Investigation Of Strengthening And Ablating On Stainless Steel And Aluminum Alloy Surfaces By Femtosecond-Laser Irradiation

Femtosecond laser has brought a series of major breakthroughs in fundamental physics and engineering sciences since its inception.Due to its ultra-short pulse width and ultra-high peak photon density,femtosecond laser can easily induce the ultrafast phase transitions of materials and dynamic evolutions of particles.Mechanical enhancement and high-precision processing of metal materials are in the core areas of industrial manufacturing.However,the traditional continuous and long-pulse lasers are unable to meet the increasing demands of high precision and high quality for mechanical manufacturing.For instance,the deteriorated surface quality is a challenging issue for the relatively mature nanosecond laser peening technology.In addition,the adverse effect of thermal diffusion is an insoluble problem for continuous and long-pulse laser technologies in metal processing.Femtosecond laser has great potential applications in the field of metal manufacturing,for example,microhole drilling and nano-patterning processing,due to its high precision and low thermal effect.However,at present,there is still a lack of in-depth and systematic understanding of the ultrafast interaction mechanisms during femtosecond-laser processings on metal materials at varied power levels,especially the surface strengthening of metals induced by femtosecond lasers and the ablation mechanisms driven by high-energy multi-pulse femtosecond lasers.In view of this,we have systematically studied the strengthening and ablation mechanisms during femtosecond-laser processing on metal surfaces at multi-power levels,including the impact strengthening effect induced by low-energy femtosecond laser on metal surfaces,as well as the material removal,incubation effect and hot-melting recasting accumulation of metals driven by single-pulse and multipulse femtosecond lasers with various pulse energies,both in experiment and theory,taking stainless steel and aluminum alloy as the research objects.The main contents of this dissertation are as follows:The laser peening experiments were performed by direct irradiation on the surfaces of stainless steel and aluminum alloy samples using high-pulse-density and low-pulseenergy(relatively lower power level)femtosecond lasers.Laser-induced mechanical enhancement and surface quality change were studied by measuring surface elastic modulus,hardness,roughness,morphology and gradient micro-nano structures.Experimental results showed that the low-energy femtosecond laser achieved the enhancement of mechanical properties and high-quality surface of metals simultaneously.The enhancement of surface mechanical properties was in the same degree of that induced by traditional nanosecond lasers.For instance,the increasement of surface elastic modulus and hardness reached 22.3% and 33.6% for stainless steel,and 41% and 105% for aluminum alloy,respectively,via femtosecond laser peening.Meanwhile,the surface roughness of the samples was increased slightly,much lower than that induced by traditional nanosecond laser peening.The changes of temperature fields and dynamic evolutions of lattice defects on the surfaces of stainless steel and aluminum alloy samples during the ultrafast compressive process induced by femtosecond laser irradiation were investigated using coupling models consisting of two-temperature model,finite element analysis and molecular dynamic simulations.The coupling models based on two-temperature equation and finite element model showed that the surface temperature rise of the target material was limited during the low-energy femtosecond laser peening,thus avoiding large-scale melting at target surface.It was revealed by the coupling models based on twotemperature equation and molecular dynamic model that the dislocation nucleation,growth and inter-entanglement in metal lattices,the extension of the complex defect mesh structure and its pinning effect on dislocation movement enhanced both the linear and nonlinear mechanical properties of metal surfaces.In addition,the existence of vacancy accelerated the elastic deformation process of lattice and significantly affected the multiplication and movement of dislocations.The ultrafast phase-transition evolution mechanisms driven by single-pulse femtosecond laser ablations with various pulse energies(multi-power levels)and pulse widths were investigated by experiments,two-temperature model,finite element analysis and hydrodynamic simulations.Experimental results showed that the thermal effect was significantly enhanced with high-energy femtosecond laser pulse,which had an adverse effect on the quality control of high-precision processing of metal surfaces by femtosecond lasers.Two-temperature model coupled hydrodynamic simulations revealed the material removal mechanisms of phase explosion and critical point phase separation during femtosecond laser ablations,via monitoring the electron,ion temperature and pressure,material density,charge distribution,and phase state of the metal target surface.Additionally,pulse width had obvious effects on femtosecond laser ablation of metals.The temperature and pressure of the electrons and lattice were higher with shorter pulse width,as well as stronger material removal by gasification.Nonlinear absorption,incubation effect and the accumulation of hot-melting recasting during the ablation of metal targets by multi-pulse femtosecond lasers were studied by combining experiments and two-temperature model coupled hydrodynamic model.Incubation effect originated from the changes of the optical and thermodynamic properties of the metal surfaces during the multi-pulse ablation process.The measurements of the metallic ablation thresholds provided important experimental parameters for the induction technique of periodic micro and nano structures on metal surfaces.In multi-pulse high-energy(relatively higher power level)femtosecond laser ablations,the abruptly heat-intensified effect and serious hot-melting recasting accumulations were produced because of the defocus phenomenon occurring in ablated craters,which was adverse for the processing of high-precision micro-holes.The enhanced absorption and energy coupling effect during multi-pulse process were attributed to micro and nano structures,phase change,photoionization,and plasmas at metal surfaces.