After the sample navigation is completed, the detection can be started, as shown in Figure 1. Move the sample table so that the sample is under the objective lens, and adjust the working distance between the sample and the objective lens. Select the required aperture and probe, first minimize the magnification, use the mouse to focus, adjust the aperture alignment and astigmatism to make the aperture alignment, filament alignment and image number calibration, and then adjust the filament brightness and contrast to make the image clear gradually. The above steps are repeated many times until the clear image and the appropriate magnification are achieved.
After confirming that it is the clearest image, freeze the clearest image in the noise removal mode, and save the high-quality image to the specified hard disk directory. Then turn off the high pressure, let off the vacuum and take out the sample. The scanning and image generation process of a sample is completed. As shown in Figure 2, it is the enlarged picture of shrinkage defects formed when the internal wall thickness of the water cooling shell is uneven and crystallized.
When the clear image is reached, freeze the image, move the mouse to the image, click the right mouse button, select send to, and then select the format to save the image. Smart SEM software provides a variety of image file formats, generally JPEG or BMP. As shown in Figure 3, it is the enlarged BMP format picture of shrinkage defects formed when the isolated liquid phase position of the water cooling shell crystallizes. From the comparison between Fig. 2 and Fig. 3, it can be seen that the shrinkage defects formed in the isolated liquid phase position crystallization caused by the structural barrier of barrel cooling channel sand core are larger than those formed in the general uneven wall thickness position, so it is more necessary to take corresponding process measures to improve.
In practice, through the use of different low-pressure casting mold cooling technology schemes, and adjusting the process parameters such as pressurization speed and pressure value in each stage, different internal structure crystallization and solidification situation can be obtained, which can help the technicians to analyze and judge whether adding beryllium copper cooling insert at the corresponding position of the isolated liquid in the water-cooled shell can improve the shrinkage defects, Whether the pressure value is reasonable in the filling process of low-pressure casting provides a research basis for the optimization of low-pressureof water-cooled shell. In addition, the size of the detected defects in aluminum alloy castings can be visualized by color coding, and various parameters of different types of defects in low pressure casting can be obtained, such as the location, size and geometry of the defects; The graph tool can be used to count the total percentage and the volume histogram of shrinkage defects of aluminum alloy, to carry out local quantitative analysis on the image of tissue section of each batch of castings, to compare the geometry of shrinkage defects at the corresponding position of isolated liquid in different batches of castings, and to carry out more detailed digital quality analysis and process improvement.