7020 aluminum extrusion profile

In recent years, with the proposal and promotion of the concept of lightweight materials, aluminum alloy, as a recognized lightweight alloy, is widely used in aerospace, rail transit and other fields because of its excellent properties such as light weight, high strength, strong metal formability and weldability. At the same time, with the higher and stricter requirements for the properties of different aluminum alloys in various fields, it is necessary to analyze the change law of material properties in different production environments by means of a large number of microstructure observation.

7020 aluminum alloy belongs to Al Zn Mg alloy. 7020 aluminum extension profile is mainly used for sleeper beam, traction beam, large and small cross beams and bottom plates of high-speed train underframe. Long term service of materials in complex external environment, high load, frequent acceleration and deceleration and other environments is prone to premature fatigue and failure, which can not reach the design service life. Therefore, it is of great engineering significance to reveal the mechanism of multiphase microstructure of 7020 aluminum alloy on fatigue crack initiation, propagation and fracture, and to explore the regulation direction of multiphase microstructure to improve the fatigue resistance of 7020 aluminum alloy.

1) 7020 aluminum extension profile , when the stress ratio R is 0 and the fatigue limit life is 107 cycles, the fatigue strength is 232.9 MPa. Crack tip stress intensity factor Δ When k = 8 MPa · M1 / 2, the fatigue crack growth rate of the alloy is about 6.44 × 10-5 mm/cycle。

2) Size 3 ~ 12 μ Under the action of fatigue stress, the coarse refractory crystalline phase of M is easy to crack or separate from the matrix interface due to incongruous deformation with the matrix. When the coarse refractory crystalline phase is within the stress field at the tip of the main fatigue crack, the fatigue crack propagation will be accelerated. The small dispersed refractory crystalline phase slows down the crack growth rate due to dispersing fatigue stress, increasing crack section roughness and inducing crack closure.

3) When the loading direction of fatigue stress is the same as that of extrusion, the fatigue crack mainly propagates in transgranular mode. When the crack propagates to the recrystallized small grain with large orientation difference in the easy slip plane, the crack is easy to propagate rapidly along its large angle grain boundary. When the adjacent grains have the same orientation, the propagation direction is similar and propagates slowly in a transgranular manner. The lower the recrystallization degree and the proportion of large angle grain boundary, the slower the crack growth rate.

Through different corrosion tests on extruded 7020 aluminum extrusion profile T4 and T6, it is found that:

(1) the grain boundary, microstructure and grain size of extruded 7020 aluminum alloy T4 and T6 can be clearly observed when hydrofluoric acid solution ∶ water = 1 ∶ 1;

(2) phosphoric acid solution: water = 1 ∶ 9, sulfuric acid solution: water = 1 ∶ 9 ~ 2 ∶ 8, nitric acid solution: water = 1 ∶ 3. The size and distribution of the second phase in T4 and T6 state of extruded 7020 aluminum alloy can be clearly observed.