Comparison and optimization of ultrasonic welding parameters in time and displacement mode for CF/PEEK composite with integrated energy director

The ultrasonic welding (USW) technology is widely used as a joining technique for thermoplastics in the fields of the agri-food industry, automotive, and others. This fast and energy-efficient joining process is rarely used to assemble thermoplastic composite structures, although it provides excellent bonding strength. For this reason, aerospace companies have carefully studied USW as a promising option to assemble large parts reliably while reducing their carbon footprint. In this work, a neat thermoplastic film used as an energy director (ED) was integrated on the surface of the composite parts. The effect of integrated polyetherimide (PEI), instead of a loose film, on the continuous carbon fibre-reinforced polyetherketone (CF/PEEK) was investigated. Moreover, two process control strategies were tested: time- and displacement-controlled modes. Consequently, integrated ED provides a simple solution for welding parts whose dimensions are greater than the surface of the sonotrode. The duration of the vibration time phase in the process affects the quality of the welded specimens regardless of the control mode. The welding parameters, such as time, displacement, and pressure with a specific amplitude of vibrations, were used to define the optimum conditions for CF/PEEK. The assemblies were obtained in a single lap shear configuration to measure their mechanical resistance and characterise the interfaces. The highest lap shear strength (LSS) for integrated ED was found around 50 MPa for the displacement-controlled mode, while the highest LSS for the time-controlled mode was 44 MPa; both control modes ensure consistent quality of the welded joints. Controlling modes (time and displacement modes) in ultrasonic welding are compared to gain in reproducibility and to maximize mechanical strength of joints. From the experimental testing campaign, we conclude that the lap shear strength (LSS) of welds: ◌Changes with pressure in displacement mode. Lower pressures must be associated with longer welding times, which leads to the risk of overheating. ◌Is weakly influenced by other parameters in time mode. 20 % higher LSS is reached in displacement mode compared with time mode: at 2.5 bar, LSS=50 MPa in displacement mode and LSS=44 MPa in time mode. Short vibration time of 0.8 s ensures strong, well-consolidated welds. Analysing the surfaces after fracture reveals that below 40 MPa, failures occur in the PEI energy director, while above 40 MPa, they shift into CF/PEEK composite layers.