Interfacial thermo-mechanical interaction in RFSSW joining of 6061-T6 Al/SCF-PPS considering polymer thermophysical state: experiment and FEA

Metal-polymer hybrid structure is an effective lightweight design form, but reliable joining remains challenging because the interfacial interaction mechanism remains insufficiently explored. In this study, refill friction stir spot welding (RFSSW) of 6061-T6 aluminum alloy and short carbon fiber-reinforced poly phenylene sulfide (SCF-PPS) was investigated by experiments and an ALE-based thermo-mechanical finite element model. The model was used to analyze the interfacial temperature, contact pressure, and material flow behavior, and was validated by temperature history, plastic deformation zone (PDZ) area, and anchorage zone (AZ) geometry. The results show that interfacial temperature determines the thermophysical state of PPS and thus the mobility of molecular chains, while interfacial contact pressure reflects the softening state of the PPS and affects the stability of interfacial interaction. Appropriate heat input promotes the coordinated development of the PDZ and AZ, whereas excessive heat input causes over-softening and PPS overflow, reducing the effective joining area. At the optimal condition of 1000 rpm (heat input of about 3490 J), the joint exhibited a high AZ ratio of 10.73%, a large PDZ area of about 119 mm², and an average maximum load of 2405 N. The results indicate that the performance of Al/SCF-PPS RFSSW joints is governed by the balance between polymer molecular chain mobility and interfacial contact pressure, which provides a mechanistic basis for optimizing metal–polymer thermal joining.