An investigation of the formability of ultra-thin CP-Ti-Gr2 foils considering thickness-to-grain-size effects under controlled heat treatment in μ-ISF

Abstract

Micro-forming is an emerging micro-manufacturing process for the fabrication of miniature parts/components made of ultra-thin sheets (foils), forged billets, rods, etc. Achieving high formability in the micro-incremental sheet forming (μISF) process is difficult due to the size-effect and non-optimal selection of process parameters viz. step depth (∆z). The major sources of size-effects are pure volume source, surface-to-volume ratio, thickness-to-grain-size (t/d ratio), surface structure scalability, etc. In the micro-scale processes, studying the grain size of the material is crucial to understand the deformation behaviour. Additionally, the anisotropy is very prominent in thin foils and affects the micro-forming process adversely. In the present work, the intrinsic anisotropy of the foils is minimized through controlled heat treatment, and varying grain sizes, having different microstructures, are generated to investigate their effect on the formability of CP-Ti-Gr2 foils. Initially, the properties of the received material are tested along different directions through the uniaxial tensile test, followed by furnace annealing to produce equiaxed recrystallized grains and reduce the anisotropy of the foils. Subsequently, the specimens are heat-treated at different temperatures to generate a wide spectrum of grain sizes. Through extensive μISF experiments, it was established that higher annealing temperature and increase in grain size assisted in improving the ductility of the foils, leading to enhanced formability. Therefore, the combined effect of the grain size and step depth on the formability of the components is investigated. A relationship between formability and t/d ratio was established and its critical value was obtained. Interesting observations, contrary to those in macro-ISF process were observed, e.g. higher step depth in μISF helped in improving the formability of the micro-parts. The results were confirmed by measuring the forming forces during the process, and it presented a strong correlation with the yield stress values, further correlated with varying grain sizes of the foil. This work could pave the way for designing the μISF process parameters considering the size-effects at micro-scale deformation, through optimum t/d ratio, for maximizing the formability.