Defect structures at a glance: New study analyzes dislocation formation in 3D printing of metal alloys

A research team led by Tao Sun, Professor of Mechanical Engineering at Northwestern University, has quantified the formation and evolution of dislocations in metallic materials during the additive manufacturing process in real time for the first time. The results of the study, published in the journal Nature Communications, offer new insights into microstructural processes that are crucial for the mechanical properties of 3D-printed metal components.

Dislocations are linear crystalline defects that significantly influence the strength, ductility and fracture toughness of a material. Their formation during 3D printing, especially during production using powder bed processes based on melting, was previously insufficiently understood. Sun and his team investigated 316L stainless steel using a combination of synchrotron X-ray diffraction, neutron diffraction, electron microscopy and multi-physics simulation.

“By understanding when and why dislocations occur, we can better control them,” said Tao Sun, associate professor of mechanical engineering at Northwestern Engineering. “For the first time, we’ve quantitatively measured dislocation density and its changes during the rapid cooling process in fusion-based additive manufacturing. This study will inspire the research community to conduct similar experiments on novel and complex alloy systems.”

Contrary to previous assumptions, the study showed that a large proportion of the dislocations occur during the solidification of the molten bath – particularly as a result of eutectic reactions. Earlier models attributed the formation of such defects primarily to downstream residual stresses. The investigations also show that during the subsequent thermal cycle, some defects are reduced by recovery processes, while others re-emerge due to stresses. The result is a dynamic state that significantly influences the subsequent component properties.

“Our findings reveal the critical role of the eutectic reaction in initially generating high dislocation densities,” Sun said. “We also identify the competing effects of annealing and stress on dislocation evolution during the subsequent cooling and thermal cycling processes.”

“Dislocation structures are critical microstructural features that bridge printing parameters with the performance of final additive manufacturing products,” Sun said. “Understanding and controlling these unique dislocation structures will not only advance knowledge of structural dynamics under non-equilibrium conditions, but also accelerate the industrial adoption of additive manufacturing techniques for fabricating structural components.”

The findings are considered an important basis for the targeted control of microstructures by adjusting process parameters and alloy compositions. Sun emphasizes that controlling thermal expansion and interfacial cohesion can help minimize cracking, especially in multiphase alloys.

“We will continue to explore these research avenues to refine process innovation and alloy design, applying what we have learned to improve both the quality and functionality of 3D-printed metals,” Sun said.

Future work by the team aims to extend the methodology to more complex alloy systems and improve process control to produce resilient and reliable metal components in industrial 3D printing.