Insert part that can be infiltrated

Abstract

An insert part for a cast piston of an internal combustion engine may include a powder, such as a sintered powder material, containing at least iron or alloys thereof, and having a capacity for being infiltrated. The powder may contain particles having different grain sizes, and up to 4% by volume of the powder may include particles having a diameter smaller than 75 m.

Claims

1. An insert part for a cast piston of an internal combustion engine, comprising: a material composed of a powder containing at least iron or alloys thereof, the material having a capacity for being infiltrated, wherein the powder contains particles having different grain sizes and up to 4% by volume of the powder includes particles having a diameter smaller than 75 m, and wherein the powder further includes a fraction not exceeding 10% by volume of particles having a diameter of 75-106 m, a fraction ranging from 0.5 to 6.0% by volume of particles with a diameter of 106-150 m, and a fraction of at least 50% by volume of particles having a diameter greater than 150 m.

2. The insert part according to claim 1, wherein the fraction of particles with the diameter of 75-106 m is 2% by volume or less.

3. The insert part according to claim 1, wherein the powder contains a fraction of at least 88% by volume of particles having a diameter greater than 150 m.

4. The insert part according to claim 1, wherein the powder contains a fraction of at least 50% by volume of particles having a diameter greater than 212 m.

5. The insert part according to claim 1, wherein the powder further contains at least one of nickel or alloys thereof and copper or alloys thereof.

6. The insert part according to claim 1, wherein at least some individual particles of the powder are coated with a binder configured to facilitate a green stability suitable for handling a compacted green body before sintering and configured to be burned during sintering.

7. The insert part according to claim 1, wherein the material has a porosity of 50-80% by volume.

8. The insert part according to claim 1, wherein the insert part is in the form of a ring carrier, a bolt eye, or a depression border.

9. The insert part according to claim 1, wherein the material has a density of 2.5-4.7 g/cm.sup.3.

10. A method for producing an aluminium piston having an insert part, comprising: providing a sintered powder material containing at least iron or alloys thereof, wherein the sintered powder material includes particles having different grain sizes and up to 4% by volume of the powder includes particles having a diameter smaller than 75 m, and wherein the sintered powder material further includes a fraction of less than or equal to 10% by volume of particles with a diameter of 75-106 m, a fraction ranging from 0.5 to 6.0% by volume of particles with a diameter of 106-150 m, and a fraction of at least 50% by volume of particles having a diameter greater than 150 m; and introducing a liquid material of an aluminium alloy into a casting mould under a casting pressure of about 0.5 to 15 bar to form at least part of a cast piston, wherein the liquid material of the aluminium alloy infiltrates the sintered powder material arranged in the casting mould.

11. The method according to claim 10, wherein at least one of: introducing the liquid material of the aluminium alloy takes place under a buffer gas, and introducing the liquid material of the aluminium alloy further includes applying a counterpressure, wherein the counterpressure is 0.1 bar lower than the casting pressure.

12. The method according to claim 10, further comprising at least one of solution annealing the cast piston and overaging the cast piston.

13. The method according to claim 10, wherein providing the sintered powder material further includes: coating at least some individual particles of the powder with a binder; compacting the powder to form a compacted green body; and heating the compacted green body to burn away the binder.

14. An insert part for a cast piston of an internal combustion engine, comprising: a material having a capacity for being infiltrated and composed of a powder containing at least iron or alloys thereof, the powder containing a size distribution of particles having different grain sizes, wherein the size distribution of particles in the powder includes a fraction of 4% or less by volume of particles having a diameter smaller than 75 m, a fraction of 10% or less by volume of particles having a diameter of 75-106 m, a fraction of 0.5 to 6.0% by volume of particles with a diameter of 106-150 m, and a fraction of at least 28% by volume of particles having a diameter greater than 150 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The FIGURE shows, schematically, an exemplary piston and an insert part.

DETAILED DESCRIPTION

(2) The present invention is based on the general idea of selecting a powder with a completely novel grain composition in the manner of a new screening line for a sinter material for an insert part that can be infiltrated, so that the open porosity and thus also the capacity for being infiltrated of the insert part produced from said sinter material is improved considerably. This is achieved for example by defining the screening line more closely, that is to say the size distribution of the individual sinter particles and thus also making the sinter powder from which the sinter material is created more homogeneous than it usually is. The powder used according to the invention contains at least iron or alloys thereof, preferably also nickel, copper or alloys thereof, and at the same time has particles of different grain sizes, wherein not more than 4 percent by volume of the powder consists of particles that have a diameter smaller than 75 m. In this context, at least 28% by volume, preferably at least 50% by volume and in a particularly preferred embodiment at least 88% by volume of the powder contains sinter particles with a diameter larger than 150 m. Consequently, a powdery sinter material may be produced that is coarser than usual, wherein 90% of the sinter particles typically have a diameter smaller than 150 m. Besides limiting the particles with a diameter smaller than 75 m to a level not exceeding 4% by volume, the size distribution of the individual particles is defined much more narrowly, wherein the restriction of the grain sizes below the threshold value particularly limits the degree to which pores are clogged, as happened previously, thus rendering the pores unavailable for infiltration. Such a strict limitation of the lower boundary of the particle sizes is not provided in conventional sinter materials, which means that a significantly higher degree of filling is achieved, of the pores remaining between larger sinter particles as well.

(3) According to the invention, the powder used for the sinter material of the insert part has a fraction of 0-4.0% by volume particles with a diameter from 0-75 m. In one embodiment, particles with a diameter of 75-106 m account for not more than 10% by volume, preferably not more than 2% by volume of the powder. Additionally, in a particularly preferred embodiment, not more than 6% by volume of the powder includes particles with a diameter in the range from 106-150 m. Accordingly, in this preferred embodiment at least 88% by volume of the powder has a particle diameter greater than 150 m. Even with this narrow restriction of the finest components of the powder, it is already possible to ensure that the pores which remain between the individual particles in the sinter material and which can be infiltrated by a subsequent lightweight metal when the lightweight metal piston is cast, are not filled completely, so that these pores are available for infiltration by the lightweight metal, thereby creating a significantly improved bond between the insert part, which may have the form of a ring carrier, a depression border or a bolt eye in a piston, for example.

(4) For this purpose, in one embodiment at least 50% by volume of the powder has a particle diameter of 106-212 m. The high powder fraction within a relatively narrow grain size bandwidth encourages the formation of a high porosity and thus also of a sinter material that can easily be infiltrated. In another embodiment, particles with diameters larger than 212 m account for at least 50% by volume thereof. The high percentage of larger particles means that a structure with coarser pores is created, which also facilitates the infiltration.

(5) For practical purposes, a powder that is suitable for producing the sinter material according to the invention has a fraction from 0.5 to 6.0% by volume of particles with a diameter from 106-150 m. In particular, said lower limit clearly shows that in the case of such a screening line or grain size distribution, very fine particles for completely filling the pores required for infiltration are entirely absent or only present to an inadequate degree. In this way, it may be assured for example that the insert part produced, that is to say sintered, from the sinter material according to the invention has 50-80% pores, that is to say a porosity of 50-80%, which may optionally be filled at least partly by the lightweight metal. If the powder is relatively homogeneous in terms of particle size, not only does this raise the porosity of the sinter material produced, but the individual pores are also substantially larger, which further improves its capacity to allow the molten lightweight metal to flow through it.

(6) In a further advantageous embodiment of the solution according to the invention, at least individual sinter particles of the sinter material are coated with a binder, a resin for example, which increases the green stability and is burned during sintering. After compaction of the green body, however, the resin keeps the sinter particles pressed tightly against each other, thus improving the strength of the compacted green body. Such a resin thus increases the shape fidelity of the initially unsintered insert part, and so facilitates damage-free handling thereof. The binder or resin thus represents a coating of individual particles that reduces the porosity of the insert part, impairing the infiltration and thus also the bonding between the lightweight metal of the piston and the insert part during subsequent casting of the lightweight metal piston. However, the binder burns the resin when the insert part is sintered, making the occupied porosity free again, so that is can be used for the infiltration process. Alternatively, the binder may also be set up so that decomposition takes place in a chemical reaction other than an oxidising reaction during sintering. To this end, another suitable gas, e.g. an endogas, is introduced instead of air during the sintering.

(7) In an advantageous refinement of the solution according to the invention, a density of the insert part is in the range from about 2.5-4.7 g/cm.sup.3. The density of aluminium is in the order of about 2.7 g/cm.sup.3, for example, so that when the insert part is infiltrate with lightweight metal, aluminium for example, it is always still possible to achieve a density of less than 5 g/cm.sup.3. Thus, the high porosity and comparatively low density of the insert part increase the weight of the lightweight metal piston by a considerably smaller amount than a solid cast part manufactured from an iron alloy.

(8) The invention further relates to a method for manufacturing a lightweight metal piston, a magnesium or aluminium piston, for example, using an insert part as described previously, in which the liquid lightweight metal is introduced into a casting mould under a casting pressure of about 0.5-15 bar and the insert part arranged in the casting mould is infiltrated. In a preferred embodiment hypoeutectic alloys of aluminium with silicon and/or copper are used. This prevents the formation of Si or Cu phases, which may occur particularly in a hypereutectic Al alloy. This is undesirable because the sinter material may function like a filter whose pores do not allow these phases to pass through during infiltration, with the result that the phases collect on the surface thereof. The layer formed thereby separates the insert part from the cast piston body and forms a weak point that can result in the part being rejected, or the subsequent failure of the piston. Casting of the lightweight metal piston may be carried out with or without counterpressure, wherein the casting pressure should be at least 0.1 bar higher than the counterpressure.

(9) In a further advantageous embodiment of the solution according to the invention, the lightweight metal piston, for example the aluminium piston is cast under buffer gas, particularly with the use of nitrogen or argon. In this way, it is possible to prevent oxidation of the lightweight metal during casting, wherein such an undesirable oxidation of the lightweight metal can result in clogging of the sinter material pores with oxides, and so may make it more difficult to achieve good infiltration of the insert part and its mechanical bonding with the piston body, as described previously. The use of a buffer gas helps to prevent oxidation, which in turn improves infiltration of the insert part.

(10) It is expedient if the cast piston is solution annealed and/or over-aged. Particularly with aluminium alloys, solution annealing can result in a phenomenon called precipitation hardening, which can help to increase the strength of the lightweight metal piston. In this context, curing may theoretically take place in three stages, that is to say the actual solution annealing, quenching and subsequent aging (hot or cold). Solution annealing is carried out at temperatures from approximately 480 to over 50 C., wherein a temperature is chosen at which a sufficient quantity of the alloy elements has been dissolved in the mixed crystal, so that the hardening effect takes place after quenching and aging. Overaging of such an aluminium alloy may also be carried out in similar fashion.

(11) The casting mould is usually vented while the aluminium piston is cast, to prevent the casting mould from being filled completely, and to be able to achieve an optimised infiltration process of the insert part.