POSSIBILITIES OF STAINLESS STEEL LASER MARKING Michal ŠVANTNER, Martin KUČERA, Šárka HOUDKOVÁ University of West Bohemia, Univerzitní 8, 30614 Plzeň, msvantne@ntc.zcu.cz Abstract Laser techniques are one of the technologies that can be used for modification of material surface optical properties. Decorative or identification marking of products is one of possible applications. Basic characteristics of laser technology utilization for stainless steel marking are presented in this contribution. Possibilities and limitations of this technology are discussed, in particular from the point of view of the marked surface corrosion resistivity preservation. Example of results of performed analysis are demonstrated, e.g. the influence of laser parameters on marking contrast properties, laser treated surface roughness and phase composition or corrosion resistance of laser treated surfaces. A possibility of an additional acid picking of laser treated stainless steel surfaces was also tested and it is discussed in this contribution. Keywords: laser marking, stainless steel, corrosion resistance 1. INTRODUCTION There are many reasons for material or products surface marking. The marking can be applied for decorative or informative purposes in a form of colored text, pictures, barcodes or datamatrix codes. It can be produced by different procedures in dependence on the marked material or other requirements (production costs, wear resistance or acids resistance for example). Used techniques are for example dot peen marking, selfadhesive labels, printing, anodization or emulsion coating. A usage of the conventional methods of the surface marking can be limited in many cases, due to the technological requirements, production speed or expanses. Laser marking [1, 2] seems to be an interesting alternative for a permanent material surface marking. Laser marking is based on an interaction of laser beam with the material surface. This interaction can lead to different processes based on laser type, process parameters and treated material. Different laser systems with many possible process parameters combination can be used. This makes the laser marking technique very universal and usable for a wide range of applications and treated/marked materials, for example metals, plastics, ceramics or leather. This contribution is focused on stainless steel surface laser marking. The treated steel surfaces can be modified in such a way that it appears as colored. The final marking properties and quality depend on used laser type and processing parameters. Laser marking is the fastest and cheapest method in many applications. However, the thermally affected areas on the treated surface can loss their corrosion resistance or wear properties. An investigation of an influence of laser marking of a stainless steel surface is therefore important [3]. High quality laser marking for decorative or descriptive purposes can be produced if suitable laser equipment with optimized processing parameters is used. 2. STAINLESS STEEL LASER MARKING 2.1 Laser equipment and laser marking procedure Pulsed fiber lasers are the most popular in the industrial marking applications. They are compact, have the long service life and their main advantage is the possibility of an independent change of the adjustable parameters. We have used a pulsed fiber laser SPI G3-HM 20 W, SPI G3-HM 30 W and SPI G3-HM 40 W
for our experiments. The peak emission wavelength of this laser is 1062 nm (IR range) and the laser average output powers are 20, 30 or 40 W respectively. The laser marking was realized using ScanLab ScanCube 10 scanning head with f-theta lenses of various focal lengths. The pulse frequency could be adjusted from 1 to 500 khz, the pulse length could be 9-200 ns and the maximum scanning speed is about 10 m.s -1. The laser power, scanning velocity and line spacing set other important quantity - the heat input Es. The heat input is one of the principal quantities for the marking process and it has to be adjusted based on laser type, its output power, used lens and marking strategy. We have used the heat input in range from about 0.1 to 4 J.mm -2 in our experiments. The laser spot diameter is dependent on the used lens and laser power (it is about 90 um for 30 W laser and f160 f-theta lens). The SPI G3-HM lasers are usually used for marking (plastics, metals or poly-compounds), scribing, ablation or solar cell processing. The subject of laser marking is a change of the surface contrast properties caused by an interaction of laser beam with the marked material surface. It can be achieved by chemical, phase or surface morphology changes during the laser processing. Ablation and thermal oxidation are the most suitable processes for metals laser marking, however, other processes can be suitable for different materials as well [4]. Different processing strategies can be used for the laser color marking using a pulsed laser [3]: single lines, single line overlapping, multi line overlapping, grating surface creation. In addition to these techniques the laser ablation can be also used. All the described strategies can be used for stainless steel laser marking, however, melting, ablation or excessively heating of the surface can weaken its corrosion resistance. Fig.1: Optical microscopy pictures of surfaces treated by different laser marking strategies: A - laser ablation, B - single line overlapping, C - multi line overlapping, D - grating surface. The most important laser marking parameters are laser spot size, laser beam power, scanning velocity, scanning lines distances, pulse frequency and pulse length. Variation of these parameters leads to a presence of different processes during the laser marking and hence to different properties of the marked surface. The laser parameters affect the chemical or phase composition of the marked surface and its morphology. These parameters determine the laser marking quality - contrast, optical properties, wear properties and corrosion resistance of a marked surface, which is an important parameter for stainless steel usage applications. The laser marking processing parameters have to be optimized in such a way that the produced marking meet all the quality and technological (production rate for example) requirements. 2.2 Materials and experimental techniques Corrosion resistance of stainless steel is mainly caused by a specific chemical composition of the material. The most common alloying agents used for increasing of steel corrosion resistance properties are chromium, nickel or molybdenum. These addition agents cause origination of a thin oxide protection layer on the steel surface - so called passive layer. This layer is self regenerating in a case of damage and it protects the steel surface against corrosion. The ability of a stainless steel to renew a damaged passive layer can be limited or lost in some cases. Therefore, the creation of the protective layer can be supported by pickling and passivation procedure [5]. The corrosion resistance level of the steel in different environments is given by used addition agents and their content in the material. The surface state (roughness, residual stress, crystalline defects, cracks occurrence etc.) can play also an important role for the steel corrosion resistance.
We have used the AISI 304 equivalent steel in our experiments. Laser marking was applied on 1.5 mm thick sheets with the surface quality 2B (cold-rolled) after a pickling and passivation treatment. The microstructure of the steel is austenitic with marks of plastic deformation and it contains max. 0.08 % of carbon, 17.5-19.5 % of chromium, 8-10.5 % of nickel, less than 1 % of Si and less than 2 % of Mn. This type of steel is resistant against water, water vapor, atmospheric humidity or weak organic and anorganic acids. It is in common use in food industry or pharmaceutical industry. The optical properties of the marking can be evaluated immediately after the marking production by a visual inspection or using some special devices. We have used the COGNEX Data Man 7500 datamatrix reader in our testing for these purposes. Color stability and corrosion properties of the stainless steel marking is much more complicated to appreciate because of time and financial demands for necessary experiments. Therefore, we have performed an amount of experiments with a view to find relations between laser marking parameters, marked surface properties and corrosion resistance changes of stainless steel. We have tested different laser marking parameters combinations including variations of scanning velocity, frequency, pulse length, laser power and lens. The corrosion tests were performed according to ČSN 03 81 31 (exposure to 100% humidity in condensation chamber) and ČSN EN ISO 9227 (exposure in a saline mist with 5% NaCl solution) standards. Furthermore, we have preformed other surface and material analyses of the marked areas [6]: roughness measurement, microscopy surface analysis using optical and electron microscopes, metallography analysis, EDAX element analysis and XRD phase analysis. 3. RESUTS 3.1 Laser marking quality The scanning velocity and pulse length have only a weak influence on the marking quality. The pulse frequency and the laser power play more important role - the marking quality decreases rapidly if the laser power decreases under a certain level (about 50% of maximum power if 30W SPI G3-HM laser was used). However, other factors (line spacing, used lens, etc.) can also influence the result significantly and the laser processing parameters should be adjusted for each individual case of used laser and f-theta lens. Deviation from the focal length had a significant influence on the marking quality. The acceptable deviations for the f160 lens were about ±1 mm and the marking was practically unreadable if this limit was overstepped. The range of a possible focal length deviation can be increased by using of a greater focal length lens. However, such lens can change other processing parameters (minimum spot size for example), that influence the marking corrosion resistance of the marked areas negatively. The acceptable declination from the focal plane (that means the laser beam is perpendicular to the surface) was about 15. The marking quality decreases over this limit and the readability can be limited. The marking quality experiments gave as the first information of the usability of the laser marking on the stainless steel. Based on the obtained results, we have selected the parameters combinations, which were usable to make a good quality marking. We have prepared experimental samples with a matrix of marked areas, which were made using different parameters and which met the quality requirements. These samples were subsequently used for the corrosion tests. 3.2 Corrosion tests results The corrosion tests results showed that many of the parameters combinations used were not suitable for the laser stainless steel marking from the point of view of corrosion resistance of the laser treated surface. Most of the marked areas were attacked by pitting or surface corrosion or the color stability was not acceptable on these areas. The corrosion tests indicated that the lower heat input Es is more preferable. This result is in accordance with other published results [3]. However, these corrosion tests did not make possible to determine any dependence of the laser treated areas corrosion resistance on the laser processing parameters.
We have carried out other corrosion experiment with the aim to confirm the obtained results and their repeatability. We have prepared a set of identical samples with laser marked fields, which were made using the heat input in the range from 1.5 to 4 J.mm -2. We obtained very similar results as it is shown in fig.2. The best corrosion resistance was observed for the marked fields with the heat input 1.5 J.mm -2 (the lowest heat input used for this experiment). Moreover, we can see a perfect repeatability as the results are almost identical for all the tested samples. On the other hand, the results were not too satisfactory from the point of view of the used parameters combinations set, because most of the fields had significantly lowered corrosion resistance. Es=1.5 J.mm -2 Original sample Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Fig.2: Corrosion test results. Identical samples made by the same laser processing parameters after the corrosion test. We have adjusted the laser processing parameters in such a way that we could lower the heat input and the marking kept still readable. We have used the heat input in the range from 0.5 to 3 J.mm-2. Accordingly to previous results and other published observations, the most corrosion resistant fields were made by the lowest heat input. These results also showed the influence of the pulse frequency and pulse length. The higher pulse lengths and pulse frequencies were more advantageous from the point of view of corrosion resistance of the marked surfaces. However, higher pulse frequencies brought results with unsatisfactory contrast properties of the marking. Therefore, the optimum procedure seems to be the lowest possible heat input and suitable combination of the pulse length and pulse frequency so that both the marking corrosion and contrast properties are satisfactory. It should be noted that it is complicated to use a subsequent procedures to increase the corrosion resistance (for example pickling and passivation) in this case, because the marking quality (contrast) properties could be lost. We have introduced the experimental way how to determine the parameters for stainless steel laser marking. The disadvantage is that the parameters can change for a different laser equipment (laser wavelength, lens, etc.) and probably also for different material compositions. Therefore, we have performed other material and surface analyses to find a relationship between the corrosion resistance and some other quantity. 3.3 Surface and material analyses The surface roughness could be one of the factors influencing the surface corrosion resistance. The surface roughness Ra of laser treated fields was in the range from 0.2 to 1.6 and a clear dependence between the roughness, scanning velocity and pulse frequency was observed. The surface roughness increased with the decreasing scanning velocity and pulse frequency. These relations were valid independently of the heat
input. However, a relation between the roughness and heat input was not observed as well as an explicit relation between the corrosion properties and roughness was not observed. We have investigated the surface structure by both electron and optical microscopy. As we have tested a wide range of laser processing parameters combinations, we have obtained very different surface structures. The structures corresponded to the different laser marking strategies and processes that taken place during the laser marking. The individual line paths and small surface cracks were evident on some of marked fields if a higher magnification was used. However, even in this case we did not observe an expressive relationship between the corrosion resistance and surface morphology or cracks occurrence. The metallography analyses were made on a limited number of samples only. The steel structure was austenitic with the evident plastic deformation due to a rolling process. Although the periodical marks caused by the laser treatment were evident on the metallographic cross sections, no structural changes were observed at analyzed samples. The EDAX analyses confirmed the expected composition of the base material with about 18 % of Cr and 8 %Ni. The amount of these essential alloying elements did not vary significantly in consequence of laser marking. The considerable changes were observed for the amount of oxygen only, which was found to be dependent on the heat input. The higher used heat input led to an increased oxidation and thus to an increase of oxygen amount in the marked areas. It affected the corrosion properties of the marked surfaces negatively, as followed from other results as well. However, based on this measurement, it is not possible to determine the level of the corrosion resistance generally. The base material and the marked fields with supposed bad and good corrosion resistance were analyzed using the XRD measurement. Austenitic (cubic, face centered lattice) and ferritic (cubic, body centered lattice) phases were identified in the base material. The oxide phase (cubic diamond lattice) was found in the laser marked surfaces in addition to the austenitic and ferric phase. The performed experiments showed the clear relationship between the corrosion resistance, the ferritic phase amount and partially also the oxide phase amount. The ferritic phase content was reduced significantly (not detectable in some cases) in the samples, which had a good corrosion resistance. On the other hand, the content of the ferritic phase was higher in samples with a worst corrosion resistance. The content of the oxide phase seems to be also higher in the samples with the low corrosion resistance, but this is not as evident as in the case of the ferritic phase. The XRD analyses results did not explained exactly the reasons and principles of the corrosion resistance changes of the stainless steel material. However, it brought very clear relations between the phase changes in the material caused by the laser treatment and corrosion resistance properties of the affected surfaces. 4. CONCLUSIONS The laser marking seems to be an interesting technique for stainless steel marking. It can be used in many industrial applications and it can bring a lot of advantages - lower expenses or faster marking process for example. However, it is also connected with some technological problems including the marking quality questions or stainless steel corrosion resistance changes. We have shown that the SPI G3-HM laser is usable for stainless steel marking if the parameters are adjusted properly. The good quality marking can be made also if the processing conditions are not ideal (deviations from the focal length, declination from the focal plane, etc.), for example in industrial production lines. The processing parameters and used equipment can be adjusted in a wide range of combinations to meet production requirements (marking quality, production rate, etc.). An unchanged corrosion resistance is one of the very important requirements for stainless steel laser marking. The experiment showed that laser marking processing parameters variation can lead to very different results from the point of view of corrosion resistance of the marked surfaces. It was verified that the most important processing parameters was the heat input. The heat input influences the process principles - laser marking strategy, thermomechanical processes during the marking procedure and the final state of
marked surfaces. Even if a surface morphology can play a role in a material corrosion resistance, in this work was not confirmed a significant influence of surface roughness or cracks occurrence on the laser treated surfaces corrosion resistance properties. The most important for the stainless steel corrosion resistance is a passive oxide layer on the original material surface. However, this layer is probably damaged during the laser marking process and it cannot be renewed by an additional pickling and passivation, because these processes can lead to the created marking quality changes. The XRD diffraction analyses showed the occurrence of austenitic and ferritic phases in the original stainless steel material. The passive layer was neither detectable by the XRD, nor by other used experimental techniques. However, it was confirmed by the XRD analyses that processes during the marking procedure led to changes of the ferritic phase content in the material surface and to a creation of a new oxide layer. Furthermore, the results indicated that the ferritic phase content and the new oxide layer content in the marked material surface have a relationship with the laser affected surfaces corrosion resistance. The performed experiments did not make fully clear the base of the changes in corrosion resistance after the laser marking processes. However, it showed some relationship between the surface properties and corrosion resistance. As the corrosion tests are expensive and time consuming, the XRD analysis seems to be a promising method, which can be helpful to find out an optimum parameters combination. ACKNOWLEDGEMENT This research work has been solved at University of West Bohemia in cooperation with Lintech - industrial laser marking system integrator and with the support of the Ministry of Industry and Trade of the Czech Republic under the research and development project no. FR-TI1/255. REFERENCES [1] RUSCONI R., GOLD J., Color marking. Industrial Laser Solutions, p. 16-18, 2005. [2] CAREY A.M. et al., Laser Surface Ornamentation, Proc. of Int. Congress on Application of Lasers & Electrooptics ICALEO 1998, Orlando, FL, USA, 1998 [3] LAAKSO P. et al., Preliminary study of corrosion and wear properties of laser color marked stainless steel, Proc. of Int. Congress on Application of Lasers & Electro-optics ICALEO 2008, LIA, USA, 2008. [4] BUCHFINK, G., The Laser as a Tool, Vogel Buchvedag, Würzburg, 2007 [5] CROOKES R., Pickling and Passivating Stainless Steel, Material and Application Series, Vol. 4, Euro Inox, 2007 [6] ŠVANTNER M., KUČERA M., HOUDKOVÁ Š, ŘÍHA J.: Influence of laser ablation on stainless steel corrosion behaviour, Proc. of 20 th Int. Conference on Metallurgy and Materials METAL 2011, Brno, Czech Republic, 2011