Magnesium alloys welded with the GTAW process use techniques and equipment similar to those used for aluminum. Argon, helium, and mixtures of these can be used. The factors governing the shielding gas for magnesium alloys are the same as for other metals, particularly aluminum. Magnesium alloys may be welded with either AC or DC, but AC is generally preferred because of the good arc cleaning action. Conventional AC power of 60 Hz with arc stabilization or square wave alternating current may be used. With SWAC, the electrode positive and negative periods are adjustable within limits. This type of power can provide adequate cleaning action as well as good penetration and arc stability. DCEN-RP provides an arc with excellent cleaning action, and is used for thin sections because the welding current is limited by heating of the tungsten electrode, and penetration tends to be wide and shallow. DCEN-SP is not commonly used on magnesium alloys because of the absence of arc cleaning action, but is sometimes used for mechanized welding of square groove butt joints in sections up to 1/4" thick, with careful attention to preweld cleaning and good fitup. DCEN-SP power with helium shielding can produce narrow, deep joint penetration. Pure tungsten (EWP), tungsten-thoria (EWTh-1 or -2), and tungsten-zirconia (EWZr) electrodes can be used with magnesium alloys. The selection depends primarily on the type of welding power and the amperage to be used. It's too much to get into here, but I have a chart that shows thicknesses, joint designs, number of passes, welding currents, electrode diameter, welding rod diameters, etc. for GTAW of magnesium alloys. I can fax this to you or try to scan it and e-mail it to you if you're interested. Incidently, if it is a magnesium alloy casting, I've read that castings that may contain oil in the pores should not be welded. If this is not a concern, welding should start at the center of the joint and progress towards the ends using medium sized beads. The proper current is critical because a low welding current may cause cold laps, oxide contamination, or porous welds, and a high welding current may cause weld cracking or incipient melting in the heat affected zone.
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