1. Analysis of fracture morphology of aluminum alloy automobile gear
In order to analyze the fracture failure cause of aluminum alloy automobile gear, the fracture was tested. After the aluminum alloy automobile gear is broken, observe its damaged section. In the observation, four of the 17 teeth of the failed gear of the selected sample fell off almost as a whole, the other three teeth were partially damaged, and there was some damage to the bearing. The damaged part is mainly divided into two parts. One part is the junction of two disc bearings, which is mainly pure shear ductile fracture; The other part is located at the gear part, which is mainly characterized by local peeling or fracture and macro brittle fracture. For the two fracture characteristics in the same gear, the conditions such as fracture microstructure, grain size and defects are detected. Fracture sample of aluminum alloy automobile gear, as shown in the figure.
After observing the above samples, it can be seen that the fracture surface of the sample is relatively smooth and there are obvious rolling traces. It can be analyzed that the fracture is caused by fatigue crack propagation under the action of long-term high contact stress.
2. Metallographic examination of fracture
After grinding, polishing and corrosion treatment, the microstructure of the sample and the microstructure of carburized layer were observed, and its chemical composition was tested at the same time. Through the observation and analysis results, it can be seen that the sample penetration layer is mainly a small amount of austenite and tempered martensite, and there is a very small amount of white carbide at the same time. The microstructure distribution of carburized layer and transition zone is normal, the core structure is low-carbon lath martensite and free ferrite, and there is more free ferrite at the root of the tooth. ICP spectrometer is used for chemical detection of the sample. The chemical composition of the sample meets the provisions of structural steel with guaranteed hardenability, and its composition is shown in the table.
| Element | Mo | Cr | P | Mn | Si | C |
| Fracture specimen | 0.244 | 0.30 | 0.016 | 1.09 | 0.243 | 0.226 |
| GB/T5216~2004 | 0.02-0.30 | 1.05-1.40 | ≦ 0.035 | 0.85~1.20 | 0.17~0.37 | 0.17~0.23 |
| Standard value | 1.057 | 1.060 | 0.046 | 0.865 | 0.271 | 0.219 |
| Detection value | 0.15~0.26 | 1.00~1.30 | 0.04~0.10 | 0.80~1.10 | 0.13~0.37 | 0.17~0.23 |
Through the observation and analysis of the above metallographic structure, it can be seen that when the fracture failure of aluminum alloy automobile gear occurs, the tissue density is strengthened, there is the phenomenon of local stress concentration, and some structures change the original shape.