Study On Micro-holes Laser Chemical Composite Drilling Technology Of SiC/SiC Composites

Aero engine is known as the heart of the aircraft,its processing and manufacturing is one of the current"card neck"technology in China,restricting the development of China’s aerospace industry,improving the engine thrust-to-weight ratio is one of the important development trends of aero engines,but the improvement of the thrust-to-weight ratio puts forward higher requirements for the high temperature resistance of the engine turbine blades.In order to improve the high temperature resistance of the blades,on the one hand,it is necessary to develop materials with high heat resistance,and on the other hand,to find suitable cooling technology.Therefore,scholars at home and abroad had proposed to replace the manufacturing materials of aero-engine turbine blades with new high-temperature-resistant silicon carbide ceramic matrix composites,and at the same time use air film cooling technology,which can increase the working temperature of the engine by 300-500℃,reduce the weight of the structure by 50-70%,and increase the thrust by 30-100%.However,at present,traditional machining and classic special processing technology can′t meet the processing requirements of high precision and low damage of air film holes,so the drilling of air film holes in aero engine blades is a challenging industrial problem at present,and it is the key to aero engine manufacturing.In this paper,taking SiC/SiC composites as the research object,this paper proposed a laser-chemical composite drilling method and performed hole drilling on SiC/SiC composites.In this method,the chemical liquid was involved in the drilling in the form of a flowing liquid film,and the jet of chemical liquid reached the surface of the material to form a chemical liquid film,which had the effect of impact,cooling and corrosion,which can effectively remove the defect layer that appears in the laser drilling drill and achieve high-quality micro-holes drilling.Firstly,the SiC/SiC composite materials were briefly introduced,the research status and development trend of SiC/SiC composite materials drilling technology were analyzed,the research status of laser drilling micro-holes technology was summarized and summarized through three aspects:drill parameters,punching method and drilling environment,and the research status of liquid laser drilling micro-holes technology was summarized and analyzed.Secondly,the laser-chemical composite drilling mechanism was studied,and the analysis found that the greater the chemical liquid flow rate,the better the impact on the material,the cooling effect,but the greater the energy loss,when the offset distance was greater than 12mm,the chemical liquid film enterend the unstable flow area,and the drilling effect begined to deteriorate;the chemical liquid circulation part,the water cooling cycle part,the drilling platform and the water cooling control part of the composite drilling device were introduced,and the water cooling control system makeed the chemical liquid temperature stable and controlled at 20-25℃.Thirdly,the laws of micro-holes drilled by picosecond laser and nanosecond laser in SiC/SiC composites,as well as the influence mechanism and formation of different drill parameters on micro-holes quality,were studied.The study found that when the number of feeds was too much and the feed distance was too large,the material can′t be completely removed,on the contrary,the material would be over-drilled to cause local damage;the power increase would lead to an increase in the depth of the micro-holes,the diameter would tend to saturation after the increase,when the power was too small to increase the recoil pressure,resulting in the micro-holes can′t be through,Excessive power would cause excessive damage to the material;with the increase of the repetition frequency,the diameter of the micro-holes inlet decreased,the diameter of the outlet increased,and the thickness of the defect layer gradually increased When using concentric circle line scanning path,compared with the other two methods,the micro-holes quality was the best,the roundness can reach 96.9%,the taper was minimum 3.8-4.9°;The spot overlap rate and concentric circle line overlap rate have a great influence on the micropore depth,and have almost no effect on the micropore inlet diameter.Studying the influence of laser drill parameters on micro-holes quality not only lays a foundation for the subsequent study of laser-chemical composite drilling methods,but also provides theoretical research on single laser drilling of high-quality micro-holes.Finally,according to the composition,chemical properties and reaction mechanwasm of SiC/SiC composites,the HF solution with a chemical liquid concentration ratio of 2 mol/L and the HNO₃solution of 3 mol/L were analyzed and screened,taking into account the laser drilling efficiency and equipment cost（picosecond single hole drilling time was 8 minutes,nanosecond single hole drilling time was about 10 seconds）,nanosecond laser was used for composite drilling,and the chemical liquid flow rate in the laser-chemical composite drilling drill was taken.The three key influencing factors of the chemical liquid jet angle and the chemical liquid jet position were studied,and it was found that the flow rate of 14m/s,the angle of 45°,and the offset distance of 12mm were the best drilling effect;the products and main chemical reactions of the laser-chemical composite drilling drill were analyzed,and the defect layer composed of Si,SiC and SiO₂would be generated during the laser drilling,and these reactants could be removed to form H₂SiF₆,CO₂,etc.through chemical reaction EDS energy spectroscopy analysis confirmed that the white debris material produced during laser drilling was silica,through the comparison of the proportion of elements before and after composite drilling,it was found that the chemical liquid had a high degree of selectivity for SiC/SiC composite materials,only reacts with the defective layer after laser drilling,and the debris and recast layer attached to the micro-holes orifice and the pore wall can be completely removed,and did′t react with the matrix,through this method can finally obtain high-quality micro-holes without recast layer,no heat affected zone,and no microcracks.