EP1978120B1 - Aluminium-silicon alloy and method for production of same - Google Patents

Description

The invention relates to high and heat resistant silicon-containing aluminum casting alloys, their production and their use for the production of engine components.

To reduce emissions and fuel consumption as well as to increase engine power, the combustion pressures and combustion temperatures of fuel engines and internal combustion engines have increased in recent years, especially in the diesel engine. This led to increased demands on the thermo-mechanical loads for engine components.

In the prior art, in particular in engine construction. Al casting alloys known. Al castings are widely used because of their low specific weight, ease of molding and ease of processing. Also, through various casting methods, complicated workpieces such as e.g. Make pistons, cylinder heads, crankcases or engine blocks.

A proven alloy group for the production of engine components are Al-Si alloys. These materials are typically held with silicon between 6 and 18 wt .-%, in some cases up to 24 wt .-% and with admixtures of magnesium from 1 to 1.5 wt .-%, copper between 1 and 4 wt. % and often also nickel between 1 and 3 wt.% alloyed (catalog "Aluminum Casting Alloys", VAW-IMCO).

In order to improve the heat resistance of the Al-Si alloys, for example, according to the US Pat. No. 6,419,769 B1 recommended to adjust the copper content between 5.6 and 8 wt .-%. However, in this case, the improvement of the mechanical strength is opposed to a deterioration in ductility, increase in specific gravity, and reduced corrosion resistance.

A heat resistant alloy with reduced specific gravity is used in the DE-PS 747 355 described as particularly advantageous for pistons. This material is characterized by a magnesium content between 4 and 12 wt .-% and a silicon content between 0.5 and 5 wt .-% of. Furthermore, between 0.2 and 5 wt .-% copper and / or nickel may be alloyed. The high magnesium content leads to gas porosity due to the strong hydrogen absorption. The additional oxidation also involves the risk of oxide inclusions, which significantly degrade the mechanical properties of the casting.

Belov N. et al. in "Multicomponent Phase Diagrams: Applications for Commercial Aluminum Alloys", 2005, Elsevier, Oxford, UK, XP009104020, ISBN: 0-080-44537-3, pages 341-345 , Al-C-Si phase diagrams, which indicate the crystal structure of pure aluminum-carbon silicon phases, but no information about their influence by other alloying elements, especially not with increasing content of titanium, zirconium or iron.

Chunxian Xu et al., Described in "Modifying effect of Al-Ti CP Master Alloy on Hypereutectic Al-Si Alloy", 2006, Beijing International Materials Week, June 30, 2006 (2006-06-30), pages 947-951 the use of a very specific, crystallographically investigated Al-Ti-CP master alloy to improve the microstructure of hypereutectic Al-Si alloys. It is possible to refine the primary silicon even with only 2 wt .-% of Al-Ti-CP-Master Alloys.

From the W020071051162 A2 For example, an aluminum-silicon-magnesium alloy is known for molded parts in the automotive industry that contain about 6.0 to 8.0 wt% silicon, about 0.12 to about 0.25 wt% magnesium, less than or equal to about 0.35 wt.% Copper, less than or equal to about 4% zinc, less than or equal to about 0.6 wt.% Manganese and less than or equal to about 0.15 wt.% Iron, the casting being a T5 or T6 is subjected to heat treatment. The alloy contains no titanium and no zirconium.

From the US 2005/0238529 A1 For example, a cast alloy is known for the same purpose, this aluminum alloy being about 3.5 to 5.5% zinc, about 1 to 1.5% magnesium, less than about 1% silicon, less than about 0.3% manganese, and less than about 0.3% iron or other incidental impurities contains. The alloy contains no titanium and no zirconium.

As far as strength-increasing Mg ₂ Si and Al ₂ Cu precipitates form in the previously known Al casting alloys via a heat treatment, they are not stable above 150 ° C. and therefore can not cope with the thermomechanical loads of modern motors. By contrast, the intermetallic phases, such as Al ₆ Mn, Al ₃ Fe, Al ₇ Cr, Al ₃ Ni, Al ₈ Fe ₂ Si, Al ₇ Cu ₄ Ni, Al ₁₅ Mn ₃ Si ₂ , Al ₅ FeSi, Al ₃ Ti and Al ₃ Zr is unaffected by thermal long-term stress and, with favorable design (in quantity, size, shape and distribution), can make a considerable contribution to increasing the mechanical properties of the Al-Si alloys for engine construction. However, it is of particular importance that the homogeneous distribution and fine formation of the intermetallic phases in the cast structure is ensured in order not to impair the ductility of the alloy and its casting technology properties.

In the patent DE 101 17 298 C1 discloses a way to modify the texture of the AlSi base alloy by adding up to 3% by weight of rare earths. The additions of rare earths cause a significant refining of the intermetallic phases, which results in a considerable improvement of the mechanical properties, in particular fatigue strength at elevated temperatures. However, the rare earths are very expensive and rarely used in practice.

The invention has for its object to provide a suitable alloy for the production of engine components, which has high strength, heat resistance, good creep strength and sufficient ductility with low susceptibility to corrosion and is also inexpensive.

This object is achieved by an Al-Si casting alloy with the features of claim 1, the associated manufacturing method according to claim 6 and the associated use according to claim 12.

Preferably, the aluminum-silicon casting alloy contains 5 to 18 wt .-%, in particular 12.5 to 14.5 wt .-% silicon.

• 0,1 bis 1,5 Gew.%, insbesondere 0,1 bis 0,6 Gew.-% Magnesium;
• bis 0,5 Gew.-% Titan;
• bis 0,7 Gew.-%, insbesondere 0,4 Gew.-% Zirkonium;
• 0,001 bis 1,2 Gew.-%, insbesondere 0,001 bis 0,6 Gew.-% Mangan;
• bis 0,8 Gew.-% Eisen;
• 0,001 bis 0,5 Gew.-%, insbesondere 0,1 bis 0,4 Gew.-% Kobalt;
• 0,001 bis 0,5 Gew.-%, insbesondere 0,1 bis 0,4 Gew.-% Chrom;
• 0,0001 bis 0,1 Gew.-%, insbesondere 0,005 bis 0,01 Gew.-% Beryllium;
• 0,001 bis 2 Gew.-%, insbesondere 0,1 bis 1,5 Gew,-% Zink;
• 0,001 bis 4 Gew.-%, insbesondere 0,3 bis 1,8 Gew.-% Kupfer;
• 0,001 bis 4 Gew.-%, insbesondere 0,3 bis 3,0 Gew.-% Nickel;
• 0,001 bis 0,4 Gew.-%, insbesondere 0,05 bis 0,2 Gew.-% Vanadium;
• 0,0001 bis 1,2 Gew.-%, insbesondere 0,005 bis 0,5 Gew.-% Hafnium;
• 0,0001 bis 0,6 Gew.-%, insbesondere 0,005 bis 0,4 Gew.-% Niob;
• 0,0001 bis 0,4 Gew.-%, insbesondere 0,005 bis 0,2 Gew.-% Blei;
• 0,0001 bis 0,08 Gew.-%, insbesondere 0,005 bis 0,04 Gew.-% Strontium;
• 0,0001 bis 0,2 Gew.-%, insbesondere 0,002 bis 0,02 Gew.-% Natrium;
• 0,0001 bis 0,006 Gew.-%, insbesondere 0,002 bis 0,004 Gew.-% Calcium;
• 0,0001 bis 0.08 Gew.-%, insbesondere 0,01 bis 0,06 Gew.-% Bor;
• 0,0001 bis 0,4 Gew.-%, insbesondere 0,05 bis 0,3 Gew.-% Cer;
• 0,0001 bis 0,6 Gew.-%, insbesondere 0,05 bis 0,3 Gew.-% Scandium;
• 0,0001 bis 0,1 Gew.-%, insbesondere 0,001 bis 0,01 Gew.-% Phosphor.

Other elements are present in the alloy, as stated above. These are additives that in comparison
were added with aluminum and silicon in a minor amount. For example, the following amounts of additional alloying ingredients may be beneficial for the desired property profile:

• 0.1 to 1.5% by weight, in particular 0.1 to 0.6% by weight of magnesium;
• to 0.5% by weight of titanium;
• up to 0.7% by weight, in particular 0.4% by weight, of zirconium;
• 0.001 to 1.2 wt .-%, in particular 0.001 to 0.6 wt .-% manganese;
• up to 0.8% by weight of iron;
• 0.001 to 0.5 wt .-%, in particular 0.1 to 0.4 wt .-% cobalt;
• 0.001 to 0.5 wt .-%, in particular 0.1 to 0.4 wt .-% chromium;
• 0.0001 to 0.1 wt .-%, in particular 0.005 to 0.01 wt .-% beryllium;
• From 0.001 to 2% by weight, in particular from 0.1 to 1.5% by weight, of zinc;
• 0.001 to 4 wt .-%, in particular 0.3 to 1.8 wt .-% copper;
• 0.001 to 4 wt .-%, in particular 0.3 to 3.0 wt .-% nickel;
• 0.001 to 0.4 wt .-%, in particular 0.05 to 0.2 wt .-% vanadium;
• 0.0001 to 1.2 wt .-%, in particular 0.005 to 0.5 wt .-% hafnium;
• 0.0001 to 0.6% by weight, especially 0.005 to 0.4% by weight of niobium;
• 0.0001 to 0.4 wt .-%, in particular 0.005 to 0.2 wt .-% lead;
• 0.0001 to 0.08 wt .-%, in particular 0.005 to 0.04 wt .-% strontium;
• 0.0001 to 0.2% by weight, especially 0.002 to 0.02% by weight of sodium;
• 0.0001 to 0.006 wt .-%, in particular 0.002 to 0.004 wt .-% calcium;
• 0.0001 to 0.08% by weight, in particular 0.01 to 0.06% by weight of boron;
• 0.0001 to 0.4% by weight, in particular 0.05 to 0.3% by weight of cerium;
• From 0.0001 to 0.6% by weight, in particular from 0.05 to 0.3% by weight of scandium;
• 0.0001 to 0.1 wt .-%, in particular 0.001 to 0.01 wt .-% phosphorus.

The aluminum-silicon casting alloy according to the invention is preferably characterized by the fact that in its microstructure fine primary silicon (less than 50 μm) and refined eutectic are present simultaneously, as can be seen from the Schüff image. This condition is particularly desirable in near and hypereutectic Al-Si alloys.

The degree of finishing of the eutectic can be visually assessed by the foundry expert on the basis of the forms of formation of the eutectic silicon precipitates, for example with the aid of micrographs. For the visual assessment see also " Foundry practice "No. 11/12 - 1993, page 206-209, G. Chai, L. Bäckerud," Effective refining with strontium ".

It has shown. that the carbon content according to the invention causes a change in the overall solidification behavior of the Al-Si casting alloys and brings about an excellent microstructure modification. As essential features of the structure modification by carbon are a considerable refining and homogeneous distribution of the intermetallic phases, a good refinement of the Al-Si eutectic and good refining of the primary silicon crystals. This results in a significant improvement of the mechanical and casting technology properties.

By adjusting the carbon content according to the invention, it is also possible to shift the concentration limits of important alloy components, such as titanium, zirconium, iron, manganese, chromium, cobalt, molybdenum and, depending on the application of other transition elements, to higher values without impairing alloy quality.

In known Al-Si alloys with zirconium contents of more than 0.3% by weight, titanium contents of more than 0.3% by weight or iron contents of more than 0.6% by weight, the structure is very long acicular brittle phases.

The formation of these coarse intermetallic phases, as would be expected in the conventional aluminum alloys, ie especially long needles of intermetallic phases with transition elements such. As Al ₃ Zr, Al ₃ Ti and Al ₅ FeSi, is suppressed by carbon. The very long needle-shaped ones typical for high contents of transition elements. Intermetallic phases appear in the carbon-containing Al-Si alloys mostly as "Chinese writing" or as small platelets up to a maximum of 30-40 μm in length. This brings significant benefits, such. As a significant improvement in the grain refining effect of the transition elements, as well as a significant increase in the mechanical properties, hot, creep and fatigue strength of the alloy of the invention. During casting, the melt shows a significantly improved mold filling and flow behavior, and the finished castings can be a significantly increased casting quality and in particular a significantly lower gas porosity can be detected.

In hypereutectic Al-Si alloys, the simultaneous refining of the primary silicon, the refinement of the eutectic silicon, and the smallest possible formation and homogeneous distribution of the intermetallic phases are particularly important for setting the desired property profile. This long-desired modification of the structure could not be achieved so far, since the effects of strontium and phosphorus cancel each other out. By Zulegieren the Near-eutectic and hypereutectic Al-Si alloys (especially for pistons and engine blocks) with the carbon and simultaneous addition of up to 100 ppm of phosphorus have achieved the desired combined microstructural influence.

For the processing of the alloy according to the invention, basically all casting methods are suitable. These include u. a. Sand casting, full mold casting, gravity die casting, low pressure die casting, differential die casting, die casting and vacuum die casting.

For the production of the aluminum-silicon casting alloy according to the invention, it is preferably provided that the basic constituents selected for the composition are melted together. The melting temperature is preferably from 650 ° C to 1000 ° C, more preferably from 720 ° C to 950 ° C. Then it is poured into a mold. "Melted together" also covers the gradual metering of all components into a common melt. Carbon can be used as elemental carbon, e.g. Graphite, but also be added in the form of a compound or master alloy.

According to a preferred embodiment, the carbon content is achieved in particular by adding chemical carbon compounds and / or their mixtures. This can also be done by adding powdered carbides and carbonitrides, also in the form of a sintered product of carbides and carbonitrides.

Alternatively, a carbonaceous aluminum master alloy may be incorporated into the melt from the remainder of the alloyed ingredients or may be added in advance to the components to be melted.

The carbonaceous additives can contain not only carbon but also phosphorus and / or nitrogen.

A particularly preferred method of this invention is to use an aluminium-titanium-carbon master alloy.

Although is out of the DE 37 29 937 A 1 Already a refinement with Al-Ti-C alloys known, but this refers expressly to Al-Ti main alloys. In doing so, long-known difficulties in increasing the carbon content of aluminum-titanium alloys are attributed to the difficulty in these compositions in achieving wetting between the carbon and the molten aluminum. The described master alloys are particularly useful in the production of thin sheet, films or can material.

Also, the addition of carbon-containing master alloys to pure aluminum and wrought aluminum alloys with the aim of grain refining is known as such, as in " Z. Metallkd. 91 (2000) Issue 10, pp. 800-806 described. The effect of known Al-Ti-C master alloys is based on the introduction of TiC particles, which serve as nucleating agent for the α-mixed crystal, in the melt and requires in the prior art strict compliance with certain parameters in the melt management, such as z. As low as possible melt temperatures and the smallest possible silicon contents to ensure the stability of the TiC particles in the melt and to avoid their reaction with the other alloy components. At silicon contents of more than 3 wt .-%, it is reported by Greer et al. (Advanced Engineering Materials (2003) No. 1-2, pages 81-91 and " Continuous Casting, Ed. by K Ehrke and W. Schneider, DGM (2000 ) A. Tronche and AL Greer, "Effect of Solute Elements on the Grain Structures of Al-Ti-P and Al-Ti-C Grain-Refined Al Alloys", pp. 218-222; 221, 222 ) for poisoning of TiC by silicon, so that the grain refining effect of TiC on the α-aluminum mixed crystal phase in the Al-Si casting alloys is prevented. The effect of carbon on the intermetallic compounds, the primary silicon and the eutectic phases in Al-Si casting alloys without effect on the α-mixed crystal phase (ie without grain refining effect by carbon or carbide in this phase) has not been known.

Although good mechanical values already exist in the cast state. For example, castings produced from the alloy according to the invention can be subjected to all heat treatments.

The aluminum-silicon GUS alloys according to the invention are particularly suitable for casting pistons and other machine parts for internal combustion engines, for cylinder heads, crankcases, liners or engine blocks. The solution of the object of the invention therefore also includes these uses.

With reference to the figures and examples, the invention is to be further illustrated, without the examples being limiting. The person skilled in the art can easily carry out the invention with the aid of what is clearly illustrated in examples and figures throughout the scope given above.

• Fig. 1 zeigt das Mikrogefüge einer AlSi₁₂CuNiMg-Sekundärlegierung, x 500
• Fig. 2 zeigt das Mikrogefüge einer erfindungsgemäß legierten Al-Si₁₂CuNiMgSekundärlegierung unter Zugabe von Kohlenstoff, x 500
• Fig. 3 zeigt das Mikrogefüge einer AlSi₁₄Cu₃Mg-Legierung mit Phosphorzugabe. x 100
• Fig. 4 zeigt das Mikrogefüge einer erfindungsgemäß legierten Al-Si₁₄Cu₃MgLegierung unter Zugabe von Kohlenstoff und Phosphor, x 100

(Maßstab in den Figuren = 50 µm)The figures show structure for the preliminary tests

• Fig. 1 shows the microstructure of an AlSi ₁₂ CuNiMg secondary alloy, x 500
• Fig. 2 shows the microstructure of an alloyed Al-Si ₁₂ CuNiMg secondary alloy according to the invention with the addition of carbon, x 500
• Fig. 3 shows the microstructure of an AlSi ₁₄ Cu ₃ Mg alloy with phosphorus addition. x 100
• Fig. 4 shows the microstructure of an alloyed Al-Si ₁₄ Cu ₃ Mg alloy according to the invention with the addition of carbon and phosphorus, x 100

(Scale in the figures = 50 μm)

Example 1 (preliminary experiment not according to the invention) Near-eutectic Al-Si casting alloy with carbon

The secondary alloy AlSi ₁₂ CuNiMg was selected as representative of the large group of Al-Si casting alloys. The experimental alloy was in cylindrical specimens with a casting temperature of 780 ° C in a reduced to 300 ° C. heated steel mold. Carbon was added using the self-made Al-Ti-C master alloy.

A medium-frequency induction furnace was used to produce the AlTi ₆ C ₁ alloy. In a graphite crucible, 2000 g of AlTi ₆ pre-alloy was first melted at 1400 ° C. To this melt was added 30 grams of graphite powder wrapped in aluminum foil. The casting of the Al-Ti-C master alloy thus produced was carried out after a holding time of about 30 in a copper mold, the master alloy consists of an aluminum matrix, are embedded in the Al ₃ Ti and TiC particles.

Table 1 shows the composition of the alloys investigated. <u> Table 1 </ u> Al-Si Cast Alloy Composition,% by Weight Si C Cu Ni mg Fe Mn Cr Ti Zn Erfg. Leg. 1 12.4 0,003 1.3 0.8 1.4 1.2 0.3 0.15 0.07 0.3 Compare Leg.2 12.6 - 1.5 0.9 1.6 1.3 0.4 0.13 0.05 0.4

The results of the metallographic investigations are summarized in Table 2. <u> Table 2. </ u> The degree of improvement and the length of the intermetallic particles in the investigated alloy AlSi <sub> 12 </ sub> CuNiMg finishing degree Length of intermetallic particles, μm Erfg. Leg. 1 Grade 6: well finished 20-30 Comp. Leg. 2 Grade 3: partially finished 100-600

The secondary alloy AlSi ₁₂ CuNiMg has very coarse needle-shaped iron-containing phases in the cast structure (predominantly Al ₅ FeSi needles), Fig. 1 , In contrast, alloying with the carbon causes both a well-refined eutectic and precipitates of small intermetallic phases in a very uniform distribution, Fig.2 ,

Example 2 Overutectic Al-Si alloy with carbon

In this example, a hypereutectic Al-Si casting alloy was compared to the alloy of the invention having an approximately similar composition, Table 3. Both alloys were treated with an equal amount of phosphorus. <u> Table 3 </ u> Composition of hypereutectic Al-Si casting alloys, wt% Si Cu C mg Fe Mn Ti Zn P Erfg. Leg. 3 14.1 3.7 0.02 0.33 0.94 0.29 0.21 0.33 0,006 Comp. Leg. 4 14.6 4.1 - 0.32 0.73 0.28 0.22 0.35 0,006

A simultaneous filing of the primary silicon and the Al-Si eutectic was not possible until now, Fig. 3 , Our results show that phosphorus and carbon do not interfere with each other in their effect. Figure 4 shows that with a carbon content of 0.005% by weight a good refining of the primary silicon and an acceptable degree of finishing of the eutectic silicon can be achieved. The compact design and homogeneous distribution of the intermetallic phases in the alloy according to the invention is also of great technological advantage.

Example 3 Improvement of the mechanical properties of Al-Si cast alloys

The positive influence of carbon on Al-Si alloys also clearly reflects the mechanical properties already achieved in the cast state, Table 4 and Table 5. <u> Table 4. </ u> Chemical composition of the alloys studied Si C Cu Ni mg Fe Mn Zr Ti Zn Erfg. Leg. 5 11.8 0,025 1.8 0.8 0.7 0.7 0.3 0.1 0.18 0.4 Comp. Leg. 6 12.5 - 1.5 1.1 1.1 0.4 0.3 - 0.15 0.1 Mechanical properties Testing at RT Mechanical properties after pre-storage at 259 ° C / 100 h test at 250 ° C R _m, MPa R _p0.2 , MPa A ₅ ,% R _m, MPa R _p0.2 MPa A _5, % Erfg. Leg. 5, F 241 171 0.6 147 105 3.2 Comp. Leg. 6, F 225 152 0.7 112 83 9.3 R _m - tensile strength (MPa); R _p0.2 - yield strength (MPa), A ₅ % - elongation at break in%

The alloy 5 according to the invention has a good strength for a casting alloy, as results from the above-mentioned tabular data. In addition, the alloy 5 according to the invention has a significantly better heat resistance than the comparative alloy 6, the R _p0.2 value of which drops sharply at 250 ° C. after a preliminary storage at 250 ° C. when the mechanical properties are measured at 250 ° C.

By "heat-resistant" we mean here an alloy whose R _p0.2 value after storage at 250 ° C. for at least 50 h, tested at 250 ° C., is above 55 MPa.

Claims (12)

1. Aluminium-silicon casting alloy having the following composition:

   - from 5 to 25% by weight of silicon,

   - from 0.0007 to 0.1% by weight of carbon,

   - from 0 to 4% by weight of each of the following alloy constituents, with preference being given to at least one of these constituents being present and their total being up to 10% by weight, preferably up to 6% by weight, more preferably up to 4% by weight:

   magnesium, manganese, iron, cobalt, copper, zinc, nickel, vanadium, niobium, molybdenum, chromium, tungsten, beryllium, lead, yttrium, cerium, scandium, hafnium, silver, zirconium, titanium, boron, strontium, sodium, potassium, calcium, antimony, sulphur, barium, phosphorus, with the proviso that

   - more than 0.3% by weight of Zr or more than 0.3% by weight of Ti or more than 0.6% by weight of Fe is present

   - and at least 65% by weight of aluminium including unavoidable impurities is present as balance to 100% by weight.
2. Aluminium-silicon casting alloy according to Claim 1, characterized in that the alloy contains from 5 to 18% by weight, in particular from 12.5 to 14.5% by weight, of silicon.
3. Aluminium-silicon casting alloy according to either Claim 1 or 2, characterized in that fine primary silicon and modified eutectic are simultaneously present in its microstructure.
4. Aluminium-silicon casting alloy according to either Claim 2 or 3, characterized in that intermetallic phases are present as needles or small platelets having a length of up to 40 µm in its microstructure.
5. Aluminium-silicon casting alloy according to any of Claims 1 to 4 which can be obtained by targeted setting of the carbon content to from 0.0007 to 0.1% by weight.
6. Process for producing an aluminium-silicon casting alloy according to any of Claims 1 to 4, characterized in that the carbon content is set in a targeted manner by addition of carbon in any form.
7. Process according to Claim 6, characterized in that the base constituents selected for the composition are melted together while being heated to a temperature in the range from 650°C to 1000°C, preferably from 720°C to 950°C, and are poured into a casting mould.
8. Process according to Claim 6 or 7, characterized in that the carbon content is achieved by addition of chemical carbon compounds and/or mixtures thereof, in particular by addition of pulverulent carbides and carbonitrides, including in the form of a sintered product composed of carbides and carbonitrides.
9. Process according to Claim 6 or 7, characterized in that a carbon-containing aluminium prealloy is introduced into the melt composed of the other constituents provided for the alloy or is added beforehand to the constituents to be melted.
10. Process according to Claim 9, characterized in that an aluminium-titanium-carbon prealloy is used.
11. Process according to any of Claims 6 to 10, characterized in that the prealloy also contains phosphorus in addition to carbon.
12. Use of the aluminium casting alloy according to any of Claims 1 to 5 for casting highly thermally stressed machine elements, in particular pistons, cylinder heads, crankcases, bushes or engine blocks, in each case for internal combustion engines.

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