Analysis of Coarse Grain Defects in Aluminum Alloy Die Forgings

Coarse grain defects are one of the common defects in aluminum alloy die forgings, which reduce the strength of forgings. In the coarse-grained structure in the forging and the transition zone where the coarse-grained structure changes sharply to the fine-grained structure, the fatigue strength of the forging is reduced. jiahangaero mainly discusses the measures to avoid and reduce coarse grain defects in the production process of aluminum alloy die forgings.

• 1 Appearance mechanism of coarse grains

After the metal undergoes plastic deformation, the free energy increases, and the structure is in an unstable state. When it is heated to an appropriate temperature, the crystal nucleus is re-formed and grows. 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 conducive to grain growth or the heating and holding time is too long, the recrystallized grains will grow into coarse grains. jiahangaero indicates that the process of grain growth can be divided into two types: one is gradual growth, and the relative size between each grain is basically close; the other is abnormal growth, and the performance is The relative size of each grain is very different, and some grains grow very thick. In the coarse grain defects of aluminum alloy die forgings, the probability of abnormal growth of recrystallized grains is greater.

• 2 Avoid or reduce coarse grain defects in die forgings

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

2.1 Material of forging

The grades of aluminum alloys used to manufacture die forgings are different, and the probability of coarse grains in their products varies greatly. Forgings of Al-Zn-Mg-Cu alloys are less likely to have coarse-grain defects, while Al-Cu-Mg and Al-Mg-Si alloys have relatively more coarse-grain defects in forgings.

2.2 Forging process parameters and mold temperature

(1) Choose a reasonable final forging temperature

If the final forging temperature is too low, the forgings are prone to coarse grains. Jiahangaero claims that the final forging temperature must be strictly controlled especially for aluminum-copper-magnesium and aluminum-magnesium-silicon alloys, such as 2A11 (LY11) alloy blades. The final forging temperature of die forgings must be higher than 390°C, otherwise coarse grains will easily appear. Different materials of forgings have different requirements for the final forging temperature, but all aluminum alloy die forgings require that the final forging temperature is not lower than 370°C.

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

If the mold temperature is too low, the cooling rate of the metal in the cavity will be accelerated, so that the deformation temperature of the metal is too low, making it difficult for the metal to fill the cavity and may form coarse grains on the surface of the forging. The preheating temperature of the mold is related to factors such as the shape of the forging and the grade of the aluminum alloy, and is generally required to be controlled at 300°C to 400°C.

(3) The degree of deformation should not be too small

In particular, the degree of deformation in the last fire should not be too small. If the degree of deformation is small, the recrystallization nuclei are few, and the incubation period is long, coarse grains will form after reheating (or heat treatment) of the die forging. If the degree of deformation is small to the critical degree of deformation (about 3% to 15%), the recrystallized grains will grow rapidly, causing coarse grain defects to appear in the forging. In production, the die forging fire times and the reduction per fire of die forgings should be strictly controlled to avoid the forging being in a critical deformation state due to too many die forging fires and too small a reduction. In the case of ensuring that the metal can finally fill the mold cavity and the requirements of low flow line, the number of die forging fires should be reduced as much as possible.

(4) The wool allowance should not be too large

Jiahangaero said that 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 move closer together, the excess metal at the web will move along the root of the rib to the nearest The route flows directly into the burr groove, which may cause local coarse grains to appear here due to excessive deformation.

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