Sintered magnesium-based composite material and process for preparing same

Abstract: A magnesium-based composite material having an improved mechanical strength, particularly the modulus of elasticity thereof, with a relatively low density is provided by pressing and sintering a mixture of magnesium or magnesium-based alloy particle or a metal particle mixture of magnesium with other metal(s) with a reinforcement that may be of boron, or boron-coated BC₄, Si₃N₄, SiC, Al₂O₃ or MgO.

Description: The present invention relates to a sintered magnesium-based composite material and a process for preparing the same. Magnesium alloys have attracted attention as a light-weight, high mechanical strength, metal. They are used in aircraft and space equipment and components and in electronics equipment and components. In the field of electronics equipment and components, mechanical parts for magnetic recording, particularly a head arm, often comprise a diecast article made of a magnesium alloy. The important characteristics of the material for a head arm include low density and high mechanical strength, particularly the Young's modulus of elasticity. Magnesium and magnesium-based alloys are good candidates for such a head arm due to their low density, but they have a low Young's modulus of elasticity. It would therefore be desirable to be able to provide a magnesium, or magnesium-based, alloy material that has increased modulus of elasticity without significant increase in density. If a head arm were made of such a material it would be possible to obtain an improvement in the performance of a magnetic recording as a result of an increase in the speed of movement of the head. A method of improving the modulus of elasticity of a magnesium alloy is known, in which a very small amount of zirconium or a rare earth metal is added to prevent a growth of the crystal grains of the magnesium, but this provides only a low modulus of elasticity of about 4500kgf/mm². In Japanese Unexamined Patent Publication (Kokai) No. 55-161495 published on December 16, 1980, H. Inoue et al., disclose a vibrating plate for a sonic converter, comprising a fused alloy of magnesium and boron. A fused or cast alloy of magnesium and boron, however, does not provide a uniform composition due to the difference of the densities of the magnesium and the boron, and therefore, does not provide the expected improved properties. Sintering magnesium powders in the form of a shape to obtain a sintered body of that shape is known, but do not provide a body having a sufficient Young's modulus of elasticity. A sintered material prepared according to the invention has a matrix of magnesium or a magnesium-based alloy and is characterised in that it includes reinforcement dispersed in the matrix. The amount of the reinforcement is selected in order that the sintered material has the desired properties, and in particular generally in order that the modulus of elasticity of the material is substantially greater than it would be in the absence of the reinforcement, although the density is not significantly increased. The reinforcement should be distributed substantially uniformly throughout the matrix. The reinforcement is normally magnesium oxide formed by oxidation within the matrix. As explained in more detail below, the matrix may be magnesium or a magnesium-based alloy that is formed mainly of magnesium, for instance being formed of at least 88% magnesium. Magnesium aluminium alloys are particularly suitable. The materials used in the invention are the materials that have a reinforcement comprising magnesium oxide. The properties of the materials are shown in Table 1, which also shows the properties of magnesium. Material Density (g/cc) Modulus of elasticity (kgf/mm²) Magnesium 1.74 4.5 x 10³ Magnesium oxide 3.65 2.5 x 10⁴ The matrix of magnesium or magnesium-based alloy is not particularly limited, in that a magnesium-aluminium system (particularly 3-12 wt% Al), a magnesium-aluminium-zinc system (particularly 3-9 wt% Al and 0.1-3.0 wt% zinc), and a magnesium-zirconium-zinc system may be used as this magnesium-based alloy. In the present invention, there is provided a process for preparing a sintered magnesium-based composite material according to claim 1. Preferred embodiments of the invention are shown in claims 2-4. In this process, the sintered magnesium-based body containing a magnesium oxide therein is subjected to a plastic deformation process to increase the relative density thereof, and as a result, the magnesium matrix and magnesium oxide are made into a composite without heating or a reaction therebetween, i.e., without mechanically weakening the composite. The starting magnesium-based particle may be a particle of magnesium, a magnesium alloy, or a mixture of magnesium and another metal or metals forming a magnesium alloy. The above particle typically has a size of 1 to 100µm. The pressing is carried out at a pressure of 0.5 to 4 tons/cm² to form a porous body having a relative density of 50% to 93%, and the sintering is carried out at a temperature of 500 to 600°C in an oxidising atmosphere, for example, an argon atmosphere containing 50 to 1,000 ppm of oxygen, for 10 minutes to 10 hours. The plastic deformation of the sintered body may be carried out by, for example, pressing, rolling swagging, etc.; for example, it may be pressed at a pressure of 1 to 8 tons/cm². A magnesium-based material produced by the claimed process has an improved mechanical strength, particularly the modulus of elasticity thereof, and no substantial loss of the small density thereof, as shown in the following Example. The sintered magnesium-based composite material has an additional advantage in that the thermal expansion coefficient of the magnesium-based material can be adjusted by an appropriate selection of the composition of the composite. This ability to adjust the thermal expansion coefficient prevents a mismatch of the thermal expansion coefficient of a head arm with a recording disc, so that a deviation of the head from the tracks formed on a disc of e.g., aluminum, can be prevented. Example 1 A -200 mesh magnesium powder was pressed at 2 tons/cm² to form a porous magnesium shaped body having a relative density of 85%. The porous magnesium body was heat treated in a gas flow of argon containing 200 ppm of oxygen, at 500°C for 1 hour, and a sintered magnesium body containing a thickness of 0.1 to 2µm of magnesium oxide inside pores of the body, a relative density of the sintered body being 87%, was obtained. This sintered magnesium body containing magnesium oxide was pressed again at 4 tons/cm² to obtain a shaped body of Mg-MgO composite. This composite shaped body had a relative density of 96%, and the properties shown in Table 2. Reinforcing Material Density (g/cm³) Modulus of elasticity (kgf/mm²) Tensile strength (kgf/mm²) Mg-MgO composite 1.76 5400 11.5 Sintered Mg 1.69 3800 8.0

Claims: A process for preparing a sintered magnesium-based composite material, comprising the steps of: pressing magnesium-based particles to form a porous magnesium-based body; heating porous shaped body in an atmosphere of an inert gas, preferably argon, containing 50 to 1,000 ppm of oxygen to form a sintered magnesium-based body containing magnesium oxide therein; the magnesium oxide being present as a coating having a thickness of 0.1 to 2µm; and subjecting the sintered magnesium body to a plastic deformation process without heating, so as to increase the relative density of the sintered magnesium-based body due to reinforcement by the magnesium oxide. A process according to claim 1 in which the porous shaped body is heated in an atmosphere of an inert gas containing 50 to 1,000 ppm of oxygen at 500 to 600°C. A process according to claim 1 or claim 2 in which the plastic deformation is conducted by pressing, rolling or swagging at a pressure of 1 to 8 tons/cm². A process according to any preceding claim in which the magnesium-based particles comprise magnesium-aluminium alloy.

Inventors: HORIKOSHI EIJI, IIKAWA TSUTOMU, SATO TAKEHIKO, HORIKOSHI, EIJI, IIKAWA, TSUTOMU, SATO, TAKEHIKO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1988
Main classification: C22C32
IPC codes: C22C32