EP1815036A2

EP1815036A2 - Aluminium alloy for component with high hot process mechanical strength

Info

Publication number: EP1815036A2
Application number: EP05820753A
Authority: EP
Inventors: Siegfried Mielke; Gérard Laslaz; Charlotte Gautier-Picard
Original assignee: KS Kolbenschmidt GmbH; Aluminium Pechiney SA; Peugeot Citroen Automobiles SA
Current assignee: KS Kolbenschmidt GmbH; PSA Automobiles SA; Rio Tinto France SAS
Priority date: 2004-11-26
Filing date: 2005-11-23
Publication date: 2007-08-08
Also published as: WO2006056686A3; FR2878534B1; FR2878534A1; WO2006056686A2

Abstract

The invention concerns an aluminium alloy for a component having to exhibit high hot process mechanical resistance in the 230-430 °C temperature range consisting (in wt. %) of: Mg: < 0.5; Si: 8.0 - 13; Cu: 4.0 9.0; Ni: 1.0 4.0; Fe: < 1.0; Mn: < 1.0; Zn: < 1.0; V: < 0.3; Ti < 0.3; Zr < 0.05; P: 0.001 0.05; other elements < 0.15 each and < 0.30 in total, the rest being aluminium. More particularly, the invention concerns alloys for internal combustion engine pistons.

Description

Aluminum alloy for high mechanical resistance parts

Field of the invention

The invention relates to aluminum alloys for parts subjected to high thermal and mechanical stresses, in particular the pistons of internal combustion engines, and more particularly turbo-charged gasoline or diesel engines.

State of the art

Unless otherwise stated, all values relating to the chemical composition of alloys are expressed in percent by weight.

In the manufacture of engine pistons, aluminum alloys with a high silicon content (10 to 24%) are usually used to facilitate their moldability and to give them good wear resistance. To allow structural hardening, their composition systematically includes an addition of magnesium and copper hardeners, soluble at high temperature, but poorly soluble at room temperature. This double addition gives the alloy, by a complete heat treatment in the T6 or T7 state or a simple T5 state, good mechanical strength at room temperature. The typical magnesium contents are between 0.3 and 1.5% and those in copper between 0.3 and 5%.

It is also known to add less soluble elements such as nickel between 0.5 and 4%, cobalt, zirconium, vanadium, or even silver, in an attempt to improve the mechanical properties when hot. The most common alloys are 1Α-S12UNG and 1Α-S12U3-5N3G. More or less recent publications indicate compositional variants, in particular with very high levels of copper associated with significant levels of magnesium.

Thus, patent FR 2690927 in the name of the applicant, filed in 1992, describes creep resistant aluminum alloys containing from 4 to 23% of silicon, at least one of magnesium elements (0.1 - 1%) , copper (0.3-4.5%) and nickel (0.2-3%), and 0.1 to 0.2% titanium, 0.1 to 0.2% zirconium and 0.2 to 0.4% vanadium. An improvement in the creep resistance at 300 ° C was observed without significant loss of elongation measured at 250 ° C.

Another patent, filed by Toyota in 1996 (JP9272939), discloses an alloy for application to a piston of an internal combustion engine, composed of 13 to 25% of silicon and particularly charged with copper and magnesium (8 to 22% copper, 0.2 to 1.5% magnesium) with additionally 0.3 to 1.5% iron, 0.1 to 2.5% manganese, 0.01 to 1.5% titanium, 1 to 5% nickel, 0.002 to 0.4% phosphorus, other impurities , aluminum balance. A patent application also filed by Toyota, but in 1998 (JP2000 / 054053) discloses a method of manufacturing alloy for piston leading to a maximum grain size of 200 microns, and a composition also loaded both copper and aluminum. 'magnesium (3.0 to 10.0% copper and 0.7 to 1.3% magnesium with 1.5 to 3.0% nickel, 1.0% iron at most, 0.5 to 1.0% manganese, 0.05 to 0.3% vanadium and 12 to 14% of silicon).

"NASA" has more recently filed several patents also on alloy compositions with high silicon content, copper and magnesium also containing titanium, zirconium, vanadium ....

Thus, the application US 2001/0010242 A1 relates to a process for manufacturing a part molded from an alloy of composition: Si: 14.0-25.0%, Cu: 5.5-

8.0%, Mg: 0.5-1.5%, Ni: 0.05-1.2% Ti: 0.05-1.2%, Zr: 0.12-1.2%, V: 0.05-1.2%, P: 0.001-0.1%, aluminum balance; the ratio% Si /% Mg is between 15 and 35; the ratio% Cu /% Mg is between 4 and 15.

- US Pat. No. 6,399,020 for its part gives another variant of composition, namely: Si: 11.0-14.0%, Cu: 3.6-8.0%, Fe: 0-0.8%, Mg: 0.5-1.5%, Ni: 0.05-0.9

%, Mn: 0-1.0%, Ti: 0.05-1.2%, Zr: 0.12-1.2%, V: 0.05-1.2%, Sr: 0.001-0.1%. Other prior patents relate to alloys with a high content of copper and magnesium as well as nickel: this is the case of the Alcan-Gmbh patent, EP0714479, which describes an alloy of composition: Cu: 2-6% , Ni: 2-6%, Si: 11-16%, Mg: 0.5 - 2.0%, Fe <0.7%, Mn <0.5%, ...

Another route is the selection of low magnesium alloys associated with relatively moderate levels of copper.

Thus, the recent patent application of Toyota (JP2002 / 249840) relates to an alloy with low magnesium content and moderate copper, and a grain size α-Al limited, of composition: Mg <0.2%, Ti: 0.1-0.3%, Si: 11-15%, Cu: 2-3.5

%, Fe: 0.2-1%, Mn: 0.2-1%, Ni: 1-3%, P: 0.001-0.015%, remains aluminum and impurities, and hypereutectic structure.

An even more recent request, still from Toyota, but filed in the USA and Germany (US 2004/057865 A1) relates to a piston also made of an aluminum alloy with low magnesium and moderate copper contents, but without accuracy on grain size: Mg <0.2%, Ti: 0.05-0.3%, Si: 10-21%, Cu: 2- 3.5%, Fe: 0.1-0.7%, Ni 1-3%, P: 0.001- 0.02%.

The object of the present invention is to further improve the mechanical strength and the creep resistance, in the temperature range 230-380 ° C., of parts subjected locally to such temperatures, in particular the pistons of internal combustion engines, including turbo-charged and direct injection engines.

Object of the invention

The subject of the invention is an aluminum alloy for a piece having to have a high mechanical resistance when hot, of composition (% by weight):

Mg <0.5

Si: 8.0 - 13 Cu: 4.0 - 9.0

Ni: 1.0 - 4.0

Fe <1.0

Mn <1.0

Zn <1.0 V <0.3

Ti <0.3

Zr <0.05

P: 0.001 - 0.05 other elements <0.15 each and 0.30 in total, aluminum balance.

Description of figures

Figure 1 shows a section of a piston with the piston head at 1, the piston skirt at 2, the piston bowl at 3, the bowl edge at 4, and the spindle boss at 5. FIG. 2 represents the percentage of pistons having a crack of more than 1.5 mm at the edge of the bowl (in the ordinate), as a function of the number N of thermal cycling of 380 to 150 ° C. (in the abscissa) during the simulator tests. the thermal fatigue stress at the edge of the bowl. FIG. 3 shows in ordinate the limit pressure, in bars (10 ⁵ N / m ² ), deduced from the simulation of mechanical stress stressing of the pin bosses (in English "pin boss pulsator") on pistons cast with the various alloys indicated on the abscissa, these being maintained at a temperature of 250 ° C. for 20 h. Figure 4 shows the mechanical tensile characteristics at room temperature, obtained on dissecting specimens in pistons poured with the various alloys indicated on the abscissa, on the ordinate and in N / mm ² on the left for the yield strength, in% and in the right for the elongations at break.

Description of the invention

The invention is based on the finding made by the applicant that it is possible to obtain heat-resistance properties, especially between 230 ° C. and 430 ° C., which are significantly improved over existing alloys by combining, in an alloy Al-Si type molding, 4 to 9% copper content and low magnesium content.

The alloys according to the invention have, as their main characteristics, a relatively moderate silicon content of 8 to 13%, associated with a high copper content (4 to 9%), a relatively high nickel content (1.5 to 4% ) and a very limited zirconium content. More specifically, the alloys according to the invention have the following composition: Si: 8 to 13%, Fe <1.0%, Cu: 4 to 9%, Mg: <0.5%, Mn <1%, Zn <1, 0%, Ni: 1 to 4%, Ti <0.3%, P: 0.001 to 0.05%, V <0.3%, Zr <0.05%.

This type of composition compared to those described in the prior art, in particular in the case of application to pistons, provides a solidification structure of the eutectic intermetallic phases AlCuNi, A13Ni, ... little segregated and good homogeneity throughout the room and, with the conventional addition of phosphorus, a hypereutectic structure of primary silicon of slightly elongated shape and distributed regularly. ,

These structures improve the thermal fatigue behavior at very high temperatures (300 to 430 ° C) of the bowl edges at the top of the piston. What is more, this type of composition also significantly improves the resistance to hot mechanical fatigue, which is necessary in particular in the regions of the axis bosses, by virtue of obtaining, during the heat treatment, a microstructure comprising phases metastable copper type θ'- θ "derived from the precipitation system Al ₂ Cu.

These phases are more stable at moderate and high temperatures (180 to 300 ^° C.) than the β'β "binary phases based on Mg ₂ Si and the quaternary phases λ'λ" AlCuMgSi, which are formed during the incipient income. high amounts of magnesium and limited copper contents.

By choosing the copper content, it is possible to access different compromises between the mechanical properties at high temperature and the ductility. As most of the alloys for the manufacture of engine pistons, the iron content is kept below 1%, which means that they may be alloys of first or second fusion; this limit can be lowered below 0.3% (first melting), and preferably below 0.2% when a high elongation at break is desired. The titanium content is maintained between 0.03 and 0.3%, which is quite usual for this type of alloy.

An advantage of the compositions chosen according to the invention is indeed the possibility of obtaining α-Al dendritic structures that are varied and controllable according to the zones of the piston.

The Applicant has verified the possibility of promoting, by playing on the solidification thermal and on the composition, either columnar structures, even herbaceous, or on the contrary, equiaxes:

If an equiaxed structure is desired in all zones of the piston, the additions of titanium (growth retarding and refining element) up to 0.3%, possibly combined with additions of titanium-based refining agents and / or boron are preferred. - If, on the contrary, it is desired to obtain, in certain parts of the room, such as the head and the bowl edge, a columnar structure, even herbaceous, and elsewhere, especially in the axis bosses, an equiaxial structure. it is important to limit the content of peritectic elements, ie Ti <0.10%, preferentially <0.05%, and Zr <0.05%. The best results of thermal fatigue tests to date have been obtained with columnar structures at the edge of the bowl.

On the other hand, an equiaxed dendritic structure refined at the bottom of the piston (axis bosses) favors the resistance to hot mechanical fatigue of this zone. Thus, the alloys according to the invention make it possible, by adapting the local dendritic structure, to optimize the resistance of the piston both in thermal fatigue and mechanical fatigue (bowl edge and axis boss).

The zirconium content is kept below 0.05%, since this element refines the grain and prevents obtaining a columnar or herbaceous structure precisely sought in hot areas.

As with many piston alloys, the alloy contains nickel at a high level of 1 to 4%, which contributes to the heat resistance. It may also comprise vanadium at a content of less than 0.30%, and preferably between 0.04 and 0.20%. At a content of more than 0.1%, manganese has a positive effect on the mechanical strength between 230 ° C and 43O ⁰ C, but this effect then caps, which explains its limitation to 1% maximum.

In addition to its effects on the structure, the limitation of the magnesium content to less than 0.5%, preferably 0.4% and even more advantageously 0.3%, has the advantage of substantially reducing the oxidation of the alloy, this facilitating 1 Obtaining particularly sound parts (few inclusions of oxides, shrinkage, gassing porosity), resulting in a much better consistency of performance in thermal fatigue and mechanical fatigue

According to a preferred embodiment, the limitation of the magnesium content to less than 0.1% confers additional advantages: -It allows obtaining at the top of the piston, subjected to high loads of thermal fatigue at high temperature (350 ° C-430 ° C), a structure comprising a large fraction of eutectic phases Al-Cu and Al-Ni-Cu without magnesium, particularly favorable to good mechanical behavior at high temperature. With the choice of a high copper content according to the invention, it has no negative impact on the level of curing at room temperature, nor on the fatigue strength, thermal and mechanical.

Moreover, unlike conventional piston alloys, where the presence of magnesium is desired or allowed, the alloys with a very low magnesium content, according to the preferred invention, have a solidus temperature and a temperature of burn above 507 ° C. They can therefore be heat treated in the T6 or T7 state with a dissolution temperature of between 515 and 525 ^° C. depending on the copper content, and this without any particular precaution, that is to say without necessity. a slow rise in temperature or an intermediate stage, while the alloys of the same type with more than 0.5% magnesium form a quaternary eutectic invariant with the risk of a burn at 508 ^° C.

The possibility of carrying out a heat treatment at more than 515 ° C has several advantages: it is possible to obtain a further homogenization of the copper phases, a better globulization of the silicon phases and a more complete precipitation of the titanium and vanadium peritectic phases. .

Finally, another significant advantage of the combination of high levels of copper and low magnesium contents is to make the structural hardening less sensitive to quenching speed after dissolution than in the case of alloys of the Al-Si type. -Mg or Al-Si-Cu-Mg. Therefore, although water-hardenable according to the usual techniques, these alloys allow the practice of heat treatments T6 or T7 with quenching speeds significantly slowed compared to those usually used (especially by quenching with water), thus reducing the internal stresses in the rooms. This is also true for the T5 states since curing in this case does not require rapid quenching at the mold.

The alloys according to the invention are suitable for the usual molding processes, in particular gravity mold casting and low pressure shell molding, but also sand casting, squeeze casting (particularly in the case of insertion of composites) and lost-foam molding. They may be suitable for other forming processes such as, but not limited to, stamping, forging, "casting-molding" (Cobapress®) or molding in the semi-solid state.

Finally, they can also be used for inserts in the hot parts of a traditional alloy part, or for the hot parts of parts made of two different alloys ("dual casting").

The heat treatment of the T7 type may comprise dissolution typically from 15 minutes to 10 hours at a temperature of between 515 and 525 ^° C., quenching preferably with cold water or softened quenching, and over-tempering. 0.5 to 10 h at a temperature between 220 and 28O ⁰ C. If, for particular reasons, it is desired to avoid too much globulization of the eutectic silicon network, it is preferable to carry out the dissolution at a lower temperature, between 490 and 515 ° C.

The alloys according to the invention, and in particular in the case of application to pistons of automobile or aircraft engines, have excellent mechanical resistance to heat and creep resistance greater than that of the alloys of the prior art in the temperature range 230-430 ° C.

Finally, it should be noted that the alloys according to the invention can obviously contain impurities at the level encountered either in first fusion (alloys resulting from electrolysis without recycling), second melting (alloys resulting from recycling), or at a purity level. intermediate between the two previous ones.

Examples

In an industrial furnace, 7 alloys were prepared whose composition (% by mass) is indicated in Table 1. These compositions were measured by spark emission spectrometry.

Table 1

These compositions have been used to produce, by molding according to the process

"Gravity shell" means "turbo diesel" type pistons for passenger car engines. Said pistons have been:

- a mechanical characterization (cutting of dissection specimens in the heads of the pistons and tensile tests at ambient temperature),

- structural characterizations (grain size, examination of intermetallic phases),

- Experiments on a simulator of the stress in thermal fatigue (in English "Thermo-shock" or "Test of the resistance of the bowl rim"): This test, via an induction heating system and a controlled cooling of skirts in water bath, causes a thermal cycling between 350 ° C and 15O ⁰ C in the bowl edges. The heating inductor has a diameter of 1 to 2 mm smaller than that of the bowl edge in which it is positioned. Heating is usually done for 3 to 4 seconds and cooling for 6 to 10 seconds. This stress generates successively compressive and tensile stresses on the bowl edge and thus makes it possible to compare the thermal fatigue resistance of the alloys of different compositions. The criterion of ruin is the appearance of a crack of more than 1.5 mm from the edge of the bowl.

- Experiments on a simulator (in English "Pin boss pulsator") of the stress in mechanical fatigue axis bosses. The piston is positioned in a sealed cylindrical chamber adjusted to its outer lateral dimensions. An alternating oil pressure, at a frequency of 20 to 30 Hz, during 2.10 ⁶ cycles, is applied at the piston head to obtain a reaction pressure at the bottom of the constant skirt (generally 40 bars or 40 10 ⁵ N / m ² ). The temperature is generally set at 50 ° C. Depending on the average and maximum pressure values at the piston head and the temperatures usually encountered in the bosses, a calculation makes it possible to determine the maximum combustion pressure at which the bosses can withstand. It is the latter which appears on the ordinate in FIG. 3, in this case for a hold of 20 h at 250 ° C. For the various characterizations and experiments, the pistons were previously subjected to a heat treatment comprising a dissolution solution with a plateau of 60 minutes at 500 ° C., quenching and a tempering of 5 h at 230 ^° C.

The results of the thermal fatigue tests are shown in FIG. 2 and those of the fatigue tests of the axis bosses in FIG. 3: the best results are applied at the top of the piston to obtain a reaction pressure at the bottom of the constant skirt (generally 40 bars or 40 10 ⁵ N / m ² ).

The temperature is generally set at 50 ° C. Depending on the average and maximum pressure values at the piston head and the temperatures usually encountered in the bosses, a calculation makes it possible to determine the maximum combustion pressure at which the bosses can withstand. It is the latter which appears on the ordinate in FIG. 3, in this case for a hold of 20 h at 250 ^° C. For the various characterizations and experiments, the pistons were previously subjected to a heat treatment involving solution with a plateau of 60 minutes at 500 ° C., quenching and tempering for 5 hours at 230 ^° C.

The results of the thermal fatigue tests are shown in FIG. 2 and those of the fatigue tests of the axis bosses in FIG. 3: the best results are obtained in mechanical fatigue for the compositions C and F (FIG. in the composition field of the alloys according to the invention, these two compositions also leading to excellent results in thermal fatigue.

Figure 4 shows the tensile test results at room temperature on dissection specimens. The alloy C according to the invention has characteristics of the same level as those of the standard alloy A reference.

Claims

claims

1. Aluminum alloy for a part with high mechanical resistance to heat, of composition (% by weight): Mg <0.5 Si: 8.0 - 13 Cu: 4.0 - 9.0 Ni: 1.0 - 4.0

Fe <1.0 Mn <1.0 Zn <1.0 V <0.3 Ti <0.3

Zr <0.05 P: 0.001 - 0.05 other elements <0.15% each and 0.30% in total, aluminum balance.

2. Alloy according to claim 1, characterized in that the magnesium content is less than 0.4%.

3. Alloy according to claim 1, characterized in that the magnesium content is less than 0.3%.

4. An alloy according to claim 1, characterized in that the magnesium content is less than 0.1%.

5. Alloy according to one of claims 1 to 4, characterized in that the iron content is less than 0.60% and the Zn content less than 0.40%.

6. Alloy according to one of claims 1 to 5, intended to present a structure equiaxed in almost the entire room, characterized in that the titanium content is between 0.10 and 0.20%.

7. Alloy according to one of claims 1 to 5, for presenting a marked columnar structure in selected areas of the room and optionally equiaxed in other areas, characterized in that the titanium content is less than 0.10 %.

8. Alloy according to one of claims 1 to 7, characterized in that the vanadium content is between 0.04 and 0.20%.

9. An alloy according to one of claims 1 to 8, characterized in that the manganese content is between 0.15 and 0.40%.

10. Alloy according to one of claims 1 to 9, characterized in that it is used for a hot part insert of a molded part.

11. Alloy according to one of claims 1 to 10, characterized in that it is used for an internal combustion engine piston.