Microstructure and Mechanical Properties of Al Tube Processed by Friction Stir Tube Back Extrusion (FSTBE)

Friction stir tube back extrusion is presented as a method to produce fine-grained tubes based on friction stir back extrusion process. In this method, a tubular aluminium alloy specimen whose inner diameter is less than the diameter of the rotating tool, is placed on a cylindrical die and penetrated by a rotating tool. Mechanical properties and microstructural evaluation of the processed tube have been investigated. The optical micrographs of the microstructure demonstrate strain gradient along the tube wall. Reduction in grain size and grain refinement occurs across the inner wall of the formed tube as well. Hence, the grain size with an initial value of similar to 100 mu m is decreased to similar to 39 mu m in the inner side of the formed tube wall. The obtained results from tensile tests illustrate that the ultimate strength and elongation at failure increases from the initial value of 183-225 MPa and 19-22 %, respectively. Microhardness assessments represent an increase in initial value from 74 to 86 Hv. Moreover, the microhardness values of the inner surface of the formed tube are more than the outer surface. The results show that this method is suitable for fabricating a kind of ultrafine-grained tube which possess an acceptable combination of strength and ductility.