Working Principle of the Vacuum Consumable Arc Furnace
An electric arc is a form of gas discharge characterized by a low voltage but a very high current passing through an ionized gas channel, producing intense light and extremely high localized temperatures (approximately 5000 K). This high current density originates from thermionic and field electron emission from the cathode. A cloud of positive ions near the cathode creates a strong electric field, inducing significant electron emission. These electrons collide with and ionize gas molecules between the electrodes, generating more positive ions and secondary electrons. The subsequent collisions of these charged particles with the anode and cathode under the influence of the electric field release substantial energy as heat. Notably, the cathode operates at a lower temperature than the anode due to energy consumed in electron emission. Additional heat is generated by the recombination of ions and electrons within the arc plasma. An industrial furnace utilizing this principle for melting metals is an Electric Arc Furnace (EAF). When this process is conducted in a vacuum environment, it becomes a Vacuum Arc Furnace. Vacuum arc melting operates with high current and low voltage, characteristic of a short arc, typically with arc voltages of 22–65 V and corresponding arc lengths of 20–50 mm (longer for larger ingots).
The concept dates back to experiments with platinum wire in 1839, with over a century of subsequent research focused on melting refractory metals. Industrial application of vacuum arc furnaces began in 1953. Initially, until around 1956, titanium was melted in non-consumable electrode furnaces in the US and Europe, while consumable electrode melting for steel started around 1955. By approximately 1960, consumable furnaces were producing ingots over 30 tons, marking the maturation of the technology. Modern developments are exemplified by furnaces such as those from the American company Consarc. To enhance productivity and equipment utilization, designs often feature two furnace heads sharing a single main power supply, vacuum system, and automated control system.
The process involves melting a consumable electrode under vacuum in a water-cooled copper crucible (mold). A DC arc is struck between the tip of the electrode (cathode) and the base of the crucible or a starting stub (anode). The intense heat of the arc melts the tip of the electrode. Droplets form, detach, and fall through the vacuum into a molten pool below, which solidifies into a high-purity ingot as the electrode is progressively consumed.
Key aspects of the process:
Purification: As droplets form and fall, specific physical and chemical reactions occur under high temperature and vacuum, removing a portion of gas impurities (e.g., hydrogen, nitrogen, oxygen) and allowing some non-metallic inclusions to decompose or be reduced.
Contamination-Free Melting: The use of a water-cooled copper crucible eliminates contamination from refractory materials typical of other furnaces.
Controlled Solidification: The rapid cooling provided by the water-cooled crucible promotes directional solidification from the bottom up, resulting in a dense, homogeneous ingot with a fine, uniform grain structure, minimal segregation, and reduced shrinkage voids.
This refined structure, combined with effective degassing and inclusion removal, significantly improves the material's hot and cold workability, plasticity, mechanical properties, and physical characteristics. A critical benefit is the reduction of anisotropy—the difference between longitudinal and transverse properties—ensuring greater stability, consistency, and reliability in the final product's performance.
To produce a high-quality ingot, a stable melting power supply with constant-current characteristics is essential. The DC power supply system typically features:
Summary of Vacuum Consumable Arc Furnace Characteristics:
