WO1992003585A1

WO1992003585A1 - Composite material containing spinel in a metal matrix and process for its preparation

Info

Publication number: WO1992003585A1
Application number: PCT/CA1991/000292
Authority: WO
Inventors: Mukesh Jain; Sadashiv Kashinath Nadkarni; Alan David Mcleod
Original assignee: Alcan International Limited
Priority date: 1990-08-17
Filing date: 1991-08-19
Publication date: 1992-03-05
Also published as: AU8331891A; ZA916428B

Abstract

An aluminum matrix composite material is produced containing spinel as the reinforcing material. The spinel which is used has particle sizes in the range of 5-20 νm and may be of variable composition of MgO:Al2O3 containing 50 to 95 mole % Al2O3 and 50 to 5 mole % MgO. These composites have properties superior to those of the same alloy reinforced with alumina particles.

Description

Composite Material Containing Spinel in a Metal Matrix and Process for its Preparation

Technical Field .

This invention relates to metal matrix composite materials and, more particularly, to the preparation of aluminum matrix composite materials containing spinel. Background Art

Metal matrix composites typically are composed of reinforcing particles such as fibres, particulates, powder or the like that are embedded within a metallic matrix. The reinforcement imparts strength, stiffness and other desirable properties to the composite, while the matrix protects the reinforcement and transfers load within the composite. The components, matrix and reinforcement, thus cooperate to achieve results improved over what either could provide on its own.

There has been much development of met_^l matrix composites in recent years and an important improvement is described in Skibo et al, U.S. Patent 4,786,467, issued November 22, 1988. That patent describes a method for overcoming a problem of wetting the reinforcing particles with the metal matrix material. In that process, the non- metallic reinforcing particles are added to the molten metal and these are mixed together to wet the molten metal to the particles, under conditions that the particles are distributed throughout the volume of the melt and the particles and the metallic melt are sheared past each other to promote wetting of the particles by the melt. The mixing occurs while minimizing the introduction of gas into, and while minimizing the retention of gas within, the mixture of particles and molten metal, and at a temperature at which the particles do not substantially chemically degrade in the molten metal in the time required to complete the step of mixing. The resulting mixture is cast at a casting temperature sufficiently high that substantially no solid metal is present.

The metallic material is typically an aluminum alloy and the non-metallic particles are typically metal oxide, metal nitride, metal carbide or metal suicide. The most preferred composite materials are silicon carbide and aluminum oxide particulate reinforcement.

Aluminum alloys reinforced with oxides such as aluminum oxide produce a stiffer and stronger alloy.

However there is a problem with the use of aluminum oxide in that it reacts with magnesium present in the alloy to form spinel (MgO.Al₂0₃) at the interface. The reaction involved is as follows: 3Mg + 4A1₂0₃ → 3(MgO.Al₂0₃) + 2A1

The free energy of reaction is AG° = -215 kJ at 727°C.

The loss of Mg impairs the age hardening response of the matrix and results in loss of compositional control. The presence of the spinel reaction product also acts to increase the viscosity of a composite melt, and make recycling more difficult.

M.E. Fine et al., "Investigation and Synthesis of High Temperature and Increased Stiffness RSP Aluminum Alloys", AFOSR-TR-89-0081, 1989, describes composites having an aluminum-magnesium alloy matrix reinforced by particulate spinel of very small particle size in the order of 30-150 nm. However, this is concerned with mechanical alloying to produce the ultrafine reinforcement and is not adapted to the production of composites of the characteristics of the present invention. The mechanism of such ultrafine reinforcement is essentially one of dispersion strengthening whereby the main reported effect. is an increase in creep resistance. An object of the present invention is to overcome the problem of the magnesium loss by reaction with reinforcement materials when aluminum oxide particles are used as reinforcement in aluminum alloys. Disclosure of the Invention According to one aspect of the present invention, it has been discovered that spinel powder of appropriate characteristics is a highly effective reinforcement for magnesium or aluminum alloys and overcomes the problem inherent in the use of aluminum oxide powders. The reinforcement is in the form of particles consisting of fused or sintered material or single crystals of spinel within a size range of 5-20 μm. The spinel used according to this invention may be either stoichiometric spinel (MgO.Al₂0₃) or non-stoichiometric spinel containing 50 to 95 mole % A1₂0₃. Thus, the spinels of this invention may be broadly defined as MgO:Al₂0₃ containing 50 to 95 mole % A1₂0₃ and 50 to 5 mole % MgO.

It has surprisingly been found that when the above spinels are used in the form of particles consisting of fused material or single crystals of magnesium aluminate within the size range of 5-20 μm, a reinforced composite material is obtained having superior properties as compared to composites of the same alloy matrix reinforced with particulate alumina. In particular, the ultimate tensile strength (UTS) and yield strength (YS) are both superior to those of composites reinforced with alumina. The composite of the invention also has a desirable low density.

The spinel powder can be purchased from commercial sources, or a highly pure form of spinel may be produced from aluminum industry wastes containing alumina or an alumina precursor, such as aluminum dross residue.

Particles of the waste material are mixed with particles of magnesium oxide and the mixture is then calcined to form spinel. The spinel particles are collected in the desired size range. Such process is described in Nadkarni et al, U.S. Application Serial No. 07/546,003, filed June 28, 1990.

It is also possible to produce the spinel reinforcement particles of this invention by electrofusion of an aluminum dross residue, e.g. the product available from Alcan Chemicals North America under the trade mark NOVAL. The NOVAL may be subjected to electrofusion as is or it may be calcined prior to electrofusion. Pre- calcination is preferred to minimize gas evolution during fusion. During calcination A1N present oxidizes to form alumina.

A typical NOVAL product when subjected to electrofusion results in non-stoichiometric spinel, e.g. Mg0.2Al₂0₃. If stoichio etric spinel (MgO.Al₂0₃) is desired, then MgO may be added to the NOVAL prior to the electrofusion.

As the aluminum matrix, a wide range of standard wrought, cast, or other aluminum alloys may be used, such as those having the Aluminum Association designations 6061, 2024, 7075, 7079, etc. Such alloys typically contain a substantial amount, e.g 1-4%, magnesium, as well as other alloying elements. The preferred procedure for producing the composite material of the invention reinforced with particles of spinel comprises melting the metallic material; adding particles of spinel to the molten material; mixing together the molten metal and the particles of spinel to wet the molten metal to the particles, under conditions that the particles are distributed throughout the volume of the melt and the particles and metallic melt are sheared past each other to promote wetting of the particles by the melt, this mixing to occur while minimizing the introduction of gas into, and while minimizing the retention of gas within, the mixture of particles and molten metal, and casting the resulting mixture at a casting temperature sufficiently high that substantially no solid metal is present. The composite material made by the method of the invention comprises a cast microstructure of the metallic matrix, with particulate distributed generally evenly throughout the cast volume. The particulate is well bonded to the matrix, since the matrix is made to wet the particulate during fabrication. The cast composite is particularly suitable for processing by known primary forming operations such as rolling and extruding to useful shapes. The properties of the cast or cast and formed composites are excellent, with high stiffness and strength, and acceptable ductility and toughness. Composite materials have been prepared with volume 5 factions of particulate ranging from about 5 to about 40 percent, so that a range of strength, stiffness and physical properties of the composite are available upon request. The absence of any reaction of the reinforcement with magnesium enables the composition range of preferred 10 and useful products to be widened.

Brief Description of the Drawings In the drawings which illustrate this invention: Fig. 1 is a photomicrograph of stoichiometric spinel powder according to the invention; 15 Fig. 2 is a plot of UTS as a function of aging time for composites containing 15 vol% of respectively alumina and spinel reinforcement,

Fig. 3 is a plot of YS as a function of aging time for composites containing 15 vol% of respectively alumina 20 and spinel reinforcement,

Fig. 4 is a plot of modulus as a function of aging time for composites containing 15 vol% of respectively alumina and spinel reinforcement,

Fig. 5 is a plot of elongation as a function of aging 25 time for composites containing 15 vol% of respectively alumina and spinel reinforcement,

Fig. 6 is a photomicrograph of an extruded rod of AA- 6061/15% spinel composite of the present invention,

Fig. 7 is a plot of yield stress, UTS and elongation 30 as a function of age time for an AA-6061 aluminum - 15% spinel composite,

Fig. 8 is a photomicrograph of non-stoichiometric spinel; and

Fig. 9 is a photomicrograph of an extruded rod of AA- 35 6061/15% non-stoichiometric spinel composite of the present invention. Best Modes for Carrying Out the Invention The following examples serve to illustrate aspects of the invention, which should not be taken as limiting the scope of the invention in any respect. Example 1

Fine spinel powder (-325 mesh) was obtained from Muscle Shoal Minerals, Tuscumbia, Alabama and was classified using a Nisshin 15 M air classifier. Figure 1 is a photomicrograph of the classified powder and other details are given below.

Av. Particle Size: 12 μm (by Microtrac) fines <5 μm Z.5%) Chemical Analysis A1₂0₃ 71.45%

MgO 27.36% Si0₂ 0.58%

CaO 0.38%

Fe₂0₃ 0.23% XRD analysis indicated presence of small quantities of free MgO and A1₂0₃. The above spinel powder was used to produce AA 6061 aluminum-spinel composites. This was done using a mixing furnace of the type described in U.S. Patent 4,786,467.

That furnace includes a crucible which is resistively heated to melt the aluminum alloy, with the crucible positioned within a chamber which may function as a vacuum chamber. A connector provides for the ingress and egress of argon.

The mixer comprises a dispersion impeller positioned vertically along the centerline of the crucible and adapted to operate so as to induce high shears within the melt but a small vortex at the surface of the melt. A typical impeller for the purpose is a vertical rotor shaft with a plurality of flat blades. The blades are not pitched with respect to the direction of rotation, but are angled from about 15° to about 45° from a line perpendicular to the rotor shaft.

The furnace was set at a temperature of about 850- 870°C and 6061 aluminum alloy stock was charged to the furnace with an argon cover gas. As the aluminum began to melt, the temperature was reduced to about 680°C and argon was blown into the melt to displace any absorbed hydrogen, and bringing oxide particles to the surface, which were 5 then skimmed off.

The above described spinel powder was then added to the melt in an amount of 15% by volume and a mixing assembly was put in place and a vacuum was pulled on the furnace. The mixer impeller was rotated at 750 rpm for a 10 total of about 45 minutes under vacuum, after which the mixing chamber was slowly brought back to atmospheric pressure and after a total mixing time of about 55 minutes, the mixer was stopped. The composite material thus obtained was cast into billets, which were then 15 extruded to an extrusion ratio of 16:1. The bars obtained were solution treated, followed by aging at 175°C. Mechanical properties of samples aged for 0, 2, 4, 6 and 16 hours were measured using standard techniques.

Figures 2 to 5 show the ultimate tensile strength, 20 yield strength, modulus and elongations obtained. Data for A1₂0₃ are included for comparison.

These results show that spinel gives superior ultimate tensile strength and yield strength to A1₂0₃ (15% by volume) (Figures 2 and 3) . The modulus of spinel and 25 A1₂0₃ reinforced composites are similar (Figure 4) .

Elongation for spinel, as shown in Figure 5, is somewhat lower than A1₂0₃ but the difference disappears at high aging times.

These improvements in mechanical properties, combined 30 with a lower density (3.6 g/cc) for MgAl₂0₄ compared with 3.98 for A1₂0₃, result in a substantially improved and highly desirable product. Example 2

Following the same procedure as in Example 1, an AA 35 6061 aluminum/15% by volume spinel composite was produced. This composite was cast into 57 mm diameter extrusion billets and then extruded to 9.5 mm diameter rod using a 550'C billet temperature.

The microstructure of the extruded rod is shown in Figures 6a and 6b. The rod was sound with good distribution of spinel particles. The high magnification micrographs reveal that many of the particles have an interaction layer at the interface, resulting from the presence of a small amount of excess A1₂0₃ which has reacted to spinel.

The extruded material was solution treated for 1 hour at 550"C, then water quenched, naturally aged for two days and then artificially aged at 175°C. Data for the aged samples are shown in Figure 7. The four hour aging (T4) and six hour aging (T6) properties compare very favourably to equivalently processed AA6061-Al₂O₃ composites. The Young's modulus for the T4 and T6 conditions are given in Table 1 below, where A and B indicate duplicate tests:

Table I

The above values are slightly lower than comparable ones for equivalently processed AA6061-A1₂0₃ composites. This is expected in view of the fact that the modulus of spinel is around 260 GPa while that of A1₂0₃ is about 375 GPa.

Fractographs of T4 tensile samples also revealed that the fracture surfaces of the composite of the invention have comparable features to A1₂0₃ reinforced composites, consisting of ductile matrix dimpling between particles. Example 3

Test samples of spinels were produced from NOVAL in a small electric arc furnace with a 19 litre size cast iron crucible. The furnace included graphite electrodes 3.2 cm in diameter and about 1.2 m in length. These were mounted in the center of the crucible about 5 cm apart and were equipped with a mechanism to lower or lift while the fusion was in progress. The power source was two 500 Ampere transformers similar to those employed in welding. A typical chemical analysis for NOVAL is shown in Table II below:

Table II

After being calcined at 900°C for about 4 hours, the NOVAL product had the chemical analysis shown in Table III below:

Table III

(A) Ingot #1

About 27 kg of the calcined NOVAL was placed in the crucible and electric power was supplied to the electrodes. As the temperature rose, fusion started and this resulted in compaction of the powdered bed. Subsequently, the electrodes were lowered and more material was moved to the center of the crucible where the melting took place. The test was continued for about 1 1/2 hours until a 15 to 20 cm diameter of molten pool was obtained. The material outside this pool was still solid and acted as an insulator to the iron crucible. The crucible was allowed to cool for two hours and then emptied onto an iron plate. The ingot (Ingot #1) was allowed to cool overnight on its own.

(B) Ingot #2

The above fusion procedure was repeated, but with 6.8 kg of MgO being added to the calcined NOVAL prior to starting. The same fusion procedure as above was used and this formed Ingot #2.

The two ingots were broken with a hammer and then crushed in a jaw crusher to about -6 mesh size. Part of this material was ground with a roller mill to -100 mesh. The chemical analysis of NOVAL before and after fusion was performed by atomic absorption spectroscopy and by XRF. The results are shown in Table IV below. TableIV

Both Ingot #1 and 2 (-100 mesh) powders were then air jet-milled and classified to obtain particles with average particle size of 12-13 μm and Figure 8 is a photomicrograph of the classified non-stoichoimetric spinel of Ingot #1.. Samples of classified powder for Ingot #1 and Ingot #2 were respectively combined in an amount of 15% by volume with AA-6061 in the same manner as described in Example 1. The samples of composite material which were obtained were cast into billets, which billets were then extruded to form 9.5 mm diameter rods. The rods obtained were solution treated followed by aging at 175"C. Mechanical properties for samples aged four hours under natural conditions or six hours under artificial conditions are given in Table V below:

Table V

The microstructure of a longitudinal section of a rod formed with the spinel of Ingot #1 is shown in Figure 9.

Claims

Claims :

1. A metal matrix composite material comprising a metal selected from the group consisting of aluminum, magnesium and alloys thereof having uniformly dispersed therein particles of spinel (Mg:Al₂0₃) containing 50 to 95 mole % A1₂0₃ and 50 to 5 mole % MgO and having sizes in the range of 5-20 μm.

2. A composite according to claim 1 wherein the spinel is stoichiometric spinel (MgO.Al₂0₃) .

3. A composite according to claim 1 wherein the spinel is non-stoichiometric spinel.

4. A composite according to claim 1 containing about 5 to 40 percent of said spinel.

5. A composite according to claim 4 wherein the metal matrix is an aluminum alloy.

6. A composite according to claim 5 wherein the spinel is in the form of single crystals of magnesium aluminate.

7. A composite according to claim 5 wherein the spinel is in the form of fused particles of magnesium aluminate.

8. A composite according to claim 5 wherein the metal matrix comprises aluminum alloyed with magnesium.

9. A composite according to claim 1 wherein the metal matrix has an as-cast microstructure.

10. A metal matrix composite material comprising an aluminum alloy reinforced with particles of spinel produced by the steps of: melting the aluminum alloy; adding thereto particles of spinel (MgO:Al₂0₃) containing 50 to 95 mole % A1₂0₃ and 50 to 5 mole % MgO and having particle sizes in the range of 5-20 μm; mixing together the molten metal and particles of spinel to wet the molten metal to the particles, under conditions that the particles are distributed throughout the volume of the melt and the particles and the metallic melt are sheared past each other to promote wetting of the particles by the melt, said mixing to occur while minimizing introduction of any gas into, and while minimizing the retention of any gas within, the mixture of the particles and molten metal, and casting the resulting mixture.

11. A metal matrix composite according to claim 10 wherein the method for production includes casting the mixture at a casting temperature sufficiently high that substantially no solid metal is present.

12. A metal matrix composite according to claim 10 wherein the method for production includes the additional step, after the step of casting, of working the cast composite material.

13. A process for producing a spinel reinforcing material, which process comprises subjecting an aluminum dross residue to electrofusion.

14. A process according to claim 13 wherein the residue is a residue from a plasma dross treatment procedure.

15. A process according to claim 14 wherein the residue is calcined prior to electrofusing.

16. A process according to claim 13, 14 or 15 wherein MgO is added to the aluminum dross residue in an amount whereby the product of the electrofusion is stoichiometric spinel (MgO.Al₂0₃) .