EP1328666B1 - Preparing aluminium-silicon alloys - Google Patents

Abstract

A method for preparing Al-Si alloys by introducing into the molten aluminum, at a temperature of between 700 and 850° C., metallurgical silicon particles having a granulometry of less than 10 mm. The silicon particles, upon reaching the temperature of the molten aluminum, have the property of fragmenting into smaller particles.

Description

Field of the invention

The invention relates to a process for the production of aluminum-silicon alloys, more particularly alloys with more than 7% silicon, by introducing silicon metallurgical in liquid aluminum.

State of the art

Silicon is a fairly common addition element in aluminum alloys, in particular Al-Si-Mg alloys (6000 series) and Al-Si alloys (4000 series). In this last category of alloys, used mainly for the manufacture of molded parts, the content in silicon can be important, and sometimes exceed the content of the eutectic, which is around 13%. These alloys can contain other elements of addition such as magnesium, copper, manganese, zinc or nickel.

The production of these alloys is generally done in flame ovens or in kilns with induction, at temperatures of around 700 to 800 ° C. To the aluminum charge is added as soon as the start of the operation a charge of metallurgical silicon corresponding to approximately 75 to 90% of the quantity required. At this stage, the silicon is loaded into pieces and its dissolution in aluminum takes place gradually during the melting of the charge, which does not constitute in no way a brake on the productivity of the oven. Once the charge has melted, a sample is taken for analysis and a complementary addition of silicon is carried out for final title, operation whose duration, conditioned by the kinetics of dissolution of the silicon in the alloy mainly aluminum-based, is likely to limit the productivity of the oven in which the operation is carried out.

In the technique practiced until now, this final addition is in the form of silicon obtained from ingots, of mass always greater than 10 kg, crushed then crushed to obtain pieces of less than 10 mm, and, after sieving to 1 mm, a product of grain size 1-10 mm.

The kinetics of dissolution of solid silicon in aluminum and its alloys is relatively slow, and despite the granulometry of introduction chosen for the silicon, the operation can easily last an hour. The mixing of the bath, for example with a doctor blade, is a general practice to accelerate the dissolution of addition elements including silicon. he has the major drawback of destroying each layer of alumina protective that forms on the surface of the aluminum-based liquid alloy, and lead thus aluminum losses of the order of 2 to 3% of the metal in the oven.

The difference in density between solid silicon and the current liquid aluminum alloy elaboration is very low, so that the silicon introduced tends to float on the surface of the alloy bath. The surface exposed to the atmosphere of the furnace is thereby increased, which has the effect of increasing the oxidation of the metal elements placed in the oven and the formation of dross at the expense of performance.

Subject of the invention

The object of the invention is a process for the production of alloys of the Al-Si type, in particular alloys between 7 and 13% silicon, in a flame or induction furnace, allowing a rapid dissolution of silicon, a reduction in the number of bath stirrings and a lesser formation of dross.

The subject of the invention is a process for the preparation of Al-Si alloys by introduction into liquid aluminum, at a temperature between 700 and 850 ° C, of silicon grains metallurgical material with a particle size of less than 10 mm, in which the silicon grains, when they reach the temperature of liquid aluminum, have the property of fragmenting into smaller grains.

Preferably, the metallurgical silicon grains used are prepared by granulation at water from molten silicon.

Description of the invention

The invention is based on the finding made by the plaintiff of different behavior, during the development of aluminum-silicon alloys, between the silicon usually used and obtained by casting ingots, crushing and grinding, and the silicon obtained by granulation at the water. The latter, under certain conditions of use, makes it possible to reduce both the duration of dissolution of silicon in liquid aluminum, and losses of metal by oxidation.

Water-granulated metallurgical silicon is used for the synthesis of halosilanes which are used for the preparation of silicones, as indicated in patents EP 0610807 (Wacker Chemie) or EP 0673880 (Pechiney Electrométallurgie). A water granulation process of silicon is described for example in patent FR 2723325 (Pechiney Electrométallurgie).

The Applicant has sought to analyze the differences between these two types of grains of silicon. A first difference concerns the content of fine particles. We note indeed the presence in the crushed silicon in grains of significant quantities of particles of size less than 5 µm. Experience shows that sieving a powder to extract the fraction below 50 µm is almost ineffective in removing the most fines, for example the fraction less than 5µm. These very fine particles are probably generated during product packaging and observation of the powder under a microscope confirms existence. The assessment of their relative mass quantity can be determined by laser particle size. We always find in the size range 1-10 mm of silicon prepared by the dry route, mass fractions of particles of size less than 5 μm of the order of at least 0.5%.

On the other hand, in water-granulated silicon, the method of preparation can be used of the product to insert into the process a step of rinsing with water which makes it possible to eliminate the most of the particles smaller than 5 µm. We can thus obtain a granule containing less than 0.1% of particles smaller than 5 µm, or even less than 0.05% by performing two successive rinses. It is also interesting to note that in the product thus prepared, the levels of particles less than 50 µm and 5 µm respectively remain practically unchanged after its subsequent rise in metal temperature liquid.

Another difference was highlighted during the aluminum introduction tests liquid, carried out in the laboratory by the plaintiff. Indeed, these tests have shown a special behavior of water-granulated silicon compared to crushed silicon. placed on the surface of the molten aluminum bath, the grains suddenly burst and break in smaller grains, which are projected a few tens of centimeters. One would have thought that this behavior was the result of traces of residual moisture. To elucidate this point, the applicant has made tests in a laboratory oven heated between 700 ° C and 850 ° C, but empty, and therefore without molten aluminum. The behavior of granulated silicon introduced into this furnace under these conditions was the same as in the presence of aluminum, which excludes the explanation by a reaction between aluminum and possible traces of humidity.

The bursting of the grains does not concern only a few grains of granulated silicon, but the majority of them, which excludes the explanation of a sudden volatilization of inclusions of water incidentally present in some of these grains.

grains de taille supérieure à 5 mm : 37%

grains de taille comprise entre 2 et 5 mm : 47%

grains de taille comprise entre 1,6 et 2 mm : 7%.

The bursting of the largest grains remains relatively superficial and leaves mechanically stable nuclei. On the contrary, for grains of size less than 10 mm, each grain fragments, giving little more than 2 to 4 particles. The product obtained is free from fines, both below 50 µm and below 5 µm. Thus, when we test on a sample of grains of size between 5 and 6.7 mm, we find after heat treatment the following composition expressed in number of grains:

grains larger than 5 mm: 37%

grain size between 2 and 5 mm: 47%

grain size between 1.6 and 2 mm: 7%.

The cause of this behavior of granulated silicon is probably to be sought in the internal mechanical tensions accumulated in the metal during its rapid solidification, and which are released during the thermal shock caused by their introduction into aluminum liquid.

For grain sizes above 10 mm, the phenomenon is less marked and the behavior of grains obtained by reconditioning and grinding the largest grains water granulation tends to be confused with that of silicon cast in ingots, crushed and ground. This behavior may be due to the poor thermal conduction of the silicon, which has the consequence, during water granulation, of limiting the quenching effect to the husk of the grains, while the interior will see its temperature drop only much more slowly.

As the granulation with water of liquid silicon can give products whose particle size is between 0 and 30 mm, it is necessary to select from silicon granulated, by sieving for example, a finer grain size, limiting itself to the slice less than 10 mm.

To obtain a satisfactory yield of silicon when introduced into aluminum liquid, certain operating conditions must be observed. The difference in density between the solid granulated silicon and the liquid aluminum being very weak, the granulated silicon, like crushed silicon, tends to float on the surface of the bath and can be found preferably in dross. It is therefore necessary to properly clean the bath surface in fusion before addition of the granulated silicon. In addition, it is preferable to work on a temperature between 800 ° C and 850 ° C, or about 50 ° C at least above the temperature adopted under current operating conditions.

• que la cinétique de dissolution du silicium granulé est plus rapide que celle du silicium concassé, et ce pour une granulométrie comparable. le gain que permet le silicium granulé sur la vitesse de dissolution est plus important que celui que permet une hausse de température, sans en avoir les inconvénients en terme d'oxydation du bain.
• que les brassages de bain nécessaires avec un produit qui se dissout rapidement peuvent être moins fréquents et moins importants qu'avec un produit qui ne se dissout que lentement.

Under these conditions, we note:

• that the kinetics of dissolution of granulated silicon is faster than that of crushed silicon, and this for a comparable particle size. the gain which the granulated silicon allows on the speed of dissolution is greater than that which allows a rise in temperature, without having the disadvantages in terms of oxidation of the bath.
• that the bath mixings required with a product that dissolves quickly can be less frequent and less important than with a product that dissolves only slowly.

It is thus possible to reduce the duration of the development of the alloy and the number of stirrings, which can significantly reduce losses by oxidation. There is thus a gain of 1% on the metal yield at the level of operations of the order of 100 kg, this gain possibly reach 3% on operations of 5 t.

The method according to the invention makes it possible to obtain Al-Si alloys of a quality at least as good as those prepared with crushed and ground silicon. The inclusion quality of the alloys is at the same level, the number of inclusions detected in the alloy not varying significantly. The hydrogen contents measured on the liquid alloy are of the order of 0.1 to 0.2 cm ³ of hydrogen per 100 g of alloy. When adding silicon, these contents vary by plus or minus 10% whatever the type of silicon used, which confirms that the granulated silicon does not constitute a significant supply of hydrogen.

Examples

In the following examples, the inclusion quality control of the liquid metal was made by K-Mold and LIMCA (Liquid Metal Cleanliness Analysis) tests whose object is to quantify the concentrations of oxide inclusions through results expressed in units specific to each of these tests.

The K-Mold test consists of counting the number of inclusions detected on the fracture surface of a test piece poured into a mold of defined shape. The results are expressed in number of inclusions reduced to the fracture surface of the test piece. This test can detect large inclusions, typically in the 50 µm - 300 µm range.

LIMCA control implements hardware related to the Coulter Counter and allows to evaluate the concentration in the metal of solid inclusions of size between 20 μm and 150 µm; the results are expressed in number of inclusions per kg of metal. For some Al-Si alloys, the observed values can range from 1000 inclusions per kg for a alloy considered clean at 100,000 inclusions per kg for a very dirty alloy.

The hydrogen content is checked by means of an ALSCAN device which allows immediate measurement on the liquid alloy. The results are expressed in cm ³ of hydrogen gas, brought under normal conditions of temperature and pressure, per 100 g of alloy.

Example 1

The production of a silicon oven, treated in a pocket to mainly eliminate the calcium, was poured into cast iron ingot molds into ingots about 10 cm thick.

Fe: 0,27%; Ca : 0,045 % ; Al : 0,12% ; C : 0,08 % ; P : 12 ppm

Mn : 0,07% ; Cr : 3 ppm ; Cu : 1 ppm ; Ti :12 ppm ; Ni : 4 ppm ; V : 8 ppm

Analysis of the metal gave:

Fe: 0.27%; Ca: 0.045%; Al: 0.12%; C: 0.08%; P: 12 ppm

Mn: 0.07%; Cr: 3 ppm; Cu: 1 ppm; Ti: 12 ppm; Ni: 4 ppm; V: 8 ppm

This production was ground to a maximum particle size of 10 mm, then sieved to 1 mm to separate the fraction 1-10 mm. To assess the particle size quality of this product, a sample was taken and then washed with water.

The washing water was then evaporated to collect the fine entrainments which were analyzed. by means of a laser granulometer. We were able to reconstruct the real analysis particle size of the original product, which was found to contain 0.51% size fines less than 5 µm.

This classic silicon poured into ingots, crushed then crushed and sieved to 1-10 mm, was separated in four identical batches, one of which was used in the test workshop for a stake under Al-Si alloy baths before casting. The operations carried out consisted of going up by 1 point the silicon content of Al-Si alloys with 0, 6 and 12% Si respectively. These operations were carried out in an electric resistance furnace, at 750 ° C, on crucibles of 100 kg of alloy.

The times required to dissolve the addition of silicon were 10 to 12 minutes.

Tests performed on the metal before and after the addition of silicon showed an increase average K-Mold index of around 10.

The hydrogen contents measured on the molten metal before and after addition of the silicon produced practically constant results neighbors of 0.18 cm ³ / 100g. The metal yield was estimated at 98.3%.

Example 2

The second batch of ground silicon prepared in Example 1 was used during a workshop test of manufacture of alloy A-S13 for placing under the bath before casting. The operation was performed in a 5 ton flame oven whose temperature was regulated with as 750 ° C set point. For the title, 245 kg of product were added, and between at the time of this addition and the final pouring, 47 minutes have passed. Two brews of bath were carried out and at the end of the operation 16 kg of slag were recovered.

The silicon yield calculated according to the rise in the titer following the addition, was 93%.

Qualité inclusionnaire évaluée par la méthode LIMCA : 1100 inclusions/kg.

Teneur en hydrogène : 0,20 cm³/100 g.

The quality control of the AS13 alloy gave the following elements:

Inclusion quality assessed by the LIMCA method: 1100 inclusions / kg.

Hydrogen content: 0.20 cm ³ / 100g.

Example 3

The third batch of ground silicon prepared in Example 1 was used to repeat the experiment in Example 1 by controlling the oven temperature to 810 ° C. The times required for the dissolution of the silicon additions were 8 to 10 minutes, which allowed to assess at about 20% the gain due to the effect of the temperature rise.

Tests performed on the metal before and after the addition of silicon showed an increase average K-Mold index of around 15.

The hydrogen contents measured on the molten metal before and after addition of the silicon produced practically constant results, neighbors of 0.22 cm ³ / 100g.

The metal yield was estimated at 96%.

Example 4

The fourth batch of ground silicon prepared in Example 1 was used during a workshop test of manufacture of alloy A-S13 for the setting under the bath before casting. The operation was performed in a 5 ton flame oven whose temperature was regulated with as set point 810 ° C. For the title, 179 kg of product were added, and between at the time of this addition and the final pouring, 28 minutes have passed. We did two bath mixes and at the end of the operation 12 kg of slag were recovered.

Qualité inclusionnaire évaluée par la méthode LIMCA : 1400 inclusions/kg

Teneur en hydrogène : 0,20 cm³/100 g.

The silicon yield calculated according to the rise in the titer following the addition was 94%. The quality control of the AS13 alloy gave the following elements:

Inclusion quality assessed by the LIMCA method: 1400 inclusions / kg

Hydrogen content: 0.20 cm ³ / 100g.

Example 5

A test for the production of granulated silicon was carried out on the same industrial installation than that which was used to prepare the ground silicon of Example 1, without changing or charging the silicon oven, nor the operating conditions of the treatment in bags for refining. The contents of a ladle of molten silicon at 1530 ° C was poured onto an installation of water granulation in tanks.

The product recovered in the granulation pool was rinsed with water sprayed before being dried and then sieved to 10 mm. The fraction greater than 10 mm has been eliminated and assigned to other applications. No 1 mm sieving was done.

The 0/10 mm granule obtained was subjected to a particle size control in the same conditions as in Example 1. The rate of fines smaller than 5 μm was 0.03%.

Fe: 0,28%; Ca : 0,038 % ; Al : 0,14 %; C : 0,08 % ; P : 12 ppm

Mn : 0,07% ; Cr : 3 ppm ; Cu : 1 ppm ; Ti :14 ppm ; Ni : 4 ppm ; V : 7 ppm

The chemical analysis of the metal gave:

Fe: 0.28%; Ca: 0.038%; Al: 0.14%; C: 0.08%; P: 12 ppm

Mn: 0.07%; Cr: 3 ppm; Cu: 1 ppm; Ti: 14 ppm; Ni: 4 ppm; V: 7 ppm

The metal thus prepared was separated into two identical batches, one of which was used in the workshop tests for an Al-Si alloy bath before casting. As in example 1, the operations carried out consisted of raising the silicon title by 1 point of Al-Si alloys with 0, 6 and 12% Si respectively. These operations were carried out in a resistance furnace, at 750 ° C, on crucibles of 100 kg of alloy.

The times required to dissolve the addition of silicon were 10 to 12 minutes.

Tests performed on the metal before and after the addition of silicon showed an increase average K-Mold index of around 12.

The hydrogen contents measured on the molten metal before and after addition of the silicon produced practically constant results neighbors of 0.20 cm ³ / 100g. The metal yield was estimated at 99.0%.

Example 6

The second batch of granulated silicon prepared in Example 5 was used during a workshop test for manufacturing alloy A-S 13 for placing under the bath before casting. The operation was performed in a 5 ton flame oven whose temperature was regulated with as set point 810 ° C. For the title, 256 kg of product were added. The merger and mixing this addition was very quick; only one bath brewing was carried out and the pouring started only 19 minutes after the addition of silicon. At the end of the operation only 3.5 kg of slag was recovered.

The silicon yield, calculated according to the rise in the titer following the addition, was 98%.
Inclusion quality assessed by the LIMCA method: 800 inclusions / kg
Hydrogen content: 0.18 cm ³ / 100g.

Claims (7)

1. Process for making an Al-Si alloy by adding metallurgical silicon grains with a size grading of less than 10 mm into liquid aluminium at a temperature of between 700 and 850°C, characterised in that these silicon grains were obtained by granulation with water and that, when they reach the temperature of liquid aluminium, they have the property of breaking into smaller grains.
2. Process according to claim 1, characterised in that silicon is added at a temperature of between 800 and 850°C.
3. Process according to one of claims 1 and 2, characterised in that the silicon used contains less than 0.1% of particles smaller than 5 µm.
4. Process according to claim 3, characterised in that after fragmentation, the content of particles smaller than 5 µm in the silicon remains less than 0.1%.
5. Process according to one of claims 1 to 4, characterised in that the silicon used contains less than 0.05% of particles smaller than 5 µm.
6. Process according to one of claims 1 to 5, characterised in that silicon is obtained by selection of the 1 - 10 mm size grading range prepared by screening, without crushing or grinding later.
7. Process according to claim 6, characterised in that the silicon used was rinsed once or several times with water to eliminate the finest particles therefrom before final drying.