A comparative study on forming limit curves of Ti-6Al-4V alloy obtained by nakajima and elliptical hydraulic bulge tests under the effect of pulsating loading

Abstract Titanium alloys are increasingly employed in industrial applications due to their high specific strength, thermal stability, and excellent corrosion resistance. Among these, Ti-6Al-4V is the most widely used alloy, particularly in aerospace, gas turbine, and biomedical components. Nevertheless, the limited room-temperature formability of Ti-6Al-4V sheets remains a major challenge for sheet metal forming processes. To overcome this limitation, alternative forming strategies, such as hydroforming and pulsating loading, have been proposed to delay localised necking and improve strain distribution. Pulsating forming, based on cyclic or intermittent loading, has been reported to enhance formability through stress relaxation and suppression of plastic instability. Despite these advantages, to the authors’ knowledge, no systematic investigation has been reported on determining forming limit curves (FLCs) for Ti-6Al-4V sheets under pulsating loading conditions in hydroforming processes. Therefore, this study aims to experimentally determine and compare the FLCs of Ti-6Al-4V alloy obtained using the Nakajima test and an elliptical hydraulic bulge test, with particular emphasis on the effect of pulsating loading. In this study, the FLC of the Ti-6Al-4V alloy was determined using the Nakajima and hydraulic bulge tests, with pulsating loading applied during the bulge experiments. The results reveal that pulsating loading causes an approximately 23.4% upward shift in the overall FLC level measured in the elliptical bulge tests, indicating a significant improvement in formability. Compared with the bulge tests, the Nakajima tests resulted in higher strain levels, primarily due to the combined effects of tool–sheet friction, contact pressure, and differences in strain-path evolution. The experimentally determined FLCs were further validated through strain measurements conducted on female-die hydroformed cylinders and spherical components. The results indicate that FLCs obtained from the Nakajima test are more representative for sheet metal forming processes dominated by frictional effects. In contrast, FLCs derived from hydraulic bulge tests are more suitable for hydroforming applications. Overall, pulsating forming is an effective approach for significantly enhancing the formability of Ti-6Al-4V alloy sheets.