Research on the static ultrasonic welding of thermoplastics reinforced with unidirectional continuous fibers is limited. This research aims to optimize the ultrasonic static welding process of polyamide 6 reinforced with continuous unidirectional glass fibers (GF/PA6) by examining the primary process parameters—time, pressure, and power—and their interactions on the shear strength of lap joints to achieve a high shear strength in the weld. Using the response surface methodology (RSM), a suitable quadratic equation \(({\mathrm{R}}^{2} \, \mathrm{=} \, \mathrm{0.9625}\mathrm{)}\) was derived to model the weld shear strength data. Additionally, the effect of a 0.3-mm layer of polyamide 6 as an energy director was investigated through SEM imaging of weld fracture surfaces and energy consumption measurements. The results indicated that welding power and time are the most influential factors on weld shear strength. By setting time, pressure, and power to values of 1.8 s, 2 MPa, and 100%, respectively, the maximum shear strength was estimated at 28.8 MPa, which differs by approximately 6.7% from the experimental validation tests. Incorporating an energy director resulted in an average reduction of 8% in welding energy consumption and a 14% increase in weld shear strength. Finally, it can be concluded that performing static ultrasonic welding at the optimal point, along with using a 0.3-mm-thick energy director, creates a joint with a lap shear strength of 28.4 MPa. This strength is very close to the shear stress in the overlapping area of the samples under the composite’s ultimate fracture load, with only a 3.7% difference.
