Papers by Author: G. Phanikumar

Abstract: 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.

Abstract: Weld distortion and residual stresses are two major issues in the fabrication process. Numerical techniques are being tried out to accurately predict the structural integrity of the welding. Interpass time in the multipass welding is an important parameter which influences the weld distortion and residual stresses. In this study two pass tungsten inert gas (TIG) welding of 6 mm mild steel plates has been analyzed using Finite element analysis (FEA) software Sysweld and parametric study is conducted with different interpass time. The temperature distribution, distortion and residual stresses are calculated using three dimensional finite element model (FEM) considering phase transformations in the material. The transient thermo-metallurgical analysis followed by elasto-plastic analysis is carried out using temperature dependent and phase dependent material properties. The material deposition in the multipass welding is numerically simulated using chewing gum method, where dummy phase and dummy material are assigned for the element activation. The phase proportions are calculated by assigning suitable phase kinetics parameter extracted from continuous cooling transformation (CCT) diagram of a given material. Experiments are conducted for validation after given edge preparation and using same material as filler wire. The FEM analysis is carried out for eight cases with different time interval between passes, starting from 30 s to 240 s in the steps of 30 s. FEM results are verified with experimentally measured values. It is found that the time interval between passes has less influence on the residual stresses but significantly affects the distortion and phase proportion due to the first pass preheating effect on second pass and second pass postheating effect on first pass.

Abstract: Microstructure of rapidly solidified Ni2MnGa ferromagnetic shape memory alloy has been investigated experimentally by melt-spinning technique. At a constant ribbon width of 3 mm, two speeds of melt spinning 17m/sec and 30m/sec at the extrema of conditions for a good quality of ribbon resulted in two thicknesses of the ribbon, viz., 62 μm and 44 μm, respectively. TEM and AFM analysis reveals the formation of very fine clusters of Ni2MnGa at lower wheel speeds. However at higher wheel speeds nanocrystalline Ni2MnGa particles of size about 10-20 nm and martensitic phases were confirmed.

Abstract: Friction surfacing is a novel solid state surface coating process with several advantages over conventional fusion welding based surfacing processes. In this work, austenitic stainless steel (AISI 310) and tool steel (H13) coatings were friction deposited on mild steel substrates for corrosion and wear protection, respectively. Microstructural studies were carried out by using optical and scanning electron microscopy. Shear tests and bend tests (ASTM A264) were conducted to assess the integrity of the coatings. This study brings out the microstructural features across the coating/substrate interface and its mechanical properties, showing good metallurgical bonding between stainless steel and tool steel coating over mild steel.

Abstract: This study reports detailed microstructural and magnetic characterization of rapidly solidified Ni2(Mn,Fe)Ga heusler alloys processed using the melt spinning technique. Series of Ni50Mn(25-x)Fe(x= 2, 5, 8, 11)Ga25 alloys were prepared by vacuum arc melting and then melt spun at constant wheel speed of 20 m/sec to obtain samples in the form of ribbons. X-ray diffraction analysis of as-cast Ni2(Mn,Fe)Ga alloy with different ‘Fe’ concentrations revealed austenite phase with L21 Heusler atomic order at room temperature. Transmission electron microscopy of melt spun ribbons reveals a precursor tweed structures due to magnetic tweed contrast when the ‘Fe’ concentrations are 8% and 11%. In case of 11% ‘Fe’ substituted alloy martensite phase was found to from at the grain boundary triple junctions. Thermo magnetic measurements determine that, as the ‘Fe’ concentration increases from 2 to 11%; it enhances the magnetic transition temperature from 375 to 403 K.