Fig. 1. Laser cleaning platform. (a) Principle diagram of laser cleaning; (b) experimental equipment for laser cleaning; (c) scanning electron microscope; (d) confocal laser microscope; (e) ultra-depth-of-field microscope; (f) laser cleaning test bench for pipe thread

Fig. 2. Surface morphologies of rust layer. (a) Macro-morphology of petroleum pipe thread; (b) micro-morphology of peeling layer easy to fall off in rust layer; (c) micro-morphology of dense layer; (d) element content distribution of rust layer

Fig. 3. Thread surface after laser cleaning. (a) Overall morphology; (b) local amplification at A′

Fig. 4. Sample after wire cutting

Fig. 5. Micro-morphologies under different laser powers. (a) 600 W; (b) 550 W; (c) 500 W; (d) 450 W; (e) 400 W

Fig. 6. Surface roughnesses under different laser powers

Fig. 7. Contents of oxygen elements under different laser powers

Fig. 8. Micro-morphologies under different defocusing amounts. (a) +4 mm; (b) +3 mm; (c) +2 mm; (d) +1 mm; (e) 0 mm

Fig. 9. Surface roughnesses under different defocusing amounts

Fig. 10. Contents of oxygen elements under different defocusing amounts

Fig. 11. Micro-morphologies under different scanning speeds. (a) 3000 mm/s; (b) 2500 mm/s; (c) 2000 mm/s; (d) 1500 mm/s; (e) 1000 mm/s

Fig. 12. Surface roughnesses under different scanning speeds

Fig. 13. Contents of oxygen elements under different scanning speeds

Fig. 14. Residual plot of mathematical model

Fig. 15. Flow chart of improved particle swarm algorithm

Fig. 16. Convergence curves of two algorithms

Fig. 17. Contents of elements after cleaning under optimized process parameters

Fig. 18. Molten pool morphology after cleaning under optimized process parameters