Effect of cooling rate on the phase transformation and post strength of Ti-6Al-4V under hot forming conditions: experiments and modelling

A systematic experimental and numerical study of the cooling rate effects on the post-formed strength and microstructure evolutions of a dual phase Ti-6Al-4V alloy under hot forming conditions was performed in this study. A series of cooling treatment tests were designed to achieve typical cooling scenarios in current hot forming processes, i.e. air cooling, water quenching and die quenching. The post-treated mechanical properties and phase volume fractions were characterized and quantified. The results show that higher temperatures and higher cooling rate would facilitate the β phase transformation and martensite transformation of α′ phases, leading to the higher yield strength in the formed part. Based on the macro- and micro- properties relations and observed data, a unified model for the hot forming and cooling of Ti-6Al-4V was proposed and calibrated, enabling the accurate prediction of both the macro-strength and microstructures under different cooling conditions of the material. The proposed model was further implemented into FE simulation for practical forming process simulations. A typical hot gas forming process of a rectangular cross-sectioned titanium alloy hollow tubular part was investigated. Experiments of hot gas forming with different cooling conditions, i.e. forming temperatures and cooling rates, were carried out and used to validate the established material model and simulation model. Good prediction accuracy was obtained under all the tested conditions, with the maximum prediction errors of 5.55% for macro-strength and of 9.79% for average grain size. The results and the model developed in this paper provides an effective way to facilitate the process designs and optimization for hot forming titanium alloy parts in various applications.