| Laser drilling technology has the advantages of low cost,high efficiency and wide processing range.With the increasing demand for micro and multiple holes processing in the equipment manufacturing industry,laser drilling technology will have broad application prospects.Because of the complexity of laser drilling mechanism,the theoretical research of laser drilling is not yet clearly understanding,especially that of laser drilling based on fluid heat transfer and hydrodynamics theories.In addition,the existing laser drilling technology still has some problems,such as recasting layer,taper hole and surface spatters,so the further processing optimization of laser drilling technology is also needed.In order to study these problems,SUS304 stainless steel which is widely applied in the industries of healthcare,nuclear power,aviation,and shipbuilding is chosen to investigate the laser drilling problems from theories,numerical simulations and experiments based on the theoretical research of the laser interaction with metals.The main research work and conclusions of this paper are as follows:(1)The numerical model of laser drilling is established based on fluid heat transmission and hydrodynamics theories,as well as the modified Level-Set interface tracking method and solid-liquid mixing theories.The numerical model considers the influence of recoil pressure,surface tension,thermal capillary force,thermal buoyancy,gravity and other forces.The influence of latent heat of melting and evaporation is also considered based on the equivalent heat capacity method.(2)The evolution processes of the hole free interface,including the variation regulations of hole diameter and hole depth with laser duration are analyzed based on this model.The temperature and velocity fields of the laser drilling process are also analyzed.The simulation results reflect the melting and gasification of the material and the movement of metal vapor in the laser drilling process.The effects of recoil pressure and thermal capillary force on the movement of hole free interface are discussed.The results show that the recoil pressure plays an important role in the formation of deep holes in laser drilling,and the thermal capillary force is the main reason that causes the molten metal to overflow from the edge of the hole and form the bump.(3)The laser drilling experiments based on the parameters used in numerical simulations are carried out.The morphology of the cross section and the accumulation of spatters around the hole are observed,and the formation reasons of the taper holes and spatters are analyzed based on the simulation and experimental results.The experimental results agree well with the simulation results,which verifies the validity of the model and shows that the Level-Set method has good adaptability in the free interface tracking of laser drilling.The results show that the formation of the taper holes may be related to the Gauss distribution of laser heat source.The formation of spatters around the hole is related to the viscous force between the high velocity metal vapor and molten metal.(4)The processing optimization experiments based on the orthogonal experimental method are carries out to research the impact importance of pulse energy,pulse width,repetition rate,defocus distance and pulse numbers on the results(including entrance diameter,exit diameter and hole taper)of laser drilling on SUS304 stainless steel.The results show that the optimization holes obtained based on the parameters combination of the minimum hole taper satisfy the requirements of the demand of minimum hole entrance,hole exit and hole taper.The optimal experimental parameters for laser drilling of SUS304 stainless steel are obtained,which are 0.5 ms of pulse width,2.5 J of pulse energy,40 Hz of repetition rate,0 mm of defocus distance and 200 of pulse number. |
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