Influence Of Thermohydrogen Processing On Microstructure And Machinability Of Titanium Alloys

Thermohydrogen processing of titanium alloys is a positive hot forming technique, which can improve the microstructure of titanium alloys, thus efectively enhance it performance characteristics and processing characteristics with the hydrogen acting as a reversible alloying element. After hydrogenation treatment, the phase constituent and microstructure of TC4 and BT25y titanium alloys are studyed by X-ray difraction, optical microscopy, SEM and TEM. Effects of hydrogen on mechanical properties, physical properties and machinability of titanium alloys are studyed by hardness test, thermal conductivity test and cutting test. The effect of vacuum dehydrogenation processing on the microstructure and phase constituent of alloys are studyed too.The results show that: the hydrogen contents of TC4 and BT25y alloys increase with the increase of temperature. They both reach maximum when the hydrogenation temperature is 750℃at 1.35% and 1.05%, and then decrease with increasing of hydrogenation temperature. The hydrogen contents of alloys increase gradually with the rise of hydrogenation time, and then approach to a stable value after about 4 hours at 1.36% and 1.17%. The number ofαphase reduces after hydrogenation while the microstructure take on a tendency of refinement with increase of hydrogen contents, and the grain boundary become fuzzy. The hydrides are mainly TiH2 which are nanoscale particles when the hydrogen content reaches about 0.5%, what can not only precipitates from the grain boundaries, but also from the intragranular. Moreover there exists a certain orientation relationship between the hydrides and the matrixα-Ti. The hardness increases with the increase of hydrogen contents. The thermal conductivity is maximum when hydrogen content of TC4 alloy reaches 0.7% and BT25y alloy reaches about 0.6%, increase by 16% and 20% respectively.Mechinability of the two kind alloys are be greatly improved by thermohydrogen processing. TC4 alloy can get the hydrogen contents of 0.4%～0.5% that are best for cutting, when the thermohydrogen process is 750℃×1h. At this time the cutting force reduce effectively by about 10%, the thermal conductivity increase by about 10%, the surface roughness reduce by about 27%, the curl-shaped chips are transformed into the chip-breaking that can easy to exclude. BT25y alloy can get the hydrogen contents of 0.4%～0.5% that are best for cutting, when the thermohydrogen process is 750℃×0.5h. At this time the cutting force reduce effectively by about 63%, the thermal conductivity increase by about 4%, the surface roughness reduce by about 20%, the curl-shaped chips are also transformed into the chip-breaking that can easy to exclude.Hydrogen can be removed to a safe concentration by vacuum annealing at 750℃for 8 hour. The hydrogen content of TC4 alloy and BT25y alloy can reduce form 0.433% and 0.596% to 0.019% and 0.010% that both in the security level of concentration by vacuum annealing at 750℃for 8 hour. The phase consists ofαphase andβphase that in accord with original structures, but microstructures of TC4 and BT25y alloys can be refined after dehydrogenation.