EP0462850A1 - Method of making aluminium alloy castings containing an insert and cylinder head of internal combustion engine obtained thereby - Google Patents

Abstract

The present invention relates to a method of making aluminium alloy castings (9) containing an insert (1). This insert is composed of a composite material with metallic matrix, which is constituted by a reinforcement based on ceramic fibres such as fibres of alumina or alumina/silica forming a felt, impregnated under high pressure by an aluminium alloy other than or identical to the cast alloy. In order to achieve the subsequent chemical fixing between the insert and the casting, the surface of the insert is subjected to a chemical treatment of the zincate type, followed by an electrolytic coating treatment. The invention applies to the automobile industry for making aluminium alloy cylinder heads (9) comprising an insert (1) in at least one of its zones (4,5) between valve seats. <IMAGE>

Description

The invention generally relates to a method for manufacturing cast aluminum alloy parts comprising an insert.

The present invention finds application in particular in the manufacture of an internal combustion engine cylinder head, provided with an insert.

In the metallurgy of foundry alloys, a method of manufacturing a casting comprising a ceramic insert is known. It consists of depositing on the insert, a flexible mattress of ceramic fibers, then placing the insert in a mold and finally pouring the metal into the mold.

However, the parts thus obtained are often subjected in their use to thermal cycles; and under the effect of the high temperatures, these parts quickly reach their breaking limit, their limit to thermal fatigue and their elastic limit at temperature of the material considered.

In the particular case of engine cylinder heads, the alloy part forming the cylinder head undergoes large temperature variations during engine operation, the maximum temperature being able to reach 350 ° C. The part of the cylinder head forming the inter-valve seat areas deteriorates by cracking, deforms when hot and its service life decreases accordingly.

The present invention aims to overcome the drawbacks mentioned above by proposing a method for manufacturing cast aluminum alloy parts comprising an insert, where the insert is composed of a composite material with a metallic matrix, which is made up by a reinforcement based on alumina or alumina-silica fibers forming felt, impregnated under high pressure with an aluminum alloy different from or identical to the casting alloy.

Furthermore, in this process, the fixing, in particular chemical, between the insert and the cast alloy part must be perfect. Indeed, an impermeable and non-fusible alumina layer covers the entire surface of the insert and presents an obstacle to the creation of a chemical bond between the insert and the casting alloy part.

The invention therefore also aims to create a chemical bond at the insert - part interface.

To this end, the invention provides a method for manufacturing cast aluminum alloy parts comprising an insert, which more precisely consists in subjecting a matrix alloy surface of at least one face of the insert, to a treatment zincate type chemical, followed by at least one electrolytic treatment such as a nickel coating.

According to a characteristic of the invention, the chemical treatment with zincate forms on the aforementioned surface a layer of zinc or of zinc-iron alloy containing approximately 15% of iron.

According to another characteristic of the invention, the electrolytic treatment is carried out from a nickel sulphate or sulphamate solution.

According to a variant of the invention, the aforementioned surface of the insert undergoes a second electrolytic treatment such as a silver coating.

The invention also relates to an internal combustion engine cylinder head, provided with at least one insert, the cylinder head made of cast aluminum alloy being produced by the method according to the invention.

Other characteristics and advantages of the invention will appear better on reading the detailed description which follows and which refers to the appended drawings, given solely by way of example, in which.

Figure 1 shows a schematic section of the insert formed of a composite material before the treatment of its surface.

FIG. 2 represents a schematic section of this same insert after the treatment of its surface.

FIG. 3 represents a plan view of the insert after impregnation and before introduction into the mold.

Figure 4 is a sectional view of the insert along line IV-IV.

FIG. 5 represents a partial view from below of an internal combustion engine cylinder head.

Figure 6 shows a partial section of the cylinder head shown in Figure 5, along the line VI-VI.

7 shows a partial perspective view of the cylinder head shown in Figure 5 in the direction VII.

According to the method for manufacturing cast alloy parts according to the invention, an insert is placed within the part.

This insert is made beforehand from composite material which consists on the one hand of a reinforcing material based on ceramic fibers such as for example alumina or alumina-silica fibers, which are entangled with the way of a felt whose cohesion is ensured by an inorganic binder, such as for example a silica binder, and on the other hand of an aluminum-based matrix and more precisely of an aluminum alloy different from or identical to the aluminum alloy of the casting. This aluminum alloy forming the matrix is also called a matrix alloy. This composite material is produced by impregnating the reinforcing material under high pressure with the aluminum alloy or matrix alloy in the liquid state.

A method of manufacturing this composite material is described in particular in the European patent application filed under No. 89402986.7.

An example of a material of this type includes a reinforcement of short alumina fibers, an AS5U4G2 matrix according to AFNOR standards.

• une partie centrale a, qui est formée de matériau composite renforcé de fibres céramiques imprégnées par un alliage d'aluminium ;
• une face b en matériau composite où les fibres céramiques affleurent, c'est la face utile de l'insert qui permet l'exploitation optimale des propriétés du matériau composite, cette face n'entrera pas en contact avec l'alliage de coulée ;
• une face c et une face d qui sont constituées par une surépaisseur d'alliage d'aluminium ou encore d'alliage matriciel, l'alliage d'aluminium sans fibres affleure donc les faces c et d.

Referring to FIG. 1, the insert 1 comprises:

• a central part a, which is formed of composite material reinforced with ceramic fibers impregnated with an aluminum alloy;
• a face b of composite material where the ceramic fibers are flush, it is the useful face of the insert which allows the optimal exploitation of the properties of the composite material, this face will not come into contact with the casting alloy;
• a face c and a face d which are constituted by an excess thickness of aluminum alloy or even of matrix alloy, the aluminum alloy without fibers is therefore flush with the faces c and d.

The side c by its undercut, will only be used for mechanical attachment between the insert and the casting.

As for the face d, it will be used as described below to create a chemical bond at the insert-part interface, which allows chemical attachment between the part and the insert.

The insert thus produced is then subjected to chemical treatments.

More specifically, the surface of the face d of the insert will undergo chemical treatment and at least electrolytic treatment.

Indeed, the face d described above has on its surface an impermeable and non-fusible alumina layer which hinders the subsequent chemical fixation between the insert and the part.

To do this, it is necessary to remove this layer of alumina.

To this end, the surface d is subjected to a chemical treatment of the zincate type and preferably of the zinc-iron type.

This treatment is carried out using a solution containing zinc oxide, sodium hydroxide, double potassium and sodium tartrate and ferric chloride.

The main mechanisms used in the reaction are as follows.

$$\text{Al(OH)₃ → AlO₂ ⁻ + H₂O + H⁺,}$$

permettent de détruire la couche d'alumine, et les réactions du type cathodique : $$\text{H⁺ + 1e ⁻ → 1/2 H₂}$$

$$\text{Zn(OH)₄ ⁻⁻ → Zn²⁺ + 40H⁻}$$

$$\text{Zn²⁺ + 2e⁻ → Zn}$$

forment alors une couche de zinc, ou d'alliage zinc - fer qui remplace la couche d'alumine.Anodic type reactions: $$\text{Al + 30H⁻ → Al (OH) ₃ + 3e⁻}$$

$$\text{Al (OH) ₃ → AlO₂ ⁻ + H₂O + H⁺,}$$

used to destroy the alumina layer, and cathode-type reactions: $$\text{H⁺ + 1st ⁻ → 1/2 H₂}$$

$$\text{Zn (OH) ₄ ⁻⁻ → Zn²⁺ + 40H⁻}$$

$$\text{Zn²⁺ + 2e⁻ → Zn}$$

then form a layer of zinc, or zinc-iron alloy which replaces the alumina layer.

In the case of the zinc - iron alloy, this layer has an iron content of around 15%.

Referring to Figure 2, this layer shown in f is very thin, having a thickness of less than 1 micron.

The role of this layer f is to prevent the alumina layer from reforming on the surface of the face d of the insert, it therefore prevents surface reoxidation of the face d of the insert.

However, this thin layer is fragile and must be protected in order to facilitate handling and the preservation of the state of the surface of the insert.

Therefore, the surface of the insert, that is to say this face f, undergoes an electrolytic treatment to form a coating.

A conventional coating is for example an electrolytic deposit of nickel.

The treatment is carried out in a bath containing nickel sulphate or sulphamate, nickel chloride, and boric acid.

A layer g a few microns thick is formed, thereby protecting layer f and limiting the surface reoxidation of the insert before casting the part. In addition, this layer f allows the diffusion of nickel in the aluminum casting alloy thus creating the chemical bond in the casting step of the part.

Finally, under difficult casting conditions, it is possible to place on the layer g, an additional coating by an electrolytic deposit of silver.

This coating is produced from a bath typically containing: double cyanide of silver and potassium, potassium cyanide and potassium carbonate.

The silver layer h thus obtained, a thickness of a few microns, better protects the surface of the insert than nickel by more effectively limiting the reoxidation of the surface of the insert before casting. In addition, this silver layer h improves the chemical bond at the insert-part interface because the diffusion coefficient of silver in aluminum is higher than that of nickel in aluminum.

The faces of the insert thus prepared and treated ensure both mechanical attachment and a chemical bond at the insert - part interface.

All forms of insert can be considered.

The insert which will be placed in the mold for the casting of the part, may be in a temporary form, sometimes necessary to maintain it motionless during casting. In this case, it will be machined after the casting step, in order to reach its final shape.

Then, the actual insertion step is considered.

All configurations of parts are possible, the molds being prepared accordingly.

The insert comprising all the coating layers f and g and optionally h on its face d, is positioned inside the mold. The latter is then closed and the casting of the part can begin.

All current and future light alloy casting techniques can be applied here.

The aluminum alloy of the part is cast according to specific parameters such as the temperature of the insert in the mold, the temperature of casting of the liquid alloy and the quantity of alloy which, in contact with the insert , heats it for the duration of the casting. All of these parameters taken in combination acts on the creation of a chemical bond at the insert - part interface. An optimal combination of parameters generates a very good chemical bond.

The part thus cast comprising an insert is then removed from the mold.

The method of manufacturing an alloy part comprising an insert according to the invention finds application in the automotive industry.

In particular cylinder heads of internal combustion engine, made of aluminum alloy, are thus produced.

An example of an insert intended to be introduced into a cylinder head is shown in FIG. 3. It has lateral faces 2 and 3 of concave shape, where the valves will be housed in the inter-seat zones 4 and 5. The insert in cross section according to Figure 4, has a trapezoidal section. The undercuts 6 and 7 are used for the mechanical fixing of the insert, and the face 8 which will undergo the chemical and electrolytic treatments according to the invention, is used for the chemical fixing between the insert and the part.

The aluminum alloy of the cylinder head is then poured, for example by gravity, onto this insert of composite material which has undergone the chemical and electrolytic treatments mentioned above.

In the case of cylinder heads, an insert (or more) is (are) for example disposed (s) at the areas between the valve seats.

With reference to FIG. 5, the insert 1 is housed between two inter-seat zones of valves 4 and 5 inside the cylinder head 9. Valves will be housed in the spaces 10 and 11.

Figure 6 is a partial section of the cylinder head shown in Figure 5, shows the insert 1 fixed in particular by its face 8 to the cylinder head 9 by the chemical bond generated by the method of manufacturing the cylinder head.

The insert 1 is positioned between the spaces 10 and 11 or also valve ducts, one inlet duct, the other exhaust duct.

The valve heads 12 and 13 respectively are received against the insert 1 in the conduits 10 and 11, coming into contact with the insert 1 in the zones 4 and 5.

Likewise, the perspective view of FIG. 7 illustrates the positioning of the insert fixed within the cylinder head. The valve heads 12 and 13 can move on either side of the insert 1.

The thus produced aluminum alloy cylinder head comprising an insert in at least one of its inter-valve seat areas, prolongs the life of the cylinder head by clearly improving the life in thermal fatigue and the elastic limit in temperature of these inter-valve seat areas.

The cylinder head then withstands temperature variations much better during engine cycles, compared to conventional cylinder heads.

The invention also finds application in the manufacture of other parts made of cast aluminum alloy.

Claims (9)

Method for manufacturing cast aluminum alloy parts comprising an insert composed of a metal matrix composite material, which consists of a reinforcement based on ceramic fibers such as alumina or alumina-silica fibers forming felt , impregnated under high pressure with an aluminum alloy different from or identical to the casting alloy, characterized in that it consists in subjecting a matrix alloy surface to at least one face (d) of the insert (1) mentioned above, a chemical treatment of the zincate type, followed by at least one electrolytic treatment such as a nickel coating to create a chemical bond at the insert-part interface. Method according to claim 1, characterized in that the zincate treatment forms on the above-mentioned surface a layer (f) of zinc or of zinc-iron alloy containing about 15% of iron. Process according to claim 2, characterized in that the chemical treatment with zincate is preferably a treatment of the zinc-iron type in a solution containing zinc oxide, sodium hydroxide, ferric chloride and potassium double tartrate and sodium. A method according to claim 1, 2 or 3, characterized in that the electrolytic treatment is carried out from a solution of nickel sulphate or nickel sulphamate, forming a layer (g) of nickel. Process according to claim 4, characterized in that the solution of nickel sulphate or nickel sulphamate additionally includes nickel chloride and boric acid. Method according to one of claims 1 to 5, characterized in that the aforementioned surface of the insert undergoes a second electrolytic treatment such as a coating of silver to form a layer (h) of silver. Method according to one of claims 1 to 6, characterized in that the insert thus treated is introduced into a mold where the part is cast and in that the part comprising the insert is then removed from the mold. Cylinder head (9) of internal combustion engine, provided with at least one insert (1), characterized in that said cylinder head (9) made of cast aluminum alloy, is manufactured by the process according to one of claims 1 to 7. Cylinder head according to claim 8, characterized in that said insert (1) is provided in areas (4,5) between the valve seats.

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