Molybdenum alloy and graphite brazing
Because molybdenum alloy has excellent high temperature strength, low thermal expansion coefficient, sputtering yield and excellent thermal and electrical conductivity, it is widely used in X-ray targets, discharge lamps, welding electrodes, radiation shielding and plasma surface components. However, the application of molybdenum alloys is still limited due to the difficulty of machining and welding.
Another material, graphite, has high thermal conductivity, low coefficient of thermal expansion, low density and self-lubricating properties. Therefore, it has been used in applications such as radiators, as a material in mold sintering furnaces and carbon piston production and engine cylinder blocks and cylinder liners. But graphite also has disadvantages. They are high porosity, very low strength, poor homogeneity and easy oxidation.
In modern manufacturing, taking advantage of the superior properties of molybdenum and graphite and overcoming their disadvantages respectively, attempts have been made to combine molybdenum and graphite to obtain structural materials such as targets with high-power CT machines, as well as aerospace thermal erosion and heat transfer components for nuclear fusion reactors.
At present, brazing is the most widely used joint mode of graphite and molybdenum alloy, among which the solder is the most important factor affecting the joint performance. The commercial filler metals mainly include silver base, copper base, gold base and nickel base. In terms of high temperature performance, the brazing joint performance of silver base and copper base solder is relatively poor. The brazing joint of gold base filler metal has good performance but high cost. The joints brazed with nickel-based filler metal have better high temperature performance, high melting point and good high temperature performance. At the brazing temperature, the overflow and gap filling is very good; Nb and Co added in the alloy effectively improve the strength of the brazed joint, but nickel reacts with Mo at high temperature and forms a layer of continuous intermetallic compounds on the crystal surface, which makes the brazed joint brittle and reduces the strength of the joint.
There is a shortage of high temperature solder for brazing graphite and molybdenum alloy at home and abroad. Therefore, finding a solder with excellent performance and low cost is the key to realize the wide application of molybdenum alloy/graphite joint. A high temperature solder used for brazing molybdenum alloys and graphite consisting of 1-3% C, 10-15% Mo, with allowances of Ti and V by mass percentage.
Tungsten block is well used in projects due to its excellent mechanical properties and chemical composition.
The high temperature solder for molybdenum alloy and graphite brazing is applied to the welding surface of the treated molybdenum alloy, and the thickness of the coating is controlled to be 190-210um. Then graphite is put on, and tungsten block is placed on the graphite to provide pressure to obtain the assembled workpiece. The pressure provided by the tungsten block is 4.5-5.5KPa.
So the tungsten block needs very good mechanical properties to provide good pressure. We can arrange to produce high quality tungsten block according to customer's requirement.
