EP0313271A1

EP0313271A1 - Metal matrix composite with silicon-free reinforcing preform

Info

Publication number: EP0313271A1
Application number: EP88309592A
Authority: EP
Inventors: Willard Mark Truman Gallerneault; Joseph Hron
Original assignee: Alcan International Ltd Canada
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 1987-10-20
Filing date: 1988-10-13
Publication date: 1989-04-26
Also published as: JPH01147031A; KR890006842A; BR8805398A

Abstract

A process is described for forming a composite cast article comprising an aluminum-silicon alloy matrix and a preform of bonded together Al₂O₃ fibres incorporated in the matrix, in which the preform is infiltrated under pressure by a melt of the alloy and the composite article thus formed is allowed to solidify by cooling. According to the novel feature, the fibres of the preform are bonded together by colloidal alumina rather than by the usual SiO₂. The result is a composite cast article which is free of silicon associated with decomposition of SiO₂ in the preform, such that a product of superior strength characteristics is obtained by using an infiltrating alloy containing the usual amounts of modifier to reduce the particle sizes reached by the silicon of the alloy during eutectic solidification. The novel preform is also described.

Description

Background of the Invention

This invention relates to the production of metal matrix composites, and more particularly to preforms of reinforcing material used in such composites.
Among metal matrix composites (MMC) having important commercial utility are fibre-reinforced articles of aluminum and its alloys, particularly aluminum-silicon alloys. One of the most popular techniques used to manufacture metal matrix composites is melt infiltration. In this procedure a preform of preferably fibrous reinforcing material is infiltrated under pressure by liquid metal. The preform must maintain its physical integrity, i.e., it must not fracture throughout the infiltration procedure. In order to accomplish this, chopped fibres of the reinforcing material are coated with a binder, such as a SiO₂ based suspension, filtered into a cake and then calcined to drive off the moisture and form a rigid preform.
The rigid preform thus formed contains solid SiO₂ and infiltrating liquid aluminum reacts with the SiO₂, reducing it to free silicon. The result is the formation of large silicon particles adjacent the fibres. These silicon particles have poor physical properties and generally degrade the ultimate performance of the composite. For many uses of the metal matrix composites, this loss of properties can be tolerated. However, the loss cannot be tolerated when the metal matrix composites are used in high temperature applications where thermal fatigue is an important consideration.
It has been known for many years to obtain a fine eutectic structure in Al-Si alloys containing about 5 to 15% silicon by the use of additives and, thus, to improve the mechanical properties of these alloys. For instance, it is well known to use alkali metals and alkaline-earth metals, e.g. sodium or strontium, as additives in Al-Si alloys. These chemical additions to a melt reduce the silicon size by affecting the normal growth kinetics of the solidification process. It would, therefore, be expected that in a similar manner additives such as sodium or strontium would suitably modify the metal matrix microstructure of a metal matrix composite. However, when the melt contains a fibrous preform reinforcement bonded with SiO₂, sodium and strontium are remarkably ineffective in modifying the metal matrix microstructure of the metal matrix composite.
It is an object of the present invention to develop a procedure for forming composite cast articles in which silicon is substantially eliminated from the preform.

Summary of the Invention

According to the present invention it has been found that a satisfactory preform can be produced by bonding reinforcing fibres together using colloidal alumina as the bonding agent in place of SiO₂. When this preform is infiltrated with liquid aluminum, the absence of SiO₂ in the preform means that a composite is formed which is free of silicon associated with the decomposition of SiO₂. Moreover, when the infiltrating melt is a silicon-containing alloy, a composite is obtained in which there is no preferential nucleation of the silicon phase at the fibres of the preform. The result is that by using the preform of this invention, a high quality composite cast article is obtained when the preform is infiltrated with a melt of aluminum-silicon alloy containing known modifiers to reduce the particle sizes reached by the silicon in the melt during eutectic solidification.
Thus, one embodiment of this invention comprises a novel preform in which randomly oriented reinforcing fibres are bonded together using colloidal alumina as the bonding agent. The reinforcing fibres are preferably δ-alumina fibres, such as Saffil® fibre. While any colloidal alumina may be used, a preferred colloidal alumina is a product of chi-alumina rehydration, which is formed during attritor grinding. This colloidal alumina has a pH of 2-4 with pseudo-boehmite-like structure and, after calcination at 500-550°C, shows a gamma-alumina structure. The preform may be prepared by mixing the colloidal alumina in an aqueous solution with chopped alumina fibre and filtering the slurry into a cake. The cake is then calcined to drive off the moisture and produce a rigid preform.
The bulk density of the preform can be controllably varied within wide limits, typically over a range from about 5% to about 50% of the density of the component fibres, by appropriate selection of compacting pressure. The compacting pressure may be exerted by vacuum drawing the slurry of fibres against a perforate wall or screen, and/or by a press. These preforms develop sufficient strength not only to render them fully self-sustaining in shape and dimensionally stable during handling, but also to withstand infiltrating pressures in the order of 100 p.s.i. and post infiltration hydrostatic pressures of about 3000 p.s.i. without significant change in dimensions or total pore volume.
A further embodiment of the invention is a process for forming a composite cast article comprising an aluminum-silicon alloy matrix containing a modifier to reduce the particle sizes reached by the silicon during eutectic solidification and a preform of bonded together Al₂O₃ fibres incorporated in the matrix. By bonding the Al₂O₃ fibres together with colloidal alumina, a composite cast article having superior physical properties is obtained when the preform is infiltrated under pressure by a melt of the modified aluminum-silicon alloy and the composite article thus formed is allowed to solidify by cooling.
Excellent composite articles are obtained when the usual additives for Al-Si alloys, e.g. alkali metals or alkaline earth metals, such as sodium or strontium, are used in the usual amounts. Typical aluminum silicon alloys for this purpose contain about 5 to 15 percent by weight silicon and such alloys are typically modified by addition of about 0.03 to 0.07 percent by weight strontium or about 0.0005 to 0.001 percent by weight sodium.
The procedure of the present invention is particularly effective when used in the method of producing composite cast articles described in EP-A-0271222.
The invention will now be further explained by means of the following non-limitative examples.

Example

A preform of reinforcing material was prepared from 3 µm diameter alumina fibre (Saffil ® fibre available from ICI). The fibre was chopped into lengths of about 200 µm and an aqueous slurry was formed containing 5% by weight coiloidal alumina and 100 g/l of the Saffil fibre. The colloidal alumina was a product of chi-alumina rehydration formed during attriter grinding, having a pH of 2-4 with pseudo-boehmite-like structure and, after calcination at 500-550°C, showing a gamma-alumina structure. The slurry was mixed and poured into a suction filter to form a cake of coated fibre. The cake was then placed in an oven and heated at 500°C for 4 hours to drive off the moisture and produce a 30 volume % rigid preform having a height of 30 mm and a diameter of 70 mm. A further firing at 1000°C for 2 hours was used to remove remaining water of hydration.
The above preform was heated to 800°C and placed into a 75 mm diameter die preheated to 500°C. A melt of super purity aluminum (Alcan 99.87%) was immediately poured on top of the hot preform and a cold ram (25°C) was used to force the molten aluminum into the porous preform. The infiltration pressure was nominally 20 MPa and sufficient of the melt was used to totally infiltrate the preform and result in a composite with free matrix aluminum both above and below the preform. The composite thus formed was allowed to solidify by cooling to obtain the desired composite cast article. A cross section of the composite cast article was subjected to metallographic examination by means of optical microscopy, scanning electron microscopy and differential scanning calorimetry and there was no evidence of silicon formation.
It is to be understood that the invention is not limited to the procedures and embodiments hereinabove specifically set forth, but may be carried out in other ways without departure from its spirit.

Claims

1. In a process for forming a composite cast article comprising an aluminum-silicon alloy matrix containing a modifier to reduce the particle sizes reached by the silicon during eutectic solidification and a preform of bonded together Al₂O₃ fibres incorporated in the matrix, wherein the preform of reinforcing fibres is infiltrated under pressure by a melt of said alloy and the composite article thus formed is allowed to solidify by cooling,
the improvement which comprises utilizing a preform in which the fibres are bonded together by means of colloidal alumina.

2. A process according to claim 1 wherein the modifier is sodium or strontium.

3. A process according to Claim 1 wherein the alloy contains about 5 to 15 percent by weight silicon.

4. A rigid preform of reinforcing material for metal matrix composites comprising a porous mass of randomly oriented alumina fibres bonded together by means of colloidal alumina as bonding agent.

5. A rigid preform according to Claim 4 having a bulk density of about 5% to about 50% of the density of the component fibres.