PSI - Issue 47

Mario A. Sánchez Miranda et al. / Procedia Structural Integrity 47 (2023) 310–324 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Discussion The union of dissimilar thermoplastics using FSW implies different physic-mechanical phenomena, which include: molecular diffusion in liquid state as the main mechanism of thermoplastic bonding; the glass transition temperature and melting temperature of both thermoplastics, conditioning the transition from rigid state to flexible state; the mixing capacity of both materials and the differentiation of mass flow in the advance side and retreating side around the pin, among others. The rheology of two dissimilar thermoplastics controls the welding efficiency when using FSW; for the present results, it was observed that pre-heating at 80  C does not improve tensile strength of this dissimilar joint. This behavior may be related to two factors: a) the low glass transitions temperature of both thermoplastics (Table 1), which don’t require additional energy by pre -heating to carry out FSW on these dissimilar polymers, b) Molecular diffusion seems adequate with the heat generation during FSW, without need of pre-heating.

Figure 9. Fracture images of tensile specimens, corresponding to testing modalities in Figure 8.

Tensile results in Figure 8 and Table 4, show higher welding efficiency for the lower values of rotating and advanced speeds; close to 80% in regard the parent material (HMWPE), for the 1112 test modality. The fracture results of tensile tests are illustrated in Figure 9a, 9b, 9c, and 9d, for the specimens in Figure 8a, 8b, 8c, and 8d, respectively. All fractures depicted in Figure 9 were developed at the stir zone (SZ), close to PP zone, particularly for the low speeds’ parameters corresponding to tests modalities: 1112 and 1525. This fracture localization is related to good molecular diffusion and mixture and to improved mechanical properties [37]; improved mechanical properties have been observed when the two dissimilar thermoplastics show similar viscosity during the FSW process [38]. The tensile behaviour for all specimens in Figure 8 is characterized by a continue increase on load with elongation, without decrease of tensile stress and increase on elongation, as in the case of parent materials, Figure 7. Dissimilar thermoplastics joining by FSW can lead to different mechanical properties in regard the parent materials; the blending capacity and modifications on the polymeric chains after FSW process, are at the origin of modifications on the physic-mechanical properties. On the other hand, the rheological properties of both thermoplastics determine the flow behaviour of mixture and the transition from laminar to turbulent regime during the welding process [39,40]. Figure 10a presents the laminar flow traces on the fracture surface, corresponding to specimen 1112, characterized by flat surface for both thermoplastics; whereas Figure 10b shows the turbulent behaviour on the fracture surface of specimen 2525, with zones of separation and discontinuities generated by the turbulent activity. Then, in increasing the turbulent flow of the mixture during FSW, the generation of defects such as tunnel or root are increased at the same time. Additional analysis using scanning electron microscope (SEM), on the fracture surface of parent materials and the UHMWPE PP joints, reveal the transition from ductile to brittle fracture of parent materials and the joined dissimilar thermoplastics. In Figure 11a and 11b are depicted the fracture surfaces of parent polymers UHMWPE and PP,