Faculty Publications
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Item Evaluation of functionally graded YSZ - IN625 clad without bond coat using laser directed energy deposition(Elsevier B.V., 2023) Likhwar, J.; Thanumoorthy, R.S.; Bontha, S.; Balan, A.S.S.M-CrAlY or M-Cr-based bond coats are used as a buffer layer to apply ceramic thermal barrier coatings (TBCs) to metal turbine blades. However, due to oxygen diffusion, thermally grown oxides grow over the bond coat material, leading to coating failure in the components. Therefore, this study attempts to fabricate a novel TBC-coated IN625 without bond coat material using a Functional grading approach. The findings from this study may pave the way for processing functionally graded ceramic materials using Laser Additive manufacturing techniques. This study also evaluates the performance of functional grading in joining dissimilar materials using small spot-size laser sources. In the present study, samples were fabricated for three sample conditions: S1: direct clad, S2: 25% linear grading, and S3: 50% linear grading. The interface between Yttria Stabilized Zirconia (YSZ) and IN625 for the S1 sample showed discontinuous and horizontal cracks along the interface due to steep variations in thermal properties. However, the interface of S2 and S3 samples showed good adhesion and a smooth transition in microstructure between IN625 and YSZ as a result of functional grading. SEM micrographs showed homogeneous YSZ distributions without segregation within the IN625 matrix. This was attributed to strong Marangoni flow as a result of the small spot-size laser beam used in this study. © 2023 Elsevier B.V.Item Hybrid additive manufacturing of ER70S6 steel and Inconel 625: A study on microstructure and mechanical properties(Elsevier Ltd, 2023) Rodrigues, J.P.; Thanumoorthy, R.S.; Manjhi, S.K.; Sekar, P.; Arumuga Perumal, D.A.; Bontha, S.; Balan, A.S.S.Hybrid Additive Manufacturing (HAM) is currently being explored because of its potential to achieve trade-off between build capacity and feature resolution. The present study aims at fabricating ER70S6-Inconel 625 (IN625) bimetallic clad using hybrid Wire Arc Additive Manufacturing (WAAM) and Laser Directed Energy Deposition (LDED) processes. Microstructure evaluation was performed at the cross section of bimetallic clad for distinct materials as well as the interface. WAAM built ER70S6 revealed equiaxed ferritic grains, whereas laser deposited IN625 region showed columnar dendrites with under developed secondary arms. However, the first layer of IN625 exhibited columnar dendrite with secondary arms due to the influence of diffused Fe from the base ER70S6 steel under the action of concentrated laser heat source, which was revealed by energy dispersive spectroscopy (EDS) maps. The measured microhardness across the cross section of the deposit showed values corresponding to inherent material system. The interface did not reveal presence of any intermetallic phases which was confirmed by hardness results and X-Ray diffraction. Shear test revealed superior bond strength between the two materials, maintaining average strength of 452 MPa. The fractography images exhibited fine dimples along with cleavages indicating mixed fracture characteristics. This additive manufacturing method explores a new dimension in multi-material fabrication which, when customized for different materials, serve critical areas in the aerospace and defence sector. © 2023 Elsevier LtdItem Thermal life assessment of laser powder-directed energy deposited NiCrAlY/CuCrZr bimetallic clad for rocket nozzle applications(Elsevier B.V., 2024) Thanumoorthy, R.S.; Urs, S.S.; Bontha, S.; Balan, A.S.S.To enhance the thermal life of rocket exhaust nozzles, the hot side of copper liners is coated with thermal barrier coatings (TBCs) to provide thermal insulation and oxidation resistance. However, interface failures often occur between M-CrAlY bond coats and nozzle liners due to significant differences in their thermal expansion coefficients (CTE). This study explores the use of Laser Powder-Directed Energy Deposition (LP-DED) to clad NiCrAlY onto a CuCrZr substrate, as the process offers localized heating which can offer better bond strength. Optimization trials were conducted using single and multi-track studies to identify optimal parameters. Due to the low energy absorption of the CuCrZr substrate to 1070 nm laser sources, cladding was performed at a high energy density of 135 J/mm2 with a 1.2 g/min feed rate to achieve defect-free clads with sufficient diffusion. The bulk of the NiCrAlY clads showed ??-Ni3Al, ?-NiAl, and ?-Ni phases, while Y4Al2O9 and Y2O3 oxides formed on the top surface due to aluminum and yttrium depletion at high temperatures. The clads exhibited cellular dendritic microstructures at the bulk region, and planar microstructures were observed at the dilution zone. EBSD-KAM maps showed higher dislocation density near the interface due to CTE mismatch across substrate and clad. Scratch tests confirmed strong adhesion with no interface cracks, though crack propagation was observed from the edges after 50 isothermal cycles, driven by copper erosion. With Cu diffusion, interface region exhibited a graded microstructure which could enhance CTE, improving compatibility compared to standard NiCrAlY alloys. © 2024 Elsevier B.V.Item Surface Characteristics of Low Plasticity Burnished Laser Directed Energy Deposition Alloy IN718(Springer, 2024) Mohanraj; Thanumoorthy, R.S.; Sekar, P.; Muthuchamy, A.; Bontha, S.; Balan, A.S.S.The research work focuses on a novel post-processing sequence to improve the surface integrity and residual stress characteristics of as-printed Inconel718 (IN718) samples. The as-printed IN718 samples are subjected to solution treatment at 1050 °C, two-step precipitation hardening (@ 720 °C for 8 h and @ 620 °C for 8 h), and low plasticity burnishing. Two different sequences were attempted. Sequence-1 involves solutionizing ? low plasticity burnishing followed by precipitation hardening, and sequence-2 includes solutionizing ? precipitation hardening followed by low plasticity burnishing. The experimental observations and detailed investigations revealed that the samples processed via sequence 2 exhibited a better surface finish. The microhardness of the samples of sequence 2 is 10% higher than their counterparts in sequence 1. The maximum residual stress of ?1375.33 MPa is obtained in sequence 1 as compared to the residual stress of ?1100.67 MPa in sequence 2. The influence of the processing sequences on the surface properties has been discussed in detail using the XRD and microstructural characterization supported with EBSD analysis. Graphic Abstract: (Figure presented.) © The Indian Institute of Metals - IIM 2024.Item Effect of age hardening precipitates on the corrosion performance of laser Powder-Directed energy deposited CuNi2SiCr(Elsevier B.V., 2024) Urs, S.S.; Thanumoorthy, R.S.; Aswith Babu, I.; Doddamani, M.; Bontha, S.; Balan, A.S.S.This study explores Laser Powder ? Direct Energy Deposition (LP-DED) processing of CuNi2SiCr and the effect of heat treatment on corrosion behavior. The findings pave the way to increasing the life of the components and the possibility of refabrication upon failure. LP-DED manufactured CuNi2SiCr was subjected to solution treatment followed by age-hardening at 500? for 1,3,5 and 7 h. The microstructure analysis showed the formation of Cr3Ni precipitates due to a higher cooling rate in the LP-DED process. Upon aging, Ni3Si, Ni2Si, and CrSi2 precipitates evolved. Due to the Orowan phenomenon, microhardness increases with the aging time as the number of precipitates along the grain boundary increases with the aging time. The 5-hour aged sample exhibited the best corrosion resistance due to precipitation coherency in the matrix and the medium-sized precipitates with uniform precipitation-free zones (PFZ) in the grain boundary. © 2024 Elsevier B.V.Item A novel NiCrAlY-Cu based bond coat for rocket nozzle applications through LP-DED process(Elsevier Ltd, 2025) Thanumoorthy, R.S.; Vijay, A.; Bontha, S.; Balan, A.S.S.This study explores the development of a novel bond coat for copper-based substrates with the goal of minimizing thermal expansion mismatch and enhancing thermal life in rocket nozzle applications. The effect of copper (Cu) addition on the microstructure, phase evolution, and thermo-mechanical behavior of NiCrAlY clads fabricated via laser powder-directed energy deposition (LP-DED) is systematically investigated to optimize their performance. SEM and elemental mapping reveal a shift from columnar to cellular substructures with Cu additions up to 20 wt%, while higher Cu contents lead to coarse dendritic growth and Cu segregation at grain boundaries, inducing localized strain and crack formation. XRD and DFT analyses indicate that Cu suppresses the ?-NiAl phase and stabilizes the ?-Ni matrix due to its limited solubility in ? and preferential partitioning into ?. High-temperature XRD and EDS analyses show that while pure NiCrAlY forms a continuous alumina scale, Cu-enriched clads develop fragmented and crack-prone thermally grown oxides (TGOs), compromising the oxidation resistance. KAM analysis suggests reduced lattice strain at 10 wt% Cu, followed by increased dislocation density at higher concentrations. Thermal expansion measurements indicate a significant increase in the coefficient of thermal expansion (CTE) at 10 wt% Cu, improving compatibility with Cu-based substrates. However, further Cu additions yield minimal CTE benefits while degrading mechanical strength. Microhardness declines from ?406 Hv (0 % Cu) to ?251 Hv (40 % Cu) due to solid solution softening and ?-phase suppression. A radar plot comparing key metrics identifies 10 wt% Cu as the optimal composition, offering a balanced property set for regeneratively cooled rocket nozzle systems. © 2025 Elsevier B.V.Item Unveiling the role of heat treatment approaches in tailoring corrosion performance of laser powder-directed energy deposited SS304(Elsevier B.V., 2025) Sasindran, J.; Narayanan, J.A.; Babu, I.A.; Balan, A.S.S.; Hebbar, H.S.; Bontha, S.This study investigates the effect of different post-processing heat treatment approaches on the corrosion behavior of Laser Powder-Directed Energy Deposited (LP-DED) SS304. Samples were fabricated using optimized LP-DED parameters: laser power of 400 W, scan speed of 800 mm/min, feed rate of 4 g/min, 0.6 mm beam diameter, and 33 % track overlap.Electrochemical analysis revealed significant differences in corrosion performance depending on the applied heat treatment approach. This treatment resulted in changes to residual stress, microstructure, and oxide layer characteristics which together influence the corrosion rate (CR). The stress-relief annealed (SRA) samples significantly improved corrosion resistance by up to 90 % compared to the as-built condition, while preserving the fine microstructure formed during the LP-DED process and maintaining a stable protective oxide layer. Solution treated and different quenched samples exhibited varying CR depending on the cooling rate and resulting coarse grain structure. These findings highlight the significant influence of post-processing heat treatments on corrosion behavior and microstructural characteristics. © 2025 Elsevier B.V.