Cold expansion process on hard alloy holes-experimental and numerical evaluation

This paper presents an evaluation of the influence of the cold expansion process on the fatigue performance of holes in hard alloys, as such materials are involved in an increasing number of aeronautical applications. Fatigue enhancement could bring significant savings for numerous possible industrial applications. However, although the cold expansion of aluminium holes has been studied widely, there have been few publications concerning this process in hard alloys and there is currently very few research activities on technique dedicated to high strength metallic holes. Thus, this work aims to define an approach to understand why processes and methodologies are suitable for obtaining effective expansion of these materials. In this article, the response of expanded holes is studied, considering various experimental parameters and for a wide range of expansion ratios. The influence on the fatigue strength of Ti-6Al-4V tensile specimens is also reported for various expansion ratios, more precisely "standard" expansion ratios and high expansion ratios. Then, a specific numerical modelling of the process is presented, which enables us to understand the impact of high expansion ratios in titanium holes and the influence on fatigue performance. A very good correlation between experimental and numerical results is observed.