Study On Mechanism And Process Of Nanosecond Pulse Laser Derusting

Ships and marine engineering equipment have been serving in corrosive environments with high salinity and oxygen enrichment in long-term.The corrosion problem was an important factor affecting the life of ships,the protective coating was an important protective measure to prevent further corrosion of steel.The rust removal was the pretreatment procedure before painting and it was the premise to ensure good adhesion of paint layer.The traditional methods of rust removal mainly includes mechanical grinding,sand blasting and high pressure water jet cleaning,which are impossible to meet the green and efficient industrial development model because its disadvantages of polluted environment,high labor intensity and low degree of automation.As a new rust removal technology of non-mechanical contact,high efficiency,flexible and green environmental protection,laser drusting has received extensive attention from the researchers in the enterprises and domestic related fields.In the future,laser derusting was undoubtedly the most effective and reliable solution in the process of seeking alternative traditional cleaning.The interaction mechanism between nanosecond pulsed laser and rusty layer on Q235 and 45 steel was firstly studied,the mechanism of nanosecond laser derusting was revealed by analyzing the phase transition process of the corrosion layer under different laser flunce,the dynamic characteristics of the corrosion layer removal under the multiple-pulse cumulative effect and the changes in the chemical composition.In addition,the ablation threshold of the corrosion layer and the base material was obtained by the calculation of the ablation area.The influence of the pulse repetition rate and laser flunce on the ablation rate was analyzed by the method of the equivalent pulse numbers.At last,the rust removal experiment of high speed surface scanning were carried out,the influence of the technological parameter includes laser flunce,pulse width,laser overlap rate and defocus amount on the surface color and roughness of the rust removal surface was studied by single factor analysis.The quality of laser derusting was evaluated by the national standard of rust removal and the anti-corrosion test.The concrete research methods and conclusions are as follows:1.The high-speed photography was used to observe the effect of laser on corrosion.The removal characteristics of the corrosion layer at different the laser fluence were compared,and the phase change process of the corrosion layer at different laser fluences was analyzed by microscopic inspection.The experimental results show that ablation and thermal action were the main mechanisms of nanosecond pulse laser corrosion removal.The laser fluences determine the removal mechanism of the corrosion layer.When the laser fluence was less than the erosion threshold of the corrosion layer,the corrosion layer was mainly removed by thermal elastic expansion.When the laser fluence reaches the ablation threshold,the phenomenon of melting and gasification phase transition occurs.In addition,the gas phase explosion impact force generated during the ablation process accelerates the removal of loose layers in the corrosion layer.In addition,the vapor explosion impact generated during the ablation process accelerates the removal of loose layers in the corrosion layer.In the process of pulsed laser,the increase of the repetition frequency leads to an increase in the number of pulses per unit time.The multi-pulse cumulative effect leads to an increase in the impact generated by gas phase explosion and accelerates the removal of the corrosion layer.The corrosion layer in the area before and after laser irradiation were detected and analyzed by EDS and LRS.The main components in the corrosion layer were?-Fe₂O₃ and Fe₃O₄ formed the dehydration of?-Fe OOH.It indicated that the corrosion layer decomposes under laser ablation,and Fe in carbon steel reacts with O₂ in the air under the action of ablation and then forms a?-Fe₂O₃ film layer on the surface of the substrate.2.The ablation threshold for Q235 steel and the 45 steel rust layer were 3.21 J/cm²and 2.51 J/cm²at the pulse width of 200 ns,which was measured by calculating the ablation area.The ablation thresholds of the substrates were 1.76 J/cm²and 2.32 J/cm²respectively.Since the removal threshold of the rust layer was greater than the damage threshold of the substrate,this indicated that the process of pulse laser derusting was difficult to avoid the ablation of substrates.The only to reduce the ablation area was changing the process parameters.Secondly,by studying the effect of repetition frequency and laser flunce on the ablation rate we found that the laser fluence was the main factor affecting the ablation rate,and the repetition frequency was only a secondary factor.By using the method of equivalent pulse number,when the pulse width was 200 ns,the laser fluence was 20.7 J/cm²,and the ablation rates were measured 9.56?m/pulse,9.48?m/pulse,and 11.25?m/pulse at 2.5 k Hz,10 k Hz and 20 k Hz respectively.But,when pulse width was 200ns,repetition frequency was 10k Hz,the laser fluence was 20.7 J/cm²,42.7 J/cm²,64.5 J/cm²,the ablation rate were measured 9.56?m/pulse,15.49?m/pulse,17.54?m/pulse respectively.3.In the process of scanning derusting,the result concluded that:?1?With the increasing of the the laser fluence,the removal of the corrosion layer and the roughness of the base material increase.The rust layer were completely removed and the surface of the substrate exhibited a metallic color with the roughness about 1.6?m,as the the laser fluence was 53 J/cm².?2?The interaction time between laser and material was depended on the pulse with,and the blue spots would appear on the surface of the substrate with the increasing of the pulse with,but did not affect the property and roughness of the material.When the pulse width was 200 ns,the pulse energy was adjusted more widely and reached the limit pulse energy in other pulse width modes.?3?When the repetition frequency was 10 kHz,the roughness started to reduce with the increase of the rate of overlap;when the rate of overlap was 40%?scanning speed was 240mm/s,scanning space was 0.024mm?,the stacked re-condensates distributed on the surface evenly and it was not difficulty in serious heat accumulation.?4?When the positive defocus amount was0.2mm,derusting quality can be obtained the reduction of heat accumulation in focul point.?5?The corrosion resistance of substrate and base material after rust removal at 6mol/L Na Cl are comparative analyzed through the experiment,and Visual evaluation of surface cleanliness after laser derusting was carried out.It was indicated that the corrosion resistance of the substrate was enhanced after laser derusting,and surface color almost achieving GB/T 8923 standard Sa2.5.