Welding of Magnesium-base Alloys

Abstrakt:

Magnesium can be welded by many of the arc and resistance welding processes, as well as by the oxyfuel gas welding process, and it can be brazed.Magnesium like aluminum is produced with different tempers. These are based on heat treatment and work hardening. The strength of a weld joint is lowered in base metal, in the work-hardened condition, as a result of recrystalization and grain growth in the heat-affected zone. This effect is minimized with gas metal arc welding because of the higher welding speed utilized. This is not a factor in the base metals that are welded in the soft condition.

Magnesium alloys containing small amounts of aluminum, manganese, zinc, zirconium, etc., have strength equaling that of mild steels. They can be rolled into plate, shapes, and strip. Magnesium can be cast, forged, fabricated, and machined.

As a structural metal it is used in aircraft. It is used by the materials-moving industry for parts of machinery and for hand-power tools due to its strength to weight ratio. Magnesium can be welded by many of the arc and resistance welding processes, as well as by the oxyfuel gas welding process, and it can be brazed.

Magnesium like aluminum is produced with different tempers. These are based on heat treatment and work hardening. The strength of a weld joint is lowered in base metal, in the work-hardened condition, as a result of recrystalization and grain growth in the heat-affected zone. This effect is minimized with gas metal arc welding because of the higher welding speed utilized. This is not a factor in the base metals that are welded in the soft condition.

Magnesium possesses properties that make welding it different than the welding of steels. Many of these are the same as for aluminum. These are:

  1. Magnesium oxide surface coating.
  2. High thermal conductivity.
  3. Relatively high thermal expansion coefficient.
  4. Relatively low melting temperature.
  5. The absence of color change as temperature approaches the melting point.
The normal metallurgical factors that apply to other metals apply to magnesium as well.

Magnesium is a very active metal and the rate of oxidation increases as the temperature is increased. The melting point of magnesium is very close to that of aluminum, but the melting point of the oxide is very high. In view of this, the oxide coating must be removed.

Magnesium has high thermal heat conductivity and a high coefficient of thermal expansion. The thermal conductivity is not as high as aluminum but the coefficient of thermal expansion is very nearly the same. The absence of color change is not too important with respect to the arc welding processes.

The welds produced between similar alloys will develop the full strength of the base metals, however, the strength of the heat-affected zone may be reduced slightly. In all magnesium alloys the solidification range increases and the melting point and the thermal expansion decrease as the alloy content increases.

In the magnesium-aluminum-zinc alloys (AZ31B, AZ61A, AZ63A, AZ80A, AZ81A, AZ91 and AZ92A), aluminum content up to about 10% aids weldability by helping to refine the grain structure, while zinc content of more than 1% increases hot shortness, which may cause weld cracking.

The high zinc alloys (ZH62A, ZK51A, ZK60A and ZK61A) are not recommended for arc welding because they are highly susceptible to cracking and have poorer weldability. Magnesium, containing small amounts of thorium, possesses excellent welding qualities and freedom from cracking. Weldments of these alloys do not require stress relieving.

Certain magnesium alloys are subject to stress corrosion. Weldments subjected to corrosive attack over a period of time may crack adjacent to welds if the residual stresses are not removed. For weldments intended for this type of service stress relieving is required.

The gas tungsten arc welding process and the gas metal arc welding process are the two recommended processes for joining magnesium. Gas tungsten arc is recommended for thinner materials and gas metal arc is recommended for thicker materials, however, there is considerable overlap. The equipment for applying these processes has been previously described.

Filler Metals

The four most commonly used electrode wires for gas metal arc welding (GMAW) and filler metals (when used) for gas tungsten arc welding (GTAW) are ER AZ61A, ER AZ101A, ER AZ92A and ER EZ33A. The choice of electrode wire or filler metal is governed by the composition of the base metal.

Electrode wires or filler metals having composition conforming to ER AZ61A or ER AZ92A (Mg-Al-Zn) are considered satisfactory for welding alloys AZ10A, AZ31B, AZ31C, AZCOML, AZ61A, AZ80A, ZE10A and ZK21A to themselves or to each other. ER AZ61A is usually preferred for welding aluminum-containing wrought products because of tendency to resist crack sensitivity. The ER AZ92A filler metal shows less crack sensitivity for welding the cast magnesium-aluminum-zinc and magnesium-aluminum alloys.

The same electrode wires or filler metals are used for joining any one of the above alloys to high-temperature alloys HK31A, HM21A, and HM31A. However, when the high temperature alloys are joined to each other, ER EZ33A is recommended. Joints of wrought or cast alloys welded with ER EZ33A filler metal exhibit good mechanical properties at high temperatures.

The choice of electrode wire or filler metal for welding wrought alloys to cast alloys should be based on the recommendations outlined above, except that ER AZ101A may be used instead of ER AZ61A or ER AZ92A.

When aluminum-containing cast alloys are joined to aluminum-containing cast alloys, ER AZ101A or ER AZ92A electrode wire or filler metal is usually recommended. However, for joining HK31A and HZ32A to themselves or to each other, ER EZ33A is preferred, for joining HK31A and HZ32A to any of the other cast alloys, ER AZ101A is used. Filler rod of the same composition as the base metal should be used for most welds.

Gas Tungsten Arc Welding

All the precautions mentioned for welding aluminum should be observed. A short arc should be used and the torch should have a slight leading travel angle. The cold wire filler metal should be brought in as near to horizontal as possible (on flat work). The filler wire is added to the leading edge of the weld puddle.

High frequency current should be used for starting the direct current and arc with alternating current high frequency should be used continuously. Runoff tabs are recommended for welding any except the thinner materials. Uniform travel speed and weld beads are recommended.

The shielding gas is normally argon. However, a mixture of 75% helium plus 25% argon is used for thicker materials. For heavy thicknesses 100% helium can be used, more helium is required than argon to do the same job.

Gas Metal Arc Welding of Magnesium

The gas metal arc welding process is used for the medium to thicker sections. It is considerably faster than gas tungsten arc welding. Special high-speed gear ratios are usually required in the wire feeders since the magnesium electrode wire has an extremely high meltoff rate. The normal wire feeder and power supply used for aluminum welding will be suitable for welding magnesium.

The different types of arc transfer can be obtained when welding magnesium. This is primarily a matter of current level or current density and voltage setting. The short-circuiting transfer and the spray transfer should be used on material 3/16 inch and thicker and the short-circuiting arc used for thinner metals.

Other Welding Processes

The resistance welding processes can be used for welding magnesium, including spot welding, seam welding, and flash welding. Magnesium can also be joined by brazing. Most of the different brazing techniques can be used. In all cases, brazing flux is required and the flux residue must be completely removed from the finished part. Soldering is not too popular since the strength of the joint is relatively low.

Magnesium can be stud welded, gas welded, and plasma welded. Finely divided pieces of magnesium such as shavings, fillings, etc, should not be in the welding area since they will burn. Magnesium castings, or wrought materials do not create a safety hazard since the possibility of fire caused by welding on these sections is very remote. The producers of magnesium provide additional data for welding magnesium.

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