Hot Isostatic Pressing on Gas Holes in Aluminum Alloy Castings (Part Two)
1. Results and Analysis
In summary, this paper analyzes the generation and mechanism of the Y-shaped linear defects that appear at the original hole locations after the sample is hot-pressed and heat-treated.
2.1 The formation of Y shaped linear defects
Figure 8 shows a schematic diagram of the sample. The dotted line in the figure outlines the gas holes before hot pressing, and the arrow indicates the direction of stress on the sample during the hot pressing process. Under the action of high temperature and high pressure, the parts around the gas hole begin to plasticize, deform, and extend into the gas hole, eventually forming a Y-shaped bridging interface inside the gas hole. When X-ray inspection is performed, it appears as a Y-shaped linear defect.
Figure 8 Schematic diagram of the formation of Y-shaped linear defects
2.2 Formation mechanism of Y shaped linear defects and joint surface cracks
During the production process of the sample, an oxide film forms on the surface after the gas hole’s surface comes into contact with air. When the sample is butt welded, air is contained in the closed gas holes, and the high temperature during the hot pressing process also promote the reaction between oxygen in the air and the gas hole’s surface to produce an oxide film. The composition of the oxide film is generally aluminum oxide (Al2O3), with a melting point of 2054°C. It is distributed on the surface of the gas holes as an inert compound. When hot isostatic pressing is performed, the alloy structure around the gas holes undergoes plastic deformation under high temperatures and high pressures, and then extends and creeps into the defects, filling the gas holes and bridging them, forming a bridging interface. However, due to the oxide film, complete chemical diffusion between the elements on the interface formed during bridging was not achieved, metallurgical bonding was not achieved, the interface did not completely disappear, and the defects formed linear closed cavities. During the heat treatment, the alloy is kept at a high temperature, but there is no external high pressure. The gas that originally diffused into the alloy precipitates again, and the volume of the closed cavity at the bridging interface increases, appearing as linear defects macroscopically. Similarly, the butt surfaces are bridged under pressure, but due to the oxide film, metallurgical bonding is not achieved. During the heat treatment, gas precipitates again, causing the bridged surfaces to separate, appearing as a crack defect macroscopically.
2.1 The formation of Y shaped linear defects
Figure 8 shows a schematic diagram of the sample. The dotted line in the figure outlines the gas holes before hot pressing, and the arrow indicates the direction of stress on the sample during the hot pressing process. Under the action of high temperature and high pressure, the parts around the gas hole begin to plasticize, deform, and extend into the gas hole, eventually forming a Y-shaped bridging interface inside the gas hole. When X-ray inspection is performed, it appears as a Y-shaped linear defect.
Figure 8 Schematic diagram of the formation of Y-shaped linear defects
2.2 Formation mechanism of Y shaped linear defects and joint surface cracks
During the production process of the sample, an oxide film forms on the surface after the gas hole’s surface comes into contact with air. When the sample is butt welded, air is contained in the closed gas holes, and the high temperature during the hot pressing process also promote the reaction between oxygen in the air and the gas hole’s surface to produce an oxide film. The composition of the oxide film is generally aluminum oxide (Al2O3), with a melting point of 2054°C. It is distributed on the surface of the gas holes as an inert compound. When hot isostatic pressing is performed, the alloy structure around the gas holes undergoes plastic deformation under high temperatures and high pressures, and then extends and creeps into the defects, filling the gas holes and bridging them, forming a bridging interface. However, due to the oxide film, complete chemical diffusion between the elements on the interface formed during bridging was not achieved, metallurgical bonding was not achieved, the interface did not completely disappear, and the defects formed linear closed cavities. During the heat treatment, the alloy is kept at a high temperature, but there is no external high pressure. The gas that originally diffused into the alloy precipitates again, and the volume of the closed cavity at the bridging interface increases, appearing as linear defects macroscopically. Similarly, the butt surfaces are bridged under pressure, but due to the oxide film, metallurgical bonding is not achieved. During the heat treatment, gas precipitates again, causing the bridged surfaces to separate, appearing as a crack defect macroscopically.
3. Actual verification
To further verify the above test results, a sample with a gas hole (a diameter of about 10 mm) was hot-pressed. The X-ray inspection results and the actual object before hot pressing, during hot pressing, and after heat treatment are shown in Figure 9. No gas hole was found in the X-ray examination after hot pressing, but after heat treatment, the original gas hole locations became linear defects. The sample was cut open at the defect location, revealing linear defects visible to the naked eye. This means the gas holes are flattened by hot pressing, but the complete diffusion of each element does not occur between the interfaces, and metallurgical bonding is not achieved.
(a) Before heating pressing (b) After heating pressing (c) After heat treatment (d) The actual object
Figure 9 X-ray detection results and physical images before and after hot pressing and after heat treatment
4. Conclusion
Through the hot isostatic pressing simulated gas hole test and actual verification results of the ZL205A alloy, it was found that hot isostatic pressing treatment can bridge the gas hole defects inside the aluminum alloy to a certain extent. However, due to the presence of oxide films on the gas hole surface, various elements on the interface are hindered. The metallurgical bonding state is not reached at the interface, so the gas hole defects that are flattened after heat treatment become linear defects, which are similar to crack defects and are more harmful to castings. In other words, hot isostatic pressing is not suitable for eliminating gas hole defects inside aluminum alloys. When hot isostatic pressing is used to eliminate porosity and other hole defects in aluminum alloy castings, the existing gas hole defects need to be repaired in advance using traditional methods to prevent them from turning into linear defects and increasing the hazards to aluminum alloy castings.
4. Conclusion
Through the hot isostatic pressing simulated gas hole test and actual verification results of the ZL205A alloy, it was found that hot isostatic pressing treatment can bridge the gas hole defects inside the aluminum alloy to a certain extent. However, due to the presence of oxide films on the gas hole surface, various elements on the interface are hindered. The metallurgical bonding state is not reached at the interface, so the gas hole defects that are flattened after heat treatment become linear defects, which are similar to crack defects and are more harmful to castings. In other words, hot isostatic pressing is not suitable for eliminating gas hole defects inside aluminum alloys. When hot isostatic pressing is used to eliminate porosity and other hole defects in aluminum alloy castings, the existing gas hole defects need to be repaired in advance using traditional methods to prevent them from turning into linear defects and increasing the hazards to aluminum alloy castings.
