EP0521580A1

EP0521580A1 - Process for the preparation of a grain refiner

Info

Publication number: EP0521580A1
Application number: EP92202013A
Authority: EP
Inventors: Antonius Kuster; Cornelis Jacobus Robert Groenenberg; Christiaan Hendrikus Van Kleef; Wim Albert Van Ravenswaay
Original assignee: KBM-Metaalindustrie BV; Shell Internationale Research Maatschappij BV
Current assignee: KBM-METAALINDUSTRIE B.V.
Priority date: 1991-07-05
Filing date: 1992-07-02
Publication date: 1993-01-07
Also published as: NO922637L; BR9202457A; AU1943892A; JPH05195108A; GB9114586D0; NO922637D0

Abstract

The invention concerns a process for the preparation of a grain refiner contained within an aluminum matrix, comprising intimate mixing of at least one reaction product of titanium and aluminum contained within an aluminum matrix and at least one reaction product of boron and aluminum contained within an aluminum matrix thereby obtaining solid state reaction products by reacting the reaction products by solid state reaction.

Description

The present invention concerns a process for the preparation of a grain refiner contained within an aluminum matrix, to a grain refiner produced by said process and to the use thereof.
It is well known that in the metals industry for the production of semi finished products and castings the grain size is an important factor to control since it influences many mechanical and chemical properties of the cast material. In general, the molten metal must have sufficient crystal nuclei to obtain the desired grain size of the cast products. It is often necessary to increase the number of crystal nuclei through additions to the melt. This is usually achieved by adding to the melt a master alloy containing a very large number of nucleating particles, which disperse in the aluminum melt.
Titanium is the most common additive for grain refining of aluminum, and also a very efficient additive in this regard. At normal melting and casting temperatures, titanium concentrations above 0.2% (m/m) form with aluminum the intermetallic phase Al₃Ti, although lower concentrations will also give a grain refining effect. In the production of a master alloy containing 1-15% Ti in aluminum, particles of Al₃Ti form together with some Ti in the solution, in accordance with generally accepted phase diagrams. Also it is generally known that addition of boron to master alloys containing Ti will considerably improve the grain refining effect, especially when the Ti/B ratio is between 2.2 and 25. Boron forms particles of the type TiB₂ and/or TiB₁₂ which are assumed by some researchers to constitute crystal nucleation sites, while others claim that in the aluminum melt boron causes a decrease in the Al₃Ti dissolution to Al and Ti, and thereby creates a more effective and durable grain refining action in which the intermetallic phase Al₃Ti is involved.
In order to prepare a suitable grain refiner or master alloy to be added to aluminum melts, it is suggested in DE-A 23 07 250 to add fine particles of a titanium alkali fluoride complex and an alkaliboron fluoride complex to an aluminum melt. With such a process a suitable grain refiner can be obtained although the process has some disadvantages.
In the first place it should be remarked that the starting products are relatively expensive, which makes the process economically less attractive. In addition thereto the reaction between the components is an exothermic one, which reaction is rather difficult to control. Furthermore minor amounts of said salt complexes lead to a decreased cleanliness of the grain refiner.
Object of the invention is to provide a process for the preparation of a grain refiner from relatively simple starting products, without the disadvantages of the prior art processes.
The invention is based thereon that it has been found that an excellent grain refiner can be prepared by solid state reaction of a reaction product of titanium and aluminum and a reaction product of boron and aluminum contained within at least one aluminum matrix.
Therefore the invention concerns a process for the preparation of a grain refiner contained within an aluminum matrix, comprising intimate mixing of at least one reaction product of titanium and aluminum contained within an aluminum matrix and of at least one reaction product of boron and aluminum contained within an aluminum matrix thereby obtaining solid state reaction products by reacting the reaction products by solid state reaction.
The reaction product of titanium and aluminum contained within an aluminum matrix is preferably Al₃Ti in aluminum, whereas the reaction product of boron and aluminum is preferably AlB₂ and/or AlB₁₂. In view of reactivity and efficiency the use of AlB₂ is preferred. However the presence of AlB₁₂ in combination with AlB₂ can also lead to a good grain refiner.
The solid state reaction mentioned above is defined to cover a reaction process starting with joining and intimately mixing said solid reaction products, i.e. Al₃Ti and AlB₂ (and/or AlB₁₂) both contained in an aluminium matrix, and leading to a certain amount of TiB₂ besides a rest of only Al₃Ti. However, it is not fully understood yet how said reaction products are reacting. Up to now it is thought that said solid state reaction covers, at least partly dissolving of the reaction products to their separate components, physical diffusion of said components, and chemically reacting of Ti an B to TiB₂ (or TiB₁₂).
The preparation of the reaction product of titanium and aluminum contained within an aluminum matrix and of the reaction product of boron and aluminum contained within an aluminum matrix can easily be done in a manner known in the art. The technique to be used can be the same as the technique for the preparation of Al/Ti and Al/B master alloys. Generally the preparation is done by adding titanium containing raw materials (such as titanium sponge or titanium salts) and boron containing raw materials (such as boron salts) to molten aluminum.
According to the invention the titanium component, Al₃Ti and the boron component, AlB₂ and/or AlB₁₂, are preferably used in the form of their alloys with aluminum, i.e. in the form of Al₃Ti in aluminum and AlB₂ and/or AlB₁₂, in aluminum. These alloys are subsequently intimately mixed in the solid state during or after which the materials are heated to the temperature required for the solid state reaction.
The intimate mixing can be carried out in different ways. According to one embodiment particles of the two materials, such as granules, powder, flakes, needles or likewise shaped fine particles are intimately mixed, followed by heating of the resulting mixture. This heating can take place by compacting or extruding the mixture with sufficient shear or force to cause the temperature to rise to the required level. It is however preferred to anneal the material after compacting or extruding as this gives a better control of the process and thus a more reproducible product. Another possibility is formed by the heating during extruding of the material.
According to another embodiment the materials are coextruded at a temperature below the melting point of the materials, optionally with heating. If necessary this process can be repeated a few times until sufficient mixing has been obtained. Depending on the temperature used the materials may have reacted, but generally it will be preferred to heat the mixture after mixing to let the materials react.
In case a powder-mixture of the alloys is prepared, the particle size and the state of the powders should be such as to promote solid state diffusion and reaction to occur. In the case of oxidised powders the particle sizes should not be too small, as this has been found to result in a less improved effectiveness, possibly due to a too high oxygen content caused by the higher surface area. Suitable particle sizes are thus between 50 µm and 1 mm.
The heating or annealing of the materials should also be such that the solid state reaction as defined above can proceed to a suitable degree. Suitable temperatures are from 400 °C. An upper limit is formed by the melting temperature of the materials. The temperature also depends on the length of time, longer heating times requiring lower temperatures. The heating time usually ranges from 0.1 to 5 hours. Longer times do not provide any advantage, whereas the lower limit is generally necessary to obtain sufficient reaction.
The amount of titanium in the grain refiner is preferably between 0.2 and 15% by weight, whereas the amount of boron in the grain refiner is preferably between 0.02 and 10% by weight. The ratio of titanium to boron (m/m) is between 1 and 80, more in particular between 2.5 and 35.
It is remarked that the amounts of boron and titanium are dependent on the effectiveness of the grain refiner and on the amount of grain refiner to be used in the final product.
Although the mechanism of grain refining the alloy in relation to the composition of the active reaction components as obtained by said solid state reaction is not absolutely certain, the solid state reaction products containing said components at least comprise Al₃Ti and TiB₂, having particle sizes of up to 30 µm and up to 5 µm respectively. Preferred solid state reaction products contain fine and many phase particles. Preferred particle sizes are respectively 10-30 µm and 0.5-5 µm. Said component product particle sizes are determined microscopically, which means that at a particular size at least 80% of the particles observed will have said size.
With these particle sizes of the two compounds it has been observed that an optimal grain refining effect is obtained. It has appeared that advantageously a minimal amount of the grain refiner of the invention can be used in the aluminum. Generally these amounts are up to 5 kg, preferably between 0.1 and 5 kg per 1000 kg of aluminum. More specifically it has appeared that, compared to the case where for example 2 kg of a grain refiner according to the prior art having a Ti-content of 5% w and a B-content of 1% w should be used, when supplying 2 kg of the grain refiner of the present invention, a Ti-content of only 3% w and a B-content of only 0.5% w are sufficient. Furthermore, for reason of clarity it is noted that the amount of grain refiner (in kg) is calculated on the basis of the total weight of the grain refiner and not on the amount of titanium and boron.
The grain refiner according to the invention can be used in various shapes, such as powder, needles, flakes, granules and the like. Preferably the grain refiner is produced in the shape of wire, as is conventional in the art of aluminum casting.
The grain refiner can be used for preparing aluminum intermediate products, such as ingots, but it can also be used for the production of castings.
The invention is elucidated on the basis of the following examples, which are intended to serve as an illustration of the invention.

EXAMPLES

Examples 1-3

Granules of reaction products of titanium and aluminum contained within an aluminum matrix containing about 3.4 to 3.6% (m/m) of titanium in the form of Al₃Ti, were mixed with granules of reaction products of boron and aluminum contained within an aluminum matrix having boron contents of 2.2, 1.2 and 0.4%(m/m), mainly in the form of AlB₁₂ (2.2%) or AlB₂ (1.2 and 0.4%).

The mixture of granules was first compacted to an extrusion billet and subsequently extruded. The extruded material was annealed at 640 °C for the time given in the table. The various products were tested for the grain refining response, i.e. the grain size (in µm) of the aluminum wherein the grain refiner has been included. Grain sizes are determined in accordance with the so-called Kawecki-Billiton grain refinement test, or ring test (see article by Vader et al., "Interrelations between aluminium grain refining by means of aluminium titanium boron alloys and the number of growth centers", Internationale Leichtmetalltagung, Leoben-Wien 1987, 22-26 June, 1987). Conventional operating conditions were applied for the the molten alloy into which the grain refiner was supplied. The results are given in Table 1.

TABLE 1

Example	Ti% (m/m)	B% (m/m)	Ann.time (hours)	Grain refining response (µm) 2 kg/ton 6 kg/ton
1A	1.7	1.1	0	550	500
1B			1	360
1C			4	330
1D			16	230
1E			100	250	250
2A	1.8	0.6	0	500	330
2B			1	330
2C			4	300
2D			16	300
2E			100	330	230
3A	1.9	0.2	0	450	280
3B			1	280
3C			4	300
3D			16	360
3E			100	330	250

Examples 4-6

Using the same procedure as in examples 1-3, various products were prepared having different titanium and boron contents. The results are given in Table 2.

TABLE 2

Example	Ti% (m/m)	B% (m/m)	Ann.time (hours)	Grain refining response (µm) 2 kg/ton
4A	3.4	0.05	0	650
4B			0.5	450
4C			1	400
5A	3.3	0.08	0	550
5B			0.5	330
5C			1	300
6A	3.0	0.16	0	650
6B			0.5	360
6C			1	360

Example 7

A mixture of granules of a reaction product of titanium and aluminum contained within an aluminum matrix containing about 3.4% (m/m) of titanium in the form of Al₃Ti, and of granules of a reaction product of boron and aluminum contained within an aluminum matrix having a boron content of 1.4% (m/m) was prepared. This mixture was subsequently extruded into a wire and annealed at 640 °C. The results of the grain refining response are given in Table 3. TABLE 3

Example Ti% (m/m) B% (m/m) Ann.time (hours) Grain refining response (µm) 2 kg/ton

7A 1.75 0.71 0 500

7B 0.25 360

7C 0.5 330

7D 1 280

7E 2 280

Example 8

A mixture of granules of a reaction product of titanium and aluminum contained within an aluminum matrix containing about 3.4% (m/m) of titanium in the form of Al₃Ti, and of powder of a reaction product of boron and aluminum contained within an aluminum matrix having a boron content of 1.4% (m/m) was prepared. This mixture was subsequently compacted and multiply extruded into a wire and annealed at 640 °C. In particular the compacted material was extruded five times, i.e. after extrusion the wire was cut into pieces of wire which were extruded again to obtain a wire of grain refiner. The results of the grain refining response are given in Table 4.

TABLE 4

Example	Ti% (m/m)	B% ((m/m)	Ann.time (hours)	Grain refining response (µm) 2 kg/ton
8A	1.2	0.55	0	900
8B			1	300
8C	1.4	0.62	0	400
8D			1	550
8E			2	450
8F	2.3	0.5	0	800
8G			1	450
8H			2	500

Claims

Process for the preparation of a grain refiner contained within an aluminum matrix, comprising intimate mixing of at least one reaction product of titanium and aluminum contained within an aluminum matrix and at least one reaction product of boron and aluminum contained within an aluminum matrix, thereby obtaining solid state reaction products by reacting the reaction products by solid state reaction.
Process according to claim 1, wherein as reaction product of titanium and aluminum Al₃Ti is used.
Process according to claim 1 or 2, wherein as reaction product of boron and aluminum AlB₂ and/or AlB₁₂ is used.
Process according to claim 1-3, wherein said reaction product of titanium and aluminum and said reaction product of boron and aluminum each contained in an aluminum matrix are mixed.
Process according to claim 4, comprising preparing a mixture of particles of the reaction product of titanium and aluminum contained in an aluminum matrix and of particles of the reaction product of boron and aluminum contained in an aluminum matrix, followed by heating under conditions that solid state reaction occurs.
Process according to claim 5, wherein said particles are chosen from the group of powder particles, flakes, granules and needles.
Process according to claim 1-6, wherein the reaction product of titanium and aluminum and the reaction product of boron and aluminum, each contained in an aluminum matrix are extruded together to obtain intimate mixing and are subsequently heated to obtain said solid state reaction.
Process according to claim 1-6, wherein the reaction product of titanium and aluminum and the reaction product of boron and aluminum, each contained in an aluminum matrix are extruded together to obtain mixing and are heated during said extruding.
Process according to claims 1-8, wherein the amount of titanium in the grain refiner is between 0.2 and 15% by weight.
Process according to claims 1-9, wherein the amount of boron in the grain refiner is between 0.02 and 10% by weight.
Process according to claims 1-10. wherein the ratio of titanium to boron (m/m) is between 1 and 80.
Process according to claim 1-11. wherein the solid state reaction products at least comprise Al₃Ti and TiB₂, having particle sizes of 15-30 µm and 1-5 µm respectively.
Process according to claims 1-12, wherein the grain refiner is produced in the shape of wire.
Process for the preparation of a grain refiner contained within an aluminum matrix, substantially as described hereinbefore, especially with reference to the examples.
Grain refiner produced by the process of any one of the claims 1-14.
Grain refiner contained in an aluminum matrix, comprising at least Al₃Ti and TiB₂, the amounts of Ti and B being between 0.2 and 15% (w) and between 0.02 and 10% (w) respectively, the particle sizes of Al₃Ti and TiB₂ being between 15 and 30 µm and between 1 and 5 µm, and the ratio of titanium to boron (m/m) being between 1 and 80.
Process for the preparation of aluminum semi finished products and castings, wherein the grain refiner of claims 15 or 16 is used.