EP2471966B1 - Easily castable, ductile AlSi alloy and method for producing a cast component using the AlSi cast alloy - Google Patents

Description

The invention relates to a readily castable, ductile AlSi alloy consisting of 6-11.8% silicon, 0.02-0.5% magnesium, 0.005-0.7% manganese, 0.0005-0.6% copper, 0.001-0.06% titanium, 0.03-0.3% iron, and max. 0.2% molybdenum and max. 0.2% zirconium, optionally 70-400 ppm strontium,

Rest of aluminum and production-related impurities according to DIN EN 1676, issue June 2010, as well as a process for their preparation. All information on the composition is in% by weight.

The invention further relates to a method for producing a thin-walled casting with a wall thickness of 1.0 - 50 mm.

Basically, unrefined AlSi materials solidify either lamellar or grainy, with a lamellar structure often resulting in surface defects and possibly also sticking in the mold. Therefore, the granular structure which can be refined by adding strontium or sodium is preferred.

It is also known that a fine-grained or refined structure is associated with better casting properties, such as better feeding properties, reduction of voids and a finer distribution of microporosities. Be analogous to improving the castability also general improvements in the strength properties by grain refining or by refining identified, but there are also outliers here, which can lead to significant damage due to poor ductility in the production of highly stressed high-volume parts and as a result of strength fluctuations.

This is where the invention begins. The task was to develop a highly castable, ductile AlSi alloy for highly stressed high-volume parts, which already has a minimum strength Rp _0.2 of 60 MPa and an elongation of greater than 7% in the cast state, or a finite strength greater in the finished state 100 MPa or an elongation greater than 10% achieved.

This object is achieved by the features specified in the claims 1 to 3 features. In a detailed study of the fracture pattern of highly stressed castings, the inventors have found that primary silicon may play a crucial role in the ductility and setting behavior of AlSi materials.

On the basis of this finding, several tests were carried out with different alloys, but it has been shown that it is not only important to suppress the suppression of the primary silicon, but also that the other alloy constituents have to be strictly limited in order to achieve the desired ductility and castability. It was also surprising that, in the case of narrowly limited, low phosphorus contents under the conditions described in patent claims 1 and 2 described conditions a non-lamellar structure could be achieved with particularly favorable properties.

Even with small amounts of additional alloy constituents or exceeding the phosphorus content of more than 5 ppm, primary silicon and a lamellar structure were found. Apparently, even the lowest levels of phosphorus together with aluminum as aluminum phosphide can nucleate primary silicon and other intermetallic phases.

1. a) Si = 6 - 11,8 %
2. b) Mg = 0,02 - 0, 5 %
3. c) Fe = 0,03 - 0,3 %
   wobei das Mg/Si-Verhältnis im Bereich gehalten wird
4. d) Mg ≤ 1,018 - 0, 083 % Si

It has further been recognized that iron-silicon based intermetallic phases, for example so-called FeSi phases, can be advantageously prevented from growing under the following conditions:

1. a) Si = 6 - 11.8%
2. b) Mg = 0.02-0.5%
3. c) Fe = 0.03 - 0.3%
   wherein the Mg / Si ratio is kept in the range
4. d) Mg ≤ 1.018-0.083% Si

Under these conditions it is achieved that the FeSi phases crystallize later, and only when the AlSi eutectic solidifies. The FeSi phase becomes liquid / solid at the phase transition (see FIG. 10 ) pressed into the eutectic and thus successfully suppresses growth of the intermetallic phase.

As an additional measure e) it is recommended, according to the experience of the inventors, to monitor the casting conditions with appropriate setting of the casting parameters, so that the dendrite arm spacing DAS is below 60 μm, preferably below 40 μm. Under these conditions, even with high-volume parts a constant castability of the ductile AlSi alloy with high strength values is guaranteed. In this way, high-stress automotive parts, such as cylinder heads of diesel engines, structural components or rims of car or truck wheels can be produced.

The structure produced under the conditions described does not contain large or even plate-like precipitates, but is characterized by many small finely divided particles interdendritically incorporated into the AlSi structure. By "large particles" is meant a particle diameter of about 5 - 10 microns.

For an explanation of the processes, please refer to the attached thermal analysis curve (see FIG. 10 ), which shows only a short exothermic reaction in the relevant temperature range during the cooling and the phase transition from "liquid" to "solid". Thus, in addition to the pure α-aluminum, the microstructure consists only of the eutectic and, together with it, the most finely divided intermetallic phases crystallized.

This is also evidenced by the measurements of the electrical conductivity carried out on numerous samples. The increase of the conductivity, in particular in the case of unrefined alloys, by more than 10% shows how much influence the alloy composition of the invention has on the Training a fine-celled, particularly uniform structure has.

The mode of action of the other alloy components could not be clarified in detail, however, outside of the areas mentioned in the claims an influence on the crystal structure, which was accompanied by a deterioration of the ductility. It was also surprising that, contrary to the assumption that a lamellar structure was obtained at low levels of finishing additives, a granular structure appeared in the field of application according to the invention with correspondingly favorable processing properties. Thus, a quasi-finished casting structure with very low additions according to claims 1 and 2 can be achieved.

The patents DE 10323741 B and EP 1687742 B disclose Al-Si alloys.

FIG. 1:

Detail des Versuchsaufbaus für Kokillenguss

FIG. 2a:

Ausbildung der Probestäbe (Gusszustand)

FIG. 2b:

Ausbildung der Probestäbe nach mechanischer Bearbeitung

FIG. 3:

CAD-Zeichnung einer Probenplatte nach der Entnahme aus der Druckgussform

FIG. 4:

bearbeitete Zugprobe

FIG. 5:

Legierungstabelle

FIG. 6a, b:

Versuchstabellen mit Angaben zum Gießverfahren, Wärmebehandlung, Festigkeitswerten und Dehnungswerten für Legierungen nach dem Stand der Technik und für die erfindungsgemäßen Legierungen

FIG. 7a, b:

Schliffbilder von Vergleichslegierungen Nr. 1 und Nr. 5 gemäß Legierungstabelle

FIG. 8a, b:

Schliffbilder der Legierungen Nr. 7 und Nr. 11 gemäß Legierungstabelle

FIG. 9a, b:

Tabelle mit Angaben der elektrischen Leitfähigkeit für Legierungen nach dem Stand der Technik und für die erfindungsgemäßen Legierungen

FIG. 10:

Graphische Darstellung der thermischen Analyse

In the following the invention will be explained in more detail with reference to several embodiments. Show it:

FIG. 1:

Detail of the experimental setup for chill casting

FIG. 2a:

Training of the test bars (casting condition)

FIG. 2 B:

Training of the test bars after mechanical processing

FIG. 3:

CAD drawing of a sample plate after removal from the die

FIG. 4:

processed tensile test

FIG. 5:

alloy table

FIG. 6a, b:

Tables listing the casting method, heat treatment, strength values and elongation values for alloys after State of the art and for the alloys of the invention

FIG. 7a, b:

Micrographs of comparative alloys Nos. 1 and 5 according to the alloy table

FIG. 8a, b:

Micrographs of alloys Nos. 7 and 11 according to the alloy table

FIG. 9a, b:

Table containing electrical conductivity data for prior art alloys and alloys of the invention

FIG. 10:

Graphical representation of the thermal analysis

FIG. 1 shows a plan view of a gray cast iron mold as a detail of the experimental setup for the production of test specimens by means of chill casting, consisting of the mold 1, a cutout for chill casting with length L, height H, and feeder. 2

FIG. 2a shows a Kokillengussstab 3 before processing in side view and as a cross section AA with height H and width B, FIG. 2 B the formation of the chill casting rod in the form of a test bar 4 after mechanical processing.

Out FIG. 3 is a Druckgussprobeplatte 5 with the longitudinal side LS and the broad side BS immediately after removal from the die mold visible. The sample plate 5 comprises front overflows 5.1 to 5.4 with ejector openings 7.1 to 7.4 and rear overflows 6.1, 6.2. Furthermore, a sprue 8 and a vacuum connection 9 can be seen. From the die casting plate, the flat tensile bars for the later taken from tensile tests. The flat sides of the sample page are not processed.

In FIG. 4 the processed tensile test is shown. It was removed from the die casting sample plate in the manner described above.

The alloy table in FIG. 5 shows the composition in weight percent of 6 prior art alloys (Nos. 1-6) and 6 alloys with the composition of the present invention (Nos. 7-12). The alloy contents were specified for the alloy constituents specified in the patent claims, the remainder being aluminum and production-related impurities to be completed in accordance with DIN EN 1676, June 2010 edition.

FIG. 6a, b contains the test tables with information on the casting process, the heat treatment, the strength values and the elongation values for alloys according to the prior art and for the alloys according to the invention. Details of the experiments are given at the end of the description.

The FIG. 7a . b and 8a . b show micrographs of prior art alloys (Alloy Nos. 1 and 5) and alloys having the composition of the present invention (Alloy Nos. 7 and 11). For those skilled in the art, it can be seen that the micrographs of the alloys according to the invention (Alloy No. 7, AlSi 7 with 4 ppm phosphorus and die casting alloy No. 11 with 1 ppm phosphorus) have a refined structure, although the alloys have no Refining additives included. In contrast, the standard AlSi 7 alloy (alloy No. 1) exhibits a microstructure with primary silicon particles which has a lamellar structure and therefore can be expected to have unfavorable processing properties (micrograph FIG. 7a with dark primary silicon particles in the middle). Even a standard AlSi 11 die-cast alloy shows an unfavorable lamellar structure (see micrograph FIG. 7b with primary silicon particles at the lower right edge) (Alloy No. 5).

The inventively found favorable mechanical properties in a suppression of Primärsiliziumphasen in the cast structure can be achieved not only in unrefined, but also in refined casting alloys, although the increase in elongation and strength in the refined alloys is not as clear as in the unrefined. This shows the comparison of the tables in the FIG. 6a, b and FIG. 9a, b ,

From the comparison, the particularly advantageous effects on the mechanical properties of the casting alloys according to the invention can be seen. Out FIG. 7 . 8th In particular, it appears that outside the regions according to the invention a lamellar structure with the above-mentioned negative processing properties occurs.

This was also confirmed by an eddy current test with the forester probe. The principle of the test is based on the fact that due to coarsening in the microstructure the conductivity σ, measured in m / Ω mm ² , has low values, eg alloy 1 (standard AlSi 7) the value of 22.2 (see tables acc. FIG. 9a, b ) or alloy 3a of 20.8.

On the other hand, the alloys 7 and 9a according to the invention have significantly higher conductivities with 26.4 (AlSi 7) and 22.2 (AlSi 7 Mg 0.3), which makes it possible to conclude a fine and uniform microstructure. This result was based on micrographs, eg FIG. 8a . b , checked. It can be seen from this that the alloys composed according to the invention without finishing additives, for example Nos. 7 and 8, had the typical structural features of a finished structure.

In the micrographs of the alloy AlSi 7 according to the invention (test number 7) and the alloy AlSi 11 according to the invention (test number 11), relatively uniformly shaped, rounded dendrite arms of the α-aluminum can be seen (see FIG. 8a . 8b ), which are embedded in a fine-celled eutectic.

The unrefined AlSi 7 alloy (test number 7) shows in the micrograph FIG. 8a naturally less quantitatively eutectic than the AlSi 11 alloy according to microsection FIG. 8b but both show a pronounced refined structure.

The tests have shown that a minimum content of 0.1 ppm phosphorus is required in order to avoid the risk of Envelope in the growth morphology during the cooling phase to meet with the formation of unwanted primary silicon particles.

Phosphorus favors the early nucleation of silicon and leads to a granular solidification morphology. To form a fine eutectic phase with a small proportion of finely divided FeSi phases, however, excessive overcooling of the melt must be prevented.

According to the invention, for this purpose, in an alloy according to claim 1 or 2, a control of the cooling rate according to claim 4 is made.

This is based on FIG. 10 explained below, wherein the curve C represents a typical cooling curve in the system AlSi 7 to 11.

If one follows the cooling curve C along the time axis at decreasing temperatures, the formation of α-mixed crystal begins with the onset of solidification. In this case, the deliberately influenced exothermic growth of the dendrites slows down the cooling rate and thus the curve in the region a, b form a steeper or shallower inclination angle γ with the time axis.

With a content of 0.1-5 ppm of phosphorus and the adjustment of the alloy composition according to claim 1, it is possible to suppress the precipitation of the FeSi phases by a steeper course of the cooling curve, but nevertheless to produce a silicon skeleton for the formation of the eutectic. This could be confirmed by taking pictures with the raster electron microscope.

The so far regarded as mandatory progressive conversion of a framework-forming silicon first into a lamellar cast structure and further into a granular to compact structure with the formation of FeSi phases. Could be stopped by targeted adjustment of the alloy contents. In addition to the α-aluminum, this resulted in a pure eutectic, into which any remaining finely divided crystallized intermetallic phases are incorporated.

Subsequently, the casting conditions are compared, so that it can be seen that the castings with inventive structure structure according to claim 3 - manufactured under the conditions specified in claim 4 - bring particularly advantageous processing properties, especially in the production of mass production parts.

chill casting

The alloy was melted in a 60 kg crucible and heated to a temperature of about 730 ° C. This was followed by a degassing treatment by means of a rotor. Nitrogen was introduced as the gas. After a standing time of approx. 20 min, samples were taken according to FIG. 2a in an open mold according to FIG. 1 decanted. These samples were then analyzed according to FIG. 2 B Tension rods were removed and either cast as cast to EN 10002-1: 2001 (D) or subjected to a T6 heat treatment (535 ° C / 6h, water quenched, 180 ° C / 6h air cooled) and then tested.

diecast

In a 200 kg oven, the individual alloy variants were melted and brought to a temperature of 720 ° C. Casting was done on a 850 t die casting machine. The filling chamber was filled manually. The mold was provided with a vacuum vent to prevent air or mold inclusion in the sample.

FIG. 4 shows a tensile test made from a die casting plate FIG. 3 , These test pieces were made according to EN 10002-1: 2001 (D) with a thickness of 3 mm from the die casting plate according to FIG. 3 machined and torn on a tensile testing machine according to EN 10002-1: 2001 (D).

Results:

It is surprisingly found in the variants according to the invention that the test bars with a P content <5 ppm were always far superior in terms of elongation.

The metallographic evaluation showed that at P-contents P> 5 ppm primary silicon also occurred at 7% silicon. The samples with P <5 ppm even showed a grafted texture, although no finishing agent was added.

Claims (3)

1. Readily castable, ductile AlSi alloy comprising

   6-11.8% of silicon,

   0.02-0.5% of magnesium,

   0.005-0.7% of manganese,

   0.0005-0.6% of copper,

   0.001-0.06% of titanium,

   0.03-0.3% of iron,
   as well as max. 0.2% of molybdenum and max. 0.2% of zirconium and optionally 70-400 ppm of strontium,
   balance aluminium and impurities conditional to manufacturing according to DIN EN 1676, June 2010, characterized in that, to suppress the primary silicon phase, the alloy has a phosphorus content of 0.00001 - 0.0005%, and the respective content of magnesium is linked to the content of silicon by the following relation:

   Mg ≤ 1.018 - 0.083% Si.
2. Readily castable, ductile AlSi alloy which in the cast state and unrefined has a minimum strength Rp 0.2 of 60 MPa and an elongation of greater than 7%, according to claim 1, characterized in that the conductivity in the cast state, as measured by the Förster probe, is above 26 m/Ω mm² in the AlSi alloys and above 22 m/Ω mm² in the AlSiMg alloys.
3. A process for producing a thin-walled casting (wall thickness 1.0 to 50 mm) with a readily castable, ductile AlSi alloy according to any one of the preceding claims 1 to 2, characterized in that the alloy is cast by permanent mould casting at a wall temperature of the permanent mould in the range of 80 to 250 °C and with a local solidification time in the range of 5 to 30 sec, or by pressure die casting with a local solidification time in the range of 5 to 40 ms with a dendrite arm spacing DAS of 5 µm to 60 µm.

Images