A Novel Severe Plastic Deformation Technique with Potential for Producing Relatively Long Ultrafine Grained Tubes

In present study, hydrostatic tube cyclic extrusion compression process is introduced as a novel severe plastic deformation process for grain refining and improving mechanical properties of tubular components. Also, this process has the potential to produce relatively long and large tubes. In this process, because of the utilization of pressurized hydraulic fluid between the tube and die, there is nearly no contact friction. This leads to a significant reduction in pressing load. In this research, after applying hydrostatic tube cyclic extrusion compression process on pure copper tube, the microstructure evolution and the mechanical properties improvement were examined. The results denoted that this process was successfully performed on pure copper tube. In this way, the microstructure and mechanical properties were improved significantly. For example, after this process, the ultimate strength of pure copper, the yield strength and the value of hardness became 1.57, 1.85 and 1.86 times higher, respectively, and a low loss of ductility was achieved. Also, after this process, an ultrafine cellular microstructure with average size of about 990 nm were observed. While, the average value of grain size for the unprocessed tube was about 40 μm. The stages of the formation of the observed microstructure are as follows: the creation of a high density of dislocations, the dislocations coalescence with each other and the formation of tangled structures, the formation of ordered arrangements of dislocations and low angle boundaries, the formation of dislocation cells to diminish strain energy, the creation of new dislocations and their movement to boundaries.