On the Microstructural Homogeneity and Mechanical Properties of Al 6063 Alloy Processed by the Cyclic Expansion Extrusion Process

This work reports the severe plastic deformation processing of aluminum alloy 6063 (AA 6063) at a temperature of 130 °C (0.2 Tm of AA 6063) up to 6 passes using the cyclic expansion extrusion (CEE) process at a pressing velocity of 0.5 mm s−1. The CEE die had an extrusion semi-die angle (α) of 30°. The effects of plastic deformation on the microstructure and mechanical properties were studied after every two passes by determining the micro-hardness, and tensile properties, and performing an electron backscatter diffraction (EBSD) analysis. The severe strain introduced by plastic deformation gives rise to ultrafine-grained structures with improved strength and hardness properties. The EBSD analysis revealed the refinement of (average) grain size from the starting material grain size of 22 ± 5 to 3 ± 1 µm. The EBSD analysis and transmission electron microscope (TEM) observations confirmed the formation of fine grains with high-angle grain boundaries (HAGBs). Nano-sized grains of d ~ 50 nm and ultrafine grains (up to d ~ 300 nm) could also be seen at the end of 6 passes which, however, occupied only 8% of the total volume. Likewise, the micro-hardness value showed a significant increase through grain refinement. The ultimate tensile strength of AA6063 improved by ~ 87% compared to parent material. In addition, the role of HAGBs and LAGBs in the strengthening mechanisms is discussed.