Laser marking has significant advantages in terms of productivity and costs. It does not hurt when it produces good quality as well. Contributed by Petrina Heng, sales manager, Laser Division of Trumpf Asia Pacific.
Laser marking units use pulsed nanosecond lasers generating a variety of wavelengths, from infrared to green and ultraviolet. The units best suited for marking metals are diode-pumped, solid-state lasers with yttrium aluminium garnet or vanadate as the active laser medium. Fibre lasers are also appropriate.
Different kinds of lasers offer a variety of advantages in differing applications and when marking any of a number of metals. Not only does the type of laser make a difference; the pulse duration, peak pulse power, beam quality and mean power will have an influence on the marking results.
Finding The Range
That is why developers are offering a broad range of marking lasers exhibiting various power classes, pulse lengths and wavelengths. Higher mean power will usually achieve higher velocities.
However, not only power is important, but the beam’s quality too. High quality makes it possible to focus the laser beam on a very small spot. In this way, great power densities can be achieved on the surface of the workpiece.
Laser Marking On Metal
Lasers are suitable marking tools for metals. When marking steel, infrared light at a wavelength of 1,064 nm is normally used for processing. Metals with higher reflectivity — such as aluminium, copper and gold — will absorb only a fraction of the laser power.
It is said that lasers with excellent beam quality and high peak pulse power offer advantages. Over and above that, selecting a short wavelength (green) can also bring about good marking results, since these metals will absorb a shorter wavelength more readily.
When it comes to precision, shorter wavelengths are suitable in these situations, since shorter wavelengths can be focused better. The marking on the workpiece itself is in the form of a series of dots, either adjacent or overlapping, which the pulsed laser beam leaves on the material. Pulsing makes for a more carefully defined introduction of energy and subsequently the heat into the material.
Only a very small area is subjected to thermal and mechanical effects. In principle, there are three processes widely used in industry to mark metals. These are engraving, ablation and annealing.
Engraving With Laser
When engraving, the laser beam strikes the surface of the metal, fusing and vapourising the material. This leaves recesses in the surface of the component. One usually distinguishes between ‘black’ and ‘white’ engraving. The former is associated with higher power levels and lower advancing speeds.
Black engraving leaves a depression with a dark, oxidised surface and a build-up of the flux at the edge. White engraving only roughens the surface slightly and therefore alters the visual impression. Light falling on the object is diffused when reflected and appears to be brighter than the untreated metal surface on the component.
For example, a QR-code can be applied with a combination of black and white engraving. This brings about greater contrast and improves legibility. Engraving is preferable where components are susceptible to corrosion and whenever coating is to follow marking. The deeper the engraving, the longer the laser will take, since it will have to make several passes to achieve the desired depth.
Ablating With Laser
A further technique used for laser marking is the ablation of a layer on the workpiece. Repeated passes will result in differing ablation depths. Once the layer has been removed, the substrate becomes visible. This is the situation for anodised aluminium, painted metals, or where there is a film covering.
Marking lasers can, however, also be used to prepare for other steps in the production process. Laser welding and bonding are just two examples. If, for instance, a component with an oily, dirty or oxidised surface is to be joined, then the laser beam can be used to remove the contaminant. In this way, a uniform and defined surface can be prepared so that the subsequent welding procedure can be carried out correctly.
Applications such as these are being requested more than ever. This is because not only good joints are achieved, but cleaning agents can be eliminated. That is easy on both the environment and the wallet.
Annealing With Laser
One property of metals is that they will discolour when heated and subsequently cooled. Laser annealing makes use of this phenomenon. The laser can be used to induce heat in a closely defined area on the surface of a workpiece.
An oxide layer is formed and it determines what colour will be seen. Differences in annealing hues arise from differing temperatures on the surface of the workpiece. This process leaves the surface of the part intact — making it suitable for labeling surgical instruments or implants.
Meeting Requirements In Medical Technology
The surface remains smooth and clean and complies with the stringent requirements prevailing in medical technology with regard to biocompatibility and corrosion resistance. Here, the annealing procedure utilises its advantages to the full. Metals like titanium and stainless steel are suitable for marking by annealing.
An essential prerequisite for success is closely defined energy introduction. This is guaranteed by the precise power regulation incorporated into marking lasers.
It is even possible during annealing marking to achieve certain nuances in the change of colour. In addition to its uses in the fields of medical technology, annealing is increasingly being used to create design elements on metallic surfaces and to enhance and individualise products.
With the examples discussed in this article, it is easy to see that laser marking is one reliable technology that not only helped produce quality parts, it is a good cost saver as well.
APMEN Fast Fab