View source for Vacuum arc remelting – Process capabilities and Modeling approaches


=Process description=
Vacuum arc re-melting(VAR) is a process to obtain materials with high degree of homogeneity for
applications in Aerospace, Aviation and Bio-Medical Engineering<ref>M. McLean. "High Performance Materials in Aerospace, chapter Nickel-based
alloys: recent developments for the aero-gas turbine," pages 135-154. Springer
Netherlands, 1995.</ref><ref>A. Kermanpur, P. D. Lee, M. McLean, and S. Tin. "Integrated modeling for the
manufacture of aerospace discs: Grain structure evolution." JOM, 56(3):72-78,
2004</ref>. It differs from the traditional
metallurigal techniques and is able to control the micro-structure more accurately. Typically the
alloy to undergo VAR is formed into a cylindrical consumable electrode and is placed in a crucible
which is at metallurgical vacuum(0.0010.1 mmHg or 0.113.3 Pa). A small amount of alloy to be
remelted is placed at the bottom of the crucible. And the top electrode is brought in proximity to this
material kept for remelting. Arcing between these two portions is brought about by passing several
kiloamperes of DC current between the two pieces. The metal melts continiously and the top
electrode is gradually moved downwards by a mechanical ram in such a way that the arc is
maintained steadily between the electrodes. The arcing between the crucible and the electrode is
controlled by designing the crucible to have a larger diameter than the electrode. The crucible is
cooled with water jackets and solidification of the material is closely controlled. The quality of the
resulting alloy is dependant on the cooling rate, gap between the electrodes and current<ref>Zhi-jun Yang, Hong-chao Kou, Xiao-hua Zhao, Jin-shan Li, Rui Hu, Hui Chang,
and Lian Zhou. "Effect of remelting current on molten pool profile of titanium
alloy ingot during vacuum arc remelting process." Journal of Shanghai Jiaotong
University (Science), 16(2):133-136, 2011</ref>. Process
parameter control is significant and can be directly linked to the material’s homogeneity and
properties<ref>Hafid El Mir, Alain Jardy, Jean-Pierre Bellot, Pierre Chapelle, David Lasalmonie,
and Jean Senevat. Thermal behaviour of the consumable electrode in the vacuum
arc remelting process. Journal of Materials Processing Technology, 210:564-572,
2010</ref>. Nickel, titanium and special steels are often processed using VAR.

[[Image:VAR.png|A visual depiction of VAR process|thumb|right|325px]]

=Process handling=
Steady state melting and solidifications conditions are essential to maintain Bouyancy and Lorentz
forces required to drive fluid flow in the metal pool ahead of the dendritic solidification front. The
melting current and electrode velocity are the two handles available for the operator to maintain the
process under steady state condition. The electrode gap must be small and constant. Intermediate
and long electrode gaps can create a non-uniform current distribution on the anode or the pool
surface and influence the fluid flow in the melt pool. The electrode gap is usually controlled by drop
short frequency, anode spike frequency and furnace voltage. Under small electrode gap metal is
transfered by formation and subsequent rupture of molten metal columns(drop shorts). These
columns form a low resistance bridge between the cathode(electrode) and the anode(ingot) and the
arc is extinguished temperarily which is observed as a voltage drop across the electrodes<ref>F. J. Zanner. "Vacuum consumable arc remelting electrode gap control strategies based on drop short properties." Metallurgical and Materials Transactions B,
12:721-728, December 1981.</ref>. Transfer
of metal during vacuum arc remelting causes a pattern on the voltage waveform called drop shot
and the subsequent arc reignition creates an anode spike which are used to control the electrode gap
around a nominal value. Electrode gap and arc voltage in turn affect the melt rate. Melt rate control
is essential to obtain the desired microstructure and minimize the segregation related defects.
=Modeling Approaches=
To model the temperature distribution in the solidifying ingot and to calibrate the fluid flow in the
melt pool conventional finite element codes are used. Approaches can vary from 2-D Finite element
based solvers to 3-D Axisymmetric Finite element models<ref>Kanchan M Kelkar, Suhas V Patankar, Alec Mitchell, Osamu Kanou, Nobuo
Fukada, and Kenji Suzuki. "Computational modeling of the vacuum arc remelting
(var) process used for the production of ingots of titanium alloys.," In 11th World
Conference on Titanium (Ti-2007), pages 3-7, Kyoto, Japan, 2007</ref>. Simple two equation based K-<math>\epsilon</math>
models can be used to consider turbulent mixing in the melt pool. Latent heat associated with
melting and solidification is also taken into account in most cases by using ethalpy method<ref>V. R. Voller. "An implicit enthalpy solution for phase change problems:with application to a binary alloy solidification." Applied Mathematical Modelling, 1987.</ref>. Heat
removal by water jackets is modeled by newton’s law of cooling. Dirichlet and Neumann based BCs
are also frequently used to model temperature on the outer surface of the ingot. Mass and
momentum is conserved to determine the radial and axial velocities. Effect of buoyancy and
Lorentz forces are also introduced to account for velocity due to magnetic stirring. Steady state
assumptions are not valid in the case of VAR modeling in general due to the complex nature of the
process and the interplay between various process parameters. A transient heat transfer model is
used based on this rational<ref>Nastac. "Modeling and Simulation of Microstructure Evolution in Solidifying Alloys."
Kluwer Academic Publishers, 2004</ref>. Cellular Automaton which is a stochastic model is often coupled with
FE code to predict the grain structure at different position of the ingot, grain size and parameters
related to columnar to equiaxed transition(CET)<ref>R. C. Atwood, P. D. Lee, R. S. Minisandram, and R. M. Forbes Jones. "Multiscale
modelling of microstructure formation during vacuum arc remelting of titanium
6-4." Journal of Materials Science, 39(24):7193-7197, 2004.</ref><ref>Kou Hongchao, Zhang Yingjuan, Li Pengfei, Zhong Hong, Hu Rui, Li Jinshan,
Zhou Lian" Numerical Simulation of Titanium Alloy Ingot Solidification Structure during VAR Process Based on
Three-Dimensional CAFE Method" Rare Metal Materials and Engineering, 2014, 43(7): 1537-1542</ref>. Heterogeneous nucleation assumed to start
randomly at predefined number of sites with predefined nuclei radii are described by a continious gaussian distribution. This coupling between meso and macro scales helps us to calibrate the
dendrite tip velocity.
[[Image:VAR_Model.png|Proposed VAR model|thumb|left|500px]] [[Image:VAR_temp.png|Temperature distribution in VAR|thumb|center|580px]]

=References=
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[[Category:Macroscale]]
[[Category:Microscale]]
[[Category:Mesoscale]]
[[Category:Metals]]