Figure 3. The two setups of figure 2 were tested on a cranial implant to create different surface finishes in each quadrant

Figure 4. 3D surface maps of small (top row) and large (bottom) as-built and laser polished dental samples. The upper part of each sample is the original as-built surface, and only the lower area was laser polished

the dental implants, the researchers needed to rotate and move them under a stationary laser spot. To do this, a rotary arrangement was developed that uses a DC motor combined with a linear stage. The rotation was controlled to maintain the required energy density, while the line overlap was controlled using the linear stage travelling at a constant velocity, set to have moved one hatch spacing in the time for one full rotation of the part. This maintained a continuous spiral along the entire length of the component. For this experiment, the

researchers trialled replacing the fibre laser source with a fibre delivered diode array laser with a maximum power of 150W to achieve a more cost-effective solution. The wavelength of this diode array was 915nm, compared with the 1,064nm of the fibre laser. Despite resulting in a lower beam

quality, the almost top-hat shaped beam profile of the multimode fibre

delivery aided the laser polishing process. This is because it provided a more uniform temperature gradient in the melt pool, compared with the Gaussian shaped beam of the fibre laser. The researchers found that the diode array laser setup delivered comparable results to the fibre laser setup.

The size and thickness of dental implants can vary depending on the tooth that is being replaced. This can affect the heat accumulation and thus the difference in laser parameters required to polish each implant. Two variants (smaller and larger) of cobalt chrome implants were therefore studied by the researchers, as shown in figure 4, using the rotational setup for processing cylindrical parts. They found that the smaller implant required lower energy density (2kJcm−2

) and less processing time

(60s) to polish compared with the larger implant (4kJcm−2

,120s). A line overlap of 50 per cent was used WWW.LASERSYSTEMSEUROPE.COM | @LASERSYSTEMSMAG

LBP Optics offers an in- house precision lapping and polishing service for flaw-free, ultra smooth surfaces on many materials including stainless steel, titanium, graphite, brass, ceramics, mild steel, bronze, arcap alloy, electroless nickel and tungsten. We can produce parts with just 1 micron variation in edge thickness. The typical flatness achieved is 0.5 micron or better on lapped parts up to 220mm diameter.

Our chemically polished

in both cases. The as-built surface roughness, Sa, was reduced from 5.6 to 0.45μm after laser polishing for the smaller implant and from 4.9 to 0.63μm for the larger implement.

Out of reach Additive manufacturing can be used to produce components with complex internal structures such as cooling channels. These internal surfaces, however, pose a challenge for laser polishing, as there is often no direct line of sight. The researchers therefore

experimented with polishing the internal surface of a 30mm diameter hollow cylinder by mounting a right- angled prism within the structure on a lens tube attached to a rotation stage. This enabled the laser light to be aligned to the centre of the lens tube, with the rotating prism providing the spot 360° of rotation around the internal surface.

mirrors have the ultra smooth surfaces needed for very high quality UV grade mirrors. The surface quality is high enough, and the roughness so low, that they can be used in the near IR, visible and the UV. We also regularly polish metal parts that are not being used as traditional mirrors, but still need optical precision. For example applications such as alignment and positioning systems, test equipment and scientific instruments.

An improvement of surface

roughness from 6.3μm to 0.5μm was achieved, however during processing, some back spatter was observed, which could damage the prism and highlights possible limitations of the process in how close the prism can be to the processed surface. McDonald explained that implementing a smaller prism and lens tube would enable smaller cylinders to be processed; also, it is planned to further reduce this diameter by using an optical fibre with an angled end-face to replace the prism. l

More information on this topic can be read in Journal of Laser Applications 32, 042019 (2020); ‘Practical implementation of laser polishing on additively manufactured metallic components’:


McDonald et al.

McDonald et al.

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