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Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser
Materials Today: Proceedings xxx (xxxx) xxx
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Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser A. Jayanthi a,b, K. Venkatramanan a, K. Suresh Kumar c,⇑ a
Department of Physics, SCSVMV University, 631 561, India Department of Physics, Jeppiaar Institute of Technology, 631604, India c P.T.Lee Chengalvaraya Naicker College of Engineering and Technology, 631502, India b
a r t i c l e
i n f o
Article history: Received 27 June 2019 Received in revised form 26 July 2019 Accepted 30 July 2019 Available online xxxx Keywords: Pulsed laser welding 316L Stainless steel Moving heat source Heat transfer Melt flow Peclet Number
a b s t r a c t Conductive and convective heat transfers during an autogenous butt joint welding of AISI 316L stainless steel sheets using pulsed Nd: YAG laser have investigated. An FEA based three-dimensional model was developed to receive the transient thermal responses across the weld pool and the predicted results are compared with experimental observations. The heat transfer in the keyhole region, melt flow directions around the keyhole and heat affected zone are computed for consecutive pulse irradiation during laser welding and the same was investigated on far with experiment. The temperature distribution have predicted in terms of inward heat flux and Paclet number to investigate the heat and mass transportation in the weld pool. These results have taken for discussion and found that has a close association with experimental results. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials Engineering and Characterization 2019.
1. Introduction The steel has great versatility made possible by the thermodynamic stability of austenite at high temperatures and the thermodynamic forces that drive austenite transformation to more stable, lower-energy microstructures on cooling. The transformations to lower-energy microstructures are dependent on kinetic factors, such as processing times and temperatures that enhance or restrict diffusion-controlled mechanisms of microstructure change [1–4]. Therefore, the heat and mass transportation of the molten metal during welding need to be addressed that affect the weld pool, microstructure and hence, mechanical properties of the weld produced [5]. In laser welding, melting occurs in the entire joint is mainly depending upon the input power and pulse duration. When penetration increased above certain depth, a thin cylindrical/conical hole produced surrounded by molten metal, known as Keyhole. The keyhole helps to reach maximum penetration depth quickly and ensures good quality of joint by the transportation of mixture of spices/atoms between the parent materials. To achieve the desired microstructure and mechanical properties of a weld joint,
⇑ Corresponding author. E-mail address: [email protected] (K. Suresh Kumar).
controlled and minimum melting and heat-affected zones become necessary to construct strong and repeatable welds. Therefore, it is necessary to have an investigation of the transportation of molten material that takes place in the weld pool due to irradiation of consecutive laser pulses during welding.
2. Literature survey The developments of austenitic stainless steel type 316 have been briefed up to date in [1–3]. The most heat transfer models predicted through numerical methods on laser welding its related process was reviewed up to date [6,7]. From those reviews, it has found that many models have been developed and simulated successfully to investigate the thermal profile, Liquefied flow and heat transfer in the weld pool; for instance, evaluate temperature distribution, residual stress and distortion distributions during the welding process of structural parts of austenitic stainless steel [8]. A 2D model was developed to investigate the interaction of a pulsed laser [6], a 3D analytical solution of the Fourier heat equation for laser-material interaction for spatial and temporal distributed heat source [9], A 3D nonlinear heat transfer model, based on keyhole and a coupled transient thermo-mechanical analysis [10], a transient axis symmetrical solution of the Navier stokes
https://doi.org/10.1016/j.matpr.2019.07.721 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials Engineering and Characterization 2019.
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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equation and the equation of conservation of energy for the GMAW [11], the generalised Navies-stokes equation and electric transportation equation in pulsed current gas tungsten arc (GTAW) welding [12], on the sensitization of butt welds using Gas Tungsten Arc Welding (GTAW) and Laser Beam Welding (LBW based on a moving heat source model [13], Two sets of 3D (with Turbulence and without turbulence) was developed for turbulence on the transportation characteristics associated with typical laser welding process [14], Inhomogeneous thermal-mechanical analysis of 316L butt joint in laser welding [4], a control volume method approach was introduced to discrete governing equation for laser heating of semi-infinite solid with consecutive pulses [5]. Some case studies as butt-welding of plates, circumferential welding of pipes, and multipass welding of plates by various welding methods were briefed [15]. Similarly, many important commercial and free software’s have been developed to simulate the variety of materials processing applications including laser welding based on finite difference, finite element or control volume methods, for instance, newly developed multiphysics software’s like LUMET (for laser ultrasonic metallurgy), open FOAM and COSMOL, those allow to simulate the laser welding and its process dynamics [16]. The survey of works of literature on characteristics of many materials dealt with the fundamental science of heat transfer by experimental, numerical, and analytical works have reviewed. Especially, the survey of presented exclusively on the thermal modelling and its prediction during laser welding in metals including conventional welding process in addition to related processes such as alloying, cladding and surface hardening was given up to the year 2016 in [17–20]. However, it has found that very few reports about the heat and melts flow during pulsed laser welding on AISI 316L stainless steel Sheets. The main objective of this article is to investigate conductive and convective heat transportation during pulsed laser welding of a pair of 2 mm thick AISI 316 L stainless steel sheets and the same is simulated using FEA based COMSOL multiphysics code. The transient thermal responses recorded across the weld joint at the top and bottom, the results are discussed. Temperature distributions predicted as a function of inward heat flux and Paclet number during the pulsed laser welding process to investigate the heat and mass transportation in and around the weld pool. The melt flows in the weld pool and heat transfer in the heat-affected zone investigated for sequential pulses and the results compared experimental observations.
3. Materials and methods A set of AISI 316L stainless steel plates and optimized operating parameters are as followed in [17,21]. Shielding gas (Ar), supplied at the upper and lower faces of the plates at an angle 45°, the nozzle placed behind the laser irradiation, and the operating parameters given in table 1. The techniques followed to prepare the sample for metallographic analysis is given in [22,23].
Table 1 Operating parameters for Nd: YAG pulsed laser welding. Parameters
Values
Units
Average Peak Power Density Focussed beam waist Defocus Position Pulse Duration Frequency Shielding Gas Shielding Gas Flow Rate Welding Velocity Ambient Temperature
2100 451 0 (on the surface) 12 15 Argon 20 180 293
Watts mm Mm mS Hz Ar Litre/min mm/min Kelvin
4. Model definition This model enables an observer can estimate the temperature distributions in either side of the plates along centreline weld, while a moving laser during the welding process with the optimized parameters. The physics of heat conduction principle, boundary conditions, temperature depended thermo-physical properties was took for the consideration as followed in [7,17,18,21–23]. The range thermo-physical properties of 316 L stainless steel are taking from ambient temperature to the melting point. The laser beam profile modelled as a Hermit-Gaussian source assumed that spatially distributed at TEM00 mode and the global parameters used for plotting the heat source given in [22]. The procedure of optimization of operating parameters for autogenous welding of 316L stainless steel through pulsed Nd: YAG laser has discussed in [21]. The effect of shielding gas is not considered. 5. Results and discussions When a real weld started, the weld pool not formed instantly. First, a Gaussian flux distribution heats the specimen to the melting point, then a thin layer of molten metal forms that grow in width and depth with time. Further increase in penetration gives rise to a long, thin cylindrical hole surrounded by molten metal. While increasing depth, the weld pool surface is depressed and stirring is induced by electro-magnetic, buoyancy, vapor pressure, and surface tension gradient forces [14]. Fig. 1 shows the photograph of butt-welded 316 L stainless steel sheets using pulsed Nd: YAG laser. We investigated the heat conduction and convection during welding from the observations of the weld bead and the longitudinal and transverse cross-sectional view of the weld joint. The heat transfer in and around the melt pool are compared with simulated results based on the temperature depended on thermo physical properties. Further, the melt flow in the weld pool had examined in terms of Paclet number using a transient threedimensional model. 5.1. Conductive heat transfer The predicted temperature distributions and the isothermal surface contours during welding are levied on an actual keyhole are presented as shown in Fig. 2. We may witness that the temperature profile is symmetric and maximum on both sides of the sheets from the laser spot diameter on the butt joint. Since it is a similar weld; the isosurface contour has symmetric in forms on both sides from the centre line weld. The temperature distribution in front of the laser beam observed very narrowly and wider behind it, because of, the translating motion of the weld piece relative to the beam at the welding velocity 3 mm/sec. However, the intensity of flux distribution in radial distances is limited to very few millimeters since the 316L stainless steel is showing high resistance to heat. At the same stage, most of the beam energy transferred in the direction of laser irradiation rather than conducts radial. Images of Fig. 2 together reveal the conductive and convective heat transfer took place around the keyhole where temperatures very close to the boiling point, whereas laser pulses, are acting as pierce head of a weld pool that melting and ejecting the molten material behind it during welding. Thus, an oval-shaped weld pool observed and it has been validated with the computed results of the temperature distribution through thermal diffusivity, thermal conductivity, density, and total enthalpy. The Fig. 2 shows that the heat transports radially from the keyhole to the pool edge and results in a wider pool at the top; on the other hand, the inward heat flow in the direction of laser delivers heat to the entire thickness that involved in various phenomenon during laser-
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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Fig. 1. Photograph of AISI 316L stainless steel bead profile in butt joint welding.
Fig. 2. Simulated results of (a) distribution of total inward heat flux and (b) Comparison of simulated isothermal surfaces with actual keyhole of 316 L stainless steel sheets during pulsed laser welding.
material interaction, results in a keyhole and full penetration. The predicted isothermal surface in terms of temperature gradient has a close association with an actual keyhole. 5.2. Convective heat transfer Laser operating parameters such as spot size, traverse speed, and input power increase the heat flow and its sideways effects increase with pulse duration in terms of the penetration depth, therefore 12 ms giving the adequate flux to generate melt pool and the keyhole, since this pulse has longer heating effects [24]. Because of this, the profile of the weld pool is wider at the top and deeper in the bottom than the top. As we discussed above, laser pulses are acting as pierce head of weld pool that melts and ejecting molten material behind it during welding resulting in an oval-shaped weld pool. Under these circumstances, the hydrodynamics in this zone is very complex due to the temperature dependence of thermo physical properties such as thermal conductivity and density. Further, Kerr effect and linear electro-optic effect or Pockel’s effect leads to the strong convectional movements in the melt pool that has significant influence on heat transfer called ‘‘Marangoni effect” [25] as shown in Fig. 3, i.e., cross-sectional image of fusion zone around keyhole imposed on the projected melt flow direction that developed based on literature. The Marangoni numbers are different for conduction and conductive region; the volume of the molten material increased quite large compared with the conduction region. Therefore, ejected molten liquids solidified and formed humps (weld beads) behind the heat source as shown in Fig. 1. However, the characteristics of the conductive
Fig. 3. Longitudinal cross section of fusion zone around keyhole in a butt joint welding of 316L Stainless Steel using pulsed Nd: YAG laser beam levied on the schematic of projected melt flow direction developed based on literature.
and convective heat transfer during the pulse and absence of pulses would be reasonably different; hence, it is interesting to study the melt flow vectors during pulses and intervals between them. To find the convective heat transfer/melt flow, the range of temperature and its oscillations on the entire bead where the laser pulses directly irradiated and where no direct laser pulse irradiation on butt joint are predicted as shown in Fig. 4. These fluctuations result in ups and downs in the formation of weld bead as we can see in Fig. 1 and the predicted temperature fluctuation at the top and bottom joint in Fig. 4 show the butt joint welding took
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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Fig. 4. Computed temperature oscillations during pulsed laser welding (a) on the entire bottom bead where the laser pulses directly irradiated and (b) on the entire top bead where no direct laser pulse irradiation on 316 L stainless steel sheets [22,23].
its threshold time to achieve the keyhole to obtain full penetration. This can be observed from Fig. 4 that the temperature range close to the keyhole surface reaches close to the vaporization point on both the sides that reveals the keyhole reaches full penetration ensures the good quality weld. 5.3. Heat and mass transportation in and around keyhole The combination of a thermal gradient, density gradient, and diffusion takes place near the keyhole wall around the vapor/ plasma by the theory of irreversible thermodynamics. That crosslinking the process of heat conduction in the boundary of vapor/ plasma- inner keyhole wall boundary has a very weak dependence on the thermal gradient of the order of 109 K/s [26]. The surface temperature of the liquid layer is usually higher than the boiling temperature of the molten metal. Therefore, very strong recoil pressure generated due to the strong temperature dependence of the saturated vapor pressure. This high recoil pressure pushes the vapor away from the vaporizing surface and at the same time squeezes the liquid maintaining the thin liquid layer as shown in Figs. 5 and 6, thus convection takes place between the inner and outer wall of the keyhole. Thus, transport of ionization energy in
the plasma with recombination and the consequent release of energy occurring in a thin sheath at the keyhole wall. From here, the heat conduction perpendicular to the liquid region is the dominant heat transfer in the melt pool [27,28]. The heat flux in the liquid side at the solid-liquid interface (Outer keyhole wall) is equal to the heat flux going into the liquid region at the liquid-vapor interface (Inner keyhole wall), it can be obtained by subtracting the vaporization energy from the absorbed laser energy at the liquid-vapor interface. In this simulation work, thermo physical properties of 316 L stainless steel accounted only for the solidstate region, however, the temperature predicted in around the weld joint help us to identify the keyhole and weld pool regions. When the presence of a pulse, cooling occurs that, disperse and decrease the heat conduction and convection motion in the welding pool. The backward flow of the molten material behind the moving keyhole took place due to the thermo-capillary convection. This flow can appear if the rear wall became hotter than the front wall, the front wall cools off faster than the rear wall because of the rear wall in the heated part of the specimen, while in front of the front wall was in the cold metal. The backward motion of the melt begins when the maximum heat flux displaced to the rear wall as shown in Fig. 6. When the laser pulse is drop-down, the supply of
Fig. 5. Simulated keyhole shapes with arrow volume directions of heat flow in AISI 316 L stainless steel sheets in terms of temperature gradient when the presences of pulse (a) side view (b) front view.
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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Fig. 6. Isosurface and heat flux direction in terms of temperature gradient during pulsed laser welding.
heat energy is taken off during the dwelling time of the pulse, then, the hot plasma radiation is the only heat input source for the keyhole wall. However, the temperature of the plasma drops very quickly due to no heat input to the plasma and the heat capacity of plasma is very small [22]. Meanwhile, the heat conduction from the keyhole wall to the surrounding metal remains due to the high aspect ratio and high-temperature gradient, but the orientation of convective heat transfer was perturbed as shown in Fig. 7. Thus, the size and density of the vapor/plasma plume are dependent on several process variables such as laser power, incident angle, and spot size, shielding gas, ambient temperature, and pressure that affect the keyhole depth [27]. As we discussed above, a cycle of conductive and convective heat transfer took place during laser welding, however, in case of pulsed laser welding, the arrival of new pulses at the impacted surface can easily get on to melt pool leads to the local ablation generates a recoil pressure [29]. If this local recoil pressure exceeds the
surface tension, the keyhole wall propagates inside the sample because of sideways melt expulsion. A similar process occurs when this pressure exerted on the rear side. In that case, the melt pool collapse, and encloses the top surface of the keyhole, therefore keyhole prevented. In this way, each translating pulse irradiated on the melt pool and makes propagation inside the preserved keyhole repetitively [30]. 5.4. Heat and mass transportation in terms of Peclet number In the complex structures like a keyhole, where the heat convection is the dominant mechanism for heat transfer, dimensionless numbers such as Marangoni number, Peclet number, Prandtl number, Reynolds number, Nusselt number, and Weber number can help someone to investigate the way of heat and melt flow that takes place in entire weld pool. The Peclet number (Pe), (a product of Reynolds number and Prandtl number), defined as an estimation
Fig. 7. Simulated keyhole shape with arrow volume directions of heat flow in AISI 316 L stainless steel sheets in terms of temperature gradient during welding when the absence of pulse.
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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of convective heat transfer relative to conductive heat transfer, which helps to expose the melt flow through an appropriate thermal gradient near the solid-liquid boundary. Fig. 8 shows the melt flow at the top and bottom regions predicted in terms of Peclet number as a function of surface temperature. It shows a strong correlation between the Paclet number and temperature throughout the volume of the weld pool, the Paclet number value decreases while an increase in temperature. Fig. 9 shows the computed convective heat flux contour in terms of the Peclet number during pulsed laser welding. It shows the compressed contour lines ahead of the heat source has a much higher temperature gradient and rarefactions in the order of the contour lines behind the pulse. It confirms the temperature gradient ahead of the heat source is much higher than that behind it. The laser irradiation spot reach their maximum temperature at the same instant; hence, we have almost constant Paclet number at the keyhole center as can be seen in Fig. 10. However, it has observed some droplet-shaped contours on the same region; it may because of the preservation of the vapor region inside the keyhole depth. The main advantage of the predicted result provided the information on accurate penetration depth. Also, defines the
weld pool and its shape due to the input power of the laser pulse. Where the Pe 1, moderate temperature gradients are observed, simultaneous transport by diffusion and loss by radiation, as shown in Figs. 9 and 10 where density is approximately constant in the radial direction than in the depth direction. If Pe 1, temperature gradients can be very large, and density is highly timedependent, hence, all transportations are dominated by melt flow: convection is insignificant around the keyhole in direction of irradiation compared to the radial direction. After every pulsation during, the initial phase of solidification, islands, or non-uniform boundary layers formed around the keyhole by nucleation, growth, and coalescence of clusters of primary dendritic as contour lines shown in Fig. 10. It shows impressions of solidified layers behind the keyhole front that formed by the pulses irradiated during a time delay between the pulse intervals. The solidified lines of pulses may be similar to the phenomenon of the Vollmar-Weber nucleation mechanism [31]. The dark region observed at top of the keyhole may be the Marangoni flow (surface-tension-driven convection) that brought the ejected molten material flow in sideways around the keyhole as shown; consequently, the molten pool becomes wider and deeper. Since, the
Fig. 8. Computed results of convective melt flow at the top (left) and bottom region (right) in terms of Peclet number during pulsed laser welding.
Fig. 9. Computed convective heat flux contour in terms of Peclet number during pulsed laser welding (Side view).
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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Fig. 10. Simulated contours in terms of convective heat transfer in terms of Paclet number levied on the actual pulse trials of an actual keyhole of 316 L stainless steel sheet joint [18].
surface tension depends on temperature, the large temperature gradient results at top of the weld pool surface in large surface tension gradients. As a result, liquid metal flows from near the keyhole to the edge of the weld pool owing to surface tension thereby enhancing the heat transfer [32]. The remaining part of Fig. 10 shows the simulation of very small and compressive contours lines at keyhole front that has a very high-temperature gradient than the rear side. In addition, it exposes the keyhole geometry with broadened top, trapped vapor region and downward expanded vapor region when the laser pulse was not present. Turbulence observed above downward expanded vapor region because the convection slows down with an increase in penetration depth. Furthermore, the thermal conductivity increases and the effect of convection on the pool shape thus decreases.
6. Conclusion Conductive and convective heat transfers during an autogenous butt joint welding of AISI 316L stainless steel sheets using pulsed Nd: YAG laser have investigated through the simulated results of FEA based three-dimensional models. The discussion made for the results recorded for transient thermal responses across the weld joint at the top and bottom. Temperature distributions predicted as functions of inward heat flux and Paclet number during the pulsed laser welding process to investigate the heat and mass transportation in and around the weld pool. The melt flows in the weld pool and heat transfer in the heat-affected zone have investigated for sequential pulses and the results compared experimental observations. These computed and experimental results have a close association with experimental results. Acknowledgements The authors wish to thank Dr. S. Venugopal, The Director, National Institute of Technology, Nagaland, India and Dr. S. Murugan, Associate Director ‘‘Indira Gandhi Centre for Atomic Research, India for granting permission and providing facilities to carry out the experimental work.
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Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser A. Jayanthi a,b, K. Venkatramanan a, K. Suresh Kumar c,⇑ a
Department of Physics, SCSVMV University, 631 561, India Department of Physics, Jeppiaar Institute of Technology, 631604, India c P.T.Lee Chengalvaraya Naicker College of Engineering and Technology, 631502, India b
a r t i c l e
i n f o
Article history: Received 27 June 2019 Received in revised form 26 July 2019 Accepted 30 July 2019 Available online xxxx Keywords: Pulsed laser welding 316L Stainless steel Moving heat source Heat transfer Melt flow Peclet Number
a b s t r a c t Conductive and convective heat transfers during an autogenous butt joint welding of AISI 316L stainless steel sheets using pulsed Nd: YAG laser have investigated. An FEA based three-dimensional model was developed to receive the transient thermal responses across the weld pool and the predicted results are compared with experimental observations. The heat transfer in the keyhole region, melt flow directions around the keyhole and heat affected zone are computed for consecutive pulse irradiation during laser welding and the same was investigated on far with experiment. The temperature distribution have predicted in terms of inward heat flux and Paclet number to investigate the heat and mass transportation in the weld pool. These results have taken for discussion and found that has a close association with experimental results. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials Engineering and Characterization 2019.
1. Introduction The steel has great versatility made possible by the thermodynamic stability of austenite at high temperatures and the thermodynamic forces that drive austenite transformation to more stable, lower-energy microstructures on cooling. The transformations to lower-energy microstructures are dependent on kinetic factors, such as processing times and temperatures that enhance or restrict diffusion-controlled mechanisms of microstructure change [1–4]. Therefore, the heat and mass transportation of the molten metal during welding need to be addressed that affect the weld pool, microstructure and hence, mechanical properties of the weld produced [5]. In laser welding, melting occurs in the entire joint is mainly depending upon the input power and pulse duration. When penetration increased above certain depth, a thin cylindrical/conical hole produced surrounded by molten metal, known as Keyhole. The keyhole helps to reach maximum penetration depth quickly and ensures good quality of joint by the transportation of mixture of spices/atoms between the parent materials. To achieve the desired microstructure and mechanical properties of a weld joint,
⇑ Corresponding author. E-mail address: [email protected] (K. Suresh Kumar).
controlled and minimum melting and heat-affected zones become necessary to construct strong and repeatable welds. Therefore, it is necessary to have an investigation of the transportation of molten material that takes place in the weld pool due to irradiation of consecutive laser pulses during welding.
2. Literature survey The developments of austenitic stainless steel type 316 have been briefed up to date in [1–3]. The most heat transfer models predicted through numerical methods on laser welding its related process was reviewed up to date [6,7]. From those reviews, it has found that many models have been developed and simulated successfully to investigate the thermal profile, Liquefied flow and heat transfer in the weld pool; for instance, evaluate temperature distribution, residual stress and distortion distributions during the welding process of structural parts of austenitic stainless steel [8]. A 2D model was developed to investigate the interaction of a pulsed laser [6], a 3D analytical solution of the Fourier heat equation for laser-material interaction for spatial and temporal distributed heat source [9], A 3D nonlinear heat transfer model, based on keyhole and a coupled transient thermo-mechanical analysis [10], a transient axis symmetrical solution of the Navier stokes
https://doi.org/10.1016/j.matpr.2019.07.721 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials Engineering and Characterization 2019.
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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equation and the equation of conservation of energy for the GMAW [11], the generalised Navies-stokes equation and electric transportation equation in pulsed current gas tungsten arc (GTAW) welding [12], on the sensitization of butt welds using Gas Tungsten Arc Welding (GTAW) and Laser Beam Welding (LBW based on a moving heat source model [13], Two sets of 3D (with Turbulence and without turbulence) was developed for turbulence on the transportation characteristics associated with typical laser welding process [14], Inhomogeneous thermal-mechanical analysis of 316L butt joint in laser welding [4], a control volume method approach was introduced to discrete governing equation for laser heating of semi-infinite solid with consecutive pulses [5]. Some case studies as butt-welding of plates, circumferential welding of pipes, and multipass welding of plates by various welding methods were briefed [15]. Similarly, many important commercial and free software’s have been developed to simulate the variety of materials processing applications including laser welding based on finite difference, finite element or control volume methods, for instance, newly developed multiphysics software’s like LUMET (for laser ultrasonic metallurgy), open FOAM and COSMOL, those allow to simulate the laser welding and its process dynamics [16]. The survey of works of literature on characteristics of many materials dealt with the fundamental science of heat transfer by experimental, numerical, and analytical works have reviewed. Especially, the survey of presented exclusively on the thermal modelling and its prediction during laser welding in metals including conventional welding process in addition to related processes such as alloying, cladding and surface hardening was given up to the year 2016 in [17–20]. However, it has found that very few reports about the heat and melts flow during pulsed laser welding on AISI 316L stainless steel Sheets. The main objective of this article is to investigate conductive and convective heat transportation during pulsed laser welding of a pair of 2 mm thick AISI 316 L stainless steel sheets and the same is simulated using FEA based COMSOL multiphysics code. The transient thermal responses recorded across the weld joint at the top and bottom, the results are discussed. Temperature distributions predicted as a function of inward heat flux and Paclet number during the pulsed laser welding process to investigate the heat and mass transportation in and around the weld pool. The melt flows in the weld pool and heat transfer in the heat-affected zone investigated for sequential pulses and the results compared experimental observations.
3. Materials and methods A set of AISI 316L stainless steel plates and optimized operating parameters are as followed in [17,21]. Shielding gas (Ar), supplied at the upper and lower faces of the plates at an angle 45°, the nozzle placed behind the laser irradiation, and the operating parameters given in table 1. The techniques followed to prepare the sample for metallographic analysis is given in [22,23].
Table 1 Operating parameters for Nd: YAG pulsed laser welding. Parameters
Values
Units
Average Peak Power Density Focussed beam waist Defocus Position Pulse Duration Frequency Shielding Gas Shielding Gas Flow Rate Welding Velocity Ambient Temperature
2100 451 0 (on the surface) 12 15 Argon 20 180 293
Watts mm Mm mS Hz Ar Litre/min mm/min Kelvin
4. Model definition This model enables an observer can estimate the temperature distributions in either side of the plates along centreline weld, while a moving laser during the welding process with the optimized parameters. The physics of heat conduction principle, boundary conditions, temperature depended thermo-physical properties was took for the consideration as followed in [7,17,18,21–23]. The range thermo-physical properties of 316 L stainless steel are taking from ambient temperature to the melting point. The laser beam profile modelled as a Hermit-Gaussian source assumed that spatially distributed at TEM00 mode and the global parameters used for plotting the heat source given in [22]. The procedure of optimization of operating parameters for autogenous welding of 316L stainless steel through pulsed Nd: YAG laser has discussed in [21]. The effect of shielding gas is not considered. 5. Results and discussions When a real weld started, the weld pool not formed instantly. First, a Gaussian flux distribution heats the specimen to the melting point, then a thin layer of molten metal forms that grow in width and depth with time. Further increase in penetration gives rise to a long, thin cylindrical hole surrounded by molten metal. While increasing depth, the weld pool surface is depressed and stirring is induced by electro-magnetic, buoyancy, vapor pressure, and surface tension gradient forces [14]. Fig. 1 shows the photograph of butt-welded 316 L stainless steel sheets using pulsed Nd: YAG laser. We investigated the heat conduction and convection during welding from the observations of the weld bead and the longitudinal and transverse cross-sectional view of the weld joint. The heat transfer in and around the melt pool are compared with simulated results based on the temperature depended on thermo physical properties. Further, the melt flow in the weld pool had examined in terms of Paclet number using a transient threedimensional model. 5.1. Conductive heat transfer The predicted temperature distributions and the isothermal surface contours during welding are levied on an actual keyhole are presented as shown in Fig. 2. We may witness that the temperature profile is symmetric and maximum on both sides of the sheets from the laser spot diameter on the butt joint. Since it is a similar weld; the isosurface contour has symmetric in forms on both sides from the centre line weld. The temperature distribution in front of the laser beam observed very narrowly and wider behind it, because of, the translating motion of the weld piece relative to the beam at the welding velocity 3 mm/sec. However, the intensity of flux distribution in radial distances is limited to very few millimeters since the 316L stainless steel is showing high resistance to heat. At the same stage, most of the beam energy transferred in the direction of laser irradiation rather than conducts radial. Images of Fig. 2 together reveal the conductive and convective heat transfer took place around the keyhole where temperatures very close to the boiling point, whereas laser pulses, are acting as pierce head of a weld pool that melting and ejecting the molten material behind it during welding. Thus, an oval-shaped weld pool observed and it has been validated with the computed results of the temperature distribution through thermal diffusivity, thermal conductivity, density, and total enthalpy. The Fig. 2 shows that the heat transports radially from the keyhole to the pool edge and results in a wider pool at the top; on the other hand, the inward heat flow in the direction of laser delivers heat to the entire thickness that involved in various phenomenon during laser-
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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Fig. 1. Photograph of AISI 316L stainless steel bead profile in butt joint welding.
Fig. 2. Simulated results of (a) distribution of total inward heat flux and (b) Comparison of simulated isothermal surfaces with actual keyhole of 316 L stainless steel sheets during pulsed laser welding.
material interaction, results in a keyhole and full penetration. The predicted isothermal surface in terms of temperature gradient has a close association with an actual keyhole. 5.2. Convective heat transfer Laser operating parameters such as spot size, traverse speed, and input power increase the heat flow and its sideways effects increase with pulse duration in terms of the penetration depth, therefore 12 ms giving the adequate flux to generate melt pool and the keyhole, since this pulse has longer heating effects [24]. Because of this, the profile of the weld pool is wider at the top and deeper in the bottom than the top. As we discussed above, laser pulses are acting as pierce head of weld pool that melts and ejecting molten material behind it during welding resulting in an oval-shaped weld pool. Under these circumstances, the hydrodynamics in this zone is very complex due to the temperature dependence of thermo physical properties such as thermal conductivity and density. Further, Kerr effect and linear electro-optic effect or Pockel’s effect leads to the strong convectional movements in the melt pool that has significant influence on heat transfer called ‘‘Marangoni effect” [25] as shown in Fig. 3, i.e., cross-sectional image of fusion zone around keyhole imposed on the projected melt flow direction that developed based on literature. The Marangoni numbers are different for conduction and conductive region; the volume of the molten material increased quite large compared with the conduction region. Therefore, ejected molten liquids solidified and formed humps (weld beads) behind the heat source as shown in Fig. 1. However, the characteristics of the conductive
Fig. 3. Longitudinal cross section of fusion zone around keyhole in a butt joint welding of 316L Stainless Steel using pulsed Nd: YAG laser beam levied on the schematic of projected melt flow direction developed based on literature.
and convective heat transfer during the pulse and absence of pulses would be reasonably different; hence, it is interesting to study the melt flow vectors during pulses and intervals between them. To find the convective heat transfer/melt flow, the range of temperature and its oscillations on the entire bead where the laser pulses directly irradiated and where no direct laser pulse irradiation on butt joint are predicted as shown in Fig. 4. These fluctuations result in ups and downs in the formation of weld bead as we can see in Fig. 1 and the predicted temperature fluctuation at the top and bottom joint in Fig. 4 show the butt joint welding took
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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Fig. 4. Computed temperature oscillations during pulsed laser welding (a) on the entire bottom bead where the laser pulses directly irradiated and (b) on the entire top bead where no direct laser pulse irradiation on 316 L stainless steel sheets [22,23].
its threshold time to achieve the keyhole to obtain full penetration. This can be observed from Fig. 4 that the temperature range close to the keyhole surface reaches close to the vaporization point on both the sides that reveals the keyhole reaches full penetration ensures the good quality weld. 5.3. Heat and mass transportation in and around keyhole The combination of a thermal gradient, density gradient, and diffusion takes place near the keyhole wall around the vapor/ plasma by the theory of irreversible thermodynamics. That crosslinking the process of heat conduction in the boundary of vapor/ plasma- inner keyhole wall boundary has a very weak dependence on the thermal gradient of the order of 109 K/s [26]. The surface temperature of the liquid layer is usually higher than the boiling temperature of the molten metal. Therefore, very strong recoil pressure generated due to the strong temperature dependence of the saturated vapor pressure. This high recoil pressure pushes the vapor away from the vaporizing surface and at the same time squeezes the liquid maintaining the thin liquid layer as shown in Figs. 5 and 6, thus convection takes place between the inner and outer wall of the keyhole. Thus, transport of ionization energy in
the plasma with recombination and the consequent release of energy occurring in a thin sheath at the keyhole wall. From here, the heat conduction perpendicular to the liquid region is the dominant heat transfer in the melt pool [27,28]. The heat flux in the liquid side at the solid-liquid interface (Outer keyhole wall) is equal to the heat flux going into the liquid region at the liquid-vapor interface (Inner keyhole wall), it can be obtained by subtracting the vaporization energy from the absorbed laser energy at the liquid-vapor interface. In this simulation work, thermo physical properties of 316 L stainless steel accounted only for the solidstate region, however, the temperature predicted in around the weld joint help us to identify the keyhole and weld pool regions. When the presence of a pulse, cooling occurs that, disperse and decrease the heat conduction and convection motion in the welding pool. The backward flow of the molten material behind the moving keyhole took place due to the thermo-capillary convection. This flow can appear if the rear wall became hotter than the front wall, the front wall cools off faster than the rear wall because of the rear wall in the heated part of the specimen, while in front of the front wall was in the cold metal. The backward motion of the melt begins when the maximum heat flux displaced to the rear wall as shown in Fig. 6. When the laser pulse is drop-down, the supply of
Fig. 5. Simulated keyhole shapes with arrow volume directions of heat flow in AISI 316 L stainless steel sheets in terms of temperature gradient when the presences of pulse (a) side view (b) front view.
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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Fig. 6. Isosurface and heat flux direction in terms of temperature gradient during pulsed laser welding.
heat energy is taken off during the dwelling time of the pulse, then, the hot plasma radiation is the only heat input source for the keyhole wall. However, the temperature of the plasma drops very quickly due to no heat input to the plasma and the heat capacity of plasma is very small [22]. Meanwhile, the heat conduction from the keyhole wall to the surrounding metal remains due to the high aspect ratio and high-temperature gradient, but the orientation of convective heat transfer was perturbed as shown in Fig. 7. Thus, the size and density of the vapor/plasma plume are dependent on several process variables such as laser power, incident angle, and spot size, shielding gas, ambient temperature, and pressure that affect the keyhole depth [27]. As we discussed above, a cycle of conductive and convective heat transfer took place during laser welding, however, in case of pulsed laser welding, the arrival of new pulses at the impacted surface can easily get on to melt pool leads to the local ablation generates a recoil pressure [29]. If this local recoil pressure exceeds the
surface tension, the keyhole wall propagates inside the sample because of sideways melt expulsion. A similar process occurs when this pressure exerted on the rear side. In that case, the melt pool collapse, and encloses the top surface of the keyhole, therefore keyhole prevented. In this way, each translating pulse irradiated on the melt pool and makes propagation inside the preserved keyhole repetitively [30]. 5.4. Heat and mass transportation in terms of Peclet number In the complex structures like a keyhole, where the heat convection is the dominant mechanism for heat transfer, dimensionless numbers such as Marangoni number, Peclet number, Prandtl number, Reynolds number, Nusselt number, and Weber number can help someone to investigate the way of heat and melt flow that takes place in entire weld pool. The Peclet number (Pe), (a product of Reynolds number and Prandtl number), defined as an estimation
Fig. 7. Simulated keyhole shape with arrow volume directions of heat flow in AISI 316 L stainless steel sheets in terms of temperature gradient during welding when the absence of pulse.
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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of convective heat transfer relative to conductive heat transfer, which helps to expose the melt flow through an appropriate thermal gradient near the solid-liquid boundary. Fig. 8 shows the melt flow at the top and bottom regions predicted in terms of Peclet number as a function of surface temperature. It shows a strong correlation between the Paclet number and temperature throughout the volume of the weld pool, the Paclet number value decreases while an increase in temperature. Fig. 9 shows the computed convective heat flux contour in terms of the Peclet number during pulsed laser welding. It shows the compressed contour lines ahead of the heat source has a much higher temperature gradient and rarefactions in the order of the contour lines behind the pulse. It confirms the temperature gradient ahead of the heat source is much higher than that behind it. The laser irradiation spot reach their maximum temperature at the same instant; hence, we have almost constant Paclet number at the keyhole center as can be seen in Fig. 10. However, it has observed some droplet-shaped contours on the same region; it may because of the preservation of the vapor region inside the keyhole depth. The main advantage of the predicted result provided the information on accurate penetration depth. Also, defines the
weld pool and its shape due to the input power of the laser pulse. Where the Pe 1, moderate temperature gradients are observed, simultaneous transport by diffusion and loss by radiation, as shown in Figs. 9 and 10 where density is approximately constant in the radial direction than in the depth direction. If Pe 1, temperature gradients can be very large, and density is highly timedependent, hence, all transportations are dominated by melt flow: convection is insignificant around the keyhole in direction of irradiation compared to the radial direction. After every pulsation during, the initial phase of solidification, islands, or non-uniform boundary layers formed around the keyhole by nucleation, growth, and coalescence of clusters of primary dendritic as contour lines shown in Fig. 10. It shows impressions of solidified layers behind the keyhole front that formed by the pulses irradiated during a time delay between the pulse intervals. The solidified lines of pulses may be similar to the phenomenon of the Vollmar-Weber nucleation mechanism [31]. The dark region observed at top of the keyhole may be the Marangoni flow (surface-tension-driven convection) that brought the ejected molten material flow in sideways around the keyhole as shown; consequently, the molten pool becomes wider and deeper. Since, the
Fig. 8. Computed results of convective melt flow at the top (left) and bottom region (right) in terms of Peclet number during pulsed laser welding.
Fig. 9. Computed convective heat flux contour in terms of Peclet number during pulsed laser welding (Side view).
Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721
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Fig. 10. Simulated contours in terms of convective heat transfer in terms of Paclet number levied on the actual pulse trials of an actual keyhole of 316 L stainless steel sheet joint [18].
surface tension depends on temperature, the large temperature gradient results at top of the weld pool surface in large surface tension gradients. As a result, liquid metal flows from near the keyhole to the edge of the weld pool owing to surface tension thereby enhancing the heat transfer [32]. The remaining part of Fig. 10 shows the simulation of very small and compressive contours lines at keyhole front that has a very high-temperature gradient than the rear side. In addition, it exposes the keyhole geometry with broadened top, trapped vapor region and downward expanded vapor region when the laser pulse was not present. Turbulence observed above downward expanded vapor region because the convection slows down with an increase in penetration depth. Furthermore, the thermal conductivity increases and the effect of convection on the pool shape thus decreases.
6. Conclusion Conductive and convective heat transfers during an autogenous butt joint welding of AISI 316L stainless steel sheets using pulsed Nd: YAG laser have investigated through the simulated results of FEA based three-dimensional models. The discussion made for the results recorded for transient thermal responses across the weld joint at the top and bottom. Temperature distributions predicted as functions of inward heat flux and Paclet number during the pulsed laser welding process to investigate the heat and mass transportation in and around the weld pool. The melt flows in the weld pool and heat transfer in the heat-affected zone have investigated for sequential pulses and the results compared experimental observations. These computed and experimental results have a close association with experimental results. Acknowledgements The authors wish to thank Dr. S. Venugopal, The Director, National Institute of Technology, Nagaland, India and Dr. S. Murugan, Associate Director ‘‘Indira Gandhi Centre for Atomic Research, India for granting permission and providing facilities to carry out the experimental work.
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Please cite this article as: A. Jayanthi, K. Venkatramanan and K. Suresh Kumar, Conductive and convective heat transfer during welding of AISI316L stainless steel using pulsed Nd: YAG laser, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.721