Volumetric Heat Source Model Research Articles

Improvement of welding heat source models for TIG-MIG hybrid welding process

Tungsten inert gas-metal inert gas (TIG-MIG) hybrid welding process is an effective way to improve welding productivity and quality due to advantages of the two processes. Mathematical analysis is crucial to fundamentally understand this synergetic welding process. In this study, based on experimental visualization of arc behaviors, some assumptions are proposed to deduce adaptive plane and volumetric heat source models separately for each involved welding method first. The influence of torch angles on distribution of temperature and geometry of weld bead are calculated and compared with experimental results. It shows that this developed algorithm of heat source can be employed to accurately predict welding process whether the electrode gun is slanted backward or forward to the direction of welding. Then TIG-MIG hybrid welding process is simulated and analyzed without considering the attractive or repulsive force of two arcs. The characteristic of TIG-MIG welding process is discussed compared to single MIG. It lays the foundation for the further research on the interaction of the two arcs during TIG-MIG hybrid welding.

A study on an efficient prediction of welding deformation for T-joint laser welding of sandwich panel Part II: Proposal of a method to use shell element model

I-core sandwich panel that has been used more widely is assembled using high power CO2 laser welding. Kim et al. (2013) proposed a circular cone type heat source model for the T-joint laser welding between face plate and core. It can cover the negative defocus which is commonly adopted in T-joint laser welding to provide deeper penetration. In part I, a volumetric heat source model is proposed and it is verified thorough a comparison of melting zone on the cross section with experiment results. The proposed model can be used for heat transfer analysis and thermal elasto-plastic analysis to predict welding deformation that occurs during laser welding. In terms of computational time, since the thermal elasto-plastic analysis using 3D solid elements is quite time consuming, shell element model with multi-layers have been employed instead. However, the conventional layered approach is not appropriate for the application of heat load at T-Joint. This paper, Part II, suggests a new method to arrange different number of layers for face plate and core in order to impose heat load only to the face plate.

A Coupled Thermo-Hydro-Mechanical Model of Jointed Hard Rock for Compressed Air Energy Storage

Renewable energy resources such as wind and solar are intermittent, which causes instability when being connected to utility grid of electricity. Compressed air energy storage (CAES) provides an economic and technical viable solution to this problem by utilizing subsurface rock cavern to store the electricity generated by renewable energy in the form of compressed air. Though CAES has been used for over three decades, it is only restricted to salt rock or aquifers for air tightness reason. In this paper, the technical feasibility of utilizing hard rock for CAES is investigated by using a coupled thermo-hydro-mechanical (THM) modelling of nonisothermal gas flow. Governing equations are derived from the rules of energy balance, mass balance, and static equilibrium. Cyclic volumetric mass source and heat source models are applied to simulate the gas injection and production. Evaluation is carried out for intact rock and rock with discrete crack, respectively. In both cases, the heat and pressure losses using air mass control and supplementary air injection are compared.

A study on an efficient prediction of welding deformation for T-joint laser welding of sandwich panel PART I : Proposal of a heat source model

The use of I-Core sandwich panel has increased in cruise ship deck structure since it can provide similar bending strength with conventional stiffened plate while keeping lighter weight and lower web height. However, due to its thin plate thickness, i.e. about 4~6 mm at most, it is assembled by high power <TEX>$CO_2$</TEX> laser welding to minimize the welding deformation. This research proposes a volumetric heat source model for T-joint of the I-Core sandwich panel and a method to use shell element model for a thermal elasto-plastic analysis to predict welding deformation. This paper, Part I, focuses on the heat source model. A circular cone type heat source model is newly suggested in heat transfer analysis to realize similar melting zone with that observed in experiment. An additional suggestion is made to consider negative defocus, which is commonly applied in T-joint laser welding since it can provide deeper penetration than zero defocus. The proposed heat source is also verified through 3D thermal elasto-plastic analysis to compare welding deformation with experimental results. A parametric study for different welding speeds, defocus values, and welding powers is performed to investigate the effect on the melting zone and welding deformation. In Part II, focuses on the proposed method to employ shell element model to predict welding deformation in thermal elasto-plastic analysis instead of solid element model.

A study on an efficient prediction of welding deformation for T-joint laser welding of sandwich panel PART I : Proposal of a heat source model

The use of I-Core sandwich panel has increased in cruise ship deck structure since it can provide similar bending strength with conventional stiffened plate while keeping lighter weight and lower web height. However, due to its thin plate thickness, i.e. about 4~6 mm at most, it is assembled by high power CO2 laser welding to minimize the welding deformation. This research proposes a volumetric heat source model for T-joint of the I-Core sandwich panel and a method to use shell element model for a thermal elasto-plastic analysis to predict welding deformation. This paper, Part I, focuses on the heat source model. A circular cone type heat source model is newly suggested in heat transfer analysis to realize similar melting zone with that observed in experiment. An additional suggestion is made to consider negative defocus, which is commonly applied in T-joint laser welding since it can provide deeper penetration than zero defocus. The proposed heat source is also verified through 3D thermal elasto-plastic analysis to compare welding deformation with experimental results. A parametric study for different welding speeds, defocus values, and welding powers is performed to investigate the effect on the melting zone and welding deformation. In Part II, focuses on the proposed method to employ shell element model to predict welding deformation in thermal elasto-plastic analysis instead of solid element model.

Melting process and mechanics on laser sintering of single layer polyamide 6 powder

In this paper, polyamide 6 single layer laser sintering process was investigated. A moving volumetric heat source model that involves the energy function morphology was simulated, and the temperature field distribution of a single layer of polyamide 6 powder was investigated using the finite element method, with different scan space and preheating temperature. The sintering processes were conducted on several batch of single layer with original thickness of 0.5 mm, which consists of 80 mesh polyamide 6 powders. Then the percentage of porosity was calculated and tensile stress of was tested. The results indicate that the scanning space strongly affect the percentage of porosity. For example, the percentage of porosity reaches 45.1 % when the scanning space increases from 0.25 to 0.45 mm. The results also indicate that the preheating and holding temperature strongly affect the mechanical properties. For example, the max tensile test stress increased from 2.04 to 4.10 MPa when the temperature rises from 50 to 150 °C, while the percentage of porosity shows very limited shift, reducing by only 9.54 %.

Numerical analysis of the heat and fluid flow in a weld pool with a dynamic keyhole

In keyhole plasma arc welding, the interaction between the keyhole and the weld pool occurs when the keyhole appears inside the weld pool. The change of the keyhole shape and dimensions has direct effect on the heat and fluid flow in the weld pool, and the latter will also influence the keyhole geometry. In this study, the coupled behaviors of weld pool and keyhole are treated to develop a three-dimensional model for analyzing the heat and fluid flow inside a weld pool with a dynamic keyhole. In view of the characteristics of PAW process, a combined volumetric heat source model (double-ellipsoid plus conic body source) is established, and one of its distribution parameters is adjusted dynamically with the variation of the depth of keyhole. The physical phenomena, such as the weld pool development, the keyhole formation, the evolution of fluid flow and thermal field, the full-penetration of the test plates, and the transformation from a blind keyhole to an open keyhole, are quantitatively analyzed. The numerical results reveal the regularity of fluid flow in weld pool with a keyhole. The calculated keyhole shape and the fusion zone of plasma arc welds are compared with the experimental measurements. Both agree with each other generally. It lays foundation for optimizing the welding process parameters and improving the stability of plasma arc welding process.

Residual Stress and Bead Profile Analysis of Pulsed Nd:YAG Laser Lap Welding

The weld bead temperature distribution and shape during pulsed Nd:YAG laser lap welding are studied. A volumetric heat source model is derived to include the surface flux and the keyhole heat transfer effects in the pulsed laser lap welding process. The proposed pulsed laser heat transfer mode is employed in a simulation with the commercial finite element software Marc. The numerically computed results of the weld pool dimensions are compared with the experimental results. The comparison shows a good agreement between the simulated and measurement results, indicating that the proposed model is feasible. The results reveal that the pulse duration and spot pitch have considerable influence on the temperature field distribution and the residual stress distribution.

Monte Carlo simulation of grain growth in heat-affected zone of 12 wt.% Cr ferritic stainless steel hybrid welds

The grain structure in the heat-affected zone (HAZ) of 12wt.% Cr ferritic stainless steel under laser+pulsed gas metal arc welding (laser+GMAW-P) hybrid welding was studied experimentally and theoretically. First, an adaptive combined volumetric heat source model was used to calculate the temperature field in laser+GMAW-P hybrid welding. Then a three dimensional Monte Carlo method using the computed thermal cycles was applied to simulate the evolution of grain structure in the hybrid welding HAZ of the ferritic stainless steel. The predicted fusion zone geometry and the mean grain size agree with the measured results for different welding conditions in laser+GMAW-P hybrid welding of ferritic stainless steel. The simulate results showed that in hybrid welding the width of coarse grain heat-affected zone and the grain structure varied dramatically along the fusion line direction, the maximum mean grain size near the top surface was much larger than that close to the weld root.

Theoretical investigations into heat transfer in laser-welded steel sheets

This study contains mathematical modelling and numerical analysis of heat transfer in laser beam welding process. The temperature field was obtained on the basis of numerical solution into unsteady heat transfer equation with convective term and volumetric heat sources taken into account. Volumetric heat source model describing laser beam power distribution in combined truncated cone–cylinder volume was developed. Due to the wide range of temperatures appearing in the process latent heat of fusion, evaporation as well as latent heat of phase transformations in solid state were taken into account in the solution algorithm. On the basis of developed numerical algorithms an analysis of heat transfer in laser butt-welded steel sheets as a three-dimensional initial-boundary problem was performed.

FINITE ELEMENT ANALYSIS OF TEMPERATURE FIELD IN LASER+GMAW HYBRID WELDING FOR T-JOINT OF ALUMINUM ALLOY

T-welded structures of aluminum alloy are increasingly used in automotive,railway vehicles,aerospace and bridges.However,compared with the simple joint,the T-joint of aluminum alloy is more difficultly welded due to its complex temperature distribution and fluid flow mode in the weld pool.Whether using laser welding or the conventional arc welding process,aluminum alloy T-wleded joint is more prone to welding defects such as crack,pore,undercutting,joint softening, and so on.As a promising joining technology,laser+gas metal arc welding(laser+GMAW) hybrid welding not only combines the advantages of laser welding with those of GMAW,but also overcomes their shortcomings,thus having great potential to achieve high efficiency and high quality welding of aluminum alloy T-joint.So far,however,there is a lack of fundamental investigations involving mathematical modelling and understanding of the hybrid welding process of aluminum alloy T-joint.As key factors determining the weld quality,thermal field has a significant influence on microstructure and properties of T-welded joint.In this work,using the numerical simulation method,the temperature distribution in laser+GMAW hybrid welding of aluminum alloy T-joint was studied.Considering the influence of joint form on welding heat flux,an adaptive combined volumetric heat source model for laser+GMAW hybrid welding for T-joint is developed based on macroscopic mechanism of heat transfer. The arc heat flux and heat content of overheated droplet are described using an double ellipsoid body heat source model,and the laser power is regarded as peak density exponentially increasing-conic body distribution.To take into account the effect of inclination of welding gun on heat flow distribution in T-joint welding,the heat source model is rotated by way of coordinate transformation,thus deducing the formula of combined heat source model suitable to hybrid welding for T-joint.The built model is used to calculate the geometry and dimensions in laser+GMAW hybrid both-sided welding for T-joint of aluminum alloy under different welding conditons,and the simulated resluts agree well with the experimental ones,which indicates the accuracy and applicability of the combined model. Besides,the thermal cycles at different positions in hybrid welding for T-joint of aluminum alloy are computed,and the characteristics of the thermal cycles are analyzed,which will lay the foundation for prediction of microstructure and properties of welded joint.

MODELLING FLUID FLOW AND HEAT TRANSFER PHENOMENA IN KEYHOLING STAGE OF PLASMA ARC WELDING

Because of its high gas velocity and heat input,plasma arc welding(PAW) can penetrate thicker workpieces with a single pass because PAW can operate in the keyhole mode. Compared with electron beam and laser beam welding,keyhole PAW is more cost effective and more tolerant of joint preparation,so that it is widely used in manufacturing structures with medium thickness. However,the keyhole establishment and sustainment during the initial stage of PAW process, i.e.,the keyholing process,has a critical effect on the process stability and the weld quality.Thus, modelling and simulating of the keyholing process and its influence on fluid flow and heat transfer in keyhole PAW process is of great significance to completely understand the process mechanism.With considering the interaction between weld pool and keyhole,a three dimensional transient model of fluid flow and heat transfer in weld pool is developed for numerical analysis of keyholing process in PAW.The volume of fluid method(VOF) is used to track the keyhole shape and size.The latent heat and momentum sink due to solidifying and melting are dealt with by enthalpy-porosity technique. Considering the larger ratio of PAW weld depth to width,a combined volumetric heat source model is established,and one of its distribution parameters is adjusted dynamically with the variation of keyhole depth.The evolution of fluid flow and thermal field in weld pool,and the keyholing process are quantitatively analyzed on the stainless steel plates of thickness 8 mm.The feature of fluid flow in weld pool is revealed.The predicted keyhole size at bottom side of workpiece and fusion line at transverse cross-section of welds agree with the experimentally measured results.

Finite-element thermal analysis of laser welding of galvanized high-strength steel in a zero-gap lap joint configuration and its experimental verification

A three-dimensional (3D) finite element (FE) model is applied to predict the temperature evolution in the laser welding of galvanized high-strength steels in a zero-gap lap joint configuration. A rotary Gaussian volumetric heat source model is introduced to simulate the laser energy input. The temperature data measured by thermocouples are used to verify the boundary conditions of the thermal model. The experimentally verified finite element model is then used to predict the weld pool size and the area of vaporized zinc at the faying surface. A feature parameter that represent the ratio of the zinc coated vaporized area to the area of molten pool at the faying surface has been established. The acquired values of the feature parameters could be used to numerically estimate the weld quality with respect to the different laser welding parameters. A series of tensile shear tests for the lap-jointed DP980 steel coupons has been performed correspondingly. The experimental results show that failure load of lap joint of galvanized DP980 steel increases with a lower welding speed or higher laser power. It is found that there is a potential relationship between the area of vaporized zinc at the faying surface of lap joint of galvanized steel and the final weld quality, and a decrease in the welding speed or increase in laser power mitigates to a certain extent the defects caused by zinc vapor in the weld bead.

Análisis térmico de soldadura GTAW sobre placa de acero AISI 316L empleando el método de elementos finitos

En el presente trabajo se realiza la modelación térmica de soldadura GTAW sobre placa de acero inoxidable AISI 316L. Se analizan los aspectos teóricos más relevantes a considerar durante la ejecución de un análisis de este tipo. En la simulación se emplea un software de análisis por elementos finitos de uso general y se enriquece una metodología, previamente desarrollada, para la modelación en 3D no lineal transitoria del proceso de soldeo. En dicha metodología se implementa una subrutina en lenguaje APDL, programada con el modelo volumétrico de fuente calor de doble elipsoide, en un sistema de coordenadas cartesiano. Se analiza la influencia del paso de tiempo seleccionado sobre los resultados de la simulación. Finalmente se valida la metodología enriquecida, en base a la correlación de los resultados del modelo respecto a los resultados experimentales.

Adaptive volumetric heat source models for laser beam and laser + pulsed GMAW hybrid welding processes

Laser + pulsed gas metal arc welding (GMAW) hybrid welding process is an attractive joining technology in industry due to its synergy of the two processes. It is of great significance to conduct fundamental investigations involving mathematical modeling and understanding of the hybrid welding process. In this study, an adaptive heat source model is first developed for laser beam welding. Through combining the ray-tracing method with the keyhole profile determination technique based on the local energy balance, the keyhole shape and size are calculated and correlated to the distribution parameters of the volumetric heat source model. Then, thermal action characteristics in laser + pulsed GMAW hybrid welding are considered from viewpoint of macro-heat transfer, and a combined volumetric heat source model for hybrid welding is developed to take consideration of heat input from laser, pulsed gas metal arc, and overheated droplets. Numerical analysis of thermal conduction in hybrid welding is conducted. The shape and size of fusion zone and weld dimension in the quasi-steady state are calculated for various hybrid welding conditions, which have a fair agreement with the experimental results.

Numerical analysis of temperature profile and weld dimension in laser+pulsed gas metal arc welding hybrid welding

Increasing the number of process parameters allows a flexible adjustment of the laser+pulsed gas metal arc welding (GMAW) hybrid welding process; however, it places higher demands on the technology development and the process optimization. It is therefore important to conduct fundamental investigations involving mathematical modelling to understanding the hybrid welding process. In this study, thermal action characteristics in laser+pulsed GMAW hybrid welding are considered from the viewpoint of macro heat transfer, and two kinds of adaptive combined volumetric heat source models are developed for numerical analysis of heat flow in the hybrid welding. Based on the appropriate description of heat input from a laser beam, pulsed GMAW and heat content of overheated droplets, the adaptive heat source models are developed for laser+pulsed GMAW hybrid welding through combining three contributions to the heat density, i.e. double elliptic planar distribution of arc heat for both peak and background durations, uniformed distribution of droplet heat content in a double ellipsoid body, and logarithm or parabolic curve-rotated body distribution of laser beam with linearly enhanced peak density along the central axis. The interaction between laser and arc is taken into account indirectly by appropriately varying the distribution parameters of heat sources. The shape and size of the weld pool and the weld reinforcement in the quasi-steady state are calculated for three hybrid welding conditions, which show a fair agreement with the experimental results. The temperature profiles, cooling rates, and widths of the heat-affected zone in hybrid welding under different welding conditions are also computed and compared with those of pulsed GMAW, and the differences among them are discussed and explained.

STUDY ON THE THERMAL LAG EFFECT OF KEYHOLE IN CONTROLLED PULSE KEY--HOLING PLASMA ARC WELDING

The behaviors of keyhole formation determine to a great degree the deep penetration welding process and the weld depth-width ratio in welding of medium and large thickness plates. Obtaining a deep insight into the dynamic keyholing process in the weld pool in plasma arc welding is of great significance for widening the process parameter-widow and improving the process robustness as well as weld quality stability. Based on the Level-Set theory, a numerical model is developed to describe the keyhole behaviors in the weld pool in stationary plasma arc welding (PAW), and employed to track the evolution process of keyhole boundary. In this paper, the fluid flow fields in both the plasma and the keyhole regions are constructed according to the experimental and simulation data in the previous literatures. The combined volumetric heat source model is used to numerically analyze the transient temperature field and then to determine the weld pool geometry. The algorithm of Level-Set theory combined with the transient thermal conduction model is used to determine the evolution of both keyhole and weld pool geometry at different time steps. The dynamic information on the weld pool and keyhole geometry and sizes under a few process conditions is obtained by the numerical simulation of keyholing phenomena in welding of stainless steel plates. It is found that a complete keyhole is established at 2.7 and 2.5 s for the current levels of 170 and 180 A, respectively. During the stationary PAW process, the cross-section geometry of keyhole transforms from U-shape at initial stage to V- shape later and shows finally a hyperbola shape after a complete keyhole is formed. The numerical analysis of keyhole formation is verified by measuring the efflux plasma voltage signals at the moment of keyholing.

Laser pulse heating and phase change process: A comparison of volumetric heat source models

In the laser heating process, irradiated energy is absorbed on the surface skin of the substrate material. This results in excess computational efforts due to grid arrangement in the irradiated region and the remaining region in the solution domain due to the fine grid spacing in the irradiated region. However, consideration of the surface heat source minimizes this problem, since it does not require fine grid spacing in the skin of the surface. In the present study, laser heating and phase change in the irradiated region are modelled. The laser heating situation is modelled after considering the volumetric heat source incorporating an absorption process (Beer—Lambert's Law) and the surface heat source model. The temperature distribution, melt layer, and solid—liquid zone (mushy zone) formed in the heated region are predicted for the volumetric and surface heat source heating models. This study is extended to include the influence of spatial distribution of the laser pulse on temperature rise and phase change processes. It is found that the surface heat source model predicts higher values of temperature than those corresponding to the volumetric heat source in the surface vicinity. As the depth increases, temperature distributions predicted from both models become almost identical. In addition, the melt layer thickness and mushy zone predicted from both models are almost identical.

On the Modeling of Laser as a Moving Distributed Volumetric Heat Source for Laser Cutting Simulation

Laser processing of sheet metals (such as cutting or welding) involves heating of the substrate material by laser beam with temperature in the substrate materials reaching the melting temperature. Therefore, such laser processes consist of heating, melting and solidification of the substrate metal. An important topic in laser processing simulation is the modeling of the heat source (distribution of heat input). The interaction of laser beam with a molten metal pool is a complex physical phenomenon that still cannot be modeled rigorously. Laser beam as a heat source causes highly non-linear temperature distribution across the cut or weld and various heat source modeling approaches have been reported in the literature. In general, the distribution of heat input can be classified as superficial and volumetric. In this paper, a moving volumetric heat source model is presented. Using the proposed heat source model, laser cutting process is simulated and residual stresses generated in the cutting region are predicted.

Thermomechanical analysis for Laser+GMAW-P hybrid welding process

A three-dimensional transient model is developed for numerical analysis of the coupled heat conduction and residual stress and distortion phenomena in laser beam welding plus pulsed gas metal arc welding (GMAW-P) hybrid process. Based on an adaptive combined volumetric heat source model, the transient temperature field is computed, and then the thermal analysis data are utilized as input of subsequent thermomechanical analysis. The residual stress distributions of test plates are calculated for both GMAW-P and Laser + GMAW-P hybrid welding processes. Experiments are made to measure weld dimensions and residual stresses. The weld residual stress distributions of GMAW-P and Laser + GMAW-P hybrid welding are compared under the conditions of the nearly same weld penetrations. The predicted results of the weld shapes and residual stresses are in agreement with the experimental measurements. The simulated results show that the residual stresses and distortion of test plates made by Laser + GMAW-P hybrid welding is lower than those by GMAW-P.