Analysis of microstructure and strengthening in pure titanium recycled from machining chips by equal channel angular pressing using electron backscatter diffraction

Highlights: ► Ti machining chips were solid-state recycled using equal channel angular pressing. ► An ultrafine grain structure was produced by continuous dynamic recrystallization. ► Enhanced strength was attributed to both high and low angle grain boundaries. ► The dependence of yield strength on grain size follows the Hall–Petch relationship. ► Equal channel angular pressing is a promising technique for recycling Ti chips. - Abstract: Equal channel angular pressing (ECAP) was used to realise solid-state recycling of pure titanium machining chips. Fully dense bulk Ti with ultrafine grain sizes was obtained after ECAP at 450 °C although the grains were coarser at the higher consolidation temperature of 590 °C. The yield strength of the recycled Ti obeys the Hall–Petch relationship and reaches 650 MPa after consolidation at 450 °C for two passes. The strengths achieved are comparable to those observed in other Ti produced by ingot metallurgy with similar grain sizes. Analyses show that the strengthening is primarily due to grain size refinement rather than oxide dispersion, and low angle grain boundaries make significant contributions especially at the lower processing temperature.