Bikash Kumar

Bio: Bikash Kumar is an academic researcher from Indian Institute of Technology Guwahati. The author has contributed to research in topics: Welding & Residual stress. The author has an hindex of 5, co-authored 13 publications receiving 55 citations.

Influence of the mode of laser welding parameters on microstructural morphology in thin sheet Ti6Al4V alloy

Abstract: The transients and gradients generated due to localized heating and cooling by a laser source governs the thermal-metallurgical-mechanical performance of the welded structure. An attempt is made to investigate the significance of the pulse parameter over continuous mode in Yb-fiber laser welding of 800 μm thin Ti6Al4V alloy. The full-depth of penetration with minimum weld width is achieved at the lowest heat input of 12 J/mm for the pulse laser. The behaviour pattern of the thermal history is critically assessed with the aid of FE based heat transfer model and corresponding relation with evolved microstructural morphologies are systematically investigated. Several stages of transformation occurred in the weld zone such as α -phase dissolution, β -transus in the heating cycle, and diffusionless β → α ' / α martensitic transformation during the cooling cycle are well explained by thermal history. A relatively high amount of α ' -martensite is observed in the pulse laser welding whereas the transformed β -phase fraction gradually reduces further away from the weld line. Blocky plate-shaped α ' -martensite within the coarse β -grain boundary is apparent at higher heat input (26–80 J/mm), whereas acicular morphology within the fine β -grain boundary is observed at a low heat input of 12–19 J/mm. The dimensional variation of α ' -lath at the fusion zone has a significant influence on strength. Weld metal containing very fine α ' -lath have comparable strength to that of the base metal. The images of the fractured surface shows the dimples as well as microspores in continuous mode, whereas combinations of large and small dimples are observed for pulse mode of welding. The level of contamination is examined by surface discoloration technique and found to be the highest for the heat input of 80 J/mm.

On the interaction of microstructural morphology with residual stress in fiber laser welding of austenitic stainless steel

Abstract: The development of compressive residual stress is governed by the metallurgical transformation and associated microstructure, which in turn influences the mechanical properties of a welded structure. In the present work, the microstructural interaction with the thermo-mechanical performance of Yb-Fiber laser-welded austenitic stainless steel (SS304) is evaluated at different heat inputs and varying defocus distances. The interrelation between microstructural morphology and the typical pattern of residual stress is systematically investigated. The development of the thermal-metallurgical-mechanical model particularly considers the effect of solid-state phase transformation (SSPT) such that the model predicts the phase fraction and its influence on residual stress distribution. The present study exclusively focuses on the mechanism of residual stress generation as a consequence of dual-phase microstructural evolution during solidification. Low heat input (45 J/mm) leads to a high cooling rate, excellent tensile strength, and low tensile residual stress. The finite element-based thermo-mechanical model predicts the residual stress with a maximum deviation of ± 50 M P a compared to the experimental value. The Ferritic-austenitic (FA) mode of solidification results in the combination of skeletal and lathy δ -ferrite morphology. Increasing peak intensity of δ {110} with a cooling rate confirms the enhancement of δ -phase within the austenite matrix and restricts the complete transformation to the austenitic phase. A relatively high amount of δ -ferrite due to enriched Cr and lean Ni content at the dendritic core with fine or acicular morphology at a high cooling rate reduces the longitudinal and transverse residual stress significantly. Besides, the increasing trend of primary dendritic arm-size of δ -ferrite yields a gradual increase in residual stress.

Microstructure evolution in thin sheet laser welding of titanium alloy

Abstract: The effect of cooling rate on microstructural morphology and mechanical properties of laser welded thin sheet Ti6Al4V alloy is studied. The numerical investigation has been performed to predict the weld pool geometry at different heat input by pulse Nd:YAG laser. The cooling rate is estimated from simulated time-temperature history. The solidified structure is complex and may acquire various microstructural transformations with different morphology of mainly α and β phases depending upon the particular cooling rate followed. Diffusional, α′-martensitic and mixed structures are found in the welded joint. Massive diffusion-controlled α lamellae has found in the range of 52-325 K/s. Volume fraction of α′-martensitic phase in the fusion zone increases with cooling rate. It shows that the dimensional variation of α lamellae plays an important role on mechanical properties. Substantial improvement of the mechanical properties with increase in cooling rate is characterized by the volume fraction of primary α-phase and the α + β lamellae spacing.

Influence of Different Frequency Pulse on Weld Bead Phase Ratio in Gas Tungsten Arc Welding by Ferritic Stainless Steel AISI-409L

Abstract: The objective of the present experimental work is to obtain an appropriate welding parameter on pulsed current gas tungsten arc welding (GTAW) ferritic stainless steel AISI 409L with a thickness of 4.5 mm. Frequency affected penetration, and the ratio of bead width to penetration (aspect ratio) is the main objective of the research. A Taguchi L9 orthogonal array with three level four factors was chosen to execute the bead on plate welding. It leads to optimize the input process parameters on main effect plot via analysis of variance, and it has imposed to determine their contribution level of each parameter with respect to responses. Taguchi optimized conditions for butts weld with the pulsed TIG and its surface morphology, and mechanical characteristics of AISI 409L weld was investigated. A full penetration with an optimal aspect ratio was accomplished using high-frequency pulsing, according to the findings. The mechanical properties were characterized using Vickers microhardness and tensile tests on the base material and weld metals. Microstructural study revealed that these variables have a greater impact on the bead profile. Pulsed TIG showed maximum UTS of 445 MPa and a minimum of 385 MPa, with an average of three samples of 415 MPa. The weld metal in all of the zones had influenced superior tensile strength (UTS) as compared to base metal, according to the findings. The obtained strain percentage for both butt and TIG welds, however, was smaller than that of the parent metal. The formation of martensitic was attributed for the higher tensile strength with minimized ductility of the pulsed TIG welds. Furthermore, results on the hardness are in concurrence with the tensile experiments, as the zone for fusion has a higher hardness than the base metal. Corrosion behavior of the parent metal and welded specimen was analyzed using a potentio-dynamic polarization technique. The electrochemical behavior of base material and weld samples confirmed that the overall corrosion resistance is better in parent material than the other zones.

Influence of the mode of laser welding parameters on microstructural morphology in thin sheet Ti6Al4V alloy

Abstract: The transients and gradients generated due to localized heating and cooling by a laser source governs the thermal-metallurgical-mechanical performance of the welded structure. An attempt is made to investigate the significance of the pulse parameter over continuous mode in Yb-fiber laser welding of 800 μm thin Ti6Al4V alloy. The full-depth of penetration with minimum weld width is achieved at the lowest heat input of 12 J/mm for the pulse laser. The behaviour pattern of the thermal history is critically assessed with the aid of FE based heat transfer model and corresponding relation with evolved microstructural morphologies are systematically investigated. Several stages of transformation occurred in the weld zone such as α -phase dissolution, β -transus in the heating cycle, and diffusionless β → α ' / α martensitic transformation during the cooling cycle are well explained by thermal history. A relatively high amount of α ' -martensite is observed in the pulse laser welding whereas the transformed β -phase fraction gradually reduces further away from the weld line. Blocky plate-shaped α ' -martensite within the coarse β -grain boundary is apparent at higher heat input (26–80 J/mm), whereas acicular morphology within the fine β -grain boundary is observed at a low heat input of 12–19 J/mm. The dimensional variation of α ' -lath at the fusion zone has a significant influence on strength. Weld metal containing very fine α ' -lath have comparable strength to that of the base metal. The images of the fractured surface shows the dimples as well as microspores in continuous mode, whereas combinations of large and small dimples are observed for pulse mode of welding. The level of contamination is examined by surface discoloration technique and found to be the highest for the heat input of 80 J/mm.

Finite-element inverse analysis of residual stress for laser welding based on a contour method

Abstract: The thermal effect of laser welding degrades the local material properties, and this inevitably leads to thermal deformation and thermal residual stress in welded joints. In this study, the residual stress distribution of laser-welded Al–Li alloy parts was measured by a combination of the contour method and finite-element simulation. First, the contour deformation of the cutting surfaces of welded parts resulting from the release of residual stress was measured by a coordinator. Then, the reverse contour deformation was applied to the finite-element model as the displacement boundary condition to invert the full-field residual stress of the cutting surface. Furthermore, a thermal/structural sequential coupling analysis method was used to establish a complete three-dimensional finite-element model of a laser-welded plate and calculate the residual stress field, taking the actual weld morphology as the characteristic parameter of the heat source, using the improved conical heat source model of laser welding. The result is consistent with the results of the contour method.