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How to Prevent Coarse Grain Defects of Aluminum Alloy After Heat Treatment?

Coarse grains often appear in annealed and quenched plates, and “orange peel” and cracking defects appear on the surface during stamping.

Quenching and annealing temperatures are too high, and the holding time is too long, especially slow heating can easily cause coarse crystals. Anti-rust aluminum, which cannot be strengthened by heat treatment, will have coarse crystals when the annealing temperature is slightly higher. The material composition is not uniform. For example, Mn forms intragranular segregation, which makes the recrystallization temperature of the same part inconsistent. At the same annealing temperature, the nucleation rate of the area with higher Mn content is less, and the crystal grains are coarse.

Plastic deformation is in critical deformation, where the annealed grains grow up sharply. The rolling temperature of aluminum alloy profile is too low, and the deformation is uneven everywhere. Coarse crystals are formed in the parts with small deformation after heat treatment. If the deformation is too small, the inner and outer parts of the product will be deformed differently, forming part or all of the coarse crystals. For extruded aluminum parts, the outer surface has high resistance and slow deformation. After annealing, a coarse crystal ring is formed on the outer surface.

In order to prevent coarse crystals, the critical deformation degree (5%~15%) should be avoided during plastic deformation. Before deformation processing, stress relief annealing is used to eliminate the stress of grain growth. Pre-drawing of the blank, multiple forming and other processes can make the amount of deformation exceed the critical degree of deformation.

The Fe and Ti in the ingot have the effect of refining the crystal grains. The uniform annealing of the ingot and reducing the influence of Mn segregation can control the coarse crystal. When annealing in a nitrate bath, increasing the annealing heating rate, such as high-temperature rapid annealing, is extremely beneficial to the grain refinement of various aluminum alloys.

For different aluminum alloys, the quenching temperature should be controlled. For example, forged aluminum 6A02 (LD2), although the melting point temperature reaches 590℃, the crystal grains will grow when the temperature reaches 540℃, so the commonly used quenching temperature is (520±10)℃.