Effect of energy director thickness on thermal diffusion and joint quality during ultrasonic welding of CF/PEEK composites

Abstract

Thermoplastic composites offer new possibilities for the aerospace industry. Large and thin structural parts can be processed rapidly and more cost-effectively than thermoset composites since the latter need to undergo lengthy curing reactions. An additional advantage is the possibility of being assembled by welding processes. However, joining thermoplastic composites remains challenging due to the lack of knowledge of the impact of process parameters (time, pressure, temperature) on the welded joints. A polymer film named energy director (ED) is usually set up at the interface to enhance the weld resistance. A combined experimental-numerical approach was carried out to estimate the influence of ED thickness on the thermal diffusion within the assemblies and the mechanical resistance of welds in the ultrasonic welding (USW) process. The studied materials are unidirectional carbon fibre-reinforced polyetheretherketone (CF/PEEK) and 50 µm and 250 µm polyetherimide (PEI) films as ED. Thermocouples (TCs) are inserted into the PEI layer to measure the temperature evolution during USW. Besides, the thermal profile is obtained based on the simulation of conduction phenomena through the thickness of the material using the finite element method (FEM). The effect of the ED through-thickness model points out the evolution of the temperature profile with time during the USW process for different PEI thicknesses. For both ED thicknesses, the temperature reached is above the PEI glass transition (220 °C) allowing macromolecular interdiffusion. According to our results, only the first ply reaches the melting temperature of PEEK, showing that the composite parts are not thermally affected during USW. Because of the high conductivity of unidirectional CF/PEEK composites, a thinner ED leads to a too-short heating time at the interface. Associated with too high power in a short time, a lower weld resistance of APC-2 parts is obtained by single lap shear for 50 µm PEI layer, 12 MPa, compared to 43 MPa obtained for 250 µm. A higher PEI thickness at the interface slows down the cooling step, resulting in a deeper interdiffusion of PEI and so, a stronger weld.