NL1009258C2 - Process for the preparation of an Al-Ti-B grain refiner for aluminum-containing products, and a method for casting aluminum products. - Google Patents

Description

Process for the preparation of an Al-Ti-B grain refiner for aluminum-containing products, and a method for casting aluminum products

The present invention relates to a method for preparing an Al-Ti-B grain refiner for cast aluminum-containing products.

In the known prior art method, titanium and boron-containing salts, such as K2TiF6 and KBF4, are added to molten aluminum at 800-900 ° C. The salts decompose and titanium and boron are taken up in the aluminum, after which Al3Ti and TiB2 are formed which are contained in an aluminum matrix.

This method has the drawback that undesired by-products are formed, such as slag on the molten aluminum and gaseous boron fluoride. The latter is undesirable from an environmental and health point of view. Furthermore, titanium and boron can only be added in limited quantities, otherwise the viscosity of the slag and the molten aluminum will increase to such an extent that they can no longer be easily separated from each other. As a result, the grain refiner can only contain a maximum of 5% by weight of titanium and 1% by weight of boron, the remainder comprising aluminum matrix 20.

The present invention aims to overcome the above-mentioned drawbacks to a large extent. In particular, it is contemplated to provide a more environmentally friendly method, which makes it possible to obtain a grain refiner with higher concentrations of titanium and boron in the aluminum matrix. More particularly, it is an object of the present invention to provide a grain refiner which has the property that when the grain refiner is introduced into an aluminum melt, the grain size, and hence the quality of melt-produced aluminum products remains constant, even if the melt stands for a long time. That is, for example, in the manufacture of aluminum products in which a container with the aluminum melt is only emptied slowly, the microstructure of later cast products 1009258 2 is essentially the same as that of previously cast products. Second, the present invention aims to provide a method by which a grain refiner can be prepared which does not have to be used immediately after its preparation.

To this end, the process of the present invention is characterized in that the Al-Ti-B grain refiner is prepared by: i) mixing powders selected from the group consisting of aluminum, titanium, boron, and alloys and intermetallic compounds thereof; ii) pressing the mixed powders; iii) heating the pressed mixture in an inert environment; and iv) cooling the grain refiner thus prepared.

Surprisingly, the grain finer thus prepared has the property that the quality of cast products remains essentially constant during long-term casting processes. In the present application, an inert environment is understood to mean an inert atmosphere, for instance formed by a noble gas, or vacuum.

When reference is made in the present application to grain refinement, it is understood to mean the reduction of the crystalline aluminum grains that arise during solidification of aluminum in the aluminum melt. A small grain size brings with it a range of beneficial properties; in particular, toughness and strength are increased. In addition, castability (during continuous casting) and extrudability are improved.

U.S. Patent 4,710,348 is directed to the manufacture of metal-ceramic composites. This I publication contains a sideways comment that a bar is of honor. such an unreacted mixture could be used as a grain refiner for aluminum. When the rod is added to an aluminum melt, the constituent constituents of the mixture react to form seeds to achieve (aluminum) grain refinement. In other words, the grain refiner itself 1009256 3 is prepared in situ. The applicant has attempted to verify this suggestion too experimentally, but has failed to achieve grain refinement.

According to a very favorable embodiment, the reaction parameters are adjusted such that the ratio of elongated and cubic Al3Ti crystals is as small as possible.

Despite many years of research, the way in which grain refiners work is not fully understood. Surprisingly, the applicant has found that it is in particular the cubic Al3Ti crystals that contribute to a further grain refinement and that it is possible to change the ratio between elongated and cubic Al3Ti crystals. This finding may contribute to a better understanding of the action of grain paint irons.

According to a favorable embodiment, the ratio is changed by choosing the molar ratio of B / Ti such that it is less than 1.9, in particular less than 1.7 and more preferably less than 1.5.

The powders used in step i) are preferably less than 100 μτη.

This allows the powders to be mixed well, which promotes a good and fast reaction of the starting materials, yielding a grain refiner in which Al3Ti and TiBj are more homogeneously distributed in the aluminum matrix.

Regarding the amounts, it is noted that titanium is preferably present in the mixture from step i) in an amount of at least 10 wt% and preferably at least 25 wt%.

Furthermore, boron in the mixture from step i) is preferably present in an amount of at least 3 wt% and preferably at least 5 wt%.

A good grain refiner can thus be prepared. At lower percentages of titanium, it is preferred that the percent of boron is higher.

According to a favorable embodiment, AlB is used as the source of boron for the preparation of the grain refiner.

AlB12 is known to have an adverse impact on the quality of molded products, especially films. By not reacting the reactants in situ (ie in an aluminum melt), and the fact that A1B12 appears to react completely completely in the process according to the invention, an excellent grain refiner can be prepared in a simple manner, of which less is needed when used. This has the further advantage that filters used during the casting process wear less quickly. Without wishing to be bound by any theory in this respect, it is thought that during the preparation of a grain refiner starting from A1B12 there is a different reaction kinetics, as a result of which the borides and intermetallic compounds formed are smaller. The fact that less is needed for the grain refiner offers two advantages for aluminum recycling. A high titanium content adversely affects the quality of the cast aluminum products for many applications. Since less is needed for the grain refiner according to the invention, the composition of the aluminum melt changes and the expected material properties change minimally. Furthermore, such a small increase also makes it possible to recycle aluminum more often.

According to a preferred embodiment, the prepared grain refiner is subjected to an extrusion process before being contacted with an aluminum melt.

A grain refiner with further improved grain refining properties can thus be obtained.

The invention further relates to a method for casting aluminum products in which an Al-Ti-B grain refiner is added to an aluminum melt.

This method is characterized in that a grain refiner prepared according to the invention is added.

A product manufactured by this method has improved strength and toughness properties due to the fine-grained structure. This allows them to be stronger and / or lighter. Also, with this method, less grain refiner suffices, which limits wear of filters used during the casting process.

The invention will now be elucidated on the basis of an exemplary embodiment.

Preparation of a grain refiner

For the preparation of a grain refiner of the present invention, 15.86 g of Al, 7.50 g of Ti and 1.82 g of AlB12, each as a powder, are mixed. The powder sizes of the starting materials were:

Al powder ("Type 1" Metalloys Ltd. (Sheffield, UK)): <4.5 μτη;

Ti powder ("ΜΡ-020" Micron Metals Inc. (Utah, 10 V.S.)): <4.5 μτη;

AlB12 powder (Grade "A" H.C. Starck GmbH & Co. KG (Goslar, B.R.D.)): <3 μτη.

After mixing, the powder mixture is cold pressed under such a high pressure that a solid is formed. Then the solid is placed in an oven and heated under an argon atmosphere, raising the oven temperature at a rate of 20 ° C / min. After it has been determined with the aid of a temperature sensor arranged in the oven that an exothermic reaction occurs in the solid body, the heating is stopped. After cooling in the oven and still under an argon atmosphere, the grain paint is ready for use.

Test the grain refiner

The grain refiner obtained above was tested by examining its effect on the casting structure of an aluminum melt. The aluminum melt composition was: Fe: 0.16 wt%; Si: 0.05 wt%; Sn, Pb, Zn, Cu,

Ni, Mn, Cr, Ti, Mg <0.01 wt%. At the temperature of the aluminum melt of 750 ° C, as much grain refiner 30 was added as corresponded to 0.03 wt% titanium based on the final aluminum melt. After addition, stirring was done manually for 15 seconds. Within 30 seconds after the completion of the stirring, a sample was poured into a preheated sand mold at a temperature of 50 °. After the first casting, the temperature of the aluminum melt was kept at 740 ° C and more samples were poured after 8 hours and 24 hours; also at a temperature of 740 ° C.

The average grain size was determined by etching a casting structure with king water (25% HNO 3 + 75% HCl) 1009258 6 using the line interception method known in the art. The number of times a line crosses a grain boundary is measured. The ratio of the line length and the measured number of grain boundaries gives the mean-5 grain size.

1009258 7

The results are shown in the table below. TABLE

grain size 5 20 sec. 200 μτη 8 hours 165 μτη 24 hours 14 0 μτη

Control Experiment The following control experiment was conducted using data from U.S. Patent 4,710,348, which claims that the mixture described there would be suitable as grain refiner for aluminum.

Using a powder mixture of iron, titanium and boron, the composition of which was the same as described above, a solid was pressed. The solid was added to an aluminum melt under the same conditions as described above but without the solid being heated in an over in an inert atmosphere. No grain refinement was observed.

1009258

Claims (9)

A method for preparing an Al-Ti-B grain refiner for cast aluminum-containing products, characterized in that the Al-Ti-B grain refiner is prepared by: 5 i) mixing powders selected from the group consisting of aluminum, titanium , boron, and alloys and intermetallic compounds thereof, - ii) pressing the mixed powders; iii) heating the pressed mixture in an inert environment; and iv) cooling the thus prepared grain refiner. 2. Process according to claim 1, characterized in that the reaction parameters are adjusted such that the ratio of elongated and cubic Al3Ti crystals is as small as possible. Method according to claim 1 or 2, characterized in that the ratio is changed by choosing the molar ratio of B / Ti such that it is less than 1.9, in particular less than 1.7 and with more preferably less than 1.5. A method according to any one of the preceding claims, characterized in that the powders used in step i) preferably have a size of less than 100 µm. A method according to any one of the preceding claims, characterized in that the prepared grain refiner is subjected to an extrusion process before being contacted with an aluminum melt. 6. A method according to any one of the preceding claims, characterized in that titanium is present in the mixture from step i) in an amount of at least 15% by weight and preferably at least 25% by weight. A method according to any one of the preceding claims, characterized in that boron is present in the mixture from step i) in an amount of at least 3 wt% and preferably at least 5 wt%. 1009258 » Process according to one of the preceding claims, characterized in that AlB12 is used as the source of boron for the preparation of the grain refiner. 9. A process for casting aluminum products in which an Al-Ti-B grain refiner is added to an aluminum melt, characterized in that a grain refiner prepared according to any one of claims 1 to 8 is added. 1009258