MAGNESIUM ALLOYS
General
Magnesium and its alloys are used in the automotive, aerospace and computer industries, and in the manufacture of apparatus for which light weight is a major factor like mechanical saws, mowers and certain hand tools.
Magnesium is used for its light weight, its density is one quarter that of steel and two thirds that of aluminum. Moreover it has good atmospheric corrosion resistance and high mechanical strength.
Magnesium alloys are weldable using the GTAW, GMAW and even OFW processes with the help of Sodel 0681 flux. Some alloys weld better than others, but an acceptable weld can be made with the majority of these alloys.
Classification
The magnesium alloy designation system was set up in 1948 by ASTM (American Society for Testing and Materials). The alloy is classified according to its main alloy elements and their approximate composition.

Example : AZ61A – F
A and Z : indicates that the alloy contains aluminum and zinc
61 : approximately 6% aluminum and 1 % zinc in the alloy.
A : specify the proportion of secondary elements : 0.015% min manganese, 0.05% max copper and indicate that the alloy is in its fabricated state.
F : indicates that the alloy is in as fabricated state.

Weldability of magnesium alloys
Surface preparation
To minimize weld defects, all trace of oil, hydrocarbons, or contaminants must be removed, as well as the oxide layer coating the alloy. This layer can be removed using a grinder, a stainless steel brush, stainless steel wool, machining, or by using an appropriate chemical solution. In addition, for flame welding, Sodel 0681 flux should be applied just prior to starting the weld.
Preheating
The melting point of magnesium alloys is between 600 and 650°C (1110 and 1200°F). The degree of preheating is determined by the thickness of the alloy and the amount of restraint. For very thick pieces subject to low restraint, preheating is often not required. Thin and greatly restrained pieces have a greater tendency toward cracking, particularly with high zinc content. Maximum preheat temperatures vary according to the alloy usually between 500 and 750°F (260 and 400°C). Postheating is sometimes required after welding to reduce residual stresses.
Welding method and processes
The GMAW process is often used to weld very thick pieces of magnesium. However, the GTAW process provides a definite advantage for thin pieces or when high quality welds are required, because it provides better heat input control. As a last resort, when it is not possible to use an electric arc process, some pieces can be assembled using the OFW process.
In GTAW, an AC welding current with high frequency is preferred to stabilize the arc. DC welding current with a positive electrode (DC+) can also be used if the pieces are thin. For pieces with thicknesses greater than 3/16 inch (5 mm), AC will provide better penetration.
Argon is the most frequently used shielding gas. Argon-helium mixtures can also be used. Helium provides good penetration for thick pieces, but pure helium is not recommended because it leads to a lot of spatters during the welding process. Also helium is a very light gas, and three times as much helium is needed compared to argon to obtain the same shielding.
To minimize cracking when welding inside a groove, the weld can be started and stopped on extension pieces (run-in or run-out tabls) that can be removed after welding. For fillet welds, you can start in the centre of the joint and work toward one end; then re-strike in the centre on top of the bead you just finished and work toward the other end (magnesium 1 diagram).
To repair cracks, the edges must be grooved with a grinded. The crack should be welded starting at the end closest to the centre of the piece and moving toward the other end.

PRACTICAL TIPS FOR WELDING MAGNESIUM ALLOYS
1.The GTAW process is the most preferable for making quality welds.
2.Using a foot or hand control for GTAW welding helps prevent overheating that may cause burn through.
3.The cracking tendency increases as the proportion of zinc increases.
4.Preheat pieces (and fixtures, when used) to between 200 and 300°F (95 and 150°C) to minimize cracking.
5.If you preheat the pieces in an oven, use a protective atmosphere to reduce the likelihood of oxidation at temperatures above 700°F (370°C).
6.If the difference in thickness between the pieces is greater than 1/4 inch (6.3 mm), preheat the thicker piece to around 300°F (150°C).
7.Excessively high welding currents can cause weld cracking or hot-cracking in the thermally affected area.
8.Excessively low welding currents can cause oxide weld contamination or porosity.
9.If start-stop cycles are required, you can re-strike on the bead you just laid down at about 1/2 inch (13 mm) from the end to minimize cracking.
10.Use a grinder to remove cracks prior to welding.