WO2003040423A1

WO2003040423A1 - Aluminum-silicon alloys having improved mechanical properties

Info

Publication number: WO2003040423A1
Application number: PCT/AT2002/000309
Authority: WO
Inventors: Erhard Ogris; Peter Uggowitzer; Josef Wöhrer
Original assignee: Salzburger Aluminium Aktiengesellschaft
Priority date: 2001-11-05
Filing date: 2002-11-05
Publication date: 2003-05-15
Also published as: EP1442150A1; CA2465683A1; CA2465683C; JP2005508446A; EP1442150B1; ATE350507T1; SI1442150T1; HK1071171A1; KR20050043748A; DK1442150T3; DE50209192D1; US20050000608A1; US20100193084A1; HUP0401962A2; ES2280578T3; PT1442150E; AT411269B; ATA17332001A; CN100366782C; CN1602368A

Abstract

The invention relates to a heat treatment method for articles composed of substantially Al-Si alloys that contain a eutectic phase, and to articles that consist of these alloys. In order to improve the ductility of the material or to increase the elongation after fracture, an annealing process is carried out in the form of shock annealing, said process comprising the following steps: rapidly heating the material to an annealing temperature of from 400 to 555 DEG C, maintaining it at this temperature for a period of not more than 14.8 minutes, force-cooling it, and then aging the article. The inventive article comprises spheroidized silicon precipitations in the eutectic phase portion with an average plane of section ASi of less than 4 mu m2 and/or an average distance between the silicon particles lambda Si of less than 4 mu m and/or an average spheroidization density xi Si of greater than 10.

Description

Aluminum-silicon alloys with improved mechanical

characteristics

The invention relates to a method for improving the mechanical properties of aluminum-silicon alloys. Specifically stated, the invention relates to a heat treatment process for improving the material ductility of objects consisting of a preferably refined or refined aluminum-silicon, optionally containing a further alloy and / or contaminant casting or wrought alloy with a eutectic phase component, which objects are subjected to an annealing treatment with subsequent aging.

Furthermore, the invention relates to an object made of preferably at least one refinement element, optionally containing magnesium and further alloy and / or impurity elements containing aluminum-silicon alloy with a eutectic phase component consisting essentially of an <X-Al matrix and silicon precipitates.

Aluminum forms a simple eutectic system with silicon, the eutectic point being at a Si concentration of 12.5% by weight and a temperature of 577 ° C.

By alloying magnesium, which can be dissolved up to a maximum of 0.47% by weight at a temperature of approx. 550 ° C in the α- _A i matrix, by means of a heat treatment, ..and the. Mg≥Si precipitates formed a considerable. Strength increase of the material can be achieved.

When an Al-Si-Mg melt cools, the residual melt can solidify eutectically, with silicon precipitating out in a rough, plate-like form. It has long been state of the art to add sodium or strontium to such alloys and thereby to hinder the growth of the silicon crystals during solidification, which is called refining or refining and is carried out consistently to improve the mechanical properties, in particular the elongation at break.

The mechanical properties of semi-finished products or objects made of aluminum alloys can be significantly influenced by heat treatment processes and the heat treatment conditions are defined in European Nϋxm EN 515. According to the standard, the letters F = state of manufacture and T = heat-treated to stable states. The respective heat treatment status is identified in detail by the number after the letter T.

Furthermore, the following heat treatment conditions are described in the description

Materials indicated with the abbreviations:

F State of manufacture

T5 Quenched from the manufacturing temperature and stored under warm conditions

T6 Solution annealed and aged warm

T6x heat treated according to the invention

T4x heat treated according to the invention

For the marketing or the industrial use of objects made of Al-Si alloys, the material properties on the one hand, but on the other hand also the costs or the economic conditions of the production are important, because longer annealing treatments at higher temperatures as well as necessary message processes caused by the so-called gravitational creeping may be necessary in the case of long-term annealing.

In principle, it can be stated that an Al-Si alloy in state F mostly has low material strength values R _p and relatively high elongation at break A.

With a heat treatment condition T5, i.e. quenched from the manufacturing temperature and aged, e.g. at 155 ° C to 190 ° C with a time period of 1 to 12 hours, higher strength values R _p are achieved with lower elongation at break A of the samples.

In the case of a heat treatment condition corresponding to T6 with solution annealing at a temperature of 540 ° C for a period of 12 hours and a subsequent heat aging, a substantial increase in the strength of the material with approximately the same elongation at break of the samples or ductility of the material can be compared with the condition F, can be achieved. The long solution annealing time enables, for example, an advantageous diffusion of magnesium atoms in the material, whereby fine, evenly distributed Mg ₂ Si precipitates are formed in the α- _A ι matrix after quenching and hot aging of the object, which precipitates decisively increase the material strength.

Solution heat treatments at high temperatures with a long duration, however, have the disadvantage of, as mentioned earlier, gravitational creeping of the part and a complex temperature-time treatment process. For economic reasons, it is therefore often avoided to achieve the highest strength and good ductility of the material with T6, and a treatment state T5 is selected for the object. The significantly lower material strength due to T5 may have to be compensated for by design changes to the component.

The invention now aims to provide a new economical method of heat treatment, with which the ductility of the material can be increased significantly without large drops in material strength compared to T6 or a significantly higher ductility and higher material strength compared to T5 is reached.

Furthermore, it is an object of the invention to provide a microstructure of an object of the type mentioned at the outset, which brings about advantageous mechanical material properties.

The objective of the process is achieved in that the solution heat treatment as an impact heat treatment consisting of rapid heating to an annealing temperature of 400 ° C.-555 ° C., holding at this temperature with a holding time of at most 14.8 minutes and a subsequent forced cooling is carried out essentially at room temperature.

The advantages achieved with the invention are essentially to be seen in the fact that the highest ductility values of the material can be achieved with a simple high-temperature, short-time annealing. Furthermore, a so-called impact annealing causes little or no component warping or warping of the object, so that it may also not be necessary to straighten it. The short-term annealing treatment is also highly economical and can be easily integrated into a manufacturing sequence, for example by means of a continuous furnace, an adjustment of the material strength can then mostly be done using a coordinated technology for arrnaiis storage. If, as can be advantageously provided, the shock annealing treatment is carried out with a holding time of less than 6.8 minutes, preferably with a time period of 1.7 to possibly at most 5 minutes, the majority of the Al-Si alloys are greatest increases in ductility.

If the article is warmed up after the annealing, it is advantageous to fix it at a temperature in the range between 150 ° C and 200 ° C for a period of 1 to 14 hours.

It can also be advantageous from a material technology point of view if the article, following the impact annealing, is cold-aged at substantially room temperature.

The further object of the invention is achieved in that the silicon precipitates are spheroidized in the eutectic phase portion and have an average cut area, Asi, of less than 4 μm.

The cut surface determination is formally shown below, with the factors identified:

^A sι = - n ∑ii ^A ≤ ⁴ μ ™ ²

Asi = average area of the silicon particles in μm ²

A = average area of silicon particles per image in μm ² n = number of measured

The advantages of such a microstructure can essentially be seen in the fact that crack initiation in the material is substantially reduced by the spheroidization of the Si precipitates and their fineness, and the material ductility is improved. In other words: the spheroidization and the small size provide a favorable morphology of the brittle eutectic silicon and lead to significantly higher elongation at break values of the material. The stress peaks at the phase interface Si-Al are reduced under mechanical load. A transcrystalline fracture of the material was also found in tests, which indicates its highest ductility. In terms of process technology, but also for high elongation at break values of the material, it can be advantageous if the silicon deposits in the eutectic phase portion are spheroidized and have an average cutting area of less than 2 μm ² .

If, according to the task, as was shown in the development work, the inventive solution is achieved in that the mean free path between the silicon particles, λ _Sj , in the eutectic phase component, defined as the root of a square measuring area divided by the number of silicon particles contained in it has a size of less than 4 μm, preferably less than 3 μm, in particular less than 2 μm, a particularly homogeneous stress distribution is achieved with the lowest stress peak values in the loaded material, because the distance between the small-area silicon particles essentially depends on the flow behavior of the Material affects in a corresponding stress state. The determination of the mean distance between the Si particles, λsi, is again shown formally below.

λsi = average distance between the Si particles

AQua rat ⁼ square reference area in μm ²

N silicon = number of Si particles n = number of images measured

A solution annealing according to the prior art, which is provided as a long-term annealing with 2 to 12 hours for a diffusion of the alloy components effective for hardening and their enrichment in the mixed crystal, also brings about a spheroidization of the silicon particles as a side effect, but these particles are due to the long Annealing time very large and roughly distributed, which can have an adverse effect on the fracture behavior of the material. It was quite surprising that a eutectic silicon network can be spheroidized invention by a brief shock annealing even in small periods of a few minutes to give a favorable micro ^S tructure of the material is achievable. It is important that the temperature for the shock annealing is as high as possible, but below the lowest melting phase, preferably 5 to 20 ° C below. The silicon particles are subjected to diffusion-controlled growth with increasing glow time, whereby the initially favorable high spheroidization density, ξsi, decreases.

Highest ductility of an Al-Si alloy object was found in a solution to the object of the invention if the mean spheroidization density, ξsi, defined as the number of spheroidized eutectic silicon particles per 100 μm ^{2 has} a value greater than 10, but preferably 20 ,

ξsi = average spheroidization density of the eutectic Si particles

Nsiii _z ium ⁼ number of Si particles A = reference area in μm ² n = number of images measured

As a precaution, the relationship was formally stated again.

The work has shown that essentially any of the Al-Si alloys containing eutectic can be provided with a structure according to the invention and that the objects formed therefrom have high ductility values of the material. Increasing the quality and improving the elongation at break are particularly efficient if the article is produced using the thixocasting process.

The invention is described in more detail below on the basis of examination results and images. Show it

Fig. 1 Bar graph: Mechanical material values depending on the heat treatment condition

Fig. 2 Likewise

Fig. 3 SEM image of a cut

Fig. 4 Likewise

Fig. 5 dependence of the average area of the Si precipitates on the annealing time

Fig. 6 Likewise

Fig. 7 Average free path between the Si particles

Fig. 8 Average spheroidization density Fig. 9 Bar chart: Mechanical material properties of various Al-Si alloys

Tab. 1 Numerical values from Fig. 9

In Fig. 1, the R _p o ^ yield strength values and the elongation at break values A of samples from a test component made of an alloy AlSi7MgO, 3, which part was produced in the thixocasting process, are shown in a bar graph: The values of the heat treatment state T6 (12 hours 540 ° C + 4 hours 160 ° C) of the material are compared to those using the method T6x according to the invention after an impact glow time of 1 minute (T6xl), after 3 minutes (T6x3) and after 5 minutes (T6x5) at a temperature of 540 ° C have been achieved. All samples were aged for 4 hours at a temperature of 160 ° C. The: Results of the tensile tests show that the samples have a significantly higher elongation at break after an impact heat treatment, the state T6x3 causing an increase in A of around 60% compared to T6.

In FIG. 2, with the same sample preparation, the state values F, T4x3, T5, T6x3 and T6 are again compared with respect to the R _p o_ ₂ and the elongation at break A in bar form. Striking increases in elongation at break values are considered comparatively ^'turn given. As can be seen from FIG. 2, the material can be aged cold (T4x3) or warm (T6x3) after an impact anneal with 3 minutes in order to obtain superior elongation at break properties according to the invention.

3 and 4 show scanning electron microscopes - images of Si precipitates. Regarding the recording and evaluation process, it should be noted: In order to be able to evaluate the micrographs quantitatively, suitable binary images must be available. Up to a glow time of 2 hours inclusive, the images were taken with the scanning electron microscope after the sections had been etched for 30 seconds with a solution of ^ 9.5% water and 0.5% hydrofluoric acid. After 4 hours of annealing, the sections were etched with the cellar solution and the images could be taken with the light microscope. All the images were then digitally post-processed with the Adobe Photoshop 5.0 program and evaluated with the Leica QWin V2.2 image analysis program, the minimum detection area being 0.1 μm ² . 3 shows the material AlSi7MgO, 3 after a usual T6 annealing time of 12 hours by means of an SEM image. 4 is the Microstructure of the same material according to a Stoßguart-se -aϊϊaitfflg "by _. IviTnuten reproduced. A spheroidization of the silicon precipitates is clearly visible after a short time (Fig. 4) and the diffusion-controlled growth thereof after long annealing times (Fig. 3).

5 and FIG. 6 show the average sectional area A _S J of the silicon particles during the grinding test as a function of the annealing time at 540 ° C. 4 with logarithmic time axis, the increase in the average sectional area of the Si particles, which characterizes the particle size, can be clearly seen. 6 shows the increase in the average silicon areas within the first 60 minutes due to diffusion. The mean size of the silicon particles, which increases with the annealing time, depends to a large extent on the initial size of the Si particles in the eutectic. Since in the case mentioned there is an extremely well refined and finely distributed silicon, the time in which a critical mean silicon area, Asi, of approx ,

The change in the mean distance between the Si particles as a function of the annealing time is shown on the basis of test results in FIG. 7. An increase in the mean distance between the Si inclusions is clearly evident.

Finally, the drop in the mean spheroidization density, ξsi, as a function of the glow time is shown in FIG. 8. The steep drop in the mean spheroidization density begins at 1.7 minutes and leads to a pronounced loss of ductility from a value of ξsi <10. At higher annealing temperatures, this value can be reached after 14 to 25 minutes, whereby a density value of greater than 20 must be provided for superior elongation at break values.

j In FIG. 9, the measured values with regard to the elastic limit and

Elongation at break, which are shown in Table 1, of 8 differently composed Al-Si

Alloys reproduced. According to the invention, an increase in material ductility is achieved for all alloys.

Tab. 1

Claims

claims

1.Heat treatment process to improve the material ductility of objects consisting of a preferably refined or refined aluminum-silicon, optionally further alloying and / or contaminating elements such as magnesium, manganese, iron and the like containing casting or wrought alloy with a eutectic phase component, which are objects of an annealing treatment with: Subsequent outsourcing, characterized in that the annealing treatment as a shock annealing treatment consisting of rapid heating to an annealing temperature of 400 ° C to 555 ° C, a holding at this temperature with a holding time of, preferably at least 1.7 minutes to a maximum of 14.8 Minutes and a subsequent forced cooling to essentially room temperature is carried out.

2. The method according to claim 1, characterized in that the shock annealing treatment is carried out with a holding time of less than 6.8 minutes, preferably with a period of at least 1.7 to optionally at most 5 minutes.

3. The method according to claim 1 or 2, characterized in that the subsequent annealing treatment of the article is carried out as a hot aging at a temperature in the range between I50 ° C and 200 ° C with a period of 1 to 14 hours.

4. The method according to claim 1 to 3, characterized in that the subsequent annealing treatment of the article takes place as cold aging at substantially room temperature.

5. Item from a preferably a refining element, optionally further alloy and / or impurity elements such as magnesium, manganese, iron. and the like containing aluminum-silicon alloy with a eutectic phase component, consisting essentially of an α _A ι matrix and silicon precipitates, characterized in that the silicon precipitates are spheroidized in the eutectic phase component and an average cutting area, A _S j, of less than 4 μm ² have. A _si = -∑ A <4μm ^: n k-1

As; = average area of the silicon particles in μm ²

6. Article according to claim 5, characterized in that the silicon deposits are spheroidized in the eutectic phase portion and have an average cutting area of less than 2 microns.

7. Object from a preferably having a finishing element, optionally containing further alloy and / or ner impurity elements such as magnesium, manganese, iron and the like, containing aluminum-silicon alloy with a eutectic phase component, consisting essentially of a CCAI matrix and silicon precipitates, characterized in that the mean free path length, λ _si , between the _: silicon particles in the eutectic phase component, defined as the root of a square measuring area divided by the number of silicon particles contained therein, has a size of less than 4 μm.

λsi = average distance between the Si particles

AQua rat - square reference area in μm ²

N silicon = number of Si particles n = number of images measured

8. The article of claim 7, characterized in that the mean free path has a size of less than 3 microns, preferably less than 2 microns.

9. Article made of a preferably a refining element, optionally containing further alloy and / or impurity elements such as magnesium, manganese, iron and the like, containing aluminum-silicon alloy with a eutectic phase component consisting essentially of an α _Λ ι matrix and silicon precipitates, characterized that the mean spheroidization density, ξ _{si> is} defined as the number of spheroidized eutectic silicon particles per 100 μm ² a Wt ^ fö & eHl ^ Θ bWzϊ.

ξsi = average spheroidization density of the eutectic Si particles

Silicon ⁼ number of Si particles

A = reference area in μm ² n = number of images measured

10. The article of claim 9, characterized in that the average spheroidization density has a value greater than 20.

1 1. Object according to claims 5-10, produced by the method 1 to 4, characterized in that it is produced in the thixocasting process.