Welded Mo Crucible

Molybdenum is a refractory and brittle metal. It exhibits significant brittleness at room temperature, is prone to oxidation at high temperatures, and is highly susceptible to cracking during welding. This is mainly due to three major factors: the inherent difficulties of the material, the challenges in the welding process, and the difficulties in post-forming control.As a result, the product scrap rate is high, the yield is low, and the cost increases

1. Extremely brittle at room temperature: Pure molybdenum has almost no plasticity at room temperature, and welding stress can easily directly tear the weld seam; the grain size in the heat-affected zone of welding coarsens, impurities (O, C, N) segregate at the grain boundaries, and the weld seam becomes severely brittle. Even slight vibrations or temperature fluctuations after welding can cause cracking.

2. High temperature is highly prone to oxidation and failure: When the temperature exceeds 400℃ and comes into contact with air, it undergoes slow oxidation. When the temperature exceeds 700℃, it rapidly forms MoO₃ white powder oxide. After the weld is oxidized, a brittle layer is directly formed, resulting in a sharp drop in weld strength, the appearance of pores and贯穿 cracks. Therefore, the welding process must be protected by high vacuum / high-purity argon gas throughout, and the investment threshold for the equipment is extremely high.

3. Potential problems with powder metallurgy base materials: Most molybdenum plates are made by sintering molybdenum powder. The fine pores inside the base material expand due to the heat from the welding molten pool, which can easily form pores and inclusions in the weld seam, reducing the sealing performance of the weld seam. The crucible is prone to leakage when it is later filled with molten materials.

1. Difficult to control thermal deformation

Molybdenum has a fast heat conductivity and a low thermal expansion coefficient. The high temperature of the local molten pool during welding can cause huge temperature difference stress. The welding of thin-walled molybdenum boats and special-shaped molybdenum crucibles is prone to warping deformation. For high-end displays and semiconductor evaporation, the molybdenum boats require a long-term deformation of ≤ ±0.05mm at 1700℃. Ordinary argon arc welding cannot meet the accuracy and can only use vacuum electron beam welding (EBW), which is expensive.

2. The weld strength fails to meet the standard

The strength of the weld produced by ordinary argon arc welding is only 60% to 75% of the base material, which often leads to weld fracture. In high-end scenarios, electron beam deep penetration welding must be used, with the width of the heat affected zone controlled to be ≤ 0.5mm, in order to achieve a weld strength of more than 95% of the base material.

3. Welding of thin-walled non-standard components becomes extremely difficult

The thickness of the molybdenum boats used for photovoltaic HJT evaporation and the molybdenum crucibles for square welding is generally 0.3 to 1.0 mm. Welding of ultra-thin molybdenum materials is prone to penetration and collapse; the corners of the square crucibles with multiple folds have concentrated stress, which are high-risk locations for cracking. Therefore, the entire area of the workholding fixtures needs to be used for global positioning and constraint to prevent deformation.

1. The elimination of residual stress is complex: After welding, there are a large amount of internal stresses remaining. Without annealing treatment, the component will slowly crack after being subjected to high-temperature service for a few weeks; it is necessary to perform vacuum annealing at 1000-1300℃ to relieve the stress, which increases the process and cost.

2. The cleanliness control is strict

In the semiconductor and panel industries, the oxygen content of the molybdenum boat should be less than 30ppm. Any oil stains or oxidation spots on the weld will volatilize and pollute the chips and display yield during high-temperature evaporation, and after welding, a complete set of cleaning processes such as vacuum acid washing and plasma cleaning are required.

Pure molybdenum will undergo recrystallization and become brittle under repeated high-temperature conditions. In continuous high-temperature operations, TZM and MoLa lanthanum-molybdenum alloy plates must be used. The welding process of alloy molybdenum is even more demanding, and the welding parameters need to be re-adjusted.

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