Hot extrusion of aluminum powder at low reduction ratios: Analytical and metallographic studies
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I.
INTRODUCTION
M E T A L powder extrusion is one of the relatively new compacting methods which converts metal powder compacts into fully dense material requiring no further treatment. It has been observed experimentally ~ that the extrusion pressure for aluminum powder billet is lower than that for extrusion of cast billet and that powder extrusion is less strain rate sensitive. The conventional method to analyze the flow of metal through the die is by slip-line field and load bounding techniques. These obviously have some inherent drawbacks. Based on these techniques and incorporating some modifications, solutions for extrusion pressures have been reported by Sheppard and McShane, 2 for reduction ratios ranging from 10:1 to 80: 1. Although good agreement between theoretical and experimental results has been reported, this was only in a narrow reduction ratio range (25 : 1 to 40: 1). In a further modification of an axisymmetric upper bound solution McShane et al. 3 used an equation expressing mean flow stress, ~ = K.~g"-g" incorporating mean strain, g, and mean strain rate, g. The. constants, K~, m, and n, were determined by minimizing the function using a penalty function approach. It was reported that this analysis facilitated satisfactory prediction of extrusion pressure for powder compact. The present investigation examines the extrusion behavior for low reduction ratios. The experimental studies backed by metallographic observations are presented together with analytical results. The reasons for using Finite Element Method 4'~ (FEM) for analyzing powder extrusion were two-fold: firstly to predict extrusion pressure and its comparison with experimental values and secondly to analyze velocity vectors, average effective strain rate contours, and average pressure contours in the billet being extruded, in the light of metallographic observations and to correlate them with microstructural changes.
ASHOK KUMAR is Senior Research Engineer, Rolling Mills Group, R & D Centre for Iron and Steel, SAIL, Ranchi, India. P.C. JAIN and M.L. MEHTA are Professors with the Department of Civil Engineering and Metallurgical Engineering, University of Roorkee, Roorkee, India. Manuscript submitted September 14, 1984. METALLURGICALTRANSACTIONS B
II.
EXPERIMENTAL PROCEDURE
The air-atomized aluminum powder (99.5 pct pure) particle size distribution given in Table I was cold-compacted in a cylindrical die to produce a preform of 27 mm diameter, 50 mm length, and 85 to 86 pct theoretical density. The die was lubricated with zinc stearate. The experimental apparatus consisted of an extrusion assembly surrounded by a band heater capable of heating the assembly up to 550 °C. The extrusion was carried out under axisymmetric condition at four different temperatures, v i z . , 300, 400, 450, and 500 °C. The extrusion assembly, consisting of container and die punch along with powder preform, was heated to extrusion temperature through band heater before starting the extrusion. The lubricant, colloidal graphite powder in acetone, was appli
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