Cold metal forming has received considerable attention in recent years. Considering that in most metal forming processes, there exists a compressive stress state, the study of deformation behavior of metals under this condition is of great importance. In this study, the cold upset of aluminum and brass solid cylindrical specimens with different aspect ratios (1.5, 1.0 and 0.5) under different frictional conditions has been investigated. The upsetting tests have been carried out at two different strain rates of 10-2 and 10-3 s-1 and under lubricated and unlubricated conditions. Using PTFE lubricant sheets decreases the friction and thus eliminates barreling phenomenon. In both of the materials under unlubricated condition, decreasing the aspect ratios decreases the stress required for deformation at a constant strain while in the lubricated condition, the behavior is reversed. Using the difference between the stress–strain curves in the lubricated and unlubricated conditions, the contribution of friction and redundant work to the deformation has been studied. For brass, the curves for the stress differences are closer to each other. This may be caused by the presence of undissolved lead in the brass specimens which act as an internal lubricant, causing a lower friction level and thus, a lesser difference between the curves. The calculated friction coefficients were found to have constant values of 0.15 and 0.08 for aluminum and brass, respectively. These values were independent of strain rate and aspect ratio. It has been also found that the value of this coefficient for brass specimens is lower than that of aluminum. It has been observed that change of strain rate does not affect the stress–strain curves for both aluminum and brass specimens.
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