Why is titanium alloy framework cracked
There is one skeleton sample of titanium alloy casting. The casting treatment process is: hot isostatic pressing → nondestructive testing → repair welding → heat treatment → final testing. Cracks are found in the subsequent processing, and the causes of cracks are required to be analyzed.
Physical and chemical examination
Table of Contents
The macro morphology of casting cracks is shown in Fig. 1a. The outer surface near the crack has obvious metallic luster, the crack can not be closed, and there is a large gap in the middle. No obvious free surface was found by naked eye observation. The fracture was observed under three-dimensional video microscope, and a bright area was found on one side of the fracture. The morphology is shown in Fig. 1b and Fig. 1C.
Fig. 1 macroscopic appearance of sample
The fracture surface was observed under scanning electron microscope. Some areas of fracture surface are similar to free surface, while others are dimples. The morphology is shown in Fig. 2. Energy spectrum analysis was carried out for the similar free surface area and dimple area, and the main element content was basically the same, and no oxygen was found. The results of energy spectrum analysis are shown in Table 1.
Fig. 2 SEM fracture morphology
Table 1 results of energy spectrum analysis (mass fraction) (%)
The fracture specimen is split half in the middle for metallographic analysis, as shown in Fig. 3. It is found that there are weld area in the upper part of the right fracture, columnar crystal in the lower part, heat affected zone in the upper and lower parts of the left and right fracture, and obvious welding characteristics near the defects.
Figure 3 macromorphology of fracture section
The results of metallographic observation are as follows: the matrix is widmanstatten structure; The heat affected zone is needle shaped α+ Intergranular β+ Partial residual grain boundary α； The weld is needle shaped α+ Intergranular β+ Very little residual grain boundary α， The microstructure is shown in Figure 4.
Fig. 4 metallographic structure
Analysis and discussion
Shrinkage cavity and shrinkage porosity are common defects in titanium alloy casting process. Shrinkage porosity of titanium alloy can be eliminated by hot isostatic pressing (hip). However, after hip treatment, large-size spongy shrinkage porosity often occurs on the surface of casting, which usually needs repair welding. Compared with other metal casting shrinkage, the surface of titanium alloy casting shrinkage is not oxidized, but still metallic luster.
Macroscopic metallographic examination showed that repair welding was carried out near the crack of the wing skeleton after casting, but the original defects were not completely removed during the repair welding.
Scanning electron microscopy showed that the fracture surface was dimple fracture, and only a few areas were found to be similar to free surface. There was a gap that could not be closed when the fracture surface was observed by naked eye, which should be the original shrinkage defect produced by casting; The matrix of the casting is composed of coarse grains and coarse overheated widmanstatten structure. This structure determines that there is a large internal stress in the casting, and the stress at the original defects is easy to concentrate. At the same time, the plasticity of the casting itself is low, which easily leads to further cracking of the casting.
The reason for the cracking of titanium alloy framework is that there are shrinkage defects in the original casting, and the defects are not completely removed in the repair welding process, resulting in stress concentration. In the subsequent processing, the joint action of welding stress and processing stress leads to cracking, and the direction of cracking is from the defect to expand outward.
Authors: Pan hengpei, Xu kuilong, Li Zhihao, etc
Source: Network Arrangement – China Titanium Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.titaniuminfogroup.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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