How does laser ablation work when cleaning metal?

What matters is the interplay of material, layer structure, laser source, pulse energy, focus, processing speed and the desired surface condition. Only when these factors fit together does a stable and cost-effectively sensible process emerge.

This article therefore does not look at the possible uses of the technology in general, but specifically at the factors that influence removal on metal surfaces.

What decides whether a layer is removed?

During processing, laser radiation meets an existing surface layer and the substrate beneath it. The two can differ markedly in absorption, thermal conduction, reflection and thermal behaviour.

The physical basis is explained in the Beamlux glossary under laser ablation. For practical use, what matters above all is whether enough energy reaches the layer to be removed to produce controlled removal. Several variables interact here:

• Laser power, pulse energy and pulse duration
• Frequency and beam diameter
• Focal position and scan width
• Travel speed and number of passes
• optical properties of the coating
• thermal properties of the base material

The Fraunhofer Institute for Laser Technology ILT expressly describes laser-based processes for cleaning, coating removal and the pre-treatment of surfaces. Both pulsed and continuous beam sources can be used for this.

For users, an important conclusion follows: there is no universal parameter set for every metal surface. Thin flash rust on a steel sheet is technically a different task from a multi-layer paint system on a machine component or a thin oxide layer on stainless steel.

Considering base material and top layer separately

With a new application, at least two levels should be considered: the base material and the layer to be removed. Possible base materials include steel, stainless steel, aluminium or copper. On top of these there may be rust, paint, scale, oxides, production residues or other layers.

For the process, what is to be removed is not the only thing that matters. Equally important is how the surface should look after processing and how it is to be processed further. A solid steel beam that is recoated afterwards places different demands on the process than a visible stainless steel surface. A welding surface, too, may require a different target condition from a bonding surface.

Asking whether a laser can process a particular metal therefore falls short. More useful technically is: which layer is to be removed from which base material, and which surface condition is required afterwards? Only with this information can a process be assessed sensibly.

Energy density and material removal threshold

Two important variables are energy density and the material removal threshold. Stating the laser power alone does not yet describe how concentrated the energy actually is when it arrives at the surface.

If the energy is concentrated on a small area, the local loading changes. If the beam is moved faster, the interaction time on any single area is reduced. With several passes, by contrast, the total energy input rises. This is why two operators can achieve different results with the same machine if they work with different distances, speeds or scan widths.

In a stable industrial process, therefore, it is not only a wattage that is set. Several parameters are defined and matched to one another. This is particularly important for recurring components: once a suitable process window has been established, parameters can be documented and used again. That turns manual processing into a reproducible process.

Why pulsed lasers work differently from CW systems

When selecting a system, power figures are often the first thing compared. A system with 2,000 watts inevitably looks more powerful on the data sheet than a machine with 200 or 300 watts. This view is incomplete, however, when different types of laser are being compared.

A pulsed laser transfers energy in short pulses. High peak powers can arise within these pulses. A CW laser, by contrast, works with continuous power output. The interactions with the surface differ as a result.

Pulsed systems are often of interest where controlled and precise removal is required. CW systems can play to their strengths where robust surfaces and larger areas are to be processed. Fraunhofer IWS likewise shows that both pulsed systems and high-power continuously operating lasers can be used for surface pre-treatment.

The power classes should therefore not be understood as a simple ranking. The right question is rather: which technology produces the required result on the specific component within the necessary process time? This comparison is examined in more depth in the article on the energy input of the two designs.

Pulse energy, frequency and processing speed

With pulsed systems in particular, wattage alone is not enough. Alongside the average power, pulse energy, pulse duration and frequency play a central role. Pulse energy describes the energy of a single pulse. The frequency indicates how many pulses are produced within one second.

At the same time, the beam moves across the surface. This movement creates an overlap of individual processing spots. If the work is carried out slowly, certain areas can receive energy more often. At a higher travel speed, the exposure time per unit area falls. Different combinations can therefore lead to very different results.

A thin surface film can possibly be processed with a single fast pass. A thicker coating may require several passes. In industrial applications, quality and cycle time therefore always have to be weighed against each other. A technically excellent result is not sufficient for series production if the required cycle time is exceeded considerably.

Focal position and scan width

The focal position influences how the energy of the beam is distributed over the workpiece surface. Even a change in working distance can therefore affect the result.

Then there is the scan width. A wide scan covers a larger area per movement. At the same time, however, the energy distribution changes. “Wider” is therefore not automatically “faster”: a large scan width with too low an effective energy density can mean that a coating is not removed completely. Too small a scan width, on the other hand, can cost unnecessary time.

For a high area coverage rate, scan width, power, distance and travel speed have to match. With hand-held systems in particular, how evenly the operator moves also plays a part. In automated processes, such movement sequences can be reproduced very precisely.

Rust, paint and oxides require different processes

The term contamination covers technically very different materials.

Corrosion layers

Rust can occur as light flash rust or as a heavy corrosion layer. With deep-seated corrosion, it has to be borne in mind that although the laser can remove existing corrosion products, it does not restore base material that has already been lost. For the general application there is a separate page, Rust processing with a laser for different metal surfaces. This article looks exclusively at the technical influencing variables.

Paint systems

An industrial paint system can consist of a primer, intermediate layers and a topcoat. The individual layers can react differently to the energy applied. The process can therefore change at the transition from one paint layer to the next.

Oxide layers

Oxides can be extremely thin or occur as pronounced scale. Heat tint after welding processes also belongs in this category. A thin oxide layer does not necessarily require the same parameters as heavy scale on a hot-rolled component.

Steel, stainless steel and aluminium

Steel

Steel is one of the materials most frequently processed. Besides the rust or the coating present, what happens after processing is important. Is the component to be coated, welded, bonded or merely cleaned? This subsequent process defines the desired surface condition.

Stainless steel

With stainless steel, oxides, discolouration and industrial residues can be the main concern. On visible surfaces in particular, it should be checked not only whether a layer is removed, but also how texture and appearance change.

Aluminium

Aluminium has different reflection and thermal conduction properties from steel. Parameters should therefore not be transferred from a steel component to aluminium without checking. Different aluminium alloys also vary. With high-value components, a test on the original material is therefore particularly worthwhile.

Why a material test is decisive for the process

Theoretical technical data provide initial orientation. For a real investment or process decision, a processing trial is often considerably more meaningful. A material test shows, for example:

• whether the desired layer is completely removed
• what the surface looks like afterwards
• how many passes are necessary
• what speed can be achieved
• whether a pulsed or a CW system is more suitable
• which power class appears sensible

For companies, the real process time is particularly important. A machine can achieve a technically very good result and still be too slow for their own production volume. Conversely, a lower power class can be entirely sufficient in economic terms for small, high-quality components. Beamlux offers material tests and live demos for specific applications.

When higher power actually makes sense

More power can bring an advantage above all where larger surfaces or larger quantities of a layer to be removed have to be processed in a short time. That applies, for example, to robust metal components in maintenance.

The highest power is only an advantage, however, if the desired surface condition is still reliably achieved. At least three factors should therefore be taken into account for the comparison: Is the defined target condition achieved? Is processing fast enough? Can the result be produced repeatably? Only when all three points fit can a power class be assessed sensibly.

The Beamlux system overview for professional applications shows the available designs and power ranges.

Automation and reproducible processes

Laser-based surface processes are generally well suited to automated sequences. Scanners, robots or axis systems can follow defined processing paths reproducibly. That is of interest, for example, for:

• Welding surfaces and bonding surfaces
• Contact areas
• local coating areas
• recurring tool geometries

Fraunhofer ILT expressly names the integration of corresponding cleaning and coating removal processes into industrial production sequences as a field of application. Fraunhofer IWS likewise describes automatable surface pre-treatments and scanner-based systems. Once a stable process window has been determined, movement, speed and laser parameters can be documented. This increases repeatability. Further fields of use are summarised on the page Application areas for components and surfaces.

Occupational safety and extraction are part of the process

Technical process optimisation must not be considered separately from occupational safety. Hand-held high-power systems can be class 4 lasers. The DGUV guidance document FBHM-139 “Strahlarbeiten – Reinigen und Entschichten mit Laserstrahlung”, edition 06/2024, expressly deals with hand-held class 4 laser equipment.

Alongside laser radiation, it also covers the hazardous substances produced and the capture of process emissions. Suitable process extraction is therefore part of a professional workplace concept. Further points can be:

• Risk assessment and access control
• Shielding of the laser area
• Instruction and suitable eye protection
• Consideration of reflections
• Fire hazard and assessment of the removed substances

The TROS Laserstrahlung are also relevant for assessing the hazard and the protective measures. BAuA lists the corresponding parts in the July 2018 version. Where sites of use change in particular, the specific surroundings have to be assessed as well. The page Safety information on laser class 4 summarises the most important points.

Conclusion: the process window matters more than the wattage

How does laser ablation work when cleaning metal? The short answer is: through a controlled interplay of laser energy, layer, base material and movement. For industrial practice, what matters is developing a reproducible process from it.

No single figure decides the result. Material, coating, laser type, pulse parameters, energy density, focal position, scan width and speed act together. Two technically very different systems can therefore both be right — for different tasks. A pulsed system can be sensible for precise processing. A powerful CW system can offer advantages on robust, larger surfaces.

Anyone who wants to master the process has to know what controls the removal. The sensible approach is: determine the base material, analyse the layer to be removed, define the desired surface condition, define the necessary area coverage rate, compare suitable laser types, test on the original material, assess quality and process time together — and then select the matching power class. That is how a machine specification becomes a robust B2B process.

Sources

• Fraunhofer Institute for Laser Technology ILT — “Cleaning”: information on laser-based cleaning, coating removal and surface pre-treatment.
• Fraunhofer Institute for Material and Beam Technology IWS — information on surface pre-treatment, joining technology and scanner-based laser processes.
• DGUV — FBHM-139 “Strahlarbeiten – Reinigen und Entschichten mit Laserstrahlung”, edition 06/2024.
• BAuA — TROS Laserstrahlung, July 2018 version.

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