Effect of Tooling Material on the Internal Surface Quality of Ti6Al4V Parts Fabricated by Hot Isostatic Pressing
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-net shape hot isostatic pressing (NNS-HIP), combined with tooling development, has been widely applied to fabricate fully dense parts with complex geometry from metal powders.[1–4] In this process, a shaped capsule with an internal core (usually made of steel) is filled with powder, followed by outgassing and sealing, and then isostatically pressed under high pressure and temperature.[5,6] The geometry of the internal core is designed via computer modeling, predicting the shrinkage of the compact during the densification process occurring in the HIP process.[7] NNS-HIP technology could be used to fabricate complex-shaped titanium alloy parts with good mechanical properties by controlling the final microstructure via changing HIP processing parameters (temperature and pressure). Therefore, it has been widely exploited in aero and rocket engines.[8,9] For instance, a cryogenic impeller for the liquid hydrogen turbo-pump used in aerospace was fabricated from Ti6Al4V powder using
CHAO CAI and WU YAN, Ph.D. Students, BO SONG, Associate Professor, QINGSONG WEI and YUSHENG SHI, Professors, and PENGJU XUE, Postdoctoral Researcher, are with the State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China. Contact e-mails: [email protected] and [email protected] Manuscript submitted February 25, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
the NNS-HIP process and exhibited excellent mechanical properties comparable with those of parts fabricated by forging technology.[10] In our previous work, a complex Ti6Al4V disk was successfully fabricated using NNS-HIP and its relative density reached 99.5 pct.[11] Nevertheless, because these parts contain a complex structure (such as the blades of impeller and turbine disk) and it is extremely difficult to improve the internal surface quality by subsequent machining, causing most of the as-fabricated internal surface has to be designed to remain.[12] On the other hand, surface roughness is vital for a final part as it could affect their performance such as fatigue. Davidson et al.[13] pointed out that most failures of engineering components initiated on the surface, especially in the case of fatigue failure. It was also found that the fatigue strength of the HIPped part could be improved by approximately 200 MPa due to the improvement in as-HIP surface quality. Zhang et al. reported that a HIP process, in which temperature was first increased and then the pressure was raised, could decrease the surface roughness.[14] Moreover, the fatigue strength of as-fabricated parts was about 100 MPa higher than that of components fabricated under a conventional HIP condition where the component surfaces are rougher. Therefore, it is obvious that improving surface quality is essential for the fatigue property of HIPped components and their service life. Nevertheless, up to now, to the best of the authors’ knowledge, current work mainly focused on optimization of HIP processing parameters[15–20] and simulation of shape ch
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