Defects of aluminum alloy forgings during heating
1. Gallium oxide: Gallium oxide not only loses a lot of steel, but also reduces the surface quality and service life of forgings. If pressed into the metal, the forging will be scrapped. If gallium oxide is not removed, it will affect automobile processing.
2. Decarburization: Decarburization refers to the burning of all or part of carbon on the steel surface. Decarburization causes soft spots on the surface of the workpiece, reducing the surface hardness, wear resistance and fatigue strength.
3. Overheating and overburning: overheating refers to the grain thickening of steel after exceeding the allowable temperature during heating. Overheating is not conducive to heat treatment and reduces the brittleness and mechanical properties of steel, but can be eliminated by normalizing or annealing after forging. Overburning refers to metal oxide or partial melting due to long heating time and high temperature. Overheating cannot be remedied.
4. Stress: due to the different heating inside and outside the metal, the expansion is uneven, resulting in internal stress, which is called thermal stress. The change of metallographic structure caused by heating also causes stress, which is called structural stress. This will cause cracks in the heating process, resulting in cracks and scrapping after automobile processing.
5. Cross section: this defect destroys the chemical composition and structure uniformity of the steel, and reduces the quenching hardness and mechanical properties. If the temperature of graphite section is too high during annealing, it is not easy to cut, overheat and deform during quenching. However, if there is lack of heat or low temperature during annealing, pearlite will not be fully globalized, which is not conducive to cutting and subsequent heat treatment.
6. Hard and brittle network carbide: it weakens the binding force between crystals, significantly reduces mechanical properties, especially impact toughness, but can be improved or eliminated by normalizing. If there are banded carbides, the hardness and structure after quenching and tempering are uneven and easy to deform. This is also the banded structure defect of pearlite and ferrite along the processing deformation direction. At the same time, it will also reduce the plasticity and toughness of steel, make the automobile processing size unstable, and the tool wear rapidly.