Special Issue on Heat Transfer, February 2011
Manuscript received and revised January 2011, accepted February 2011 Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved
257
Simulation of Thermal Transfer Process in Cast Ingots at Electron Beam Melting and Refining
Katia Zh. Vutova, Elena G. Koleva, Georgi M. Mladenov
Abstract – Computer simulations of the heat transfer processes at electron beam melting and refining is a tool for better understanding and choice of optimal regimes of application of this
expensive modern technology. The heat exchange at different interfaces between the casting ingot and both the water-cooled crucible and the pulling mechanism is studied. Radiation and
evaporation losses from the top molten surface of drip cast ingots are also evaluated. Regression equations for the heat flows through the concrete boundary surfaces, as well as for the volume of
the molten metal pool have been created. They are considered as functions of the coefficients of the heat transfer and the width of the heat contact ring between the top part of the casting ingot
and the copper crucible. The response surface plots are obtained to visualize these dependences. They can be used for choosing proper process conditions and for the optimization of the heat
flows according to the process requirements. Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved.
Keywords:
Electron Beam, Heat Transfer, Crucible, Molten Pool, Cast Ingot
I. Introduction
The electron beam melting and refining EBMR is a widely used method of the special electric-metallurgy,
due to the vacuum environment, the use of water-cooled crucibles and the possibility to elevate the metal
temperature to high values 30–50 higher, than the melting temperature of the metal T
m
. The energy of the heating source in this method can be controlled
independently from the melted material properties, from the cross-section of the re-melted material, from the
volume of the melted metal pool and from the design peculiarities of the crucible used. This extends the ability
to control the speed of melting or the speed of crystallization of the casting ingot and in this way the
quality of the manufactured metal.
In comparison with other metallurgic methods the EBMR provides a better refining of the casting metal
from solvable gases and non-metal inclusions, as well as from easily evaporated metal components. The blocks
produced by EBMR have more isotropic mechanical properties and the density of the produced metal is
comparable with the densities of deformed metals. Metals such as W, Ta, Ti, Nb, Mo, Zr, Hf, V, Pt, Cu,
alloys, steels and precision alloys based on Fe, Ni, Co and Cu are produced by this flexible technology. The up
to date EBMR plants manufacture samples ranging from small quantities such as pure metal tablets of a few grams
to large scale ingots weighting up to 5 000–25 000 kg. At the same time, the composition control and the
inclusion cleanliness of the metal alloys remain a delicate problem not completely solved.
The optimization of the process and equipment designs is hard task due to unknown process parameters
that are difficult to be directly measured. Computer simulation can give a better understanding of the coupled
mechanisms heat and mass transfer involved in the melting operation.
Previous numerical study of heat flows during EBMR was made in [1] – [8]. The calculations reveal the
importance of the some process characteristics that values are not exactly known. A need for more clear
understanding of the EBMR process details was indicated.
In this paper results based on our heat transfer computer tool [1]– [3] and regression analysis [9]-[11] of
the obtained, by numerical experiments, data are given. The aim is to clarify the influence of the heat exchange at
different interfaces between the casting ingot and both the water-cooled crucible and the pulling mechanism as
well as to get some data for evaporation and irradiation losses from the top surface of the melting pool at
electron beam drip melting and refining because in the case of use of hearth the main transfer processes are
similar.
II. Process Description
Two kinds of electron beam melting and refining EBMR could be distinguished: i drip EBMR and ii
hearth EBMR. The drip melting is the classical method for processing of refractory metals. Raw material in form
of bars is fed horizontally or vertically in vacuum
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
258 environment and is drip-melted directly into the
withdrawal bottom of a copper water-cooled crucible Fig. 1. The refined metal is extremely pure, due to lack
of contact with a ceramic pot surface, typical for conventional metallurgical re-melting technologies.
During the EBMR droplets, created on the front surface of the feeding rod, fall on the surface of the molten pool,
situated on the upper part of the cast ingot. At the same time the electron beams heat this surface, keeping it
molten. The liquid pool surface is maintained on a constant level, suitable to be viewed by the operator and
to adequate withdrawal continuously or by small steps of the bottom of the growing cast ingot. Refining is
based on degassing and selective evaporation of metallic and non-metallic constituents with vapor pressures
higher than the base material. Hard inclusions that can exist there could floats or being solute in the molten
metal. Mostly repeated re-melting of the first-melt-ingots is applied to achieve the required final quality of the
performed metal. Often, for repeated re-melting vertical feeding is applied due to more uniform irradiation of the
molten pool surface.
Fig. 1. Drip melting
A principle scheme of electron beam drip-melting- furnace is shown in Fig. 1. The process is held in a
vacuum chamber, one or several electron-optical systems 2 are mounted, and in which intense electron beams 3 are
generated and directed to interaction zones on the re- melting bar and on the molten pool on the top surface of
the cast ingot. The electrons that fall on the refining metal 4, heat it and the molten metal in the form of drops
fall in a copper crucible 5 with water cooling 6 and cast ingot 7, solidified on a moving down bottom. The
surface of the molten metal in the crucible is also heated by the electrons. The electron beam and the high vacuum
provide degassing and high level of refining, as well as homogeneity of the chemical composition and optimal
structure of the cast ingots. The method provides independent control of the melting speed and the
crystallization speed; the power distribution and overheating of the molten material can be optimal, there
are no impurities from the crucible and there is possibility to realize electro-dynamic movement of the
molten metal in the crucible. Second important scheme of EBMR is shown in Fig.
2. An intermediate water cooled copper pot, termed hearth, is used to reach higher refining level due to the
higher ratio between the surface area and molten metal depth there and due to the additional possibility to stop
transportation of refractory nonmetallic inclusions that form slag on the top of the molten pool in the hearth by a
cooled surface barrier on its surface or heavier metallic inclusions such as tungsten carbide chips from
machining instruments in titanium. After refining in the intermediate cold hearth the molten metal goes to the
water cooled crucible, as it was in the drip melting. This technology method is applied in the case of performing
extremely pure metals at higher energy consumption.
Fig. 2. Electron beam melting and refining with a hearth: 1-electron guns, 2- electron beams, 3-feeding mechanism for scrap or compacted
fed ingot, 4- hearth, filled with molten metal, 5- crucible, 6- water cooling, 7- cast ingot pulled down
Often the raw material is fed into the melting space after compacting by cold pressing without oil or after
casting and welding of tablets as compact ingots 4 in Fig. 1 or 3 in Fig. 2.
Another important possibility is when the re-melted scrap or raw material for example sponge is fed as
small particles utilizing a vibrating or screw-like feeding mechanism.
The main characteristics of the melting process which the operator can control are: beam power and beam
current, because usually the accelerated voltage is constant during the melting; beam focusing current that
defines the average power density in the beam spot on the molten material; melting rate andor casting velocity,
or speed of feeding and pulling mechanisms together with the cross section and densities of the transported
metal that define the residual times of molten metal in the hearth and in the molten pool. Using a deflection
control unit the operator moves the beams on the molten surface.
The frequency and the trajectory of the beam deflection, together with the beam energy values and
chosen energy densities as well as the material thermo- physical parameters and the dimensions of hearth and
crucible determine the heating flows, mass transport in the molten pool and evaporation rates.
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
259
III. Heat Model for Solidification of
