Thermal engineered laser shock peening driven nanostructures and their effects on mechanical applications
Abstract
Warm laser shock peening (WLSP) is a thermal-mechanical processing technique, which integrates the advantages of laser shock peening (LSP), dynamic strain aging (DSA), and dynamic precipitation (DP) to enhance the fatigue performance. Compared with LSP, WLSP results in a better stability of microstructures and mechanical properties due to the dislocation pinning effect of nano-precipitates. However, the efficiency of WLSP on mechanical performance is relatively low, and the microstructure after WLSP is not optimized. In this study, thermal engineered LSP is proposed and evaluated in order to obtain the optimal microstructures with the optimized pinning strength for mechanical applications. This technique extends the precipitation kinetics from the nucleation stage to the coarsening stage by combining WLSP with a following post-shock heat treatment. Experimentally, AISI 4140 steel and aluminum alloy 6061 (AA6061) have been selected as the materials to conduct thermal engineered LSP; the optimal mechanical properties, including the surface hardness, the stability of surface strength, the stability of compressive residual stress, and the bending fatigue performance have been obtained by carefully manipulating processing conditions; the resulting microstructures after various processing conditions have been characterized by TEM. The mechanism of the extended fatigue performance after thermal engineered LSP has been theoretically investigated; an analytical model has been proposed to estimate the nucleation rate during processing; multiscale discrete dislocation dynamics (MDDD) simulation has been conducted to investigate the interaction of dislocations and precipitates during the shock wave propagation. Through this study, it has been found that: (1) after WLSP, the unique microstructures with highly dense nano-precipitates surrounded by highly dense dislocations is generated; (2) during post-shock heat treatment, the coarsening of precipitates is dislocation-assisted, and the growth of precipitate size is accompanied with the decrease of precipitate number density; (3) the dislocation pinning effect makes the major contribution to the enhanced mechanical properties and microstructure stability after processing, and the pinning strength is highly dependent on the precipitate parameters, including the size, number density, distribution, and volume fraction; (4) by carefully adjusting the processing condition (including WLSP laser intensity and temperature, and post-shock heating temperature and time), the optimal microstructure for optimal mechanical properties of metallic components could be obtained.
Degree
Ph.D.
Advisors
Cheng, Purdue University.
Subject Area
Engineering|Industrial engineering
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