EP3048179B1 - Method for forming complex cast parts and cast part consisting of an AlCu alloy - Google Patents

Description

The invention relates to a method for producing complex shaped castings from an AlCu alloy.

When referring to the content of alloying elements, these are based on the weight of each alloy, unless expressly stated otherwise.

AlCu alloys of the type in question existing castings have particularly high strengths, especially at elevated temperatures of more than 250 ° C. However, this is offset by poor casting properties, which complicate the casting-technical production of components that are characterized by a complex shape.

Typical examples of such castings are cylinder heads intended for internal combustion engines, which on the one hand are exposed to high temperatures in practical use and, on the other hand, have a compact design in which filigree shaped mold elements, such as cooling and oil passages, recesses, lands, guides and the like, are molded.

A significant problem in the processing of essentially Si-free AlCu alloys results from their high susceptibility to hot cracking and a refilling behavior that is significantly worse than with conventional AlSi alloys.

From the WO 2008/072972 A1 For example, a method of making complex shaped castings from an AlCu alloy is known which consists of (in wt%) 2 - 8% Cu, 0.2 - 0.6% Mn, 0.07 - 0.3% Zr , up to 0.25% Fe, up to 0.3% Si, 0.05-0.2% Ti, up to 0.04% V and the remainder being Al and unavoidable impurities, the sum of the contents being Impurities is not more than 0.1%. The presence of Zr is attributed a special significance with regard to the production of a fine microstructure with particle sizes of at most 100 μm.

In order to improve the fineness of the casting structure, a grain refining agent, such as, for example, TiC in a dosage of typically 2 kg per ton of melt, may additionally be added prior to casting in carrying out the known method of a correspondingly composed melt. The casting obtained after casting and solidification is subjected to a heat treatment, in which it is first solution-annealed at 530-545 ° C. From the solution annealing temperature, the casting is accelerated by means of water or in the air stream accelerated, in particular, the quench with water is considered to be advantageous in terms of the desired high strength, but the cooling in the air stream is recommended in the case that the casting, due to its complex shape tends to crack during faster cooling. After quenching, the casting is held at a temperature of 160-240 ° C over a period of 3 to 14 hours to increase the hardness of the structure.

Tests for the practical implementation of the known method have shown that although the known alloy has advantages in terms of material properties, which makes it particularly interesting for the casting production of cylinder heads for internal combustion engines. However, it does not succeed on an industrial scale with the necessary reliability to produce castings from this alloy with the known method, which withstand the demands placed on them in practical use.

Thus, it has been shown that the grain size of each casting obtained in fact varies extremely depending on the casting. Thus, for example, a mean particle size of about 100 μm could be determined on a very large specimen which has solidified very slowly. If, however, a smaller piece is removed from this test piece, it melts again and then solidifies again very quickly, so despite the fast solidification rate, contrary to expectations, grain sizes of 500 to 900 μm are reached. Castings with such a coarse structure are completely inadequate for the application to which the methods in question are directed.

Against the background of the prior art, therefore, the object was to provide a method which allows in a practical, reliable manner, the production of castings from an AlCu alloy of the known type.

With respect to the method, the invention has achieved this object in that in the production of castings made of an AlCu alloy, the steps specified in claim 1 are completed.

Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

1. a) Erschmelzen einer AlCu-Legierung, die aus (in Gew.-%)
   • Cu: 6 - 8 %,
   • Mn: 0,3 - 0,55 %,
   • Zr: 0,15 - 0,25 %,
   • Fe: bis zu 0,25 %,
   • Si: bis zu 0,125 %,
   • Ti: 0,05 - 0,2 %,
   • V: bis zu 0,04 %,
   • Rest Al und unvermeidbare Verunreinigungen, besteht;
2. b) Halten der Schmelze bei einer 730 - 810 °C betragenden Haltetemperatur über eine Haltedauer von 4 - 12 Stunden;
3. c) Durchmischen der Schmelze;
4. d) Entnehmen einer Schmelzenportion aus der Schmelze;
5. e) Vergießen der aus der Schmelze entnommenen Schmelzenportion zu dem Gussteil;
6. f) Lösungsglühen des Gussteils bei einer 475 - 545 °C betragenden Lösungsglühtemperatur über eine Lösungsglühdauer von 1 - 16 Stunden;
7. g) Abschrecken des Gussteils von der Lösungsglühtemperatur auf eine höchstens 300 °C betragende Abschreckstopptemperatur, wobei das Gussteil mindestens im Temperaturbereich von 500 - 300 °C mit einer Abkühlrate von 0,75 - 15 K/s abgeschreckt wird;
8. h) Warmauslagern des Gussteils, wobei das Gussteil während des Warmauslagerns über eine Dauer von 1 - 10 Stunden bei einer 150 - 300 °C betragenden Warmauslagerungstemperatur gehalten wird;
9. i) Abkühlen des Gussteils auf Raumtemperatur.

A method according to the invention for casting filigree castings thus comprises the following steps:

1. a) melting of an AlCu alloy consisting of (in% by weight)
   • Cu: 6 - 8%,
   • Mn: 0.3-0.55%,
   • Zr: 0.15-0.25%,
   • Fe: up to 0.25%,
   • Si: up to 0.125%,
   • Ti: 0.05-0.2%,
   • V: up to 0.04%,
   • Rest Al and unavoidable impurities, consists;
2. b) holding the melt at a hold temperature of 730 - 810 ° C for a holding period of 4 - 12 hours;
3. c) mixing the melt;
4. d) removing a melt portion from the melt;
5. e) casting the melt portion removed from the melt into the casting;
6. f) solution annealing the casting at a solution annealing temperature of 475-545 ° C for a solution annealing time of 1-16 hours;
7. g) quenching the casting from the solutionizing temperature to a quenching stop temperature of at most 300 ° C, wherein the casting is quenched at a temperature of at least 500-300 ° C at a cooling rate of 0.75-15 K / s;
8. h) hot aging of the cast part, wherein the cast part is kept during the aging process for a period of 1 - 10 hours at a 150 - 300 ° C amount of hot aging temperature;
9. i) cooling the casting to room temperature.

The inventive method is based on that of the already mentioned WO 2008/072972 A1 known AlCu alloy and provides a casting that meets the highest demands on its performance in practical use.

Copper is present in the inventively processed alloy in contents of 6-8 wt .-%, in order to achieve the required heat resistance of the casting to be produced. Optimum properties in this respect are achieved when the Cu content of the alloy processed according to the invention is 6.5-7.5% by weight.

Manganese in contents of 0.3-0.55% by weight supports the diffusion of Cu in the Al matrix of the structure of a component produced according to the invention and thus stabilizes the strength of the alloy according to the invention even at high operating temperatures. This effect is achieved particularly reliably when the Mn content is 0.4-0.55% by weight.

Zirconium has a special significance for the heat resistance of castings produced according to the invention. For example, Zr contents of 0.15-0.25% by weight favor the formation of disperse precipitates which ensure that the castings cast from casting alloys according to the invention have a fine microstructure which is optimally uniform over the casting volume Distribution of mechanical properties and a minimized tendency to crack has. These advantages can be achieved particularly reliably if the Zr content of the alloy processed according to the invention is 0.18-0.25% by weight, in particular 0.2-0.25% by weight.

Iron is undesirable in an alloy according to the invention because it tends to form brittle phases.

Therefore, the Fe content is limited to a maximum of 0.25 wt .-%, preferably 0.12 wt .-%.

The content limit prescribed for the Si content according to the invention is at most 0.125% by weight, because with higher contents of Si the risk of the formation of hot cracks increases. Negative effects of Si on the properties of an alloy according to the invention can be safely excluded by limiting the Si content to at most 0.06% by weight.

Ti at levels of 0.05-0.2% by weight, especially 0.08-0.12% by weight, like Zr also contributes to grain refining. Grain refining can also be supported by adding up to 0.04% by weight V. This applies in particular when 0.01-0.03% by weight of V is present in the alloy processed according to the invention.

The sum of the contents of impurities due to melting and production unavoidable impurities should be kept low as in the prior art, in particular not exceed 0.1 wt .-%.

The invention is based on the recognition that it is necessary for the production of reliable defect-free complex shaped castings, such as cylinder heads for gasoline or diesel-powered internal combustion engines, from an AlCu alloy to modify the parameters of the manufacturing process over the already known measures. Only in this way can be produced reliably according to the invention composite castings, the have a particle size of less than 100 μm, ideally less than 80 μm, over their entire volume.

As a first step in this direction, the melt must be kept warm for a sufficiently long duration in a suitable temperature range.

Extensive investigations have shown that this requires a holding period of 4 to 12 hours and a holding temperature of 730 to 810 ° C., in particular 750 to 810 ° C., the desired results being particularly reliable when the holding period 6 - 10 hours and the holding temperature 770 - 790 ° C amount.

The mechanism of action associated with the holding provided in the above-mentioned time and temperature ranges (step b) of the method according to the invention) could not yet be conclusively clarified. However, here the presence of Zr, Ti and optionally V in the quantities provided according to the invention seems to have a decisive influence. These elements, together with aluminum as the main constituent of the alloy at high temperatures, form precursors that are activated by the long holding time and then effectively function as a grain refining agent.

Likewise, it has been found that for a consistently high casting yield over many casts, the melt is required before the start of the Mix well each casting campaign at least once.

Subsequently, the actual casting operation begins with step d). The working steps d) -i) of the method according to the invention are then repeated until the number of castings intended for the respective casting campaign has been produced.

If necessary, the mixing between two portion withdrawals can be repeated. The thorough mixing carried out, for example, as intensive stirring can be carried out in the course of a conventional degassing treatment, as is customarily used in the production method of the type in question before the start of the actual casting operation starting with the first removal of a melt portion.

The formation of a particularly fine structure in the castings produced according to the invention can be further supported by the fact that the respective melt portion, for example, on its way to the casting mold, is optionally subjected to a grain refining treatment prior to casting to the casting. Such a treatment can be used in the application of the method according to the invention produce castings in which the structure of an average grain size of less than 60 microns can be guaranteed.

Grain refining agents optionally added according to the invention are suitable for this purpose already known compounds, such as TiC or TiB, which can be added in each case in a dosage of 1 - 10 kg per ton of melt. Experiments have shown that an optimum grain refining effect results when the dosage of grain refining agent is 4 - 8 kg per ton of melt.

For the casting of the casting (step e) of the method according to the invention) is in principle any conventional casting process. This includes the possibility of conventional gravity casting.

Practical testing of the method according to the invention has shown, however, that parts cast from the alloy processed according to the invention, even when a fine microstructure has been achieved in the casting due to the measures taken during the preparation of the casting, are absent due to the absence of Si in its alloy are sensitive to the temperature gradient that occurs during their cooling. This sensitivity can be counteracted by a casting process, which causes the best possible directional solidification.

If particularly filigree-shaped components with optimized properties are to be produced, a so-called "dynamic casting method" should therefore be used. These are understood to mean those processes in which the casting mold is moved during the filling with melt, in order to ensure, on the one hand, a smooth, low-flow inflow of the melt and, consequently, equally quiet filling of the casting mold and, on the other hand, to achieve an optimum solidification profile after filling.

A common characteristic of the dynamic casting process, which is also known by the term "tilt casting method", is that the casting mold is filled via a melt container docked to it by a melt container from a starting position, in which the melt container is filled with the melt to be cast, around a Swivel axis is rotated to an end position, so that flows as a result of this pivotal movement, the melt in the mold. Examples of such methods are in the EP 1 155 763 A1 , of the DE 10 2004 015 649 B3 , of the DE 10 2008 015 856 A1 , of the DE 10 2010 022 343 A1 and the hitherto unpublished German patent application DE 10 2014 102 724.8 described.

By the above-explained measures (steps a) - e) and additionally performed if necessary grain refining treatment) is already a casting after casting and solidification, the structure of which meets the requirement of its fine grain requirement (average particle size <100 microns).

In order to adjust its further use properties, the casting according to the invention now undergoes a heat treatment in which it first undergoes solution heat treatment at a solution annealing temperature of 475-545 ° C. over a solution annealing time of 1-16 hours. In order to achieve the highest possible Cu concentrations in the Al matrix and thus to exploit the full potential of the alloy, the solution temperature can be set to 515-530 ° C.

The duration of solution heat treatment has no significant influence. It is to be set within the framework according to the invention so that the copper content present is optimally dissolved in the Al matrix. In practice, it is typically possible here to dissolve at least 60% of the existing Cu content, with the aim of dissolving the highest possible proportions, for example at least 70% or more, of the Cu content present. For this purpose, a solution annealing time of 2 to 6 hours can be provided in practice in the casting production of components for internal combustion engines.

After the solution heat treatment, the respective casting is accelerated from the solution annealing temperature to a quench stop temperature of at most 300 ° C. Here, the quenching rate is of decisive importance.

The quench rate is limited at the bottom by the fact that too slow cooling results in too low strengths. It can thus be seen that with conventional air quenching, the tensile strength and yield strength of castings consisting of the alloy processed according to the invention are lower than those of castings consisting of standard alloys. Therefore, in step g), the invention provides a quench rate of at least 0.75 K / s on average over the entire casting.

In the case of too rapid cooling after solution annealing, on the other hand, there is a risk of cracks. These can occur, for example, when the casting is quenched in less than 70 ° C warm, applied as a jet, wave or dip tank water. By the Deterrence is carried out with water heated to at least 70 ° C, the cracking can be sufficiently reliably avoided.

Alternatively, it is also possible to make the deterrent with a spray. In a spray mist quenching, the cooling is so gentle that it does not crack even when the spray is applied at room temperature.

Regardless of how the quenching is carried out, the upper limit of quenching rate achieved over the entire casting in the inventively made deterrent in step g) of the inventive method is limited to 15 K / s to avoid cracking.

Ideal is an average cooling rate of 1.5 - 7.5 K / s achieved over the entire casting. For example, a water quenching with 90 ° C warm water gives a cooling rate of about 7.5 K / s and led to the best results in the trial of the method according to the invention.

The quenching agent, as mentioned, for example, be applied as a wave or spray. When spray mist cooling is used, it is possible to cool the parts by pressurizing their outside or inside by passing the quencher through channels in the casting, such as a cylinder head through the water jacket. For this purpose, measures are, for example, in the DE 102 22 098 B4 described. Cooling from the outside results a cooling rate of approx. 2 - 2.5 K / s, with an internal quenching the quenching rates are 1.5 - 3.75 K / s.

In step g), the casting is quenched to a temperature that is less than or equal to the subsequent aging temperature. The aging according to the invention lasts 1 - 10 hours at a 150 - 300 ° C, in particular 200 - 260 ° C, amounting Wärmauslagerungstemperatur. The thermal aging is thus based on the conventional procedure, unlike there, however, the invention expressly does not provide for aging.

The duration of artificial aging has no significant effect on the treatment outcome. In order to achieve a stable condition of the casting, however, it has proved expedient to carry out the aging for at least 2 hours. In practice-oriented design, the duration provided for hot aging is typically 2 to 4 hours.

Castings produced according to the invention are thus characterized in that they consist of an AlCu alloy with (in% by weight) 6 - 8% Cu, 0, 3 - 0.55% Mn, 0, 15 - 0.25% Zr, up to 0.25% Fe, up to 0.125% Si, 0.05-0.2% Ti, up to 0.04% V and as consisting of balance Al and unavoidable impurities and having a structure which has a mean grain size of less than 100 microns, especially less than 80 microns, has.

In this case, according to the invention manufactured and manufactured castings have minimized susceptibility to cracking even after at least 400 h use at temperatures of at least 250 ° C, as are typical for applications in internal combustion engines for automobiles, at a test temperature of 250 ° C, a tensile strength of at least 160 MPa , typically at least 200 MPa, and a yield strength of at least 100 MPa, typically at least 150 MPa.

The invention will be explained in more detail by means of exemplary embodiments.

To test the process according to the invention, experimental melts S1, S2, S3 have been melted in a conventional melting furnace whose composition is given in Table 1.

In this case, the melts S1, S2, S3 have been kept at a holding temperature TH in the melting furnace for a duration tH in each case.

Subsequently, prior to the start of the actual casting campaign, a conventional degassing treatment in which the respective melt S1, S2, S3 was additionally vigorously stirred in order to achieve a thorough mixing.

In the respective casting campaign starting thereupon, the castings G1-G4 (melt S1), G5 (melt S2) and castings G6, G7 (melt S3) have been cast from the melts S1, S2, S3. For castings G1 - G5 were cylinder heads for diesel internal combustion engines, whereas cast parts G6, G7 were cylinder heads for gasoline internal combustion engines.

For the casting of the cast parts G1-G7, sufficiently sized portions of the respective melt S1, S2, S3 have been taken from the melting furnace in the respective casting campaign by means of a conventional ladle.

The melt portion contained in the ladle has been added to each TiB in a dosage of DKF.

The casting of each melt portion was carried out using the known under the keyword "Rotacast" Rotationsgießverfahrens in a conventional rotary caster, as for example in the EP 1 155 763 A1 is described.

After solidification and demolding, the resulting castings have been solution annealed at a solution annealing temperature TLG for a solution annealing time tLG.

After the end of the solution annealing, the castings have been quenched from the respective solution annealing temperature TLG to a quench stop temperature TAS at a cooling rate dAS.

This was followed by heat aging of castings G1 - G7. The castings are over a duration tWA on the respective thermal aging temperature TWA.

In Table 2, for each of the castings G1-G7 thus obtained are the melt from which they were respectively cast, and the parameters holding time tH, holding temperature TH, dosage DKF, solution annealing temperature TLG, solution annealing time tLG, quench stop temperature TAS, cooling rate dAS, warm aging time tWA and aging temperature TWA.

The average grain size of the microstructure, tensile strength Rm, yield strength Rp0.2 and elongation A determined after cooling at room temperature are listed in Table 3.

It turns out that the casting G3 quenched after the solution heat treatment has achieved a significantly lower tensile strength Rm and also a significantly lower yield strength Rp0.2 than the castings G1, G2 and G4 heat-treated in accordance with the invention and consisting of the same Melt S1 have been poured. Table 1 melt Cu Mn Zr Fe Si Ti V According to the invention? S1 6.52 0,455 0.206 0.074 0,095 0.086 0.0093 Yes S2 6.34 0.433 0,189 0.094 0.10 0.085 0.0095 Yes S3 6.47 0.453 0.198 0,089 0,051 0.091 0.0101 Yes In% by weight, balance Al and unavoidable impurities casting melt TH tH DKF TLG TLG TAS the TWA tWA According to the invention? [° C] [Hours] [kg per ton of melt] [° C] [Hours] [° C] [K / s] [° C] [Hours] G1 S1 780 12 8th 530 4 100 6.9 240 4 Yes G2 S1 780 12 8th 530 4 100 13.8 240 4 Yes G3 S1 780 12 8th 530 4 150 0.70 240 4 No G4 S1 780 12 8th 530 4 150 2.45 240 4 Yes G5 S2 775 8th 7 530 4.5 75 2.03 240 4 Yes G6 S3 779 8.5 8th 530 4 90 7.5 240 4 Yes G7 S3 779 8.5 8th 530 4 90 7.5 240 4 Yes casting mean grain size rm Rp0.2 A According to the invention? [.Mu.m] [MPa] [MPa] [%] G1 53.0 324 203 3.89 Yes G2 54.3 333 218 3.43 Yes G3 52.8 270 137 7.03 No G4 55.2 297 173 4.58 Yes G5 46.5 336 212 4.79 Yes G6 39.5 329 196 4.99 Yes G7 36.8 329 198 5.55 Yes

Claims (14)

1. Method for producing complex formed castings comprising the following working steps:

   a) melting an AlCu alloy which consists of (in% by weight)

   Cu: 6 - 8%,

   Mn: 0.3 - 0.55%,

   Zr: 0.15 - 0.25%,

   Fe: up to 0.25%,

   Si: up to 0.125%,

   Ti: 0.05 - 0.2%,

   V: up to 0.04%,

   remainder Al and unavoidable impurities;

   b) keeping the melt at a holding temperature of 730 - 810°C for a holding period of 4 - 12 hours;

   c) mixing of the melt;

   d) removal of a portion of melt from the melt;

   e) casting of the portion of melt removed from the melt into the casting;

   f) solution annealing of the casting at a solution annealing temperature of 475 - 545°C for a solution annealing period of 1 - 16 hours;

   g) quenching of the casting from the solution annealing temperature to a maximum quenching stop temperature of 300°C, wherein the casting is quenched at least within a temperature range of 500 - 300°C at a cooling rate of 0.75 - 15 K/s;

   h) artificial ageing of the casting, wherein during artificial ageing the casting is kept at an artificial ageing temperature of 150 - 300°C for a period of 1 - 10 hours;

   i) cooling of the casting to room temperature.
2. Method according to claim 1, characterised in that the portion of melt removed from the melt undergoes a grain refinement treatment before being cast into the casting.
3. Method according to claim 2, characterised in that for the grain refinement treatment, TiC or TiB is added as grain refiner in a dosage of 1 - 10 kg per ton of melt.
4. Method according to claim 3, characterised in that the dosage is 4 - 8 kg per ton of melt.
5. Method according to any one of the preceding claims, characterised in that a dynamic casting method is used when casting the portion of melt into the casting.
6. Method according to any one of the preceding claims, characterised in that the holding period (working step b) lasts 6 - 10 hours.
7. Method according to any one of the preceding claims, characterised in that the holding temperature (working step b) is 770 - 790°C.
8. Method according to any one of the preceding claims, characterised in that the mixing (working step c) is carried out in the course of a degassing treatment of the melt.
9. Method according to any one of the preceding claims, characterised in that the solution annealing temperature is 515 - 530°C.
10. Method according to any one of the preceding claims, characterised in that the solution annealing period lasts 2 - 6 hours.
11. Method according to any one of the preceding claims, characterised in that for quenching the casting (working step g), a quenching medium is used which is heated to a temperature of at least 70°C.
12. Method according to any one of the preceding claims, characterised in that the quenching medium is directed onto the casting as an atomised spray.
13. Method according to any one of the preceding claims, characterised in that the artificial ageing temperature is 200 - 260°C.
14. Method according to any one of the preceding claims, characterised in that the duration of the artificial ageing period (working step h) is 2 - 4 hours.