Laser polishing works by melting the surface of a material to reduce the average height of any peaks while being sure not to melt the material too deeply, so as to avoid microstructural changes deeper into the workpiece.
Laser polishing processes are affected by the type of laser radiation (whether pulsed, continuous or a mixture of both) as well as by the process parameters, including the laser power, feed rate, scanning velocity, diameter of the laser beam, the focal point of the laser, and the actual workpiece surface itself.
The process can be undertaken to reduce the initial roughness of a range of metals and metal alloys, including stainless steel, as well as for some ceramics and glass. Laser polishing can be performed at a micro or a macro level, although the outcome is the same in that it reduces either macro or micro roughness on the surface of the workpiece.
Laser sources are used to melt a thin layer on the surface of the material, with the surface tension causing the molten material to flow from the peaks to fill the valleys. This reallocation of material provides improved process times compared to manual polishing, offering a typical processing speed of one minute per cm2 - or up to 30 times faster.
The process is used for the polishing of metal and other materials for industries ranging from moulding through to medical, while the appearance of design surfaces can be further enhanced through the creation of a dual gloss effect through selective laser polishing, which creates a change in surface roughness on the laser polished surface.
Laser polishing is typically carried out on a five-axis mechanical handling system coupled with a three axis laser scanning system and fibre laser. The two techniques used can be identified as shallow surface melt (SSM) and surface over melt (SOM), depending on how the laser is deployed:
- Shallow Surface Melt (SSM): This technique uses the dynamic behaviour of the liquid metal at high temperatures, which flows into the surface micro-asperities. This fills up the valleys, typically to a depth lower than the total peak-valley distance.
- Surface Over Melt (SOM): By increasing the energy density of the laser beam it is possible to change how the melted material or melt pool behaves. As the melted material thickens it can exceed the peak-valley distance, thereby turning the entire metal surface into a melt pool. At higher laser densities the molten metal will be pulled away from the solidifying front, creating ripples across the surface of the metal. Understanding the effects of laser density parameters is important in reducing waves in a final, polished surface.
Laser polishing processes can use either; pulsed laser sources; continuous laser sources; a combination of pulsed and continuous laser sources; selective laser polishing techniques.
1. Pulsed Sources:
Pulsed sources are low-power pulses that heat the metal surface for a set duration and at a set frequency. They tend to be used for micro-surface polishing. The molten metal pool is created at around 10–100 nm, with the material resolidifying between pulses. This process can be controlled by adjusting the length and frequency of the pulses.
2. Continuous Sources:
Used for polishing macro-surfaces, this process uses a continuous laser beam that irradiates the surface of the material. The diameter and scanning speed of the beam as well as the power of the laser determines the size and depth of the molten metal pool. This process typically uses higher power levels than pulsed sources, but usually vary from 70-300W. The continuous source laser melting will cover 20-200 microns of surface and obtain an average roughness Ra reduction from 2-16 microns to 0.1 microns. Scanning speeds for this process can exceed 100mm per second.
3. Combination Sources:
With the use of a Q-Switch, it is possible to use a combination of pulsed and continuous laser sources. The polishing will usually begin in continuous mode to homogenise the metal surface before a series of pulsed mode interventions to improve the surface finish. By using a combination of sources it is possible to achieve finishes that cannot be obtained through a single laser source.
4. Selective Laser Polishing:
Laser polishing can also be performed selectively, whereby the laser beam is only applied to areas that need improvement. This can be performed without the need for masking, unlike with mechanical techniques that require masking.
As noted above, the primary advantage of laser polishing is to improve the surface of a worked material, however, there are other advantages to using this process too:
1. Surface Morphology and Microstructure:
The average surface roughness of a material is greatly improved by laser polishing. This is due to the uniform distribution of the melt pool caused by the laser pressure gravity and surface tension as it solidifies. The process creates three main zones; the re-melted layer, the heat affected zone (HAZ), and the original material composition. The re-melted layer will have finer grains compared to the rest of the material as a result of the high cooling rate. The HAZ has coarser grain sizes as it has not been exposed to the laser beam, although it has still been affected by the melt pool. The original material layer will have the largest grain sizes, especially with additively manufactured parts. Despite this heating, there is very little change to the material properties of the workpiece or at the microstructural level.
2. Tensile Properties:
Laser polished surfaces show increases in tensile strength, although the total elongation before failure decreases. This decrease in elongation is a result of densification and the improved adhesion between the materials. In addition, the melting of the material and its subsequent flowing from the peaks to the valleys removes many defects, while improving the strength of the materials. The polished surface also benefits from increased micro-hardness, as well as improved corrosion and wear resistance
3.Fracture Behaviour:
Laser polishing leads to reduced defects and increased resistance to crack propagation for materials including pure metals, non-metals, alloys, polymers, ceramics, amorphous solids and composites. However, these improvements are not universal, since unaffected areas of workpiece material may still include defects.
Laser polishing is ideal for finishing small components with complex geometries, offering advantages over other techniques such as sand blasting, grinding, tumbling or electrochemical polishing as no chips are formed and there are no by-products. Because laser polishing doesn’t involve tools that require frequent replacement there is no chance of tracks or scratches appearing on the material surface.
Laser polishing is also desirable for different applications as it can be automated, creating a high level of repeatability for industrial applications.
Lasers can be used to polish surfaces and achieve an improved smoothness on the material. Because lasers have a range of adjustable parameters, such as energy, power, pulse width, frequency, and energy profiling, it is possible to adapt the process to achieve results with a range of different materials.
Because the process can be automated it can also deliver long-term repeatability, which makes it an attractive solution for many industries, and especially those dealing with small and complex components.