EP1877589A1

EP1877589A1 - Grain refinement agent comprising titanium nitride and method for making same

Info

Publication number: EP1877589A1
Application number: EP06743761A
Authority: EP
Inventors: Michel Jehan; Gérard Bienvenu
Original assignee: Closset Bernard
Current assignee: Closset Bernard
Priority date: 2005-05-06
Filing date: 2006-05-03
Publication date: 2008-01-16
Also published as: WO2006120322A1

Abstract

The invention relates to a grain refinement agent for treating a liquid metal selected from aluminium, magnesium or a respective alloy thereof. The agent comprises at least 5 wt % of titanium nitride and at least one powder additive having a melting point lower than the treatment temperature of the metal or alloy of which the grain is to be refined. The powder additive is preferably suitable for inclusion in the composition of the metal or alloy to be treated. The refinement agent is preferably in the form of a compressed blend such as a pellet so that it is easier to add and dissolve the refinement agent during the treatment of the liquid metal or alloy, and the titanium nitride can be quickly and efficiently dispersed in the molten metal or alloy.

Description

Grain refining agent comprising titanium nitride and method of manufacturing such an agent

Technical field of the invention

The invention relates to a grain refining agent for the treatment in the liquid state of a metal selected from aluminum and magnesium or a respective one of their alloys and to a process for producing the same.

State of the art

In the presence of an insufficient number of inoculant crystals, aluminum, magnesium and their respective alloys develop, during the solidification step, grains of heterogeneous shape and of coarse dimensions.

Thus, it is often necessary to increase the number of inoculant particles by the addition of grain refining agents to the metal or liquid alloy. This is generally achieved by the addition of parent alloys containing intermetallic compounds serving as nucleation seeds. The addition of grain fining agents indeed improves the heterogeneous solidification and results in a fine macrostructure characterized by an equiaxial grain orientation. One of the main effects is an improvement in the mechanical properties of the refined material.

For several decades now, titanium alloys have been used for refining aluminum and aluminum alloy grains. Indeed, at a content of the order of 0.2%, the titanium can form, during the solidification of an aluminum alloy, the intermetallic compound AI ₃ Ti which acts as an inoculant of aluminum. Al-Ti binary master alloys generally have a titanium content of between 5% and 15%. According to the phase diagram of the Al-Ti alloy, the intermetallic compound AbTi is present in this type of grain refining agent. Ternary parent alloys such as Al-Ti-B and Al-Ti-C have also been developed. For example, in an Al-Ti-B parent alloy, the Ti: B ratio can vary from 1: 1 to 50: 1. The presence of boron and carbon significantly increases the refining power of these parent alloys. This is certainly due to the formation respectively of TiB2 and TiC particles which combine their refining power with that of the AI ₃ Ti compound also present.

In practice, Al-Ti-B parent alloys are the most frequently used, especially for the treatment of foundry aluminum alloys. Because of their manufacturing process; which consists in reacting a mixture of K ^ TiFe-KBF ₄ salts with pure aluminum melted at around 75O ⁰ C, then bringing the bath to a temperature close to 1250 ⁰ C; Al-Ti-B parent alloys have a titanium content generally less than 10%. The parent alloy AI-5% Ti-1% B contains intermetallic compounds such as AbTi and TiB ₂ , the size of which can vary greatly depending on the method of preparation of the parent alloy and the shape of the product (son, bars , ingots, etc ...). For example, the TiB ₂ particles may have a size ranging from 1 to 10 microns while the AbTi compound, present in the form of needles, may have a larger size ranging from 20 to 60 microns.

In the case of pure aluminum or low alloyed alloys, it is sometimes preferred to use Al-Ti-C type parent alloys having a carbon content of less than 1%.

The addition of 0.01 to 0.02% titanium from a parent alloy such as Al-5% Ti-1% B or Al-3% Ti-0.3% C is required to obtain solid state a fine macroscopic structure of aluminum and aluminum alloys. However, relatively large amounts of parent alloys are often required to achieve the desired refining effect. So, using an AI-5% Ti-1% B parent alloy, it is generally necessary to add 4 kg of parent alloy per 1000 kg of molten aluminum in order to increase the titanium content by nearly 0.02%. in addition, even preheated, the parent alloys initially dissolve at a relatively slow rate. In addition to the low content of active compounds, such as Al ₃ Ti and TiB ₂ , in the mother alloy Al-5% Ti-1% B, microscopic observation often shows an irregular dispersion of these compounds in a matrix of primary aluminum. The hyperperitectic phase of AbTi is present as large needles in the primary aluminum while the TiB ₂ precipitates are located mainly at the grain boundaries.

In addition, any presence of oxides or other inclusions in the treated alloys is likely to have negative effects on the performance of these parent alloys used as refining agents. On the other hand, any contamination, added to that of the presence of dissolved hydrogen in the parent alloys, will be reflected after the refining treatment on the quality of aluminum and molten aluminum alloys. Therefore special precautions must be taken during the preparation, often at temperatures above 1000 ⁰ C, of the parent alloys. At these high temperatures, production costs are substantial and the risk of contamination of parent alloys becomes very important.

Finally, in the alloys-mothers, it should be noted that aluminum, which serves as a vector for the affinants, is present in large quantities. For example, the AI-5% Ti-1% B parent alloy contains 94% aluminum. In addition, for rapid dispersion of the active compounds, it is recommended to use the parent alloys at relatively high temperatures.

Until recently, magnesium and some of its alloys were refined with hexachloroethane (C ₂ Cl ₆ ). For environmental reasons, this product is no longer recommended for grain refining. Object of the invention

The object of the invention is to avoid the drawbacks relating to the production and use of currently available grain fining agents and, more particularly, to propose a novel and highly efficient grain refining agent for treatment in the liquid state, aluminum, magnesium or one of their respective alloys, before casting, without the need to produce a parent alloy.

According to the invention, this object is achieved by the appended claims.

Brief description of the drawings

Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention given by way of non-limiting example and represented in the accompanying drawings, in which:

FIGS. 1 and 2 show polished samples observed under an optical microscope and corresponding to a cast Al-Si cast aluminum alloy, respectively before and after the use of a grain refining agent according to FIG. prior art. FIGS. 3 and 4 represent polished samples observed under an optical microscope and corresponding to said cast aluminum alloy cast respectively before and after the use of a grain fining agent according to the invention. Description of particular embodiments

According to the invention, a grain refining agent, suitable for use in the liquid state treatment of aluminum, magnesium or one of their respective alloys, comprises at least titanium nitride TiN. The titanium nitride content in said agent is at least 5% by weight. It is preferably between 5% by weight and 90% by weight and more particularly between 15% by weight and 75% by weight.

In addition, the grain refining agent comprises a powder additive having a melting temperature below the temperature at which the metal or alloy, whose grain is to be refined, is treated in the liquid state. The powdery additive is, more particularly, chosen so that the constituent chemical element (s) are capable of entering into the composition of the metal or alloy whose grain is to be refined. The powdery additive preferably has an average size of between 0.5 mm and 5 mm.

The powder additive may be of any type and more particularly it may be in the form of a powder of one or more pure chemical elements or of one or more compounds such as metal alloys or intermetallic compounds. The element or elements of said additive are likely to enter the composition of cast aluminum and magnesium products. By way of example, the powdery additive may consist of:

by at least one chemical element chosen from titanium, manganese, boron and carbon and preferably from aluminum, magnesium, copper, silicon and zinc

or by at least one metal alloy chosen from alloys of aluminum and magnesium, aluminum and silicon, aluminum and copper, magnesium and copper, magnesium and zinc, aluminum and aluminum. zinc - or by at least one intermetallic compound selected from titanium aluminide, aluminum boride, titanium carbide, aluminum carbide, titanium boride, aluminum nitride, zirconium boride. Preferably, when the powdery additive is constituted by at least two chemical elements or when it is a metal alloy or an intermetallic compound, it is chosen so as to have a composition close to that of a corresponding eutectic and more particularly close to of the composition of a corresponding eutectic whose melting point is lower than the treatment temperature of the metal or alloy whose grain is to be refined. The composition is preferably chosen to be close to that of a eutectic whose melting point is as low as possible. Thus, by way of example, the pulverulent agent may be formed of 40% by weight of aluminum and 60% by weight of magnesium, this agent possibly being a 40% Al-60% Mg alloy in the form of a powder. a mixture of 40% pure aluminum in powder form and 60% pure magnesium in powder form. This agent has a melting temperature of the order of 455 ° C, which is close to the melting point of the 32% AI-68% Mg eutectic compound, which is 437 ° C. Such an agent can then be mixed with titanium nitride, so as to obtain, for example, the following grain refining agent: 25% TiN-30% AI-45% Mg.

The fining agent is preferably in a compact form. It may, for example, be in the form of tablets, disks, bars, pellets, bricks. Thus, it can be manufactured by mixing at least 5% by weight of titanium nitride in the form of a fine powder with at least one predetermined powdery additive. The choice of the powdery additive is, more particularly, carried out so that it consists of one or more chemical elements capable of entering into the composition of the metal or alloy whose grain is to be refined. Said mixture can then be compacted so as to obtain a compressed mixture of any type of shape. This makes it possible to treat aluminum, magnesium and alloys of aluminum and / or magnesium when these are maintained at a relatively low temperature. By way of example, the refining agent can be produced according to the following main steps, carried out at ambient temperature: a) use of a fine TiN powder whose average particle size varies, preferably between 0.1 and 20 microns and more particularly between 0.1 microns and 5 microns and advantageously between 0.1 and 2 microns, b) use of at least one more or less fine powder of additive such as aluminum, magnesium, copper, silicon, zinc, titanium and boron or any other powder obtained from a pure metal or an alloy, the element or elements of which may enter into the composition of cast products based on aluminum and magnesium; c) mechanical mixing and / or mechanical grinding, normal and / or intense in air or inert gas, of a predetermined amount of TiN powder and a predetermined amount of the additive in the form of of powder, so as to obtain a mixture of powders at least 5% by weight of TiN and for example between 10% and 90% by weight of TiN. d) compacting said mixture, with or without additional binder, for example by exerting a uni-axial or isostatic pressure so as to obtain a compressed mixture, for example in the form of tablets, discs, bars, pellets, briquettes or any other forms allow the treatment of metals and alloys of aluminum and magnesium, maintained in the liquid state.

Grain refining tests were carried out with an Al-Si alloy casting alloy whose composition is given below:

For each series of tests, a quantity of about 10 kg of alloy was melted in a silicon carbide crucible. The temperature of the liquid alloy is stabilized beforehand in a range between 720 and 70 ^° C., before the introduction of different grain refining agents. Each type of grain refining agent is then added to the surface of the molten alloy, which is maintained at a temperature close to 730 ^° C. One minute after the addition of the refining agent of grain, the alloy maintained in the liquid state is slightly stirred to ensure a homogeneous distribution of the grain refining agent in the liquid alloy.

Before and one hour after the addition of the grain fining agent in the molten alloy bath, samples were cast in a graphite mold specially designed for these tests and maintained at a temperature of about 250 ^° C. C, before casting said alloy. The samples are polished and then observed under an optical microscope to observe the size of primary aluminum grains in the hypoeutectic alloy Al-Si foundry.

Three tests were carried out, respectively with the grain refining agent AI-5% Ti-1% B described in the prior art (Example 1) and with first and second refining agents according to the invention ( examples 2 and 3).

The Al-5% Ti-1% B alloy, the grain refining agent currently the most commonly used, is here used as a reference for the tests carried out with the refining agents according to the invention. The first fining agent according to the invention consists of a pellet of 5.5 g of a binary mixture of 25% of TiN and 75% of Al while the second agent is formed by a mixture of powders. compressed in the form of a pellet of 5.5 g whose composition is 25% TiN, 30% AI and 45% Mg. Example 1

50 g of Al-5% Ti-1% B parent alloy are added in a 10 kg bath of molten alloy.

By comparing the two molded and polished samples respectively before and after the addition of the parent alloy, it is found that the titanium content in the alloy goes from 0.068% for the untreated sample to 0.088% for the treated sample while the magnesium content remains unchanged. In addition, for the sample not treated with the master alloy, the size of the primary aluminum grains varies from 600 to 800 microns, whereas for the sample cast and polished one hour after the refining treatment with the parent alloy, the grain size is in the range of 100 to 200 microns. The reduction in the size of the primary aluminum grains by adding a parent alloy is confirmed by the observation of the micrographs obtained by optical microscopy and represented in FIGS. 1 and 2. However, FIG. 2 also confirms that the compounds The AbTi and TiE4 intermetallic dispersions are irregularly dispersed in the primary aluminum matrix.

Example 2

A pellet of 5.5 g of a 25% TiN-75% AI binary mixture is used for the refining treatment of about 10 kg of the molten aluminum alloy.

An analysis of the two alloy samples, performed after casting, shows that the addition of grain refining agent in the molten alloy bath does not significantly increase the titanium content in the alloy. alloy after casting. Indeed, the alloy treated with the grain refining agent has a titanium content only 20ppm (0.002%) higher than the untreated alloy. The size of the primary aluminum grains passes an average size of 700 microns for the non-alloy alloy sample treated at 200 microns for the sample taken one hour after the beginning of the refining treatment.

Example 3

The alloy is treated with 5.0 g of a mixture of compressed pellet powders whose composition is: 25% TiN -30% AI-45% Mg. As indicated above, the powdery additive is thus formed by a mixture of 40% by weight of aluminum and 60% by weight of magnesium, with a melting temperature of 455 ^° C., close to that of the European Union. % AI- 68% Mg (437 ⁰ C) and lower than the treatment temperature of the aluminum alloy whose grain is to be refined (treatment temperature between 720 ⁰ C and 74O ⁰ C). The analysis of the respectively untreated and treated samples does not show an increase in the respective titanium and magnesium contents in the alloy. On the other hand, as shown in FIGS. 3 and 4, the addition of approximately 100 ppm (0.01%) of TiN in the molten alloy results in a marked decrease in the size of the primary aluminum grains compared to an untreated alloy. Thus, the grains go from an initial average size of 800 microns to a final average size of about 100 microns, one hour after the start of treatment. Thus, the refining agent in the form of a compressed mixture comprising at least 5% by weight of titanium nitride is easily added and dissolved during the treatment in the liquid state of the alloy and it allows rapid dispersion and effective titanium nitride in the molten alloy, which allows to obtain a grain refining with low levels of addition.

Claims

claims

Grain refining agent for the treatment in the liquid state of a metal chosen from aluminum and magnesium or from one of their respective alloys, characterized in that it comprises at least 5% by weight titanium nitride and at least one pulverulent additive having a melting temperature below the processing temperature of the metal or alloy whose grain is to be refined.

2. Agent according to claim 1, characterized in that it consists of a compressed mixture of titanium nitride with the powdery additive.

3. Agent according to one of claims 1 and 2, characterized in that the powdery additive consists of:

by at least one chemical element chosen from aluminum, magnesium, titanium, copper, manganese, silicon, boron, carbon and zinc

or by at least one metal alloy chosen from alloys of aluminum and magnesium, aluminum and silicon, aluminum and copper, magnesium and copper, magnesium and zinc, aluminum and aluminum. zinc

or by at least one intermetallic compound chosen from titanium aluminide, aluminum boride, titanium carbide, aluminum carbide, titanium boride, aluminum nitride and zirconium boride.

4. Agent according to claim 3, characterized in that the powdery additive consisting of at least two chemical elements, a metal alloy or an intermetallic compound, it has a composition close to the composition of a corresponding eutectic whose melting point is less than the processing temperature of the metal or alloy whose grain is to be refined.

5. Agent according to any one of claims 1 to 4, characterized in that the powdery additive has an average size of between 0.5mm and 5mm.

6. Agent according to any one of claims 1 to 5, characterized in that the titanium nitride is in the fining agent in the form of fine particles having an average size between 0.1 microns and 20 microns.

7. Agent according to claim 6, characterized in that the average size is between 0.1 microns and 5 microns.

8. Agent according to any one of claims 1 to 7, characterized in that the titanium nitride content in the refining agent is between 5% by weight and 90% by weight.

9. Agent according to claim 8, characterized in that the titanium nitride content in the refining agent is between 15% by weight and 75% by weight.

10. A process for producing a grain fining agent for the treatment in the liquid state of a metal selected from aluminum and magnesium or for one of their respective alloys, characterized in that it comprises at least one less:

the selection of at least one pulverulent additive consisting of one or more chemical elements suitable for entering into the composition of the metal or alloy whose grain is to be refined, and the mixture of at least 5% by weight of titanium nitride with the selected powder additive.

11. Method according to claim 10, characterized in that the mixture is produced by mechanical mixing and / or mechanical grinding.

12. Method according to one of claims 10 and 11, characterized in that the mixing step is followed by a compaction step.

13. The method of claim 12, characterized in that the compaction is carried out by exerting on the mixture, a uni-axial or isostatic pressure.

14. Process according to any one of Claims 10 to 13, characterized in that the selection of the powdery additive is carried out from pulverulent additives consisting of at least two chemical elements, a metal alloy or an intermetallic compound and having a composition close to the composition of a corresponding eutectic whose melting point is lower than the treatment temperature of the metal or alloy whose grain is to be refined.