Knowledge of heat treatment of aluminum alloy
The metallographic structure of cast aluminum alloy is larger than that of wrought aluminum alloy, so it is different during heat treatment. The former has a long holding time, generally over 2h, while the latter has a short holding time, as long as tens of minutes. Because the metallurgical structure of metal castings, low-pressure castings, and cast aluminum alloys is coarser than that of wrought aluminum alloys, they are different during heat treatment. The former has a long holding time, generally over 2h, while the latter has a short holding time, as long as tens of minutes. Because metal castings, low pressure castings, and differential pressure castings are crystallized and solidified under a relatively large cooling rate and pressure, their crystalline structure is much thinner than that of gypsum and sand casting, so their heat preservation during heat treatment is also short a lot of. Another difference between cast aluminum alloy and deformed aluminum alloy is that the wall thickness is uneven, and there are complex structural shapes such as special-shaped surfaces or internal channels. In order to ensure that there is no deformation or cracking during heat treatment, special fixtures are sometimes designed for protection and quenching medium. The temperature is also higher than that of deformed aluminum alloy, so artificial aging is generally used to shorten the heat treatment cycle and improve the performance of the casting.
Purpose of heat treatment
The purpose of heat treatment of aluminum alloy castings is to improve the mechanical properties and corrosion resistance, stabilize the size, and improve the machining performance of cutting and welding. Because the mechanical properties of many as-cast aluminum alloys cannot meet the requirements of use, except for the ZL102 of the Al-Si series, the ZL302 of the Al-Mg series and the ZL401 alloy of the Al-Zn series, the rest of the cast aluminum alloys must be further improved by heat treatment. The mechanical properties and other service properties of the casting are as follows:
- 1) Eliminate the uneven cooling rate of the casting caused by the uneven cooling rate during the crystallization and solidification due to the structure of the casting (such as uneven thickness of the wall and thick transition) 2) Improve the mechanical strength and hardness of the alloy, improve the metallographic structure, and ensure that the alloy has a certain degree of plasticity, cutting performance, and welding performance;
- 3) Stabilize the structure and size of the casting, prevent and eliminate high-temperature phase transformations. The volume changes;
- 4) Eliminate intergranular and component segregation, and homogenize the structure.
The effect of annealing treatment is to eliminate the casting stress of the casting and the internal stress caused by machining, stabilize the shape and size of the processed part, and spheroidize part of the Si crystals of the Al-Si alloy to improve the plasticity of the alloy. The process is: heating aluminum alloy castings to 280-300°C, holding for 2-3 hours, and cooling to room temperature with the furnace, so that the solid solution slowly decomposes, and the precipitated second particles gather, thereby eliminating the internal stress of the casting and achieving a stable size , The purpose of improving plasticity, reducing deformation and warpage.
Quenching is to heat the aluminum alloy castings to a higher temperature (usually close to the melting point of the eutectic, mostly above 500 ℃), and hold for more than 2 hours to fully dissolve the soluble phase in the alloy. Then, it is quenched into water at 60-100℃ to make the castings cool rapidly, so that the strengthening components can be dissolved to the maximum extent in the alloy and stored at room temperature. This process is called quenching, also called solution treatment or cold treatment.
Aging treatment, also known as low temperature tempering, is a process that heats the quenched aluminum alloy castings to a certain temperature, heats them out for a certain period of time and air-cools them to room temperature to decompose the supersaturated solid solution and stabilize the alloy matrix structure.
During the aging treatment process of the alloy, with the increase of temperature and the extension of time, after the recombination of atoms in the supersaturated solid solution lattice, the solute atom enrichment zone (called G-PI zone) and G-PⅠ zone disappear. The atoms of the second phase segregate according to a certain rule and form the G-PII region, and then a metastable second phase (transition phase) is formed. A large number of G-PII regions are combined with a small amount of metastable phase and the metastable phase is transformed into a stable phase. , The second phase particles gather in several stages.
The aging treatment is divided into two categories: natural aging and artificial aging. Natural aging refers to aging strengthening at room temperature. Artificial aging is divided into three types: incomplete artificial aging, complete artificial aging, and overaging.
- 1) Incomplete artificial aging: heat the casting to 150-170°C for 3-5h to obtain better tensile strength, good plasticity and toughness, but a heat treatment process with lower corrosion resistance;
- 2) Complete artificial aging: heat the casting to 175-185°C and keep it for 5-24 hours to obtain sufficient tensile strength (that is, the highest hardness) but a heat treatment process with lower elongation;
- 3) Over-aging: heat the casting to 190-230°C and keep it for 4-9 hours to reduce the strength and increase the plasticity to obtain better resistance to stress and corrosion. It is also called stabilized tempering.
Cool the aluminum alloy casting to a certain temperature below zero (such as -50°C, -70°C, -195°C) and keep it for a certain period of time, and then heat the casting to below 350°C to make the alloy’s moderate solid solution lattice repeatedly shrink and expand. And make the crystal grains of each phase move a small amount, so that the atomic segregation zone and the particles of the intermetallic compound in the solid solution crystal lattice are in a more stable state, so as to achieve the purpose of increasing the size and volume of the product parts. This heat treatment process of repeated heating and cooling is called cyclic treatment. This kind of treatment is suitable for parts that require very precise and stable dimensions in use (such as some parts on testing instruments). Generally castings are not treated in this way.
Code and meaning of heat treatment state of cast aluminum alloy
Alloys cast in metal molds or green sand molds have obtained a certain degree of supersaturated solid solution due to the faster cooling rate, even if there is a partial quenching effect. After artificial aging and demelting strengthening, the hardness and mechanical strength can be improved, and the machinability can be improved. Effective for improving the strength of alloys such as ZL104 and ZL105.
The main function is to eliminate the internal stress of the casting, stabilize the size of the casting, and spheroidize the Si crystals of the Al-Si series alloy to improve its plasticity. The effect on Al-Si series alloys is obvious, the annealing temperature is 280~300 ℃, and the holding time is 2~4h.
T4——Solid solution treatment plus natural aging
Through heating and holding, the soluble phase is dissolved, and then quenched, so that a large amount of the strengthening phase is solid-dissolved in the α solid solution to obtain a supersaturated solid solution to improve the hardness, strength and corrosion resistance of the alloy. For Al-Mg series alloys, it is final treatment, and for other alloys that require artificial aging, it is preliminary treatment.
T5——Solid solution treatment plus incomplete artificial aging
It is used to obtain higher strength and plasticity, but the Cornish property will decrease, especially the intergranular corrosion will increase. The aging temperature is low, the holding time is short, the aging temperature is about 150-170 ℃, and the holding time is 3 to 5 hours.
T6——Solid solution treatment plus complete artificial aging
Used to obtain the highest strength, but the plasticity and corrosion resistance are reduced. Perform at higher temperature and longer time. Suitable for parts that require high load, the aging temperature is about 175~185 ℃, and the holding time is more than 5h.
T7-solution treatment plus stabilized tempering
Used to stabilize the size and structure of castings, improve corrosion resistance, and maintain high mechanical properties. It is usually carried out at close to the working temperature of the part. Suitable for parts that work at high temperatures below 300 ℃, tempering temperature 190-230 ℃, holding time 4-9h.
T8 solution treatment and softening and tempering
The solid solution is fully decomposed, and the precipitated strengthening phase is aggregated and spheroidized to stabilize the size of the casting and increase the plasticity of the alloy, but the tensile strength decreases. Suitable for castings that require high plasticity, the tempering temperature is about 230-330 ℃, and the holding time is 3-6h.
Used to further stabilize the size and shape of the casting. The repeated heating and cooling temperature and the number of cycles should be determined according to the working conditions of the parts and the properties of the alloy. Suitable for parts requiring precise and stable appearance.
Heat treatment process parameters of cast aluminum alloy
|Alloy code||Heat treatment||Quenching process||Aging or annealing process|
|Heating temperature (℃)||Holding time (h)||Cooling medium (water)||Heating temperature (℃)||Holding time||Cooling method|
Technical points of heat treatment operation
- 1) Before heat treatment, check whether the heat treatment equipment, auxiliary equipment, instruments, etc. are qualified and normal, and whether the temperature difference across the furnace is within the specified range (±5°C);
- 2) Before installing the furnace, sand blowing or washing should be carried out, and there should be no oil, dirt, and soil, and alloy grades should not be mixed;
- 3) Castings that are prone to warping in shape and shape should be placed on a dedicated chassis or bracket, and suspended cantilever parts are not allowed;
- 4) Single casting or attached casting test bars for checking the performance of castings should be treated together with the parts in the same furnace to truly reflect the performance of the castings;
- 5) During the heat preservation period, check and correct the temperature around the furnace at any time to prevent local high temperature or burning;
- 6) When it cannot be recovered in a short time after the power is cut off, the castings in the heat preservation should be quickly out of the furnace for quenching, and after the return to normal, the furnace is installed, warmed and heat treated;
- 7) Castings that have been quenched in the nitrate bath should be rinsed with hot water immediately after quenching to eliminate residual salt and prevent corrosion;
- 8) If the casting is found to be deformed after quenching, it should be corrected immediately;
- 9) The parts to be aging treatment should be aging treatment within 0.5h after quenching;
- 10) If the performance is found to be unqualified after heat treatment, the heat treatment can be repeated, but the number of times shall not exceed 2;
- 11) The heat treatment should be carried out according to the heat treatment process established by the casting structure shape, size, alloy characteristics, etc.
Causes of heat treatment defects and their elimination and prevention methods
The annealing state δ5 is low, and the strength and elongation after quenching or aging treatment are unqualified. The annealing temperature is too low or the holding time is insufficient, or the cooling is too fast; the quenching temperature is too low or the holding time is not enough, or the cooling rate is too slow (the temperature of the quenching medium is too high); the incomplete artificial aging and the complete artificial aging temperature are too high, or the holding If the time is too long, the chemical composition of the alloy will deviate. Re-anneal, increase the temperature or extend the holding time; increase the quenching temperature or extend the holding time, and reduce the temperature of the quenching medium; if quenching again, adjust the subsequent aging temperature and time; if there is a deviation in the composition, it must be based on the specific deviation element , Deviation, change or adjust repeated heat treatment parameters.
The size and shape change of the casting reflected in the heat treatment or subsequent machining. The heating rate or quenching cooling rate is too fast (too intense); the quenching temperature is too high; the design structure of the casting is unreasonable (for example, the wall thickness of the two connecting walls is too different, and the ribs in the frame structure are too thin or too small; during quenching Improper launching direction of the workpiece and improper charging method. Reduce the heating rate, increase the temperature of the quenching medium, or change to a quenching medium with a slower cooling rate to prevent residual stress in the alloy; apply paint or use asbestos to thick or thin walls Fibers and other thermal insulation materials cover the thin-walled parts; choose a reasonable launching direction according to the structure and shape of the casting or use special anti-deformation fixtures; the parts with small deformation can be corrected immediately after quenching.
The obvious cracks on the surface of the quenched casting that can be seen with the naked eye or the micro cracks that can not be seen by the naked eye through fluorescent inspection. The cracks are often tortuous and not straight and appear dark gray. The heating speed is too fast, and the cooling is too fast during quenching (the quenching temperature is too high or the temperature of the quenching medium is too low, or the speed of the quenching medium is too fast); the design of the casting structure is unreasonable (the wall thickness difference between the two connecting walls is too large, and the middle of the frame Reinforcing ribs are too thin or too small); the method of loading the furnace or the direction of launching is wrong; the temperature of the furnace is uneven, which makes the temperature of the casting uneven. Slow down the heating rate or adopt an austempering process; increase the temperature of the quenching medium or replace it with a quenching medium with a slow cooling rate; apply paint on the wall thickness or thin-walled parts or cover the thin-walled parts with heat insulation materials such as asbestos; use special anti-corrosion materials Cracked quenching fixture, and choose the correct direction of launching.
There are nodules on the surface of the casting, and the elongation of the alloy is greatly reduced. The content of low-melting impurity elements in the alloy such as Cd, Si, Sb, etc. is too high; the heating is uneven or too fast; the local temperature in the furnace exceeds the overburning temperature of the alloy; the instrument for measuring and controlling the temperature fails, making the furnace actual The temperature exceeds the value indicated by the meter. Strictly control the content of low-melting alloy elements not to exceed the standard; slowly increase the temperature at a rate of no more than 3°C/min; check and control the temperature of each zone in the furnace to not exceed ±5°C; regularly check or calibrate the measurement and control instrument to ensure that the instrument temperature, temperature, display, The temperature control is accurate.
The surface of the casting appears streaked or lumpy, which is different from the surface of the aluminum alloy casting. The chloride content in the nitrate solution exceeds the standard (>0.5%) and causes corrosion to the surface of the casting (especially the loose and shrinkage parts); after being taken out from the nitrate tank, it is not sufficiently cleaned, and the nitrate adheres to the surface of the casting ( Especially narrow gaps, blind holes, channels) cause corrosion; nitrate salt solution is mixed with acid or alkali or castings are corroded around concentrated acid or alkali. Try to shorten the time for castings to move from the furnace to the quenching tank; check whether the chloride content in the nitrate salt exceeds the standard, if it exceeds the standard, the content (or concentration) should be reduced, and the castings heated from the nitrate salt tank should be immediately heated with warm water or Rinse with cold water; check the content of acid and alkali in the nitrate salt. If there is acid or alkali, neutralize or stop using it; do not place aluminum alloy castings around where there is concentrated acid or alkali.
The elongation and hardness of the thick parts of the castings are low (especially the inner center), and the thin-wall parts have high hardness (especially the surface layer). The heating and cooling of the castings are uneven, the thick parts cool slowly, and the heat permeability is poor. Re-heat treatment, reduce the heating rate, extend the heat preservation time, and balance the temperature of the thick and thin parts; apply heat-insulating paint or cover heat-insulating materials such as asbestos to the thick-walled parts to try to cool all parts of the casting at the same time; make the thick parts Start the water first; change to an organic quenching agent to reduce the cooling rate.
Heat treatment equipment and materials
Main technical requirements for heat treatment equipment
- 1) Because the temperature difference between the quenching and aging temperature of aluminum alloy is not large (because the quenching temperature is close to the melting point of the low melting eutectic component in the alloy), the temperature difference in the furnace should be controlled at ±5°C;
- 2) The temperature measurement and temperature control instruments are required to be sensitive and accurate to ensure that the temperature is within the above-mentioned error range;
- 3) The temperature of each zone in the furnace should be uniform, and the difference should be within the range of 1-2°C;
- 4) The quenching tank has a heating device and a circulation device to ensure the heating and temperature of the water is uniform;
- 5) The contaminated cooling water should be regularly checked and replaced.
Quenching medium is an important factor to ensure the realization of various heat treatment purposes or functions. The higher the cooling rate of the quenching medium, the more intense (faster) the cooling of the casting, the higher the degree of supersaturation of the α solid solution in the metal structure, and the better the mechanical properties of the casting, because a large amount of intermetallic compounds and other strengthening phases are solid solution Into the Al α solid solution. Quenching medium according to the cooling rate of the casting is: dry ice and acetone mixture (-68℃), ice water, water at room temperature, water at 80-90℃, water at 100℃, water after atomization , Various oils (rapeseed oil, etc.), various oils heated to 200-220℃, air, etc.
Temperature measurement and temperature control instruments and materials
The accuracy of temperature measurement and temperature control instruments should not be lower than 0.5. The heat treatment furnace should be equipped with automatic recording, automatic alarm, automatic power-off, and power-up devices and meters that can automatically measure and control temperature to ensure the temperature in the furnace. Accurate display and control and uniform temperature.
The thermocouple uses nickel-chromium-nickel-silicon, nickel-chromium-nickel-aluminum couple wires with a diameter of 2.0-0.5mm. In order to improve the sensitivity of the thermometer and reduce the temperature fluctuation range, it is best to use the above-mentioned material of Ф0.5-1.0mm. And before use and during use (once every 3 months) test and calibration once.
What are the differences between aluminum alloy heat treatment characteristics and steel heat treatment?
As we all know, for steels with higher carbon content, high hardness is obtained immediately after quenching, while plasticity is very low. However, this is not the case for aluminum alloys. After quenching, the strength and hardness of aluminum alloys do not immediately increase. As for the plasticity, instead of decreasing, it increases. But this kind of quenched alloy, placed for a period of time (such as 4-6 days and nights), the strength and hardness will be significantly improved, while the plasticity will be significantly reduced. The phenomenon that the strength and hardness of the aluminum alloy after quenching increases significantly with time is called aging. Aging can occur at room temperature, which is called natural aging, or it can occur in a certain temperature range (such as 100 to 200°C) higher than room temperature, and is called artificial aging.
The age hardening of aluminum alloy is a very complicated process, which not only depends on the composition and aging process of the alloy, but also depends on the defects caused by the shrinkage of the alloy during the production process, especially the number and distribution of vacancies and dislocations. At present, it is generally believed that age hardening is the result of the segregation of solute atoms to form a hardened zone.
The size and number of hardened zones depend on the quenching temperature and quenching cooling rate. The higher the quenching temperature, the greater the vacancy concentration, the greater the number of hardened zones, and the size of the hardened zone decreases. The greater the quenching cooling rate, the more vacancies fixed in the solid solution, which is beneficial to increase the number of hardened zones and reduce the size of the hardened zones.
A basic feature of precipitation hardening alloys is the equilibrium solid solubility that changes with temperature, that is, the solid solubility increases with the increase of temperature. Most aluminum alloys that can be heat treated and strengthened meet this condition.
Source: China Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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