INFLUENCE OF LASER ABLATION ON STAINLESS STEEL CORROSION BEHAVIOUR

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1 INFLUENCE OF LASER ABLATION ON STAINLESS STEEL CORROSION BEHAVIOUR Michal ŠVANTNER a, Martin KUČERA b, Šárka HOUDKOVÁ c, Jan ŘÍHA d a University of West Bohemia, Univerzitní 8, Plzeň, msvantne@ntc.zcu.cz b University of West Bohemia, Univerzitní 8, Plzeň, kucera82@ntc.zcu.cz c University of West Bohemia, Univerzitní 8, Plzeň, houdkov@ntc.zcu.cz d University of West Bohemia, Univerzitní 8, Plzeň, janriha@ntc.zcu.cz Abstract Laser ablation is the method for material removing from a surface using pulsed lasers. It is used for surface structuring, laser drilling or laser marking. A change of the corrosion behavior of a base material caused by thermal effects during laser processing is an important problem of this technology. Possibilities of surface modification of stainless steel using laser ablation are presented in this contribution. The most important processes that take place during laser treatment of material surface are described. The influence of laser processing parameters on resulting material surface state and a possible connection of laser parameters with corrosion tests results are discussed. Keywords: laser ablation, stainless steel, corrosion resistance 1. INTRODUCTION Laser technologies are progressive methods of material treatment. These methods are based on an incidence of a high-energy laser beam on solid (or occasionally liquid) material surface. Interaction between the laser beam and the material surface is the base of the process. The properties of the material and the laser beam determine what kind of process occurs. The material absorbs one part of laser radiation when the second part reflects and the third part passes through the material. In case of metals is the third part zero. The absorptivity is the most important material parameter in laser material interaction. For each configuration is the absorptivity given by combination of laser parameters wavelength, angle of incidence, polarization of the laser radiation and of the material radiative properties state, geometry of the surface and temperature. The higher value of the resulting absorptivity means that the more laser radiation is used for the processing. The heat conductivity of the material determines if the material is heated rather locally (conductivity lower values) or if the heat dissipates to material (conductivity higher values) [1]. The absorbed laser radiation causes different processes on material surface. The interaction of laser beam with the material can lead to blenching, foaming, ablation, thermal oxidation, etc. The decisive parameters are the power density and the interaction time. These parameters determine usability for different technologies hardening, welding, cutting, drilling, engraving etc Laser ablation Laser ablation is a technology of surface material removing due to the laser beam incidence. The laser energy causes vaporization and partial ionization of the material. The goal of the process is to evaporate the material and simultaneously thermally affect (melting, structural changes etc.) as few as possible the neighborhood of the treated area. This is achieved by introducing of the sufficient energy in a short time interval. The required power density is greater than 10 MW/mm 2 and the interaction time in nanosecond for metals [1].

2 The pulsed lasers are used for the laser ablation. It can be used for permanent marking of products in one step without added materials with high efficiency. The mark is induced by interaction of the focused laser beam with the material surface. The laser beam is deflected with galvanometric scanner. Therefore, it can reach each point of the scanned area and a defined track can be created Laser marking This basic principle of the laser marking is uniform for all materials. Different additional processes can take place during the ablation and can lead to different origins of the material contrast changes. For each material is suitable different process and strategy. For marking of metals are suitable ablation and thermal oxidation. For this purpose are suited the pulsed lasers q-switched YAG lasers and pulsed fiber lasers. The pulsed fiber lasers conquer the industrial marking application currently. They are compact, have long service life and their main advantage is the possibility of independent change of the adjustable parameters. The pulsed fiber lasers pulse repetition frequencies are typically in range from 1 khz to 1 MHz, maximum pulse energy are mj and the pulse length can be tuned approximately from ns. It is possible to focus the laser radiation in spot of size typically microns by using a pulsed fiber laser together with a galvanometric scanner with the f-theta objective. The power density then reaches 10 MW/mm 2 and it is suitable for ablation. The spot size can be easily changed by changing the beam expansion. For laser color marking of stainless steel with pulsed fiber lasers it is possible to use many different process strategies [2]: single lines, single line overlapping, multi line overlapping, grating surface creation. The fastest methods are single line and overlapping the single lines. Each line is produced by given thermal load and the oxides on the surface are formed after the process during cooling. The color of the marked lines or areas can be varied by the thickness of the oxide layer. The thickness of the oxide layer can be controlled by energy per unit of area J/mm 2 introduced to the surface (the heat input). The heat input can be controlled by changing the power and the scanning speed in these methods. The other method to create the color marking is the overlapping the melting pools created after pulsed laser irradiation. The distances between the individual melting pools edges are comparable with the visible light wavelength. A diffracting surface is created if the surface area is filled with these melting pools. The melting of the surface, which is typical for three previous methods, can easily weaken the corrosion resistance. The most promising method to create the uniform oxide layer on the surface without melting is the multi line overlapping. In this case, the scanning velocity is very high (1 m.s -1 ) and the lines are very close. The area is scanned several times the line spacing is in ten microns and the line overlap is about 90%. The pulse repetition frequencies are in range 100 khz and the times between the pulses are therefore microseconds. The laser operates rather as a area source. The heat input can be in this method varied by changing the power, scanning speed and line spacing. In this method the surface is not melted and the oxide layer is formed by laser enhanced diffusion and by the reactions within the laser irradiated area. Strong temperature gradients due to laser irradiation will enhance the transport of species by formation of stresses, strains, cracks and other defects. In the case of pulsed-laser oxidation the short irradiation times involves to remain the thermodynamically unstable phases while other phases cannot nucleate [3] Corrosion properties of laser treated surfaces The corrosion resistivity of the surfaces affected by laser ablation can be changed due to the above described thermo-chemical processes. With the multi line overlapping method it is possible to obtain the uniform, contrast and color mark on steel surface. In dependence on the various parameters which determines the thermal load (J.mm -2 ) - line spacing, laser power, scanning speed and their combination with subsequent parameters as pulse length, pulse frequency, pulse shape, etc. we can obtain the different corrosion resistance of marked surface [2]. The increasing the amount of the oxygen in the ambient

3 atmosphere will increase the oxide layer growth. On the contrary in the inert atmosphere or under liquid layer will not the oxide layer growth. The change of the corrosion properties of the materials after surface laser treatment can be one of the significant parameters of this technology. Reducing of the corrosion resistivity of the materials can be fundamental in some applications and therefore limiting for laser ablation technology using. We have analyzed changes of the corrosion properties of stainless steel after laser marking, which is a special case of laser ablation. Some experiments, material analyses and results are presented in this contribution. 2. EXPERIMENTAL The aim of experiments was to find out a connection between laser color marking parameters, surface or structural changes and corrosion properties of stainless steel. The pulsed fiber laser SPI G3-HM [4] with the ScanLab ScanCube 10 [5] scanning head with f160 f-theta was used for laser marking. The average output power of the laser is 30W. The peak emission wavelength of this laser is 1062 nm and maximum pulse energy is 1 mj. The pulse frequency could be tuned from 30 to 500 khz and the pulse length could be ns. The laser spot diameter in the focus distance is about 90 µm and the maximum scanning speed is 10 m.s -1. The SPI G3-HM lasers are usually used for marking (plastics, metals or poly-compounds), scribing, ablation, solar cell processing and other. Scanning f requency variants Increasing scanning speed Increasing pulse length Energy Es = 3 J.mm -2 Energy Es = 1.5 J.mm -2 Fig.1. The stainless steel sample with laser color marking prepared by varying parameters The influence of laser marking on corrosion resistance and related properties of stainless steel equivalent to the AISI 304 steel were studied. Laser marking with different parameters was applied on thin sheets samples of dimensions 150 x100 mm and thickness 1.5 mm. Samples surface conforms to quality 2B cold-rolled. As expected, the microstructure of the steel was austenitic, with the marks of plastic deformation. The AISI 304 steel contains max % of carbon, % of chromium, % of nickel, less than 1 % of Si and less than 2 % of Mn. The laser parameters varied in terms of scanning velocity, pulse frequency and pulse length. It was chosen 36 different combinations of parameters for laser marking and a square shaped areas of dimensions 6 x 6 mm was marked on the sample by each of this parameters combination. The appearance of the steel sample with the laser color marked areas is shown in the Fig. 1. All squares were marked with constant energy per unit of area (Es), the experiments were carried out for energies 3 J.mm -1 and 1.5 J.mm -1. The surface morphology of the laser color marked areas was evaluated by optical microscope and scanning electron microscope. The roughness Ra of the laser treated surface was measured by HOMMEL TESTER T 1000 profilometer in the direction perpendicular to the laser trajectory. The corrosion tests were realized in the SVÚOM Praha s.r.o. The samples were exposed to 100% humidity in condensation chamber for 21 days (the test according to ČSN ) and to the saline mist with 5% NaCl solution spraying for 120 h (the test according to ČSN EN ISO 9227). The corrosion of laser color marked areas was evaluated by optical

4 microscope after 7, 14 and 21 days of exposition in the case of the test in condensation chamber and after 24, 48, 72, 96 and 120 hours in the case of the saline mist test. To study the relationships between the laser parameters, properties of color marked areas and their corrosion resistance, the laser parameters corresponding to the areas that show the sufficient corrosion resistance and to the areas with poor corrosion resistance, was chosen and together with unaffected surface, further analyzed by EDAX and XRD analyses. The EDAX analyses were realized by SEM Quanta 200 from FEI with EDAX EPMA with EDS detector. For the XRD measurements the automatic powder diffractometer Panalytical X Pert Pro with copper X-ray tube (λ CuKα = nm) was used. For the measurement, the Grazing Incidence X-ray Diffracton GIXRD method, suitable for thin film measurement, was used. The constant incident-beam angle was 1 w hich corresponds to depth of radiation penetration 1 µm. 3. RESULTS AND DISCUSSION 3.1. Surface morphology The surface morphology, studied by the optical microscope and surface roughness measurement, showed the unambiguous dependence on the laser parameters. The laser color marked areas surface roughness Ra decreased together with increasing pulse frequency and scanning velocity. Simultaneously, the scatter between the Ra values of areas, marked by the same parameters with varying pulse frequency, is decreasing with increasing of scanning velocity. These observations are valid regardless the total heat input and pulse length. In the Fig. 2, the surface appearance for the lowest used pulse frequency and scanning velocity (Fig. 2a) and highest pulse frequency and scanning velocity (Fig. 2b) can be seen. a) b) Fig. 2. The SEM of area created by a) lowest pulse frequency and scanning velocity and b) highest pulse frequency and velocity Corrosion tests The corrosion tests led to two detectable changes of laser color marked areas. First, some of the areas changed the color all around their surface. Such effect, even though it did not caused the corrosion defects, can be limiting for further use of laser color marking for example in the case of laser bar coding. Secondly, some of the areas underlaid to a corrosion attack, usually preceded by a lightning of color in a localized

5 spots. The corrosion attack had the form of orange-brown colored spots and stains, that spread progressively. In the Fig. 3, the evolution of corrosion in saline mist is shown. a) b) c) d) Fig. 3. The OM of laser marked area, representing the spreading corrosion in saline mist a) before exposition, b) 24 h exposition, c) 48 h exposition, d) 120 h exposition. The dependence of corrosion resistance on the laser parameters was not satisfactory validated by performed experiments. In agreement with [2], the better corrosion resistance was achieved using lower heat input. On the other hand, the dependence on scanning speed, referred in [2], was not confirmed in this study. The relationship between the corrosion resistance and surface roughness cannot be straightly explained. The measured results indicate that lower Ra and smoother surface lead to the better corrosion resistance, but there are many exceptions in the set of experimental data. Probably, the surface roughness is not the only parameter that plays its role EDAX and XRD analyses To evaluate the differences between marked areas prepared by different laser parameters in more detail, the EDAX and XRD analyses were made on several selected areas. The EDAX analyses confirmed the expected composition of base material with 18 % of Cr and 8 %Ni. The amount of these essential alloying elements did not vary significantly in consequence of laser color marking. The considerable changes was observed for the amount of oxygen, which was found to be dependent on the heat input the higher used heat input leads to an increase of oxygen amount in tested areas. The XRD results are shown in the Fig. 4 for base material, for marked area with expected poor corrosion resistance and for area with expected good corrosion resistance. The XRD results not only identify the type of created oxides on the color marked areas, but also point to a differences in the steel microstructure. In the base Intensity (cts) Oxides Austenite Ferrite Oxides ϑ ( ) Fig.4. The XRD measurement results. Base material Poor corrosion resistance Good corrosion resistance Ferrite

6 material, two basic phases was identified: the austenitic phase (cubic, face centered lattice) and ferritic phase (cubic, body centered lattice). Both laser color marked areas contain three phases: austenitic, ferritic and oxidic phase. The oxidic phase could correspond to Fe2NiO4, FeCr2O4, MnCr2O4 or FeFe2O4 (cubic diamond lattice). Comparing the XRD data it can be noticed, that the intensity of ferritic phase differs for the area with poor corrosion resistance, created using higher heat input. It could indicate two effects: the influence of deformation texture, or, with regard to the corrosion tests results, the lower amount of ferritic phase in the area with sufficient corrosion resistance. 4. CONCLUSIONS The performed experiments showed the relationships between the changes of corrosion resistance caused by the laser color marking and the other connected surface and structural changes. Based on the results, it could be stated, that the laser marking influences the corrosion resistance of stainless AISI 304 steel. The experiments revealed that the lower heat input leads to reduction of the oxygen amount in the laser treated area (thinner oxidic layer) and also to a better corrosion resistance. Similar dependences were also found out in [2]. The lower surface roughness produced by the laser treatment contributes to a better corrosion resistance, however, other effects can play more significant role than roughness. An amount of dissolved chromium in the austenitic and ferritic phases and the mutual ratio of these phases can also influence the corrosion properties, but this is not confirmed by performed experiments. The described effects can be controlled by combination of laser parameters. In this study, the unambiguous relationship between laser parameters and the surface roughness was found, while the relationships of other studied features and laser parameters are not so straightforward. The more detailed corrosion tests and following analyses will be provided to explain these relationships in the future. REFERENCES [1] BUCHFINK, G., The Laser as a Tool, Vogel Buchvedag, Würzburg, 2007 [2] LAAKSO, P. et al., Preliminary study of corrosion and wear properties of laser color marked stainless steel, In Conf. Proc. ICALEO 2008, LIA, USA, 2008, pp [3] BAUERLE, D., Laser processing and Chemistry, 3rd Edition, Springer, Berlin-Heidelberg, 2000, pp [4] [5]