US 3849123 A
A master alloy comprising an intermetallic grain refiner carried in an aluminum matrix is added in small particle size form to an aluminum melt.
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Description (OCR text may contain errors)
States Eaten] 1 Webster, Jr.
[ NOV. 19, 1974 INCORPORATION OF SOLID ADDITIVES INTO MOLTEN ALUMINUM 1/1961 Towner et a1 75/138 1/1972 Hoff 75/138  Inventor: Earle R. Webster, Jr., 213 Harvard Ave. Palmerton Pa. 18071 Primary Exammer-R. Dean  Filed: Nov. 7, 1972 Attorney, Agent, or Firm-Synnestvedt & Lechner  Appl. No.: 304,330
 U.S. C1. 75/138, 75/68 R 7 A T  Int. Cl. C220 1/06 [5 1 BSTRAC  Field of Search 75/138, 135, 68 R, 122,
75 93 A master alloy comprising an mtermetalllc gram refiner carried in an aluminum matrix is added in small 5 References Cited particle size form to an aluminum melt.
UNITED STATES PATENTS 2,595,292 5/1952 Reece 75/122 8 Claims, 1 Drawing Figure g, 252:? I j 16 1 1 a f 5 if I8 i 6 7 2| 5 i I \i I i 1 11 i 9 W \l] a e I 32 L II" n 3 l IIIII'IIII"" 8 0o "o II a", j
INCORPORATION OF SOLID ADDITIVES INTO MOLTEN ALUMINUM FIELD OF THE INVENTION This invention relates to the introduction of additives into an aluminum melt. More specifically, this inven tion relates to the incorporation of solid additives, such as intermetallic grain refiners, into an aluminum melt.
When used herein, the term aluminum melt means a substantially pure aluminum melt, for example, a melt containing 99 wt. or more of aluminum, or a melt containing aluminum which is alloyed with a metal or metals which modify the properties of the aluminum. Examples of such metals are copper, magnesium, manganese, silicon, zinc, bismuth, lead, beryllium, chromium and nickel. The aluminum melt can contain other alloying constituents, the aforementioned list being exemplary.
lt is known to incorporate into molten aluminum materials which are effective in reducing the grain size of the solid aluminum which is formed from the aluminum melt as it cools. Such materials are referred to usually as grain refiners. In general, the smaller the grain size of the solid aluminum, the better the physical properties thereof. Although there are different theories as to the mechanism by which the grain refiner functions, it is generally accepted that the grain refiner, directly or indirectly, provides sites about which the molten aluminum nucleates as it crystallizes. This results in a finer grained aluminum solid. Aluminum solids comprised of relatively large or coarse grains have poorer strength and have more of a tendency to crack during continuous casting than aluminum solids comprised of relatively small or fine grains.
One type of grain refiner that is used popularly in the aluminum industry is an intermetallic compound such as an alloy of titanium and aluminum. Such an alloy or intermetallic grain refiner is combined with aluminum, the host metal, wherein the aluminum functions as a matrix or carrier for the intermetallic grain refiner. The solid product comprising the intermetallic grain refiner carried in the aluminum matrix is referred to often as a master alloy of the grain refiner. Upon adding the master alloy to the aluminum melt, the aluminum carrier melts and releases the intermetallic grain refiner into the aluminum melt to perform its grain refining function.
The constituents comprising the aforementioned type of master alloy, upon being placed in the aluminum melt, do not volatize and are not violently reactive therewith. Also, the density of the intermetallic grain refiner is not significantly different from the density of the aluminum melt. Nevertheless, and as will be discussed in more detail hereinbelow, problems have been encountered in introducing the master alloy into the aluminum melt. For best results, the grain refiner must be distributed thoroughly and uniformly throughout the melt. If the grain refiner is not so distributed, the molten aluminum, as it crystallizes or solidifies, forms a product which is not homogeneous, that is, one comprised of relatively coarse grains and/or of relatively fine grains of varying sizes. This is not desirable because the product does not have uniform properties.
This invention relates to an improved method forintroducing into an aluminum melt a master alloy of an intermetallic grain refiner in a manner such that the grain refiner is distributed thoroughly and uniformly throughout the aluminum melt.
REPORTED DEVELOPMENTS As will be seen from the discussion which follows, heretofore reported methods for adding the aforementioned type of master alloy to an aluminum melt have one or more disadvantages.
One method used widely for incorporating the aforementioned type of master alloy into an aluminum melt consists of adding thereto slabs or ingots of the master alloy. Such slabs or ingots are quite heavy and weigh approximately 16 lbs. Often the slabs are notched so that they can be broken readily :into smaller chunks weighing about 1 lb. A serious disadvantage inherent in the use of such slabs or ingots, in addition to their having a relatively low solution rate in the aluminum melt, is that the intermetallic grain refiner tends to gravitate to the bottom of the furnace holding the aluminum melt. This problem can become particularly severe when, for one reason or another, the holding time of the aluminum melt in the furnace is prolonged. Although a sufficient number of slabs or ingots can be added to the melt in an effort to uniformly distribute sufficient grain refiner throughout the melt, that portion of the grain refiner which settles to the bottom of the furnace is waste which eventually builds up into excess amounts of sludge. When this occurs, the sludge is removed from the furnace. This is a time consuming and inefficient work step.
lt has been reported also that there can be added to an aluminum melt the aforementioned type of master alloy in the form of small discrete masses which weigh approximately a few ounces. It has been proposed to add such relatively small chunks of the master alloy to a stream of the aluminum melt as it is fed from the holding furnace to other containers where it is cast into its desired form. The preparation of such relatively low weight chunks of the master alloy within a weight tolerance which permits accurate amounts of the grain refiner to be introduced into the aluminum melt is difficult. Also, such chunks have a relatively low rate of solution in the aluminum melt. This creates the risk that the grain refiner will not be distributed uniformly throughout the melt.
A more recent method for adding the aforementioned type of master alloy to an aluminum melt consists of feeding master alloy in the form of a rod-like body, such as a wire or strip, into a :stream of the aluminum melt as it is fed from the furnace to other containers where it is cast into its desired form. A disadvantage of this method is that the wire or other rod-like object is relatively costly to fabricate.
In view of the above, it is an object of the present invention to provide an improved method for incorporating into an aluminum melta master alloy comprised of an intermetallic grain refining agent carried in an aluminum matrix.
BRIEF DESCRIPTION OF THE INVENTION In accordance with this invention, there is provided a method for incorporating into an aluminum melt a master alloy comprising aluminum and an intermetallic grain refiner comprising forming a stream of. the aluminum melt and adding thereto said master alloy in the form of small particles, for example, powders or granules thereof.
In preferred form, the small particles of the master alloy are introduced into a supported flowing stream of the aluminum melt below the surface thereof and in a manner such that the aluminum oxide layer which forms on the surface of the stream of the aluminum melt is not significantly disturbed.
Also, in preferred form, the particles of the master alloy, which are added to the melt, are prepared in a manner such that each individual particle contains many relatively small particles of the intermetallic grain refiner distributed homogeneously throughout the aluminum matrix which functions as a carrier for the grain refiner. As the small'particles of the master alloy are fed into the flowing stream of the aluminum melt beneath the surface thereof, the aluminum matrix of each particle melts almost instantaneously thereby releasing many minute grain refining particles into the molten stream of aluminum throughout which the grain refining particles are distributed. It should be appreciated that as the total of the small particles of master alloy are placed in the molten stream, and as each particle thereof releases many minute particles of the grain refiner, there is introduced into or produced in the aluminum melt a vast number of very small sites about which the aluminum can crystallize. This can result in an improved fine grained aluminum solid.
The introduction into the aluminum melt of a vast number of very small particles of the grain refiner is one of the principal characteristics of the present invention which distinguishes it from heretofore known methods for incorporating a grain refiner/master alloy into an aluminum melt. In heretofore used forms of the master alloy (slabs, ingots, chunks, wire, etc.), the grain refiner is present in the aluminum matrix in relatively large particles. (The methods used to prepare such forms of the master alloy produce relatively large particles of the grain refiner in the master alloy.) Per unit weight or volume of this type of master alloy, the number of grain refining particles introduced into the melt is far below the number that can be introduced when practicing the present invention in its preferred form.
It is noted also that in preparing master alloys in the form of slabs, ingots, etc., there is a tendency for the intermetallic compound to gravity segregate from the molten aluminum in which it is dispersed. When this occurs, the particles of the intermetallic compound are not dispersed homogeneously throughout the master alloy. On the other hand, the small particle form of the master alloy used in the present invention can be prepared readily in a manner such that the particles of intermetallic compound are distributed substantially homogeneously throughout the individual particle.
Another advantage of the present invention is that the small sized master alloy particles, with their inherent high surface area, go into solution more quickly when added to the aluminum melt than other forms of master alloy heretofore used. Due to the quick and almost instantaneous solution rate, the particles can be added to the aluminum melt just prior to casting without fear of gravity segregation. Waste of the grain refiner is avoided. Compared to the cost of other forms of the master alloy which can be fed continuously or semi-continuously into the aluminum melt, the small particle form is a less costly form to prepare.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of an illustrative system for feeding small particles of a grain refiner/- master alloy into a stream of aluminum melt in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION It is believed that the present invention will be used most widely in a system of the general type shown in FIG. 1 of the drawing. With reference to FIG. I, there is shown a holding furnace 2 containing aluminum melt 4. In a typical industrial process, molten metallic aluminum is transported from reduction cells in which alumina (M 0 is reduced to metallic aluminum to a first furance, not shown. If the aluminum is to be alloyed, alloying metals are generally added to the aluminum melt while it is contained in the furnace. From this first furnace, the molten aluminum is generally allowed to flow to a second or holding furnace, such as the furnace 2. When the aluminum melt 2 is ready to be cast, it is drawn off from furnace 2 through spout 5 in the form of a stream 6, supported in the trough 8, which flows to a suitable vessel or vessels where it is cooled and formed into desired forms such as, alloy ingots, rolling ingots, and billets.
The addition of the master alloy in the form of small particles to the stream 6 of the aluminum melt can be effected as follows.
Master alloy granules 14 from the hopper 16 are metered into an enclosed inclined chute 18 by means of a feedwheel 20 which controls the rate of flow of the granules. The granules 14 flow through a tube 21, comprised of graphite, or other metal which is non-reactive with the aluminum melt, and are forced into the molten aluminum stream 6 by the rotating screw 22 positioned in the tube 21. It can be seen that the granules are fed into the stream beneath the surface thereof without disturbing the aluminum oxide skin 7 which naturally forms on the surface of the stream. Breach of the aluminum oxide skin would cause undesirable oxygen pickup and the formation of additional aluminum oxide, which might be carried into the stream, along with pieces of the originally formed skin. The introduction of the aluminum oxide, which has a density similar to that of the aluminum melt, into the stream can affect adversely the properties of the cast aluminum. The system shown in FIG. 1 for feeding the master alloy granules into the stream of aluminum melt is such that little if any aluminum oxide is introduced into the stream of molten aluminum.
As the granules 14 are introduced beneath the surface of the stream 6, the aluminum matrix of the master alloy melts almost instantaneously and releases the intermetallic grain refiner into the stream. The temperature of the aluminum melt is sufficiently high to melt the aluminum matrix of the master alloy, but is below the temperature at which the intermetallic grain refiner melts or decomposes. As the grain refiner is released into the stream, it is carried along by the flow of the stream and distributed substantially uniformly throughout the stream. The flow of the stream 6, aided by the almost instantaneous dissolution of the aluminum matrix, keeps the released grain refiner from settling in the trough 8. The molten stream of aluminum containing the grain refiner is transferred to the molds in which the melt is solidified and formed into the desired shape.
Various modifications can be made to the feeding system described above. For example, there can be used other methods for introducing granules or powdered master alloy into the molten aluminum stream. This can be accomplished by blowing the granules or powder through a tube by means of a compressed gas which is nonreactive with the granules and aluminum melt. The tube through which the granules are fed, as shown in the drawing, is in a vertical position, but it may be also placed in a horizontal position, or at any angle to the flowing stream.
The aluminum melt to which the small particle master alloy is added can be substantially pure aluminum or an aluminum alloy.
Any suitable intermetallic grain refiner can be used in the practice of the present invention. Examples of such grain refiners include: alloys of titanium and aluminum; alloys of titanium, boron and aluminum; and alloys of boron and aluminum. It should be understood that other grain refiners can be used also. It is believed that the invention will be most widely used in adding to the aluminum melt a grain refiner comprising an aluminum/titanium alloy, the grain refiner presently used most widely throughout the industry.
As mentioned above, the grain refiner is incorporated in an aluminum matrix, thereby providing a master alloy. The amount of grain refiner comprising such master alloys will tend to vary depending on the particular metal or metals incorporated in the master alloy. By way of example, it is noted that the master alloys presently being used comprise about 2 to about 30 wt. of the alloy metal or metals, the remainder of the master alloy comprising aluminum. Examples of master alloys are the following: about 5 to about 20 wt. titanium, balance aluminum; about 5 to about 20 wt. titanium, about 1 to about wt. boron, and balance aluminum; and about 2 to about 20 wt. boron and balance aluminum. Specific examples of such master alloys include: 6 wt. titanium, balance aluminum; 5 wt. titanium, 1 wt. boron and balance aluminum; and 4 wt. boron and balance aluminum. It is noted that zirconium can be substituted for titanium in the aforementioned examples thereby providing a zirconium based grain refiner/master alloy.
The master alloy in small particle size form can be prepared according to any of the many available methods for preparing metals or metal alloys in granular or powdered form. Speaking generally, the aluminum is melted and then superheated to a temperature at which the intermetallic grain refiner goes into solution in the aluminum melt or to a temperature at which the intermetallic grain refiner is chemically formed in situ in the aluminum melt. The resulting liquid alloy is then atomized with a liquid or gas, thereby providing liquid particles which upon cooling comprise a solid aluminum matrix in which particles of the grain refiner are dispersed.
In a preferred method for producing the small particles of master alloy, the liquid solution of aluminum and grain refiner is very quickly chilled after it is atomized. (For example, see the method described in U.S. Pat. No. 2,967,351.) This decreases the size of the grain refiner particles in the aluminum matrix of each particle. The advantages of having each particle of master alloy comprised of many minute particles of the intermetallic compound have been discussed above.
The particle size of the master alloy can vary over a relatively wide range. Generally speaking, the smaller the particles, the more readily they are dissolved and dispersed in the aluminum melt. Thus, there are advantages to be gained by having the particles as small as possible, for example, on the order of 325 mesh (U.S. Standard Sieve Series). However, there is a risk of explosion with very small sized particles or powders. To avoid or minimize this risk, the average particle size of the master alloy should be no smaller than on the order of about 100 mesh. Larger sized particles can be used, for example, on the order of 5 mesh.
The particles of master alloy can be of any shape. Spherically shaped particles have the advantage that they can be gravity fed to the aluminum melt very easily.
The master alloy should be added to the aluminum melt in amounts such that a sufficient amount of the grain refiner is present in the aluminum melt to perform its grain refining properties. The amount of grain refiner can vary over a wide range depending on numerous factors such as the specific grain refiner used, the particular type of aluminum melt which is treated with the grain refiner, the particular casting method used in shaping the aluminum melt, and the desired grain size of the cast aluminum object. By way of example, it is noted that the cast aluminum shape can comprise about 0.002 wt. to about 0.2 wt. titanium or about 0.01 to about 0.1 wt. boron.
EXAMPLES U.S. Standard Sieve Series, No. Wt. /r passing The molten aluminum, which is alloyed with 1.25 wt. manganese, is fed from a holding furnace to a casting station at a rate of 500 lb. per minute. The temperature of the aluminum melt, as it exits from the furnace is about 1,350F; the temperature upon reaching the casting station is about 1,300F. The master alloy granules are added to the flowing stream of aluminum melt at the rate of 0.05 lb. per minute. The aluminum cast that is produced contains 0.005 wt. residual titanium. The aluminum cast has a fine grained structure.
1. A method for forming a fine grained aluminum solid by incorporating a master alloy containing aluminum and an intermetallic grain refiner into an aluminum melt comprising forming a stream of the alumi num melt and adding thereto beneath the surface of said stream said master alloy in the form of small particles and thereafter solidifying said aluminum melt, and wherein said particles are prepared by quickly chilling an atomized liquid solution of a master alloy containing aluminum and an intermetallic grain refiner, thereby providing particles, each of which contain many minute particles of the grain refiner carried in an aluminum matrix.
2. A method according to claim 1 wherein the size of said particles is such that substantially all of said particles pass through about 5 mesh (US. Standard Sieve Series).
3. A method according to claim 1 wherein the size of said particles is such that substantially all of the particles pass through about 5 mesh and substantially all are retained on about 100 mesh (US Standard Sieve Series).
4. A method according to claim 1 wherein said particles are spherically shaped.
5. A method for forming a fine grained aluminum solid from an aluminum melt containing a master alloy comprising aluminum and intermetallic grain refiner comprising: forming a supported stream of said aluminum melt, adding to said stream beneath the surface thereof said master alloy in the form of particles having a size of about 5 to about 100 mesh (US. Standard flowing liquid form for a sufficient period of time to allow said grain refiner to be distributed substantially uniformly throughout said stream; and thereafter solidifying said melt; wherein said particles are prepared by quickly chilling an atomized liquid solution of a master alloy containing aluminum and an intermetallic grain refiner, thereby providing particles, each of which contain many minute particles of the grain refiner carried in the aluminum matrix of said master alloy.
6. A method according to claim 5 wherein said master alloy comprises about 2 to about 30 wt. of intermetallic grain refiner, balance aluminum.
7. A method according to claim 6 wherein said master alloy comprises about 5 to about 20 wt. titanium, balance aluminum.
8. A method according to claim 5 wherein said parti-