A PADS (Plasma Assisted Debinding and Sintering) reactor developed by Lupatech S.A.
has been employed to MIM process two Ni-based superalloys under Argon, at temperatures in the
1280 -1310 °C range, and for 2 to 3 hours. Both materials have chemical compositions similar to that
of standard Nimonic® 90 but differ considerably in their powder characteristics. One type of powder
was gas-atomized whereas the other was water-atomized. Samples of both materials in as-sintered
states as well as subject to different HIP and heat treating conditions have been characterized
mechanically in tensile tests and by HV measurements. The best overall results are attained by the
water-atomized material sintered at lower temperatures. The PADS processing of these superalloys
shows marked advantages over more conventional PM processing technologies. These results are
particularly relevant to the development of turbine components for the automotive and aerospace
industries.
The segmented expanding m&ire1 test is modified and improved to yield more quantitative information on the local stress and strain concentrations in Zircaloy tubing samples. The experimental apparatus has undergone changes in segment design, end restraint fixture design and the method of sample heating, These changes result in localization of stress and strain concentrations to known locations in the tubing, the introduction of a small degree of biaxiality and a more uniform sample temperature, respectively. A chamfered specimen design has been introduced for the purpose of producing a near plane-strain condition. Finite-element analysis has been added to provide information on the magnitude of localii stresses and strains given the sample geometry, test procedure and measured diametral strains. The resulting test represents a si~ficant increase in the quantitative information produced in the segmented expanding mandrel technique.
The gelcasting process is a forming technique originally developed for the shaping of advanced ceramics into final products in attempts to overcome some of the limitations of conventional forming techniques used in powder metallurgy. It is based on preparing a high solids loading suspension of powder dispersed in an aqueous organic monomers solution, which is poured into a mold and gelled through a chemically initiated polymerization. This work describes the gelcasting of HK-30 stainless steel, a type of powder commonly processed by injection molding. Large (70 x 30 mm) and geometrically complex green compacts with outstanding form retention and stability were obtained. Sintered parts showed good surface finishing and reached 96% of theoretical density, yield strength of 418.5 MPa, and ultimate strength of 701.5 MPa. These results compare favorably with those typically obtained through conventional powder injection molding of HK-30 feedstocks. They support the growing view that gelcasting may soon become an industrial, low cost alternative for near net shaping metallic powders into small or large parts with complex geometries
The segmented expanding mandrel test is modified to provide additional information on the localized stresses and strains during ramp-hold tests. A chamfered specimen is designed with four flats on the outer diameter for the purpose of creating sectors of variable wall thickness which establishes a more nearly plane strain state in the thinnest-gauge sections. Ramp-hold tests conducted in 40 MPa of iodine at 32S'C show that at a measured diametral strain of 0.88, chamfered specimens fail while regular tubing specimens require diametral strains of 2% or greater for failure. Calculations made with a two-dimensional, finite elements code show that at a measured diametral strain of 0.84, the local strains in the clad ID over the segment interfaces are nearly identical for the regular and chamfered samples while the local stress in the chamfered sample is 1.5% greater. Results suggest that the smaller diametral failure strain of the chamfered specimen at low measured diametral strains is mainly due to its near plane strain condition and the higher hoop stresses resulting therefrom.
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