EP1516072B1 - Part cast from aluminium alloy with high hot strength - Google Patents

Abstract

Cast part with high creep resistance, made of an alloy with a composition comprising (% by weight): Si: 5-11 Fe<0.6 Mg: 0.15-0.6 Cu: 0.3-1.5 Ti: 0.05-0.25 Zr: 0.05-0.25 Mn<0.4 Zn<0.3 Ni<0.4 other elements<0.10 each and 0.30 total, remainder aluminium.

Description

Field of the invention

The invention relates to molded aluminum alloy parts subjected to high thermal and mechanical stresses, including cylinder heads and crankcases of internal combustion engines, and more particularly turbocharged gasoline or diesel engines. There are also, outside the automobile parts subject to the same types of constraints, for example in the field of mechanics or aeronautics.

State of the art

1. 1) les alliages contenant de 5 à 9% de silicium, de 3 à 4% de cuivre et du magnésium. Il s'agit le plus souvent d'alliages de seconde fusion, avec des teneurs en fer comprises entre 0,5 et 1%, et des teneurs en impuretés, notamment en manganèse, zinc, plomb, étain ou nickel, assez élevées. Ces alliages sont généralement utilisés sans traitement thermique (état F) ou simplement stabilisés (état T5). Ils sont plutôt destinés à la fabrication de culasses de moteurs à essence assez peu sollicités thermiquement. Pour les pièces plus sollicitées destinées aux moteurs diesel ou turbo-diesel, on utilise des alliages de première fusion, avec une teneur en fer inférieure à 0,3%, traités thermiquement à l'état T6 (revenu au pic de résistance mécanique) ou T7 (sur-revenu).
2. 2) Les alliages de première fusion contenant de 7 à 10% de silicium et du magnésium, traités à l'état T6 ou T7, pour les pièces les plus sollicitées comme celles destinées aux moteurs turbo-diesel.

In the manufacture of engine cylinder heads, two families of aluminum alloys are usually used:

1. 1) alloys containing 5 to 9% silicon, 3 to 4% copper and magnesium. It is most often secondary alloys, with iron contents between 0.5 and 1%, and levels of impurities, including manganese, zinc, lead, tin or nickel, quite high. These alloys are generally used without heat treatment (state F) or simply stabilized (state T5). They are rather intended for the manufacture of cylinder heads of gasoline engines with little thermal stress. For more stressed parts for diesel or turbo-diesel engines, primary alloys with an iron content of less than 0.3% are used, heat treated in the T6 state (at peak strength) or T7 (over-income).
2. 2) Primary alloys containing 7 to 10% silicon and magnesium, treated in the T6 or T7 state, for the most stressed parts such as those intended for turbo-diesel engines.

• Al-Si5Cu3MgFe0,15 T7 : bonne résistance - bonne ductilité
• Al-Si5Cu3MgFe0,7 F : bonne résistance - faible ductilité
• Al-Si7Mg0,3Fe0,15 T6 : faible résistance - extrême ductilité

These two major families of alloys lead to different compromises between the various properties of use: mechanical strength, ductility, creep and fatigue resistance. This problem has been described for example in the article of R. Chuimert and M. Garat: "Choice of Heavy Duty Cast Aluminum Casting Aluminum Alloys", published in the March 1990 SIA Review . This article summarizes the properties of 3 alloys studied:

• Al-Si5Cu3MgFe0,15 T7: good resistance - good ductility
• Al-Si5Cu3MgFe0.7 F: good strength - low ductility
• Al-Si7Mg0.3Fe0.15 T6: low strength - extreme ductility

The first and third alloy-state combinations can be used for heavily loaded cylinder heads. However, the search continued for an improved compromise between strength and ductility. The patent FR 2690927 in the name of the applicant, filed in 1992, describes creep resistant aluminum alloys containing from 4 to 23% of silicon, at least one of magnesium elements (0.1 - 1%), copper (0.3 - 4.5%) and nickel (0.2 - 3%), and 0.1 to 0.2% titanium, 0.1 to 0.2% zirconium and 0.2 to 0.4 % of vanadium. An improvement in the creep resistance at 300 ° C was observed without significant loss of elongation measured at 250 ° C.

The article by F. J. Feikus "Optimization of Al-Si cast alloys for cylinder head applications" AFS Transactions 98-61, pp. 225-231, studies the addition of 0.5% and 1% copper to an alloy AlSi7Mg0.3 for the manufacture of cylinder heads of internal combustion engines. After a conventional T6 treatment comprising a dissolution of 5 h at 525 ° C., followed by quenching with cold water and a 4 hour yield at 165 ° C., it does not observe any gain in limiting the temperature. elasticity, neither in hardness at room temperature, but at operating temperatures above 150 ° C, the addition of copper brings a significant gain in yield strength and creep resistance.

The patent EP 1057900 (VAW Aluminum), filed in 1999, is a development in the same way and describes the addition to an Al-Si7Mg0.3Cu0.35 alloy of tightly controlled amounts of iron (0.35 - 0.45%), manganese (0.25 - 0.30%), nickel (0.45 - 0.55%), zinc (0.10 - 0.15) and titanium (0.11 - 0.15%). This alloy exhibits in the T6 and T7 states good creep resistance, high thermal conductivity, satisfactory ductility and good resistance to corrosion.

The object of the present invention is to further improve the mechanical strength and the creep resistance of AlSiCuMg alloy castings in the temperature range 250-300 ° C, without degrading their ductility, and avoiding the multiplication of elements. addition that may be problematic for recycling.

Object of the invention

The object of the invention is a molded part with high mechanical resistance to heat and high resistance to creep alloy composition (% by weight): If: 5 - 11 and preferably 6.5 - 7.5 Fe <0.6 and preferably <0.3 Mg: 0.15 - 0.6 «« 0.25 - 0.5 Cu: 0.3 - 1.5 «« 0.4 - 0.7 Ti: 0.05 - 0.25 «« 0.08 - 0.20 Zr: 0.05 - 0.25 «« 0.12 - 0.18 Mn <0.4 «« 0.1 - 0.3 Zn <0.3 «« <0,1 Ni <0.4 «« <0,1 other elements <0.10 each and 0.30 in total, remain aluminum.

The part is preferably treated by dissolution, quenching and tempering at T6 or T7.

Description of the invention

The invention is based on the finding by the applicant that adding a small amount of zirconium to a silicon alloy containing less than 1.5% copper and less than 0.6% magnesium, could be obtained on parts moldings treated in the T6 or T7 state, good mechanical strength and good creep resistance in the range 250-300 ° C, without loss of ductility. This result is achieved without having to use elements such as nickel or vanadium that pose problems with recycling. In addition, nickel has the disadvantage of reducing the ductility of the part.

Like most of the alloys for the manufacture of motor cylinder heads, the alloy contains from 5 to 11% silicon, and preferably from 6.5 to 7.5%. The iron is maintained below 0.6%, and preferably below 0.3%, which means that it may be first or second fusion alloys, with a preference for the first one. melting when a high elongation at break is desired.

Magnesium is a common addition element for cylinder head alloys; at a content of at least 0.15%, and in combination with copper, it improves the mechanical properties at 20 and 250 ° C. Beyond 0.6%, there is a risk of reducing the ductility at room temperature.

The addition of 0.3 to 1.5%, and preferably 0.4 to 0.7%, of copper makes it possible to improve the mechanical strength without affecting the corrosion resistance. In addition, the Applicant has found that, within these limits, the ductility and the hot resistance of the parts in the T6 or T7 state were not lowered. In addition, it has been found, surprisingly, that when the Cu and Mg% contents increase together within the limits indicated above by following the condition: 0.3Cu + 0.18 <Mg <0.6, we improve significantly heat resistance and creep resistance at 250 ° C.

At a level of more than 0.1%, manganese also has a positive effect on mechanical strength at 250 ° C, but this effect peaks above a content of 0.4%.

The titanium content is maintained between 0.05 and 0.25%, which is quite usual for this type of alloy. Titanium contributes to the refining of the primary grain during solidification, but, in the case of the alloys according to the invention, it also contributes, in connection with zirconium, to the formation, during the dissolution of the piece molded, very fine dispersoids (<1 μm) AlSiZrTi located at the heart of the solid solution α-Al which are stable above 300 ° C, unlike phases Al ₂ CuMg, AlCuMgSi, Mg ₂ Si and Al ₂ Cu which coalesce from 150 ° C.

These dispersoid phases are not embrittling, unlike the large AlSiFe and AlSiMnFe iron phases (20 to 100 μm), as well as the nickel phases, which are formed during casting in the interdendritic spaces.

The parts are made by the usual molding processes, including gravity mold casting and low pressure casting for the cylinder heads, but also sand casting, squeeze casting (especially in the case of composite insertion) and lost foam molding.

The heat treatment comprises a dissolution typically of 3 to 10 h at a temperature of between 500 and 545 ° C, a quench preferably with cold water, a quench between tempering and income of 4 to 16 h, and an income from 4 to 10 h at a temperature between 150 and 240 ° C. The temperature and the duration of the income are adjusted to obtain either an income at the peak of mechanical strength (T6) or an over-income (T7).

The parts according to the invention, and in particular the cylinder heads and crankcases of an automobile or aircraft, exhibit both high mechanical strength, good ductility, higher mechanical strength and creep resistance than pieces of the prior art.

examples Example 1

• Si=7,10 Fe=0,15 Mg=0,37 Ti=0,14 Sr=170 ppm

100 kg d'alliage B de même composition avec une addition complémentaire de 0,49% de cuivre
100 kg d'alliage C de même composition que B avec une addition complémentaire de 0,14% de zirconium.In the silicon carbide crucible of an electric furnace, 100 kg of alloy A of composition (% by weight) were produced:

• Si = 7.10 Fe = 0.15 Mg = 0.37 Ti = 0.14 Sr = 170 ppm

100 kg of alloy B of the same composition with a supplementary addition of 0.49% of copper
100 kg of alloy C of the same composition as B with a complementary addition of 0.14% zirconium.

These compositions were measured by spark emission spectrometry, except for Cu and Zr which were measured by induced plasma emission spectrometry.

50 AFNOR tensile shell specimens of each alloy were cast. These specimens were subjected to a heat treatment comprising a dissolution of 10 h at 540 ° C, preceded for copper alloys B and C by a 4 hour step at 500 ° C to avoid burns, a quenching. cold water, ripening at room temperature for 24 h and 5 h at 200 ° C.

From these specimens, tensile specimens and creep specimens were machined to measure the mechanical properties (tensile strength R _m in MPa, yield strength R _p0,2 in MPa and elongation at break A%) at room temperature, 250 ° C and 300 ° C. The results are shown in Table 1: Table 1 R _m R _p0,2 AT R _m R _p0,2 AT R _m R _p0,2 AT Temp. Amb. Amb. Amb. 250 ° C 250 ° C 250 ° C 300 ° C 300 ° C 300 ° C AT 299 257 9.9 61 55 34.5 43 40 34.5 B 327 275 9.8 73 66 34.5 44 40 34.6 VS 324 270 9.8 68 63 34.5 45 42 35.0

It is found that the addition of copper to the alloy A is favorable to the mechanical strength, both cold and hot, without modifying the elongation, and that the addition of zirconium to B is practically without influence on the mechanical characteristics.

The elongation (in%) after 100 h at 250 ° C. and 300 ° C. under different stress levels (in MPa) was then measured on the creep specimens, for alloys B and C. The results are shown in Table 2: Table 2 Temperature (° C) 250 250 300 Constraint (MPa) 45 40 22 A (%) alloy B 2.43 0.134 0,136 A (%) alloy C 0.609 0.079 0.084

It is found that at identical temperature and stress, the alloy C with zirconium addition has a significantly improved creep behavior, the deformation under constant load being reduced, as the case may be, from 40 to 75%.

Example 2

Under the same conditions as for the alloy C of Example 1, 10 test pieces of each of the 5 alloys D to H were prepared by varying the content of copper and magnesium within the aforementioned preferential composition limits. upper. The compositions of the alloys are shown in Table 3: Table 3 Alloy Yes Cu mg Zr Ti D 7.1 0.4 0.3 0.14 0.12 E 7.1 0.4 0.4 0.14 0.12 F 7.1 0.5 0.35 0.14 0.12 BOY WUT 7.1 0.65 0.3 0.14 0.12 H 7.1 0.65 0.4 0.14 0.12

The mechanical characteristics at 20 ° C and 250 ° C were measured in the same manner. The results, corresponding to the average of the values obtained on the test pieces of each alloy, are shown in Table 4: Table 4 Alloy R _m (MPa) R _0.2 (MPa) AT (%) R _m (MPa) R _0.2 (MPa) AT (%) 20 ° C 20 ° C 20 ° C 250 ° C 250 ° C 250 ° C D 301 250 8.9 69 60 44.5 E 325 282 7.6 77 66 36.3 F 320 271 8.7 74 63 41.5 BOY WUT 315 259 9.1 71 60 45.2 H 339 291 8.7 81 69 39.6

It is found that, within the composition limits tested, the tensile strength and the yield strength increase when the Cu and Mg contents increase, but also that the elongation is not affected. At 250 ° C., the increase of 0.3 to 0.4% in the Mg content has a very favorable effect on the tensile strength and the yield strength, in particular for the alloy with the highest copper content (H ).

On the other hand, with an equal copper content, the increase of 0.3 to 0.4% of the magnesium content improves the creep resistance at 250 ° C, as shown by the results of the stress creep tests. 40 MPa after 100, 200 and 300 h for G and H alloys, as shown in Table 5: Table 5 duration 100 h 200 h 300 h ε (%) G 0.098 0.48 1.20 ε (%) H 0.078 0.18 0.31

Example 3

In the same way as for alloy C of Example 1, test pieces of 6 alloys 1 to N were prepared, the composition of which is shown in Table 6: Table 6 Alloy Yes Cu mg mn Zr Ti I 7 0.5 0.3 - 0.14 0.12 J 7 0.5 0.3 0.15 0.14 0.12 K 7 1 0.3 - 0.14 0.12 The 7 1 0.3 0.15 0.14 0.12 M 7 1 0.3 0.25 0.14 0.12 NOT 7 1 0.5 0.25 0.14 0.12

The mechanical characteristics were measured at 250 ° C. and the results are shown in Table 7: Table 7 Alloy R _m (MPa) R _0.2 (MPa) AT (%) I 73 62 45 J 76 65 37 K 70 59 46 The 77 62 47 M 77 62 46 NOT 90 75 33

It is found that the addition of 0.1 to 0.3% of manganese increases by at least 5% the strength at 250 ° C. On the other hand, there is no increase between 0.15 and 0.25%. Finally, for the high copper N alloy, the increase in magnesium content from 0.3 to 0.5% leads to a spectacular and unexplained increase in the mechanical strength when hot.

Claims (13)

1. Cast part with high creep resistance, made of an alloy with composition (% by weight):

   Si: 5 - 11

   Fe < 0.6

   Mg: 0.15 - 0.6

   Cu: 0.3 - 1.5

   Ti: 0.05 - 0.25

   Zr: 0.05 - 0.25

   Mn < 0.4

   Zn < 0.3

   Ni < 0.4

   other elements < 0.10 each and 0.30 total, remainder aluminium.
2. Part according to claim 1, characterised in that its silicon content is between 6.5 and 7.5%.
3. Part according to either claim 1 or 2, characterised in that its iron content is less than 0.3%.
4. Part according to one of claims 1 to 3, characterised in that its copper content is between 0.4 and 0.7%.
5. Part according to one of claims 1 to 4, characterised in that its magnesium content is between 0.25 and 0.5%.
6. Part according to one of claims 1 to 4, characterised in that the contents of magnesium and copper in % are such that 0.3Cu + 0.18 < Mg < 0.6.
7. Part according to one of claims 1 to 6, characterised in that its titanium content is between 0.08 and 0.20%.
8. Part according to one of claims 1 to 7, characterised in that its zirconium content is between 0.12 and 0.18%.
9. Part according to one of claims 1 to 8, characterised in that its manganese content is between 0.1 and 0.3%.
10. Part according to one of claims 1 to 9, characterised in that its zinc content is less than 0.1%.
11. Part according to one of claims 1 to 10, characterised in that its nickel content is less than 0.1%.
12. Part according to one of claims 1 to 11, characterised in that it is solution heat treated, quenched and tempered to T6 or T7.
13. Part according to one of claims 1 to 12, characterised in that it is a cylinder head or a crankcase of an automobile or aircraft engine.