Coarse grain defect is one of the common defects in aluminum alloy precision forgings, which will reduce the strength of forgings. In the transition zone where the coarse-grained structure and coarse-grained structure change sharply to fine-grained structure in forgings, the fatigue strength of forgings decreases.

After plastic deformation, the free energy of metal is improved, and the structure is in an unstable state. When it is heated to a proper temperature, it will re-form crystal nuclei and grow. The microstructure composed of new grains is called recrystallization. Fine and uniform equiaxed grains can generally be obtained after recrystallization, but if the heating temperature is favorable for grain growth or the heating and holding time is too long, the recrystallized grains will grow into coarse grains. The process of grain growth can be divided into two types: one is gradual growth, which shows that the relative sizes of each grain are basically close; The other is abnormal growth, which shows that the relative size of each grain is extremely different, and some grains grow very coarse.

In the waste products with coarse grain defects of aluminum alloy precision forgings, the abnormal growth of recrystallization grains is more likely. 

Heat treatment purpose of all-aluminum light alloy high temperature forgings

The main purpose of heat treatment of all-aluminum light alloy high temperature forgings in forging workshop is to improve the cutting performance of forgings; The heat treatment after forging processing is carried out in the heat treatment workshop. The heat treatment of non-ferrous metal forgings is mostly completed by forging workshop.

An important link to ensure the quality of heat treatment is the uniformity of heating temperature of forgings. For quenching of most low alloy steels. The temperature precision of austenitizing should be controlled within 15℃. However, the heating temperature accuracy of nickel-based alloy, especially its aging treatment, should reach 15℃. For the heat treatment of high-strength aluminum alloy forgings, such high temperature control accuracy is also needed. In order to ensure such high accuracy, the so-called "block thermocouple" method is often used. That is, the test block is made of the same material as the forging, and the size of the test block is generally 40 mm× 25 mm× 25 mm. A thermocouple is inserted in it, and then the test block is placed in the middle of the forging to record the temperature of the furnace.

In order to control the heat treatment quality of forgings, the workshop should take a set of supporting measures, including marking the batch number of the heat treatment furnace, spot checking the hardness, and regularly checking the furnace temperature calibrator. 

For reference, here are some basic items:

1, the structure of the heating furnace should be reasonable. The uniformity of furnace size, furnace temperature and temperature in the furnace can meet the needs of heat treatment.

2. The size of the quenching tank is large enough. Ensure fast and uniform cooling. In order to control the temperature of quenching liquid, a heat exchanger should also be equipped.

3. The potentiometer should be checked regularly. The heating furnace should be equipped with potentiometers to control and record the temperature, and the records should be checked by special personnel.

4, check and spot check hardness. Hardness inspection is an integral part of heat treatment. In the tempering process of many steel grades, a forging is often selected for hardness inspection to determine whether it is necessary to adjust the temperature.

5, temperature uniformity inspection. The furnace temperature uniformity should be checked under normal working conditions once every six months.

6, according to the chemical composition modulation heat treatment system. For some superalloys or alloy steels, slight changes in composition require corresponding changes in the optimal aging or tempering temperature.

7. Make records. The records of the heat treatment furnace must correspond to the batch number of the heat treatment furnace for forgings. The batch number of the heat treatment furnace is good, which is next to the melting furnace number on the forging.

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