Conventional processes often reach their limits in the manufacture of turbines. Complex components with delicate structures and curved shapes are therefore increasingly being produced using additive manufacturing. Various test methods are used to find defects inside the component. A research team from the Technical University of Munich (TUM) has now tested several methods. The best defect detection was achieved with neutrons from the Heinz Maier-Leibnitz Research Neutron Source (FRM II).

Laser beam melting is a common additive manufacturing process for turbine blades with internal cooling channels. In this process, a laser melts a thin layer of metal powder at certain points. Layer by layer, the component is created in a bed of powder. Like an archaeological excavation, the component is then uncovered and the remaining powder can be reused for the next component.

However, process instabilities can lead to defects in the component and thus reduce the strength of the component. Typical defects are pores and cracks. Individual layers can even become partially or completely detached from one another.

In the case of safety-relevant components such as turbine blades, such defects can have serious consequences. "We therefore have to inspect critical components after the manufacturing process - non-destructively, of course," explains Cara Kolb from the Institute for Machine Tools and Industrial Management at TUM.

A look inside

For their experiments, the researchers produced test specimens with defects of different sizes and depths and attempted to detect them using non-destructive testing methods. They used active infrared thermography (aIRT), ultrasonic testing (UT), X-ray computed tomography (CT) and neutron grating interferometry (nGI).

PhD student Tobias Neuwirth carried out the tests on the ANTARES instrument at the research neutron source. "We test components with neutron grating interferometry. We observe the scattering and absorption of neutrons with spatial resolution. If this changes, it provides information about the type and size of the defects," he explains.

Deeper penetration and better resolution with neutrons

Each of the methods tested has its potential and challenges. Although neutron grating interferometry is complex and more expensive than the other test methods examined, it detects the most and smallest defects of all the methods.

"Neutrons can penetrate deep into the material and enable a high resolution of the inner component structure. They are particularly suitable for nickel-based alloys, which are extremely important for the additive manufacturing of aerospace structural components," concludes Cara Kolb.

Research into test methods that ensure the quality of additively manufactured components in a non-destructive manner is very important. Such test procedures indicate how likely a component is to fail during operation. And with the increasing use of additive manufacturing in aircraft or cars, for example, the test procedures are also becoming more important.