The Origin of Microstructural Diversity, Texture, and Mechanical Properties in Electron Beam Melted Ti-6Al-4V
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INTRODUCTION
WHILE the production of net/near-net shape metallic components is currently achievable in terms of shape, microstructures obtained via additive layer manufacture (ALM) are not, from a conventional viewpoint, favorable for the production of engineering materials. The imposed solidification conditions result in microstructures and textures unsuitable for dynamically loaded, structure-critical applications. Nonetheless, research into ALM continues with advantages such as reduced material waste, shorter lead times, zero tooling cost, and high component complexity of great interest to aerospace, automotive, and power generation sectors. The ‘‘freeform’’ nature of the process is also beneficial for the production of medical implants[1,2] and lattice and novel structures[3] for applications such as load absorption and heat exchangers. Although current ALM microstructures are not ideal, control of solidification conditions would allow control over microstructural morphology and broaden the range of potential applications. Such microstructural control is possible. Ga¨umann et al.[4] document how deposition of single crystal superalloys can be achieved via control of process parameters during laser cladding, closely analogous to ALM. The authors go on to produce processing maps that allow the microstructure to be determined (as a function of the imposed processing conditions). Such a maturity of understanding is key to future development S.S. AL-BERMANI, Postgraduate Student, Innovative Metals Processing Centre (IMPC) and the Institute for Microstructural and Mechanical Process Engineering (IMMPETUS), and I. TODD, Reader in Metallurgy and Director, IMPC, and Member, IMMPETUS, are with The University of Sheffield, Sheffield S1 3JD, United Kingdom. Contact e-mail: s.al-bermani@sheffield.ac.uk M.L. BLACKMORE, Design Engineer, and W. ZHANG, Postdoctoral Research Assistant, are with the IMPC, Engineering Materials, The University of Sheffield. Manuscript submitted December 10, 2009. Article published online August 31, 2010 3422—VOLUME 41A, DECEMBER 2010
and uptake of ALM technology. However, the sheer number of variables involved in current powder bed ALM processing makes this a challenge. Moat et al.[5] comment that the wide scale adoption of ALM will require that processing maps are available; this may be necessary for each alloy system or for each ALM technique. Kobryn and Semiatin[6] have produced a solidification map, for Ti-6Al-4V, in terms of thermal gradient (G, K m1) and growth rate (R, m s1), onto which solidification conditions of any process may be plotted in order to determine grain morphology; such maps are extremely useful and have been used to great effect by Ga¨umann et al.[4] to produce both columnar and equiaxed microstructures during laser surface remelting of the superalloy CMSX-4. In order to attempt to tailor microstructural morphology, a sound understanding of the ALM process is required. The aim of this work is to provide an understanding of material response to what are, today, considered to be standard
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