MACHINEABLE METAL-MATRIX COMPOSITE AND METHOD FOR MAKING THE SAME
Field of the Invention
The present invention relates to metal-matrix composites and methods for their manufacture and more particularly to metal-matrix composites including uniformly distributed ceramic particles wherein at least about 50 percent of the particles are free of bonding to one another.
BACKGROUND FOR THE INVENTION
Metal-matrix composites and methods for producing those composites are disclosed in U.S. Patent Nos. 5,511,603 and 5,702,542 which are incorporated herein in their entirety by reference. As disclosed therein the manufacturing methods include providing a ceramic preform having a uniform distribution of ceramic particles sintered to one another at the points of contact. The particles have an average particle size of no greater than 1.0 μ, and at least one-half of the volume of the preform is occupied by porosity. The preform is then placed into a mold and contacted by molten metal. The molten metal is forced into the pores of the preform and permitted to solidify to form a solid metal-matrix composite. This composite is machineable with a high-speed steel (HSS) bit for greater than about one minute without excessive wear occurring to the bit. Metal-matrix composites may include Al., Li., Be., Pb., Au., Sn., Mg., Ti., Cu., and Zn. Preferred ceramics include oxides, borides, nitrides, carbides, carbon or a mixture thereof. Inert gas pressures of less than about 3,000 psi can be used to infiltrate the preform.
' The metal-matrix composites disclosed in the aforementioned patents combine the high-strength stiffness and wear resistance of ceramics with the machineability, toughness and formability of metals. A small characteristic reinforcement size of less than about 1 μ in conjunction with a large volume of porosity and a substantially uniform distribution of ceramic particles in a sintered preform are employed to provide improved temperature strengths increased modulus and excellent machineability and ductility even at high ceramic loadings. Such compacts have been machined using only high-speed steel milling, drilling
and tapping tooling without experiencing difficulty. Excellent surface finishes were produced.
The metal-matrix composites, exhibit high-strength at room and elevated temperatures since the small reinforcement size and interparticle spacing meets the criteria for dispersion strengthening. The small uniformly distributeα-Ger-amic particles permit the composite to behave more like a metal than atypical metal-matrix composite permitting their use in applications requiring greater ductility, toughness and formability. The particular metal infusion procedures are adaptable to multiple alloy and ceramic pairings and permit greater latitude for increasing the tensile modulus as loadings approach 50 volume percent. Specific reinforcement ceramics and volume fractions can be selected which will permit designable engineered properties dictated by the application including high elastic modulus, strength and ductility.
Notwithstanding the advantages of metal-matrix composites, of the aforementioned metal-matrix composites, it has been found that a significant improvement can be made in metal forming with a product and process in accordance with the present invention.
SUMMARY OF THE INVENTION
In essence the present invention contemplates a method for producing a metal-matrix composite comprising a uniform distribution of sintered ceramic particles having a particle size of no greater than about one micron and a metal or alloy substantially uniformly distributed with the ceramic particles. Those particles comprise at least 15 volume percent of the metal matrix which is capable of being machineable with a high-speed steel (HSS) bit for greater than about one minute without excessive wear to the bit. In a metal-matrix composite in accordance with the present invention at least 50 percent and preferably about 80% of the particles are free of bonding to another particle.
The invention also contemplates a method for forming a metal-matrix composite comprising the steps of forming a sintered ceramic preform including a network of uniformly
distributed ceramic particles having a particle size of one micron or less and being bonded together at their points of contact by sintering to provide a preform. After forming a preform, the preform is placed in a mold and infiltrated with molten metal. The molten metal is then solidified to form a shaped body. This shaped body is then subjected to sufficient strain to eliminate at least 50 percent of the bonds in the network. For example, the shaped body may be subjected to strain by upsetting, extrusion, twisting or possibly by heating and cooling or by passing a sheer wave such as an acoustic wave through the body. The shaped body is then subjected to a metal forming step such as forging or semisolid forming.
In one preferred embodiment of the invention, a metal matrix composite is produced by forming a sintered ceramic preform which includes a network of uniformly distributed particles having a particle size of between about 0.01 and 0.5 μ. These particles are then bonded together by sintering to thereby form a ceramic preform. The ceramic preform is then placed in a mold and infiltrated with molten aluminum. The infiltrated aluminum is then solidified to form a shaped body or billet. This billet is sliced into discs and the discs heated to a temperature of between about 450°C and about 600°C and subsequently subjected to sufficient strain to eliminate at least about 50% of the sintered bonds as for example by compressing the discs by between about 2% to 10% along its axis to form a first product. The compressed discs or first product is then semisolid or wrought formed into a piston shape or other preselected shape which is net formed and ready for final machining.
DETAILED DESCRIPTION OF THE INVENTION
Machineable metal-matrix composites (MMCs) are derived from combining ceramic particles of no greater than about one micron with molten metal in an extremely uniform manner. By employing smaller ceramic particles of submicron size and distributing them throughout the metal matrix so as to avoid agglomeration, both high ductility and strength can be provided to the composite without limiting machineability. In one embodiment of the invention at least 80 percent of the ceramic particles are uniformly distributed on a scale of 3 times the particle size and more preferably at least 90 percent of the ceramic particles are uniformly distributed on a scale of twice the particle size. This degree of fine particle
distribution virtually eliminates large inclusions and agglomerates which detract from the ductility, strength and machineability of the composite.
The MMCs can be. made from many different combinations of matrix material and reinforcing particles to develop whatever special set of properties is required for each application. This, invention-contemplates employing ultra high-strength metal matrixes including those having yield strengths, of about 70 to 2000-MPa. Such metals include for example cobalt and its' alloys, martensi tic stainless steels, nickel and its alloys and low-alloy hardening steels. High-strength metals and alloys are also potential candidates for the matrixes including tungsten, molybdenum and its alloys, titanium and its alloys, copper casting alloys , bronzes, coppers, niobium and its alloys and super alloys containing nickel, cobalt and iron. Medium strength metals and alloys can also be considered including hafnium, austenitic stainless steels, brasses, aluminum alloys between 2000 and 7000 series, beryllium-rich alloys, depleted uranium, magnesium alloys, silver, zinc, casting alloys, coppers, copper nickels, copper nickel zincs and other materials having yield strengths of about 40 to 690 MPA. Finally the invention optionally employs low-strength, low-density alloys for the matrixes of the invention. Such metals are represented by gold, cast magnesium alloys, platinum, aluminum alloys of the 1,000 series, lead and its alloy and tin and its alloys. These materials have a yield strength of only about 5 to 205 MPA. Most desirably, the invention employs lightweight metals and those which are relatively inexpensive and widely available such as aluminum, lithium, beryllium, lead, tin, magnesium, titanium and zinc and metals which have superior electrical properties such as copper, silver and gold. All of these selections can be provided in commercially pure or alloyed form. Specific alloys which have be recognized to have particular usefulness in MMCs include Al-lMg-0.6Si., Al-7 Si-lMg., Al-4.5Cu., Al-7 Ng-2 Si., and Al-Fe-B-Si.
Although alloys and commercially pure metals can be employed to produce the matrixes, a pure metal is generally the matrix of choice since ceramic dispersion strengthening is desired. A pure metal also offers enhanced corrosion resistance as compared to alloys and eliminates the effect of overaging of precipitates. Pure metals also boost elevated temperature capability by increasing the homologous melting point over comparable
alloys. Finally, pure metals eliminate the difficulties associated with microsegregation and macrosegregation of the alloying elements in non-eutectic alloys during solidification.
The ceramic or second phase constituents of the metal-matrix composites are desirably of a size which does not interfere with machining by HSS tooling. For example, machineability can be preserved only if the particles are less than about one micron although a range of about ,0.01 to, 0.5. microns is preferred. The ceramic particles should be thermally and chemically stable for the time and temperature of the particle fabrication process and environmental conditions observed.
These ceramic particles should not be decomposed at high temperatures, nor react with a metal matrix. If they tend to diffuse into the matrix, diffusion of the reinforcement must be slow so that the strength of the composite does not seriously degrade. Ultrafine reinforcement particles having a volume fraction of about 20 to 40 percent are particularly advantageous in yielding composites with improved Young's modulus, ductility and machineability.
Examples of second phase ceramic candidates include borides, carbides, oxides, nitrides, silicates, sulfides and oxysulfides of elements which are reactive to form ceramics including, but not limited to, transition elements of the third to sixth groups of the periodic table. Particularly useful ceramic-forming or intermetallic-compound forming constituents include aluminum, titanium, silicon, boron, molybdenum, tungsten, niobium, vanadium, zirconium, chromium, hafnium, yttrium, cobalt, nickel, iron, manganesium, tantalum, thorium, scandalum, lanthanum and rare earth elements. More exotic ceramic materials include titanium diboride, titanium carbide, zirconium diboride, zirconium disilicide and titanium nitride.
Carbon-based ceramics can also be useful as the ceramic phase including natural and
) synthetic diamonds, graphite, fuUerenes, diamond-like graphite etc. Certain ceramics because of their availability, ease of manufacturing, low cost or exceptional strength in inducing properties are most desirable. These include Al2O3, SiC, B4C, MgO, Y2O3, TiC, graphite,
diamond, SiO2, ThO2, and TiO2. These ceramic particles desirably have an aspect ratio of no greater than about 3 : 1 and preferably no greater than about 2:1 but can be represented by fibers, particles, beads and flakes for example. However, particles are preferred for machineability.
Alternatively, the ceramic reinforcement can have aspect ratios ranging from equiaxed to platelets and spheredized configurations. The particle size distribution can range from mono-sized to a gausean distribution or a distribution having a wide tail at fine sizes. These particles can be mixed using a variety of wet and dry techniques including ball milling and air abrasion.
The preferred binders employed in connection with the ceramic reinforcements can include inorganic colloidal and organic binders such as sintered binders, low temperature and high temperature colloidal binders. Such binders may include polyvinyl alcohol, methal cellulose, colloidal alumina and graphite.
A composite material was prepared having a commercially pure aluminum matrix including 25 volume percent Al2O3, about 0.2 micron average particle size on a population basis. As a preliminary step, the raw materials were weighed out as follows:
Reinforcement :A 16SG , calcined Al2O3ι Alcoa Industrial Chemical Division, 259.8 grams.
Carrier: POLAR distilled Water, Polar Water Company, 1205.8 grams.
Filler: Micro 450 (M-450) graphite, Asbury Graphite Mills, Inc., 184,6 grams.
Colloidal Binder: Inorganic NYACOL, AL20, high temperature coating/binder,
Nyacol Products, Inc., 86.0 grams.
This mixture was combined in a mill using the following mill parameters: slurry solids content of 10% and mill fill level of 30%. The slurry batch was milled for about 23 to 25 hours, removed from the mill, and disposed in a pressure filtration unit. The slurry was filtrated at 350 psi for about 36 to 60 hours. When filtration was complete, the green preform was removed from the filtration unit. It was measured to have dimensions of about 4.9 cm in diameter xl2 cm long. The green preform had a reinforcement loading of about 22 vol. %.
The green preform was then dried at ambient conditions until a weight loss of at least about 25 wt. % had been achieved.
The dry preform was then placed in a furnace and fired according to -the following scheduled:
Ramp Ramp . Hold Hold , ...
Ramp Rate Time Temp Time
Seq. (°C/hr) (hr) ( Q (hr)
3/4 50 12 900 30
5/6 50 6 1,200 1.5
7/8 100 12 22 24
The fired preform had a loading of about 25 vol. % of sintered ceramic particles. It was removed and inspected, and a weight loss of about 40 wt. % was noted. This weight loss insured that all filler material had been removed.
A mild steel infiltration crucible was then prepared by coating with a graphite wash coating DAG 154 Graphite Lubricating/Resistance Coating, available from Achesion Colloids Company. The interior of the crucible was then lined with GRAFOIL graphite paper, Grade GTB available from UCAR Carbon Company, Inc. The fired preform was inserted into the lined crucible and a preform support rod was inserted to prevent floating. The crucible was then inserted into the pressure infiltration unit, which was custom built. The pressure infiltration unit was evacuated , and then preheated using the following heat cycle:
Ramp Hold Hold
2 200 0.05
3/4 8 700 2
Approximately 650 grams of commercially pure aluminum (99.9% aluminum, 2 to 5 shot available from Alcoa) was then melted in an electrical resistance furnace and covered with Flux No. 770 cover Flux, available from Asbury Graphite Inc. The infiltration unit was then back-filled with argon. The crucible was removed from the pressure, infiltration unit, and the molten alloy was poured into the crucible which caused the argon to bubble to the top of the crucible. The crucible was then placed into the pressure infiltration unit, and it was again evacuated. After evacuation, the unit was pressurized with argon to about 2,150 psi in about 40 to 80 seconds and held for five minutes. The unit was then vented, and the crucible was placed onto a water-cooled chill at the bottom of the pressure infiltration unit. The unit was once again repressurized to 1,000 psi for solidification. The mixture was permitted to cool for about one hour until directionally solidified. The sample was removed from the pressure infiltration unit, the crucible was cut off, and the alloy head was removed.
Under a scanning electron microscope, a fracture surface of one sample of the above composite was visually inspected at 35,000x. The observed particle size was found to be about 0.05 to 0.4 microns, with 0.2 microns being typical, and an interparticle spacing of about 0.05 to 0.4 microns was measured.
In the practice of the present invention, it is important to break up the sintered network, i.e. the bonds at the points of particle to particle contact before wrought forming or semisolid forming. As used herein, wrought forming means forging, extrusion, hot rolling and related processes. This step is typically done by a so-called upset step wherein a billet of a metal matrix composite such as the one prepared above, is heated to a temperature below the melting temperature of the metal and then deformed with a component of shear deformation. For example, a pure aluminum based metal matrix composite is heated to about 450 - 600° C. The matrix billet is then compressed along its longitudinal axis by about two to about 10% . hi practice, the matrix is subjected to a strain rate of about 10"1 to a cost limited strain rate and preferably a strain rate of between about 4.44 x 10"3/sec. to about 6.0 x 10"2 sec. For example, at a strain rate of 4.44 x 10"3/sec, l V2 inch diameter billet underwent a 9.29% reduction in height over about 19 seconds. For aluminum, at a strain rate of about 1/sec. or greater significant problems with cracking occurred at a deformation of about 20%.
For manufacturing purposes, it is desirable to complete the so-called upset or other steps in breaking the particle to particle bonds as quickly as possible. However, it is presently believed that the strain rate can be tailored to provide the amount of prior strain needed for subsequent wrought or semisolid forming within an optimum time schedule. It is believed that an optimal strain rate can be applied to provide sufficient deformation without cracking for any particular application. It is also believed that the break-up of the bonds can be accomplished by deformation of a body so that the bonds are broken throughout the body.
It has also been found, that without preliminary strain, a billet as described above will exhibit severe cracks upon deformation of about 10 to 20%>. By contrast, the same materials are capable of as much as 80 to 90% deformation without cracking after being subjected to sufficient strain to break-up essentially all of the bonds as for example 50 to 80% or more. Therefore, final products can be produced when the net shape includes displacements of at least 25%> without cracking and when portions of the final shape have been displaced by at least 50% and up to 80%-90% all without cracking.
The degree of break-up of the bonds was shown by removing the aluminum metal from the matrix by dissolving the aluminum in sodium hydroxide solution . After removal, the ceramic particles remained and upon examination showed no perceptible bonding. By contrast, removal of the metal from billets which had not been upset or otherwise subjected to strain and which had not been subjected to subsequent forming techniques left a residue of bridged particles and in essence a return to the original preform.
Example 1 Material: 2024 aluminum with 30 volume percent sub-micron alumina.
Weight: 427 grams
Diameter: 1.590 inches
Length: 4.140 inches
Volume: 134.7 cc
Density: 3.170 gram cc
Upset temperature: 450 C
Upset strain rate: 4.44x1 ONsec.
Upset total strain: 9.29%
Semi-solid forming temperature: 725C
Semi-solid ram rate: 80 inches /sec.
Semi-solid die temperature: between 350 and 600C
Semi-solid flow stress: between 270 and 3730 psi
Yield Strength: 441 MPa
Ultimate tensile strength: 538 MPa
Tensile elongation: 2.5%
Young's modulus: 144 GPa
Thermal expansion: 16.5 ppm/C '
For comparison, the handbook of mechanical performance of 2024-T4 aluminum is:
2024 yield strength: 325 MPa 2024 ultimate tensile strength: 470 MPa 2024 tensile elongation: 20.0% 2024 Young's modulus: 70 GPa 2024 thermal expansion: 22.5 ppm/C
Example 2
Material: 6061 aluminum with 30 volume percent sub-micron alumina.
Weight: 375 grams
Diameter: 1.573 inches
Length: 3.816 inches
Volume: 121.5 cc
Density: 3.09 gram/cc
Upset temperature: 450C
Upset strain rate: about 6x10"2/sec
Upset total strain: about 10%
Semi-solid forming temperature: 725C
Semi-solid ram rate: 80 inches/sec
Semi-solid die temperature: between 350 and 600C
Semi-solid flow stress: between 270 and 3730 psi
Yield Strength: 255 MPa
Ultimate tensile strength: 400 MPa
Tensile elongation: 5.0%
Young's modulus: 124 GPa
Thermal expansion: 16.5 ppm C
For comparison, the handbook of mechanical performance of 6061-T4 aluminum is:
oυoi yieiα strength: 145 MPa
6061 ultimate tensile strength: 240 MPa
6061 tensile elongation: 22.0%
6061 Young's modulus: 70 GPa
6061 thermal expansion: 22.3pρm C
Example 3
Material: 1090 (pure) aluminum with 30 volume percent sub-micron alumina.
Weight: 354 grams
Diameter: 1.570 inches
Length: 3.700 inches
Volume: 117.4 cc
Density: 3.02 gram/cc
Upset temperature: 600C
Upset strain rate: about 6 x 10"2/sec.
Upset total strain: about 10%
Semi-solid forming temperature: 725C
Semi-solid ram rate: / 80 inches/sec.
Semi-solid die temperature: between 350 and 600C
Semi-solid flow stress: between 270 and 3730 psi
Yield Strength: 207 MPa
Ultimate tensile strength: 225 MPa
Tensile elongation: 2.5%
Young's modulus: 83 GPa
Thermal expansion: 16.5 ppm/C
For comparison, the handbook of mechanical performance of 1090 (pure) aluminum is:
2024 yield strength: 35 MPa 2024 ultimate tensile strength: 90 MPa 2024 tensile elongation: 70.0% 2024 Young's modulus: 70 GPa 2024 thermal expansion: 23 ppm/C
While the invention has been disclosed in connection with its preferred embodiments, it should be recognized and understood that changes and modifications may be made therein without departing from the scope of the appended claims.