Miniaturised experimental simulation and combined modelling of open-die forging of Ti-6Al-4V titanium alloy

This study demonstrates the application of a new experimental technique for laboratory-scale simulation of the open-die forging process, known as cogging, an intermediate hot-working process necessary to design an optimised microstructure in the advanced engineering titanium alloy Ti-6Al-4V. Small test-bars of Ti-6Al-4V alloy were subjected to multi-directional cogging operations at elevated temperatures (950–1050 °C). The as-received material, prior to forging, underwent heat treatments to coarsen the initial grain structure, to better simulate the industrial-scale intermediate microstructure (i.e., β recrystallised) and to help prove the capability of the set-up to achieve microstructure modification via globularisation (below β-transus), and recrystallisation (dynamic and static) and recovery mechanisms (above β-transus) within the cogged material. The influences of hot working parameters on deformation localisation, width of α platelets, and globularisation within the resulting microstructure variation have been investigated using light microscopy (LM), Vickers hardness (HV) testing, and electron backscatter diffraction (EBSD). The cogged Ti-6Al-4V alloy specimens underwent various microstructural evolution stages after hot forging, thus indicating successful application of the designed miniaturised open-die forging apparatus for high temperature experimentation and characterisation studies. This will be suitable for low-cost small-scale trials to determine the key process parameters affecting the onset of microstructure evolution during open-die forging (e.g., ingot-to-billet conversion) of the Ti-6Al-4V alloy, prior to large-scale trials which are rather more expensive.