Argon arc welding in vacuum chamber

    Argon arc welding is suitable for welding various stainless steel components in ultra-high vacuum systems. It is arc welding with argon gas protection. Since the weld is protected by argon, the degree of oxidation of the molten metal is reduced, so the quality of the weld is relatively high. At the same time, due to the concentration of the heat source, the heat-affected zone during welding is small, and the deformation of the workpiece is small, thereby ensuring the air tightness and mechanical strength of the workpiece.

    The general principles of argon arc welding structure design are:

    ①The design of the weldment joint must ensure that the weld can be penetrated once, and the two welding will cause harmful space, which is not conducive to leak detection;

    ②Design the welding on the vacuum side of the container as much as possible, and the part that cannot be welded on the inside of the vacuum can adopt an external one-time penetration welding structure to avoid false leakage;

    ③If welding needs to be carried out twice, internal air-tight welding should be guaranteed to be leak-free, and external strength welding must be reserved for drilling and plug holes or intermittent welding, which is convenient for vacuum leak detection of welds;

    ④ Structural welds should not intersect with airtight welds. When designing the welds of each component, care should be taken to facilitate these welds to be tested separately at the manufacturing stage, and they can be corrected before final assembly;

    ⑤ Avoid welding deformation caused by the welding of thin-walled and thick plates, which is beneficial to eliminate the defects of thick steel or other materials.

    The problem encountered when welding the domestic 1Cr18Ni9Ti stainless steel is the cracking of the weld under the hot state. Residues of copper plating and silver copper solder are not allowed near the stainless steel weld. The surface to be welded can be cleaned with a clean stainless steel brush. Try to avoid using abrasives such as alumina, silicon carbide or steel grit. Abrasives must be used. When using, clean the abrasive on the surface to be welded with acid after use to minimize the porosity of welding. If the stainless steel is plated with a nickel layer before welding, the strength and air tightness of the argon arc welding seam can be improved.

    The vacuum flange welded to the pipe will be warped and deformed due to the influence of welding stress. When designing the processing technology, it is necessary to reserve enough machining allowance, and use the flange sealing groove and sealing surface after welding to be processed as much as possible. Avoid flange deformation affecting the vacuum seal.

    For welding of different materials, such as argon arc welding of stainless steel flanges and Kovar pipes and copper pipes, the welding structure or welding process should combine the thermal expansion characteristics, melting temperature, thermal conductivity and shrinkage rate under cooling conditions of different materials. Comprehensive consideration can get a good airtight weld.

    After welding, a highly sensitive nitrogen mass spectrometer leak detector must be used to check the leak of each weld. When the leak rate of the weld is found to exceed the standard, all the welds in the exceeding section should be worn out to expose the original base metal and re-weld. Pay attention to the overlap length at both ends of the repair welding. It is wrong to repair welds on the original welds without grinding away the welds where the original leakage rate exceeds the standard section, hoping to plug the leaks, because the stresses that may be generated by the repair welding will cause new cracks. The more it will make up for the leakage.

(Huiputech)