As a result of demand for metal parts with complex shapes and stringent mechanical property requirements, fabricators have sought to make many parts by powder metallurgy processes. Attainment of necessary dimensional control can be difficult in such processes, especially when making large parts.
United Kingdom published patent specification No. 2,005,728 A describes a particularly useful powder metallurgy process for making precision parts from spherical non-refractory metal powders by molding in a flexible mold a plastic mixture of such powders and heat-fugitive binder comprising thermoplastic material to form a green article of predetermined shape and dimensions, heating the green article to remove the binder and consolidate the non-refractory spherical powders in the form of a porous, monolithic skeleton of necked particles of non-refractory metal, infiltrating the skeleton with a molten metal having a melting point that is at least 25.degree. C. less than the melting point of the lowest melting of said spherical, non-refractory metal powders, and cooling the infiltrated skeleton, thereby forming a homogeneous, void-free, non-refractory metal article of two intermeshed metal matrices. In practice, cobalt alloy-containing spherical non-refractory metal powders have proven themselves especially useful in such process because articles made from such powders have greater wear and corrosion resistance than iron-base articles made according to the same process and hardened to an equivalent hardness level.
Articles produced according to the process described in said patent specification have very low dimensional change during processing. With adjustment of the size of the master, a precision tolerance from blueprint specification of better than .+-.0.2% can be obtained with said process. Included among the examples in said patent specification are articles (made without adjustment of the master) having shrinkage of between 0.40% and 1.98% based on a comparison of the dimensions of the green molded article and the infiltrated final article. Also included among the examples in said patent specification are articles having shrinkage of between 0.25% and 0.32% based on a comparison of the dimensions of the lightly sintered skeletal preform and the infiltrated final article.
The dimensions of hard metal parts such as tools and die cavities are generally specified in the trade on an absolute basis (e.g., as plus or minus a specific lineal dimension) rather than being specified on a relative basis (e.g., as plus or minus a specific percentage of total lineal dimension). Therefore, a powder metallurgy process which results in even very low dimensional change on a relative basis may be unacceptable for use in the manufacture of large precision parts because the extent of dimensional change encountered during processing of such parts using powder metallurgy techniques may exceed the required lineal tolerance for such parts. Also, when articles having unequal length and width are prepared, dimensional change during processing can lead to anisotropic lineal shrinkage, thereby rendering it difficult to accurately replicate such articles using powder metallurgy processes. Accordingly, it is always desirable to reduce the extent of dimensional change in a powder metallurgy process because such reduction in dimensional change may thereby enable the processing of large parts, or parts with unequal length and width, while remaining within specified lineal dimensional tolerances.
Shrinkage is the most common form of dimensional change occurring during processing of precision molded articles using the method described in said U.K. Patent Specification. In conventional compressed powder metallurgy compaction processes, a variety of types of metal powder additives have been added to the powder compact in order to further densify the compact. Because an increase in densification of a powder metallurgical article represents a form of shrinkage, the use of such metal powder additives in the process of said patent specification would not be expected to result in shrinkage retardation or expansion.
Carbonyl nickel is a powdered, finely divided metal which has been utilized in conventional compressed powder metallurgy compacts to promote densification thereof, see "INCO Nickel Powders, Properties and Uses", 11 (International Nickel Company, Inc., 1975). Carbonyl nickel powder has also been reported as an infiltrant additive in the processing of iron compacts using conventional compressed powder metallurgy techniques, see Snape, "Infiltration of Iron Compacts with Ni-Containing Copper Infiltrants", Powder Metallurgy International, 6, 1, pp. 20-22 (1974) and U.S. Pat. Nos. 3,459,547 and 3,708,281 to Andreotti et al. Snape infiltrated an iron compact with copper and observed that expansion occurred during infiltration. Addition of carbonyl nickel powder to the infiltrant reduced the expansion, thereby providing a compensatory shrinkage. The nickel-containing infiltrated iron compact described by Snape had increased yield strength but decreased elongation compared to an iron compact made without carbonyl nickel powder addition to the infiltrant. After heat treating, yield strength increased and elongation decreased for iron compacts prepared with or without a carbonyl nickel powder addition to the infiltrant.