The present invention relates to a method of refining grains of primary silicon in hypereutectic aluminum-silicon alloys upon solidification of the melt.

In the art of casting alloys, Al-Si systems have been widely used for their high fluidity in a molten state and their low shrinkage upon solidification. In particular, hypereutectic Al-Si alloys having a high silicon content have been suitably applied as casting alloys which are excellent in wear resistance.

In conducting a continuous casting operation to obtain an ingot of such hypereutectic alloys of a silicon content higher than the eutectic point (generally, Si = 12.7%), it is a common practice to introduce the Al-Si melt into a casting mold from a holding furnace through a pouring trough, so that the melt is cast into the ingot of a desired shape within the mold. According to this casting process, primary silicon crystals of the hypereutectic Al-Si melt grow into relatively coarse grains of more than 40 microns, sometimes even 100 microns or larger, during the solidification of the melt, unless a particular grain-refining treatment is effected. As a result, the obtained cast ingot tends to have a coarse-grain structure, exhibiting reduced toughness. To avoid this phenomenon, phosphorus is generally added to a hypereutectic Al-Si melt for the purpose of refining the primary phase of silicon. The addition of phosphorus produces compounds of AlP, which have a nucleation behavior in such a way as to promote the grain refinement of the primary silicon.

The aforementioned addition of phosphorus to the melt of hypereutectic Al-Si alloy systems is conventionally achieved by introducing into the Al-Si melt accommodated in a holding furnace, a suitable grain-refining agent to refine the primary silicon, for example, a grain refiner in the form of Cu-8%P or Cu-15%P master alloy, or a flux containing phosphorus. However, this manner of introduction- of a grain refiner directly into the holding furnace, suffers difficulty in attaining uniform or even distribution of phosphorus throughout the entire mass of the Al-Si melt in the holding furnace. Further, the direct introduction method is not satisfactory for its low yield of phosphorus introduced in the furnace. More specifically described, since the amount of phosphorus to be introduced is considerably small with respect to the volume of the molten alloy mass, phosphorus is likely to be locally concentrated in the melt in the furnace even if the melt is stirred. Further, phosphorus may not be effectively utilized because of its partial sedimentation on the bottom of the furnace or its floating to the liquid surface while the melt is stirred.

According to the conventional method indicated above, the grain-refining effect of phosphorus is varied with a pouring time of the melt. Namely, a bottom part of the obtained ingot which is formed of an initially poured portion of the melt, tends to have a different grain size of the primary silicon, from that of a part of the ingot which is formed of a portion of the melt which is introduced later in the furnace. Consequently, the property of the ingot as an end product is varied from one part to another. Described in more detail, the grain-refining effect of phosphorus as a grain refiner for primary silicon is high enough for the initially poured portion of the melt, but the effect is reduced with a time lapse after the phosphorus addition to the melt, whereby the primary silicon in the subsequently cast part of the ingot is given a larger grain size than the initially cast part. That is, the grain refiner added to the melt is not sufficiently effective for the terminal portion of the melt mass.

It is accordingly an object of the present invention to provide a method of refining grains of primary silicon in an aluminum-silicon alloy upon solidification of its melt, which is effective for overcoming the inconveniences encountered in the prior art discussed above.

According to the present invention, there is provided a method of refining grains of primary silicon of a hypereutectic aluminum-silicon alloy which is cast into an ingot by introducing a melt of the alloy into a casting mold from a holding furnace which accommodates the melt, comprising the step of introducing a grain-refining agent for primary silicon containing phosphorus, continuously into a flow of the melt from the holding furnace to the casting mold through a passage communicating with the furnace and the mold, and thereby refining the grains of the primary silicon in the hypereutectic aluminum-silicon alloy, upon solidification of the melt in the casting mold.

Contrary to the conventional method wherein a batch of primary-silicon refining agent required for the entire volume of the alloy melt is introduced at one time into the holding furnace accommodating the melt, the aforementioned method of the present invention requires the grain-refining agent to be continuously fed into a continuous flow of the melt from the furnace through the pouring passage, so that a suitable amount of the grain-refining agent is applied to a unit volume of the melt.

The instant method makes it possible to distribute the grain-refining agent evenly throughout the mass of the hypereutectic aluminum-silicon alloy melt, whereby phosphorus of the added grain-refining agent may be used in effect with an improved yield.

According to the method of the invention, the continuous addition of the grain-refining agent to the melt is effected a comparatively short time before the melt is poured into the casting mold. Therefore, the grain-refining effect of phosphorus is maintained at a relatively high level, and the time between the addition of phosphorus and the pouring of the melt into the mold is held constant. Thus, the instant method overcomes the conventionally experienced inconvenience that the obtained ingot suffers variations in property from one part to another, due to reduction of the grain-refining effect of the agent with a time lapse after the addition of the agent until individual portions of the melt mass are poured into the casting mold.

According to one advantageous embodiment of the invention, the grain-refining agent consists of a mass of particles containing phosphorus, and the method further comprises the step of charging an aluminum tube having a small diameter with the particles of the grain-refining agent to prepare a grain-refining rod for primary silicon. The leading end of the aluminum tube is located in the flow of the melt flowing through the passage, to thereby distribute the grain-refining agent in the melt of the hypereutectic aluminum-silicon alloy.

In the above embodiment of the invention, the grain-refining agent in the form of particles containing phosphorus is continuously introduced into a continuous flow of the melt from the furnace into the mold, by holding the currently leading end of the grain-refining rod in the flow of the melt. This arrangement permits uniform distribution of the grain-refining agent throughout the mass of the melt.

According to another advantageous embodiment of the invention, the grain-refining agent consists of a mass of particles containing phosphorus, which particles are mixed with a mass of aluminum particles or an aluminum matrix. In this instance, the method further comprises the step of charging an aluminum shell with a mixture of the particles of the grain-refining agent and the aluminum particles or the aluminum matrix, so as to prepare a grain-refining rod. The leading end of the grain-refining rod is located in the flow of the melt flowing through the passage, to thereby distribute the grain-refining agent in the melt of the hypereutectic aluminum-silicon alloy.

In the above embodiment, too, the grain-refining rod is disposed so that its leading end is located in the continuous flow of the melt from the furnace into the mold. Unlike the grain-refining rod used in the preceding embodiment, however, the grain-refining rod consists of a mixture of the grain-refining agent and a mass of aluminum particles or an aluminum matrix, and an aluminum shell which is charged with the mixture.

The outer aluminum shell of the grain-refining rod contributes to easy melting of the rod in the aluminum-silicon melt. Further, since the grain-refining agent in the form of particles is distributed in the aluminum particles or aluminum matrix, the wettability of the grain-refining agent with respect to the molten hypereutectic aluminum-silicon alloy is increased. Accordingly, the grain-refining agent such as copper-phosphorus alloy particles or phosphorus particles may be efficiently distributed and melted in the Al-Si melt. Since such grain-refining agent is surrounded by the aluminum particle mass or aluminum matrix and further covered by the aluminum shell, a loss of the agent due to its oxidization is minimized, whereby the yield of the agent is improved.

As the grain-refining agent containing phosphorus, the present method generally uses particles (powder) of _Cu-P alloys, or fluxes, for example of ALPO₄ or a combination of red or yellow phosphorus and chloride such as KCl. The content of phosphorus of the Cu-P alloy is suitably selected, so as to give the finally processed grain-refining rod a suitable strength for easy positioning and orientation of the rod in the flow of the aluminum-silicon melt. It is generally recommended that the content of phosphorus of the Cu-P alloy be held within a range between 0.5% and 15%, by weight, preferably about 8% by weight at which the eutectic phase of the Cu-P alloy is established. With the content of phosphorus exceeding 15% by weight, phosphorus vaporizes and a Cu-P alloy can not exist.

The grain-refining rod containing phosphorus and aluminum may be preferably prepared with one of the following two methods:

• According to the first method, a mass of particles of a selected grain-refining agent is mechanically mixed with a mass of aluminum particles. Then, a suitable aluminum tube (having a small diameter) is charged or filled with the mixture mass of the grain-refining particles and the aluminum particles. Thus, a grain-refining rod is prepared.

According to the second method, a mixture mass is similarly obtained by mechanically mixing particles of the grain-refining agent and the aluminum particles. This mixture mass is compacted, so that the compacted mass is subsequently inserted into a suitable hollow billet or pipe which is made of aluminum. Then, the hollow billet charged with the compacted body of the above-indicated mixture of particles is closed at its opposite ends and is then subjected to a hot-extruding process at a temperature of 350-500^oC. In this manner, a desired grain-refining rod is obtained. According to this method, the aluminum particles are formed into an integral aluminum matrix body. The obtained grain-refining rod contains the particles of the grain-refining agent such that they are distributed within the aluminum matrix.

The grain sizes and forms of Cu-P alloy particles or phosphorus particles used as a grain-refining agent, and of aluminum particles mixed with such grain-refining particles, and the diameter and shape of the grain-refining rod to be prepared, are suitably selected depending upon: a rate of feed of the grain-refining rod into the flow of the aluminum-silicon alloy melt; a casting method; a size of an ingot to be cast; a kind of aluminum-silicon alloy; a distance between the location of the leading end of the rod and the casting mold; and other conditions. It will be obvious that the grain-refining rod may have any suitable cross sectional shapes other than a circular shape. For example, the grain-refining rod may be provided in the form of a strip or tape.

The thus prepared grain-refining rod which contains a grain-refining agent for refining primary silicon in a hypereutectic aluminum-silicon alloy, is continuously fed by suitable means into the melt of the hypereutectic Al-Si alloy while it is flowing through a passage leading to a casting mold.

The foregoing and other objects, features and advantages of the present invention will be better understood from reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawing, in which:

• Fig. 1 is a schematic view of a casting system, illustrating a method of refining grains of primary silicon according to one embodiment of the invention;
• Fig. 2 is a fragmentary perspective view of an example of a grain-refining rod which consists of a grain-refining agent and an aluminum tube, and which is used in the method of Fig. 1; and
• Figs. 3 and 4 are views corresponding to Fig. 2, showing modified forms of a grain-refining rod which are used according to modified embodiments of the invention.

To clarify the concept of the present invention, preferred embodiments of the invention will be described in detail, referring to the accompanying drawing.

There is first shown in Fig. 1 a casting system, wherein reference numeral 10 designates a holding furnace for holding a melt 12 of a hypereutectic aluminum-silicon alloy having a silicon content exceeding the eutectic point but not exceeding 30% by weight. The holding furnace 10 has an outlet 14 through which the alloy melt 12 is led into a passage in the form of a pouring trough 16. The melt 12 flowing through the pouring trough 16 is introduced into a casting mold 20 through a spout 18 formed at the end of the trough 16. In this arrangement, the molten hypereutectic Al-Si alloy is continuously cast into an intended ingot 22.

According to a method of the present invention, a grain-refining agent containing phosphorus (P) is added to a mass 15 of the melt 12 flowing through the trough 16, in order to refine grains of primary silicon in the hypereutectic Al-Si alloy. In this embodiment, the grain-refining agent is supplied in the form of a grain-refining rod 26 which consists of a mass of particles 30 of a Cu-8%P alloy, and an aluminum tube 31 which has a wall thickness of 0.5-2 mm and an outside diameter of 5-15 mm. The bore of the aluminum tube 31 is filled or charged with the mass of Cu-8%P alloy particles 30 in a relatively dense manner, as shown in Fig. 2. The grain-refining rod 26 is wound as a coil ₂4, and is continuously fed by a feeding device 28 into a continuous flow of the melt mass 15 in the pouring trough 16, as illustrated in Fig. 1, so that the leading end of the grain-refining rod 26 is held in the flow of the melt mass 15. With the leading end of the rod 26 located within the melt mass 15, the corresponding leading portion of the aluminum tube 31 is melted under heat of the melt mass 15, while the mass of Cu-8%P particles 30 within the tube 31 is melted into the flow of the melt 15. The grain-refining rod 26 is continuously fed into the melt mass 15 at a rate which corresponds to a flow rate of the melt mass 15 from the furnace 10 toward the casting mold 20. In this manner, the grain-refining agent 30 accommodated in the aluminum tube 31 is introduced into a continuous flow of the melt mass 15 through the pouring trough 16.

The amount of feed of the grain-refining particles 30 per unit volume of the melt mass 15 may be adjusted by changing the diameter and/or feed rate of the rod 26, so that the effective concentration of phosphorus in the melt mass 15 is held within a range of 10-1000 ppm.

The location at which the currently leading end of the grain-refining rod 26 is positioned within the melt mass 15 in the trough 16, is selected so that the rod 26 is sufficiently melted in the melt mass 15 to permit the grain-refining agent 30 to give an intended grain-refining or nucleation effect. While the location is generally determined by the form and composition of a grain-refining agent used, the present embodiment is adapted to locate the leading end of the rod 26 at a position near the outlet 14 of the holding furnace 10, at which the temperature of the melt mass 15 is comparatively high, and from which it takes more than one minute for the melt mass 15 to leave the trough 16. In the case where the casting system is equipped with a degasing.device disposed over the pouring trough 16 to remove gases from the melt mass 15, the leading end of the grain-refing rod 26 is preferably located upstream of such a degasing device, as viewed in the direction of flow of the melt mass 15, so that the grain-refining agent 30 introduced into .the melt mass 15 may be effectively distributed by the degasing device.

. As described above, the present method comprises a step of introducing the grain refiner or grain-refining agent 30, in a continuous fashion by a small amount per unit time, into the melt mass 15 of hypereutectic Al-Si alloy which is flowing through the pouring trough 16. This arrangement is effective to avoid inconveniences of sedimentation and/or floating of the grain-refining agent within the holding furnace 10, which are encountered in a conventional batch method in which a required total amount of the grain-refining agent 30 is introduced at one time into the mass of the melt 12 in the furnace 10. Consequently, the manner of introducing the grain-refining agent 30 according to the present method can make an effective use of the agent 30, and permits an improved yield of the agent.

Since the grain-refining agent 30 (particles of Cu-8%P alloy) is added continuously to a continuous flow of the melt mass 15 flowing through the pouring trough 16, the agent 30 is uniformly or evenly distributed throughout the flowing mass 15, whereby the concentration of phosphorus (P) is kept constant at an optimum level.

In addition, the instant method is adapted to introduce the grain-refining agent 30 into the melt mass 15 which will be poured into the casting mold 20 in a relatively short time, the agent 30 maintains its intended grain-refining effect until the melt mass 15 is poured into the mold 22. Further, a time between the addition of the agent 30 to a given portion of the melt mass 15 and the pouring of that portion into the mold 20 is held constant. This constant time lapse contributes to improved grain refinement of the primary silicon in the Al-Si alloy, and to prevention of a variation in the degree of refinement between the bottom (leading) and top (trailing) parts of the ingot 22 which correspond to initially and finally poured portions of the melt 12.

According to the instant method, the mass of the grain-refining agent 30 is introduced into the melt mass 15 while the agent 30 is covered with the annular wall of the aluminum tube 31. This aluminum tube 31 protects the agent 30 from oxidization due to heat from the melt mass 15 immediately before the agent 30 is delivered into the mass 15. Further, since the agent 30 is provided in the form of particles, the agent 30 is rapidly melted into the melt mass 15 and evenly distributed after the leading portion of the aluminum tube 31 has been melted. In addition, the grain-refining rod 2C may be easily fed by a suitable feeding device as indicated at 28 in Fig. 1. Since the rod 26 is stored as a coil, it is easily handled.

EXAMPLE 1

To confirm better results of the present method as compared with a conventional batch method, the grain-refining agent 30 in the form of Cu-8%P alloy particles was. introduced into the flow of the melt mass 15 of a hypereutectic Al-18%Si alloy in the pouring trough 16, which was cast into the ingot 22 having a diameter of 200 mm. In the conventional method, the same grain-refining. agent 30 was introduced into the holding furnace 10 in a batch manner. In the present and conventional methods, the agent 30 was added so that the effective concentration of phosphorus in the melt 12 or 15 was 100 ppm. Table 1 shows the results of the above experiment.

It will be understood from the above table that the present method provides a higher yield percent of phosphorus, and permits a smaller grain size and a less variation in the grain size between the bottom and top parts of the ingot 22, as compared with the known batch method.

Referring next to Figs. 3 and 4, there are shown modified grain-refining rods 32 and 40 which are used in modified embodiments of the present invention. Like the grain-refining rod 26 used in the preceding embodiment, these grain-refining rods 32, 40 also contain particles 34 of a suitable grain-refining agent such as a Cu-8%P alloy.

More specifically _described, the grain-refining rod 32 of Fig. 3 consists of a mixture of the grain-refining particles 34 and an aluminum matrix 36, and an aluminum shell in the form of an aluminum tube 38 (having a wall thickness of 0.5-2 mm and an outside diameter of 5-15 mm). The aluminum tube 36 is charged with the mixture mass 34, 38 such that the particles 34 are distributed in the aluminum matrix 36. In the meantime, the grain-refining rod 40 of Fig. 4 consists of a mixture of the grain-refining particles 34 and aluminum particles 42, and an aluminum shell 44 which is charged with the mixture of the particles 34, 42.

In the above embodiments of Figs. 3 and 4, too, the grain-refining agent in the form of the particles 34 which are distributed in the aluminum matrix 36 or in the aluminum particle mass 42, are added to the melt mass 15 by locating the leading end cf the grain-refining rod 32, 40 into a flow of the melt mass 15. Therefore, the grain-refining agent 34 is given improved wettability with respect to the melt of the aluminum-silicon alloy, and is rapidly distributed and melted in the melt 15. The present method makes it possible to introduce the grain-refining agent at a relatively low temperature of the aluminum-silicon alloy melt, for example, in the neighbourhood of 720°C. Further, the present method provides considerable improvements in the yield and distribution uniformity of phosphorus of the grain refiner, and permits improved uniformity of the grain size of the primary silicon throughout the entire mass of the cast aluminum-silicon ingot, from its bottom to its top.

Further, the grain-refining agent 34 is protected from oxidation, by the aluminum matrix 36 or aluminum particles 42, and by the aluminum shell 38, 44. In this sense, the yield of phosphorus added to the Al-Si melt is further improved.

EXAMPLE 2

To continuously cast a hypereutectic Al-18%Si alloy into an ingot of a 150 mm diameter, a grain-refining agent in the form of particles of a Cu-8%P master alloy was introduced into the melt of the hypereutectic Al-18%P alloy, according to the known method, and according to the two different methods of the invention using the grain-refining rods of Figs. 2 and 3, respectively.

According to the known method, a required total amount of the grain-refining agent was introduced at one time into the holding furnace 10. In the method of Fig. 2, the grain-refining rod 26 with the aluminum tube 31 charged with the Cu-8%P alloy particles 30 was continuously fed into a continuous flow of the melt mass 15, as shown in Fig. 1.

According to the method of Fig. 3, one part of aluminum particles and one part of grain-refining Cu-8%P alloy particles were mixed with each other, and the obtained mixture mass was compacted. In the meantime, a hollow billet (pipe) of aluminum was prepared and charged with the compacted mixture mass. The charged hollow billet was hot-extruded into the intended grain-refining rod 32 wherein the grain-refining Cu-8%P alloy particles were distributed in the aluminum matrix. The thus prepared rod 32 was continuously fed into a continuous flow of the melt mass 15, as shown in Fig. 1.

The Al-18%Si alloy ingots 22 obtained according to the aforementioned three different methods were examined in terms of yield percent of phosphorus, and grain size of the primary silicon in the bottom, intermediate and top parts of the ingots 22. The results of examination are indicated in Table 2. In all of the three methods, the grain-refining agent was introduced so that the effective concentration of phosphorus in the melt 12 or 15 was 100 ppm.

As indicated in Table 2, the method of the present invention practiced by using the grain-refining rod 32 of Fig. 3 which contains the Cu-P alloy particles in the aluminum matrix, exhibited considerably better results than the method by using the rod 26 of Fig. 2, as well as than the conventional batch method, in terms of the yield of phosphorus, grain-refining capability and uniformity of grain size of the primary silicon between the bottom and top parts of the cast ingot.

While the present invention has been described in its preferred embodiments, it is to be understood that the invention is not confined to the precise disclosure of the illustrated embodiments, but may be otherwise embodied.

For example, Cu-15%P and other compositions may be used as a grain-refining agent for refining the primary silicon according to the invention. Although the grain-refining agent in such an alloy form is desired for elimination of otherwise required removal of a reaction residue, it is possible to use a grain-refining agent in the form of a flux containing phosphorus. If the grain-refining agent is provided in the form of particles, it can be directly added to the melt from the furnace, by using a suitable device such as a vibration feeder. While the illustrated embodiments are adapted to locate the leading end of the grain-refining rod at a position in the trough near the outlet of the furnace, the location at which the grain-refining agent is added may be suitably selected depending upon the composition and the form of the grain-refining agent.

It will be obvious that the present invention may be embodied with other changes and modifications which may occur to those skilled in the art, within the scope of the invention defined in the appended claims.