Vacuum consumable electrode melting (VAR) is a process that involves melting metal via electromagnetic induction under vacuum conditions.
This process is particularly useful for metals and alloys that have an affinity for oxygen and nitrogen, making it impossible to melt them in air.
7 Steps Explained
1. Loading the Electrode
The electrode to be melted is loaded into the furnace.
For specialty steels and superalloys, the electrode is previously cast in air or vacuum.
For reactive metals like titanium, the electrode is fabricated from compacted sponge and/or scrap, or from a hearth melt process like Plasma or Electron Beam.
2. Vacuum Vessel Setup
Two major mechanical assemblies combine to form the vacuum vessel in which melting occurs - the movable furnace head and the fixed melt station.
The movable furnace head is the upper section of the vessel and supports and controls the movement of the electrode.
The fixed melt station, which forms the lower half of the vessel, consists of a removable copper crucible placed into a fixed stainless steel water jacket.
3. Establishing a Vacuum
Once the electrode is clamped to the ram assembly, the ram lifts the electrode while the furnace head is lowered to create a vacuum seal on top of the crucible.
Once a vacuum is established, the DC power supply is activated.
4. Arc Melting
The control system automatically strikes a high current arc between the consumable electrode (cathode -) and the crucible base (anode +), quickly forming a molten pool of metal.
The gap between the melting electrode and the metal pool (arc gap) is precisely maintained, and a controlled melt rate is established.
5. Vacuum Benefits
The metal droplets falling through the arc gap are exposed to the vacuum environment and the extreme temperatures of the arc zone.
This causes the removal of dissolved gases, vaporization of tramp elements, and improvement in oxide cleanliness.
The water-cooled crucible allows for directional solidification, preventing macro segregation and reducing micro segregation.
6. Controlled Solidification
The molten pool of metal formed by the metal droplets is solidified in a directional fashion.
When the melt rate and arc gap are correctly controlled, this directional solidification prevents segregation and enhances the material properties of the ingot.
7. Gradual Power Reduction
Towards the end of the process, the power is gradually reduced to provide a controlled hot top, maximizing the yield of useful product.
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