Experimental and numerical study of friction stir back extrusion process for producing ultra-fine-grained tubes

In this paper, friction stir back extrusion process is proposed for producing ultra-fine grain (UFG) tubes. In this process, a rotating tool is plunged into round bar specimens and then forming tubes. The influence of design parameters such as heat treatment, applied strain and material flow behavior was determined by finite element simulation. Various tests including optical microscopy, tensile test and microhardness test were carried out to evaluate the metallurgical and mechanical properties of processed tubes. The optical micrographs of the microstructure demonstrate that this process is capable to form a tube with a significant fine grained structure resulting from dynamic recrystallization phenomenon. Reduction in grain size and grain refinement occur across the inner wall of the formed tube as well. Hence, the grain size with an initial value of ~ 110 µm was decreased to ~ 33 µm in the inner side of formed tube wall. The obtained results from tensile tests illustrate that the ultimate strength and elongation to failure were increased from the initial value of 153 MPa to 195 MPa and 10% to 13%, respectively. Thus the ultimate strength increased by 13%. The average hardness along thickness has reached to 82 Hv from 63 Hv. The results show that this method is suitable for fabricating a kind of ultrafine-grained tube which own an acceptable combination of strength and ductility.