Abstract
In this paper, the microstructure and mechanical properties of AA7075 with a coarse-fine-grained laminated microstructure produced by spark plasma sintering (SPS) and the cyclic extrusion severe deformation (KOBO) technique were investigated. It was found that an inhomogeneous grain microstructure was formed from coarse and fine grains by the SPS process and then was transformed into a coarse-fine-grained laminated microstructure by means of KOBO extrusion at room temperature. The grain refinement during KOBO extrusion resulted in a fine grained laminated microstructure created due to the formation of low-angle grain boundaries (LAGBs), followed by dynamic recrystallization, leading to high-angle grain boundaries (HAGBs). The EBSD analysis results reveal the formation of a deformed and partially recrystallized ultrafine grain microstructure owing to the generation and development of shear bands during KOBO extrusion. The ultimate tensile strength (UTS) of the AA7075 alloy rose after SPS-KOBO severe deformation up to 422 MPa, with high strains of about 33%. The obtained results clearly show that the SPS-KOBO extrusion technique allows a bimodal laminated fine gradient grain microstructure to be obtained due to deformation and dynamic recrystallization, which result in both high strength and good ductility. The new heterogeneous AA7075 alloys obtained by the SPS-KOBO combined techniques demonstrate that microstructural heterogeneities can assist in overcoming the strength–ductility trade-off.
Highlights
The spark plasma sintering (SPS) of Al alloys followed by post-sintering by means of KOBO extrusion is a pathway for producing ultrafine-grained materials by imposing severe plastic deformation [1]
A similar behaviour in the compressive strength is observed. These results demonstrate that softening processes such as dynamic recrystallization and dynamic recovery are the main microstructure formation mechanisms during KOBO extrusion due to the high content of point defects generated by severe deformation and significant enhancement of diffusion [12,13,14]
It was found that the microstructure synthesized by the SPS process was inhomogeneous, formed by coarse and fine grains randomly distributed within the matrix, and developed into a coarse- and fine-grained laminated microstructure by a subsequent KOBO extrusion process at room temperature
Summary
The spark plasma sintering (SPS) of Al alloys followed by post-sintering by means of KOBO extrusion is a pathway for producing ultrafine-grained materials by imposing severe plastic deformation [1]. During SPS, powder particles are compacted in graphite die, heated by a high-intensity and low voltage pulsed DC electric current, and deformed due to shear stresses generated by the mechanical field. This results in some advantages such as high heating and cooling rates, short sintering times, and the formation of specific microstructures with enhanced mechanical properties. The authors of [4] demonstrated that SPS at optimized regimes led to disruption of the film on the surface of the Al particles, which allowed the wrought-like ductility of the sintered compacts to be achieved (tensile elongation of 40–50%). The research of severe deformation methods to increase the ductility without compromising the material strength is of great interest
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