Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
2003-06-09
2004-05-11
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C075S604000, C075S684000, C164S055100, C164S123000, C420S590000
Reexamination Certificate
active
06733566
ABSTRACT:
FIELD OF THE INVENTION
The instant invention relates to a method of preventing metal loss due to oxidation in molten aluminum and magnesium alloys. Specifically, covering the molten alloy surface with a layer of petroleum coke, which subsequently oxidizes at high temperatures to form a layer of carbon dioxide that insulates the alloy melt from ambient air oxygen, significantly reduces metal loss from the molten alloys due to oxidation.
BACKGROUND OF THE INVENTION
Many metals and metal alloys, used especially in the foundry industries, suffer from loss of metal due to oxidation while in the molten state (“melt” for brevity). This is particularly true for many aluminum-magnesium alloys. Significant and costly magnesium weight loss in the melt can be experienced. This leads to difficulty in controlling alloy composition, which is required to meet mechanical property specifications of the solidified alloy. As magnesium in the melt is depleted by loss to oxidation, magnesium concentration in the melt needs to be monitored and restored to desired levels by new magnesium additions.
Further, the significant metal weight loss in the melt due to oxidation results in the formation of dross on the surface of the melt, and oxide inclusions in the melt. Dross, as used herein, refers to undesirable oxides that form on the surface of a melt. Prior to casting the alloy, dross must be skimmed off of the melt surface, and disposed of in landfills. Oxide inclusions result in degradation of mechanical properties, such as ultimate tensile and yield strengths, of the alloys.
Previous methods that are employed to limit metal melt oxidation attempted to either blanket the melt in an atmosphere of inert gas, thus preventing oxidation of the melt surface, or to cover the melt surface with a solid inert barrier material that could physically prevent oxygen in the furnace atmosphere from reacting with the melt surface.
U.S. Pat. No. 6,024,779 describes a means to minimize oxidation of copper melts. Carbon sand, consisting of coke particles, is layered on top of a copper melt in thicknesses from about 1 millimeter to about 4 inches to prevent oxidation of the melt. The carbon sand forms a protective layer over copper and copper alloy melts and can be re-used. U.S. Pat. No. 6,485,541 teaches a method to reduce oxidation of aluminum melts. Inert refractory material is allowed to float on the melt, thus isolating the melt from oxygen in the air. U.S. Pat. No. 5,415,220 teaches the use of a salt covering for DC casting aluminum-lithium alloys. A mixture of lithium chloride and potassium chloride is preferred. The need for an inert atmosphere is obviated by use of the salt cover. U.S. Pat. No. 5,421,856 discloses the use of gas injectors in the melt that utilize inert gases to remove oxygen from the atmosphere above the melt.
Using an inert atmosphere to minimize oxidation requires specialized furnaces that seek to maintain an inert atmosphere directly above the melt surface. Such furnaces are costly to install, to maintain, and to use. However, the exclusion of practically all of the oxygen from the furnace atmosphere provides for low oxide formation.
Placing physical barriers on the melt surface allows for the use of ordinary furnaces that operate in ambient air. However, because of the high thermodynamic driving force for the reaction of certain metals, such as aluminum and magnesium, with oxygen, it is difficult to eliminate oxide formation simply by using physical barriers.
Accordingly, it would be advantageous to have an economical process that would combine the efficiency of inert atmosphere with the economy and the facility of using a physical barrier.
In addition to magnesium metal loss due to oxidation and general dross formation, it is well known in the aluminum alloy casting art that melt surface oxidation can result in various surface imperfections in cast ingots such as pits, vertical folds, oxide patches and the like, which can develop into cracks during casting or in later processing. A crack in an ingot or slab propagates during subsequent rolling, for example, leading to expensive remedial rework or scrapping of the cracked material.
The casting of alloys may be done by any number of methods known to those skilled in the art, such as direct chill casting (DC), electromagnetic casting (EMC), horizontal direct chill casting (HDC), hot top casting, continuous casting, semi-continuous casting, die casting, roll casting and sand casting. Each of these casting methods has a set of its own inherent problems, but with each technique, surface imperfections can still be an issue. One mechanical means of removing surface imperfections from an aluminum alloy ingot is scalping. Scalping involves the machining off a surface layer along the sides of an ingot after it has solidified.
Certain alloys, such as 7050 and other 7xxx alloys as well as 5182 and 5083 are especially prone to surface defects and cracking. In the past, beryllium has been added, usually at part per million (ppm) levels to some of these alloys to control melt surface defects, and to prevent magnesium loss due to oxidation. However, beryllium has been banned from aluminum products used for food and beverage packaging. Further, there have been increased concerns over the health risks associated with factory workers using beryllium and products containing beryllium. For this reason, although beryllium is effective at controlling surface defects and melt surface oxidation in aluminum cast ingots, a suitable replacement is needed.
U.S. Pat. No. 5,469,911 to Parker discloses a method for improving the surface quality of electromagnetically cast aluminum alloy ingots, which includes the addition of 0.01 to 0.04 wt. % calcium prior to the ingot head of an ingot mold. These levels of calcium are significantly higher than the ppm levels employed with beryllium. Such high levels of calcium can adversely affect the properties of the alloy.
U.S. Pat. No. 4,377,425 to Otani et al. discloses using calcium in high iron containing direct chill cast aluminum alloy ingots to minimize the occurrence of dendritic or so-called “fir tree” crystal structures with a grain size of less than 150 microns. This method was particularly useful for AA1000 and AA5000 series aluminum alloys. The effect, if any, of calcium on the surface quality of the resulting ingots was not disclosed by Otani et al.
There remains a need for an effective alternative to beryllium to prevent surface imperfections, such as vertical folds, pits, oxide patches and the like from forming during aluminum ingot casting, and to prevent melt surface oxidation. Such a method would be instrumental in preventing cracks, which can form during casting or can develop in later processing. Finally, the method preferably would have no adverse affect on alloy properties.
The primary object of the present invention is to provide an economical and efficient method for minimizing metal loss due to oxide formation on an aluminum alloy or magnesium alloy melt surface in furnaces operating in ambient air.
Another object of the present invention is to provide an in situ method of generating an inert atmosphere above an aluminum alloy or magnesium alloy melt surface without the requirement of specialized furnaces that are adapted to displace oxygen during operation.
A further object of the present invention is to provide a method of minimizing oxide formation on an aluminum alloy or magnesium alloy melt surface with the method having the efficiency of using an inert atmosphere and the facility of using a physical barrier placed on the melt surface.
Yet another object of the present invention is to provide a method to minimize dross formation on an aluminum alloy or magnesium alloy melt surface.
Still another object of the present invention is to provide a method to minimize oxide inclusions in an aluminum alloy or magnesium alloy solid, thereby maximizing the mechanical properties of that solid.
Still a further objective to this invention is to provide an effective alternative to beryllium alloying additions to alum
Fang Que-Tsang
Haddenhorst Holger
Kuhn Timothy A.
Lin Jen C.
Alcoa Inc.
Andrews Melvyn
Beiriger Tracey D.
Nitowski Gary A.
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