substance of a technology: Electron-beam melting and refining, The
substance of a technology: Electron-beam melting and refining, The
THE BEGINNINGS
If
a technology can have a birthday, then the birthdate of the electron
beam is March 26, 1907, which is when a U.S. patent was issued to
Marchelo von Pirani (Figure 1).2
Fifty years passed before von Pirani's electron-beam concept saw
true commercial realization. This occurred in the late 1950s when
Temescal Metallurgical Corporation, under the leadership of Hugh Smith,
began to perform electron-beam melting by using transverse guns. This
was the commercial beginning of electron beam melting.3-5 By 1957,
Temescal was operating facilities capable of melting titanium ingots of
1.5 m in length and 80 mm in diameter; work on furnaces capable of
melting larger ingots was in progress. By the early 1960s, Temescal was
able to melt 80 mm tantalum and tungsten ingots and 127 mm in diameter
titanium ingots weighing several hundred kilogramsall by use of
transverse electron guns. Correspondingly, laboratory-scale furnaces
evolved into industrially sized furnaces in the 50-150 kW power range.
PROCESS CHARACTERISTICS
Electron-beam melting technology is powered by an energy-conversion
process. Highly accelerated electrons are formed into a beam by an
electron gun. The high kinetic energy of the beam is converted into
heat on impact with a surface. As the vibrational energy of the metal
lattice increases, melting and, ultimately, evaporation occurs.
Electron-beam heating has no upper limit. Today, electron-beam heating
is practiced in two modes: drip melting (the original technique) and
electron-beam cold-hearth remelting (EBCHR). Depicted in Figure 2, drip
melting is a purification technique. Metal droplets are produced by
melting exposed feedstock in the furnace vacuum. This permits the
evaporative removal of most of the absorbed gases and virtually all
elements having evaporation temperatures greater than that of the metal
being refined. It is the only technology available for refining
reactive and refractory metals.
Like drip melting, EBCHR (Figure 3) also removes gases and higher
volatility impurities in the material being processed. It also removes
inclusions. While the liquid metal travels along the hearth, the
heavier-than-the-metal inclusions settle to the bottom of the hearth
and are removed with the skull. The lighter components either dissolve
in the metal or float to the surface and are removed by a dam before
the melt flows into a watercooled ingot mold.
Both modes can be classified as crucible-free melting techniques,
with the "skin" of the molten metal forming on water-cooled copper to
become the crucible. The selective removal of the highervapor-pressure
elements-the key to refining reactive and refractory metalsbecomes a
liability when electron-beam processing alloys that contain components
with widely divergent vapor pressures. While such alloys are EBCHR
melted in large quantities, the processing costs are higher. This is
because more precise control of the feedstock and melt pool are needed
to compensate for the loss of the high volatility elements while still
providing the desired composition. At present, electron-beam
drip-melting processes account for virtually 95% of the 2,7003,600
tonnes of reactive metals produced in the United States, as well as
some of the refractory metals (titanium production is in addition to
this).
Recently, electron-beam processing has been applied to some
platinum-group metals,6 and this technique is likely to become the
process of choice for uranium scrap recovery.7 It is also used to
process silicon for solar energy applications, vanadium, and some
ceramics. EBCHR has become the most economical technique for converting
titanium scrap and/or scrap and sponge mixtures; in the United States,
this approach accounts for more than 35% of the 48,000 tonnes of
titanium produced. It is also used to produce ultraclean alloys,
specifically ultraclean titanium alloys and superalloys for aircraft
engine components. It is also used to produce directionally solidified
jet engine blades and reactive metal castings.
From an analytical perspective, electron-beam melting is virtually
the only process that makes it possible to monitor metal cleanliness.
Using a specially designed electron-beam button melter, an alloy sample
is drip melted under precisely controlled conditions. Via this method,
the inclusions float to the surface for analysis by scanning electron
microscopy.8 The use of such a technique can be a critically important
tool for aircraft engine manufacturers, for whom performance is
critical. As of the late 1950s, annular electron guns have made it
possible to grow refractory-metal single crystals and, in turn, study
the plastic deformation of these materials. In this regard, the latest
equipment was placed in service in 1993.9
TECHNOLOGY EVOLUTION
In general, three factors have spurred the success of electron-beam
melting technology: advances in vacuum technology, advances in computer
technology, and advances in the reliability and performance of electron
guns.
This article is the only place where the name marchelo pirani is found on the internet. There is a person named Marchello pirani, is that the same person?
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