Research On Cold-welding Of Nano-metallic Materials Based On Molecular Dynamics Simulations And Experiments

The welding of metals at the nanoscale is likely to have an important role in the bottom-up fabrication of electrical and mechanical nanodevices.Existing welding techniques use local heating,requiring precise control of the heating mechanism and introducing the possibility of damage.The welding of metals without heating(or cold welding)has been demonstrated,but only at macroscopic length scales and under large applied pressures.In this dissertation,the cold-welding behavior of several nanoscale materials were studied via large-scale molecular dynamics(MD)simulations and experiments.Firstly,the cold-welding behavior of single-crystalline nanowires were studied.To explore the welding mechanism of nanoscale structures,here,molecular dynamics was performed on single-crystalline nanowires under different welding conditions and various original characteristics to obtain an atomic-level depiction of their cold-welding behavior.By analyzing the mechanical properties of as-welded nanowires,the relations between welding quality and welding variables are revealed and identified.Secondly,the cold-welding behavior of fivefold twinned and poly-crystalline structure were studied.According to the Voronoi construction,the poly-crystalline nanowire was generate to the molecular dynamic simulations.Four samples of nanowires were successfully welded through limited loadings with similar stress-time responses toward cold welding.The stress shows a low average value during welding with oscillations,indicating the existence of relaxation stages.Herein,the number of defects in the welded nanowires could be controlled by varying the loading time.Thirdly,we explored the bearing assembling using cold-welding via molecular dynamics simulations.We demonstrate that it is also feasible to assemble bearing and bearing at the nanoscale to form a stable mechanism.The effect of temperature was also investigated.With an increase in temperature,the weld stress and the mechanical strength of the nanobearing-nanobearing structures significantly decreased as an increase in disorder magnitude was observed.Fourthly,the cold-welding behavior of nanoporous metals were studied.A phase field method was presented to generate the bi-continuous open-cell porous microstructure.This study shows that it is possible to cold-weld two nanoporous metals to form a novel composite material.The influence of temperature on the mechanical properties of the resultant composite material was investigated.With an increase in temperature,the weld stress and the mechanical strength of the nanoporous structures significantly decreased as an increase in disorder magnitude was observed.Molecular dynamic simulations on samples with various ligament diameter and relative density were also carried out to study how these factors affect the assembly process.Meanwhile,the nanoporous gold was prepared by the dealloying method,and the corresponding cold welding experiment was carried out at room temperature.The composite structure after cold welding was characterized by scanning electron microscope and nanoindentation experiments respectively.