Sat, 29 Sep 2018 03:40:24 +0000

Heat treatment of aluminum alloy

The metallographic structure of cast aluminum alloy is thicker than that of deformed aluminum alloy, so it is different in heat treatment. The former has long holding time, generally more than 2h, and the latter holding time is short, as long as dozens of minutes. Because of metal casting and low pressure casting parts.
The metallographic structure of cast aluminum alloy is thicker than that of deformed aluminum alloy, so it is different in heat treatment. The former has long holding time, generally more than 2h, and the latter holding time is short, as long as dozens of minutes. Because metal mold castings, low pressure castings and differential pressure castings crystallize and solidify under relatively large cooling rate and pressure, their crystalline structure is much finer than that of castings cast by plaster mold and sand mold, so their heat preservation is much shorter during heat treatment. Another difference between cast aluminium alloy and deformed aluminium alloy is that the wall thickness is not uniform, there are complex structures such as special-shaped surface or inner passage. In order to ensure no deformation or cracking during heat treatment, special clamp is sometimes designed to protect them, and the temperature of quenching medium is also higher than that of deformed aluminium alloy, so artificial aging is usually used to shrink them. Short heat treatment cycle and improved casting performance.

Heat treatment of aluminum alloy

I. purpose of heat treatment

The purpose of heat treatment of aluminium alloy castings is to improve the mechanical properties and corrosion resistance, stabilize the size, and improve the machinability of cutting and welding. Because the mechanical properties of many as-cast aluminium alloys can not meet the requirements of use, except Al-Si ZL102, Al-Mg ZL302 and Al-Zn ZL401, the other casting aluminium alloys have to be further improved by heat treatment mechanical properties and other performance of the castings, the specific aspects are as follows: 1) Eliminate the castings due to casting. The internal stress caused by inhomogeneous cooling rate during crystallization and solidification is caused by the structure (such as uneven thickness and large thickness of transfer joint); 2) the mechanical strength and hardness of the alloy are increased, the metallographic structure is improved, and the plasticity, machinability and weldability of the alloy are guaranteed; 3) the structure and size of the casting are stabilized. 4) Eliminate the segregation of intergranular and component, so as to homogenize the microstructure.

Two. Heat treatment methods

1. Annealing treatment
The effect of annealing treatment is to eliminate the casting stress and the internal stress caused by mechanical processing, stabilize the shape and size of the workpiece, and make some Si crystals spherical in Al-Si alloys, and improve the plasticity of the alloy. The process is: heating the aluminum alloy castings to 280-300 C, holding 2-3 hours, cooling to room temperature with the furnace, so that the solid solution decomposes slowly, the precipitated second particle aggregation, thereby eliminating the internal stress of the castings, to achieve the goal of stable size, improving plasticity, reducing deformation and warpage.
2, quenching
Quenching is to heat the aluminum alloy castings to a higher temperature (generally close to the eutectic melting point, mostly more than 500 degrees Celsius), holding more than 2 hours, so that the alloy fully soluble phase. Then, the castings are quenched rapidly into water at 60-100 degrees Celsius so that the strengthened components are dissolved to the maximum extent and stored at room temperature. This process is called quenching, also called solid solution or cold treatment.
3. Aging treatment
Aging treatment, also known as low temperature tempering, is the process of heating Quenched Aluminum alloy castings to a certain temperature, holding a certain time out of the furnace air-cooled until room temperature, so that supersaturated solid solution decomposition, so that alloy matrix structure stability.
During aging treatment, the solute atom enrichment region (called G-P I region) and G-P I region disappear after rearrangement of atoms in the supersaturated solid solution lattice, and the second phase atoms segregate and form G-P II region according to certain regularity, and then the metastable second phase (excess) is formed. In the transition phase, a large number of G-P II regions combine with a small number of metastable phases, and the metastable phase transforms into stable phase and the second phase particle aggregates.
Aging treatment is divided into two categories: natural aging and artificial aging. Natural aging refers to aging at room temperature. Artificial aging is divided into 3 types: incomplete artificial aging, artificial aging and over aging.
1) Incomplete artificial aging: heating the castings to 150-170 C for 3-5 h to obtain better tensile strength, good plasticity and toughness, but lower corrosion resistance heat treatment process; 2) complete artificial aging: heating the castings to 175-185 C for 5-24 h to obtain sufficient tensile strength (i.e. the highest hardness) but delayed Heat treatment process with low elongation; 3) Overaging: heating the castings to 190-230 C for 4-9h, resulting in a decrease in strength and an increase in plasticity, in order to obtain a better resistance to stress and corrosion, also known as stabilization tempering.
4. Cycle processing
Aluminum alloy castings are cooled to a certain temperature below zero (e.g. – 50 -, – 70 -, – 195 -) and kept for a certain period of time. Then the castings are heated to below 350 -, resulting in repeated contraction and expansion of the lattice of the medium solid solution of the alloy and a small displacement of the grains of each phase, so as to cause the atom segregation zone and metal in the lattice of the solid solution crystallization. Inter-compound particles in a more stable state, to improve the size of product parts, more stable volume. This repeated heating and cooling heat treatment process is called cycle treatment. This process is suitable for parts that require very precise and stable sizes in use (e.g. some parts on a testing instrument). General castings are not disposed of.

5. Code and meaning of heat treatment for cast aluminum alloy
The role or purpose of code state alloy heat treatment
The alloy cast by T1 artificial ageing in metal mold or wet sand mold has obtained a certain degree of supersaturated solid solution, i.e. partial quenching effect, because of its rapid cooling rate. After artificial aging and dehardening, the hardness and mechanical strength can be improved and machinability improved. It is effective to improve the strength of Zl104 and ZL105 alloys.
T2 annealing is mainly used to eliminate the internal stress (casting stress and stress caused by machining), stabilize the casting size, spheroidize the Si crystals of Al-Si alloys and improve their plasticity. The effect of Al-Si alloy is obvious. The annealing temperature is 280-300 C and the holding time is 2-4h.
T4 solution treatment (quenching) and natural aging can dissolve the soluble phase by heating and heat preservation, then quench, so that a large number of strengthening phase can be dissolved in a solid solution, and supersaturated solid solution can be obtained to improve the hardness, strength and corrosion resistance of the alloy. The Al-Mg alloy is final heat treatment, and other alloys requiring artificial aging are prepared for heat treatment.
T5 solution treatment (quenching) plus incomplete artificial aging is used to obtain higher strength and plasticity, but the corrosion resistance will decline, especially intergranular corrosion will increase. The aging temperature is low, the holding time is short, the aging temperature is about 150-170 C, and the holding time is 3-5h.
T6 solution treatment (quenching) and full artificial aging are used to obtain the highest strength, but the plasticity and corrosion resistance are reduced. At higher temperature and longer time. It is suitable for parts requiring high load. The aging temperature is about 175-185 C and the holding time is above 5h.
T7 solid solution treatment (quenching) and stabilization tempering are used to stabilize the size and structure of castings, improve corrosion resistance (especially stress corrosion resistance) and maintain high mechanical properties. Most of them work near the working temperature of parts. It is suitable for parts with high temperature below 300 C. the tempering temperature is 190-230 C and the holding time is 4-9h.
T8 solid solution treatment (quenching) and softening tempering make the solid solution decompose sufficiently, the precipitated strengthening phases aggregate and spherical to stabilize the casting size and improve the plasticity of the alloy, but the tensile strength decreases. Suitable for castings with high plasticity, tempering temperature is about 230-330 C, holding time is 3-6h.
T9 cycle treatment is used to further stabilize the size and shape of the castings. The reheating and cooling temperatures and cycles are determined by the working conditions of the parts and the properties of the alloys. It is suitable for parts with high accuracy and stability in size and shape.

Three. Heat treatment process

1. Heat treatment parameters of cast aluminum alloy
Alloy designation, heat treatment, solid solution treatment, aging treatment (after insulation, air cooling)
Heating temperature (?) holding time (h) quenching temperature (?) heating temperature (?) holding time (h)
2. Key points of heat treatment operation
1) Before heat treatment, it is necessary to check whether the heat treatment equipment, auxiliary equipment and instruments are qualified and normal, and whether the temperature difference in each part of the furnace is within the prescribed range (+5).
2) the sand should be blown or washed before loading. There should be no oil pollution, dirt and soil. Alloy grades should not be confused.
3) The castings which are prone to warpage should be placed on the special chassis or bracket, and the cantilever part is not allowed.
4) The test rods of single or attached castings for checking castings’properties should be processed together with the parts in the same furnace to truly reflect the properties of the castings.
5) during the thermal insulation period, the temperature of the furnace should be checked and corrected at any time to prevent local high temperature or burning.
6) When the castings can not be restored for a short time after power failure, they should be quenched rapidly out of the furnace in the heat preservation process, and then installed, kept warm and heat treated after the castings are restored to normal.
7) Castings quenched and overheated in nitrate tank should be washed with hot water immediately after quenching to eliminate residual salt and prevent corrosion.
8) found that the deformation of castings after quenching should be corrected immediately.
9) aging parts should be aged within 0.5h after quenching.
10) If the performance is found to be unqualified after heat treatment, the heat treatment may be repeated, but the number of times shall not exceed 2;
11) Heat treatment shall be carried out according to the heat treatment process formulated according to the structural shape, size and alloy characteristics of the castings.
3, causes of heat treatment defects and elimination and prevention measures.
Defect name defect performance causes and preventive measures
Unqualified mechanical properties of the annealed state of low delta 5, quenching or aging treatment of the strength and elongation of unqualified. Low annealing temperature or insufficient holding time, or too fast cooling; low quenching temperature or insufficient holding time, or too slow cooling rate (quenchant temperature is too high); incomplete artificial aging and full artificial aging temperature on the high side, or holding time on the long side, the chemical composition of the alloy deviation. Re-annealing, increase the temperature or prolong the holding time; increase the quenching temperature or prolong the holding time, reduce the quenching medium temperature; if re-quenching, then to adjust the subsequent aging temperature and time; if the composition of the deviation, according to the specific deviation elements, deviation, change or adjust the parameters of repeated heat treatment.
The changes of size and shape of castings reflected by deformation, warping heat treatment or subsequent mechanical processing. The heating rate or quenching cooling rate is too fast (too intense); the quenching temperature is too high; the design structure of castings is unreasonable (such as the wall thickness difference between the two connecting walls is too large, the stiffener in the frame structure is too thin or too small; the workpiece is not properly loaded in the direction of water when quenching. Reduce the heating rate, increase the quenchant temperature, or replace the quenchant with a slower cooling rate to prevent residual stress in the alloy; coat the thick or thin-walled parts with paint or asbestos fiber and other thermal insulation materials to cover the thin-walled parts; according to the casting structure, shape, choose a reasonable direction of water or adopt a special anti-deformation. The parts with little deformation can be corrected immediately after quenching.
The surface of the quenched castings is markedly cracked with the naked eye or fine cracks invisible to the naked eye by fluorescence examination. The cracks are often tortuous and dark gray. The casting structure design is unreasonable (the thickness difference between the two connecting walls is too big, the reinforcing ribs in the middle of the frame are too thin or too small); the furnace loading method is improper or the water direction is not correct; the furnace temperature is not uniform. The casting temperature is uneven. Slow down the heating rate or adopt isothermal quenching process; raise the temperature of Quenchant or replace quenchant with slow cooling rate; coat the wall thickness or thin-walled parts with paint or asbestos and other insulation materials; adopt special anti-cracking quenching fixture, and choose the correct direction of water.

There are nodules on the surface of the overburnt castings, and the elongation of the alloy is greatly reduced. The content of impurities such as Cd, Si, Sb in the alloy is too high; the heating is not uniform or too fast; the local temperature in the furnace exceeds the super-firing temperature of the alloy; the instrumentation for measuring and controlling the temperature fails to make the actual temperature in the furnace exceed the indicated temperature. Strictly control the content of low melting point alloy elements do not exceed the standard; with a speed of not more than 3 C / min slow heating; check and control the furnace temperature does not exceed 5 C; regularly check or calibrate the measuring and controlling instruments to ensure the accuracy of temperature measurement, indication and control.
The surface of the surface of the casting is different from that of the surface of the aluminum alloy. The chloride content in the nitrate solution exceeds the standard (>0.5%) and causes corrosion to the surface of the castings (especially at the loose and shrinkage holes); the surface of the castings is not cleaned sufficiently after removal from the nitrate tank, and the nitrate salts adhere to the surface of the castings (especially in narrow cracks, blind holes and channels); the nitrate solution is mixed with acid or alkali or the castings are placed in concentrated acid. Or alkali is corroded all around. Shorten the time when the castings are moved from the furnace to the quenching tank as far as possible; check whether the chloride content in the nitrate salt exceeds the standard; if it exceeds the standard, reduce its content (or concentration). Castings heated from the nitrate tank should be washed immediately with warm or cold water; check the acid and alkali content in the nitrate salt, if there is acid or alkali, should be neutralized or stopped. Use; do not place aluminum alloy castings around concentrated acids or bases.
The elongation and hardness of the thick and large parts of the castings quenched unevenly are low (especially the inner center), and the hardness of the thin-walled parts is high (especially the surface layer). The heating and cooling of the casting are uneven, the cooling of the thick parts is slow, and the heat penetration is poor. Re-heat treatment, reduce the heating rate, prolong the holding time, so that the thickness of the temperature balance; in the thick-walled parts of the coating insulation or coated with asbestos and other insulation materials, as far as possible to make all parts of the casting cooling at the same time; so that the thick parts of the first water; replace with organic quenchants, reduce the cooling rate.

Four. Heat treatment equipment and materials

1. Main technical requirements of heat treatment equipment
1) Because the temperature difference between quenching and aging of aluminium alloys is not large (because the quenching temperature is close to the melting point of eutectic composition at low melting point in the alloy), the temperature difference in the furnace should be controlled at 5.
2) the temperature and temperature control instruments are required to be sensitive and accurate so as to ensure that the temperature is within the error range mentioned above.
3) the temperature in each area of the furnace should be uniform, with a difference of 1-2 degrees.
4) quenching tank has heating device and circulation device to ensure water heating and temperature uniformity.
5) regular inspection and replacement of contaminated cooling water should be carried out regularly.
2. Quenching medium
Quenching medium is an important factor to ensure the purpose or function of various heat treatment. The higher the cooling rate of quenchant, the more intense the cooling of the casting, the higher the degree of supersaturation of the alpha solid solution in the metal structure, the better the mechanical properties of the casting, because a large number of intermetallic compounds and other strengthening phases are dissolved into the alpha solid solution of Al. According to the cooling rate of the quenching medium, the quenching medium is the mixture of dry ice and acetone (- 68), ice water, water at room temperature, water at 80 – 90, water at 100, atomized water, various oils (rapeseed oil, etc.), oil and air heated to 200 – 220, etc.
In recent years, the cooling rate of aluminum alloy quenchant developed in China is between water and oil. For example, the water-soluble quenchant of De Runbao Co., Ltd. can dissolve with water in any proportion. Its mixing ratio is different, and the cooling rate is different. Therefore, it is very convenient to adjust the cooling rate according to the quenching object. It does not need to be washed after quenching and its surface is clean, pollution-free, non-toxic, and rust-proof. Its main technical indicators are: appearance: light yellow to yellow viscous homogeneous liquid, density: 1.085-1.1234 mg /, viscosity Y38: > 154 MPa s, reverse melting point: 80-87, refractive n: 1.4138-1.4450, critical cooling rate: > 260 260 s (450-260 c).
The reason why the aqueous solution of Delubao organic quenchant has excellent quenching characteristics is that the solution can precipitate and decompose the organic components from the aqueous solution when the quenching temperature rises to a certain value, and form a uniform conductive film on the surface of the workpiece. The quenching bubble acts directly on the workpiece. This film is used instead of directly acting on the workpiece, thereby reducing the direct hammering effect of forming quenching stress, thereby reducing the deformation and cracks of the workpiece. After quenching, when the water solution is cooled to a certain temperature, the organic film is dissolved in the water solution and returns to the original uniform water solution state. It does not impede the effect of repeated use.
3. Temperature measurement, temperature control meters and materials.
The precision of temperature measuring and controlling instruments should not be lower than 0.5 grade. Heat treatment heating furnace should be equipped with automatic recording, alarming, power-off and power-restoring devices and instruments which can automatically measure and control temperature to ensure accurate temperature display and control and uniform temperature.