A.

Base metal	Typical form	Recommended filler	Why
AZ31B	Sheet, plate, formed brackets	AZ61A or AZ92A (prefer AZ92A)	Wrought-to-wrought pair, best crack resistance
AZ61A / AZ80A	Extrusions, tube, structural shapes	AZ61A or AZ92A (prefer AZ92A)	Matches wrought AZ series; AZ80A strength does not change the filler choice
AZ91D / AZ91E	High-pressure die castings	AZ92A	Castings are crack sensitive, need the tougher filler
AZ92A	Sand and permanent-mold castings	AZ92A	Matching chemistry
EZ33A, HM21A, HK31A, WE43	Elevated-temperature and aerospace parts	EZ33A	Rare-earth filler preserves elevated-temperature properties

Cast magnesium alloys such as AZ91 and AZ92 are more crack sensitive than wrought grades, so they call for conservative heat input, uniform preheat and a crack-resistant filler. AZ92A answers those requirements at once, which is why it keeps surfacing as the default for casting repair. The CK Worldwide technical guide carries useful filler chemistry comparisons, and the EWI light paper on arc welding aluminum and magnesium covers the same matching logic in a broader context.

Three mistakes that wreck magnesium joints

Assuming designations are interchangeable. AZ31B, AZ61A, AZ91D and AZ92A look similar on a bench tag and behave very differently. Identify the alloy before you buy rod, and confirm the filler designation against the manufacturer datasheet rather than trusting a label or a supplier’s description.

Storing rod like steel filler. Filler metal must arrive dry, free of oxides, grease and foreign matter. Damp filler should be baked before use per the CK Worldwide technical guide, and oxide-laden rods rank among the top causes of porosity. Keep rod in its original sealed packaging, or in a heated rod oven or a dedicated container that stays dry and clean. On magnesium, the consumable is part of the shielding system, not an afterthought.

Machine Setup: AC Balance, Tungsten, Torch and Argon Shielding

Every magnesium problem that shows up a month later starts as a setup problem. The weld looks fine, the part ships, and then the customer calls about a cracked corner. The culprit is usually something you can dial in before the arc is even struck: the balance setting on your inverter, the condition of your tungsten, or the argon that is not actually flowing pure and dry over the joint. Magnesium TIG is a forgiving-looking process with very little margin, so let’s get the machine right first.

Start with an AC power source and an adjustable balance

Magnesium TIG requires alternating current. DC does not cathodically clean the oxide film, and you will spend the whole job fighting inclusions. What you want is a modern inverter TIG welder with a user-adjustable electrode-negative to electrode-positive balance, which means the machine controls how much of each half cycle you get.

The balance control is the single most important setting on the machine. A longer electrode-negative portion gives you deeper penetration, a narrower bead profile, less heat on the tungsten, and travel speeds up to about 20 percent faster. Modern machines let you set EN anywhere from roughly 50 to 99 percent, according to Miller Electric’s AC TIG guidance. For thin sheet, a higher EP (shorter EN) percentage adds scrubbing action that helps break up the oxide and gives you a wider, flatter bead. For thicker plate and castings, go EN-heavy for the penetration you need. If your shop is also runningaerospace TIG work, you already know that balance settings are documented per procedure, not per habit.

Tungsten: use rare earth, and shape it correctly

The standard AC choices are 2 percent ceriated and 2 percent or 1.5 percent lanthanated. Zirconiated is a workable alternative. Thoriated tungsten is unnecessary here and raises radiation handling questions nobody needs in a general fabrication shop. Whichever you pick, grind it to a truncated point rather than a needle point. The flatter tip spreads the arc, resists wandering, and holds up under the reversing load that AC puts on the electrode.

Tungsten diameter	Typical AC current range	Notes
1/16 in (1.6 mm)	roughly 60 to 100 A	Thin sheet and small repair work
3/32 in (2.4 mm)	roughly 115 to 150 A	The workhorse size for most magnesium jobs
1/8 in (3.2 mm)	Higher current,200 A and up	Heavy plate and thick sections

One caution: AC loads the electrode harder than DC, so size up compared to what you would use for steel at the same amperage. And if you notice a tungsten dipping into the pool, stop, grind, and re-shape. A contaminated tip is the fastest route to porosity.

Torch, cup and argon shielding

Your gas cup should be roughly 1/16 in to 1/4 in larger than the tungsten diameter, and bigger is generally better than too small. The cup has to deliver enough argon coverage to keep the puddle and the cooling weld face completely shielded. A too-small cup on a too-big electrode is the classic cause of a white, oxidized bead.

Argon is the standard shield gas. AWS C5.10/C5.10M lists 100 percent argon as the shielding gas for nickel, copper, aluminum, titanium and magnesium alloy base metals. Argon also makes arc starting easier because its ionization energy is 15.7 eV against helium’s 24.5 eV, and argon carries only about a tenth of helium’s thermal conductivity, which produces that concentrated, finger-like penetration profile that suits thin magnesium sections. Adding helium to the blend raises heat input for heavier material but demands more flow.

Material thickness	AC amperage	Argon flow
1/16 in	about 60 A	13 CFH
1/8 in	about 115 A	19 CFH
1/4 in	100 to 130 A	25 CFH
1/2 in	about 260 A	35 CFH

Those numbers come from the CK Worldwide TIG technical guide and are starting points, not answers. Verify on a coupon.

Gas purity, leaks, and backing

Buy the best argon you can afford, from a reputable supplier like an Airgas welding supply dealer, and check the dew point on the cylinder or bulk tank. Moisture in the shielding gas becomes porosity in the pool. Purge the gas lines, check the regulator for a stuck moisture trap, inspect hoses for leaks, and never weld with a cup that has a chipped ceramic or a cracked nozzle. Argon flow is not optional on magnesium, so keep a spare regulator and hose on hand.

Ventilation matters too, andAWS F3.2M covers fume control guidelines for magnesium welding. If you are wondering how argon practice differs on aluminum, the comparison in the aluminum MIG welding guide is a useful reference, since both metals share the 100 percent argon convention but differ sharply in filler behavior.

Once the machine is set, the argon is verified, and the balance is dialled in for your section thickness, the next decision is which filler rod and which alloy you are actually joining.

Surface Preparation, Joint Design, Fit-Up and Preheat

Most magnesium weld failures blamed on the machine began before the arc was struck. Magnesium is the most reactive of the common structural metals and forms a tenacious oxide film in minutes, not hours. A dirty, wet, loosely fitted or cold joint fails no matter how the amps, balance percentage or filler rod are set.

Clean Within One Inch of the Joint

The rule from the Edison Welding Institute’s light paper on arc welding of aluminum and magnesium is straightforward: clean surfaces within about an inch of the joint before welding, and keep that cleaned zone clean until the arc passes through it.

Suitable cleaning equipment:

• Rotary powered stainless steel wire wheels
• Stainless steel wire brushes
• Carbide burrs
• Carbide-tipped wheels

Steel brushes and most grinding wheels and pads are not recommended because they embed iron and steel grit in a surface far softer than the tool, and that grit later rusts, lifts and ends up in the weld as inclusions and porosity.

Use a dedicated stainless brush for magnesium and never let it touch steel; one nick of carbon steel in the bristles contaminates the workpiece. Keep it sealed. Degrease with a residue-free cleaner, wipe dry and weld promptly.

Hydrated oxides are the hidden hazard. As EWI explains, hydrated oxides on the joint surface and the filler wire introduce hydrogen into the weld pool, and hydrogen causes porosity in magnesium welds. Wipe the rod with an acetone-dampened cloth before feeding.

Magnesium TIG is a no-flux process, so shops accustomed to aluminum flux welding must un

Magnesium TIG is a no-flux process, so shops accustomed to aluminum flux welding must unlearn the assumption that a chemical cleaner does the work. The aluminum cleaner guide on WeldZen covers solvent selection and contaminant removal, and the same residue-free logic applies to magnesium.

Joint Design: Wide Grooves Beat Tight Ones

Prefer a wide V-groove or U-groove with a generous root opening. A broader groove means less filler metal and less total heat in the joint, and less filler means less shrinkage stress on a heat-affected zone prone to cracking.

Avoid an excessively narrow root face and sharp notches at the groove root. A knife edge or unfilled corner is a stress raiser and a ready-made crack site. Use a generous groove radius blended into the plate.

Restraint is the other half. Magnesium’s coefficient of thermal expansion, about 26 micrometers per meter per kelvin, is higher than steel’s, so a joint moves more as it heats and cools. Blocking that movement turns expansion into stress, and stress plus a narrow solidification range equals cracking.

Fit-Up, Tacking and Fixturing

Hold an even root gap along the whole joint and check both ends, because a fit-up that starts tight and ends wide gives a bead that varies in penetration. Plan for shrinkage: the puddle pulls as it solidifies and the joint closes slightly. Build in a hair more root gap than you think you need, and use a generous groove angle.

Space tacks generously. A tack is a mini weld that shrinks, and tightly spaced tacks lock that shrinkage into the part and build restraint right where you do not want it. A few inches apart on a simple bracket is plenty, so each tack can relax.

Clamp thin magnesium so it is supported rather than squeezed. Squeeze clamps and hold-downs bow thin sheet, and a bowed part will not flatten out after welding. Use aluminum or copper backing bars under the joint as support and heat sinks, with clamps light and spread out.

Preheat: The Right Kind, Not Just Any

Light wrought magnesium sheet generally needs no preheat. Thick plate, restrained joints and most castings benefit from a uniform preheat that slows cooling through the heat-affected zone, giving solidifying metal time to relieve stress.

Uniform is the operative word. A torch played over one corner creates a thermal gradient, and thermal gradients produce distortion and HAZ cracking. Heat the whole joint evenly with a heat lamp, hot plate or oven, keeping the part moving through it.

Do not invent the numbers. Take preheat and interpass temperatures from the alloy producer or from published data such as the CK Worldwide technical guide or the ASM Handbook, then verify the part with a temperature crayon or infrared thermometer. Control interpass temperature so you are not stacking heat into one zone pass after pass.

The order of operations is worth memorizing: clean, degrease, dry, fit up, tack, preheat, verify temperature, weld promptly, clean up as you go. Every step here is cheap. The arc is the expensive part, and it comes last.

Step-by-Step: A Practical Magnesium TIG Welding Procedure

Here is a complete job: a 3 mm AZ31B bracket flange joined to a 3 mm AZ31B gusset with a 60 degree butt joint. Every number below is a published starting point for AC TIG on magnesium, and starting points only.

Before You Strike an Arc

1. Confirm the alloy and pick the filler. Read the stamp or cert. AZ31B sheet takes AZ61A or AZ92A filler under AWS A5.19. The rod must be clean, bright and dry; dullness or a white powdery film means porosity.
2. Set the machine. AC, electrode-negative heavy balance. EN-heavy settings penetrate better and heat the tungsten less, so start near 80 to 85 percent EN and adjust from there.
3. Machine the tungsten. 2 percent ceriated or lanthanated, 1/16 in for this sheet. Grind on a dedicated wheel and break the tip to a truncated point; a needle point on AC wanders and spits. Re-shape it if you nick it.
4. Fit the cup and set gas. A cup 1/16 in larger than the tungsten covers thin sheet; more coverage is cheap insurance. Argon near 13 CFH for 1/16 in material per the CK Worldwide technical guide. Line pressure around 15 psi; leak check cup, hose and regulator.
5. Clean and dry the joint. Degrease, then brush with a dedicated stainless wire brush within one inch of the joint. Steel brushes and most grinding wheels embed contaminants that cause porosity. Weld while the metal is bright; magnesium oxide reforms instantly.
6. Backing or purge. A copper backing bar behind the root controls the puddle and sinks heat. Magnesium needs positive argon pressure behind the weld, so without a solid bar, purge the back with a bleed hose and argon blanket.
7. Run a coupon first. Strike a stringer on scrap of the same alloy and thickness. It proves your balance, gas coverage and feed angle before you touch the real part.

Starting Parameters

Thickness	Tungsten	Filler Rod	Amperage (AC)	Argon Flow
1/16 in	1/16 in	1/16 in	about 60 A	about 13 CFH
1/8 in	3/32 in	1/8 in	about 115 A	about 19 CFH
1/4 in	3/16 in	5/32 in	about 100 to 130 A	about 25 CFH
1/2 in	1/4 in	3/16 in	about 260 A	about 35 CFH

These are the published magnesium practice ranges from the CK Worldwide guide, a starting dial position and not a spec. Verify every one on a coupon of your actual alloy and thickness, and record what you found. Filler diameter matters, because a heavier rod on thin material dumps too much heat into the puddle. Our guide to position welding rod selection covers matching rod size to joint design.

Making the Weld

Start with the root pass. Hold the torch nearly vertical with a small weave, 20 to 30 percent travel across the joint, enough to keep the puddle moving and work the filler in. Feeder angle 60 to 80 degrees, rod pointing along travel direction into the leading edge of the puddle. Push the rod into the middle of the pool and you shove metal around, chill the joint, and risk tungsten contamination. Touch the leading edge, where the puddle is hottest, and the filler melts cleanly into the bead. Keep the rod out of the arc, or the tip contaminates and the arc wanders.

Arc length stays short, about 2 to 3 mm, and the arc must never touch the molten metal. Puddle size is your best heat control: keep it small and consistent, roughly the width of the filler rod, and let it freeze behind you. If it grows and flattens, you are running too much heat, and that means burn-through on thin material and hot cracking on anything restrained.

Check fusion on a broken coupon: a clean, well-fused root, no lack-of-fusion lines, no white oxide film at the boundary. Poor fusion means a dirty joint, too much travel speed, or too little electrode-positive cleaning from the balance setting.

Then fill passes. Keep interpass temperature under control and never let heat stack in one spot. On a 3 mm bracket a single root plus a single fill is often enough. Wider sections get a wider V or U groove, which lowers filler volume and heat input, the best defense against cracking.

Crater Fill, Cleanup and Inspection

End every pass with a crater fill: tap the amperage down or add extra filler as you break the arc so the puddle closes gradually.

On a cosmetic joint, a light back-strike or small foot pass over the crown improves the reinforcement. Keep the heat low and the arc short.

Let the part cool, and do not quench it or force it flat. Scrub the bead with a stainless brush to remove flux residue and loose oxide, rinse with clean water, and dry it fully.

Inspect the bead: uniform reinforcement, a slightly darker heat-affected zone, no white powdery residue, no undercut at the toes, no visible cracks. A magnesium crack is usually a hot-cracking problem, forming in the last stages of solidification when the last liquid metal is squeezed between locked grains, which is why it appears in the last inch of the bead.

If a crack appears, stop and find the cause before adding more filler. More heat in a cracked magnesium weld means more cracking, and you cannot weld out of a restraint problem. Weld a fresh coupon, section it, and confirm your procedure before rework.

Clean every magnesium chip, dust and grinding fine before you put the torch down. A part that passes inspection and then sits in a pile of swarf is a fire waiting for the next job. Our aluminum MIG welding guide covers the spray-transfer setup shops reach for when volume matters more than control.

Safety First: Magnesium Fire Risk, Fume and PPE

Every technique in this guide can be picked up in an afternoon. The safety rules cannot, because magnesium is one of very few industrial metals that can punish a mistake with a second, much larger event. A botched bead is scrap. A magnesium fire in a cluttered shop is a catastrophe that burns above the melting point of the alloy, reacts with water, and spreads to every combustible surface nearby.

The Fire That Killed a Welder Named Ben

NIOSH tells this story inToolbox Talk 2022-137, and it is the single most important paragraph in this entire guide. Ben was welding steel angle iron to a joist. Nothing about his job involved magnesium. But magnesium shavings were lying in his work area from an earlier operation, and falling sparks and hot slag landed in them. The shavings ignited. Ben suffered severe burns, inhaled the smoke, and died.

Read that again. He was weldingsteel. He was killed by magnesium that was not even part of his work. That is the core of the hazard: the fuel for a magnesium fire does not have to be magnesium on the torch. It can be machining swarf, grinding fines, dust from a filing operation, or chips swept under a bench where nobody thinks to look.

Why a Magnesium Fire Is Different

Magnesium chips, dust, and grinding fines are classified as combustible metal dust. They ignite readily when exposed to sparks, slag, hot metal, or an open flame. Once burning, magnesium combustion temperatures run well above the alloy’s melting point, roughly 650 degrees Celsius, so the material keeps burning as a molten, intensely bright mass that sticks to whatever it lands on.

Then there is the water reaction. OSHA’s combustible dust guidance is blunt about it: applying water or any water-based agent such as foam to burning magnesium, aluminum, or titanium can cause an explosible reaction. Water intensifies the fire by supplying oxygen and by generating hydrogen. A stream from a hose can turn a small burn into a steam or hydrogen explosion that scatters burning metal across the room. That is why every instinct that says “grab the hose” is wrong here.

How to Put Out a Magnesium Fire

Agent	On a magnesium fire
Water	Never. Reacts with hot magnesium to release hydrogen and can cause an explosible reaction.
Foam, CO2, halon, ABC dry chemical	Not for magnesium. They cool rather than smother the burning metal and can leave it burning.
NRTL-listed Class D extinguishing agent	Correct choice. Rated specifically for combustible metal fires.
Dry sand	Acceptable. Smothers the burning metal and separates it from air.
Magnesium foundry flux	Acceptable. Covers the burning metal and isolates it from oxygen.

NIOSH’s guidance is straightforward: use a Class D fire extinguisher, or cover the fire with sand or magnesium foundry flux. Note the emphasis on dry sand. Damp sand carries moisture into the hot metal.

Hot Work Discipline: The Part Shop Owners Miss

OSHA’s guidance for die casting operations covers hot work performed in areas where combustible metal dust, powder, fines, or sponge are present, and it is the best checklist available for magnesium shops. According to that guidance, hot work in such areas requires:

• A hot work permit system, so that every shift knows when and where an arc is burning.
• Cleanup of the area before work begins, including accumulated dust, fines, and residue.
• Protection of combustibles within 35 feet of the hot work location.
• A fire watch kept in service for at least 30 minutes after the work is finished.

That last point catches people out. The fire is not the only danger. Molten magnesium that lands in a pile of shavings can smolder for a while before the fire becomes obvious, so a fire watch that clocks out the moment the arc stops is not a fire watch. NFPA 484, the standard for combustible metals, exists for the same reason: this is a material that needs its own rules rather than a generic hot work program.

The Fume Nobody Sees

The fire is the dramatic hazard. The fume is the everyday one. Welding magnesium produces magnesium oxide fume, and the NIOSH Pocket Guide for magnesium oxide fume lists an OSHA permissible exposure limit of 15 mg/m3 as an 8-hour TWA and an IDLH value of 750 mg/m3. Symptoms include irritation of the eyes and nose and metal fume fever: cough, chest pain, and a flu-like fever that typically shows up several hours after exposure and can last a day or two.

Ventilation is the control, not respirators. AWS F3.2M covers ventilation for welding and cutting operations, and AWS Z49.1 requires adequate ventilation for all welding operations, with respiratory protective equipment used when adequate ventilation is not practical.

Magnesium deserves extra attention here because it wants less shielding gas flow than steel, which sets up a bad habit. A shop that runs 30 CFH on a steel job may dial the argon back to 10 on a thin magnesium bracket and wonder why the fume hangs in the air. Use a local exhaust hood or a flexible exhaust arm, keep the hood a few inches from the arc, and ventilate the whole area. It is also worth reviewing how cylinders and regulators are handled and secured, which our guide to air gas welding supply, cylinders and safety covers in more detail.

PPE and Keeping an Extinguisher in Reach

• Shade lens: the correct filter shade for the amperage you are running, no darker “just in case.” A too-dark lens means the welder works with a raised helmet and worse visibility of the puddle, which is exactly where magnesium welds go wrong.
• Flame-resistant clothing with sleeves closed, leather gloves, and nothing loose that can catch slag.
• No bare skin exposed to arc light. Magnesium’s bright burn is a UV source, and the plume from a magnesium weld is a serious irritant.
• A Class D extinguisher on hand, not across the shop. OSHA 29 CFR 1910.157(d)(6) requires portable extinguishers or other containers of Class D extinguishing agent placed so that travel distance from the combustible metal working area to the agent is 75 feet or less, and closer is better in a magnesium shop.

Those75 feet are a legal maximum, not a design goal. If the extinguisher is a minute’s walk and half the walk crosses a cluttered aisle, someone will not make it under stress. Keep a Class D unit mounted within a few steps of the welding bay, and confirm the service date on the tag. For shops running hot work near any aluminum or magnesium work, the same discipline applies to the gas supply and consumables handling described in our AllGas welding supply, gases and safety guide.

Common Magnesium TIG Defects and How to Fix Them

Magnesium TIG fails in a handful of characteristic ways, and nearly every one of them traces back to something you can see, hear, or measure. The real skill is learning which symptom points to which cause, because a pitted bead, a chalky white bead, and a bead with a crack running through it all get filed under “bad weld” when you are guessing.

One bit of physics explains most of the list. The magnesium oxide layer melts at roughly 2,800 degrees C, while AZ31B turns fully liquid near 630 degrees C, so the oxide never melts away during a TIG pass. It has to be removed mechanically before the arc is struck, or broken up cathodically during the electrode-positive half cycle. Every defect below is, in one way or another, a story about that oxide, your shielding gas, or the heat you put into the joint.

Porosity: holes in the puddle

Porosity is the most common magnesium TIG defect and the easiest to misdiagnose. Documented causes in arc welded magnesium include inadequate gas shielding, hydrated oxides on both the filler wire and the joint surfaces, and contamination introduced by improper pre-cleaning, as detailed in the EWI review of aluminum and magnesium arc welding. On top of those three, shop reality adds two more: condensation in the gas lines and a regulator that is leaking.

• Clean and dry everything. The joint, the filler rod, the tungsten, and the inside of the gas line.
• Bake the filler. Rods that have sat in a damp shop absorb moisture. A modest bake and a sealed storage box prevent a whole class of failures.
• Check gas purity and dew point. Shielding gas has to meet the required dew point, and condensation forming in the delivery line is a classic, easily fixed cause of porosity in argon-shielded nonferrous work. The CK Worldwide technical guide covers the setup details.
• Purge the lines. Blow argon through the hose and regulator before opening the valve on the tank.
• Increase argon coverage. A larger cup and slightly more flow cover the puddle better and reduce turbulence around the arc.

Oxidation and white powdery residue

A bead that comes out of the weld looking dull, dusty, or chalky white was under-shielded or exposed too long to the air. Usual suspects are a cracked or badly worn gas cup, too little argon flow, an arc stretched longer than it needs to be, contaminated gas, and an AC balance set too far toward electrode-negative so the surface is not being cleaned cathodically. Check the cup for cracks and carbon build-up, confirm flow at the cup with a flowmeter, tighten the balance slightly toward electrode-positive, and keep the arc short. If the residue keeps coming back, verify the gas itself rather than assuming the machine is at fault.

Lack of fusion and cold lap

If the bead rolls on top of the base metal instead of merging into it, you have incomplete fusion, and in magnesium it traces directly back to residual surface oxide. Because the oxide melts far above the base alloy, a dirty joint simply cannot fuse. Causes also include insufficient electrode-positive cleaning time, travel speed that is too fast for the heat you are putting in, too little total heat, and the wrong filler for the parent alloy.

Fixes: clean within an inch of the joint with a magnesium-dedicated stainless brush or carbide tools, re-verify the AC balance, slow your travel speed, and confirm the filler matches. If you work both aluminum and magnesium in the same shop, keep the tools separate. Our guide to prepping aluminum right makes the same dedicated-tool argument for the other metal, and cross-contaminating a magnesium brush with steel or aluminum residue is a fast way to ruin a joint.

Cracking

Magnesium welds in two distinct failure modes. Hot cracking happens while the metal is solidifying and shows up mostly in castings and heavily restrained joints. It is driven by too much heat input, high restraint, a narrow groove, and the wrong filler. AZ61A filler is explicitly documented as subject to weld cracking, which is why AZ92A is the standard recommendation for castings and restrained joints, and it is the safer rod to reach for when you are unsure. Cold cracking appears later, often days after the weld, and is driven by residual stress combined with moisture.

Both respond to the same countermeasures: uniform preheat to slow cooling through the heat-affected zone, a more generous groove that reduces filler volume, better fit-up with even gaps, less restraint from clamping, and better filler selection. For a fuller look at picking rods by joint condition, the all-position welding rod guide explains how filler geometry and joint position change your choices.

Undercut, tungsten contamination, burn-through and sagging

• Undercut (a groove melted along the toe of the bead) comes from excessive travel speed, too much amperage for the thickness, or a poor filler angle. Slow down and add filler sooner so the arc is not chewing past the edge of the puddle.
• Tungsten contamination happens when the electrode touches the pool or the filler rod, or when the electrode-positive portion of the cycle is excessive. It is a serious defect, because tungsten particles act as crack initiation sites in a metal that is already crack sensitive. Keep the arc short, restore the tip shape, and grind the tungsten back into form rather than welding on with a contaminated point.
• Burn-through and sagging on thin sheet or on a root pass mean too much heat. Reduce amperage, add a copper or aluminum backing bar, break long passes into shorter ones, and watch the interpass temperature.

For arc shape and electrode setup, the Miller guide to selecting tungsten for AC TIG is a good reference for keeping the tip pointed and the arc stable.

Symptom to cause at a glance

Symptom	Most likely cause	First thing to check
Bubbles or pinholes in the bead	Moisture on base metal, filler, or in gas lines	Dry the joint, bake the rod, purge the lines
White powdery film over the bead	Under-shielding, long arc, dirty gas	Cup condition, flow rate, gas dew point
Bead rolls on the surface	Residual oxide, cold joint, travel too fast	Clean within one inch, verify AC balance, slow down
Crack in a casting or fillet	Hot cracking from heat and restraint	Uniform preheat, open the groove, use AZ92A
Crack appears days later	Cold cracking from stress and moisture	Reduce restraint, control moisture
Groove along the bead toe	Too fast or too hot for the thickness	Slow down, add filler earlier
Erratic arc, pits, bright specks	Electrode touching the pool or rod	Re-shape the tungsten, shorten the arc
Dripping, collapsed bead on thin sheet	Excess heat	Lower amperage, add backing, shorter passes

Magnesium TIG vs. MIG, Laser, Brazing and Adhesives

TIG is the default answer when someone says “magnesium,” and for most repair shops and one-off jobs that default is right. But magnesium is one of the few metals where the obvious process is not always the best. A die-cast housing with a hairline crack may not be worth a weld at all.

AC TIG (GTAW)

Gas tungsten arc welding in AC remains the general-purpose answer for magnesium: thin to medium sheet, repair work, low volume, and any joint where filler goes on one small increment at a time. Alloy selection guidance describes GTAW as manual AC welding with argon for arc stability, adding helium for heavier sections, with helium raising the required gas flow rate. CK Worldwide’s technical guide covers this alongside practical parameter data.

The trade-off is straightforward. TIG gives the best control of filler addition, the cleanest cosmetic bead, and the widest range of joints a shop can repair without special tooling. It is also slow and labor-intensive, and the skill cost is real. Welders who have only run TIG on steel or aluminum need coaching before they can hold a magnesium puddle, so a refresher on aerospace TIG welding standards and techniques is worth the time.

GMAW (MIG) with spray or pulsed-spray transfer

Where productivity decides, GMAW is the production answer. High deposition rates and continuous filler feeding make spray or pulsed-spray transfer the operating modes of choice for magnesium, while GTAW is reserved for low-volume work demanding superior weld quality. EWI’s technical paper on arc welding aluminum and magnesium lays out the transfer behavior.

One catch trips up people who order a machine expecting a turnkey setup: few welding equipment manufacturers offer spray transfer and pulsed-spray programs for magnesium. That is a practical reason fabricators lean on TIG for short runs and custom programs for production.

On shielding, argon gives excellent cleaning action on the puddle, and helium may be added to improve filler flow and penetration for heavier sections, but pure helium is not recommended for GMAW on magnesium. AWS C5.10 covers the gas selection logic. If you know MIG but not magnesium, our complete aluminum MIG welding guide covers the shared fundamentals of machine setup, gas flow, and spray mode behavior that carry over directly.

Laser welding

Laser welding is excellent on thin magnesium: very precise, very fast, minimal heat affected zone, and easy to automate. It shines in high-volume thin-sheet applications and joints that need repeatable narrow seams. The drawbacks are capital cost and the fact that a laser system is close to useless for field repair. A cracked bracket at a customer site is not coming back to a fiber laser. Our guide to laser welding systems, uses, and safety goes deeper on the technology.

Brazing, soldering, adhesives and mechanical fastening

Brazing and soldering suit thin, low-stress assemblies and one job TIG cannot do easily: joining magnesium to a dissimilar metal without melting the base metal. Braze filler and flux chemistry must be matched to the service temperature, and flux residue has to be cleaned off thoroughly because magnesium is porous enough to trap corrosive residue. The principles are the same as torch brazing on any metal, with far less tolerance for a poor joint.

Adhesive bonding and mechanical fastening suit magnesium housings where a structural weld is not needed, such as camera bodies, electronics enclosures, and instrument housings. Adhesives add no heat and create no stress concentration, and cast bosses with threaded inserts or self-tapping fasteners take mechanical loads cleanly. Plan for galvanic corrosion between magnesium and dissimilar fasteners, usually handled with isolation washers, sealant, or a compatible coating.

Friction stir welding

Friction stir welding is the outlier because it is a solid-state process. The joint never melts, so there is no solidification step and none of the porosity or hot cracking that comes with it. That makes it attractive for structural magnesium where fatigue performance matters. It is limited to specific geometries, mainly lap, butt, and tubular configurations, and it demands dedicated tooling with a shoulder and pin, so it is a production process, not a shop repair technique.

Process comparison

Process	Best use	Advantages	Limitations	Typical applications
AC TIG (GTAW)	Thin to medium sheet, repairs	Precise filler control, cleanest bead, widest joint range	Slow, high skill cost	Crack repairs, brackets, prototypes
GMAW spray / pulsed-spray	High-volume production	High deposition rate, repeatable	Few magnesium spray programs	Production housings, frames
Laser welding	Thin, precise, high-speed joints	Fast, narrow heat zone, automatable	High capital cost, poor field repair	Thin sheet enclosures
Brazing / soldering	Thin, low-stress and dissimilar joints	Base metal not melted	Lower strength, filler and flux critical	Heat sinks, magnesium to copper or steel
Adhesives / fasteners	Housings needing no structural weld	No heat or stress riser	Not structural, galvanic risk	Electronics and instrument housings
Friction stir welding	Lap, butt and tubular geometries	Solid state, no solidification porosity	Dedicated tooling, limited geometries	Aerospace structural panels

Advanced Techniques, Real Repairs and Your Next Steps

Once the basics are mastered, the advanced side of magnesium TIG is less about adding heat and more about controlling it. For thin sheet, small castings, or service critical repairs, precision with pulse, shielding, and procedure qualification will make or break the joint.

Pulsed AC and Square-Wave Control

Modern AC TIG inverters give far more control than simple polarity switching. By adjusting wave frequency and utilizing pulsed AC, the peak and background currents can be tailored to match the puddle to the work at hand.

Higher frequencies constrict the arc into a tighter column, keeping the puddle small and fluid. Lower frequencies spread the arc out, widening the puddle with more penetration. For thin magnesium, the high frequency, tight puddle is often the only way to avoid burn through. For castings with uneven section thickness, pulsing becomes invaluable by applying more heat to cold sections during peak current, and less to already hot sections during the background. For a breakdown of these features, Miller’s guidance on AC TIG inverter features is a solid reference.

Backing and Trailing Shields

Hot magnesium weld metal oxidizes extremely fast as it cools. Without coverage behind the torch, the reinforcement and root are vulnerable to a white, powdery oxide. To combat this, a few setups are worth the extra time:

• Trailing shield: A hood that follows the torch to blanket the cooling bead with argon, ideal for cosmetic or corrosion critical work.
• Copper backing strip: A chilled copper bar clamped under thin sheet to pull heat out of the puddle. When properly insulated, it greatly reduces burn through and produces a cleaner root.
• Purge backing: A backing bar with an argon port to shield the underside of the root, preventing oxidation there entirely.

Repairing Cracked Die Castings

Not every crack is worth welding. Before striking an arc, assess the job with three points in mind: location, contamination, and value. A crack through a load bearing boss is far harder to contain than one on a non structural face. Any coatings, platings, or sealants must be stripped to bare metal in and around the weld area. Most importantly, ask if the part is worth the repair to begin with.

Many magnesium die castings are not weldable without risk. Their cast microstructure, segregation, and residual stresses don’t always play well with added heat and restraint. As outlined by EWI’s technical paper on arc welding of aluminum and magnesium, filler selection and tight heat input control are critical for cast to cast joints. On top of this, AZ91 loses creep resistance as service temperatures climb into the low hundreds of degrees Celsius, meaning a repaired part may not regain its original properties.

In most cases, for badly cracked AZ91 castings, replacement is the most realistic option. Only consider repair for expensive parts with a non structural crack, a qualified procedure, and a means of inspection.

Dissimilar Metal Repair and Transition Inserts

Welding magnesium directly to steel or aluminum is not a viable fix. Vast differences in melting points, thermal expansion, and galvanic potential make it prone to cracking and accelerated corrosion. The correct route is a transition insert, most often aluminum, mechanically fastened to the magnesium and joined to the steel side. If isolation is required in a wet environment, seal the joint. For work on the aluminum side of an insert, aluminum TIG and MIG fundamentals are applicable, however magnesium’s narrow thermal window will always dictate the assembly’s heat input.

Inspection and Qualification

Inspection should always build upon itself. Start with visual inspection, followed by dye penetrant for surface cracks on clean, uncoated metal. For code required work, radiography or ultrasonic testing may be needed. Above all, macro-etch a test coupon. Cutting and etching a coupon is the cheapest way to verify penetration, fusion, and the absence of lack of fusion before touching the actual part.

For documentation, AWS B2.1 and ASME Section IX provide the framework to qualify a GTAW procedure, and AWS A5.19 governs magnesium filler metals. For aerospace or pressure work, follow the governing code as aerospace TIG welding standards and training requirements will be well above shop practice alone.

Your Pre-Weld Checklist and Final Take

Run the following every time:

• Alloy identified and filler matched to it.
• Joint clean and dry, brushed with a dedicated magnesium stainless brush and degreased.
• AC balance set for the alloy and thickness, confirmed on a coupon.
• Argon verified for flow, purity, and leaks.
• Class D extinguisher within reach with fines cleared from the area and a fire watch assigned.
• Coupon welded and sectioned prior to the real part with all parameters recorded.

Magnesium TIG is at its best for thin to medium repairs, prototypes, and small housings where control outweighs cycle time. It is the wrong answer for high volume production or for badly cracked, high stress die castings, where replacement is often cheaper than verification. With clean metal, dry filler, verified argon, an accurate balance, and a fire plan, the most demanding magnesium repairs become calculated, not reckless.

Frequently Asked Questions

What polarity should I use for TIG welding magnesium?

Use alternating current (AC). The electrode-negative half of the cycle drives penetration, while the electrode-positive half breaks up the surface oxide through cathodic cleaning. This is the same reason aluminum is normally AC welded, and it is why magnesium should not be run on straight DC with a modern inverter.

What filler rod do I use for AZ31B magnesium?

AZ61A or AZ92A rod, per AWS A5.19, with AZ92A preferred on castings, restrained joints, and anywhere crack resistance matters. AZ61A is cheaper but documented as more subject to weld cracking. Never use an aluminum filler rod on magnesium.

What shielding gas and flow rate do I need for magnesium TIG?

Use 100 percent argon. Published starting points are roughly 13 CFH for 1/16 in, 19 CFH for 1/8 in, 25 CFH for 1/4 in, and 35 CFH for 1/2 in material at 15 psi. Add argon/helium blends only for heavier sections, and remember that magnesium wants a positive pressure of argon behind the weld on the root side.

Why does my magnesium weld have white powdery residue on it?

That is magnesium oxide, which forms when the weld is not properly shielded. Common causes are insufficient argon flow, a worn or too-small gas cup, too long an arc, contaminated gas lines, and insufficient electrode-positive cleaning on the AC cycle. Shorten the arc, check the cup and flow, and verify the balance setting.

Can I use water to put out a magnesium fire?

No. Water reacts violently with hot magnesium, releasing hydrogen, and can cause a steam or hydrogen explosion. OSHA states that water intensifies the fire by supplying oxygen and generating hydrogen. Use a NRTL-listed Class D extinguisher rated for magnesium, or cover the fire with dry sand or magnesium foundry flux.

Why do my magnesium welds crack but the same filler works on aluminum?

Magnesium alloys have a narrow solidification range and a high coefficient of thermal expansion, so restraint and heat input produce solidification cracking. Use a crack-resistant filler such as AZ92A, add uniform preheat for thick or cast sections, use a wider groove to reduce filler volume, improve fit-up, and reduce restraint during cooling.

Is it safe to grind or machine magnesium in a workshop?

It can be, but only with discipline. Magnesium chips, dust, and fines are flammable and ignite readily from sparks. Grind magnesium on a dedicated setup, keep fines from accumulating, store them in a covered metal container, and never let steel sparks fall into magnesium dust. Clean the area before any hot work.

Should I TIG weld magnesium or use MIG for my job?

TIG is the better choice for low-volume work, repairs, thin to medium sheet, and any joint where control of filler addition matters. GMAW in spray or pulsed-spray transfer wins on production volume and thicker sections, but few power sources ship with magnesium spray programs, so those jobs often need custom setups. Weld a coupon first either way.

Can a cracked magnesium die casting be repaired by welding?

Sometimes, but be cautious. Cast magnesium is more crack sensitive than wrought grades, and die castings can change properties locally, so repairs need uniform preheat, a crack-resistant filler such as AZ92A or EZ33A, and a qualified procedure. For structural or safety-critical parts, weigh the repair against replacement, and check what the design authority and governing code allow.

What causes porosity in magnesium TIG welds?

Moisture is the usual culprit, whether on the joint, on the filler rod, or condensed in the gas lines. Hydrated oxides, inadequate shielding, and contamination from improper cleaning also contribute, because magnesium has high hydrogen solubility in the molten state and low solubility in the solid state. Clean and dry everything, bake the filler, purge the gas lines, and confirm shielding coverage.