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In this study, a finite element model of ultrasonic welding of continuous carbon fiber (CF) reinforced polyetheretherketone (PEEK) laminates is established based on the viscoelastic dissipation theory. The impacts of the size and apex angle of the energy director on the heating process during ultrasonic welding are discussed. Simulation results show that the apex angle of the energy director has a more significant effect on the temperature rising rate than size of the energy director. The apex angle also greatly affects the temperature field profile of the energy director. The highest temperature region of energy directors with 30 and 60° apex angle presents a butterfly shape and deviates a distance from the lower laminate. The ‘wings’ of the highest temperature region of the 90° energy director nearly merge together and make contact with the lower laminate. The highest temperature region of 120° energy director becomes a closed ellipse area but is wrapped in the energy director tip. As the cross-sectional area of the energy director is smaller than 0.25 mm2, the heated region on the energy director and lower laminate decreases with the cross-sectional area. As the cross-sectional area exceeds 0.25 mm2, the heated area on the energy director and lower laminate remains constant. Therefore, an energy director with a 90° apex angle and about 0.25 mm2cross-sectional area may be the most suitable for ultrasonic welding of CF/PEEK laminates.

The surface phase constitution and the kinetics of layer growth in pure iron pulse plasma nitrided at 520 • C for different times were systematically studied, and the relative expressions of the nitrogen concentration profiles in nitrided layers have been deduced. The results show that the nitrided layer consists of ε-Fe 2−3 N (ε), γ -Fe 4 N (γ ) and α-Fe (α) phases, and its growth in thickness follows a parabolic law. In addition, the growth of a compound layer and diffusion layer and the nitrogen concentration distribution in the nitrided layers can be precisely predicted by employing the mathematical models obtained.

The ultrasonic welding process of thermoplastic composite with different shapes of energy director (ED) was simulated using finite element model. The results show that the highest temperature zone locates at the tip for the semicircular and triangular ones, and locates at the middle height for the trapezoid one. But it does not locate at the body of ED for the rectangular one. Energy director with different shapes lead to the temperature rising rate at different order of amplitude. The welding amplitude has same influence on the four shapes of ED. The temperature distributing profiles of semicircular, triangular and trapezoid ED keep constant from the initial welding time to that when the highest temperature on joints arrives the temperature of glass transformation (Tg), but the profile for rectangular ED changes greatly.

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