Faculty Publications
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Item Effect of heat treatment on microstructure, corrosion, and shape memory characteristics of laser deposited NiTi alloy(Elsevier Ltd, 2018) Marattukalam, J.J.; Balla, V.K.; Das, M.; Bontha, S.; Kalpathy, S.K.The aim of this work is to study the effect of heat treatment on the microstructure, phase transformations, shape memory characteristics and corrosion behaviour of laser deposited equiatomic NiTi alloy. Dense samples of NiTi alloy were fabricated using Laser Engineered Net Shaping (LENS™) with two different laser energy densities by varying the scan speed and laser power. These samples were annealed for 30 min at 500 °C and 1000 °C in flowing argon, followed by furnace-cooling to room temperature. The resulting microstructures and properties were compared with the corresponding as-deposited samples. Microstructural analysis after heat treatment showed needle-shape martensite in the samples processed at lower laser energy density of 20 J/mm2, and lenticular or plate-like martensite in the samples processed at 80 J/mm2. The XRD results revealed relatively high concentration of martensite (B19?) in heat-treated NiTi alloy compared to as-processed samples. Furthermore, the heat treatment decreased the forward and reverse transformation temperatures of NiTi alloy from 80 – 95 °C to 20–40 °C, presumably due to annihilation of thermally induced defects. Interestingly, the samples annealed at 500 °C showed a measurable increase of 1–2% in the shape memory recovery, from the net recovery of 8% exhibited by the as-processed NiTi alloy. The corrosion resistance of laser-processed NiTi alloy decreased upon annealing. © 2018 Elsevier B.V.Item Laser surface melting of ?-TiAl alloy: An experimental and numerical modeling study(Institute of Physics Publishing helen.craven@iop.org, 2019) Mallikarjuna, M.; Bontha, S.; Krishna, P.; Balla, V.K.The objective of present work is to study the evolution of thermal stresses during laser surface melting (LSM) of ?-TiAl alloy using experimental and numerical modeling approaches. LSM of ?-TiAl alloy samples were carried out at different processing conditions in a controlled atmosphere. Material characterization of the melted region was investigated using scanning electron microscope. It was found that fully lamellar microstructure was transformed into predominantly ?-TiAl with little amount of ?2-Ti3Al. A maximum improvement in hardness of over 72% was noticed in the melted region compared to that of the substrate. Three-dimensional thermomechanical finite element analysis of LSM of ?-TiAl alloy was carried out. Melt pool dimensions, temperature history, and residual stresses were predicted from the finite element models. Measured and predicted values of melt pool depth were in good agreement with a maximum error of 13.6% at P=400Wand V=10mms-1. Predicted residual stress in the melted region exceeded the yield strength of ?-TiAl alloy and resulted in cracking of the melted region at all process conditions. ©2019 IOP Publishing Ltd.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 Laser directed energy deposited Ti-48Al-2Cr-2Nb alloy: An investigation of high temperature oxidation behavior(Elsevier B.V., 2024) Gurugubelli, R.C.; Balla, V.K.; Rajasekaran, B.; Krishna, P.; Bontha, S.This study investigates high-temperature oxidation behavior and kinetics of Laser Directed Energy Deposited (LDED) Ti-48Al-2Cr-2Nb (Ti-48-2-2) alloy at 750 °C, 850 °C, and 950 °C, for 30, 60 and 100 h. Results reveal that the oxide-scale consists of alternating bands of TiO2 and Al2O3 and its stability is strongly dependent on the oxidation temperature and duration. At 850 °C and 950 °C, the oxide-scale delaminated following 100-h exposure. LDED Ti-48-2-2 exhibited an oxidation rate constant of 0.984 mg2 cm−4 h−1 at 850 °C (100 h) and 2.09 mg2 cm−4 h−1 at 950 °C (100 h), and an activation energy of 83.7 kJ mol−1 (850°–950 °C). LDED Ti-48-2-2 exhibited poor oxidation resistance compared to conventionally processed Ƴ-TiAl alloys. This can be attributed to the absence of N-rich layer and the typical nano-scale α2/γ banded lamellar microstructure observed in other processing routes. Post-process heat treatments can be utilized to obtain the desired microstructural features. © 2024 Elsevier B.V.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 Isothermal oxidation behavior of As-deposited and HIPed Ti-48Al-2Cr-2Nb alloy processed using Electron Beam Powder Bed Fusion(Elsevier Ltd, 2025) Gurugubelli, R.C.; Balla, V.K.; Rajasekaran, B.; Krishna, P.; Bontha, S.This work focuses on oxidation behavior of Electron Beam Powder Bed Fusion (EB-PBF) processed Ti-48Al-2Cr-2Nb at elevated temperatures. Two different sample conditions were considered: As-deposited (AD) and post-processed by hot isostatic pressing (HIPed). The oxidation studies were carried out at 750 °C, 850 °C, and 950 °C for 30, 60, and 100 h. The oxidized samples were analyzed for oxide layer growth and kinetics using Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDS), Raman Spectroscopy, and X-ray Diffraction (XRD) techniques. Results indicate that oxide layers are composed of alternative bands of TiO2 and Al2O3. These oxide layers spalled at 850 °C and 950 °C after an exposure of 100 h. The HIPed samples exhibited superior oxidation resistance when compared to AD samples, with an oxidation rate constant of 0.134 mg2 cm4 h?1 at 950 °C (100 h). The presence of homogenized microstructure with large nano-scale lamellar colonies aided in uniform oxide layer growth. EB-PBF samples exhibit fine fully lamellar microstructure due to the rapid heating and cooling cycles. Hence EB-PBF (AD and HIPed) samples exhibited better oxidation resistance when compared to conventionally processed Ti-48Al-2Cr-2Nb. © 2025 Elsevier B.V.Item Influence of bulk post processing techniques on anisotropy of microstructural and tribological properties of L-DED produced Ti64 alloy(Elsevier Ltd, 2025) Suresh, S.; Joshy, J.; Kuriachen, B.; Gurugubelli, R.C.; Kumar, V.; Bontha, S.Laser-Direct Energy Deposited (L-DED) Ti64 alloy is known to have high anisotropy, and low wear resistance which reduce the longevity of artificial bone joints. Thus, the primary objective of this study is to compare and contrast the effect of bulk treatments to mitigate these inherent limitations. Keeping printing parameters constant, the printed samples were put through different post-treatments, namely, super-? annealing (1050 °C, 1 h) and deep cryogenic dipping (?196 °C, 48 h). Electron back scatter diffraction (EBSD) and x-ray diffraction (XRD) analysis revealed differences in grain morphology and phase distributions in the treated samples. A linear reciprocating wear test is conducted with Al2O3 as the counter body to mimic the artificial hip socket. The super-? annealing process reduced the anisotropy in wear rate from 76 % to 60 % but did not show an overall betterment. On the other hand, the cryo-treatment showed an 83 % reduction in wear and a slight reduction in anisotropy compared to the as-build sample. The coefficient of friction (COF) plots also displayed an increase for annealed samples (15.4%–31.5 % higher) while showing a major reduction in cryo-treated samples (42.8%–54.7 % reduction). © 2025 Elsevier B.V.Item Investigation of high-temperature oxidation behavior of Ti-48Al-2Cr-2Nb fabricated using electron beam powder bed fusion and feature added with laser directed energy deposition(Elsevier B.V., 2025) Gurugubelli, R.C.; Balla, V.K.; Krishna, P.; Bontha, S.This study investigates the microstructure and high-temperature oxidation behavior of Ti-48Al-2Cr-2Nb (Ti-4822) alloy at 950°C processed using Electron Beam Powder Bed Fusion (EB-PBF) with feature addition using Laser Directed Energy Deposition (LDED). The EB-PBF substrates in As-Built (AB) and EB-PBF + Hot Isostatic Pressing (HIP) conditions were used for feature addition using LDED. Oxidation studies revealed oxide scales mainly comprising alternating bands of TiO2 and Al2O3. The oxidation rate constant of AB + LDED samples was 1.223 mg2 cm?4 h?1 and AB+ HIP + LDED samples was 0.874 mg2 cm?4 h?1. The oxide scale thickness on the LDED-feature side was higher than that observed on the AB, /AB + HIP side (of the AB+LDED, AB+HIP+LDED) samples suggesting their poor oxidation resistance. This can be attributed to the ultra-fine massive-like ?-TiAl microstructure in LDED compared to the nano-sized ?2/? lamellae seen in EB-PBF (AB/ AB+HIP). Delamination and spallation of the oxide scale were observed at the interface between the substrate and LDED feature owing to the difference in oxide-scale growth rates and the presence of ?2-Ti3Al. © 2025 Elsevier B.V.