When Two Metals Become One

Looking inside the melt pool

Metal additive manufacturing, often referred to as metal 3D printing, builds components layer by layer by selectively melting metal powder with a high-power laser. This technology has transformed manufacturing by enabling complex geometries that would be difficult—or even impossible—to produce using conventional methods. Going one step further, multi-material additive manufacturing combines different metals within a single component, opening the possibility of creating parts with locally tailored properties.

However, bringing two different metals together is far from straightforward. Within a fraction of a millisecond, the materials melt, flow, mix, and solidify while temperatures exceed several thousand degrees. These rapid processes determine the local composition of the material and whether defects such as pores become trapped inside the final component. Yet, because they happen beneath the surface and on extremely short timescales, they have remained largely hidden.

To reveal these processes, we used high-speed synchrotron X-ray imaging to observe laser powder bed fusion in real time. By printing copper onto aluminium, we were able to directly follow how the two materials mixed during processing. Rather than blending smoothly, copper entered the melt pool through a series of discrete mixing events driven by the complex motion of the liquid metal. These observations showed that melt pool flow—not diffusion—is the dominant mechanism controlling how composition evolves during printing.

From observation to understanding

Experiments reveal what happens, but they cannot measure every physical quantity inside the melt pool. To complete the picture, this project brought together complementary expertise from the Paul Scherrer Institute (PSI) and ETH Zurich. While the PSI team captured the process in real time using synchrotron X-ray imaging, researchers at ETH Zurich developed high-fidelity multiphysics simulations that accurately reproduced the observed melt pool dynamics. Together, these approaches provided an unprecedented view of the physics governing multi-material additive manufacturing.

This combination allowed us to investigate how the evolving material composition influences the stability of the laser-induced keyhole. We discovered that mixing between the two metals fundamentally changes the behaviour of the keyhole, creating new pathways for pore formation. Large pores form when the keyhole becomes unstable and collapses, while smaller pores originate from gas entrapment and local melt-flow instabilities. Together, the experiments and simulations provide the first quantitative framework linking material mixing, melt pool dynamics, and defect formation in multi-material laser powder bed fusion.

Towards reliable multi-material manufacturing

Understanding these mechanisms is an important step towards making multi-material metal additive manufacturing more reliable. By revealing how processing conditions control both composition evolution and defect formation, our work provides the knowledge needed to optimize printing strategies and engineer components with locally tailored properties.

Beyond the specific aluminium–copper system studied here, the combination of operando synchrotron X-ray imaging and predictive numerical simulations provides a powerful framework for understanding complex manufacturing processes. By bringing together complementary expertise in advanced experiments and computational modelling, the collaboration between PSI and ETH Zurich is helping to build the scientific foundations needed for the next generation of reliable, multi-material metal additive manufacturing.