Continuous casting aluminum alloy

A method is provided for continuously casting aluminum alloys having more than two percent total alloying elements by a combination of casting at thinner gauges and much higher speeds than usual. Since remelting in the caster is avoided, sheet quality is vastly improved, surface ripples are avoided, and casting rate increased as much 50%. The method is characterized by the thickness of the cast sheet being in the range of from 4 to 5.8 millimeters and the casting rate being in the range of from 1.3 to 1.9 meters per minute. Chlorides in the molten metal are coalesced and oxides filtered to keep the inclusion rate in the cast sheet very low.

FIELD OF THE INVENTION 
This invention relates to a process for high production rate continuous 
casting of aluminum base alloys, with two percent or more of total 
alloying elements, and is particularly useful for magnesium containing 
alloys used for magnetic recording disk substrates. 
BACKGROUND 
The hard magnetic disks used as memory media for storage of data in 
computers require an extremely high quality aluminum alloy substrate. The 
substrate depends on production of an especially high quality aluminum 
alloy sheet commonly referred to as disk stock. The magnetic disk 
substrate is blanked from this sheet, then processed through various 
thermal flattening, machining, lapping, polishing, chemical and anodizing 
operations before being coated with a thin film of magnetizable material. 
For example, such coatings may be applied by electroless or electrolytic 
plating or sputtering of cobalt-phosphorus or cobalt-nickel-phosphorus 
alloys directly on the aluminum, alloy substrate, or by coating the 
substrate with iron oxide or other magnetic powder. 
The magnetic transducer that reads and writes on such a disk "flies" within 
a micron or less of the rotating disk surface. An extremely high 
uniformity of surface is required to avoid crashes of such a flying head 
and to prevent dropouts of magnetic recorded data due to pinholes or the 
like in the recording film. 
In recent years there has been an emphasis on producing disks with higher 
information density in order to increase their capacity. A higher density 
inherently necessitates a decrease in the area for each bit of magnetic 
information on the disk. The increased resolution requires decreasing 
thickness of the magnetizable film and reducing the distance from the 
flying head to the magnetizable film surface. These requirements can only 
be met on a surface which has minimal micro roughness and no asperities. 
Hence, a substrate material with excellent surface is a prerequisite. 
The surface layers of the substrate must be mechanically, chemically and 
microstructurally homogeneous, thus assuring that after polishing and 
electrochemical treatments, the surface of the disk is extremely smooth 
and flat and has high magnetic uniformity. The surface layers should be 
free from defects, inclusions and segregation which may cause 
discontinuities in the surface topography or magnetic characteristics. 
To make magnetic memories economically in commercial quantities, industrial 
scale melting and casting conditions must be used, and conventional 
aluminum plant rolling and heat treating equipment are important. The 
substrate must have suitable mechanical strength, corrosion resistance, 
modulus of elasticity, density, heat resistance and magnetic properties 
for reliable magnetic memory disks. 
At present most disk stock is produced by classical methods involving 
casting of large direct chill ingots 300 to 600 millimeters thick and 
sufficiently wide to be rolled to sheet having a width of 1.1 meters. The 
cast ingot is hot rolled, followed by cold rolling and annealing 
operations to obtain the desired thickness and width. 
Exemplary alloys for magnetic memory disk stock are 5082 with a magnesium 
content of about 4%, and 5086 having a magnesium content of about 4% and a 
manganese content of from 0.2 to 0.7%. These intentionally added alloying 
elements, along with some impurity elements typically present in the 
alloy, tend to form intermetallic compounds during the solidification 
process, the most prominent of these being various forms of Al-Fe-Mn and 
Mg-Si phases. Because of the relatively slow cooling rate with large 
ingots, the intermetallic compounds tend to be rather coarse with 
dimensions generally exceeding ten microns. These large intermetallic 
compound particles can be quite deleterious to the quality of a magnetic 
memory disk substrate. The intermetallic compounds are invariably harder 
than the aluminum alloy matrix and do not exhibit the same degree of 
plastic flow during rolling operations, hence they have a tendency to 
separate from the matrix, forming microscopic voids. The machining and 
lapping operations may leave the intermetallic particles as protuberances 
from the surface or may pull them out from the surface, leaving voids. 
Such surface particles or voids cause an electrochemical discontinuity 
which tends to disrupt the formation of a smooth, continuous anodic film 
during the electrochemical treatments. Discontinuities in the anodizing 
can be mimicked in the magnetic film applied to the substrate. 
Grain refining materials can be added to the alloy used for casting of 
large ingots to produce a fine grain size. However, the intrinsically slow 
cooling rate produces a comparatively large dendrite arm spacing, allowing 
microsegregation to occur and producing microheterogeneity, particularly 
in the intermetallic compound distribution. This microsegregation is 
difficult to eliminate during subsequent processing and may result in 
uneven surface in the final disk substrate. 
Another proposed technique for producing disk stock for magnetic media 
starts with continuous casting of aluminum alloy sheet. Techniques have 
been developed for continuously casting a variety of aluminum alloys into 
sheet less than 10 millimeters thick by introducing the metal through a 
pouring tip made of insulating material, into the nip of continuously 
rotating casting rolls which are water cooled, thereby freezing and 
somewhat hot rolling the cast sheet. This technique has proved rapid and 
economical for casting commercial purity aluminum sheet and a variety of 
aluminum alloy. However, continuous casting of aluminum alloys has not yet 
had an impact on the disk stock market. 
The alloys of choice for making disk stock are 5082, 5086 and 5182 or the 
like. These alloys have proved particularly difficult to continuously cast 
with consistently high quality. No suitable technique has been developed 
for making production quantities of disk stock of these materials. Only 
narrow width, pilot plant scale quantities of metal have been produced. 
Even so, the method has been dependent on tight control of alloy 
chemistry, which would be difficult to achieve in production conditions. 
Intermetallic segregation remains a problem since the largest particles 
are still of sufficient size to either protrude from the surface or leave 
voids, which in either case disrupt the formation of the anodic and 
magnetic films during electrochemical treatment. 
Most significantly, prior continuous casting techniques for these alloys 
have not produced a completely homogeneous surface structure in the cast 
strip. Fluctuations during the casting process result in heterogeneity 
which results in heterogeneity which results in a rippled appearance on 
the surface. Heterogeneity in the cast sheet may require a high 
temperature annealing treatment to ameliorate its effects. 
Although particularly troublesome in making computer disk stock, the 
appearance of ripple on the surface of aluminum alloys can be quite 
troublesome when the alloys are used for other purposes, as well. Ripple 
seems to be a problem in many alloys having more than about 2% of alloying 
elements in the aluminum. It is not generally regarded as a problem with 
the 1000 series of wrought aluminum materials, which are effectively 
commercially pure aluminum having 99% or more aluminum. 
The reason for appearance of ripple on continuously cast aluminum alloy 
sheet has not previously been understood. It has been known to be 
associated with appearance of contamination on the surface of the casting 
rolls. Efforts have been made to avoid the appearance of ripples by 
mounting wire brushes to continually scrape such contamination from the 
roll surfaces. This has not proved satisfactory since such mechanical 
abrasion of the roll surface may lead to sticking, where the cast aluminum 
sheet adheres to the roll surface, causing quite several damage to the 
sheet. 
Surprisingly it is found that when casting aluminum alloys in practice of 
this invention, rippling can be avoided and quite substantial increases 
can be made in the output of the casting machine. This effect is obtained 
not only with the magnesium-bearing alloys but also with other 
continuously castable alloys having more than about 2% of total alloying 
ingredients. 
SUMMARY OF THE INVENTION 
There is, therefore, provided in practice of this invention an improved 
method for casting aluminum alloy having more than about 2% of alloying 
elements in the aluminum wherein the molten aluminum alloy is continuously 
introduced through an insulated pouring tip into the entrance to the nip 
of the rotating rolls and a cast sheet is continuously withdrawn from 
between the rolls. The method, according to a presently preferred 
embodiment, is characterized by the thickness of the cast sheet being in 
the range of from 4 to 5.8 millimeters, and the casting rate being more 
than 1.3 meters per minute. Preferably, the thickness of the cast sheet is 
in the range of from four to five millimeters, and the casting speed is in 
the range of from 1.5 to 1.9 meters per minute. 
A variety of practices provide high quality cast sheet in accordance with 
such a process. Among other things, careful attention to filtering the 
alloy upstream from the caster to remove insoluble materials is important. 
The caster tip should be free from baffles on which insoluble materials 
can collect.

DETAILED DESCRIPTION 
The process provided in practiced of this invention may be conducted by way 
of a continuous roll caster of a type commonly used for casting 
aluminum-base alloys. Such an apparatus is described in U.S. Pat. No. 
4,054,173 by Hickam, the subject matter of which is hereby incorporated by 
reference. In such an apparatus a pair of water-cooled, parallel casting 
rolls are positioned one above the other. These rolls are spaced apart a 
distance corresponding to the thickness of a sheet being cast. A pouring 
tip fits snugly into the converging space between the casting rolls on the 
entrance side. In an exemplary caster, each of the rolls is about one 
meter in diameter, and they have a length in the order of 1.5 meters. 
Preferably, the plane in which the roll axes lie is not vertical, but 
instead is tilted backward by about 15.degree.; that is, the plane is 
tilted so that the upper roll is about 15.degree. nearer the entrance side 
than the lower roll. The metal thus tends to move somewhat upwardly into 
the nip of the rolls. This is referred to as a tilt caster. A so-called 
horizontal caster has the rolls in a vertical plane with metal flowing 
horizontally into the nip of the rolls. Early casters for aluminum had the 
rolls in a horizontal plane with metal flowing vertically into the rolls. 
FIG. 1 illustrates schematically in transverse cross section a fragment of 
an exemplary horizontal roll caster. It will be understood that in this 
drawing the aforementioned 15.degree. tilt is not illustrated merely for 
convenience in drafting. Thus, in the drawing the upper roll 10 is 
illustrated as directly above the lower roll 11. During use, the rolls are 
rotated at a selected speed in the direction of the arrows. A pouring tip 
12 is positioned between the rolls on the entrance side of the nip between 
the rolls. The pouring tip 12 is positioned between the rolls on the 
entrance side of the nip between the rolls. The pouring tip is made of a 
ceramic insulating material such as Marinate or a fibrous material as 
described in U.S. Pat. Nos. 4,232,804 and 4,303,181. The pouring tip 
comprises, in effect, a pair of parallel, spaced-apart slabs of such 
material extending in the direction of the length of the rolls, a distance 
corresponding to the width of the sheet to be cast. For example, if it is 
desired to cast a sheet 1.2 meters wide the inside length of the pouring 
tip would be about 1.2 meters. 
The front of the pouring tip has a pair of lips 13 spaced apart to define a 
tip orifice 14 therebetween. Inside the pouring tip the walls 16 are 
parallel to each other for a distance rearwardly from the lips. On the 
outside the pouring tip has curved faces 17 with a curvature about the 
same as the curvature of the adjacent faces of the rolls 10 and 11. At a 
location rearwardly from the lips of the pouring tip where the wall 
thickness is increased to provide a desired strength, the interior walls 
of the casting tip diverge toward an interior plenum 18. Additional 
details of a casting tip suitable for use in practice of this invention 
are described hereinafter in relation to FIGS. 4 and 5. 
The front of the pouring tip is inserted into the space between the rolls 
so that the lips are a specified distance from the central plane 19 that 
includes the axes of the rolls. It is, of course, at this plane that the 
spacing distance between the rolls is at a minimum. The distances from the 
central plane to the nearest edge of the lips 13 on the pouring tip is 
referred to as the setback. 
During operation of the caster, molten aluminum alloy is fed from a headbox 
(not shown) to the rear of the pouring tip. The molten metal passes 
through the interior plenum 18 and out of the orifice 14 into the space 
between the rolls. When the metal contacts the water-cooled rolls, 
freezing occurs and solidification progresses from the roll surfaces 
toward the center of the metal. In an exemplary casting operation 
solidification is complete before the advancing metal reaches the center 
plane of the mill and some hot working of the solidified metal occurs as 
the metal advances toward the center plane of the rolls. The cast sheet 21 
is withdrawn from the exit side of the rolls. 
Molten metal exiting from the orifice of the pouring tip advances to the 
moving roll surfaces in an envelope of a thin oxide film that forms on the 
molten metal surface. Hence, the lips need not contact the roll surfaces, 
and in fact a small space exists between the lip and the roll to avoid 
wear of the tip. 
A broad variety of casting parameters have been employed in the past, but 
no combination of the conventional parameters proved satisfactory for 
casting the magnesium bearing alloy used for computer memory disks. 
Previously, attempts have been made to cast this alloy with a conventional 
cast sheet thickness of about 7.7 millimeters and a casting speed of about 
0.58 meters per minute, or a production rate of about 710 kilograms per 
meter of width per hour. The quality of the product has been quite poor, 
with excessive surface ripple, shiny spots after anodizing, and inclusions 
that cause surface roughness and dropouts in recording film. 
These casting parameters are about the same as used for a variety of other 
alloys used for a variety of purposes. Commercially pure aluminum can be 
cast at rates as high as 1.5 meters per minute, but much lower casting 
rates are required for alloys. Commercial casting of aluminum alloy sheets 
is typically in the thickness range of 7 millimeters or more. Some alloys 
have been cast as thin as 6 millimeters. 
Generally speaking pure aluminum can be cast at a high rate since it has a 
sharp melting point. Alloys must be cast at a lower rate since there is 
often a substantial difference between the liquidus and solidus, and 
controlled freezing is important. Alloys are often more difficult to cast 
because of alloy segregation between the surface and center of the sheet. 
Other problems may be caused by hot working an alloy sheet between the 
locus of solidification and the minimum clearance between the casting 
rolls. 
It is found in practice of this invention that satisfactory quality can be 
consistently obtained when casting aluminum alloys having more than about 
2% total alloying elements when the thickness of the cast sheet is in the 
range of from 4 to 5.8 millimeters and casting rates are in the range of 
from 1.3 to 1.9 meters per minutes. Higher casting speeds can be used when 
careful control is maintained. For example, a magnesium containing alloy 
has been cast as fast as 2.1 meters per minute. Increasing the thickness 
and decreasing casting speed to conventional ranges results in surface 
ripple and other objectionable defects. Increasing casting speed without 
decreasing thickness may yield incomplete solidification and severe 
defects. Decreasing thickness without increasing casting speed can lead to 
premature solidification and excessive hot working. The separating force 
between the rolls also increases as the thickness is decreased and high 
bearing loads can result. 
It is found that a combination of casting speed in the range of from, 1.3 
to 1.9 meters per minute and a sheet thickness of 4 to 5.8 millimeters is 
important for reliably and reproducibly obtaining cast sheet of aluminum 
alloy with minimal surface and internal defects. 
If the casting speed is less than 1.3 meters per minute, the desired 
microstructural refinement from rapid solidification seems to be lost for 
most alloys. If the casting speed is more than 1.9 meters per minute the 
ability to control the caster is jeopardized. Caster control is based on 
control of current to run the caster. There is minimum current required to 
run the caster without any metal being cast. When metal is being cast, the 
sheet is conveyed to a coiler which is driven to apply a tension to the 
sheet to cinch it tightly onto the coil being formed. Such tension is also 
important to prevent sticking of the sheet onto the rolls of the caster. 
When the casting speed is more than 1.9 meters per minute, the tension on 
the sheet approximately balances the retarding forces due to rolling 
solidified metal in the nip of the rolls, and the current required to run 
the caster is about the same as when no metal is being cast. This makes 
reliable control difficult. 
Excellent quality can reliably be obtained by casting sheet in a thickness 
in the range of from 4 to 5 millimeters at casting speeds of 1.5 to 1.8 
meters per minute. A production rate of 1000 kilograms per meter of width 
per hour or more is readily obtained. By going to such thinner sheet and 
higher casting speeds, productivity has been increased by as much as 50% 
and objectionable surface ripple has disappeared. These ranges are 
preferred because of the enhanced reliability and ease of control of the 
casting process. 
When the casting speed is at least 1.5 meters per minute good 
microstructural refinement is obtained and it is preferred to assure such 
refinement for alloys by casting at a speed at lest this high. It is 
preferred that the maximum casting speed be about 1.8 meters per minute 
since this allows some perturbations in casting conditions (e.g., change 
in headbox temperature) without degrading quality of the cast sheet. In 
other words, some leeway in control of casting conditions is available and 
at higher speeds the caster is not as tolerant of variations. 
The aforementioned casting speeds are appropriate for casting rolls having 
a diameter of about one meter. Roll casters for aluminum have been built 
with roll diameter from about 2/3 meter to about 1.5 meter. Suitable 
adjustments in casting speed are made for these larger and smaller 
machines. Casting speeds are generally lower for smaller diameter rolls 
and higher for larger diameter rolls to obtain equivalent results. 
Preferably cast sheet thickness is at least 4 mm. so that the sheet can be 
subjected to some cold work for finishing the disk stock with fine grain 
size. Further, if the thickness is less than four millimeters, the orifice 
of the ceramic casting tip becomes so small that starting molten metal 
flow becomes quite difficult. For reasons not fully understood, it is 
quite difficult to obtain consistently good quality in the cast sheet when 
the thickness is more than 5.8 mm. 
It is particularly preferred that the magnesium containing aluminum alloy 
sheet to be used as stock for making magnetic recording disks be cast in a 
thickness in the range of from 4 to 5 millimeters at a casting speed of 
from 1.3 to 1.8 meters per minute. 
At high casting speeds there is rapid solidification. Cooling rates may be 
in excess of 1000.degree. C. per second (as compared with about 
300.degree. C. per second in conventional continuous casting) which 
considerably refines the particle size of intermetallic compounds and 
virtually suppresses formation of such particles in the surface layers. 
Rapid solidification technology refers to processes where the cooling rate 
is in excess of 1000.degree. C. per second. New metallurgical phenomena 
occur and in the aluminum alloys non-equilibrium phases may occur. It is 
not known exactly what microstructural phenomena are occurring but it is 
known that excellent memory disks can be made from sheet cast at 
thicknesses less than and speeds greater than conventional practice. The 
high speed of the casting process also almost completely eliminates 
temperature fluctuations during solidification, thereby avoiding the 
heterogeneity associated with surface ripples and ameliorating need for 
subsequent homogenization heat treatment. 
It has often been a characteristic of a cast aluminum sheet from a 
continuous roll caster, particularly with alloys having more than about 2% 
total alloying elements that there is a repetitive heterogeneity that 
manifests itself as a series of ripples perpendicular to the casting 
direction. The severity of these ripples varies with the alloy being cast. 
In many alloys, the ripple may be sufficiently severe that it leaves a 
"zebra stripe" appearance on finished products. 
Such ripple is largely avoided in practice of this invention. Surface 
solidification of the cast sheet progresses without remelting by heat 
transferred from the solidifying center of the strip. A balance of casting 
speed and sheet gauge to achieve the desired result is important. Sheet 
thickness in the range of from 4 to 5.8 millimeters is cast with a speed 
more than about 7.3 meters per minute and preferably in the range of from 
1.3 to 1.9 meters per minute. Preferably sheet thickness is less than 5 
millimeters and casting speed is in the range of from 1.5 to 1.8 meters 
per minute. Other parameters that help achieve a ripple-free casting 
include the casting temperature, tip design, setback, and metal head. 
Although the reasons for ripple may not be fully understood, a reasonable 
hypothesis can be stated. In a paper entitled "A Steady State Model for 
Roll Casting" presented at a Conference on Materials Casting at Santa 
Barbara, Ca., in Jan. 1986, M. J. Bagshaw, J. D. Hunt, and R. M. Jordan 
postulate three different heat transfer regions as metal solidifies in a 
roll caster. Heat is removed from the sheet by the roll along the sheet 
roll contact length at a rate characterized by a heat transfer 
coefficient. This coefficient varies along the contact length as the sheet 
passes between the rolls. 
It is postulated that initially there is a region of relatively high heat 
transfer coefficient along the region of the contact length corresponding 
to the intimate contact of the molten metal with the roll. This is 
followed by a second region of lower heat transfer coefficient due to 
shrinkage and bucking of the sheet away from the roll. Finally, as more 
solid forms at the center of the sheet and the alloy gains in strength in 
the center, a greater pressure is exerted by the rolls, on the sheet, thus 
obtaining intimate contact once again and achieving a higher heat transfer 
in a third region. These authors postulate heat transfer coefficients in 
these three regions as 3.53, 0.105, and 20.0 J/cm.sup.2 s.degree. C. These 
authors conclude that "heat lines", regions of extremely bad surface 
extending along the length of the sheet may result when the casting speed 
is increased above the usually acceptable limits. 
In that paper experimental sheet exit temperatures were reported as a 
function of casting speed. The highest recorded speed for each alloy was 
very close to the speed at which heat line formation first occurs, i.e., a 
practical speed limit. The cast strip thickness was about six millimeters 
and the maximum speeds recorded were less than 0.84 meters per minute, 
except for commercially pure aluminum and alloy 8006 which contains 1.2 to 
2.0% iron, 0.3 to 1.0% manganese and up to 0.4% silicon. The maximum 
casting speed recorded for that alloy was less than 1.14 meters per 
minute. There was one test of alloy AA-1100, which is a minimum of 99% 
aluminum, at a casting speed of less than 1.44 meters per minute. 
We believe that ripple may occur as a consequence of the differing heat 
transfer coefficient between the first two postulated regions. In the 
first region the surface of the metal being cast solidifies as heat is 
rapidly extracted by the rolls. The remaining molten metal has appreciable 
latent heat of fusion. When the metal enters the second region with low 
heat transfer coefficient, heat transferred from the center of the sheet 
may remelt surface metal, particularly that portion with compositions near 
the solidus, such as in grain boundaries. The remelted metal has low 
strength, and since there has been an opportunity for oxidation of the 
metal after leaving the casting tip, some of the oxidized surface material 
may be preferentially transferred to the roll surface. Any such material 
transfer would be minute. 
Thereafter, when the caster roll has made a full revolution any residual 
transferred material adhering to the roll intervenes between the roll and 
the sheet being cast. This results in still lower heat transfer 
coefficient in both the first and second regions, and more extensive 
remelting. Minute variations in heat transfer coefficient can thereby be 
reinforced as remelting is exacerbated by oxidation products adhering to 
the roll in some areas, while other areas retain a somewhat higher heat 
transfer coefficient. The accuracy of such a model is supported by the 
observation that ordered nonuniform deposits can be seen on a used caster 
roll which produces sheet having surface ripple. Such observations led to 
the attempts to avoid ripple by wire brushing the rolls. 
It is believed that in practice of this invention a sufficiently high 
cooling rate is achieved with the thinner sheet and faster casting speed 
that remelting of the surface in the postulated second region is avoided. 
The enhanced surface strength helps avoid accumulations of contamination 
on the roll surface by retaining any oxidation products on the cast sheet. 
The ripple on the sheet surface is believed to be a consequence of the 
nonuniform cooling and freezing and the resultant strength variations of 
the sheet as it is deformed in the nip of the rolls. By significantly 
reducing sheet thickness and increasing casting speed, remelting is 
avoided, accumulation of oxidation products on the roll is minimized, 
uniform cooling is promoted, and ripple-free sheet is obtained. As a 
substantial additional benefit, the production rate of cast sheet is 
increased by as much as 50%. The high speed cooling also helps avoid 
segregation in the higher alloy content materials and yields a fine grain 
structure with small dendritic arm spacing. 
Problems with high speed casting in the past have included sticking of the 
cast sheet to the roll surface, resulting in severe damage to the cast 
sheet, often making it completely unusable. Since it has been believed 
necessary to run the casting machine slowly to avoid sticking, relatively 
thick sheets (typically from 7 to 10 millimeters) have been cast to obtain 
a production rate as high as possible. Thick sheets can be accommodated in 
conventional casting because the longer time of the sheet in contact with 
the rolls can result in sufficient cooling to produce a completely solid 
sheet at or before the nip of the rolls. 
There are several additional benefits from casting much thinner sheet than 
previously believed feasible for aluminum alloys. The cast sheet is 
typically wound into a coil as it comes out of the caster. The coiling 
machine, shears, and other sheet handling equipment must be heavier and, 
hence, more expensive for handling conventional thick sheet than the 
thinner sheet provided in practice of this invention. Later the sheet is 
unwound from the coil and rolled to the desired thickness for a finished 
product. This equipment must also be heavier and more expensive. The 
amount of final thickness reduction can also be minimized in practice of 
this invention, thereby reducing subsequent processing costs. Segregation 
of alloying elements in sheet cooled with rapid solidification technology 
may be so much reduced that a costly homogenization heat treatment is 
avoided. Most significant are suppression of ripple and increased 
production. 
The temperature of the molten metal should be sufficiently above the 
liquidus temperature to avoid premature solidification in the casting tip 
and may differ somewhat depending on heat losses in the pouring tip, 
casting rate, gauge of the cast strip, etc. Temperature is typically 
measured in the headbox or tundish that feeds molten metal to the pouring 
tip. For a 5082, 5086 or 5182 alloy, temperature in the headbox is 
preferably in the range of from 675.degree. to 725.degree.. If temperature 
is too low, small areas of solidification may occur in the casting tip, 
leading to imperfections in the cast sheet. If the molten metal 
temperature is too high, the metal may not completely solidify between the 
rolls and the cast sheet is defective. In a process as herein described 
the temperature of the molten aluminum alloy is preferably held about 
20.degree. above the liquidus temperature of the alloy, or 680.degree. to 
690.degree. C. for the 5086 alloy, for example. 
The level of liquid metal in the headbox is preferably maintained at an 
elevation in the range of from -4 to +22 millimeters from the elevation of 
the intersection of the centerline of the pouring tip with the center 
plane of the rolls. Since the preferred caster is tilted backwards about 
15.degree., this intersection is at a higher elevation than the end of the 
tip. If the head is more than 22 mm, smooth flow of metal from the tip to 
the rolls may be disrupted and surface irregularities may result. A slight 
negative head can be maintained since metal is continually withdrawn from 
the nip of the casting rolls. Preferably a head of about +1 millimeter is 
maintained above the tip. 
It has been found quite significant to maintain a very low level of 
insoluble inclusions in the alloy being cast. An inclusion level of less 
than 0.008% by weight should be maintained for casting disk stock for 
making magnetic memories. Cleanliness and filtering the molten alloy are 
keys for maintaining a low inclusion level in the cast sheet. The design 
of the pouring tip is also significant with respect to preventing 
inclusions. 
An aluminum alloy preferred for casting disk stock in practice of this 
invention is similar to alloy 5086. The magnesium content is in the range 
of from 2 to 5% by weight. At least 2% magnesium needs to be present to 
impart the necessary mechanical strength in the fully annealed disk 
substrate. Additions of magnesium in excess of 5% may cause excessive 
oxidation of the melt and preferably are avoided. Preferably the manganese 
content is in the range of from 0.07 to 0.15%, however, minor excursions 
outside these limits may be acceptable. It is preferable that the 
manganese content be at least 0.07% to increase the mechanical strength, 
modulus, and corrosion resistance of the substrate. It is preferable that 
the manganese content be less than about 0.15% by weight to minimize 
segregation due to formation of Al-Fe-Mn intermetallic compounds. 
Iron and silicon are usually present as impurities and tend to aggravate 
centerline segregation in the cast strip. The iron and silicon content 
should be held below 0.2% each to minimize segregation that may perturb 
subcutaneous magnetic characteristics of the substrate. 
Chromium in the range of from 0.05 to 0.10% by weight may be beneficial for 
grain size control during annealing. This has not proved to be a critical 
parameter and lower chromium levels can be acceptable. Amounts of chromium 
substantially above the preferred range are not recommended since chromium 
contents in excess of about 0.35% tend to promote growth of large 
intermetallic particles. 
Lithium and beryllium may be employed to retard oxidation of the molten 
alloy. These materials aid in maintenance of a continuous tough oxide skin 
on the melt. It is therefore desirable to include these elements in the 
range of up to 0.04%A by weight. It is believed that such additions of 
lithium and beryllium tend to decrease the formation of non, metallic 
inclusions. 
Small amounts of calcium may be included in the composition to control 
dendritic segregation, although in the high speed casting process, this 
use appears to be optional. The calcium, content is preferably less than 
about 0.05% by weight. Small additions of strontium may refine 
intermetallic particle size. The high cooling rate in this process, 
however, tends to render such additions virtually unnecessary. Preferably 
the strontium content is less than about 0.05% by weight. 
Hydrogen in the melt should also be kept to a minimum. Its presence can 
cause porosity in the cast sheet and it may also progressively accumulate 
within the pouring tip, eventually causing a disturbance of the metal 
flow. It is therefore preferred that hydrogen be kept below 0.2 PPM and it 
is particularly preferred that hydrogen be kept below 0.1 PPM when several 
days of continuous operation are desired. 
Addition of a grain refiner appears beneficial in suppressing segregation. 
In an exemplary embodiment the grain refiner contains both titanium and 
boron. The exact composition of the grain refining addition is not of 
particular importance. It is preferable that the grain refining addition 
activate before reaching the point where solidification occurs and remains 
active through solidification. The grain refining addition should not 
introduce particles that rapidly cluster or settle in the molten alloy. 
Preferably an aluminum-titanium-boron master alloy wire is added to the 
melt just before the caster to act as a grain refiner. The addition rate 
of grain refiner may be determined by grain size evaluations of the cast 
strip or by test castings of the melt taken immediately prior to entering 
the casting machine. The preferred addition rate is that at which further 
increases in the addition rate of grain refiner case no significant 
further decrease in grain size. 
The high speed thin gauge casting process provided in practice of this 
invention is so tolerant of alloy chemistry that acceptable quality disk 
stock has been produced with no additions of grain refiner. Moderate 
variation of the content of other alloying ingredients and some tolerance 
of impurities are also hallmarks of the high speed casting process. 
FIG. 2 illustrates in block diagram the preparation of metal for casting. 
Due to the demanding nature of disk stock, it is important that 
non-metallic inclusions be kept to a minimum. They can be deleterious in a 
number of ways, not the least of which is that such particles can be 
carried through in the molten metal, resulting in a defect in the 
substrate surface. Non-metallics can disrupt the casting process by acting 
as nucleation sites for premature solidification, thus disturbing 
microstructure of the disk stock. Thus, a careful metal preparation is 
important. Ingots of metal are melted in a melter 25 and the molten metal 
is passed through a series of cleaning steps before reaching the caster 27 
which produces the final cast sheet. The individual components, with 
exception of a coalescer, are conventional in that they are commercially 
available, but so far as is known they have not been employed as described 
herein. 
The melting furnace is kept thoroughly clean and is regularly drained and 
cleaned to avoid accumulations of insoluble material that might be carried 
through the system with the molten metal to appear in the cast sheet. It 
is preferable to form the desired alloys by melting 99.98% pure aluminum 
ingots plus suitable master alloys to minimize contamination. Recycled 
scrap is preferably avoided. The melt is continuously covered by a 
suitable flux such as a conventional mixture of chloride and fluoride 
salts. The melt in the furnace is skimmed to remove insolubles and 
chlorine or an argon-chlorine mixture may be bubbled through the melt to 
help remove metallic and non-metallic impurities and reduce dissolved 
hydrogen. Further, beryllium or lithium master alloy may be added to the 
melt to assist in deoxidation. 
Since the caster may operate continuously for several days, additional 
alloy is melted in the melting furnace. The molten metal is transferred to 
a holding furnace 26 when the melt chemistry has been verified, so as to 
maintain a steady supply of molten metal for the caster. Fluxing is 
continued to the holding furnace. 
Enroute to the caster the molten metal is passed through a ceramic foam 
filter 28 having about 30 pores per inch for minimizing oxide particles in 
the melt. The filtered metal then goes to a spinning nozzle inert 
flotation filter 39, commonly referred to as a SNIF unit. A nozzle in the 
SNIF unit is rotated at about 350 RPM to sparge a mixture of argon and 
chlorine into the metal. About 2.5 Nm.sup.3 /hr of high purity argon with 
about 0.015 Nm.sup.3 /hr of chlorine is injected into the molten metal. 
Very fine bubbles of gas ascending through the molten metal tend to sweep 
solid particles to the surface and remove dissolved hydrogen or other 
gases. The chlorine combines with some impurities and the resultant 
chlorides tend to float out as well. 
The degassed metal from the SNIF unit 29 is then passed through a coalescer 
31. The purpose of the coalescer is to coalesce extremely fine droplets of 
molten chlorides in the metal to form larger droplets which float from the 
melt. The chlorine sparged into the molten metal in the SNIF unit reacts 
with metals in melt to produce primarily magnesium chloride, but also 
chlorides of sodium, potassium lithium, and calcium which are impurities 
to be removed. These liquid chlorides pass through ceramic filters with 
great ease and tend to carry oxide particles through such filters as well. 
Thus, the filters are ineffective and oxide particles may appear as 
inclusions in the cast sheet. Removal of such chlorides prior to 
filtration is therefore desirable, if not essential. 
The chloride droplets downstream from the SNIF unit are too small to float 
out in a reasonable time. Techniques have therefore been proposed for 
coalescing these particles, but without successful commercial 
implementation. For example, one such coalescer described in U.S. Pat. No. 
4,390,364, employs a very large "box" having inclined plates from 12 to 50 
millimeters apart between which the molten metal flows. Although 
coalescence can be achieved in such a unit, its very large size has made 
it unacceptable and such units are not in industrial use. 
The preferred coalescer 31 employed in practice of this invention comprises 
an extended "honeycomb" of rigid ceramic such as alumina, mullite, or 
other inert ceramic, 50 by 100 millimeters wide in the direction 
transverse to liquid metal flow and having a thickness of 12 to 15 
millimeters in the direction of metal flow. The honeycomb chosen is 
extruded with square "honeycomb" cells 22 extending in the direction of 
thickness of the coalescer as illustrated in the fragmentary view of a 
corner of such a coalescer in FIG. 3. Each cell opening is two millimeters 
by two millimeters with a thin wall 23 between adjacent openings. Thus, 
the aluminum alloy flows through a plurality of parallel passages two 
millimeters square and twelve to fifteen millimeters long. In an exemplary 
embodiment about 1000 kilograms of alloy per hour is passed through such a 
coalescer, 50 millimeters by 100 millimeters and having almost 1000 such 
passages. It is found that such a coalescer is extremely effective in 
causing coalescence of the chloride droplets which float out so that a 
filter downstream from the coalescer effectively removes oxide particles. 
The extruded ceramic honeycomb employed in the coalescer was originally 
developed to serve as a substrate in automotive exhaust catalytic 
converters. It is also used for filtering cast iron as it is poured into a 
mold. Such material is commercially available from a variety of vendors, 
including Ringold Ceramics, Corning Glass, Foseco and others, and in a 
variety of ceramic materials. It is available in a variety of cell 
geometries, including hexagons, squares, and triangles, and in a variety 
of cell sizes and lengths. 
It is preferred to employ a coalescer having a cell opening in the range of 
from 0.5 to 5 millimeters and a length in the range of from 5 to 50 
millimeters for coalescing chloride droplets. Preferably the length of the 
passages through the honeycomb are in the range of from four to ten times 
the width of the passage to assure that the liquid droplets have 
sufficient residence time in the coalescer to approach a wall of the 
coalescer and contact other droplets. Thus, the dimensions of the 
coalescer are in part determined by the flow rate of metal. It has been 
calculated that flow through the narrow passages is laminar with a 
Reynolds number of about 200. The extremely low flow rate and Reynolds 
number through the coalescer explain the great effectiveness of the 
preferred embodiment with two millimeter wide passages only twelve 
millimeters long. the molten aluminum does not readily wet the ceramic and 
must be urged through the passages to get flow through the coalescer 
started. Typically the coalescer can be started by heating it to somewhat 
higher than the casting temperature of the aluminum, applying molten 
aluminum to one face of the honeycomb and vibrating the honeycomb to 
initiate flow through it. 
If the honeycomb passages are significantly smaller than 0.5 millimeters, 
difficulty in starting flow of molten aluminum through the coalescer may 
be encountered. If the passages are significantly larger than five 
millimeters, adequate coalescence to remove sufficient chlorides for good 
filtration may not be obtained. If the passages are shorter than about 
five millimeters, residence time of alloy in the coalescer may be too 
short to provide adequate coalescence. If the passages through the 
honeycomb are significantly longer than 50 millimeters, starting flow 
through the coalescer is more difficult. Long lengths have not proved 
necessary since excellent coalescence is obtained with a flow through only 
12 to 15 millimeters of coalescer. The dimensions of the coalescer are 
chosen to be large enough to avoid plugging by occasional large particles 
of oxide that may be present in the melt and to minimize head loss in the 
coalescer. The furnace and caster are ordinarily arranged with a fall or 
decrease in height of only about 1% in the trough between the furnace and 
the headbox. Substantial obstruction by the coalescer is therefore to be 
avoided. The short narrow passages in the preferred embodiment have 
negligible head loss. 
The coalescer is preferably tilted so that the molten metal flows 
downwardly through it at an angle of up to 45.degree. from the horizontal. 
This helps assure that coalesced droplets float out on the upstream face 
of the coalescer and is believed to improve chloride removal. Good 
coalescence and removal have been obtained with the coalescer passages 
horizontal, or even tilted upwardly so that droplets float out on the 
downstream side of the coalescer. The coalescer is positioned in a flow 
trough downstream from the SNIF or other unit for sparging chlorine 
containing gas in the melt and is completely immersed in the liquid metal. 
A baffle above the coalescer assures that metal passes through the 
coalescer only from the portion of the trough below the floating oxide 
film. 
After flotation of coalesced chloride droplets, any remaining oxides are 
removed by passing the molten metal through a rigid, porous medium filter 
32. An exemplary filter is made of sintered silicon carbide grit which is 
highly effective for removing fine particles form molten aluminum. A 
typical filter has a first layer of sintered six mesh grit and a second 
layer of eight mesh grit. The coarser grit side of the filter is placed 
upstream so that coarser particles are removed first, followed by removal 
of finer particles in the pores of the smaller grit size layer of the 
filter. 
Downstream from the final ceramic filter a grain refining wire is 
introduced by a wire injector 33. An exemplary grain refiner comprises 
aluminum wire containing about 5% by weight titanium and 0.2% by weight 
boron. The boron content can be as much as 1% by weight. Sufficient grain 
refining alloy is added to bring the titanium content up to about 0.02% by 
weight. It is found desirable to inject the grain refining wire downstream 
from the sintered silicon carbide grit filter to avoid removal of titanium 
boride by the very effective filter. A pair of woven ceramic fiber filter 
trough "socks" 34 in series are used just prior to the molten metal 
entering the casting machine for removing any oxide particles entrained 
into the melt by the grain refining wire. 
The molten aluminum alloy then passes into the pouring tip of the casting 
machine. It is found that careful attention should be given to the pouring 
tip for practice of this invention. The high speed casting process is 
particularly sensitive to any disruption of flow within the pouring tip. 
Any accumulation of non-metallic inclusions can disturb the planar, 
non-turbulent flow exiting the tip orifice. Smooth flow is important for 
producing a homogeneous cast strip. Thus, it is important to minimize 
inclusions in the molten alloy and to avoid accumulation of inclusions in 
the pouring tip. 
FIGS. 4 and 5 illustrate an exemplary casting tip made to be used in 
practice of this invention. FIG. 4 is a longitudinal cross section of the 
casting tip taken along line 4--4 in FIG. 5 in the direction of the width 
of the cast sheet. Only a little more than half of the tip is illustrated 
in FIG. 4. The other half being the same as the part illustrated. FIG. 5 
is a transverse cross section perpendicular to the center plane of the 
casting machine. 
As mentioned above, such a pouring tip can be made of Marinite or a rigid 
ceramic fiber as described in the aforementioned patents, or other 
insulating material with good dimensional stability and durability. Molten 
metal enters a distribution plenum 36 at the rear of the pouring tip 
through a central opening 37 connected to the headbox. A baffle 38 extends 
across the plenum and has a plurality of holes 39 through which metal 
passes enroute to the tip. The holes are smaller near the center of the 
pouring tip and become increasingly larger toward the edges to assure 
metal distribution across the full width of the pouring tip. The pouring 
tip 41 is sealed to the plenum chamber by a thin gasket 42. 
The feed tip 41 is assembled from two long, more or less flat, slabs 43 of 
Marinite or rigidized ceramic fiber. These slabs are secured together by 
bolts 44 passing through upstream spacers 46 between the two slabs. The 
upstream spacers are in a row parallel to the plenum. A second row of 
downstream spacers 47 between the slabs hold an upstream portion of the 
slabs parallel to each other. 
Each of the spacers has a teardrop shape with the tail pointing downstream. 
This shape is employed to minimize turbulence in the metal flowing through 
the tip. It has been found that solid inclusions tend to accumulate in the 
wake of spacer or other baffle in the tip and when a sufficient quantity 
of such insolubles accumulate, they may break away and appear in the cast 
sheet. Such accumulations of solids in the tip may, in an aggravated 
situation, result in partial plugging of the tip and require shutdown of 
the caster due to defective sheet. It is also significant that the number 
of spacers in the tip is minimized so as to have only enough spacers to 
maintain the structural integrity of the tip. This reduces the local 
velocity of the molten metal, thereby reducing turbulence. 
The spacers 46 and 47 are in an upstream portion of the tip where the 
inside walls are parallel to each other. Downstream from this portion 
there is a tapered portion 48 where the walls converge. Still further 
downstream, the inside faces of the walls are again parallel to each other 
in the region immediately upstream from the orifice 49 through which the 
metal flows into the space between the rolls. 
The exterior faces of the slabs 43 are parallel to each other in the 
upstream portion. Toward the downstream portion there is an arcuate face 
51 on each slab to provide clearance from the rolls when the tip is 
inserted into the gap between them. The exterior arcuate face 51 converges 
toward the inside face of each slab to leave a thin lip 52 on each slab 
along the orifice 49. Preferably the inside of the lips adjacent the 
orifice have a small bevel (not shown) to minimize abrupt changes in the 
direction of metal flow and minimize defects due to tip erosion. Molten 
metal coming out of the orifice between the lips is contained by the 
adjacent rolls. At each end of the pouring tip there is a short wing 53 
which prevents metal from flowing longitudinally along the rolls until 
frozen into the cast sheet. 
In an exemplary embodiment the throat of the pouring tip, that is, the 
distance between the wings at each end, is about 1.2 meters. An exemplary 
distance from the gasket 42 to the lips 52 is about 35 centimeters. The 
width of the upstream portion of the interior of the tip where the spacers 
46 and 47 are located may be about 18 millimeters. Such dimensions are in 
the range of conventional practice. 
A portion of the tip that is not conventional is adjacent the orifice 49 
through which the molten metal is cast toward the rolls. It is preferred 
that the width of the orifice be in the range of from 50 to 130 percent of 
the thickness of the sheet being cast. Thus, in an exemplary embodiment 
the width of the orifice is five millimeters for casting sheet having 
thickness in the range of from 4 to 5 millimeters. Preferably the width of 
the tip orifice is in the range of 100 to 110% of the thickness of the 
sheet being cast. 
It is significant that the thickness of the lip 52 on each slab is less 
than two millimeters, as contrasted with a thickness of about 4 
millimeters in conventional practice. A thin lip is important even though 
structurally fragile so that a minimal setback between the orifice and the 
center plane of the rolls can be used. Preferably the tip is set back from 
the center plane of the rolls in the range of from 35 to 60 millimeters, 
and preferably in the range of from 45 to 50 millimeters. The spacing 
between the exterior of the tip and the rolls should be as small as 
feasible, preferably less than one millimeter and most preferably as 
little as 0.1 millimeter. Conventional setback in continuous less than one 
millimeter and most preferably as little as 0.1 millimeter. Conventional 
setback in continuous casters has been in excess of 60 millimeters and is 
ordinarily greatly in excess of 60 millimeters. 
Tip setback is an important parameter. Increasing the setback increases the 
area of contact by the roll with the solidifying metal. It also increases 
the volume of metal being solidified at any instant. Within limits, 
increasing the setback increases the maximum speed at which "hard" sheet 
is cast, since there is more mechanical working of the sheet after 
complete solidification. For thin sheet cast in practice of this 
invention, however, a large setback is undesirable since it extends the 
depth of the solidification front. 
At large setbacks and high speeds the center of the strip may still be 
solidifying at the exist of the rolls. This casting condition, in 
combination with the high metallostatic forces developed in the roll bite, 
can result in inverse segregation near the surface. It may also increase 
the tendency of the strip to stick to the casting rolls, leading to severe 
defects. Reducing the casting speed is no answer since production rate is 
decreased and roll separating force increased. Setback is a compromise 
between speed and segregation. Preferably parameters are adjusted so that 
the extrusion value of the sheet being cast is about 110%, that is the 
sheet exiting from between the rolls is travelling about 10% faster than 
the roll surface speed, which is a consequence of hot working the metal 
after solidification. 
EXAMPLE 
A molten metal aluminum alloy having the following composition was cast 
into sheet suitable for high quality disk stock: 
______________________________________ 
SiFeCu MnMgCr ZnBeTi Al 
______________________________________ 
.10% .25% .009% 
.13% 4.01% .004% 
.01% .003% .02% 
Bal. 
______________________________________ 
The molten metal was passed through a ceramic foam filter having thirty 
pores per inch. It was then further purified in a spinning nozzle 
degassing unit operating with about 2.5 Nm.sup.3 /hr argon and 0.015 
Nm.sup.3 /hr chlorine with the nozzle rotating at about 350 RPM. The 
molten metal passed through a honeycomb coalescer and rigid media 6/8 grit 
ceramic filter as hereinabove described. Sufficient aluminum alloy wire 
having 5% titanium and 0.2% boron was added as a grain refiner to bring 
the titanium content up to 0.02%. A woven ceramic fiber trough sock was 
used to filter the metal just prior to the headbox. 
Typical headbox temperature was 685.degree. to 687.degree. C. and a head of 
metal was maintained five millimeters about the center line of the tip 
orifice. The tip orifice was 4.3 millimeters high and had a width of 1206 
millimeters. The lip thickness was 1.5 millimeters and the lip to roll 
distance was 0.5 millimeters. A tip setback of 50 millimeters was used. 
Roll diameter was about one meter. The sheet was cast to a thickness of 
4.8 millimeters and a width of 1220 millimeters. The resultant sheet was 
smooth and free of ripple with a surface substantially free of inclusions 
and areas of segregation or premature solidification of the alloy in the 
casting tip. 
The sheet was rolled to form disk stock without a homogenization heat 
treatment. The sheet was rolled in two passes to 3.7 millimeters and 2.7 
millimeters, respectively. It was then edge trimmed and annealed at 
380.degree. C. for two hours. It was again cold rolled in two passes to 
2.12 millimeters and 1.45 millimeters respectively. The edge was again 
trimmed and the sheet was annealed at 340.degree. C. for two hours. After 
tension levelling the sheet, circular disk substrates were blanked from 
the sheet. These disks were thermally flattened and upon inspection found 
to be satisfactory for forming computer memory disks. 
If desired a thermal homogenization treatment may be used on the as cast 
sheet to eliminate any minor areas of segregation caused by imperfections 
in the casting conditions. A reason for doing this is to allow the machine 
operator a somewhat larger margin of variation in casting parameters in a 
production operation. An exemplary homogenization maintains the 
temperature of the as cast sheet in the range of 485.degree. to 
500.degree. C. for about sixteen hours. 
A technique is provided in practice of this invention for production of 
high quality aluminum alloy sheet by continuous casting. This sheet is 
cast in thinner gauges than previously considered feasible and with a 
substantially higher casting speed than previously employed for alloys. In 
addition to providing sheet with a surface substantially free of ripples, 
it is found that a production rate increase of almost 50% is obtained. 
Thus, instead of being a particularly intransigent material to cast 
continuously, the magnesium bearing alloys can be cast with high quality 
and substantially higher productivity than ever before obtained. 
Although one example of a technique for the casting of aluminum alloys has 
been described in detail herein, it will be apparent that principles of 
this invention are applicable to other alloys. Variations in the casting 
parameters to obtain desired results can also be practiced. For example, 
when sufficient cold work can be applied to the cast sheet to make a 
finished product and the cast sheet is narrow enough to facilitate 
starting the casting process, the cast sheet thickness can be as little as 
3 millimeters, and casting speed concomitantly higher. Casting speeds may 
also be higher when microsegregation is less of a problem than in disk 
stock. It is therefore to be understood that within the scope of the 
appended claims, the invention may be practiced otherwise than as 
specifically described.