Processing map for hot working of powder
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I.
INTRODUCTION
A L U M I N U M alloy-based metal matrix composites (MMCs) find important applications where specific stiffness is important and are produced by casting as well as powder metallurgy (PM) routes. As these MMCs have limited ductility, mechanical processing is a critical step in component development. The choice of temperature and strain rate for deformation processing is generally done by trial and error methods, although recently, the use of processing maps for the optimization of processing parameters has been found to be very useful, m Tuler and Klimowicz t2] developed processing maps for aluminaaluminum alloy cast MMCs and conducted successful forging trials validating the maps. The aim of the present investigation is to study the constitutive flow behavior of 2124 aluminum alloy-20 vol pct silicon carbide particulate MMC under hot-working conditions and to generate a processing map for optimization of hot workability. The map is interpreted on the basis of the Dynamic Materials Model (DMM), t3j which is reviewed by Gegel et al. tt] In this model, the workpiece subjected to hot working is considered as a nonlinear dissipator of power and the instantaneous power dissipated at a given strain rate may be considered to consist of two parts: G content, representing the temperature rise, and J co-content, representing the dissipation through metallurgical processes. The factor that partitions power between J and B.V. RADHAKRISHNA BHAT and Y.R. MAHAJAN, Scientists, are with the Defence Metallurgical Research Laboratory, Hyderabad 500 258, India. H. Md. ROSHAN, Professor, is with the Department of Metallurgical Engineering, Indian Institute of Technology, Madras 600 036, India. Y.V.R.K. PRASAD, Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted May 16, 1991. METALLURGICAL TRANSACTIONS A
G is the strain-rate sensitivity (m) of flow stress, and the J co-content is given by tu J = o ' ~ m / ( m + 1)
[1]
For an ideal linear dissipator, J = Jmax = 0"6/2. The efficiency of power dissipation of a nonlinear dissipator may be expressed as a dimensionless parameter r/ = J/Jmax" The variation of r/ with temperature and strain rate represents the characteristics of power dissipation through microstructural changes in the workpiece and constitutes a processing map. The different domains in the map may be correlated with specific microstructural mechanisms, and the "safe" mechanisms may be chosen for processing. In this study, the hot deformation behavior of the matrix material (2124 PM compact) is also evaluated for the purpose of comparison with that of the MMC. This would help in understanding the microstructural mechanisms involved in the hot working of the MMC. II.
EXPERIMENTAL
2124 aluminum alloy powder was blended with 20 vol pct silicon carbide particulates with an average size of 14.5/zm. The blend was degassed and compacted using cold isostatic pressing. The billets were vacuum hot pressed, and cylindrical compression specimens o
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