Nov. 28: Wanting’s poster is the MRS 1st place poster presentation.

Wanting’s poster “Micro-ballistic Experiment of Single Aluminum Particles for Cold Spray Additive Manufacturing” was selected as the 1st Place Poster Presentation in the Materials Research Society (MRS) Meeting in Boston.

Abstract: Cold spray (CS), an additive manufacturing process utilizing extreme plastic deformation arising from high-speed collision events between fast-moving powders and substrates, has been highlighted as a unique manufacturing and repair solution for high-performance mechanical parts. Although many metals and alloys have been successfully processed using the CS techniques, the accurate dynamic responses of individual metallic particles related to the consolidation and deformation characteristics are still largely unknown. Although various numerical approaches have been expected to facilitate fundamental understanding of the extreme deformation mechanisms, their results are still limited due to the lack of referenceable experimental results with precise experimental parameters.

We conducted the laser induced single particle impact experiments to study the extreme dynamics of aluminum 6061 particles during individual impacts, and to provide precisely defined critical parameters for computational analysis. Single aluminum particles around 20 ?m in diameter were accelerated up to 1 km/s using high-power laser ablation. Light pulses, generated by a femtosecond oscillator were used to take ultra-high-speed photographs (maximum 80 million frames per second) and accurate kinetic information of the impacting aluminum particle was acquired from the photographs.

Two different target substrates, sapphire and aluminum 6061, were used to study the deformation of the impacting particles. Since the entire plastic deformation occurred solely on aluminum spheres due to the high modulus of sapphire, aluminum-sapphire collisions provided a very simple environment for numerical simulations and reference data for the more complicated aluminum-aluminum collisions. As an in-situ characterization, we measured the rebound speed of aluminum particles as a function of its impact speed. The coefficient of restitution, which depends on the impact speed, evidently showed two different transition points related to the high-strain-rate and hydrodynamic regimes in aluminum deformation. In the aluminum-aluminum case, one more transition point around 860 m/s was the critical velocity, over which no rebounding particle was observed. The xenon-plasma focused ion beam cross sectioning of deformed aluminum particles was performed without critical gallium contamination and the electron diffraction backscatter diffraction mapping of the cross sections provide the entire deformation field depending on the collision speeds. Moreover, cross sectional images allow us to investigate the microstructural changes of the particle and the target under high-speed impact so as to study the phenomena that are responsible for the dynamic consolidation.