DE9422167U1 - Cylinder liner made of a hypereutectic aluminum / silicon alloy cast into a crankcase of a reciprocating piston engine - Google Patents

Description

Aluminium/silicon alloy"

The innovation is based on a cylinder liner cast into a reciprocating piston engine made of a hypereutectic aluminum/silicon alloy according to the generic term of claim

From EP 367 229 Al, a cylinder liner is known which is made from metal powder and mixed graphite particles (0.5 to 3%; grain diameter maximum 10 μιη or less, measured in a plane measured transversely to the cylinder axis) and hard material particles without sharp edges (3 to 5%; grain diameter maximum 30 μιη, average 10 μιη or less), in particular aluminum oxide. The metal powder is initially produced on its own, i.e. without mixed non-metallic particles, by air atomization of a hypereutectic aluminum/silicon alloy with the following composition - with the remainder being aluminum (data in percent by weight based on the total metal content of the alloy, i.e. without the hard material particles from outside the melt and

&igr; Graphite content):

Silicon 16 to 18 %, '

iron

copper

Magnesium 0.5 to 2% and manganese 0.1 to 0.8%.

4 to 6%, 2 to 4%,

The metal powder is mixed with non-metallic particles and this powder mixture is pressed at about 2,000 bar into a preferably tubular body. This powder metallurgy

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The mechanically produced blank is inserted into a piece of soft aluminum pipe of the same shape and the double-layered pipe thus obtained is sintered and formed together in an extrusion process, preferably at elevated temperatures, to form a tubular blank from which the individual cylinder liners can be made. The embedded hard material particles should give the cylinder liner good wear resistance, while the graphite particles serve as a dry lubricant. To avoid oxidation of the graphite particles, hot extrusion should take place in the absence of oxygen. There is also a risk that at high processing temperatures the graphite will react with the silicon and hard SiC will form on the surface, which will impair the dry lubricating properties of the embedded graphite particles. Since the powder mixture is always more or less perfect, it can never be completely ruled out that there will be local, more or less large fluctuations in the concentration of hard material particles and/or graphite particles on the surface of the workpiece. Due to the embedded hard material particles, the hot pressing tool wears out relatively quickly because the hard material particles are still highly abrasive despite their rounded edges; only a partial rounding of the edges on the particles created by crushing can be achieved with reasonable effort. The subsequent mechanical processing of the running surface of the cylinder liner is also associated with high tool wear and thus with high tool costs. The hard material particles exposed in the running surface are sharp-edged after surface processing and exert relatively high wear on the piston skirt and the piston rings, so that these must be made from a wear-resistant material or provided with a correspondingly wear-resistant coating. The well-known cylinder liner is relatively expensive overall, not only in terms of the raw materials with several separate components, but also the high tool costs in connection with the plastic and machining processes drive up the unit costs. Apart from that, the type of production of the

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The known cylinder liner made of a heterogeneous powder mixture has the risk of inhomogeneities, which may impair functionality, i.e. result in rejects, but in any case require complex quality control. In addition, it requires complex piston designs in engine operation, which make the reciprocating piston machine more expensive overall.

Also worth mentioning is US-PS 4,938,810, which also discloses a cylinder liner produced using powder metallurgy. A large number of alloy examples are given here, as well as measurement and operating data for the cylinder liners produced using them. The silicon content of the examples given is in the range of 17.2 to 23.6%, although the protection claim of this document recommends a broader range of 10 to 30%, which extends into the hypoeutectic range. At least one of the metals, namely nickel, iron or manganese, should also be contained in the alloy, namely at least 5% or (iron) at least 3%. As a representative, only an alloy composition in weight percent is given here, the rest is aluminum; zinc and manganese contents are not given, which suggests that these metals, apart from traces, should not be contained: silicon: 22.8%,
Copper: 3.1%,
Magnesium: 1.3%,
Iron: 0.5% and
Nickel: 8.0%.

The nickel content in the alloy example mentioned is very high. A blank for a cylinder liner is hot-extruded from the powder mixture.

Finally, US-PS 4 155 756 should be mentioned, which deals with the same topic; there, among other things, the following composition of a powder-metallurgically manufactured cylinder liner is mentioned as one example of several - the remainder being aluminum:
Silicon: 25%,

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·■ 4,

Copper: 4, 3 Magnesium: , 65 Iron: 0, 8th

and

The aim of the innovation is to improve the cylinder liner used in terms of wear resistance and lubricating oil consumption, while at the same time reducing the risk of wear for the piston; when reducing lubricating oil consumption, the focus is less on the lubricating oil itself, but rather on its combustion residues - essentially hydrocarbons, which have an adverse effect on the exhaust gases emitted by the internal combustion engine.

This task is solved according to the invention by the characterizing features of claim 1, based on the generic reciprocating piston machine. Due to the special alloy composition of the material for the cylinder liner, silicon primary crystals and intermetallic phases are formed directly from the melt; the addition of separate hard particles is therefore not necessary. In addition, spray compacting of the alloy, which is easy to control and comparatively inexpensive, is used, followed by energy-saving extrusion of the blank. The combination of these processes results in particularly low oxidation of the droplet surfaces and particularly low porosity of the liner. The alloy compositions A and B mentioned are optimized for use with an iron-coated piston (alloy A) and with an uncoated aluminum piston (alloy B). On the one hand, the melt-born hard particles have a high level of hardness and give the running surface good wear resistance, and on the other hand, these melt-born hard particles do not affect the processing of the material too much, so that the running surface can be machined sufficiently well. Due to the formation of the primary crystals and intermetallic phases in each individual sprayed and then solidified on the growing blank,

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Melt droplets result in a very even distribution of the hard particles in the workpiece due to the process. The melt-born particles are also less angular and tribologically not as aggressive as broken particles. In addition, the melt-born, metallic hard particles are more intimately embedded in the basic alloy structure compared to mixed, non-metallic broken particles, so that the risk of cracks forming at the hard material boundaries is less. In addition, the melt-born hard particles show better running-in behavior and less abrasive aggressiveness towards the piston and its rings, so that longer service lives are achieved or - if conventional service lives are accepted - less complex designs can be permitted on the piston side.

Appropriate embodiments of the innovation can be found in the subclaims. The innovation is further explained below using an embodiment shown in the drawing; in which:

Fig. 1 is a partial sectional view of a reciprocating piston engine with cast-in cylinder liner,

Fig. 2 a greatly enlarged section of a cross-section taken parallel to a cylinder surface line through a region of the cylinder liner close to the surface,

Fig. 2a is a further enlarged detail of a detail from Figure 2,

Fig. 3 is a bar chart illustrating the grain sizes of the various melt-born hard particles and

Fig. 4 a converted honing machine for mechanically exposing the hard particles from the surface of the cylinder liner.

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The reciprocating piston engine partially shown in Figure 1 contains a crankcase 2 made of die-cast metal, in which cylinder jackets 4 are arranged to accommodate a cylinder liner 6, in which a piston 3 is guided so that it can move up and down. A cylinder head 1 with the devices for charge exchange and charge ignition is mounted on top of the crankcase 2. Inside the crankcase, a cavity is provided around the cylinder jacket 4 to form a water jacket 5 for cylinder cooling.

The cylinder liner 6 is manufactured as a single part using a process described in more detail below in a hypereutectic composition, which will also be discussed in more detail below, then cast as a blank into the crankcase 2 and machined together with the crankcase. To do this, the running surface of the cylinder liner is first roughly pre-machined and then finely machined by machining in the sense of drilling or turning. The running surface 7 is then honed in at least one stage. After honing, the particles in the running surface that are harder than the basic structure of the alloy, such as silicon crystals and intermetallic phases, are exposed from the running surface in such a way that the plateau surfaces of the particles protrude from the rest of the surface of the basic structure of the alloy.

In order to improve the cylinder liners in terms of wear resistance as well as lubricating oil consumption and thus the emission of hydrocarbons by the internal combustion engine, the invention provides for a bundle of measures which work together in this sense.

First of all, an optimization of the composition of the alloy should be mentioned here, whereby two alternative alloy types were found to be optimal, whereby one alloy type A is recommended for use with iron-coated pistons, the other alloy type B has been optimized in conjunction with uncoated aluminum pistons.

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The following percentages are percentages by weight. Alloy A is composed as follows:

Silicon 23.0 to 28.0%, preferably about 25%, magnesium 0.80 to 2.0%, preferably about 1.2%, copper 3.0 to 4.5%, preferably about 3.9%, iron max. 0.25%
Manganese, nickel and zinc max. 0.01% and the rest aluminum.

Alloy B, designed for use with uncoated aluminum pistons, has the same composition as alloy A in terms of silicon, copper, manganese and zinc content; only the iron and nickel contents are slightly higher, namely
Iron 1.0 to 1.4% and
Nickel 1.0 to 5.0%.

By finely spraying the melt in an oxygen-free atmosphere and precipitating the melt mist to form a growing body, a billet with fine-grained formation of the silicon primary crystals 8 and intermetallic phases 9 and 10 is first produced from the aluminum/silicon alloy. Intermetallic phases are formed between magnesium and silicon (Mg ₂ Si) and between aluminum and copper (Al ₂ Cu). The atomized melt is cooled very quickly in a nitrogen jet, whereby cooling rates in the range of 10^ K/sec. are achieved. By this so-called spray compacting, a very narrow-band structure with approximately ±5 ... 10 μ΄α can be produced around an average value, whereby the average value can be adjusted within a relatively wide grain size spectrum of approximately 7 to 200 μ΄α . In this case, a very fine grain setting - grain size of 2 to 10 μιη - is used, so that a correspondingly fine structure with fine and even silicon distribution is obtained. Each powder particle has the full alloy components. The powder particles are sprayed onto a rotating plate on which the aforementioned billet with a diameter of, for example, 300 mm and about 1000 mm

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length. This depends on the system design. The billets must then be pressed into tubes on an extrusion press. It is also conceivable that the billet is not allowed to grow axially on a rotating plate, but rather the atomized melt is allowed to grow radially on a rotating cylinder, so that an essentially tubular pre-product is created.

The melt is atomized so finely during spraying that the silicon primary crystals 8 and the intermetallic phases 9 and 10 forming in the growing ingot are obtained with very small grain sizes with the following dimensions: Si primary crystals: 2 to 15, preferably 4 to 10 μηη, Al2Cu phase: 0.1 to 5.0, preferably 0.8 to 1.8 μηη, Mg2Si phase: 2.0 to 10.0, preferably 2.5 to 4.5 μηη.

This fine grain size results in a finely dispersed distribution of the hard particles within the basic alloy structure and a homogeneous material. Since the atomization is carried out from a melt, no inhomogeneities in the mixture can form. Due to the compaction of the atomized melt droplets, the droplets also bond very closely to one another and porosity is largely avoided.

The process of spray compacting aluminum alloys is known per se and should only be used here in an advantageous manner. The extrusion of ingots produced in this way into tubes, from which individual sleeves can then be cut to length, is also known per se. For this reason, it will not be discussed further here.

The cylinder liner blanks produced in this way and possibly machined to a certain level of further processing are cast into a crankcase made of an easily castable aluminum alloy, whereby a die-casting process is recommended. For this purpose, the prefabricated cylinders to be cast

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The cylinder liners are pushed onto a guide pin with the die-casting tool open, the mold is closed and the die-casting material is injected. Due to the rapid cooling time and the possibility of cooling the cylinder liner to be cast via the guide pin, there is no danger of the melt of the die-cast workpiece thermally affecting the cylinder liner material in an uncontrolled manner. The alloy used for die-casting is hypoeutectic and is therefore easy to process in casting technology. On the other hand, the thermal expansion of the alloy of the die-cast workpiece on the one hand and the cylinder liner on the other hand is almost the same, so that uncontrolled thermal stresses do not occur between the two.

After the cylinder liner has been cast into the crankcase, the crankcase is machined on the required surfaces, in particular on the running surfaces 7 of the cylinder liner 6. These machining processes - only drilling and honing are mentioned here - are known in themselves, which is why they will not be discussed here. After honing, the silicon primary crystals 8 embedded on the surface and the particles from intermetallic phases 9 and 10 must be exposed. This exposure is usually carried out chemically by etching, which is not only time-consuming, but also involves a certain amount of stress on the work environment due to the evaporation of etching fluid. In addition, a certain inhomogeneity during etching cannot be avoided because the etching conditions are not completely the same everywhere. For this reason, a certain minimum exposure depth must be aimed for in order to achieve a certain minimum exposure depth in every case, even in the most unfavorable places. The time required for etching, the safety precautions at the workplace and the ongoing operating costs, primarily chemical, waste water and disposal costs, add up to very significant amounts per cylinder liner. The present innovation takes a different approach here, namely the primary crystals 8 or particles 9 and

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10 is mechanically exposed by a grinding or polishing process using flexible polishing or grinding molds 16. This not only avoids the disadvantages and costs of etching, but above all also achieves usage and functional advantages of the running surface 7 of the cylinder liner, which will be discussed in more detail below. The costs per cylinder liner caused by the mechanical exposure are no higher than the costs of a honing process.

In connection with the mechanical exposure, the honing machine shown in Fig. 4, which is also to be used in this polishing process, is discussed in more detail. The honing machine 13 shown there has a movable machine table 18, which holds the crankcase 2 to be machined in a collecting tray 19. Above the machine table 18, there is at least one vertically extending honing spindle 14 with a honing tool 15 accommodated therein, which can be lowered into a cylinder bore of the crankcase. The special feature of the honing machine is that the honing tool 15 is not equipped with hard honing stones, but with several axially aligned felt strips 16 on the circumference, which automatically adapt cylindrically to the inner surface of the cylinder liner due to the flexibility of the felt on the outside. These serve as shape-adapted polishing or grinding bodies. The design of the honing tool includes metal grinding wheel carriers that can be moved radially in the honing tool and can be pressed against the inner surface of the cylinder liner with adjustable force. The metal grinding wheel carriers were flat on the side facing radially outwards, i.e. not cylindrical. Cut pieces of felt mat 9 mm thick were glued onto these flat surfaces, whereby the glued felt pieces were not machined on the outside to give them a cylindrical surface. Rather, the required cylindricity was automatically achieved when the honing-like polishing or grinding was started under the pressure of the felt pieces on the inner surface of the cylinder liner.

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The felt material was a felt with the designation piece felt Tm 30 - 9, DIN 61206; a felt with the designation piece felt Tm 32 - 9, DIN 61206 would also certainly be suitable. The individual data in the designation mean: m - > mottled,

3 0 -» 0.3 g/cm ³ bulk density or 32 ->· 0.32 g/cm ³ bulk density, 9 —> 9 mm thickness.

The hardness of the felt pieces used was M6 (medium 6) according to DIN 61200; for the piece felt Tm 32 - 9, DIN 61206, a hardness according to DIN 61200 with the designation Fl (solid) would be recommended.

Since this mechanical exposure takes place in the presence of an abrasive, amorphous grinding or polishing medium containing hard material particles, provisions are also made on the honing machine 13 for feeding the grinding medium. A collecting container 20 is set up on the machine to hold a slurry 23 of fine hard material particles, preferably silicon carbide particles in honing oil. To prevent sedimentation of the hard material particles, an agitator 21 is provided in the collecting container. A circulation pump 22 conveys the slurry from the collecting container 20 to an annular shower head 17, which surrounds the honing tool above the cylinder liner and supplies it with plenty of grinding fluid. During mechanical exposure, the rotating honing tool performs an axially oscillating up and down movement in a known manner, so that all parts of the running surface 7 are reached by the felt strips 16. In a known manner, the honing tool is designed so that the felt strips can be pressed against the running surface 7 with adjustable pressure. The mechanical exposure is preferably carried out under a pressure of about 3 to 5 bar, preferably about 4 bar. This type of processing removes some of the material of the alloy base material located between the individual harder particles on the surface, so that the harder particles protrude with a plateau surface 11 relative to the removed base material 12. This dimension t represents the exposure depth. With this

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In this way, the edges of the plateau surfaces 11 are rounded into a crown so that they merge smoothly into the alloy base material 12. This design of the plateau surfaces II is very advantageous for the piston or piston rings sliding over it because it is tribologically less aggressive in contrast to the sharp-edged hard material particles in the case of chemical exposure. The extent of the exposure depth t can be determined - in addition to the contact pressure of the felt strips - above all by the duration of the honing-like process during mechanical exposure. It is true that the longer the exposure time, the more rounded the plateau surfaces 11 become and are worn away like a dome. It has therefore proven to be expedient to expose mechanically in the manner mentioned for about 20 to 60 seconds, preferably about 40 seconds. This produces an exposure depth of about 0.2 to 0.3 [im. However, this exposure depth is superimposed by a surface roughness that is at least of the same order of magnitude, if not greater, which is not shown in Fig. 2a. The roughness of the surface is essentially determined by the grain size of the hard material particles in the slurry 23; the roughness values on machined cylinder surfaces are in the range of 0.7 to 1.0 µm. These roughness values and the low exposure depth made it possible to achieve very low oil consumption and therefore very low emissions of hydrocarbons. In addition, the wear resistance and sliding properties of the cylinder liners produced in this way are excellent.

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Claims (3)

Daim 23 964G/4 Daimler-Benz Aktiengesellschaft FTP/P -Pö Stuttgart 04.05.1998 1) Cylinder liner cast into a reciprocating piston engine made of a hypereutectic aluminum/silicon alloy, characterized by the commonality of the following features: t> the aluminium/silicon alloy of the cylinder liner (6) which is free of melt-independent hard material particles is composed as follows in the two alternatively usable alloy types A and B, where the numbers mean the content in weight percent: &Aacgr; : Silicon 23.0 to 28.0%, preferably about 25%, Magnesium 0.80 to 2.0%, preferably about 1.2%, Copper 3.0 to 4.5%, preferably about 3.9%, Iron maximum 0.25%, Manganese, nickel and zinc maximum 0.01% each, balance aluminium or Silicon 23.0 to 28.0%, preferably about 25%, Magnesium 0.80 to 2.0%, preferably about 1.2%, Copper 3.0 to 4.5%, preferably about 3.9%, Iron 1.0 to 1.4 %, Nickel 1.0 to 5.0 % Manganese and zinc maximum 0.01% each, balance aluminium, > in the cylinder liner (6) there are silicon primary crystals (8) and intermetallic phases (9, 10) with the following grain sizes: Daim 23 964G/4 _# ·"_# ·* · · sizes, where the numbers indicate the average grain diameter in μ&tgr;&eegr;: Si primary crystals: 2 to 15, preferably 4.0 to 10.0 μιη, Al ₂ Cu phase: 0.1 to 5.0, preferably 0.8 to 1.8 μιη, Mg ₂ Si phases: 2.0 to 10.0, preferably 2.5 to 4.5 μιη, > the superficially embedded silicon primary crystals (8) and particles of intermetallic phases (9, 10) partially protrude from the finely machined running surface (7) of the cylinder liner (6), the protruding parts of the primary crystals (8) or particles (9, 10) having a plateau surface (11) located in the running surface (7), which at its edges is crowned or rounded into the alloy base material (12) passes over. 2) Cylinder liner according to claim 1,
characterized in that the extent of protrusion (exposure depth t) of the plateau surfaces (11) of the primary crystals (8) or particles (9, 10) relative to the surrounding alloy base material (12) is approximately 0.2 to 0.3 fim. 3) Cylinder liner according to claim 1,
characterized in that the protruding primary crystals (8) or particles (9, 10) have a roughness of Rz = 0.7 to 1.0 μιη on their exposed plateau surface (11). -.ooo·-