Aluminum alloy heat treatment process
2023-02-09 01:06:27
Aluminum alloy heat treatment principle
The heat treatment of aluminum alloy castings is to select a heat treatment specification, control the heating speed to a certain temperature and heat for a certain time and cool at a certain speed to change the structure of the alloy. The main purpose is to improve the mechanical properties of the alloy and enhance the resistance. Corrosion performance, improved processing ability, and dimensional stability.
Aluminum alloy heat treatment characteristics
It is well known that for steels with a high carbon content, high hardness is obtained immediately after quenching, and plasticity is low. However, the aluminum alloy is not. After the quenching of the aluminum alloy, the strength and hardness do not immediately increase. As for the plasticity, the plasticity has not decreased, but has increased. However, after the quenched alloy is placed for a period of time (for example, after 4 to 6 days and nights), the strength and hardness are remarkably improved, and the plasticity is remarkably lowered. After the quenching, the strength and hardness of the aluminum alloy increase remarkably with time, which is called aging. The aging can occur at room temperature, called natural aging, or it can occur in a certain temperature range above room temperature (such as 100 ~ 200 ° C), called artificial aging.
Aluminum alloy ageing strengthening principle
Age hardening of aluminum alloy is a rather complicated process, which depends not only on the composition of the alloy, the aging process, but also on the defects caused by the alloy during the production process, especially the number and distribution of vacancies and dislocations. Age hardening is generally believed to be the result of segregation of solute atoms to form a hardened zone.
When the aluminum alloy is quenched and heated, vacancies are formed in the alloy. When quenching, these vacancies are too late to be removed due to rapid cooling, and are "fixed" in the crystal. These vacancies in supersaturated solid solutions are mostly combined with solute atoms. Since the supersaturated solid solution is in an unstable state, it will inevitably transition to an equilibrium state, and the presence of vacancies accelerates the diffusion rate of solute atoms, thereby accelerating the segregation of solute atoms.
The size and number of hardened zones depend on the quenching temperature and the quenching cooling rate. The higher the quenching temperature, the greater the vacancy concentration, the greater the number of hardened zones, and the smaller the size of the hardened zone. The greater the quenching cooling rate, the more vacancies are fixed in the solid solution, which is beneficial to increase the number of hardened zones and reduce the size of the hardened zone.
An essential feature of precipitation hardening alloys is the equilibrium solid solubility as a function of temperature, ie, the increase in solid solubility with increasing temperature, and most heat treatable aluminum alloys meet this condition. The solubility-temperature relationship required for precipitation hardening can be explained by the composition and structure of the alloy aging by the Al-4Cu alloy of aluminum-copper system. Figure 3-1 Binary phase diagram of aluminum-rich copper-rich portion, eutectic transformation L→α+θ(Al2Cu) at 548 °C. The ultimate solubility of copper in the alpha phase is 5.65% (548 ° C). As the temperature decreases, the solid solubility decreases sharply, and is about 0.05% at room temperature.
During the aging heat treatment, the alloy structure has the following changes:
Formation of solute atomic segregation zone -G·P(I) zone In the newly quenched supersaturated solid solution, the distribution of copper atoms in the aluminum lattice is arbitrary and disordered. At the initial stage of aging, when the immediate effect temperature is low or the aging time is short, the copper atoms aggregate on some crystal planes on the aluminum matrix to form a solute atom segregation zone, which is called the G·P(I) region. The G·P(I) region maintains a coherent relationship with the matrix α. These polymers constitute a coherent strain region for improving deformation resistance, so that the strength and hardness of the alloy are increased.
Ordering of G·P region-forming G·P(II) region As the aging temperature increases or the aging time prolongs, the copper atoms continue to segregate and become ordered, ie, the G·P(II) region is formed. It still maintains a coherent relationship with the matrix α, but the size is larger than the G·P(I) region. It can be regarded as the intermediate transition phase, which is usually expressed by θ". It is larger than the distortion around the G·P(I) region, and the hindrance to the dislocation motion is further increased, so the aging strengthening effect is greater, and the θ" phase is precipitated. The stage of maximum strengthening for the alloy.
Forming a transition phase θ'
With the further development of the aging process, the copper atoms continue to segregate in the G·P(II) region, and when the ratio of copper atoms to aluminum atoms is 1:2, the transition phase θ′ is formed. Since the lattice constant of θ′ changes greatly, when it forms, the coherent relationship with the matrix begins to break, that is, from the complete coherence to the local coherence, so the coherent distortion of the matrix around the θ′ phase is weakened. The hindrance of the dislocation motion is also reduced, and the hardness begins to decrease in the performance of the alloy. It can be seen that the existence of coherent distortion is an important factor in the ageing strengthening of the alloy.
Forming a stable θ phase
The transition phase is completely desolvated from the aluminum-based solid solution, forming an independent stable phase Al2Cu with a clear interface with the matrix, which is called θ phase. At this time, the coherent relationship between the θ phase and the matrix is ​​completely destroyed, and has its own independent lattice. The distortion also disappears, and the temperature is increased or the time is prolonged. The concentration of the θ phase grows, the strength and hardness of the alloy further decrease, and the alloy softens and is called “overagedâ€. The θ phase aggregates and grows to become coarse.
The aging principle and general rule of aluminum-copper binary alloys are also applicable to other industrial aluminum alloys. However, the types of alloys are different, and the stability of the formed G·P region, the transition phase, and the final precipitation are different, and the aging strengthening effect is also different. It can be seen from the table that the aging process of different alloys does not completely undergo the above four stages, and some alloys do not pass through the G·P(II) zone and form a transition phase directly. That is, the same alloy has different temperature and time due to aging, and does not completely undergo the entire aging process in sequence. For example, some alloys only end up in the G·P(I) zone to the G·P(II) zone during natural aging. In artificial aging, if the aging temperature is too high, the transition phase can be directly precipitated from the supersaturated solid solution without passing through the G·P region, and the degree of time-effect is directly related to the structure and properties of the alloy after aging.
Factors affecting aging
Effect of residence time from quenching to artificial aging
It has been found that some aluminum alloys, such as Al-Mg-Si alloys, are artificially aged after being left at room temperature. The strength index of the alloy does not reach the maximum value, and the plasticity increases. For example, ZL101 cast aluminum alloy, after quenching, stay at room temperature for one day and then artificial aging, the strength limit is 10-20Mpa lower than the immediate aging after quenching, but the plasticity is improved compared with the aluminum alloy which is immediately aged.
Effect of alloy chemical composition
Whether an alloy can be strengthened by ageing depends first on whether the elements constituting the alloy can be dissolved in the solid solution and the degree of solid solubility changes with temperature. For example, the solid solubility of silicon and manganese in aluminum is relatively small, and it does not change much with temperature, while magnesium and zinc have a large solid solubility in aluminum-based solid solution, but the structure and matrix of the compound formed with aluminum The difference is small and the enhancement effect is minimal. Therefore, binary aluminum-silicon, aluminum-manganese, aluminum-magnesium, and aluminum-zinc are generally not subjected to aging strengthening treatment. And some binary alloys, such as aluminum-copper alloys, and ternary alloys or multi-alloys, such as aluminum-magnesium-silicon, aluminum-copper-magnesium-silicon alloys, etc., have solubility and solid phase transition during heat treatment. It can be strengthened by heat treatment.
Effect of alloy solution treatment process
In order to obtain a good aging strengthening effect, the quenching heating temperature is higher under the conditions of no overheating, over-burning and grain growth, and the holding time is longer, which is favorable for obtaining a uniform solid solution with maximum supersaturation. In addition, the second phase is not precipitated during the quenching and cooling process, otherwise, in the subsequent aging treatment, the precipitated phase will act as a nucleus, causing local uneven precipitation and reducing the aging strengthening effect.
Effect of aging temperature
When aging at different temperatures, the critical nucleus size, number, composition and rate of aggregation growth of the precipitation phase are different. If the temperature is too low, the G·P region is difficult to form due to diffusion difficulties, and the strength and hardness after aging are low. When the temperature is too high, the diffusion is easy to proceed, and the critical nucleation size of the precipitated phase in the supersaturated solid solution is large, and the strength and hardness are low after aging, that is, overaging is caused. Therefore, various alloys have optimum aging temperatures.
Regression of aluminum alloy
After quenching and natural aging, the aluminum alloy (such as aluminum-copper) is reheated to 200-250 ° C, and then quickly cooled to room temperature, the alloy strength decreases, softens again, and the performance returns to the quenched state; if placed at room temperature, As with the new quenched alloy, normal natural aging can still be performed. This phenomenon is called regression. The explanation about the regression phenomenon is that when the alloy is naturally aged at room temperature, the size of the G·P region is small. When heated to a higher temperature, these small G·P regions are no longer stable and are re-dissolved into the solid solution. When it is cooled to room temperature, the alloy returns to the new quenching state and can be re-naturally aged. In theory, the regression treatment is not limited by the number of treatments. However, in practice, it is difficult to completely re-dissolve the precipitated phase during the regression treatment, resulting in local precipitation in the subsequent aging process, and the aging strengthening effect is gradually weakened. At the same time, in the repeated heating process, the solid solution grains have an increasing tendency, which is unfavorable for performance. Therefore, the regression treatment is only used to repair the rivet alloy for the aircraft, and this phenomenon can be utilized to rive the joint at any time, but has no use value for other aluminum alloys.
Solution treatment and quenching
In order to utilize the precipitation hardening reaction, a supersaturated solid solution is first formed by heating and rapid cooling. The process of forming a solid solution is called solution heat treatment. The purpose is to dissolve the alloy's maximum amount of hardenable hardening elements in the solid solution. This process involves heating the alloy to a sufficiently high temperature for a sufficient period of time and then rapidly cooling the water.
In summary, the heat treatment for improving the strength and hardness of the aluminum alloy includes a three-step process: (1) solution heat treatment - dissolution of the soluble phase. (2) Formation of quenching-supersaturated solid solution. (3) Aging - Precipitation of solute atoms at room temperature (natural aging) or at high temperatures (artificial aging or precipitation heat treatment).
The heat treatment of aluminum alloy castings is to select a heat treatment specification, control the heating speed to a certain temperature and heat for a certain time and cool at a certain speed to change the structure of the alloy. The main purpose is to improve the mechanical properties of the alloy and enhance the resistance. Corrosion performance, improved processing ability, and dimensional stability.
Aluminum alloy heat treatment characteristics
It is well known that for steels with a high carbon content, high hardness is obtained immediately after quenching, and plasticity is low. However, the aluminum alloy is not. After the quenching of the aluminum alloy, the strength and hardness do not immediately increase. As for the plasticity, the plasticity has not decreased, but has increased. However, after the quenched alloy is placed for a period of time (for example, after 4 to 6 days and nights), the strength and hardness are remarkably improved, and the plasticity is remarkably lowered. After the quenching, the strength and hardness of the aluminum alloy increase remarkably with time, which is called aging. The aging can occur at room temperature, called natural aging, or it can occur in a certain temperature range above room temperature (such as 100 ~ 200 ° C), called artificial aging.
Aluminum alloy ageing strengthening principle
Age hardening of aluminum alloy is a rather complicated process, which depends not only on the composition of the alloy, the aging process, but also on the defects caused by the alloy during the production process, especially the number and distribution of vacancies and dislocations. Age hardening is generally believed to be the result of segregation of solute atoms to form a hardened zone.
When the aluminum alloy is quenched and heated, vacancies are formed in the alloy. When quenching, these vacancies are too late to be removed due to rapid cooling, and are "fixed" in the crystal. These vacancies in supersaturated solid solutions are mostly combined with solute atoms. Since the supersaturated solid solution is in an unstable state, it will inevitably transition to an equilibrium state, and the presence of vacancies accelerates the diffusion rate of solute atoms, thereby accelerating the segregation of solute atoms.
The size and number of hardened zones depend on the quenching temperature and the quenching cooling rate. The higher the quenching temperature, the greater the vacancy concentration, the greater the number of hardened zones, and the smaller the size of the hardened zone. The greater the quenching cooling rate, the more vacancies are fixed in the solid solution, which is beneficial to increase the number of hardened zones and reduce the size of the hardened zone.
An essential feature of precipitation hardening alloys is the equilibrium solid solubility as a function of temperature, ie, the increase in solid solubility with increasing temperature, and most heat treatable aluminum alloys meet this condition. The solubility-temperature relationship required for precipitation hardening can be explained by the composition and structure of the alloy aging by the Al-4Cu alloy of aluminum-copper system. Figure 3-1 Binary phase diagram of aluminum-rich copper-rich portion, eutectic transformation L→α+θ(Al2Cu) at 548 °C. The ultimate solubility of copper in the alpha phase is 5.65% (548 ° C). As the temperature decreases, the solid solubility decreases sharply, and is about 0.05% at room temperature.
During the aging heat treatment, the alloy structure has the following changes:
Formation of solute atomic segregation zone -G·P(I) zone In the newly quenched supersaturated solid solution, the distribution of copper atoms in the aluminum lattice is arbitrary and disordered. At the initial stage of aging, when the immediate effect temperature is low or the aging time is short, the copper atoms aggregate on some crystal planes on the aluminum matrix to form a solute atom segregation zone, which is called the G·P(I) region. The G·P(I) region maintains a coherent relationship with the matrix α. These polymers constitute a coherent strain region for improving deformation resistance, so that the strength and hardness of the alloy are increased.
Ordering of G·P region-forming G·P(II) region As the aging temperature increases or the aging time prolongs, the copper atoms continue to segregate and become ordered, ie, the G·P(II) region is formed. It still maintains a coherent relationship with the matrix α, but the size is larger than the G·P(I) region. It can be regarded as the intermediate transition phase, which is usually expressed by θ". It is larger than the distortion around the G·P(I) region, and the hindrance to the dislocation motion is further increased, so the aging strengthening effect is greater, and the θ" phase is precipitated. The stage of maximum strengthening for the alloy.
Forming a transition phase θ'
With the further development of the aging process, the copper atoms continue to segregate in the G·P(II) region, and when the ratio of copper atoms to aluminum atoms is 1:2, the transition phase θ′ is formed. Since the lattice constant of θ′ changes greatly, when it forms, the coherent relationship with the matrix begins to break, that is, from the complete coherence to the local coherence, so the coherent distortion of the matrix around the θ′ phase is weakened. The hindrance of the dislocation motion is also reduced, and the hardness begins to decrease in the performance of the alloy. It can be seen that the existence of coherent distortion is an important factor in the ageing strengthening of the alloy.
Forming a stable θ phase
The transition phase is completely desolvated from the aluminum-based solid solution, forming an independent stable phase Al2Cu with a clear interface with the matrix, which is called θ phase. At this time, the coherent relationship between the θ phase and the matrix is ​​completely destroyed, and has its own independent lattice. The distortion also disappears, and the temperature is increased or the time is prolonged. The concentration of the θ phase grows, the strength and hardness of the alloy further decrease, and the alloy softens and is called “overagedâ€. The θ phase aggregates and grows to become coarse.
The aging principle and general rule of aluminum-copper binary alloys are also applicable to other industrial aluminum alloys. However, the types of alloys are different, and the stability of the formed G·P region, the transition phase, and the final precipitation are different, and the aging strengthening effect is also different. It can be seen from the table that the aging process of different alloys does not completely undergo the above four stages, and some alloys do not pass through the G·P(II) zone and form a transition phase directly. That is, the same alloy has different temperature and time due to aging, and does not completely undergo the entire aging process in sequence. For example, some alloys only end up in the G·P(I) zone to the G·P(II) zone during natural aging. In artificial aging, if the aging temperature is too high, the transition phase can be directly precipitated from the supersaturated solid solution without passing through the G·P region, and the degree of time-effect is directly related to the structure and properties of the alloy after aging.
Factors affecting aging
Effect of residence time from quenching to artificial aging
It has been found that some aluminum alloys, such as Al-Mg-Si alloys, are artificially aged after being left at room temperature. The strength index of the alloy does not reach the maximum value, and the plasticity increases. For example, ZL101 cast aluminum alloy, after quenching, stay at room temperature for one day and then artificial aging, the strength limit is 10-20Mpa lower than the immediate aging after quenching, but the plasticity is improved compared with the aluminum alloy which is immediately aged.
Effect of alloy chemical composition
Whether an alloy can be strengthened by ageing depends first on whether the elements constituting the alloy can be dissolved in the solid solution and the degree of solid solubility changes with temperature. For example, the solid solubility of silicon and manganese in aluminum is relatively small, and it does not change much with temperature, while magnesium and zinc have a large solid solubility in aluminum-based solid solution, but the structure and matrix of the compound formed with aluminum The difference is small and the enhancement effect is minimal. Therefore, binary aluminum-silicon, aluminum-manganese, aluminum-magnesium, and aluminum-zinc are generally not subjected to aging strengthening treatment. And some binary alloys, such as aluminum-copper alloys, and ternary alloys or multi-alloys, such as aluminum-magnesium-silicon, aluminum-copper-magnesium-silicon alloys, etc., have solubility and solid phase transition during heat treatment. It can be strengthened by heat treatment.
Effect of alloy solution treatment process
In order to obtain a good aging strengthening effect, the quenching heating temperature is higher under the conditions of no overheating, over-burning and grain growth, and the holding time is longer, which is favorable for obtaining a uniform solid solution with maximum supersaturation. In addition, the second phase is not precipitated during the quenching and cooling process, otherwise, in the subsequent aging treatment, the precipitated phase will act as a nucleus, causing local uneven precipitation and reducing the aging strengthening effect.
Effect of aging temperature
When aging at different temperatures, the critical nucleus size, number, composition and rate of aggregation growth of the precipitation phase are different. If the temperature is too low, the G·P region is difficult to form due to diffusion difficulties, and the strength and hardness after aging are low. When the temperature is too high, the diffusion is easy to proceed, and the critical nucleation size of the precipitated phase in the supersaturated solid solution is large, and the strength and hardness are low after aging, that is, overaging is caused. Therefore, various alloys have optimum aging temperatures.
Regression of aluminum alloy
After quenching and natural aging, the aluminum alloy (such as aluminum-copper) is reheated to 200-250 ° C, and then quickly cooled to room temperature, the alloy strength decreases, softens again, and the performance returns to the quenched state; if placed at room temperature, As with the new quenched alloy, normal natural aging can still be performed. This phenomenon is called regression. The explanation about the regression phenomenon is that when the alloy is naturally aged at room temperature, the size of the G·P region is small. When heated to a higher temperature, these small G·P regions are no longer stable and are re-dissolved into the solid solution. When it is cooled to room temperature, the alloy returns to the new quenching state and can be re-naturally aged. In theory, the regression treatment is not limited by the number of treatments. However, in practice, it is difficult to completely re-dissolve the precipitated phase during the regression treatment, resulting in local precipitation in the subsequent aging process, and the aging strengthening effect is gradually weakened. At the same time, in the repeated heating process, the solid solution grains have an increasing tendency, which is unfavorable for performance. Therefore, the regression treatment is only used to repair the rivet alloy for the aircraft, and this phenomenon can be utilized to rive the joint at any time, but has no use value for other aluminum alloys.
Solution treatment and quenching
In order to utilize the precipitation hardening reaction, a supersaturated solid solution is first formed by heating and rapid cooling. The process of forming a solid solution is called solution heat treatment. The purpose is to dissolve the alloy's maximum amount of hardenable hardening elements in the solid solution. This process involves heating the alloy to a sufficiently high temperature for a sufficient period of time and then rapidly cooling the water.
In summary, the heat treatment for improving the strength and hardness of the aluminum alloy includes a three-step process: (1) solution heat treatment - dissolution of the soluble phase. (2) Formation of quenching-supersaturated solid solution. (3) Aging - Precipitation of solute atoms at room temperature (natural aging) or at high temperatures (artificial aging or precipitation heat treatment).
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