Faculty Publications
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Item Prediction and validation of residual stresses generated during laser metal deposition of ? titanium aluminide thin wall structures(Institute of Physics Publishing helen.craven@iop.org, 2019) Mallikarjuna, M.; Bontha, S.; Krishna, P.; Balla, V.K.The focus of the current work is to predict and validate residual stresses developed during Laser Metal Deposition (LMD) of Gamma Titanium Aluminide (?-TiAl) alloy by using a combination of numerical modeling and experimental methods. Laser Engineered Net Shaping (LENS), which is one of the commercially available LMD techniques, was used to fabricate ?-TiAl alloy thin wall structures at various processing conditions. These deposits are expected to develop residual stresses due to the rapid heating and cooling cycles involved in the LMD process. 3D transient thermomechanical finite element analysis was used to simulate the LMD process. Thermal gradients and residual stresses were predicted from the thermomechanical models. It was found that the magnitude of thermal gradients increases with the addition of each deposited layer. Tensile residual stresses were observed at the edges of the thin-wall, while compressive residual stresses were observed at the center of the wall as well as in regions away from the edges. Residual stresses in the deposited samples were also measured using the x-ray diffraction technique. Reasonable agreement was observed between the predicted and measured values of residual stresses. © 2019 IOP Publishing Ltd.Item Characterization and thermal analysis of laser metal deposited ?-TiAl thin walls(Elsevier Editora Ltda, 2021) Mallikarjuna, B.; Bontha, S.; Krishna, P.; Balla, V.K.The present work focuses on investigating the effect of process variables (power, travel speed, powder flow rate) on microstructure and mechanical properties of Laser Metal Deposited (LMD) ?-TiAl thin walls. To this end, LMD technique was used to deposit ?-TiAl thin walls at different processing conditions. Microstructures of as-deposited samples were investigated using both optical and scanning electron microscopy. X-ray diffraction (XRD) technique was used to determine the phases present. Microhardness measurements were carried out along both longitudinal and build directions. Microstructural analysis of as-deposited samples revealed a fine lamellar structure comprising of ? and ?2 phases. Colony size of 30–60 ?m and lamellar spacing between 0.1 and 0.7 ?m were observed. XRD analysis confirmed the presence of ? and ?2 phases. Comparison of elemental analysis results on both powder and as-deposited samples revealed a negligible loss of Al and no oxygen pick up in the deposited thin walls. Hardness values were found to decrease with an increase in wall height, and hardness values increased marginally (5%) with an increase in travel speed. Further, 3D transient thermal analysis was also carried out to complement the LMD of thin walls in terms of melt pools and cooling rates. It was found that the melt pool depth (MPDc = 0.266 mm) is smaller at the centre than the edge (MPDe = 0.513 mm) of the wall. A higher cooling rate of 1.05 × 105 °C/s near the wall substrate was found for 200–12. © 2021Item 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 Effect of deposition strategy and post processing on microstructure and mechanical properties of serviced Inconel 625 parts repaired using laser directed energy deposition(Elsevier Ltd, 2024) Chaurasia, J.K.; Jinoop, A.N.; Paul, C.P.; Bindra, K.S.; Balla, V.K.; Bontha, S.In the present work, an attempt is made to understand and explore the repair capabilities of the Laser Directed Energy Deposition (LDED) process on Nickel based superalloy Inconel 625 (IN625). Samples were extracted from a wrought plate of IN625 and then were subjected to a fatigue test to mimic a component in service for repairing. Further, deposition was carried out on these fatigued tensile sample surfaces i.e., Top, Top & bottom, One side and Both sides. The samples were also solution-treated at 1200 °C for 90 mins. Microstructure and mechanical properties were evaluated and then compared between the different deposition strategies and sample heat-treatment conditions. Tensile properties were compared for all the three sample conditions viz. wrought alloy, as repaired and solution treated. Results indicate sound deposition with minimal porosity in all the four deposition strategies using the LDED process with a mean deposit height of 1.02 ± 0.25 mm. Microstructural analysis revealed mixed dendrite and columnar structure in the case of as-deposited samples whereas, solution treated samples exhibited recrystallized equiaxed grains with the presence of annealing twins. The as-deposited samples show a ductile mode of failure with a maximum ultimate strength of 830 MPa, yield strength of 350 MPa and elongation of 72%. For solution treated samples, a maximum ultimate tensile strength of 620 MPa, yield strength of 270 MPa and elongation of 73% were observed. The strength of the material was found to be highly influenced by the solution treatment. © 2023 Elsevier LtdItem Effect of CMT-WAAM Process Parameters on Bead Geometry, Microstructure and Mechanical Properties of AZ31 Mg Alloy(Springer, 2024) Manjhi, S.K.; Sekar, P.; Bontha, S.; Balan, A.A.S.Fabrication of Mg alloys using the additive manufacturing process is quite challenging owing to high oxidation and volatile nature at high temperatures. The present study investigates the effect of wire feed speed (WFS) and travel speed (TS) on single tracks of AZ31 Mg alloy fabricated using the cold metal transfer wire arc additive Manufacturing (CMT-WAAM) process. The WFS and TS of CMT-WAAM are optimized to achieve better deposition quality. An increase in WFS increased the width, height, penetration depth, and heat-affected zone of single tracks. In addition, increasing TS decreased the deposited tracks' contact angle and height. The average grain size at the interface zone, center and top portion of single tracks are 35, 42, and 60 μm. The x-ray diffraction results show only the presence of primary phase α-Mg; interestingly, the β-Mg17Al12 and η-Al8Mn5 secondary phases are identified by SEM + EDS and TEM images. The microhardness increased from the substrate to the top section of single tracks due to the increased volume fraction of secondary-phase particles. Based on the best-chosen process parameters obtained from single-track deposition, a multilayer AZ31 Mg thin wall is deposited. The UTS, YS, and % EL of the deposited thin wall in travel direction (TD) are 222 MPa, 102 MPa, and 18%, while in build direction are 202 MPa, 110 MPa, and 14%, respectively. The tensile strength and elongation % of TD and BD samples exhibited comparable properties and were higher than cast AZ31 Mg alloy. © ASM International 2023.Item Process parameter optimization for laser directed energy deposition (LDED) of Ti6Al4V using single-track experiments with small laser spot size(Elsevier Ltd, 2024) Gonnabattula, A.; Thanumoorthy, R.S.; Bontha, S.; Balan, A.A.S.; Anil Kumar, V.A.; Kanjarla, A.K.Single-track experiments are routinely used in the optimization of process parameters in additive manufacturing processes. Most of the process parameter optimization studies use a laser spot size of 1 mm or more. Since laser spot size affects the input energy density and in turn the efficiency of the deposition process, it is important to develop process maps every time a laser of different spot sizes is used. In this work, we determine the process maps for a laser of 0.6 mm spot size. By combining the process maps and the metallographic inspection, we estimate the optimum process parameters (laser power, scan speed, powder feed rate) for building Ti6Al4V components using powder-based laser-directed energy deposition(LDED). Single-tracks corresponding to 64 different parameter combinations are deposited. After eliminating the process parameter combinations resulting in defective tracks, the optimum process parameters of 300 W laser power and 720 mm min−1 scan speed is established by considering the relationship between the process parameters and the geometrical features of the deposit. The experimental results are then used to calibrate the modeling parameters of a three-dimensional finite element model for simulating the deposition process. © 2024 Elsevier LtdItem Effect of Heat Treatment on Microstructure and Dry Sliding Wear Behavior of Laser Directed Energy Deposited Inconel 625(Springer, 2025) Praharaj, A.K.; Chaurasia, J.K.; Suvin, P.S.; Narayanan, J.A.; Paul, C.P.; Balla, V.K.; Chakrapani, S.K.; Bontha, S.Laser directed energy deposition (LDED) is a promising technology for manufacturing and repair of Inconel 625 (IN625) components used in critical sectors requiring enhanced tribological performance due to harsh operating environments. Hence, the current work focuses on the evaluation of the tribological performance of LDED-built IN625 with the implementation of different heat treatment methods, i.e., solution treatment (ST), direct aging (AG), and solution treatment + aging (ST + AG). A detailed microstructural analysis, hardness, and wear testing were performed for the as-deposited (AD) and heat-treated (HT) samples, and the results were compared. The analysis revealed coarser grains in the case of ST and ST + AG samples, whereas finer grains for AD and AG samples, indicating grain coarsening after solution treatment. Further, the brittle laves phase gets dissolved after ST, whereas the AG and ST + AG samples resulted in the precipitation of metal carbides and strengthening phases. The microhardness of the ST sample (193.2 HV) was lower compared to the AD (211.6 HV) sample, whereas the AG and ST + AG samples exhibited 25.6 and 9.3% higher hardness than the AD sample. Considering tribological performance, the AG sample illustrated a maximum reduction of 61.4% in the coefficient of friction (COF) and 36.5% in wear rate when compared to the AD sample. This could be attributed to the presence of finer grains and strengthening phases. © ASM International 2025.Item Tailoring the Microstructure and Mechanical Properties of Laser Powder-Directed Energy-Deposited Inconel 625 Using Scan Strategies(Springer, 2025) Aromal, S.S.; Malathesh, P.B.; Thanumoorthy, R.S.; Agasti, S.K.; Praharaj, A.K.; Anil Kumar, V.A.; Sudarshan Rao, G.S.; Bontha, S.The current study is focused on the influence of different scan strategies on the microstructural evolution, crystallographic texture, and mechanical properties of the Inconel 625 (IN625) fabricated using the laser powder-directed energy deposition (LP-DED) process. Prior to the deposition of the bulk specimens, an optimized set of process parameters (laser power (P), scan speed (v), and feed rate (f)) was selected through analysis of single-track deposits. The single tracks were thoroughly analyzed based on the aspect ratio, track stability, dilution, and shape of the melt pool. Further, six rectangular blocks of IN625 with different scan strategies (unidirectional ? 0°, bidirectional ? 0°, 45°, 67°, 90°, and spiral) were fabricated using the optimized process parameters for deposition. Samples with a 0° unidirectional scan strategy exhibited higher yield strength values but lower ductility. Notably, the sample with a scan orientation of 67° exhibited superior isotropic properties that are required to bear intense multi-axial loads when compared to other samples. The results indicated that the sample with a 67° scan orientation has the best combination of both strength and ductility. This can be attributed to finer cells/grains, which occur due to fragmentation of cells/grains during their growth across the successive layers, a higher fraction of low-angle grain boundaries (LAGBs), and variation of vector length within a layer. EBSD analysis revealed that samples with a 67° scan orientation exhibited a random crystallographic texture (MUD = 2.2), which suggests isotropic behavior compared to other samples. © ASM International 2025.