EP2516687B1 - Casting made from copper containing aluminium alloy with high mechanical strength and hot creep - Google Patents

Description

Field of the invention

The invention relates to copper aluminum alloy castings subjected to high mechanical stresses and working, at least in some of their areas, at high temperatures, including cylinder heads supercharged diesel or gasoline engines.

State of the art

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

The alloys commonly used for the cylinder heads of automotive mass-produced vehicles are essentially silicon alloys (5 to 10% Si in general) often containing copper and magnesium in order to increase their mechanical characteristics, especially when hot. . The main types used are as follows: AlSi7Mg, AlSi7CuMg, AlSi (5 to 8) Cu3Mg, AlSi10Mg, AlSi10CuMg. These alloys are used with different methods of heat treatment: sometimes in the F-state without any treatment, sometimes in the T5 state with a simple income, sometimes in the T6 state with dissolution, quenching and drying. at the peak of hardening or slightly below, and often in the T7 state with dissolution, quenching and over-tempering or stabilization.

The reason for using aluminum silicon alloys is the superiority of their foundry properties, in particular lack of creasability, high flowability, good power of the shrink. Only these silicon alloys greater than or equal to 5% lend themselves to shell molding, by gravity or low pressure, which is the dominant process for large series cylinder heads.

For low-volume manufacturing generally made in sand casting, such as cylinder heads of high-performance vehicles or hot-working parts for armament and aeronautics, copper alloys of the AlCu5 type are also sometimes used. added with elements promoting the heat resistance such as Ni, Co, Ti, V and Zr: there is particularly in this category AlCu5NiCoZr and AlCu4NiTi. These alloys are very resistant to heat, especially at 300 ° C where they clearly outperform the silicon aluminum mentioned above, but suffer from two serious weaknesses: their high cracks, combined with a bad shrinkage behavior, which makes them very difficult. to flow in shell in mass production, and also the mediocrity of their mechanical characteristics at ambient temperature: they have in particular a very low elongation, which makes them fragile and not very efficient in mechanical fatigue. Table 1 summarizes the characteristics at ambient temperature of these two sand-cast alloys heat-treated in the T7 state (Rp0.2 (or 0.2% TYS) being the elastic limit in MPa; Rm (or UTS) being the breaking strength in MPa, and A (or E) being elongation at break in%: Table 1 Alloy Rp0.2 (MPa) Rm (MPa) AT (%) AlCu4NiTi Unmeasurable 343 0.11 AlCu5NiCoZr 270 295 1

There is also an alloy formerly standardized by the Aluminum Association (hereafter referred to as "AA" for convenience) under number 224, which is of the AlCu5Mn VZr type. It has been declared "inactive" by this association, which has been withdrawing it for years from its periodically updated "Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot". This alloy 224 does not contain magnesium (this element falling into the category of impurities, with a maximum of 0.03% each, 0.10% total), and old characterization results on sand-cast plates have shown the characteristics to the T7 state described in Table 2: Table 2 Alloy Rp0.2 (MPa) Rm (MPa) AT (%) 224 280 360 4.8

Problem

Since, in future common-rail or gas-supercharged diesel engines, the cylinder head combustion chambers, and in particular the inter-valve bridges, will reach or exceed 300 ° C and will experience higher pressures than in generations of previous engines in use today, the use of copper aluminum alloys is a solution "break" compared to the incremental progress made by the optimization of silicon aluminum alloys.

• hautes propriétés mécaniques à température ambiante,
• hautes propriétés mécaniques dans le domaine 250 - 300°C,
• et haute résistance au fluage à 300°C, température caractéristique notamment des pontets inter-soupapes, éléments particulièrement sollicités thermo-mécaniquement.

But we still have to find an alloy of this family that combines:

• high mechanical properties at room temperature,
• high mechanical properties in the range 250 - 300 ° C,
• and high resistance to creep at 300 ° C, characteristic temperature including inter-valve bridges, particularly thermomechanically stressed elements.

Conventional AlCu5Mg alloys such as AlCu5MgTi (designated as 204 AA), and A206 and B206 (AA), for room temperature or moderate working parts do not meet these requirements, particularly 300 ° C.

The alloys AlCu4NiTi and AlCu5NiCoZr (203 following AA) mentioned above are too weak and fragile at room temperature.

The AlCu5MnVZr (formerly 224 following AA) for hot workpieces has a more interesting combination of properties but still lacks yield strength at room temperature compared to the desired improved properties: it gives, in state T7 , a yield strength Rp0.2 = 280 MPa, compared with 275 MPa for AlSi7Cu0.5Mg0.3 T7 and 311 MPa for AlSi5Cu3Mg T7 (values measured by the applicant and published respectively in the articles "alloys of 'Enhanced Aluminum for Diesel Cylinders' (Men and Foundry - February 2008- No. 382) and 'Aluminum Casting Alloys for Highly Stressed Diesel Cylinder Heads', (3. International Symposium Aluminum + Automobil); Düsseldorf; FRG; 3-4 Feb 1988, pp. 154-159, 1988).

It has therefore been sought to achieve considerable progress with respect to the former 224 in terms of yield strength and ultimate strength from ambient temperature to 250 - 300 ° C. It has also been sought to improve the creep resistance at 300 ° C. of this old alloy.

Object of the invention

• Si : 0.02 - 0.50 %, de préférence 0.02 - 0.20 % et plus préférentiellement 0.02 - 0.06%,
• Fe : 0.02 - 0.30 %, de préférence 0.02 -0.20 %, plus préférentiellement 0.02 - 0.12 % et mieux 0.02 - 0.06%,
• Cu : 3.5 - 4.9 %, de préférence 3.8 - 4.9 % et plus préférentiellement 4.0 - 4.8 %,
• Mn : < 0.70 %, de préférence 0.20 - 0.50 %,
• Mg : 0.05 - 0.20 %, de préférence 0.07 - 0.20 %, et plus préférentiellement 0.08 - 0.20 % et enfin de façon très préférentielle 0.09 - 0.13%,
• Zn : < 0.30 %, de préférence < 0.10 % et plus préférentiellement < 0.03%,
• Ni : < 0.30 %, de préférence < 0.10 % et plus préférentiellement < 0.03%,
• V : 0.05 - 0.30 %, de préférence 0.08 - 0.25 %, et plus préférentiellement 0.10 - 0.20%,
• Zr : 0.05 - 0.25 %, de préférence 0.08 - 0.20 %,
• Ti : 0.01 - 0.35 %, de préférence 0.05 - 0.25 % et plus préférentiellement 0.10 - 0.20%, autres éléments au total < 0.15%; et 0.05 % chacun,
• reste aluminium.

The subject of the invention is therefore a molded part with high static resistance at ambient temperature and with high heat resistance and high creep resistance, in particular at 300 ° C. and above, cast in aluminum alloy of the following chemical composition , expressed in percentages by weight:

• If: 0.02 - 0.50%, preferably 0.02 - 0.20% and more preferably 0.02 - 0.06%,
• Fe: 0.02 - 0.30%, preferably 0.02 -0.20%, more preferably 0.02 - 0.12% and better 0.02 - 0.06%,
• Cu: 3.5 - 4.9%, preferably 3.8 - 4.9% and more preferably 4.0 - 4.8%,
• Mn: <0.70%, preferably 0.20 - 0.50%,
• Mg: 0.05 - 0.20%, preferably 0.07 - 0.20%, and more preferably 0.08 - 0.20% and finally very preferably 0.09 - 0.13%,
• Zn: <0.30%, preferably <0.10% and more preferably <0.03%,
• Ni: <0.30%, preferably <0.10% and more preferably <0.03%,
• V: 0.05 - 0.30%, preferably 0.08 - 0.25%, and more preferably 0.10 - 0.20%,
• Zr: 0.05 - 0.25%, preferably 0.08 - 0.20%,
• Ti: 0.01 - 0.35%, preferably 0.05 - 0.25% and more preferably 0.10 - 0.20%, other elements total <0.15%; and 0.05% each,
• remains aluminum.

Description of figures

• The figure 1 represents a cluster of four shell cast specimens from the Rio Tinto Alcan company with a diameter of ¼ "(6.35 mm).
• The figure 2 represents differential enthalpy analysis curves for AlCu4.7MnVZrTi alloys with magnesium content of 0%, 0.09% and 0.13%.
• The figure 3 shows results of creep tests at 300 ° C on T7-treated AlCu4.7MnVZrTi and AlSi7Cu3.5MnVZrTi alloys also treated T7 with variable magnesium content respectively from 0% to 0.13% and from 0.1% to 0.15%.

Description of the invention

The invention is based on the finding by the applicant that it is possible to make very significant improvements to the characteristics mentioned above of the old alloy 224 (according to the AA), and thus to solve the problem posed, in particular by the addition of a limited amount of magnesium.

Indeed, the addition of a small amount of magnesium, of the order of 0.10 to 0.15%, makes it possible to considerably increase the yield strength and the resistance of the alloy not only at room temperature but also hot, especially at 250-300 ° C and above. It is at room temperature that the relative gain is the most important: as explained in the following examples and Tables 6, 7, 8, the elastic limit goes from about 190 MPa without magnesium to about 340 MPa with only 0.09% and then at over 390 MPa with 0.13%. If we consider the average results obtained with 0.09% and 0.13% magnesium, the gains observed on the yield strength and the resistance at ambient temperature are remarkable: respectively + 96% and + 29% in relative terms.

However, the elongation is substantially reduced by half but still retains a suitable level of 6 to 8%.

At high temperature, 250 then 300 ° C, the gains brought by the addition of magnesium remain even if they decrease. The observed gains in yield strength and strength are respectively 35 and 13% in relative terms at 250 ° C, and 27 and 8% in relative terms at 300 ° C. Far from impairing the hot stability of the hardening phases as might have been envisaged, the addition of magnesium remains beneficial at least up to 300 ° C., especially as the loss of elongation fades at these high temperatures.

In addition, the addition of magnesium considerably improves the hot creep resistance, reducing by approximately 2, for example, the deformation observed after 300 h at 300 ° C. under a stress of 30 MPa. The addition of magnesium therefore does not affect the hot stability, contrary to the philosophy that led to the definition of alloys AlCu5NiCoZr (203 following the AA) and AlCu5MnVZr (224 following the AA) conventional that are devoid of magnesium .

It is interesting to locate the average level of performance of the alloy according to the invention (for simplicity have been assigned the average of the characteristics of alloys at 0.09% and 0.13% magnesium the alloy designated "AlCu4.7MnMg _moy VZrTi ") compared to some aluminum-silicon-based cylinder head alloys. Table 3 summarizes the mechanical characteristics. Table 3 Alloy / heat treatment Ambient temperature 250 ° C 300 ° C Rp0.2 rm AT% Rp0.2 rm AT% Rp0.2 rm AT% AlCu4.7MnMg _Avg VZrTi T7 369 451 7.4 182 226 9.8 125 158 14.8 AlSi5Cu3Mg F 172 237 2.1 107 133 5.8 60 86 12 AlSi7Mg0.3Ti T7 257 299 9.9 55 61 34.5 40 43 34.5 AlSi7Cu0.5Mg0.3Ti T7 275 327 9.8 66 73 34.5 40 44 34.6 AlSi7Cu3.5Mg0.15 MnVZrTi T7 306 392 5.2 101 115 27 60 70 31

With regard to the creep resistance at 300 ° C., the alloy according to the invention treated T7 can be compared with the AlSi7Cu3.5Mg0.15MnVZrTi also treated T7, which was also developed by the applicant and is its most creep-resistant knowledge of the series of aluminum silicon alloys considered in the previous table. The curve of the figure 3 shows the very great superiority of the AlCu4.7MnMgVZrTi, which deforms substantially 4 times less under the same conditions.

It thus appears that the objective of "breaking" progress with respect to existing alloys is achieved by the addition of magnesium to a base of AlCu5MnVZrTi type.

Although the addition of magnesium gradually lowers the off-equilibrium burn temperature, it is still possible to dissolve the alloy at 525 ° C. or 528 ° C., as is done, for example, with conventional alloys A206 and B206. A step treatment will eventually allow the alloy to be processed at a slightly higher final temperature, but this stepwise treatment is not essential, given the very high results achieved with isothermal treatment under the burn temperature.

The magnesium content can be increased beyond the area already experienced in the examples. If only very high strength and hardness are sought, with a reduced ductility requirement, a maximum level of 0.38% can be envisaged, knowing that the burn temperature will be lowered and the heat treatment will have to be adapted. The minimum to obtain a significant curing effect is of the order of 0.05%. A smaller range is from 0.07% to 0.30% and the preferred range, corresponding to the resistance-ductility-creep tradeoffs quantified in the examples while having an industrially acceptable width is 0.08-0.20%, or even 0.09-1.03%.

• Silicium : il est généralement néfaste à la ductilité et peut abaisser la température de brûlure. Par contre, il améliore les propriétés de fonderie et en particulier est susceptible, même à faible niveau, de réduire la criquabilité, comme décrit dans l' ASM Handbook, volume 15, édition 2008 . Un niveau minimum de 0.02% est nécessaire. Un niveau maximum de 0.50% est pensable pour des pièces solidifiées très rapidement ou ne nécessitant guère d'allongement, mais on préfèrera généralement moins de 0.20%, voire de 0.06%.
• Fer : il est néfaste à la ductilité, mais diminue par contre la criquabilité, comme également décrit dans l' ASM Handbook, volume 15, édition 2008 . De plus le limiter à un très bas niveau augmente évidemment le coût de la pièce. Un niveau minimum de 0.02% est donc avantageux. Un niveau maximum de 0.30% est pensable pour des pièces solidifiées très rapidement ou ne nécessitant guère d'allongement, mais on préfèrera généralement moins de 0.20% pour des grandes séries automobiles, voire de 0.12% ou même 0.06% pour des pièces extrêmement sollicitées.
• Cuivre : il durcit l'alliage, augmentant limite d'élasticité et résistance mais diminuant l'allongement. La fourchette de l'ancien alliage 224 était de 4.5 à 5.5%. L'expérience acquise par la demanderesse avec le B206 indique qu'il est bon de limiter le cuivre à un maximum de 4.9% car au-delà il est très difficile de remettre tout le cuivre en solution. Comme les présents résultats, obtenus avec un cuivre de 4.7 à 4.8%, montrent que la résistance à température ambiante obtenue avec addition de magnésium est très élevée mais que l'allongement est réduit par rapport à l'ancien alliage 224 sans magnésium, il est logique de prévoir la possibilité de réduire le cuivre en dessous de 4.5%, et plus particulièrement jusqu'à 3,5%. La demanderesse a effectué des travaux sur l'alliage B206 pour lesquels elle estime que les résultats qui sont transposables à l'alliage selon l'invention et montrent que qu'un abaissement du cuivre de 5.0% à 4.0% permet de gagner notablement en allongement au prix d'une perte de résistance, mais que celle-ci reste supérieure à 400 MPa. Dans l'optique de certaines culasses, il est même concevable d'accepter une baisse un peu plus importante de la résistance pour privilégier l'allongement et de réduire le cuivre jusqu'à 3.5%. On pourra choisir des sous-domaines entre 3.5% et 4.9% en fonction du compromis de caractéristiques visées pour la pièce spécifique. D'une façon générale, des sous domaines centrés sur 4.3% ou 4.4% tels que 3.8 - 4.9% et mieux 4.0 - 4.8% conduisent à un compromis assez équilibré.
• Manganèse: cet élément ne doit pas excéder 0.70% sous peine de risquer de former des phases intermétalliques grossières. Comme il améliore généralement les propriétés mécaniques, particulièrement à chaud, un domaine de 0.20 - 0.50% analogue à celui des alliages du type 206 est préféré.
• Zinc: cet élément est une impureté qui, à haute teneur, peut diminuer les propriétés mécaniques et rendre le bain liquide plus oxydable. On peut envisager de tolérer jusqu'à 0.30% dans le but de faciliter l'emploi de métal de recyclage, mais on préfère moins de 0.10% et mieux moins de 0.03% pour des pièces à hautes performances.
• Nickel: il contribue en général à la résistance mécanique à chaud mais réduit considérablement l'allongement. Comme la résistance à chaud est assurée dans l'invention par l'addition d'autres éléments, cuivre, magnésium, vanadium et zirconium, le nickel est considéré ici comme une impureté, qu'on limite au maximum à 0.30% dans le but de faciliter l'emploi de métal de recyclage, et de préférence à 0.10% et encore mieux à 0.03% pour des pièces à hautes performances.
• Vanadium: Cet élément péritectique améliore en particulier la résistance au fluage à chaud. La demanderesse a observé que, dans une autre base d'alliage contenant du silicium, la résistance au fluage était fortement améliorée entre 0 et 0.05%, puis s'améliorait ensuite plus progressivement de 0.05% à 0.17% et était au-dessus de 0.17% stable à un excellent niveau. Limiter le niveau maximum de vanadium à 0.15% comme dans l'ancien 224 ne paraît donc pas souhaitable. Dans l'alliage suivant l'invention, un niveau de 0.05 à 0.30% est prévu, qui pourra être resserré à des sous-domaines plus étroits de 0.08 - 0.25% et préférentiellement 0.10 - 0.20%.
• Zirconium: cet élément péritectique améliore également en particulier la résistance au fluage à chaud, et son effet est additif à celui du vanadium. Une teneur de 0.05 - 0.25% et de préférence 0.08 - 0.20% est retenue.
• Titane: cet élément péritectique a deux effets différents : d'une part, il est souvent utilisé comme élément affinant du grain, souvent en combinaison avec un ajout d'alliage mère ou de sel ajoutant du titane et du bore. Cependant, il existe d'autres pratiques d'affinage consistant à n'ajouter que des produits introduisant du titane et du bore, voire même du bore seul, et dans ce dernier cas la présence de titane n'est pas favorable. D'autre part, le titane contribue à la bonne résistance au fluage à chaud, quoi que moins fortement que vanadium et zirconium, comme la demanderesse l'a observé. On a donc retenu une teneur maximum de 0.35%, mais on préférera en général une addition de 0.05 à 0.25% et encore mieux de 0.10 à 0.20%.

With regard to the other elements constituting the type of alloy according to the invention, their contents are justified by the following considerations:

• Silicon: It is generally harmful to ductility and can lower the burn temperature. On the other hand, it improves the casting properties and in particular is capable, even at a low level, of reducing the creasability, as described in FIG. ASM Handbook, volume 15, edition 2008 . A minimum level of 0.02% is necessary. A maximum level of 0.50% is conceivable for parts solidified very quickly or requiring little elongation, but one will generally prefer less than 0.20%, or even 0.06%.
• Iron: it is harmful to the ductility, but decreases against the crackability, as also described in the ASM Handbook, volume 15, edition 2008 . Moreover, limiting it to a very low level obviously increases the cost of the piece. A minimum level of 0.02% is therefore advantageous. A maximum level of 0.30% is conceivable for parts solidified very quickly or not requiring much elongation, but one will generally prefer less than 0.20% for large automotive series, or even 0.12% or even 0.06% for highly stressed parts.
• Copper: It hardens the alloy, increasing yield strength and strength but decreasing elongation. The range of the old alloy 224 was 4.5 to 5.5%. experience acquired by the applicant with the B206 indicates that it is good to limit the copper to a maximum of 4.9% because beyond it is very difficult to put all the copper in solution. As the present results, obtained with a copper of 4.7 to 4.8%, show that the resistance to ambient temperature obtained with the addition of magnesium is very high but that the elongation is reduced compared to the old alloy 224 without magnesium, it is It is logical to foresee the possibility of reducing copper below 4.5%, and more particularly up to 3.5%. The Applicant has carried out work on alloy B206 for which it considers that the results which are transferable to the alloy according to the invention and show that a lowering of copper from 5.0% to 4.0% allows to gain significantly in elongation at the cost of a loss of resistance, but that it remains higher than 400 MPa. In view of some cylinder heads, it is even conceivable to accept a slightly larger drop in resistance to favor elongation and reduce copper up to 3.5%. We can choose subdomains between 3.5% and 4.9% depending on the trade-off of characteristics targeted for the specific part. In general, subdomains centered on 4.3% or 4.4% such as 3.8 - 4.9% and better 4.0 - 4.8% lead to a fairly balanced compromise.
• Manganese: this element must not exceed 0.70% otherwise the risk of forming coarse intermetallic phases. Since it generally improves the mechanical properties, especially when hot, a range of 0.20 to 0.50% similar to that of 206 type alloys is preferred.
• Zinc: this element is an impurity which, at high content, can decrease the mechanical properties and make the liquid bath more oxidizable. It may be envisaged to tolerate up to 0.30% in order to facilitate the use of recycle metal, but less than 0.10% and better still less than 0.03% is preferred for high performance parts.
• Nickel: it generally contributes to the mechanical strength when hot but considerably reduces elongation. As the hot resistance is ensured in the invention by the addition of other elements, copper, magnesium, vanadium and zirconium, nickel is considered here as an impurity, which is limited to a maximum of 0.30% for the purpose of facilitate the use of recycling metal, and preferably at 0.10% and even better at 0.03% for high performance parts.
• Vanadium: This peritectic element in particular improves the resistance to creep when hot. Applicant has observed that in another alloy base containing silicon, the creep resistance was greatly improved between 0 and 0.05%, then improved more gradually from 0.05% to 0.17% and was above 0.17%. % stable at an excellent level. Limiting the maximum level of vanadium to 0.15% as in the old 224 does not therefore seem desirable. In the alloy according to the invention, a level of 0.05 to 0.30% is provided, which can be tightened to narrower subdomains of 0.08 - 0.25% and preferably 0.10 - 0.20%.
• Zirconium: this peritectic element also improves in particular the resistance to hot creep, and its effect is additive to that of vanadium. A content of 0.05 - 0.25% and preferably 0.08 - 0.20% is retained.
• Titanium: This peritectic element has two different effects: on the one hand, it is often used as a refining element of the grain, often in combination with an addition of parent alloy or salt adding titanium and boron. However, there are other refining practices consisting of adding only products introducing titanium and boron, or even only boron, and in the latter case the presence of titanium is not favorable. On the other hand, titanium contributes to good resistance to creep hot, although less strongly than vanadium and zirconium, as the applicant has observed. A maximum content of 0.35% has therefore been retained, but an addition of 0.05 to 0.25% and even more preferably of 0.10 to 0.20% is preferred.

The other elements are to be considered as impurities. In order to facilitate the recycling, we can tolerate for some parts a maximum total level of 0.50%, but preferably for the parts solicited we will adopt maxima of 0.15% in total and 0.05% each.

Examples

A series of three alloy compositions described in Table 4 was developed in a 35 kg electric furnace, all elements being expressed in% by weight. Table 4 landmark Yes Fe Cu mn mg Ti V Zr 0 Mg 0.09 0.14 4.83 0.34 0.00 0.18 0.21 0.14 0.09 Mg 0.08 0.14 4.74 0.33 0.09 0.22 0.17 0.13 0.13 Mg 0.09 0.14 4.81 0.33 0.13 0.20 0.17 0.13

These alloys were refined by addition of AlTi5B (30 ppm of titanium thus added) and degassed by a 10 minute treatment using a graphite rotor rotating at 300 rpm with an argon flow of 5. liters / minute, all under cover of a MgCl ₂ 60% wash stream - 40% KCl.

Du "(6.5 mm) diameter shell specimens of the Rio Tinto Alcan type, represented in FIG. figure 1 for tensile tests and ASTM B108 ½ "(12.7 mm) shell test pieces to be used as blanks for 4 mm diameter creep specimens. figure 1 is more particularly a cluster 10 of 4 test tubes 11 of Rio Tinto Alcan cast into shell with a diameter of the barrel ¼ "(6.35 mm) .This cluster 10 takes, scale 1/2, the design of the ASTM specimen B108.

The burning temperature of the various compositions was first determined by performing differential enthalpic analyzes (AED) on pellets machined in the cast specimens. The rate of rise in temperature was 20 ° C / minute. The AED curves are represented at figure 2 . The burn temperatures observed corresponding to the melting peaks obviously depend on the magnesium content as shown in Table 5: Table 5 Content in Mg (%) Burning temperature (° C) 0 542.7 0.09 538.2 0.13 533.9

The burn temperature gradually shifts to lower temperatures when the Mg content increases from 0% to 0.09% and then to 0.13%.

These 3 alloys were then heat treated by applying them to a solution comprising a preliminary stage of 2 hours at 495 ° C. and then a main stage of 12 hours at 528 ° C., followed by a quenching with water at 65 ° C. C and an income of 4h at 200 ° C. An alloy in the T7 state is thus obtained.

The blanks intended for the creep tests were subjected, prior to this heat treatment, to hot isostatic compaction at 1000 bar at 485 ° C. for 2 hours in order to eliminate any microporosity which could seriously affect the tests given the small diameter of the specimen.

Static mechanical characteristics were measured at room temperature and at 250 ° C and 300 ° C. In the latter two cases, the specimens were preheated for 100 hours at the temperature before being tracted.

The results are shown in Tables 6, 7 and 8: Table 6: Mechanical characteristics at room temperature Alloy Rp0.2 rm AT Mg (%) MPa MPa % 0 187.8 349.3 15.3 0.09 344.5 435.0 8.2 0.13 393.4 466.4 6.6 Alloy Rp0.2 rm AT Mg (%) MPa MPa % 0 134.7 199.5 10.7 0.09 172.2 223.7 7.3 0.13 191.4 228.8 12.2 Alloy Rp0.2 rm AT Mg (%) MPa MPa % 0 98.3 147.1 14.5 0.09 130.2 167.2 11.2 0.13 120.0 149.4 18.3

• Les éprouvettes de diamètre 4 mm dans la zone utile, usinées dans les ébauches de diamètre 12.7 mm, ont d'abord été préchauffées 100 h à 300°C dans un four séparé, puis placées sur la machine de fluage et stabilisées à nouveau ½ h à 300°C avant de les mettre sous une charge constante de 30 MPa. La déformation en % est alors enregistrée continûment pendant une durée de 300 h à 300°C. Le critère principal utilisé pour l'interprétation des essais est la déformation obtenue après 300 h.

Creep tests were conducted at 300 ° C under the following conditions:

• The test pieces with a diameter of 4 mm in the working zone, machined in blanks with a diameter of 12.7 mm, were first preheated for 100 h at 300 ° C. in a separate oven, then placed on the creep machine and stabilized again for ½ hour. at 300 ° C before putting them under a constant load of 30 MPa. The% strain is then recorded continuously for 300 hours at 300 ° C. The main criterion used for the interpretation of the tests is the deformation obtained after 300 h.

Table 9 summarizes the results: Table 9: Creep at 300 ° C. under 30 MPa Magnesium content (%) Deformation (in%) after 300h 0 0.26 0.09 0.13 0.13 0.14

These results are reported in the figure 3 where also appear by reference the results obtained by the applicant with a series of AlSi7Cu3.5MnVZrTI type alloys with different Mg content.

A part may then be molded from the advantageous alloy defined above, this part may in particular be a cylinder head or an insert of a cylinder head or of another part requiring a high static mechanical resistance at room temperature and at room temperature. hot and high resistance to creep when hot, in particular at 300 ° C.

The part is advantageously treated T7, even if a T6 treatment is also possible.

Also, recently, a new foundry process called "Ablation Molding" has been introduced in North America. This process has been described in the article "Ablation Casting" by J.Grassi, J.Campbell, M.Hartlieb and F. Major presented at TMS 2008 . This process involves first pouring the piece into a sand mold + enough insulating binder, then when it has reached at least locally a sufficient solid fraction, to water the mold with one (or more) water jet that dissolves instantly bind the sand and cause the collapse of the mold. The part being solidified is then directly exposed to the impact of the water which extracts the calories very quickly (in a similar way to that observed for example in vertical continuous casting of aluminum billets). This leads to a very fast solidification of the alloy and to obtaining fine structures having high mechanical characteristics equal to or even greater than those obtained in shell casting with a metal mold.

Ablation molding is particularly suitable for molding high-tread alloys. Initially, it is sand casting that does not much upset the withdrawal, and then after removal of the mold the end of the solidification is carried out without rigid mold at all. In addition to providing a high solidification rate, the process also leads to high temperature gradients because the spray is generally progressive, starting on selected areas and advancing towards the end points of solidification where it is possible to attach. the weights. This advantageously also favors the use of alloys with low feed capacity of the shrink, such as copper aluminum alloys, including the alloy according to the invention.

• fournir un moule formé à partir d'un agrégat et d'un liant hydrosoluble ;
• couler l'alliage dans le moule ;
• projeter de l'eau sur le moule de manière à désagréger le moule et à refroidir l'insert ou la culasse pour accélérer la solidification de l'alliage.

Also, the invention also relates to a method for molding a part from the alloy according to the invention, in particular an insert or a cylinder head, comprising the steps of:

• providing a mold formed from an aggregate and a water-soluble binder;
• pour the alloy into the mold;
• projecting water on the mold so as to disintegrate the mold and cool the insert or the cylinder head to accelerate the solidification of the alloy.

The implementation of this method advantageously allows the mass production of molded parts with the alloy according to the invention having much higher mechanical properties than aluminum silicon alloys.

However, the prospects for using high-strength copper aluminum alloys are not restricted to the ablation process: there are other ways, including conventional sand casting, possibly combined with metal chillers, and mold casting. metallic shell, possibly with modifications of the part layout to accept the less good foundry properties of this family of alloys.

Claims (17)

1. Cast part with high static mechanical strength at ambient and hot temperatures and high creep strength at high temperature, especially at 300° C and above, cast in aluminum alloy with the following chemical composition, expressed as percentages by weight:

   Si: 0.02 - 0.50%

   Fe: 0.02 - 0.30%

   Cu: 3.5 - 4.9%

   Mn: < 0.70%

   Mg: 0.05 - 0.20%

   Zn: < 0.30%

   Ni: < 0.30%

   V: 0.05 - 0.30%

   Zr: 0.05 - 0.25%

   Ti: 0.01 - 0.35%

   other elements in total < 0.15%; and less than 0.05% each,

   the remainder being aluminum;
2. Cast part according to claim 1, characterized in that the magnesium content of the alloy is between 0.07 and 0.20%.
3. Cast part according to either of claims 1 or 2 characterized in that the magnesium content lies between 0.08 and 0.20% and preferably between 0.09 and 0.13%.
4. Cast part according to one of claims 1 to 3 characterized in that the copper content lies between 3.8 and 4.9% and preferably between 4.0 and 4.8%.
5. Cast part according to any of the previous claims characterized in that the vanadium content is between 0.08 - 0.25% and preferably between 0.10 - 0.20%.
6. Cast part according to any of the previous claims characterized in that the zirconium content lies between 0.08 and 0.20%.
7. Cast part according to any of the previous claims characterized in that the titanium content lies between 0.05 and 0.25% and preferably between 0.10 and 0.20%.
8. Cast part according to any of the previous claims characterized in that the silicon content lies between 0.02 and 0.20% and preferably between 0.02 and 0.06%.
9. Cast part according to any of the previous claims characterized in that the iron content lies between 0.02 and 0.20%, preferably between 0.02 and 0.12% and more preferably between 0.02 and 0.06%.
10. Cast part according to any of the previous claims characterized in that the manganese content lies between 0.20 and 0.50%.
11. Cast part according to any of the previous claims characterized in that the zinc content is less than 0.10% and preferably less than 0.03%.
12. Cast part according to any of the previous claims characterized in that the nickel content is less than 0.10% and preferably less than 0.03%.
13. Cast part according to any of the previous claims having undergone T7 or T6 type heat treatment.
14. Insert comprising a cast part according to any of claims 1 to 13.
15. Insert according to claim 14, characterized in that said insert is essentially made up of the cast part.
16. Cylinder head comprising a cast part according to any of claims 1 to 13 or an insert according to any of claims 14 and 15.
17. Method for casting an insert according to any of claims 14 to 15 or a cylinder head according to claim 16, comprising stages of:

    - providing a mold formed from an aggregate and a water-soluble binder;

    - casting the alloy in the mold;

    - spraying water on the mold so as to break up the mold and cool the insert or cylinder head.

Images