Theoretical criteria are proposed for the plastic cohesion of aluminum alloys during hot extrusion.it is postulated that the governing influence is the tangential force field at the surfaces in contact. The critical value of the energy liberated at the surfaces by the actionof these forces depends on the temperature, the pressure, the yield stress and the physical state and degree of intimacy of the contacting surface. A mathematical formulation is proposed, which is a function of the emperature, the yield stress, the normal stress field at the surface, the deformation rate and certain parameters describing the geometry and the physical state of the surface. The theory has been tested experimentally using a silicone polymer as a substitute material and a transparent plastic die such that thch velocity fields at the surfaces could be observed and measured during plastic flow, allowing the empirical coefficients in the mathematical formulation to the estiamted. On the basis of the theory an optimal die chamber was designed for a die with a compelx shape.
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