The process of forging aluminum alloy blank in forging press is called aluminum alloy die forging.

Advantages of aluminum alloy die forging:

1, the production efficiency is high

2. Low labor intensity

3, the size is accurate and the machining allowance is small

4. Aluminum alloy forgings with complex shapes can be forged.

5, suitable for mass production.

6. Brass die forgings have high mechanical properties.

7. Service life factory.

Disadvantages of aluminum alloy die forging;

1, the mold cost is high.

2. Special die forging equipment is required.

3, not suitable for single piece or small batch production.

Measures to prevent aluminum alloy forging oxidation and reduce decarburization

When aluminum alloy is heated in the forging process, the phenomenon that iron and alloy on the surface of material steel react with elements and oxygen, carbon dioxide and water vapor in the medium to form oxide film is called oxidation. The dimensional accuracy and surface brightness of high temperature workpiece deteriorate after oxidation, and steel parts with poor hardenability of oxide film are prone to quenching soft spots.

In order to prevent aluminum alloy forging oxidation and reduce decarburization measures are:

1, the workpiece surface coating, sealed with stainless steel foil heating.

2, using salt bath furnace heating.

3. Use protective atmosphere for heating.

4, using flame burning furnace.

When raw materials are heated in the forging process of aluminum alloy, the surface carbon reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium, which reduces the surface carbon concentration, which is called decarburization. After decarbonization, the surface hardness, fatigue strength and wear resistance are reduced, and residual tensile stress on the surface is easy to form surface reticular cracks.

Coarse grain defect is one of the common defects of aluminum alloy die forgings, which reduces the strength of forgings. The fatigue strength of forgings decreases in the coarse-grained structure and the transition zone from coarse-grained structure to fine-grained structure.

1 coarse crystal formation mechanism

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 [3-4]. 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. The abnormal growth of recrystallization grains is more likely to occur in the waste products with coarse grain defects of aluminum alloy die forgings.

2 to avoid or reduce the coarse grain defects of die forgings

The coarse grain defects of aluminum alloy die forgings are related to the forging material, forging process parameters, forging shape, die temperature and heat treatment process parameters.

2.1 Material of forgings

The probability of coarse grains in die forgings varies greatly with different aluminum alloy brands. Forgings of Al-Zn-Mg-Cu alloys are less prone to coarse grain defects, while forgings of Al-Cu-Mg and Al-Mg-Si alloys are more likely to have coarse grain defects.

2.2 forging process parameters and die temperature

(1) Select a reasonable final forging temperature.

If the final forging temperature is too low, coarse grains will easily appear in forgings, especially in Al-Cu-Mg series and Al-Mg-Si series alloys. For example, the final forging temperature of 2A11(LY11) alloy blade die forgings must be higher than 390℃, otherwise coarse grains will easily appear. Different materials of forgings have different requirements for final forging temperature, but all aluminum alloy die forgings are required to have a final forging temperature not lower than 370℃.

(2) The preheating temperature of the mold should not be too low.

Too low die temperature will accelerate the cooling rate of the metal in the die cavity, so that the deformation temperature of the metal is too low, which makes it difficult for the metal to fill the die cavity and may form coarse grains on the surface of the forging. The preheating temperature of the die is related to the forging shape and aluminum alloy brand, and it is generally required to be controlled at 300℃ ~ 400℃.

(3) the deformation degree should not be too small

Especially, the deformation degree of the last fire should not be too small. If the deformation degree is small, the recrystallization crystal nucleus is less and the incubation period is long, the die forgings will form coarse grains after reheating (or heat treatment). If the deformation degree is as small as the critical deformation degree (about 3% ~ 15%), the recrystallized grains will grow rapidly, resulting in coarse grain defects in forgings. It is necessary to strictly control the number of die forging heats and the reduction per heat in production, so as to avoid the forging being in a critical deformation state because of too many die forging heats and too small reduction at one time. Under the condition of ensuring that metal can finally fill the die cavity and meet the requirements of low streamline, the number of die forging fires should be reduced as much as possible. Forgings with simple shape and easy forming can be formed in one fire, and forgings with complex shape and difficult forming should not be forged for more than three times as far as possible.

(4) the wool allowance should not be too large

Especially for forgings with high ribs, if there is still excess metal after the metal has filled the cavity, if the upper and lower dies continue to close together, the excess metal at the web will flow directly into the burr groove along the root of the rib in the shortest route, which may cause local coarse grains here due to excessive deformation.

2.3 Selection of heat treatment parameters

Too high quenching temperature and too long holding time during heat treatment are easy to cause coarse grains in aluminum alloy forgings. Especially for Al-Mg-Si and Al-Cu-Mg alloys, the quenching temperature and holding time must be strictly controlled.

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