Heat Treating Magnesium Alloys: T6 Aging, Fire Risk, and Castings vs Wrought AZ31B

Magnesium's ignition temperature is uncomfortably close to its solution heat treatment range. AZ91D solution treats around 780 to 790F, and the alloy can ignite if local temperatures overshoot or if oxidation runs away, which is why magnesium heat treatment furnaces use protective atmospheres of sulfur dioxide, sulfur hexafluoride, or carbon dioxide to suppress oxidation and ignition. Tight temperature uniformity and overshoot protection are mandatory, not optional. The fire hazard also dictates emergency procedure: a magnesium fire cannot be put out with water, which reacts with hot magnesium to release hydrogen and intensify the fire, so shops keep Class D dry-powder extinguishers and dry sand on hand. This is a major reason not every heat treater accepts magnesium, you need the right atmosphere capability and safety protocol. For buyers, the practical implication is to use a heat treater experienced specifically with magnesium and to expect the protective-atmosphere requirement to add cost. Sending magnesium to a general steel heat treater is both a quality and a safety problem.

The cast magnesium alloys are where heat treatment delivers real strength gains. AZ91D responds to T4 (solution treated, around 780F for many hours, giving maximum toughness and ductility), T5 (artificial aging only, for a modest strength bump and dimensional stability), and T6 (solution treat plus age, for peak strength). The long solution soak, sometimes 16 to 24 hours, is needed to dissolve the coarse magnesium-aluminum intermetallic (Mg17Al12) into solution before aging precipitates it in a fine, strengthening form. WE43 is the high-performance rare-earth alloy (magnesium with yttrium and neodymium) used in aerospace and motorsport for its excellent strength retention up to around 480F and good creep resistance, properties the common AZ alloys lose at elevated temperature. WE43 is typically used in the T6 condition, solution treated and aged, and its heat treatment is what unlocks the elevated-temperature performance that justifies its high cost. It is also notable for biomedical interest as a bioresorbable implant material. Buyers specifying cast magnesium should call out the T-temper, because the difference between T4, T5, and T6 is a real strength-versus-ductility trade, and the long solution cycles drive both cost and the importance of the protective atmosphere holding stable for many hours.

AZ31B is the common wrought magnesium alloy, supplied as sheet, plate, and extrusion, and unlike the cast alloys it is not significantly age-hardenable, its strength comes from its composition and from the wrought processing and any temper. The heat treatment that matters for AZ31B is stress relief, used to remove residual stress from forming, welding, or machining so parts stay dimensionally stable, with stress-relief temperatures and times differing for the O (annealed) versus H24 (strain-hardened) tempers because you must avoid recrystallizing and softening the strain-hardened material. Magnesium is sensitive to stress-corrosion cracking, particularly the AZ alloys with higher aluminum content, so stress relief on welded or heavily formed AZ31B serves the same protective purpose that it does in brass, relieving the residual tensile stress that drives SCC. Welded AZ31B assemblies are routinely stress relieved for this reason. The buyer guidance: don't expect a hardening response from AZ31B, specify stress relief after welding or heavy forming, and respect the temper-specific stress-relief schedule so you don't accidentally anneal away the strength of H-temper material.