Full length articleNumerical simulation of the laser welding process for the prediction of temperature distribution on welded aluminium aircraft components
Introduction
Laser welding technology is used widely in many industrial fields. The process of laser welding offers a great potential for new product design [1], [2], [3]. Compared to other welding processes (arc welding, solid-state welding, induction welding, etc.) less heat is coupled into the work piece, resulting in a small heat affected zone (HAZ).
Aluminium alloys are widely used in the automotive, aerospace and other industries because of their high strength/weight ratio. A large number of aerospace components have complex shapes and are manufactured in several steps, often using fusion joining (particularly low heat input LB welding) and solid state joining operations, i.e. friction stir welding (FSW) which is widely used to join Al-alloys [4], [5], [6], [7], [8], [9], [10], [11].
During laser welding, complicated phenomena such as temperature dependency of material properties phase transition, (i.e. melting and evaporation), laser light absorption and reflection in a plasma occur in a very short time. In deep penetration laser welding, when a laser beam with high intensity irradiates the workpiece, a keyhole is formed in the workpiece, which enables the laser beam to penetrate deeply into the workpiece; while in conductive laser welding, the laser energy is absorbed on the surface of the workpiece.
Studies on the thermal cycles and temperature distribution during welding are very important, as the thermal cycle data form the input for many other analysis like the prediction of the microstructures in weld and HAZ and susceptibility of the weld for cracking. Traditional trial and error approaches based on welding experiments have encountered many difficulties to optimise the laser welding process and avoid the crack initiation. In order to extend the industrial applications of laser welding and make the process more reliable, it is necessary to develop appropriate control techniques based mainly on numerical simulation. After the advent of high steed computers, numerical techniques like FEM have acquired importance as they have capability to model different material types, heat sources, boundary conditions and structures.
Numerical simulation of the welding process has been one of the major topics in welding research for several years as presented in [12], [13], [14]. The results of simulations can be used to explain the physical essence of some complex phenomena in the welding process explicitly and can be also used as the basis for optimising the welding parameters. Simulation of the laser welding process enables estimation of the temperature distribution during welding. In order to determine the thermal field, an accurate description of the heat source should be provided. The keyhole phenomenon is the principal contribution to the non-homogeneous heating along the thickness. In deep penetration welding the keyhole shape its nearly conic and its vertex angle decreases as keyhole depth increases.
Several approaches to the mathematical modelling of keyhole formation in laser welding can be found in the literature. A number of researchers have developed mathematical models for the shape and location of the weld pool and the keyhole, by setting appropriate energy and pressure balances [15], [16], [17], [18], [19]. Other authors for similar purposes used alternative heat source approaches
A number of analytical and numerical local models of welding processes have been developed to evaluate temperature distribution during the welding process of structural components [20], [21], [22], [23], [24], [25], [26], [27]. In the above 3D models, the investigations on aluminium alloys components are limited [28], [29], [30], [31]. However, to the author’s knowledge, no investigations about new aircraft aluminium alloys such as 6056 and 6013 have been presented until today.
In the present investigation, a local three-dimensional finite element model for the laser welding simulation, using the finite element software SYSWELD, was developed. The model considers a Gaussian distribution of heat flux using a moving heat source with a conical shape as evaluated by the keyhole formation model. A non-linear thermal analysis was performed using temperature dependent thermal material properties. The developed model has been applied on aircraft components building on 6056-T78 and 6013-T4 aluminium alloys. The applications of the model were verified with experimental investigations.
Section snippets
Investigation cases
The simulation of the laser beam welding process of a skin-clip aircraft component has been succuded in order to predict the temperature distribution during process. The F.E. results have been compared with experimental measurements presented by research centre Helmholtz-Zentrum Geesthacht (formerly GKSS Research Center) in the frame of the programme “Development of short distance WELding concepts for AIRframes” (WEL-AIR).
The simulated geometry of the structure is show in Fig. 1. A clip with
Finite element modelling
A 3D finite element model was developed to simulate the T-joint laser beam welding process for using the commercial code SYSWELD. The geometry of the weld structure, shown in Fig. 1, was modelled using two types of elements; 3D volume elements with eight nodes and 2D membrane elements with four nodes. The mesh created for the geometry above is show in Fig. 2.
The 3D elements were used for the basic body structure and the 2D elements for the surface of the welded parts in order to simulate the
Results and discussion
The F.E. results have been compared with experimental measurements of temperature on different locations of the specimen during laser welding process. Fig. 5 shows the locations of the measured temperature. The temperatures were measured using thermocouples on the upper surface of the skin.
Experimental measurements on skin have been done for both sides of the clip. The side 2 is the welded side. In Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11 the experimentally measured time-dependent
Conclusions
A three-dimensional finite element model has been developed to simulate the laser welding process and predict the temperature distribution of laser beam welded aluminium alloys structures. The finite element calculations were performed using the SYSWELD F.E code. Nonlinear thermal analysis and temperature dependency of the thermal materials properties were taken into account. The developed model considers a moving heat source in a conical shape, in order to simulate the laser beam and the
Acknowledgements
The major part of this work was conducted in the frame of the programme “Development of short distance WELding concepts for AIRframes” (WEL-AIR). The financial support of the European Union under contract AST3-CT-2003-502832 is gratefully acknowledged.
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