The fracture is located at the root of the support arm on the casting, and the overall paint surface of the broken support arm is complete without peeling.
Figure 1 shows the fracture morphology of the arm. It can be seen from Figure 1 that the fracture is relatively straight, but the overall contrast of the fracture is different. The contrast in the red dotted line is significantly deeper. The fracture morphology at the edge of the corner was further observed by magnification. Figure 2 shows the fracture edge morphology of the arm. It can be seen from Figure 2 that the fracture edge of the arm presents obvious non-metallic luster, indicating oxidation.
The fracture edge morphology of the arm was further observed by scanning electron microscope. Fig. 3 shows the fracture morphology observed by scanning electron microscope. It can be seen from Fig. 3A that no metallurgical defects such as inclusions and slag inclusions are found on the fracture surface. It can be seen from Fig. 3B that the fracture surface presents cleavage fracture characteristics, and a small number of dimples are observed. Further zoom in to observe the morphology of the edge. Figure 4 shows the morphology of the arm fracture edge at 900 times magnification. It can be seen from Figure 4 that flocculent nonmetallic substances are attached to the fracture surface. It is found that flocculent substances are mainly rich in O, s and Cr by EDS analysis of scanning electron microscope. The cast zl116 alloy does not contain Cr element according to the chemical composition in Table 1. X-ray fluorescence spectrometer is used to analyze the chemical composition of the fracture sample. Table 2 shows the measured chemical composition of the fracture arm. Comparing the chemical composition in Table 1 and table 2, it is found that the chemical composition of ZL116 alloy meets the design composition requirements, and no Cr element is found in the matrix. Samples were taken near the fracture of zl116 alloy arm to observe its microstructure. Fig. 5 shows the microstructure near the fracture of the arm. It can be seen from the observation in Figure 5 that the alloy structure is grayish white α- Al matrix, eutectic Si and Mg2Si strengthening phase are typical zl116 alloy after heat treatment. The sample was cut from the matrix near the fracture for Brinell hardness test. The hardness value test result was HB 102, meeting the requirements of technical conditions.
Through the above analysis of the structure and properties of the arm, it is shown that theand heat treatment process of the alloy meet the production process specifications, and the fracture of the arm is not caused by the defects introduced in the process of casting and heat treatment. According to EDS analysis at the fracture edge, there are flocs of O, s and Cr elements. In combination with the process flow of the arm preparation, no crack defects were found in the NDT after casting, while CR could only be introduced from the anodic oxidation process, because dichromate needs to be added to the electrolytic cell solution during the anodic oxidation process, and it is precisely in this process that cracks were formed in the edge area of the arm corner area due to excessive processing stress during machining, It causes Cr ions to enter the fracture surface. It is the cracks formed in the machining process that lead to the rapid propagation of cracks in the aluminum alloy arm components during service, and finally lead to fracture.
According to the confirmation of the cracking cause of the support arm, the nondestructive testing process is added after the casting machining process to determine whether there is a fine crack during the machining process. If there is a crack in the flaw detection, the part needs to be scrapped. By adding non-destructive testing after machining, no arm fracture occurred in the subsequent ZL116 aluminum alloy arm components, which improved the safety and reliability of the casting in service.