EP2171115B1 - Process for the heat treatment of cylinder heads made of an aluminium-based alloy, and cylinder heads having improved fatigue resistance properties - Google Patents

Process for the heat treatment of cylinder heads made of an aluminium-based alloy, and cylinder heads having improved fatigue resistance properties Download PDF

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Publication number
EP2171115B1
EP2171115B1 EP08761236.2A EP08761236A EP2171115B1 EP 2171115 B1 EP2171115 B1 EP 2171115B1 EP 08761236 A EP08761236 A EP 08761236A EP 2171115 B1 EP2171115 B1 EP 2171115B1
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Prior art keywords
heat treatment
quenching
temperature
fluidized bed
tempering
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German (de)
French (fr)
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EP2171115A1 (en
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Philippe Meyer
Denis Massinon
Julien Morichon
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Montupet SA
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Montupet SA
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
    • C22F1/05—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys of the Al-Si-Mg type, i.e. containing silicon and magnesium in approximately equal proportions
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C21/00—Alloys based on aluminium
    • C22C21/02—Alloys based on aluminium with silicon as the next major constituent
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
    • C22F1/043—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with silicon as the next major constituent

Definitions

  • the field of the invention is that of the heat treatments of castings made of aluminum-based alloys.
  • the invention more specifically relates to a heat treatment process of cast aluminum alloy cylinder heads, and cylinder heads resulting from such a process.
  • the cast aluminum alloy cylinder heads are mostly used in high-performance automotive engines.
  • Aluminum casting alloys have different families of compositions, most of which are suitable for structural hardening by heat treatment. In particular, mention may be made of the aluminum / silicon / magnesium family and the aluminum / silicon / copper / magnesium family.
  • alloys of type A356 further comprising 0.5% copper are frequently used for heavily loaded diesel engine cylinder heads.
  • the above treatments are typically carried out for periods of the order of 5 to 6 hours of residence at temperature in the oven, the rise in temperature of the charge being usually of the order of 1 to 2 hours for yokes.
  • the document GB 2,361,710 proposes to limit the dissolution time to a maximum of three hours, and preferably to a maximum of two hours.
  • the treatment recommended by this document results, for a lower treatment time, in mechanical characteristics similar to that of the T7 treatment.
  • this document GB 2,361,710 states that the fatigue properties are only slightly influenced by the type of heat treatment and indicates that, as a result, the fatigue properties of the parts subjected to the T7 treatment or to the treatment according to this document GB 2,361,710 with a shorter dissolution in solution should not be significantly different.
  • the document US 6,752,885 also provides a heat treatment shorter than the T7 treatment, this treatment comprising a dissolution of a duration of between one hour thirty minutes and two hours, and an income at a temperature of 250 ° C.
  • the document US 2004/265163 discloses a heat treatment comprising a solution solution, tempering and tempering step, by recommending the choice of over-tempering for parts of the part for which good dimensional stability is required.
  • the document US 2007/051336 discloses a heat treatment comprising a solution solution step, a water quenching step and an overflow step of the workpiece.
  • This treatment has the advantage of eliminating the residual stresses, but does not make it possible to take full advantage of the hardening potential of the alloy.
  • the present invention aims to overcome these disadvantages and to reduce the residual stresses of the alloy, while making better use of the hardening potential of the alloy in order to maximize the functional performance of the cylinder head, especially the resistance to hot fatigue - cold which is the most severe test for the development of driven cylinder heads.
  • the invention proposes a cylinder head made of an alloy based on aluminum, silicon or magnesium, characterized in that it exhibits a fatigue resistance evaluated by the "hot-air” fatigue test. cold ", and expressed in number of cycles before fatigue failure at +/- 200 cycles, greater than 4800 cycles, preferably close to 9500 cycles.
  • the invention is applicable to structural hardening aluminum alloys regardless of their chemical composition, and to parts made from these materials.
  • the invention relates in particular to castings made of alloy based on aluminum, silicon, and magnesium.
  • the invention is more particularly applicable to heat treating automotive components molded in such aluminum-based alloys, and subjected to very high loads in service.
  • the invention proves particularly advantageous for automobile engine cylinder heads which, as previously mentioned, are highly stressed casting parts.
  • We will take the example of such cylinder heads in the following description.
  • the dissolution can be carried out in a fluidized bed furnace, or in a conventional furnace, or even a succession of both.
  • the dissolution is performed is at the usual temperature of dissolution of the alloy.
  • the dissolution is carried out for a period of between 3 hours and 10 hours. According to an advantageous variant of the invention, the dissolution is carried out for a period of between 5 hours to 10 hours, including for fluidized bed treatments.
  • the dissolution comprises successively in a fluidized bed dissolution (for example a quench in a short fluidized bed, of a duration less than or equal to 30 minutes), and a complementary dissolution in conventional oven, the duration of the dissolution of complementary solution is then adapted to achieve a total duration of 3 hours and 10 hours according to the invention.
  • a fluidized bed dissolution for example a quench in a short fluidized bed, of a duration less than or equal to 30 minutes
  • the dissolving is applied to the cylinder head after a complete grit removal which rids it of all its internal cores, in particular by mechanical means (hammering and vibration typically), and after elimination of the flyweights and casting systems, which maximizes quench cooling rate.
  • the dissolution can be performed on the molded output parts, the realization of the dissolution in a fluidized bed in particular for an effective degritting of the cylinder head.
  • the cylinder head still has its weights, or even its casting system during quenching, which is not optimal as just noted.
  • the cylinder head After dissolution, the cylinder head is quenched not in water, but in air or in a fluidized bed.
  • the cooling rate of the quenching is adapted so that the cooling, as measured by one or more thermocouple (s) positioned in the cylinder head, lies under the curve SUP represented on the figure 1 , and preferably under the curve INF, represented on the figure 1 .
  • Such a cooling rate according to the invention implies in particular that the temperature of the cylinder head in its most critical areas from the functional point of view, ie the fire side, goes from 495 ° C. to 200 ° C. less than 7 minutes 30 seconds (curve SUP), and preferably less than 4 minutes 30 seconds (curve INF).
  • Such a cooling rate can be obtained by quenching the cylinder head in a fluidized bed consisting of a diphasic mixture of particles, for example a mixture of siliceous sand and air, which allows high quenching speeds.
  • the temperature of the fluidized bed is below 40 ° C, for example at a value of 30 ° C.
  • this quenching medium in particular its good thermal conductivity, the level of the residual stresses after the quenching operation is practically negligible.
  • a possible alternative to the fluidized bed quenching treatment described above is quenching in air.
  • the Applicant has developed a process which allows, under the conditions described by the invention, quenching speeds that are almost equivalent to those recorded for the fluidized bed, and such that the cooling curve of the cylinder head during quenching at air is also under the curve SUP, preferably under the curve INF represented on the figure 1 .
  • a high cooling rate can also be achieved by quenching in air, when performing air quenching at room temperature of pieces arranged vertically or horizontally in a single layer.
  • the parts of this single layer are separated from each other by a distance of the order of 30 mm (with inter-room separators not included in this distance) and traversed by a flow of cooling air.
  • the air flows to be considered for the cooling air flow are preferably greater than 1000 m 3 / h and per cylinder head, and preferably greater than or equal to 1700 m 3 / h and head.
  • the air is at room temperature.
  • the air speed is of the order of 23 m / s for a flow rate of 1000 m 3 / h and per cylinder head, and of the order of 45 m / s for a flow rate of 1700 m 3 / h and per cylinder head.
  • the determination of the peak of resistance can be done experimentally and conventionally, for example by first choosing the temperature of income to be considered, then by submitting parts previously dissolved and quenched at variable periods of income at this temperature of returned. These parts are then characterized for example by taking tensile test pieces in the functional critical areas of the cylinder head, for example the fire side.
  • the peak of resistance is defined by characterizing the various test pieces in tension, then by plotting the curve "mechanical strength in tension as a function of the duration of income", for the temperature of income considered.
  • the peak of resistance is defined as the maximum of this curve.
  • This peak strength can also be determined using hardness tests instead of tensile tests, but the technique using the tensile test pieces is more accurate and is therefore recommended by the Applicant.
  • a hard peak income is used, according to which the alloy is brought to the resistance peak determined according to the method previously described.
  • the state commonly referred to as "T6" for the aluminum alloys corresponding to the peak strength of the alloy for this given temperature is thus obtained.
  • an income is obtained near the peak, according to which the alloy is subjected to a suitable income to obtain a level of tensile strength of at least 85%, preferably at least 90%, more preferably at least equal to 95%, of the maximum resistance level at the considered tempering temperature.
  • the income according to the invention is therefore produced according to a temperature / duration pair adapted to reach, or at least approach, the highest possible T6 state after air quenching or quenching in a fluidized bed according to the temperature-time curves. described by the Applicant, and corresponding to the maximum strength of the alloy, according to the method just described.
  • the hard income according to the invention can be achieved by maintaining a temperature of 240 ° C for 1:30 for an alloy of type AA 319 in a treatment of income. fluidized bed, or else by maintaining a temperature of 180 ° C for 5 h for an AA 356 type alloy in a conventional oven tempering treatment.
  • in-line five-cylinder diesel cylinder heads were cast in static gravity in a metal mold, fire-facing downwards, with a steel plate cooled energetically so as to obtain a very fine microstructure that the 'SDD (' Secondary Dendrite Arm Spacing ') can be characterized with values of the order of 23 microns in the area where the tensile test pieces used to characterize the material are taken.
  • 'SDD ' Secondary Dendrite Arm Spacing '
  • the casting metal temperature is 710-715 ° C at the inlet in the casting cup of the mold, from which feed channels leave to fill the mold through attacks located at the foot of the piece.
  • the millet setting, ratio between the weight cast (piece plus feeding system, plus weights) and the weight of the piece is 1.66.
  • the molded piece weighs 18.6 kg.
  • the molding cycle time is of the order of 6 minutes from room to room.
  • the alloy is of AA 319 type, selected second melting, with a chemical composition given below in percentage by weight: Yes Cu Fe mn mg Ti Zn al 6.87 3.10 0.45 0.20 0.21 0.14 0.20 rest
  • the alloy has its eutectic structure modified by addition of strontium.
  • the bench test was performed under conditions that reproduce the thermal stresses of a severe hot-cold test on the engine test bench, the cylinder head being fixed by its fixing screws on a steel plate replacing the engine block, and provided with bores reproducing the bores of the engine cylinders.
  • Gas burners are housed in these bores.
  • the cylinder head is fitted with open exhaust valves and closed intake valves.
  • the hot cycle consists in heating the combustion side with the aid of gas burners, the circulation of water being traversed by the coolant, so that the temperature in the inter-valve bridges reaches the value of 250 ° C. .
  • the cold cycle consists in interrupting the heating of the combustion side, the circulation of water being still traversed by the coolant, so that the temperature in the inter-valve bridges reaches the value of 40 ° C.
  • the temperature of the inter-valve bridges is measured at 1 mm from the fire-facing surface in a cylinder head identical to that of state T5 (test no. 1) and provided with thermocouples, in order to carry out the adjustments of the burners and flow rates of coolant to achieve these temperatures.
  • the duration of the hot cycle is 40 seconds; that of the cold cycle of 25 seconds, which gives a total unit cycle time of 65 seconds.
  • the test is regularly interrupted to examine the bridges on the combustion side, to determine the crack initiation phase.
  • the fatigue failure is determined as soon as a through-bridge crack leads to a leakage of water from the water circulation to the combustion face is observed.
  • the cylinder heads have also been characterized at room temperature in traction and hardness.
  • the tensile properties are measured according to the AFNOR EN 10002-1 standard in the fire face by tensile specimens with a diameter of 6.18 mm and a calibrated length of 36.2 mm. Each measurement is the average of 5 test pieces per piece, for 3 pieces.
  • Brinell hardness is measured according to the AFNOR EN ISO 6506 - 1 and ASTM E10-06 standards in the fire side as well. One measurement is made per piece, for five pieces.
  • the piece was extracted from the mold and cooled in a forced air tunnel so that it was cooled to a temperature of 50 ° C in a time of about 60 minutes.
  • the cylinder head then underwent the usual operations of completion and then income: 4 hours at 210 ° C in a conventional income oven, then complete machining, before undergoing the bench test.
  • the result of the resistance test expressed as the number of cycles before the fatigue failure, is 3600 cycles (+/- 200 cycles, double repetition) for the cylinder head treated by the T5 heat treatment.
  • the test was carried out on a cylinder head having undergone the following process.
  • the result of the resistance test expressed as the number of cycles before the fatigue failure, is 4800 cycles (+/- 200 cycles, double repetition) for the cylinder head treated according to test No. 2.
  • the test was carried out on a cylinder head having undergone the following process.
  • the result of the strength test expressed as the number of cycles before the fatigue failure, is 9500 cycles (+/- 250 cycles, double repetition) for the cylinder head treated according to test No. 3.
  • in-line four-cylinder diesel cylinder heads were cast in static gravity in a metal mold, fire-facing down, with a steel plate cooled energetically so as to obtain a fine microstructure that the it is possible to characterize by measuring the SDAS ("Secondary Dendrite Arm Spacing"), with values of the order of 30 microns in the area where the tensile specimens used to characterize the material are taken.
  • SDAS Serial Dendrite Arm Spacing
  • the metal temperature at casting is 720 ° C at the inlet in the casting cup of the mold, from which feed channels leave to fill the mold through attacks located at the foot of the room.
  • ratio between the weight cast (piece plus feeding system, plus weights) and the weight of the piece is 1.7.
  • the molded piece weighs 14.1 kg.
  • the molding cycle time is of the order of 5 minutes from room to room.
  • the alloy is of AA 356 type, of first fusion, with a chemical composition given below in weight percentages: Yes Fe mn mg Ti Zn al 7.4 0.12 0.02 0.30 0.11 0.02 rest
  • the alloy has its eutectic structure modified by addition of strontium.
  • the part was extracted from the mold and cooled in a forced air tunnel so that it was cooled to a temperature of 50 ° C. in a time of about 120 minutes.
  • Traction test pieces were taken from the yokes of tests 4, 5 and 6, similarly to what was carried out in the context of experiment 1, and the mechanical traction characteristics were measured on these test pieces. .
  • Example 2 In a manner similar to Example 1, the hardness was also measured on the fire side of the cylinder heads.
  • the yokes were cut progressively by milling from the fire side.
  • the yokes are, before cutting, then at each stage of the cutting, at 13 and 30 mm depth with respect to the fire face, measured dimensionally so as to quantify the maximum deflection of the yoke with respect to three reference ranges Pr of the fire side, and the variation of the average length of the bolt on the sides A and B.
  • the treatment according to the invention comprising air quenching under the operating conditions of the process according to the invention is the only one able to offer a significant reduction of residual stresses, while maintaining a good level of mechanical characteristics.
  • This effect on the residual stresses is probably a discriminating element in the unexpected improvement of the behaviors on the bank of thermally treated cylinder heads by the method according to the invention.

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Description

Le domaine de l'invention est celui des traitements thermiques des pièces de fonderie réalisées en alliage à base d'aluminiumThe field of the invention is that of the heat treatments of castings made of aluminum-based alloys.

L'invention concerne plus précisément un procédé de traitement thermique de culasses moulées en alliages d'aluminium, et les culasses résultant d'un tel procédé.The invention more specifically relates to a heat treatment process of cast aluminum alloy cylinder heads, and cylinder heads resulting from such a process.

Les culasses moulées en alliages d'aluminium sont très majoritairement utilisées dans les moteurs automobiles performants.The cast aluminum alloy cylinder heads are mostly used in high-performance automotive engines.

L'augmentation des puissances spécifiques de ces moteurs, et la recherche de meilleures performances en émissions polluantes conduisant à accroitre la sollicitation du matériau de la culasse, l'homme de l'art utilise classiquement des alliages de moulage à durcissement structural.The increase of the specific powers of these engines, and the search for better performance in pollutant emissions leading to increase the loading of the material of the cylinder head, the person skilled in the art conventionally uses structural hardening molding alloys.

Les alliages d'aluminium de moulage comportent différentes familles de composition dont la plupart sont aptes au durcissement structural par traitement thermique. En particulier, on peut citer la famille Aluminium/Silicium/Magnésium et la famille Aluminium/Silicium/Cuivre/Magnésium.Aluminum casting alloys have different families of compositions, most of which are suitable for structural hardening by heat treatment. In particular, mention may be made of the aluminum / silicon / magnesium family and the aluminum / silicon / copper / magnesium family.

Notamment, pour la fabrication en grande série de composants automobiles, par exemple des culasses soumises à de très fortes sollicitations en service, l'homme de l'art utilise notamment :

  • les alliages de type A319 selon l'appellation de l'Aluminium Association, comprenant de 5,0% à 9,0% de silicium, de 2,0% à 3,5% de cuivre, de 0,2 à 0,6% de magnésium,
  • les alliages de type A356 ou A357 selon l'appellation de l'Aluminium Association, comprenant de 6,5% à 7,5% de silicium, de 0,2 à 0,7% de magnésium,
  • voire des alliages intermédiaires ou proches de ces deux familles.
In particular, for the mass production of automotive components, for example cylinder heads subjected to very high loads in service, those skilled in the art use in particular:
  • alloys of type A319 according to the name of the Aluminum Association, comprising from 5.0% to 9.0% of silicon, from 2.0% to 3.5% of copper, from 0.2 to 0.6% % magnesium
  • alloys of the A356 or A357 type according to the name of the Aluminum Association, comprising from 6.5% to 7.5% of silicon, from 0.2% to 0.7% of magnesium,
  • even intermediate alloys or close to these two families.

En particulier, des alliages de type A356 comprenant en outre 0,5% de cuivre sont fréquemment utilisés pour les culasses de moteur diesel fortement sollicitées.In particular, alloys of type A356 further comprising 0.5% copper are frequently used for heavily loaded diesel engine cylinder heads.

Afin de maximiser les propriétés mécaniques de ces alliages, il est d'usage, du moins pour les cas de sollicitations les plus sévères, de réaliser un traitement thermique complet associant dans l'ordre :

  • une mise en solution à haute température, typiquement entre 490°C et 550°C ;
  • une trempe à l'eau entre la température de mise en solution et la température de l'eau de trempe (typiquement entre 20°C et 95°C ; on peut également utiliser de l'eau bouillante) ;
  • un refroidissement dans l'ambiance de l'atelier avant un traitement de revenu de durcissement à des températures de l'ordre de 120°C à 250°C.
In order to maximize the mechanical properties of these alloys, it is customary, at least for the most severe stress cases, to carry out a complete heat treatment combining in the following order:
  • solution at high temperature, typically between 490 ° C and 550 ° C;
  • water quenching between the solution temperature and the quenching water temperature (typically between 20 ° C and 95 ° C, boiling water may also be used);
  • cooling in the shop environment before hardening treatment at temperatures in the range of 120 ° C to 250 ° C.

Les traitements ci-dessus sont typiquement réalisés sur durées de l'ordre de 5 à 6 heures de séjour à température dans le four, la montée en température de la charge étant usuellement de l'ordre de 1 à 2 heures pour des culasses.The above treatments are typically carried out for periods of the order of 5 to 6 hours of residence at temperature in the oven, the rise in temperature of the charge being usually of the order of 1 to 2 hours for yokes.

Cependant, ces pratiques pour des pièces de géométrie aussi complexe que celle des culasses conduisent lors de la trempe à générer des contraintes résiduelles importantes qui peuvent atteindre localement le niveau de limite élastique à froid du matériau.However, these practices for parts of geometry as complex as that of the cylinder heads lead during quenching to generate significant residual stresses that can locally reach the cold limit level of the material.

L'augmentation de la température de l'eau de trempe est un moyen bien connu de l'homme de l'art pour réduire ces contraintes résiduelles.Increasing the temperature of the quenching water is a means well known to those skilled in the art for reducing these residual stresses.

Toutefois, l'usage de l'eau bouillante ne permet pas de supprimer complètement ces contraintes résiduelles. En outre, la trempe à l'eau bouillante conduit à des abattements significatifs des caractéristiques mécaniques.However, the use of boiling water does not completely eliminate these residual stresses. In addition, quenching with boiling water leads to significant reductions in mechanical characteristics.

L'homme de l'art remédie partiellement à la persistance des contraintes résiduelles après la trempe en pratiquant des revenus de durcissement à des températures élevées, typiquement au delà de 200 °C, conduisant ce faisant l'alliage à un état de sur-revenu (passage au delà du pic de résistance maximale). Ces types de traitements sont communément appelés T7.Those skilled in the art partially remedy the persistence of residual stresses after quenching by practicing hardening incomes at elevated temperatures, typically above 200 ° C, thereby conducting the alloy to a state of over-income (passage beyond the peak of maximum resistance). These types of treatments are commonly called T7.

Il a également été proposé de réduire le temps nécessaire pour réaliser l'ensemble du traitement thermique (mise en solution, trempe et revenu). Le document GB 2 361 710 propose ainsi de limiter la durée de la mise en solution à un maximum de trois heures, et de préférence à un maximum de deux heures. Le traitement préconisé par ce document résulte, pour un temps de traitement inférieur, en des caractéristiques mécaniques similaires à celle du traitement T7.It has also been proposed to reduce the time required to achieve the entire heat treatment (solution, quenching and tempering). The document GB 2,361,710 proposes to limit the dissolution time to a maximum of three hours, and preferably to a maximum of two hours. The treatment recommended by this document results, for a lower treatment time, in mechanical characteristics similar to that of the T7 treatment.

On relèvera que ce document GB 2 361 710 précise que les propriétés de fatigue ne sont que peu influencées par le type de traitement thermique et indique que de ce fait, les propriétés de fatigue des pièces soumises au traitement T7 ou au traitement selon ce document GB 2 361 710 avec une mise en solution plus courte ne doivent pas être significativement différentes.It will be noted that this document GB 2,361,710 states that the fatigue properties are only slightly influenced by the type of heat treatment and indicates that, as a result, the fatigue properties of the parts subjected to the T7 treatment or to the treatment according to this document GB 2,361,710 with a shorter dissolution in solution should not be significantly different.

Le document US 6 752 885 prévoit également un traitement thermique plus court que le traitement T7, ce traitement comprenant une mise en solution d'une durée comprise entre une heure trente minutes et deux heures, et un revenu à une température de 250°C.The document US 6,752,885 also provides a heat treatment shorter than the T7 treatment, this treatment comprising a dissolution of a duration of between one hour thirty minutes and two hours, and an income at a temperature of 250 ° C.

Le document US 2004/265163 décrit un traitement thermique comprenant une étape de mise en solution, une trempe et un revenu, en préconisant le choix d'un sur-revenu pour des parties de la pièce pour lesquelles une bonne stabilité dimensionnelle est exigée.The document US 2004/265163 discloses a heat treatment comprising a solution solution, tempering and tempering step, by recommending the choice of over-tempering for parts of the part for which good dimensional stability is required.

Le document US 2007/051336 décrit un traitement thermique comprenant une étape de mise en solution, une étape de trempe à l'eau et une étape de sur-revenu de la pièce.The document US 2007/051336 discloses a heat treatment comprising a solution solution step, a water quenching step and an overflow step of the workpiece.

La demanderesse a également mis au point et utilise commercialement depuis de nombreuses années un traitement de type T5 consistant à enchaîner :

  • une solidification rapide à l'intérieur du moule, généralement métallique, au moins pour la partie moulant la face feu de la culasse,
  • un refroidissement rapide à l'air forcé après éjection de la pièce hors du moule, la pièce étant encore munie de ses noyaux intérieurs en sable et de ses masselottes, jusqu'à la température ambiante,
  • le tout suivi d'un revenu de durcissement.
The Applicant has also developed and has been using commercially for many years a T5-type treatment consisting in linking together:
  • fast solidification inside the mold, generally metal, at least for the molding part the fire side of the cylinder head,
  • rapid cooling with forced air after ejection of the piece out of the mold, the piece still having its inner sand cores and weights, to ambient temperature,
  • all followed by a hardening income.

Ce traitement présente l'avantage de supprimer les contraintes résiduelles, mais ne permet pas de tirer tout le bénéfice du potentiel de durcissement de l'alliage.This treatment has the advantage of eliminating the residual stresses, but does not make it possible to take full advantage of the hardening potential of the alloy.

La présente invention vise à pallier à ces inconvénients et à permettre de réduire les contraintes résiduelles de l'alliage, tout en exploitant mieux le potentiel durcissant de l'alliage en vue de maximiser les performances fonctionnelles de la culasse, notamment la tenue en fatigue chaud-froid qui est le test le plus sévère pour le développement de culasses sollicitées.The present invention aims to overcome these disadvantages and to reduce the residual stresses of the alloy, while making better use of the hardening potential of the alloy in order to maximize the functional performance of the cylinder head, especially the resistance to hot fatigue - cold which is the most severe test for the development of driven cylinder heads.

Elle propose à cet effet, et selon un premier aspect, un procédé de traitement thermique d'une pièce de fonderie de type culasse réalisée en alliage d'aluminium, de silicium et de magnésium, caractérisé en ce qu'il comprend les étapes suivantes :

  • mise en solution de la pièce pendant une durée comprise entre trois et dix heures ;
  • trempe de la pièce à l'air ou dans un lit fluidisé ;
  • revenu de la pièce au pic de résistance, ou à proximité du pic de résistance pour atteindre un niveau de résistance de la pièce au moins égal à 85% du niveau de résistance maximal à la température du revenu considérée.
It proposes for this purpose, and according to a first aspect, a heat treatment process of a breech type casting piece made of aluminum alloy, silicon and magnesium, characterized in that it comprises the following steps:
  • dissolution of the piece for a period of between three and ten hours;
  • quenching of the workpiece in the air or in a fluidized bed;
  • the workpiece returns to the peak of resistance, or near the peak of resistance to reach a resistance level of the piece at least equal to 85% of the maximum level of resistance to the temperature of the considered income.

Certains aspects préférés, mais non limitatifs, de ce procédé sont les suivants :

  • la mise en solution est réalisée pendant une durée comprise entre cinq heures et dix heures ;
  • la mise en solution est réalisée à une température comprise entre 490°C et 550°C ;
  • la mise en solution est effectuée dans un four conventionnel ;
  • la mise en solution est effectuée dans un lit fluidisé ;
  • la mise en solution comprend :
    • o une mise en solution en lit fluidisé pendant une durée inférieure ou égale à 30 minutes, appliquée en sortie de moulage de manière à dessabler la pièce,
    • o une mise en solution complémentaire en four conventionnel.
  • la mise en solution est appliquée à la pièce après un dessablage complet adapté pour débarrasser la pièce de ses noyaux internes, et après élimination des masselottes et des systèmes de coulée ;
  • la mise en solution est appliquée à la pièce en sortie de moulage ;
  • le refroidissement de la pièce au cours de la trempe est réalisé selon une courbe de refroidissement située sous la courbe de refroidissement SUP de la figure 1, et de préférence sous la courbe de refroidissement INF de la figure 1 ;
  • la trempe est effectuée dans un lit fluidisé à une température à moins de 40°C ;
  • la trempe est une trempe à l'air à température ambiante de pièces disposées en une couche unique traversée par le flux d'air ;
  • les débits d'air sont supérieurs à 1000 m3/h/pièce, et de préférence supérieurs ou égaux à 1700 m3/h/pièce ;
  • le revenu est réalisé au pic de résistance maximale pour la température de revenu considérée ;
  • le revenu est réalisé en four conventionnel ;
  • le revenu est réalisé en lit fluidisé ;
  • l'alliage est du type AA 319 et le revenu est réalisé à 230°C pendant 1h30 heures en lit fluidisé, ou à 210°C pendant 4 heures en four conventionnel ;
  • l'alliage est du type AA 356 et le revenu est réalisé à 180°C pendant 5 heures en four conventionnel ;
  • la pièce est une culasse.
Some preferred, but not limiting, aspects of this method are as follows:
  • the dissolution is carried out for a period of between five hours and ten hours;
  • the dissolution is carried out at a temperature between 490 ° C and 550 ° C;
  • the dissolution is carried out in a conventional oven;
  • the dissolution is carried out in a fluidized bed;
  • the dissolution in solution comprises:
    • a dissolution in a fluidized bed for a duration of less than or equal to 30 minutes, applied at the molding outlet so as to strip the part,
    • o Complementary solution in a conventional oven.
  • the dissolving is applied to the workpiece after a complete desiccation adapted to rid the part of its internal cores, and after removal of the weights and casting systems;
  • the dissolution is applied to the molding die;
  • the cooling of the part during the quenching is carried out according to a cooling curve located under the cooling curve SUP of the figure 1 , and preferably under the cooling curve INF of the figure 1 ;
  • quenching is carried out in a fluidized bed at a temperature below 40 ° C;
  • quenching is an air-quenching at room temperature of pieces arranged in a single layer through which the airflow passes;
  • the air flows are greater than 1000 m 3 / h / piece, and preferably greater than or equal to 1700 m 3 / h / piece;
  • the income is made at the peak of maximum resistance for the income temperature considered;
  • the income is made in conventional oven;
  • the income is made in a fluidized bed;
  • the alloy is of the AA 319 type and the product is produced at 230 ° C. for 1h30 hours in a fluidized bed, or at 210 ° C. for 4 hours in a conventional oven;
  • the alloy is of the AA 356 type and the product is produced at 180 ° C. for 5 hours in a conventional oven;
  • the room is a breech.

Selon un deuxième aspect, l'invention propose une culasse réalisée en alliage à base d'aluminium, de silicium, de magnésium, caractérisée en ce qu'elle présente une résistance à la fatigue évaluée par le test de résistance à la fatigue « chaud-froid », et exprimée en nombre de cycles avant la rupture de fatigue à +/-200 cycles, supérieure à 4800 cycles, de préférence voisine de 9500 cycles.According to a second aspect, the invention proposes a cylinder head made of an alloy based on aluminum, silicon or magnesium, characterized in that it exhibits a fatigue resistance evaluated by the "hot-air" fatigue test. cold ", and expressed in number of cycles before fatigue failure at +/- 200 cycles, greater than 4800 cycles, preferably close to 9500 cycles.

D'autres aspects, buts et avantages de la présente invention apparaîtront mieux à la lecture de la description détaillée suivante de formes de réalisation préférées de celle-ci, donnée à titre d'exemple non limitatif, et faite en référence aux dessins annexés sur lesquels :

  • la figure 1 représente des courbes de refroidissement sous lesquelles la trempe selon l'invention doit être réalisée, ainsi que des courbes de refroidissement appliquées à des culasses lors de la trempe selon des exemples de réalisation de l'invention ;
  • la figure 2 est relative à la caractérisation d'une culasse en traction et en dureté ;
  • la figure 3 représente le principe de mesure des déformations dues à la relaxation d'une culasse ;
  • la figure 4 est un diagramme température-temps illustrant de manière schématique le traitement thermique du procédé selon le premier aspect de l'invention.
Other aspects, objects and advantages of the present invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and with reference to the appended drawings in which: :
  • the figure 1 represents cooling curves under which the quenching according to the invention is to be carried out, as well as cooling curves applied to yokes during quenching according to embodiments of the invention;
  • the figure 2 relates to the characterization of a yoke in tension and in hardness;
  • the figure 3 represents the principle of measuring the deformations due to the relaxation of a cylinder head;
  • the figure 4 is a temperature-time diagram schematically illustrating the heat treatment of the process according to the first aspect of the invention.

L'invention s'applique aux alliages d'aluminium de moulage à durcissement structural quelque soit leur composition chimique, et aux pièces réalisées à partir de ces matériaux.The invention is applicable to structural hardening aluminum alloys regardless of their chemical composition, and to parts made from these materials.

L'invention concerne en particulier les pièces de fonderie réalisées en alliage à base d'aluminium, de silicium, et de magnésium.The invention relates in particular to castings made of alloy based on aluminum, silicon, and magnesium.

L'invention trouve plus particulièrement application pour traiter thermiquement des composants automobiles moulés dans tels alliages à base d'aluminium, et soumis à de très fortes sollicitations en service.The invention is more particularly applicable to heat treating automotive components molded in such aluminum-based alloys, and subjected to very high loads in service.

L'invention s'avère en particulier particulièrement avantageuse pour les culasses de moteurs automobiles qui comme on l'a vu précédemment sont des pièces de fonderie fortement sollicitées. On prendra l'exemple de telles culasses dans la suite de la description.In particular, the invention proves particularly advantageous for automobile engine cylinder heads which, as previously mentioned, are highly stressed casting parts. We will take the example of such cylinder heads in the following description.

Le procédé selon l'invention comprend les étapes consistant à soumettre la culasse :

  • à un traitement de mise en solution aux températures habituelles de mise en solution, mais pendant une durée plus importante,
  • à une trempe à l'air ou en lit fluidisé, ne générant pas de contraintes résiduelles,
  • à un revenu « dur » au pic de dureté de l'alliage.
The method according to the invention comprises the steps of subjecting the breech:
  • to a solution treatment at usual dissolution temperatures, but for a longer period,
  • in an air quench or in a fluidized bed, which does not generate residual stresses,
  • at a "hard" income at the peak of hardness of the alloy.

Mise en solutionDissolution

La mise en solution peut être effectuée dans un four à lit fluidisé, ou dans un four conventionnel, voire une succession des deux.The dissolution can be carried out in a fluidized bed furnace, or in a conventional furnace, or even a succession of both.

La mise en solution est réalisée est à la température usuelle de mise en solution de l'alliage.The dissolution is performed is at the usual temperature of dissolution of the alloy.

La mise en solution est réalisée pendant une durée comprise entre 3 heures et 10 heures. Selon une variante avantageuse de l'invention, la mise en solution est réalisée pendant une durée comprise entre 5 heures à 10 heures, y compris pour les traitements en lit fluidisé.The dissolution is carried out for a period of between 3 hours and 10 hours. According to an advantageous variant of the invention, the dissolution is carried out for a period of between 5 hours to 10 hours, including for fluidized bed treatments.

On comprendra que lorsque la mise en solution comprend en succession une mise en solution en lit fluidisé (par exemple une trempe en lit fluidisé courte, d'une durée inférieure ou égale à 30 minutes), et une mise en solution complémentaire en four conventionnel, la durée de la mise en solution complémentaire est alors adaptée pour atteindre une durée totale comprise en 3 heures et 10 heures conforme à l'invention.It will be understood that when the dissolution comprises successively in a fluidized bed dissolution (for example a quench in a short fluidized bed, of a duration less than or equal to 30 minutes), and a complementary dissolution in conventional oven, the duration of the dissolution of complementary solution is then adapted to achieve a total duration of 3 hours and 10 hours according to the invention.

On relèvera que l'invention va à l'encontre des tendances du métier, illustrées notamment par les documents GB 2 361 710 et US 6 752 885 , qui préconisent une réduction de la durée du traitement thermique complet, via notamment une réduction de la durée de la mise en solution.It should be noted that the invention goes against the tendencies of the profession, illustrated in particular by the documents GB 2,361,710 and US 6,752,885 , which advocate a reduction in the duration of the complete heat treatment, in particular by reducing the duration of dissolution.

La Demanderesse a pu constater que contrairement à ce qu'indiquent les propriétés mécaniques statiques (dureté, traction sur éprouvettes, par exemple), il s'avère intéressant d'accroitre la durée de mise en solution pour ce qui concerne les propriétés fonctionnelles des culasses (et cela même lorsque cette mise en solution est effectuée en lit fluidisé).The Applicant has found that, contrary to what is indicated by the static mechanical properties (hardness, traction on test specimens, for example), it proves interesting to increase the dissolution time with regard to the functional properties of the cylinder heads. (And this even when this dissolution is carried out in a fluidized bed).

Selon une variante de réalisation de l'invention, la mise en solution est appliquée à la culasse après un dessablage complet qui la débarrasse de tous ses noyaux internes, notamment par des moyens mécaniques (martelage et vibration typiquement), et après élimination des masselottes et des systèmes de coulée, ce qui permet de maximiser la vitesse de refroidissement à la trempe.According to an alternative embodiment of the invention, the dissolving is applied to the cylinder head after a complete grit removal which rids it of all its internal cores, in particular by mechanical means (hammering and vibration typically), and after elimination of the flyweights and casting systems, which maximizes quench cooling rate.

En variante, la mise en solution peut être réalisée sur les pièces en sortie de moulage, la réalisation de la mise en solution dans un lit fluidisé notamment permettant un dessablage efficace de la culasse. Cependant, la culasse comporte encore ses masselottes, voire son système de coulée lors de la trempe, ce qui n'est pas optimal comme on vient de le noter.Alternatively, the dissolution can be performed on the molded output parts, the realization of the dissolution in a fluidized bed in particular for an effective degritting of the cylinder head. However, the cylinder head still has its weights, or even its casting system during quenching, which is not optimal as just noted.

Trempetemper

Après mise en solution, la culasse est soumise à une trempe qui n'est pas effectuée dans l'eau, mais à l'air ou en lit fluidisé.After dissolution, the cylinder head is quenched not in water, but in air or in a fluidized bed.

On a représenté sur la figure 1 des courbes température-temps représentant l'évolution au cours de la trempe de la température de la culasse dans le domaine le plus critique de température, c'est-à-dire entre 500°C et la température ambiante.We have shown on the figure 1 temperature-time curves representing the evolution during the quenching of the temperature of the cylinder head in the most critical temperature range, that is to say between 500 ° C and the ambient temperature.

Dans le cadre de l'invention, la vitesse de refroidissement de la trempe est adaptée pour que le refroidissement, tel que mesuré par un (des) thermocouple(s) positionné(s) dans la culasse, se situe sous la courbe SUP représentée sur la figure 1, et de préférence sous la courbe INF, représentée sur la figure 1.In the context of the invention, the cooling rate of the quenching is adapted so that the cooling, as measured by one or more thermocouple (s) positioned in the cylinder head, lies under the curve SUP represented on the figure 1 , and preferably under the curve INF, represented on the figure 1 .

Une telle vitesse de refroidissement conforme à l'invention implique en particulier que la température de la culasse dans ses zones les plus critiques du point de vue fonctionnel, c'est à dire la face feu, passe de 495°C à 200°C en moins de 7 mn 30 s (courbe SUP), et de préférence en moins de 4mn 30 s (courbe INF).Such a cooling rate according to the invention implies in particular that the temperature of the cylinder head in its most critical areas from the functional point of view, ie the fire side, goes from 495 ° C. to 200 ° C. less than 7 minutes 30 seconds (curve SUP), and preferably less than 4 minutes 30 seconds (curve INF).

Une telle vitesse de refroidissement peut être obtenue en trempant la culasse dans un lit fluidisé constitué par un mélange diphasique de particules, par exemple un mélange de sable siliceux et d'air, qui autorise des vitesses de trempe élevées.Such a cooling rate can be obtained by quenching the cylinder head in a fluidized bed consisting of a diphasic mixture of particles, for example a mixture of siliceous sand and air, which allows high quenching speeds.

La température du lit fluidisé est inférieure à 40°C, par exemple à une valeur de 30°C.The temperature of the fluidized bed is below 40 ° C, for example at a value of 30 ° C.

Grâce aux propriétés physiques uniques de ce milieu de trempe, notamment sa bonne conductivité thermique, le niveau des contraintes résiduelles après l'opération de trempe est pratiquement négligeable.Thanks to the unique physical properties of this quenching medium, in particular its good thermal conductivity, the level of the residual stresses after the quenching operation is practically negligible.

De tels traitements de trempe en lit fluidisé sont actuellement disponibles sur le marché, tout comme ils le sont pour la mise en solution.Such fluidized bed quenching treatments are currently available on the market, just as they are for dissolution.

Une alternative possible au traitement de trempe en lit fluidisé décrit ci-dessus consiste à faire une trempe à l'air.A possible alternative to the fluidized bed quenching treatment described above is quenching in air.

L'homme de l'art utilise rarement la trempe à l'air pour le traitement thermique de culasses automobiles bien qu'il se traduise par une quasi absence de contraintes résiduelles. En effet, l'application de cette technique résulte souvent dans un abattement excessif des caractéristiques du matériau.Those skilled in the art rarely use air quenching for the heat treatment of automobile heads although it results in a virtual absence of residual stresses. Indeed, the application of this technique often results in an excessive reduction of the characteristics of the material.

La demanderesse a toutefois développé un procédé qui autorise, dans les conditions décrites par l'invention, des vitesses de trempe quasi équivalentes à celles enregistrées pour le lit fluidisé, et telle que la courbe de refroidissement de la culasse lors de la trempe à l'air se situe également sous la courbe SUP, de préférence sous la courbe INF représentées sur la figure 1.The Applicant, however, has developed a process which allows, under the conditions described by the invention, quenching speeds that are almost equivalent to those recorded for the fluidized bed, and such that the cooling curve of the cylinder head during quenching at air is also under the curve SUP, preferably under the curve INF represented on the figure 1 .

Ainsi, selon des travaux originaux menés par la Demanderesse, et de façon tout à fait surprenante, une vitesse de refroidissement élevée peut également être atteinte par une trempe à l'air, lorsque l'on réalise une trempe à l'air à température ambiante de pièces disposées verticalement ou horizontalement en une couche unique. Les pièces de cette couche unique sont séparées l'une de l'autre d'une distance de l'ordre de 30 mm (avec des séparateurs inter-pièces non compris dans cette distance) et traversées par un flux d'air de refroidissement. Les débits d'air à considérer pour le flux d'air de refroidissement sont de préférence supérieurs à 1000 m3/h et par culasse, et de préférence supérieurs ou égaux à 1700 m3/h et par culasse. L'air est à température ambiante. A titre d'exemples, la vitesse de l'air est de l'ordre de 23 m/s pour un débit de 1000 m3/h et par culasse, et de l'ordre de 45 m/s pour un débit de 1700 m3/h et par culasse.Thus, according to original work conducted by the Applicant, and quite surprisingly, a high cooling rate can also be achieved by quenching in air, when performing air quenching at room temperature of pieces arranged vertically or horizontally in a single layer. The parts of this single layer are separated from each other by a distance of the order of 30 mm (with inter-room separators not included in this distance) and traversed by a flow of cooling air. The air flows to be considered for the cooling air flow are preferably greater than 1000 m 3 / h and per cylinder head, and preferably greater than or equal to 1700 m 3 / h and head. The air is at room temperature. By way of example, the air speed is of the order of 23 m / s for a flow rate of 1000 m 3 / h and per cylinder head, and of the order of 45 m / s for a flow rate of 1700 m 3 / h and per cylinder head.

RevenuReturned

Dans les deux cas (trempe en lit fluidisé, trempe à l'air), les contraintes résiduelles sont minimisées par rapport à une trempe à l'eau (et cela quelque soit la température de l'eau de trempe). Cette minimisation des contraintes résiduelles permet de se dispenser du sur-revenu à haute température (de type T7) que l'homme de l'art applique en général aux culasses après trempe à l'eau pour relâcher les contraintes résiduelles, mais qui se traduit par une réduction des caractéristiques mécaniques par rapport au maximum de dureté.In both cases (quenching in a fluidized bed, quenching in air), the residual stresses are minimized compared with quenching with water (and whatever the temperature of the quenching water). This minimization of the residual stresses makes it possible to dispense with the over-tempering at high temperature (of the T7 type) that the person skilled in the art generally applies to the cylinder heads after quenching with water to release the residual stresses, but which results in by a reduction of the mechanical characteristics with respect to the maximum of hardness.

Ce faisant, il est possible dans le cadre de l'invention d'appliquer à la culasse un revenu « dur », proche du pic de résistance, ce qui permet de maximiser les caractéristiques mécaniques du matériau, notamment la limite élastique et la charge de rupture.In doing so, it is possible in the context of the invention to apply to the yoke a "hard" income, close to the peak of resistance, which makes it possible to maximize the mechanical characteristics of the material, in particular the elastic limit and the load of break.

La détermination du pic de résistance peut se faire de façon expérimentale et conventionnelle, par exemple en choisissant d'abord la température de revenu à considérer, puis en soumettant des pièces préalablement mises en solution et trempées à des durées de revenu variables à cette température de revenu. Ces pièces sont ensuite caractérisées par exemple en prélevant des éprouvettes de traction dans les zones critiques fonctionnelles de la culasse, par exemple la face feu. Le pic de résistance est défini en caractérisant les différentes éprouvettes en traction, puis en traçant la courbe « résistance mécanique en traction en fonction de la durée de revenu », pour la température de revenu considérée. Le pic de résistance est défini comme le maximum de cette courbe.The determination of the peak of resistance can be done experimentally and conventionally, for example by first choosing the temperature of income to be considered, then by submitting parts previously dissolved and quenched at variable periods of income at this temperature of returned. These parts are then characterized for example by taking tensile test pieces in the functional critical areas of the cylinder head, for example the fire side. The peak of resistance is defined by characterizing the various test pieces in tension, then by plotting the curve "mechanical strength in tension as a function of the duration of income", for the temperature of income considered. The peak of resistance is defined as the maximum of this curve.

La durée de revenu conférant à l'alliage ses caractéristiques mécaniques maximales, associée à la température de revenu considérée, constituent ainsi les conditions de revenu au pic de résistance.The duration of income conferring on the alloy its maximum mechanical characteristics, associated with the temperature of income considered, thus constitute the conditions of income at the peak of resistance.

Ce pic de résistance peut aussi être déterminé en utilisant des tests de dureté en lieu et place des essais de traction, mais la technique utilisant les éprouvettes de traction est plus précise et est de ce fait recommandée par la demanderesse.This peak strength can also be determined using hardness tests instead of tensile tests, but the technique using the tensile test pieces is more accurate and is therefore recommended by the Applicant.

Selon un mode de réalisation préférentiel de l'invention on met en oeuvre un revenu dur au pic, selon lequel l'alliage est porté au pic de résistance déterminé selon la méthode préalablement décrite. On obtient ainsi pour cette température de revenu donnée, l'état communément désigné par « T6 » pour les alliages d'aluminium correspondant au pic de résistance de l'alliage pour cette température donnée.According to a preferred embodiment of the invention, a hard peak income is used, according to which the alloy is brought to the resistance peak determined according to the method previously described. For this given tempering temperature, the state commonly referred to as "T6" for the aluminum alloys corresponding to the peak strength of the alloy for this given temperature is thus obtained.

En variante, on réalise un revenu à proximité du pic, selon lequel l'alliage est soumis à un revenu adapté pour obtenir un niveau de résistance en traction au moins égal à 85%, de préférence au moins égal à 90%, encore de préférence au moins égal à 95%, du niveau de résistance maximal à la température de revenu considérée.As a variant, an income is obtained near the peak, according to which the alloy is subjected to a suitable income to obtain a level of tensile strength of at least 85%, preferably at least 90%, more preferably at least equal to 95%, of the maximum resistance level at the considered tempering temperature.

Le revenu selon l'invention est donc réalisé selon un couple température/durée adapté pour atteindre, ou tout du moins approcher, l'état T6 le plus élevé possible après trempe à l'air ou trempe en lit fluidisé selon les courbes température-temps décrites par la Demanderesse, et correspondant à la résistance maximale de l'alliage, selon la méthode qui vient d'être détaillée.The income according to the invention is therefore produced according to a temperature / duration pair adapted to reach, or at least approach, the highest possible T6 state after air quenching or quenching in a fluidized bed according to the temperature-time curves. described by the Applicant, and corresponding to the maximum strength of the alloy, according to the method just described.

A titre d'exemples, et comme cela sera détaillé par la suite, le revenu dur selon l'invention peut être réalisé en maintenant une température une température de 240°C pendant 1h30 pour un alliage de type AA 319 dans un traitement de revenu en lit fluidisé, ou bien encore en maintenant une température de 180 °C pendant 5 h pour un alliage de type AA 356 dans un traitement de revenu en four conventionnel.As examples, and as will be detailed later, the hard income according to the invention can be achieved by maintaining a temperature of 240 ° C for 1:30 for an alloy of type AA 319 in a treatment of income. fluidized bed, or else by maintaining a temperature of 180 ° C for 5 h for an AA 356 type alloy in a conventional oven tempering treatment.

La mise en oeuvre du procédé selon l'invention résulte en des performances de durée de vie de la culasse tout à fait exceptionnelles, telles que mesurées sur une culasse complète dans un essai de fatigue dit « chaud-froid » du type de celui décrit dans la demande de brevet EP 1 090 278 de la Demanderesse. On notera que ce test présente l'avantage d'être parfaitement corrélé avec les essais moteurs, tout en étant accéléré, pour une durée totale d'essai de l'ordre de 1 à 2 semaines.The implementation of the method according to the invention results in quite exceptional cylinder head life performance, as measured on a complete cylinder head in a so-called "hot-cold" fatigue test of the type described in US Pat. the patent application EP 1 090 278 of the Applicant. It should be noted that this test has the advantage of being perfectly correlated with engine tests, while being accelerated, for a total test duration of the order of 1 to 2 weeks.

ExpériencesExperiences Expérience n°1Experience # 1

Dans tous les exemples ci-dessous, des culasses diesel cinq cylindres en ligne ont été moulées en gravité statique en moule métallique, face feu vers le bas, avec une semelle en acier refroidie de façon énergique de façon à obtenir une microstructure très fine que l'on peut caractériser par la mesure du SDAS (« Secondary Dendrite Arm Spacing »), avec des valeurs de l'ordre de 23 microns dans la zone où sont prélevées les éprouvettes de traction servant à caractériser le matériau.In all the examples below, in-line five-cylinder diesel cylinder heads were cast in static gravity in a metal mold, fire-facing downwards, with a steel plate cooled energetically so as to obtain a very fine microstructure that the 'SDD (' Secondary Dendrite Arm Spacing ') can be characterized with values of the order of 23 microns in the area where the tensile test pieces used to characterize the material are taken.

La température métal à la coulée est de 710 à 715°C à l'arrivée dans le godet de coulée du moule, d'où partent les chenaux d'alimentation pour remplir le moule au travers des attaques situées au pied de la pièce.The casting metal temperature is 710-715 ° C at the inlet in the casting cup of the mold, from which feed channels leave to fill the mold through attacks located at the foot of the piece.

La mise au mil, ratio entre le poids coulé (pièce plus système d'alimentation, plus masselottes) et le poids de la pièce est de 1,66. La pièce moulée pèse 18,6 kg.The millet setting, ratio between the weight cast (piece plus feeding system, plus weights) and the weight of the piece is 1.66. The molded piece weighs 18.6 kg.

Tout le noyautage est réalisé en procédé de type « boite froide », pour la réalisation des formes intérieures : conduits d'admission, d'échappement, de circulation d'eau, d'huile et pour la réalisation du noyau contenant les masselottes, réserve de métal située au dessus de la pièce elle-même et permettant l'alimentation en métal liquide pendant la solidification et la contraction de la pièce.All coring is carried out in cold box type process, for the realization of the inner forms: intake ducts, exhaust ducts, circulation of water, oil and for the realization of the core containing the weights, reserves of metal located above the piece itself and allowing the supply of liquid metal during the solidification and contraction of the piece.

Le temps de cycle de moulage est de l'ordre de 6 minutes de pièce à pièce.The molding cycle time is of the order of 6 minutes from room to room.

L'alliage est de type AA 319, de deuxième fusion sélectionné, avec une composition chimique donnée ci-après en pourcentage pondéraux : Si Cu Fe Mn Mg Ti Zn Al 6,87 3,10 0,45 0,20 0,21 0,14 0,20 reste The alloy is of AA 319 type, selected second melting, with a chemical composition given below in percentage by weight: Yes Cu Fe mn mg Ti Zn al 6.87 3.10 0.45 0.20 0.21 0.14 0.20 rest

L'alliage a sa structure eutectique modifiée par ajout de strontium.The alloy has its eutectic structure modified by addition of strontium.

L'essai sur banc a été réalisé dans des conditions permettant de reproduire les sollicitations thermiques d'un essai chaud-froid sévère sur banc moteur, la culasse étant fixée par ses vis de fixation sur une plaque d'acier remplaçant le bloc moteur, et munie d'alésages reproduisant les alésages des cylindres du moteur.The bench test was performed under conditions that reproduce the thermal stresses of a severe hot-cold test on the engine test bench, the cylinder head being fixed by its fixing screws on a steel plate replacing the engine block, and provided with bores reproducing the bores of the engine cylinders.

Des brûleurs au gaz sont logés dans ces alésages. La culasse est montée soupapes d'échappement ouvertes et soupapes d'admission fermées.Gas burners are housed in these bores. The cylinder head is fitted with open exhaust valves and closed intake valves.

Le cycle chaud consiste à chauffer la face combustion à l'aide de brûleurs à gaz, la circulation d'eau étant parcourue par le liquide de refroidissement, de telle sorte que la température dans les pontets inter-soupapes atteigne la valeur de 250°C.The hot cycle consists in heating the combustion side with the aid of gas burners, the circulation of water being traversed by the coolant, so that the temperature in the inter-valve bridges reaches the value of 250 ° C. .

Le cycle froid consiste à interrompre la chauffe de la face combustion, la circulation d'eau étant encore parcourue par le liquide de refroidissement, de telle sorte que la température dans les pontets inter-soupapes atteigne la valeur de 40°C.The cold cycle consists in interrupting the heating of the combustion side, the circulation of water being still traversed by the coolant, so that the temperature in the inter-valve bridges reaches the value of 40 ° C.

La température des pontets inter-soupapes est mesurée à 1 mm de la surface face feu dans une culasse identique à celle de l'état T5 (essai n°1) et munie de thermocouples, ceci afin d'effectuer les réglages des brûleurs et des débits de liquide de refroidissement permettant d'atteindre ces températures.The temperature of the inter-valve bridges is measured at 1 mm from the fire-facing surface in a cylinder head identical to that of state T5 (test no. 1) and provided with thermocouples, in order to carry out the adjustments of the burners and flow rates of coolant to achieve these temperatures.

La durée du cycle chaud est de 40 secondes; celle du cycle froid de 25 secondes, ce qui donne un temps de cycle total unitaire de 65 secondes.The duration of the hot cycle is 40 seconds; that of the cold cycle of 25 seconds, which gives a total unit cycle time of 65 seconds.

L'essai est régulièrement interrompu pour examiner les pontets coté face combustion, pour déterminer la phase d'amorçage des fissures. La rupture en fatigue est déterminée dès qu'une fissure traversante de pontet conduit à une fuite de l'eau de la circulation d'eau vers la face de combustion est observée.The test is regularly interrupted to examine the bridges on the combustion side, to determine the crack initiation phase. The fatigue failure is determined as soon as a through-bridge crack leads to a leakage of water from the water circulation to the combustion face is observed.

Ces conditions expérimentales de test de rupture à la fatigue ont d'abord été appliquées aux culasses traitées à l'état T5 (essai n°1). Elles ont ensuite été reproduites dans les mêmes conditions de puissance de chauffe, de puissance de refroidissement et de cyclage à des culasses soumises à d'autres traitements thermiques (essais n°2 et 3).These experimental fatigue test conditions were first applied to T5-treated cylinder heads (Test # 1). They were then reproduced under the same conditions of heating power, cooling power and cycling to cylinder heads subjected to other heat treatments (tests No. 2 and 3).

Les culasses ont aussi fait l'objet de caractérisation à l'ambiante en traction et en dureté.The cylinder heads have also been characterized at room temperature in traction and hardness.

Les propriétés de traction sont mesurées selon la norme AFNOR EN 10002-1 dans la face feu par des éprouvettes de traction de diamètre 6,18 mm de longueur calibrée 36,2 mm. Chaque mesure est la moyenne de 5 éprouvettes par pièce, pour 3 pièces.The tensile properties are measured according to the AFNOR EN 10002-1 standard in the fire face by tensile specimens with a diameter of 6.18 mm and a calibrated length of 36.2 mm. Each measurement is the average of 5 test pieces per piece, for 3 pieces.

La dureté Brinell est mesurée selon les normes AFNOR EN ISO 6506 - 1 et ASTM E10-06 dans la face feu également. Une mesure est réalisée par pièce, pour cinq pièces.Brinell hardness is measured according to the AFNOR EN ISO 6506 - 1 and ASTM E10-06 standards in the fire side as well. One measurement is made per piece, for five pieces.

On a représenté sur la figure 2 les zones de prélèvements T1-T5 des éprouvettes de traction, la position HB1 des mesures de dureté et la position A-E des examens micrographiques pour la mesure du SDAS.We have shown on the figure 2 the T1-T5 sampling zones of the tensile test specimens, the HB1 position of the hardness measurements and the AE position of the micrographic examinations for the measurement of the SDAS.

✔Essai n°1 : Culasse soumise au traitement de référence T5✔ Test No. 1: Cylinder Head Subject to T5 Reference Treatment

Après coulée, la pièce a été extraite du moule et refroidie dans un tunnel à air forcé de telle sorte qu'elle soit refroidie jusqu'à la température de 50 °C en un temps de l'ordre de 60 minutes.After casting, the piece was extracted from the mold and cooled in a forced air tunnel so that it was cooled to a temperature of 50 ° C in a time of about 60 minutes.

La culasse a ensuite subi les opérations habituelles de parachèvement puis de revenu : 4 heures à 210°C dans un four de revenu conventionnel, puis d'usinage complet, avant de subir l'essai sur banc.The cylinder head then underwent the usual operations of completion and then income: 4 hours at 210 ° C in a conventional income oven, then complete machining, before undergoing the bench test.

Le résultat du test de résistance, exprimé en nombre de cycles avant la rupture en fatigue, est de 3600 cycles (+/- 200 cycles ; répétition deux fois) pour la culasse traitée par le traitement thermique T5.The result of the resistance test, expressed as the number of cycles before the fatigue failure, is 3600 cycles (+/- 200 cycles, double repetition) for the cylinder head treated by the T5 heat treatment.

✔ Essai n°2 ✔ Test n ° 2

L'essai a été réalisé sur une culasse ayant subi le procédé suivant.The test was carried out on a cylinder head having undergone the following process.

Après coulée, la pièce a été extraite du moule et soumise, après avoir été débarrassée de ses systèmes de coulée, à un traitement thermique avec les paramètres suivants :

  • Mise en solution 2 h à 498°C dans un lit fluidisé ;
  • Trempe dans un lit fluidisé à 30°C. La courbe de refroidissement est conforme à celle portant la référence E2/E3 sur la figure 1 et située sous la courbe de refroidissement INF. On relèvera en particulier que suivant cette courbe E2/E3, la durée pour passer de 495°C à 200°C est de l'ordre de 3mn 30s, par conséquent inférieure à 4mn 30s ;
  • Revenu 4 h à 210°C dans un four conventionnel ventilé, correspondant au pic de résistance maximale de l'alliage.
After casting, the part was extracted from the mold and subjected, after having been stripped of its casting systems, to a heat treatment with the following parameters:
  • Solution for 2 hours at 498 ° C. in a fluidized bed;
  • Quenching in a fluidized bed at 30 ° C. The cooling curve is in conformity with that bearing the reference E2 / E3 on the figure 1 and located under the cooling curve INF. It will be noted in particular that following this curve E2 / E3, the time to pass from 495 ° C to 200 ° C is of the order of 3mn 30s, therefore less than 4mn 30s;
  • 4 hours at 210 ° C in a conventional ventilated oven, corresponding to the peak of maximum strength of the alloy.

La culasse a ensuite fait l'objet d'un usinage complet, avant de subir l'essai sur banc.The bolt was then machined completely before being bench tested.

Le résultat du test de résistance, exprimé en nombre de cycles avant la rupture en fatigue, est de 4800 cycles (+/- 200 cycles ; répétition deux fois) pour la culasse traitée selon l'essai n°2.The result of the resistance test, expressed as the number of cycles before the fatigue failure, is 4800 cycles (+/- 200 cycles, double repetition) for the cylinder head treated according to test No. 2.

✔ Essai n°3 ✔ Test n ° 3

L'essai a été réalisé sur une culasse ayant subi le procédé suivant.The test was carried out on a cylinder head having undergone the following process.

Après coulée, la pièce a été extraite du moule et soumise, après avoir été débarrassée de ses systèmes de coulée, à un traitement thermique conforme à l'invention avec les paramètres suivants :

  • Mise en solution longue : 8 h à 498°C dans un lit fluidisé ;
  • Trempe dans un lit fluidisé à 30°C constitué d'un mélange diphasique d'air à température ambiante et de sable siliceux de granulométrie 50 AFS, le tout fluidisé dans un bac de trempe muni de plaques de refroidissement parcourues d'eau froide à 20-23°C. La courbe de refroidissement est conforme à celle portant la référence E2/E3 sur la figure 1, située sous la courbe de refroidissement INF ;
  • Revenu 1h30 à 230°C dans un lit fluidisé, ce qui correspond au pic de résistance de l'alliage à cette température de revenu.
After casting, the part was extracted from the mold and subjected, after being freed from its casting systems, to a heat treatment according to the invention with the following parameters:
  • Long dissolution: 8 h at 498 ° C in a fluidized bed;
  • Dipping in a fluidized bed at 30 ° C consisting of a two-phase mixture of air at room temperature and silica sand with a particle size of 50 AFS, all of which is fluidized in a quenching tank provided with cooling plates traversed with cold water at 20 ° C. -23 ° C. The cooling curve is in conformity with that bearing the reference E2 / E3 on the figure 1 , located under the cooling curve INF;
  • Income 1:30 at 230 ° C in a fluidized bed, which corresponds to the peak strength of the alloy at this temperature of income.

Le résultat du test de résistance, exprimé en nombre de cycles avant la rupture en fatigue, est de 9500 cycles (+/- 250 cycles ; répétition deux fois) pour la culasse traitée selon l'essai n°3.The result of the strength test, expressed as the number of cycles before the fatigue failure, is 9500 cycles (+/- 250 cycles, double repetition) for the cylinder head treated according to test No. 3.

L'ensemble de ces résultats montrent clairement l'intérêt de l'association, représentée de manière schématique sur le diagramme température-temps de la figure 4, d'une mise en solution longue L (L pour « Long »), d'une trempe ne générant pas de contraintes résiduelles S (S pour « Soft »), et d'un revenu dur H (H pour « Hard »).All these results clearly show the interest of the association, represented schematically in the temperature-time diagram of the figure 4 , a long solution L (L for "Long"), a quench not generating residual stresses S (S for "Soft"), and a hard income H (H for "Hard") .

On observe également que le traitement thermique selon l'invention est nettement plus performant que le traitement T5 tel que pratiqué par la demanderesse, ce traitement T5 étant lui même plus performant sur essais moteurs que les traitements thermiques conventionnels de type T7, pour la culasse considérée.It is also observed that the heat treatment according to the invention is clearly more efficient than the T5 treatment as practiced by the Applicant, this T5 treatment itself being more efficient on motor tests than conventional heat treatments of T7 type, for the cylinder head considered. .

Les traitements conventionnels T7 consistant en une mise en solution de l'ordre de 5 heures à 495°C, une trempe dans de l'eau à 70°C typiquement, et des revenus de l'ordre de 5 heures à 230°C (sur-revenu) ont été caractérisés par ailleurs par la demanderesse sur des culasses similaires. En particulier, la publication "A Phenomenological Model for Fatigue Life Prédiction of Highly Loaded Cylinder Heads" (SAE Technical Paper n° 2006-01-0542) met en évidence que pour certaines applications moteur très sollicitées, les alliages de type 319 traités à l'état T5 - bien que n'atteignant pas le potentiel de durcissement maximal de l'alliage - peuvent offrir des performances sur culasses en service supérieures à celles d'alliages soumis à des traitements thermiques complets type T7.Conventional T7 treatments consisting of a dissolution of the order of 5 hours at 495 ° C, quenching in water at 70 ° C typically, and incomes of the order of 5 hours at 230 ° C ( sur-revenu) have been characterized elsewhere by the plaintiff on similar heads. In particular, the publication "A Phenomenological Model for Fatigue Life Prediction of Highly Loaded Cylinder Heads" (SAE Technical Paper No. 2006-01-0542) shows that for certain high-demand engine applications, alloys of the type T5 conditions - although not reaching the maximum hardening potential of the alloy - can offer superior in-cylinder head performance compared to alloys subjected to T7 type full heat treatments.

On relèvera par ailleurs que les caractéristiques mécaniques statiques, sur les culasses des essais n°1, 2 et 3, et reportées dans le tableau ci-dessous, n'ont pas montré de différences significatives entre les pièces de l'essai n°2 et celles de l'essai n°3. Ces pièces sont pourtant très différentes en comportement fonctionnel sur banc (4600 cycles avant rupture pour l'essai n°2 ; 9500 cycles avant rupture pour l'essai n°3). Essai Limite à la rupture Rm (MPa) Limite élastique Rp02 (MPa) Allongement Ap (%) Dureté HB N°1 280 218 1,7 101 N°2 346 282 1,9 107 N°3 349 281 1,9 107 It should also be noted that the static mechanical characteristics, on the cylinder heads of the tests n ° 1, 2 and 3, and reported in the table below, did not show any significant differences between the parts of the test n ° 2 and those of Trial 3. These parts are however very different in functional behavior on bench (4600 cycles before rupture for the test n ° 2, 9500 cycles before rupture for the test n ° 3). Trial Limit at break Rm (MPa) Resistance limit Rp 02 (MPa) Lengthening at p (%) Hardness H B # 1 280 218 1.7 101 # 2 346 282 1.9 107 # 3 349 281 1.9 107

L'ensemble de ces observations amène à conclure que les caractérisations de traction et de dureté ne permettent pas de prédire les performances fonctionnelles de la culasse sur banc.All these observations lead to the conclusion that the tensile and hardness characterizations do not make it possible to predict the functional performance of the bolt head.

On comprend qu'au vu des propriétés mécaniques statiques observées sur ces échantillons éprouvettes, rien ne laissait présager que le traitement thermique conforme à l'invention de l'essai n°3 puisse permettre d'obtenir des performances fonctionnelles sur culasse en service tant supérieures à celles de l'essai n°2.It is understood that in view of the static mechanical properties observed on these specimen samples, there was no reason to believe that the heat treatment according to the invention of test No. 3 could make it possible to obtain functional performance on cylinder head in service both to those of Trial 2.

Il s'agit là de résultats observés de manière surprenante par la Demanderesse, lorsque cette dernière a réalisé une caractérisation sur banc de la résistance à la fatigue des culasses complète.These are results observed in a surprising way by the Applicant, when the latter has carried out a characterization on bench of the complete fatigue resistance of the yokes.

On relèvera par ailleurs, que dans le domaine technique de l'invention, on partage le sentiment général selon lequel la durée de la mise en solution n'a pas d'impact considérable sur la fatigue, pour peu qu'une mise en solution soit appliquée. Il en découle, comme cela est par exemple décrit dans le document GB2361710 , un encouragement à réduire la durée de la mise en solution.It should also be noted that, in the technical field of the invention, there is a general feeling that the duration of solution dissolution does not have a considerable impact on fatigue, provided that a dissolution is applied. It follows from this, as is for example described in the document GB2361710 , an encouragement to reduce the duration of dissolution.

La Demanderesse a toutefois pu constater, alors même que les caractéristiques mécaniques statiques (traction, dureté) sont globalement équivalentes et que rien n'incitait par conséquent à faire des expériences supplémentaires, notamment sur les pièces complètes, et alors même que les préjugés du domaine tendait à l'éloigner de la solution de l'invention en préconisant une mise en solution courte, que de manière surprenante on peut aboutir à une forte augmentation des performances fonctionnelles de la culasse en mettant en oeuvre le procédé selon l'invention associant une mise en solution longue, une trempe ne générant pas de contraintes résiduelles, et un revenu dur.The Applicant has however been able to observe, even though the static mechanical characteristics (traction, hardness) are globally equivalent and that there was therefore no incentive to carry out additional experiments, especially on complete parts, and even as the prejudices of the field tended to move away from the solution of the invention by advocating a short dissolution, that surprisingly can lead to a sharp increase in the functional performance of the cylinder head by implementing the method according to the invention associating a setting in long solution, a quenching that does not generate residual stresses, and a hard income.

Expérience n°2Experience n ° 2

Dans tous les exemples ci-dessous, des culasses diesel quatre cylindres en ligne ont été moulées en gravité statique en moule métallique, face feu vers le bas, avec une semelle en acier refroidie de façon énergique de façon à obtenir une microstructure fine que l'on peut caractériser par la mesure du SDAS (« Secondary Dendrite Arm Spacing »), avec des valeurs de l'ordre de 30 microns dans la zone où sont prélevées les éprouvettes de traction servant à caractériser le matériau.In all the examples below, in-line four-cylinder diesel cylinder heads were cast in static gravity in a metal mold, fire-facing down, with a steel plate cooled energetically so as to obtain a fine microstructure that the it is possible to characterize by measuring the SDAS ("Secondary Dendrite Arm Spacing"), with values of the order of 30 microns in the area where the tensile specimens used to characterize the material are taken.

La température métal à la coulée est de 720°C à l'arrivée dans le godet de coulée du moule, d'où partent les chenaux d'alimentation pour remplir le moule au travers des attaques situées au pied de la pièce.The metal temperature at casting is 720 ° C at the inlet in the casting cup of the mold, from which feed channels leave to fill the mold through attacks located at the foot of the room.

La mise au mil, ratio entre le poids coulé (pièce plus système d'alimentation, plus masselottes) et le poids de la pièce est de 1,7. La pièce moulée pèse 14,1 kg.Putting millet, ratio between the weight cast (piece plus feeding system, plus weights) and the weight of the piece is 1.7. The molded piece weighs 14.1 kg.

Tout le noyautage est réalisé en procédé de type « boite froide », pour la réalisation des formes intérieures : conduits d'admission, d'échappement, de circulation d'eau, d'huile et pour la réalisation du noyau contenant les masselottes, réserve de métal située au dessus de la pièce elle-même et permettant l'alimentation en métal liquide pendant la solidification et la contraction de la pièce.All coring is carried out in cold box type process, for the realization of the inner forms: intake ducts, exhaust ducts, circulation of water, oil and for the realization of the core containing the weights, reserves of metal located above the piece itself and allowing the supply of liquid metal during the solidification and contraction of the piece.

Le temps de cycle de moulage est de l'ordre de 5 minutes de pièce à pièce.The molding cycle time is of the order of 5 minutes from room to room.

L'alliage est de type AA 356, de première fusion, avec une composition chimique donnée ci-après en pourcentage pondéraux : Si Fe Mn Mg Ti Zn Al 7,4 0,12 0,02 0,30 0,11 0,02 reste The alloy is of AA 356 type, of first fusion, with a chemical composition given below in weight percentages: Yes Fe mn mg Ti Zn al 7.4 0.12 0.02 0.30 0.11 0.02 rest

L'alliage a sa structure eutectique modifiée par ajout de strontium.The alloy has its eutectic structure modified by addition of strontium.

Après coulée, la pièce a été extraite du moule et refroidie dans un tunnel à air forcé de telle sorte qu'elle soit refroidie jusqu'à la température de 50 °C en un temps de l'ordre de 120 minutes.After casting, the part was extracted from the mold and cooled in a forced air tunnel so that it was cooled to a temperature of 50 ° C. in a time of about 120 minutes.

Les culasses ont ensuite été soumises aux opérations habituelles de parachèvement puis :

  • ✔ Essai n°4 : le traitement thermique de référence comprenant :
    • Une mise en solution 6 h à 540°C dans un four conventionnel
    • Une trempe dans de l'eau chaude à 70°C
    • Un revenu de 6h à 200°C dans un four conventionnel
  • ✔Essai n°5 : un traitement thermique hors du périmètre de l'invention, mettant en oeuvre une trempe à l'eau, et comprenant :
    • Une mise en solution 6h à 540°C dans un four conventionnel
    • Une trempe dans de l'eau proche de l'ébullition à 93 °C
    • Un revenu 6h à 200°C dans un four conventionnel
  • ✔Essai n°6 : un traitement thermique conforme à l'invention, comprenant :
    • Une mise en solution 6 h à 540°C dans un four conventionnel
    • Une trempe à l'air dans les conditions opératoires exposées précédemment, avec une courbe de refroidissement conforme à celle portant la référence E6 sur la figure 1, située sous la courbe de refroidissement INF. On relèvera en particulier que suivant cette courbe E6, la durée pour passer de 495°C à 200°C est de l'ordre de 4mn, par conséquent inférieure à 4mn 30s.
The cylinder heads were then subjected to the usual operations of completion and then:
  • ✔ Test No. 4 : the reference heat treatment comprising:
    • Dissolving for 6 h at 540 ° C. in a conventional oven
    • Quenching in hot water at 70 ° C
    • An income of 6 hours at 200 ° C in a conventional oven
  • Test No. 5 : a heat treatment outside the perimeter of the invention, using a quenching with water, and comprising:
    • Dissolving 6h at 540 ° C in a conventional oven
    • Quenching in water close to boiling at 93 ° C.
    • An income 6h at 200 ° C in a conventional oven
  • Test No. 6 : a heat treatment according to the invention, comprising:
    • Dissolving for 6 h at 540 ° C. in a conventional oven
    • Quenching in the air under the operating conditions described above, with a cooling curve in accordance with that with the reference E6 on the figure 1 , located under the INF cooling curve. It will be noted in particular that according to this curve E6, the time to pass from 495 ° C to 200 ° C is of the order of 4 minutes, therefore less than 4 minutes 30 seconds.

La trempe à l'air de la courbe E6 correspond à une trempe des culasses disposées verticalement et individuellement sur une couche unique, les culasses étant séparées de 30 mm les unes des autres (sans compter les intercalaires), et posées dans des paniers traversés par un flux d'air présentant un débit de 3000 m3/h.

  • Un revenu 5h à 180°C dans un four conventionnel, correspondant au pic de résistance maximale de l'alliage.
The air quenching of the curve E6 corresponds to a quenching of the yokes arranged vertically and individually on a single layer, the yokes being separated by 30 mm from each other (not including tabs), and placed in baskets crossed by a flow of air with a flow rate of 3000 m 3 / h.
  • An income of 5h at 180 ° C in a conventional oven, corresponding to the peak of maximum resistance of the alloy.

Des éprouvettes de traction ont été prélevées dans les culasses des essais n°4, 5 et 6, de façon similaire à ce qui a été réalisé dans le cadre de l'expérience 1, et les caractéristiques mécaniques de traction ont été mesurées sur ces éprouvettes.Traction test pieces were taken from the yokes of tests 4, 5 and 6, similarly to what was carried out in the context of experiment 1, and the mechanical traction characteristics were measured on these test pieces. .

De façon similaire à l'exemple 1 la dureté a aussi été mesurée sur la face feu des culasses.In a manner similar to Example 1, the hardness was also measured on the fire side of the cylinder heads.

Ces caractéristiques sont reportées dans le tableau suivant Essai Limite à la rupture Rm (MPa) Limite élastique Rp02 (MPa) Allongement Ap (%) Dureté HB N°4 283 245 6,7 97 N°5 271 233 6,2 95 N°6 290 238 4,4 106 These characteristics are reported in the following table Trial Limit at break Rm (MPa) Resistance limit Rp 02 (MPa) Lengthening at p (%) Hardness H B # 4 283 245 6.7 97 # 5 271 233 6.2 95 # 6 290 238 4.4 106

On observera que le traitement thermique conforme à l'invention et mettant en oeuvre une mise en solution longue, une trempe à l'air et un revenu à proximité du pic de résistance (essai n°6) induit un jeu de propriétés mécaniques comparables à celles du traitement de référence de type T7 effectué avec une trempe à l'eau à 70° C, au prix d'un léger abattement sur les caractéristiques d'allongement par rapport au traitement T7 mettant en oeuvre une trempe à l'eau classique (essai n°4) et donne des résultats de caractéristiques mécaniques supérieurs à ceux obtenus avec le traitement T7 mettant en oeuvre une trempe à l'eau à une température voisine du point d'ébullition (essai n°5).It will be observed that the heat treatment in accordance with the invention and implementing a long dissolution, an air quenching and an income near the peak of resistance (test No. 6) induces a set of mechanical properties comparable to those of the reference treatment of the T7 type performed with water quenching at 70 ° C., at the cost of a slight reduction in the elongation characteristics compared with the T7 treatment using a conventional water quench ( test no. 4) and gives results of mechanical characteristics superior to those obtained with the treatment T7 implementing quenching with water at a temperature close to the boiling point (test No. 5).

Les niveaux globaux de contraintes résiduelles ont par ailleurs été caractérisés pour les trois essais n°4, 5 et 6, de la manière suivante.The overall levels of residual stresses were furthermore characterized for the three tests 4, 5 and 6, as follows.

Comme cela est représenté de manière schématique sur la figure 3, les culasses ont été sectionnées progressivement par fraisage en partant de la face feu. Les culasses sont, avant découpe, puis à chaque étape de la découpe, à 13 et à 30 mm de profondeur par rapport à la face feu, mesurées dimensionnellement de façon à quantifier la flèche maximale de la culasse par rapport à trois plages de référence Pr de la face feu, et la variation de longueur moyenne de la culasse sur les faces A et B.As shown schematically on the figure 3 the yokes were cut progressively by milling from the fire side. The yokes are, before cutting, then at each stage of the cutting, at 13 and 30 mm depth with respect to the fire face, measured dimensionally so as to quantify the maximum deflection of the yoke with respect to three reference ranges Pr of the fire side, and the variation of the average length of the bolt on the sides A and B.

On a ainsi reporté dans le tableau ci-après les valeurs des déformations dues à la relaxation de la culasse ; ces valeurs représentant une mesure qualitative et globale de l'état de contraintes résiduelles de la culasse. Essai Flèche maximale face feu (mm) Contraction moyenne en longueur (A/B) (mm) N°4 0,14 0,10 N°5 0,15 0,26 N°6 0,05 0,05 The values of the deformations due to the relaxation of the cylinder head are thus reported in the table below; these values representing a qualitative and global measurement of the state of residual stresses of the cylinder head. Trial Maximum arrow facing fire (mm) Mean contraction in length (A / B) (mm) # 4 0.14 0.10 # 5 0.15 0.26 # 6 0.05 0.05

On observe que parmi ces trois essais, le traitement selon l'invention comportant une trempe à l'air dans les conditions opératoires du procédé selon l'invention est la seule capable d'offrir un abattement significatif de contraintes résiduelles, tout en conservant un bon niveau de caractéristiques mécaniques. Cet effet sur les contraintes résiduelles est probablement un élément discriminant dans l'amélioration inattendue des comportements sur banc des culasses traitées thermiquement par le procédé selon l'invention.It is observed that among these three tests, the treatment according to the invention comprising air quenching under the operating conditions of the process according to the invention is the only one able to offer a significant reduction of residual stresses, while maintaining a good level of mechanical characteristics. This effect on the residual stresses is probably a discriminating element in the unexpected improvement of the behaviors on the bank of thermally treated cylinder heads by the method according to the invention.

Claims (13)

  1. A method for heat treatment of a casting part of the cylinder head type made in an alloy of aluminium, silicon and magnesium, characterized in that it comprises the following steps:
    - solution heat treatment of the part for a duration comprised between three and ten hours, at a temperature comprised between 490°C and 550°C;
    - quenching of the part with air or in a fluidized bed, at a cooling rate such that the temperature of a fire face of the part passes from 495°C to 200°C in less than 7 min 30 s;
    - tempering of the part at the resistance peak, or in proximity to the resistance peak, tempering being performed according to a suitable duration in order to attain a resistance level of the part at least equal to 85% of the maximum resistance level at the relevant tempering temperature.
  2. The method according to claim 1, wherein solution heat treatment is performed for a duration comprised between five hours and ten hours.
  3. The method according to any of claims 1 or 2, wherein solution heat treatment is carried out in a conventional oven or in a fluidized bed.
  4. The method according to any of claims 1 or 2, wherein the solution heat treatment comprises:
    - a solution heat treatment in a fluidized bed for a duration of less than or equal to 30 minutes, applied upon exiting molding so as to clean the part,
    - complementary solution heat treatment in a conventional oven.
  5. The method according to any of claims 1 to 3, wherein the solution heat treatment is applied to the part after complete blast cleaning adapted for ridding the part of its internal cores, and after removal of the feeder heads and the casting systems.
  6. The method according to any of claims 1 to 3, wherein the solution heat treatment is applied to the part upon exiting molding.
  7. The method according to claim 1, wherein quenching is carried out in a fluidized bed at a temperature of less than 40°C.
  8. The method according to claim 1, wherein the quenching is air quenching at room temperature of parts positioned in a single layer crossed by the air flow, wherein the air flow rates are greater than 1000 m3/h/part, and preferably greater than or equal to 1700 m3/h/part.
  9. The method according to any of claims 1 to 8, wherein tempering is performed at the peak of maximum resistance for the relevant tempering temperature.
  10. The method according to any of claims 1 to 9, wherein tempering is performed in a conventional oven or in a fluidized bed.
  11. The method according to any of claims 1 to 9, wherein the alloy is of the AA 319 type and tempering is performed at 230°C for 1h30 min in a fluidized bed or at 210°C for 4 hours in a conventional oven.
  12. The method according to any of claims 1 to 9, wherein the alloy is of the AA 356 type and tempering is performed at 180°C for 5 hours in a conventional oven.
  13. A cylinder head made in an alloy of aluminium, silicon and magnesium characterized in that it has a fatigue resistance evaluated by the hot-cold fatigue resistance test, and expressed as a number of cycles before fatigue breaking to within +/- 200 cycles, greater than 4800 cycles, preferably close to 9500 cycles.
EP08761236.2A 2007-06-22 2008-06-19 Process for the heat treatment of cylinder heads made of an aluminium-based alloy, and cylinder heads having improved fatigue resistance properties Active EP2171115B1 (en)

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FR0755973A FR2917751B1 (en) 2007-06-22 2007-06-22 PROCESS FOR THE HEAT TREATMENT OF ALUMINUM-BASED ALLOY CUPS AND CULONS WITH IMPROVED FATIGUE RESISTANCE PROPERTIES
PCT/EP2008/057811 WO2009000749A1 (en) 2007-06-22 2008-06-19 Process for the heat treatment of cylinder heads made of an aluminium-based alloy, and cylinder heads having improved fatigue resistance properties

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US9595533B2 (en) 2012-08-30 2017-03-14 Micron Technology, Inc. Memory array having connections going through control gates
WO2017152397A1 (en) * 2016-03-09 2017-09-14 中南大学 Method for testing phase transformation point of aluminium alloy
DE102019208807A1 (en) * 2019-06-18 2020-12-24 Audi Ag Process and system for batch heat treatment of light metal castings

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MX339962B (en) 2016-06-20
US20110011501A1 (en) 2011-01-20
WO2009000749A1 (en) 2008-12-31
US9303303B2 (en) 2016-04-05
ES2624598T3 (en) 2017-07-17
EP2171115A1 (en) 2010-04-07
PL2171115T3 (en) 2017-09-29
FR2917751A1 (en) 2008-12-26
SI2171115T1 (en) 2017-07-31
FR2917751B1 (en) 2011-04-01

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