Frontiers of Materials Science (v.5, #2)

Simulation of welding by Chuan-Song Wu; Michael Rethmeier; Christopher Schwenk (77-78).

Virtual welding equipment for simulation of GMAW processes with integration of power source regulation by Uwe Reisgen; Markus Schleser; Oleg Mokrov; Alexander Zabirov (79-89).
A two dimensional transient numerical analysis and computational module for simulation of electrical and thermal characteristics during electrode melting and metal transfer involved in Gas-Metal-Arc-Welding (GMAW) processes is presented. Solution of non-linear transient heat transfer equation is carried out using a control volume finite difference technique. The computational module also includes controlling and regulation algorithms of industrial welding power sources. The simulation results are the current and voltage waveforms, mean voltage drops at different parts of circuit, total electric power, cathode, anode and arc powers and arc length. We describe application of the model for normal process (constant voltage) and for pulsed processes with U/I and I/I-modulation modes. The comparisons with experimental waveforms of current and voltage show that the model predicts current, voltage and electric power with a high accuracy. The model is used in simulation package SimWeld for calculation of heat flux into the work-piece and the weld seam formation. From the calculated heat flux and weld pool sizes, an equivalent volumetric heat source according to Goldak model, can be generated. The method was implemented and investigated with the simulation software SimWeld developed by the ISF at RWTH Aachen University.
Keywords: GMAW; process simulation; regulation algorithm; power source

It is of great significance to obtain a thorough understanding of the physical mechanisms responsible for humping bead phenomenon in high speed gas metal arc welding (GMAW) in order to raise welding efficiency. Experiments were conducted to observe the weld pool behaviors in high speed GMAW, and it was found that both the severely deformed weld pool surface and strong backward flowing play a dominant role in humping bead formation. In this study, a mathematical model is developed to quantitatively analyze the forming mechanism of humping beads for high speed GMAW through considering both the momentum and heat content distribution of the backward flowing molten metal inside the weld pool. The transient development of temperature profiles in the weld pool with severe deformation demonstrates the humping bead forming process under some welding conditions. The predicted and measured humping bead dimensions are in agreement.
Keywords: numerical simulation; humping weld bead; high-speed GMAW

Numerical investigations of arc behaviour in gas metal arc welding using ANSYS CFX by M. Schnick; U. Fuessel; M. Hertel; A. Spille-Kohoff; A. B. Murphy (98-108).
Current numerical models of gas metal arc welding (GMAW) are trying to combine magnetohydrodynamics (MHD) models of the arc and volume of fluid (VoF) models of metal transfer. They neglect vaporization and assume an argon atmosphere for the arc region, as it is common practice for models of gas tungsten arc welding. These models predict temperatures above 20 000 K and a temperature distribution similar to tungsten inert gas (TIG) arcs. However, current spectroscopic temperature measurements in GMAW arcs demonstrate much lower arc temperatures. In contrast to TIG arcs they found a central local minimum of the radial temperature distribution. The paper presents a GMAW arc model that considers metal vapour and which is in a very good agreement with experimentally observed temperatures. Furthermore, the model is able to predict the local central minimum in the radial temperature and the radial electric current density distributions for the first time. The axially symmetric model of the welding torch, the work piece, the wire and the arc (fluid domain) implements MHD as well as turbulent mixing and thermal demixing of metal vapour in argon. The mass fraction of iron vapour obtained from the simulation shows an accumulation in the arc core and another accumulation on the fringes of the arc at 2000 to 5000 K. The demixing effects lead to very low concentrations of iron between these two regions. Sensitive analyses demonstrate the influence of the transport and radiation properties of metal vapour, and the evaporation rate relative to the wire feed. Finally the model predictions are compared with the measuring results of Zielińska et al.
Keywords: arc welding; numerical simulation; GMAW; ANSYS CFX

The mechanism of penetration increase in A-TIG welding by Rui-Hua Zhang; Ji-Luan Pan; Seiji Katayama (109-118).
The mechanism of the increasing of A-TIG welding penetration is studied by using the activating flux we developed for stainless steel. The effect of flux on the flow and temperature fields of weld pool is simulated by the PHOENICS software. It shows that without flux, the fluid flow will be outward along the surface of the weld pool and then down, resulting in a flatter weld pool shape. With the flux, the oxygen, which changes the temperature dependence of surface tension grads from a negative value to a positive value, can cause significant changes on the weld penetration. Fluid flow will be inward along the surface of the weld pool toward the center and then down. This fluid flow pattern efficiently transfers heat to the weld root and produces a relatively deep and narrow weld. This change is the main cause of penetration increase. Moreover, arc construction can cause the weld width to become narrower and the penetration to become deeper, but this is not the main cause of penetration increase. The effects of flux on fluid flow of the weld pool surface and arc profiles were observed in conventional TIG welding and in A-TIG welding by using high-speed video camera. The fluid flow behavior was visualized in realtime scale by micro focused X-ray transmission video observation system. The result indicated that stronger inward fluid flow patterns leading to weld beads with narrower width and deeper penetration could be apparently identified in the case of A-TIG welding. The flux could change the direction of fluid flow in welding pool. It has a good agreement with the simulation results.
Keywords: A-TIG welding; penetration increase; numerical simulation; X-ray; high-speed video camera

The paper presents bounded volume heat sources and the corresponding functional-analytical expressions for the temperature field. The power density distributions considered here are normal, exponential and parabolic. The sources model real heat sources like the welding arc, laser beam, electron beam, etc., the convection in the weld pool as well as the latent heat due to fusion and solidification. The parameters of the heat source models are unknown a priori and have to be evaluated by solving an inverse heat conduction problem. The functional-analytical technique for calculating 3D temperature fields in butt welding is developed. The proposed technique makes it possible to reduce considerably the total time for data input and solution. It is demonstrated with an example of laser beam welding of steel plates.
Keywords: laser beam welding; volume heat source; functional-analytical solution; inverse modelling

Estimation of a source term in a quasi steady two-dimensional heat transfer problem: application to an electron beam welding by Jia-Lin Guo; P. Le Masson; E. Artioukhine; T. Loulou; P. Rogeon; M. Carin; M. Dumons; J. Costa (126-134).
In previous work, we have analyzed the feasibility of the estimation for a source term S(x, y, z) in a transversal section. The present study is concerned with a two-dimensional inverse phase change problem. The goal is the estimation of the dissipated heat flux in the liquid zone (reconstruction of a source term in the energy equation) from experimentally measured temperatures in the solid zone. This work has an application in the electron beam welding of steels of thickness about 8 cm. The direct thermometallurgical problem is treated in a quasi steady two-dimensional longitudinal section (x, y). The beam displacement is normally in the y direction. But in the quasi steady simulation, the beam is steady in the study section. The sample is divided in the axial direction z in few sections. At each section, a source term is defined with a part of the beam and creates a vaporized zone and a fused zone. The goal of this work is the rebuilding of the complete source term with the estimations at each section. In this paper, we analyze the feasibility of the estimation. For this work, we use only the simulated measurements without noise.
Keywords: estimation; source term; quasi steady; the iterative regularization method

The results of the accurate experimental observations on binary Al-Si alloys are presented, which clearly demonstrate that the solidification cracking is a result of the accumulation of macroscopic tensile displacement in a microscopic intergranular liquid film of segregates at the final stage of the weld metal solidification. The reconstructed mechanism of crack initiation provides a clear phenomenological interrelation between the cracking susceptibility, parameters of the welding process and properties of the base and filler material. The correspondent numerical model takes into account the effects of displacement accumulation as well as the influence of thermodynamical and thermo-mechanical properties of the welded material. It is successfully applied for development of technological means for elimination of the solidification cracking during welding of aluminium alloys AA6056, such as a multi-beam welding.
Keywords: welding; solidification cracking; numerical simulation

Low-expansion superalloys are susceptible to weld solidification cracks and heat-affected zone (HAZ) microfissures. To predict solidification cracking, QBasic procedures were developed and solidification reaction sequence, type, and amount of eutectic product were calculated. As manifested, primary solidification is followed by L → (γ + NbC) and L → (γ + Laves) eutectic reaction sequentially for GH903 and GH907; hence, the terminal eutectic constituents are made up of γ/NbC and γ/Laves. While for GH909, only reaction L → (γ + Laves) occurs and more γ/Laves eutectic forms. Therefore, GH909 is more sensitive to solidification cracking. To predict HAZ liquation, cracking Visual FORTRAN procedures were developed, and constitutional liquation of NbC was simulated. As shown, solid dissolution of NbC prior to liquation decreases, and initial liquid film increases with the rate of thermal cycle. Higher rate of thermal cycle promotes the melting of the matrix adjacent to the liquid film and postpones the solidification of the liquid by the liquid-to-γ mode. Thus, more residual liquid film remains at the eutectic point, which will promote HAZ microfissuring. The increase in original grain size and peak temperature also promotes liquation. Finally, these conclusions were verified indirectly by hot ductility tests.
Keywords: solidification cracking; HAZ microfissuring; constitutional liquation; low-expansion superalloy; numerical simulation

Development and application of software packages in welding engineering by Yan-Hong Wei; Xiao-Hong Zhan; Zhi-Bo Dong (160-167).
The weldability of some material is analyzed with simple calculating program in this paper, and weldability testing data are shared through database system. The welding procedures are designed with help of expert systems, and the knowledge is shared among welding engineers. Not only the preparing progress of the welding documents is completed with database systems but also the complex decision on the necessity of the qualification test according to the present procedure qualification records (PQRs) and manufacture codes is made. Moreover, the artificial neural network (ANN) technique is proven to be one of the effective ways to predict mechanical properties of welded joints when there are enough tested data to train the models. Finally, the achievements in modeling microstructure of welded joints are introduced, especially in solid transformation and grain growth in both heat-affected zone (HAZ) and welded molten pool.
Keywords: welding software package; finite element method (FEM); welding procedure qualification; mechanical properties of welded joints; expert system; database system

The usage of continuous cooling transformation (CCT) diagrams in numerical welding simulations is state of the art. Nevertheless, specifications provide limits in chemical composition of materials which result in different CCT behavior and CCT diagrams, respectively. Therefore, it is necessary to analyze the influence of variations in CCT diagrams on the developing residual stresses. In the present paper, four CCT diagrams and their effect on numerical calculation of residual stresses are investigated for the widely used structural steel S355J2 + N welded by the gas metal arc welding (GMAW) process. Rather than performing an arbitrary adjustment of CCT behavior, four justifiable data sets were used as input to the numerical calculation: data available in the Sysweld database, experimental data acquired through Gleeble dilatometry tests, and TTT/CCT predictions calculated from the JMatPro and Edison Welding Institute (EWI) Virtual Joining Portal software. The performed numerical analyses resulted in noticeable deviations in residual stresses considering the different CCT diagrams. Furthermore, possibilities to improve the prediction of distortions and residual stress based on CCT behavior are discussed.
Keywords: welding simulation; GMAW; CCT sensitivity; welding residual stress

Node dynamic relaxation method: principle and application by Hong-Yuan Fang; Tao Wang; Jun-Feng Hu; Jian-Guo Yang (179-195).
Two main methods, inactive element method and quiet element method, to simulate the process of multilayer and multipass welding were reviewed, and the shortcomings of both methods were discussed as well. Based on these analyses, a method called node dynamic relaxation method was put into forward to simulate the multilayer and multipass welding process, and the principle and application of this method were discussed in detail. The simulating results show that using the node dynamic relaxation method can decrease mesh distortion, improve calculation efficiency, and obtain good simulation results. This method can also be used in the field of simulation addition or removing materials in finite element analysis.
Keywords: inactive element method; quiet element method; node dynamic relaxation method; multilayer and multipass welding; finite element analysis (FEA)

The possibilities of predicting welding distortions are extensive. The boundary conditions used in industrial production play an important role in choosing the right strategy. Not only the right abstraction of the welding process is essential for correct and useful results, the clamping conditions and pre-tack welding are also very important. This article reviews experiments and FEM calculations of welded complex structures of industrial relevance. The examined structure comes from a railway vehicle and contains u-profiles with a sheet thickness of 4 mm. The review starts with the explanation of the researched structure and shows different welding situations, like unclamped and clamped manufacturing. Then the FE model with several weld seams is explained and the used boundary conditions are shown. Finally, the measured and calculated distortions are compared and discussed.
Keywords: welding simulation; FEM; distortion; boundary condition; clamping

In this paper, Ductility Dip Cracking (DDC) susceptibility in Inconel600 companion Filler Metal 82 (FM82) under different stress states is investigated. Inconel600 is a Ni-Cr-Fe alloy with excellent resistance to general corrosion, localized corrosion, and stress corrosion, which has been widely used in nuclear power plants. However, the companion FM82 has been shown to be susceptible to DDC in welding process. To resolve the problem, this work is mainly focused on evaluating DDC susceptibility in FM82 in welding process. First of all, Strain to Fracture (STF) test is used to achieve the DDC criterion under simple stress state, and the formation mechanism of DDC was explained. Real welding is a process with complex stress state. Later, to get the DDC susceptibility under complex stress state, models about multi-pass welding were built up by means of finite element method. According to numerical simulation results, relationship of deformation and temperature history is achieved. Moreover, susceptible locations and moments could be determined associated with STF results. The simulation results fairly agree with welding experiment from another research.
Keywords: Ductility Dip Cracking (DDC); welding; Inconel600; numerical simulation

Hybrid model for prediction of welding distortions in large structures by Vesselin Michailov; Nikolay Doynov; Christoph Stapelfeld; Ralf Ossenbrink (209-215).
The paper presents a short overview of the contemporary approaches for calculating welding distortions. In order to meet the existing challenges, an advanced hybrid model for prediction of welding distortions in large structures is described. For the purpose of illustrating the capability of this model, a simulation case is put into discussion. The results are validated by comparison with experimental data, as well as with common simulation technique. Analysis of the calculation costs is also presented. The directions for development of calculation technique, based on the presented model, are also suggested.
Keywords: hybrid model; welding distortion; calculation

Residual stress characteristics of butt-welded flange by finite element analysis by Yong-Lun Song; Xiao-Hong Yang; Guo-Wei Ran; Tian-Jiao Xiao; Si-Bo Yan (216-223).
Finite element simulation is utilized in an aluminum alloy 2014 butt-welded flange under AC Tungsten Inert Gas (AC-TIG) welding condition. The simulated results are in good agreement with the residual stress for the plate test using the actual welding parameters. Furthermore, characteristics of residual stress could be investigated in detail in several aspects, such as the welding structures, the welding sequences, the time intervals, preheating, and repair weld. The intermittent welding may be more convenient and advantageous for the practical applications to reduce the stress, and the local repair welding may cause more stress within the repairing region obviously.
Keywords: finite element method; high-strength aluminum alloy; butt weld; residual stress; weld processes

Numerical methods are nowadays a useful tool for the calculation of distortion and residual stresses as a result from the welding process. Modern finite element codes not only allow for calculation of deformations and stresses due to the welding process but also take into account the change of microstructure due to different heating and cooling rates. As an extension to the pure welding simulation, the field of welding mechanics combines the mechanics and the material behaviour from the welding process with the assessment of service behaviour of welded components. In the paper, new results of experimental and numerical work in the field of welding mechanics are described. Through examples from automotive, nuclear and pipe-line applications it is demonstrated that an equilibrated treatment and a close interaction of “process”, “properties” and “defects” are necessary to come up with an advanced fitness-forservice assessment of welded components.
Keywords: welding simulation; residual stress; defect assessment; fracture mechanics

Modeling of friction stir welding process for tools design by San-Bao Lin; Yan-Hua Zhao; Zi-Qiu He; Lin Wu (236-245).
A three-dimensional friction stir welding (FSW) process model has been developed based on fluid mechanics. The material transport in the welding process has been regarded as a laminar, viscous, and non-Newtonian liquid that flows past a rotating pin. A criterion to divide the weld zone has been given on the basis of cooperation of velocity field and viscosity field. That is, the η 0-easy-flow zone that existed near the tool pin corresponded to the weld nugget zone; the area between the η 0-easy-flow zone and η 1-viscosity band is corresponded to the thermal-mechanical affected zone (TMAZ). The model gives some useful information to improve the understanding of material flow in FSW through the simulation result of velocity distribution. In order to appraise the friction stir pin design, three kinds of pin geometry, one is column pin, the second is taper pin, and the last one is screw threaded taper pin, were used in the model. The pin geometry seriously affected the simulation result of velocity distribution in the η 0-easy-flow zone. The velocity distribution in the η 0-easy-flow zone can be considered as the criterion of optimizing friction stir tool design. This study will benefit to direct the friction stir tool design.
Keywords: friction stir welding (FSW); 2014 Al alloy; model; flow; friction stir pin