Magnesium alloys are used in a wide variety of applications where light weight is important. Structural applications include industrial, materials-handling, commercial, and aerospace equipment. In industrial machinery, such as textile and printing machines, magnesium alloys are used for parts that operate at high speeds and must be lightweight to minimize inertial forces. Materials-handling equipment examples are dock boards, grain shovels, and gravity conveyors; commercial applications include such items as luggage and ladders. Good strength and rigidity at both room and elevated temperatures, combined with light weight, make magnesium alloys useful for some aerospace applications.

Alloy Systems

Most magnesium alloys are ternary types. They may be considered in four groups based on the major alloying element: aluminum, zinc, thorium, or rare earths. There are also binary systems employing manganese and zirconium. Magnesium alloys may also be

grouped according to service temperature. The magnesium-aluminum and magnesium-zinc alloy groups are suitable only for room-temperature service. Their tensile and creep properties decrease rapidly when the service temperature is above about 150°C (300°F).

The magnesium-thorium and magnesium-rare earth alloys are designed for elevated-temperature service. They have good tensile and creep properties up to

370°C (700°F). Designation Method. Magnesium alloys are designated by a combination letter-number system composed of four parts. Part 1 indicates the two principal

alloying elements by code letters arranged in order of decreasing percentage. The code letters are listed in Table M- 1.

Part 2 indicates the percentages of the two principal alloying elements in the same order as the code letters. The percentages are rounded to the nearest whole number. Part 3 is an assigned letter to distinguish different alloys with the same percentages of the two

principal alloying elements. Part 4 indicates the condition of temper of the product. It consists of a letter and number similar to those used for aluminum, as shown in Table M-2. They are separated from Part 3 by a hyphen.

An example is alloy AZ63A-T6. The prefix AZ indicates that aluminum and zinc are the two principal alloying elements. The numbers 6 and 3 indicate that the alloy contains nominally 6% aluminum and 3% zinc. The following A indicates that this is the first standardized alloy of this composition. The fourth part, T6, states that the product has been solution heat-treated and artificially aged.

Commercial Alloys. Magnesium alloys are produced in the form of castings and wrought products including forgings, sheet, plate, and extrusions. A majority of the alloys produced in these forms can be welded. Commercial magnesium alloys are designed for either room-temperature or elevated-temperature service. Some of the more important magnesium alloys for room temperature service are listed in Table M-3. Those for elevated temperature service are listed in Table M-4.

Wrought Alloys. Welded construction for room-temperature service is frequently designed with AZ3 1B alloy. It offers a good combination of strength, ductility, toughness, malleability, and weldability in all wrought product forms. The alloy is strengthened by

work hardening. AZ80A and ZK60A alloys can be artificially aged to develop good strength properties for room temperature applications.

Weldments made with AZlOA, MIA, and ZK21A alloy are not sensitive to stress-corrosion cracking, so postweld stress relieving is not required for weldments made of these alloys. They are strengthened by work hardening for room-temperature service. HK31A, HM21A, and HM31A alloys are designed for elevated-temperature service. They are strengthened by a combination of work hardening followed by artificial

aging.

Cast Alloys. The most widely used casting alloys for room-temperature service are AZ9 1C and AZ92A. These alloys are more crack-sensitive than the wrought Mg-Al-Zn alloys with lower aluminum content. Consequently, they require preheating prior to fusion welding.

EZ33A alloy has good strength stability for elevated-temperature service and excellent pressure tightness. HK31A and HZ32A alloys are designed to operate at higher temperatures than is EZ33A. QH21A alloy has excellent strength properties up to 260°C

(500°F). All of these alloys require heat treatment to develop optimum properties. They have good welding characteristics.

Mechanical Properties. Typical strength properties at room temperature for magnesium alloys are given in Table M-5. For castings, the compressive yield strength is about the same as the tensile yield strength. However, the yield strength in compression for

wrought products is often lower than in tension.

The tensile and creep properties of representative magnesium alloys at a service temperature of 315°C (600°F) are given in Table M-6. The alloys containing thorium (HK, HM, and HZ) have greater resistance to creep at 3 15°C (600°F) than do the Mg-Al-Zn alloys.

Major Alloying Elements. With most magnesium alloy systems, the solidification range increases as the alloy addition increases. This contributes to a greater tendency for cracking during welding. At the same time, the melting temperature as well as the thermal conductivity and electrical conductivity decrease. Consequently, less heat input is required for fusion welding as the alloy content increases.

Aluminum and zinc show decreasing solubility in solid magnesium with decreasing temperature. These elements will form compounds with magnesium. Consequently, alloys containing sufficient amounts of aluminum and zinc can be strengthened by a precipitation-hardening heat treatment. Other alloying elements also behave similarly in ternary alloy systems. Beryllium, manganese, silver, thorium and zirconium are major alloying elements in magnesium alloys.

Weldability. The relative weldability of magnesium alloys by gas shielded arc and resistance spot welding processes is shown in Table M-7. Castings are not normally resistance welded. The Mg-Al-Zn alloys and alloys that contain rare earths or thorium as the major alloying element have the best weldability. Alloys with zinc as the major alloying element are more difficult to weld. They have a rather wide melting range,

which makes them sensitive to hot cracking. With proper joint design and welding conditions, joint efficiencies will range from 60 to 10096, depending on the alloy and temper.

Most wrought alloys can be readily resistance spot welded. Due to short weld cycles and heat transfer characteristics, fusion zones are fine-grained, and heat-affected zones experience only slight degradation.

 

Arc Welding

Applicable Processes. The gas tungsten arc and gas metal arc welding processes are commonly used for joining magnesium alloy components. Inert gas shielding is required with these processes to avoid oxidation and entrapment of oxide in the weld metal. Processes that use a flux covering do not provide adequate oxidation protection for the molten weld pool and the adjacent base metal. Procedures for arc welding magnesium are similar to those used for welding aluminum.

Filler Metals. The weldability of most magnesium alloys is good when the correct filler metal is

employed. A filler metal with a lower melting point and a wider freezing range than the base metal will provide good weldability and minimize weld cracking. The recommended filler metals for various magnesium alloys are given in Table M-8.

Casting repairs should be made with a filler metal of the same composition as the base metal when good color match, minimum galvanic effects, or good response to heat treatment is required. For these unusual service requirements, the material supplier should be consulted for additional information.

Safe Practices. The welding fumes from all commercial magnesium alloys, except those containing thorium, are not harmful when the amount of fumes remains below the welding fume limit of 5 mg/m3. Welders should avoid inhalation of fumes from the thorium-containing alloys because of the presence of alpha radiation in the airborne particles. However, the concentration of thorium in the fumes is sufficiently low so that good ventilation or local exhaust systems will provide adequate protection. The radiation concern, however, is primarily responsible for the decline in use of the thorium-containing alloys. No external radiation hazard is involved in the handling of the thorium containing alloys.

The possibility of ignition when welding magnesium alloys in thicknesses greater than 0.25 mm (0.01 in.) is extremely remote. Magnesium alloy product forms will not ignite in air until they are at fusion temperature. Then, sustained burning will occur only

if the ignition temperature is maintained. Inert gas shielding during welding prevents ignition of the molten weld pool. Magnesium fires may occur with accumulations of grinding dust or machining chips. Accumulation of grinding dust on clothing should be

avoided. Graphite-based (G- 1) or proprietary salt-based powders recommended for extinguishing magnesium fires should be conveniently located in the work area. If large amount of fine particles, or fines,are produced, they should be collected in a waterwash-type dust collector designed for use with magnesium. Special precautions pertaining to the handling of wet magnesium fines must be followed.

The accumulation of magnesium dust in a water bath also can present a hazard. Dust of reactive metals like magnesium or aluminum can combine with the oxygen in the water molecule, leaving hydrogen gas trapped in a bubbly froth on top of the water. A heat source may cause this froth to explode.

The possibility of ignition when welding magnesium alloys in thicknesses greater than 0.25 mm (0.01 in.) is extremely remote. Magnesium alloy product forms will not ignite in air until they are at fusion temperature. Then, sustained burning will occur only

if the ignition temperature is maintained. Inert gas shielding during welding prevents ignition of the molten weld pool. Magnesium fires may occur with accumulations of grinding dust or machining chips. Accumulation of grinding dust on clothing should be

avoided. Graphite-based (G- 1) or proprietary salt-based powders recommended for extinguishing magnesium fires should be conveniently located in the work area. If large amount of fine particles, or fines,are produced, they should be collected in a waterwash-type dust collector designed for use with magnesium. Special precautions pertaining to the handling of wet magnesium fines must be followed.

The accumulation of magnesium dust in a water bath also can present a hazard. Dust of reactive metals like magnesium or aluminum can combine with the oxygen in the water molecule, leaving hydrogen gas trapped in a bubbly froth on top of the water. A heat source may cause this froth to explode.

 

 

 

 

 

 

 

 

 

 

 

 

 

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