In the past decades, the laser marking industry has developed rapidly. Nowadays, there are a large number of laser marking system suppliers serving various industries around the world. The main change in this market is the introduction of low-power pulsed fiber lasers, which have now developed to the point that almost all suppliers provide this type of fiber laser marking equipment within their product range.
These lasers are usually near infrared (NIR) near 1070 nm. Compared with CO2 lasers with longer wavelengths, they have lower reflectivity, so they are very suitable for marking most metal products.
But even in this wavelength range, the complexity of labeling different metals is different. Aluminum, copper and their alloys are widely used in almost all industries. These materials can be used for laser marking, but it is sometimes difficult to see black spots on these low-temperature metals with the naked eye. In addition, proven methods have shown that highly permeable materials (such as B markings and textures) with minimal damage during the duration of the pulse can usually be treated without unexpected nonlinear behavior.
Laser surface treatment:
In the broad field of industrial laser material processing, the term "laser surface treatment" is usually used to describe various processing types using multi kilowatt continuous wave (CW) laser sources in the near infrared range. However, the above method is quite different from the method described here, and it can be considered that it is applicable to micro and nano scale surfaces. Several methods have been identified and published using ultrashort (10-12) and femtosecond (10-15) pulsed lasers.
The main disadvantage of these methods is that even in such low-power laser series, their investment and operating costs are still high. Because the processing speed usually depends on the average laser power, for most industrial laser users, the cost of laser processing under real surface coating conditions may be prohibitive.
Recently, the pulse width range of known nanosecond pulsed fiber lasers has been extended to the sub nanosecond range, where the peak power has increased by a corresponding order of magnitude. This makes it possible to develop a new laser surface treatment process using an economical long picosecond laser source.
Although these technologies are usually called laser surface, they are closely related to laser marking in machinery, because they are limited to the surface treatment of parts, and usually require the combination of laser ablation and melting processes. Figure 1 attempts to classify these various processes using industry standard terminology and key physical mechanisms.
Analysis of surface texture and laser marking
Laser marking plays an important role in changing the laser marked surface area in some way to provide visual contrast with the unmarked area.
Pen profilers are probably the most famous and commonly used method for measuring relevant data, which is why they were selected for the initial evaluation of laser therapy. Surface morphology provides a more general qualitative and quantitative description of surface characteristics and shapes, of which imaging technology is the most appropriate. Therefore, 2D and 3D images are selected from the confocal laser scanning microscope.
Advanced spectrophotometers are often used to quantify surface color. This is achieved by analyzing the light reflected from different points in the visible spectrum to produce a unique reflection curve that reflects the characteristics of any surface, regardless of the presence of specular elements. These devices are also often used to measure the L * value of the surface or the hue of the surface. Today, this technology is an important tool to determine the effectiveness of laser marking on various consumer goods. These reflectivity curves and L * values are used to quantify the benefits of a powerful short pulse fiber laser for three difficult to handle materials: aluminum, copper, and glass.
1. Laser texture of metal and aluminum surfaces.
In aluminum, the natural oxide layer is hygroscopic, and its thickness will increase with time. Therefore, it may be sufficient to remove this coarse, contaminated oxide layer to expose the aluminum below to produce sufficient contrast. Another complicating factor is that the degree of melting or erosion of the underlying aluminum can significantly affect the appearance of the mark.
By carefully adjusting the laser settings, you can get a brighter surface with a blending effect that enhances contrast. However, if you want to get a lower L * value, you can create a darker, highly oxidized surface on the aluminum with a pulse energy of about 1 mJ, but it still has a solid, non brittle surface, so that the appearance of the logo does not change Perspectives require careful management of the process. Increasing the ablation level to produce a slightly roughened surface also results in darker surfaces, greater absorption, and higher L * values.
Removing anodized layer from aluminum surface is a common method, and the same rule applies to laser engraving: more melting means more reflective surfaces. Whether it is pure aluminum or anodized aluminum, the marking speed has reached a high level of 1-2m/s. Recently, laser marking technology has been developed for some anodic oxide coatings to achieve L *<30 using fiber lasers with short nanosecond and subnanosecond durations, but it is much slower than the above marking speed.
Laser texturing of copper surface
Laser polishing metal copper to produce contrast is a relatively well-known process, but due to the inherent high reflectivity of metal, dark spots are usually more difficult to achieve.
By comparing it with the surface roughness before polishing, the difference of laser cutting surface roughness (<1 µ mRa) can be observed. However, the surface structure is more complex, and the surface is significantly improved, resulting in a super absorbent surface.
There is still the assumption that the nonlinear plasma process is not the traditional process for removing hot materials. Another important proof is that under the same laser parameters, 20 µ m thick copper plates can be processed without material deformation, but the average power of 28.5 W sub nanosecond laser is used.
Surface texture with laser or glass marking
Surprisingly, almost the same parameters as the copper material can be used to mark the top and bottom of the uncoated borosilicate glass. This again confirms the hypothesis that nonlinear absorption is related to the performance of peak power fiber lasers. By observing the removal area, it can be seen that "cracks" are very limited, with cracks<10 µ m and surface roughness<5 µ m Ra.
This paper discusses how to quantify laser marking and surface pattern, and applies these methods to laser marking aluminum. The technology of marking more complex uncoated copper and glass surfaces indicates that there will be more extensive application of surface patterns in the future.
Laser welding between copper and other metals has always been a problem in low power hot wire welding due to the combination of inherent high reflection and dispersion coefficient and volatility of oxide layer. This dark marking method has been proven to improve the consistency of copper welding. These fine structures can also improve the adhesion between copper or aluminum and other metals as part of ongoing research to improve and standardize the surface absorption of laser beams.