The properties of magnesium alloy at room temperature and high temperature are improved by precipitation hardening caused by solid strengthening and aging treatment.

The commonly used heat treatment methods of magnesium alloy mainly include solution treatment, aging treatment and annealing treatment, etc. The choice of heat treatment method mainly depends on the type of alloy and service conditions. In the actual production, the appropriate treatment process should be selected according to the requirements of mechanical properties, such as solid solution treatment can improve the yield strength and tensile strength of alloy plastic solid solution aging treatment.

() Annealing
The plastic deformation ability of magnesium alloy is reduced by residual stress and uneven distribution of microstructure. These defects can be effectively reduced or eliminated by annealing process to improve the plasticity of magnesium alloys and prepare for subsequent processing.
Fully annealed (0)
To eliminate the deformation strengthening effect in the plastic deformation process, restore and improve its plasticity for subsequent deformation processing.
Full annealing is very effective in eliminating work hardening and can restore and improve the working plasticity of the material. Since the deformation of magnesium alloy is generally carried out at high temperature, the deformed alloy is rarely completely annealed. This process can be used as the final heat treatment for some magnesium alloys with not obvious heat treatment strengthening effect.
The temperature range of complete annealing is 300~400℃, the holding time is 3~8h, and the process parameters are selected appropriately according to the type of alloy and the degree of deformation.

Stress relief annealing (T2): eliminate the residual stress caused by the casting and processing of deformed magnesium alloy products.
Stress relief annealing is widely used in production practice. For example, the optimal annealing process for AZ31 magnesium alloy sheet is about 1h at 300~350℃, and the strength and toughness of the alloy are well matched after this process.
In addition, magnesium alloy will also produce residual stress during machining, and necessary annealing treatment is conducive to the machining accuracy and service life of the workpiece.

() Solid solution treatment
The properties of magnesium alloy are improved by solution strengthening and aging strengthening.
Solution treatment (T4):
The alloy is heated to the appropriate temperature in the single-phase solid solution phase area, and the corresponding time is kept warm, so that some of its elements dissolve in the matrix to form a supersaturated solid solution, which is called solution treatment. It is a heat treatment process of natural aging of magnesium alloy after heating and holding, which can improve the tensile strength and plasticity of the material at the same time, and obtain greater impact resistance.
For example, the optimal solution treatment process of Mg-4Y-2Nd-1Gd-0.4Z magnesium alloy is 525℃X8h, and its elongation increases significantly after heat treatment.

() prescription treatment
Aging: the yield strength is increased, and the plasticity is slightly reduced.
Artificial aging (T5): is an important strengthening method of magnesium alloy, the process is relatively simple, there is no need for fixing or annealing treatment before aging, the aging temperature is generally about 200℃, and the holding time is about 20b, for the processed alloy containing precipitated phase, after artificial aging treatment, high strengthening effect can be obtained.

() Solution aging treatment
Artificial aging (T6) after solid solution treatment: the supersaturated solid solution breaks down and precipitates the second phase during artificial aging. The combination of alloy system, aging temperature and added elements affects the aging precipitation process and the characteristics of the precipitated phase.
It is found that the as-cast ZA72 alloy is mainly composed of a-g matrix and bulk second phases (Mg32(A1, Zn)49 and g7Zn3). After solution treatment, fine granular second phases are distributed on the grain boundaries and grains, and the size and quantity of the second phase gradually decrease with the increase of solid solution. After T6 treatment, the fine and dispersed secondary strengthened phase is dispersed out in the product boundary and inside the particle, and the longer the T6 treatment time, the more the amount of precipitated phase: the mechanical properties of the alloy after T6 treatment are significantly improved, and the tensile strength and microhardness of the alloy first increase and then decrease with the increase of T6 treatment time. The tensile strength of 28h alloy is 308MPa, which is 52.4% higher than that of as-cast alloy.

() hydrogenation treatment
Oxidation treatment: For magnesium alloys containing large bulk compounds in the tissue, the compound phase is very stable, and it is difficult to dissolve or break up by conventional methods. The alloying elements (such as Zn, Sn, etc.) are reduced from the compound by daily, so that the alloying elements are solidly dissolved into the magnesium matrix again, and the solid solution strengthening effect is increased. And H oxidizes with some elements in the insoluble compound to generate discontinuous compounds, which are dispersed in the tissue and play a dispersion strengthening effect.

() Surface heat treatment
Laser surface alloying makes the remelting of the surface slow, which is conducive to the full dissolution of alloying elements, and the surface quickly forms a compact and uniform structure.

() Protect the atmosphere
Generally, a protective atmosphere is used in solution treatment of magnesium alloys. According to the relevant standards for the heat treatment operation of magnesium alloy castings, a protective atmosphere must be used when the solution temperature exceeds 400 ° C to prevent the surface oxidation of magnesium alloy castings (the strength of the castings will be reduced when the surface oxidation is serious) and combustion.
Heat treatment furnaces need to be equipped with high-speed fans or other devices that can be used to circulate gas to improve furnace temperature uniformity.
The protective gas circulates in the heat treatment furnace, and its circulation rate is fast, so that the temperature distribution of all workpieces is uniform, and the minimum circulation rate varies with the design of the heat treatment furnace and the actual loading situation.

() Heat treatment defects and elimination methods thereof
(1) Oxidation If the magnesium alloy workpiece is heat treated without the use of protective gas, local oxidation will occur and even fire in the furnace. Usually into the heat treatment furnace (0.5-1.5)Vo1.%S0, or (3-5)Vo1.%C0, or containing (0.5-1.5)Vo1.%SF: 00, protective gas, or inert gas to avoid oxidation of magnesium alloy workpieces. In addition, it is necessary to ensure that the furnace is clean, dry and sealed.
(2) When the overburning heating speed is too fast, the heating temperature exceeds the solution temperature limit of the alloy, and there are more low melting point substances in the alloy, the magnesium alloy workpiece is prone to overburning. It is usually used to avoid overburning of magnesium alloy workpieces by segmenting heating from 260 ° C to more than 2 hours of solidification treatment temperature, controlling furnace temperature fluctuation within 5 ° C, and reducing zinc content to the prescribed lower limit.