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Application Focus: Polishing

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Florent Husson, of Alphanov, describes how large parts can be polished using robotics and high-power lasers

Laser polishing is gaining increasing attention due to the rapid development of laser additive manufacturing (AM). 

AM can be used to create both large and thin structures with optimised topologies, which can be used to decrease part weight in, for example, biomechanical or aeronautic applications. Nevertheless, despite the widely recognised advantages of AM, its wide diffusion in industry is currently limited by the low surface quality of completed parts. 

While this issue can be addressed using conventional post-processing strategies, such as abrasive blasting and/or mechanical polishing, these techniques do suffer from their own drawbacks: material wastage, long process times and mechanical tool wear (leading to frequent tool replacement).

In the complex thermodynamic process of laser polishing, a high-intensity laser beam impacts the surface of a part, melting a thin layer of material. This melt pool is then redistributed around the adjacent area, driven by the multidirectional action of surface tension. 

Laser polishing can be used for almost all metals, and has also been used to polish ceramics and glasses, thus proving it to be one of the most promising polishing technologies currently available. 

When compared to conventional polishing methods, laser polishing shows numerous advantages: zero material removal, zero scratches left on the part, lower processing times, and the ability to reach areas of parts with low-accessibility.

An example can be seen in figure 1, where the roughness of a stainless steel (316L) surface was successfully reduced by a factor of 10 using laser polishing.

Fibre lasers are the preferred type of laser used for laser polishing, due to their low cost, high efficiency, high beam quality, high reliability, and their ability to melt metal surfaces with ease. Spot diameters between hundreds of microns up to around 1mm, as well as laser powers between 40 and 500W, are generally used. Processing times are usually between 10 to 200s/cm2, depending on the type of material being polished, its initial surface roughness, and the desired final roughness.

Figure 1: Surface topography before (left) Sa = 6µm and after (middle-left) Sa = 0.3µm laser polishing 10mm-thick stainless steel (316L). Middle-Right: A photo of the polished surface. Right: An SEM image of the polished surface.

Some industries already use fully automated laser polishing machines that include a five-axis CNC machine, a scan head and a gas chamber in order to protect the sample from oxidation during the process. A setback of this, however, is that the gas chamber has limited dimensions, and thus the maximum part size that can be processed is around (400 x 400 x 400)mm3.

While larger parts could be polished using similar setups with larger gas chambers, this would require a re-design of industrial systems, and would result in systems of even higher cost. In addition, even if such systems were designed, the laser parameters used by their smaller predecessors would result in long processing times when polishing larger parts.

The question therefore is: How do we decrease processing times when laser polishing large-scale components? 

First, a higher laser power and thus a larger spot diameter than those used by existing systems can be wielded. This allows more surface to be covered with the beam, which decreases polishing times greatly. Higher scan velocities and a higher hatch distance can also be used. However, increasing all these parameters can lead to unwanted ripples occurring on polished surfaces. This can be avoided by controlling these parameters very precisely. 

Next, by using a robotic arm with scanner mirrors moving in sync, it is possible to overcome the limited scanning area of current laser polishing systems, and thus process parts in excess of (1,000 x 1,000 x 1,000)mm3. The arm is set up in a tank flooded with argon gas, with the tank being both easier and more cost effective to modify the size of – when accommodating larger parts – than the gas chambers of existing industrial laser polishing machines.

In addition to being able to process larger parts than current laser polishing systems, this solution can also process heavier parts. Current systems feature a five-axis platform that enables parts of up to 100kg to be fixed and moved in three dimensions during processing. As our system does not require this platform, it is able to process parts heavier than 100kg.

The seperate components of the Alphanov team's setup for large-scale laser polishing.

Our team at Alphanov put this into practice using a 10kW, 1,070nm YLR 10,000 continuous-wave (CW) laser from IPG, along with a galvanometric IntelliWeld FT scanner by Scanlab that was mounted on a robotic arm from Fanuc. A ScanControlUnit (SCU), from Blackbird Robotics, was also used to monitor and control the synchronisation of the movement between the scan unit and the industrial robot. This avoids any stitching errors and enables the continuous processing of large 3D parts.

Figure 2 shows the schema of communication in between the different elements. In order to control laser power in co-ordination with the scanner and/or robot movement during operations, an interface to the laser source must be implemented. Unlike static processing, here the robot movement takes place in sync with the process (beam scanning) using bus communication between the robot unit and SCU. This process is also called on-the-fly (OTF) processing, and makes it possible to achieve maximum process efficiency with minimised cycle times.

Figure 2: A schematic of the communication and synchronisation in between the elements of the Alphanov team’s laser polishing setup

For the moment we have studied the upscaling of process speed with spot diameters up to 5mm. With it, laser polishing has been conducted with power up to 2.5kW and demonstrated for a one-pass process. It was shown it is possible to laser polish an initial surface Sa = 6µm down to Sa = 0.5µm in 3.25s/cm². With 10kW power reserve, we can use bigger spot size up to 15mm, and in doing so will be able to reduce processing times even further. High-speed beam scanning has not yet been investigated, but with it we envisage being able to reduce processing times further still.

The achieved processing speed outperforms polishing systems using CW lasers. Our system cannot be compared to systems using pulsed lasers, as these are instead used to polish from Sa = 1µm down to Sa = 30-50nm, while CW lasers are used to polish from Sa > 10µm down to Sa = 300-200nm. In order to bring this process and setup further to successful application, adjustments in thermal accumulation control are needed.

The potential of this robot-based setup is not only limited to laser polishing. As we are capable of melting the surface to control its roughness using melt pool movement by fast laser power modulation, we are also able to move this melted material across the surface and shape it with different geometries. A remodeling process uses the same principle as laser polishing (melting a surface without material waste), therefore, our setup can also be adapted for the laser remodeling of large 3D surfaces. Applying this technology, we have fabricated defined topographies with astonishing accuracy for mould generation, an example of which can be seen in figure 3.

Figure 3: The Alphanov team’s laser polishing setup can also be used to remodel surfaces. Pictured is a reflector mould for vehicles.

To conclude, laser polishing can be upscaled using a high-power continuous-wave laser, a large spot diameter and a robotic arm. 

The setup used by our Alphanov team enables the continuous processing of large surfaces without the need to use a stitching method. We are aiming to further decrease the processing time, not only with a bigger spot diameter, but also with an optimised and efficient process strategy using the full capability of OTF processing, enabling us to polish large-scale additively manufactured parts in almost all kinds of material. 

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