The Effectiveness of Hot Isostatic Pressing for Closing Porosity in Titanium Parts Manufactured by Selective Electron Be
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ditive manufacturing (AM) processes allow components to be directly produced from CAD models by dividing them into thin 2D slices, which are built sequentially on top of one another. In powder bed processes, such as selective electron beam melting (SEBM), material is added by spreading a thin layer of powder across the build area. A rapidly moving focused electron beam is then used to selectively melt each powder layer, to densify the required cross-section profile, and fuse it to the previously deposited layer.[1,2] A distinguishing feature of the SEBM process is that the whole build takes place at an elevated controlled temperature that is maintained by pre-heating through rapid scanning of the defocused electron beam across
SAMUEL TAMMAS-WILLIAMS, formerly Ph.D. Student with the School of Materials, University of Manchester, Manchester M13 9PL, UK, and also Visiting Student with the Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, UK, is now Postdoctoral Research Associate with the Department of Materials Science and Engineering, University of Sheffield. Contact e-mail: s.tammas-williams@sheffield.ac.uk IAIN TODD, Professor, is with the Department of Materials Science and Engineering, University of Sheffield. PHILIP J. WITHERS and PHILIP. B. PRANGNELL, Professors, are with the School of Materials, University of Manchester. Manuscript submitted July 9, 2015. Article published online March 16, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
the powder bed prior to melting each layer, which reduces the build-up of residual stresses in the part.[1] The static mechanical properties of Ti-6Al-4V components produced by SEBM have been shown to be comparable to those of conventional wrought material, but the high-cycle fatigue life can exhibit considerable scatter even when testing polished samples.[1,2] This scatter is primarily caused by the presence of pores in the consolidated material, which acts as fatigue crack initiation sites, due to the stress concentration they generate in the surrounding material.[3] Three types of defect with separate origins and appearance have been previously identified in Ti-6Al-4V material produced by the Arcam SEBM process.[4–10] Most commonly observed are gas pores, which are caused by small bubbles of argon trapped inside the feedstock that arises as an artifact of the atomization process used to produce the powder. If the argon is unable to escape the melt pool during the SEBM process, this results in small (5 to 100 lm) near spherical voids in the solid material.[4–7] More irregular lack of fusion defects are caused by the electron beam failing to fully melt and consolidate the powder layer.[4,7–9] Thirdly, and more rarely, in samples built with older generation machines and control software,[4] or with a non-optimum-reduced energy density,[10] tunnel defects that span multiple deposited layers have been observed. The origin of such defects has been elegantly simulated using lattice Boltzmann modeling by Bauereiß et al.[10] who demonstrated that they d
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