Faculty Publications
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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 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 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.