1. Features of casting inspection

Castings are now widely used in aviation and aerospace, including aluminum alloys, magnesium alloy titanium alloys and high-temperature alloys. Compared with casting and blank processing to form workpieces, castings have low cost and can form very complex shapes, which is difficult for processing technology. Most of the castings have defects, and some are even so serious that they affect the performance of the entire casting. Therefore, non-destructive testing must be performed to ensure its quality.

For the internal quality inspection of castings, the mature and conventional method is film radiography. Commonly found internal defects of castings include shrinkage porosity, shrinkage porosity, bubbles and inclusions.

According to the radiographic results, the internal defects of the castings are classified, and the judgment of qualified or unqualified is made.

However, for castings with complex and irregular shapes and internal structures, X-ray radiography (RT) or ultrasonic methods (UT) are no longer suitable for internal defect detection. Industrial CT can not be affected by the shape of the specimen structure, CT and RT usually provide more defect information.

This is because there will be overlap of information in RT, which reduces the sensitivity, while in CT, these factors can be eliminated. CT can determine the size and location of the looseness, pores, shrinkage holes and cracks inside the test piece. For the classification and evaluation of defects, in-depth information is very useful.

Since all the spatial information and defect information of the test piece can be obtained through the industrial CT, the defect can be judged more accurately, and the misjudgment or missed judgment can be reduced.

However, in the current CT non-destructive testing, the damage location is mainly found by observing a set of two-dimensional slice images, which often requires the experience of engineers to determine. As for the accurate determination of the spatial location, size, and geometric shape of the injury site, it is difficult to achieve only by observing the two-dimensional slice image. At present, the software for post-processing industrial CT images, especially 3D reconstruction, is mostly foreign products. Because the industrial CT equipment itself is very expensive, the price of the post-processing software for 3D reconstruction is also very high, which limits the application of 3D reconstruction.

2. Three-dimensional reconstruction

The purpose of 3D reconstruction is to better realize the special requirements of inspection, and facilitate the observation of defect space shape and specific density components. Three-dimensional imaging research can be divided into two categories. One is to study direct projection data for three-dimensional reconstruction, or called true three-dimensional reconstruction technology, which refers to the use of obtained two-dimensional projection data to achieve direct three-dimensional imaging. The other is the stacking of multiple 2D CT images to produce 3D images of the sample, such as surface display method, triangulation method, Delaunay triangulation method, etc., which use limited tomographic data to obtain a smooth object surface closer to the actual .

3. Application in rapid manufacturing technology

Analyzing and researching existing parts, especially advanced devices, is an effective means to learn from advanced design, and it is also a shortcut to manufacturing products and innovation.

However, how to perform physical measurement quickly and accurately has always been the “bottleneck” of this shortcut. Traditional measurement methods such as manual measurement, projection measurement, three-coordinate measurement, etc. all have measurement circumferences, large degrees of manual intervention, and difficulty in adapting to modern products. The rhythm of upgrading. The interface between industrial CT and rapid manufacturing system is an effective way to solve this “bottleneck” problem. 4. Application in 3D structural analysis

Three-dimensional reconstruction can not only realize the application of industrial CT in copy manufacturing, but also obtain cross-sectional views in any direction, so as to realize accurate analysis and measurement of internal structure dimensions.

5. Main conclusion

(1) CT has a high resolution for the detection of castings, and is currently one of the most accurate and reliable non-destructive evaluation methods;

(2) Three-dimensional imaging detection can observe the spatial shape of the internal defects of the casting, realize the measurement of arbitrary cross-sectional density and internal structure size, and solve the limitation of the scanning fault direction and fault discontinuity of the two-dimensional tomographic imaging. A very important computer-aided evaluation method.

(3) The problem of the interface with the rapid prototyping machine is solved, so as to realize the application in reverse engineering and shorten the aerospace mold design and product development and production cycle.