Methodology to improve applicability of welding simulation

Abstract

The objective of this paper is to demonstrate a new simulation technique which allows fast and automatic generation of temperature fields as input for subsequent thermomechanical welding simulation. The basic idea is to decompose the process model into an empirical part based on neural networks and a phenomenological part that describes the physical phenomena. The strength of this composite modelling approach is the automatic calibration of mathematical models against experimental data without the need for manual interference by an experienced user. As an example for typical applications in laser beam and GMA–laser hybrid welding, it is shown that even 3D heat conduction models of a low complexity can approximate measured temperature fields with a sufficient accuracy. In general, any derivation of model fitting parameters from the real process adds uncertainties to the simulation independent of the complexity of the underlying phenomenological model. The modelling technique presented hybridises empirical and phenomenological models. It reduces the model uncertainties by exploiting additional information which keeps normally hidden in the data measured when the model calibration is performed against few experimental data sets. In contrast, here the optimal model parameter set corresponding to a given process parameter is computed by means of an empirical submodel based on relatively large set of experimental data. The approach allows making a contribution to an efficient compensation of modelling inaccuracies and lack of knowledge about thermophysical material properties or boundary conditions. Two illustrating examples are provided.     

Application of process modelling - an industrial view

Process modelling has become accepted as a normal part of the design and improvement of industrial processes. A rapid increase in the use and application of computer modelling has occurred over the past decade due to the increase in capability of desk-top computers. The Steel industry now has an array of powerful computer-based modelling and simulation tools at its disposal, and these are widely used.

This paper presents an overview of the use of modelling and simulation techniques applied to steel mill processes in an industrial context. Modelling techniques include finite element methods, finite difference applications, and the use of neural networks and expert systems. The application of these methods to flat and long product production processes within a ‘through-mill’ modelling capability are demonstrated to show the potential of computer models to process design and simulation.

While indicating the state of the art in application of this technology, some indications of future requirements and goals from an industrial viewpoint are given.     

The investigation of a novel approach in the simulation of the trap levels effect on the conduction in a GaAs P+νN+ diode

A numerical simulation using C⁺⁺ language, allowing the store of big size information (presented as (X × Y × Z) matrix) is realized, and a theoretical modelling of the charges transport in the semiconductor devices made in materials presenting a significant concentration of impurities whose energy levels are deep is implemented. The results are validated on a semi-insulating gallium arsenide (SI-GaAs) PIN structure. The numerical simulation is made by the resolution of the phenomenological transport's equations by adopting a new approach permitting to study the conduction phenomena through all the structure. From the 3D modelling, the variation of the potential profile, the free carriers and the space charge distributions, and the electrical field inside the structure are simulated. The effects of the traps are taken into account through the mechanisms of recombination and storage of a significant space charge, where the influence of the presence of these levels on the above factors is seen. Through this work a physical model intended for the study of the influence of the trap levels on the potential and the free carrier density distributions for weak lifetime semiconductors, more particularly on semi-insulating GaAs is introduced. Taking into account the trap centres is essential for a good comprehension and explanation of the transport phenomena. The present study consists to do a simulation and a modelisation of structures based on materials presenting a deep and a trap centres. A trapping model based on a combined numerical method Gummel and Newton is adopted to reduce both the run time and to reach convergence in the process when working not with one junction but with two.     

Quantitative simulations of microstructure evolution in single crystal superalloys during solution heat treatment

Abstract

This work presents a modelling approach for the simulation of the microstructure evolution during solution heat treatment in technical, multi component superalloys. This model is based on a theoretical approach and calclations. All calculations start with the simulation of solidification, as the as-cast microstructure is the starting condition to simulate the subsequent heat treatment. The model takes into account typical length scales, diffusion, morphological and constitutional aspects of the alloys. Simulations are performed for an alloy with several alloying elements. It is proposed that with the aid of the simulations optimised solution heat treatments can be derived.     

Hybrid forming processes for production of lightweight high strength automotive panel parts

Abstract

The research concentrates on a heat treatable AA 6082 aluminium alloy. A set of unified constitutive equations has been developed and determined from experimental data. In addition to modelling viscoplastic flow of the material at different temperatures, the equations contain other two specific features. One is to predict the failure of the material under various deformation conditions based on continuum damage mechanics theories. The other is to model the precipitation formation and growth under straining and aging conditions; thus, the strength distribution of formed parts can be predicted via process modelling. The determined unified constitutive equations are then implemented into the commercial finite element code ABAQUS/Explicit via the user defined subroutine, VUMAT. A finite element process simulation model and numerical procedures are established for the modelling of a hot stamping and cold die quenching processes for a spherical part with a central hole. To validate the simulation results, a test programme is developed, a test rig has been designed and manufactured and tests have been carried out under different forming rates. It has been found that very close agreements between experimental and numerical process simulation results are obtained for the range of forming rates carried out.     

A holistic integrated dynamic design and modelling approach applied to the development of ultraprecision micro-milling machines

Ultraprecision machines with small footprints or micro-machines are highly desirable for micro-manufacturing high-precision micro-mechanical components. However, the development of the machines is still at the nascent stage by working on an individual machine basis and hence lacks generic scientific approach and design guidelines. Using computer models to predict the dynamic performance of ultraprecision machine tools can help manufacturers substantially reduce the lead time and cost of developing new machines. Furthermore, the machine dynamic performance depends not only upon the mechanical structure and components but also the control system and electronic drives. This paper proposed a holistic integrated dynamic design and modelling approach, which supports analysis and optimization of the overall machine dynamic performance at the early design stage. Based on the proposed approach the modelling and simulation process on a novel 5-axis bench-top ultraprecision micro-milling machine tool – UltraMill – is presented. The modelling and simulation cover the dynamics of the machine structure, moving components, control system and the machining process, and are used to predict the overall machine performance of two typical configurations. Preliminary machining trials have been carried out and provided the evidence of the approach being helpful to assure the machine performing right at the first setup.     

Flow stress modeling for copper under changing process conditions

Realistic simulation of metal forming processes requires constitutive equations that describe the behavior of a material under varying process conditions. The equations that have hitherto been developed to address this problem are generally too involved and require the determination of many constants. In this paper, a simple approach based on the representative nature of the work hardening rate, which enables the use of an elementary rate equation in such modelling, is introduced. A prediction methodology based on the concept of fading memory is developed and is found to give good predictions in the case of copper. A mechanistic interpretation of the approach is proposed.     

On the evaluation of the global heat transfer coefficient in cutting

The use of numerical simulations for investigating machining processes is remarkably increasing because of the simulation cost is lower than the experiments and the possibility to analyze local variables such as pressures, strains, and temperatures is allowable. Process simulation is very hard from a computational point of view, since it frequently requires remeshing phases and very small time steps. As a consequence, the simulated cutting time is usually of the order of few milliseconds and no steady cutting conditions are generally achieved, at least as far as thermal conditions are concerned. Therefore, nowadays numerical prediction of cutting temperatures cannot be considered fully reliable. In the paper this issue was taken into account: a mixed Lagrangian-Eulerian numerical approach was utilized and the global heat transfer (film) coefficient at the tool-chip interface was derived through an inverse approach. Finally, the dependence of the film coefficient on pressure and temperature on the rake face was investigated.     

Modeling mechanical alloying: Advances and challenges

While mechanical alloying is a commercial entity for oxide-dispersion-strengthened superalloys, its application to other systems has run into a number of scientific and commercial barriers. In part, this is due to the inadequate scientific underpinning. This article reviews the status of the modeling of the mechanical alloying processes and suggests an approach to improving current knowledge of the process.     

Life Cycle Simulation System for Life Cycle Process Planning

To realize closed loop manufacturing, it is essential to design product life cycles and to plan life cycle processes property. Life cycle simulation has been recognized as an effective tool in this direction. In this paper, we present a life cycle simulation system developed as a general tool for life cycle design and management. The system includes functions for modelling and controlling each life cycle process in a flexible manner. The system maintains usage history of products and parts independently taking the reuse of parts into account. Examples of the simulation are shown for both a rapid life cycle scenario and for a part sharing scenario over the product generations.     

Riveted assembly modelling: Study and numerical characterisation of a riveting process

In the field of structural stress analysis and especially in transitory dynamics (crash and impact simulations), study/design accuracy requires increasingly predictive models. A compromise between cost and precision entails modelling and simplification of all the link elements. Riveting is a particularly sensitive case. The characteristics needed to ensure effective modelling are indeed difficult to measure. This paper presents adjustment of a numerical model simulating a riveted link using a number of different approaches. The results analysis considerably improve knowledge about the riveting process and behaviour of riveted links. This study will pave the way for resistance tests to be conducted in order to numerically characterise riveted links under load. The aim is to develop an approach that reduces the model size and calculation time without adversely affecting the validity of the simulation results, and to show the effect of strains and residual stresses on the link in post-riveting.     

A new one-phase material model for the numerical prediction of critical material flow conditions in thixoforging processes

Thixoforging allows one-step forming processes of near-net shape components having excellent mechanical properties. However, the high sensitivity of thixoforging regarding process conditions requires precise modelling and determination of process related parameters. At the same time, simple numerical design proves challenging because of the inaccuracy of existing one-phase material models regarding the shear thinning flow behaviour of semi solid metals. Consequently, this paper deals with the development of a new one-phase material model providing a more precise simulation of materials’ shear rate dependency. By using this model, simulations could be performed, which allowed the prediction of solidification and flow-related component defects.     

Optimization method for stamping tools under reliability constraints using genetic algorithms and finite element simulations

Controlling variability and process optimization are major issues of manufacturing processes which should be tackled together since optimal processes must be robust. There is a lack of numerical tool combining optimization and robustness. In this paper, a complete approach starting from modelling and leading to the selection of robust optimal process parameters is proposed. A model of stamping part is developed through Finite Element simulation codes and validated by experimental methods. The search for optimal tool configurations is performed by optimizing a desirability function and by means of a genetic algorithm based optimization code. Several tool configurations are selected from the resulting solutions and are observed through robustness analysis. Noise parameters relating to friction and material mechanical properties are taken into consideration during this analysis. A quadratic response surface developed with design of experiments (DOE) links noise parameters to geometrical variations of parts. For every optimal configuration, the rate of non-conform parts which do not satisfy the design requirements is assessed and the more robust tool configuration is selected. Finally, a sensitivity analysis is performed on this ultimate configuration to observe the respective influence of noise parameters on the process scattering. The method has been applied on a U-shape part.     

Uncertainty quantification in modelling welding residual stress and distortion

Abstract

Currently available welding simulation methodologies provide deterministic results for the best estimate of the input parameters, such as part geometry, processing conditions and material properties. If there is an uncertainty in any of the input parameters, then a reanalysis needs to be performed with perturbed values of each uncertain variable. However, there can be several hundred input parameters; therefore, the use of reanalysis in uncertainty quantification in welding modelling can be time consuming or computationally prohibitive, especially for three-dimensional modelling. This paper explores the application of design sensitivity analysis in quantifying uncertainty in welding residual stress and distortion computations. Analytic sensitivities are computed by direct differentiation, resulting in a very efficient computational approach. The variation of temperature, welding residual stress and distortion with respect to processing parameters is computed from a first order Taylor expansion of the model output. The approach is demonstrated in a three-dimensional model of a singe pass weld and validated by comparing sensitivity analysis results to reanalyses.     

Simulation of workpiece forming and centre displacement in plunge centreless grinding

By reducing the set-up time of centreless grinding system, higher process flexibility and productivity can be obtained. This goal is approached by developing a simulation model that assists in efficient centreless grinding system set-up. In this paper, an analytical modelling of plunge centreless grinding is described. The developed model incorporates the workpiece forming mechanism along with workpiece dynamics and facilitates the simulation of workpiece form and its centre displacement. The prototype simulation model has been embedded within a spreadsheet environment and experimentally verified.     

Modelling Dynamics of Autonomous Logistic Processes: Discrete-event versus Continuous Approaches

For developing and benchmarking autonomous logistic processes, dynamic models are essential. The paper investigates two different modelling approaches regarding their abilities to describe an exemplary scenario -an autonomously controlled shop floor. A discrete-event simulation model is compared to a continuous System Dynamics model. An autonomous control strategy is developed and its effectiveness and robustness are investigated by analysing the dynamic behaviour and the logistic performance in cases of work load fluctuations and unexpected disturbances.     

Elasto-visco-plastic analysis of welding residual stress

Abstract

In the past three decades, extensive research has focussed on the application of numerical methods for the computation of residual stress. Most commonly, the simulations involved performing weakly coupled thermal mechanical finite element analyses in Lagrangian reference frames assuming rate independent elasto-plastic material response. Nearly all approaches assumed rate independent elasto-plastic material response, which is most appropriate at low to moderately elevated temperatures. At, the high temperatures near the fusion zone, the material response becomes rate dependent and an elasto-visco-plastic model would be more suitable. In 1989, Brown et al. (Int. J. Plast., 1989, 5 , 95–130) proposed a rate dependent constitutive equation (commonly known as Anand's model) to describe the plastic evolution of metals at high temperatures. The objective of this work is twofold: evaluate the suitability of Anand's elasto-visco-plastic model in computing welding residual stress and investigate the feasibility of an Eulerian implementation of Anand's model in modelling welding residual stress. Such an implementation has the potential to reduce computational cost in modelling welding processes, since it is a steady state analysis as compared to the common time incremental Lagrangian analyses. An Eulerian reference frame is also more advantageous in modelling processes with large deformation such as friction stir welding, rolling and extrusion since excessive mesh distortion and re-meshing are no issues as the case of Lagrangian models (Int. J. Mater. Form., 2008, 1 , 1287–1290).     

Modelling of residual stress field in spherical mandrelling process

Fatigue life of structural elements with bolt holes depends mainly on residual stress distribution law around these holes. Residual compressive circumferential normal stresses around the hole reduce operating stress magnitudes to minimum values for cyclic tension and this enhances fatigue life and load-carrying capacity of structures. The presence of residual stresses is a result of the manufacturing process. Residual stresses can also be induced deliberately around the holes by means of appropriate working with suitably chosen parameters. Quantitative knowledge of residual stresses is necessary to model the stress field after applying an external load to a structural element in order to assess static or dynamic strength, fatigue strength including. This paper presents a combined approach consisting of experimental and numerical modelling of residual circumferential normal stress distribution when forming holes in workpieces of medium carbon steel by spherical mandrelling (SM). Since the object of study is residual macrostresses (stresses of first type), a mechanical method of their determination has been employed. On the basis of experimental outcomes, a mathematical model has been built and it predicts mean integral value distribution of residual circumferential normal stresses. Since the range of the experimental technique employed is limited by the wall thickness of the bushing being worked, numerical modelling of residual stresses by means of FE simulation has been performed. The numerical results obtained allow this mathematical model to be applied to various wall thicknesses by introducing correction factors for the polynomial coefficients. At the same time, the adequacy of the proposed FE model can be evaluated only by the experimentally obtained mathematical model. The SM efficiency for enhancing the load-carrying capacity of structural elements with cylindrical holes subjected to tension has been proved by means of FE simulation.     

Finite element analysis of the ring rolling process with integrated closed-loop control

In FEA of ring rolling processes the tools’ motions usually are defined prior to simulation. This procedure neglects the closed-loop control, which is used in industrial processes to control up to eight degrees of freedom (rotations, feed rates, guide rolls) in real time, taking into account the machine's performance limits as well as the process evolution. In order to close this gap in the new simulation approach all motions of the tools are controlled according to sensor values which are calculated within the FE simulation. This procedure leads to more realistic simulation results in comparison to the machine behaviour.     

板料成形模拟中惯性效应的探讨

为了研究冲压成形过程动力显式有限元分析中惯性效应对板材变形的影响 ,针对圆筒件拉深建立了一个力学模型 ,采用能量法对该模型进行了分析 ,探讨了决定惯性效应对板材变形影响程度的主要因素 ,提出了合理选择模具运动速度的方法。通过数值计算对力学模型分析结果进行了验证