The Crucible’s role in the vacuum coating equipment
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Crucible, also known as crystallizer, is the core of vacuum arc melting. Here metals are melted, superheated, refined and crystallized into ingots. A large amount of heat exchange is carried out here, and the heat transfer intensity of the crucible wall can reach tens of thousands to millions of W/m2.
When an abnormal phenomenon occurs in the electric arc, it will even burn through the crucible wall. So, there's a set of requirements for the crucible.
In view of the importance of crucible, the main points of crucible design are as follows:
1. Crucible shall be made of materials with good thermal conductivity, good electrical conductivity and not easy to be bonded by molten metal. The material that accords with this kind of requirement has red copper, anaerobic copper to wait. The vast majority of crucible wall thickness of 12 ~ 24mm copper tube and copper alloy, (manganese bronze) : flange welded.
Because red copper thermal conductivity and electrical conductivity is good, and does not bond with the smelted metal, but also cheaper than oxygen-free copper, better than steel heat transfer, electrical conductivity.
2. The inner surface of the crucible shall be smooth, with a consistent section shape and no serious pitting, so as to facilitate the ingot removal. The main manufacture methods of crucible tube are extrusion, electrodeposition and centrifugal casting. Among them, the quality produced by extrusion method is good, but it is difficult to produce large size (diameter > 500mm) crucible tube by this method.
The electrodeposition method has unlimited specifications and wide application. In addition, there are copper crucible tube rolled welding, but the inner surface of the welding seam must be precision machining. In the process of arc melting, the inner surface of the crucible will be fouled, so the inner surface of the crucible should be cleaned carefully before every melting.
3. The crucible shall have enough stiffness and strength to support the mass of large ingots in the melting condition, and it shall not be damaged by short circuit collision of electrodes.
4. The wall thickness of the crucible should be thinner if the strength allows. The wall of the crucible is mainly determined by the diameter and load of the spindle. Generally, the wall thickness of the small furnace is 8 ~ 10mm, that of the large furnace is 12 ~ 24mm, and that of the steel crucible is 8 ~ 12mm.
However, some people advocate a thicker crucible wall (greater than 25mm). They believe that heat can also be transferred along the longitudinal direction of the crucible wall, which is very useful for cooling the melting zone at high temperature.
Others believe that as long as the cooling water is enough, the longitudinal heat transfer effect of the wall is small, so a thinner wall can be used. Moreover, the thickness of the wall is not the factor that determines the difficulty of burning through the crucible. In the case of insufficient cooling water, no matter the thickness of the wall, the copper with lower melting point can hardly cope with the high temperature.
5. The structure of the crucible shall allow the thermal expansion and deformation of the crucible. The temperature curve of the outer wall of the crucible is the measured temperature curve of the outer surface of the 8-inch (200mm) copper crucible cooled by vacuum technology company of Japan on the water of the serpentine tube.
It can be seen that in the melting process, although there is water cooling, the temperature of the wall is still higher. According to the measured data, the walls of the water cooled copper crucible (pool) high temperature is 230 ~ 280 ℃; Calculate the z * high temperature is about 400 ℃, the average temperature is about 150 ℃.
Because the crucible is long and the longitudinal thermal expansion is large, it must be considered in the design. For example: 2 m long copper crucible, the average temperature is 150 ℃, the longitudinal thermal expansion amount & Delta; L = (150-20) & times; 18 & times; 10-6 & times; 2000 = 4.68 mm. It can be seen that if the crucible is fixed up and down, the huge internal stress caused by such a large amount of expansion will damage the crucible.
6. Crucible with diameter less than 150mm usually has a certain taper, because the crucible needs to be deingot.Large ingots are easy to be removed in condensation because of the large total shrinkage.However, small ingots need crucible to have a certain degree of taper, so as to facilitate ingot removal. Generally, the taper is within 0.5%.