Induction Welding of Composite Materials

Joining together of composite materials like Carbon Fibre Reinforced Polymer (CFRP) is an important technological problem in many applications. Due to its high strength-to-weight ratio, CFRP is increasingly widely used in aerospace, automotive, civil engineering, consumer products and other industries. As the number of applications increases, it becomes necessary for every new case to optimise the technological parameters of the welding process and achieve satisfactory results at low cost.

In this document we describe how Opera FEA software can be used to simulate the induction welding process by considering an example of joining together two thin plates of CFRP.

Figure 1. Induction welding of composite materials.

Figure 1. Induction welding of composite materials.

FEA model of the welding system

The process of induction welding of sheets of CFRP consists of moving the laminates on a carrier below an induction coil at a certain speed. The parts are heated by emitted electromagnetic field. A pressure roller applies welding force and removes excessive heat from the material. Remaining heat is removed by air ventilation.

Unlike carbon fibres which can be heated to very high temperatures without melting, typical bonding materials, like epoxy resins or ceramics, have much lower melting points. Therefore the heating process should be capable of heating parts to be jointed above the melting point of the bonding material. This could be as low as 50°C, but for some cured high strength epoxy resins could be more than 300°C.

We show in this example how finite element analysis (FEA) using Opera 18R1 package can help identify induction welding process control parameters needed to create defect-free bonding of CFRP parts.

As with every model, accuracy of FEA results relies on knowledge of material properties and properties of the interfaces. Contacts at the interfaces between CFRP plates are affected by temperature, pressure and fibre structure, which determine whether the main heating mechanism is associated with conductive Joule losses along the fibres generated by Eddy currents, dielectric heating at fibre junctions or contact resistance at junctions [1,2].

In this model we assume that there is no heat barrier at the interface between the plates. The material properties of the CFRP are considered isotropic, with the exception of the thermal conductivity, which is isotropic in-plane, but has significantly lower conductivity out-of-plane. The coil is driven at 1 MHz with a peak current of 193.5 A. We take into account some heat dissipation from the surfaces of the plates into surrounding air. For a more accurate result we include the temperature dependence of electrical conductivity of the plates.

The continuous induction welding process is modelled as a multiphysics problem involving harmonic electromagnetic and transient thermal analysis available in Opera. An important feature of Opera in this simulation is the facility in the Opera-3d Post-Processor to exchange data between simulation types and even allow geometric shifts in the data (in this case losses and temperature) to account for motional changes.

Computational time is significantly reduced by using a regular hexahedral mesh and an analytical representation of the source field produced by the conductor, which is mapped onto the FE mesh. This provides a significantly quicker solution and avoids re-meshing when relative motion of the plates with respect to the coil is modelled. Computational time is further reduced by updating the electromagnetic simulation only when the temperature has changed sufficiently.

Results of simulations

Figure 2 shows the temperature ‘trace’ of the moving coil on the surface of the CFRP when the coils move parallel to the surface of the plate at a speed of 2 mm/s, starting from X=-200 mm. The temperature at the hottest spot on the top surface is about 588°C.

Figure 2. Temperature distribution on a surface of a CFRP plate after 200 seconds of induction heating. The induction coil moves at a speed of 2 mm/s parallel to the surface of the plate.

Figure 2. Temperature distribution on a surface of a CFRP plate after 200 seconds of induction heating. The induction coil moves at a speed of 2 mm/s parallel to the surface of the plate.

Whereas it is useful to know the surface temperature of the material, from the point of view of designing the manufacturing process it is more important to know the temperature at the boundary between the two plates. It is difficult to measure this temperature experimentally but easy to predict it from the FEA simulation as shown in Figure 3. We can see that the maximum temperature at the interface between the two 2 mm thick plates is 407°C, i.e. significantly lower than the maximum surface temperature of 588°C.

Figure 3. Temperature distribution near the interface of two CFRP plates after 200 seconds of induction heating. The induction coil moves at a speed of 2 mm/s parallel to the surface of the plate.

Figure 3. Temperature distribution near the interface of two CFRP plates after 200 seconds of induction heating. The induction coil moves at a speed of 2 mm/s parallel to the surface of the plate.

Knowledge of temperature distributions in different directions predicted by the analyses, like the one in Figure 4, can be used, for example, to design jet cooling of the top surface, optimise the speed of motion or the width of the materials and set up the pressure roller.

Figure 4. Temperature variation along X, Y and Z directions at different positions of the plate.

Figure 4. Temperature variation along X, Y and Z directions at different positions of the plate.

References

1. T. Bayerl, M. Duhovic, P. Mitschang et al., Composites, A57, 27 (2014).
2. T.L. Ahmed et al., Composites, A37, 1638 (2006).