Heat Input Model Research Articles

A Proposal of a Heat Input Model, for Heating Correction, on Welded Steel Structural Members

A Proposal of a Heat Input Model, for Heating Correction, on Welded Steel Structural Members

Track-scale Anisotropic Thermal Material Model as a Viable Substitution In Selective Laser Melting

High temperature gradients and resulting residual stresses are the main sources of defects such as cracks, distortion and fatigue failure is selective laser melting. In this paper, a new material model utilizing anisotropic thermal conduction model and track-scale heat input model is used to predict the melt pool geometry, material state and thermal history during the SLM process of SS316L in a large range of laser parameters. The model takes into consideration both the material state and the issue of phase transition during the track-scale simulation.. The simulated melt pools in the beam-scale and track-scale simulations are compared with experimental measurements in different laser parameters. It is found that the proposed material model is able to maintain accuracy between the beam-scale and track-scale simulations at an average of 5μm regarding melt pool dimensions. Furthermore, It can be inferred that both track-scale and beam-scale models exhibit the capability to provide precise predictions of melt pool geometries when compared to experimental measurements while the former being about 100 times faster than the latter. An average error of 10% was concluded for the material state comparison of the two models while the track-scale model was able to capture the temperature profile and cooling rate accurately in comparison with the beam-scale model.

Single-track study of A20X aluminum alloy fabricated by laser powder bed fusion: Modeling and experiments

Optimum process parameter window for recently developed metal powders used for laser powder bed fusion (LPBF) is strongly correlated with characteristics of each single track and single layer. In the present research, the influence of LPBF process parameters on geometrical and microstructural characteristics of A20X aluminum single tracks was studied theoretically and experimentally. Increasing the laser scan speed led to formation of non-homogenous, irregular single tracks. Moreover, enlarging the powder layer thickness to 120 µm induced balling phenomenon owing to a poor wetting of substrate by melt pool. Experimental measurements indicated that width and depth of melt pools decreased up to 53 % and 68 %, respectively by increasing the scan speed from 500 to 1700 mm/s and layer thickness from 40 to 120 µm. These findings were in agreement with predictions of an exponentially decaying heat input model used in this study. The model took into account thermo-physical properties of the alloy at different states. Bead height was observed to increase with the powder layer thickness, but remain unchanged with the scan speed. The process parameter window (energy density range) resulting in the conduction melting mode for the A20X alloy was found to be more than two times wider compared to conventional LPBF aluminum alloys. For all process parameters, a fine equiaxed grain structure was observed in the vertical section of the single tracks. Enhancing the scan speed and reducing the powder layer thickness resulted in a substantial grain refinement. This was well explained by the simulated cooling rates. In contrast to cast Al-Cu alloys, shape, size, and volume fraction of the second phase precipitates were determined to be independence of cooling rate over the investigated cooling rate range (1.4 × 106 to 7.5 × 106 °C/s).

A macroscale thermal simulation strategy with track-scale resolution for laser powder bed fusion

Numerical modelling is an effective tool to investigate the rapid temperature cycling in laser powder bed fusion. However, the high computational cost limits the modelling of part-scale components due to the significantly different scales between the highly local energy input region and the global field response region. This study presents a macroscale finite element thermal simulation strategy with a track-scale resolution by combining the recently developed characteristic time-based heat input (CTI) model with a currently proposed adaptive mesh re-mapping method. Due to the uniformly distributed power density along the scan direction in the CTI model, the optimized mesh strategy allows one element to capture the temperature solutions over the majority of a scan track except at the two ends. The adaptive mesh re-mapping method is used to map simulated results from a previous scan track model to a current scan track model with a new mesh configuration. This strategy has been implemented for a Ti-6Al-4V part with the dimensions of 20×40×3mm3. The predicted temperature histories agree well with the in-situ measurements at the points in both the printed part and powder bed. Three case studies demonstrate that our strategy can improve computational efficiency between 6 and 30 times.

Development of an Elongated Ellipsoid Heat Source Model to Reduce Computation Time for Directed Energy Deposition Process

Directed Energy Deposition (DED) Additive Manufacturing process for metallic parts are becoming increasingly popular and widely accepted due to their potential of fabricating parts of large dimensions. The complex thermal cycles obtained due to the process physics results in accumulation of residual stress and distortion. However, to accurately model metal deposition heat transfer for large parts, numerical model leads to impractical computation time. In this work, a 3D transient finite element model with Quiet/Active element activation is developed for modeling metal deposition heat transfer analysis of DED process. To accurately model moving heat source, Goldak’s double ellipsoid model is implemented with small enough simulation time increment such that laser moves a distance of its radius over the course of each increment. Considering thin build-wall of Stainless Steel 316L fabricated with different process parameters, numerical results obtained with COMSOL 5.6 Multi-Physics software are successfully validated with experiment temperature data recorded at the substrate during the fabrication of 20 layers. To reduce the computation time, elongated ellipsoid heat input model that averages the heat source over its entire path is implemented. It has been found that by taking such large time increments, numerical model gives inaccurate results. Therefore, the track is divided into several sub-tracks, each of which is applied in one simulation increment. In this work, an investigation is done to find out the correct simulation time increment or sub-track size that leads to reduction in computation time (5–10 times) but still yields sufficiently accurate results (below 10% of relative error on temperature). Also, a Correction factor is introduced that further reduces computation error of elongated heat source. Finally, a new correlation is also established in finding out the correct time increment size and correction factor value to reduce the computation time yielding accurate results.

Numerical modelling of electron beam welding (EBW) of Zhs6u superalloy and its experimental validation

ABSTRACT In this paper, the electron beam welding of Zhs6u superalloy is modelled numerically by a combined heat input model. Also the welding process was performed experimentally on Zhs6u samples to validate the numerical results. The results showed that weld pool shape obtained by the numerical model was in accordance to the experimental analysis of the weld cross section. Also the numerical model was able to predict the susceptibility to formation of liquation cracks in the heat-affected zone based on the study of the temperature gradient during the solidification stage. It was found that by increasing the heat input the temperature gradient decreases and it takes more time for the workpiece to solidify which decreases the susceptibility to liquation crack formation.

An Efficient Track-Scale Model for Laser Powder Bed Fusion Additive Manufacturing: Part 1- Thermal Model

This is the first of two manuscripts that presents a computationally efficient full field deterministic model for laser powder bed fusion (LPBF). A new Hybrid Line (HL) heat input model integrates an exponentially decaying (ED) heat input over a portion of a laser path to significantly reduce the computational time. Experimentally measured properties of the high gamma prime nickel-based superalloy RENÉ 65 are implemented in the model to predict the in-process temperature distribution, stresses, and distortions. The model accounts for specific properties of the material as different phases. The first manuscript presents the HL heat transfer model, which is compared with the beam-scale exponentially decaying model, along with the melt pool geometry obtained experimentally by varying the laser parameters. The predicted melt pool geometry of the beam-scale ED model is shown to have good agreement with experimental measurements. While the proposed HL model exhibits lesser accuracy in predicting the melt pool geometries, it can predict the cooling rates and nodal temperatures as accurately as to the ED model. Moreover, under large time integration steps, the HL model becomes more than 1,500 times faster than the ED model.

An Efficient Track-Scale Model for Laser Powder Bed Fusion Additive Manufacturing: Part 2—Mechanical Model

This is the second of two manuscripts that presents a computationally efficient full-field deterministic model for laser powder bed fusion (LPBF). The Hybrid Line (HL) thermal model developed in part I is extended to predict the in-process residual stresses due to laser processing of a nickel-based superalloy, RENÉ 65. The computational efficiency and accuracy of the HL thermo-mechanical model is first compared to the exponential decaying heat input model on a single-track simulation. LPBF thin-wall builds with three different laser powers and four printing patterns are evaluated in this study and compared with part-scale simulations. The simulations show good agreements with the experimental X-Ray diffraction measured residual stresses. Compared to the laser power, the scanning pattern is demonstrated to have significant effects on residual stresses. Laser scan patterns utilizing short laser paths generate lower tensile stress along the longitudinal direction of the part and higher compressive stress along the build direction.

A path generation method for wire and arc additive remanufacturing of complex hot forging dies

Wire and arc additive remanufacturing (WAAR) technology has become a new solution for hot forging dies repair and remanufacturing. In this study, a path generation method is proposed for WAAR of hot forging dies. At first, a WAAR process of the hot forging die is presented, and considering the characteristics of large welding heat input and complex 3D digital model, the hybrid path planning strategy is confirmed as an appropriate strategy for WAAR. The developed hybrid path generation method for WAAR consists of three main steps: determine the direction of the scan line; divide and fill the internal area; and connect the sub-paths. The relatively optimal scanning direction is determined by calculating the length and inclination angle of each line segment in the contour lines, which reduces the possibility of sharp angles. The internal region is divided according to the position of the selected extreme points, and the path space is adjusted to avoid the occurrence of the unfilled phenomenon. At the stage of sub-path connection, some criteria are proposed to reduce the number of sub-paths. At last, the effectiveness and robustness of the proposed method are validated through the planar deposition experiment and the WAAR process of four damaged hot forging dies.

A characteristic time-based heat input model for simulating selective laser melting

The residual stress induced by rapid temperature cycling in a component during a selective laser melting (SLM) process has a significant detrimental effect on the component’s quality and may cause component failure during or after the printing process. Predicting and therefore actively controlling the residual stress are important research topics in SLM. Numerical simulation is an effective tool to predict the residual stress and distortion throughout the printing process. Goldak heat input model has been commonly used to model the heat source in an SLM process. However, it incurs an unpractically high computational cost when simulating building a part with dimensions in centimetres due to the huge scale gap between the component size and the laser spot size. In the present work, a characteristic time-based heat input (CTI) model has been developed and implemented in a finite-element thermal-mechanical model to significantly reduce the process simulation time while satisfactorily predicting the temperature and residual stress state of SLMed parts. This characteristic time-based heat input model speeds up the computation by applying the integrated energy along the scan path over a characteristic heating time, which is defined as the ratio of the axis of the ellipsoidal heat source in the laser scanning direction to the scanning speed. The characteristic heating time ensures that the peak temperature and subsequent heat transfer of each deposited track can be satisfactorily captured in just one numerical step, respectively. The present model was calibrated and validated by identified isothermal curves on the cross section of a printed Ti-6Al-4V track. Case studies demonstrate that the simulated temperature profile and stress field are in a good agreement with Goldak model. The maximum distortion of a 32 mm single-track sample predicted by CTI model is within 6% of that predicted by Goldak heat input model while CTI’s computational time is just 0.4% of Goldak’s. • A characteristic time-based heat input (CTI) model was developed by integrating Goldak model over a characteristic time. • The characteristic heating time was defined as a ratio of the axis of the heat source to the laser scanning speed. • Residual stress and displacement predicted by CTI model agreed well with those by Goldak model. • CTI model can greatly reduce the simulation time and it is only 0.4% of Goldak model in simulating a 32 mm single-track.

Effect of Weld Heat Input Modeling and the Material Hardening Model for Gas Tungsten Arc Welding Finite Element Analysis

The effects of weld heat input modeling and the material hardening model on gas tungsten arc welding were evaluated by implementing different input models in a finite element analysis (FEA). For the comparison with FEA results, an experiment was conducted on an SA-516 Gr.70 plate specimen with a part-through wall groove. The comparison with the experimental results confirmed that the shape of the heat distribution during the welding process was changed according to each heat input method. For heat input models based on heat flux, the heat distribution of the weld was dependent on the volume and shape of the finite element model. For the welding deformation results, the FEA results obtained when using the temperature boundary condition method differed from the experimental data. The difference in the FEA results of deformation and residual stress according to the heat input method and material kinematic hardening model was analyzed as a complex phenomenon in which the distribution of the equivalent plastic deformation during the welding process and the hardening behavior can be treated differently. Key words: Welding residual stress, Heat input model, Material hardening model, Finite element analysis, Isotropic hardening, Kinematic hardening

Effect of Tool Shoulder and Pin Cone Angles in Friction Stir Welding using Non-circular Tool Pin

In friction stir welding frictional heat is generated by the rotating tool, sliding over the stationary plate along the weld centre. Tool being the only source of heat producing member, its geometrical design influences the heat generation rate. In this present work, effects of variation in tool shoulder and tool pin taper angles on thermal history during joining are analysed. Tools with triangular and hexagonal tool pins are used to understand the influence of tool pin shape on process temperature. An analytical heat input model is developed for tools with non-circular tool pins and a comparative study is carried out between the hexagonal and triangular tool pins on temperature distribution using a three dimensional Matlab model. Proposed model is validated through experimental analysis. Apart from this, regression model based comparative study is carried out on the variation in temperature response to the change in tool pin shape, tool shoulder and tool pin taper angle.

Microstructure, mechanical properties and pitting corrosion of TIG weld joints alternative low-cost austenitic stainless steel grade 216

This present work aims to study 200 series austenitic stainless steel as a replacement for 300 series austenitic stainless steel. This is due to 200 series austenitic stainless steel being inexpensive and also due to its popularity in Southeast Asia. This study investigated the behavior and properties of the weld metal produced from 216 grade austenitic stainless steel. The microstructure, mechanical properties and pitting corrosion resistance that were produced by the tungsten inert gas welding process (TIG) with filler metal ER316 are studied in this paper. The variable parameters of the welding process were heat inputs at high (0.78 kJ/mm), medium (0.52 kJ/mm) and low (0.39 kJ/mm) ranges. It was found that the weld metal with a high heat input level increased the width of the face weld, root weld, PMZ and HAZ. However, the opposite result was achieved when the heat input was reduced. The microstructure of all the weld metals formed delta-ferrite content in the austenite matrix. This was shown to be greater in the weld metal with a low heat input, under the solidification mode FA. The results were shown to agree with the predictions outlined in the Schaeffler diagram. In addition, it was discovered that a high heat input level produced a more discontinuous form of delta-ferrite content than for the low heat input model. This corresponded to the dendrite length and inter-dendrite spacing in the weld zone. The hardness, ultimate strength and elongation tended to decrease when the heat input increased. The fractures for all the weld metals were found to occur in the location of the weld zone. The pitting corrosion resistance increased when the heat input was reduced, which related to the volume of delta-ferrite content in the weld metal zone. This corresponds to the pitting corrosion resistance in the weld metals being higher than for the base metal material. Furthermore, it was found that pitting occurred initially at the interface between the delta-ferrite and austenite phase and the corrosion then spread into the austenite phase.

Model ulaza prolazne toplote kod zavarivanja trenjem sa mešanjem korišćenjem nekružne igle alata

Frictional heat source in the contact surface of a rotating tool and a stationary work piece is used to join metal pieces along the weldline in the friction stir welding process. Temperature profile developed during welding decides the post weld properties on the joined work pieces not only along the weld line but also in the nearby layers of heat affected zone. As the heat input depends on the contact surface geometry in the tool/matrix interface, tool shape has a higher influence on peak temperature developed during the process and on the other hand it influences the post weld properties also. In this paper, a simplified analytical heat input model has been derived and validated for noncircular pin profile by introducing a new multiplication factor in the circular pin profile heat input model. Transient thermal numerical models are developed for, triangular, square and pentagonal pin profiles using analytical heat input results to analyse the change in temperature profiles with respect to the change in tool pin geometry. Furthermore, a detailed analysis is made to understand the percentage contribution of tool pin on the total heat supply.

A Computational approach in optimizing process parameters influencing the heat input and Depth of Penetration of Tungsten Inert Gas welding of Austenitic Stainless Steel(AISI 316L) using Response Surface Methodology

Austenitic stainless steel (AISI 316L) possesses high strength, and corrosion resistance finds application as a major base material for the manufacturing of boilers, chemical reactors, etc. Tungsten Inert Gas Welding (TIG) is a common process used for making joints in the fabrication industries like chemical, railway coaches, and automobile parts. In this work, Austenitic Stainless steel plates of Size 200 x 100 x 5 mm undergoes conventional TIG process. The input parameters set for the study was Welding Current, Welding Speed and Gas flow rate and the output response chosen for the study was Depth of Penetration (DOP) and Heat Input. Central Composite design, a common response surface methodology of Design Experimentation was used to conduct the welding trials. ANOVA was conducted to find a relation between the input and the output with a certainty level of 95%. A full quadratic response surface regression model was developed between Depth of Penetration (DOP), Heat Input and welding current, Welding Speed, and gas flow rate. It was found DOP there exists a linear relation for the input parameters. The Heat input model shows a quadratic relation exists between welding current, welding speed, and output response. The model accuracy was 95.29% for DOP and 99.43% for Heat input. The difference between R-sq. and R-sq. (adj) was less than 2% for both the cases which indicates the model accurately resembles the experimental parameters chosen for the study. Desired depth & heat were accomplished at an optimum current of 185 A, welding speed of 50.11 mm/min and gas flow rate of 13.1 l/min.

An energetically consistent heat input model for additive manufacturing

AbstractAdditive manufacturing offers the ability to design parts more freely and to realize complex geometries that would not have been manufacturable with conventional processes. In powder bed‐based beam melting processes parts are build in a layer‐by‐layer fashion by selectively melting the powder material to generate the part. The energy input of the laser beam and the temperature distribution during the process strongly influence the quality of the produced part and can be studied by means of macroscopic thermal simulations. Due to the non‐linearity of the material behaviour and the diverging time and length scales huge computational costs arise. In the present contribution a temporal integration of the heat input model is used to allow for larger time step sizes. In order to reduce the errors due to the spatial discretization in the heat affected zone, a correction term is introduced, rendering an energetically consistent heat input. A numerical example shows the success of this strategy.

Domain decomposition applied to the thermal model of selective beam melting processes

AbstractThe utilization of additive manufacturing enables the realization of rather complex part geometries. In the context of powder bed‐based selective beam melting processes, parts are built in a layer‐by‐layer fashion by locally fusing powdered material using either a laser or electron beam. The simulation of powder bed‐based additive manufacturing processes for both, polymers as well as metals, is usually a computationally expensive task. This is especially due to the different involved time and length scales, highly non‐linear material behaviour and dynamic growth of the simulation domain. Hence, several numerical methods are ought to be combined in order to reduce the associated computational cost. Therefore we employ adaptivity in the spatial and temporal domain. This allows to have locally refined regions that are currently exposed to the beam, while coarsening the discretization in areas that are currently not exposed to the beam. In addition the path of the heat input is integrated in time, which renders a line heat input model. Furthermore, domain decomposition (Multi‐Time‐Stepping) is applied to account for the different requirements regarding the temporal discretization of different parts of the considered domain. This renders a time integration scheme with distinct time step sizes for different partitions of the domain.

Effects of heat source geometric parameters and arc efficiency on welding temperature field, residual stress, and distortion in thin-plate full-penetration welds

The Goldak’s double-ellipsoid volumetric heat source model is commonly used for arc welding in numerical simulation. Usually, heat source geometric parameters and arc efficiency need to be calibrated in thermal analysis until a good agreement with experiments is achieved. However, the influence of individual heat source geometric parameters and arc efficiency on welding temperature field, residual stress, and distortion is still unclear. In this study, this influence has been systematically investigated in a thin-plate full-penetration weld by methods of both numerical simulation and experimental method. The predicted and measured results indicate that the location of HAZ boundary and the Δt8/5 time can only be influenced by arc efficiency (significantly) and heat source width (slightly) in thin-plate full-penetration welds. Furthermore, the variation of arc efficiency can significantly affect welding residual stress and distortion, while the reasonable change of heat source geometric parameters has a slight effect. In this study, a calibration procedure of heat input model in FE simulation of thin-plate full-penetration welds is provided.

Numerical study of the influence of gap between plates on weld pool formation in arc spot welding process

Arc spot welding is a low heat input process that it is suitable for lap joint welding of thin plates. In a typical arc spot welding process, a controlled short-circuit transfer process, such as cold metal transfer, is used to control the weld pool and/or the heat input to the base metal. In this study, the arc spot welding process is modeled in two dimensions to clarify the metal transfer and weld pool phenomena during the process. In this model, the arc plasma is not calculated and a pseudo arc is set as the conductor of current between the wire and the base metal; the heat input from the arc plasma is given by a simplified heat input model. Droplet formation, droplet transfer, and weld pool formation are included in the proposed model, and the influence of the welding conditions is investigated numerically. First, welding parameters, such as current and the wire feeding speed, are obtained from experimental observation as the input parameters of the model. The calculation result of the weld shape shows good agreement with the experimental result with adjustment of the heat input parameters. The influence of the gap between thin plates is investigated numerically. When a gap exists between the plates, the weld shape differs from that without a gap, and the numerical result is similar to the experimental result. This result shows that the heat conduction between the plates strongly affects the weld shape.

Thermal modelling of selective beam melting processes using heterogeneous time step sizes

In the context of powder bed-based additive manufacturing, complex part geometries are realised in a layer-by-layer fashion and by the fusion of powdered material in locally defined regions by means of e.g. a laser or electron beam. Simulating these manufacturing processes from a macroscopic point of view may lead to computationally expensive models, due to different scales in space and time, highly non-linear material behaviour and methods to account for the layer-wise build, i.e. dynamic growth of the computational domain. Aiming towards a reduction of the computational cost multiple numerical methods are therefore combined.One approach in the context of Finite Element simulations is given by adaptive mesh refinement and coarsening. This allows to locally refine the mesh where it is needed, e.g. in the region currently exposed to the beam, while keeping it relatively coarse in other regions. Yet, the time step size of the model is constrained by the resolution of the beam path, which clearly limits the size of the problem under investigation. Within the present contribution we employ heterogeneous time step sizes for the arising thermal problem in different parts of the computational domain in order to account for the distinct scales in time. In addition, a line heat input model is utilised to further lower the computational cost.