In order to avoid pores in the welding of molybdenum crucibles, it is necessary to start from multiple links such as raw material preparation, welding process, welding environment, and post-weld treatment, and systematically control the source and formation mechanism of pores. The following is a detailed analysis of the causes and preventive measures:
- Causes of pores in molybdenum crucible welding
| Stomatal origin | illustrate |
| Material gas impurities | Molybdenum contains gas impurities such as oxygen, hydrogen, and nitrogen, which escape under high welding temperature to form bubbles. |
| Oxygen /humidity in welding environment | The water vapor or oxygen in the air is decomposed by the welding arc, producing H₂ / O₂ which enters the molten pool |
| Improper welding current parameters | The molten pool is too deep/cools too quickly, so the gas does not have time to escape |
| Welding wire or workpiece surface contamination | Oil stains, water stains, oxide films or dust will release gas when heated |
| The shielding gas purity is poor or the flow is unstable | Impurities in argon or helium or interruption of gas supply led to oxidation of the molten pool |
- Preventive measures to avoid porosity in molybdenum crucible welding
- Raw material preparation
Use high purity molybdenum material (Mo ≥ 99.95%);
The welding wire should be dried and degassed (150–200 ° C for 2–4 hours);
The workpiece surface should be polished clean, oil-free and free of oxide scale;
It is recommended to perform vacuum annealing before welding to remove internal gas.
- Optimization of welding process parameters
| parameter | Recommended value |
| Welding method | TIG welding (gas tungsten arc welding) or vacuum electron beam welding |
| Current | Medium to low current (to avoid overheating and triggering ignition) |
| Welding speed | Slightly slower (to facilitate gas escape) |
| Interlayer temperature control | ≤200 ° C (to prevent heat build-up) |
| Welding Angle | Proper tilt (to guide gas out) |
- Welding atmosphere and protection
| condition | Require |
| Protective gas | ≥99.999% pure argon or helium |
| Airflow direction | Blowing from front to back is beneficial to molten pool protection |
| Gas flow | Stable, usually 8–15 L/min |
| Welding environment | It is recommended to operate in a vacuum or inert atmosphere box (especially precision crucibles) |
| Cooling method | Slow cooling helps release gas |
- Structural design improvements
The joints should be designed as overlaps or bevels as much as possible to facilitate gas discharge;
Avoid the formation of a closed weld pool, especially in bottom welding;
For multi-layer welds, grind and remove residue in between.
- Post-weld treatment
Perform vacuum annealing (such as 1100 ° C × 2 h, vacuum degree ≤10 ⁻ ³ Pa), which helps to release the gas in the micropores;
Surface flaw detection: such as ultrasonic, penetrant, and X-ray testing to detect defects in a timely manner;
Carry out air tightness test when necessary to check the sealing of welding points.
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