APPLICATION FOCUS: POLISHING


LASER POLISHING FOR LARGE 3D SURFACES


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. 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.


Application Focus sponsored by


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


“Process parameters used by industrial polishing


machines result in long processing times when polishing larger parts”


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


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


32 LASER SYSTEMS EUROPE SPRING 2020


to process larger parts than current laser polishing systems, this solution can also process heavier parts. Current systems


@LASERSYSTEMSMAG | WWW.LASERSYSTEMSEUROPE.COM


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36