Welding magnesium is performed for primary manufacturing or repair.
Magnesium alloys with a density of about 1.74 g per cubic centimeter (0.063 lb. per cu in.), when in cast form alloyed with Aluminum, Manganese, Rare Earths, Thorium, Zinc or Zirconium, display high strength to weight ratio making them materials of choice whenever weight reduction is important or when it is imperative to reduce inertial forces (for rapidly moving machine parts). Magnesium is roughly 20% the weight of steel and 67% the weight of aluminum. Magnesium castings exhibit remarkable damping capacity.
Safety precautions must be understood and followed. Magnesium oxidizes easily. If ignited when it is in the form of machined turnings or powders, it burns intensely. Machining must be performed under controlled conditions, with extinguishing agents on hand.
Welding magnesium alloys require lower amounts of heat to melt than other materials. However, they are susceptible to distortion, due to the high thermal conductivity and coefficient of thermal expansion. Adequate precautions must be taken.
Because magnesium is too mechanically weak to be used as is, it must be alloyed with other elements which confer improved properties. The Mg-Al-Zn group of alloys, contains Aluminum, Manganese and Zinc, which are the most common alloying elements for room temperature applications. Alloying elements Thorium, Cerium and Zirconium (without Aluminum) are used for elevated temperature, forming the Mg-Zn-Zr group.
An increase in alloy content depresses the melting point, enlarges melting range and increases weld cracking tendency. High alloy content needs less heat for melting and limits grain growth, showing higher welding magnesium efficiency.
Welding magnesium is generally performed with arc processes using direct current with reverse polarity (electrode positive). Wrought alloys are usually more weldable than certain cast alloys.
Metal Transfer Modes for Gas Metal Arc Welding magnesium (GMAW) or Metal Inert Gas (MIG)
Gas Tungsten Arc for Welding magnesium (GTAW) also known as Tungsten Inert Gas (TIG)
Electron Beam welding magnesium has been used for repairing expensive casting on alloys containing less than 1% Zinc. The relative weldability of the different magnesium alloys is similar to that displayed for the more common arc processes.
The conditions have to be strictly monitored because of the danger of developing voids and porosity due to the low boiling point of Magnesium and the still lower one of Zinc. A slightly defocused beam may help to obtain sound welds.
Laser Beam is a preferred method for welding magnesium because of its low heat input, elevated speed and limited deformation. However this method has a tendency of developing porosity.
Resistance welding magnesium for either spots or seams is performed on wrought alloys like sheets and extrusions, essentially with equipment and conditions similar to those used for aluminum.
Repairing Castings: One of the most common Welding magnesium applications is repairing castings either as cast or after service. Preparation is important and should exclude contamination from extraneous materials. Generous bevels should be prepared to allow for full penetration.
Preheating: The need for preheating when welding magnesium is dictated by the degree of joint restraint and by metal thickness: for thick walls and a short welding bead, it may not be required. Preheating should be performed in a furnace with a protective atmosphere for reducing oxidation. One of the recommended procedures to minimize weld cracking is to weld from the center towards the sides (one half after the other). Thermal shocks should be avoided.