EP2097551A1

EP2097551A1 - Aluminium casting alloy, method for the manufacture of a casting and cast component for combustion engines

Info

Publication number: EP2097551A1
Application number: EP07860907A
Authority: EP
Inventors: Franz Josef Feikus; Annegret Franke
Original assignee: Hydro Aluminium AS
Current assignee: Hydro Aluminium AS
Priority date: 2006-12-13
Filing date: 2007-12-10
Publication date: 2009-09-09
Also published as: EP2097551A4; NO20065767L; WO2008072972A1

Abstract

The invention provides an aluminium casting alloy from which even complex-shaped castings can be cast with no problem and at high températures has good mechanical properties. To achieve this, an aluminium casting alloy according to the invention consists of (in wt.-%) : Cu: 2 - 8 %, Mn: 0.2 - 0.6 %, Zr: 0.07 - 0.3 %, Fe: ≤ 0.25 %, Si: ≤ 0.3 %, Ti: 0.05 - 0.2 %, V: ≤ 0.04 %, remainder Al and unavoidable impurities, wherein the total content of impurities amounts to a maximum of 0,10 wt.-%. In addition to this, the invention provides a method for the manufacture of castings which allows for the operationally reliable manufacture of complex. shaped castings for combustion engines in particular. Advantageous uses of the casting alloy according to the invention are disclosed, too.

Description

Aluminium casting alloy, method for the manufacture of a casting and cast component for combustion engines

The invention relates to an aluminium casting alloy, which as well as aluminium, contains substantial constituents of Cu, Mn, Zr and Ti in respect of its properties. In addition to this, the invention relates to a method for the manufacture of castings from such an aluminium casting alloy. Finally, the invention relates to a cast component for internal combustion engines and preferred uses of the inventive alloy.

From US 2,706,680 an alloy of the type described in the preamble is known, which (in wt.-%, which means % by weight) contains 5 - 13 % Cu, 0.15 - 1.7 % Mn, wherein the Mn-content in each case amounts to 3 - 13 % of the Cu content, 0.05 - 0.30 % Zr, 0.01 - 0.25 % of at least one element from the group "Co, Ni, Mo, W, Cr, Ti, B, Ta, Nb" , wherein the sum of the contents of these elements does not exceed 0.25 %, up to 0.75 % Fe and up to 0.40 % Si.

The alloy composed in this way is intended to be easy to cast and nevertheless have particularly good strength values, such as high tensile strength, low inclination to material fatigue, and good fatigue and endurance strength at high temperatures . The combination of the said contents of Mn, V, and Zr in this case is ascribed as having a particularly favourable influence on the said properties.

To prove the mechanical properties of the known alloy, according to the method described in US 2,706,680 four aluminium forging alloys with Cu-contents of 5.98 - 12.14 % by weight, Mn-contents of up to 1.45 % by weight, V-contents of 0.10 - 0.14 % by weight, Zr-contents of 0.23 - 0.25 % by weight, Fe-contents of 0.11 - 0.20 % by weight and Si-contents of 0.07 - 0.20 % by weight were smelted and cast into bars, which were then formed by forging. In comparison, an aluminium casting melt, which contained 6.0 % by weight Cu, 0.3 % by weight Mn, 0.10 % by weight V, 0.25 % by weight Zr, as well as 0.02 % by weight Ti and 0.005 % by weight B, were smelted and cast to make a bar, which was then not subjected to any further forming. The bars forged from the aluminium forging alloy are solutionized over a period of one to two hours at temperatures in the range from 510 - 537²C, while during the solutionizing of the bar produced from the casting alloy the annealing temperature was about 510²C. After the solutionizing, the bars were in each case quenched in water. All the bars were then hardened at a temperature of 190^aC for 12 hours. For the forged bars obtained in this way, at a temperature of 315²C tensile strengths from 62 - 138 MPa and yield strengths of 49 - 105 MPa could be proved, while at the same temperature and at an elongation of 9 %, the tensile strength determined for the cast bar was 172 MPa.

Practice on casting of components for internal combustion engines has shown that the demands on modern casting alloys cannot be fulfilled with the alloy known from US 2,706,680, despite its comparable hot strength. This applies in particular if it is intended that cylinder heads or engine blocks should be cast from the known alloy which in operation are to be subjected to both high thermal loads as well as high mechanical loads.

Against this background, the object of the invention was to provide an aluminium casting alloy from which even complex-shaped castings can be cast with no problem and at high temperatures has good mechanical properties. In addition to this, it is intended that a method be described for the manufacture of castings which allows for the operationally reliable manufacture of complex shaped castings for combustion engines in particular. Finally, it is intended that a particularly advantageous uses be described of a casting alloy of the type according to the invention.

With regard to the aluminium casting alloy, the object previously mentioned is resolved in that such a casting alloy according to the invention contains (in wt.-%) 2 - 8 % Cu, 0.2 - 0.6 % Mn, 0.07 - 0.3 % Zr, maximum 0.25 % Fe, maximum 0.3 % Si, 0.05 - 0.2 % Ti, maximum 0.04 % V, and as the remainder Al and unavoidable impurities, wherein the total content of impurities amounts to a maximum of 0.1 % by weight. Preferably, the content of each of the impurities is restricted to a maximum of 0.03 wt.-%.

With an aluminium alloy according to the invention, the contents of Cu, Mn, Zr, and Ti are restricted in each case to a range which is perceptibly more closely restricted than, for example, the prior art known from US 2,706, 680.

At the same time, vanadium, as an impurity, is present only in a content which is of no effect with regard to influencing the properties of the alloy. Surprisingly, it has turned out that, with the maintaining of these narrow alloy limits specified by the invention, an aluminium casting alloy is obtained which allows for the production of castings with excellent strength properties .

The presence of Mn in the contents specified according to the invention leads to a perceptible rise in the hot strength of the alloy according to the invention. The presence of Mn in the contents according to the invention in this situation impedes the speed with which the copper is diffused into the α-aluminium. As a result, Mh in this way stabilizes the strength of the alloy according to the invention even at high operating temperatures. This effect reliably occurs when, with an alloy according to the invention, the content of Mn lies within the range from 0.3 - 0.55 % by weight, in particular more than 0.3 % by weight. Particularly high strength values can be guaranteed in this situation, if the Mn content of the casting alloy according to the invention is 0.4 - 0.55 % by weight .

The presence of Zr in the content ranges specified by the invention also has a favourable effect on the high temperature strength. Thus, Zr contents of 0.07 - 0.3 % by weigh favour the occurrence of disperse precipitations which, with castings cast from the casting alloys according to the invention, in addition to the effect of Mh, guarantee that the alloy according to the invention has high tensile strengths even at high temperatures and a minimised inclination to crack formation.

At the same time, Zr has a favourable effect on the creation of a fine-grain cast structure, which ensures that castings cast from the alloy according to the invention have good elongation and uniform property distribution. Accordingly, for castings cast from the alloy according to the invention a grain size of less than 100 μm can be guaranteed, without additional grain fining means needing to be added to the melt.

The positive effects of the presence of Zr in the alloy according to the invention can be achieved in an optimized manner if the Zr content lies in the range of 0.18 - 0.25 % by weight.

The formation of a fine-grain structure can also be supported in that titanium carbide in small amounts such as 2 kg Al3%TiO,15%C rod per ton Al-meltis added to the melt as a grain fining means . In this context the addition of TiC is particularly advantageous if the Zr content of an alloy composed according to the invention is located in the range of 0.07 - 0.15 wt.-% . Practical studies have shown that the addition of TiC has no effect if the Zr-content of inventive alloy amounts at least to 0.15 wt.-%. The alloy according to the invention does not contain any vanadium because it has been surprisingly ascertained that taking into consideration the alloy prescribed by the invention the presence of Vanadium does not increase the mechanical properties of an aluminium alloy according to the invention in an noteworthy manner. Accordingly the invention proposes to abstain from adding V as an alloying element in order to provide a lower-cost product, which is easy to cast and has very good mechanical properties . This awareness and proposal is in direct contrast to the skilled knowledge documented in US 2,706,680 for example.

The effect of Cu, which enables the precipitation hardening by forming AI₂CU and enhances the strength at high temperatures in an aluminium casting alloy according to the invention reliably occurs when the Cu content is 6 - 8 % by weight, in particular 6.5 - 7.5 % by weight. It was proved in practical tests, for example, that in particular with Cu contents of 7 + 0.5 % by weight, in combination with the other alloy elements, with good casting capacity, even after long application at high temperatures, high strength values are guaranteed with components cast from an alloy according to the invention.

Ti is contained in an alloy according to the invention in contents from 0.05 - 0.2 % supports the precipitation hardening of the inventive alloy. In addition to that it enhances the grain refinement in combination with Zr.

Surprisingly, it has been shown that the aluminium casting alloy according to the invention, after solutionizing at a solutionizing temperature of more than 485²C, controlled cooling, and, following the cooling, soaking of the casting at a temperature of 160 - 240 ^aC over an annealing time of 3 - 14 h, it has a superior tensile strength at room temperature of at least 270 MPa, a yield strength of at least 250 MPa, and an elongation of 0,5 - 10 %.

Particularly worth noting in this context is the fact that the alloy according to the invention, after an application lasting at least 400 h at a temperature of at least 250^aC, at a test temperature of 250^aC has a tensile strength of at least 160 MPa, in particular of at least 200 MPa, and a yield strength of at least 100 MPa, in particular of at least 150 MPa.

Additionally, with regard to the properties being striven for of the alloy according to the invention, it is advantageous if it is made on the basis of primary aluminium. The reduction to a minimum of the impurities which is achieved with the use of primary aluminium as a basis for the alloy according to the invention additionally supports the development of the superior mechanical properties being striven for.

With regard to the method for the manufacture of a casting, the object indicated previously is resolved in that, in the course of such a method according to the invention, the following working steps are run through:

- Smelting of an aluminium casting alloy composed according to the invention, - Casting of the aluminium casting alloy to form the casting,

- Heat treatment of the casting, wherein the heat treatment comprises:

- a single-stage solution annealing at a solutionizing temperature of more than 485²C,

- a controlled cooling of the solutionized casting, and

- after cooling, maintaining the casting over an annealing time of 3 - 14 h at a temperature of 160 - 240 ³C.

so that the heat-treated casting, at an elongation of 0.5 - 10 %, has a tensile strength of at least 270 MPa and a yield strength of at least 250 MPa.

With the method according to the invention, economical castings can be manufactured which, even after a long period of use at high temperatures, still have superior strength and elongation values. Particular significance is attached in this situation to the solutionizing annealing. Inasmuch as this is carried out as one step, only one furnace passage needs to be run through for the solutionizing .

The effort associated with the method according to the invention is minimised accordingly. Surprisingly, it has turned out that, despite the single-step performance of the solutionizing, the castings manufactured from a casting alloy alloyed according to the invention have superior strength and elongation properties . Tests have shown that this effect can surprisingly be particularly effectively used if the solutionizing temperature amounts to at least 530 - 545³C, especially in the range of 538 - 542 ⁰C.

After the solutionizing, the solutionized casting is cooled under controlled conditions. This controlled cooling can, for example, be carried out with a suitable quenching liquid, such as water, or with the aid of an air flow, in which the respective casting is cooled in an accelerated manner.

Quenching with water or comparable media leads to a particularly strong structure. Surprisingly, it has transpired that, for castings cast from an alloy according to the invention and quenched with water, even after an application lasting for at least 400 h, during which the castings are subjected to a temperature of at least 250²C, at a test temperature of 250²C a tensile strength of at least 200 MPa and a yield strength of at least 150 MPa is guaranteed.

If, with particularly complex-shaped components, with forced cooling the risk pertains of the occurrence of stresses or cracks, then, as an alternative to quenching with a liquid, regulated cooling can take place in an air flow. In this case too, castings composed and manufactured according to the invention attain a high strength level, which is retained even at high temperatures. Thus, for castings produced in accordance with the invention and cooled in an airflow, after an application of at least 400 h duration, during which they are likewise subjected to a temperature of at least 250³C, at a test temperature of 250^aC a tensile strength of at least 160 MPa and a yield strength of at least 100 MPa can be guaranteed.

A temperature loading of 250²C over, for example, 500 h, or 290 ^aC over 400 h, corresponds to the thermal load which the components of a typical car internal combustion engine are subjected to overall in the course of their average service life. Accordingly, particularly high- value castings for internal combustion engines can be manufactured from the alloy according to the invention and in the manner according to the invention, for which it is guaranteed over their entire service life that they will satisfy even the most stringent demands on their mechanical properties .

In particular, because of its good casting capacity and other property profile, the alloy according to the invention can be used for the manufacture of filigree- shaped cylinder heads or engine blocks for internal combustion engines, with which, even after a long period of use at high temperatures, superior high strength values and elongation properties are still guaranteed. This applies in particular if the method according to the invention is applied during the casting production of these engine castings .

For hardening, the castings cast from the alloy according to the invention are soaked after controlled cooling at temperatures of 160 - 260 ⁰C. Particularly good results from this precipitation hardening are achieved if the cast component is soaked for an annealing time of 4 to 12 h, at a temperature of 180 - 220 ²C. With regard to the saving of costs short soaking times of 4 to 10 h are preferred.

To prove the effect of the invention, an AlCu7MnTiZr- alloy, of which the composition is indicated in table 1 was melted and cast into bars .

The bars were re-melted, degassed and cleaned in a commercial rotary melt cleaning unit.

18 cylinder heads Cl/1, Cl/2 , Cl/3 to C6/1, C6/2 , C6/3 were cast in six castings Cl to C6 from the melt obtained without adding a refining agent.

After the sixth casting, 2 kg/t AlTiC were added to the melt for grain refinement. Due to the low addition of AlTiC grain refiner the Ti-content of the melt was not raised. Accordingly, the alloy composition kept unaffected during all casting trials.

After the addition of the TiC grain refiner additional 12 cylinder heads C7 /1,C7 /2 , C7 /3 to ClO/1, CIO/2 , ClO/3 were cast in additional four castings.

The casting temperature was constantly kept at 750 ⁰C for all castings. After complete solidification the cylinder heads Cl/1 to CIO/3 were given a solution annealing at 540 ⁰C for 8 hours .

After solutionizing the cylinder heads Cl/1 to CIO/3 undergo a controlled cooling. For the cylinder heads Cl/1 to C3/3 and C7/1 to C8/3 this controlled cooling was performed as quenching in water, which had a temperature of 80 ⁰C, whereas the cylinder heads C4/1 to C6/3 and C9/1 to CIO/3 were cooled in an air stream.

Following to the cooling step, aging of the cylinder heads Cl/1 to ClO/3 to cause precipitation hardening was performed during which the cylinder heads Cl/1 to ClO/3 were held at 190 ⁰C for 12 hours.

To simulate the thermal stress the cylinder heads would be exposed during there life cycle as part of a combustion engine for a vehicle the cylinder heads C3/1 to C3/3 and C6/1 to C6/3 were subjected to a post-aging heat treatment during which they were held at 250 ⁰C for 500 hours.

For cylinder head Cl/1 which was cast from the melt before the addition of a grain refiner as well as for the cylinder heads Cl/1, C8/1 which were cast after the TiC addition as grain refiner the grain sizes were determined. It has been found that as indicated in Table 2 neither TiC addition nor the time passed between the addition of the grain refiner and the casting have an influence on the grain size. The differences in grain sizes given in Table 2 are within the scatter of results. For all specimens the grain size was at a remarkable low level, namely between 84 μ.m and 98 μm. It is assumed that the Zircon contained in the melt already refines the grain size efficiently so that at higher Zr-contents of for example 0.15 - 0.25 wt.-% the addition of a grain refiner to the melt is not necessary.

For the cylinder heads Cl/1 to Cl/3, C4/1 to C4/3, C7/1 to Cl /3 and C9/1 to C9/3 the mechanical properties were determined at room temperature. Furthermore, for the cylinder heads C2/1 to C2/3, C5/1 to C5/3, C8/1 to C8/3 and ClO/1 to ClO /3 the mechanical properties were determined at 250 ⁰C. Subsequently the mechanical properties have been determined for the post-aged cylinder heads C3/1 to C3/3 and C6/1 to C6/3 were determined at a test temperature of 250 ⁰C. The obtained results are summarised in Table 3.

To summarise the results of the analyses in Table 4 the average mechanical properties at 250 ⁰C before and after TiC addition are indicated. It can be concluded that for the alloy given in Table 1 the addition of TiC does not have an influence on the mechanical properties. This corresponds to the results from grain size measurements.

The average values of the mechanical properties at room temperature without post-aging, 250 ⁰C without post-aging and at 250 ⁰C after post-aging are summarised in Table 5.

Remainder: primary aluminium and unavoidable impurities

Table 1

*) tC = Time passed between TiC-addition and casting

Table 2

¹) "HT" = Heat Treatment: "A" cold with an air stream;

"W" quenched in water;

²) Aging: "P" Post-aging; "N" none;

³) "T_test" = Test temperature; "RT": Room temperature;

⁴) "YS" = Yield Strength; "UTS" = Ultimate Tensile Strength; ^■El" =

Elongation;

Table 3

^■"^■) "tC" = Time passed between TiC-addition and casting 2) "HT" = Heat Treatment: "A" cold with an air stream; "W" quenched in water of 80 ⁰C;

3\ ) "T_fc_est" = Test temperature; 4) "YS" = Yield Strength; "UTS" = Tensile Strength; 'El" = Elongation;

Table 4

j.) n_HTn _ Heat Treatment: A" cold with an air stream;

"W" quenched in water;

²) Aging: "P" Post-aging; "N¹ none ;

³) "T_test" = Test temperature; "RT": Room temperature;

⁴) "YS" = Yield Strength; "UTS" = Tensile Strength; "El" = Elongation;

Table 5

Claims

C L A I M S

1. Aluminium casting alloy, consisting of (in wt.-%) : Cu: 2 - 8 %,

Mn: 0.2 - 0.6 %,

Zr: 0.07 - 0.3 %,

Fe: ≤ 0.25 %,

Si: < 0.3 %,

Ti: 0.05 - 0.2 %,

V: < 0.04 %, remainder Al and unavoidable impurities, wherein the total content of impurities amounts to a maximum of 0,10 wt.-%.

2. Aluminium casting alloy according to Claim 1, c h a r a c t e r i s e d i n t h a t its content of Mn amounts to 0.3 - 0.55 % by weight.

3. Aluminium casting alloy according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t its content of Zr amounts to 0.15 - 0.25 % by weight .

4. Aluminium casting alloy according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t its content of Cu amounts to 6 - 8 % by weight .

5. Aluminium casting alloy according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t the Si content is in each case restricted to a maximum of half of the Fe content.

6. Aluminium casting alloy according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t it is made from primary aluminium.

7. Aluminium casting alloy according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t after a solution annealing at a solutionizing temperature of more than 485²C, controlled cooling, and, following the cooling, soaking at a temperature of 180 - 240 ²C over an annealing time of 3 - 14 h, it has a tensile strength at room temperature of at least 270 MPa, a yield strength of at least 250 MPa, and an elongation of 0.5 - 10 %.

8. Aluminium casting alloy according to Claim 7, c h a r a c t e r i s e d i n t h a t after an application in which it is subjected for at least 400 h to a temperature of at least 250^sC, at a test temperature of 250⁹C it has a tensile strength of at least 160 MPa and a yield strength of at least 100 MPa.

9. Aluminium casting alloy according to Claim 8, c h a r a c t e r i s e d i n t h a t after an application in which it is subjected for at least 400 h to a temperature of at least 250²C, at a test temperature of 250²C it has a tensile strength of at least 200 MPa and a yield strength of at least 150 MPa.

10. Method for the manufacture of a casting from an aluminium casting alloy produced according to one of Claims 1 to 9 , comprising the following working steps :

- Smelting of the aluminium casting alloy,

- Casting of the aluminium casting alloy to form the casting,

- Heat treatment of the casting, wherein the heat treatment comprises

- a controlled cooling of the solutionized casting and

- after cooling, soaking of the casting over an soaking time of 3 - 14 h at a soaking temperature of 160 - 220 ²C,

- so that the heat-treated casting at room temperature has a tensile strength of at least 270 MPa, a yield strength of at least 250 MPa and an elongation of 0.5 - 10 %.

11. Method according to Claim 10, c h a r a c t e r i s e d i n t h a t the heat- treated casting has a fine-grained structure with grain sizes < 100 μm.

12. Method according to one of Claims 10 or 11, c h a r a c t e r i s e d i n t h a t for grain refinement TiC is added to the melt.

13. Method according to claim 12, c h a r a c t e r i s e d i n t h a t the Zr content of the melt is 0.07 - 0.15 % by weight.

14. Method according to one of Claims 10 to 13, c h a r a c t e r i s e d i n t h a t the solutionizing temperature is in the range of

530 - 545⁰C, especially in the range of 538 ²C to 542 ⁰C.

15. Method according to one of Claims 10 to 14, c h a r a c t e r i s e d i n t h a t the solutionized casting is quenched with water during the controlled cooling.

16. Method according to Claim 15, c h a r a c t e r i s e d i n t h a t the casting, after an application in which it is subjected for at least 400 h to a temperature of at least 250^aC, at a test temperature of 250²C has a tensile strength of at least 200 MPa and a yield strength of at least 150 MPa.

17. Method according to one of Claims 10 to 14, c h a r a c t e r i s e d i n t h a t, during the controlled cooling, the solutionized casting is cooled in an accelerated manner in an air flow.

18. Method according to Claim 17, c h a r a c t e r i s e d i n t h a t the casting, after an application in which it is subjected for at least 400 h to a temperature of at least 250²C, at a test temperature of 250^aC has a tensile strength of at least 160 MPa and a yield strength of at least 100 MPa.

19. Method according to one of Claims 10 to 18, c h a r a c t e r i s e d i n t h a t, in the course of the heat treatment, after the controlled cooling, the casting is soaked over a soaking time of 4 - 12 h at a temperature of 180 - 220 ²C.

20. Casting for an internal combustion engine, cast from an aluminium casting alloy composed according to one of Claims 1 to 9.

21. Casting according to Claim 20, c h a r a c t e r i s e d i n t h a t it is manufactured by the method defined according to one of Claims 10 to 19.

22. Casting according to one of Claims 20 or 21, c h a r a c t e r i s e d i n t h a t at room temperature it has a tensile strength of at least 270 MPa, a yield strength of at least 250 MPa, and an elongation of 0.5 - 10 %.

23. Casting according to one of Claims 20 - 22, c h a r a c t e r i s e d i n t h a t, after an application in which it is subjected for at least 400 h to a temperature of at least 250²C, at a test temperature of 250²C it has a tensile strength of at least 160 MPa and a yield strength of at least 100 MPa.

24. Casting according to Claim 23, c h a r a c t e r i s e d i n t h a t, after an application in which it is subjected for at least 400 h to a temperature of at least 250²C, at a test temperature of 250²C it has a tensile strength of at least 200 MPa and a yield strength of at least

150 MPa.

25. Use of an aluminium casting alloy procured according to one of Claims 1 to 9 , for the manufacture of castings for internal combustion engines.

26. Use according to Claim 25, c h a r a c t e r i s e d i n t h a t the casting is a cylinder head for an internal combustion engine .

27. Use according to Claim 25, c h a r a c t e r i s e d i n t h a t the casting is an engine block for an internal combustion engine .