Electricity: measuring and testing – Determining nonelectric properties by measuring electric... – Particle counting
Reexamination Certificate
1999-04-13
2002-01-08
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Determining nonelectric properties by measuring electric...
Particle counting
Reexamination Certificate
active
06337564
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a method and apparatus for sensing inclusions in molten metal.
2. Background
Inclusions in molten or liquid metal are impurities found in the liquid metal. The presence of inclusions can affect the quality of ingots cast from that liquid metal. The quality of the ingot affects the quality of the subsequent sheet and plate products fabricated from that ingot.
High concentrations of inclusions in molten or liquid metal, e.g., such as inclusions in molten aluminum typically at sizes of 15 to 120 microns and in amounts over about three kilocounts per kilogram, adversely affect metal quality.
INTRODUCTION TO THE INVENTION
The impurities forming the inclusions in the liquid metal, in contrast to elemental impurities, are particles of separate phases suspended in the metal.
The inclusions in the liquid metal can be classified into one of two types of inclusions. The two types are hard and soft inclusions.
Hard inclusions primarily are attributable to oxides or other non-deformable particles. Examples of hard inclusions include magnesium oxide, aluminum oxide, spinel (magnesium aluminum oxide or MgAl
2
O
4
), silicon oxide, aluminum carbide, silicon carbide, titanium diboride, and vanadium diboride in certain circumstances.
The hard inclusions either come from the metal source itself, whether from ore or recycled metal, when it gets melted, or are created by oxidation during the melting process or elsewhere in the process because of oxygen present in the air or because of water vapor present, because of cascading or turbulence in the furnace, e.g., because of a fixed tap hole making for a lot of turbulence in the metal at the furnace outlet as liquid metal flows into the trough.
Oxide inclusions arise from the oxidation of aluminum or aluminum alloy at some point in its processing. Melting of scrap metal can result in oxide films, formed on the surface of the aluminum, being entrained in the molten metal. Alloying molten aluminum with magnesium can produce magnesium oxide inclusions from oxidation of the magnesium. Oxidation of molten metal as it flows through the launder system also can produce oxide inclusions. Turbulence at the molten metal surfaces and cascading of molten metal through air promote the formation of inclusions.
Aluminum carbide inclusions arise from the Hall-Heroult electrolytic cells in which primary aluminum is produced. Aluminum reacts with carbon cathodes in these cells to form aluminum carbide.
Silicon oxides arise from refractories used in furnaces, launder systems, and casting spouts.
Boride inclusions arise from grain refiners used to control the grain size in the solidified metal. Inclusions of borides would be a much larger size than boride particles that are effective as nucleating sites for aluminum grains.
Soft inclusions are attributable to molten salt droplets, gas bubbles, agglomerates of other very small particle types, or other deformable inclusions. Examples of soft inclusions include magnesium chloride, sodium chloride, potassium chloride, calcium chloride, aluminum chloride, cryolite salts, calcium fluoride, and liquid solutions containing these compounds or a combination of these compounds. Agglomerates of borides are another example of soft inclusions.
The soft inclusions come from using chlorine and chloride to treat the metal, e.g., in an in-line degassing process wherein even small amounts of chlorine gas form aluminum chloride and magnesium chloride. The aluminum chloride is not stable and tends to react to form magnesium chloride.
Fluxing molten metal with chlorine or a combination of an inert gas with chlorine removes hydrogen, alkali metals, alkaline earth metals, and hard inclusions from the molten metal.
Chlorine is used to react chemically with alkali and alkaline earth metals. The chlorine aids wetting of hard inclusions by flux gas bubbles and allows for removal by flotation. The chlorine aids the separation of skim from molten metal. The formation of molten chloride inclusions is a by-product of using chlorine to treat the metal. Fluxing is carried out in furnaces and in-line during the casting process.
The soft inclusions also come from using granular salts in the furnace, e.g., magnesium chloride, calcium chloride, sodium chloride, and potassium chloride, and combinations of magnesium chloride, calcium chloride, sodium chloride, and potassium chloride. These salts are used for reacting to remove sodium and calcium from the metal, to minimize melt loss, or to keep the furnace clean. Such granular salts in the furnace cause some carryover of molten salt inclusions.
The hard inclusions and soft inclusions range in size from about 1 micron to several hundred microns.
The total concentration of hard inclusions and soft inclusions is about 0.05-0.1 kilocount per kilogram, i.e., 50-100 particles per kilogram of metal, to about 150 kilocount per kilogram, i.e., 150,000 particles per kilogram of metal. The total concentration of hard inclusions and soft inclusions depends on the source of the metal from scrap type, solid primary metal, molten primary metal, or remelted beverage cans. The total concentration of hard inclusions and soft inclusions depends on settling time allowed in the furnace, the cleanliness of the furnace, the in-line treatment of the metal, the in-line filtration of the metal, and the design of the launder system.
Inclusions cause problems which depend on the type of product and the gauge of the product. For example, inclusions affect three inch plate differently from 6 micron thick foil. The inclusions will cause a hole or pinhole in the 6 micron thick foil. In beverage can or food can sheet, for example, pinholes are serious concerns.
Inclusions cause pinholes in foil and rigid container sheet such as food can sheet or beverage can sheet. Inclusions cause breakage of wire during drawing operations. Inclusions cause surface imperfections such as streaking in bright products such as reflector sheet or automobile trim. Inclusions also cause surface defects during extrusion processes. Inclusions also serve as nucleating sites for the formation of gas bubbles during solidification and thereby affect the fatigue life of certain products.
The two different inclusion types affect metal quality in different ways. It is difficult to know whether hard inclusions or soft inclusions cause particular problems because of an inability to distinguish between different inclusions.
Inclusions in the metal are analyzed by destructive methods of analysis. Currently, inclusions in the metal are analyzed and classified by destructive testing by taking a sample of the metal, solidifying the metal sample, cutting open the solid metal, looking at it under a microscope, and classifying or identifying the inclusion in the solid metal sample to determine qualitatively, and semi-quantitatively, whether the inclusion is a hard or soft inclusion.
Currently, inclusions in the molten metal are analyzed and classified by destructive testing by taking a sample of the molten metal and metallographically analyzing the sample for inclusions. The inclusions in the metal are concentrated in the sample by passing the molten metal through a filter or frit and then metallographically analyzing the sample to search for the inclusions at the leading edge of the filter or frit. Podfa and LAIS are trade marks of two commercially available sampling systems based on metallographic analysis. The metallographic analysis identifies the inclusion types and distinguishes between what are hard and soft inclusions at molten metal temperatures. However, the metallographic analysis is only semi-quantitative and does not provide results in real time.
Currently, inclusions in the metal are analyzed by non-destructive testing by ultrasonic testing. However, ultrasonic testing is performed only on the metal after it is solidified into a solid part. Moreover, ultrasonic testing provides a resolution of only about {fraction (1/64)} inch. Much smaller inclusions cause problems in certain products. For exampl
De Young David H.
Manzini Richard A.
Alcoa Inc.
Glantz Douglas G.
Kerveros J
Metjahic Safet
Pearce-Smit David W.
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