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R ELATION OF LASER PARAMETERS IN COLOR MARKING OF STAINLESS STEEL
P. Laakso1, S. Ruotsalainen1, H. Pantsar2, R. Penttilä1
Abstract Color marking of stainless steels as a process is known for sometime but still it has not been used widely in the industry. Some industrial applications have been seen. New MOPA fiber lasers allow independent tuning different laser parameters and the marking process can be optimized for producing colors with better quality and visual appearance. Laser processing of metal surfaces creates an oxide layer on the surface. The thickness of this layer defines how white light is reflected from the sample. What in principle is only a thin oxide layer on the surface can be seen as different colors by the viewer.  In this study, the visual appearance of laser marked surfaces was optimized by varying different laser parameters. The aim was to create a uniform oxide layers on the surface which would appear as a high quality color marking. Marking quality was evaluated by visual examination. Relation of different laser parameters to produced color is discussed based on these results. Few application samples are also presented. Keywords: laser marking, color, thin film effect

1      Introduction

Color marking of metal surfaces is conventionally done by printing, anodization or emulsion coating.  However,  the  scratch  properties  of  printing  are  limited  and  anodizing  more  than  a single color precisely is not easy. Emulsion coatings are more expensive and they require one step more to produce colors. Lasers can be used to create a permanent color mark on a metal surface as a one step process with a high throughput. Laser color marking of metals has been used  for  more  than  ten  years  with  a  variety  of  different  laser  sources  /1,2,3,4/. Despite the possibility to vary processing parameters typically in a wide range, some lasers are more suitable for color marking than others and the pulse width seems to be an important issue in defining the marking quality and contrast. Typical laser of choice for marking is a q- switched crystal laser which produces pulses in the nanosecond regime. These lasers do not allow independent adjustment of the processing parameters, but the pulse width is dependent on the repetition frequency and as the frequency changes, so does the pulse width. Therefore lasers that allow adjustment of the pulse width regardless of the frequency might give an advantage in marking. In most applications laser marking is the fastest and the cheapest method. Flexibility of laser marking is based on writing with the laser beam, which interacts with the material surface creating the mark. Unlike most of the other marking techniques, laser marking does not use any chemicals or tools. Some metals can be marked in a way that the surface appears colored. This is based on oxidation and the following thin film effect. In order to create a uniform and high quality mark, the used laser must have a good enough beam quality and stability. New fiber lasers are well suited for this method and their affordable price opens up new applications for laser marking. Color marking with lasers is easy and makes the surface visually more attractive. Suitable colors can be chosen by proper parameter control and if needed, the marking can be made on the fly on a moving object.

2      Principle of laser color marking

Surface  oxidation  of  metals  in  an  oxidizing  agent  is  a  well  known  phenomenon.  Clean surfaces of many materials spontaneously react in air to form thin native oxide layers. Light- enhanced and in particular laser-enhanced material oxidation is based on thermal or non- thermal molecule surface excitations /5, p. 535/. Thin oxide films can be formed by heating the surface uniformly using a laser beam.  Oxygen must be present in the ambient atmosphere when marking with a laser. Air is sufficient for thin film growth, but a higher concentration of oxygen can be used to enhance film growth. Oxide film growth cannot be started in an inert atmosphere. Most important parameters in laser marking are the focal spot diameter, power on sample, marking speed, line spacing, marking direction, repetition rate and pulse length. Gaussian beam profile of these fiber lasers might not be the best option for marking, but it would be modified into a top hat mode beam using a simple beam homogenizer. The surface temperature and the thermal load,  or the input  energy over an area,  can be adjusted by varying the beam power, scanning velocity or the line spacing. The aim is to maintain  a  constant  surface  temperature  in  order  to  create  a  uniform  color  surface.  The following oxide layer will grow to a certain thickness and create the thin film effect. The process is self terminating because it comes less likely with increasing layer thickness /5, p. 536/. The thickness of this layer defines how white light is reflected from the sample. What in principle is only a thin oxide layer on the surface can be seen as different colors by the viewer. If this oxide layer is thick and solid it will have also good corrosion and scratch properties, which are essential in consumer products. Variation in the oxide layer thickness and the surface roughness will have an effect to the resulting color seen by the viewer (Figure 1). Depending on the oxide layer quality, the color may change when viewed from different angles.

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