HK1071171B - Aluminum-silicon alloys having improved mechanical properties - Google Patents

Description

The invention relates to a method for improving the mechanical properties of aluminium-silicon alloys.In a more precise manner, the invention relates to a heat treatment method for improving the ductility of articles consisting of a cast or kneaded aluminium alloy with eutectic phase, preferably containing further alloying and/or impurity elements, which are subjected to a filament treatment followed by extrusion.

The invention also relates to an article of aluminium-silicon alloy with a eutectic phase component, preferably containing at least one refining element, and possibly containing magnesium and other alloying and/or contaminating elements, consisting essentially of an α-Al matrix and silicon secretions.

Aluminium forms a simple eutectic system with silicon, with a eutectic point at a Si concentration of 12,5% by weight and a temperature of 577 °C.

By alloying magnesium, which can be dissolved in the α-Al matrix at a temperature of approximately 550 °C to a maximum of 0.47% by weight, a significant increase in the strength of the material can be achieved by heat treatment and the resulting Mg2Si excretions.

The remaining molten material may solidify euthetically when an Al-Si-Mg alloy is cooled, leaving behind a rough, plate-like form in this silicon.

The mechanical properties of semi-finished products or articles of aluminium alloys can be significantly affected by heat treatment processes and the heat treatment conditions are defined in the European standard EN 515.

The following heat treatment states of the materials with the abbreviations are further specified in the description: F Manufacture stateT5 Removed from manufacture temperature and hot-storageT6 Solution heated and hot-storageT6x heat treated according to the inventionT4x heat treated according to the invention

For the marketing or industrial use of articles made of Al-Si alloys, the material properties are important, but also the cost and economic conditions of production, especially because of the long heat treatments at higher temperatures and the necessary messaging operations that may be required by the so-called gravity crunch in a long-term heating process.

In principle, it can be stated that an Al-Si alloy in the state F usually has low material strength values Rp and relatively high refractive index A.

At a heat treatment state T5, i.e. at a temperature which is not the manufacturing temperature and which is heat-treated, for example at 155°C to 190°C for 1 to 12 hours, higher strength values Rp are achieved, but at lower refractive indices A of the samples.

In a heat treatment state corresponding to T6 with a solution glow at a temperature of, for example, 540°C with a duration of 12 hours and subsequent heat storage, a substantial increase in the strength of the material can be achieved with approximately the same fracture expansion of the samples or ductility of the material compared to state F. The long solution glow time allows, for example, a favourable diffusion of magnesium atoms in the material, which after a shrinkage and heat storage of the object, produces uniformly fine Mg2Si matrix sections in the α-Al matrix, sections which significantly increase the material strength.

However, long-term high-temperature solution heat treatments have the disadvantage of gravity creep of the part and an expensive temperature-time treatment process, as mentioned earlier, so that in many cases, for economic reasons, T6 is not used to achieve the highest strength and ductility of the material and a T5 treatment state is chosen for the object.

The present invention aims at creating a new economic heat treatment process which can significantly increase the ductility of the material without significantly reducing the strength of the material compared to T6 or achieving a much higher ductility and strength of the material compared to T5.

The purpose of the invention is also to specify a microstructure of an object of the type described above which gives an advantageous mechanical material property.

The objective of the procedure is to achieve the solution heat treatment as a shock heat treatment consisting of a rapid heating to a heat temperature of 400°C to 555°C, a holding at this temperature for a holding time of not more than 14,8 minutes and then a forced cooling to substantially room temperature.

The advantages of the invention are essentially that a simple high temperature - short-term heat treatment achieves the highest ductility of the material. Furthermore, a so-called shock heat treatment causes little to no delay in the component or the object, so that no adjustment is necessary. The short-term heat treatment also has a high cost-effectiveness and can be easily integrated into a manufacturing sequence, for example by means of a flow furnace. If, as can be expected, the treatment with a holding time of less than 6,8 minutes, preferably between 1,7 and 5 minutes, the greatest ductility increases are achieved for the majority of AlSi alloys.

If the object is to be heated after the shock heating, it is preferable to set it at a temperature of between 150°C and 200°C with a duration of 1 to 14 hours.

It may also be advantageous from a material point of view if the material is stored cold at essentially room temperature after the shock heating.

The further problem of the invention is solved by the fact that the silicon secretions are spheroidized in the eutectic phase and have a mean cross-sectional area, ASi, of less than 4 μm2. The following is a formal representation of the interface determination, indicating the factors: $A_{\mathrm{si}}=\frac{1}{n}\sum _{k=1}^{n}A\le 4\mathrm{\hspace{1em}\mu }{m}^2$ ASi = mean area of silicon particles in μm2A = mean area of silicon particles per image in μm2n = number of measured

The advantages of such a microstructure are essentially that the spheroidisation of the Si secretions and their fineness significantly reduces the initiation of cracks in the material and improves the material ductility. In other words, the spheroidisation and the small size give a favourable morphology of the brittle eutectic silicon and lead to much higher refractive indices of the material.

In the process engineering, but also for high material refractive rates, it may be advantageous if the silicon secretions are spheroidized in the eutectic phase and have an average cross-sectional area of less than 2 μm2.

If, as shown in the development work, the inventive solution is obtained by the mean free path length between the silicon particles, λSi, in the eutectic phase ratio, defined as the root from a square measuring surface divided by the number of silicon particles contained in it, having a size of less than 4 μm, preferably less than 3 μm, and in particular less than 2 μm, a particularly homogeneous stress distribution is achieved at the lowest stress peaks in the loaded material, because the distance between the small-surface silicon is largely dependent on the flow distribution of the material at a corresponding stress ratio. ${\lambda }_{\mathrm{si}}=\frac{1}{n}\sum _{k=1}^n\sqrt{\frac{A_{\mathrm{Quadrat}}}{N_{\mathrm{Silizium}}}}\le 4\mathrm{\mu m}$ λSi = mean distance between Si particlesA Square = square reference surface in μm2

A solution annealing according to the state of the art, which is intended as a long-term annealing with a duration of 2 to 12 hours for diffusion of the alloying components active for curing and their enrichment in the mixed crystal, has as a side effect spheroidization of the silicon particles, but these particles are very large and coarse due to the long duration of the annealing, which can have a negative effect on the fracture behaviour of the material.

The silicon particles undergo diffusion-controlled growth as the ignition time increases, with the initially favourable high spheroidization density, ξSi, decreasing.

The highest ductility of an Al-Si alloy material was found in a solution to the problem of the invention when the mean spheroidization density, ξSi, defined as the number of spheroidized eutectic silicon particles per 100 μm2 has a value greater than 10, but preferably 20. ${\xi }_{\mathrm{si}}=\frac{1}{n}\sum _{k=1}^n\frac{N_{\mathrm{si}}}{A}\times 100\ge 10$ ξSi = mean spheroidization density of eutectic Si particlesNSilicium = number of Si particlesA = reference area in μm2n = number of images measured

As a precaution, the relationship was formally announced again.

The work has shown that essentially all the al-Si alloys containing eutecticum can be given a structure according to the invention and that the articles formed from them have high ductility values of the material.

The following is a detailed description of the invention, using test results and illustrations. Figure 1Ball representation: Mechanical material values depending on the heat treatment stateFig. 2AlsoFig. 3REM- image of a grinding machineFig. 4AlsoFig. 5Dependency of the mean surface area of the Si emissions from the ignition timeFig. 6AlsoFig. 7Mean free path length between the Si particlesFig. 8Mean spheroidization densityFig. 9Ball diagram: Mechanical material properties of various Al-Si alloysFig. 1Figure 9Number values

The values of the heat treatment state T6 (12 hours 540°C + 4 hours 160°C) of the material are compared with those obtained with the T6x method of the invention after a shock ignition time of 1 minute (T6x1), after 3 minutes (T6x3) and after 5 minutes (T6x5) at a temperature of 540 °C. All the samples were cooled for 4 hours at a temperature of 160°C. The results of the tests show that the samples show a significantly higher rate of ignition after a shock treatment of T6x3 compared to a steady-state ignition rate of 60%.

In Fig. 2 the state values F, T4x3, T5, T6x3 and T6 are again compared in the form of a bar with respect to Rp0.2 and the fracture length A in the same sample production.

Fig. 3 and Fig. 4 show the scattering of Si emissions by the raster electron microscope. As regards the method of capture and evaluation, it should be noted that suitable binary images must be available to quantify the scattering images. Up to and including a duration of 2 hours, the scattering was done with the scattering microscope after the scattering had been estimated 30 seconds before with a solution of 99.5% water and 0.5% fluidic acid. From 4 hours of duration, the scattering was evaluated with the cellar solution and the images could be taken with the Adobe light microscope for 12 hours. All images were then reprocessed with the Photoshop Q5.0 after processing and scattered with the medium-speed V2.2F diffusion analysis of the image, with the minimum detection value of 0.2 μF. The scattering of the liquid is the same after 3 minutes of processing (Fig. 4 shows the same amount of liquid after 3 hours of exposure to the liquid.

In Fig. 5 and Fig. 6 the mean area ASi of the silicon particles in the grinding test is shown as a function of the flash time at 540°C. The logarithmic time axis of Fig. 5 shows the increase in the mean area ASi of the Si particles, which characterizes the particle size. The diffusion-related increase in the mean silicon areas within the first 60 minutes can be seen in Fig. 6. The mean size ASi of the silicon particles increasing with the flash time is largely dependent on the initial size of the Si particles in the eutectic. In the case of the extremely thin silicon particles, which are very fine and well distributed, a critical size of AS2 can be reached at a time of 4 μs, which is much less than the critical size of the silicon particles in the eutectic.

The change in the mean distance between the Si particles depending on the ignition time is shown by the test results in Figure 7.

Finally, Fig. 8 shows the decrease in the mean spheroidization density, ξSi, depending on the ignition time. The steep decrease in the mean spheroidization density starts as early as 1.7 minutes and leads to a marked loss of ductility from a value of ξSi < 10. At higher ignition temperatures this value can be reached after 14 to 25 minutes, with a density value of greater than 20 for superiorly high refractive indices.

In Fig. 9 the measured values of the limit of tensile strength and refractive index shown in Table 1 are represented by a bar diagram of 8 AlSi alloys of different compositions, all of which have an increase in ductility according to the invention.

	F		T5		T6x3		T6
Varianten	Rp [MPa]	A [%]	Rp [MPa]	A [%]	Rp [MPa]	A [%]	Rp [MPa]	A [%]
Alsi7Mg03	121.7	13.0	167.5	9.9	228.5	16.7	259.8	10.6
Alsi7Mg05	143.9	10.4	175.8	9.3	240.2	13.9	311.7	9.1
Alsi7Mgx	159.8	8.3	197.2	6.8	265.2	10.1	322.9	7.6
Alsi6Mgx	159.7	10.2	195.3	7.8	250.6	8.9	318.6	6.5
Alsi5Mgx	154.9	10.1	189.6	7.5	240.6	9.5	313.6	8.7
+Mn04	157.1	10.6	183.7	6.9	252.7	7.4	322.7	7.6
+Mn08	154.8	9.9	184.0	6.6	255.9	6.7	324.4	4.9
AlSi5Mgxx	211.7	3.5	256.4	2.5	242.1	5.1	291.6	5.3

Claims (11)

1. A thermal-treatment process for improving the ductility of articles consisting of a cast or wrought, preferably enriched or purified aluminum-silicon alloy witih a eutectic phase, which alloy optionally contains other alloying and/or contaminating elements such as magnesium, manganese, iron and the like, said articles being subjected to an annealing treatment and subsequent aging, characterized in that the annealing is effected as shock annealing comprising rapid heating of the material to an annealing temperature of 400°C to 555°C, maintaining it at this temperature for a holding period from at most 14.8 minutes, and subsequently force cooling it to essentially room temperature.
2. The process as defined in claim 1, characterized in that the shock annealing is effected with a holding time of less than 6.8 minutes, preferably with a time span from at least 1.7 minutes to a maximum of 5 minutes if necessary.
3. The process as defined in any one of claims 1 or 2, characterized in that the aging of the article, which follows the shock annealing, is effected as thermal aging at a temperature in the range between 150°C and 200°C with a period from 1 to 14 hours.
4. The process as defined in any one of claims 1 to 2, characterized in that the aging of the article that follows the shock annealing is effected as cold aging at essentially room temperature.
5. An article of an aluminum-silicon alloy, optionally containing other alloying and/or contaminating elements such as magnesium, manganese, iron and the like, with a eutectic phase, consisting essentially of an α_Al matrix and silicon precipitates, characterized in that the silicon precipitates in the eutectic phase are spheroidized and have an average cross-sectional area A_Si of less than 4µm², whereby $A_{\mathrm{si}}=\frac{1}{n}\sum _{k=1}^{n}A\le 4\mathrm{\hspace{1em}\mu }{m}^2$

   A_Si = average area of the silicon particles in µm²

   A = average area of the silicon particles per image, in µm²

   n = number of images sampled
6. The article as defined in claim 5, characterized in that the silicon precipitates in the eutectic phase are spheroidized and have an average cross-sectional area of less than 2µm².
7. An article of an aluminum-silicon alloy, optionally containing other alloying and/or contaminating elements such as magnesium, manganese, iron and the like, with a eutectic phase, consisting essentially of an α_Al matrix and silicon precipitates, characterized in that the average free path length λ_Si between the silicon particles in the eutectic phase defined as the root of a square measured area divided by the number of silicon particles contained within it is of a size that is less than 4µm, whereby ${\lambda }_{\mathrm{si}}=\frac{1}{n}\sum _{k=1}^n\sqrt{\frac{A_{\mathrm{Quadrat}}}{N_{\mathrm{Silizium}}}}\le 4\mathrm{\mu m}$ wherein

   λ_Si = average spacing between the silicon particles in µm²

   A_Quadrat = square reference area, in µm²

   N_Silicon = number of silicon particles

   n = number of images sampled
8. The article as defined in claim 7, characterized in that the average free path length is are of a size that is less than 3µm, and preferably less than 2µm.
9. An article of an aluminum-silicon alloy, which preferably has an enriching element and optionally contains other alloying and/or contaminating elements such as magnesium, manganese, iron and the like, which alloy has an eutectic phase, consisting essentially of an α_Al matrix and silicon precipitates, characterized in that the average spheroidization density ξ_Si, defined as the number of spheroidized eutectic silicon particles per 100µm² is greater than 10. ${\xi }_{\mathrm{si}}=\frac{1}{n}\sum _{k=1}^n\frac{N_{\mathrm{si}}}{A}\times 100\ge 10$

   ξ_Si = Mean spheroidization density of the eutectic Si particles

   N_Silicon = Number of silicon particles

   A = Reference area in µm²

   n = Number of images sampled
10. The article as defined in claim 9, characterized in that the average spheroidization density is greater than 20.
11. The article as defined in any one of claims 5 to 10, produced according to the method set out in any one of claims 1 to 4, characterized in that this is manufactured by the thixocasting method.