Have you used aluminum pipes?
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As the title suggests, have you used aluminum pipes? What specifications and standards? The piping cannot be located.
All the pipes in cryogenic air separation units are made of aluminum; you might want to consult them.
There are corresponding specifications for many aluminum alloy pipes in air separation systems
1 Welding defects: The welding porosity in aluminum and aluminum-magnesium alloys is mainly hydrogen porosity. Aluminum can absorb and dissolve large amounts of hydrogen in its liquid state; its solubility at the molten state is 0.0069 ml/g, while it is 0.00036 ml/g when solidified at high temperatures, representing a difference of nearly 20 times. Aluminum has a high thermal conductivity; under the same welding conditions, its cooling rate is 4 to 7 times that of steel, which accelerates metal crystallization. During this rapid cooling process, the solubility of hydrogen decreases sharply, resulting in the formation of large amounts of supersaturated gas. Hydrogen does not have time to escape, and thus pores form within the weld metal. The alignment quality of the welded parts and the welding position are particularly important for the welding quality of aluminum-magnesium alloy fittings. Too small a fitting gap, excessive blunt edges, too small an groove angle, and excessive misalignment can all easily lead to incomplete penetration at the root in vertical and overhead welding positions. Most air separation pipelines consist of elbows, and their thickness is generally greater than that of the straight sections. Although there is a thickness transition, the volume per unit area on each side differs, which results in different heat transfer and heat dissipation rates. If proper adjustment of the welding torch angle and the wire position is not ensured, it is easy to end up with incomplete welding at one side of the root.
1 Welding defects: The welding porosity in aluminum and aluminum-magnesium alloys is mainly hydrogen porosity. Aluminum can absorb and dissolve large amounts of hydrogen in its liquid state; its solubility at the molten state is 0.0069 ml/g, while it is 0.00036 ml/g when solidified at high temperatures, representing a difference of nearly 20 times. Aluminum has a high thermal conductivity; under the same welding conditions, its cooling rate is 4 to 7 times that of steel, which accelerates metal crystallization. During this rapid cooling process, the solubility of hydrogen decreases sharply, resulting in the formation of large amounts of supersaturated gas. Hydrogen does not have time to escape, and thus pores form within the weld metal. The alignment quality of the welded parts and the welding position are particularly important for the welding quality of aluminum-magnesium alloy fittings. Too small a fitting gap, excessive blunt edges, too small an groove angle, and excessive misalignment can all easily lead to incomplete penetration at the root in vertical and overhead welding positions. Most air separation pipelines consist of elbows, and their thickness is generally greater than that of the straight sections. Although there is a thickness transition, the volume per unit area on each side differs, which results in different heat transfer and heat dissipation rates. If proper adjustment of the welding torch angle and the wire position is not ensured, it is easy to end up with incomplete welding at one side of the root.
The surface preparation of the workpiece before welding is also a factor that affects welding quality. Inadequate removal of the oxide layer on the workpiece surface, as well as the presence of contaminants such as oil, along with a low welding current, an excessively long arc, or too fast welding speed, can all result in insufficient melting temperature during welding, thereby preventing proper penetration at the root of the joint. In aluminum and aluminum-magnesium alloys, as the temperature rises at high temperatures, their tensile strength decreases. At 400°C, their tensile strength is only 9.8 N/mm2. When the temperature rises to the melting point, the tensile strength is almost 0, and it can no longer support its own weight. Near the melting temperature, aluminum itself has almost no plasticity. When it reaches the molten state, liquid aluminum has extremely high fluidity. The coexistence of these three elements determines that it will inevitably develop a certain degree of inward curvature. The thicker the wall, the more severe the inward indentation. Therefore, during vertical welding, blockages are also very likely to occur, and many weld beads are formed during the welding process; in severe cases, this can lead to the wall of the tube being burned through.
Aluminum alloys have a high thermal conductivity, allowing for rapid cooling, as well as a fast rate of primary crystallization in the molten pool. Its linear expansion coefficient is twice that of steel, and its volume contraction rate during solidification is around 6.5%. Therefore, both the internal stress in aluminum welds and the rigid restraint forces at the welded joints are high; if the resulting stresses are too great, the complex stress conditions can cause thermal cracks in the welds. The presence of oxide films and other inclusions can reduce the strength of its welds. Too slow a welding speed and excessively high pool temperature can cause the loss of magnesium and zinc. Welding small-diameter and thin-walled tubes can lead to the loss of alloying elements; such over-burning results in oxidation and porosity of the crystalline structure.