Understanding Magnesium's Unique Properties
Magnesium is the lightest structural metal, with a density only two-thirds that of aluminum and one-quarter that of steel. This exceptional lightness makes magnesium alloys attractive for aerospace, automotive, and electronics applications where weight reduction is critical. However, magnesium's unique properties—including high thermal conductivity, low melting point, and high chemical reactivity—create significant welding challenges that require specialized techniques and precautions.
Magnesium alloys typically contain aluminum, zinc, manganese, and rare earth elements to improve strength, castability, and corrosion resistance. Common wrought alloys include AZ31B (3% aluminum, 1% zinc, 0.2% manganese) and AZ61A (6% aluminum, 1% zinc). These alloys can be welded successfully with proper procedures, though their weldability is generally considered fair to good rather than excellent.
The primary welding challenges for magnesium include its low melting point (1202°F, 650°C) combined with high thermal conductivity, which makes burn-through easy on thin sections. The metal's high chemical reactivity requires excellent shielding to prevent oxidation. Additionally, magnesium's hexagonal close-packed crystal structure provides limited slip systems at room temperature, making it susceptible to cracking under stress.
Safety Considerations for Magnesium Welding
Fire and Explosion Hazards
Magnesium presents significant fire hazards when melted or machined. Molten magnesium can ignite in air if overheated, burning with an intense white flame that reaches temperatures over 5000°F. Magnesium fires cannot be extinguished with water—water reacts violently with burning magnesium, producing hydrogen gas that can explode.
Preventing magnesium fires is essential. Never exceed recommended welding temperatures, and ensure adequate shielding gas coverage to prevent oxidation. Keep the welding area clear of combustible materials, and have appropriate fire extinguishing materials readily available.
If magnesium does ignite, use Class D dry powder fire extinguishers specifically designed for metal fires. Dry sand or foundry flux can also smother magnesium fires. Never use water, CO2, or halon extinguishers on magnesium fires—these can intensify the fire or cause explosions.
Fume Hazards
Welding magnesium produces fumes that can cause metal fume fever, a flu-like condition characterized by fever, chills, and muscle aches. While typically not life-threatening, metal fume fever is unpleasant and indicates inadequate ventilation.
Adequate ventilation is essential for magnesium welding. Local exhaust ventilation capturing fumes at the source is most effective. General area ventilation may be insufficient for production welding. Respiratory protection may be required if ventilation cannot control fume levels adequately.
Handling Precautions
Magnesium alloys should be stored in dry conditions to prevent corrosion. Moisture on magnesium can cause problems during welding, including porosity and hydrogen embrittlement. Preheating can help drive off moisture before welding.
Clean magnesium carefully before welding. Abrasive cleaning can generate fine magnesium dust that presents fire hazards. Use non-flammable cleaning methods, and collect any dust or chips in closed containers. Never use steel wool on magnesium—it can cause galvanic corrosion and contamination.
Shielding Gas Requirements
Argon Shielding
Argon is the preferred shielding gas for magnesium MIG welding, providing excellent coverage and arc stability. Argon's density (heavier than air) helps blanket the weld pool and prevent atmospheric contamination. High-purity argon (99.995% or better) should be used for critical applications.
Flow rates for magnesium welding are typically higher than for aluminum—35-50 CFH is common. The higher flow rates ensure adequate coverage and help sweep away any air that might enter the shielding zone. Gas lenses in the torch can improve coverage and allow lower flow rates.
Argon-Helium Mixtures
Argon-helium mixtures increase heat input for thicker magnesium sections. Helium's higher ionization potential creates a hotter arc that can improve fusion on thick materials. Mixtures of 25-50% helium with argon are used for sections over 1/4" thick.
Pure helium is rarely used for magnesium welding due to cost and arc starting difficulties. The benefits of helium additions must be weighed against the additional expense for each application. For most magnesium welding, pure argon is adequate.
Back Purging Requirements
Back purging protects the root side of magnesium welds from oxidation. Even if the root won't be visible in service, oxidation embrittles the weld and reduces corrosion resistance. Back purging is recommended for all but the thinnest magnesium welds.
Purging methods include removable backing bars with gas channels, ceramic backing tape with purge capability, or sealed enclosures for small parts. The goal is to displace air from the back side with inert gas before welding begins.
Filler Metal Selection
Matching Composition Fillers
Matching composition fillers are preferred for magnesium welding to ensure consistent properties. AZ61A (ER AZ61A) filler matches AZ31B base metal well and provides good crack resistance. AZ92A (ER AZ92A) filler contains more aluminum and zinc, providing higher strength but slightly reduced ductility.
Filler metal diameter selection affects welding characteristics. Smaller diameters (0.030-0.045") provide better control for thin materials. Larger diameters (1/16") increase deposition rates for thicker sections. Most magnesium MIG welding uses 0.045" or 1/16" wire.
Magnesium filler wire must be clean and dry. Store in original packaging until use, and handle with clean gloves. Moisture or oxide on filler wire causes porosity and contamination.
Alternative Fillers
For some applications, EZ33A filler (3% rare earths, 3% zinc, 0.7% zirconium) provides improved elevated temperature properties. This filler is used for applications requiring service above 200°F where standard fillers would soften.
When welding dissimilar magnesium alloys, select a filler compatible with both base metals. AZ61A is generally compatible with most common magnesium alloys and provides good crack resistance.
Equipment for Magnesium MIG Welding
Power Sources
Standard constant voltage DC power sources work for magnesium MIG welding. Pulsed MIG capability is beneficial for controlling heat input and reducing burn-through on thin materials. Modern synergic pulsed systems simplify parameter selection.
Use DC electrode positive (DCEP) polarity for magnesium MIG welding. The positive electrode provides good cleaning action and stable arc characteristics. Some applications may use AC for special effects, but DCEP is standard.
Wire Feeders and Guns
Magnesium wire is softer than aluminum and requires careful feeding to prevent buckling and bird-nesting. Push-pull systems or spool guns are recommended, especially for soft wires and long cables. These systems have drive rolls at both ends to ensure positive feeding.
Teflon or nylon liners help reduce feeding friction for soft magnesium wire. Steel liners can be used but must be clean and properly sized. Liner diameter should match wire diameter closely—oversized liners allow buckling.
Gun contact tips must be properly sized and in good condition. Worn tips cause arc instability and wire feeding problems. Use tips specifically sized for the wire diameter being used.
Trailing Shields
Trailing shields are beneficial for magnesium welding, extending shielding gas coverage behind the torch as it moves. The hot magnesium behind the torch oxidizes quickly without continued shielding. Trailing shields are particularly important for thick sections that cool slowly.
Simple trailing shields can be fabricated from copper or stainless steel with argon distribution channels. Commercial shields are also available. Gas flow through trailing shields is separate from torch shielding, typically 20-30 CFH.
Welding Techniques for Magnesium
Parameter Selection
Magnesium MIG welding parameters are similar to aluminum for equivalent thicknesses. Typical parameters for 0.045" wire:
- Thin sheet (0.040-0.063"): 80-120 amps, 20-24 volts
- Medium thickness (1/8-1/4"): 120-180 amps, 23-27 volts
- Thick sections (over 1/4"): 180-250 amps, 26-30 volts
Pulsed parameters use similar average values with pulse frequency of 100-200 Hz. Pulsed MIG helps control heat input and reduces burn-through on thin materials.
Travel speed should be consistent and appropriate for the material thickness. Too slow causes burn-through or excessive buildup; too fast causes lack of fusion. Watch the pool and adjust speed to maintain consistent penetration.
Gun Angle and Technique
Gun angle affects shielding effectiveness and penetration. A slight push angle (5-15 degrees forward) helps the shielding gas flow ahead of the weld pool. Excessive push angle reduces penetration and may cause lack of fusion.
Stringer beads are preferred for magnesium welding. Weaving increases heat input and can cause excessive buildup or burn-through. If weave patterns are necessary for wide joints, keep them narrow (under 3/8").
Maintain consistent stick-out of 3/4" to 1" for magnesium welding. Longer stick-out increases resistance heating and can improve deposition but may reduce arc stability. Shorter stick-out provides more stable arc characteristics.
Joint Preparation
Joint design for magnesium should provide adequate access for the welding torch while minimizing the amount of welding required. Butt joints in thin material can be welded square-grooved with no gap to 1/16" gap. Thicker material benefits from beveled joints (30-45 degree bevels).
Clean joints thoroughly before welding. Remove all oxides, oils, and contaminants back at least 1" from the weld line. Stainless steel brushes dedicated to magnesium are effective for oxide removal. Solvent cleaning removes oils and greases.
Clamping and fixturing should minimize restraint while maintaining alignment. Magnesium's limited ductility makes it susceptible to cracking from welding stresses. Allow some movement during welding to relieve stresses.
Common Magnesium Welding Problems
Porosity
Porosity is the most common defect in magnesium welds. Sources include moisture, oxides, contaminated filler wire, and inadequate shielding. Prevention requires meticulous cleanliness and proper shielding.
Ensure base metal and filler are clean and dry. Preheating can help drive off moisture. Verify adequate shielding gas flow and coverage—magnesium requires more gas than steel. Check for gas leaks that could aspirate air.
If porosity persists, check shielding gas purity. Moisture in gas lines from condensation is a common cause of porosity. Purge lines thoroughly and consider using gas dryers for critical applications.
Burn-Through
Burn-through occurs easily in magnesium due to its low melting point and high thermal conductivity. Thin sections are particularly susceptible. Prevention requires careful parameter control and technique.
Reduce parameters if burn-through occurs. Increase travel speed to reduce heat input per unit length. Use pulsed MIG to provide cooling periods between pulses. Backing bars can support the root side and prevent drop-through.
Fit-up affects burn-through tendency. Excessive root opening allows the arc to impinge directly on the back side. Maintain consistent, appropriate gaps for the material thickness.
Cracking
Magnesium alloys are susceptible to hot cracking, particularly those with higher aluminum content. Cracking results from low-melting constituents segregating to grain boundaries during solidification.
Use crack-resistant fillers like AZ61A rather than higher-aluminum compositions. Control heat input to minimize grain size and segregation. Minimize restraint through proper joint design and fixturing.
If cracking occurs in highly restrained joints, consider stress relief heat treatment after welding. Heating to 300-400°F and holding for several hours can reduce residual stresses.
Applications for Magnesium Welding
Aerospace Components
Aerospace applications use magnesium for aircraft gearboxes, transmission housings, and structural components where weight savings are critical. Welds in aerospace applications must meet stringent quality requirements.
Most aerospace magnesium welding uses TIG welding for maximum control, but MIG welding finds use in thicker sections and production applications. Procedures must be qualified to aerospace standards with extensive documentation.
Repair welding of magnesium aircraft components requires special procedures to ensure that repairs don't compromise airworthiness. Heat treatment after welding may be required to restore mechanical properties.
Automotive Applications
Automotive applications include transmission cases, steering components, and structural elements. Magnesium's weight reduction benefits fuel efficiency, driving increased automotive use.
Production welding of automotive magnesium components often uses automated MIG welding for consistency. Robotic welding cells provide high-quality welds at production rates.
Repair of magnesium automotive components requires identification of the specific alloy and appropriate welding procedures. Many automotive magnesium parts are die castings with different welding characteristics than wrought alloys.
Electronics and Consumer Products
Magnesium's light weight and EMI shielding properties make it popular for laptop computers, cameras, and other portable electronics. These applications involve thin sections requiring careful welding technique.
MIG welding of thin magnesium sheet requires precise parameter control to prevent burn-through. Pulsed MIG is particularly valuable for these applications, providing good penetration with controlled heat input.
Cosmetic appearance is important for consumer products. Welds must be clean and consistent, often requiring post-weld finishing to achieve desired appearance.
Conclusion
MIG welding magnesium alloys requires understanding the material's unique properties and challenges. The combination of low melting point, high thermal conductivity, and chemical reactivity demands specialized techniques and precautions. However, with proper equipment, procedures, and safety measures, quality magnesium welds can be produced consistently.
For fabricators serving aerospace, automotive, and electronics industries, magnesium welding capability provides access to high-value markets where weight reduction is critical. The investment in equipment and training pays dividends through expanded capabilities and new business opportunities.
Whether you're welding aerospace gearboxes, automotive components, or consumer electronics, the principles in this guide provide a foundation for successful magnesium MIG welding. Respect the material's unique characteristics, maintain proper safety precautions, and follow proven procedures to produce magnesium welds that meet demanding requirements.
