U.S. patent application number 16/138756 was filed with the patent office on 2019-01-24 for systems and methods for forming a layer onto a surface of a solid substrate and products formed thereby.
The applicant listed for this patent is ATS MER, LLC. Invention is credited to Jowie Abcede, Lori Bracamonte, James Withers.
Application Number | 20190024738 16/138756 |
Document ID | / |
Family ID | 58717982 |
Filed Date | 2019-01-24 |
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United States Patent
Application |
20190024738 |
Kind Code |
A1 |
Bracamonte; Lori ; et
al. |
January 24, 2019 |
SYSTEMS AND METHODS FOR FORMING A LAYER ONTO A SURFACE OF A SOLID
SUBSTRATE AND PRODUCTS FORMED THEREBY
Abstract
A method for forming a vehicular brake rotor involving loading a
shaped metal substrate with a mixture of metal alloying components
and ceramic particles in a dieheating the contents of the die while
applying pressure to melt at least one of the metal components of
the alloying mixture whereby to densify the contents of the die and
form a ceramic particle-containing metal matrix composite coating
on the metallic substrate; and cooling the resulting coated
product.
Inventors: |
Bracamonte; Lori; (Tucson,
AZ) ; Withers; James; (Tucson, AZ) ; Abcede;
Jowie; (Tucson, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ATS MER, LLC |
Tucson |
AZ |
US |
|
|
Family ID: |
58717982 |
Appl. No.: |
16/138756 |
Filed: |
September 21, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15822023 |
Nov 24, 2017 |
10107344 |
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16138756 |
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15357730 |
Nov 21, 2016 |
9933031 |
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15822023 |
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62265765 |
Dec 10, 2015 |
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62258448 |
Nov 21, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F16D 65/127 20130101;
B32B 15/01 20130101; F16D 65/125 20130101; C22C 1/0416 20130101;
B22D 27/15 20130101; C22C 32/0063 20130101; B22D 19/08 20130101;
F16D 2200/003 20130101; C22C 19/05 20130101; F16D 2200/0086
20130101; B22F 7/04 20130101; C23C 24/103 20130101; B22F 2007/045
20130101; B22D 23/06 20130101 |
International
Class: |
F16D 65/12 20060101
F16D065/12; B22D 23/06 20060101 B22D023/06; B22D 27/15 20060101
B22D027/15; B22F 7/04 20060101 B22F007/04; C23C 24/10 20060101
C23C024/10; C22C 19/05 20060101 C22C019/05; B22D 19/08 20060101
B22D019/08; B32B 15/01 20060101 B32B015/01; C22C 32/00 20060101
C22C032/00 |
Claims
1. A method for integrating a liquid metal matrix composite to a
surface of a metal substrate, the method comprising: enabling a
reaction involving the liquid metal matrix composite; and heating
and densifying the combination of the liquid metal matrix composite
and the metal substrate during at least a portion of the reaction,
and cooling the metal matrix composite from liquid to solid while
applying pressure to the composite, and optionally heat treating
the formed product.
2. The method of claim 1, further comprising densifying the
combination of the liquid metal matrix composite and the metal
substrate during at least a portion of a duration of the
reaction.
3. The method of claim 1, wherein the volume of the liquid metal
matrix composite is less than the volume of the substrate.
4. The method of claim 1, wherein the substrate has a melting
temperature that corresponds to the melting point of an element of
the group consisting of aluminum, iron and titanium, or an alloy
thereof, preferably an aluminum alloy comprises aluminum and one or
more elements of the group consisting of cobalt, copper, iron.
nickel, titanium, vanadium, zinc, chromium, magnesium, manganese,
niobium, and silicon
5. The method of claim 1, characterized by one or more of the
following features: a. wherein the reaction involves an exothermic
reaction of two or more metals selected from the group consisting
of aluminum, boron, cobalt, copper, iron. nickel, palladium,
titanium, vanadium, zinc, chromium, magnesium, manganese, niobium,
and silicon; b. wherein the reaction involves a first reactant of
the liquid metal matrix of the composite and a second reactant of
the liquid metal matrix of the composite; and c. wherein the liquid
metal matrix composite comprises an aluminum alloy and a ceramic
particulate, preferably silicon carbide.
6. The method of claim 1, wherein: a. the substrate comprises a
metal selected from the group consisting of aluminum, iron and
titanium; and b. the metal matrix composite comprises silicon
carbide, and an alloy of aluminum and silicon, or an alloy of
aluminum, silicon, and magnesium, or an alloy of aluminum and
titanium, or an alloy of aluminum, silicon and titanium, or a
prealloyed powder selected from Ti--Al or Ti--Al--Cr--Nb.
7. The method of claim 1, wherein from 25% to 65% of an exterior
surface of the layer, preferably 35% or more of an exterior surface
of the layer, comprises ceramic particulate.
8. The method of claim 1, wherein a porosity of the formed product
is less than 2% by volume, preferably less than 1% by volume, more
preferably less than 0.25% by volume.
9. The method of claim 1, further comprising enabling the reaction
in less than ambient atmospheric pressure.
10. The method of claim 1, wherein the substrate comprises a
hub-disc assembly for a vehicle brake.
11. A method for integrating a metal matrix composite to a surface
of a metal substrate, the method comprising: enabling a reaction
involving the metal matrix composite; and heating and densifying
the combination of the metal matrix composite and the metal
substrate during at least a portion of the reaction, and cooling
the metal matrix composite and metal substrate while applying
pressure to the composite, and optionally heat treating the formed
product.
12. The method of claim 11, further comprising densifying the
combination of the metal matrix composite and the metal substrate
during at least a portion of a duration of the reaction.
13. The method of claim 11, wherein the volume of the metal matrix
composite is less than the volume of the substrate.
14. The method of claim 11, wherein the substrate has a melting
temperature that corresponds to the melting point of an element of
the group consisting of aluminum, iron and titanium, or an alloy
thereof, preferably an aluminum alloy comprises aluminum and one or
more elements of the group consisting of cobalt, copper, iron.
nickel, titanium, vanadium, zinc, chromium, magnesium, manganese,
niobium, and silicon
15. The method of claim 11, characterized by one or more of the
following features: a. wherein the reaction involves an exothermic
reaction of two or more metals selected from the group consisting
of aluminum, boron, cobalt, copper, iron. nickel, palladium,
titanium, vanadium, zinc, chromium, magnesium, manganese, niobium,
and silicon; b. wherein the reaction involves a first reactant of
the metal matrix of the composite and a second reactant of the
metal matrix of the composite; and c. wherein the metal matrix
composite comprises an aluminum alloy and a ceramic particulate,
preferably silicon carbide.
16. The method of claim 11, wherein: a. the substrate comprises a
metal selected from the group consisting of aluminum, iron and
titanium; and b. the metal matrix composite comprises silicon
carbide, and an alloy of aluminum and silicon, or an alloy of
aluminum, silicon, and magnesium, or an alloy of aluminum and
titanium, or an alloy of aluminum, silicon and titanium, or a
prealloyed powder selected from Ti--Al or Ti--Al--Cr--Nb.
17. The method of claim 11, wherein from 25% to 65% of an exterior
surface of the layer, preferably 35% or more of an exterior surface
of the layer, comprises ceramic particulate.
18. The method of claim 11, wherein a porosity of the formed
product is less than 2% by volume, preferably less than 1% by
volume, more preferably less than 0.25% by volume.
19. The method of claim 11, further comprising enabling the
reaction in less than ambient atmospheric pressure.
20. The method of claim 11, wherein the substrate comprises a
hub-disc assembly for a vehicle brake.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/822,023, filed Nov. 24, 2017, which in turn is a
divisional of U.S. patent application Ser. No. 15/357,730, filed
Nov. 21, 2016, now U.S. Pat. No. 9,933,031, granted Apr. 3, 2018,
which claims priority from U.S. Provisional Application Ser. No.
62/258,448, filed Nov. 21, 2015, and from U.S. Provisional
Application Ser. No. 62/265,765, filed Dec. 10, 2015, the contents
of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to the formation of
substantially pore free wear resistant metal ceramic particulate
composites on metal substrates, systems for forming such composite
layers, and products made. The invention has particular utility in
connection with vehicular brake rotors and will be described in
connection with such utility, although other utilities are
contemplated.
BACKGROUND OF THE INVENTION
[0003] Land vehicles including cars, trucks, trains, and mass
transit systems use disc brakes to slow and/or stop the vehicle.
Disc brake systems generally include a rotor and caliper. The rotor
is mounted to turn with the wheel of the vehicle. The caliper
includes brake pads that are forced into frictional contact with
the rotor to slow and or stop rotation of the wheel. Conventional
cast iron brake rotors are relatively heavy. These rotors wear
during braking, generating dust. Alternatives to conventional cast
iron brake rotors can reduce weight as well as contribute to better
fuel economy, reduced both air and water pollution, and enhanced
vehicle acceleration. Reduced weight materials to cast iron include
aluminum and titanium; however, their surface tribology in a
friction application, in contrast to cast iron, lacks the necessary
performance to function as a brake rotor. It has been known to add
ceramic particulate to a metal matrix to increase friction for
improved stopping power and to enhance wear resistance, which also
has the advantage of producing little to no dust in a friction
application.
[0004] Many conventional processes have drawbacks when applied to
forming brake rotors with desirable features. For example, some
rotors formed of aluminum with a ceramic coating applied by a
conventional plasma spray technique have unsatisfactory residual
porosity as well as insufficient adhesion of the coating to the
rotor substrate for vehicular rotor applications. Brake rotors
mostly free of porosity are enabling for reliability and good heat
transfer.
[0005] Conventional stir casting used to produce composites and
particularly aluminum-ceramic particle composites inherently
results in unacceptable porosity for most vehicular rotor
applications. In addition, ceramic particle loading of at least 30
to 50% by volume is desirable for high wear resistance and braking
performance of vehicles with relatively high momentum. Conventional
stir casting is not known to produce pore free composites at this
necessarily high particle loading. In conventional stir casting,
ceramic particles are incorporated throughout a molten aluminum
alloy. As the insoluble particulate is stirred into the liquid
aluminum, the viscosity of the mixture increases with the volume of
particulate added. The increase in viscosity is also related to the
size of the particulate. Generally, the smaller the particulate,
the greater the increase in viscosity. However, fine particle sizes
are used, preferably below approximately 50 .mu.m for good braking
performance. A conventional stir process to incorporate fine
ceramic particles into the aluminum matrix for brake rotor
applications is limited to approximately 25% by volume, depending
on the actual particle size, because stirring and casting of higher
volume loading and viscosity mixtures become impractical.
SUMMARY OF THE INVENTION
[0006] It is therefore an object of the present invention to
provide a method and apparatus for forming substantially porosity
free metal/ceramic composite bonded on support substrates that
overcomes the aforesaid and other problems with the prior art.
[0007] According to various aspects of the present invention, a
composite layer formed on a surface of a substrate (e.g., a
vehicular brake rotor), using the systems and methods discussed
herein, is substantially free of porosity and includes ceramic
particles of desirable size and composite density. A relatively
high density of ceramic particles within the composite layer has a
capability to retard heat flow into the core substrate. The layer
may encompass the entire substrate surface or be formed in one or
more selected regions (e.g., one or more circular bands) of an
external surface of the substrate. The composite layer may also
provide reinforcement to the substrate, so that the combination of
the layer and substrate provides an improved combination of
mechanical and thermal properties (e.g., friction coefficient, wear
rate, thermal conductivity, thermal capacity, specific heat,
specific gravity, density, compressive strength, ductility,
stiffness) compared to the substrate without the layer.
[0008] In an application of the inventions discussed herein related
to vehicular brake rotors, the layer depending on particle volume
loading has higher friction and inherently has higher heat capacity
than cast iron. The higher heat capacity provides a greater heat
storage capacity than iron, a ceramic particle loading higher than
approximately 30 volume percent provides improved braking
performance, and low wear characteristics results in little or no
dust generation, and thus less air pollution than cast iron.
[0009] The composite friction layer may be graded as to density or
volume of ceramic particles as a function of distance from the
outer surface of the layer toward the substrate. More than one
layer may be formed. For example, when the ceramic particle volume
is varied from the outer surface to the substrate core, the
temperature of the substrate may be less than the temperature of an
exterior surface. A thin coating on the external surface by
conventional means that includes a relatively low thermal
conductivity ceramic (e.g., oxides) may provide excellent thermal
insulation of heat flow to the substrate.
[0010] Substrates of aluminum, aluminum alloy, titanium, and/or
titanium alloy are much lighter than cast iron and are preferred
because of their light weight and high heat capacity for vehicular
brake rotor applications. Titanium, similar to aluminum, does not
provide a tribological surface suitable for vehicular brake rotor
applications. A layer of ceramic particles in a metal matrix of one
or more metals (e.g., metals of the substrate, or metals having
lower melting points than the substrate melting temperature, and
metals serving as reactants in an intended exothermic reaction) may
provide a wear resistant composite braking surface capable of
withstanding vehicular braking temperature extremes.
[0011] In the description below, reference is made to vehicular
brake rotors as an example of a metal substrate coated with a metal
matrix composite layer. The metal substrate is also called a core.
In operation, an external surface of the layer provides a braking
surface. When such a brake rotor is mounted on a wheeled vehicle,
in a conventional fashion, as discussed above, friction between the
braking surface and one or more brake pads slows and/or stops the
vehicle.
[0012] A system for forming a layer onto a surface of a solid metal
substrate may be assembled from conventional manual and/or
automated processing and process controlling equipment and
facilities configured according to the disclosures herein.
Automation may be accomplished by installing in the equipment, or
linking the equipment for access to, firmware and/or software
prepared using conventional programming technologies. In one
implementation, a system includes an integration of a chamber, a
heater, a press, and a vacuum pump of conventional design. A system
may be of the type known as a vacuum hot press. Commercial vacuum
hot presses of the type supplied by Thermal Technology LLC of Santa
Rosa, Calif. and The Furnace Source LLC of Terryville, Conn. may be
used, or custom made systems can meet the objectives of producing
metal matrix composite brake rotors.
[0013] A chamber includes any enclosure insulated to economically
maintain a relatively high temperature within and relatively low
atmospheric pressure within, while located in ambient conditions
without. The furnace operation may also be performed under positive
pressure or with a gas flow, as long as an inert gas such as argon
or a gas that is substantially unreactive with the brake materials
such as nitrogen is used. Generally, a chamber includes a closable
opening through which opening a die may be placed in the chamber to
be subject to the higher temperature and applied pressure for a
desirable period and later removed from the chamber. Additional
passages through the enclosure are generally useful for supporting
the heating, pressing, and evacuating functions; as well as
instrumentation and controls. A chamber may include a structure to
accommodate a press. Conversely, a chamber may be moveable and
lowered or alternatively raised over a base with ceramic composite
layers to be processed into a brake rotor.
[0014] A heater includes any apparatus (e.g., electrical, chemical,
magnetic) that provides energy to heat the contents of the die.
Heating may be conducted into the die (e.g., pressing a hot object
against the die, electrical conduction through the resistance of
the die and/or its contents, magnetic induction and heating of the
type called spark plasma sintering also referred to as field assist
sintering technology (FAST) or current activated pressure assisted
densification) and/or radiated toward the die (e.g., microwave
heating, electron beam heating, laser light heating, radiant heat
source within the chamber, and conduction or convection via a gas
that surrounds the die prior to evacuation of the chamber). The
heater may include a source of energy used for heating. The heater
may have access to a source of energy used for heating or energy
conversion. The heater may convert energy from one form to another
(e.g., electrical power to RF for magnetic induction, pulsing of
power, etc.)
[0015] A press includes any equipment that applies a mechanical
force onto a die within the chamber. A press may include one or
more rams (e.g., moving piston, fixed backstop) that come in
contact with the die to apply the force. The press may include
heating, cooling, and/or insulation to reduce thermal variation
between the press and the die.
[0016] A vacuum pump includes any mechanical equipment that removes
gas from a chamber. A vacuum pump may also replace the gas in the
chamber with another gas. A vacuum pump may govern a pressure of
gas surrounding the die.
[0017] Integrating control of the temperature, mechanical pressure,
and extent of surrounding gas within the chamber may be
accomplished by one or more programmed controllers. Programming may
include sequences for treating the contents of the die and
sequences for maintaining safe operating conditions for persons,
equipment, and the contents of the die. Sequences, in any
conventional programming language, may include instructions (e.g.,
commands, parameters, arguments, statements) that define set
points, rates of change, durations at set points, durations for
accomplishing changes, limit conditions for exceptional actions
when the limit is crossed (e.g., return chamber to ambient
conditions, return chamber to known safe conditions, sound alarm
for operator to make adjustment, end sequential control), and/or
program sequence controls (e.g., branching, looping, subroutines,
error handling). Programming may be centralized or distributed.
When program control is distributed among specialized controllers,
signals conveying status and controls (e.g., feedback, commands,
requests for information, requests for resources, commitments) may
be sent and received using conventional technologies to accomplish
the effect of centralized program control.
DESCRIPTION OF THE DRAWINGS
[0018] Further features and advantages of the present invention
will be seen from the following detailed description, wherein:
[0019] FIG. 1 is a functional block diagram of a system that can be
used to produce a dense, essentially porosity free, metal ceramic
particulate composite in the architecture of a composite coating on
a core metal or as a component that encompasses the entire
cross-section of the composite according to one aspect of the
present invention; FIG. 1 includes a cross sectional view of a die
and its contents, according to various aspects of the present
invention
[0020] FIG. 2 is a flow chart of a method for forming a layer onto
a surface of a substrate, according to one aspect of the present
invention;
[0021] FIG. 3 is top view of an empty die for use in the system of
FIG. 1 and/or with the method of FIG. 2; and
[0022] FIG. 4 is a top view of a press fit base for use with the
die of FIG. 3; a press fit top having similar construction.
[0023] FIG. 5 is a perspective view of a press fit tip and bottom
assembled together; and
[0024] FIG. 6 is a view similar to FIG. 2 of an alternative method
for forming a layer onto a surface of a substrate according to
another aspect of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0025] Referring to the drawings, FIG. 1 shows a system that can be
used to produce a dense essentially porosity free metal ceramic
particulate composite coating on a metal core in accordance with
the present invention. As used herein "porosity free" means that
the coating comprises generally not more than about 0.05-2% by
volume pores, more particularly about 0.5-1% by volume pores, even
more particularly about 0.1-0.25% volume pores.
[0026] System 100 of FIG. 1 includes chamber 102, controller 110,
heating subsystem 112, pressing subsystem 114, and evacuating
subsystem 116. These components of system 100 may be arranged in
any convenient orientation to each other and to gravity. As shown,
die 150 is located in chamber 102 for processing contents of die
150. Die 150 defines interior 166. Die 150 includes peripheral
component 152, press fit base 156, and press fit top 154. Contents
of die 150 are placed in die interior 166 and include solid
substrate 162 and powder 164 and 165. Placement of the contents
into interior 166 is referred to herein as loading die 150. Loading
may be accomplished with any conventional technologies manually
and/or automatically.
[0027] Chamber 102 represents a chamber as discussed above. Chamber
102 provides a thermally insulated interior 104. Chamber 102
isolates the interior 104 from ambient atmosphere. Chamber 102 has
a door and a doorway (not shown) through which a loaded die may be
placed into the chamber for processing with the door closed.
Placement may be by manual and/or automatic means of conventional
design (e.g., conveyer, hoist, elevator, turntable, web). When
processing is completed, the die may be removed and unloaded to
release a product comprising a substrate with a composite layer as
discussed above.
[0028] Controller 110 represents a programmable controller as
discussed above. Portions of controller 110 may be integral to one
or more of heating subsystem 112, pressing subsystem 114, and/or
evacuating subsystem 116 (e.g., efficient packaging of
instrumentation and/or circuitry for performing instructions, for
reporting status, for feedback control, for monitoring limit
conditions, for alarms, for safety of personnel and equipment, for
cooperation with materials handling equipment). Controller 110
includes conventional memory (not shown) and a conventional
sequential machine that reads the memory to perform a sequence as
discussed above. In an implementation, controller includes a
personal computer, Microsoft Windows operating system, and process
control software. The personal computer includes a user interface
for receiving program instructions to store one or more sequences
in the memory; and further includes displays for monitoring
progress of the process applied to contents of die 150, observing
conditions as described by instrumentation that may be located
within heating subsystem 112, pressing subsystem 114, evacuating
subsystem 116, chamber 102, and/or die 150. Controller 110
communicates with heating subsystem 112 via signals 122 for status
and control using conventional technologies. Controller 110
communicates with pressing subsystem 114 via signals 124 for status
and control using conventional technologies. Controller 110
communicates with evacuating subsystem 116 via signals 126 for
status and control using conventional technologies.
[0029] Heating subsystem 112 includes a heater as discussed above.
Heating subsystem 112 responds to controller 110 to provide heat
132 to the contents of die 150 and/or ram 134. Heat from subsystem
112 melts powder 164 and 165 as discussed below. Heating subsystem
112 may further include conventional cooling technologies
controlled by controller 110 to cool the contents of die 150 at any
suitable time in the sequence.
[0030] Pressing subsystem 114 includes a press as discussed above.
Pressing subsystem 114 responds to controller 110 to apply a
mechanical force to the contents of die 150 via ram 134 and press
fit top 154. As shown, pressure from ram 134 is passed through
standoff 170 to avoid mechanical interference from hub 158. The
force from pressing subsystem 114 densifies the contents of die 150
as discussed below. The mechanical force applied by pressing
subsystem 114 is matched by a reaction force. The reaction force
may be provided by an opposing ram (not shown). For simplicity,
FIG. 1 indicates the reaction force provided by earth ground
106.
[0031] Standoff 170, cover 154, and press fit base 156 are designed
to evenly distribute the mechanical force of the ram onto the
contents of die 150. In an implementation according to various
aspects of the present invention, a uniform thickness of the layer
is achieved by assuring that the contents of die 150 are maintained
throughout the process relatively close to level.
[0032] Evacuating subsystem 116 includes a vacuum pump as discussed
above. Evacuation subsystem 116 is in fluid communication by
coupling 136 to die interior 166 via vent 172 through hub 158,
through standoff 170, and chamber interior 104. Evacuation
subsystem 116 responds to controller 110 to control the kinds of
gases (if any) and gas pressure in die interior 150. Evacuation
subsystem 116 may similarly control chamber interior 104, for
example, to indirectly control die interior 166 via vent 172.
Evacuation subsystem 116 may introduce and/or pressurize one or
more gases in die interior 166 and/or chamber interior 104.
[0033] A die includes any structure that contains contents subject
to temperature and pressure. At least some of the contents may be
in liquid phase. For example, die 150 contains (e.g., encloses)
substrate 162 and powder 164 and 165 so that any liquid and/or
gaseous phase of these materials that may exist during processing
will not leak from die 150. For example, when powder 164 and 165
includes aluminum alloy, aluminum alloy in liquid phase (e.g.,
resulting from an elevated temperature caused by heating subsystem
112) is contained (e.g., enclosed) by die 150 so as not to leak
into chamber interior 104. Die 150 may be formed of several
components with dimensions suitable for sealing at interfaces
between its components.
[0034] Sealing prevents the low viscosity liquid metal phase of the
contents of die 150 from leaking out of die 150. Seals described
herein inhibit leaking of metals at the temperatures and pressures
discussed below. According to various aspects of the present
invention, the amount of liquid phase contents of die 150 is
relatively small compared to liquefying the entire content of die
150.
[0035] Die 150 may be formed of graphite, ceramic, and/or metal
(e.g., steel, titanium) with a coating (e.g., carbon, graphite,
boron nitride).
[0036] In one implementation, according to various aspects of the
present invention, die 150 is formed of press fitted graphite
components, sized to be forced together in a friction fit of a
graphite surface of a first component against a graphite surface of
a second component.
[0037] Components may be friction fit into such materials to form
suitable seals between components that prevent any liquid metal
under pressure from leaking through such press fitted
components.
[0038] Member 154 completes the enclosure of die 150 with respect
to contents in interior 166. Member 154 conducts mechanical
pressure from ram 134 via standoff 170 to contents of die 150.
Member 154 also slides within die interior 166 as urged by ram 134.
In a first position, as shown, member 154 does not obstruct vent
172. In this position of member 154, vent 172 provides fluid
communication between die interior 166 and chamber interior 104.
Evacuation of chamber interior 104 consequently evacuates die
interior 166 via vent 172. Introduction of a gas into chamber
interior 104 also introduces the gas into die interior 166 via vent
172 by filling, mixing, and diffusion.
[0039] In one implementation, ram 134 applies a force only in one
direction. Ram 134 may be used for three functions. First, ram 134
may be used to introduce member 154 into die 150 to close die 150,
applying a force to overcome the friction fit of member 154 and
peripheral component 152. Member 154 is then in a first position.
Second, ram 134 may be used to move member 154 away from the first
position, in the same direction (further into die 150) to obstruct
vent 172 to prevent molten metal from escaping through the vent as
pressure is applied. Member 154 is then in a second position.
Third, ram 134 may be used to provide a force in the same direction
for densification of the contents of die 150.
[0040] When ram 134 urges member 154 away from the first position
(as shown) and further into die 150, member 154, in a second
position, completely obstructs vent 172, providing a gas and liquid
seal so that contents of die 150 do not leak into chamber interior
104. In one implementation, according to various aspects of the
present invention, member 154, a component of die 150, is formed of
graphite. Dimensions of member 154 provide a friction fit inside
peripheral component 152 as discussed above.
[0041] A substrate or core provides structural rigidity to the
intended product. The substrate has a melting temperature that is
higher than a melting temperature of at least one of the components
forming the composite surface layer so that by applying an elevated
temperature that is less than the substrate melting temperature, to
the contents of the die, the surface layer material will form a
melt while the bulk of the substrate will remain solid, i.e., not
melt. There will be melting on the substrate surface in contact
with the composite layer that results in excellent bonding. A
substrate may be formed of structural metal (e.g., aluminum,
cobalt, copper, iron, nickel, titanium, vanadium, zinc) in
commercially pure form or in some cases, an alloy. Alloys may
include any structural metal(s) and/or alloying material(s) (e.g.,
chromium, magnesium, manganese, niobium, silicon). The substrate
can be commercial plate material precut to a brake rotor which
exhibits inherently low porosity or substrate 162 may be a cast or
rolled disc of commercially pure aluminum or aluminum alloy formed
with low porosity.
[0042] A surface layer material which typically is in the form of a
powder may include at least one reactant for an exothermic reaction
and may further include material to perform a function and/or
enhance performance of a function of the product. The function may
rely on structural properties. The powder and/or surface layer may
be referred to as a reinforcement. The product of substrate and
surface layer may be referred to as a core with surface
reinforcement. A powder, which may also take the form or a metal
ingot may include particles of one or more materials, each material
being a component material of the powder. A powder when heated may
participate in an exothermic reaction. Preferably, the exothermic
reaction does not produce products in a gas phase. For example,
powder 164 and 165 are identical in composition. Powder 164
provides a surface that is integral with substrate 162 as a result
of the methods discussed herein. Powder 164 and 165 includes a
ceramic particulate (e.g., silicon carbide) to improve the friction
and wear properties of a vehicular brake rotor formed with
substrate 162 as a core. Powder 164 or 165 may include two
reactants. The first reactant may be a structural metal and/or an
alloying material. The second reactant may be boron, silicon,
cobalt, iron, nickel, magnesium, palladium, titanium, or a
combination thereof. The exothermic reaction will not begin until
the powder is heated to an elevated temperature, determined by the
intended reaction and the reactants using conventional analysis,
simulation, or test.
[0043] For vehicular brake rotors, silicon carbide is a preferred
ceramic particle included in the metal powder. Other ceramic
particles that may be used, in place of silicon carbide or in
addition to silicon carbide, in the powder include metal carbides,
nitrides, silicides, oxides, and intermetallic compositions. The
metal component of these ceramic particles include aluminum,
titanium, boron iron, nickel, cobalt, copper, and zinc solely or in
any combination. In an implementation for the fabrication of
vehicular brake rotors, the powder may include an aluminum alloy
and a ceramic particulate. The aluminum alloy may include Si from
zero percent up to 6% to 19% by weight, preferably 7 to 18% by
weight, more preferably 9 to 12% by weight and Mg from zero up to
0.1% to 15% by weight, preferably 0.2 to 12% by weight, more
preferably 0.4 to 1% by weight. The silicon of the alloy provides
excellent wettability and/or bonding to most ceramic particles
including silicon carbide. Silicon coupled with SiC reduces the
formation of aluminum carbide. Aluminum carbide can corrode in the
presence of moisture, thereby losing structural integrity. In a
preferred vehicular brake rotor, according to various aspects of
the present invention, the composition of the layer is 25% to 65%
by volume or more preferably 30 to 65% by volume, and more
preferably 35 to 50% by volume of single or mixed ceramic
particulate.
[0044] A layer refers to a result of the bonding of the metal
matrix and the substrate. A cross section of a product of a
substrate and a layer formed according to various aspects of the
present invention may reveal a graded concentration of a component
material of the powder beginning at the surface of the product and
extending into the substrate. The layer corresponds to the portion
of the product having more than or equal to a concentration of the
component material that is significant to serving a function of the
product. The interior boundary of the layer may be difficult to
ascertain by visual inspection. However, based on the intended
function, concentration of component material needed to
significantly perform that function, and electron dispersive
spectrometry, a boundary (e.g., depth from the surface) may be
determined. Because the boundary may not be apparent to visual
inspection, the boundary may be determined by analysis, simulation,
and/or test and specified as a dimension (e.g., thickness) of the
product. For example, for a vehicular brake rotor as the product, a
layer formed by powder 164 may extend into the product from the
surface of the product to a boundary defined with reference to a
concentration of ceramic particles associated with an end of useful
braking life of the vehicular brake rotor. A concentration of
silicon carbide from the powder may be determined by test at
various depths of a prototype product so as to define the braking
layer. A method for forming a layer onto a surface of a solid
substrate, according to various aspects of the present invention,
produces the layer substantially absent of porosity as discussed
above. This result is obtained, in some implementations, by a
combination of one or more of evacuating the die cavity, heating
the contents of the die to a temperature between the melting point
of the metal powder and the melting point of the substrate, and
applying pressure for a period that may begin before any potential
exothermic reaction between the powder and the substrate and may
end after the layer has cooled below the melting point of the
powder. In an implementation of the method to produce a vehicular
brake rotor, the layer comprises a ceramic joined to the substrate
by integration with the substrate to form a metal matrix composite.
The layer may have superior friction, wear, and thermal capacity
characteristics compared to the substrate.
[0045] An implementation of such a method includes in any practical
order, loading contents into a die where the contents include a
solid substrate and a powder; heating the contents to a temperature
that enables melting of the surface of the substrate and the
powder; and densifying the contents. In another implementation
heating in the absence of an exothermic reaction is performed that
achieves bonding between the surface layer and the substrate. The
method may further include evacuating the die before
densifying.
[0046] For example, method 200 of FIG. 2, begins with assembling
(202) a die (150), coating the interior of the die with a mold
release compound, and placing the assembled die within the chamber.
Assembly may be omitted (e.g., unnecessary) when a die of unitary
construction is used, for instance a die capable of containing the
contents (in any phase occurring during the process) and product,
as discussed above.
[0047] Depending on the materials used for the die, the powder, and
the substrate, a suitable release compound may be applied to coat
the interior of the die. Such a coating facilitates removal of the
product from the die. For example, a dry film lubricant formed of
graphite and thermoplastic resin of the type used as a billet
lubricant in hot forging of aluminum, orbital forging, and cold
extrusion as well as mold pre-treatment in metal casting may be
used. For instance, DAG 386 marketed by Acheson may be used. Boron
nitride release compounds also may be used.
[0048] Loading the die with contents may be accomplished by
spreading (204) a powder (164) inside the die (150), placing (206)
a substrate (162) (e.g., a brake rotor core) onto the powder (164),
and spreading (208) powder (165) to cover the substrate (162).
Aluminum ingot in solid form or in powdered form such as A356 or
A359 may also be used in place of the mixed metal powders. The two
powder distributions or two ingots or both powders and ingots and
the substrate constitute the contents of the die.
[0049] In another implementation, one of the spreading steps may be
omitted for forming a layer on only one side of the substrate.
[0050] Prior to loading the die, the powder may be prepared as a
uniform mix of two or more powdered materials. Uniformity of
particle size distribution as well as uniformity of particle
material distribution within the mix may be accomplished by
preparing the powder at least in part using conventional
pulverizing, screening, and mixing technologies.
[0051] After loading, the die (150) may be covered (154) as
discussed above. In one implementation, the die (150) is closed
leaving a vent (172) of the die open for evacuation of residual
atmospheric gases from the interior of the die. The die may be
placed in a vacuum hot press, a system (100) as discussed above, or
a vacuum chamber for evacuation.
[0052] In the case where the die includes a vent and the vent is
open, gases in the interior of the die may be removed by evacuation
(210) and/or purging in one or more cycles to obtain a suitable
absence of atmospheric or other gas that could otherwise contribute
to porosity in the layer. Any introduced gas may remain to
participate in a desired chemical reaction or, preferably, be
evacuated from the die. After evacuation, the vent is closed, for
example, by movement of the ram as discussed above. In other
implementations, the vent is closed by covering the vent using
conventional techniques (e.g., introducing a plug into the vent,
applying a band around the die to secure the plug, applying a band
around the die to cover the vent).
[0053] In one implementation, evacuation (210) is accomplished in a
vacuum chamber (as opposed to a vacuum hot press) equipped to close
the vent. In another implementation, the die is loaded (204, 206,
208) and closed in an evacuated chamber. In still another
implementation, the die does not include a vent and the die is
loaded (204, 206, 208) and closed (e.g., cover 154 installed) in an
evacuated chamber. The die is then placed in a conventional hot
press or a system of the type discussed above with reference to
FIG. 1 except that the evacuation subsystem is omitted with
consequent simplifications to the controller and chamber).
[0054] One or more loaded dies (150) may be prepared in advance,
transported, and/or stored indefinitely. Loaded dies may be
combined into a web for batch processing. Batch processing may be
accomplished with conventional materials handling technologies
between any steps of method 200. The contents of the loaded die are
heated (212) to an elevated temperature sufficient to melt the
metal alloy in the surface layers. Any exothermic reaction between
component materials of the powder will further increase temperature
in the surface layer so that less external heating is required,
producing what is herein called a melt. Generating a higher
temperature at the interface between the surface layers and the
core will also contribute to better adhesion between the surface
layers and the core. However, for certain component materials such
as Al--SiC powder, the exothermic reaction will either be very
small or not occur at all unless e.g., Ti is added. The heating may
be continued until the entire die contents are at the elevated
temperature. Heating may be discontinued after a period sufficient
to fully melt the surface layers and bond to the core and for
substantially completing the exothermic reaction of the contents of
the die (e.g., time required for 90% to 100% of the intended
reactants to be consumed by the reaction). The exothermic reaction
may also involve a surface of the substrate (162) adjacent to the
powder. Enablement generally includes exceeding the melting point
of a component material of the powder.
[0055] In the absence of the substrate, the powder has a melting
temperature at which at least one component material of the powder
exists in a liquid phase. The powder melting temperature may be
defined as a temperature at which one or more component materials
of the powder exist in a liquid phase. When more than one component
is melted, the melted components may form an alloy or a new
compound. For example, molten aluminum alloy may react with
titanium powder to form one or all the intermetallics TiAl,
Ti.sub.3Al, TiAl.sub.3. Remaining component materials of the powder
(generally, ceramic particulate) may continue in a solid phase.
Exothermic reactions may improve the wetting of the solid materials
of the powder with the liquid phase materials of the powder.
[0056] The surface of the substrate when not in contact with the
powder has a melting temperature, herein called the substrate
melting temperature. The surface of the substrate may be subject to
the heating and chemical effects of the exothermic reaction and
experience localized melting at its surface or that in contact with
the lower melting temperature surface layer. For example, silicon
in aluminum alloys lowers its melting temperature. If Si is present
in the surface layer with Al, the Si could begin to react with the
surface of the core and diffuse into the core to cause further
melting. Thus, the time at the melting temperatures during
processing is important and must be optimize to avoid melting the
core beyond the surface of the core. Extended times at the melting
temperature could also cause porosity at the interface, which is
detrimental to mechanical and thermal properties.
[0057] In an important class of implementations, according to
various aspects of the present invention, the bulk of the substrate
does not exist as a liquid at any time during the process. To avoid
bulk melting, the elevated temperature discussed above, must be
below the substrate melting temperature. To avoid bulk melting, the
elevated temperature, duration of maintaining the elevated
temperature, extent of any exothermic reaction, and the thermal
capacity of the substrate are selected by analysis, simulation,
and/or test.
[0058] Before, during, and/or after enabling any potential
exothermic reaction, the contents of the die may be densified.
Densification may be accomplished using techniques somewhat similar
to hot press techniques. Hot press techniques apply pressure to
solid materials. In contrast, pressure, according to various
aspects of the present invention, is applied to a melt that
includes liquid. For example, pressure may be applied (214) to the
contents before and/or during any exothermic reaction that may
occur. The pressure (i.e., a mechanical force) may contribute to
initiation and/or maintenance of the exothermic reaction. The
pressure may be applied to a component of the die (e.g., cover 154,
base 156, both cover and base). The pressure may be applied to
force the cover and base of the die toward each other. Pressure may
continue at the same force or at a series of forces. Pressure may
be interrupted briefly and reinstated, for example, to facilitate
movement of the die out of a region for heating (104) and into a
region for cooling.
[0059] By applying pressure to the melt against the substrate
possibly during the exothermic reaction, at least one material of
the melt (e.g., a metal matrix composite, an intermetallic
composite, a liquid metal alloy, a product of the reaction) bonds
with the substrate by forming a metal matrix composite, an
intermetallic composite, and/or an alloy of the substrate. By
applying pressure to the melt, solid particulate material of the
melt may be more thoroughly wetted. Consequently, when the product
is in use, the layer may retain the solid particulate in adverse
conditions.
[0060] The exothermic reaction, presence of the melt against the
surface of the substrate, and/or pressure of the melt against the
substrate may integrate the melt and the surface of the substrate.
Integrating may include local melting of the surface of the
substrate.
[0061] Preferably, densification continues as the contents of the
die cool from a melted liquid component to a solid. For some
substrate and powder combinations, significant shrinking of the
product may occur during cooling from a liquid to a solid. Porosity
may be avoided by continuing densification pressure during cooling.
For example, pressure is maintained on the contents of the die
while the contents of the die are cooled (216). Accelerated cooling
may be accomplished using conventional technologies (e.g.,
introducing chilled gas into the chamber such as from a supply of
liquid nitrogen or merely a gas flow through the chamber).
[0062] After a risk of further significant shrinkage is low,
pressure may be removed (218). Thereafter, the chamber (102) may be
prepared for opening. The die may be removed from the chamber to
facilitate removal of the product from the die. Finally, the
product is removed (220) from the die. Removal may include
disassembly of the die. Removal may include removing a peripheral
component from the remaining combination of the die and the
product, then separating the base and cover components from the
product. The components of the die may be refurbished for reuse or
reused as-is.
[0063] In one implementation, according to various aspects of the
present invention, the product is ready for installation in a next
assembly or ready for use. To meet product specifications,
post-process machining may be unnecessary. In other
implementations, the product is in near-net shape, ready for
post-process machining.
[0064] In an implementation for forming a vehicular brake rotor, a
die serves as a containment vessel for molten metal. All components
of the die may be formed of similar constitution to avoid
differences in thermal expansion. For example, die 150 of FIG. 3
includes peripheral component 152, bottom base component 156, hub
component 158, and top cover component 154 of FIG. 4. In one
implementation, the components have the dimensions and
characteristics described in Table 1 below.
[0065] A peripheral component provides one or more exterior side
walls for a die. The exterior side walls may correspond to a net
shape desired for the product. Circular and oval shapes are
preferred to simplify removal of the product from the die and for
better containment of molten metals. For example, peripheral
component 152 is formed as an empty cylinder having an axis of
circular symmetry through center 302. Peripheral component 152
defines at radius 386 an exterior surface of die 150.
[0066] A base component serves to at least partially close a
peripheral component so that the die may be loaded. A base
component may provide a surface for the application of mechanical
force against the contents of the die. For example, base component
156 is formed as a disc having an axis of circular symmetry through
center 302 that corresponds to the axis of symmetry of peripheral
component 152. Base component 156 forms a circular friction fit
seal at radius 384 with peripheral component 152. Base component
156 includes a circular orifice sized to form a friction fit seal
against hub component 158 at radius 382. The friction fit seal
between components is adjusted such that at operating temperature
expansion does not result in relaxing the friction fit that allows
liquid metal under pressure to leak through the friction fit.
[0067] A hub component provides one or more interior side walls for
a die. The interior side walls may correspond to a net shape
desired for the product. Circular and oval shapes are preferred to
simplify removal of the product from the die. For example, hub
component 158 is formed as a cylinder having an axis of circular
symmetry through center 302. Hub component 158 forms a circular
friction fit seal at radius 382 with base component 156. Hub
component 158 includes a vent 172.
[0068] Vent 172 may be formed by drilling one or more holes into
hub 158. In other implementations, hub component 158 includes more
than one vent of similar construction and orientation.
[0069] A cover component for a die cooperates with one or more
other components of the die to fully contain the contents within
the die. A cover may provide a surface for the application of
mechanical force against the contents of the die. For example,
cover component 154 is formed as a disc having an axis of circular
symmetry through center 401 that corresponds to the axis of
symmetry of peripheral component 152. Cover component 154 forms a
circular friction fit seal at radius 384 with peripheral component
152. Cover component 154 includes a circular orifice 402 sized to
form a friction fit seal against hub component 158 at radius 404
and 382.
TABLE-US-00001 TABLE 1 Component Dimensions (inches) Peripheral
component 152 Wall thickness 3.0 Height 3.0 Outside diameter 18.0
Base component 156 Thickness 1.0 Diameter 12.0 Orifice diameter 6.0
Hub component 158 Diameter 6.0 Height 3.0 Cover component 154
Thickness 1.0 Diameter 12.0 Orifice diameter 6.0 Radius 382 of hub
component 158 1.5 Radius 384 of base component 156 6.0 Radius 386
of peripheral component 152 9.0 Radius 404 of orifice in cover
component 3.0 154 Radius 406 of cover component 154 6.0 Vent 172
diameter 0.0625 inch to 0.25 inch
[0070] Friction fit seals may be formed by mismatching the radii of
the components to be friction fit. The mismatch may be from 0.002
inch to 0.010 inch, preferably about 0.003 to 0.009 inch, more
preferably 0.004 to 0.008 inch. A mismatch of about 0.005 inch is
particularly preferred. The mismatch remains constant at elevated
temperatures with section of the same composition and
cross-section.
[0071] Further features of the invention will be seen from the
following working examples.
Example I
[0072] As a first example, a vehicular brake rotor may be formed
using method 200 as follows. At steps 204 and 208 the powder
comprises powdered aluminum silicon alloy, silicon carbide powder,
and titanium powder. There is no binder in the powder. The AlSi
alloy component material is 59.5% by weight of the type marketed by
Valimet as Grade S-9 having a distribution of particle sizes
defined by D10 4.82 microns, D50 15.23 microns, and D90 37.56
microns. The SiC component material is 38.0% by weight of the type
marketed by ESK as Grade F600-D having particle size from 10
microns to 20 microns. The titanium component material is 2.5% by
weight of the type marketed by ADMA having particle size of about
44 microns specified as 325-mesh. The powder is mixed until uniform
in material distribution. Each powder portion 164 and 165 is spread
to a uniform thickness of 0.010 inch to 0.25 inch, preferably 0.060
to 0.125 inch, more preferably 0.040 to 0.080 inch, most preferably
about 0.050 inch.
[0073] The titanium component material of the powder serves several
purposes. Titanium and aluminum react when the aluminum is in
liquid phase, producing heat (i.e., an exothermic reaction). The
reaction of titanium and aluminum may form titanium aluminide
having a melting point much higher than the elevated temperature of
the chamber. Consequently, some of the aluminum of the powder, now
as titanium aluminide, is no longer in liquid phase, reducing the
risk of leaks through the seals of die 150. The surface of the
layer is subject to the formation of piping structures as the melt
cools, a result of surface deformation due to shrinking. The
formation of piping structures may be inhibited by including the
titanium component material in the powder and applying pressure
during the cooling of the aluminum alloy component.
[0074] At step 206 the substrate is commercially pure aluminum
(wrought aluminum) also referred to as 1100 aluminum in the shape
of a disc about 12.0 inches in diameter and about 0.5 inch thick.
The disc has a central circular aperture about 6.0 inches in
diameter for mounting the disc onto a conventional hub of an
automobile. The commercial aluminum disc has very low porosity.
[0075] At step 210 the die is evacuated using a conventional pump
down scenario to a pressure less than 100 torr. The chamber is back
filled with nitrogen or argon gas and then pumped down a second
time. The chamber is filled with nitrogen or argon gas and then
pumped down a third time. The resulting pressure is less than 100
torr. Prior to evacuation, the powder in the die may be dried. To
accomplish drying, the heating subsystem 112 is programmed with a
set point of between 150 degrees and 250 degrees Celsius,
preferably about 200 degrees Celsius and a hold time of up to 10
hours to 20 hours. The vent is closed by programming the pressing
subsystem to a set point of from 20 psi to 100 psi to smoothly move
the cover from the first position to the second position as
discussed above.
[0076] In another implementation, the powder is pre-dried by any
conventional technique (e.g., vacuum, temperature) before being
spread into the die (204, 208). Consequently, step 210 may be
simplified by omitting the temperature set point and hold time.
[0077] At step 212 the heating system is programmed with a set
point of about 625 degrees Celsius and a hold time of about up to
1.5 hours. A set point of 550 degrees Celsius and a hold time of
about 2.0 hours may also be used. A set point of 750 degrees
Celsius may be used for a period that ends when a temperature of
hub component 158 reaches a temperature from 550 degrees to 625
degrees Celsius; followed at the end of the period by a set point
of 625 degrees Celsius and a duration of from 1.5 hours to 2.0
hours. As discussed above, the substrate comprises a metal in pure
or alloy form having a substrate melting temperature that is higher
than a temperature that melts a component material of the powder.
The contents of the die are heated in a manner that does not result
in bulk melting of the substrate.
[0078] Steps 212 and 214 are performed concurrently.
[0079] At step 214 pressing subsystem 114 is programmed to a set
point of from 250 psi to 600 psi, preferably from 350 psi to 600
psi more preferably 500 psi and a rate to achieve a transition time
of from 0.25 to 1.0 hour, preferably about 0.5 to 1.0 hour, more
preferably about 0.5 hour. The linear displacement of ram 134
within die 150 is limited to the calculated thickness of the
contents when in liquid form. After cooling begins in step 216, an
additional linear displacement into die 150 may be calculated to
account for shrinking of the product during cooling.
[0080] At step 216 the heating system is programmed with a set
point of ambient temperature. Cooling proceeds for about 2.5-3
hours without forced cooling at which time the die is about 300
degrees Celsius. Preferably, densification continues through
cooling in the evacuated chamber.
[0081] In another implementation, after a final soak at 625 degrees
Celsius, the evacuating subsystem is programmed with a set point of
ambient pressure and further cooling is accelerated by introducing
nitrogen into the chamber from a liquid nitrogen source.
[0082] At step 218, the pressing subsystem is programmed with a set
point of zero psi or the pressure is simply released manually. When
the set point is achieved, the die is removed from the chamber.
[0083] At step 220 die 150 is disassembled to remove the formed
disc, which may then be welded to a rotor hub for installation on a
vehicle. The resulting vehicular brake rotor product has a solid
aluminum core about 8 mm thick and a layer about 3 mm thick on each
side comprising silicon carbide in an aluminum matrix for the
braking surface. From 35% to 60% of the braking surface area on
each side of the rotor is a silicon carbide metal matrix composite.
The porosity of the rotor is preferably less than 0.5-1% by volume.
The bond formed between the metal matrix layer and the core is a
metallurgical diffusion bond which eliminates delamination and
spalling. As a result of the method, evacuation degassed the melt,
applied mechanical force maintained the solid silicon carbide
particles in the melt, and applied mechanical force contributed to
bonding of the melt to the surface of the substrate.
[0084] Vehicular brake rotors, formed using the systems and methods
discussed herein, may include a ceramic composite through the full
thickness of the rotor that may include an exothermic component of
the formulation. In other implementations no exothermic component
is utilized in the formulations and the ceramic composite portion
of the rotor exhibits a gradation of the ceramic particle from a
relatively high concentration on the friction surface to a
relatively lower concentration through the thickness of the
rotor.
Example II
[0085] As a second example, a vehicular brake rotor may be formed
in a variation of method 200 as follows. At steps 204 and 208, the
powder comprises aluminum alloy, titanium, and silicon carbide. The
aluminum alloy is of the type generally referred to as 356. Silicon
carbide is included at 35% of the powder. Ninety percent of the
silicon carbide particles are 15 micrometers (.mu.m). The particles
are dry mixed by any conventional mixing technology (e.g., ball
mill with steel balls or ceramic balls, attrition milling,
rotational tumbling, pulse fluid bed). After mixing to a uniform
material consistency, the powder is poured into a die to contain
the mixture when heated to above the melting point of the aluminum
alloy of the powder.
[0086] At step 206, a plate of pure aluminum produced by casting or
rolling is used as the substrate and aluminum alloy 356 used as the
surface layers.
[0087] At step 210, the loaded die is placed in a chamber capable
of evacuation of air. A vacuum pump is used to pump down the
pressure in the chamber to under 100 Torr. The chamber is back
filled with an inert gas (e.g., nitrogen, argon, carbon dioxide,
ammonia). The pump down and back filling cycle is repeated two
additional times.
[0088] Steps 212 and 214 are performed concurrently.
[0089] At step 212 the temperature is raised to the melting point
of aluminum alloy 356, thereby melting the aluminum alloy of the
powder and melting the surface. The titanium powder reacts with the
molten aluminum of the powder and the substrate exothermically (a
thermite type reaction) producing heat that contributes to the
melting of the aluminum alloy of the powder. The heat also enhances
the wetting and/or bonding of the aluminum with silicon carbide
particles and the wetting and/or bonding of silicon with the
silicon carbide particles. The titanium component material of the
powder reacts with the molten aluminum alloy of the powder to form
titanium aluminides (e.g., TiAl, Ti3Al and TiAl3). Some of the
titanium component material of the powder may react with silicon
from the aluminum alloy of the powder to form titanium silicides
(e.g., TiSi2, Ti5Si3).
[0090] At step 214, pressure is applied to the die with a leak
proof fitted ram that prevents any squeeze out of liquid aluminum
metal. Pressures from approximately 1.4 MPa to 35 MPa (200 psi-5
ksi) may be used. The selected pressure is sufficient to cause the
liquid aluminum alloy of the powder to infiltrate the remaining
component material of the powder. The pressure of the ram on the
molten aluminum alloy presses onto the silicon carbide particles.
In a first implementation, sufficient pressure and duration (e.g.,
suitable set points) concentrate the silicon carbide particles at
the surface adjacent to base 156 to provide relatively higher
friction on that surface as compared to the surface adjacent to
cover 154. In a second implementation, a lower pressure set point
and/or shorter duration set point is used compared to the first
implementation. In the second implementation, silicon carbide
particles are more uniform throughout the cross-section of the
product.
[0091] At step 216, heating is discontinued. Cooling proceeds to
ambient temperature.
[0092] At step 220 the product, a vehicular brake rotor, is removed
from the die. The product preferably exhibits substantially no
porosity. No subsequent machining is required for use of the
product in a vehicular brake system.
[0093] The product consists of a composite formed in part by an
exothermic reaction of titanium particles and molten aluminum
alloy, where the composite is a result of heat from the reaction,
auxiliary heating, and pressure pushing ceramic particulate through
molten aluminum.
[0094] In a first implementation, steps 210 continues concurrently
with steps 212, 214 and 216. In a second implementation steps 212,
214, and 216 are performed in ambient atmospheric pressure.
Although both products contain substantially low porosity, lower
porosity can result from the first implementations, compared with
the second.
[0095] Several variations of this second example are
implementations according to various aspects of the present
invention. Any aluminum alloy may be used for the powder and the
substrate as long as the melting temperature of the substrate is
higher than the aluminum alloy used for forming the layers. The
amount of SiC powder spread in the die at steps 204 and 208 may be
increased to increase the surface concentration or loading of
ceramic in the product. Pressing may be performed under reduced
atmospheric pressure, or at ambient atmospheric pressure of an
inert non-reactive gas. Heating may be accomplished using
conventional electrical conduction, magnetic induction, radiant
microwave technologies or resistant heating of the die contents
through an upper and lower ram. The powder may include reactants in
place of or in addition to titanium to support exothermic reactions
with aluminum involving cobalt, iron, magnesium, and/or nickel. The
powder may include ceramic particles of size suitable for the
product function. For vehicular brake rotors, ceramic particles may
be used having size less than 500 microns, preferably less than 100
microns and most preferably 50 microns and below. Any ceramic
particle may be used in place of or in addition to silicon carbide.
In other implementations, ceramic particles are a mix of different
conventional materials (e.g., oxides, carbon, titanium carbide,
chromium nitride, chromium carbide, titanium diboride).
Example III
[0096] As a third example, the process of Example II was repeated,
except the substrate was formed of aluminum alloy 5052, and the
metal alloy powder used in the surface layers was type 4032.
Example IV
[0097] As a fourth example, a vehicular brake rotor may be formed
by a method 200 as follows. At steps 204 and 208, a mixture of
aluminum powder and silicon carbide powder is used.
[0098] At step 206, the substrate is a disc of commercially pure
titanium, pre-cut to dimensions for a vehicular brake rotor.
[0099] Steps 212 and 214 are performed concurrently.
[0100] At step 212, resistance heating includes connecting an
electric current source (e.g., DC, AC, pulse) supply to the ram and
the base component of the die. Electrical current through the die
raises the temperature, melts the aluminum powder, and generates
the plasma. Heat causes the aluminum to melt. The bulk substrate
does not melt because the temperature experienced by the substrate
does not exceed the melting temperature of the substrate. However,
the surfaces of the substrate will start to react with the elements
in the metal matrix composite surface layers, which is beneficial
as it promotes strong bonding at the interface. The melted aluminum
exothermically reacts with the titanium of the substrate to form
gamma titanium (i.e., TiAl) or other aluminides of Ti.sub.3Al or
TiAl.sub.3. During the liquid phase of the aluminum, a fully dense
surface layer forms, consisting of silicon carbide particles in
titanium aluminide (e.g., TiAl, Ti3Al, TiAl3).
[0101] At step 214, pressure is applied in the range of
approximately 10 MPa to 35 MPa (1.45 ksi-5 ksi) to densify the
contents of the die. With pressure applied, the silicon carbide
particulate reinforced titanium aluminide is made nearly fully
dense, substantially free of porosity.
[0102] Several variations of this third example are implementations
according to various aspects of the present invention. The powder
may further include other alloying elements known to produce good
gamma alloys (e.g., chromium, manganese, niobium, silicon). The
feed powder may be prealloyed powders such as TiAl, Ti--Al--Cr--Nb,
etc. in contrast to the initial feed to Ti and Al powders with the
SiC or other ceramic particles such as TiC, TiB.sub.2, B.sub.4C,
etc. The substrate may be formed of another alloy of titanium
instead of the commercially pure titanium such as alloy Ti-6Al-4V
Heating can be supplemented by radiation (e.g., microwave),
magnetic induction, or other technologies as discussed above. The
resistance heating technology between the rams may be replaced with
one or more of the heating technologies discussed above. Any
ceramic particle or combination of ceramic particles may be used in
the powder. The powder mixture may be free of exothermic reaction
components or may include other reactants for exothermic
reaction(s) such as intermetallic formers (e.g., copper, cobalt,
iron, nickel).
Example V
[0103] As a fifth example, a vehicular brake rotor may be formed by
method 200 as follows. Steps 204 through 220 are the same as for
the third example, discussed above with a few exceptions. At step
210 the die is not evacuated. Instead the die and the atmosphere of
the chamber are filled with an ionizing gas (e.g., argon, helium,
nitrogen).
[0104] At step 212 resistive heating is applied for a duration
until (and including) the time the aluminum material component of
the powder reaches its melting temperature. This duration may be
determined by analysis, simulation, and/or test to account for the
mass and heat transfer characteristics of the die and contents of
the die, and any effects of the other system components (e.g., the
ram). Concurrent with the resistant heating melting the aluminum
component a plasma is generated through the melt due to the voltage
which may be pulsed as required to melt the aluminum. The plasma
relatively quickly raises the temperature of the disc and the melt,
facilitating densification. The plasma cleans the surfaces of the
particles of the aluminum before melting and the un-melted, silicon
carbide ceramic, facilitating consolidation and densification.
[0105] The plasma may be generated by conducting a high energy
discharge of electricity from additional power supplies and/or a
capacitor bank through the ram that ionizes the gas in the die.
[0106] In one implementation, resistive heat is omitted from step
212 and melting is accomplished solely by heat associated with the
plasma.
[0107] Several variations of this fourth example are
implementations according to various aspects of the present
invention. The substrate may be any structural metal or alloy of
structural metal as discussed above. Any ceramic particle or
combination of ceramic particles may be used in the powder. The
powder may include other reactants for exothermic reaction(s) such
as intermetallic formers (e.g., copper, cobalt, iron, nickel). The
pressure of the ionizing gas within the die may be below ambient,
ambient, or greater than ambient pressure.
Example VI
[0108] A mixture of aluminum alloy 356 in powder form was mixed
with powders of titanium that was 2% of the sum of the aluminum and
35% SiC with the latter in a particle size with 90% at 15
micrometers (.mu.m). The particles were dry mixed by suitable
mixing including ball mill with steel balls or ceramic balls,
attrition milling, rotational tumbling, pulse fluid bed and poured
into a die to contain the mixture when heated to above the melting
point of the aluminum. On top of the dry mix a solid plate of 356
aluminum alloy was placed. The loaded die was placed in a chamber
that was capable of evacuation of air. A vacuum pump was utilized
to reduce the pressure in the chamber to under 100 Torr. The
chamber was back filled with an inert gas which included nitrogen,
argon, CO.sub.2 and ammonia, and repumped down three times.
Pressure was applied to the die with a fitted ram and the
temperature raised to the melting point of the 356 aluminum alloy.
Pressures from approximately 10 MPa to 35 MPa (1.45 ksi-5 ksi) are
satisfactory to infiltrate the molten 356 alloy through the dry
powder mixture wherein the titanium powder reacts with the molten
aluminum exothermically (a thermite type reaction) producing
internal heat that contributes to the melting of the 356 alloy
powder and enhancing the wetting/bonding of the aluminum and
silicon with the SiC particles as well as the titanium reacting
with the molten alloy that forms titanium aluminides that can
include TiAl, Ti.sub.3Al and TiAl.sub.3, and the silicides
TiSi.sub.2 and Ti.sub.5Si.sub.3 from the Si in the 356 alloy. The
pressure of the ram on the molten 356 alloy presses onto the SiC
particles that if high enough pressure and held long enough will
concentrate the SiC particles on the bottom surface that can
subsequently provide higher friction on that surface. If this
pressure is in the lower range and the pressure removed after
sufficient time for the molten top 356 layer to infiltrate the SiC
particle bed, the SiC will be uniform through the cross-section of
the formed composite. Heat power is turned off and the molten 356
alloy cooled to room temperature and the rotor removed from the
die. Typically the surface of the melt molded rotor is sufficient
and the molding to tolerance that no subsequent machining is
required for mounting the rotor in the brake system.
[0109] The exothermic heating between the titanium particles and
the molten aluminum coupled with the auxiliary heating and the ram
pressure pushing the molten aluminum that is melted on top through
the composite results in a composite with a residual porosity less
than 2% and most often less than 1%, and typically less than 0.5%
which is an ideal composite for brake rotors.
[0110] One iteration that enhances the probability of low porosity
is to perform the pressing under reduced pressure. Removing any
residual gas by reducing the pressure contributes to no residual
gas pockets being trapped leading to residual porosity.
[0111] Any aluminum alloy can be used as the matrix, any volume
loading of SiC or any ceramic particle composition can be used, and
the pressing operation can be performed under reduced pressure or
at ambient pressure of an inert non-reactive gas. The heating
method can be by conduction, induction, microwave or any suitable
means to raise the mixture in the die containment to above the
melting point of the aluminum. Also, reactions other than the
titanium and aluminum can be used to generate an exothermic heat
generating reaction. Some example exothermic reactions with the
molten aluminum include Fe, Ni, Co, and Mg. The particle size of
the ceramic filler can be any size, but smaller size particles
perform best in braking operations, therefore particle sizes under
approximately 500 .mu.m should be used and preferably under 100
.mu.m and most preferably under 50 .mu.m. Also, mixtures of ceramic
particles can be used to achieve custom properties as the friction
surface of the composite rotor. For example, oxide particles aid in
resisting heat flow into the body of the rotor in emergency braking
scenarios, carbon can also react with the titanium to form a TiC
particle, chromium nitride or carbide are known reducers for
friction while TiB.sub.2 increases thermal conductivity. It is
therefore possible to customize the alloy and particle compositions
to produce brake rotors with customized properties that includes
friction coefficient and thermal conductivity of the surface.
Example VII
[0112] Referring to FIG. 6, a disc of CP titanium precut to brake
rotor dimensions was produced, and a mixture of aluminum and SiC
powder as previously described was applied to both sides at step
602. Additional mixture was placed in the bottom of a die, the CP
Ti disc placed in the die, and a layer of the mixture was added to
the top surface of the CP Ti disc in the die at step 604. A ram was
added to fit the die, in a chamber, and the chamber was evacuated
and back filled with inert gas at step 606. Heating was performed
at step 608, while pressing, by connecting a high amperage, e.g.,
25,000 amp, alternating electrical current power supply to the top
ram and the bottom of the die which may also be a fitted ram. The
electrical power applied raised the temperature on the surfaces of
the CP Ti disc that melted the aluminum in the powder mix. Heating
was continued to cause the aluminum to exothermically react with
the titanium while pressure was continued to be applied in the
range of approximately 10 MPa to 35 MPa (1.45 ksi-5 ksi). During
the liquid phase of the aluminum, a fully dense surface layer
consisting of SiC particle in titanium aluminide that can include
TiAl, Ti.sub.3Al or TiAl.sub.3 which the TiAl is referred to as
gamma titanium was formed. Other alloying elements known to produce
good gamma alloys such as Nb, Cr, Mn, Si, etc. can be added to the
Al--SiC powder mix. The melting of the aluminum reacts with the CP
Ti disc to form a metallurgical bonded surface layer to the CP Ti
disc. With the applied pressure the SiC particulate reinforced
titanium aluminide is fully dense and free of porosity, a highly
desirable feature for brake rotors. The entire heating process was
quite rapid, taking only about 60 seconds. The electrical current
power supply was disconnected, and the die assembly containing the
coated brake rotors was blasted with cooling inert gas or air,
depending on temperature, for about 5 minutes at step 610.
[0113] The substrate can be CP Ti disc or any titanium alloy and
heating can be performed by pulsed or continuous electrical
current, microwave, induction, radiation, etc. as an alternate to
the resistant heating between the rams. Also any ceramic particle
or combination of ceramic particles can be utilized and that other
exothermic heating reactions which can be intermetallic formers
other than aluminum can be used such as Cu, Fe, Ni, Co. It is also
recognized that the resistant heating can be accomplished with any
frequency current, direct current as well as pulsing the
current.
Example VIII
[0114] The same set-up as Example VII was utilized. When the
resistive heating had brought the aluminum to its melting point, a
high discharge of power from additional power supplies or capacitor
bank was induced and with a residual atmosphere of argon or helium
a plasma becomes generated that instantaneously raised the
temperature of the disc and the surface coating. With pressure
applied the aluminum reacted with the titanium surface that
produced a fully dense pore free surface conversion layer. This
process is often referred to as spark plasma sintering (SPS). The
plasma generated cleans the particle surfaces and enhances
consolidation that is well known in the art. If sufficient pulse of
power is initially added, no prior resistant or other form of
heating is required that shortens the overall consolidation time to
produce the fully dense pore free surface/coating composite with
the SiC.
[0115] The titanium can be any alloy, the ceramic particles can be
singular or mixed to produce a custom surface property and that
heat generating reactions including intermetallics other than
aluminum such as Cu, Fe, Ni, Co can be used and the ionizing gas
can be other than argon or helium to produce the plasma, and the
process can be carried out at reduced pressure, ambient pressure or
even positive pressure.
Example IX
[0116] As yet another example, a vehicular brake rotor is formed
using an integrated one piece hub-disc. The cost of the brake discs
becomes enabling to their use in automobiles. After a composite
disc is fabricated as a sandwich of a composite layer on each side
of a monolithic core or as a composite through the entire thickness
of the disc, a hub must be welded to the disc that permits mounting
to the wheel assembly. The separate making of the hub and its
welding to the disc which may require a subsequent machining
operation for any loss in parallelism due to the welding operation
are all costly procedures. An alternative process that reduces cost
is to utilize a casting, forging, or alternative process that
results in a disc and hub produced in one piece which is then
coated with a layer of composite on the disc portion of the
assembly. The integrated hub-disc one piece is placed in a die set
such as graphite that contains an outer ring with a solid graphite
disc for the base. A mixture of aluminum-silicon carbide (Al--SiC)
powder is placed in the bottom of the die, on top of the solid
graphite disc piece with an additional graphite plug for the hole
on the bottom of the hub-disc opposite the side with the protruding
hub. After the one piece hub-disc is placed in the die, an
additional mixture of Al--SiC is placed on the disc portion of the
one piece hub-disc followed by a die top consisting of a hole to
cover the hub. The loaded die is placed in a chamber which is
closed to the atmosphere. The residual atmosphere is exchanged via
purging with an inert gas such as argon or nitrogen, or evacuated
by applying a vacuum. The die is heated to a temperature that the
Al alloy metal matrix composition melts but the one piece hub-disc
does not melt. Pressure is applied in the range of (1 to 8 MPa) to
the die that consolidates the Al--SiC alloy and metallurgically
bonds to the core one piece hub-disc. Pressure remains applied
throughout the cooling cycle that produces a layer of Al--SiC alloy
with a porosity of less than 2%. The percent of SiC in the
composite layer can be varied from approximately 25% to 65% and
preferably 35% to 50%.
Example X
[0117] As another example, the process of example XI was repeated,
except the formed brake disc was subjected to heat treatment to
increase hardness and improve its mechanical properties. Preferably
the heat treatment comprises a so-called T6 temper also known as
solution heat treating plus aging in which the sandwich composite
is first allowed to cool naturally. The sandwich composite is then
heated to a temperature of 510.degree. C. in a high temperature
oven. After 4 hours, the casting is removed from the oven and
quickly quenched. The casting is then moved to a low temperature
oven and held at 170.degree. C. for 12 hours and then air
cooled
Example XI
[0118] An additive manufacturing (AM) process can be utilized
following the teachings, for example of our prior U.S. Pat. No.
8,203,095, the contents of which are incorporated herein by
reference, to form a composite that is the full thickness of a
rotor or as a surface reaction or cladding. Additive manufacturing
consists of a CAD driven power beam source that melts powder or
wire in a continuously moving spot that builds material
layer-by-layer or transforms a surface that can build-up a clad or
coating layer. The processing to produce a power beam include
electron beam (e-beam), laser or plasma transferred arc (PTA)
wherein the work piece is one electrode and a torch head is the
other electrode that ionizes a gas such as argon or helium that
forms the plasma beam between the head and the part. A brake rotor
can be built with any of the power beam processes by co-feeding the
metal and ceramic particle to build layer-by-layer. For example,
aluminum wire or powder can be fed to the melt pool spot with a
ceramic powder also co-fed to produce the composite. The CAD
program can be set to build an entire rotor in the thickness
desired at virtually any ceramic particle volume desired that
includes that most desirable for brake rotors of 35-60%. The
ceramic can be varied in concentration at the start of the build
and decreased until the final layer thickness is built and again
added at a high concentration to provide high wear resistance on
each surface of the rotor.
[0119] Any power beam process can be used to build the brake rotor
layer-by-layer in any desired ceramic particle loading including
graduations to each surface in aluminum or its alloys, titanium or
its alloys, or other structural metals that can be used as rotors
such as Cu, Fe, Ni, Co, Zn.
[0120] In a specific case pure 1100 aluminum alloy powder can be
used as the matrix and SiC particles as the ceramic filler. The PTA
system can be utilized as the power beam source. A feed mixture of
1100 aluminum powder and SiC powder can be used to produce a
composite composition of 40 volume % SiC in the aluminum matrix.
The PTA system can be set for a build rate of 5 lbs/hr to build the
entire rotor in one hour since a typical rotor of the approximate
size of 310 mm diameter with a 150 mm open center hole weighs less
than pounds in an aluminum-SiC composite.
Example XII
[0121] The PTA AM system can be utilized to apply an aluminum-SiC
composition to the surface of a CP Ti disc precut to size. The CAD
system and power to the PTA can be set to just melt a few mm of the
surface of the disc with a powder feed mixture of 356 aluminum
alloy and SiC powder to produce a layer of titanium aluminide
containing 50 volume % SiC. The PTA beam power can be controlled to
compensate for the exothermic reaction of the aluminum with the
titanium so as not to melt too deep a layer of the CP Ti disc.
[0122] This power beam processing can form the surface clad layer
in base materials other than titanium. Base materials that include
aluminum and cast iron or steels can be used. If aluminum were the
base rotor material, titanium powder can be used to produce the
exotherm to form the composite clad layer. Standard cast iron
rotors can be used to form surface layers of FeAl, Fe.sub.3Al or
FeAl.sub.3 as well as titanium to form the titanium intermetallic
TiFe.sub.2 or TiFe.
[0123] Any power beam process can be used to melt the surface of
the titanium disc that could be any titanium alloy. Intermetallics
other than aluminum such as Cu, Fe, Ni, Co, Zn could be used to
form the clad reactive layer surface along with any single or mixed
composition ceramic particle as well as base alloys of cast iron or
steels onto which the intermetallic-ceramic particle alloy may be
formed.
Example XIII
[0124] A friction stir welding system can be modified to provide a
powder feed into the rotating head that is controlled with a CAD
system that provides controlled downward pressure on the rotating
head as well as controlled speed forward in a tool path. A mixture
of 50 volume % SiC powder and aluminum powder can be fed to the
rotating head that has sufficient downward pressure that the
friction causes the surface of the titanium substrate to melt with
the aluminum in the feed, generating exothermic heat contributing
to the melting under the friction stir process rotating head that
encapsulates the SiC particles in the molten layer that on
solidification as the rotating head moves forward in its CAD guided
tool path, a surface of titanium aluminide containing SiC
particulate is produced free of porosity.
[0125] To speed the forward movement of the rotating head with
controlled downward pressure, in front of the head, additional heat
can be applied such as a flame torch, laser beam, electron beam,
plasma transferred arc (PTA) beam, induction coupling, or a focused
radiation beam such as from a tungsten-quartz lamp. The rotational
speed and downward pressures rotating head can be somewhat relieved
to generate a molten path on the titanium rotor with the additional
heat applied in front of the FSP processing. With the CAD control
of the FSP the composite layer formed can be over the entire
surface of a CP Ti disc precut to size for a brake rotor, or just
in a path that is the width of the brake pad.
[0126] The thickness of the titanium aluminide-SiC layer can be
controlled by the depth of the melt generated by the rotating head
speed and its downward pressure, the exotherm from the Al--Ti
generated heat as well as any auxiliary heat supplied in front of
the forward moving rotating head. Surface layer thickness can be as
little as approximately 0.25 mm up to approximately 5 mm. It is
possible with a very hard and strong rotating head with sufficient
downward pressure and rotating speed to melt depths of 10 or more
mm. Such a friction stir capability permits the entire
cross-section of a brake rotor to be produced as a composite by
friction stir processing (FSP) which is possible in a single pass
or multiple passes analogous to layer-by-layer as delineated in
additive manufacturing.
[0127] While this example is given for a CP Ti base rotor, other
base metals such as aluminum, cast iron, steels, copper, and zinc
can be utilized to produce a composite with single or mixed ceramic
particles by friction stir processing (FSP).
[0128] The foregoing description discusses implementations and
preferred embodiments of the present invention, which may be
changed or modified without departing from the scope of the present
invention as defined in the claims. The examples listed in
parentheses may be alternative or combined in any manner. The
invention includes any practical combination of the structures and
methods disclosed. As used in the specification, the words
`process` and `method` are synonymous. As used in the specification
and claims, the words `having` and `including` in all grammatical
variants are open-ended and synonymous with `comprising` and its
grammatical variants. While for the sake of clarity of description
several specific implementations and embodiments of the invention
have been described, the scope of the invention is intended to be
measured by the claims as set forth below.
* * * * *