Views:7 Author:Site Editor Publish Time: 2019-10-19 Origin:Site
After heat treatment, the inner surface of 2A12 aluminium alloy shell was processed by numerical control, and the longitudinal crack defect on the inner surface of the shell was found by inspection.
The aluminium alloy is a kind of strengthened aluminium alloy which can be heat treated. Its strength is higher after artificial aging, natural aging or solution heat treatment. It has good machinability and plastic forming ability. Because it can obtain different types of aluminum alloy products, it has been widely used in aerospace and other industries.
By analyzing the macro and micro characteristics of the inner surface cracks of alloy shell parts and the processing process of the parts, the failure causes are found out and the improvement measures are put forward.
By analyzing the chemical composition of the fracture surface of the fault part, the content of each element meets the requirements of relevant standards, so the possibility of cracking on the inner surface of the part caused by the mixture of raw materials is excluded.
The results of low-magnification corrosion test of faulty parts exclude the possibility of cracking on the inner surface of parts caused by serious metallurgical defects of raw materials.
Aiming at the crack defect in the inner wall of the shell part, the processing flow was analyzed in detail. The original processing technology of 2A12 aluminium alloy shell parts is as follows: blanking car center hole (_65) heat treatment (quenching + natural failure) car inner hole (_140) failure. Before heat treatment, the wall thickness of parts reaches 92.5mm.
The metallographic analysis of the failure parts after heat treatment (quenching + natural failure) shows that the reason for the cracks in the inner wall of the shell parts is that the tube wall is too thick and the heat treatment is not enough.
According to the actual situation of raw materials for aluminium alloy shell parts, further optimization of heat treatment process, quenching + artificial aging heat treatment. The well-type furnace is heated to 495 +5 C, kept for 180-210 min, and water-cooled. No over-burning was found in the microstructure, and the agglomeration of strengthening phases along grain boundaries was obviously improved. It is proved that the crack initiation on the inner surface of the part is caused by inadequate heat treatment.
By analyzing the processing technology of the faulty parts, it is concluded that when the heat treatment is insufficient and the cutting speed and cutting quantity are maintained, the stress at the tool tip concentrates on the agglomeration of grain boundary strengthening phases, which leads to the rapid propagation of cracks along the zonal distribution of strengthening phases.
Axial cracks appear on the inner surface of aluminium alloy shell during quenching and tempering heat treatment. Scanning electron microscopy (SEM) analysis of the cracks was carried out, and the results showed that the cracks were caused by insufficient heat treatment at the center of the part and excessive cutting speed and quantity during machining.
In order to avoid the inner wall cracks of aluminum alloy shell parts in the process of processing, it is suggested that the size of the center hole should be enlarged before heat treatment in the processing procedure. In the process of heat treatment, the cooling speed of the parts should be accelerated so as to promote the complete transformation of the structure.