Fig. 1. Typical heat-treat processing oven (Photograph courtesy of Wisconsin Oven Corp.)


The world is becoming lighter, faster and more energy efficient. For all these reasons, and more, the spotlight is turning toward the use of magnesium and magnesium alloys, especially in the aerospace and automotive industry. We need to better understand these alloys and how to heat treat them. Let’s learn more.

Alloying

Magnesium is rarely used as an engineered material in its unalloyed form. Its hexagonal close-packed (HCP) lattice structure promotes alloying with many elements including aluminum, zinc, lithium, cerium, silver, zirconium and thorium. Magnesium alloys are available as cast and wrought products.

Techniques for producing wrought alloys include rolling (sheet, plate), extrusion and forging. They are designed with properties such as low-to-medium-to-high strength, weldability, corrosion and creep resistance, and ultra lightweight. Wrought magnesium alloys that can be strengthened by heat treatment are grouped according to composition with examples in parentheses.
  • Aluminum-manganese (LA141)
  • Aluminum-manganese-zinc (AZ31, AZ61, AZ80)
  • Manganese (M1)
  • Manganese-zinc (ZM21)
  • Thorium-zirconium (HK31)
  • Thorium-manganese (HM21)
  • Zinc-zirconium (ZK31, ZK61)
  • Zinc-zirconium-thorium (HZ11)
Various casting techniques (sand cast, chill cast) and temper conditions (e.g. T4, T5, T6) produce castings of various compositions having characteristics such as good room-temperature strength and ductility, good creep resistance and proof stress, and they are easily weldable. Magnesium-alloy castings that benefit from heat treatment are grouped based on the alloying elements they contain:
  • Aluminum-manganese
  • Aluminum-zinc
  • Aluminum-zinc-manganese (AZ63, AZ81, AZ91, AZ92)
  • Rare earth metal-zinc-zirconium (ZE41, ZE 63, EZ33)
  • Rare earth metal-silver-zirconium, with or without thorium (OE22, EQ 21, QH21)
  • Rare earth metal-yttrium-zirconium (WE43, WE54)
  • Thorium-zirconium, with or without zinc (HK31, HZ32)
  • Zinc-zirconium (ZK51, ZK61)


Types of Heat Treatment

The types of heat treatments applied to magnesium and its alloys are:
  • Annealing of wrought magnesium alloys is done to negate the effects of strain hardening or tempering.
  • Solution heat treatment yields proper mechanical properties such as increased strength and ductility.
  • Stress relief, as applied to castings, helps avoid warping and distortion in subsequent heat treatments and reduces the risk of stress-corrosion cracking (SCC) in welded components.
  • Stress relief of wrought alloys, though not common, is used to counteract stress induced by cold and hot working, shaping, forming, straightening and welding.
Specifications such as ASTM B661-06 (Standard Practice for the Heat Treatment of Magnesium Alloys) and AMS 2768 (Heat Treatment of Magnesium Alloy Castings) are often referenced for specific treatment details.

Solution Heat Treating

Solution heat treatment of magnesium alloys results in high tensile strength and maximum ductility. Magnesium alloys tend to reach temperature quickly due in part to their high thermal conductivity and low specific heat. In normal practice, the soak time begins when the furnace reaches set point.

The time at temperature is a function of the section thickness of the material and may vary from several minutes to many hours. A good “rule of thumb” is to double the time at solution heat-treatment temperature for thick-sectioned castings. For example, AZ63A castings normally require 12 hours at 725°F (385°C) but require 24+ hours when section thickness exceeds 2 inches (50 mm). Similarly, the suggested solution treating time for AZ92A castings is about 6 hours at 760°F (405°C) followed by 2 hours at around 660°F (350°C) and 10 hours again at 760°F (405°C) to prevent excessive grain growth. But for castings with sections more than 2 inches (50 mm) thick, it is recommended that the last soak be extended from 10 to 20 hours. Checking the part microstructure is by far the best way to determine whether or not additional solution treating time is required.

Normal oven requirements (Fig. 1) for temperature uniformity are ±10°F (±5.5°C), but certain alloys require tighter tolerances. The time required to heat a load to the treatment temperature also increases with section thickness and loading arrangement. Thus, the total cycle time must take into consideration these factors.

Certain alloys like magnesium-aluminum-zinc require precautions such as loading into the oven, which is at a preheat temperature typically around 500°F (260°C), and then slowly ramping to solution-treating temperature to avoid fusion of eutectic compounds and formation of voids. The time required to bring the load from 500°F (260°C) to the solution-treating temperature is determined by the size of the load and by the composition, size, weight and section thickness of the parts.

Other special cases apply. Alloys containing large percentages of thorium, rare-earth metals (yttrium, hafnium, etc.) and zirconium, used in the T5 or T6 temper tend to shrink rather than grow at solution heat-treatment temperatures. Still, other castings of aluminum-manganese or aluminum-manganese-zinc exhibit permanent growth if subjected to long exposure to temperature.

Protective Atmospheres

Above 750°F (400°C), protective atmospheres are used in solution heat treatment so as to prevent surface oxidation and as a safety measure due to the unpredictable combustive nature of magnesium, especially if the processing temperature is exceeded. Ovens and furnaces used for solution heat treatment must therefore have gas-tight construction.

Sulfur dioxide (SO2) and carbon dioxide (CO2) are the most common gases used. Inert gas such as argon or nitrogen may be used if one can ensure that oxygen is not allowed to enter the furnace atmosphere. Sulfur dioxide has a pungent odor (rotten-egg smell) and can be corrosive to certain materials (e.g. nylon). A typical concentration of 0.7% (0.5% minimum) is used to prevent the material from self-igniting up to a temperature of 1050°F (565°C), provided melting has not occurred. Carbon dioxide in a concentration of 3% will prevent the pyrophoric reaction to 950°F (510°C), and a carbon dioxide concentration of 5% will provide protection to about 1000°F (540°C).

Safety

In large sections, magnesium’s high thermal conductivity makes it difficult to ignite and under normal heat-treat conditions prevents it from burning. Magnesium will not burn until it reaches its melting point of 1204°F (651°C), but when it does, the result is extremely high temperatures and, quite literally, blinding white light. Magnesium in the form of fine chips or dust, however, is easily ignited. Should a fire occur, it can be extinguished with special powders such as soapstone or graphite. Water or any standard liquid or foam fire extinguishers cause magnesium to burn more rapidly and may cause an explosion.

Fig. 2. Magnesium casting after solution treating

Quenching

Unlike aluminum alloys, most magnesium alloys (Fig. 2) are quenched in air following solution heat treatment. For dense loads or heavy castings, fans are used to accelerate the cooling process.

Aging (Age Hardening)

The response to age hardening by magnesium alloys is significantly less than with aluminum alloys. When aging, parts should be loaded into the furnace at the treatment temperature, held for at least one hour per inch of cross-sectional thickness and then cooled in still air.

Stress Relief (Wrought Alloys or Castings)

Magnesium castings do not normally contain a high level of residual stresses and do not generally require stress relief. However, precision machining of castings to close dimensional limits and the low modulus of elasticity of magnesium alloys means that comparatively low stresses can produce appreciable deformation. Stress relief is also used to prevent stress-corrosion cracking in, for example, welded magnesium-aluminum casting alloys. Residual stresses may arise from contraction due to mold restraint during solidification, from no uniform cooling after heat treatment or from quenching. Machining operations can also produce residual stress and require intermediate stress relieving prior to final machining.

Annealing

Annealing can be used for both heat-treatable and non-heat-treatable alloys to increase ductility with a slight reduction in strength. A typical annealing cycle for wrought magnesium alloys is to heat them to 550-850°F (290-455°C) for at least one hour. Since most forming operations are done at elevated temperature, most wrought material is already fully annealed.

Other Thermal Treatments

Additional thermal treatments are seldom necessary. If the heat-treated microstructure is not optimum, or if a casting has not been properly cooled after solution treating, for example, a reheat process is necessary.

Preparation of Samples for Analysis of Microstructures

If you are going to look at the microstructure of these alloys, a few tips are in order. Pure magnesium is difficult to polish because it is very soft and is attacked by many dilute organic acids, slowly attacked by water (many magnesium alloys are rapidly attacked) and mechanical twinning may be produced during cutting and (abrasive) grinding. Fine magnesium dust is also a fire hazard.

Macroetchants reveal flow lines, grain structure and internal defects. Other etchants (Ref. 3) are used to reveal microstructure, grain contrast, grain boundaries and compositional variation within grains.

Chemical and electrolytic polishing and attack etching are methods used to prepare magnesium alloys. Attack etching (etch polishing or etch attack) involves adding a dilute chemical etchant to the polishing medium to increase polishing rate and change to dissolution-predominate polishing mechanisms. It is also performed when removal of passivating films is necessary. IH