Recycling of titanium alloys from machining chips using equal channel angular pressing
thesisposted on 18.11.2015, 14:30 authored by Qi (Alex) Shi
During the traditional manufacturing route, there are large amount of titanium alloys wasted in the form of machining chips. The conventional recycling methods require high energy consumption and capital cost. Equal channel angular pressing (ECAP), one of the severe plastic deformation techniques, has been developed to recycle the metallic machining chips. The purpose of the PhD work is to realize the ECAP recycling of titanium alloys, in particular Ti-6Al-4V and Ti-15V-3Cr-3Al-3Sn, and investigate the effects of processing parameters on the resultant relative density, microstructure evolution, texture development and microhardness homogeneity. The microstructures of Ti-6Al-4V and Ti-15V-3Cr-3Al-3Sn machining chips obtained from conventional turning (CT) and ultrasonically assisted turning (UAT) were initially investigated. It was found that ultrafine grains were formed in the primary and secondary shear zones. For Ti-6Al-4V chips, the β phase in the shear zones was refined into nano-sized equiaxed grains and aligned up to form banded structures. For Ti-15V-3Cr-3Al-3Sn chips, the nano-crystalline grains were enveloped in the shear zones and have clear boundaries to the surrounding matrix. It was observed that in terms of microstructure, there is no significant difference between CT and UAT chips. Recycling of Ti-6Al-4V machining chips was carried out at moderate temperatures with various back-pressures. For single-pass samples, the relative density was increased with the applied back-pressure and operating temperature. It was found that after multiple passes, near fully dense recycled Ti-6Al-4V can be fabricated. The microstructure observations showed that the nano-sized equiaxed and elongated grains co-existed with relatively coarser lamellar structures which were initially refined after the first pass. In the subsequent passes, the fraction of equiaxed nano-grains increased with the number of passes. The original β phase banded structures were fragmented into individual nano-sized grains randomly distributed within α matrix. The chip boundaries were eliminated and nano-crystalline microstructure region was observed at the chip/chip interface after multiple passes. In the sample processed at 550 °C, type dislocations were observed and oxide layer at chip/chip interface was detected. The texture evolution was investigated using electron backscatter diffraction. It was found that the recycled samples performed a strong basal texture along the normal to ECAP inclination direction after the first pass. After multiple passes, in addition to the normal to inclination direction, the recycled Ti-6Al-4V exhibits a basal texture towards the transverse direction. Microhardness mapping showed that the average hardness and degree of homogeneity were increased with number of passes, while the imposed back-pressure had little effect on the average value and homogeneity. Recycling of Ti-15V-3Cr-3Al-3Sn machining chips was implemented using similar ECAP conditions. The effects of processing parameters, such as back-pressure, operating temperature and number of passes, on the relative density were similar to those for Ti-6Al-4V. Microstructural characterization showed that equiaxed instead of needle shaped α precipitates formed in the β matrix due to the high dislocation density and sub-grain boundaries introduced during ECAP. In terms of microhardness, the maximum hardness was obtained at the specimen pressed at 450 °C. It was found that the applied back-pressure and number of passes enabled to improve the homogeneity, but had little effect on the average hardness.
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