Tech tips for welding magnesium — Part 2: Shielding and filler requirementsShawn Martin | December 30, 2019
This article is Part 2 of Welding Digest's two-part series on magnesium welding. Read Part 1 on pre-welding preparations here.
With proper setup, magnesium alloys exhibit favorable weldability. Joints should be clean and free of any oxides, oils or foreign debris, and thicker workpieces will typically require preheating to help alleviate thermal stress during welding operations. Gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) are the two most common arc welding processes for welding magnesium and should be completed with well-controlled, low power input. Laser beam welding has also emerged as a viable method for joining magnesium workpieces. In all cases, gas shielding is required as fluxed electrodes alone do not provide adequate shielding.
GTAW direct current with electrode positive (DCEP) is the most common arc welding technique for joining magnesium alloys, although material thickness in excess of 3/16 in may require an alternating current setup or may be easier to accomplish with GMAW. The following recommendations improve weld quality and are typically defined by welding procedures for each specific alloy and process.
Magnesium has high oxidation potential. This quality is part of the reason why joint setup and cleaning methods are paramount to magnesium welding procedures. It is also the reason why an externally introduced inert gas shield is required to protect the weld pool.
Shielding gases used for welding magnesium include argon, helium and argon-helium mixtures. Argon is most commonly used for arc welds. It has a low ionization potential, facilitating arc ignition and arc stability. It also supports shallow penetration, which aids in limiting heat input. A flux may also be introduced to improve penetration when welding thicker geometries. In other cases a mixture of 75% helium plus 25% argon is used for thicker materials.
Pure helium is not recommended as it introduces excess heat input to the workpiece. However, helium is preferred for laser beam welding techniques. Laser beam welding limits the heat affected zone, and helium shielding has shown to provide the best surface roughness, penetration depth and seam shape for these applications, although flow rates are generally double the recommendation for argon, as helium is a lighter gas that tends to dissipate.
Filler metals for magnesium alloys
Alloying elements present in magnesium alloys either improve or decrease weldability. Alloys with higher concentrations of aluminum up to 10% are easier to weld, while higher zinc concentrations in the range of 5 to 6% increase hot shortness and sensitivity to weld cracks. Nomenclature for magnesium alloy die castings is defined by ASTM B94 where the first two letters designate alloying elements present followed by alloying percentages rounded off to whole numbers. The alloying element listed first is present in higher concentrations. The most common alloying elements include the following: A – aluminum; E – rare earths; H – thorium; K – zirconium; L – lithium; M – manganese; Q – silver; S – silicon and Z – zinc. For example, AZ101 contains 10% aluminum and up to 1.25% zinc.
While autogenous GTAW processes may be performed on magnesium, filler metals introduce alloying elements to the weld pool that improve joint strength and durability. AZ61A, AZ101A, AZ92A and EZ33A are the most common filler alloys for joining magnesium and should be selected to match base metal composition.
AZ61A is recommended for welding aluminum-containing wrought products. AZ92A decreases crack sensitivity when joining cast magnesium-aluminum-zinc and magnesium-aluminum alloys. EZ33A is preferred for joining high temperature alloys. In cases where welding procedures are not defined, AZ101 magnesium filler rod is most common and is also used as an alternative to AZ61A and AZ92A. It is forgiving when welding magnesium as it enriches the weld pool in aluminum without introducing large concentrations of zinc that could increase the possibility for weld cracks. Ultimately, electrode wire or filler metal is governed by the composition of the base metals.
The weldability of magnesium alloys is dependent on alloy constituents. Alloys with lower zinc concentrations and with higher aluminum concentrations are easier to weld. Its high oxidation potential, low melting temperature and high thermal expansion coefficient introduce nuances that can be addressed by selecting the proper welding technique, shielding gas and filler alloy to produce high-strength, durable joints.