Hypereutectic aluminium-silicon alloys

Abstract: A hypereutectic aluminium-silicon alloy obtainable by a powder metallurgy technique which comprises 12 to 50% by weight of silicon and 0.01 to 0.05% by weight of phosphorus, the content of Ca as an impurity being 0.03% by weight or less.

Description: The present invention relates to hypereutectic aluminium-silicon alloys obtainable by powder metallurgy techniques. More specifically, it relates to hypereutectic aluminium-silicon alloys with refined primary silicon particles, which have improved machinabilities and mechanical properties. Hypereutectic aluminium-silicon alloys have been produced by casting methods. Hypereutectic aluminium-silicon casting alloys have been expected to be used in various fields due to their low coefficient of thermal expansion, high modulus and good wear resistance, but in practice they are not used. The main reason is that they contain coarse primary silicon particles which give the alloys poor machinabilities and poor mechanical properties. To improve the machinability and the mechanical strength, refinement of the primary silicon particles in the hypereutectic aluminium-silicon casting alloy is effected by adding a modifier for refining the primary silicon particles, particularly a modifier containing phosphorus. Unfortunately, the addition of the modifier cannot give well-refined primary silicon particles. In particular when the hypereutectic aluminium-silicon casting alloy contains 20% by weight or more of silicon, coarse primary silicon particles are found. JP-A-62/112,706 describes a hypereutectic Si-Al alloy powder for use in manufacturing wear-resistant sliding parts. This powder may be, for example, as Al-25Si-3Cu-0.5% Mg alloy powder. Recently it has been proposed to produce a hypereutectic aluminium-silicon alloy by a rapid solidification method. According to this method a hypereutectic aluminium-silicon alloy with refined primary silicon particles can be obtained, even if it contains 20% by weight or more of silicon. In this case, the improvement of the machinability is satisfactory to a certain extent, but the improvement of the mechanical properties is limited. The addition of the modifier for refining the primary silicon particles does not improve the mechanical properties to a satisfactory extent. We have found that the reason there is an insufficient improvement in the mechanical properties, especially the mechanical strength, of the hypereutectic aluminium-silicon alloy produced by a powder metallurgy technique even if the modifier for refining the primary silicon particles is added in an adequate amount is because there is also present more than 0.03% by weight of calcium as an impurity. The calcium is derived from the aluminium and silicon raw materials. The present invention provides a hypereutectic aluminium-silicon alloy obtainable by a powder metallurgy technique which comprises 12 to 50% by weight of silicon, 0.01 to 0.05% by weight of phosphorus, and, optionally, 1.0 to 5.0 % by weight of copper, 0.5 to 2.0 % by weight of magnesium and/or 0.2 to 2.0 % by weight of manganese, the content of calcium as an impurity being 0.03% by weight or less, the balance being Al and incidental impurities. The present invention further provides a process for the preparation of a hypereutectic aluminium-silicon alloy as defined above which comprises subjecting appropriate amounts of aluminium, silicon and a phosphorus-containing modifier for refining the primary silicon particles to a powder metallurgy technique, the calcium content of the raw materials being such that the alloy comprises 0.03% by weight or less of calcium. The present invention additionally provides a process for the preparation of a consolidated product which comprises subjecting a hypereutectic aluminium-silicon alloy as defined above to cold shaping followed by hot working while heating in air or an inert gas. The hypereutectic aluminium-silicon alloy of the present invention comprises well refined primary silicon particles and has excellent machinability and mechanical properties. The hypereutectic aluminium-silicon alloy of the present invention comprises 12 to 50% by weight of Si. When the Si content is less than 12% by weight, the primary Si particles are not crystallized. On the other hand, when it is above 50% by weight, the amount of primary Si particles is too great. Thus the machinability and the mechanical strength are poor, even if the primary Si particles are well-reined. The preferred Si content is 20 to 30 % by weight. The hypereutectic aluminium-silicon alloy of the present invention contains 0.01 to 0.05 % by weight of P. P is contained so as to refine the primary Si particles. Thus a hypereutectic aluminium-silicon alloy with uniform dispersion of the well-refined primary Si particles is obtained. When the P content is less than 0.01 % by weight, the refinement of the primary Si particles is not good and therefore coarse primary Si particles are observed and the improvement in the machinability is not satisfactory. On the other hand, when it is above 0.05 % by weight, the primary Si particles cannot be further refined. The preferred P content is 0.015 to 0.05%, especially 0.02 to 0.05 %, by weight. In the hypereutectic aluminium-silicon alloy of the present invention, the content of Ca as impurity is controlled to 0.03 % by weight or less. When the Ca impurity is contained in an amount of above 0.03 % by weight in the hypereutectic aluminium-silicon alloy containing the above-defined amounts of Si and P, the improvement of the mechanical properties, especially the mechanical strength,is not satisfactory. This is shown in the Examples given hereinafter. Preferably, the Ca content is controlled to 0.01 % by weight or less. If desired, the hypereutectic aluminium-silicon alloy of the present invention may contain 1.0 to 5.0 % by weight of copper 0.5 to 2.0 % by weight of magnesium and/or 0.2 to 2.0 % by weight of manganese, to improve further the mechanical strength. The hypereutectic aluminium-silicon alloy of the present invention is produced by the powder metallurgy technique. In the production of the hypereutectic aluminium-silicon alloy of the present invention, the use of Al and Si raw materials whose Ca contents are suitably controlled is essential. As the modifier for refining the primary Si particles, a P containing modifier is used, such as Cu-8 % by weight of P, Cu-15 % by weight of P, PCl 5 and a mixture mainly composed of red phosphorus. When the hypereutectic aluminium-silicon alloy of the present invention is produced by, for example, atomization, it can be obtained in the form of an atomized powder. It is desirable to sieve the resultant atomized powder so as to obtain an atomized powder of not more than 350 µm particle size, which is suitable for practical use. When the hypereutectic aluminium-silicon alloy of the present invention is produced by a method other than atomization, it can be obtained in the form of flakes or ribbons. The hypereutectic aluminium-silicon alloy of the present invention is mainly used for the preparation of consolidated products. Generally, the consolidated product is prepared by subjecting the alloy to cold shaping followed by subjecting it to a hot working, such as hot extrusion or hot forging, while heating in air or an inert gas such as argon or nitrogen. The thus-prepared consolidated products are applied in various fields. Examples of consolidated products prepared from the hypereutectic aluminium-silicon alloy of the present invention include automobiles, electrical parts and mechanical parts. Examples The present invention is further described in the following Examples. Examples 1 to 4 and Comparative Examples 1 to 4 Atomized powders were produced by subjecting molten aluminium alloys having the compositions shown in Table 1 to air atomization. Then they were sieved to have a particle size of 100 to 150 mesh (105 to 149 µm) so that the cooling rate is controlled to be constant. The size of the primary Si particles in the atomized powders is determined under an optical microscope. Further, the atomized powders were sieved to have a particle size of -100 mesh (not more than 149 µm). Then, the sieved atomized powders were cold pressed at 3 tons per cm 2 into rods (30 mm in diameter and 80 mm in length) followed by subjecting them to hot extrusion at a temperature of 480°C and at an extrusion ratio of 10:1 into plates (20 mm in width and 4 mm in thickness). After the resultant plates were subjected to T6 treatments, their flexural strengths were determined in accordance with JIS Z2203. The distance between two marks was set to be 30 mm. The results are shown in Table 1. The hypereutectic aluminium-silicon alloys produced in Examples 1 to 4 of the present invention had well-refined primary Si particles and high flexural strengths. The hypereutectic aluminium-silicon alloy produced in Comparative Example 1 in which P was not substantially contained had coarse primary Si particles. The hypereutectic aluminium-silicon alloy produced in Comparative Example 2 in which the P content was not enough to refine the primary Si particles had primary Si particles whose refinement was improved as compared with those in Comparative Example 1, but not greatly. The hypereutectic aluminium-silicon alloy produced in Comparative Example 3 in which the P content was enough to refine the primary Si particles had well-refined primary Si particles, but its flexural strength was poor because of its higher Ca content. The hypereutectic aluminium-silicon alloy produced in Comparative Example 4 in which the P content was not enough to refine the primary Si particles showed results similar to those of Comparative Example 2. As clear from the above results, the well-refined primary Si particles are uniformly dispersed in the hypereutectic aluminium-silicon alloy produced by the powder metallurgy technique according to the present invention. Thus, the hypereutectic aluminium-silicon alloy according to the present invention has excellent machinability. Further, the Ca content in the hypereutectic aluminium-silicon alloy produced by the powder metallurgy technique according to the present invention is controlled. Thus the hypereutectic aluminium-silicon alloy according to the present invention has excellent mechanical strength.

Claims: A hypereutectic aluminium-silicon alloy obtainable by a powder metallurgy technique which comprises 12 to 50% by weight of silicon, 0.01 to 0.05% by weight of phosphorus, and optionally 1.0 to 5.0% by weight of copper, 0.5 to 2.0% by weight of magnesium and/or 0.2 to 2.0% by weight of manganese, the content of Ca as an impurity being 0.03% by weight or less, the balance being Al and incidental impurities. An alloy according to claim 1 which comprises 20 to 30% by weight of silicon. An alloy according to claim 1 or 2 which comprises 0.015 to 0.05% by weight of phosphorus. An alloy according to any one of the preceding claims wherein the content of Ca is 0.01% by weight or less. An alloy according to any one of claims 1 to 4 which comprises 1.0 to 5.0% by weight of copper, 0.5 to 2.0% by weight of magnesium and/or 0.2 to 2.0% by weight of manganese. An alloy according to any one of claims 1 to 5 which is in the form of an atomized powder of not more than 350 µm particle size. A process for the preparation of a hypereutectic aluminium-silicon alloy as defined in claim 1 which comprises subjecting appropriate amounts of aluminium, silicon and a phosphorus-containing modifier for refining the primary silicon particles to a powder metallurgy technique, the calcium content of the raw materials being such that the alloy comprises 0.03% by weight or less of calcium. A process for the preparation of a consolidated product which comprises subjecting a hypereutectic aluminium-silicon alloy as defined in any one of claims 1 to 6 to cold shaping followed by hot working while heating in air or an inert gas. A process according to claim 8 wherein the consolidated product is an automobile part, an electrical part or a mechanical part.

Inventors: KUBO KOHEI, KUSUI JUN, TANAKA AKIEI, WATSUJI TAKASHI, YOKOTE TAKAMASA, KUBO, KOHEI, KUSUI, JUN, TANAKA, AKIEI, WATSUJI, TAKASHI, YOKOTE, TAKAMASA
Applicants: SUMITOMO ELECTRIC INDUSTRIES, TOYO ALUMINIUM KK, SUMITOMO ELECTRIC INDUSTRIES, LTD., TOYO ALUMINIUM KABUSHIKI KAISHA
Application year: 1992
Main classification: C22C21
IPC codes: C22C21, C22C1