Flow stresses in metal laminates and pure metals during high temperature extrusion

The flow stresses developed during elevated temperature extrusion in laminated metal composites are analyzed. The laminates consist either of ultrahigh carbon steels (UHCSs) or NiAl jacketed in a mild steel can. Flow stresses in the individual components of the laminate were determined by utilizing an isostrain model and by establishing the true coefficient of friction during extrusion. The average coefficient of friction was found equal to about 0.38 for UHCSs and 0.32 for NiAl. The results reveal that plastic flow of the UHCSs is a diffusion-controlled dislocation creep process. An increase in carbon content leads to a decrease in the flow stress because iron atom mobility is increased. An increase in substitutional solid solution alloy additions leads to an increase in the flow stress because of a decrease in the stacking fault energy. The influence of strain rate sensitivity of each component on the extrusion of laminated composites is modeled. It is shown that striking differences in extrusion response are achieved by altering the volume fraction and strain rate sensitivity of the component materials. The analysis of extrusion is extended to interpret Pearson and Smytheʹs 1931 data on extrusion of lead, tin and cadmium. These data are also shown to be well characterized by a diffusion-controlled dislocation creep process. Coefficients of friction for extrusion were determined for these three metals. These results, together with those for the UHCSs and NiAl, indicate that the friction coefficient is a function of the flow stress of the metal during extrusion, increasing in value as the flow stress decreases. It is proposed that this trend is principally related to microstructural changes that are unique to the process of extrusion.