AEROSPACE


relying heavily on shaped cooling holes that are generated by laser processing. Te laser allows for tailoring of shapes for each region or component without high consumable cost, as well as generating those shaped features in short cycle times.’


Pulsing for precision According to GE Aviation, the drive towards more shaped holes will require post-process inspection of the shapes to ensure the required performance of the hole. Higher-quality laser processing would allow for reduced inspection of the laser generated shapes. ‘Te growth of diode pumped lasers, both fibre and disk laser designs, will improve both the performance and stability of the beam quality, and the ability to have better control of pulse widths and frequencies, which should improve the capability to create shapes that are closer to idealised designs,’ explained Benvie. ‘Tese aspects should help to reduce post-process inspection and also reduce or eliminate cleaning of laser generated by-products. Pulsed disk lasers have some very interesting promise in this respect, but are probably still several years away from introduction into the value stream.’ Richard Baxter, global capital systems and


marketing manager for Winbro Group Technologies, said: ‘It’s all about using new technology to produce these turbine engine blades more rapidly and in a more efficient way.’ In addition to the holes, further thermal protection is provided by a thermal barrier coating (TBC), but this ceramic layer makes the holes more difficult to drill. A high-peak pulsed laser can be used to drill through the ceramic coating and create the hole on the


Results from PIV at Iowa State University. Colour map to show the ensemble-averaged adiabatic film cooling effectiveness.


Possible minimum = 0, Possible maximum = 1. (a) Single cylindrical hole


(b) Central cylindrical hole in an array of three holes spaced laterally by 10 diameters centre-to-centre


(c) Central cylindrical hole in an array of five holes spaced laterally by 5 diameters centre-to-centre


(d) Central three cylindrical holes in an array of five holes spaced laterally by 3 diameters centre-to- centre


Dr Blake Johnson, Iowa State University


turbine blade. However, if the peak power is too high, the thermal shock of the beam hitting the TBC can cause cracking or even removal of the coating. Tis is particularly apparent in the case of percussion drilling, where the laser stays in the same position to deliver a number of pulses to the material until the hole is finished. For manufacturing companies that produce parts for turbine blades, this type of drilling could result in failure to meet the strict standards laid down for these engine components.


Reducing thermal schock Laser ablation removes the TBC at the location of the hole in a controlled manner, without damaging the surrounding coating. Tis process employs a fibre laser, which has a much lower energy and much shorter pulses than a traditional high peak pulse laser used for drilling. Te TBC is


is a good way of creating both two- and three- dimensional shaped cooling holes


Laser ablation


broken down a thin layer at a time so there is reduced thermal shock to the material. Laser ablation can also be applied to the re-opening of cooling holes that become blocked during re-coating of the engine parts aſter repair. According to Winbro’s Baxter: ‘Te materials of the engine parts are changing in that they are coated with TBC. So, we have had to develop ways of machining those materials, which laser ablation does very effectively.’ As it happens, laser ablation


is also a good way of creating both two- and three- dimensional shaped cooling holes. A three-dimensional model of the shape is first designed, and then the shape is recreated on the surface of the


engine part, by taking away the material layer by layer. ‘Laser ablation is a more efficient way of producing shaped cooling holes on turbine blades. Shaped holes allow cool air to be spread over the surface of the part more effectively,’ said Baxter. Manufacturers such as Winbro produce


machines that can combine laser ablation and drilling processes into a single machine. So, on a turbine blade, a fibre laser can be used for removal of the TBC and creation of 3D shapes, and a high-power pulse laser can then drill the holes. ‘Tis makes the whole operation more efficient as it eliminates alignment issues of switching between two processes,’ said Baxter.


Particle image velocity at Iowa State University used light from a green 532nm Nd:YAG laser to show the coolant flow


www.lasersystemseurope.com | @lasersystemsmag


Measured approach Laser technology is also useful in researching how the geometry and distribution of cooling holes can improve cooling performance. GE Global Research is funding researchers from the aerospace engineering department at


ISSUE 21 • WINTER 2013 LASER SYSTEMS EUROPE 15


Dr Blake Johnson, Iowa State University


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