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Overview work laser welding

By Rowshni Jabeen

The objective of the worktasks by IMT Lille Douai and ARMINES is the numerical modelling of Laser Transmission Welding (LTW) process for bio-sourced fibres reinforced thermoplastic composites.

Of two composite parts that are to be welded by LTW, the one should be (semi-) transparent to laser transmission and the other opaque (absorbent) to the same. During the welding, the two parts are clamped together and a laser beam is transmitted on the interface of the two parts, passing through the transparent part (see Fig. 1). At the interface, the laser energy is absorbed by the absorbent part and the material is heated and melted locally. The heat is transferred to the transparent part by conduction. This results in a joint after solidification. Material properties, laser characteristics and welding time influence the interfacial adhesion strength established by the laser welding process. The semi-transparent part used in this project contains short flax fibres (less transparent to laser transmission) which scatter the laser rays, causing a loss of energy reaching the weld interface.

Fig1 eng

The main subject in the numerical modelling of LTW process is to predict the temperature field at the weld interface, especially after laser ray scatter and energy absorption in the semi-transparent part. Hence, the laser transmission and ray scattering depend on the morphology of fibres such as their shape, size, orientation and volume fraction as well as the optical properties of fibres and matrix.

The morphology of flax fibres reinforced thermoplastic composites has been analysed and a numerical algorithm to generate a periodic Representative Volume Element (RVE) representing the microstructure of the composite material (see Figs 2 and 3) has been developed with non-overlapping inclusions (i.e. short fibres) using MATLAB and COMSOL. The developed algorithm can generate inclusions with any regular shape. Another algorithm to describe the fibre orientation state by fibre orientation tensor and orientation distribution function has been developed and verified. The fibre orientation tensor data is provided by KU Leuven, generated from injection moulding simulation software, MoldFlow.

Fig2 eng
Fig3 eng

The developed microstructure is discretised into a numerical mesh and ray tracing simulation is carried out. During the ray tracing simulation, the path of a ray is followed in the microstructure based on the ratio of refractive index of the fibre and the matrix. Fig. 4 shows that a laser beam with initial radius of 1E3 m is distributed at the weld interface. This distribution data is being used to perform heat transfer simulations at the weld interface.

Fig4 eng