Hydrostatic Tube Cyclic Extrusion Compression as a Novel Severe Plastic Deformation Method for Fabricating Long Nanostructured Tubes

In this study, hydrostatic cyclic extrusion compression (HTCEC) is introduced as a novel severe plastic deformation technique for producing the relatively long and large ultrafine grained (UFG) and nano-grained tubes. In HTCEC technique, the utilization of pressurized hydraulic fluid between the piece and the die leads to the reduction of the friction force and the required processing load. All of these conditions facilitate the processing of the long and large tubes. Also, during HTCEC process, a movable mandrel is placed inside the hollow tube, which plays a main role in the reduction of the required hydrostatic pressure. To investigate the efficiencies of HTCEC process, aluminum 5052 alloy tubes were processed by HTCEC up to two passes. Then, the mechanical properties and microstructure evolution were examined. After the first pass of HTCEC, tangled and diffused arrangements of dislocations were formed. Then, the second pass of HTCEC resulted in a more refined and more homogeneous UFG microstructure with an average subgrain size of 636 nm, while the average grain size of the initial sample was ~ 360 μm. Tensile tests and hardness measurements revealed that two passes of HTCEC led to an increase in the yield strength from 98 MPa to 254 MPa (~ 2.6 times higher), the ultimate strength from 178 MPa to 306 MPa (~ 1.72 times higher) and the microhardness from ~ 56 to ~ 120 Hv (~ 2.14 times higher). Simultaneously, a decrease in the elongation to failure from 31.6 to 15.4% was observed, which is a small amount compared to the results of other studies. Also, in comparison to one-pass, a more homogeneous microhardness distribution through the thickness was detected in the two-pass processed tube. Fractography evaluations by SEM indicated that predominantly ductile fracture occurred in all samples. Furthermore, the equivalent strain and the pressing load of HTCEC process was evaluated by FEM simulation. It seems that HTCEC process, by utilizing its potentials, can produce the relatively long and large UFG tubes having a simultaneous high strength and good ductility. Graphic Abstract