Note: Descriptions are shown in the official language in which they were submitted.
1~00391
BACKGROUND OF THE INVENTION
6201 Aluminum Alloy is a high stren~th aluminum mag-
nesium silicon alloy which in wire~form and in the heat treated
condition has a tensile strength of over 46,000 PSI, elongation
greater than 3 p~rcent, and an electrical collductivity ~reater
than 52.5 percent IACS. In the past, 6201 Aluminum Alloy redraw
rod and similar aluminum alloy redraw rods have been manufactured
for commercial use by a plurali~y of separate steps which include
DC casting an aluminum ingot reheating the ingot to about 700 to
850F, hot rolling the cast ing~t to redraw rod and solutionizing
the rod at a temperature of approximately 1000F and water
quenching the rod. ~he rod is cold drawn to form wire, and the
wire is artiicially aged at temperatures between 250 and
450F. This procedure is cap~ble of producing wire having
tensile strength and electrical conductivity characteristics
which are similar to or in excess of those for 6201 Aluminum.
While the foregoing proceduîe produces an acceptable
pro~uct, such a batch process or non-continuous casting process,
is capable of prodllcing only a limited amount of rod; that is,
the given size billet will produce only a correspondin~ mass of
rod, and the leng~hs o~ separa~ely produced rod must be welded
together to form longer lengths of rod. I~l~en the billet is
reheated and rolled to form rod, it is customary to crop the
leading end of the rod since it is of an inferior (luality. ~hus
a substantial amount of waste is experienced in the former
procedure. An elonga~ed rod which comprises several lengths of
ba~ch produced prcducts welded together will include poor grain
-- 2 --
391
structure at the places where it is welded together, which
affects tensile strength and conductivity. Furthermore, it is
virtually impossible to create identical conditions in the
reheating and rolling of different billets, and the lengths of
rod welded together will usually have different grain
characteristics.
Not only did the prior art batch process require
extremely careful handling of the rod in the solutionizing oven
in order to achieve the uniform heating necessary to produce a
uniform product, which process was time-consuming and required
extra equipment, but it additionally provided a substantial
amount of time in which the aluminum could oxidize, as when the
cast ingot was cooled or was being reheated, when the rod from
the rolling mill cooled or was being reheated for solutionizing,
and when the solutionized rod from the reheating oven cooled.
The result was that the rod became substanitially oxidized, which
made it relatively hard for redrawing purposes, and which caused
the rod to have a relatively dull finish. Further, a highly
oxidized and hard rod is more difficult to draw and the dies
used for drawing deteriorate rapidly.
An improved method for continuously casting and rol-
ling 6201 Aluminum Alloy was described in U.S. Patent No.
3,613,767. The invention of U.S. Patent No. 3,613,767 comprised
a method of continuously manufacturing aluminum base alloy rod,
such as 6201 Aluminum Alloy rod, without the necessity of re-
heating the ingot or the rod during the process. The bar emerging
from a continuous casting machine was passed through a rolling
mill, a quench tube, and then cooled in a continuous process.
The heat of the cast bar emerging from the continuous casting
machine was not dissipated and the bar temperature was maintained
3~ .
in the solutionizing temperature range o~ the metal as a rod
was passed to the rolling mill. The rod was hot worked in th~
rolling mill and quenched immediately as it emerged from the
rolling mill so that the time lapse from the point where the bar
entered the rolling mill to where the rod was quenched to a
temperature level below the crystallization temperature o~ th~
alloy metals was less ~han th~ time required for the alloy
metals to pr~cipitate to the graln boundaries o~ the metal.
After the rod was quenched it was at a temperature below the
temperature where immediate and substantial precipitation occurs.
When the rod was subsequently cold drawn into wire it had an
unusually high tensile strength and a relatively high electrical
conductivity and an unusually bright appearance. Thus, the major
problems of separate handling between each o~ the steps in the
prior art process were eliminated by the practice of the invention
disclosed in U.S. Patent No. 3,613~767.
While the process of continuously casting and rolling
6201 Aluminum Alloy descrihed in U.S. Pater,t No. 3,613,767 was a
great improvement over the prior art non-continuous batch process
it nevertheless created entirely new problems which were not
associated with the batch processing system~ Essentially,
inasmuch as the process of U.S. Patent No. 3,613,767 required
that the temperature of the bar entering the rolling mill be
maintained in the solutionizing temperature range of the metal,
it was necessary that the casting rate proceed at a speed
suficiently high such that the cast bar emerging from the cast-
ing machine could travel the short distance into the rolling mill
without a substantial decrease in temperature therebetween. ~ow-
ever, when casting at this rate, a condition known as"solidifica-
tion shrinkage'l would occur in the casting mold. This condition
results in voids or cracks orming on the outer surface of
0391.
the cast bar as it shrinks away from the mold walls during
solidification. In addition, solidification shrinkage voids can
occur within the interior of the cast bar. During subsequent
rolling of a cast bar containing such solidification shrinkage
voids, oxide inclusions will become trapped within the voids
created on the exterior portions of the cast bar causing the
rolled rod to become brittle and significantly decreasing the
drawability of the rod. In addition, the internal voids will
cause internal microcracking which significantly affects the
elongation of the rod thereby directly affecting the post cold
wor~ing characteristics of the rod.
Furthermore, inasmuch as the process of U.S. Patent
No. 3,613,767 requires the cast bar to exit from the casting
wheel at extremely elevated temperatures, whatever precipitation
of the alloying elements occuring at this point will result in
large precipitates on the order of 20,000 angstrom units. The
presence of such large precipitates has been found to be ex-
tremely detrimental to the physical properties of the finished
product.
It has also been found,in accordance with the present
invention, that the solutionizing temperature of 6201 alloy
varies according to the concentration of alloying elements
present within the alloy in that the higher the concentration
of alloying elements present the lower the solutionizing tempera-
ture range of the alloy~ Therefore given the range of concen-
trations acceptable wi~hin 6201 allOy the solutionizing tempera-
ture may vary from about 850F to about 1140Y. On the other
hand, the process according to U.S. Patent No. 3,613,767 assumed
that solutionizing temperatures up to l,000F would be satis~
factory for all purposes, and, in fact, as a practical matter
39~
could not maintain the temperature of the bar above approxi-
mately 940F as it entered the rolling mill. Accordingly,
U.S. Patent No. 3,613,767 does not provide an acceptable method
for continuously producing 6201 alloy rod having alloying
element concentrations in the range which cause the alloy to
solutionize at temperatures in the-upper portion of the 6201
alloy solutionzing temperature range.
For the purpose of clarity, heat treatable aluminum
alloys as used in this specification shall mean those aluminum
alloys which contain alloying elements which have a high solid
solubility in aluminum at high temperatures and low solid
solubility in aluminum when cooled to room temperature. These
alloys harden by precipitation of a second phase during heat
treatment and the alloying elements are kept in solution by
rapid quenching from high temperatures5
For the purpose of clarity, wrought aluminum alloys
are used in this specification shall mean those aluminum alloys
which contain alloying elements which have low solid solubility
in aluminum at high temperatures as well as at low temperatures.
These alloys normally harden by work hardening which is a harden-
ing mechanism which operates during cold working of the alloy.
STATEMENT OF T~E INVE~TION
In view of the foregoing, it should be apparent that
there still remains significant improvements to be made in the
continuous casting and rolling of heat treatable aluminum alloy
rod from aluminum alloys such as 6201 Aluminum Alloy. Specifi-
cally, there is a need for a method of continuously processing
6201 Aluminum Alloy rod whichavoi~ls the problems of solidifica-
tion shrinkage and formation of massive precipitate particles
0391
associated with the process of U.S. Patent No. 3,613,767, and
which will additionally permit rolling of the rod at a tempera-
ture within the upper portion of the solutionizing range of
6201 Aluminum Alloy.
Accordingly, there is provided in accordance with
this invention a method of continuously casting an aluminum ,
base alloy rod containing from about 0.5 to about O.9 weight
percent silicon, about 0.6 to about O.9 weight percent magnes-
ium and the remainder essentially aluminum comprising the steps
of:
(a) pouring a molten aluminum base allOy contain-
ing from about 0.5 to about O.9 weight percent silicon, about
0.6 to about O.9 weight magnesium and the remainder essentially
aluminum into the casting groove of a continuous casting wheel
at a temperature above the melting point of the aluminum base
alloy,
(b) casting the molten metal by cooling the
aluminum base alloy in the casting wheel to form a cast bar and
removing the bar from the casting groove;
(c) passing the bar into a rolling mill and con-
tinuously hot forming the cast aluminum base metal to form a rod
at a temperature above the temperature at which the alloying
metals precipitate to the grain boundaries of the aluminum base
metal; and
(d) continuously quenching the rod to a tempera-
ture level below the temperature level at which immediate sub-
stantial precipitation of alloying metals occur, and completing
the cooling of the cast metal from the beginning of the hot
forming step to the end of the quenching step within a time
interval before which any substantial precipitation of the
alloying metals occur;
110~391
characterized in that the steps of casting the
molten metal and removing the bar from the casting groove are
performed at a rate and at a temperature such that solidifi-
cation shrinkage will not occur, and thereafter raising the
temperature of the bar as it travels between the casting wheel
and rolling mill to a temperature above the temperature at
which the alloying metals would precipitate to the grain boun-
daries of the aluminum base metal.
More particularly, contrary to the teaching of the
aforementioned U.S. Patent No. 3,613,767, the cast bar is re-
moved from the casting groove at a temperature below 940F,
preferably 800F to 940F, and then raising the temperature of
the bar, preferably betwèen 850F to 1140F, by passing the bar
through a heater and therein transferring heat to the bar.
It should be apparent that by the method of this in-
vention the bar can be removed from the casting wheel at a
temperature below which any solidification shrinkage would
occur, and thereafter raise~ to a temperature within the upper
portion of the solutionizing range for 6201 Aluminum Alloy,
if desired, prior to entry into the rolling mill. Accordingly,
a substantially superior product can be obtained as compared
with the product which results from the process of the afore-
mentioned prior art patent.
BRIEF DESCRIPTION OF THE DR~WINGS
Fig. 1 is a schematic side elevational view of a
casting machine,rolling mill, quenching tube, and coiler utilized
in the procedure herein set forth.
Fig. 2 is a ternary diagram which graphically represents
the solubility of magnesium, silicon and the intermetallic com-
~10~391
pound magnesium silicide in aluminum at various temperatures.
Fig. 3 is a graphical representation of the effectof heat treating 6201 Aluminum Alloy by the present invention
compared to prior art methods of preparing 6201 Aluminum Alloy.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now in particular to the drawings, in which
like numerals indicate like parts throughout the several views,
Fig. 1 shows a casting machine 10, a heater 11, a rolling mill
12, a quench tube assembly 13, and coiler 14. In summary, the
process of the present invention comprises pouring molten metal
from a furnace (not shown) into a casting wheel 10a of casting
machine 10. The molten metal is cooled and solidified in cast-
ing wheel 10a and extracted as a solid bar 15 at a temperature
below 940F and guided toward and through heater 11 wherein the
solid bar 15 is continuously heated until the temperature of the
bar is within the range of from about 850F to about 1140F.
The heated bar 15 is then guided toward and through rolling mill
12. The product is lengthened and reduced in its cross-sectional
area within rolling mill 12, and emerges as a wrought rod 17.
Rod 17 is passed through quench tube assembly 13 which includes
first stage quench tube 18, pinch rollers 19, second stage quench
tube 20, pinch rollers 21, and rod conduit 22. The rod emerges
from rod conduit 22 and is formed into coils by coiler 14. Pump
23 receives the quenching liquid from sump 24 and pressurizes
first stage quench tube 18. The quenching liquid is passed
through quench tube 18 in a direction of flow which is along the
path of travel of rod 17 and is passed through a conduit system
to cooling tower 26, where it is cooled and recirculated back
39~
to sump 24. Pump 27 receives quenching liquid from sump 28 and
pressurizes second stage quench tube 20. The quenching liquid
of the second stage quench tube is passed through quench tube
20 in a counterflow relationship with respect to the movement
of-the rod 17, and is passed through a conduit system to cool-
ing tower 31 where it is cooled and recirculated back to sump
28. Thus, the quenching liquids are maintained at controlled
temperatures during the quenching process.
In more detail, the molten metal processed through the
apparatus is a heat treatable aluminum alloy. If the product to
be formed is to be 6201 Aluminum Alloy, the range~ of silicon and
magnesium contents are from about 0.50 to about 0.90 percent,
and from about 0.60 to about 0.90 percent, respectively. The
range of silicon and magnesium alloys can vary in this metal
beyond the range for 6201 alloy to 0.2 to about 1.3 percent and
to 0.3 to about 1.4 percent, respectively, if desired. The metal
in its molten state is poured through a fiberglass screen into
a holding pot maintained at a temperature above 1200F, ususally
at about 1270F. From a holding pot, the metal is poured into
casting wheel lOa where it is cooled and solidified into a cast
bar 15 at a rate at which solidification shrinkage will not occur.
The cast bar is stripped from casting wheel lOa at a temperature
of from about 800F to about 940F and passed to and through
heater 11 wherein the temperature of the cast bar is increased to
a point at which the alloying elements are solutionized. Heater
11 continuously supplies energy to the bar thereby increasing
the temperature of bar 15 to from about 850F to about 1140F,
usually to from about 950F to about 1020F and depending upon
the alloy composition to from about 1020F to about 1140F. As
the cast bar exits heater 11 it is guided toward and through
-- 10 --
110~391
rolling mill 12, the bar is hot formed and coated with a soluble
oil concentration maintained at about 40 percent and at a
temperature below 200F, usually at about 160F. Rolling mill
12 includes a plurality of roll stands which compress the cast
bar alternatively from top to bottom and side to side, which
functions to lengthen the cast bar. and reduce the cross-sectional
area of the cast bar, so that the cast bar is progressively
formed into redraw rod 17. The volume of the soluble oil con-
centration in rolling mill 12 is maintained at a level of about
two-thirds the volume in a typical continuous casting system for
EC rod. The temperature and volume of the coolant applied to
the rod in the rolling mill are adjustable so that whenthe rod
17 emerges from rolling mill 12, the temperature of the rod is
at a level so that the rod is still within its hot forming tem-
perature range, which is usually above 650F, so that the alloy
metals have not precipitated from the aluminum. The low volume
of coolant applied to the rod in the rolling mill requires a
higher concentration of lubricant, approximately 40 percent
solution as compared to approximately 10 percent for an EC rod
system; and the flow is adjusted so that approximately equal flow
of coolant is maintained at each roll stand.
Fig. 2 is a Ternary diagram which graphically represents
the solubility of magnesium, silicon and magnesium silicide in
aluminum at various temperatures ranging from 440C or 825F to
535C or 995F.
Straight line 40 represents the increase in solubility
of magnesium silicon and magnesium silicide in the 6~01 alloy
system as temperature increases to approximately 995F. Point
42 on straight line 40 represents the amount of magnesium,
silicon and magnesium silicide which is in solution in a continu-
--11--
11(~0391
ously cast rod of 6201 Aluminum Alloy when the rod has beentreated by the prior art method of heat treating continuously
cast 6201 alloy. Point 43 represents the amount of magnesium,
magnesium silicide and silicon which is retained in the 6201
alloy system in solution when a continuously cast rod of 6201
Aluminum Alloy is heat treated by the present invention. As can
be seen from the diagram of Fig. 2, there is a 162 percent
increase in the amount of magnesium silicide in solution in
the 6201 alloy system when the alloy is continuously cast and
rolled into a rod and is heat treated according to the present
invention during the continuous casting and rolling operation.
As example of the improved properties result from the
increased amount of magnesium silicide in solution in the alloy
matrix prior to aging and precipitation follows. A cast bar
was continuously cast using the prior art method for heat
treating continuously cast 6201 Aluminum Alloy and the following
results were obtained. Ultimate tensile strength of the final
wire was 45,700 PSI with an elongation of 8.3 percent and a
conductlvity of 52.5 percent IACS. After establishing the above
properties as a base line, the bar temperature at a point between
the casting machine and the entry of the bar into the rolling
mill was raised from 900F to 1020F by the method of the present
invention. The bar was then rolled into a rod and made into
wire and the physical properties of the wire made from the bar
treated according to the present invention were as follows:
Ultimate tensile strength - 50,800 PSI
Elongation - 7.9~
Conductivity - 52.5%
Fig. 3 is a graphical representation of properties
resulting from prior art heat treatment of continuously cast
- 12 -
391
6201 Aluminum Alloy rod and 6201 alloy rod continuously cast
and heat treated according to the present invention wherein
curve 50 depicts the relationship between the conductivity and
ultimate tensile strength of wire fabricated from 6201 Aluminum
Alloy rod processed by prior art techniques and curve 52 depicts
the relationship between the conduetivity and ultimate tensile
strength of wire fabricated from 2601 aluminum alloy rod pro-
cessed by the method of the present invention.
While this invention has been described in detail with
particular reference to preferred embodiments thereof, it will
be understood that variations and modifications can be effective
within the spirit and scope of the invention as described herein-
before and as defined in the appended claims.
