The Mechanics And Numerical Simulation Of Femtosecond Laser Ablation Of NiTi Shape Memory Alloy

The deep understanding of the basic physics mechanism involved inthe femtosecond laser ablation of metals is of great importance in termsof improving the efficiency and quality of laser micro-machining， andoptimizing the processing technology. In this paper, a hybrid simulationcombined two-temperature model and molecular dynamics simulation isapplied to investigate femtosecond laser interaction with NiTi shapememory alloy （SMA）. The physical process including temperatureevolution, pressure distribution and atomic configuration is described indetail. The main contents in this dissertation are classified as follows.(1) The general physical process is analyzed and summarized in thedomain of femtosecond laser interaction with metals, which includes thecoupling process of optic energy and metals. The characteristic time isresolved in the special physical phenomenon. The basic calculationmethods have been performed in detail and the extraction process ofinterested amounts.(2)We use a conceptually simple and easy-to-implement methodnamed pressure-transmitting boundary conditions in NiTi shape memoryalloy for the first time.The boundary conditions allows that minimizepressure wave reflection at the boundary of a molecular-dynamicssimulation volume. The results show that the method is easily adapted tothe NiTi shape memory alloy.(3) The modeling of femtosecond laser ablation of NiTi shape memory alloy is resolved by combined two-temperature model andmolecular dynamics simulation. The results show that the peaktemperature of electron in center irradiation zone appears lower with theincrease of pulse durations at the same laser fluence, but the peaktemperature of lattice in center irradiation zone and the heat affect zoneappears greater. At low laser fluence, ablated materials is form of a bigliquid cluster, however, at high laser fluence, the ablated componentconsists of small cluster, liquid droplets and single particles. There is nofocused region which is formed by negative pressure in the substrate.