As an advanced welding method, the diffusion welding is widely applied in aerospace, instrumentation industry, engine manufacturing, etc. Not only the diffusion welding can realize the connection of different metals, but also it solves the difficult welding problem between metals and ceramics. Aiming to research the diffusion process during welding, this study investigates the diffusion behavior and mechanisms of Cu and Al under different pressures and surface roughnesses in the aspect of atomic level by using the molecular dynamics (MD) method. From the results, we can find that the pressure influences obviously the second thickness growth stage, which is one of the three stages in the whole diffusion process. The grooves generated by the surface roughness, is filled mainly by the deformation of Al, which fits the initial surface profile. The diffusion is promoted as the contacting area increases. Whether the surface is smooth or rough, the pressure has little impact on the final thickness. This study succeeds in explaining the diffusion mechanism, which is significant for the experiments of diffusion welding in practice.

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