Estimation of Melt Pool Dimensions, Thermal Cycle, and Hardness Distribution in the Laser-Engineered Net Shaping Process
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LASER engineered net shaping (LENS) process, developed by Sandia National Laboratory and commercialized by Optomec Inc., (Albuquerque, NM) facilitates building of metallic parts layer by layer by melting and deposition of metal powders using a focused, high-powered laser beam.[1–3] Powders are carried through an inert gas and delivered concentrically to the laser spot through suitably angled four nozzles placed in the deposition head. After deposition of each layer, the laser head and the powder delivery nozzles are moved upward to deposit the subsequent layers from bottom to top. LENS process is widely used in manufacturing complex parts of titanium alloy,[4,5] stainless steel,[6,7] and H13 tool steel.[8,9] The deposited layers in LENS experience high peak temperature with repeated and steep thermal cycles, V.D. MANVATKAR, Graduate Student, and A. DE, Professor, are with the Mechanical Engineering Department, Indian Institute of Technology Bombay, Mumbai 400076, India. Contact e-mail: [email protected] A.A. GOKHALE, Scientist ‘‘H’’, G. JAGAN REDDY, Scientist ‘‘F’’, and A. VENKATARAMANA, Scientist ‘‘C’’, are with the Defence Metallurgical Research Laboratory, Hyderabad 500058, India. Manuscript submitted November 11, 2010. Article published online July 19, 2011 4080—VOLUME 42A, DECEMBER 2011
which influence the microstructural features and mechanical properties of the final part. The real-time monitoring of the temperature fields and the layer dimensions during LENS process is difficult due to small melt pool size and steep gradient in the temperature field.[10–12] Attempts are made to numerically predict the thermal cycles[13] and to establish the influence of process parameters on thermal cycles, residual stress levels,[14] microstructure,[15] and microhardness[6,7,16] in deposits of SS304, SS316, and various tool steels. Zheng et al.[16] found the solidification structure of SS316 deposits to be primarily cellular without prominent secondary branching and attributed it to the high cooling rate during solidification. Smugeresky et al.[17] reported that secondary dendrite arm spacing (DAS) and cell spacing in LENS deposited structures would be the same, and in the range of 7 to 10 lm. Zheng et al.[16] used the measured DAS (i.e., cell spacing) to estimate the corresponding cooling rate using an empirical relation k2 ¼ AðCR Þn
½1
where CR refers to the cooling rate, k2 the DAS, and A and n the materials constants. They found these estimates to be comparable to the corresponding numerically computed values.[16] METALLURGICAL AND MATERIALS TRANSACTIONS A
There is no unique approach to correlate solidification structure of the deposited layers to their final hardness distributions. In multiphase materials such as martensitic steels, volume fractions of the analytically estimated phases and their corresponding hardness values are used to compute the overall hardness following rules of mixture.[6,7,18,19] In single-phase materials, on the other hand, grain size is used to estimate hardness.[14] To estimate yield strength
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