Note: Descriptions are shown in the official language in which they were submitted.
1136~79
This invention relates to a metal alloy automotive
wheel and particularly to a metal alloy automotive wheel of
complex configuration produced as an integral product by a
press forging process.
Automotive wheels and particularly those referred
to as "styled" wheels, are relatively complex shapes and
accordingly must normally be produced by permanent mold cast-
ing or die casting techniques. Such wheels are therefore
limited by the relatively lower properties available from
the casting alloys used in such forming processes. The
wheels may be forged from wrought alloys, but with serious
limitations on their geometry or configuration. The wheels
may also be fabricated from multiple forged pieces which are
welded together, but only certain alloys can be welded and
the fabrication technique also imposes serious styling
limitations. It would, therefore, ~e desirable to produce
wheels of complex shape having the properties of wrought
alloys.
There has recently been developed certain alloys
having a microstructure such that they may be cast from a
liquid-solid mixture rather than a liquid and thus solidi-
fied from a lower temperature than conventional casting alloys.
Such alloys and their preparation are disclosed, for example,
in U.S. patent 3,948,650 which issued on April 6, 1976 and
U.S. patent 3,954,455 which issued on May 4, 1976. ~s there
disclo~éd, the partially solidified metal alloys, in the
form of slurries, can be shaped into alloy parts by a variety
of metal forming processes, including die casting, permanent
mold casting, closed die forging, hot pressing and other
known techniques. ~ ~-
11366~'9
It is a primary object of this invention to provide
an automotive wheel having the complex shape normally
characteristic of cast wheels with properties approxi-
mating those of wheels produced from wrought alloys.
S It is an additional object of this invention to
provide complex, shaped automotive wheels produced as
an integral product to close tolerances by a low pres-
sure press forging process.
.
It is a more specific object of this invention to
provide a highly styled aluminum automotive wheel com-
bining excellent properties with relatively complex
configuration.
The invention comprises an aluminum alloy automotive
wheel of complex configuration produced as an integral
product to close tolerances by shaping under pressure a
semi-solid aluminum alloy charge in a closed die cavity,
said aluminum alloy charge containing discrete degenerate
dendritic primary solid particles suspended homogeneously
in a secondary liquid phase having a lower melting point
20- than said primary solid particles, the properties of
said automotive wheel being isotropic and the tensile pro-
perties at least meeting the minimum specifications for
forged aluminum alloys. Aluminum alloy wheels of the
invention combine the complex configuration normally
associated with a cast product with tensile properties
which meet or surpass minimum specifications for forged
aluminum alloys. The process of producing the wheels of
this invention is the subject of my copending Canadian
application No. 332,032, filed concurrently herewith.
A ~
fi~
The invention will be better understood from the
following description taken in connection with the accom-
panying drawing in which
FIG. 1 is a vertical crossectional view of dies in
closed position in a press suitable for use in the invention;
FIG. 2 is an elevational view of an automobile
wheel produced in the press of FIG. l; and
FIG. 3 is a plan view of the wheel shown in FIG. 2.
The metal charge or preform used in preparing the
wheels of the invention is semi-solid -- a part liquid and
part solid mixture. The solid particles, between 30% and
90~ of the total volume, are rounded in shape and are nor-
mally between about 20 and 200 microns in diameter. This is
the result of a prior treatment of the metal in which the
metal is melted and then during freezing, is vigorously
stirred. This breaks up the grain formation into the generally
rounded particles. The resulting metal composition is char-
acterized by discrete degenerate dendritic primary solid
particles suspended homogeneously in a secondary phase having
a lower melting point than the primary particles. Both the
primary and secondary phases are derived from the metal alloy
which has been vigorously agitated during freezing. The pro-
cess and the resulting alloy are more fully disclosed in the
aforesaid U.S. patents 3,948,650 and 3,954,455, reference to
which should be made for a more complete description thereof.
The generally rounded nature of the discrete de-
generate dendritic particles permits the solid particles to
il366~
flow in a viscous fashion in a continuous liquid matrix. This
permits the relatively low pressure forming of the wheel. The
pressures used in the process range from about 25 to 5000 psig
which permits the forming of parts as large as a full sized
(14") automobile wheel to be formed in a 250 ton press as
compared to a 1200 ton die casting machine or an 8000 ton
press used for conventional forging.
The largely solid nature of the charge, which ranges
from 30 to 90~, but preferably over 70%, by volume solids,
permits very rapid solidification with a minimum of liquid/
solid shrinkage. This, in turn, permits forming wheels with-
out large "feed reservoirs" or risers and allows very short
residence in the dies. The latter point is vital to the high
production rates attainable with this process, e.g. a realis-
tic rate of 240 automobile wheels an hour.
The rapid solidification means that nearly all sec-
tions of the wheel, of equal section thickness, will solidify
at the same time and thus may be ejected very rapidly, and
usually in less than 4 seconds after forming for high con-
ductivity alloys such as aluminum. For ferrous alloys or for
wheels of larger crossection, solidification time may extend
to 15 to 20 seconds, but in any event, will always be less
than a minute and usually substantially less. The rapid
ejection releases the part from many of the constraints of
the solid state contraction which normally occurs with de-
creasing temperature. Such contraction can progress to the
point at which binding on the dies causes high stresses and
resulting hot tears or cracks in the shaped ~heel.
1~36679
Wheels produced in accordance with the invention
possess many of the properties of a forging, but contain the
complex shapes and shape tolerances typical of a casting.
The wheels may be produced using nominally wrought aluminum
or ferrous alloys having the levels of tensile strength,
fatigue strength, ductility and corrosion resistance comparable
to forged or wrought products produced from these alloys.
Automobile wheels have been prepared having many of the
characteristics of forged wheels, utilizing considerably
simplified pressing equipment in a considerably more efficient
manner than conventionally forged wheels.
In the process of the invention, a preform is heated
until 10-70~ of its volume becomes liquid. As indicated above,
the preform or charge has previously been produced by vigorous
agitation of a liquid-solid mixture of the selected alloy
which was then rapidly cooled. The temperature to which the
preform is heated is between the liquidus and solidus temper-
ature for the particular alloy and will vary from heat to heat
within a given alloy system depending on the particular chem-
istry. Since there is no specific temperature at which themetal will form properly, the viscosity as measured by the
resistance to penetration of a probe into the semi-solid, may
be used as an indicator of the ~ liquid present in the mixture.
Generally the range of S psig to 15 psig will be used, the
2; exact pressure being selected to suit the conditions of the
part to be formed. It is possible to avoid cooling and re-
heating of the preform by using as the charge the vigorously
agitated slurry directly - i.e. before it is cooled to form a
billet or preform.
_ 4~
113661~9
Low pressures may be used to shape the preheated
billet providing no significant additional solidification
occurs during the shaping step. Thus, in order to insure the
use of low pressures, a shaping time in the die cavity of less
than one second is required. The die cavity is preheated to
a temperature of from 100 to 450C., depending primarily upon
part configuration, in order to prevent significant solidi-
fication during the forming or shaping step. If temperatures
are too high, there is a tendency for adhesion of the preform
to the die, known as die soldering, to occur. During the
forming stroke, the pressure raises from zero to the pressure
used for solidification. By the end of the forming stroke,
the pressure has accordingly risen from about 25 to 5000 psig,
usually 500 to 2500 psig, and solidification of the liquid
phase begins. Thus, the pressure gradually rises during the
shaping stroke and remains at a peak of from 25 to 5000 psig
during solidification. The applied pressure enhances heat
transfer from the metal alloy to the die and feeds solidifica-
tion shrinkage. If the pressure is too low, porosity may be
at an unacceptable level or complex molds may fill incompletely.
Pressures above 5000 psig may be used, but they are not necess-
ary. Moreover, higher pressures may create a venting problem.
It is desirable to form the part at as low a pressure as
possible for reasons of process economy, simplicity of pressing
equipment and for die life.
Residence time in the die cavity, subsequent to the
shaping step, should be short enough, under one minute and
preferably less than 4 seconds, to avoid hot cracking of the
1~366~9
shaped part from thermal contraction stresses but long enough
to complete solidification of the liquid phase of the alloy.
Specific times will depend on part thickness. The tendency
for hot cracking to occur is a function of alloy composition,
fraction solids percent, die temperature and part configur-
ation. Within the ranqes of forming and solidification times
herein set forth, times should, of course, be kept as short
as possible to maximize part-ma~ing productivity. ~s is
apparent from the foregoing discussion, times, pressures,
temperatures and alloy solid fraction are a combination of
critical variables which together function to achieve the
significant process economies and product improvements here-
in set forth.
The shaping process may be carried out, for example
in a 150-250 ton hydraulic press equipped with dies or molds
of the type illustrated in FIG. 1 of the drawing. The specific
die set there shown is contoured to produce a highly styled
automobile wheel. The die set comprises a movable top die or
ram 1, two side dies 2 and 3 and bottom die 4. The dies are
shown in closed position, the alloy metal 5 having been shaped
into the contour of an automobile wheel.
Another aspect of the process involves the manner in
which the dies are vented. The length and diameter of venting
channels must be of adequate size to provide ample venting. On
the other hand, the channels must normally be sufficiently
narrow and long to avoid spraying the molten metal to the ex-
terior of the dies. Venting channels of conventional size, of
-- o --
~13~67g
a diameter used for example in die casting, have proven too
narrow to eliminate air pockets in the present press forming
process. It has been found, however, that the high solids
fraction present during the pressing cycle of the present
invention permits wider and shorter venting channels to be
used. The result is not only the absence of air pockets in
the shaped product, but fewer limitations on die design, the
latter because less area is needed to achieve adequate venting.
Four such vents, 6, 7, 8 and 9, are shown in crossection in
FIG. 1. It will be seen from FIG. 1 that the shaping operation
actually involves a concurrent forward extrusion of semi-solid
metal into the narrow channels opening into vents Ç and 7, a
backward extrusion of semi-solid metal into the channels leading
to vents 8 and 9 and a forging stroke against the central
portion of the metal in the press. Reference herein to
"complex" shapes is intended to identify parts which require
such concurrent forward and backward extrusion combined with
a forging step in the process herein set forth.
The following example is illustrative of the practice
of the invention. Unless otherwise indicated, all parts are
by weight.
Example
An 18 pound billet of 6061 wrought aluminum alloy
was cast, substantially as set forth in U.S. patent 3,948,650,
from a semi-solid slurrv containing approximately 50% by vol-
ume degenerate dendrites. The billet, approximately six
inches in diameter, had the following composition:
Si Cr l~n Fe ~Ig Ti Cu B Al
0.63 0.06 0.06 0.22 0.90 0.012 0.24 0.002 3alance
~136679
The billet, contained in a stainless steel canister,
was placed within a resistance furnace set at a temperature of
677C. This temperature, approximately 28C. above the liquidus
temperature of the alloy, was sufficient to induce partial
melting of the alloy without creating significant variations
in fraction liquid within the billet. At a temperature of
632C., corresponding to a fraction solid of approximately
0.80, as detected by the penetration of a weighted probe,
the billet in its canister was transferred to the closed bottom
half of a cast iron die set, of the type shown in FIG. 1, main-
tained at 315C. and ejected from the canister to the bottom of
the die. The die set was coated with a graphite base lubricant.
The top die, also maintained with a surface temperature of
approximately 315C., was then closed at a speed of 20 inches
per second, resulting in a preform shaping time of about 0.2
seconds, the die reaching a maximum pressure of 2100 psig such
that the cavity so formed was filled with alloy. After a
holding time under pressure of 2.4 seconds, during which the
liquid phase of the part solidified, the die set was opened
and the shaped part extracted.
The shaped part, an aluminum wheel, was sectioned
and specimens for mechanical property measurement were taken.
Room temperature properties were measured. Ultimate tensile
strength was 47,000 psi, yield strength was 43,000 psi and
elongation in a 1" gauge length was 7~. ~inimum specifica-
tions for closed die forgings of 6061 aluminum alloys as setforth in Aluminum Standards and Data 1976, Fifth ~dition,
1976 are 38,000 psi ultimate tensile strength, 35,000 psi
yield strength and 7% elongation. Representative minimum
~13667~
specifications of an automobile manufacturer for cast aluminum
wheels are 31,000 ultimate tensile strength, 16,500 yield
strength and 7% elongation.
Unlike wrought products whose properties are direct-
ional, the products of the invention are isotropic - their
properties are equal in all directions. The metallurgical
structure of the wheel of the example consisted of randomly
oriented, equiaxed grain structure without the "texture"
associated with wrought components having similar properties.
A finished wheel generally identified by the numeral
lO produced in accordance with the invention is shown in ele-
vation in FIGS. 2 and 3. The plan view of FIG. 3 shows the
wheel as viewed from the direction of the bottom die in FIG. 1.
The wheel contains a plurality of roughly rectangular contours
ll around the periphery, each of the contours containing a
punched or machined hole 12 therethrough. A hub area 13 con-
tains four cored and tapped holes 14 and four larger punched
or machined holes 15. A wheel configuration of this complexity
is normally produced by permanent mold or die casting tecnniques
and is accordingly limited in its properties to the relatively
inferior properties of cast alloys. Material properties are
tnus a limiting factor on wneel weight. Lower properties must
be compensated by greater bulk in a cast wheel. ~oreover,
larger crossections are normally necessary in casting because
of li~itations inherent in casting techniques - it is difficuit
to fill a permanent mold with thin sections. Thus, tne ~heels
of the invention nave the very important capabilit~,~ of being
lighter in ~eight than comparable wheels of the prior art.
