EP0022869B1 - Process for producing graphite-containing aluminum alloy - Google Patents

Abstract

A process for producing graphite-containing aluminum alloy by introducing graphite particles into a molten aluminum. Graphite particles are prevented from floating on the molten aluminum upon being introduced thereinto, by adding titanium, chromium, zirconium, nickel, vanadium, cobalt, manganese, niobium, or phosphorus to the molten aluminum to disperse therein prior to the introduction of graphite particles. The alloy is suited for use as dry frictional members of bearings or the like.

Description

Technical area

The present invention relates to a process for the preparation of aluminum alloys containing graphite, which comprises adding and dispersing particles of raw natural graphite, not covered with a metal, in a casting of aluminum or an alloy of this type. latest.

Prior art

In many structural sliding contact elements used in internal combustion engines, such as bearings, gears, pistons, cylinders, sliders and the like, metal alloys containing a solid lubricant have been used. This method is used to compensate for a loss of lubrication by providing a self-lubricating action of the solid lubricant when a film of a lubricating oil is destroyed. We know that graphite is very suitable as a solid lubricant. Consequently, numerous alloys containing graphite particles have been proposed and manufactured to date. However, most of these metal alloys containing graphite particles are prepared according to a spray metallurgy, so that the resulting sintering products do not have sufficient mechanical properties.

In the case of large products, the manufacturing costs are much higher than in the case of cast or forged products. Also, a serious effort has been made to develop a casting technique capable of uniformly dispersing graphite particles in metal alloys without the need to float these graphite particles.

More particularly, the following methods have recently been proposed as a technique for dispersing graphite particles in an aluminum alloy casting (lower graphite solubility, by weight, at 0.01%) with which graphite is incompatible in metallurgy, without resorting to the flotation of graphite particles.

A method has therefore been proposed according to which a mixed powder of graphite particles coated with nickel and a halide is incorporated into a casting of an AI-Si hypereutectic alloy and vortices are formed in the casting by an agitator for uniformly dispersing the graphite particles, and another method in which graphite particles coated with a metal and suspended in a carrier gas are blown in a cast of an aluminum alloy, method described in the publication JP- B-45-13224.

However, these methods have the problems and shortcomings described below. In each of these methods, it is essential that the surfaces of the graphite particles to be dispersed are covered with a metal.

A metallic coating can be formed on the surfaces of the graphite particles by chemical plating or the like. However, this process involves complicated stages, the installations for treating waste water and the like pose major problems and consequently the cost prices of these products are unfavorably increased.

In addition, since the surfaces of the graphite particles coated with metal are in the oxidized state, even if these particles are thrown and dispersed in a casting, they are likely to rise to the surface of the casting due to the low wettability with that -this and it is impossible to disperse the graphite particles uniformly in the casting. It has therefore been proposed to improve this wettability by treating the graphite particles in a hydrogen atmosphere.

However, in this case, many blisters are formed by hydrogen discharge from the core of the graphite particles and it is practically impossible to obtain valid products.

It is necessary to incorporate a quantity varying from 4 to 30%, by weight, of graphite in aluminum or its alloy in order to obtain a sufficient lubricating effect of graphite in dry friction. The use of graphite particles coated with a metal is not suitable for throwing and dispersing such a large amount of graphite particles in a short-lived fusion with high yield.

In addition, when it is desired to throw and disperse a large quantity of graphite particles coated with metal in the fusion at a precise instant, the heat necessary to melt the metal is taken from the fusion as matrix, and the temperature of the latter. lowers rapidly to reduce the smoothness of the melt, and graphite particles coated with the added metal may float on the surface of the melt. The metal coated graphite particles floating on the surface of the casting can no longer be redispersed in this casting due to surface oxidation. Consequently, if it is desired to disperse a large quantity of graphite particles in the casting, it is necessary to discard and disperse these particles little by little and in increasing quantity, which requires a sufficiently long time to disperse the predetermined quantity of graphite.

When a long time is thus required to effect the dispersion of the graphite particles thrown and dispersed in the casting, the initial granular graphite begins to float on the surface of the casting and consequently, the efficiency of use of this graphite deteriorates greatly. .

The process using the mixed powder requires considerable time for this mixing, and it is very difficult to choose a suitable particle size to mix the graphite particles to be dispersed in the casting. If a carrier gas is used, the graphite particles which can be used are limited to very fine particles, and a long time is required to determine the dispersion of a predetermined amount of the graphite particles.

In view of the above, it was desired to develop a process for the preparation of aluminum alloys containing graphite, which would use graphite particles not coated with a metal.

Statement of the invention

The object of the present invention is to propose a process for the preparation of aluminum alloys containing graphite, according to which graphite particles of 2 to 30% by weight are thrown and dispersed in a very short time in aluminum castings or alloys of the latter, with adequate efficiency of use.

Another object of the invention is to propose a process for the preparation of aluminum alloys containing graphite, which uses graphite particles not covered with a metal so that it is possible to use crude graphite particles for reduce manufacturing costs.

Another object of the invention is to propose a process for the preparation of aluminum alloys containing graphite, according to which the casting structure is made fine and the graphite particles are not likely to float on the surface of the casting. One of the characteristics of the invention resides in a process for the preparation of aluminum alloys containing graphite, which comprises the following steps: incorporation, for example by throwing away from 1.5 to 20%, by weight, of at least one additive metal chosen from the group: titanium (Ti), chromium (Cr), zirconium (Zr), nickel (Ni), vanadium (V), cobalt (Co), manganese (Mn) and niobium (Nb), in a casting aluminum or an alloy thereof, after introduction of said metal, launch and dispersion of 2 to 30%, by weight, of particles of raw natural graphite without metallic coating inside the casting and after that, solidification of casting aluminum or aluminum alloy containing these graphite particles.

Instead of the additive metal mentioned above, as a protective agent for the flotation of graphite, it is possible to prevent as much as possible, that is to say to reduce, the flotation of graphite particles by adding 0.1 4%, by weight, of phosphorus (P).

Another characteristic of the invention lies in the stage of solidification of the casting under a pressure of 400 to 1000 kg / cm ² to make the sintered structure very fine and to suppress the flotation of the graphite particles.

According to the invention, it is possible to prepare an aluminum casting alloy in which the graphite particles are substantially and uniformly dispersed throughout the structure of the refined ingot, the metallic coating on the surface of the graphite particles is removed and the buoyancy of the latter is reduced. In addition, even if the resulting aluminum alloy containing the graphite particles is melted again, these particles do not float on the surface of the casting.

Brief description of the drawing

The drawing is a simple figure showing the relationship between the dispersed amount of the graphite particles and the particle size thereof when additive metals are incorporated into a casting of aluminum alloy by varying the amount of these additive metals.

Best way to realize the invention

We will explain below, in detail, the best way to carry out the invention.

It is desirable that an aluminum alloy in which graphite particles are thrown and dispersed contains at least one of the following elements: tin (Sn), copper (Cu), lead (Pb) and silicon (Si). The reason for using such alloys is to further improve the usability of these, when graphite particles are dispersed in alloys of AI-Sn, AI-Cu, AI-Pb and AI-Si , alloys widely used so far in bearings or the like.

Before throwing the particles of raw natural graphite in the casting of aluminum or its alloy, at least one element, chosen from the group: Ti, Cr, Zr, V, Nb, Ni, Co, Mn and P is incorporated into said casting. These elements were chosen on the basis of experimental results.

In addition to these 9 elements, tests were carried out on 11 other elements, namely: barium (Ba), beryllium (Be), cerium (Ce), iron (Fe), cesium (Cs), potassium (K), neptunium (Np), calcium (Ca), tungsten (W), hafnium (Hf) and antimony (Sb), but all 11 have been found to be ineffective in suppressing the buoyancy of graphite particles. The elements tested are usually known as carbide-forming elements, and only the 9 elements mentioned first can prevent the floating of the graphite particles. In the case of these elements, when we examine the textures of the resulting products under the electron microscope (x 1000), there is no layer of carbide between the graphite particles and the aluminum alloy.

If graphite particles are incorporated in an amount varying by weight, from 2 to 30%, the highest lubrication effect can be obtained when the product is used under dry friction. It is difficult to obtain a sufficient lubricating effect with an incorporation of less than 2% by weight of the graphite particles. While, when the graphite particles are used in an amount greater than 30% by weight, the mechanics also decrease.

If the graphite particles are incorporated in the range of 2 to 30% by weight, it is desirable that at least one of the elements: Ti, Cr, Zr, Ni, V, Co, Mn or Nb is first incorporated in the casting in an amount varying, by weight, from 1.5 to 20%. If such elements are incorporated in a total amount greater than 20% by weight, although the effect of preventing the flotation of graphite can be achieved, there is a risk of seeing some unexpected defects appear if the resulting molded alloy is used for a bearing or a piston.

Also, it is not recommended to incorporate the total amount of such elements in the range greater than 20% by weight.

Instead of these elements, 0.1 to 4% by weight of phosphorus (P) can be incorporated into the casting to obtain an identical effect.

If the graphite is incorporated in an amount of 20 to 30% by weight, the resulting aluminum alloys containing the graphite are suitable as metallic elements to be used at low load and at high speed.

If the graphite is incorporated in an amount of 2 to 15% by weight, particularly between 3 and 5%, the resulting aluminum alloys are suitable as metallic elements to be used under conditions of friction by lubricant, because the parts containing graphite are effective in providing an oil sink.

The graphite can also advantageously be incorporated in an amount of 15 to 20% by weight.

It is even more desirable that the temperature of the casting into which the graphite particles are thrown is between a value greater than 50 ° C relative to the liquidus and approximately 900 ° C. When the temperature is not maintained above this level 50 ° C higher than the liquid, the fluidity of the casting degrades and faults such as blowing are likely to form.

It is not desirable that the temperature of the casting be higher than 900 ° C, because the graphite particles are likely to float. It is possible to use natural graphite particles. The liquid is approximately at 570 ° C with an AI-Si alloy containing 12%, by weight, of Si, at 700 ° C with an Ai-Si alloy containing 20%, by weight of Si, at 640 ° C with an alloy AI-Sn containing 10%, by weight, of Sn and at 650 ° C. with an AI-Cu alloy containing 4%, by weight, of Cu. It is recommended to add Cu, Mg, Ni, Zn, Mn or Pb, and similar alloying elements in small amounts to these two element-matrix systems to strengthen the matrix. The liquidus temperature changes according to the quantity of elements added to suppress the buoyancy of the graphite particles and if these graphite particles are added adequately to prevent them from floating, the temperature changes only by ± 200 ° C.

The casting, immediately before the incorporation of the graphite particles, is maintained either at rest or agitated. When the casting is kept at rest, it must be stirred after incorporating the graphite particles. In any case, once the graphite particles are incorporated, they are suspended in the eddies of the casting, produced by stirring, so as to facilitate their dispersion.

This operation is very important because, otherwise, one cannot obtain a molded ingot in which the graphite particles are uniformly dispersed. When the stirring of the pouring is finished and this pouring is left to stand, it is solidified under pressure. This solidification under pressure results in a rapid solidification of the casting. The heat transfer between the casting and the mold is improved by pressurization, the solidification of the casting is accelerated and a precise molding structure is obtained.

In addition, the imperfections of the ingot also disappear. A pressure of 400 to 1000 kg / cm ² is desirable to achieve solidification. If this pressure is less than 400 kg / cm ² , you cannot extract enough gas. If, on the contrary, it is greater than 1000 kg / cm ² , such a high pressure requires too large a device, thereby increasing the costs of this apparatus.

It is also possible to shape an ingot in which the graphite is uniformly dispersed, by varying the shape of the metal mold used for this purpose, for example by making the diameter of the mold long and narrow, and by using a water cooling system.

In the aluminum alloy containing graphite, the latter generally acts as a solid lubricant and greatly contributes to improving the abrasion resistance. This action is influenced by the size of the graphite particles used.

When the size of the graphite particles is too small, adhesion occurs in these friction particles and the graphite adheres to the friction surface of a contact element. This phenomenon is often observed when the size of the graphite particles is between 20 and 50 µm. If this size is less than these values, the graphite which adheres to the contact is expelled from the friction system.

In view of the above, it is desirable for the mean diameter of the graphite particles to be used to be 50 μm. The degree of dispersion of the graphite particles is influenced by the speed of agitation of the casting. For example, an aluminum alloy containing, by weight, 12% of Si and 3% of Cr is melted and maintained at a temperature of 700 ° C in a graphite crucible of 90 mm in diameter. The casting is then stirred using paddles at different speeds, and powder is added. natural graphite of 60 to 80 meshes in the casting, in an amount equal to 9%, by weight, then the dispersion of the graphite particles is observed. At a rotational speed of less than 50 rpm, no vortex occurs in the flow which is only stirred, so it takes a long time before the graphite particles are dispersed in the flow. In addition, a small part of these graphite particles does not disperse in the casting despite a fairly long stirring, given the stains on the surface layers.

At a stirring speed greater than 500 revolutions / minute, numerous disordered vortices are observed and the incorporated graphite particles float on the surface of the casting. Between 50 and 500 revolutions / minute, normal vortices occur and the graphite particles are dispersed in the casting.

Possibility of industrial exploitation

We will now explain, according to comparative examples, certain embodiments of the invention.

Embodiment 1

700 g of an AI-Si alloy containing 20%, by weight, of Si are melted in a graphite crucible 90 mm in internal diameter and a temperature of 690 ° C. is maintained. An element in the form of a pallet is introduced into the crucible which will make the AI-Si alloy flow turn and agitate at 100 revolutions / minute to form vortices.

Next, pulverized natural graphite 177 to 250 μm in size is added to the casting, in an amount of 9% by weight. One of the following elements is incorporated into the casting: Ti, Cr, Zr, V, Ni, Co, Mn and Nb, and the quantity of such an incorporated additive element is changed to determine the quantity of the necessary additive element to disperse up to 30% by weight of graphite particles without causing them to float. The measured results are shown in Table 1. It can be seen there that if the casting contains one of these elements in an amount of 1 to 20%, by weight, the graphite particles can be incorporated between 2 and 30% in weight. In this process, solidification under pressure takes place at 600 kg / cm ² .

Again ingots an ingot with incorporated graphite particles which contains an element effective in suppressing the buoyancy of graphite, but the graphite particles do not float. No difference was observed in dispersing the graphite particles based on the difference of the additive element.

Comparative example 1

700 g of an AI-Si alloy containing 20%, by weight, of Si is melted in a graphite crucible with an inside diameter of 90 mm, and the temperature of the casting is maintained at 850 ° C. A pallet-shaped element is introduced into this crucible which will rotate and stir the flow of AI-Si alloy at 100 revolutions / minute to form vortices. Then 9%, by weight, of pulverized natural graphite of 177 to 250 µm in thickness are added to this flow and solidified under pressure of 600 kg / cm 2. However, the graphite floats on the surface of the flow and does not disperse in it.

Comparative example 2

700 g of an AI-Sn alloy containing 10%, by weight, Sn is melted in a graphite crucible with an internal diameter of 700 mm and the temperature of this casting is maintained at 650 ° C. A paddle-shaped element is introduced into the crucible which will rotate and stir the AI-Sn alloy at 100 revolutions / minute to form vortices. Then 9% by weight of pulverized natural graphite of 177 to 250 _μm in thickness is added to this flow and solidified under pressure of 600 kg / cm ² . However, graphite particles float on the surface of the casting and do not disperse in the latter.

Comparative example 3

Under the same conditions as in Comparative Example 1, an AI-Si alloy casting is produced and the elements Ba, Be, Ce, Hf, Cs, Fe, K, Ca, Mg, Np and Sb are added to it individually. . Then, the flow is rotated to cause vortices. Under these conditions, pulverized natural graphite of 177 to 250 μm in thickness is added to this flow. However, the graphite particles float on the surface of the casting and do not disperse in the latter.

(See Table 1, page 6)

Embodiment 2

700 g of pure aluminum are melted in a graphite crucible 90 mm in internal diameter and the temperature of the casting is maintained at 710 ° C. A pallet-shaped element is introduced into the crucible casting which will make this aluminum casting rotate and agitate at 100 revolutions / minute to form vortices. Then 9% by weight of pulverized natural graphite of 177 to 250 μm in thickness is added to this flow. However, the graphite particles float on the surface of the casting and do not disperse in the latter. On the contrary, if a flow of AI-Ti alloy containing 5%, by weight, of Ti is maintained at a temperature of 1100 ° C. and under the stirring conditions mentioned above, the same amount is added particles of graphite, which disperse in the casting and do not float on its surface.

The aluminum casting containing the graphite is then solidified under a pressure of 600 kg / cm ² and an aluminum alloy containing graphite is thus produced.

Embodiment 3

An AI-Cu-Zr alloy containing, by weight, 50% Cu and 3% Zr is melted in a graphite crucible with an internal diameter of 90 mm inside diameter and the resulting casting is maintained at a temperature of 750 ° C. A pallet-shaped element is introduced into the crucible with which the AI-Cu-Zr alloy will be rotated and stirred at 100 revolutions / minute to form vortices in the casting. Then 2% by weight of pulverized natural graphite, the size of which varies from 150 to 105 µm, from 177 to 150 µm, from 250 to 177 µm, from 500 to 250 µm, 710 to 500 µm or greater than 710 µm, until the flotation of the graphite particles occurs, to determine the relationship between the quantity of dispersed graphite and the particle size thereof. Solidification is carried out under a pressure of 600 kg / cm ² . The relationship between the amount of dispersed graphite and the particle size is determined by identical methods by changing the Zr. The results are shown in the accompanying single drawing. In this figure, region 1 represents a flotation region of graphite and region II a dispersion region of graphite. It will be seen there that the quantity of dispersed graphite changes according to the quantity of the additive element added and that the graphite is likely to float on the surface of the casting according to its particle size.

Embodiment 4

An AI-Si alloy containing, by weight, 12% of Si is melted in a graphite crucible with an inside diameter of 90 mm, and 0.1, 0.5, 1.0, 2 are added to this flow, respectively. , 0, 3.0 and 4.0%, by weight, of phosphorus. Then, the flows are maintained at a temperature of 700 ° C. We introduce in the crucible a pallet-shaped element with which the AI-Si-P alloy will be rotated and stirred at 150 revolutions / minute to form vortices.

Graphite particles from 177 to 250 µm in size are added to the casting, at a rate of 2% by weight, in order to determine the quantitative limit of the dispersed graphite particles as a function of each casting. The quantitative limit of the graphite particles dispersed is determined by an identical process with an AI-Si alloy containing, by weight, 20% of Si, an AI-Sn alloy containing, by weight, 5% of Sn and an AI-Cu alloy. containing, by weight, 4% Cu. The results are shown in Table 2. On the latter, it can be seen that the limit quantity of the dispersed graphite particles is influenced by the quantity of phosphorus but not by the matrix. In addition, when it is necessary to incorporate an amount greater than 30%, by weight, of graphite particles, 3.0 to 4.0%, by weight, of phosphorus can be added.

Claims (16)

1. A process for preparation of graphite containing aluminium alloys by incorporating graphite into an aluminium or aluminium alloy melt, comprising the steps of incorporating at least one additive element selected from the group consisting of titanium, chromium, zirconium, nickel, vanadium, cobalt, manganese, and niobium in the range of 1.5-20 % by weight, Into an aluminium or an aluminium alloy melt ; then, incorporating raw graphite particles without metal-coating, into the melt in an amount of 2-30 % by weight, and dispersing the raw graphite particles into the melt ; and thereafter, solidifying the aluminium or aluminium alloy melt containing the raw graphite particles.

2. A process according to claim 1, wherein the raw graphite particles are incorporated in an amount of 20-30 % by weight, and the additive element is incorporated in the melt to suppress floating of the raw graphite particles to the surface of the melt.

3. A process according to claim 1, wherein the raw graphite particles are incorporated in an amount of 15-20 % by weight, and the additive element is incorporated in the melt to suppress floating of the raw graphite particles to the surface of the melt.

4. A process according to claim 1, wherein the raw graphite particles are incorporated in an amount of 2-15 % by weight, and the additive element is incorporated in the melt to suppress floatting of the raw graphite particles to the surface of the melt.

5. A process according to claim 4, wherein the raw graphite particles are incorporated in an amount of 3-5 % by weight.

6. A process according to claim 1, 2, 3 or 4, wherein the average particle size of the raw graphite particles is larger than 50 µm in diameter.

7. A process according to claim 1, 2, 3 or 4, wherein the aluminium alloy is an Al-Sn alloy, an AI-Cu alloy, an Al-Pb alloy or an AI-Si alloy.

8. A process according to claim 1, 2, 3 or 4, wherein the temperature of the melt is held between a temperature 50 °C higher than the liquidus of the melt and 900 °C.

9. A process according to claim 1, 2, 3 or 4, wherein the aluminium or aluminium alloy melt containing raw graphite particles is solidified under the pressure of 400-1 000 kg/cm².

10. A process according to claim 1, 2, 3 or 4, wherein the aluminium alloy melt containing the raw graphite particles is solidified by water cooling.

11. A process for preparation of graphite-containing aluminium alloys by incorporating raw graphite particles Into an aluminium or aluminium alloy melt, comprising the steps of incorporating phosphorus Into the aluminium or aluminium alloy melt in an amount of 0.1-4% weight ; then, incorporating and dispersing the raw graphite particles without metal-coating in an amount of 4-30 % by weight ito the melt ; and thereafter, solidifying the aluminium or aluminium alloy melt containing the raw gra_l hite.

12. A process according to claim 11, wherein the average size of raw graphite particles is larger than 50 µm.

13. A process according to claim 11, wherein the aluminium alloy is an Al-Sn alloy, an AI-Cu alloy, an Al-Pb alloy or an Al-Si alloy.

14. A process according to claim 11, wherein the temperature of the melt is held between a temperature 50 °C higher than the liquidus of the melt and 900 °C.

15. A process according to claim 11, wherein the aluminium or aluminium alloy melt containing the raw graphite particles is solidified under the pressure of 400-1 000 kg/cm².

16. A process according to claim 11, wherein the aluminium or aluminium alloy melt containing the raw graphite particles is solidified by water cooling.

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