Impact of Interlayer Dwell Time on Microstructure and Mechanical Properties of Nickel and Titanium Alloys
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IN laser-based additive manufacturing (AM), Ti and Ni base alloy net shape and near net shape components are fabricated using powder bed fusion (PBF) and directed energy deposition (DED) processes, respectively.[1–9] While PBF processes can produce highly complex net shape components, there are limits on component size due to the available build volume and the amount of powder required to fill the build chamber. DED processes, on the other hand, are well suited to the fabrication of large components. The process is capable of producing near net shape components with lower levels of geometric complexity, requiring post-deposition
B.K. FOSTER, J.S. KEIST, E.T. McHALE, and T.A. PALMER are with the Applied Research Laboratory, Pennsylvania State University, P.O. Box 30, State College, PA 16804. Contact e-mail: [email protected] A.M. BEESE is with the Department of Materials Science & Engineering, Pennsylvania State University, University Park, PA 16802. Manuscript submitted January 24, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
machining to achieve final geometries, but without the same size limitations as the PBF process. In both DED and PBF processes, each location within a build experiences complex thermal histories caused by the addition of new layers of material and the reheating of previously deposited ones. This layer-bylayer fabrication process can also result in a significant buildup of heat in the part,[10–13] and this heat buildup also impacts the microstructures and resulting properties. As component size increases, path planning becomes an important factor in controlling the magnitude of this temperature increase and the localized thermal histories, especially for the DED process where higher laser powers and deposition rates are possible. Changes in path planning typically involve the alteration of the deposition pattern and direction or the addition of an interlayer dwell time, during which the deposition process was halted in order to allow the build to cool. Changing the processing route can alter the cooling rate and distortion of the build.[14,15] For example, scan direction has been shown to influence the growth direction of the microstructure in both Ti-6Al-4V[16] and InconelÒ 625.[10] It has also been shown that by altering the placement of consecutive deposition paths, the maximum build temperature and
orientation of prior beta grains in Ti-6Al-4V can be controlled and more uniform alpha lath widths and microhardness values achieved for low heat inputs and small sample sizes.[17] The impact of varying dwell times in the build path is an important and largely unexplored parameter for DED processes. As component size and the deposition time per layer increase, the simple ordering of deposition passes cannot entirely control the resulting thermal cycles or provide comparable microstructures in materials independent of the size of the component. As a result, interlayer dwell time becomes more of an issue as component size increases. Changes in dwell time have been shown to affect the heat accum
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