FEATURE ADDITIVE MANUFACTURING AM systems are now being used in service bureaus as well as in-house production ➤
plastic powder, the company uses a high power CO2
laser to melt the powder, which typically ranges in power from 60 to 100W. Laser sintering machines melting metals require even more energy and typically use a Yb-fibre laser from 200W to 1kW. Jon Blackburn, section manager of the laser and sheet processes department at TWI in the UK, explained how the metal sintering machines manipulate the beam with a galvanometer scanner and therefore the
better. However, we have found that for certain applications a CO2
laser is still better due to less
dilution of the substrate or the parent part.’ By choosing the laser to suit the materials, the
We use completely different parameters that depend on materials and application
beam quality of the laser is important. Blackburn said: ‘You tend to have quite long focal lengths, so to maintain a small spot size you need a good beam quality. This means you tend to be limited to very high beam quality fibre, disk or CO2
laser sources.’
He continued: ‘For DMD, you typically don’t need as high power densities, and consequently don’t necessarily need to use laser sources with excellent beam quality. This enables you to consider diode laser sources, as well as fibre, disk and CO2
efficiency of the machine will be improved. Dutta explained: ‘Each material has its own absorptivity; copper or aluminium absorbs less laser energy, while steel, Inconel, titanium or cobalt alloys absorb more. We use a completely different set of process parameters that depend on materials
and application. These proprietary recipes and the properties of DMD materials are part of our database.’ Payne, whose PhD project is based
on improving the efficiency of AM processes, explained that there is no one set of parameters, as the optimum setup depends on both the size of the powder grains and the composition of the material. So, to give an example of the balance that has to be met, smaller grains are easier to melt, but if they are too small then static electricity can be a problem, affecting the powders flow characteristics to the detriment of the process. Payne described the delicate balance when designing an AM system and how careful consideration should be given to the thermal stresses from the laser that could damage the part. For example, when trying to build a part with an overhang, the thermal conductivity of the un-melted powder in the previous layer is different to the melted powder. ‘This difference in conductivity leads to differential cooling which can cause the part to curl up and cause deformation or even failure if the raised part is knocked by the roller,’ he said. The part has to remain in a fixed position for the layers to build up accurately. The three main laser parameters are power, spot size and scanning speed. These three things combined essentially determine the energy density – how much energy is put in per unit area. They are delicately balanced and a lot of work has gone into calculating the optimum set-up. Building high-end parts with complex geometries is where additive manufacturing is most suited, because to make these parts by traditional means would be too costly and take too much time. And the components now being made with AM are production parts rather than prototypes.
As the laser processing parameters become better defined for different materials, the use of the technique is certain to grow. l
laser sources, for DMD.’ Dr Bhaskar Dutta of DM3D Technology, a direct metal deposition company based in Michigan, USA, said that ‘different lasers are used by different companies. CO2
is an old work horse in
industrial lasers, while diode and fibre lasers are gaining popularity very quickly in the cladding market.
This is because metals absorb more diode or fibre laser energy than from a CO2
laser due to their shorter wavelengths, so the efficiency is 18 ELECTRO OPTICS l MARCH 2014 A scale model of an F1 endplate for testing purposes @electrooptics |
www.electrooptics.com
Graphite Additive Manufacturing
Graphite Additive Manufacturing
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