Experimental investigation and microstructural characterization of pulsating hydroforming

The main purpose of this dissertation is to increase the limited formability of Ti 6Al 4V alloy sheet at room temperature using the pulsating hydraulic bulge test. The secondary purpose of this research is to investigate the microstructural reasons for the increased formability. During the experimental phase, hydraulic bulge tests were conducted, and the results were compared with the monotonic bulge test specimens concerning thickness distribution and bulge height. In the characterization phase, dislocation density estimations, fracture surface analysis, the thickness and micro hardness distributions were used to associate the increased formability with the microstructural changes. The results have indicated that the pulsating pressure introduces a higher thickness reduction with a more homogenous thickness distribution. The bulge height and the burst pressure increased by 15.4 % and 17 %, when compared to monotonic forming which means failure of the material is delayed. Test results with pulsation suggest that the formability of the Ti 6Al 4V sheet at room temperature is increased by 47% in terms of elongation. The reported results demonstrate that pulsating pressure improves the formability of difficult-to-form materials at room temperature leading to a more homogeneous thickness distribution as compared to the monotonic pressure. Results of the material characterization suggests that stress relaxation phenomenon took place during the pulsating HBT. For the microstructural characterization; the decrease in the dislocation density in the early stage of forming prevented the locking of the dislocations and delayed the occurrence of the damage, i.e., leading to increased bulge height. The more plastic deformation resulting from the increased formability also increased the hardness along the cross section. As a result, with pulsating forming, the formability of the Ti 6Al 4V sheets with low ductility, non-hardening, and hexagonal close packed dominant crystal structure can be significantly increased, and the microstructural reasons for this improvement has been revealed.