3D printing has opened up a whole new range of possibilities. An example is the production of new turbine buckets. However, the 3D printing process often induces internal stresses in the components which can in the worst case lead to cracks. Today, a research team has successfully used neutrons from the Technical University of Munich (TUM) research neutron source for the non-destructive detection of this internal stress – a key achievement for process improvement of production.
Gas turbine buckets have to withstand extreme conditions: under high pressure and high temperature, they are exposed to enormous centrifugal forces. In order to further maximize fuel efficiency, the buckets must withstand temperatures that are actually above the melting point of the material. This is made possible by hollow turbine buckets which are air-cooled from the inside.
These turbine buckets can be made using Laser Powder Bed Fusion, an additive manufacturing technology: Here the starter material in powder form is formed layer by layer by selective melting with a laser. Like avian bones, intricate lattice structures inside the hollow turbine cups give the part the necessary stability.
The manufacturing process creates internal stress in the material
“Complex components with such complex structures would be impossible to fabricate using conventional manufacturing methods like molding or milling,” says Dr Tobias Fritsch of the German Federal Institute for Materials Research and Testing (BAM ).
However, the highly localized heat input from the laser and the rapid cooling of the molten pool lead to residual stress in the material. Manufacturers generally eliminate these constraints in a downstream heat treatment step, which however takes time and therefore costs money.
Unfortunately, these stresses can also damage components from the production process through to post-processing. “Stress can lead to deformations and, in the worst case, to cracks,” explains Tobias Fritsch.
Therefore, he studied a gas turbine component for internal stress using neutrons from the Heinz Maier-Leibnitz research neutron source (FRM II). The component was manufactured using additive production processes by gas turbine manufacturer Siemens Energy.
Post-processing intentionally omitted
For the neutron experiment at FRM II, Siemens Energy printed a lattice structure of only a few millimeters using a nickel-chromium alloy typical of those used for gas turbine components. The usual heat treatment after production has been intentionally omitted.
“We wanted to see whether or not we could use neutrons to detect internal stresses in this complex component,” says Tobias Fritsch. He had already acquired experience in neutron measurements at the Berlin research reactor BER II, which was however shut down at the end of 2019.
“We are very happy to be able to carry out measurements in the Heinz Maier-Leibnitz Zentrum in Garching; with the equipment provided by STRESS-SPEC, we were even able to resolve internal stresses in lattice structures as complex and complex as these. ci “, says the physicist.
Even heat distribution during printing
Now that the team has successfully detected the internal stress within the component, the next step is to reduce this destructive stress. “We know that we have to change the parameters of the production process and therefore the way the component is constructed during printing,” says Fritsch. The crucial factor here is the heat input over time when building up the individual layers. “The more localized the application of heat during the melting process, the more internal stress there is.”
As long as the printer’s laser is directed at a given point, the heat of the point increases relative to adjacent areas. This results in temperature gradients which lead to irregularities in the atomic lattice.
“So we need to distribute the heat as evenly as possible during the printing process,” says Fritsch. In the future, the group will investigate the situation with new components and changed print settings. The team is already working with Siemens to plan further measurements with the TUM neutron source at Garching.