It can be seen from Fig. 1 (a) that the cracks are exposed on the forging surface, the plane composed of transverse and longitudinal cracks is parallel to the forging axial direction, and the crack plane is about in the middle region of the forging; After opening the crack, it is found that the color of the original crack position is dark gray, and the color of the manual opening position is light gray. The crack is located at the root of the 8-shaped hole and propagates along the forging axis, as shown in Fig. 1 (b).
The fracture surface was observed under scanning electron microscope, and obvious directionality was observed at the original fracture position. The fracture characteristics were fine dimples and brittle phase cracks, as shown in Fig. 2 (a). There is no obvious difference between the fracture of manual opening and that of original crack, as shown in Fig. 2 (b).
Energy spectrum analysis
The original crack fracture was analyzed by energy spectrum, and the detection position and results are shown in Figure 3. The results show that the main components of dimple area are consistent with the chemical composition of 2A14 aluminum alloy, and the main components of crack area are al, Si, Mn, Fe, etc., which are the main impurity phases of aluminum alloy. The results of energy spectrum analysis show that the chemical composition of forging is normal.
Metallographic examination was carried out on the edge and middle area of the failure part and the intact part respectively. The microstructure of the crack part of the failure part was determined by metallographic examination α It is composed of matrix, dispersed precipitated strengthening phase and chain like impurity phase. The impurity phase with cracks distributed along the chain cracks, as shown in Fig. 4 (a); The phase composition at the edge is the same as that at the crack area, but the microstructure deformation is smaller than that at the crack area, as shown in Fig. 4 (b). The same edge and center structure of the intact part and the failure part are made up of two parts α However, there is no significant difference in the degree of deformation, as shown in Fig. 5.
Brinell hardness test was carried out near the crack location of the shell, and the results meet the design requirements (≥ 130hb).
Residual stress detection
A intact part and a cracked part are selected for comparative detection of residual stress, and the detection location is shown in Figure 6. From top to bottom, they are 1 ᦇ 2 ᦇ 3 ᦇ 4 ᦇ with 1 ᦇ and 4 ᦇ at the edge, 2 ᦇ and 3 ᦇ near the crack. The stress of the intact part is small, but the tensile stress in the middle area of the cracked part is large, and the stress amplitude in the middle and edge is large.