Faculty Publications
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Item Squeeze casting process modeling by a conventional statistical regression analysis approach(Elsevier Inc. usjcs@elsevier.com, 2016) Gowdru Chandrashekarappa, G.C.; Krishna, P.; Parappagoudar, M.B.During the casting process, the alloy composition, melt treatment modification, processing method, and process variables change the microstructure, thereby affecting the mechanical properties. The hybrid squeeze casting method has been used to limit casting defects, refine the micro-structure, and enhance the mechanical properties. The process variables influence the mechanical and micro-structure properties during squeeze casting. In the present study, we established nonlinear input–output relationships and explored the physical behavior of this process based on the statistical design of experiments and using the response surface methodology. Experiments were conducted to measure the responses in terms of the density, hardness, and secondary dendrite arm spacing. Two nonlinear regression models, i.e., Box–Behnken design and central composite design, were used to conduct experiments, collect experimental data, identify significant process variables, analyze the collected data, and establish the complex input–output relationships. Surface plots were used to explore the effects of the squeeze pressure, pressure duration, pouring, and die temperature on the measured responses. Analysis of variance tests were conducted to evaluate the statistical suitability of the models developed. Furthermore, the accuracies of the predictions made by the models were investigated based on test cases. We found that both of the nonlinear models were statistically adequate and they provided complete insights into the complex nonlinear input–output relationships. Central composite design performed better for the secondary dendrite arm spacing and hardness responses, whereas its performance was the same as that of Box–Behnken design for the density response. The relationships between the responses (i.e., outputs) were established by generating large volumes of input–output data using the nonlinear regression models. We found that the density, hardness, and secondary dendrite arm spacing responses could be obtained by utilizing the nonlinear regression equations and the same set of process variables. Furthermore, the secondary dendrite arm spacing response could be expressed as third order nonlinear functions of density or hardness (structure to property relationship). The results showed that the secondary dendrite arm spacing had inverse relationships with density and hardness, whereas density and hardness had direct relationships. © 2016 Elsevier LtdItem Modelling and multi-objective optimisation of squeeze casting process using regression analysis and genetic algorithm(Taylor and Francis Ltd. michael.wagreich@univie.ac.at, 2016) Gowdru Chandrashekarappa, G.C.; Krishna, P.; Parappagoudar, M.B.In the present work, an attempt has been made using statistical tools to develop a non-linear regression model and to identify the significant contribution of squeeze cast process parameters on surface roughness, hardness and tensile strength. Microstructure examination performed on the squeeze cast samples has revealed that a maximum of 100 MPa pressure is good enough to eliminate all possible casting defects. Accuracy of the developed models has been tested with the help of ten test cases. It is important to note that the developed models predict responses with a reasonably good accuracy and the developed mathematical input–output relationship helps the foundry-man to make better predictions. The present work comprises four objectives, which are conflicting in nature. Hence, mathematical formulation is used to convert four objective functions into a single objective function. The popular evolutionary algorithm, that is genetic algorithm has been utilised to determine the optimal process parameters. © 2015 Engineers Australia.Item Synthesis of high hardness IR optical coating using diamond-like carbon by PECVD at room temperature(Elsevier Ltd, 2017) Krishna, K.; Varade, A.; Niranjan Reddy, K.; Dhan, S.; Chellamalai, M.; Balashanmugam, N.; Krishna, P.Diamond-like Carbon (DLC) for IR antireflective properties is currently being used in the coating of germanium based IR optics. These DLC coatings offer better wear resistance as compared to traditional anti-reflective (AR) coatings. The current work emphasizes the development of IR optics using germanium substrate coated with DLC which typically covers IR transmission in wavelength regions like 3–7 ?m and 9–15 ?m. In order to study IR transmission, an optimum film thickness of DLC was calculated and coated on a double sided polished germanium substrate. DLC was coated on a single side of a germanium substrate, as well as on both sides of germanium. DLC has been deposited using Radio Frequency Plasma Enhanced Chemical Vapour Deposition (RF-PECVD) at room temperature without the use of any intermediary buffer layers required for adhesion and high hardness values were achieved at room temperature as compared to existing literature. The transmission of IR through DLC coated germanium windows was measured using Fourier Transform Infra-Red (FTIR) spectroscopy. A comparison between transmission through a single side and double sided DLC coating on germanium has been demonstrated. The hardness of the film was measured using nanoindentation. Scratch test was also performed using nanoindentation. Adhesion and salt spray tests were performed as per MIL standards. With double sided DLC coating, a peak transmission value of 93% is achieved in 3–7 ?m and the average hardness of DLC is measured to be 32.74 GPa. © 2017 Elsevier B.V.Item Synthesis of high hardness, low COF diamond-like carbon using RF-PECVD at room temperature and evaluating its structure using electron microscopy(Elsevier Ltd, 2017) Krishna, K.; Varade, A.; Reddy, N.; Dhan, S.; Chellamalai, M.; Krishna, P.; Balashanmugam, N.Diamond-like carbon (DLC) coatings have been deposited on Silicon wafers using a Radio Frequency based Plasma Enhanced Chemical Vapor Deposition (RF-PECVD) at room temperature. Experiments were carried out using a flow rate of 100 sccm and 300 sccm of acetylene (C2H2) gas and the bias voltage was varied from 300 to 450 V for DLC deposition. Scanning electron microscope (SEM) and transmission electron microscope (TEM) has been used to study the structure and morphology of the DLC coating. TEM results of DLC coatings deposited at 100 sccm C2H2 flow suggest that some crystalline features of diamond are present in the disordered matrix of DLC. Mechanical properties of DLC coatings were studied using a nanoindenter. The results indicate that the hardest DLC film is obtained at 100 sccm flow rate of C2H2 deposited at 450 V bias voltage of about 32.25 GPa. The results also indicate that the lowest coefficient of friction (COF) of about 0.04 in DLC film is obtained at 300 sccm flow rate of C2H2 deposited at 400 V bias voltage. COF is found to be lower in high C2H2 flow rate, wherever relatively softer DLC was deposited. © 2017 Elsevier B.V.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. © 2021