Method for forming metal base composite

This invention relates to a method for forming a metal base composite by molten metal infiltrati- ion, according to the preamble of claim 1.

A metal base composite is a material in which a reinforcing material in shape as fiber, flake, or powder, is embedded in a matrix metal, to improve such properties of the matrix metal as strength, rigidity and heat resistance.

Conventionally, metal base composites have been manufactured by molten metal infiltration, diffusion bonding or other methods.

In the infiltration method, molten metal is made to contact and impregnate into a preform of a reinforcing material formed in required shape, density and orientation and then solidified to make a metal base composite. The reinforcing material should be preliminarily heated above a certain temperature, in order to prevent solidification of the molten matrix metal before it has made sufficient impregnation into the preform of the reinforcing material.

The density and the orientation of a reinforcing material affect performance of a composite to be fabricated. The shape of the preform of a reinforcing material is determined according to the shape of a final product of the composite. Therefore in order to obtain a metal base composite with desired performance and shape, the shape, density and orientation of the preform of the reinforcing material should be kept constant through the processes of the contact, impregnation and solidification of molten metal.

The shape, density and orientation of the reinforcing material are conventionaly retained by

• (1) preliminary forming the reinforcing material in a mat or a felt form, or
• (2) - constraining and retaining the preform within a casting mold.

However, Method (1) has a disadvantage that the material, density and orientation of the reinforcing material are limited because the reinforcing material is shaped in a mat or a felt form.

Method (2) has a disadvantage that the reinforcing material preliminarily heated may be cooled by contact with a casting mold.

Summary of the invention

Efforts are being made to develop a method, in which a suitable retaining tool is used to keep of the density, orientation and shape of the preform of the reinforcing material.

It is therefore the object of the present invention to provide a method to fabricate a metal base composite with excellent performances with excellent performances with relatively low costs, wherein the time for making said metal base composite should be shortened and the performances of the composite through use of a newly devised retaining tool should be improved.

This object is achieved by the method steps as indicated in the characterizing part of claim 1, thereby using a suitable retaining tool with favorable performances.

According to the present invention a body of water soluble salt is used for retaining and forming the reinforcing material, so that this body fulfills a function of stabilizing and controlling the impregnation step. A body of water soluble salt is principally known from GB-A-2 105 312. In this known case, however, it serves for performing a core or insert for metal or alloy castings with complicated internal shape. This body therefore has the mere function of shaping the metal. Additional functions of controlling the moulding process have no importance.

Firstly, the retaining tool does not melt or degrade in the preliminary heating made for facilitating the contact and impregnation of the molten metal against the reinforcing material after the material is retained in the tool.

Secondly, the retaining tool does not react with the molten matrix metal, during the impregnation process, so that the performance of the produced metal base composite does not degrade.

Thirdly, the retaining tool has a sufficient heat- insulating effect, to prevent cooling of the molten metal by the casting mold and to prevent solidification caused by the cooling, before the molten metal has been sufficiently impregnated into the reinforcing material.

Fourthly, the retaining tool can be easily removed after the reinforcing material and the metal are integrally formed, so that the product is not impaired or contaminated and additionally operating efficiency is improved and costs are reduced.

The retaining tool used with the method according to the invention is generally a container whose inner configuration is equal to external configuration of a composite to be produced. However, the retaining tool may have another shape than a container if it can retain the reinforcing material in the required shape, density and orientation state. A typical retaining tool may be prepared by heating and pressing such water soluble salt as sodium chloride (NaCI), potassium chloride (KCI) and barium chloride (BaCi_z), to form a container with a required shape. In order to improve the dimensional accuracy of a retaining tool or facilitate the removal of the tool, B20₃ or such alkali metal oxide as Na₂0, K₂0, Li₂0 may be added to the water soluble salt. Or, in order to reduce the amount of the salt to be dissolved away with water, a retaining tool may be formed by embedding refractory powder in the water soluble salt.

The reinforcing material is installed in the retaining tool so as to have the required shape, density and orientation in accordance with the shape and performance of a product to be manufactured. A reinforcing material may be powder, flake or fiber including carbon fiber, silicon nitride fiber or ceramic whisker.

Matrix metal may be aluminium metal, aluminium alloy, copper metal, copper alloy or any other suitable metal or alloy.

In order to prevent insufficient impregnation of the molten matrix metal into the pores of the preform of a reinforcing material due to premature solidification of the metal, it is preferable to heat the retaining tool above the melting point of the matrix metal.

It is advantageous to install the reinforcing material contained in the retaining tool (retainer) in a mold, in which the molten matrix metal is poured to impregnate into the reinforcing material. The above-mentioned mold is preferably a forming mold with a cavity to install the retainer. The mold may be a gravity casting mold, a die casting mold or a melt forging mold. It is preferred to apply pressure to the molten metal during impregnation process, in order to facilitate the impregnation. The pressure application may be achieved by mechanical means with a plunger or by gas pressurization. The molten metal is solidified in the mold to produce a formed product in which the reinforcing material and the metal are integrated.

For the step of dissolving the retainer away it is advantageous to use additives as B₂0₃, Na₂0, Li₂0 or K₂0 contained in the retainer. By this the removal may be accelerated by use of a boiling water. And in the case when a water-soluble salt used as a binder, the disintegration and removal of the retainer may be facilitated because of reduced amount of the salt to be dissolved.

One important advantage of the above described method for manufacturing the metal base composite is reduction of time and cost in the manufacture since the retainer can be removed in a shorter time. Another advantage is improved product quality since the retainer does not react with a molten matrix metal. Additionally a better dimensional accuracy of the product can be achieved since the retainer can be in a precise shape. Finally, another advantage is better performance of the metal base composite since the molten matrix metal in unlikely to be cooled by a mold due to the excellent heat insulation of the retainer and therefore the molten metal can be easily impregnated into the reinforcing material.

In the following some preferred embodiments of the invention are described with reference to accompanying drawings, in which:

• Fig. 1 shows the device for making the retainer used in the first embodiment of the present invention; Fig. 2 shows the mold used in the first embodiment, in which the molten matrix metal is contacted with and impregnated into the reinforcing material; and Fig. 3 shows an oblique view of the retainer used in the second embodiment.
• Fig. 1 shows a device utilized for forming a retainer used in the present embodiment.

Sodium chloride (NaCI), preheated up to 280°C-320°C, was set in a die 2 shown in Fig. 1, and the salt was pressed at 1000 kg/cm² with an upper die 21, to form a retainer 3 with a concavity 30 in the shape of a connecting rod.

a-alumina (AI₂0₃) filaments with a 20 ¡.1m diameter were installed and suitably oriented in the concavity of the retainer, with a fiber volume percent of 50%. A couple of retainers 3 were fixedly united, with their concavities facing each other and heated to 680°C.

Fig. 2 shows a schematic sectional view of a casting mold, in which a molten matrix metal is contacted with and impregnated into a-alumina fibers, reinforcing material.

The preheated sodium chloride retainer 3 containing a-alumina fibers was set in the casting mold 5 shown in Fig. 2. Immediately afterwards, a molten aluminum alloy (JIS AC7A) preheated at 750°C was quickly poured into the mold and a pressure of 1000 kg/cm² was applied by use of the upper mold 51 and kept until the molten metal was solidified. The interior of the casting mold 5 had been kept at 300°C until the retainer 3 was installed in the mold.

After solidification of the aluminium alloy, the product is removed from the casting mold. 5 with a knock-out plunger 6, and the extra parts of the product were cut off. Then, the product was dipped in water to dissolve the retainer 3 away, to obtain a metal base composite composed of alumina fibers and aluminium alloy.

The connecting rod made of the metal base composite produced in the above-mentioned method was confirmed to have a higher strength than the conventional steel connecting rod made by forging. Also, the shape density and orientation of the reinforcing material in the composite were found to have retained the shape, density and orientation of the reinforcement before the aluminium alloy was impregnated. The dissolved sodium chloride was found to be reusable for making retainers.

Fig. 3 shows an oblique view of the retainer used in the second embodiment. The material of the retainer was a mixture of 90 vol% of barium chloride (BaCl₂) and 10 vol% of silicon nitride (Si₃N₄). The mixture was heated to 400°C and then pressed at 1000 kg/cm² to provide the retainer as shown in Fig. 3.

In this embodiment, silicon carbide (SiC) whisker was used as the reinforcing material. The whisker was installed in the retainer in an amount to provide 30 vol% of the whisker in the composite to be finally obtained. The retainer including the whisker was preheated to 950°C.

The retainer including SiC whisker, preheated to 950°C, was installed in a casting mold and immediately afterwards a molten copper alloy (JIS high strength brass, HBs-C), heated to 960°C, was quickly poured in the mold and pressed with an upper mold and kept under a pressure of 500 kg/cm² to complete solidification.

After the copper alloy solidified, the metal composite including the SiC whisker was removed from the casting mold with a knock-out plunger. The extra parts of the retainer were cut off and the composite was dipped in water to dissolve the retainer, to obtain the neat composite consisting of SiC whisker and the copper alloy.

The composite, thus obtained, was confirmed to have high strength and wear resistance as well as good corrosion resistance. The shape, density and orientation of the reinforcing material in the composite were found to have retained the shape, density and orientation before impregnation of the molten copper alloy. The barium chloride was found to be reusable for making retainers.

As apparent from the description so far made, the present method to fabricate a metal base composite by impregnation of a molten metal involves to use a retainer mainly composed of a water soluble salt with a high melting point for constantly retaining the shape, density and orie- nation of a reinforcing material.

As may be understood by the detailed description of the preferred embodiments, the present invention facilitates the removal of a retainer. Therefore, the time for production of a composite can be shortened and no damage of the composite and of the composite quality will occur, which may originate from the removal of the retainer.

Since the retainer in accordance with the present invention has an excellent heat insulation effect, the reinforcing material is difficult to be cooled by a mold and therefore the obtained composite has good performances.

The quality of the composite is also good because the retainer in the present invention does not react with a molten matrix metal.

Also, the dimension accuracy of the composite is good due to excellent dimensional accuracy of the retainer of the present invention.

It had been shown that the water soluble salt used for the retainer could be recyled for reuse.