EP2450463B1 - Aluminium alloy - Google Patents

Description

Many components of motor vehicles are made of a steel or cast iron. Steels or cast iron have the advantage of very high strength and are also cheap base materials. A disadvantage of steels or cast iron, however, is that a casting process is usually expensive, corrosion resistance can be low and, in principle, a higher production cost is given in comparison with aluminum alloys. In addition, steels and cast iron also have a higher density, which adversely affects a weight of a vehicle component and is often undesirable in view of a comparatively higher fuel consumption.

It is therefore desirable to produce vehicle components made of light metal alloys, in particular aluminum alloys. In this regard, various alloy systems are available, each having specific advantages, but also having disadvantages. For example, alloys of the aluminum-silicon-magnesium alloy system are easy to cast, are relatively easy to machine mechanically and are generally resistant to corrosion, but often have low strength. Alloys of the aluminum-copper-titanium alloy system can achieve a high theoretical strength, but are often not resistant to corrosion and, as a rule, poorly cast.

EP 1 215 295 A1 discloses an aluminum casting alloy with 5 to 10% silicon.

EP 1136 581 discloses an aluminum alloy with 1.5-2.2 Fe.

In the aluminum-silicon-magnesium alloy system, casting alloys containing 7 to 17% by weight and up to 0.7% by weight of magnesium or wrought alloys containing less than 1% by weight of silicon and less than 1% by weight of magnesium are frequently used, depending on the component and component geometry. For example, alloys with the designations AlSi7Mg0.6 or AC72 or AlSi0.5Mg or AC04 are used for the production of vehicle components. The AlSi7Mg0.6 casting alloy, which consists essentially of about 7% silicon by weight, about 0.6% magnesium by weight, balance aluminum, has high strength, but an elongation at break is too low for some applications. In addition, points this alloy has only low heat resistance. The wrought AlSi0.5Mg alloy, which is sometimes used as a casting alloy and consists essentially of about 0.5 weight percent silicon, about 0.5 weight percent magnesium, balance aluminum, on the other hand, has a high elongation at break, but at the expense of strength , which is low and therefore unsatisfactory for many purposes.

This is where the invention starts. The object of the invention is to provide an aluminum alloy, which has a high strength with high elongation at break and good heat resistance, without a corrosion resistance is insufficient.

This object is achieved by an aluminum alloy containing (by weight)
0.3 to 1.5% cobalt
1.0 up to 2.5% nickel
more than 0 up to 1.5% magnesium
more than 0 up to 1.5% silicon
optionally more than 0 up to 1.0% silver
optionally more than 0 up to 0.20% titanium and / or boron
optionally more than 0 up to 0.003% beryllium
Remaining aluminum and production-related impurities, with a maximum iron content of up to 0.5%.

The advantages achieved with an aluminum alloy according to the invention can be seen in particular in the fact that the alloy has a high strength combined with a high elongation at break, a high heat resistance of the alloy and, moreover, that the alloy is extremely resistant to corrosion.

In the alloy according to the invention, cobalt dissolves iron present in the aluminum in formed intermetallic cobalt-aluminum phases. This leads to a good ductility of the aluminum alloy because no or only small amounts of acicular aluminum-iron phases are present. In addition, cobalt contributes to the increase in strength. Cobalt is mandatory, in concentrations of 0.3 up to 1.5%. It is preferably provided that a cobalt content is 0.30 to 0.80%, particularly preferably 0.40 to 0.60%, in particular 0.45 to 0.55%.

The aluminum alloy according to the invention further contains nickel as a mandatory constituent, with a nickel content basically being 1.0 to 2.5%, but preferably being in a range of 1.0 to 2.0%. Nickel contributes to increasing the strength of the aluminum alloy at both room temperature and high temperature, that is at more than 200 ° C, by forming an intermetallic aluminum-nickel phase. In this regard, it may be favorable that a nickel content is 1.2 to 1.6%, in particular 1.4 to 1.5%.

Magnesium is mandatory in an aluminum alloy according to the invention with a content of more than 0 up to 1.5%. Magnesium, in conjunction with the silicon also provided, serves to increase the strength in the heat-treated state, wherein a content of magnesium is oriented to a maximum solubility at an annealing temperature which is generally in the range of about 570 ° C. It is favorable if a magnesium content is 0.8 to 1.2%, in particular 1.0 to 1.1%.

Silicon is provided at levels of greater than 0 to 1.5%. As already mentioned, a content of silicon is to be considered in connection with a content of magnesium, so that the desired strength increase is achieved in the heat-treated state. A silicon content is therefore tuned to a magnesium content, however, with respect to magnesium, an excess of silicon is used. In general, an excess of silicon is provided in the amount of about one-fourth of the iron content of the alloy in order to fully exploit the precipitation potential by the excess silicon or to prevent activity reduction by the existing iron, which is usually up to 0.2%, in particular 0.1 to 0.2%, and is introduced as an impurity of the aluminum used in the alloy. A content of iron can be up to 0.5%. It is expedient if the aluminum alloy contains 0.20 to 0.80%, in particular 0.55 to 0.70%, silicon, in each case based on the preferred magnesium contents given above.

Another component which may be provided in an aluminum alloy according to the invention is silver, in a content of more than 0 up to 1.0%. Silver, like magnesium and silicon, provides high heat-treated strength but is not as temperature-sensitive as Mg ₂ Si phases.

Moreover, in comparison with copper as an alloying element, silver proves to be better in terms of corrosion resistance. It is favorable if a silver content is 0.05 to 0.70%, in particular 0.10 to 0.55%.

In addition, an aluminum alloy according to the invention may optionally also contain titanium and / or boron and also beryllium. A titanium and / or boron content may be up to 0.20% in total, but is preferably in the range of 0.001 to 0.15%, more preferably in the range of 0.001 to less than 0.02%. Titanium and boron act as grain refining elements and can be added in the production of the aluminum alloy, for example by a master alloy of the type AlTi5B1, which master alloy 5 weight percent titanium, 1 weight percent boron, balance aluminum. Beryllium serves to suppress as far as possible the evaporation of magnesium during the production of an aluminum alloy. A beryllium content can be up to 0.003%.

The alloy according to the invention is advantageously substantially copper-free and has a copper content of less than 0.005%, since copper can lead to disadvantageous, low-melting phases.

• Fig. 1 Zugfestigkeiten bzw. R_m-Werte von Legierungen;
• Fig. 2 Dehngrenzen bzw. R_p0,2-Werte von Legierungen;
• Fig. 3 Härtewerte von Legierungen;
• Fig. 4 Bruchdehnungen bzw. A₅-Werte von Legierungen;
• Fig. 5 eine vergleichende Darstellung mechanischer Eigenschaftswerte verschiedener Legierungen;
• Fig. 6 ein Schliffbild einer Aluminiumlegierung, die weder Cobalt noch Nickel enthält;
• Fig. 7 ein Schliffbild einer Aluminiumlegierung, die Cobalt und Nickel enthält;
• Fig. 8 mechanische Eigenschaften untersuchter Legierungen, die nicht oder bei unterschiedlichen Temperaturen geglüht wurden;
• Fig. 9 Korrosionskurven für eine untersuchte Legierung sowie eine Referenzlegierung;
• Fig. 10 einen metallografischen Schliff einer erfindungsgemäßen Aluminiumlegierung;
• Fig. 11 Zugfestigkeiten bzw. R_m-Werte von Legierungen bei erhöhten Temperaturen;
• Fig. 12 Dehngrenzen bzw. R_p0,2-Werte von Legierungen bei erhöhten Temperaturen;
• Fig. 13 Bruchdehnungen bzw. A₅-Werte von Legierungen bei erhöhten Temperaturen.

Further features, advantages and effects of the invention will become apparent from the embodiments illustrated below. In the drawings show:

• Fig. 1 Tensile strengths or R _m values of alloys;
• Fig. 2 _Elongation limits or R _p0.2 values of alloys;
• Fig. 3 Hardness values of alloys;
• Fig. 4 Elongation at break or A ₅ values of alloys;
• Fig. 5 a comparative representation of mechanical property values of different alloys;
• Fig. 6 a microsection of an aluminum alloy containing neither cobalt nor nickel;
• Fig. 7 a micrograph of an aluminum alloy containing cobalt and nickel;
• Fig. 8 mechanical properties of tested alloys that have not been annealed or at different temperatures;
• Fig. 9 Corrosion curves for a tested alloy and a reference alloy;
• Fig. 10 a metallographic cut of an aluminum alloy according to the invention;
• Fig. 11 Tensile strengths or R _m values of alloys at elevated temperatures;
• Fig. 12 _Elongation limits or R _p0.2 values of alloys at elevated temperatures;
• Fig. 13 Elongation at break or A ₅ values of alloys at elevated temperatures.

For comparative studies, the reference alloys AlSi7Mg0,6 or AC72 and AlSi0.5Mg or AC04 were assumed, which have a high strength and a high elongation at break. As target criteria, no or only small proportions of below the eutectic melting temperature melting phases, high eutectic temperatures and taking advantage of a full Mg ₂ Si precipitation hardening were specified.

The starting point for the comparative investigations was the binary Al-Mg ₂ Si cut in the ternary phase system aluminum-silicon-magnesium, since the eutectic temperature has a maximum in this range. Four experimental alloys were defined in the hypoeutectic region. In addition, trial alloys were defined which additionally contained cobalt or both cobalt and nickel. The chemical compositions of the trial alloys are shown in Table 1 below. As far as a T6 heat treatment was performed, this was done by heating to annealing temperature (540 ° C for AlSi7Mg0.6 or 570 ° C for AlSi0.5Mg or 565 ° C to 570 ° C for the trial alloys) for two hours, followed by quenching in a water bath a temperature of about 25 ° C, followed by heating for one hour to a hot aging temperature of 190 ° C and finally three hours holding at Ausauslagerungstemperatur and subsequent air cooling of the experimental alloys to room temperature, ie about 25 ° C. Due to the relatively short removal time, the experimental alloys were not outsourced to the maximum hardness, but to hardness values that were about 10 to 15% below the maximum achievable values. Table 1 - chemical compositions of trial alloys attempt Composition (in weight percent) T6 heat treatment mg Si Co Ni al A1 1.1 0.65 0 0 rest No A2 1.1 0.65 0 0 rest Yes A3 1.1 0.65 0.45 0 rest No A4 1.1 0.65 0.45 0 rest Yes A5 1.1 0.65 0.45 1.45 rest No A6 1.1 0.65 0.45 1.45 rest Yes B1 3.7 2.1 0 0 rest No B2 3.7 2.1 0 0 rest Yes B3 3.7 2.1 0.45 0 rest No B4 3.7 2.1 0.45 0 rest Yes B5 3.7 2.1 0.45 1.45 rest No B6 3.7 2.1 0.45 1.45 rest Yes C1 6.2 3.6 0 0 rest No C2 6.2 3.6 0 0 rest Yes C3 6.2 3.6 0.45 0 rest No C4 6.2 3.6 0.45 0 rest Yes C5 6.2 3.6 0.45 1.45 rest No C6 6.2 3.6 0.45 1.45 rest Yes D1 8.8 5.1 0 0 rest No D2 8.8 5.1 0 0 rest Yes D3 8.8 5.1 0.45 0 rest No D4 8.8 5.1 0.45 0 rest Yes D5 8.8 5.1 10.45 1.45 rest No D6 8.8 5.1 0.45 1.45 rest Yes

In Fig. 1 to 4 are mechanical property values of alloys, optionally after the T6 heat treatment as described above, for the alloys shown in Table 1, wherein Fig. 1 the tensile strengths R _m shows, Fig. 2 the yield strengths R _p0,2 , Fig. 3 the Brinell hardnesses and Fig. 4 finally, the elongations at break A ₅ . The lines 1 to 6 in Fig. 1 to 4 connect in each case those values which were obtained for the experiments A1, B1, C1, D1 to A6, B6, C6, D6, ie the numerically identically marked alloys.

As can be seen from a synopsis of Fig. 1 to 4 results, the alloys according to the experiments A5 and A6 by comparison the most balanced and thus desired property profile. In comparison with the selected reference alloys, an alloy according to experiment A6 also has a more balanced property profile, which is reflected in Fig. 5 is apparent.

In order to further improve an alloy according to experiment A5 or A6, silver was added in further experiments in amounts of 0.25% and 0.50%, respectively, whereby a further increase in strength could be effected at approximately constant elongation at break. An increase in strength was compared with that among others in Fig. 5 again about 20% for the alloy according to experiment A6.

In Fig. 6 is a microsection of an alloy shown in Experiment A2. As can be seen, one recognizes a pure mixed crystal structure and occasionally iron-rich needles. In Fig. 7 is a microsection of an alloy shown in experiment A6. It is compared with Fig. 6 recognizable that changes the structure by adding cobalt and nickel. Cobalt- and / or nickel-rich phases form at the grain boundaries and therefore give the microstructure a little more stability, which can also be understood on the basis of the mechanical properties already discussed. Iron-rich precipitates in the form of oblong needles are unlike Fig. 6 no longer present, since iron dissolves in the cobalt-rich phase. This could be verified by scanning electron microscopy. On the basis of local chemical analyzes it could be determined that selectively a cobalt content as well as an iron content far exceeded the corresponding contents in the matrix. These results confirm the assumption that a cobalt-aluminum phase has high solubility for iron. As a result, so disadvantageous iron-aluminum precipitates are avoided with unfavorable needle-shaped structure, if cobalt is provided.

A major advantage of the alloys according to experiments A5 and A6, respectively, is that higher annealing temperatures of up to about 570 ° C can be used instead of the usual annealing temperatures of about 535 ° C for standard alloys such as AC72. Due to the higher annealing temperatures more precipitation-relevant elements can be solved in mixed crystal, which leads to an increase in the achievable maximum strength. This is in Fig. 8 illustrated.

In Fig. 9 are corrosion measurements on an alloy A6Ag according to test A6, which additionally contained 0.45% silver, compared with a reference alloy AlSi7Mg0,6 or AC72 shown. To carry out the measurements, sample bodies were introduced into a test solution (0.6 g / l NaCl, diluted 1:10 in borate buffer at a pH of about 6.4) and subjected to a voltage. As counter electrode was a Platinum electrode used. As from the illustrated corrosion curves in Fig. 9 As can be seen, the corrosion behavior is comparable, but the alloy A6Ag has advantages in the passivation range. That is, this alloy has lower corrosion rates for an existing surface defect than an AlSi7Mg0.6 reference alloy.

An alloy according to the invention has, as in Fig. 10 is apparent, a globulitic structure on. Such a globulitic structure is favorable in terms of dynamic properties of an alloy.

In further experiments, the casting properties of an alloy were tested according to experiment A5. In the so-called Taturkokillenversuch a Lunkerverhalten and in a Fließlängenkokillenversuch a flow behavior was determined. Finally, a susceptibility of the alloy to hot cracks during casting was investigated in a hot-crack test specimen test. It was found that the alloy formed a large macro-shrink in the Tobby mold trial, which is more favorable than an alternative design of micro-shrinkers. The flow-length mold test showed that the alloy had a flow behavior similar to that of a AlSi7Mg0.6 reference alloy, which was assigned a good flow behavior. Only with respect to the hot tear test mold experiment, it was found that the alloy had a higher hot crack index than an AlSi7Mg0.6 reference alloy, but this can be compensated or reduced in practice by a suitable choice of the casting system.

Further investigations showed that a heat capacity of the alloy according to experiment A5 is comparable to an AlSi7Mg0.6 reference alloy, which is favorable. A thermal expansion is comparatively slightly higher. It is also favorable that a thermal diffusivity is better than with an AlSi7Mg0.6 reference alloy, which may be advantageous in particular when used as a cylinder head material, for example.

A main field of application of an alloy according to experiment A5 or A6 is in particular an application for an engine component, for example as a material for a cylinder head. In further experiments, therefore, the alloy was tested A5 compared to a common cylinder head alloy, namely AlSi7Mg0.3Cu, containing about 7 weight percent silicon, about 0.3 weight percent magnesium, about 0.5 weight percent copper, balance aluminum, as reference alloy. It was found that an alloy according to experiment A5, which was subjected to a T6 heat treatment (outsourced to hardness maximum) or a T7 heat treatment (outsourced for 500 hours at 225 ° C), less in strength at elevated temperature, which was Fig. 11 to 13 for the T7 heat treatment. An alloy A5Ag according to experiment A5, but additionally with 0.45% silver, gave even better values in terms of heat resistance.

In terms of dynamic properties, particularly thermo-mechanical fatigue, an alloy according to Experiment A6, which additionally contained 0.45% silver, exhibits better properties at higher temperatures compared to a conventional AlSi7Mg0.3Cu cylinder head alloy, and thus also lends more suitability for use purposes because the time-stability line has a larger slope, as shown in Table 2. Table 2 - Characteristics of the Wöhler Line Test temperature (° C) alloy Characteristics Wöhlerlinie Slope k of the time stability line Corner load number N _E σ _dubw (N / mm ² ) 25 A6Ag -6.21 6325571 97.90 AlSi7Mg0,3Cu -5.29 4872011 105.27 200 A6Ag -4.11 6123914 68.64 AlSi7Mg0,3Cu -5.39 6503183 78.43

Claims (16)

1. An aluminium alloy containing (as a percentage by weight)
   0.3 to up to 1.5 % cobalt
   1.0 to up to 2.5 % nickel
   more than 0 to up to 1.5 % magnesium
   more than 0 to up to 1.5 % silicon
   optionally more than 0 to up to 1.0 % silver
   optionally more than 0 to up to 0.20 % titanium and/or boron
   optionally more than 0 to up to 0.003 % beryllium
   remainder aluminium and production-related impurities, wherein an iron content is up to a maximum of 0.5 %.
2. The aluminium alloy according to claim 1, containing 0.30 to 0.80 % cobalt.
3. The aluminium alloy according to claim 1, containing 0.40 to 0.60 % cobalt.
4. The aluminium alloy according to claim 1, containing 0.45 to 0.55 % cobalt.
5. The aluminium alloy according to claim 1, containing 1.0 to 2.0 % nickel.
6. The aluminium alloy according to claim 1, containing 1.2 to 1.6 % nickel.
7. The aluminium alloy according to claim 1, containing 1.4 to 1.5 % nickel.
8. The aluminium alloy according to claim 1, containing 0.80 to 1.20 % magnesium.
9. The aluminium alloy according to claim 1, containing 1.0 to 1.1 % magnesium.
10. The aluminium alloy according to claim 1, containing 0.20 to 0.80 % silicon.
11. The aluminium alloy according to claim 1, containing 0.55 to 0.70 % silicon.
12. The aluminium alloy according to claim 1, containing 0.05 to 0.70 % silver.
13. The aluminium alloy according to claim 1, containing 0.10 to 0.55 % silver.
14. The aluminium alloy according to claim 1, containing 0.001 to 0.15 % titanium and/or boron.
15. The aluminium alloy according to claim 1, containing 0.001 to less than 0.02 % titanium and/or boron.
16. The aluminium alloy according to claim 1, which is substantially free of copper.

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