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. This effect is more pronounced on Mg-based alloys with an HCP structure than metals with an FCC structure such as Al, Cu, or Ni. With a fine microstructure the machinability of the cast products is improved, the porosity is reduced and the pores are distributed more evenly within the products, and heat treatment is more effective due to the small diffusion distances.

Unlike aluminum-based alloys, grain refining of magnesium alloys has received much less attention during the past decades. There exist several methods of grain refining which are practiced in the magnesium industry; however, the mechanism of this phenomenon has not as yet been fully understood. Two mechanisms have been proposed to explain grain refining process; solute paradigm and nucleant particles paradigm. The solute theory suggests that the grain refinement is due to the presence of alloying elements which slow down grain growth. In fact, some alloying elements segregate in the liquid/solid interface during solidification and further progress of this interface requires the diffusion of the alloying elements into the liquid. This

effect is quantified as the Growth Restriction Factor (GRF). The elements having large GRF are better grain refining agents. The nucleant particles theory, on the other hand, suggests that the grain refinement is due to the presence of solid particles on which magnesium begins to solidify. The nucleant particles should be stable at inoculation temperature and they must have certain crystallographic compatibility with magnesium.

The GRF of a number of elements has been reported by Y. C. Lee et al. in Metallurgical and Materials Transactions A, Vol. 31A, 2000, pp. 2895- 2906. Zr, Ca, Si and Ni have the highest GRF. The addition of small amounts of these elements in pure magnesium shows that the mean grain size decreases drastically, reaching a constant value at high addition levels (>0.3%). At these addition levels, a mean grain size of 100-500 llm can be obtained. Zirconium is the most efficient solute element for grain refining, but it interferes with aluminum and thus cannot be added in Mg-based alloys containing aluminum.

AZ91 alloy, containing 9 wt. % of aluminum, is a commonly used Mg-based alloy. The aluminum in this alloy has a beneficial effect on the mean grain size. This alloy, due to a high concentration of aluminum, has a very fine microstructure and a sand cast AZ91 alloy has normally a mean grain size of about 120 u. m. Further addition of refining elements such as strontium has no significant effect on the mean grain size.

Carbon inoculation is another method providing a very good refining effect and it has become the major industrial grain refining practice for Mg-based alloys. Carbon can be added in molten magnesium in form of carbon containing solids or gases. The most commonly used grain refiner for magnesium is C2C16. C2C16 is a low cost material which decomposes in liquid magnesium. The grain refinement effect of carbon is attributed to the formation of aluminum carbide particles (A14C3) which act as nucleation sites for

magnesium solidification. The problem encountered with this refiner is the emission of toxic chlorinated gases.

Particles addition in magnesium alloys is another promising method for grain refining of magnesium. Lee et al. (Magnesium Technology 2000. The Minerals, Metals & Materials Society, pp. 211-218) investigated the effect of the addition of TiC, SiC, A1N and A14C3 particles in magnesium. They showed that an addition of 1 vol. % of these particles in a Mg-Al alloy produces a composite having a mean grain size between 300 and 500 um. SiC and A14C3 exhibited a slightly better effect than other particles probably due to lower lattice disregistry between them and magnesium. The beneficial effect on grain refinement of magnesium has also been observed in Mg/SiC composites. Luo et al. (Canadian Metallurgical Quarterly, vol. 35, No. 4,1996, pp. 375-383) added 0.5 to 20 vol% SiC particles having an average particle size of 7 um into an AZ91 alloy. The mean grain size of the resulting composites were 79 and 42 pm for 0.5 and 10 vol. % addition levels, respectively. Hu et al. (Scripta Materialia, vol. 39, No. 8,1998, pp. 1015-1022) also obtained fine-grained composites (59 u. m) by the addition of 10 vol. % of SiC particles having an average particle size of 13 Fm to an AM50A magnesium matrix. Although these experimental results show that SiC particles can provide a fine-grained composite, such a high addition level does not justify their application as a grain refiner material.

Disclosure of Invention It is therefore an object of the present invention to provide an effective chlorine free and low cost grain refining agent for cast magnesium products.

According to one aspect of the invention, there is provided a grain refining agent for cast magnesium products, comprising particles formed

of a matrix of a ductile material, in which are uniformly dispersed nucleation particles of at least one element or compound selected from the group consisting of zirconium, silicon carbide, titanium carbide and mixtures of carbon with silicon or titanium, the nucleation particles having an average particle size of 0.05 to 5 am.

The expression"cast magnesium products used herein refers to a cast product comprising magnesium or an alloy thereof.

The average particle size of the nucleation particles must be within a range of from 0.05 to 5 um. When the average particle size is greater than 5 pLm, the number of nucleation 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 0.05 lem, té heterogeneous nucleation of molten magnesium is not effective.

Applicant has found quite unexpectedly that the dispersion of the nucleation particles within a matrix of ductile material improves their wettability, overcomes the problems caused by gas absorption on the surface of the particles and reduces their agglomeration during inoculation into molten magnesium or alloy thereof.

The present invention also provides, in another aspect thereof, a method of preparing a grain refining agent as defined above. The method of the invention comprises the steps of : a) mixing particles of a ductile material with nucleation particles of at least one element or compound selected from the group consisting of zirconium, silicon carbide, titanium carbide and mixtures of carbon with silicon or titanium, to form a powder mixture, the nucleation particles having an average particle size of 0.05 to 5 u. m ; and

b) subjecting the powder mixture obtained in step (a) to high- energy ball milling to uniformly disperse the nucleation particles within the ductile material, thereby obtaining particles formed of a matrix of the ductile material, in which are uniformly dispersed the nucleation particles.

Modes for Carrying out the Invention Typical examples of ductile material include aluminum, magnesium and zinc. Aluminum and magnesium are preferred.

Preferably, the particles of ductile material have an average particle size of 0.05 to 5 mm. The nucleation particles, on the other hand, preferably have an average particle size of 0.1 to 2 m, more preferably between 0.1 and 0.5 m. It is also possible to use a ductile material comprising an alloy selected from the group consisting of aluminum-based alloys, magnesium-based alloys and zinc-based alloys.

Where the nucleation particles used in step (a) of the method according to the invention do not have the desired average particle size, nucleation particles of the desired size can be obtained by subjecting coarse particles of the aforesaid element or compound having an average particle size greater than 0.05 pm to a preliminary high-energy ball milling to reduce the size of the coarse particles of the element or compound to a size from 0.05 to 5 m.

Due to impact forces during the preliminary ball milling, the particles of the aforesaid element or compound are broken to small particles with the desired average particle size. Depending on the initial particle size of the element or compound and the desired particles size thereof in the grain refining agent, the milling time can be adjusted. The milling time generally ranges from 1 hour to 20 hours. On the other hand, the impact forces during the ball milling of step (b) cause plastic deformations of the ductile material and

during these plastic deformations the nucleation particles are trapped in the ductile material to form a composite comprising a matrix of ductile material in which the nucleation particles are uniformly dispersed. The ball milling of step (b) can be carried out for a period of time ranging from 0.5 hour to 5 hours.

According to a preferred embodiment, the preliminary ball milling and the milling of step (b) are 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 these two steps 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, the preliminary ball milling and the ball milling of step (b) are 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 magnesium 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 70% Si- 30% C powder mixture in a hardened steel crucible, using a SPEX 8000 (trademark) vibratory ball mill operated at a frequency of 17 Hz. The initial particle size of the silicon powder was-100 mesh and that of the carbon powder was 1-5 um. The operation was performed under a controlled argon atmosphere to prevent oxidization. The crucible was sealed with a rubber O- ring. A milling time of 15 hours was chosen for this operation. The resulting grain refining agent in powder form having an average particle size of 0.5-3 llm was mixed with an aluminum powder in a Al/refiner ratio of 4: 1 and the mixture was milled for 1 hour under a controlled argon atmosphere, using a SPEX 8000 vibratory ball mill operated at a frequency of 17 Hz, to disperse the refiner particles within the ductile aluminum. The Al/refiner composite in powder form thus obtained was then uniaxially pressed and added into a AZ91 magnesium melt to provide 250 ppm of silicon in the melt.

Example 2 A grain refining agent was prepared by ball milling a 80% Ti- 20% C powder mixture in a hardened steel crucible, using a SPEX 8000 vibratory ball mill operated at a frequency of 17 Hz. The initial particle size of the titanium was-100 mesh and that of the carbon powder was 1-5 u. m. The operation was performed under a controlled argon atmosphere to prevent oxidization. The crucible was sealed with a rubber 0-ring. A milling time of 5 hours was chosen for this operation. The resulting grain refining agent in powder form having an average particle size of 0.5-3 um was mixed with an aluminum powder in a Al/refiner ratio of 4: 1 and the mixture was milled for 1 hour under a controlled argon atmosphere using a SPEX 8000 vibratory ball

mill operated at a frequency of 17 Hz, to disperse the refiner particles within the ductile aluminum. The Al/refiner composite in powder form thus obtained was then uniaxially pressed and added into a AZ91 magnesium melt to provide 400 ppm of titanium in the melt.

Example 3 A grain refining agent was prepared by ball milling a SiC powder in a tungsten carbide crucible, using a SPEX 8000 vibratory ball mill operated at a frequency of 17 Hz. The initial particle size of the SiC powder was-100 mesh. The operation was performed under a controlled argon atmosphere to prevent oxidization. The crucible was sealed with a rubber 0-ring. A milling time of 10 hours was chosen for this operation. The resulting grain refining agent in powder form having an average particle size of 0.5-3 p. m was mixed with an aluminum powder in a Al/refiner ratio of 4: 1 and the mixture was milled for 1 hour under a controlled argon atmosphere, using a SPEX 8000 vibratory ball mill operated at a frequency of 17 Hz, to disperse the refiner particles within the ductile aluminum. The Al/powder composite in powder form thus obtained was then uniaxially pressed and added into a AZ91 magnesium melt to provide 400 ppm of SiC in the melt.

Example 4 A grain refining agent was prepared by ball milling a TiC powder in a tungsten carbide crucible, using a SPEX 8000 vibratory ball mill operated at a frequency of 17 Hz. The initial size of the TiC powder was-325 mesh. The operation was performed under argon atmosphere to prevent oxidization. The crucible was sealed with a rubber 0-ring. A milling time of 10 hours was chosen for this operation. The resulting grain refining agent having an average particle size of 0. 5-3 um was mixed with an aluminum powder in a Al/refiner ratio of 4: 1 and the mixture milled for 1 hour under a controlled argon

atmosphere, using a SPEX 8000 vibratory ball mill operated at a frequency of 17 Hz, to disperse the refiner particles within the ductile aluminum. The Al/powder composite in powder form thus obtained was then uniaxially pressed and added into a AZ91 magnesium melt to provide 400 ppm of TiC in the melt.

Example 5 A grain refining agent was prepared by ball milling a 96 wt. % of a zirconium powder and 4 wt. % of stearic acid (lubricant) in a tungsten carbide crucible using a SPEX 8000 vibratory ball mill operated at a frequency of 17 Hz. The initial particle size of the zirconium powder was-100 mesh. The operation was performed under a controlled argon atmosphere to prevent oxidization. The crucible was sealed with a rubber 0-ring. A milling time of 10 hours was chosen for this operation. The resulting grain refining agent in powder form having an average particle size of 1-5 Fm was mixed with a zinc powder in a Zn/refiner ratio of 9: 1 and the mixture was milled for 5 hours under a controlled argon atmosphere, using a SPEX 8000 vibratory ball mill operated at a frequency of 17 Hz, to disperse the refiner particles within the ductile zinc. The resulting Zn/refiner composite in powder form thus obtained was then uniaxially pressed and added into a AZ91 magnesium melt to provide 300 ppm of zirconium in the melt.