Tungsten Cathode Manufacture by Laser Multi-layer Cladding Technology

The refractory and corrosion resistant tungsten metal is often used as cathode in the production of rare earth metals by molten salt electrolysis. Tungsten cathode usually works at the electrolyte temperature of 1000 ~ 1100 ℃, the electrolytic current of 5500 ~ 6000 A and the electrolytic voltage of 10V. The service life of a tungsten cathode with 80 mm x 780 mm is 6 to 7 months. The most serious loss of tungsten cathode is the middle part above molten salt level, which is mainly related to the oxidation of electrodes at high temperature.

Laser multi-layer cladding technology is a kind of functional gradient material and repair and remanufacturing technology. It refers to the process of pre-coating alloy powder on the original cladding layer, or directly feeding powder, repeating one or more times of cladding, increasing the thickness of cladding layer through multiple cladding, rapid parts manufacturing or preparation of functional gradient material. The tungsten cathode fabricated by this method needs the following processes:

(a)Clean and polish the "necking" part of the tungsten cathode at room temperature. The "necking" part is the surface part to be melted, and clean the oxide layer and corrosion layer.

(b)Coating materials: SiC, Pd and W powders. Among them, SiC powders with particle size ranging from - 300 to 350 mesh, Pd powders with particle size ranging from - 140 to 300 mesh and tungsten powders with particle size ranging from - 200 to 300 mesh were used.

(c)The mixture of 20% SiC and 0.5% Pd and the rest of W powder was milled by planetary high-energy ball mill in agate tank for 10 hours at a speed of 150 r/min.

(d)Using 4kW CO₂ laser, numerical control worktable, HUST-IV high precision powder feeder and coaxial powder feeding nozzle, the mixed powder is clad on the surface of the workpiece line by line under the protection of argon atmosphere.

(e)Inspection: The dimension before and after cladding is tested by micrometer, and the thickness of cladding is 10mm. Through flaw detection, the cladding layer is ensured to be free of cracks, pore and impurity defects.

(F) The cladding method is: spot diameter 3.0 mm, laser power 2000 W, scanning speed 3.0 mm/s, powder feeding rate 4.5 g/min, powder flow 200 L/h, z-axis single-layer travel 0.4/mm, 15 layers were deposited. The corrosion resistance rate of the coating reaches - 5.0 *10-4 g H-1 cm^-2, and the oxidation rate reaches 0.0007 g/10 h at 850 ℃ in air.

Compared with the conventional tungsten protection technology, its characteristic is that the 4kW CO₂ laser cladding on the "neck" of tungsten cathode line by line has a certain high temperature oxidation resistance, corrosion resistance of SiC, Pd, pure tungsten powder, thus greatly prolonging the service life.

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