Effects of processing parameters on relative density, microhardness and microstructure of recycled Ti–6Al–4V from machining chips produced by equal channel angular pressing

Ti–6Al–4V was recycled from machining chips by using equal channel angular pressing (ECAP). Samples were pressed through a die up to 8 passes at moderate temperatures in the range of 400–500°C. The ECAP process was accomplished with back-pressures ranging from 50 to 250MPa. The results demonstrate that ECAP with back-pressures allows the consolidation of machining chips to near fully dense (~99.9%) recycled products. The relative density of the as-recycled samples increased with the applied back-pressure and number of passes. The average hardness and homogeneity were improved as the number of passes increased. The transmission electron microscopy reveals a combination of equiaxed and elongated α grains after the first pass. With subsequent pressing, the samples were dominated by nano-equiaxed α grains with diameters of ~70nm. The original β phase in the machining chips was fragmented during the ECAP process.