WO2003033750A1

WO2003033750A1 - Grain refining agent for cast aluminum products

Info

Publication number: WO2003033750A1
Application number: PCT/CA2002/001549
Authority: WO
Inventors: Sabin Boily; Houshang D. Alamdari; Jacques Larouche
Original assignee: Groupe Minutia Inc.
Priority date: 2001-10-15
Filing date: 2002-10-15
Publication date: 2003-04-24
Also published as: CN1571854A; JP2005505694A; BR0213304A; CA2359181A1

Abstract

The invention relates to a grain refining agent for cast aluminum products containing titanium, comprising particles formed of a matrix of a ductile material, in which are uniformly dispersed boron particles having an average particle size of 0.1 to 10 µm. Where the cast aluminum products contain no titanium, the ductile material comprises titanium. The grain refining agent according to the invention does not require to be formed into a master alloy prior to being added to the molten aluminum to be cast.

Description

GRAIN REFINING AGENT FOR CAST ALUMINUM PRODUCTS

Technical Field

The present invention pertains to improvements in the field of cast metals and metal alloys. More particularly, the invention relates to a grain refining agent for cast aluminum products. Background Art

Grain refiners are widely used to reduce the grain size and to control the microstructure of cast metals and alloys. Adding grain refiners to molten metal or alloy during casting enhances the heterogeneous solidification and results in a fine-structured material with equiaxed grains. The resulting material shows improved mechanical properties such as high yield strength and toughness.

In the aluminum industry, different grain refiners are generally incorporated in the aluminum as master alloys which are added to the aluminum melt in solid form, for example, in the form of small ingots or a rod which is continuously fed into the melt. The master alloy can also be added in a molten state.

Typical master alloys for use in aluminum casting comprise from 1 to 10% titanium and from 0.1 to 5% boron or carbon, the balance consisting essentially of aluminum or magnesium, with particles of TiB₂ or TiC being dispersed throughout the matrix of aluminum. Master alloys containing titanium and boron are normally produced by dissolving the required quantities of titanium and boron in an aluminum melt. This is achieved by reacting molten aluminum with KBF and K₂TiF at temperatures in excess of 800°C. These complex halide salts react quickly with molten aluminum and provide titanium and boron to the melt. This technique is currently used to produce commercial master alloys by almost all grain refiner manufacturing companies. However, there is a number of disadvantages encountered with this technology. Tetrafluoroborate (KBF₄) and hexafluorotitanate (K₂TiF₆) complex salts are costly and have relatively low boron and titanium contents. KBF₄ decomposes at relatively low temperatures to give gaseous BF₃ which is toxic and thus requires special handling and filtering facilities during manufacturing of master alloys. The final master alloy contains potassium aluminum fluoride (KA1F) salt and aluminum oxide impurities giving rise to local defects in grain refined aluminum. Traces of halide salts in master alloys produced by conventional processes enhance the agglomeration of TiB₂ particles resulting in a decrease of effectiveness of the grain refiner. It is also very difficult to control the TiB₂ particle size and distribution in master alloy; these parameters are important in determining the effectiveness of grain refiner and affect the quality of the grain refined aluminum.

Many efforts have been made to overcome the above problems. Hardman et al. proposed in Materials Science Forum, Vols. 217- 222 (1966), pp. 247-252, to manufacture Al-Ti-B master alloys from low cost B₂0₃ and Ti0₂ starting materials where there is no need for KBF₄ and K₂TiF₆ salts in the manufacturing process. However, the use of cryolite (a mixture of NaF and A1F₃) is unavoidable in this process. Megy et al. proposed in Light Metals 2001. pp. 943-949, an in-situ grain refining process called the fy-Gem process. This process is carried out by introducing an Ar/BCl₃ gas mixture into molten aluminum in the form of fine bubbles using a rotating head. Borontricoloride is decomposed in molten aluminum and boron is dissolved into the aluminum and combined with titanium and other solute elements to form heterogeneous nuclei having an average particle size ranging from 0.5 and 5 μm. Although this technique does not use the complex halide salts and the problems associated with these salts are resolved, it uses borontrichloride which decomposes during the process, causing corrosive and toxic chlorine gas emission. In addition, a rotating head with special design is required to produce bubbles with optimum size within the molten aluminum in order to achieve good results. All this equipment and additional parameters to be controlled make the fy-Gem process more complicated and less interesting in practice. Disclosure of the Invention

It is therefore an object of the present invention is to overcome the above drawbacks and to provide an effective, salt free and low cost grain refining agent for cast aluminum products, especially aluminum alloys containing >0.003% Ti. According to one aspect of the invention, there is provided a grain refining agent for cast aluminum products containing titanium, comprising particles formed of a matrix of a ductile material, in which are uniformly dispersed boron particles having an average particle size of 0.1 to 10 μm. The expression "cast aluminum product" as used herein refers to a cast product comprising aluminum or an alloy thereof.

The average particle size of the boron particles must be within a range of from 0.1 to 10 μm. When the average particle size is greater than 10 μm, the number of boron particles introduced into the melt for a given addition level is too small. On the other hand, when the average particle size is smaller than 0.1 μm, the heterogeneous nucleation of molten aluminum is not effective.

Typical examples of ductile material include aluminum, titanium, chromium, copper and silicon. Aluminum is preferred. Preferably, the particles of ductile material have an average particle size of 0.5 to 5 mm. The boron particles, on the other hand, preferably have an average particle size of 0.5 to 2 μm.

When the grain refining agent of the invention is added to molten aluminum containing titanium, the ductile material melts or dissolves into the melt and boron particles are released within the melt and are combined with titanium to form heterogeneous nuclei which grain refine the aluminum during solidification.

Where the cast aluminum products contain no titanium, use is made of a ductile material comprising titanium.

Accordingly, the present invention provides in another aspect thereof a grain refining agent for cast aluminum products containing no titanium, comprising particles formed of a matrix of a ductile material comprising titanium, in which are uniformly dispersed boron particles having an average particle size of 0.1 to 10 μm.

The grain refining agent according to the invention is quite different from Al-B master alloys which are used occasionally in aluminum alloys. Al-B master alloys are produced by reacting KBF₄ salt with molten aluminum and its microstructure consists of A1B₂ or A1B₁₂ particles with extremely small amounts of boron in solid solution with an aluminum matrix. It is believed that A1B₂ particles are the effective nuclei of α-Al, however the exact grain refinement mechanism with boron addition has not as yet been clarified thoroughly. In addition, it was reported that Al-B master alloys has no effect on the grain refinement of Si-free Al alloys. In contrast, in the grain refining agent of the invention, there is no chemical reaction between the ductile material and the boron. The grain refining agent of the invention can effectively grain refine all Al alloys including Si- free Al alloys.

According to a further aspect of the present invention, there is provided a method of preparing a grain refining agent for cast aluminum products containing titanium. The method of the invention comprises the steps of: a) mixing boron particles having an average particle size greater than 0.1 μm with particles of a ductile material to form a powder mixture; and b) subjecting the powder mixture obtained in step (a) to high energy ball milling to reduce the size of the boron particles to a size ranging from 0.1 to 10 μm and to uniformly disperse the boron particles of reduced size within the ductile material, thereby obtaining particles formed of a matrix of the ductile material, in which are uniformly dispersed the boron particles having an average particle size of 0.1 to 10 μm.

As noted above, where the cast aluminum products contain no titanium, use is made of a ductile material comprising titanium.

According to yet another aspect of the invention, there is thus provided a method of preparing a grain refining agent for cast aluminum products containing no titanium, comprising the steps of: a) mixing boron particles having an average particle size greater than 0.1 μm with particles of a ductile material comprising titanium to form a powder mixture; and b) subjecting the powder mixture obtained in step (a) to high energy ball milling to reduce the size of the boron particles to a size ranging from 0.1 to 10 μm and to uniformly disperse the boron particles of reduced size within the ductile material, thereby obtaining particles formed of a matrix of the ductile material, in which are uniformly dispersed the boron particles having an average particle size of 0.1 to 10 μm.

Due to impact forces during ball milling, the boron particles are broken to small particles with the desired average particle size. Depending on the initial particle size of boron and the desired particles size thereof in the grain refining agent, the milling time can be adjusted. The milling time generally ranges from 10 minutes to 20 hours. The impact forces, in addition, cause plastic deformations of the ductile material and during these plastic deformations hard boron particles are trapped in the ductile material to form a composite comprising a matrix of ductile material in which boron particles are uniformly dispersed. According to a preferred embodiment, step (b) is carried out in a vibratory ball mill operated at a frequency of 8 to 25 Hz, preferably about 17 Hz. It is also possible to conduct step (b) in a rotary ball mill operated at a speed of 150 to 1500 r.p.m., preferably about 1000 r.p.m. According to another preferred embodiment, step (b) is carried out under an inert gas atmosphere such as a gas atmosphere comprising argon or nitrogen, in order to prevent oxidation of the grain refining agent. An atmosphere of argon is preferred.

Since the grain refining agent according to the invention is in powder form, it may be difficult to handle. Consolidation is thus preferred to facilitate manipulations and also to ensure that the grain refining agent is homogeneously dispersed in the aluminum melt to be cast. For example, the powder can be compacted to form pellets, discs or bricks by uniaxial pressing, hot or cold isostatic pressing, with or without a suitable binder. The powder can also be formed into a cored wire by wrapping the powder with a suitable foil which is preferably made of the same metal or alloy to be cast or of an element having a melting point lower than that of the metal or alloy to be cast.

The following non-limiting examples illustrate the invention. EXAMPLE 1.

A grain refining agent was prepared by ball milling a 90%A1- 10%B powder mixture in a hardened steel crucible using SPEX 8000 (trademark) vibratory ball mill operated at a frequency of 17 Hz. The initial particle size of aluminum powder was -100 mesh and that of the boron powder was l-5μm. The operation was performed under a controlled argon atmosphere to prevent oxidization. The crucible was sealed with a rubber 0-ring. The ball milling was carried for 0.5h. The resulting grain refining agent in powder form was uniaxially pressed and added into molten aluminum containing 0.15 wt% Ti. EXAMPLE 2.

A grain refining agent was prepared by ball milling a 50%Ti- 50%A1 powder mixture for 1 hour in a hardened steel crucible using SPEX 8000 (trademark) vibratory ball mill operated at a frequency of 17 Hz. Al and Ti powders with a particle size of -100 mesh were chosen as the starting materials and the operation was performed under a controlled argon atmosphere to prevent oxidization. The crucible was sealed with a rubber O-ring. Two parts of the resulting powder were rnixed with one part of the powder obtained in Example 1 and the powder mixture thus obtained was uniaxially pressed and added into a pure aluminum melt. EXAMPLE 3.

A grain refining agent was prepared starting with the same raw materials and with the same proportion as in Example 1. The ball milling was performed in a ZOZ (trademark) rotary high energy ball mill operated at 1000 r.p.m. The resulting grain refining agent in powder form was uniaxially pressed and added into molten aluminum containing 0.15 wt% Ti.

Claims

1. A grain refining agent for cast aluminum products containing titanium, comprising particles formed of a matrix of a ductile material, in which are uniformly dispersed boron particles having an average particle size of 0.1 to 10 μm.

2. A grain refining agent according to claim 1, wherein the ductile material comprises at least one element selected from the group consisting of aluminum, titanium, chromium, copper and silicon.

3. A grain refining agent according to claim 2, wherein the ductile material comprises aluminum.

4. A grain refining agent according to claim 1, 2 or 3, wherein the particles of ductile material have an average particle size of 0.5 to 5 mm.

5. A grain refining agent according to any one of claims 1 to 4, wherein the boron particles have an average particle size of 0.5 to 2 μm.

6. A grain refining agent for cast aluminum products containing no titanium, comprising particles formed of a matrix of a ductile material comprising titanium, in which are uniformly dispersed boron particles having an average particle size of 0.1 to 10 μm.

7. A grain refining agent according to claim 6, wherein the ductile material further comprises at least one element selected from the group consisting of aluminum, chromium, copper and silicon.

8. A grain refining agent according to claim 7, wherein the ductile material further comprises aluminum.

9. A grain refining agent according to claim 6, 7 or 8, wherein the particles of ductile material have an average particle size of 0.5 to 5 mm.

10. A grain refining agent according to any one of claims 6 to 9, wherein the boron particles have an average particle size of 0.5 to 2 μm.

11. A method of preparing a grain refining agent as defined in claim 1, comprising the steps of: a) mixing boron particles having an average particle size greater than 0.1 μm with particles of a ductile material to form a powder mixture; and b) subjecting the powder mixture obtained in step (a) to high energy ball milling to reduce the size of the boron particles to a size ranging from 0.1 to 10 μm and to uniformly disperse the boron particles of reduced size within the ductile material, thereby obtaining particles formed of a matrix of the ductile material, in which are uniformly dispersed the boron particles having an average particle size of 0.1 to 10 μm.

12. A method according to claim 11, wherein the ductile material comprises at least one element selected from the group consisting of aluminum, titanium, chromium, copper and silicon.

13. A method according to claim 12, wherein the ductile material comprises aluminum.

14. A method according to claim 11, wherein step (b) is carried out to reduce the size of the boron particles to a size ranging from 0.5 to 2 μm.

15. A method according to claim 11, wherein step (b) is carried out for a period of time ranging from 10 minutes to 20 hours.

16. A method according to claim 11, wherein step (b) is carried out in a vibratory ball mill operated at a frequency of 8 to 25 Hz.

17. A method according to claim 16, wherein said vibratory ball mill is operated at a frequency of about 17 Hz.

18. A method according to claim 11, wherein step (b) is carried out in a rotary ball mill operated at a speed of 150 to 1500 r.p.m.

19. A method according to claim 18, wherein said rotary ball mill is operated at a speed of about 1000 r.p.m.

20. A method according to claim 11, wherein step (b) is carried out under an inert gas atmosphere.

21. A method of preparing a grain refining agent as defined in claim 6, comprising the steps of: a) mixing boron particles having an average particle size greater than 0.1 μm with particles of a ductile material comprising titanium to form a powder mixture; and b) subjecting the powder mixture obtained in step (a) to high energy ball milling to reduce the size of the boron particles to a size ranging from 0.1 to 10 μm and to uniformly disperse the boron particles of reduced size within the ductile material, thereby obtaining particles formed of a matrix of the ductile material, in which are uniformly dispersed the boron particles having an average particle size of 0.1 to 10 μm.

22. A method according to claim 21, wherein the ductile material further comprises at least one element selected from the group consisting of aluminum, titanium, chromium, copper and silicon.

23. A method according to claim 22, wherein the ductile material further comprises aluminum.

24. A method according to claim 21, wherein step (b) is carried out to reduce the size of the boron particles to a size ranging from 0.5 to 2 μm.

25. A method according to claim 21, wherein step (b) is carried out for a period of time ranging from 10 minutes to 20 hours.

26. A method according to claim 21, wherein step (b) is carried out in a vibratory ball mill operated at a frequency of 8 to 25 Hz.

27. A method according to claim 26, wherein said vibratory ball mill is operated at a frequency of about 17 Hz.

28. A method according to claim 21, wherein step (b) is carried out in a rotary ball mill operated at a speed of 150 to 1500 r.p.m.

29. A method according to claim 28, wherein said rotary ball mill is operated at a speed of about 1000 r.p.m.

30. A method according to claim 21, wherein step (b) is carried out under an inert gas atmosphere.