- Sommitsch, Christof
- Enzinger, Norbert
- Mayr, Peter
- TitelMathematical Modelling of Weld Phenomena 12, MM 12
- LicenceCC BY
Frontmatter10.3217/978-3-85125-615-4-00 Complete 3D heat and fluid flow modeling of keyhole laser welding and methods to reduce calculation times10.3217/978-3-85125-615-4-01The fluid flow calculation inside the melt pool in welding processes is a complex challenge. It can be useful for defects prediction in the weld seam or to study the influence of some process parameters. The cost in time of these calculations makes these models not widely used, although they are rich in information. Computational prediction of penetration shapes in MIG welding of practical aluminum alloy joints10.3217/978-3-85125-615-4-02As one of the methods for simulating the molten droplet from filler wire in metal insert gas (MIG) welding, a new line-type heat source has been developed and it is added in the three-dimensional, non-stationary thermal model which can demonstrate both molten pool and penetration shape in gas metal arc welding (GMAW) process. As the result of the examination about the applicability of this combined model for the practical aluminum joint, it is found that the penetration shape in the lap joint can be fairly demonstrated by assuming the adiabatic condition due to the oxide layer and/or the physical separation. In addition, it is revealed that the overhang length of numerical joggle joint model should be appropriately shortened in order to reproduce the penetration shape of practical joggle joint. Moreover, it can be concluded that the penetration shape of the practical aluminum joints can be reproduced by defining the dominant parameters in the combined model through the examinations of the basic welding. Numerical investigation of the influence of welding parameters on the weld pool dynamics and the distribution of second phase particles10.3217/978-3-85125-615-4-03Plasma transferred arc welding (PTA) is widely used to deposit metallic materials on substrates in order to improve for example wear resistance, chemical resistance or thermal shock resistance. In terms of wear resistance, a homogeneous distribution of hard second phase particles in the deposited weld metal is favorable to achieve uniform properties. Therefore, it is important to know how the distribution of particles is affected by the welding parameters and the welding procedure in general. While experimental methods have only limited ability to provide a comprehensive insight into the weld pool dynamics, the numerical simulation of the PTA process does but is at the same time also a challenging task. Within this work, the influence of PTA welding parameters on weld pool dynamics and distribution of second phase particles in the solidified weld metal is investigated applying thermodynamic and numerical simulation. As matrix material, a CoCr alloy (Stellite 6) is used and tungsten carbide particles are included as hard second phase. The finite volume simulation using the software package ANSYS Fluent considers several physical phenomena such as solidification/melting, magnetohydrodynamics (MHD), magnetic fields, etc. The solidification behaviour is studied using the thermodynamic software package ThermoCalc. The established numerical models solve the equations of the conservation of mass, momentum and energy by means of the Euler–Euler and Euler–Lagrange method. Within this work, it was shown that thermodynamic and numerical simulations are valuable tools to describe weld pool dynamics as a function of different welding parameters for the PTA process. The distribution of second phase particles in a solidified metallic matrix was directly linked to the weld pool movement and can therefore be optimized. Simulation was verified by welding trials and subsequent metallographic characterisation. A fine modification of the double ellipsoid heat source10.3217/978-3-85125-615-4-04The analysis of residual stresses or distortion of welded structures uses the prediction of the local temperature field. The calculation of the temperature field requires the definition of heat source. The double ellipsoid Goldak equivalent heat source (EHS) is the most used heat input model for arc welding. This model distributes the heat of the arc welding process in a volume, which is equal to a melted weld pool. The heat flow calculation with the EHS reaches a compliance of the experimental and the calculated temperature field, which is far from the weld seam. Close to the weld seam and especially at the fusion zone these results are hardly reachable with the Goldak model. Another disadvantage of Goldak's EHS is a very high non-physical energy density in the centre of the heat distribution. The reasons behind these disadvantages are analysed. It is shown how to modify the equation of the double ellipsoid heat source to secure the benefits of EHS and simultaneously eliminate the disadvantages of the model. A comparison between real experiments, calculations with Goldak's EHS and the modified EHS Modification-R10 (EHS MR10) shows a significant improvement of the results using EHS MR10. The distribution of power density is described in the modified EHS by a function of the tenth power and therefore referred to as MR10. Validation of heat source model for metal active gas welding10.3217/978-3-85125-615-4-05The most sufficient welding parameters and settings of manufacturing technologies are currently mainly determined by experiments for standardised production in the practice. It is a time-consuming method, while there is a large amount of waste during the ’trial and error’ process of distinct welding technologies. Virtual fabrication and virtual testing of weldments using finite element method provide a sustainable solution for advanced applications. Calibration and validation of heat source models using finite element analysis is a crucial task, because theoretically the calibration has to be done for all the individual cases related to different welding processes and welding variables. Nevertheless, it can reduce the need of on-site experiments and waste during fabrication. A comprehensive literature review has been carried out focusing on the introduction of different heat source models. The aim of the current research is to develop a common calibration process for a wide range of heat source model parameters to ensure general applicability for a typical joint of a weldment. A systematic research program is carried out on small scale specimens using different welding input parameters. The experimental research program contains temperature measurements during welding, macrographs and deformation measurements after welding. In addition, a numerical study using uncoupled transient thermo-mechanical analysis, including sensitivity analyses and parametric studies focusing on fusion zone size, residual stresses and distortions, is performed. Based on the large number of experimental data, thermal efficiency and heat source model parameters are calibrated and verified. As a generalization, a validated heat source model is developed for a metal active gas welding power source. The developed validation process can have significant role in case of robotic welding, where welding trajectory, heat input, travel speed and quality can be controlled precisely. A study of coupled influence of evaporation and fluid flow inside a weld pool on welded seam formation in GMAW10.3217/978-3-85125-615-4-06Simulation of the gas metal arc welding (GMAW) process in the welding pool and welded plates requires to define such distributed parameters of the welding arc as heat, mass and electric current fluxes as well as arc pressure and drag forces on the free surface of the welding pool. Comprehensive approaches to define these parameters require to use three dimensional magneto-hydrodynamic arc plasma models. The high complexity of these models does not allow to use them widely for calculation. Nevertheless the most amount of available works use a simplified definition of arc source parameters as a predefined shape (e.g. circular or double ellipsoid law due to the Gauss distribution), that does not change during the calculation. In this work, a new approach is proposed to define the distributions of arc parameters not in the usual predefined shape, but in distributions, that are modified according to the calculated temperature of the free surface of the welding pool on which the arc heating, evaporation of the welding metal and hydrodynamics of the welding pool have their own coupled impact. The discussed approach was used in developing a mathematical model of GMAW process that can provide a numerical analysis of thermal, electromagnetic and hydrodynamic processes in the weld pool and welded plates. The model was used to study the proposed approach of arc parameters redistribution on the welded seam formation. Numerical simulation of ferrite/austenite phase fraction in multipass welds of duplex stainless steels10.3217/978-3-85125-615-4-07Kinetic approach to the α/γ phase transformation phenomena (α/γ phase fraction) in the heat affected zone (HAZ) and weld metal (WM) of multipass welds was made using duplex stainless steels (lean, standard and super duplex stainless steels). The kinetic equations including rate constants of the dissolution behaviour as well as precipitation behaviour of γ phase were determined by isothermal heat treatment test. Based on the kinetic equations determined, the distribution of the γ phase fraction in multipass welds of duplex stainless steels was calculated applying the incremental method combined with the heat conduction analysis during welding. The depleted zone of γ phase was formed adjacent to the fusion line in the base metal HAZs and the reheated WMs. However, the γ phase fraction in the depleted zone was increased (recovered) by the subsequent weld passes. Accordingly, the α/γ phase balance has been complexly varied in multipass welds, and the profile of the γ phase fraction was arranged in laminae in the WM roughly along the fusion lines. Furthermore, the γ phase fraction in multipass weld of standard DSS was slightly lower than those of lean and super DSSs. The over-precipitated zone, where the γ phase fraction slightly exceeded the base metal level, was not observed in the low temperature HAZ of standard DSS weld, whereas it was observed in other welds. Microstructural observation revealed that the calculated results of the γ phase fraction in multipass welds were consistent with experimental ones. It follows that the α/γ phase transformation in duplex stainless steel welds could be successfully predicted by the present approach. The influence of chemistry inhomogeneity on microstructure development and residual stress10.3217/978-3-85125-615-4-08The chemistry distribution is of importance in the welding process. By varying the chemical composition, the evolution of microstructure and the residual stress change correspondingly. To examine the effect of chemistry, a three-dimensional metallo-thermo-mechanical model is created. The model is established according to a bead-on-plate welding experiment. Samples of S700 steel are manufactured by gas metal arc welding (GMAW). In total, three welds with three heat inputs were conducted so that different chemistries are obtained. The final weld geometry and the uniform chemistry in the fusion zone (FZ) are predicted by the software SimWeld. The parameters in the double ellipsoidal heat source are also calibrated by SimWeld. An inhomogeneous chemistry field is created using the data predicted by SimWeld and the chemical composition of base material (BM), and is further imported to the coupled model by writing user subroutine in ABAQUS. The metallurgical algorithm is implemented in the same way for calculating the phase volume fraction using both the homogeneously and the inhomogeneously distributed chemistry fields. After the temperature and microstructure are determined, the mechanical analysis is conducted using linearly interpolated material properties. Finally, the results of microstructure distribution and the residual stress predicted for homogeneous and inhomogeneous field are compared to clarify the influence of chemical composition. Kinetic monte carlo simulation of pulse Cu30NI bead on plate10.3217/978-3-85125-615-4-09The purpose of this paper is to show a modelling scheme to predict rapidly three dimensionnal grain structure. The modelling is based on a plane thermal finite element for the process coupled with a monte carlo simulation for grain structure. To demonstrate the interest of this type of modelling the simulations are done for a squared pulse mode and results are compared with experimental results (EBSD) done during the work of A.Chiocca. The originality of the paper is that predictions of grain structure are investigated with current and arc voltage evolving with time. Results show the interest of using monte carlo simulation. Multi-pass ferritic steel weld modelling: phase transformation and residual stress10.3217/978-3-85125-615-4-10The solid state phase transformation (SSPT) occurring during welding thermal cycles gives rise to distinctive microstructures across the fusion zone and heat affected zone (HAZ), as well as significant effects on the residual stress generated in the weldment. We have developed a numerical model to simulate multi-pass welding in low alloy ferritic steel with consideration of SSPT. In this study, we applied a semi-empirical modelling approach to three-pass gas tungsten arc welding in a grooved plate made of SA508 steel (widely used in nuclear power plants). The microstructure, hardness and residual stress were predicted using a 2D finite element model and the predictions were compared with experimental results. We examined the sensitivity of the predicted hardness and stress to austenitisation kinetics and weld-metal plasticity. Two sets of empirical parameters were considered in the kinetic model of austenitisation to represent different levels of the heating-rate dependence of the critical temperatures (i.e. Ac1 and AC3) for austenite formation. A rule-of-mixtures method based on dilution and hardness was proposed to estimate the plastic properties of weld metal for each pass, using the predicted phase fractions and the yield stress dataset for each transformation product of base material. The modelling results show that the extent of the inter-critical HAZ, hardness and residual stress are affected by the austenitisation kinetics. The use of weld-metal plastic properties estimated by the rule-of-mixtures method can improve the residual stress prediction for the weld metal. Numerical modelling of welding of martensitic steel10.3217/978-3-85125-615-4-11The mathematical model and computer simulation for prediction of mechanical properties and microstructure composition of steel welded joint was developed. Because of wide range of applicability and ease of use of finite volume method (FVM), this numerical method was suitable to create integrated computer program for simulation of transient temperature field, microstructure transformation and mechanical properties during welding of steel. The computer simulation of mechanical properties and microstructure of welded joint is consisted of numerical calculation of transient temperature field in process of cooling, and of numerical calculation of hardness. The computer simulation of hardness of welded joint is based on both, CCT diagrams and the thermo-kinetic expressions using linear alignment with the actual chemical composition. Microstructure and hardness of welded joint has been predicted based on calculated characteristic time of cooling from 800 °C to 500 °C, (t8/5). Results of steel welding were estimated by taking into account the process of reheating of workpiece during the welding. The established procedure was applied in computer simulation of martensitic steel welded joint. Prediction of stress corrosion cracking in 304 stainless steel canisters in dry storage of spent fuel by modeling: analysis of weld residual stress and susceptible microstructure10.3217/978-3-85125-615-4-12Atmospheric chloride-induced stress corrosion cracking (CISCC) in the weldments of spent fuel canisters is one of the primary safety concerns during the dry storage of used nuclear fuel at Independent Spent Fuel Storage Installations (ISFSI) in coastal areas. For SCC to occur, three criteria must be met: an aggressive chemical environment, susceptible microstructure, and sufficient tensile stress. Instead of the environment, this paper will focus only on the material microstructure and stress state. Firstly, finite element analysis (FEA) based numerical simulation models were developed for the welding of the mockup canister. Multi-pass arc welding process was considered. The thermal history and residual stress distribution induced by the longitudinal, circumferential, and the intersection between longitudinal and circumferential welds were developed based on a thermal-mechanical coupling model using ABAQUS software. The simulation results were compared with the deep-hole drilling (DHD) and contour measurement data. Based on the model-predicted stress results, a four-point bend (4PB) setup was then designed to duplicate the weld residual stress onto the 4PB specimen, on which the maximum tensile stress is close to the predicted maximum stress level of 250 MPa in the weld heat-affected zone (HAZ). A stress gradient also existed along the thickness of the 4PB specimen which allowed for the consideration of stress variation. The designing of the 4PB test was done by FEA method using ABAQUS software, and verified by digital image correlation (DIC) measurements. Besides, to identify the most susceptible microstructure of the canister to corrosion under the weld residual stress in the controlled environment, the modified implant test (CEMIT) was designed. A FEA model considering the welding process and the following uniaxial tensile load was then developed using the SYSWELD software. The residual stress and thermal history were obtained during and after the CEMIT welding process, and after the uniaxial tensile loading process. These tests yielded instructive information for the prediction of the initiation site of the crack. The initiation of the crack observed in the experiments was explained based on the simulation data. While each of the three simulations provided an independent story, the three together produced a complete and complementary picture. Stress corrosion cracking is a series of complex interactions which simultaneously occur. The thermal mechanical model of the canister provides information for predicting the location at which stress corrosion cracking will most likely occur. The model for the 4PB test predicts the stress gradient which occurs in a bent bar. The SYSWELD CEMIT model predicts the welding cycle, the thermal history, residual stress and loading behavior of the specimen. By design, each independent CEMIT provides the location of the most crack susceptible location under a specified environment. Computational analysis of the yield stress of ultra-high strength all-weld metals10.3217/978-3-85125-615-4-13Lightweight constructions providing a high yield stress play a crucial role in transportation systems and steel constructions optimized for low energy consumption. For the fabrication of such components, the development of matching welding consumables is an essential task. In this investigation, the aim is to understand the influence of different alloying elements on the strength of all-weld metal samples of ultra-high strength filler metals with a yield strength of 1100 MPa. In the end, this should provide insight into the operating mechanisms providing the desired strength and make it possible to predict the expected yield stress with reasonable accuracy. Apart from precipitation and solid solution strengthening, special attention is paid to the contributions of dislocation hardening and grain boundary strengthening, since these are expected to be the major contributors to the overall strength in a predominantly martensitic structure. In order to apply those classical strengthening mechanisms to the specific microstructure of martensite, additional considerations have to be made concerning the effective grain size and initial dislocation density used for calculation. Finally, the developed model is tested and the results are compared with over 90 actually produced and measured alloys. Numerical Simulation of Cu-rich precipitate evolution in Cu-bearing 316L austenitic stainless steel10.3217/978-3-85125-615-4-14A numerical model based on the coupling of the thermodynamic software ThermoCalc and the Kampmann and Wagner Numerical (KWN) framework has been developed in this work to predict the evolution of Cu-rich precipitates in 316 stainless steels. The effect of precipitation hardenning on mechanical properties at 700°C is investigated as well. The predicted average particle size, volume fraction and number density of precipitates agree well with the experimental observations. In addition, the precipitation strengthening effects of Cu-rich precipitates were quantitatively evaluated and agree with experimental data as well. The slow increase in average radius of Cu-rich precipitates was consistent with the modest change in yield strength with extended aging. These cumulative results and analyses could provide a solid foundation for much wider applications of Cu-bearing stainless steels. The developed model can be used to predict the precipitation behaviour in other similar austenitic stainless steels. A coupled temperature-microstructure model for the heat-affected zone of low alloyed high strenght steel during two-pass arc welding10.3217/978-3-85125-615-4-15A coupled temperature-microstructure model was developed in order to simulate the evolution of the microstructure in the heat-affected zone during two-pass gas-metal arc welding. The model is developed to serve the steel industry's need to evaluate the weldability of new steel grades. Heat transfer and heat input models were used for modelling the arc welding and the temperature changes in the heat-affected zone. A microstructure model was fully coupled with the temperature model, including latent heat of transformation as well as the dependence of thermophysical properties on temperature and phase fractions. The microstructure model simulates phase transformations and grain growth including a simplified model for the effect of fine particles. The modeled temperature paths are in good agreement with the measured ones. The final phase fractions and grain size distribution obtained from the model correspond to the actual microstructure and the model predicts the shapes of the heat-affected zone and fusion zone with relatively good accuracy. Finite element simulation of residual stresses and distortions in selective laser melting10.3217/978-3-85125-615-4-16With additive manufacturing, the production of individual lightweight structures and complex parts with integrated functions can be realized. In selective laser melting (SLM), a highly concentrated and fast moving laser spot is used to melt the powder layers which leads to high temperature gradients during the manufacturing process. This causes thermal residual stresses and distortions which can affect the intended use of additive manufactured parts. If cracks and distortions develop during the manufacturing process, a collision of the powder coater with the manufactured part can lead to a process abortion. A simulation predicting residual stresses and distortions can be used to consider these problems before manufacturing and therefore avoid scrap parts. Parameter configurations could be developed systematically to reach specific component properties. A common approach for simulating these effects in welding is the thermomechanical finite element simulation. Using this approach with a high spatial and temporal resolution, which is necessary to accurately represent the heat source, only small additive manufactured parts can be simulated with a reasonable computational effort. In order to allow numerical simulations of parts with larger dimensions, it is necessary to use appropriate simplifications and simulation strategies. One possible simplification presented in literature is to expose a whole layer to a heat source simultaneously instead of considering the whole scan path. To evaluate the effect of this simplification, the SLM-manufacturing of cuboids is simulated with different temporal resolution. Moreover, the effects of a base plate cutting and of different scanning strategies on residual stresses and distortions are examined numerically. A shift technique for multi-pass welding simulation10.3217/978-3-85125-615-4-17The simulation of complex industrial welding processes using the Finite Elements method is not usually feasible within a reasonable time limit due to the strong non linearities of the physical models and the dimensions of the problem. To study many of these industrial cases, we would like to apply simplified methods to compute the simulations in acceptable time. We propose a simplified method called Physical Fields Shift which allows us to accelerate the computations and obtain an approximation of the strain and residual stress state at the scale of the component after the repair process. This method has been applied to an overlay welding repair with successful results. How reliable are prediction and measurement of weld residual stresses - lessons from the NeT network10.3217/978-3-85125-615-4-18The Task Groups of the NeT European Network undertake closely controlled round robin studies examining the prediction and measurement of residual stresses in thoroughly characterised welded benchmarks. Task Groups 4 and 6 are examining three-pass slot welds in plates made from AISI316L(N) and Alloy 600 respectively. A very large body of independent RS measurements and simulations have been performed, giving a unique insight into the real-world reliability of both RS measurements and finite element simulation of welding. This paper reviews NeT Task Groups 4 and 6, and considers their implications for modelling of welding processes. Process chain simulation of laser cladding and cold metal forming10.3217/978-3-85125-615-4-19Lightweight construction is still an important driver for innovations in production technologies. Recent developments in Laser Metal Deposition (LMD) and forming technology allow the production of lightweight sheet metal components with locally adapted properties, reducing the weight of parts. For example, using laser cladding, it is possible to reinforce critical areas of a sheet metal component, while other areas can still remain untouched and thus consume less material, reducing the overall weight of the component. Conventional production of such reinforced structures would require expensive and time-consuming machining steps. However, especially if it comes to sheet metal components that are to be mechanically formed, the induction of heat during laser cladding might lead to undesired properties of the final product due to distortion, residual stresses as well as changed material properties. Such effects can be investigated by means of structural welding simulation, but, if it comes to process chains, i.e. reinforced metal sheets that undergo forming processes, a process chain simulation covering forming and cladding steps is desirable. This approach would also answer the question of preferred process sequences. This article presents an approach for such a process chain simulation for a demonstrator made of aluminium EN AW 6016 undergoing one laser cladding and one cold forming process. The simulation considers the influence of the process sequence (forming-cladding vs. cladding-forming) and helps to select the proper process sequence as well as predicts properties of the final product. Evaluation of two methods for welding distortion simulation10.3217/978-3-85125-615-4-20For manufacturers of heavy steel constructions, it is a major goal to minimize the number of required prototypes within the design process of welding assemblies. Distortion minimization of complex structures usually needs several tries until the best design, welding parameters and welding sequences can be defined. ESI GmbH provides two different numerical simulations tools which can predict the distortion of welded structures. One of them is the initially introduced and more general purpose welding simulation package Sysweld®. It uses the transient method and thus allows calculations close to full physics on a global model. In this case thermo-mechanical metallurgical effects are coupled, therefore high quality meshes, and long calculation times are required compared to Weld Planner®, which is based on the shrinkage model. It requires less effort for meshing and can be computed very quickly. However, simplified models neglect some aspects like phase transformation. The present work’s objective is to figure out which of the two provided simulation tools is more applicable from a practical perspective of industrial heavy steel constructions engineers. Therefore, an exemplary welded structure of a loader crane boom, stiffener plate and console is investigated using both tools comparatively. Simulated distortions are validated against experimental results. Therefore, welding distortions are measured by means of 3D point measurement after welding experiments of the component under consideration. Based on the validated results the preferred method regarding accuracy and required effort for modelling and simulation costs is identified. Simulation of welding residual stresses - from theory to practice10.3217/978-3-85125-615-4-21The present study reviews previous and new simulations of welding residual stresses with the finite element method. The influence of modelling mechanical boundary conditions, erroneous prediction of the weld heat source coefficient, the influence of microstructural changes in aluminium welds and the consideration of strain-rate sensitivity of steel are investigated. The results are analysed so that sound conclusions regarding the investigated factors, acting as recommendations for the practitioner, can be presented. Residual stress measurements and model validation of single and double pulse resistance spot welded advanced high strength steel10.3217/978-3-85125-615-4-22Advanced high strength steels (AHSS) are increasingly used in automotive industry Numerical simulation of stress behavior during shot peening10.3217/978-3-85125-615-4-23Welding is widely used in the manufacture of steel structures. But welding causes residual tensile stress. Residual stresses due to welding may affect the initiation of fatigue cracks and stress corrosion cracking. These defects have significant effect on the integrity of products. In order to reduce or mitigate the tensile residual stress, surface treatment by peening has been proposed. To guarantee the safety of the products, it is important to evaluate the effect of peening on the surface stress in advance of fabrication. In this study, we proposed a method to predict the reduction of tensile residual stress due to shot peening using finite element method. The proposed method was applied to the analysis of Almen strip piece. In the analysis, the relation between the amount of shot projection and the arc height was discussed. In addition, the proposed method was also applied to the analysis of the shot-peened bead-on-plate specimen. As a result, we confirmed that the proposed method can quantitatively predict shot peening and it can analyse the effect of reduction of welding residual stress of tension by shot peening. Laser beam welding of steel-aluminum joints - Influence of weld metal elastic-plastic properties on the distortions10.3217/978-3-85125-615-4-24Great attention is focused nowadays on laser welding of dissimilar steel-aluminum joints in overlap configuration in key-hole mode. It was found that elastic-plastic properties of the weld metal exhibit strong difference to those of the base alloys and can be defined as a function of aluminum content in the weld metal. A developed Finite-Element simulation model allows prediction of the aluminum content as a function of welding parameters and subsequently the elastic-plastic properties of the weld metal as a function of the determined content. The main goal of the present study is to show the impact of the weld metal properties on welding distortions and residual stresses. For that purpose, a sensitivity analysis of the thermomechanical model was performed, where the distortions and residual stresses were computed as a function of welding parameters and therefore as a function of corresponding weld metal properties. The analysis showed that the influence of the weld metal is essential, and its properties should be taken into consideration in the models for better prediction accuracy. Study of solidification cracking in advanced high strength automotive steels10.3217/978-3-85125-615-4-25Advanced high-strength steels (AHSS), which are increasingly used in the automotive industry, meet many functional requirements such as high strength and crash resistance. Some of these steels contain high amounts of alloying elements, which are required to achieve the necessary mechanical properties, but render these steels susceptible to weld solidification cracking. Weld solidification cracking results from the complex interplay between mechanical and metallurgical factors. Our recent work is focused on studying solidification cracking in dual phase (DP) and transformation induced plasticity (TRIP) steels using the following modeling and experimental strategies: 1. A finite element (FE) based model was constructed to simulate the dynamic thermal and mechanical conditions that prevail during bead-on-plate laser welding. To vary the restraint, laser welding was carried out on single sided clamped specimens at increasing distances from the free edge. In TRIP steel sheets, solidification cracking was observed when welding was carried out close to the free edge and at a certain minimum distance, no cracking was observed. For the no cracking condition, in situ strain evolution during laser welding was measured by means of digital image correlation to validate the strain from the Fe-model. Subsequently, a phase field model was constructed using the validated thermal cycles from the FE-model to simulate the microstructural evolution at the tail of a weld pool, where primary dendrites coalesce at the weld centerline. From the phase field model, elemental segregation and stress concentration are used to explain the cracking susceptibility in TRIP and DP steels. For DP steel, both the experimental and modeling results indicate a higher resistance to solidification cracking. 2. A phase field model was constructed to simulate the directional solidification in TRIP and DP steels. The thermal cycle and temperature gradient were derived from the in-situ solidification experiments conducted using high temperature laser scanning confocal microscopy (HTLSCM). The model showed that longer and narrower interdendritic liquid channels exist in the case of TRIP steel. For the TRIP steel, both the phase field model and atom probe tomography revealed notable enrichment of phosphorus, which leads to a severe undercooling in the interdendritic region. In the presence of tensile stress, an opening at the interdendritic region is difficult to fill with the remaining liquid due to low permeability, resulting in solidification cracking. The overall study shows that a combination of factors is responsible for the susceptibility of a material to solidification cracking. These include particularly mechanical restraint, solidification temperature range, solidification morphology, solute segregation and liquid feeding capability. Simulation of weld solidification cracking in varestraint tests of alloy 71810.3217/978-3-85125-615-4-26Several nickel-based superalloys are susceptible to weld solidification cracking. Numerical simulation can be a powerful tool for optimizing the welding process such that solidification cracking can be avoided. In order to simulate the cracking, a crack model inspired by the RDG model is proposed. The model is based on a crack criterion that estimates the likelihood for a preexisting pore in a grain boundary liquid film to form a crack. The criterion depends on the thickness and the liquid pressure in the grain boundary liquid film, as well as the surface tension of the pore. The thickness of the liquid film is computed from the macroscopic mechanical strain field of an FE model with a double ellipsoidal heat source. A temperature-dependent length scale is used to partition the macroscopic strain to the liquid film. The liquid pressure in the film is evaluated using a combination of Poiseuille parallel plate flow and Darcy’s law for porous flows. The Poiseuille flow is used for the part of the grain boundary liquid film that extends into the region with liquid fraction less than 0.1, while Darcy’s law is used for the rest of the liquid film that extends into the regions with liquid fraction greater than 0.1. The proposed model was calibrated and evaluated in Varestraint tests of Alloy 718. Crack location, width, and orientation were all accurately predicted by the model. Use of modelling to characterize the risk of hot cracking in austenitic stainless steels during welding10.3217/978-3-85125-615-4-27Liquation cracking may occur in the heat affected zone (HAZ) during welding. Two factors influence this phenomenon: the tensile stresses generated during welding and the potential loss of ductility due to the presence of a liquid film at grain boundaries depending on their chemical composition. Gleeble hot-ductility tests have been used to study the combined effect of boron content and holding time on ductility drop in the liquation temperature range of a 316L type austenitic stainless steel. It is shown that high boron contents and short holding times promote the loss of ductility in the liquation temperature range. Secondary ion mass spectrometry (SIMS) has been used to correlate mechanical results to boron distribution either at grain boundaries or in the bulk. Other weldability tests have been performed to confirm the influence of boron content on hot cracking sensitivity of AISI 316L stainless steels. Results indicate that cracks appear on all specimens but at different strain levels. The higher boron content is, the more specimen exhibits tendency to hot cracking. Thanks to numerical modelling of these tests, a cracking criteria is proposed to quantify the risk of liquation cracking for different boron contents. 3D finite element modeling of the linear friction welding of a beta titanium alloy10.3217/978-3-85125-615-4-28In this study, a 3D numerical model for linear friction welding (LFW) of a metastable beta titanium alloy (Ti-8V-6Cr-4Mo-4Zr-3Al) with a rectangular cross-section was established and validated experimentally. The effects of welding time and oscillatory direction on the thermal profiles, burn-off rates and subsequent microstructures were investigated. The results showed that the interface temperature and width of the weld center zone (WCZ) decreased when the oscillation along the short edge of rectangle compared to that along the long edge. The temperature at the interface was quickly increased to 1000 °C at around 1s Analysis of acoustic softening, heat and material flow in ultrasonic vibration enhanced friction stir welding10.3217/978-3-85125-615-4-29To improve the welding efficiency and lower the welding loads in friction stir welding (FSW), a process variant of conventional FSW, i.e., the ultrasonic vibration enhanced friction stir welding (UVeFSW), is developed. The experimentations show that UVeFSW not only increases the welding speed and decrease the weld load, but also improve the microstructure and mechanical properties of the joints. To understand the underlying physics in UVeFSW, a mathematical model is developed to analyze the interaction of the ultrasonic vibration with the plastically deformed material around the tool as well as the heat transfer and material flow phenomena in UVeFSW. It is found that the ultrasonic vibration enhances the material flow and the strain rate in the shear layer, but does not cause evident temperature increment. The assistant softening in the material around the tool is caused by the exerted ultrasonic vibration, i.e., acoustic softening. The numerical analysis results lay solid foundation to explain the process effectiveness and the UVeFSW process optimization. Thermo-mechanical model of the friction stir welding process and its application for the aluminium alloy AA575410.3217/978-3-85125-615-4-30A thermo-mechanical model of the FSW process for the investigation of the visco-plastic flows produced in the stir zone has been developed. It takes into account the frictional and volumetric heat sources. The input characteristics of the dynamic viscosity and shear stress for AA5754 were determined during specially designed FSW-alike experiments. The obtained distributions of the temperature, relative pressure, strain rate, viscous flow velocities, as well as the values of the longitudinal and transversal tool force components agree qualitatively with the literature data. Development of a model to investigate the interaction between process and machine tool and the resulting dynamics of friction stir welding10.3217/978-3-85125-615-4-31Friction Stir Welding (FSW) is a solid state joining process where the process and the machine tool used for welding are in direct interaction with each other. This mechanical coupling means that the influence of the machine tool cannot be neglected when investigating FSW: Welds produced on different machines while keeping welding parameters and materials constant are known to show different properties. It is furthermore known that the process forces of FSW have a periodic nature. The reasons for this periodicity, however, are not fully understood yet. It is assumed that properties, like out-of-round-deviations of the spindle, the tool geometry or the stiffness of different machine parts in interaction with the welded material, lead to the characteristics of the FSW forces. Since the welding forces can presumably be employed for online process monitoring, a deeper understanding for the underlying reasons is necessary. In order to investigate this interaction of material and machine as well as the resulting dynamics, a finite element model containing both FSW process and machine, is developed in this work. The process is modeled using the coupled Eulerian-Lagrangian finite-element-formulation. The large strains and strain rates occurring during FSW make it necessary to use an Eulerian formulation for the material to avoid mesh distortion. The ability of the Eulerian formulation to describe the material flow, including free surfaces, furthermore provides the possibility to analyze the formation of volumetric welding defects, which severely reduce the strength of the welds. The tool is modeled using lagrangian elements, a contact is employed to couple the eulerian and lagrangian domains with each other. The machine is modeled in a simplified way using discrete spring elements. The developed model is validated in a first step with welding experiments that were conducted at the IMWF. Second, the model is employed to investigate the interaction between process and machine and therefore the dynamics of FSW. By varying the parameters of the model, the effects, which contribute significantly to the periodic dynamics of FSW, are identified. A simplified 2d-model is employed additionally to analyze the material flow during welding. Another focus lies on the correlation of the process forces and the tool trajectory with defect formation mechanisms. It is shown how flaws in the weld like voids or excessive flash correlate with saliences in the force measurements. These results can not only be employed for monitoring systems but also to improve existing or develop completely new process control strategies. Advances in numerical modelling of linear friction welded high strength steel chains10.3217/978-3-85125-615-4-32The linear friction welding process is an innovative solid state joining technology enabling high quality joints in chains, thus competing with the currently in use flash butt welding (FBW) process, with none of the drawbacks related to fusion welding processes. Modelling has proven to be an indispensable tool in LFW, providing necessary insight to the process due to its rapid nature. Furthermore, no need of expensive infrastructure, welding experimentsand subsequent testing is necessary. In this article, the current status of understanding and development in LFW of chains is presented via 2D and 3D models carried out in the commercial software DEFORM. The pre-processing steps are thoroughly described in terms of meshing characteristics, energy input analysis, boundary conditions, amongst others. As a result, various process outputs are presented, such as temperature evolution, flash morphology, material flow behaviour, and stress analysis. Experimental validation was carried out to assess the quality of the models. The models were able to predict the thermal evolution in the vicinity of the weld interface, as well as reproduce the phenomena behind flash formation and material flow. Finally, an outlook on numerical simulation of the process applied to chain welding is presented. A proposal for thermal computational model for API 5L-X80 steel friction stir welds based on thermocouples measurements10.3217/978-3-85125-615-4-33This work aims to validate a computational model suitable for predicting temperature distribution of API 5L-X80 steel welded joints manufactured by Friction Stir Welding. The model was verified through comparative analysis between experimental data and a computational model generated from a commercial finite element method software. The testing data was acquired by temperature measurements, using thermocouples positioned equally spaced along a workpiece plate of 12 mm thick during the welding process. The experiment was conducted with two different sets of heat inputs and rotational pin speeds: a joint with heat input of 1.69 KJ.mm-1 and 300 rpm (cold joint), and the other with a heat input of 1.91 KJ.mm-1 and 500 rpm (hot joint). Temperature data was processed and used to preview the material’s thermal cycle. The computational model was developed using the COMSOL Multiphysics® software, as the heat source was considered stationary in a Eulerian model. The model was calibrated for both joints and comparison between measured temperatures with thermocouples results have showed significant similarities when the maximum simulated thermal cycles and the experimental temperature data are compared. The thermal model was also used to predict maximum temperatures the thermal history for points of the welded region where is physically impossible to perform experimental temperature measurements due to the presence of the pin and the tool’s shoulder. Influence of the probe geometrical features on the stress condition of the tool during friction stir welding10.3217/978-3-85125-615-4-34The objective of the present paper is to investigate the effect of the tool probe features on the stress condition of the tool in friction stir welding (FSW). The friction stir welding of an Al 6082 T6 alloy was performed to provide validation data for numerical models of the process. The developed earlier 3D thermal model was improved in order to collect the data about workpiece material strength and heat generation at the probe/workpiece interface during FSW. These data were used as a boundary conditions in models for temperature in the welding tool and stress condition of the tool. Influence of temperature and tool probe features such as flat depth, thread root radius and probe root radius, on stress condition of FSW tool at different angular position was defined. Modelling approach to the microstructure evolution in commercially pure aluminium during the RFW process10.3217/978-3-85125-615-4-35The rotary friction welding (RFW) is nowadays a well-established joining technique since it is highly productive, entirely repeatable, and very economical. It belongs to the solid-state fusing methods allowing the combination of similar and for a wide spectrum of dissimilar materials. Commonly combined are aluminium and steel materials to manufacture high quality lightweight structures with excellent technological properties. Due to the fact that the welding temperature does not exceed the material’s melt temperature, microstructural transformations only appear caused by the heating and cooling rate and the amount of plastic deformation. Aluminium alloys in particular show a pronounced change in microstructure as a consequence of dynamic recrystallization evoked by the high degree of plastic deformation. The purposes of this paper is to present a modelling approach to the microstructure evolution in commercially pure aluminium, for aluminium to steel friction welded joints. The main motivation therein is the consistent prediction of the recrystallization rate of different layers in the process affected zone for an improved understanding of microstructural evolution during the RFW process. Electrical contact resistance model for aluminum resistance spot welding10.3217/978-3-85125-615-4-36Resistance spot welding is one of the most important welding processes for joining sheet metal parts in automotive industry. In the process of resistance spot welding electrical contact resistance is of critical importance. The process involves mechanical, electrical and thermal interactions and is dominated by Joule heating, generated at faying surfaces and electrode-sheet-interface. Especially for aluminum alloys, due to small bulk resistance and oxide layer, most of heat is generated at the interfaces. Up to today, barely numerical models for the dynamic contact resistance of aluminum have been published. In this work, a model for contact resistance of aluminum alloys is presented. The model describes the dynamic contact resistance as a function of pressure and temperature. Therefore, an experimental study was designed to determine the dynamic behavior of the contact resistance. Two sheets of aluminum alloy AA5182 were joined by resistance spot welding with variation of electrode force and current. In order to determine the apparent contact resistance, current and voltage differences between sheet-sheet and electrode-sheet were measured. A coupled thermal-electrical-mechanical FE-model with temperature-dependent material properties was used to simulate the experiments. Calculated contact resistances and nugget diameters were compared to the measured ones in order to calibrate the contact resistance model and to validate the simulation. Experimentally measured resistances and nugget diameters are in good accordance with numerical results. The dynamic contact resistance can be calculated by the deployed model with reasonable accuracy. Weldability of a dissymetric assembly with a very thin sheet during resistance spot welding10.3217/978-3-85125-615-4-37In automotive industry, to simultaneously impact safety and light weighting for reducing energy consumption, new families of high strength steel have been introduced in the design of body in white. Some combinations of dissimilar sheets, including a very thin sheet, cause weldability problems and difficulties to the optimization of the process parameters setting. Thanks to the progress achieved in numerical and computer engineering fields, modelling and numerical simulation is a relevant approach, to understand the difficulties encountered during resistance spot welding of these assemblies, and to search solutions to improve the weldability. This work aims at the improvement of the weldability of a dissymmetric combination of three dissimilar sheets: a very thin (0.57 mm) zinc coated low carbon steel sheet, a thick (1.47 mm) zinc coated advanced high strength steel sheet, and a thick (1.2 mm) aluminum-silicon coated press hardened sheet. A numerical axisymmetric 2D Electro-Thermo-Mechanical model developed with the software FORGE® is used to improve the knowledge about the mechanisms which influence the nugget formation and growth, and its penetration inside the cover thin sheet. Experimental evolutions of thermal and electrical contact resistances evolutions, at electrode/sheet and sheet/sheet interfaces, strongly dependent of coatings properties, are embedded in the model and considered dependent on contact temperature and normal stress. The contact radius evolutions, involving the normal contact stresses and the current density distributions in the assembly, are calculated during squeezing, welding, and forging stages. The model is consistent with several experimental observations (nugget size, contact radii, dynamic resistance) issued from welding tests. Thanks to this model, the important effect of the interfacial mechanisms on the formation and the growth of the molten pool have been highlighted. Furthermore, the influence of the process parameters (current, force) and of the curvature radius of the rounded tip electrodes on the penetration of the welding pool into the thin sheet have been investigated. Overheating induced by Al-Si coating during spot welding of a dissymmetrical three sheets assembly10.3217/978-3-85125-615-4-38In automotive industry, the lightweight of body in white is a challenge to meet environmental standards. Where some car manufacturers favour lighter materials, steel is always more economical and easy to assemble by resistance welding. Thus, the tendency is to reduce the thickness of cover sheets and use advanced high strength steel in better position for the structure. The press-hardened steels (of hot-pressed forming steel) are also a key of lightweight. The manufacturing of the body in white requires various combinations of different steel sheets that can induce weldability issues when using resistance spot welding process. This work aims at understanding the mechanisms that influence the initiation and the growing of the nugget in a specific combination with three dissimilar coated steel sheets. The stack-up includes a very thin (0.57 mm) zinc coated low carbon steel sheet, a thick (1.47 mm) zinc coated advanced high strength steel sheet, and a thick (1.2 mm) aluminum-silicon coated press hardened steel sheet. The aluminum-silicon coating is transformed during hot stamping process and becomes more resistive. A numerical 2D axisymmetric electro-thermal model developed with COMSOL Multiphysics® is used here to improve the knowledge of the thermal phenomena occurring during the first times of the welding stage. Specific attention is paid to the contact conditions at both macroscopic and microscopic scales. Contact radii evolutions issued from experimental observations are considered in the model. Thermal and electrical contact resistances evolutions experimentally measured versus stress and temperature are also considered in the model. The model is consistent with several experimental observations issued from infrared images and voltage between the electrodes. It confirms the intense heating observed by infrared camera at the level of the two interfaces with the PHS sheet, because of the high contact resistances induced by the aluminum-silicon coating. Consequently, the nugget appears quickly inside the PHS sheet. Advancing spot welding process assessment10.3217/978-3-85125-615-4-39Resistance spot welding still is one of the most important techniques for joining sheet material. Ongoing developments such as new material grades, coating technologies and process controls generate new challenges for the process. In order to overcome those challenges, numerical process simulation is an important tool in order to develop new welding strategies. Conventional simulation approaches only evaluate basic parameters like nugget diameter, temperature profiles and basic electrical parameters. Although evaluation of these parameters is very important to compare the model to experimental data, it does not do the true evaluation possibilities of numerical models justice. For the first time, new mathematical methods are presented in this work, which allow the computation of comprehensive parameters like electric potentials, effective temperatures, true electrical resistances and heat contributions as well as mechanical parameters from finite element simulation. Exemplary results of the approach on the example of a spot weld on martensitic 22MnB5 steel are discussed. Among these, for the first time, the quantitative curve of dynamic efficiency of the spot welding process is presented. Multiphysics finite element simulation of resistance spot welding to evaluate liquid metal embrittlement in advanced high strength steels10.3217/978-3-85125-615-4-40Advanced high strength steels (AHSS) open new possibilities in lightweight design in the automotive industry. However the risk of liquid metal embrittlement (LME) of modern zinc coated advanced high strength & high ductility steels (AHSS HD) during resistance spot welding (RSW) is a limiting factor for the use of such steels in the cars body in white. The goal of this work is to evaluate and exclude this risk for the high ductility dual phase steel DP1200HD. In the RSW process two electrodes are pressed to similar or dissimilar metal sheets, which are heated and joined by means of Joule heating. An experimental RSW study of similar two-sheet stack ups of DP1200HD showed zinc induced LME for certain welding conditions. Commonly trial and error testing is applied to determine proper welding conditions in order to increase the spot weld quality and to decrease LME. To avoid this extensive testing and to better understand the effect of the local physical conditions on the formation of LME a finite element model of the RSW process has been developed. This multi-physical model couples electrical, thermal, mechanical and metallurgical effects and provides local quantities for current density, temperature, strains, stresses and phase compositions. In addition a damage parameter for LME has been developed based on experimental findings and the local mechanical quantities from the simulations. The simulation results have been validated against experimental RSW tests. Magnetic pulse welding of tubular parts - process modeling10.3217/978-3-85125-615-4-41Joining of metallic tubes by fusion welding processes has remained critical due to distortion and dimensional inaccuracy of the joint. Magnetic pulse welding (MPW) facilitates joining of overlapping tubes by controlled plastic deformation under the action of a short electromagnetic impulse without bulk melting of workpiece materials. Thus, MPW can be an efficient recourse for joining of circular sections. However, the short duration and high amplitude electromagnetic forces impose significant challenge to real-time monitoring of MPW. In contrast, computer-based models can provide an opportunity to realize the causative effect of the key process variables on the joint structure and quality quickly and economically. An attempt is therefore presented in this investigation to develop and test a finite element method based numerical model for MPW process with a focus to realize the nature of the magnetic field and electromagnetic forces that facilitates plastic deformation and joining between the flyer and the target tubes. The effect of key process variables is also tested with experimentally measured results reported in independent literature. CFD simulation of particle movement during atmospheric plasma spraying10.3217/978-3-85125-615-4-42Atmospheric Plasma Spraying (APS) is a powder based coating process with versatile applications in terms of functional layers like corrosion, wear resistant or thermal barrier coatings. However, the fundamental process interactions cannot fully be described and understood experimentally. Therefore, a supportive CFD model was carried out by use of ANSYS 19.0. In detail, the CFD model consists of direct coupled electromagnetic and hydrodynamic formalisms. The particle behaviour was described by a simple multiphase reaction routine. Based on the CFD model, the resulting temperature field and the particle behaviour can be investigated. Especially, the particle trajectory, which represents the particle dwell time in the plasma stream, is of special interest for the final APS coating. Therefore, the description of a stable heat source is of major priority. This work shows a promising approach to evaluate the above mentioned particle and plasma properties, supported by a systematic parameter investigation. The obtained and experimental validated data can be used for a better process understanding as well as for further process optimisation. Correlating large sets of experimental data with high resolution computational weld mechanics models10.3217/978-3-85125-615-4-43The validity of 3D transient computer models to predict the mechanics of welds is judged by correlating predictions made by the computer model with data acquired from experiments, i.e., correlating virtual and real data. The authors suspect that by far the greatest uncertainty in the predictions from these computational models has been due to the sparsity in the available experimental data on material properties and the welding process itself. The goal of this paper is to present a few examples that demonstrate how large sets of experimental data provided by recent developments in sensor technology and data acquisition systems such as distributed fiber optic temperature and strain sensing optical fiber, in addition to full field and transient data such as thermographic and Digital Image Correlation (DIC) cameras can be correlated with predictions from VrWeld, a computer model for computational weld mechanics. A fiber optic system using Rayleigh backscattering is used to measure temperature and strain with a spatial resolution of 0.65 mm at a data acquisition rate of 10 Hz but it could be as high as 250 Hz. The experimental data has noise that must be filtered. To correlate the virtual data the 3D transient Computational Weld Mechanics (CWM) models had to use time steps smaller than 1 second and element size less than 1 mm. With this data, one can correlate the clocks for the experiment and the computer model. In addition, one can run Design of Experiments (DOEs) to correlate material properties such as temperature dependent thermal conductivity and boundary conditions such as convection coefficients. Some of the challenges in synchronizing real and virtual data in space and time are discussed. Investigation of the influence of the welding speed and current on the parameters of the adaptive function10.3217/978-3-85125-615-4-44The adaptive function developed by authors allows direct correlation between welding circumstances and temperature distribution using limited experimental data including weld pool dimensions and temperature at some arbitrary points. This paper intends to investigate the effects of welding speed and welding current on the parameters of the adaptive function. GTAW with various welding speeds and various welding currents was applied on duplex stainless steel plates. According to the experimental data, the parameters of the adaptive function were expressed as a function of welding speed and weld pool dimensions. To show the effectiveness of the new method, Rosenthal model and FEM were employed to simulate the conducted welding and the accuracy of the predicted result rather than the measured temperature were estimated by relative error. The results show that the adaptive function method is more accurate than the FEM and Rosenthal approach in all studied cases. Prediction of grain boundary evolution in an titanium alloy substrate using a novel phase field model coupled with a semi-analytical thermal solution10.3217/978-3-85125-615-4-45Grain boundary migration in the presence of concentrated sources of heat is a complex process that has a considerable impact on resultant material properties. The large thermal gradients generated during welding cause grain boundaries to migrate in order to minimise the total free energy of the system. It is important to consider both the thermal gradient driving force, as well as the local curvature driving force of the grain boundaries which both play a significant role in the evolution of the micro-structure in the weld region. In this work a multi-phase field model is used to predict the grain boundary evolution in a Ti6Al4V substrate subjected to a heat source representative of the electron beam (EB) welding process. While numerical simulations incorporating the mass transfer and complex flow dynamics associated with high energy density welding processes are favourable in that they consider the physical processes occurring in the weld explicitly, they are also extremely computationally expensive. As such, the thermal field, on which the phase field model is dependent, is computed using a semi-analytical solution technique. In this approach the complicated flow dynamics of the EB process are represented as a four-quadrant volumetric heat source, the recently published DEC heat source which has shown to be a good thermal representation of EB processes. Using a Green’s function approach, the time and position dependent thermal field is obtained for this DEC heat source in motion is found, free from numerical errors. Predicted grain size distributions are presented for various energy inputs and conclusions drawn based on the applied driving forces, captured in the phase field model Experimental validation of a simplified welding simulation approach for fatigue assessments10.3217/978-3-85125-615-4-46The paper presents the application of a simplified approach for numerical welding simulations and its validation by means of residual stress measurements. The aim of the presented work is to provide a practical calculation method for welding residual stresses to assess their influence on the fatigue strength of welded structures. Welding simulations are relatively complex while their reliability is often uncertain. On the other hand, residual stress measurements frequently show wide scatter. The paper motivates the use of a simplified approach without calibration by experimental data, as it is applicable for fore- and hindcasting of residual stresses during the design phase or for failure analysis. The simulations are divided into a transient thermal analysis followed by a mechanical analysis. A simple prescribed temperature heat source with uniform temperatures is used to apply the welding energy. The results are validated with residual stress measurements by X-ray diffraction and hole drilling on three welded geometries: longitudinal stiffeners, K-butt welds and a structure-like component. Real-time welding simulation for education10.3217/978-3-85125-615-4-47Virtual reality training systems are state-of-the-art technology in welding education and are mainly meant to teach the manual skills needed for proper welding. In combination with computational numerical welding process simulation such systems can also be used to investigate the influence of different welding parameters on close-to-reality welding results. In this work, a virtual welding training system has been combined with physical welding process simulation algorithms that are optimised for near-real-time calculations and thereby enable a trainee to visualise and learn the influence of different welding parameters in a fast and cost-efficient way. Asymptotics and blending in the modeling of welding10.3217/978-3-85125-615-4-48Important welding questions are often easy to ask and difficult to answer. For example, the question "what is the width of the weld?" is essential for understanding the strength of a weld, but it is currently answered through trial and error, or through sophisticated numerical modeling. In this work, it is proposed that there is a third approach based on a deep understanding of physics, and a basic command of mathematics. From the point of view of the practitioner, the answer can be approximated using formula, tables, and graphs of great generality. In this approach, the aspect of interest of the weld is reduced to its minimal representation, neglecting all secondary physical phenomenon. Mathematically, this corresponds to an asymptotic regime. In contrast with other asymptotic techniques such as perturbation analysis, in the proposed methodology, blending techniques are applied. The advantage of these blending techniques is that they approach the exact solutions (typically within a few percentage points) but involve only a few constants that are suitable to be transmitted in print. Much of the existing work on heat transfer outside welding is summarized in this form, but the approach has not been applied to welding yet. Some welding problems are outside the range of standard blending techniques, and an extension of the techniques will be discussed. The application of this approach will also be discussed using the width of the weld and other related problems. Process simulation of TIG welding for the development of an automatic robot torch through heat pipe - head cooling system10.3217/978-3-85125-615-4-49Modern robot welding torches for arc welding are equipped with interchangeable neck systems. Generally, in TIG welding torches, the transfer powers to the electrode and the heat dissipation from the electrode occur just through the clamping area. The clamping geometry is usually formed in a ring. Therefore, a high resistance is generated between electrode and clamping system. This system suffers from power losses as well as limited heat dissipation. Furthermore, with this system, torch head setting needs tools and consumes a relatively long time. By developing of a non-linear thermo-flow-mechanical / magneto-hydro-dynamic (MHD) FE-model, using CFX ANSYS program, a novel robotic-TIG welding torch with an interchangeable head system and heat dissipation (heat pipe) was innovated. Depending on the predefined process parameters, the resulting thermo-physical heat output effects the torch-head design were investigated and analyzed then taken into account in the design process. The influences of the welding parameters such as current, shielding gas quantity on the torch design were determined through FE calculations. This determination process was connected to the design of the torch cooling system (heat pipe). The temperature distribution and its behavior at arc contour, shielding gas nozzle, and heat pipe body within the heat output range was determined. As well as, the distribution of flow gas velocity through the arc region and its behavior within the heat output range was determined. The use of a four heat pipes cooling system is sufficient for the maximum predefined power. The designed gas supply channels system gives efficient, functioning, and error-free gas flow results during the welding process. At maximum power range, the maximum calculated temperature at the shielding gas nozzle and at heat pipe – (tube body) is ≤ 350 K. Furthermore, the simulation results show the effect of arc focusing due to the shielding gas flow as well as the effect of increasing the welding current on the arc dynamics and their geometric shape. The performed simulation process shows a good alignment between the calculations and the experiments. Finally, a robotic TIG torch with high-quality performance and easy interchangeable torch’s component was innovated and constructed, as well as high weld quality was achieved. Evaporation-determined model for arc heat input in the cathode area by GMA welding10.3217/978-3-85125-615-4-50The most used approaches for the modelling of the heat input in GMA welding process simulation usually assume an axisymmetrical Gaussian distributed heat flux in the cathode region, whereas it has been suggested that the attachment region of the arc to the cathode consists of several highly mobile elemental cathode spots. It is assumed, that the processes in each mobile spot are not stable, but are strongly influenced by the evaporation from the workpiece. Their existence is therefore transient and outside of equilibrium. To calculate the heat input in this area, a concept has been developed on the basis of a cellular automaton method, which allows to calculate the resulting heat flux distribution. At the core of this concept is a simplified model for the elementary cathode spot, which delivers the heat flux and the current density, while taking into account effects of evaporation, which are characteristic for the real GMA welding process. The cellular automaton consists of a grid, whose size is related to the typical size of the cathode spot, taken from literature. The random motion of the spots is simulated according to a probability distribution, which is dependent on the potential heat release of each cathode spot to the cathode surface and therefore also the cathode surface temperature. The number of spots is counted to satisfy a „fuzzy“ current continuity. Additionally a condition is implemented to take into account the state of the arc area. In a next step this model will be coupled to a weld pool calculation. Potentials of the ALE-method for modeling plastics welding processes, in particular for the quasi-simultaneous laser transmission welding10.3217/978-3-85125-615-4-51The Arbitrary-Lagrangian-Eulerian-Method (ALE-Method) offers the possibility to model the quasi-simultaneous laser transmission welding of plastics, in which a squeeze-flow of molten plastic occurs. It is of great interest to get a deeper understanding of the fluid-structure-interactions in the welding zone, since the occurring squeeze-flow transports heated material out of the joining zone, causing a temperature decrease inside. In addition, the numerical modelling offers the possibility to investigate the flow conditions in the joining zone. The aim of this article is to show the potentials of the ALE-Method to simulate the quasi-simultaneous laser transmission welding with the commercially available software LS-DYNA. The central challenge is to realize a bi-directional thermo-mechanically coupled simulation, which considers the comparatively high thermal expansion and calculates the interactions of solid and melted plastic correctly. Finally, the potentials of the ALE element formulations for the mathematical description of welding processes are shown, especially for those with a squeeze-flow. Improvement of numerical simulation model setup and calculation time in additive manufacturing‑laser-metal-deposition compontents with an advanced modelling strategy10.3217/978-3-85125-615-4-52Rapid localized heating and cooling during additive manufacturing using laser deposition method (LMD) lead to loss of dimensional accuracy as well as cracking of built parts. Finite-Element welding simulations allow prediction of geometrical deviations and accumulated residual stresses as well as their optimization before conducting experiments. Due to the great length of stacked welds, calculation times for fully transient thermomechanical simulations are currently long, the calculation stability suffers from the high number of contact bodies in the model and the modelling effort is high, as the geometries need to be sliced and positioned layer-wise. In this contribution, an integrated modelling approach is demonstrated for a thin-walled LMD component made from 30 layers of 1.4404 (316L) stainless steel: Instead of the layer-by-layer modelling strategy commonly found in the literature, the whole component mesh is kept in one piece and the fully transient, layer-by-layer material deposition is implemented via element sets. In contrast to prior simulations, nonlinear contact between the layers does not have to be considered, significantly decreasing calculation times. The calculated distortions are compared to recently published, in-situ digital image correlation (DIC) measurements as well as numerical simulations conducted with the established layer-wise modelling strategy to judge result quality. Finally, the improvement in calculation time and ease-of-use is compared between both modelling approaches and conclusions regarding future usage for industrial-scale components are drawn. Influence of the first weld bead on strain and stress states in wire+arc additive manufacturing10.3217/978-3-85125-615-4-53WAAM (Wire+Arc Additive Manufacturing) allows manufacturing mechanical components by adding successive layers of molten metallic wire using electrical arc. The WAAM process, compared to other processes using metallic powders, presents some advantages such as: high deposition rate (2-4kg/hour), manufacturing of large scales components and cheaper industrial installations. WAAM is then an interesting candidate for manufacturing components often CNC machined. However, the main disadvantages of this process are: high surface roughness requiring a post machining, strains and stresses states generated during the deposition process . A better understanding of the relation between the welding parameters and the state of stresses can contribute to minimize residual stresses, eventually in relation with a deposition strategy . As a first approach, the effects of the first deposition of molten metal on the base plate is investigated. This work focuses on finite element method, based on Code Aster solver, with a nonlinear thermo-mechanical model. Concerning the thermal aspects, the GMAW heat input is modeled by a Gaussian distribution . The temperature fields are used to solve the mechanical problem. The material behavior laws are assumed to be elastoplastic with different hardening configurations: no hardening, linear isotropic or kinematic hardening and non-linear isotropic hardening. Based on the results from these elastoplastic models, the influence of the hardening is presented. New approach for fast numerical prediction of residual stress and distortion of AM parts from steels with phase transformations10.3217/978-3-85125-615-4-54Selective Laser Melting (SLM) is a promising additive manufacturing technology for production of complex and highly individual parts on short lead time request. Pre-processing assisted by numerical simulation can reduce defects which occur during construction and manufacturing and hence increase the quality of the parts and the efficiency of this technology. Especially the inherent strain method  is common and well suited for the fast numerical prediction and optimization of residual stresses and distortion of AM-parts. A major disadvantage of this method is the restriction to austenitic steels without phase transformations in the solid state during the process. The inherent strain method has only a limited validity for martensitic steels because the effects of phase transformations are not taken into account. However, an increasing number of materials to be processed requires a detailed numerical forecast also for martensitic steels to guarantee high quality parts. This research work introduces an extension of the inherent strain method for the consideration of martensitic phase transformations during SLM-process. A multiscale approach, based on a coupled nonlinear thermo-mechanical finite element analysis and the further development of the inherent strain method combined with a new calibration procedure is proposed. It is demonstrated, that the developed approach can be effectively used for the forecast of structure distortions. The presented approach opens the way for the optimization of the additive manufacturing technology towards a defect-free manufacturing process. Multi-scale multiphysics simulation of metal L-PBF AM process and subsequent mechanical analysis10.3217/978-3-85125-615-4-55In this paper, a multi-physics numerical model for multi-track-multi-layer laser powder bed fusion (L-PBF) process is developed and used for analysing the formation and evolution of porosities caused by lack of fusion and improper melting. The simulations are divided into two categories: first and foremost, a multi-physics thermo-fluid model in meso-scale, and second, a mechanical model based on the concept of a unit cell. The thermo-fluid model is used to track and observe the formation of the porosities, and considers phenomena such as multi-phase flow, melting/solidification, radiation heat transfer, capillary and thermo-capillary (Marangoni effect) forces, recoil pressure, geometry dependant absorptivity, and finally evaporation and evaporative cooling. The results for the investigated process parameters indicate that the porosities are mainly formed due to improper fusion of the particles. The probability of presence of pores is also observed to be higher in the first layers. Moreover, the lack of fusion zones are seen to become smaller in the subsequent layers, largely due to better fluid flow and higher temperatures in those layers. Based on the porosity levels determined from the thermo-fluid model, a unit cell mechanical model with an equivalent amount of porosity has been made and subsequently subjected to loading for analysing the part’s mechanical behaviour. The unit cell results show that an increase in the porosity can highly affect and deteriorate the part’s elastic modulus and its yield strength, as well. The combination of the thermo-fluid and the mechanical unit cell model establishes a direct link between process parameters and mechanical properties for L-PBF. Development of a two-dimensional axial symmetry model for wire arc additive manufacturing10.3217/978-3-85125-615-4-56In this study, a numerical model taking into account electromagnetism, fluid flow, and heat transfer in the arc and the melt pool is developed using COMSOL Multiphysics®. The level set method is used to simulate the layer-by-layer addition of materials along the vertical axis to form a cylindrical rod. This shape has the advantage to be simulated with a 2D axial-symmetry model in order to reduce computation time. The implementation of the arc plasma model is first validated by comparing predictions with literature data. Then, the arc pressure, arc shear stress, Lorentz forces and heat flux are analysed and used to define source terms for the additive manufacturing model. In this model, the building of a 308 stainless steel rod is simulated by adding molten metal droplet along the vertical axis. The adding of droplets and heat source are periodically stopped in order to simulate the layer-by-layer build-up of the rod. The calculated shape and the temperature field are analysed and compared to experimental data.