Laser additive manufacturing: Listening for defects as they happen

EPFL researchers have resolved a long-standing debate about laser additive manufacturing processes with a pioneering approach to defect detection.

The progression of laser additive manufacturing, which involves 3D printing metal objects using powders and lasers, has often been hampered by unexpected defects. Traditional monitoring methods, such as thermal imaging and machine learning algorithms, have shown significant limitations. They often overlook or misunderstand defects, making precision manufacturing elusive and excluding the technique from essential industries such as aeronautics and automobile manufacturing.

But what if it were possible to detect defects in real time based on the differences in the sound the printer makes during a flawless print and another with irregularities? Until now, the possibility of detecting these defects in this way was considered unreliable. However, researchers at EPFL School of Engineering’s Laboratory of Thermomechanical Metallurgy (LMTM) have successfully challenged this assumption.

Professor Roland Logé, director of the laboratory, said: “There has been an ongoing debate about the feasibility and effectiveness of acoustic monitoring for laser-based additive manufacturing. Our research not only confirms its relevance but also underlines its advantage over traditional methods. “.

This research is of utmost importance to the industrial sector as it presents an innovative yet cost-effective solution to monitor and improve the quality of products manufactured using Laser Powder Bed Fusion (LPBF).

Principal investigator Dr Milad Hamidi Nasab commented: “The synergy of synchrotron X-ray imaging with acoustic recording provides real-time information about the LPBF process, making it easier to detect defects that could endanger the integrity of the product. In an era where industries continually strive for efficiency, precision and waste reduction, these innovations not only result in significant cost savings but also increase the reliability and safety of manufactured products.

How does LPBF manufacturing work?

LPBF is a cutting-edge method that is reshaping metal manufacturing. Basically, it uses a high-intensity laser to meticulously melt tiny metal powders, creating layer after layer to produce detailed 3D metal constructions. Think of LPBF as the metal version of a conventional 3D printer, but with an added degree of sophistication.

Instead of molten plastic, it uses a thin layer of microscopic metal powder, the size of which can vary from the thickness of a human hair to a fine grain of salt (15 to 100 μm). The laser moves through this layer, melting specific patterns based on a digital plane. This technique allows the production of complex and personalized parts, such as lattice structures or different geometries, with minimal excess. However, this promising method is not without challenges.

When the laser interacts with the metal powder, creating what is known as a melt pool, it fluctuates between liquid, vapor and solid phases. Sometimes, due to variables such as laser angle or the presence of specific geometric attributes of the powder or part, the process can fail. These cases, called “interregime instabilities,” can sometimes lead to switches between two fusion methods, known as “driving” and “keyhole” regimes.

During unstable keyhole regimes, when the pool of molten powder goes deeper than anticipated, it can create pockets of porosity that culminate in structural failures in the final product. To facilitate the measurement of melt pool width and depth in X-ray images, the Image Analysis Center at EPFL Imaging Center developed an approach that facilitates the visualization of small changes associated with liquid metal and a tool to note the geometry of the melt pool.

Detect these defects through sound

In a joint venture with the Paul Scherrer Institute (PSI) and the Swiss Federal Laboratories for Materials Science and Technology (Empa), the EPFL team formulated an experimental design that merged operational X-ray imaging experiments with acoustic emissions measurements.

The experiments were carried out on the TOMCAT beamline of the Swiss Light Source at PSI, with the miniaturized LPBF printer developed in the group of Dr. Steven Van Petegem. Fusion with an ultrasensitive microphone placed inside the build chamber identified distinct changes in the acoustic signal during regime transitions, thus directly identifying defects during manufacturing.

A crucial moment in the research was the introduction of an adaptive filtering technique by signal processing expert Giulio Masinelli of Empa. “This filtering approach,” Masinelli emphasized, “allows us to discern, with incomparable clarity, the relationship between defects and the acoustic signature that accompanies them.”

Unlike typical machine learning algorithms, which excel at extracting patterns from statistical data but are often tailored to specific scenarios, this approach provides broader insights into the physics of melting regimes while also time, offers superior temporal and spatial precision.

With this research, EPFL brings valuable knowledge to the field of laser additive manufacturing. The findings have important implications for potential industrial applications, particularly in sectors such as aerospace and precision engineering. The study, which reinforces Switzerland’s reputation for meticulous craftsmanship and manufacturing precision, underlines the need for consistent manufacturing techniques.

Furthermore, it suggests the potential for early detection and correction of defects, improving product quality. Professor Logé concludes: “This research paves the way for better understanding and refinement of the manufacturing process and will ultimately lead to greater long-term product reliability.”

The findings are published In the diary Nature Communications