Laser ablation is achieved when a very localized area of a material, which is targeted by the focused beam of an industrial laser, undergoes either fusion or sublimation. The fused material is expelled, resulting in a change of the topography of the targeted material, The depth of material affected can amount to a few tenths of millimeters.
The Laser Ablation Process
All materials have ablation thresholds. It is a property that is unique to a material. When intensity (energy per surface area) is above the ablation threshold, the laser ablation occurs. But if the intensity is below the ablation threshold, nothing happens, apart from a slight temperature increase.
Laser ablation is the mechanism through which most laser material processing occurs. Other material processing techniques also exist. The most noteworthy are: laser carbonization for plastics as well as laser annealing, laser etching and laser engraving for metals.
In the following section, you will find a list of factors that are used to optimize laser ablation and a few industrial applications of laser ablation.
Factors to Consider for a Successful Laser Ablation Process
- Wavelength: When light hits a surface it is either reflected, absorbed, transmitted or a combination of the three. Also, the reflectivity of a material varies as a function of the laser’s wavelength. Therefore, a laser that emits at a wavelength that has a low reflectivity should always be favored. For most metals, Laserax’s fiber lasers are a good choice.
- Diameter of the beam: Changing the diameter of the beam impacts the intensity of the laser that hits the targeted material. All else remaining equal, Increasing the beam diameter will reduce the intensity.
- Number of passes: With certain jobs that require deep engravings, you might get better results with several passes rather than increased intensity with one pass.
- Focal distance: As the focal distance is fixed for a given lens, some manual adjustments might be necessary. For optimal results, it is important that the targeted area of the parts being processed be at, or very close to, the focal distance.
- Line feed: The speed at which the laser beam sweeps the surface to be processed has an impact on the intensity delivered to the surface, especially when you’re using a pulsed laser. The speed of the line feed will determine the spacing between each laser pulse on the targeted materials.
- Distance between successive lines: Changing the distance between the lines has a similar effect as that of the line feed. The shorter the distance, the tighter successive lines will be; more energy will be transferred to the material per unit surface.
- Power of the laser: The more powerful a laser is, the more intensity it can deliver over a given period of time.
- Pulse frequency: It is possible to change the elapsed time between each pulse. Reducing the pulse frequency will reduce the energy delivered to the targeted material per unit of time (if we keep the energy of the pulse constant).
Increasing the pulse frequency is a little bit more tricky. When the elapsed time between each pulse is reduced, the intensity is increased up to a certain point, corresponding to the maximum optical power (PMAX) level of the laser (see the graph below).
Even if you were to double the number of pulses per unit of time (from n to 2n), the power cannot be increased past the laser’s maximum optical power; the energy per pulse delivered would therefore be reduced (from E1 to E2).
- Average laser power: It is important to closely control this parameter, because it may have the most significant impact on marking quality. You can adjust the pulse energy, the pulse frequency or both. Increasing the pulse energy or the pulse frequency can increase the average level of laser power.
Applications for Laser Ablation
Laser Direct Part Marking
Most metals, several plastics and other organic materials can be ablated with a laser. Thanks to the ability to control the path of the laser beam, it is possible to engrave complex messages on all of these materials. Messages could take the form of text, logos, barcodes, Data Matrix Codes, serial numbers, dates and time etc. Click on the image below to watch a video of laser direct part marking.
As explained earlier, every material has its own ablation threshold. The ablation threshold of rust, paint, oil and many other contaminants is lower than that of metals. It is therefore possible to set the laser’s parameter so that the intensity of the laser beam will be sufficient to remove most contaminants and, at the same time, low enough to minimize the impact on the metal to be cleaned.
Laser Rust Removal
Rust can be easily removed using a good laser ablation technique. In the following video, you can watch a demonstration of several parts being cleaned of their rust and oxidation with a laser cleaning system.
Laser Paint Stripping
By using appropriate laser ablation methods, it is possible to remove industrial paint from steel parts. The painted area being stripped of paint can easily be delimited as you can see in the paint removal video below.
Laser Ablation: An Important Material Processing Technique
It is important that the intensity of the laser beam is sufficiently high to exceed the ablation threshold; otherwise, no material processing would occur. The ablation thresholds of each material are different, which makes the use of laser ablation for laser cleaning possible.
There are different ways to change the intensity of the laser beam, which will generate different results. This means that laser ablation can be used in a wide variety of applications, such as laser marking, laser engraving and laser cleaning.