Interface investigation between dissimilar materials by ultrasonic thermal welding by the third phase

Abstract Composites of polylactide (PLA) and polyformaldehyde (POM) were employed as the interposed sheet (IPS) to weld the dissimilar materials PLA and POM, utilizing ultrasonic thermal welding by the third phase (TWTP). PLA/POM composites showed excellent plasticity and retained the thermodynamic properties of the matrices, which are requirements of the IPS. The welded strengths were enhanced when the welding times and pressures increased, although extremely long welding times or extremely high pressures resulted in excessive heat, causing defects and decreasing the welded strengths. A maximum welded strength of 52 MPa was measured for the welded specimen, which was similar to the tensile strength of the PLA/POM composite when the welding time and pressure were 4 s and 0.2 MPa, respectively. Thus, the mechanical properties of both the matrices and IPS gave affects to the welded strength due to ultrasonic TWTP. Moreover, according to the polarizing microscopy images and the functional groups in the Fourier transform infrared (FTIR) spectra, gradient interfaces were observed and the thicknesses of the weld lines were identified as 40–50 μm.