Ingot Homogenization Annealing
1 The principle of homogenization annealing
The purpose of ingot homogenization annealing before hot rolling is to make the unbalanced eutectic structure in the ingot tend to be uniformly distributed in the matrix, and supersaturated solid solution elements are precipitated from the solid solution to eliminate casting stress, improve ingot plasticity, reduce Small deformation resistance, improve the structure and performance of processed products. The reason for the non-equilibrium crystalline structure of the ingot is that the diffusion process is blocked during crystallization. This structure is thermodynamically metastable. If the ingot is heated to a certain temperature, the internal energy of the ingot is increased, and the thermal motion of the metal atoms is enhanced. Unbalanced metastable tissues gradually tend to stabilize tissues.
The ingot homogenization process is achieved by the diffusion of alloying elements. When the ingot is homogenized and annealed, the diffusion of atoms is mainly carried out in the crystal, so that the chemical composition in the crystal is uniform. It can only eliminate intragranular segregation and has little effect on regional segregation. Since the homogenization annealing is carried out below the temperature of the unbalanced solidus or eutectic line, the insoluble and non-metallic inclusion defects distributed among the grains of the ingot cannot be eliminated by the process of dissolution and diffusion. Therefore, the homogenization annealing The shape of the matrix grains in the alloy cannot be significantly changed. In the process of ingot homogenization annealing, in addition to the diffusion of atoms, it is also accompanied by changes in structure, that is, the dissolution and diffusion of soluble intermetallic compounds and strengthening phases enriched on grain and dendrite boundaries, as well as supersaturation Precipitation and diffusion of solid solution, so that the ingot structure is uniformly treated, and the processing performance is improved.
Hard aluminum alloy ingots, such as 2A11, 2A12, 7A04, 7A03, 7A05, 7A09, etc., must be homogenized before hot rolling. Soft aluminum alloys with simple composition and high plasticity, such as pure aluminum, 3A21, etc., have special requirements on the final structure and properties of the product (such as requiring fine grain size, good deep drawing energy, etc.), and the structure should also be homogenized. Otherwise, Soft aluminum alloys may not be homogenized.
2 Determination of Homogenizing Annealing Parameters
The process regime of homogenization annealing includes annealing temperature, heating rate, holding time and cooling rate. In the process parameters of homogenization annealing of aluminum alloy ingots, the annealing temperature plays a major role.
A Annealing temperature
The homogenization annealing temperature usually used in industrial production is 0.9T<T<0.95T, where T is the actual melting temperature. Sometimes it is difficult to achieve the purpose of homogenization of the structure by performing homogenization annealing at a temperature lower than the non-equilibrium solidus temperature, and even if it can be achieved, it will take a long time. In order to strengthen the homogenization process, the homogenization annealing temperature should be increased as much as possible, and high temperature homogenization annealing can be used, that is, the annealing process above the non-equilibrium solidus temperature and below the equilibrium solidus temperature. The homogenization temperature should be selected as high as possible, but should be lower than the unbalanced solidus or the low melting point eutectic temperature in the alloy by 5-40 °C. A reasonable annealing temperature is often determined through experiments.
B heating rate
The heating rate is based on the principle of not causing cracking and excessive deformation of the ingot. c holding time
The holding time should ensure a certain annealing temperature to dissolve the non-equilibrium phase and eliminate intragranular segregation, but it should be determined according to the characteristics of the alloy, the size of the ingot, the degree of segregation, the shape, size, and distribution of the second phase, heating equipment, and temperature, etc. The length of the holding time. The practice has proved that the rate of the homogenization process increases with time, and gradually decreases from large to large. Therefore, it is not suitable to prolong the holding time too much, which will increase the burning loss and energy consumption of the metal, and reduce productivity.
C cooling rate
After homogenization, the ingot can be cooled with the furnace or air-cooled from the furnace. The cooling rate should not be too fast for duralumin, so as to avoid the quenching effect.
Ingot homogenization annealing can be carried out alone or in combination with heating before hot rolling. That is, the ingot is heated to the homogenization annealing temperature, and then lowered to the hot rolling temperature after holding for a certain period of time, and then hot rolling, which not only reduces the process but also saves energy consumption. However, the homogenization annealing temperature is high, the time is long and the energy consumption is large, and the homogenization treatment is not necessary for the alloy with simple composition, less serious segregation, and good plasticity.
Generally, superhard aluminum alloy, hard aluminum, forged aluminum, and rust-proof aluminum need to be homogenized and annealed.
Homogenizing Annealing System
On the basis of a lot of research and factory practice, the optimal homogenization annealing system for aluminum and aluminum alloys has been formulated.
3 Control points of the homogenization annealing process:
(1) Before the heat treatment tooling furnace, check the operation of the equipment, the closed furnace door, the spreader, etc., and the furnace can be installed after confirming that it is normal. (2) The heat treatment worker should write down the date, furnace loading time, alloy grade, melting time, furnace number, quantity, soaking system, power transmission, power failure and change of fixed temperature and release time on the soaking record book, and sign it. (3) Ingots of the same soaking system can be loaded into the same furnace; two ingots of the same melting level must be loaded into the same furnace. Load the oven immediately after it comes out of the oven or alternately. Power outages, wind outages, and use of safety racks are required to install out of the furnace. (4) Put the unsoaked ingot into the explosion-proof frame. 5) The matching error of the instruments, galvanic couples and the entire temperature control system of the soaking furnace is within ±4°C; the temperature difference of the empty furnace is not higher than 10°C. (6) According to the temperature and time required by the heat treatment workers at work or in this class, timely and accurately power supply, power failure or change the fixed temperature. (7) According to the recorded soaking system, observe the temperature indicated by the meter in each area 1-2 hours before the fixed temperature needs to be changed. If the second zone or above reaches the fixed temperature in advance, the fixed temperature should be changed in advance, but the advanced time must be added to the holding time, that is, the holding time should be extended so that the total time remains unchanged. (8) When it is found that the surface of the ingot is abnormal, the soaking temperature of the ingot exceeds the specified temperature and time, or the surface of the ingot is found to have black precipitates or other special circumstances, the microstructure inspection should be carried out. After confirming that the structure is not over-burned, can be put into production. (9) For the 2A12 and 2024 ingots that are uniformly fired, samples shall be taken at the high temperature end for microstructure inspection, and there shall be no less than one sample per furnace. The sample location is 15-20mm away from the surface of the ingot. (10) In industrial production, it is best to use a resistance furnace with a forced hot air circulation system for uniform annealing of keys, and a sensitive temperature control system should be installed to ensure uniform furnace temperature. (11) In order to effectively utilize the electric furnace, it is required that the homogenized annealed ingots should be classified into the furnace according to the type of alloy, external dimensions and homogenization annealing temperature. When the furnace temperature is higher than 150 ℃, the furnace can be installed directly, otherwise the furnace should be preheated according to the electric furnace preheating system. When loading the furnace, there should be a gap in the position of the ingot in the furnace to ensure the smooth flow of hot air. (12) The cooling rate of the homogenized ingot is generally not strictly controlled. In actual production, it can be cooled in the furnace or stacked together in the air for cooling. However, when the cooling is too slow, the particles of the precipitated phase from the solid solution will grow very coarse. (13) During homogenization annealing, first set the temperature of the heating furnace to the homogenization temperature. After the ingot is loaded into the furnace, the holding time will not be calculated until the surface temperature of the ingot rises to the homogenization temperature. Generally, the upper limit of the use time of large ingots, the lower limit of the use time of small ingots; the lower limit of the use time of high temperature, and the upper limit of the use time of low temperature.