Background of the Invention
The present invention relates to a system for detecting
and characterizing contamination of high temperature metal melts caused by the entry
of condensed vapor matter to the molten metal pool. The invention relates generally
to the melt processing of high temperature metals, such as those with melting temperatures
above 1000°C, and particularly to those processes in which heat is applied
directly to the melt surface, as is the case with the cold-hearth refining of metals
using an electron beam heat source. This practice is commonly used for the production,
recycling and refinement of reactive and refractory metals, such as titanium alloys
and nickel-base super alloys. In the electron beam, cold-hearth refining practice
charge material is melted under a high intensity electron beam or beams in a water-cooled
copper hearth. The melting process is performed under vacuum and is characterized
by high-power densities, extremely high attainable melting temperatures, and the
ability to accept a wide range of charge materials, including bars, compacts, sponge,
scrap and chips. A thin layer of molten metal solidifies on the hearth surface creating
a "skull" that protects the hearth, prevents cross-contamination with the copper,
and contains the liquid metal pool. Refinement of the molten material is achieved
by the evaporation of volatile impurities, sedimentation and entrapment in the skull
of high density contaminants, and the dissolution of low density impurities in the
super heated surface layer of the melt pool. Molten metal may flow continuously
from the metal hearth to a secondary hearth for further refining or it may be cast
directly into a mold where it is solidified. Careful control of the melting, refining
and solidification stages of this process yields very clean, virtually defect-free
One drawback of this method, however, is that a substantial
amount of metal vapor is generated during the melting process due to the high temperatures
and vacuum environment. These vapors condense on the interior surfaces of the furnace
chamber and form solid deposits that have the potential and propensity to fall back
into the melt. This presents a major risk of contamination as the metal vapors are
typically rich in the more volatile elements of the melt. Vapor deposits from a
titanium alloy containing nominally 6% aluminum, for example, are known to contain
as much as 70% aluminum. This disparity in composition may cause significant local
property variations and lead to potentially critical defects in final products should
the condensate re-enter the melt.
Instances of condensed vapor metal flaking off of the deposition
surface and falling into the melt pool are known to occur during normal melting
practice. Steps to mitigate the effect of contamination may include temporarily
halting the cast and/or the direction of additional heat to the affected area to
promote the dissolution of the contaminating material and homogenization of the
melt chemistry. These require immediate action on the part of the furnace operator.
Any remediation must be effected prior to solidification of the contaminated region
and thus instantaneous detection, characterization and response are essential.
SUMMARY OF THE INVENTION
Detection of contaminant entries currently relies on visual observation by the furnace
operators controlling the melt process. Given that contaminant particles may be
small in relation to the melt pool and the operator's attention may be divided among
several additional tasks, it is likely that some portion of these entries go undetected
by the operators. Of greatest concern are those contaminants that fall directly
into the solidification mold and go undetected, as there is very limited capability
to refine and homogenize the melt at this late stage of the process.
Accordingly, the invention provides a system for constant,
uninterrupted visual monitoring of the molten metal bath either in a hearth or mold.
It further provides means to instantaneously detect the entry of condensed vapor
material from the furnace chamber into the melt pool. It also provides immediate
notification to furnace operators as to the presence of contaminant entries so that
corrective action may be taken, if necessary, in a timely manner. It provides characterization
and a record of the number, size and location of all contaminant entries into the
melt and makes this data available subsequently for product quality review.
The apparatus in accordance with the invention for detecting
and characterizing contaminants in high temperature metal melts includes means for
visually monitoring a bath of molten metal within a hearth or mold to detect instantaneously
entry of a condensed contaminant to the bath. Means for providing an instantaneous
signal are provided that characterizes size and location of a contaminant within
The means for visually monitoring includes an infrared
The instantaneous signal from the infrared video camera
is a gray-scale, thermal image of a selected viewing area of the bath of molten
metal. The instantaneous signal is directed to a video recorder and monitor.
The instantaneous signal may be also directed to means
for analyzing the signal to determine the number, size, location and duration of
the contaminant. Means are provided for storage of the number, size, location and
duration of the contaminant.
BRIEF DESCRIPTION OF THE DRAWINGS
The single Figure of the drawing is a schematic, block
diagram of a system for detecting and characterizing contaminants in a high temperature
DESCRIPTION OF THE EMBODIMENT
Referring to the Figure, the detection system consists
of a video camera 1 with associated lens package 2, a digital converter 3, a high
speed analyzer 4 and a video recorder 5 with an optional monitor 6. This system
is integrated with the furnace data acquisition system (DAS) 7 which provides for
interfacing with an operator interface 8 and additionally provides for automated
The camera may be any video camera that operates in the
near-infrared (NIR) region of the wavelength spectrum (400nm - 1100 nm). The camera
is affixed to an optical housing which mounts to a viewport on the furnace in such
a way as to create a vacuum tight seal and provide an unobstructed view normal to
the surface of the melt pool. The optical housing contains a fixed focusing lens,
an infrared filter to suppress radiation from outside the waveband of interest,
and a fixed aperture plate and neutral density filter to reduce the radiation intensity
at the detector. A leaded glass window is used to prevent x-ray radiation from damaging
the detector array and a disposable quartz window is placed at the very front of
the housing to protect the optics from degradation from the heat and vapors from
the furnace. The front of the housing is continuously flushed with a flowing gas,
such as argon, to prevent metal vapor deposition on the window.
The camera system produces a gray-scale, thermal image
of the viewing area. The video signal is output, either directly by high speed serial
bus (i.e. firewire) or through a frame grabber, to the digital image analyzer and
the video recorder and monitor. The analyzer, an Acuity Powervision System (Nashua,
NH) in the present configuration, performs real time, high speed inspection of the
video image. A region of interest (ROI) within the video image is defined and all
areas of the image outside of the ROI are masked from inspection. In the current
application, the ROI is the liquid metal pool contained in the solidification mold.
The analyzer characterizes the gray level intensity of the pixels within the ROI
and establishes the background intensity level. The detection threshold is determined
which defines the boundary between background and foreground pixels. The ROI is
then continuously inspected for features (groups of pixels) with intensities below
the threshold level, so called foreground pixels, which represent lower temperature,
solid contaminants in the molten metal pool. The analyzer operates at a rate of
at least 4 scans per second to ensure adequate resolution of contaminant entries,
and is able to extract a wide range of information from the image including the
number, size, location and duration of contaminant entries. This data is transmitted
to the furnace data acquisition system.
Data from the analyzer is continuously processed, compiled
and stored by the DAS. Image data is filtered to suppress transient detections,
ignore very small entries, and reduce the occurrence of nuisance alarms. In the
event of contamination, the DAS provides an instantaneous audible alarm to the furnace
operator as well as a visual prompt indicating the size and location of the entry.
The operator can then quickly evaluate the entry and, if necessary, take the prescribed
action to remediate the contamination.
Integration with the DAS enables control of and communication
with the condensate detection system through a central operator interface and eliminates
redundant data entry and setup tasks. It also allows automated execution of system
functions based on furnace and melt parameters. The DAS, for example, supplies the
mold geometry on which the ROI is based, automatically initiates image inspection
at the start of the melt process and shuts the detection system down when the melt
power is turned off at the conclusion of casting. This makes for virtually transparent
operation of the detection system. The stored data from the image analyzer becomes
a permanent record for customer and quality review.
The above description makes particular reference to application
in conjunction the solidification mold, however, it is intended that the invention
could be applied, with little modification, to the melting and/or refining hearths
in the EBM process.
It is also recognized that the data produced by the image
analyzer could be output to a computer controlled beam guidance system, in furnaces
so equipped, to automatically direct additional beam energy to the affected area
of the melt in order to enhance dissolution and diffusion of the contaminant.