Abstract
o 1.The coarse-grained material from the rim of extruded bars has the lowest strength and very low fatigue resistance by comparison with the material from the central part. A reduction of the quenching temperature from 505\dgC, at which the coarse-grained rim is formed, to 490\dgC prevents the formation of the coarse crystals at the periphery of the bar. When the unrecrystallized structure refined in extrusion is retained in the periphery of the bar and in the central part then the material has a high fatigue resistance like that of the center of bars quenched from 505°C, with minimum scattering of the fatigue test results. 2.Quenching from 490\dgC instead of 505\dgC should not be taken as a standard method of obtaining extruded products without coarse-grained rims. To prevent the formation of such rims it is necessary to select the proper extrusion conditions (temperature of the billet, temperature of the cylinder, degree of deformation, shape of the die [3], etc.) If such a rim is formed despite all this then it is expedient to reduce the quenching temperature below the temperature of maximum solubility of alloying elements in aluminum. This temperature is determined by the specific conditions in each case.
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