## 3 Experiments and Processes
### Analysis of Gas Leakage Sources and Surface Microstructure  
After completing the gas sealing performance test, the end cap at the top of the setup was opened, and an appropriate amount of water was added above the alloy in the experimental sample, as shown in Figure 8. At the bottom side of the plug, pressurization was carried out until bubbles were generated on the water surface. It could be clearly observed that the bubbles all came from the circumference of the alloy plug, which indicated that there was a micro-annular gap at the interface between the alloy and the casing, which formed a flow path for the passage of the gas. The absence of bubbles on the round surface of the alloy plug indicates that the experimental alloy plug did not generate a continuous path for fluid passage internally during the molding process.  
Figure 7: Structure of the gas sealing performance test setup.  
The alloy plug samples were wire-cut, scanned, and analyzed using an optical microscope at several sampling points of the alloy plug samples, and the results are shown in Figure 9. The 100 mm alloy plug had parts with a large gap, which was due to the fact that the upper part of the alloy plug fails to fill the surface of the threaded groove fully during the process of the alloy solidifying and expanding, while the lower part of the alloy plug fills the surface of the threaded groove fully due to the influence of the gravity of the alloy plug itself. As a result, the gap between the lower alloy plug and the casing was small and uniform. As shown in Figure 9, there was a small and uniform gap between the alloy plug and the casing in the 150 mm alloy plug. This was due to the pressure generated by the alloy 50 mm above the threaded area, so the lower part of the alloy would fill the surface of the threaded groove fully during solidification and expansion, which was in contrast to the larger gap in the 100 mm alloy plug, indicating that the weight of the alloy itself could affect the molding process of the alloy plug.  
The alloy plug was separated from the casing in the cut alloy plug sample, as shown in Figure 10. And the surface of the alloy plug and casing was measured for roughness. The results of the measurements are shown in Figure 11.  
The leakage pathway in the casing-alloy interface is also due to the poor engagement of microstructures on the contact surfaces, which results in micro-annular channels. Figure 10 indicates the poor surface engagement area, where the surface roughness of the alloy plug was low and the surface roughness of the casing at the corresponding position was high. Therefore, more gaps between the surfaces of the alloy plug and casing at this location were generated, which was the main concentration area of the micro-annular gap. Meanwhile, Figure 10 is the better surface engagement area, where the peaks and valleys on the micro-surface of the alloy plug and the corresponding micro-surface of casing formed a better engagement, and the number of micro-annular gaps generated in this area was relatively small.  
Figure 8: Observations of alloy plug gas leakage sources.  
Figure 9: Optical microscope observations of threaded casing alloy plug samples with different plugging lengths.