DE102010055011A1 - Readily castable ductile aluminum-silicon alloy comprises silicon, magnesium, manganese, copper, titanium, iron, molybdenum, zirconium, strontium, and aluminum and unavoidable impurities, and phosphorus for suppressing primary silicon phase - Google Patents

Abstract

Readily castable ductile aluminum-silicon alloy comprises (in %): silicon (6-11.8%); magnesium (0.02-0.5); manganese (0.005-0.7); copper (0.0005-0.6), titanium (0.001-0.06); iron (0.03-0.3); molybdenum (maximum 0.2); zirconium (maximum. 0.2); optionally 70-400 ppm of strontium; and aluminum and unavoidable impurities (remaining) according to DIN EN 1676. The alloy comprises phosphorus (0.00001-0.0005) for suppressing a primary silicon phase, where the respective magnesium content is linked with the silicon content by a relation including magnesium = 1.018-0.083 % of silicon. An independent claim is also included for producing a thin-walled cast component exhibiting thickness of 1-50 mm with a well-castable, ductile aluminum?silicon alloy, comprising casting the alloy in a die casting having a mold wall temperature of 80-250[deg] C and a local solidification time of 5-30 seconds, or by die casting process, at a local solidification time of 5-40 microsecond, having a dendrite arm spacing of 5-60 mu m.

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,
Balance aluminum and production-related impurities according to DIN EN 1676 , Issue June 2010, as well as a method of making them. All information on the composition are in wt .-%.

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. B. better feeding properties, reduction of voids and a finer distribution of microporosities. Analogous to improving the castability and general improvements in strength properties by grain refining or by refining are found, but there are also outliers that 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 further surprising that, with narrowly limited, low phosphorus contents under the conditions described in claims 1 and 2, a non-lamellar microstructure with particularly favorable properties could be achieved.

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

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

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

• a) Si = 6-11.8%
• b) Mg = 0.02-0.5%
• c) Fe = 0.03-0.3% keeping the Mg / Si ratio within the range
• 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 10 ) pressed into the eutectic and thus successfully suppresses growth of the intermetallic phase.

As an additional measure e), according to the experience of the inventors, a monitoring of the casting conditions with appropriate setting of the casting parameters is recommended 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. B. cylinder heads of diesel engines, structural components or rims of car or truck wheels.

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, reference is made to the attached thermal analysis curve (see 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 a-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 in the conductivity, in particular in the case of unrefined alloys, by more than 10% shows how much influence the alloy composition according to the invention has on the formation of a fine-cell, particularly uniform microstructure.

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.

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

1 : Detail of the experimental setup for chill casting

2a : Training of the test bars (casting condition)

2 B : Training of the test bars after mechanical processing

3 : CAD drawing of a sample plate after removal from the die

4 : worked train sample

5 : Alloy table

6a , b: Test tables with information on the casting method, heat treatment, strength values and elongation values for alloys according to the prior art and for the alloys according to the invention

7a , b: micrographs of comparative alloys Nos. 1 and 5 according to the alloy table

8a , b: Micrographs of alloys Nos. 7 and 11 according to the alloy table

9a , b: Table with electrical conductivity data for prior art alloys and for the alloys according to the invention

10 : Graphical representation of the thermal analysis

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

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

Out 3 is a die casting plate 5 visible with the longitudinal side LS and the broad side BS immediately after removal from the die. The sample plate 5 includes 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 8th and a vacuum connection 9 to recognize. From the die casting sample plate, the flat tensile bars are removed for the tensile tests to be carried out later. The flat sides of the sample page are not processed.

In 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 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 alloying components specified in the claims, with the balance being aluminum and production-related impurities according to US Pat DIN EN 1676 , Edition June 2010, is to be supplemented.

6a , b contains the experimental tables with information on the casting method, 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 7a , Federation 8a , b show micrographs of prior art alloys (Alloy Nos. 1 and 5) and alloys having the inventive composition (Alloy Nos. 7 and 11). It is obvious to the person skilled in the art 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 contain no finishing additives. 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 7a with dark primary silicon particles in the middle). Even a standard AlSi 11 die-cast alloy shows an unfavorable lamellar structure (see micrograph 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 6a , Federation 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 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 by coarsening in the structure, the conductivity σ, measured in m / Ω mm ² , has low values, eg. B. Alloy 1 (standard AlSi 7) the value of 22.2 (see tables acc. 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, z. B. 8a , b, checked. It can be seen from this that the inventively assembled alloys without finishing additives, z. B. No. 7 and 8, which had typical structural features of a refined structure.

In the micrographs of the alloy AlSi 7 according to the invention (test chamber 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 8a . 8b ), which are embedded in a fine-celled eutectic.

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

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

Phosphorus promotes the early rhyme formation 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 supercooling 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 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 scanning electron micrographs.

The advancing conversion of a framework-forming silicon into a lamellar casting structure, which was previously regarded as mandatory, and further into a granular to compact structure with the formation of FeSi phases could be stopped by deliberate adjustment of the alloy contents. This resulted in a pure next to the α-aluminum Eutectic, in which any existing 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 2a in an open mold according to 1 decanted. These samples were then analyzed according to 2 B Tension rods are unscrewed and checked after casting EN 10002-1: 2001 (EN) or a T6 heat treatment (535 ° C / 6 h, water quenched, 180 ° C / 6 h cooled air) 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.

4 shows a tensile test made from a die casting plate 3 , These test bars were after EN 10002-1: 2001 (EN) with a thickness of 3 mm from the die casting plate according to 3 worked out and on a tensile tester after EN 10002-1: 2001 (EN) torn.

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.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• DIN EN 1676 [0001]
• DIN EN 1676 [0032]
• EN 10002-1: 2001 (D) [0047]
• EN 10002-1: 2001 (D) [0049]
• EN 10002-1: 2001 (D) [0049]

Claims (3)

Highly 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 and optionally 70-400 ppm strontium, remainder aluminum and production-related impurities according to DIN EN 1676, issue June 2010, characterized in that the alloy for suppression of the primary silicon phase has a content of 0.00001-0.0005% phosphorus and the respective magnesium content is linked to the silicon content by the relationship: Mg ≤ 1.018-0.083% Si. A castable, ductile AlSi alloy having a minimum Rp _0.2 strength of 60 MPa and an elongation of greater than 7% in the as-cast and unfired state, according to claim 1, characterized in that the as-cast conductivity measured with the forster probe is AlSi alloys are above 26 m / Ω mm ² and for AlSiMg alloys above 22 m / Ω mm ² . Process for the production of a thin-walled cast part (wall thickness 1.0 to 50 mm) with a ductile, ductile AlSi alloy according to one of the preceding claims 1 to 2, characterized in that the alloy in the nodular cast iron at a mold wall temperature in the range of 80 to 250 ° C and a local solidification time in the range of 5 to 30 sec, or cast in the die casting process with a local solidification time in the range of 5-40 ms with a Dendritenarmabstand DA3 of 5 .mu.m to 60 .mu.m.

Images