GB2112813A - Wear-resistant aluminum base composite material suitable for casting and method of preparing same - Google Patents

Abstract

An aluminum base composite material suitable for casting comprises a base material which is a conventional aluminum casting alloy containing 4-12 wt% of Si and fine particles of alumina, preferably having a mean particle size of 0.01 to 10 mu m dispersed in the aluminum alloy in an amount of from 0.5% to 10% by weight, based on the weight of the aluminum alloy. Mixing of the alumina particles with the aluminum alloy is performed while the temperature of the aluminum alloy is maintained within a range where the solid phase and the liquid phase of the aluminum alloy coexist. Cast and heat-treated components, e.g. automobile engine parts, of the composite material are high in resistance to wear. <IMAGE>

Description

SPECIFICATION Wear-resistant aluminum base composite material for casting and method of preparing same This invention relates to a novel aluminum base composite material for casting, which is high in resistance to wear and is suitable for sliding parts in automobile engines for example, and to a method of preparing the same.

Wear-resistant materials are widely used for machine parts that make sliding contact with other parts, such as cylinders and cylinder liners in internal combustion engines and various bearings.

For example, high-silicon aluminum alloys, cast iron and die-cast aluminum alloys with a coating of a hard film formed by a surface-hardening treatment such as plasma-spraying or wire explosion can be named as wear-resistant materials for automotive engine parts.

However, most of conventional wear-resistant materials have certain disadvantages or offer some problems too in practical use. For example, high-silicon aluminum alloys are unsatisfactory in their fluidity in casting processes and, therefore, casts of alloys of this type are liable to suffer from casting defects such as cracks or insufficient run of the molten metal depending on the shapes of the casts. The surface hardening coatings on the surfaces of cast aluminum alloy or cast iron offer difficulty in finishing of the produced parts and sometimes peel away from the base metal due to a difference in thermal expansion rate between the base metal and the hard coating material.

Cast iron has commonly been used for most parts of internal combustion engines, but cast iron has a relatively high specific gravity and, therefore, has become unfavorable as a primary material for engines to serve as powerplants of vehicles such as automobiles in which the reduction of gross weight has become a matter of serious concern. Where the cylinder block of an automobile engine is formed of a casting of an aluminum alloy and a cylinder liner or sleeve formed of cast iron is tightly fitted into each cylinder bore of the cylinder block, the difference in thermal expansion rate between the aluminum alloy and cast iron becomes a cause of discontinuity of the interface between the cylinder block and the cylinder liner and often leads to seizing between the piston and the cylinder liner due to insufficient cooling of the sliding contact surfaces via the cylinder block.

It is an object of the present invention to provide a novel aluminum base composite material for casting, which has a low in specific gravity and high resistance to wear and seizing at sliding contact surfaces of the casts.

It is another object of the invention to provide a method of preparing the aluminum base composite material according to the invention.

The present invention provides a wearresistant aluminum base composite material for casting, consisting essentially of an aluminum alloy for casting, which contains 4 to 12% by weight of Si, and fine particles of alumina which are dispersed in the aluminum alloy and amount to 0.5 to 10% by weight of the aluminum alloy.

A method according to the invention for preparing the composite material of the invention is characterised by the step of mixing 0.5 to 10 parts by weight of fine particles of alumina with 100 parts by weight of an aluminum alloy for casting, the alloy containing 4 to 12% by weight of Si, while the aluminum alloy is kept heated to maintain the temperature of the alloy within a range where the solid phase and the liquid phase of the alloy coexist.

The aluminum alloy base material of the composite material according to the invention can be selected from conventional aluminum alloys for sand-, metal- or shell-mold casting processes and aluminum alloys for die casting and may contain small amounts of auxiliary alloying elements used in such aluminum alloys.

It is preferred that the mean particle size of the alumina particles added to the aluminum alloy is in the range from about 0.01 ,um' more preferably from about 0.08 ,um, to about 10 ,um.

After completion of the above stated mixing step, it is optional whether to immediately raise the temperature of the composite material above the melting point of the aluminum alloy in order to successively perform casting of the composite material or to allow the composite material to once cool and solidify. The composite material of the invention can be formed into variously shaped parts by conventional casting processes, and the casts are subjected to a heat treatment having the effect of artificial age hardening or stabilization.

The composite material of the invention has a low specific gravity since it consists of an aluminum alloy and a limited amount of alumina, but this composite material is far higher in wear resistance than conventional high-silicon aluminum alloys for casting and even comparable to cast iron in this respect. Accordingly, this composite material is quite suitable for automobile engine parts that make sliding contact with other parts, e.g. cylinder liners. Such engine parts formed of this composite material are light in weight and excellent in resistance to wear and seizing and therefore operate with decreased loss of energy at the sliding contact surfaces and exhibit good durability. Thus the present invention contributes to the reduction of gross weight and improvement in durability of the powerplants of automobiles and other vehicles.

In the accompanying drawings: Fig. 1 is a schematic and sectional view of an exemplary apparatus for the preparation of an aluminum base composite material according to the invention; Fig. 2 is a schematic and sectional view of a die-casting machine useful for casting of the composite material of the invention; Fig. 3 is a perspective view of an engine cylinder liner formed of the composite material of the invention; Fig. 4 is a schematic and sectional view of an abrasion test machine used in examples of the invention; Fig. 5 is a graph showing the relationship between the content of alumina particles in the composite material according to the invention and the wear resistance of the material observed in an example of the invention;; Figs. 6 and 7 are micrographs showing the structure of a composite material according to the invention taken before melting of the composite material and after casting and heat treatment, respectively; Fig. 8 is a graph showing the relationship between the mean particle size of alumina particles in the composite material according to the invention and the wear resistance of the material observed in an example of the invention; and Fig. 9 is a graph showing the results of an abrasion test in which an example of the composite material according to the invention was compared with conventional gray cast iron and high-silicon aluminum alloy.

The base material for an aluminum base composite material of the invention is an aluminum alloy for casting, and the aluminum alloy is required to contain 4 to 12% by weight of Si. The content of Si in the aluminum alloy must be at least 4% by weight because it is difficult to afford good castability to the composite material by using an aluminum alloy having a lower content of Si, but the content of Si in the aluminum alloy must not exceed 12% because the presence of a larger amount of Si makes it impossible to realize a hypoeutectic composition that is important to the present invention.If the temperature of a hypereutectic composition containing more than 12% of Si is maintained within a range where solid phase and liquid phase of the aluminum alloy coexist there occurs crystallization of Si as primary crystals from the liquid phase, and the silicon crystals float up since the specific gravity of the silicon crystals (2.4 g/cm3) is smaller than that of the liquid phase of the alloy (about 2.7 g/cm3) so that the composition of the base material for the composite material of the invention becomes nonuniform. Moreover, the floating and separation of the silicon crystals become an obstacle to uniform mixing of the subsequently added alumina particles with the aluminum alloy.

Besides Si, the aluminum alloy may optionally contain auxiliary alloying elements which serve for reinforcement of the alloy structure or stabilization of crystal grains, such as Cu, Mg, Zn, Fe, Mn, Ni, Sn and/or Sb, in such amounts as are usual in conventional aluminum alloys for casting or in somewhat larger or smaller amounts. For example, it is possible to select a suitable alloy from aluminum alloys for sand-, metal- or shellmold casting specified by JIS (Japanese Industrial Standard) H 5202 (Class AC aluminum alloys), which may contain up to 13% of Si, up to 4.5% of Cu, up to 1.5% of Mg and smaller amounts of Zn, Fe, Mn, Ti and some times Ni, and aluminum alloys for die casting specified by JIS H 5302 (Class ADC aluminum alloys) which may contain up to 13% of Si, up to 4% of Cu, and smaller amounts of Mg, Zn, Fe, Mn, Ni and Sn.

The percentage of alumina particles used in the present invention is limited within the range from 0.5 to 10% by weight of the aluminum alloy because composite materials obtained by adding less than 5% of alumina particles to the aluminum alloy are insufficient in resistance to wear and seizing and also because composite materials obtained by the addition of more than 10% of alumina particles become unsatisfactory in wear resistance and so brittle as to offer difficulty in machine-finishing of the casts.

As mentioned hereinbefore, it is preferred to use alumina particles having a mean particle size in the range from about 0.01 m to about 10 ym.

It is desirable that the particulate alumina for use in this invention is in the form of a-Al2O3 which is hard and stable but when mean particle size is less than 0.01 m particulate alumina exists as Awl203 which is lower in hardness. In this regard, it is most preferable to use alumina particles not smaller than about 0.08 ,um in mean particle size.

On the other hand, the use of alumina particles larger than about 10,um results in gradual lowering in the wear resistance of the composite material with enlargement of the mean particle size, and the wear resistance of the composite material becomes poor when the mean particle size of alumina particles reaches about 1 5 4m.

Probably the reason for such tendency is that large particles of alumina easily separate from the sliding contact surface of the aluminum alloy as the base metal and act on the contact surface as an abrasive.

Fig. 1 shows an exemplary apparatus for mixing fine particles of alumina with an aluminum alloy to prepare an aluminum base composite material according to the invention. The apparatus has a cylindrical enclosure 10 with a detachable lid 12. In the interior there is a cylindrical heating plate 1 6 provided with electric heater 1 8 with heat-insulating material 14 between heating plate 1 6 and the wall of the enclosure 10. A crucible 24 is fixedly placed on a plate 20 mounted on a shaft 22 which is rotatable and axially movable upwards and downwards. As a stirrer for the material in the crucible 24, a blade 26 is fixed to the lower end of a shaft 28 which extends through a hole in the lid 12 and is rotatable and axially movable downwards and upwards. A hopper 32 is attached to the lid 12 to introduce alumina particles into aluminum alloy 30 melted in the crucible 24. The lid 12 is provided with a gas inlet 34 to introduce an inert gas such as nitrogen or argon into the interior of the apparatus. In this apparatus an aluminum base composite material of the invention is prepared in the following manner.

Initially the stirrer blade 26 is held above the position of the crucible 24 as indicated by the phantom line in Fig. 1, and the crucible 24 containing a measured quantity of an aluminum alloy for casting which meets the requirement of the present invention is positioned on the rotatable plate 20. Then nitrogen gas is introduced into the apparatus to maintain an inert gas atmosphere in the apparatus during the following mixing process. In this state the heater 1 8 is energized to heat and melt the aluminum alloy 30 in the crucible 24. The current supplied to the heater 1 8 is controlled so as to maintain the temperature of the aluminum alloy 30 within a range where coexist the solid phase and liquid phase of the alloy 30.Such a range of temperature is determined in advance by thermal analysis of the aluminum alloy. However, it is preferred to first heat the aluminum alloy 30 up to a temperature above the melting point of the alloy to thereby completely melt the alloy 30 and then lower the temperature of the alloy 30 to a temperature within the aforementioned range.

The stirrer blade 26 is moved downwards to submerge in the aluminum alloy 30 while the alloy is in the completely molten state. By this procedure the submersion of the stirrer blade 26 in the alloy 30 can be done more smoothly with less resistance of the alloy to the blade than in the case of submerging the blade 26 in the alloy 30 partly in solid phase. To maintain the aluminum alloy 30 in the crucible 24 in a uniformly heated state, the shaft 22 for the support plate 20 and the shaft 28 of the stirrer blade 26 are continuously rotated. Preferably, the stirrer shaft 28 is rotated in the direction reverse to the direction of rotation of the support plate 20 with a view to effectively make shearing stirring of the partly liquid and partly solid aluminum alloy 30.

Under such stirring the aluminum alloy 30 kept heated at the above described level assumes a paste-like state.

In this state a predetermined quantity of alumina particles is introduced into the aluminum alloy 30 by using the hopper 32, and the stirring is continued in order to uniformly disperse the alumina particles in the partly molten aluminum alloy 30. In the mixing process it is desirable that 5 to 65% by weight of the aluminum alloy 30 is in solid phase, because when the solid phase is less than 5% the viscosity of the alloy 30 becomes unfavorably low for uniform dispersion of the alumina particles in the alloy but when the solid phase is more than 65% excessively high viscosity of the alloy 30 offers difficulty in stirring and hence to uniform mixing of the alumina particles with the alloy 30.

Preparatory to the above described mixing process, the alumina particles may optionally be subjected to a surface treatment such as plating for the purpose of improving wettability of the alumina particles with the aluminum alloy.

After sufficient mixing the stirrer blade 26 is pulled up, and the heating is terminated to allow the mixture of the aluminum alloy and the alumina particles to cool down and solidify.

Obtained as the result is an ingot of an aluminum base composite material according to the invention. Where it is intended to perform a casting process successively to the preparation of the composite material, it is permissible to raise the temperature of the unsolidified mixture of the aluminum alloy and the alumina particles so as to completely melt the aluminum alloy in the mixture. The melting may be performed in the mixing apparatus of Fig. 1 or alternatively in a separate apparatus located near a casting machine. Lowering in the temperature of the mixture, whether intentional or not, during the interval between the mixing process and the melting process raises no problem.In the melting process preparatory to casting, it is usually suitable to heat the composite material of the invention up to a temperature higher than the melting point of the used aluminum alloy by about 1000C, but it is a matter of course to suitably determine the heating temperature for melting depending on the type of the casting process and the casting conditions.

In the composite material of the invention, the alumina particles remain uniformly dispersed in the aluminum alloy even after complete melting of the aluminum alloy whether the composite material is first solidified or not. This is an important advantage of this composite material.

We have carried out many experiments in which various kinds of ceramics in the form of fine particles were uniformly dispersed in aluminum alloy by the above described mixing process, and we have found that complete melting of the aluminum alloy after mixing with ceramics particles results in agglomeration, sedimentation of floating up of a considerable portion of the dispersed ceramics particles except when the ceramic is alumina. Considering that the true specific gravity of a-A12O3 is 3.9 g/cm3 while the aluminum alloys used in the present invention are about 2.7 g/cm3 in specific gravity, it is certain that the continuance of uniform dispersion of alumina particles in molten aluminum alloy is to be attributed to not only the specific gravity values but also some other factors that will probably include the surface energy of the alumina particles.

Conventional casting processes for aluminum alloys can be used for casting of an aluminum base composite material according to the invention. However, gravity die casting of the composite material sometimes encounters difficulty and fails to give good casts because molten metal of this composite material has a higher viscosity than conventional aluminum alloys for casting due to the existence of alumina particles dispersed in the molten aluminum alloy.

Therefore, it is preferable to employ a pressure die casting process, centrifugal casting process or vacuum casting process for casting a composite material according to the invention.

For example, Fig. 2 shows the principal part of a pressure die-casting machine useful for casting of the composite material of the invention. The die-casting machine has a combination of a movable metal die 40 and a stationary metal die 42 so shaped as to produce a die cavity 44. In the illustrated case, the die cavity 44 has a shape corresponding to an engine cylinder liner 60, shown in Fig. 3, in the shape of a hollow cylinder flanged at one end and has slight draft or taper to facilitate parting of the cast from the metal dies.

After closing and clamping of the metal dies 40, 42, a required quantity of molten metal prepared by completely melting the aluminum alloy in the composite material of the invention is poured into an injection sleeve 48 from a sprue 50, and a plunger 52 for injection is advanced into the injection sleeve 48 to pressurize the molten metal in the sleeve 48 and to inject the pressurized molten metal into the die cavity 44 via a runner 46. After solidification of the aluminum base composite material in the die cavity 44 the die set is opened by retreating the movable metal die 40, and then the cylindrical cast is separated from the metal die by advancing ejection pins 54.

The casts obtained by casting of an aluminum base composite material according to the invention are subjected to a suitable heat treatment. Usually the casts need to be subjected to stabilizing annealing to prevent changes in the dimensions of the casts with the lapse of time. For example, stabilizing annealing is indispensable to cylinder liners formed by casting of the aluminum base composite material, because unless properly annealed the cylinder liners which are finished by machining with very strict tolerance of dimensions will possibly undergo deformation by the effect of very severe thermal environment in internal combustion engines to result in widening of the clearance between the cylinder liner and the piston slidably received therein. Besides stabilizing annealing, in many cases the casts need for certain heat treatment that affords required strength to the casts.For example, the heat treatment to improve the strength and other mechanical properties of the casts will be performed p the manner of the T5, T6 or T7 heat treatment for conventional aluminum alloys to thereby achieve artificial age hardening of the casts.

After heat treatment, and where necessary, the casts are subjected to machining for the purpose of finishing. For example, the cylindrical cast formed in the die-casting machine of Fig. 2 to produce the cylinder liner of Fig. 3 must be subjected to lathing or honing to finish the outer and inner cylindrical surfaces so as to correct the taper due to the draft of the die cavity 44. If the composite material of the casts contain an excessively large amount of alumina particles, machining of the casts for finishing will cause minute cracks in the casts or will suffer from rapid wear of the machining tools.However, we have experimentally confirmed that such problems or troubles in practical machining of the casts arise only when the alumina particles in the composite material amount to more than 11% by weight of the aluminum alloy as the base metal of the composite material.

The present invention will further be illustrated by the following nonlimitative examples.

Example 1 The aluminum alloy for casting used in this example was ADC 12 of JIS H 5302, an equivalent of ASTM SC 11 4A aluminum alloy, which contained 11.8% by weight of Si and about 2.5% of Cu. The particulate alumina used in this example had a mean particle size of 0.08 ym, and the percentage by weight of the alumina particles to the aluminum alloy was varied over the range from 0.5% to 10.0% to obtain eleven composite alloys different only in the content of alumina particles.

Each of the composite alloys was prepared by adding the given amount of alumina particles to the aluminum alloy with stirring while the aluminum alloy was maintained at 5750C, at which coexisted the solid phase and liquid phase of the alloy, and was in a paste-like state and continuing stirring to uniformly disperse the alumina particles in the aluminum alloy. After sufficient mixing the heated mixture of the aluminum alloy and alumina particles was divided into two portions. The first portion of the mixture was left to cool and solidify and then heated up to 6860C, which was higher than the melting point (5860C) of the aluminum alloy ADC 12, to become molten, whereas the second portion was immediately heated up to 6860C upon completion of mixing of the alumina particles with the aluminum alloy.By using a die-casting machine of the type shown in Fig. 2, each of the thus prepared two batches of molten metal was cast into a cylindrical body. The casting conditions were as follows.

Injection Pressure 500 kg/cm2 Injection Speed 1.2 m/sec Metal Die Temperature 200-2500C Pouring Temperature 685"C Mold Clamp Time 12 sec The cylindrical bodies cast in this example were subjected to stabilizing annealing which consisted of heating at 2400C for 3 hr and subsequent furnace cooling. After that test pieces for abrasion testing were cut out of each of the cast bodies. The test pieces were in the shape of square prisms 5 mmx5 mm wide and 10 mm long.

Fig. 4 shows an abrasion testing machine used in this example. This abrasion testing machine was of the pin-and-disc type often used for testing of alloys for automotive engine parts such as cylinder liners and piston rings. Four identical test pieces 70 were tightly set in a disc-shaped holder 72 which was fixedly mounted on a rotatable plate 74. A metal disc 76 which has hard chromium plating and was fixed to a holder 78 was pressed against the end faces of the test pieces 70 by applying a load to a compression rod 80. Indicated at 82 is a hole or oil passage through which lubricating oil was supplied to the contact surfaces of the metal disc 76 and the test pieces 70.The plate 74 was rotated such that the velocity of the sliding movement of each test piece 70 over the end face of the metal disc 76 was 3 m/sec in one case but 8 m/sec in the other case, and in either case the rotation was continued until the total slide distance travelled by each test piece 70 reached 100 km. The pressure at the contact surfaces was maintained at 50 kg/cm2, and No. 30 motor oil heated to 800C was supplied as lubricating oil to the contact surfaces at a rate of 350 400 ml/min.

For comparison, six kinds of composite alloys were prepared generally similarly to the composite alloys of this example but by increasing the percentage of the alumina particles to the aluminum alloy ADC 12 to 11 12%, 13%, 14%, 15% and 16%, respectively. These modified composite alloys were individually cast and subjected to stabilizing annealing in the above described manner, and test pieces cut out of the cast bodies were subjected to the above described abrasion test. Further, the aluminum alloy ADC 12 itself was subjected to the same test.

Fig. 5 shows the results of the abrasion tests for the composite materials of Example 1 and the materials additionally prepared for the sake of comparison.

The testing of the aluminum alloy ADC 12 without the addition of alumina particles at the sliding velocity of 3 m/sec resulted in the occurrence of seizing between the test pieces 70 and the chromium plated disc 76 during an initial stage of the test before the rise of the pressure at the contact surfaces up to the intended level of 50 kg/cm2, and the same alloy underwent a very large amount of wear when tested at the sliding velocity of 8 m/sec. In contrast, the composite material prepared by adding 0.5-11% of alumina particles to the aluminum alloy exhibited excellent wear resistance.However, the addition of still larger amounts of alumina particles caused lowering in the wear resistance to such an extent that the amount of wear of the composite material prepared by the addition of 16% of alumina particles became nearly equivalent to that of the aluminum alloy ADC 12. When 20% of alumina particles was added to the aluminum alloy to confirm such tendency, the amount of wear reached about 1 50 ym.

Also for the sake of comparison, an ordinary gray cast iron (FC 25 of JIS G 5501) was subjected to the same abrasion test under the same test conditions. The amount of wear of the cast iron was 8 Mm when tested at the sliding velocity of 3 m/sec and 1544m at 8 m/sec.

Therefore, the aluminum base composite materials of Example 1 can be evaluated as comparable to gray cast iron in wear resistance.

Of course these composite materials have an important advantage that they are much lower in specific gravity. In the abrasion test, no significant difference was observed between the samples prepared by once solidifying the mixture of the aluminum alloy and alumina particles and the samples prepared by immediately raising the temperature of the mixture beyond the melting point of the alloy.

Example 2 The aluminum alloy for casting used in this example was AC 8B of JIS H 5202, which contained 9.45% by weight of Si, about 3% of Cu and about 1% of Mg. In this example several batches of composite alloy were prepared by using several lots of alumina particles different in mean particle size. The percentage of alumina particles to the aluminum alloy was a constant 5% by weight. In the preparation of each batch of composite alloy the addition of alumina particles to the aluminum alloy AC 8B was performed in the same manner as in Example 1, while the aluminum alloy was maintained at 5600C at which coexisted the solid phase and liquid phase of the alloy.After mixing, a half portion of each batch was left to cool and solidify and then heated up to 6850C, which was higher than the melting point (5850C) of the aluminum alloy AC 8B, to become molten, whereas the remaining portion was immediately heated up to 6850C upon completion of the mixing.

Fig. 6 is a micrograph (x200) showing the structure of the composite alloy prepared in this example by using alumina particles having a mean particle size of 0.1 ym. In the photograph the white areas are given by the primary crystals of a-Al, the gray areas by Si crystallized by eutectic reaction and the black spots by Awl203. As represented by this micrograph, uniform dispersion of the added alumina particles in the resultant composite alloy was confirmed by microscopic observation for every sample prepared in this example.

By using the die-casting machine described in Example 1, each of the thus prepared batches of molten metal was cast into a cylindrical body under the same casting conditions as in Example 1. The cylindrical bodies cast in this example were subjected to a heat treatment, which was corresponding to the T6 heat treatment and was a combination of initial heating at 5200C for 2 hr as a solution treatment and subsequent tempering performed by maintaining the casts at 1 800C for 4 hr. After that, 5 mmx5 mm wide and 10 mm long test pieces for abrasion test were cut out of each of the cast bodies and subjected to the abrasion test described in Example 1 under the same test conditions.

Fig. 7 is a micrograph (x200) showing the structure of the composite material prepared in this example by using alumina particles having a mean particle size of 0.1 4m observed after the above described casting and heat treatment processes. In the photograph the white areas are given by fine dendrite structure of the primary crystals of a-Al, the gray areas by Si crystallized by eutectic reaction and the black spots by Al2O3. it is understood that the alumina particles were uniformly dispersed in the aluminum alloy of which the structure became very fine by the effect of melting after the addition of the alumina particles thereto and rapid cooling at the die casting process.By comparison with the micrograph of Fig. 6, it was confirmed that the heating of the aluminum alloy above its melting point after the addition of alumina particles, die casting of the molten alloy and heat treatment of the cast produced practically no difference in the manner of dispersion of the alumina particles in the aluminum alloy although the alloy matrix underwent some changes by reason of its thermal history.

Fig. 8 shows the results of the abrasion tests for the samples prepared in Example 2.

When use was made of alumina particles smaller than 0.01 m in mean particle size, though not shown in Fig. 8, it was difficult to achieve uniform dispersion of the alumina particles in the aluminum alloy because of the tendency of the alumina particles to agglomerate (probably by reason that alumina in the form of such fine particles exist usually as y-Al,O, that is as high as 2.28 in true specific gravity), and there occurred seizing between the test pieces 70 and the chromium-plated disc 76 during an initial stage of the test in the both cases of testing at the sliding velocities of 3 m/sec and 8 m/sec, respectively. Such seizing did not occur, and improvement in wear resistance was recognized when the mean particle size of the alumina particles added to the aluminum alloy was 0.01 or or larger.As can be seen in Fig. 8, the amount of wear was very small when the mean particle size of alumina particles added to the aluminum alloy was between 0.08 *4m and 10 jum.

However, the amount of wear increased significantly as the mean particle size of alumina particles was enlarged beyond 10 ym. When use was made of alumina particles having a mean particle size of 20 ym the amount of wear of the resultant composite material was 1 50 Mm in the case of testing at the sliding velocity of 3 m/sec and 120 m at 8 m/sec.

In the abrasion test, no significant difference was observed between the samples prepared by once solidifying the mixture of the aluminum alloy and the added alumina particles and the samples prepared by immediately raising the temperature of the mixture beyond the melting point of the alloy.

Example 3 The aluminum alloy for casting used in this example was AC 4C of JIS H 5202, which contained 7.10% by weight of Si, about 0.19/0 of Cu and about 0.3% of Mg. A composite alloy was prepared by mixing alumina particles, which had a mean particle size of 5 m, amounting to 5% by weight of the aluminum alloy with the aluminum alloy by the method described in Example 1.

During mixing the aluminum alloy was maintained at 5900C at which coexisted the solid phase and liquid phase of the alloy. The composite alloy was melted and used in the die casting process described in Example 1 to produce a cylindrical cast, which was subjected to the T6 heat treatment described in Example 2 and then to finishing machining to obtain a cylinder liner. Test pieces cut out of the cast after the heat treatment were subjected to the abrasion test described in Example 1. In this case the test was carried out at three different sliding velocities: at 3 m/sec, at 5 m/sec and at 8 m/sec. For comparison, an ordinary gray cast iron (FC 25) and a high-Si aluminum alloy (A390) containing 18% by weight of Si were tested under the same test conditions.

As can be seen in Fig. 9, the aluminum base composite material prepared in Example 3 was remarkably superior in wear resistance to the high-Si aluminum alloy and even better than the gray cast iron. Of course this composite material was far lower in specific gravity than the cast iron.

Example 4 Generally similarly to Example 3, five kinds of aluminum base composite alloys were prepared by using five different kinds of aluminum alloys for casting, respectively: namely ADC 12 (used in Example 1), ADC 10 (containing about 9% by weight of Si and about 3% by weight of Cu), AC 8B (used in Example 2), AC 4C (used in Example 3) and AC 2A (containing about 4.5% by weight of Si and about 4% by weight of Cu). For every aluminum alloy, the percentage of the added alumina particles (5 ,um in mean particle size) was 5% by weight of the alloy.During mixing with the alumina particles the aluminum alloys ADC 12 and ADC 10 were both maintained at 5750C, but the aluminum alloys AC 8B, AC4C and AC 2A were maintained at 5600C, at 5900C and at 600 C, respectively, to ensure coexistence of the solid phase and liquid phase of each aluminum alloy.

Each of these composite alloys was melted and used in the die casting process described in Example 3 to produce a cylindrical cast, which was subjected to the heat treatment mentioned in Example 3 and then to finishing machining to obtain a cylinder liner.

Test pieces cut out of each cast after the heat treatment were subjected to the abrasion test described in Example 1. In this test the sliding velocity was constantly 8 m/sec, and the pressure at the contact surfaces was gradually increased to determine the critical contact pressure at which occurred a sharp change in the torque of the shaft on which the rotating plate 74 was mounted. The sharp change in the torque was attributed to the occurrence of seizing between the test pieces 70 and the opposite disc 76, so that the critical contact pressure may be referred to as seizing load. For comparison, gray cast iron FC 25, high Si aluminum alloy for casting A390 and aluminum alloy for casting AC 8B (without the addition of alumina particles) were subjected to the same abrasion test. The results of the test are presented in the following Table.

Base Awl203 Seizing Material Particles (Wt%) Load (kg/cm2) ADC 12 5 250 ADC10 5 190 AC 8B 5 230 AC 4C 5 180 AC 5 170 FC 25 170 A390 - 80 AC 8B 100 The test results in this Table demonstrate that the aluminum base composite materials according to the invention are far superior to conventional aluminum alloys for casting, and even better than gray cast iron, in resistance to seizing and that these composite materials are quite suitable for producing lightweight cylinder liners of automobile engines.

Example 5 Using aluminum alloy for casting ADC 12, an aluminium base composite alloy was prepared by adding alumina particles 0.2 ,um in mean particle size amounting to 5% by weight of the aluminum alloy to the aluminum alloy, which was maintained at 5750C until completion of thorough mixing to ensure coexistence of the solid phase and liquid phase of the alloy. The composite alloy was heated to 6800C to prepare molten metal containing alumina particles, which was used in the die casting process described in Example 1 to produce a cylindrical cast. After heat treatment the cast was finished by machining into a cylinder liner which was 86 mm in outer diameter, 80 mm in inner diameter and 136.5 mm in height.

A plurality of cylinder liners produced in this way were fitted into a cylinder block of an automobile engine by using piston rings made of gray cast iron (FC 25) and plated with chromium.

The engine assembled by using this cylinder block was subjected to an endurance test performed as a bench test. The endurance test was continued for 5 hr at an engine speed corresponding to a car speed of 100 km/hr. The tested engine was disassembled to measure the amounts of wear of both the cylinder liner surfaces and the piston rings. The amount of wear of the cylinder liners was 8,um on an average, and the amount of wear of the piston ring was 5 ym on average. These values can be taken as indicative of very small amounts of wear and verify that cylinder liners made of an aluminum base composite material according to the invention are fully practicable in automobiles for the market.

Claims (17)

Claims

1. A wear-resistant aluminum base composite material for casting, consisting essentially of an aluminum alloy for casting, and containing from 4 to 12% by weight of Si, and fine particles of alumina dispersed in said aluminum alloy in an amount of from 0.5 to 10% by weight, based on the weight of said aluminum alloy.

2. A composite material according to Claim 1, wherein said particles of alumina have a mean particle size of from 0.01 yam to 10,t4m.

3. A composite material according to Claim 2, wherein said particles of alumina have a mean particle size of not less than 0.08 ym.

4. A composite material according to Claim 1, 2 or 3 wherein said aluminum alloy contains up to 4.5% by weight of Cu.

5. A composite material according to Claim 4, wherein said aluminum alloy contains up to 1.5% by weight of Mg.

6. A method of preparing a wear-resistant aluminum base composite material for casting, the method comprising the step of mixing from 0.5 to 10 parts by weight of fine particles of alumina with 100 parts by weight of an aluminum alloy for casting, said alloy containing 4 to 12% by weight of Si, while said aluminum alloy is kept heated to maintain the temperature thereof within a range wherein the solid phase and the liquid phase of said aluminum alloy coexist.

7. A method according to Claim 6, wherein said particles of alumina have a mean particle size of from 0.01 yam to 10,us.

8. A method according to Claim 7, wherein said particles of alumina have a mean particle size of less than 0.08 Mm.

9. A method according to Claim 6, 7 or 8 wherein said aluminum alloy contains up to 4.5% by weight of Cu.

10. A method according to Claim 9, wherein said aluminum alloy contains up to 1.5% by weight of Mg.

11. A method according to any one of Claims 6 to 10, wherein the temperature of said aluminum alloy at the mixing step is adjusted such that from 5 to 65% by weight of said aluminum alloy is in the solid phase.

1 2. A method according to any one of Claims 6 to 11 , further comprising the steps of completely melting said aluminum alloy prior to the mixing step, and lowering the temperature of the molten alloy to a temperature within said range prior to the mixing step.

13. A method according to any one of Claims 6 to 12, further comprising the step of heating the mixture of said aluminum alloy and said particles of alumina to a temperature higher than the melting point of said aluminum alloy to thereby completely melt said aluminum alloy in said mixture.

14. A method according to Claim 13, wherein said temperature in the heating step is higher than said melting point by about 1000C.

1 5. A method according to Claim 13, further comprising the step of solidifying the mixture of said aluminum alloy and said particles of alumina prior to the heating step.

1 6. An aluminum base composite material according to Claim 1, substantially as herein described in any one of Examples 1 to 5.

17. A method according to Claim 6, substantially as herein described in any one of Examples 1 to 5.