| As an important method of hole making,laser drilling is widely used in aerospace and other manufacturing industries.A typical application of laser drilling is forming film cooling holes on turbine blades.At present,film cooling holes on turbine blades are mainly processed by electromachining.However,electromachining has limitations.The resolution of electromaching can not meet the requirements of complex shaped film cooling holes due to the limitation of electrode size;turbine blades need to use nonconducting thermal barrier coatings(TBC),hole drilling by electromachining needs to divide the process into two steps,i.e."drilling before coating".However,this divided process will lead to shrinkage or even blocking of the film cooling holes.In contrast,with high speed,high resolution and high precision,laser drilling has been successfully used to drill film cooling holes in turbine blades overseas.Because of the hollow and complex blade structure,laser drilling may cause back strike of the turbine blade after laser penetrated the front of film cooling holes.As an international conundrum,up to now,back strike during laser film cooling hole drilling has not yet been completely resolved all around the world.Therefore,this thesis will mainly study the technology of laser drilling,and carry out basic experiments to decrease back strike in nanosecond laser drilling.The main completed work of this thesis:(1)The mechanism of nanosecond pulsed laser drilling was analyzed through study on macro and microscale energy-matter interaction.Based on the above analyses,defects of laser drilling were expounded,and the degree of back strike was measured and defined.(2)According to the experimental conditions,the laser ablation threshold of aluminum alloy 7075 was studied using the calculation method of area.And then the linear relationship between the logarithm of average laser power and the square of ablation aperture was deduced.The waist radius of nanosecond pulsed laser is calculated to be 10.62?m and the single-pulse ablation threshold of aluminum alloy 7075 is 10.904mJ/cm2.The accumulation effect and threshold of multi-pulse laser ablation are also obtained based on the previous experiments.(3)By means of laser trepanning of aluminum alloy 7075,the effects of different laser energy on the inner wall of the holes were analyzed by single factor experiments.It focuses on researching the effects of laser drilling parameters(laser energy density,scanning speed and defocus amount)on the processing quality(taper and roundness)of micro-holes machined by laser trepanning,and then the suitable machining parameters were selected.(4)The back strike protection experiments were carried out by selecting appropriate filling materials to block the laser energy.The problem of back strike of the real turbine blade is simplified to that of flat cavities.The damage extents of the back strike are characterized by morphology and average depth of the craters on the interior walls.The experiments of back strike were carried out under high-purity argon shielding,chosen filling materials include paraffin-graphite and polyacrylamide-silica.The patterns of laser drilling include trepanning and raster scanning.The results showed that the paraffin-graphite could,to a certain extent,block the laser beam and reduce back strike.The best case can reduce the back damage from 24.2?m to 4.2?m;Filling a cavity with 3mm-thick polyacrylamide-silica can totally block the laser energy and eliminate back strike when trepanning is used.The above results provided meaningful reference on laser drilling and laid foundation for further study on back strike protection. |
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