PICTURE: Neutrons can “see” through metal. Therefore, neutron diffraction is an ideal method for measuring the residual stress in components made by additive manufacturing. The picture shows a lattice structure in … view More
Credit: Dr. Tobias Fritsch / BAM
3D printing has opened up completely new possibilities. One example is the manufacture of new types of turbine blades. However, the 3D printing process often leads to internal stresses in the components, which in the worst case can lead to cracks. A team of researchers has now succeeded in using neutrons from the neutron source at the Technical University of Munich (TUM) for the non-destructive detection of this internal tension – an important achievement for improving production processes.
Gas turbine blades have to withstand extreme conditions: under high pressure and high temperatures, they are exposed to enormous centrifugal forces. To further maximize energy yields, the blades must withstand temperatures that are actually higher than the melting point of the material. This is made possible by hollow turbine blades that are air-cooled from the inside.
These turbine blades can be manufactured using Laser Powder Bed Fusion, an additive manufacturing technology: Here, the starter material in powder form is built up layer by layer by selective melting with a laser. Based on the model of bird bones, complicated lattice structures in the hollow turbine blades give the part the necessary stability.
The manufacturing process creates internal stresses in the material
“Complex components with such complicated structures cannot be manufactured using conventional manufacturing methods such as casting or milling,” says Dr. Tobias Fritsch from the Federal Institute for Materials Research and Testing (BAM).
However, the highly localized heat input from the laser and the rapid cooling of the weld pool lead to residual stresses in the material. Manufacturers usually remove such stresses in a subsequent heat treatment step, which, however, takes time and costs money.
Unfortunately, these stresses can also damage the components during the production process and right up to post-processing. “The stress can lead to deformations and in the worst case to cracks,” says Tobias Fritsch.
Therefore, he investigated a gas turbine component for internal stresses using neutrons from the research neutron source Heinz Maier-Leibnitz (FRM II). The component was manufactured using additive production processes from the gas turbine manufacturer Siemens Energy.
Post-processing intentionally left out
For the neutron experiment at the FRM II, Siemens Energy printed a lattice structure just a few millimeters in size with a nickel-chromium alloy typical of gas turbine components. The usual post-production heat treatment has been intentionally omitted.
“We wanted to see whether we could use neutrons to record internal stresses in this complex component,” explains Tobias Fritsch. He had already gained experience with neutron measurements in the Berlin research reactor BER II, which was shut down at the end of 2019.
“We are very pleased to be able to carry out measurements at the Heinz Maier-Leibnitz Center in Garching. With the devices provided by STRESS-SPEC, we were even able to resolve internal stresses in such complicated and complex grid structures.” Physicist says.
Even heat distribution when printing
After the team has managed to identify the internal stress within the component, the next step is to reduce this destructive stress. “We know that we have to change the production process parameters and thus the way in which the component is built up during printing,” says Fritsch. The decisive factor is the heat input over time when building the individual layers. “The more local the application of heat is during the melting process, the more internal tensions arise.”
As long as the printer’s laser is aimed at a particular point, the point’s heat will increase relative to adjacent areas. This leads to temperature gradients which lead to irregularities in the atomic lattice.
“That’s why we have 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 printing parameters. The team is already working with Siemens to plan new measurements with the TUM neutron source in Garching.
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In addition to scientists from the Heinz Maier-Leibnitz Center of the Technical University of Munich and the Federal Institute for Materials Research and Testing, a developer from Siemens Energy GmbH & Co KG and a scientist from the University of Potsdam were also involved in the research.
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