Friction Stir Back Extrusion (FSBE) of the AZ31 Magnesium Alloy: Microstructural Evolution and Mechanical Properties

The limited room-temperature ductility of lightweight alloys, particularly magnesium, inhibits their fabrication without the introduction of heat to the process; lightweight alloys are thus associated with warm or hot forming operations. Recently, friction stir back extrusion (FSBE), a hybrid between friction stirring and conventional back extrusion, was introduced as a processing technique for the manufacture of fine-grained tubes with unique microstructural features and mechanical properties, in an energy efficient way. In this work, the results of a detailed investigation on the effects of FSBE on the microstructural evolution and mechanical properties of the AZ31 magnesium alloy are discussed. Microstructural examination reveals a significant grain refinement and homogenisation within the processed zone; from a heterogeneous coarse-grained microstructure with bimodal peaks at ~55 and ~115µm, to a homogeneous fine-grained structure with ~5µm average grain size. EBSD analysis shows the development of a strong basal texture in the material due to processing. Microstructural analysis at key locations across the tubes indicates to a good thermal stability during FSBE. Tensile testing supported by digital image correlation (DIC) revealed significant favourable changes in the mechanical properties and deformation behaviour. The processed material exhibits a drop in yield strength, yet a slight increase in tensile strength and a very substantial increase in tensile ductility, strain hardening and Lankford coefficient. The evolution of the surface strain field maps clearly shows the shift from a brittle behaviour with premature localised necking in the as-received material towards a uniform ductile-like behaviour with prolonged and stable diffused necking in the processed material.