Mechanical and microstructural analysis of Ti-6Al-4V material in a wide range of superplastic forming conditions

Abstract

Abstract. In order to accurately define the superplastic forming (SPF) conditions of Ti-6Al-4V material, an understanding of the stress-strain behaviour, the initial microstructure, and their evolution during superplastic deformation are required. Ti-6Al-4V material with microstructure beneficial for SPF was superplastically tested according to the ASTM E2448 standard considering a wide range of forming conditions in terms of temperatures (750°C – 830°C) and strain-rates (seven strain-rates ranging from 5∙10^(-5) s^(-1) to 〖5∙10〗^(-2) s^(-1)) – some of the tests of the 3x7 matrix are considered “extreme” conditions from an SPF point of view. The material showed improved superplastic behaviour, which was evident from the stress levels and strain-rate sensitivity values as estimated from the flow curves obtained for the different conditions. In comparison with other commercial alloys or results from similar analyses published in the last decades, low stress values and high strain-rate sensitivity (m) values were obtained despite the low temperatures and high strain-rates used in this analysis. The tests were interrupted when 0.5 true strain (65% engineering strain) was achieved followed by quenching, as this was the maximum local strain achieved when forming the component of interest. Samples did not show any sign of premature necking or failure, with the exception of the two most “extreme” cases. Particularly for the lower strain-rates (below 10^(-4) s^(-1)), some level of material hardening associated with a minimum grain growth was observed in the flow curves. In contrast, a noticeable material softening was observed for the higher strain-rate conditions (above 5∙10^(-3) s^(-1)), associated with the microstructural changes occurring due to dynamic recrystallization. These higher strain-rates led to formation of submicron-sized grains, which could have helped in the superplastic response of the material under these strain-rate conditions. Intermediate strain-rates (5∙10^(-4) s^(-1) and 10^(-3) s^(-1)) showed a different type of response in terms of microstructural behaviour (and flow curve outline) depending on the testing temperature. A negligible amount of cavitation was observed in the samples tested under extreme conditions.