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CONTROL OF MICROSTRUCTURE AND MECHANICAL PROPERTIES BY THERMAL ASSISTED LASER SHOCK PEENING
Laser shock peening is a high strain rate plastic deformation process, and it has been widely used in automobile, aerospace, and nuclear industries for surface enhancement. Lots of new developments of the laser shock peeing process have been studied to expand its new applications such as cryogenic laser shock peeing, warm laser shock peening, laser shock peening without coating, laser shock peening without confinement. There are still some issues that has not been addressed: 1) interaction between laser shock wave and layer structured composite material has not been studied. 2) investigation on microstructure and mechanical properties of intermetallic phase strengthened composite material processed by warm laser shock peening is rare. 3) preheating method for warm laser shock peening needs improvement.
In this study, thermal and temporal modulated laser shock peening process is developed to control microstructure and mechanical properties. 1) Laser shock peening and cryogenic laser shock peeing was applied to copper graphene heterostructure. Hardness, yield strength were measured and microstructures were characterized. Shock wave propagation and its interaction with monolayer graphene was studied by finite element analysis. Results showed that the yield strength of laser shock peeing and cryogenic laser shock peeing processed copper graphene samples increased by 40%, and 76% respectively. It was found that shock wave could pass through long-distance to generate dislocation transportation from one layer to another graphene with the shock wave interaction between graphene layers separated very far away. 2) Warm laser shock peening with different preheating temperature was performed on lightweight steel. Effect of temperature on mechanical properties, precipitates and dislocation distributions are investigated. A coupled phase field-dislocation dynamics model was developed to study the precipitates and dislocation generation mechanism. The yield strength of the lightweight steel after warm laser shock peeing reaches 2030Mpa, which is the highest for lightweight mid-carbon steel (70% Fe, 1%C). Experiment results have confirmed high density dislocations and precipitates are generated by warm laser shock peeing process. And we find a new mechanism, avalanche multiplication of dislocations and precipitates, during the warm laser shock peeing: I) Dislocations assist precipitates formation. II) Precipitates boost dislocation generation. 3) A novel dual pulse laser shock peening process was developed which combines preheating and laser shock peening process.The effect of modulating pulse width and pulse duration on processing temperature and material microstructures were studied. Results showed that single pulse laser processing could successfully remelted the second phase and had much smaller grain (500nm) due to fast cooling, and dual pulse with appropriate pulse duration resulted in high density nanosized (30nm) intermetallic phase. High hardness 59 HV and yield strength 547MPa could be achieved due to the combination of grain size refinement, hard second phase and dislocations.
- Doctor of Philosophy
- Mechanical Engineering
- West Lafayette