Proposed Heat Source Research Articles

Experimental and numerical investigation of a gouging heat source model and gouging/welding residual stress and deformation

Gouging is generally used for groove preparation of multi-pass welding and repair welding. Arc gouging is one of the most efficient gouging methods which utilizes an arc heat source and a trailing air blower. To date, the heat effects from the arc gouging are not clear especially when it is coupled with subsequent welding. In this study, a new heat source model considering the blown molten metal as well as the increased heat transfer by the blowing air was proposed. A 3D finite element model for gouging and welding was employed to analyze the thermal behavior, welding distortion and residual stresses. The transient temperature predicted by the proposed heat source model has good agreement with thermocouple measurements, both featuring multiple peaks in the thermal profile. The predicted distortion and residual stress due to gouging show similar trends as those in experiment. The proposed analysis approach can provide necessary insights, with high accuracy, in the evaluation of welding distortion and residual stresses in practical manufacturing.

Study of Plasma Arc Underwater Penetration Processes in Steel Structures Covered with Non-conductive Materials

Recently, plenty of offshore platforms have faced decommissioning. Offshore steel structures covered with rust, mud, and barnacles require suitable cutting methods for their removal. The non-transferred plasma arc is highly efficient in cutting both conductive and non-conductive materials and is expected to play an important role in offshore demolition operations. This work aims to investigate the underwater penetration process of non-transferred plasma arcs on steel structures covered with non-conductive substances. The workpiece consisted of a 5 mm cement layer and a 10 mm steel plate, whose removal mass and hole morphology were recorded at several moments. It was found that the hole of the cement layer was larger than that of the steel plate, while the mass removal rate of the steel plate was much higher than that of the cement layer. The hole in the cement layer gradually changes from an inverted cone to a drum, and the hole in the steel plate is always an inverted cone. A combined heat source model with time variation is proposed based on the experimental results, and a 2D axisymmetric simulation model is developed. The error between the simulated and experimental hole dimensions was within 3.1%, proving that the proposed heat source model can reflect the heat process of the workpiece well.

Three-dimensional annular heat source for the thermal simulation of coaxial laser metal deposition with wire

Coaxial laser metal deposition with wire (LMD-w) is an innovative additive manufacturing technology in which a wire is coaxially fed through the center of a hollow laser beam into a laser-induced melt pool. This special configuration results in a direction-independent process, which facilitates the manufacturing of thin-walled metal components at high deposition rates. However, laborious experimental test series must be conducted to adjust the process parameters so that the substrate and the part do not overheat. Therefore, models are needed to predict the resulting temperature field and melt pool dimensions efficiently. This paper proposes a finite element simulation model using an innovative heat source, which considers the unique intensity distribution of the annular laser spot. The heat source parameters were calibrated experimentally based on fusion lines obtained from metallographic cross sections of aluminum alloy samples (AA5078 wire and AA6082 substrate). Subsequently, the temperature distribution in the substrate plate was measured by means of thermocouples to validate the developed model. It was shown that the proposed heat source replicates the heat input accurately. With the presented model, essential features for process development, such as the temperature field and the melt pool dimensions, can be reliably predicted. The model contributes to a better understanding of the LMD-w process and facilitates an efficient process development in future research work as well as for industrial applications. Key words: thermal simulation, annular laser spot, heat source, laser metal deposition, coaxial wire feeding, directed energy deposition

Thermal Elasto-Plastic Analysis of Large Steel Plate Cutting Using Layered Shell Elements

In this study, plasma cutting deformation of a large plate was simulated by applying a thermal elasto-plastic analysis technique using a layered shell elements. First, plasma cutting heat source model was proposed based on double Gaussian distribution model. And the proposed heat source model was compared with the experimental results through heat source parameter tuning. In addition, by applying the adaptive element division technique, we increase the heat flux input accuracy for the heat source concentration in a cutting area. And by applying the element de-activating technique, it was possible to simulate the melted part extinguished by cutting. Thermal elasto-platsic analysis of cutting of large plates was performed using the developed heat source model. To this end, layered shell element was used, and heat flux was applied to each layer of the layered shell element to improve the accuracy of analysis. The analysis result was compared with the experimental result to confirm the analysis accuracy. Through the analysis results, the occurrence of rotational deformation and bending deformation that affect the dimensional quality of large plates was verified, and its validity was reviewed by comparing them with the experimental measurement results.

A Temperature-Dependent Heat Source for Simulating Deep Penetration in Selective Laser Melting Process

Numerical methods for simulating selective laser melting (SLM) have been widely carried out to understand the physical behaviors behind the process. Numerical simulation at the macroscale allows the relationship between input parameters (laser power, scanning speed, powder layer thickness, etc.) and output results (distortion, residual stress, etc.) to be investigated. However, the macroscale thermal models solved by the finite element method cannot predict the melt pool depth correctly as they ignore the effect of fluid flow in the melting pool, especially in the case of the presence of deep penetration. To remedy this limitation, an easy-implemented temperature-dependent heat source is proposed. This heat source can adjust its parameters during the simulation to compensate for these neglected thermal effects related to the fluid flow and keyhole, and the heat source’s parameters become fixed once the temperatures of the points of interest become stable. Contrary to the conventional heat source model, parameters of the proposed heat source do not require a calibration with experiments for each process parameter. The proposed model is validated by comparing its results with those of the anisotropic thermal conductivity method and experimental measurements.

A New Heat Source Model for Keyhole Mode Laser Welding

Abstract Laser welding is a common technique for joining metals in many manufacturing industries. During welding, a weld gun traverses the interface of the parts to be joined causing them to melt, fuse, and solidify when the temperature decreases, thus joining the parts. Due to the heat input and the resulting melting and solidification, the parts deform causing residual distortion and residual stresses. To assure the geometrical and functional quality of the product, computational welding mechanics (CWM) is often employed in the design phase to predict the outcome of different design proposals. Furthermore, CWM can be used to design the welding process with the objective of assuring the quality of the weld. However, welding is a complex multiphysical process including the weld pool flow, microstructure dynamics, and structural mechanics. In a design process, it is typically not feasible, for example, to employ fluid simulation of the weld pool in order to predict deformation of a welded assembly, especially if a set of design proposals is under investigation. This is because of the high resolution needed for these fluid simulations in combination with challenges to couple fluid simulation with structural simulation. Instead, what is used is a heat source that emulates the heat input from the melt pool. An example of a heat source is the standard doubled ellipsoid. This heat source has been efficiently used for a large number of welding simulation. However, standard heat sources are typically not flexible enough to capture the fusion zone for deep keyhole mode laser welding. In this study, we presented a new heat source model for keyhole mode laser welding. In an industrial case study, a number of bead-on-plate welds have been employed to compare standard weld heat sources and develop the new heat source model. The proposed heat source is based on a combination of standard heat sources. From this study, it was concluded that the standard heat sources could not predict the observed melted zone for certain industrial application while the new heat source was able to do so. Therefore, the proposed heat source model can be employed to model keyhole mode laser welding, which enables welding simulation of a set of design proposals during the design process in a larger number of industrial cases.

An innovative heat source model for calculating wheel/rail contact temperature

Modeling an accurate and effective heat source is a challenge when describing the thermal behavior of wheel/rail contact, conventionally modeled using Goldak’s heat source. This Gaussian distributed model transplanted directly from the welding field is quite different from the ellipsoidal heat source on the contact patch. An innovative heat source model which reflects the shape and size of contact patch was created based on the energy equivalence in this study. The thermal behavior of the wheel/rail contact was described by using this heat source moving along the rail. Numerical simulations based on a three-dimensional (3-D) finite element (FE) model validate that the proposed heat source model is valid as the relative errors of the peak temperature between the proposed and previously published heat source model are less than 4.45%. The distribution of the contact temperature on the rail surface reflects the shape and size of the contact patch. The semi-ellipsoidal distributed temperature obtained by the proposed elliptic cylinder model is more precise than the Gaussian distributed temperature obtained by the conventionally used Goldak’s model. The proposed elliptic cylinder model has the advantages of simple structure and flexible model variables than the ready-made Goldak’s model in commercial software.

Development of avocado shape heat source model for finite element based heat transfer analysis of high-velocity arc welding process

The mathematical representation of the welding heat source is a critical task as the competence of the model decides the prediction accuracy of the thermal profile of the welded joint. The geometric shape of the moving heat source in fusion welding is a strong function of weld velocity. The elliptical shape conforms to the non-symmetric volumetric energy distribution by a double-ellipsoidal heat source model at low speed. However, the geometric conformity of the elongated weld pool may not be elliptical at relatively high welding speed. In the present work, a new volumetric heat source model, the so-called avocado shape, is proposed, a more realistic approximation of the applied volumetric heat flux and flexible enough to accommodate a wide variation of welding conditions. The proposed model is a modified version of ellipsoidal shape with non-symmetrical energy distribution and unequal axis length in the front and rear parts along the weld velocity vector. Thus, it is different from the double-ellipsoidal model in terms of user-defined parameters and from the egg-configuration model in terms of unequal axis lengths. The avocado shape heat source model is self-consistent since it estimates all the parameters from weld width and depth of penetration. The heat source follows the Gaussian distribution of volumetric energy flux for a moving heat source problem. A finite element based heat transfer analysis is performed using the avocado shape heat source model for gas tungsten arc welding (GTAW) and submerged arc welding (SAW) processes. Compared with the experimentally measured weld dimensions, time-temperature history and microstructure evaluate the prediction accuracy of the proposed heat source model.

Residual stresses evolution during strip clad welding, post welding heat treatment and repair welding for a large pressure vessel

Strip clad welding is a key manufacturing process of large pressure vessel and repair welding was usually used to repair the defects of strip welded structure, which generates very complex residual stresses and have a great effect on structure integrity. Therefore, in this paper, the residual stress distributions and their evolutions during strip clad welding, post-welding heat treatment (PWHT) and repair welding of a large pressure vessel were investigated. A new strip cladding heat source model considering the magnetic pinch effect was proposed. The predicted welding pool size by the proposed heat source model has a good agreement with the experiment results. And the predicted residual stresses in every stage agree very well with those by indention measurement method. The residual stresses after strip clad welding along the through-thickness direction present tension-compression-tension distribution. The PWHT has little eliminating effect on tensile residual stress on cladding layer, and even worsens the discontinuity stress distributions across the interfaces between the clad metal and base metal. After repair welding, local detrimental tensile residual stresses were generated in the repaired zone. A new heat treatment method based on distinct temperature regulation is proposed, which has a good ability to eliminate the tensile stress of the cladding layer to compressive stress and has a good application in engineering.

Probing a novel heat source model and adaptive remeshing technique to simulate laser powder bed fusion with experimental validation

A finite element method based three-dimensional heat transfer model with adaptive remeshing is presented to simulate the building of multiple tracks and layers in laser powder bed fusion of metallic powders with enhanced computational competence. A dimensional analysis is undertaken to define the heat source dimensions as function of laser parameters and properties of alloy powder. The computational model is used to calculate the melt pool cross sections for multiple combinations of laser power and scanning velocities considering multi-track-multi-layer builds of SS316L powder. The computed results are verified extensively with the corresponding experimentally measured ones. The model is utilized further to examine its ability to predict defects such as melt track discontinuity and incomplete fusion between neighboring tracks and layers. Overall, the adaptive remeshing and the proposed heat source expression could significantly enhance the model competence by reducing the computational time and memory while maintaining the accuracy of results in simulating laser powder bed fusion of multiple tracks and layers.

Simulation and analysis of electron beam welding residual stress in thin-walled high-temperature alloy aeroengine structures

Electron beam welding (EBW) is widely used to connect thin-walled high-temperature alloy structures in aeroengines. However, the residual stress and deformation caused by the high-temperature gradients generated during welding would affect the rigidity, dimensional stability, and fatigue resistance of the welded structures. The study reported here used a model combining ellipsoidal and Gaussian rotating body heat sources to undertake a numerical simulation of the temperature and residual stress generated during the EBW process. The model was systematically refined by observing and measuring the molten pool morphology. The error rates for key dimensions determining the molten pool shape were less than 6%. With a microscope-based examination, the energy distribution characteristics were detected by the microstructure analysis of grain type and size in different regions to verify the viability of the heat source model. The residual stress of a butt welding was simulated by the proposed heat source model based on the full consideration of a full-loop thin-walled combustor casing structure and material properties. It was found that the average errors for longitudinal and transverse residual stresses of welded joints and their adjacent areas were 10% and 12%, respectively, by comparing with the experimental results. Simultaneously, the low cycle fatigue life of the welded combustor casing would be decreased by 32% considering the influence of welding residual stress. These conclusions can be used as a basis for studying the integrity of thin-walled welded structures in aeroengines.

A Simplified Calculation Method of Heat Source Model for Induction Heating.

Line heating is used in forming the complex curve plates of ships, and this process is becoming integrated into automated tools. Induction heating equipment has become commonly used in automatic line heating. When applying automated equipment, it is necessary to calculate the relationship between the heating parameters and the temperature field. Numerical methods are primarily used to accomplish the calculations for induction heating. This computation process requires repeated iterations to obtain a stable heat generation rate. Once the heat generation rate changes significantly, a recalculation takes place. Due to the relative position of the coil and plate changes during heating, the grid needs to be frequently re-divided during computation, which dramatically increases the total computation time. In this paper, through an analysis of the computation process for induction heating, the root node that restricts the computation efficiency in the conventional electromagnetic-thermal computation process was found. A method that uses a Gaussian function to represent the heat flux was proposed to replace the electromagnetic computation. The heat flux is the input for calculating the temperature field, thus avoiding the calculation of the electromagnetic analysis during induction heating. Besides, an equivalence relationship for multi-coil was proposed in this paper. By comparing the results of the experiment and the numerical method, the proposed heat source model’s effectiveness was verified.

Estimation of the energy-saving effect of introducing a heat source water network system with single-loop piping utilizing hot spring heat

The introduction of a heat source water network system using heat from hot spring and drainage water for hot water supply in accommodation facilities in hot spring areas is expected to contribute to increased energy efficiency. In this study, the relationship between the temperature of the heat source water and the energy efficiency of heat pump water heaters introduced in hot spring facilities connected to a single-loop heat source water network was analyzed. The primary energy consumption of the heat source water network system was compared with that of a centralized heat supply system by conducting a dynamic simulation using Modelica in the Dymola environment. The yearly average coefficients of performance of the proposed heat source water network and centralized heat supply systems were estimated to be is 4.56 and 3.24, respectively. The seasonal performance of the proposed system was also investigated based on measurement data. In December, it was estimated that the heat source water network system would achieve a 17.6% reduction in primary energy consumption in comparison with the centralized heat supply system. However, in August, it was estimated that the primary energy consumption of the heat source water network system would be 8.9% higher than that of the centralized system.

Laser penetration welding of ship steel EH36: A new heat source and application to predict residual stress considering martensite phase transformation

A weld profile with an accurate distribution of the temperature field is very difficult to simulate, especially for laser penetration welding. In this paper, a peak index increment-double cone heat source model was proposed for laser penetration welding of ship steel EH36. This heat source model was proposed considering the actual weld profile with a geometry of a “V” combined with an inverted “V”; the adjustment parameter was rapidly selected by a simplified linear interpolation method, and the validity of this model was proven by experimental results with an average error of 11.12%. Microstructures of the weld zone, which included ferrite and martensite, were then analysed. Considering the influence of the martensite phase transformation on the strain increment, the distribution of residual stress was simulated by loading the proposed heat source model; the peak values and the high gradient behaviour of the residual stress were intensified. The proposed heat source model was proven to be valid and was applied to predict the residual stress; it can be further used in other welding processes with different metal materials.

Hybrid laser/arc girth welding of 304L stainless steel tubes, part 1 – Pore mitigation, thermal analysis and mechanical properties

Single-pass deep penetration hybrid laser arc welding (HLAW) of 8 mm thick AISI 304L stainless steel tubes with diameter of 280 mm was conducted by using a high-power disk laser combined with a gas metal arc welding machine. First, the effects of scanning speed as well as heat source arrangement on the weld integrity and pore mitigation were investigated. Then, the thermal simulation of the pipe girth welding using ANSYS was carried out to identify the heat distribution throughout the weld. Finally, the microstructural and mechanical attributes of the optimal weld were characterized by scanning electron microscopy (SEM), micro-hardness, and tensile test, respectively. The results showed that high welding speed along with the application of arc-push mode not only generated a visually sound weld but also significantly mitigated the number of keyhole-induced or gas-caused pores. The proposed heat source model for thermal simulation resulted in isotherms that were relatively consistent with the cross section of the weld for the arc-push and arc-pull modes. From the microstructural point of view, it was observed that the amount of δ-ferrite that was decomposed at the arc zone of the weld was proportional to the cooling rate. Therefore, the arc-pull mode with the higher cooling rate led to the formation of a large amount of δ-ferrite. Micro-hardness analysis of the optimal joint revealed neither softening nor hardening inside the FZ and HAZ. The optimal joint showed relatively uniform mechanical properties throughout the weld. Tensile test results showed that all joints failed in base metal confirming a stronger weld compared to the base metal.

Optimized segmented heat source for the numerical simulation of welding-induced deformation in large structures

A segmented heat source is typically used in welding processes with high energy density and high welding speed, such as laser beam welding. In such welding processes, the intense heat source runs through one segment so quickly that the heating processes in each segment could be considered as a segmented heat source with the largest heat flux heating in a short time. It is then possible to use a segmented heat source model to reduce the computation time. In the present work, a feasible and highly efficient optimized segmented heat source, in which cooling time is considered, was proposed to extend its application area. The optimized segmented heat source could be applied to simulate welding processes with various welding speeds and to simulate large structures with high efficiency. The axial strain in welding a pipe with external diameter of 510 mm and wall thickness of 13.5 mm by shielded metal arc welding was measured to verify the accuracy of the proposed heat source. The temperature filed characteristics and the prediction of welding deformation and its distribution were discussed to measure the accuracy and efficiency of the model with the proposed heat source. The model with the optimized segmented heat source predicts the welding deformation with higher accuracy compared to the model with the traditional segmented heat source; however, the efficiency was compromised. Its accuracy is in the range of 10% compared to the model with a double-ellipsoidal heat source, but it is more effective.

Some studies on temperature field during plasma arc welding of thin titanium alloy sheets using parabolic Gaussian heat source model

In this paper, a new volumetric heat source model is developed for predicting the weld bead geometry during plasma arc welding of thin sheets of titanium alloy. Numerical simulations are carried out with the proposed parabolic Gaussian heat source (PGHS) model and already prevailing familiar heat source models namely, conical heat source and modified conical heat source, using finite element package COMSOL. The temperature-dependent material properties for Ti–6Al–4V alloy are considered for performing numerical calculations, which tend to influence the temperature fields while computing. Besides, the effect of trailing gas shielding, latent heat, and radiative and convective heat transfer are taken into account while performing the transient thermal analysis which significantly alters the sensitivity and accuracy of the model. Experimental trials on thin titanium alloy sheets are carried out to enable the validation of the proposed PGHS model. Subsequently, the outcome reveals that the PGHS model is capable and proved its high degree of efficiency in predicting the weld bead geometry more accurately than the existing heat source models. The distribution of heat intensity along the thickness of thin sheet is observed to be parabolic as predicted by the proposed model. The prediction appears to have a good correlation with the experimental result and it is clearly perceptible that the parabolic shape is more reliable and yields greater accuracy of the proposed heat source model.

Development of a new combined heat source model for welding based on a polynomial curve fit of the experimental fusion line

To improve the accuracy of finite element (FE) models used to simulate electric arc and beam-based fusion welding processes, a combined heat source model is proposed. The combined model, termed the polynomial heat source, is based on a Gaussian heat density distribution and employs a disc source to account for surface heating effects while a polynomial equation is used to define the volumetric heat power source. This provides an improved capability for matching the heat source power distribution to fusion zones having variable curvature and also results in a simplified process for defining heat source parameters as only three numerical values need to be determined. To validate the heat source model, fusion zone cross sections, thermal cycles, angular distortion, and residual stresses obtained using SYSWELD were compared to measurements taken from gas metal arc (GMA) welds made on 10-mm-thick high-strength low-alloy (HSLA) plates. Predictions obtained from SYSWELD using double ellipsoid and conical heat source models were also compared. An analysis of the FE predictions obtained from each of the three heat source models showed that the best agreement between simulated and experimental values was achieved using the polynomial model. This is attributed to the fact that heat power distribution can be adjusted at the top and bottom of the fusion zone and results in an improved level of bead cross section matching. It is expected that the proposed heat source model will be applicable for simulating both shallow and deep penetration welding processes where the fusion zone is symmetric about its centerline.

Development of egg-configuration heat source model in numerical simulation of autogenous fusion welding process

The mathematical structure of the heat source models should at least approximately represent the phenomena of real heat source. In fusion welding process, only the surface heat flux fails to realize enhanced heat transfer due to predominant effect of momentum transport of the material within molten pool. The convection in the weld pool can be substituted by apparent volumetric heat source which is unrealistic to define outside the molten pool volume. The key issues to define a decent volumetric heat source are the geometric shape and size, non-symmetric heat density distribution due to moving arc or laser beam. In present work, the egg-configuration volumetric heat source model is proposed to address these issues. The proposed heat source model which is derived from ellipsoid shape provides the non-symmetry heat energy distribution with less number of model parameters and conserves the continuity in profile during moving heat source problem. The general equation of egg-configuration heat source provides the basis of Gaussian distributed disc, hemispherical, and semi-ellipsoidal volumetric heat source models which can be considered as the special cases of proposed heat source model. A 3D finite element based transient heat transfer analyses is conducted using the proposed heat source model for gas tungsten arc welding (GTAW) and diode laser welding processes. The maximum deviation of 10% in weld pool size estimation is observed as compared to experimental results. The time-temperature history confirms the peak temperatures of 940 K and 1060 K, respectively for GTAW and laser welding processes. The amount of the maximum cooling rate predicted for GTAW is 603 K/s which is significantly lower than 11.1 × 103 K/s for laser welding. Overall, the proposed heat source model generalized the applicability over a wide variety of materials and autogenous fusion welding processes.

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.