Surface Thermal Treatment of Aluminum Alloy Die Casting Parts

Posted on :Oct 25 , 2022 By  GREFEE

Surface thermal treatment of aluminum alloy die casting parts

The thermal treatment of aluminum alloy die casting parts refers to control the heating temperature, holding time and cooling rate change the microstructure of the alloy. The main purpose of it is to improve the mechanical performance, corrosion resistance, machining performance to obtain the dimensional stability.

The thermal treatment of aluminum alloy die casting parts can be divided into the following four categories:

1. After annealing, raise the temperature to heat the aluminum alloy parts, generally is around 300℃, then after holding for a period of time, the process of cooling to room temperature with the furnace is called annealing. During this process, the solid is dissolved and the second phase particle aggregation to reduce the internal stress of castings and stabilize the dimension of parts. In the meantime, it also reduces the possibility of deformation and improve the shaping performance of castings.

2. Solution treatment raise the temperature of castings as high as possible, close to the melting point of the eutectic. Holding the temperature for a certain period of time and cool the castings quickly to maximize the dissolution of the reinforcing element. This high temperature state is fixed at room temperature, which is called solid solution treatment, through which the strength and plasticity can be enhanced to improve the corrosion resistance of alloy.

Three factors determine the effect of solution treatment  

Temperature of solution treatment. The higher the temperature is, the faster the strengthening element dissolves, the better the strengthening effect is. Generally, the upper limit of heating temperature should be lower than the temperature at which the alloy begins to overheat. While, the lower limit should be able to dissolve the strengthening element in the solid as much as possible. To obtain the solid dissolution strengthening effect, the temperature of the alloy should not be too high.

Sometimes, it adopts the method of graded heating, which is to keep the heat preservation under the low melting point eutectic temperature. After the element is diffused and dissolved, the melting point eutectic does not exist, and then rises to a higher temperature for heat preservation and quenching. During the dissolving process of solid, the heating speed should not be too fast because higher heating speed will cause the deformation and melting of locally aggregated low melting point structure. What also should be noticed is that keeping the haggard fire transfer time of the solid heat dissolve treatment as short as possible (no more than 15mins）to avid the diffusion and precipitation of alloy element to lower the performance of alloy.

(2)Heat preservation time. The heat preservation time depends on the dissolving speed of strengthening elements, and the speed is related to the type, composition, structure, casting method, shape and wall thickness of the alloy. The heat preservation time for casting aluminum alloys is longer than that of deformed aluminum alloys, which is usually decided by the experiments. The time for general sand casting parts is generally 20-25% longer than that of the same type of metal castings.

(3)Cooling rate. During quenching, the higher the cooling rate of casting is, the higher the supersaturation of solid solution is under high temperature, the higher the mechanical performance of castings is, the higher the internal stress is, the higher the possibility to deform is. Cooling rate can be changed by selecting different cooling medium with heat capacity, thermal conductivity, latent heat of evaporation and viscosity. To get low internal stress, the castings can be cooled in the heat medium (boiling water, hot oil or molten salt). To ensure a high mechanical performance and low internal stress of castings after quenching, the isothermal quenching is used. That means the casting after solution treatment is quenched into the heat medium at 200-250 ℃ for a period of time, and the solution treatment and aging treatment are combined.

3. Aging treatment.

Heating the castings that have received solid solution treatment to a certain level and hold for a period of time until it cools slowly and reaches the room temperature is called aging. If the aging strengthening is underwent in the room temperature, which is called natural aging. If the aging strengthening is underwent when the room temperature is higher, which is called artificial aging. Aging is the natural process of the decomposition of supersaturated solid solution, making the lattice of the alloy matrix is restored to a relatively stable state. The temperature and time selection of aging temperature is based on the requirements for alloy properties, alloy properties, solid solution supersaturation and casting method.

Artificial aging is divided into three types: Incomplete manual timeliness, complete manual timeliness and obsolescence. The incomplete manual timeliness utilizes lower timeliness: temperature or short insulation time to obtain good integrate mechanical performance, that includes high strength, good plasticity and toughness. However, the corrosion resistance may be relatively low. The complete manual timeliness utilizes high timeliness temperature and long insulation time to obtain hardness and tensile strength, but elongation is low.

 The obsolescence is carried out under high temperature to maintain a high strength and improve the plasticity to get better stress corrosion resistance. To obtain higher organization and geometry, the timeliness should be performed under high temperature and the obsolescence generally is divided into stabilization treatment and softening treatment  based on the using requirements.

The precipitation process of alloy elements needs to go through the following four stages:

(1) The formation of G-PI region.

With the increase of lattice distortion, the recombination of atoms in solid solution lattice and the enrichment region of solute atoms improve the mechanical performance of the alloys, and reduce the electrical conductivity of the alloys.

(2) The formation of G-PII.

The atoms of alloy elements segregate in a certain proportion to form the G-PII area to prepare for the formation of the metastable phase, which further improves the strength of alloys.

(3) The formation of metastable.

Metastable, also called transition phase, is coherent with the matrix. A large G-PII area combined with a small amount of metastable phase to make the alloy to obtain strength.

(4) Aggregation of the second phase particles and the second phase particles.

The metastable phase transmits into the stable phase and the small particles are distributed in the grain,  but the coarse particles are distributed in the crystal boundary. The particles of the second phase are successively gathered, the lattice distortion is seriously weakened, the strength of the alloy is significantly reduced, and the plasticity of the alloy is improved. The above phases are not separated completely. Sometimes, they are progressing in the meantime, low temperature aging. The second phase is large, high temperature aging. The third and fourth stage are stronger.

4. Cold & heat cycle treatment

Parts after cold and heat cycle treatment will tend to shrinkage and inflate due to multiple times of heating and cooling of the solid solution lattice, making each phase lattice slightly displaced and the second phase particle in a more stable state so that the stability of the dimension of castings will be enhanced, which is especially suitable for manufacturing precise parts. The aluminum alloys are more tend to brittle fracture under low temperature. As the decrease of temperature, its mechanical performance is changed slightly, but the strength and plasticity are smaller. Therefore, to reduce the stress of castings sometimes, casted or quenched the parts and cool the parts to -50℃，-70℃ or even lower for 2-3 hrs before heating to room temperature in air or hot water, or artificial restriction, is called cold treatment.

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