Chris Bennett

Modelling and simulation of the inertia friction welding of shafts

The commercial materials forming package DEFORM-2D is used to model the inertia friction welding (IFW) process with particular reference to aero-engine mainline drive shafts. Both representative and predictive modelling techniques are presented, and models are described for the welding of identical and dissimilar material/geometry combinations. The range of material properties required for the models are discussed and details of the tests carried out to produce suitable material data are included. Case studies involving Inconel 718 and AerMet 100 are presented. The phase transformations in a high-strength aerospace steel are included in the model and their effects on residual stresses are presented. Temperature profiles are compared with experimental thermocouple measurements and the models are also compared with upset and rotational velocity data collected during welding. The DEFORM-2D software in conjunction with a friction law coded into a subroutine are shown to be suitable for modelling the IFW process between similar and dissimilar shaft materials. Results highlight the importance of the inclusion of the volume change associated with the martensite transformation on the residual stresses generated during the post-weld cooling of IFW joints.

Finite Element Modeling of the Inertia Friction Welding of Dissimilar High-Strength Steels

Finite element (FE) process modeling of inertia friction welding between dissimilar high-strength steels, AerMet® 100 and SCMV, has been carried out using the DEFORM™-2D (v10.0) software. This model was validated against experimental data collected for a test weld performed between the materials; this included process data such as upset and rotational velocities as well as thermal data collected during the process using embedded thermocouples. The as-welded hoop residual stress from the FE model was also compared with experimental measurements taken on the welded component using synchrotron X-ray and neutron diffraction techniques. The modeling work considered the solid-state phase transformations which occur in the steels, and the trends in the residual stress data were well replicated by the model.

Finite element modelling strategies of weld repair in pre-stressed thin components

A total of two computational procedures have been developed in the commercial finite element software codes Sysweld and ABAQUS to analyse and predict the residual stress state after the repair of small weld defects in thin structural components. The numerical models allow the effects of the repair to be studied when a pre-existing residual stress field is present in the fabricated part and cannot be relieved by a thermal treatment. In this work, the modelling strategies are presented and tested by simulating a repair of longitudinal welds in thin sheets of Inconel 718. Although the numerical strategies in the two codes are intrinsically different, the results show a significant agreement, predicting a notable effect imposed by the initial residual stress.

Current capabilities of the thermo-mechanical modelling of welding processes

Some fundamental principles and advanced applications of thermo-mechanical modelling techniques used for industrial welding processes are described. The paper covers a range of modelling procedures comprising welding simulation and residual stress analysis of multi-pass, thick-walled ferritic steel pipes; welding simulation and distortion analysis of nickel-based superalloy thin plates; and inertia friction welding of dual alloy drive shafts. A number of pertinent mechanical and metallurgical concepts are discussed, including material behaviour models and material properties, microstructural evolution, solid state phase transformation and post-weld heat treatment. The paper emphasises the general methodologies of thermal modelling procedures and their role in the holistic process of the life assessment and design improvement of power plant piping systems and aeroengine casings and shaft components, operating under high temperature creep and fatigue service conditions.

An experimental and numerical investigation on the process efficiency of the focused-tungsten inert gas welding of Inconel 718 thick plates

A combined experimental and numerical approach was adopted to investigate the focused-tungsten inert gas welding process by producing bead-on-plate welds in Inconel 718 plates. Experimental investigations were carried out by means of thermocouple measurements and optical macrographs of the weld cross-section. Three-dimensional finite element simulations were conducted using the commercial specialized finite element software Sysweld in order to predict the thermal field induced by the process in the plates. The work presents an approach to investigate the process efficiency and calibrate the heat source model in order to produce a full thermal characterization of the plasmatron welding apparatus.

Modelling of Inertia Friction Welding Using Finite Element Analysis and Computational Fluid Dynamics

Inertia Friction Welding (IFW) is a solid-state joining process where one rotating (connected to an inertia) and one stationary part are brought together under an axial load, causing frictional heat generation and plastic deformation at the interface; upon cooling a weld is formed between the components. There is evidence in welds between dissimilar materials which show a flow regime that may keep impurities at the weld interface and may have implications for weld strength and fatigue life. Numerical modelling of IFW using Finite Element Analysis (FEA) has allowed the successful prediction of temperature profile, upset (length loss) and flash shape and process parameters such as flywheel slowdown. However, due to the lack of knowledge of the behaviour of the severely plasticised zone (shear zone) and the fluid-like nature of the material near the interface, the use of Computational Fluid Dynamics (CFD) has been considered. This paper presents a method to utilise both FEA and CFD modelling techniques to provide a better modelling strategy for the IFW processes. By using the results of an FEA model as the boundary/initial conditions for the CFD, simple models have allowed comparison between the two numerical approaches and have validated the implementation and consistency of material properties and modelling methodology for both. A model of the interface has been produced with CFD with this method which illustrates the possible material behaviour and material flow in that zone.

Optimisation of material properties for the modelling of large deformation manufacturing processes using a finite element model of the Gleeble compression test

The finite element modelling of manufacturing processes often requires a large amount of large plastic strain flow stress data in order to represent the material of interest over a wide range of temperatures and strain rates. Compression data generated using a Gleeble thermo-mechanical simulator is difficult to interpret due to the complex temperature and strain fields, which exist within the specimen during the test. In this study, a non-linear optimisation process is presented, which includes a finite element model of the compression process to accurately determine the constants of a five-parameter Norton–Hoff material model. The optimisation process is first verified using a reduced three-parameter model and then the full five-parameter model using a known set of constants to produce the target data, from which the errors are assessed. Following this, the optimisation is performed using experimental target data starting from a set of constants derived from the test data using an initial least-squares fit and also an arbitrary starting point within the parameter space. The results of these tests yield coefficients differing by a maximum of less than 10% and significantly improve the representation of the flow stress of the material.

The Evaluation of Coefficient of Friction for Representative and Predictive Finite Element Modelling of the Inertia Friction Welding

Inertia friction welding is the process in which stored kinetic energy in a flywheel is converted to heat by relative sliding movement between surfaces of axi-symmetric components to achieve a weld in the solid-state. The work in this paper relates to the production of dual-alloy shafts for aeroengines. Frictional characteristics determine the conditions at the weld interface and these are controlled by rotational velocity and applied axial pressure. So-called representative and predictive methods have been developed to evaluate friction conditions during the process and these are discussed in this paper. Weld data for the dissimilar weld between a high strength steel and a nickel-based super-alloy were provided by Rolls-Royce and MTU Aero Engines. The finite element software package DEFORM-2D is used to develop coupled thermo-mechanical axi-symmetric models. In previous work, methods employed to evaluate the efficiency of mechanical energy utilised during a weld, a parameter of great importance for numerical analysis, are not clear. Previous predictive approaches have employed test/weld data in one way or another to obtain the interface friction coefficient. This paper proposes a formula that incorporates the value of the mechanical energy efficiency of the welding machine into the calculation of coefficient of friction for representative modelling. It also introduces a predictive approach based on sub-layer flow theory to predict frictional behaviour during the welding process that is independent of test/weld data.Copyright

Residual stress analysis and finite element modelling of repair-welded titanium sheets

An innovative finite element modelling approach has been tested to investigate the effects of weld repair of thin sheets of titanium alloy, taking into account a pre-existing stress field in the components. In the case study analysed, the residual stress fields due to the original welds are introduced by means of a preliminary sequentially-coupled thermo-mechanical analysis and considered as pre-existing stress in the sheets for the subsequent repair weld simulation. Comparisons are presented between residual stress predictions and experimental measurements available from the literature, with the aim of validating the numerical procedure. As a destructive sectioning technique was used in the reference experimental measurements, an investigation is also presented on the use of the element deactivation strategy when adopted to simulate material removal. Although the numerical tool is an approximate approach to simulate the actual material removal, the strategy appears to predict a physical strain relaxation and stress redistribution in the remaining part of the component. The weld repair modelling strategy and the element deactivation tool adopted to simulate the residual stress measurement technique are shown to predict residual stress trends which are very well correlated with experimental findings from the literature.

Numerical Simulation of Residual Stresses Induced by Weld Repair in a Stainless Steel Pipe Considering the Influence of an Initial Fabrication Weld

This work presents the application of a finite element (FE) model developed to simulate the repair process in the case of components with a pre-existing stress state. The approach is tested in the case of a repair of a laser beam weld in a stainless steel pipe with the region of repair located in the heat affected zone of the original weld. The area of the repair is removed and refilled testing different approaches in terms of the number, and direction of the repair passes. The comparison between the refilling procedures is presented with the aim of evaluating the effects on the final residual stress distribution.