Vacuum Arc Remelting

Vacuum Arc Remelting (VAR) is a secondary melting process for production of metal ingots with elevated chemical and mechanical homogeneity for highly demanding applications. ["Modeling for Casting & Solidification Processing", by Kuang-Oscar Yu,CRC; 1st edition (October 15, 2001), ISBN-10: 0824788818] 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. Because it is both time consuming and expensive, a majority of commercial alloys do not employ the process. Nickel, titanium, and specialty steels are materials most often processed with this method. The conventional path for production of Titanium alloys includes single, double or even triple VAR processing. ["Titanium: Past, Present, and Future (1983)" http://books.nap.edu/openbook.php?record_id=1712&page=65] 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 microstructure as well as the ability to minimize segregation
*The gases dissolved in liquid metal during melting metals in open furnaces, such as nitrogen, oxygen and hydrogen 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 oxygen, carbon, sulfur, and magnesium (frequently contaminants) are lowered in concentration.
*Centerline porosity and segregation are eliminated.

Process description

The alloy to undergo VAR is formed into a cylinder typically by vacuum induction melting (VIM). This cylinder, referred to as an electrode is then put into a large cylindrical enclosed crucible and brought to a rough vacuum. 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 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 Electric_arc 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. [Monitoring the vacuum arc remelting process. DA Melgaard, RG Erdmann, JJ Beaman, RL Williamson - 2007] 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.

Materials processed by VAR

Turbine disk alloys

Nuclear steels

Titanium (many grades)

Inconel

Invar

Nitinol

Further reading

[http://www.patentstorm.us/patents/5373529-description.html|A Related Recent Patent] [http://www.britannica.com/eb/article-81421/steel|Britannica]

VAR based alloy providers

Allegheny Technologies - [http://www.alleghenytechnologies.com/]

Timet - [http://www.timet.com/]

Special Metals - [http://www.specialmetals.com/]

Vale Inco - [http://www.inco.com/]

References