Vacuum Arc Remelting
Encyclopedia
Vacuum arc remelting is a secondary melting
process for production of metal
ingot
s with elevated chemical and mechanical homogeneity
for highly demanding applications. The VAR process has revolutionized the specialty traditional metallurgical techniques industry, and has made possible incredibly controlled materials used in the biomedical, aviation, and aerospace fields.
s do not employ the process. Nickel
, titanium
, and specialty steel
s are materials most often processed with this method.
The conventional path for production of Titanium alloys includes single, double or even triple VAR processing. Use of this technique over traditional methods presents several advantages:
(VIM) or ladle refining (airmelt). This cylinder, referred to as an electrode is then put into a large cylindrical enclosed crucible
and brought to a metallurgical vacuum
(1–100 micrometres of mercury i.e. millitorr; 0.13–13 Pa
). At the bottom of the crucible is a small amount of the alloy to be remelted, which the top electrode is brought close to prior to starting the melt. Several kiloamperes of DC
current are used to start an arc between the two pieces, and from there, a continuous melt is derived. The crucible
(typically made of copper
) is surrounded by a water jacket used to cool the melt and control the solidification rate. To prevent arcing
between the electrode and the crucible side walls, the diameter of the crucible is larger than that of the electrode. As a result, electrode must be lowered as the melt consumes it. Control of the current, cooling water, and electrode gap is essential to effective control of the process, and production of defect free material.
Ideally, the melt rate stays constant throughout the process cycle, but control of the process is not simple. This is because there is very complex heat transfer going on involving conduction, radiation, convection (within the liquid metal), and advection (caused by the Lorentz Force
). Ensuring the consistency of the melt process in terms of pool geometry, and melt rate is pivotal in ensuring the best possible properties from the alloy.
Note that pure titanium and most titanium alloys are double or triple VAR processed. Nickel-based super alloys for aerospace applications are usually VAR processed. Zirconium and niobium alloys used in the nuclear industry are routinely VAR processed. Pure platinum, tantalum, and rhodium may be VAR processed.
Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
process for production of metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
ingot
Ingot
An ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...
s with elevated chemical and mechanical homogeneity
Homogeneity (physics)
In general, homogeneity is defined as the quality or state of being homogeneous . For instance, a uniform electric field would be compatible with homogeneity...
for highly demanding applications. The VAR process has revolutionized the specialty traditional metallurgical techniques industry, and has made possible incredibly controlled materials used in the biomedical, aviation, and aerospace fields.
Overview
VAR is used most frequently in high value applications. Essentially it is an additional processing step to improve the quality of metal. Because it is both time consuming and expensive, a majority of commercial alloyAlloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
s do not employ the process. Nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, and specialty steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
s are materials most often processed with this method.
The conventional path for production of Titanium alloys includes single, double or even triple VAR processing. Use of this technique over traditional methods presents several advantages:
- The solidification rate of molten material can be tightly controlled. This allows a high degree of control over the microstructureMicrostructureMicrostructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
as well as the ability to minimize segregation - The gases dissolved in liquid metal during melting metals in open furnaces, such as nitrogenNitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, oxygenOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and hydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
are considered to be detrimental to the majority of steels and alloys. Under vacuum conditions these gases escape from liquid metal to the vacuum chamber. - Elements with high vapor pressure such as oxygenOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, carbonCarbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, sulfurSulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, and magnesiumMagnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
(frequently contaminants) are lowered in concentration. - Centerline porosityPorosityPorosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
and segregation are eliminated. - Certain metals and alloys, such as Ti, cannot be melted in open air furnaces
Process description
The alloy to undergo VAR is formed into a cylinder typically by vacuum induction meltingVacuum Induction Melting
Vacuum induction melting is a process for melting metal under vacuum conditions using electromagnetic induction. It works by creating electrical eddy currents in the metal which heats the "charge" to melt it...
(VIM) or ladle refining (airmelt). This cylinder, referred to as an electrode is then put into a large cylindrical enclosed crucible
Crucible
A crucible is a container used for metal, glass, and pigment production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents...
and brought to a metallurgical vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
(1–100 micrometres of mercury i.e. millitorr; 0.13–13 Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
). At the bottom of the crucible is a small amount of the alloy to be remelted, which the top electrode is brought close to prior to starting the melt. Several kiloamperes of DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
current are used to start an arc between the two pieces, and from there, a continuous melt is derived. The crucible
Crucible
A crucible is a container used for metal, glass, and pigment production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents...
(typically made of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
) is surrounded by a water jacket used to cool the melt and control the solidification rate. To prevent arcing
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
between the electrode and the crucible side walls, the diameter of the crucible is larger than that of the electrode. As a result, electrode must be lowered as the melt consumes it. Control of the current, cooling water, and electrode gap is essential to effective control of the process, and production of defect free material.
Ideally, the melt rate stays constant throughout the process cycle, but control of the process is not simple. This is because there is very complex heat transfer going on involving conduction, radiation, convection (within the liquid metal), and advection (caused by the Lorentz Force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
). Ensuring the consistency of the melt process in terms of pool geometry, and melt rate is pivotal in ensuring the best possible properties from the alloy.
Materials and applications
The VAR process is used on many different materials, however certain applications almost always use a material that has been VAR treated. A list of materials that may be VAR treated include:- SteelSteelSteel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
- 9310
- 4340 & 4330+V
- 300M
- AF1410
- Aermet 100
- M50
- BG42
- Nitralloy
- 16NCD13
- 35NCD16
- HY-100
- HY-180
- HY-TUF
- D6AC
- Maraging steelMaraging steelMaraging steels are steels which are known for possessing superior strength and toughness without losing malleability, although they cannot hold a good cutting edge. Aging refers to the extended heat-treatment process...
s - UT-18
- HP 9-4-30
- TitaniumTitaniumTitanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
- Ti-6V-4Al
- Ti-10V-2Al-3Fe
- Ti-5Al-5V-5Mo-3Cr
- InvarInvarInvar, also known generically as FeNi36 , is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion . The name, Invar, comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.It was invented in 1896 by Swiss scientist...
- Nitinol
- NickelNickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
superalloySuperalloyA superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. ...
s- InconelInconelInconel is a registered trademark of Special Metals Corporation that refers to a family of austenitic nickel-chromium-based superalloys. Inconel alloys are typically used in high temperature applications. It is often referred to in English as "Inco"...
alloys - Rene alloysRené 41René 41 is a nickel-based high temperature alloy developed by General Electric which retains high strength in the 1200/1800°F temperature range...
- RR1000
- Inconel
- ZirconiumZirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
- NiobiumNiobiumNiobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
- PlatinumPlatinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
- TantalumTantalumTantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
- RhodiumRhodiumRhodium is a chemical element that is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. It is composed of only one isotope, 103Rh. Naturally occurring rhodium is found as the free metal, alloyed...
Note that pure titanium and most titanium alloys are double or triple VAR processed. Nickel-based super alloys for aerospace applications are usually VAR processed. Zirconium and niobium alloys used in the nuclear industry are routinely VAR processed. Pure platinum, tantalum, and rhodium may be VAR processed.