Cold spray sounds almost too simple to work: fire a metal particle at a surface fast enough, and it sticks – no melting, no welding, just brute plastic deformation smashing a bond into place. That has been the textbook story for two decades. But in our latest paper with colleagues at Politecnico di Milano and Brunel University London, we show that when the particle and the surface are very different metals – here, iron onto an aluminium alloy – something the textbooks do not mention is doing a lot of the work. By pairing atomic-scale simulations with STEM imaging of real cold-sprayed samples, we caught the interface doing something unexpected: for a few billionths of a second, it melts.
It is not bulk melting – nothing one would ever see with the naked eye, and nothing that would make cold spray anything other than the “solid-state” process it has always been called. But at the particle–substrate interface, our molecular dynamics simulations show a nanoscale film flashing from crystalline to liquid and back again in picoseconds, and the microscopy backs this up beautifully: an oxygen-rich layer twice as thick as the native oxide, telltale silicon segregation banding through the heat/impacted affected zone, and fresh magnesium-silicon precipitates that simply are not there in the untouched alloy. Together, these are the fingerprints of a liquid phase that formed, redistributed atoms, and froze again almost instantly – long enough to leave a trail, too fast to ever be “seen” happening. It is a reminder that even a process built on speed and impact, not heat, can have a fleeting thermodynamic life of its own hiding in the nanoseconds.
A special word for Dr. Arash Kardani, lead author and a good friend of mine: the atomistic simulations at the heart of this study are his, and it is a real joy to see his meticulous, patient work with molecular dynamics uncover something this subtle – a physical event that lasts picoseconds and spans a few atomic layers, made visible through sheer computational care. This paper also marks something of a personal milestone for us: it is the first collaboration with Prof. Sara Bagherifard‘s team at Politecnico di Milano, a partnership that began with an informal conversation at ECF2024 in Zagreb. It is always a pleasure when a good chat at a conference turns into a paper we are genuinely proud of!
Access the paper here (pre-proof/in press):
A. Kardani, M.A. Tunes, P.J. Uggowitzer, H. Assadi, S. Bagherifard, Transient Interfacial Melting Promotes Bonding in Cold Spray Additive Manufacturing of Dissimilar Metals, Additive Manufacturing (2026), doi: https://doi.org/10.1016/j.addma.2026.105338
Materials at Extremes 