Now showing 1 - 10 of 94
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    Correlation of microstructure with HAZ welding cycles simulated in Ti-15-3 alloy using Gleeble® 3800 and SYSWELD®
    (03-08-2015)
    Rahul, M. R.
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    Metastable b-titanium alloys are finding increasingly wider applications in structural components in aerospace, energy, and chemical industries because of their formability and heat-treatment possibilites. Components from these alloys are usually welded by processes, such as gas tungsten arc welding (GTAW), electron beam welding (EBW), and laser beam welding (LBW). Post-weld heat treatment improves the strength of the weld because of the precipitation of a phase and TiCr2 particles. In b-titanium alloys, the location, distribution and morphology of a precipitates in b matrix plays an important role in the performance of the welded components. In this work, we simulate different welding processes using SYSWELDVR software and obtain realistic thermal cycles after calibrating the fusion zone dimensions with known experimental data. These cycles are then used to program the heat-affected zone (HAZ) cycles in GleebleVR 3800 to study their effect on the microstructure of the b-titanium alloy. Both continuous and pulsed welding conditions are used for the welding process. Microstructure characterization was performed using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscopy (TEM). Precipitates of a phase below 0.2 μm are seen to be uniform across the b grain but the number density is not uniform across different grains of b. We discuss the characterization results in light of existing models in the literature. The a precipitation and hardness variation are correlated with welding cycles. A combination of computational and physical simulation tools is proposed to reduce the cycle to find optimal choice in the fabrication process design space.
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    High Temperature Deformation Behavior and Processing Maps of FeCoNiCrAlTi Dual Phase High Entropy Alloy
    (01-09-2023)
    Kumar, Piyush
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    Jain, Reliance
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    Rahul, M. R.
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    Ghosh, Abhijit
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    Samal, Sumanta
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    The present study explores the high temperature deformation behavior of FeCoNiCrAlTi dual phase high entropy alloy (henceforth referred to as DP-HEA) in the temperature range of 900–1100 °C (1173–1373 K) and the strain rate varying from 0.001 to 0.1 s−1. The as-cast sample has been characterized using scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry, which reveals the presence of two phases, disordered γ and ordered γ′. A constitutive relationship between the process parameters (stress temperature, strain, and strain rate) has been drawn using the Arrhenius-type equation to recognize the high temperature deformation behavior of the DP-HEA. The optimum thermomechanical processing window of the DP-HEA has been determined by constructing multiple contour maps based on different parameters such as efficiency, strain rate sensitivity, etc. And the optimum processing domain has been found to lie approximately in the temperature range of 1260–1300 K and SR = 10–2.3–10–2 s−1, 1325–1373 K and SR = 10–1.4–10–1 s−1 & 1173–1193 K and SR = 10–1.3–10–1.55 s−1. Finally, the stable and unstable regimes in the processing maps are correlated with the microstructure of hot deformed samples. Graphical Abstract: [Figure not available: see fulltext.]
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    Improvement of mechanical properties of gas tungsten arc and electron beam welded AA2219 (Al-6 wt-%Cu) alloy
    (01-10-2007)
    Nair, Biju S.
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    Prasad Rao, K.
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    Sinha, P. P.
    Despite its excellent weldability characteristics, AA2219 suffers from poor fusion zone strength under the as welded condition. In the present work, it is attempted to increase the mechanical properties of the as welded fusion zone of this alloy by increasing the weld cooling rates and multipass welding. The cooling rate was increased with the use of high intense heat source, namely electron beam in a pulsed current mode. Multipass gas tungsten arc welding was carried out using direct current straight polarity. These techniques resulted in a significant improvement in fusion zone hardness and tensile properties, which is attributed to reduced copper segregation and natural aging as well as aging caused by heat of multipass welding. © 2007 Institute of Materials, Minerals and Mining.
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    Investigation of fusion weldments of semi-solid aluminium A356 alloy: Pool geometry and microstructure
    (01-01-2013)
    Sandhya, S.
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    A fusion welding technique to join a semi-solid processed A356 cast plate is explored using Gas Tungsten Arc Welding (GTAW). Semi-solid metal (SSM) billets of non-dendritic microstructure produced by rheocasting in a mould placed inside a linear electromagnetic stirrer were used for this study. GTAW experiments were conducted to simulate different thermal gradients near the fusion zone. The geometries of the weld pool as well as the temperature gradient in the fusion boundary were measured to understand the microstructure evolution. Simulation of the welding process was performed to aid in the analysis. Quantitative metallography provided the shape factor as a measure of globularity of the primary a-Al phase. Based on the studies, a model has been proposed to explain the observation of globular microstructure in the fusion zone of the welds. Conclusions show a positive correlation of thermal gradient with globular microstructure formation in this class of alloys. © (2013) Trans Tech Publications, Switzerland.
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    Development of ultrahigh strength novel Co–Cr–Fe–Ni–Zr quasi-peritectic high entropy alloy by an integrated approach using experiment and simulation
    (01-12-2020)
    Jain, Reliance
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    Jain, Avi
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    Rahul, M. R.
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    Kumar, Ashok
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    Dubey, Mrigendra
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    Sabat, Rama Krushna
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    Samal, Sumanta
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    For the first time, we report here that (CoCrFeNi)90Zr10 bimodal eutectics high entropy alloy (HEA) consisting of both globular eutectics (i.e., L ⟶ FCC (α) + Ni2Zr-type Laves phases) and lamellar eutectics (i.e., L ⟶FCC (α) + Ni7Zr2) is designed and developed by an integrated approach of using thermodynamic simulation and experimental techniques. The present study explores the understanding of new pseudo-quasiperitectic four-phases equilibrium reaction, i.e., L + Ni2Zr⟶FCC (α)+ Ni7Zr2 in the novel (CoCrFeNi)90Zr10 quasi-peritectic HEA (QHEA). The hot deformation behavior of QHEA has been investigated in the temperature range 1073–1323 K and different strain rates (10−3,10−1,1and 10 s−1). Arrhenius-type constitutive equation and artificial neural network (ANN) model have been used to predict the flow stress of QHEA during thermomechanical processing. The hot workability regimes of QHEA has been identified using multiple model parameters, indicating stable regimes in temperature range 1073–1323K and strain rate range 10−3–10−1.3 s−1 as well as temperature range 1073–1125 K and strain rate range10−3–10−0.75 s−1. The plastic strain field distribution and material flow behavior during hot deformation of newly developed QHEA have been predicted using Finite element simulation.
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    Modeling Microsegregation during Metal Additive Manufacturing: Impact of Dendrite Tip Kinetics and Finite Solute Diffusion
    (01-05-2023)
    Hariharan, V. S.
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    Nithin, Baler
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    Ruban Raj, L.
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    Makineni, Surendra Kumar
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    Murty, B. S.
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    Rapid solidification during metal additive manufacturing (AM) leads to non-equilibrium microsegregation, which can result in the formation of detrimental phases and cracking. Most of the microsegregation models assume a Scheil-type solidification, where the solidification interface is planar and there exists a local equilibrium at the interface along with either zero or infinite solute diffusion in the respective participating phases—solid and liquid. This assumption leads to errors in prediction. One has to account for finite solute diffusion and the curvature at the dendritic tip for more accurate predictions. In this work, we compare different microsegregation models, that do and do not consider finite diffusion and dendrite tip kinetics, against experiments. We also propose a method to couple dendrite tip kinetics with the diffusion module (DICTRA®) implemented in Thermo-Calc®. The models which accounted for both finite diffusion and dendrite tip kinetics matched well with the experimental data.
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    Microstructural evolution during friction surfacing of austenitic stainless steel AISI 304 on low carbon steel
    (01-01-2013)
    Khalid Rafi, H.
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    Kishore Babu, N.
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    Prasad Rao, K.
    Austenitic stainless steel AISI 304 coating was deposited over low carbon steel substrate by means of friction surfacing and the microstructural evolution was studied. The microstructural characterization of the coating was carried out by optical microscopy (OM), electron back scattered diffraction (EBSD), and transmission electron microscopy (TEM). The coating exhibited refined grains (average size of 5 μm) as compared to the coarse grains (average size of 40 μm) in as-received consumable rod. The results from the microstructural characterization studies show that discontinuous dynamic recrystallization (DDRX) is the responsible mechanism for grain evolution as a consequence of severe plastic deformation. © 2012 The Minerals, Metals & Materials Society and ASM International.
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    Growth kinetics, microhardness and microstructure evolution of undercooled FeCoNiCuSn high entropy alloy
    (10-03-2020)
    Rahul, M. R.
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    Multi-principle element alloy FeCoNiCu with varying Sn addition was undercooled using the meltfluxing technique to illustrate the dependence of growth kinetics on Sn addition. The alloy FeCoNiCuSn0.5 shows morphological variation in the microstructure from dendritic to equiaxed grain morphology with the increase in undercooling. The alloy FeCoNiCuSn5 shows dendrite morphology with undercooling. The dendritic growth velocity was sluggish with solute addition,i.e., at an undercooling of 200 K, the growth velocity decreased from ~25 m/s to 6 m/s while varying Sn concentration from 0.5 to 5 at % suggesting solute drag effect. The microhardness improvement could be correlated to the microstructure refinement achieved during undercooling.
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    Experimental and modelling studies for solidification of undercooled Ni-Fe-Si alloys
    (22-04-2019)
    Mohan, Dasari
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    We present experimental results, analytical calculations and phase-field simulations for undercooled Ni-Fe-Si alloy system. Undercooling experiments are performed using flux encapsulation along with in situ measurement of recalescence speed using a high-speed camera followed by microstructural characterization. Dendrite growth calculations are performed using a modified Boettinger, Coriell and Trivedi theory to incorporate constitutional undercooling due to multiple segregating elements and a modified kinetic undercooling term. Phase-field simulations are performed using a multi-component phase-field model to generate dendrites in this alloy. High growth velocities are observed and the analytical calculations are in good agreement with experiments. The microstructure evolution from the phase-field simulations indicates that there is a difference in solute segregation during growth of dendrites. This article is part of the theme issue 'Heterogeneous materials: Metastable and non-ergodic internal structures'.
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    Machine learning enabled processing map generation for high-entropy alloy
    (01-09-2023)
    Kumar, Saphal
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    Pradhan, Hrutidipan
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    Shah, Naishalkumar
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    M R, Rahul
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    Identifying optimum processing conditions is necessary for new material development. The flow curves can be used to develop the processing map for an alloy. The current study trained multiple machine learning models such as Random Forest Regressor (RFR), K Nearest Neighbors (KNN), Extra Tree Regressor (ETR) and Artificial Neural Network (ANN) to predict the flow behaviour of the material. The testing R2 fit score of more than 0.99 was obtained for all four algorithms, and trained models were used to generate the flow curves at various temperature strain rate combinations for CoCrFeNiTa0.395 eutectic high entropy alloy. A processing map was developed using the results from ANN and validated with the experimental microstructure observations.