GB2290598A - Pistons for internal combustion engines - Google Patents

Description

2290598 PISTONS FOR INTERNAL COMBUSTION ENGINES The present invention

relates generally to pistons for internal combustion engines for motor vehicles, etc. and more particularly. to piston ring grooves thereof.

In recent years, pistons for internal combustion engines for motor vehicles are made of aluminum alloy instead of cast iron so as to achieve a weight reduction in view of requirements of high power and high performance. Grooves for receiving piston rings are formed on anouter peripheral surface of the piston which faces an inner wall of a cylinder bore. A top is ring groove, the nearest one of the piston ring grooves with respect to a combustion chamber, suffers great wear by a piston ring (top ring) due to exposure to a high temperature and direct receiving of combustion pressure in particular. Thus, aluminum micro-welding Is apt to occur between the top ring groove and the top ring.

Various techniques have been proposed to prevent such aluminum micro-welding: (1) Reinforcement of the surface of the top ring groove by compounding inorganic f iber aggregate (see JP-A 59-201953); (2) Application of hybrid MMC (metal matrix composites) by In-Situ process to the pistons (see Automotive Technique No. 891,056 published in May. 1989 by Automotive Technique Society); (3) Reinforcement of the surface of the top ring groove by compounding nickel porous material (see JP-B2 3-30708); (4) Reinforcement of the surface of the top ring groove by an alumilite layer (see JP-A 1-190951); (5) Formation of a copper-alloy layer by electron beam fusion treatment on the surface of the piston ring groove (see Technical Revue No. 1 published in 1988 by Mitsubishi Motor Co., Ltd.; (6) Formation of a ring 2 support portion in the top ring groove by casting aluminum alloy around Ni-resist cast iron as alfin treated; (7) Use of a low silicon aluminum-magnesium alloy member as alumilite-treated on the surface of the top ring groove (see JP-A 1-190951).

However, the above prior arts present the following inconveniences: The prior arts(i)-3) need the use of a high-pressure solidification method in view of applied materials such as inorganic fiber, etc. with respect to a forming method. This results in a rise of manufacturing cost and a restriction of the piston shape.

The prior art (4) contributes to improvement of micro-welding resistance with the piston ring due to presence of the alumilite layer. but fails to provide sufficient wear resistance. Contrariwise, the prior art (5) may fail to provide sufficient micro-welding resistance.

The prior art (6) a technique being applied f rom long ago, ensures wear resistance and micro-welding resistance, but cannot avoid a weight increase due to cast iron making. The prior art (7) fails to provide sufficient wear resistance and micro-welding resistance due to the use of an aluminum-magnesium alloy member.

It would therefore be desirable to be able to provide pistons for internal combustion engine which provide sufficient wear resistance and micro-welding resistance without any increase in weight and manufacturing cost.

According to one aspect of the present invention, there is provided a piston for an internal combustion engine, comprising:

a main body made of aluminum alloy; groove means for defining a plurality of piston ring grooves on an outer periphery of said main body; 1 3 wear resisting means for resisting wear of said groove means, said wear resisting means being made of aluminum alloy containing particulates; and layer means for covering said wear resisting means and said main body in the vicinity thereof.

According to another aspect of the present invention. there is provided a method of manufacturing a piston for an internal combustion engine having a main body and at least one ring groove, the method comprising the steps of:

forming a wear resisting annulus out of aluminum alloy containing particulates; casting molten aluminum alloy for the main body of the piston around said wear resisting annulus. said wear resisting annulus being positioned to correspond to the ring groove of the piston; and subjecting said wear resisting annulus to anodic oxidation treatment.

BRTFF J)FSCTTON OF THF DRAWMCS Fig. 1 is a fragmentary enlarged section showing a top ring groove of a piston embodying the present invention; Fig. 2 is a longitudinal section showing the piston; Fig. 3 is a view similar to Fig. 1, showing a wear resisting annulus having an alumilite layer for the top ring groove; Fig. 4 is a view similar to Fig. 2, showing metal molds for the wear resisting annuls; Fig. 5 is a schematic drawing showing a test on wear resistance; Fig. 6 is a view similar to Fig. 5, showing a test - on micro-welding resistance; Fig. 7 is a table illustrating components of matrix aluminum alloy; Fig. 8 is a table similar to Fig. 7, illustrating results of the tests; and 4 Fig. 9 is a table similar to Fig. 8. illustrating results of the tests.

DETAILED DFRCTTON Referring to the drawings, particularly to Figs.

1 and 2. a piston includes a piston main body 1 which is made of aluminum alloy QISACM-T6) and shaped substantially like a cylinder and has a crown 2 facing a combustion chamber, three piston ring grooves 4, 5, 6 formed on an outer peripheral surface of a ring land 3 arranged below the crown 2, three piston rings 7, 8, 9 engaged with the piston ring grooves (top, second and oil ring grooves) 4, 5. 6, and a skirt 10 arranged below the piston ring grooves 4, 5, 6.

The top ring groove 4 is formed to have a middle 9 mm distant from a top face of the crown 2, and be 4 mm in width and 8 mm in depth. Additionally, only a surface of the top ring groove 4 is formed by a wear resisting annulus 11 made according to a forming method as will be described later.

Referring to Fig. 3, the wear resisting annulus 11 is made of aluminum alloy 11b containing silicon carbide (Sic) particulates 11a, and cast within the piston main body 1 so as to.Xorm the surface of the top ring groove 4.

By anodic oxidation treatment, an alumilite layer is formed on a surface of the wear resisting annulus 11, and an outer peripheral surface of the piston main body 1 in the vicinity of the crown 2 and the top ring groove 4 of the ring land 3. The alumili te layer 20 contains Sic particulates 20a in the same way as the wear resisting annulus 11. That is, during anodic oxidation treatment, sic particulates in the wear resisting annulus 11 are precipitated and included in the alumilite layer 20.

The forming method of the wear resisting annulus 11 will be described concretely. First, an aluminum alloy cast ingot containing Sic particulates of several to tens Jmicrometers in maximum diameter is melted in an inert atmosphere such as argon gas or the like and maintained at 993 K, then subjected to mechanical agitation so as to uniformly disperse SIC particulates in aluminum alloy material.

Referring to Fig. 4, a molten aluminum alloy 13 containing SIC particulates is Injected into a lower mold 12, and pressured by an upper mold 14 for solidification. After cooling. a rough section of the wear resisting annulus 11 is taken out of the lower mold 12. Then, a dead head is cut out, and machining is carried out if necessary, thus completing forming work of the wear resisting annulus 11.

The rough section of the wear resisting annulus 11 can be made according to a die casting method or a molten metal casting method in addition to the above gravity casting method. Moreover, a powder metal forging method is applicable. According to this. SIC particulates are mixed with aluminum alloy particulates, which are charged in a metal mold, and pressured by the upper mold 14 for forming. After heating, forging is carried out to increase the density. When failing to obtain the increased density, forging is repeatedly carried out after reheating. This method allows finishing of a final product shape, necessitating no subsequent machining, resulting in an improved working efficiency.

The wear resisting annulus 11 formed in such a way is cast within the piston main body 1 for fixing.

An example of this casting condition is- such that a preheating temperature of the wear resisting annulus 11 is 673 K, an injecting temperature of molten alloy of the piston main body 1 is 993 K, a temperature of the molds is 473 K, and chemical conversion treatment of the wear resisting annulus 11 is carried out in a 313 K heated Palcoat 3756 solution of Nihon Parkerizing Co., Ltd. during 60 seconds of immersion.

The reason why the wear resisting annulus 11 is previously subjected to chemical conversion treatment is as follows: Since aluminum material has a fine oxide film formed on a surface thereof. sufficient deposition cannot be obtained at a contact interface with the molten alloy, resulting in insufficient joining of an aluminum layer of the piston main body 1 formed by molten alloy with the wear resisting annulus 11 made of aluminum alloy. When raising a heating temperature of molten alloy or carrying out sufficient preheating of the wear resisting annulus 11. there appears a phenomenon of deposition.

However, due to highly restricted condition and range of this deposition, uniform joining is practically difficult to carry out.

Particularly, preheating of the wear resisting annulus 11 causes thickening of the oxide film, resulting in increasing possible difficulty in joining.

When previously carrying out chemical conversion treatment as described above. a chemical-conversion treated layer is oxidized by preheating, while aluminum alloy material of the wear resisting annulus 11 is not oxidized. Oxide of the chemical-conversion treated layer is easily eliminated by molten aluminum alloy of the piston main body 1, aluminum alloy of the piston main body 1 and that one (11b) of the wear resisting annulus 11 can be connected to each other with a high joining strength.

On the other hand, the reason why the alumilite layer 20 is formed on upper and lower surfaces of the top ring groove 4 having possible heavy wear and the outer peripheral surface of the crown 2 and the ring land 3 in the vicinity of the top ring groove 4, and not formed on the skirt 10 is as follows: If the alumilite layer 20 is formed on the skirt 10, a lack of oil film is produced thereon, resulting in easy 7 occurrence of scuffing.

The thickness of the alumilite layer 20 is prefeably within the range of 10 to 50 pm in view of results of the iests-which will be described later.

Specifically, a thickness under 10 pm may not provide sufficient wear resistance, whereas a thickness over 50 pm produces not only an increase in surface roughness. but in treatment cost.

Moreover, the size of the SiC particulates 11a, 20a Is prferfibly. within the range of 3 to 40 pm for the following reasons. In view of the results of the tests which will be described later, "a size under 3 pm has difficulty in fully supporting a load of the piston ring 7, resulting in difficulty in is providing sufficient wear resistance. On the other hand, a size over 40 pm produces not only aa increase in surface roughness after groove machining and thus af ter alumilite treatment, but also easy occurrence of cracks in the alumilite-treated layer 20 which way result in flaking thereof.

The results of the tests on characteristic variations will be described with regard to wear resistance and micro-welding resistance of the wear resisting annulus 11 formed through the above processes and having the alumilite layer 20, and machinability of a material of the wear resisting annulus 11.

Fig. 7 shows components of matrix aluminum alloy.

In the tests, samples made according to the casting method were used. An addition amount of SiC particulates having diameter of 9.3 4 pm was evaluated on seven alumilite-treated samples of 0. 5. 10, 15, 20, 25, and 30% by volume.

Next. the diameter of SiC particulates added by 10.0% by volume was evaluated on six alumilite-treated samples of 2, 5, 10, 20, 30, and 40 pm. At the same time, an evaluation was made with regard to samples 8 without SiC particulates added (0% by volume) or samples of simple matrix aluminum alloy. and samples with no alumilite treatment.

An evaluation method of wear resistance used an apparatus as shown in Fig. 5. Specifically. the piston ring 7 is fixed on a rotary bed 15 rotated by a motor (not shown). A test-piece 17 fixed at a lower end of a heater 16 is pressed against an upper portion of the piston ring 7 for abrasion. This test-piece 17 is a part of the wear resisting annulus 11 cut out of the ring groove of the piston main body 1. In this method, test conditions such as temperature, lubrication, etc. are.established to be correlative with the piston of the real engine. An evaluation is carried out on the depth of wear after testing.

An evaluation method of micro-welding resistance used an apparatus as shown in Fig. 6. Specifically, an acceleration test method is adopted in which the piston ring 7 is pressed against an underside of the top ring groove 4 of the piston main body 1, and is slid only in one direction as indicated by an arrow in Fig. 6 through actuators 18. 19. An evaluation is carried out on a rate of a welded and worn area of the ring groove 4 to a slide area of the piston ring 7.

The test is continued until the alumilite layer is fully removed.

An evaluation of machinability was carried out such that a cylindrical rough section of 70 mm diameter is machined in the following conditions, and the total machining time until a tool has a wear amount of 0.3 mm is measured:

Machining speed: 200 m/min Cut amount: 0.3 mm Feed amount: 0.03 mm/rotation Tool used: gas-phase synthetic diamond tool manufactured by Asahi Daiya Co., Ltd.

9 Figs. 8 and 9 show results of the above evaluations.

Referring to Figs. 8 and 9, wear resistance is given by a rate when the wear amount is determined at 100 without SIC particulates added (0% by volume) and with no alumilite treatment. A smaller value indicates smaller wear.

Micro-welding resistance is given by a rate when a welded area is determined at 100 without SiC particulates added (0% by volume) and with no alumilite treatment. A smaller value indicates smaller micro-welding.

Machinability is given by a tool life with respect to 100 when the rough sections without SIC particulates added are machined by using a sintered diamond tool COMPAX manufactured by General Electric Co., Ltd. It is to be noted that the rough sections with SIC particulates added are machined by the gas phase synthetic diamond tool.

Referring to Fig. 8. in case of having no alumilite treatment, even when the addition amount of SIC particulates is 5% by volume, wear resistance Is largely improved as compared with a case without addition. At 10% by volume, an effect of addition becomes substantially constant.

On the other hand, in case of having alumilite treatment, wear resistance is improved as compared with a case of having no alumilite treatment, since wear resistance of an alumilite layer is better than that of aluminum alloy.

Referring to Fig. 8, micro-welding resistance shows a similar tendency. In case of having no alumilite treatment. micro-welding resistance is remarkably improved with an increase in the addition amount of SIC particulates. At 25% by volume or more, there is no occurrence of micro-welding.

On the other hand, in case of having alumilite treatment, even when the addition amount of SIC particulates is 5% by volume, there Is no occurrence of micro-welding, having remarkably improved micro welding resistance.

As for machinability, even when the addition amount of SIC particulates is 5% by volume.

machinability deteriorates as compared with a case without addition. Moreover, with an increase in the addition amount, machinability gets worse, and at 30% by volume, the tool has an edge broken, falling in impossible machining.

Referring to Fig. 9. as for an effect of the size of SiC particulates, in case of having no alumilite treatment, wear resistance is remarkably improved when the diameter of SIC particulates is 5 pm or more (3 pm or more. to be exact). Likewise. micro-welding resistance is improved with an increase in the size of SiC particulates. which is not remarkable, however, when a volume percentage of SIC particulates is constant.

On the other hand. in case of having alumilite treatment. wear resistance is remarkably Improved. and micro-welding resistance is not produced when the diameter of SIC particulates is 5 pm or more (3 pm or more, to be exact).

Machinability gets worse with an increase in the size of SIC particulates. At 40 pm, the surf ace roughness after machining deteriorates, which becomes remarkable by subsequent alumilite treatment.

The above examinations reveal that the optimum addition amount of SIC particulates is in a range of to 25% by volume, preferably, 10 to 20% by volume, and that the optimum size of SiC particulates is in a range of 3 to 40 pm, and preferably, 3 to 20 pm.

Moreover. an operation test was carried out with regard to the alumilite-treated piston having the piston main body 1 of aluminum alloy which was cast 1 around the wear resisting annulus 11 of aluminum alloy with SiC particulates added. the piston being built in the internal combustion engine. The addition amount of SiC particulates in the wear resisting annulus 11 was set at 10% by volume. Additionally. for comparison, an operation test was carried out with regard to a piston with no alumilite treatment and a piston having a piston main body without the wear resisting annulus 11.

Operation conditions are as follows: A four- cylinder 2,00Occ-displacement gasoline engine is used, and subjected to 200 hours continuous running at a 1500 C oil temperature and a 1200 C coolant temperature.

Results are such that the piston without the wear resisting annulus 11 suffers 50 pm abrasion and produces micro-welding on 85% of the underside of the top ring groove, while the piston with the wear resisting annulus 11 and having no alumilite treatment suffers 5 pm abrasion and produces micro-welding on 50% of the underside of the top ring groove, and the piston with the wear resisting annulus 11 and having alumilite treatment suffers neither abrasion nor micro-welding.

Having described the present invention in connection with the preferred embodiment, it is noted that the present invention is not limited thereto, and various changes and modifications are possible without departing from the scope of the present invention. By way of example, instead of SiC particulates, particulates having equivalent hardness may be used such as BN, Si3N4, A120.. WC, TiC, and TiB2.

12 C1lims:- 1. A piston for an internal combustion engine, comprising a main body made of aluminum alloy including at least one ring groove provided with wear resisting means made of aluminum alloy containing particulates, the wear resisting means and the main body in the vicinity thereof being covered by a coating comprising at least one layer.

2. A piston as claimed in claim 1, wherein the said groove is defined by casting around the wear resisting means.

3. A piston as claimed in claim 1 or 2, wherein the said groove is a top ring groove.

4. A piston as claimed in any preceding claim, wherein the wear resisting means is in the form of an annulus.

5. A piston as claimed in any preceding claim, wherein the particulates in the wear resisting means comprise silicon carbide (SiC) particulates.

6. A piston as claimed in claim 5, wherein the SiC particulates amount to 5 to 25% by volume, preferably 10 to 20% by volume.

7. A piston as claimed in claim 5 or 6, wherein the SiC particulates have diameters within the range of 3 to 40 micrometers, preferably 3 to 20 micrometers.

8. A piston as claimed in any preceding claim, wherein the particulates in the wear resisting means comprise particulates of BN, Si 3 N 43 Al 2 0 31 WC, TiC, or TiB 2 9. A piston as claimed in any preceding claim, wherein the said layer is a product of anodic oxidation treatment of surfaces of the wear resisting means and the main body in the vicinity thereof.

10. A piston as claimed in any preceding claim, wherein the said coating comprises an alumilite layer containing the said particulates.

11. A piston as claimed in claim 10, wherein the alumilite layer is at least 10 micrometers thick.

12. A method of manufacturing a piston for an internal combustion engine, having a main body and at least one ring groove, the method comprising:

forming a wear resisting annulus of aluminum alloy containing particulates; casting molten aluminum alloy for the main body of the piston around the wear resisting annulus, which is positioned to correspond to the said ring groove of the piston; and subjecting said wear resisting annulus to anodic oxidation treatment.

13. A method as claimed in claim 12, wherein the particulates in the wear resisting annulus comprise silicon carbide (SiC) particulates.

14. A method as claimed in claim 13, wherein the SiC particulates amount to 5 to 25% by volume, preferably 10 to 20% by volume.

15. A method as claimed in claim 13 or 14, wherein the SiC particulates have diameters within the range of 3 to 40 micrometers, preferably 3 to 20 micrometers.

16. A method as claimed in any of claims 12 to 15, wherein the particulates in the wear resisting annulus comprise particulates of BN, Si 3 N 41 Al 2 0 3' WC, TiC, or TiB 2 17. A method of manufacturing a piston, substantially as described with reference to Figures 1 to 4 of the accompanying drawings.

18. A piston manufactured by a method according to any of claims 12 to 17.

19. A piston substantially as described with reference to Figures 1 to 3 of the accompanying drawings.