The nanocrystalline metal material has been an investigation hotspot due to its excellent mechanical property. The high content of grain boundaries (GBs) in microstructure is the key factor affecting its fracture behavior during service. Therefore, it is essential to investigate the effect mechanism of nanoscale GB on crack propagation. In this study, four molecular dynamics (MD) models of nanocrystalline aluminum (Al) with different GB contents are established. The results show that the high‐content GBs in polycrystals can increase the toughness and absorb energy, reducing the risk of brittle crack propagation. The presence of GB can reduce the stress concentration of the crack, and the dislocation emission from the crack tip can be absorbed by the front GB. The microstructure with high‐content GBs can actuate more plastic deformation mechanisms such as multiple slips, GB slip, and migration. The region with more GBs can induce a more even deformation of the whole microstructure by means of dislocation emission and GB migration to the region with fewer GBs. The purpose of this study is to provide a rational mechanism reference for the failure of nanocrystalline metal material.

The partial weak bonding phenomenon induced by an unsuitable welding parameter often occurs at the joint interface of Al/Cu dissimilar friction stir welding (FSW). Diffusion time, temperature, and pressure are the main factors that affect the diffusion behavior at the interface. Therefore, molecular dynamics (MD) simulation is used to investigate the influence mechanism of these factors from the perspectives of diffusion thickness, total energy, and volume. The diffusion time and the temperature affect the diffusion thickness significantly, whereas external pressure has little impact. The variations of total energy and volume consist of two stages, which are caused by the combined effects of lattice transformation and atomic diffusion. The variation processes of total energy and volume are accelerated with the increase in temperature due to a higher diffusion velocity, and it is delayed by external pressure at a higher temperature because the lattice transformation is inhibited. These findings are beneficial to the choice of welding parameters in actual production.

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A favorable microstructure is a perquisite for ensuring the quality of welding joints, but grain coarsing inevitably occurs in the heat‐affected zone (HAZ). Controlling the reduction of deterioration as much as possible is a key problem in welding engineering. Based on the welding experiment, a finite element model considering the contact heat transfer of water‐cooled sliders is constructed to obtain key information about the welding heat cycle, such as T8/5, and a prediction algorithm for the microstructure prediction of HAZ is developed based on the Leblond model. The results show that the proportion of ferrite gradually decreases with the increase in cooling rate. The evolution law of the model prediction is the same as that of the experimental results, and the maximum error is within 5%, indicating that the prediction is accurate and reliable. This observation has great significance for the optimization of the electro‐gas welding process.