An analysis of the isothermal hot compression test
- PDF / 1,432,313 Bytes
- 13 Pages / 630 x 792 pts Page_size
- 120 Downloads / 266 Views
I.
INTRODUCTION
THE manufacture
of advanced materials often includes a deformation process at elevated temperatures, typically in the hot working regime. To select the temperature and deformation rate for such operations, it has become common practice to utilize some form of small-scale simulative test. [1.2] By this means, the optimal conditions with regard to both workability (i.e., fracture resistance) and microstructure development are readily identified. In addition, such tests provide estimates of the flow stress dependence on level of deformation, deformation rate, and temperature, or information which is valuable in selecting equipment as well as in the design of preforms and tooling via increasingly popular numerical techniques. For bulk forming operations, such as forging, rolling, and extrusion, the isothermal hot compression test is often used as a simulative test because of its similarity to the actual forming operation in terms of stress state and achievable deformation rates. In this test, lubricated, cylindrical test samples and dies are heated to nominally the same temperature using induction or indirect resistance techniques, soaked for a fixed period of time, and then upset to a reduction typically of the order of 50 pet. Deformation nonuniformities may take a number of forms, such as barreling (usually due to friction) t3] or localized deformation at one or both sample ends, a phenomenon often ascribed to the growth of an initial inhomogeneity in materials which exhibit flow softening, whose origin can be microstructural or deformation heating in na-
S.I. OH, formerly Research Leader, Net Shape Manufacturing Group, Battelle Memorial Institute, Columbus, OH, 43201, is President, Scientific Forming Technologies Corporation, Columbus, OH, 43202. S.L. SEMIATIN, formerly Senior Research Scientist, Metalworkirig Group, Battelle Memorial Institute, Columbus, OH 43201, is Senior Scientist, Metals and Ceramics Division, Materials Directorate, Wright Laboratory, WL/MLLN, Wright-Patterson Air Force Base, OH, 45433. J.J. JONAS, CSIRA/NSERC Professor, is with the Department of Metallurgical Engineering, McGill University, Montreal, PQ Canada H3A 2A7. Manuscript submitted June 3, 1991. METALLURGICAL TRANSACTIONS A
ture. t4'51 It has been found that initial defects which are of the geometric (cross-sectional area variation) or strengtl~ type give rise to strain concentrations at a rate approximately an order of magnitude greater than so-called "hammer-blow" defects, the latter being initial deformation nonuniformities, t5'6] Impetus for the present work has come from the increasing popularity of the isothermal hot compression test, particularly for aerospace materials, which must be worked at temperatures often well above 1000 ~ Its objective has been to establish the effect of temperature nonuniformities and friction on nonuniform flow and the derivation of flow stress relationships. Temperature nonuniformities were of two t y p e s - - a n initial temperature nonuniformity and a temperature inhomogeneit
Data Loading...
