Research On Mechanisms Of Laser Rust Removal And Manufacture Of Laser Cleaning Devices

Laser cleaning as a novel surface cleaning technique, has the advantages of no mechanical contact, no substrate damage, selective cleaning, environmental friendly, et al. Laser cleaning has been widely applied in many areas including preservation of cultural relics, manufacturing of opto-electronic devices, tyre mould cleaning, et al. Scientists and engineers in the filed all over the world have been paying more and more attention to the study on the cleaning mechanism and the development of laser cleaning machine.In the thesis we have introduced the application of laser cleaning technology in rust removal from iron or steel surface, there are three main works we have done. First, based on the instruments in the laboratory, we have carried out a series of experiments on laser rust cleaning, and proposed the condition of simultaneous cleaning and passivation when quasi-continuous wave laser is irradiating on the floating rust layer, and the method of dual laser beam wet cleaning. Second, we have used the finite element method software ANSYS to simulate the processes of laser rust interactions. By analyzing the simulating results, we have put forward a physical model for laser rust removal when laser fluence is lower than the evaporation threshold of the material, which is called film-dependent model. Third, the group of laser cleaning including Peng Du and I, have constructed two low power laser cleaning machines operating in different modes. Using these two cleaning devices, we have made some attempts in the industrial applications, such as paints, rusts, oils, et al. More detailed information is listed below.(1) By reading and summarizing the published papers, we have present a summary on the mechanism of laser cleaning. The item that removed by laser cleaning can be divided into two different types, one is fine particles, and the other is thin films. For particle removal, there are three main effects that play a role in the cleaning processes, which are thermal expansion, ablation and evaporation of the adsorbed moisture in the gap between the particle and the substrate. Among these three effects, thermal expansion is dominant. In the view of laser incidence, four implementation ways can be adopted, including normal incidence, angular incidence, glancing incidence and verso incidence. By calculating the distribution of temperature and displacement caused by laser heating, we have founded the comparison between the cleaning force and the adhesive force, or between the elastic energy and the adhesive potential. Then we can confirm the cleaning condition, and in the end present the dependence of cleaning effect and efficiency on laser parameters, material properties and the environmental conditions.(2) We have proposed a dual laser beam wet cleaning method and the concerning devices. The dual laser beam consists of a pulsed CO2laser and a quasi-continuous wave Nd:YAG laser. Assisted by a liquid film depositing system, the pulsed CO2laser used in the rust removal can effectively improve the cleaning efficiency. Quasi-continuous wave Nd:YAG laser firstly swept the residue liquid on the substrate surface to avoid second corrosion, and then actively accelerated the oxidizing process to form a uniform passivation layer by adjusting the laser pulse number.(3) Quasi-continuous wave laser using line-scanning method irradiated on the floating rust layer can result in simultaneous cleaning and passivation. When we fixed the pump electric current (20A) and the repetition rate (3kHz), the pulse width of quasi-continuous wave laser is150ns, which is in the range of intermediate pulse width. Compared with short pulse, intermediate pulse could reduce the damage risk for delicate substrate and be flexibly coupled into fiber. Using intermediate pulse laser to treat the floating rust layer, when laser fluence is selected to increase the surface temperature to the melting point of the metal substrate, an iron bath could be generated and the heat and oxygen concentration could be uniformed in the bath, which is benefit for the formation of passivation layer. When laser fluence is not sufficient to increase the temperature to the melting point but the floating rust could also be removed (traditional cleaning threshold), the corrosion resistance of the iron substrate after laser irradiating is partially improved, but could not perform as a passivation layer. When laser fluence is so high that the temperature of the rust surface could reach the decomposing point (melting point), the Fe2O3would breakdown and generate Fe3O4and O2. At the same time because the depth of the bath is increased, the oxygen concentration would diffuse into the deeper layer and cause a layered distribution of oxygen concentration. The layered structure conformed by different iron oxides has no effect in protecting the inner substance. Through comparison between the experimental results and the simulating ones, we finally determine the condition for quasi-continuous wave laser to realize simultaneous cleaning and passivation.(4) We have founded a film-dependent model of laser derusting when laser fluence is lower than the evaporation threshold. When laser beam irradiated on the rust surface, the distribution of power density absorbed in the superficial layer has a profile of exponential function for Beer’s law. The temperature of the upper layer could be increased to the melting point, and the iron oxide (Fe2O3) would decompose into Fe3O4and O2. The bottom layer of the heat affected zone by one laser pulse also has some extent temperature increment. The air in the porous structure of rust could be heated. When the pressure of the expanded air overcame the bonding force, the rust layer could be crushed and ejected. The removal mechanism of the bottom layer is called phase explosion. The thickness of the rust layer was decreased pulse by pulse through melting decomposition and phase explosion. When the thickness of the remaining rust layer is lower than the length of heat diffusion, the transmitted laser energy could be absorbed directly by the metal substrate and cause thermal expansion. The mechanism of film shaking-off could take effect and the whole remaining rust layer could be removed more efficiently. We have used ANSYS software to simulate the processes related to the above three mechanisms. The phase explosion is the main regime of rust removal when laser fluence is lower than the evaporation threshold among these three mechanisms.(5) We have also proposed a online monitoring method called’optic-acoustic spectroscopy method’. In the configuration of optic-acoustic spectroscopy device, LIBS (laser induced breakdown spectroscopy) qualitative analysis is used for element determination, and TOF (time of flight) method is used to distinguish different layers. Compared with single LIBS or TOF, optic-acoustic spectroscopy method has wider applications, especially suitable for laser cleaning multi-layer samples. For example, iron-rust-paint triple layers, iron-base coat-finishing coat multiple layers, and metal-corrosion products-soil triple layers for items newly excavated. The LIBS part through qualitative element analysis could distinguish different materials from the atomic spectroscopy, and the TOF part could examine different layers by the changes of surface reflection or absorption, so the monitoring method we proposed can obtain more information to deal with more complicated cleaning situations.(6) Two laser cleaning devices have been constructed by the cleaning group including Peng Du and I. The first one is an electro-optical Q-switched laser cleaning machine, whose main use is to remove thin paints. The laser beam emitted from the laser cavity is coupled into a mechanical arm with seven junctions, and automatically scanning the sample by a3D translation stage. The second one is an acousto-optical Q-switched laser cleaning machine. The output laser beam is firstly coupled into a high power fiber and then scanning into a line by a single galvanometric scanner. The scanner and the focusing lens are packaging into an output head which could be flexibly operated manually.