1.1 Solution treatment
At this stage, the hub casting is heated to a single-phase region above the solvus line, so that the solute is dissolved into the alpha matrix to form a single solid solution phase. However, the selected temperature must not exceed the eutectic dissolution temperature, otherwise there will be dissolution along the grain boundary. After quenching, a very brittle film will be formed, which will lower the mechanical properties of the hub. Within the upper and lower limits of the solution temperature, the higher the temperature is, the faster the atomic dispersion rate is. The more thoroughly the solute dissolves into the α-base, the greater the amount of full-saturation, and the hardening of the artificial aging. The better the effect, the worse the worse.
In the resolution of the solution treatment time, it is necessary to allow the solute atoms to be dissolved into the α matrix in a proper manner, so that it has the shortest time constraint. If the time is too long, the crystal grains will grow excessively and the energy will be spoiled. Therefore, the most At a reasonable time, the shape, thickness and heating medium of other heat-treated objects will affect the heating rate and uniformity, and should also be considered.
The solution treatment in the T6 treatment procedure of the Al-Mg-Si alloy is mainly intended to dissolve the coarse Mg2Si which is separated during casting, homogenize the casting, and modify the eutectic Si shape. Therefore, the solution temperature should be close to the eutectic temperature, but it must not be exceeded to prevent the dissolution of the grain boundary and the formation of mechanical properties.
In general, the effect of the temperature at the time of solution treatment on the A356.2 alloy is that at 520 ° C or 530 ° C, the tensile strength and elongation are added as the solid solution time is added. At 540 ° C, its tensile strength and elongation reach a maximum at a short time and then gradually decrease. However, the tensile strength and the flexural strength are not changed much. In terms of elongation, the solid solution treatment at 530 ° C for 12 hours can reach an elongation of 18%; but at 540 ° C, it takes only 2 hours to reach this value. The impact strength is added according to the solution temperature and the addition of time. If the solution is 540 ° C for 9 hours, the impact strength can reach a maximum. In the solution treatment at 520 ° C, the hardness value is not changed much. In short, only the tensile strength and elongation are greatly affected by the solution treatment conditions. In economic considerations, it is feasible to reduce the time of solution treatment because the segregation of Si and Mg is not serious in the Al-Si-Mg alloy, so that homogenization and Mg2Si solid solution can be completed in a short time. In the α base, if a mutagenic agent (such as Sr, Na, Sb) is added properly, the solid solution time can be lowered without impairing the mechanical properties.
Segregation of solute elements in a dendritic coagulation arrangement can damage the mechanical properties of the material. Therefore, in the solution treatment, the intention of homogenizing the data can be achieved, and the segregation of the alloy elements is reduced. The time required for homogenization depends on the solution temperature and the solid solution time. In the case of casting, the central portion of the dendritic arrangement has the highest Si content. For this reason, the formation of this region has a higher temperature, forming Si in The solid solubility of the dendritic arrangement is also large. The segregation phenomenon of magnesium is relatively subtle. Generally, at a solid solution temperature of 550 ° C, the casting can reach the requirement of homogenization for 30 minutes. Generally, the longer the homogenization time, the better the elongation of the aluminum alloy, because the eutectic Si will be smoother. .
At this stage, the single solid solution phase after the solution treatment is quenched to a temperature below the solid solution line to obtain a saturated solid solution. However, in order to obtain the maximum amount of separation (ie, the best separation hardening effect), the cooling rate should be fast enough, especially to prevent the fastest temperature scale. If the temperature scale is too long, it will form a coarse The balance is separated. They not only have no strengthening effect, but also reduce the base's fullness and reduce the effect of artificial aging after hardening.
Therefore, there are two requirements for quenching: (1) moving from the furnace to the quenching liquid, it is necessary to prevent cooling in the air to the fastest separation scale, that is, the Keep nose portion in the TTT curve; (2) fast in the quenching medium After the fastest separation of the scale. In order to be useful to meet these two requirements, it is practical to constrain the calculation of the delay time by air opening until all the workpieces are immersed in the quenching medium.
After the Al-Si-Mg aluminum alloy is solution treated, the casting is generally quenched in water. The purpose of quenching is to suppress the separation of the Mg2Si equilibrium phase during the cooling process, leaving the Mg, Si and solute atoms in the solid solution in order to be at a low temperature. The maximum over-fullness of Mg and the solid solution of the pores are obtained. Quenching liquid and quenching time will affect the two elements of this step. The quenching liquid must have enough energy to take away the heat on the casting to achieve the fastest cooling rate, because the faster the cooling rate, the more Mg2Si remains in the solid solution. In order to achieve maximum strength. However, the quenching cooling rate cannot be infinitely fast, otherwise the casting is generally quenched with water at 25 ° C - 100 ° C.
The effect of quenching conditions on the A356 alloy is that the impact strength, elongation and tensile strength decrease with the addition of quenching temperature and quenching delay time, that is, at a faster quenching rate (lower quenching medium temperature or The quenching medium with better heat conduction can obtain greater elongation and impact strength.
In addition, in order to ensure that the Mg2Si can be evenly distributed during the aging treatment, the casting is generally quenched in 30 seconds after being taken out of the furnace, but the material with high quenching sensitivity for A356.2 is obtained. The preferred mechanical properties, the quenching delay time is preferably within 10 seconds.
1.3 aging treatment
At this stage, the over-filled solid solution obtained after quenching is placed in a constant temperature to gradually separate the separated material to form a property change, which includes strength, hardness, toughness, elongation, fatigue strength, and resistance. Corrosive, stress corrosion resistant, electrical conductivity, etc. Depending on the temperature of the aging treatment, it can be divided into natural aging and artificial aging. It is called naturalaging at room temperature; and it is called artificial aging at temperature and environment above room temperature. When aging is applied, the internal arrangement of the alloy changes with the addition of time, causing changes in physical, chemical and mechanical properties. If there is a significant addition that causes hardness or strength, it can be called age hardening.
The artificial aging variable is generally the cooperation of temperature and time, because the optimal processing temperature of each property is different from the time. Therefore, unless there is a special need, it is practical to select the best combination of important properties as consideration. The aging treatment can be carried out in an oil bath furnace, a salt bath furnace, a water heater or an air furnace, but the temperature needs to be precise and uniform. If it is not uniform, the properties may vary depending on the location, and the oil bath furnace is generally preferred.
1.3.1 Natural aging
Keeping the quenched castings at room temperature for a while is called natural aging. The effect is hardness, and the drop strength decreases with the addition of natural aging time, but the elongation and impact strength are gradually added. Especially for alloys containing high magnesium (Mg), this phenomenon is more pronounced. The effects of natural aging are generally most affected by the first three hours. If stored below 0 °C, the effect of natural aging can be prevented.
The effect of alloy composition on natural aging is also great. In alloys with higher magnesium content, the natural aging effect has a significant tensile strength. It is said in the literature that the inclusion of In, Sn, Cd or Cn in the casting may reduce the preheating. The effect of aging.
1.3.2 Artificial aging
The artificial aging treatment has nothing to do with the shape and arrangement of the Si particles, and the intention of artificial aging is to promote the separation of the solute in the saturated solid solution. The separation procedure of the Al-Si-Mg alloy is as follows: the initial separation is obtained before the needle-like growth of the β-Mg2Si in the form of a rod, and the main strengthening phase is a rod-shaped β-Mg2Si.
The temperature and time of aging treatment are the two main factors affecting the separation property. Therefore, the aging temperature of general A356.2 is about 120 ° C ~ 180 ° C, and the tensile and flexural strength is increased by the addition of the aging temperature, but the elongation and The impact strength is reduced by the addition of temperature. According to the literature, the best strength can be obtained by aging at 180 ° C for 6 hours, and the best extension can be obtained by aging at 140 ° C for 4 hours.