Air-cooled latticed turbine buckets operate at extreme temperatures and pressures within gas turbines. Their complex structure can only be created additively, with selective laser sintering the technology widely used today.
"Complex components with such intricate structures would be impossible to make using conventional manufacturing methods like casting or milling," said Dr Tobias Fritsch, from the German Federal Institute for Materials Research and Testing (BAM).
But the highly localised heat and the rapid cooling associated with laser sintering can lead to residual stress in components produced this way. During production, manufacturers usually address this issue with a downstream heat treatment process, but this additional step costs both time and money. Using the Research Neutron Source Heinz Maier-Leibnitz (FRM II) at the Technical University of Munich (TUM), Fritsch and his colleagues were able to screen components for residual stress using non-destructive neutrons.
In a paper published in the Journal of Applied Crystallography, it’s described how Siemens Energy printed a lattice structure just a few millimetres in size using a nickel-chrome alloy typically used for gas turbine components. The usual heat-treatment after production was intentionally omitted.
"We wanted to see whether or not we could use neutrons to detect internal stresses in this complex component," said Fritsch.
"We're very glad to be able to make measurements in the Heinz Maier-Leibnitz Zentrum in Garching; with the equipment provided by STRESS-SPEC we were even able to resolve internal stress in lattice structures as intricate and complex as these."
Now that the team has succeeded in detecting the internal stress within the component, the next step is to consistently reduce this destructive stress by tweaking the manufacturing process.
"We know that we have to modify the production process parameters and thus the way in which the component is built up during printing," said Fritsch. "The more localised the heat application is during the melting process, the more internal stress results."
