EP1215295B1 - Precipitation-hardening aluminium alloy and part thereof - Google Patents

Description

The invention relates to an aluminum casting alloy according to claim 1 and a method for producing a component according to claim 3.

• 8,0 bis 10,9 Gew.% Silizium,
• 0,8 bis 2,0 Gew.% Magnesium,
• 4,0 bis 5,9 Gew.% Kupfer
• 1,0 bis 3,0 Gew.% Nickel,
• 0,2 bis 0,4 Gew.% Mangan

sowie weniger als 0,5 Gew.% Eisen bekannt.
(Gew.% = Gewichtsprozent, Anteil der Einzelelemente an der Gesamtmasse der Legierung).DE 44 04 420 A1 discloses a curable aluminum alloy having a composition:

• 8.0 to 10.9 wt.% Silicon,
• 0.8 to 2.0% by weight of magnesium,
• 4.0 to 5.9 wt.% Copper
• 1.0 to 3.0% by weight nickel,
• 0.2 to 0.4% by weight of manganese

and less than 0.5% by weight of iron.
(Wt.% = Weight percent, proportion of individual elements in the total mass of the alloy).

This alloy is especially intended for pistons in internal combustion engines. The relatively high silicon content leads to good wear resistance and high strength even at high temperatures. The other alloying elements prevent the formation of sharp primary silicon crystals, which form the starting points of fatigue fractures under alternating load. However, such white components only have limited elongation at break.

• 5,0 bis 12,0 Gew.% Silizium,
• 0 bis 0,8 Gew.% Magnesium,
• weniger als 0,01 Gew.% Kupfer
• weniger als 0,2 Gew.% Eisen,
• 0,1 bis 0,5 Gew.% Kobalt.

DE 42 15 160 C2 describes an aluminum alloy for die-casting applications, which ensures good releasability of a component from the die casting mold. In addition to 99.7% pure primary aluminum, it has the following composition:

• 5.0 to 12.0% by weight of silicon,
• 0 to 0.8% by weight of magnesium,
• less than 0.01% by weight copper
• less than 0.2% by weight iron,
• 0.1 to 0.5% by weight of cobalt.

From CH-A-168 202 alloys are known having a composition in wt.% Of 8 to 15% Si, 0.1 to 0.6% Mg and up to 1% Ni or Co.

In general, iron is added to reduce adhesion between the component and the mold of the alloy, but at higher concentrations it increases the brittleness of the component. In particular, the cobalt here shows the functional property to reduce the adhesive properties of the component to the mold, without increasing the brittleness. Thus, the iron content can be greatly reduced.

The already mentioned brittleness of the alloy, which is due to different alloying elements and is acceptable as a compromise in different applications, leads to failure for certain, heavily loaded components. This applies in particular to engine components such as cylinder heads or cylinder crankcases. Here are particularly high temperatures and pressures and alternating loads. In addition, complex geometries cause significant notch effects. In these cases, an extremely high ductility of the material is required to avoid component failure. This is especially true in modern high performance engines, where the loads in the cylinder heads are steadily rising.

The object of the invention is thus to provide an alloy from which components result which have a high heat resistance, a high elongation at break, a high ductility with a low tendency to corrosion.

The object is achieved by an alloy according to claim 1 and a method for producing a component according to claim 3.

The alloy of the invention according to claim 1 has a silicon content of between 5% and 10%. A lower silicon content would affect the castability of the alloy. A higher silicon content leads to material embrittlement. More preferably, the silicon content is between 6.5% and 7.5%.

The alloying element magnesium forms, together with the silicon, Mg ₂ Si crystals (magnesium silicide), which increase the strength. With a magnesium content below the lower limit according to the invention, the resulting component has too low a strength, above 0.35% magnesium, the Mg ₂ Si crystals lead to too high brittleness.

The alloying element nickel forms together with the aluminum intermetallic phases, such as. B. Al ₃ Ni (nickel aluminide) which increase the thermal stability and melt congruently at temperatures above 800 ° C (in contrast to Al ₂ Cu (copper aluminide), which forms in copper-containing alloys and melts below 600 ° C). In addition, the phases containing aluminum and nickel do not adversely affect the ductility of the material. The nickel content of the alloy according to the invention is between 0.3% and 3%, preferably between 0.5% and 2.5%.

It is possible to add cobalt as an alloying element to the alloy according to the invention. Cobalt also forms intermetallic compounds based on aluminum and cobalt, similar to the compounds based on aluminum and nickel, which increase the heat resistance. The alloy according to the invention may contain cobalt between 0.6% by weight and 3% by weight.

Iron, which reduces the elongation at break, can be dispensed with in the alloy according to the invention. The same applies to the copper as an alloying element, which deteriorates the corrosion resistance.

Another object of the invention is a method for producing a component according to claim 3. The component is cast from an alloy, which is already described in claim 1 and has the advantages that result from this alloy.

A heat treatment of the component preferably after a solution annealing leads to a precipitation hardening (hot curing) of an Al matrix (by which the component is formed) by targeted intermetallic phases such. B. said Mg ₂ Si or Al ₃ Ni are excreted. The precipitation hardening takes place in a temperature interval between 160 ° C and 240 ° C for a period of 0.2 h to 10 h. The precipitation hardening is particularly preferably carried out in a temperature interval between 180 ° C. and 220 ° C. for a period of 0.5 h to 8 h. The duration of the temperature treatment depends on the temperature, at higher temperatures, the heat treatment is shortened considerably.

The component which is represented by the alloy according to the invention is preferably designed as a sand casting or chill casting component, since this facilitates the already mentioned heat treatment. For a component that is produced by die casting, the heat treatment due to air bubbles is not readily possible. In this case, a procedurally more complex Vakuumdruckgießverfahren would be applied.

Particularly suitably, the component according to the invention is designed as a cylinder head or as a cylinder crankcase in an internal combustion engine. In these components, especially in cylinder heads very high pressures occur at high temperatures. In addition, these components have very complex geometries such. B. on the valve webs in the cylinder head or on the cooling channels in the cylinder crankcase. These constructions act as notches and break points especially at the high temperatures, pressures and alternating loads. A particularly high elongation at break in combination with an increased heat resistance offers a considerable advantage here.

The embodiment of the invention will be explained in more detail in the following embodiment.

Fig. 1,

das schematische Aushärtverhalten eines Bauteils als Funktion der Zeit, bei einer Temperatur T1.

Fig. 2,

das schematische Aushärtverhalten eines Bauteils als Funktion der Zeit, bei einer Temperatur T2, wobei T2 größer als T1 ist.

Show it:

Fig. 1,

the schematic curing behavior of a component as a function of time, at a temperature T1.

2,

the schematic curing behavior of a component as a function of time, at a temperature T2, where T2 is greater than T1.

A cylinder head of an internal combustion engine is cast in the cube casting process with the alloy according to the invention. The casting parameters correspond to the usual process-related process control.

After casting and after cooling, the component has a coarse grain structure of mixed crystals, since aluminum has a very low solubility with respect to most alloying elements at room temperature. For this reason, a solution annealing of the component takes place for about 4 -5 h at a temperature of about 540 ° C. In this step, the alloying elements in the aluminum matrix dissolve. The component is then quenched in water leaving the alloying elements dissolved in the aluminum matrix.

This is followed by excretion hardening, in which the elements dissolved in the aluminum matrix precipitate out of the matrix in a controlled manner, forming mixed crystals. This is done at a temperature of 220 ° C for 0.5 hours. Alternatively, the precipitation hardening may be done at 180 ° C for 8 hours. The phases that form during precipitation hardening (precipitates) are intermetallic compounds, which include, among other things, Mg ₂ Si, which determines the strength of the component increases and Al ₃ Ni (or other ternary and / or quaternary intermetallic compounds based on aluminum and nickel) increases due to its high melting temperature, the heat resistance of the component.

The strength and the ductility of the component is adjustable by the temperature control and the duration of the temperature treatment, which, as mentioned, is due to the precipitated crystal (eg the intermetallic compounds Mg ₂ Si and Al ₃ Ni).

Likewise, the size of the Mg2Si and Al3Ti precipitates, which are also affected by the heat treatment, affects the component properties, as explained below.

FIGS. 1 and 2 schematically show the strength σ of the component (left y-axis) and the elongation at break ε (right-hand y-axis, dashed) as a function of the duration of the heat treatment t. Figures 1 and 2 differ in the temperature T of the heat treatments, T of Figure 1 is smaller than T of Figure 2. The solid curves 1 and 3 show schematically the course of strength σ, the dashed lines 2 and 4 the course of Elongation at break ε.

Depending on the temperature, the component strength reaches a maximum after a certain duration of the heat treatment. This state is generally called T6, the component has a very fine structure of the precipitates here. At the same time, the elongation at break in the T6 state reaches a minimum. If the heat treatment is continued after reaching the T6 state, a so-called overcure occurs, which is referred to as T7 state. The T7 state has the advantage that, due to the coarser structure of the precipitates that occurs in this state, the elongation at break increases again.

The terms T6 and T7 are fixed technical terms, T does not stand for temperature in these terms.

During the heat treatment of the component according to the invention, care must be taken that both the strength and the elongation at break meet the requirements for the component. In general, task T7 is aimed at with the highest possible elongation at break.

A comparison of Figure 1 and Figure 2 shows that the maxima and minima of the T6 state at a higher temperature (Figure 2) are much more pronounced and reached earlier than at lower temperatures (Figure 1). However, phase formation is more difficult to control at higher temperatures. The described heat treatment of 220 ° C for 1.2 h represents a compromise of these aspects.

The alloying elements silicon and magnesium cause an increase in strength and a shift of the curves 1 and 3 upwards. In turn, these elements shift the curves 2 and 4 downwards, which has a negative effect on the elongation at break. Surprisingly, it was found that both nickel and cobalt shift as the alloying elements curves 1 and 3 upwards, without exerting a negative effect on the elongation at break.

Thus, the addition of nickel and / or cobalt in itself, but especially in combination with a controlled heat treatment, which form the desired precipitates of compounds based on aluminum and nickel or aluminum and cobalt and the advantageous grain structure is adjusted, to the inventive solution of the problem.

Claims (8)

1. Age-hardenable aluminium casting alloy, characterized in that the alloy, in addition to aluminium, contains as functional elements
   5 to 7.5% by weight silicon,
   0.2 to 0.35% by weight magnesium,
   0.3 to 3% by weight nickel
   and/or 0.6 to 3% by weight cobalt,
   remainder Al and production-related impurities.
2. Age-hardenable aluminium casting alloy according to Claim 1, characterized in that the alloy, in addition to aluminium, contains as functional elements
   6.5 to 7.5% by weight silicon,
   0.2 to 0.35% by weight magnesium,
   0.5 to 2.5% by weight nickel,
   remainder Al and production-related impurities.
3. Process for producing a component from an aluminium alloy, characterized in that

   • an alloy which, in addition to aluminium, contains as functional elements
   5 to 7.5% by weight silicon,
   0.2 to 0.35% by weight magnesium,
   0.3 to 3% by weight nickel
   and/or 0.6 to 3% by weight cobalt,
   remainder Al and production-related impurities, is selected for the component, and

   • in that the component contains phases which include aluminium and nickel and/or aluminium and cobalt and are in the form of binary and/or ternary and/or quaternary intermetallic compounds.
4. Process according to Claim 3, characterized in that

   • an alloy, which in addition to aluminium, contains as functional elements
   6.5 to 7.5% by weight silicon,
   0.2 to 0.35% by weight magnesium, and
   0.5 to 2.5% by weight nickel,
   is selected for the component, and

   • in that the component contains phases which include aluminium and nickel and are in the form of binary and/or ternary and/or quaternary intermetallic compounds.
5. Process according to Claim 3 or 4, characterized in that the component is hot-age-hardened for 0.2 to 10 h at a temperature of between 160°C and 240°C.
6. Process according to one of Claims 3 to 5, characterized in that the component is heat-treated for 0.5 to 8 h at a temperature of between 180°C and 220°C.
7. Process according to one of Claims 3 to 6, characterized in that the component is produced in a sand-casting or gravity die-casting or vacuum die-casting process.
8. Process according to one of Claims 3 to 7, characterized in that a cylinder head or a cylinder crankcase of an internal combustion engine is formed.

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