Improved Microstructures and Mechanical Properties of 2024 Aluminum Alloy Produced by a Reciprocating Extrusion Method

Abstract

A reciprocating extrusion process was used to work the 2024 aluminum alloy ingots at 723 K so that refined microstructures and improved mechanical properties could be obtained. The number of extrusion passes was monitored to see their effect upon microstructure and properties. As the number of extrusion passes was increased, the grains became finer but after 5 passes a limiting grain size of about 2 μm was approached. On the other hand, the dispersoids and inclusions became smaller and more equiaxed, and more uniform in the matrix up to 20 passes. Grain refinement results from the repeated partial recrystallization and the increased number of inclusions and dispersoids to enhance recrystallization during reciprocating extrusion. The refinement of dispersoids and inclusions was attributable to three possible cracking mechanisms: bending mechanism, short-fiber loading mechanism and shear mechanism depending on the shape of a particle. Redistribution of particles is driven by the cyclic plastic flows of extrusion and compression during reciprocating extrusion.In the first 5 passes, yield strength and ultimate tensile strength showed a small decrease but elongation increased much. After more passes, their variations became small. The overall loss of strength is about 10% and the increase of elongation is about 54%. The combination of strength and elongation is still significantly improved. The decrease in strength is mainly due to the coarseness of S′ precipitates which is related to the loss of quenched vacancies. The large improvement of elongation is mainly attributable to the refinement of inclusions.