Molecular dynamics study of temperature and heating rate–dependent sintering of titanium nanoparticles and its influence on the sequent tension tests of the formed particle-chain products

Sintering of multiple single-crystal titanium (Ti) nanoparticles (NPs) during additive manufacturing by using ultrafast laser was simulated using molecular dynamics (MD). The aim was to better understand how factors such as sintering temperature and heating rate would influence the mechanical properties of the ultrafine-sized sintered products, i.e., Ti “NP-chains.” For this purpose, the effects of heating and strain rate on the tensile behavior of the final sintered products were studied in detail. Ti NP-chain precursors with weak neck connections were first created through solid-state sintering process at room temperature. They were later heated very rapidly to 800 K, 1200 K, or 1500 K with two different heating rates of 0.04 K/ps and 0.2 K/ps, and maintained at these high-temperature levels for 1 ns to mimic the fast temperature rise and short equilibration due to femtosecond/picosecond laser irradiation. The formed Ti NP-chains with different neck connection strengths were then cooled to 298 K. Those final NP-chains were subjected to uniaxial tension at three different strain rates of 0.001%/ps, 0.01%/ps, and 0.1%/ps. Our simulation results indicate a strong correlation between the tensile strength of the final NP-chain product and the heating rate during the previous short sintering process (including the ultrafast temperature rise and up to 1-ns high-temperature equilibration). A slower heating rate to a higher temperature level yields larger neck connection diameters in the final NP-chain product, resulting a higher tensile strength. Furthermore, our results demonstrate that high strain rates applied to the NP-chains with stronger neck connections result in an improvement in the tensile strength and ductility of the final products. In contrast, sintered products resulting from a lower temperature level show an elastic-brittle-damage behavior. Due to the weak neck connections and limited crystal sliding, the heating rate effect during sintering does not have a significant effect on the tensile strength.