GB600729A - Improvements relating to powder metallurgy - Google Patents

Abstract

600,729. Powder metallurgy. AMERICAN ELECTRO METAL CORPORATION. Nov. 6, 1944, No. 21777. Convention date, Oct. 30, 1943. [Class 83 (iv)] Forging dies.-In order to obtain substantial uniformity of particle distribution in a body formed by powder metallurgy and having final thicknesses which differ considerably in the direction of pressing, the metal powder is compressed in a die to form a compact of an intermediate shape and density, which compact is sintered and is then subjected to a final pressing operation at a considerably higher pressure in which it is brought to the required final shape and in which the less dense parts of the compact are more compressed than the denser portions. If a metal powder fill in a die 13 is pressed by punches 15 and 16, Fig. 3b, to its final form 25, the density of the thinner central portion will considerably exceed that of the outer portions which may crumble during subsequent handling. According to the method of the invention this difficulty is overcome by filling a die 13 whose bottom is formed by a punch 55, Fig. 7, having an upstanding central portion whose curvature is considerably greater than that of the required final body and partially compressing the powder fill between this punch and an upper punch 63 with a central portion of less curvature to form the compact 68, Fig. 8. The average density of the compact is about midway between that of the powder fill and that of the required final body. This compact is sintered for from 10 to 30 minutes in a neutral or reducing atmosphere within a temperature range of about 20 to 35 per cent below the melting point of the powder. In the subsequent final shaping step the sintered body is compressed in a die 13 between punches 73 and 74, Fig. 9. The central portion of the lower punch 74 is of less curvature than that of the compacting punch 55 so that the less dense outer portions of the body are more highly compressed than the central portion. In general the compact will be formed at a pressure of from 2 to 10 tons per sq. inch and the final shaping will be carried out at a temperature of from 40-50 tons per sq. inch. A volatile lubricant may be added to the original powder mix. As a specific example, pure spongy iron passing a 300 mesh sieve is mixed with 1 per cent lubricant of a zincstearite base and pressed at 5 tons per sq. inch. The intermediate shape was sintered for 15-20 minutes at 1100‹ C. and then pressed to final shape at 50 tons per sq. inch. The body was finally annealed at a temperature between 600‹-720‹ C.