| Turbine blade of aero-engine is listed as the first key part because it worked in the harshest environment with highest temperature and most complex stress,which bears the function of converting high-temperature gas into kinetic energy to provide power for aircraft,and is an important symbol of the advanced degree of engine.In order to improve the service life and working stability,film cooling technology is required.During the operation of the aero engine,the air flow through the holes forming a cooling gas film to protect turbine blades.The film holes are small in diameter,large in number,complex in space and angle.Traditional micro-hole processing technologies such as drill drilling,electric discharge machining and electrochemical machining are difficult to meet the current demand for high-quality air film holes.As a special processing technology,laser processing uses a high energy laser beam focusing on the surface of the material,that making it melting and even vaporizing to achieve the hole.Although the processing efficiency is high,but the hole has heat affected zone(HAZ),recast layer and micro-cracks defect layers.In order to eliminate the defect layer caused by laser processing,new processing methods must be explored to realize efficient and high-quality machining of the film holes,and promote the development of the overall manufacturing level of China's aero-engines.Aiming at the processing requirements of high-quality film holes,A new method of dynamic liquid assisted laser machining is proposed.Taking IN718 superalloy as the research object,the technology and mechanism of dynamic water flow and dynamic chemical liquid-assisted laser processing of micro-holes are studied.Through process optimization,high-quality micro-holes without defects such as recast layer and microcracks are obtained.Firstly,the research status and the theory of liquid-assisted laser processing technology are summarized and analyzed.The absorption characteristics of metal materials to laser,the interaction mechanism between laser and metal materials,and mode of laser machining micro-holes are discussed.The transmission characteristics of laser in water are studied.Then,the damage threshold of IN718 superalloy irradiated by nanosecond laser in air was investigated.The effects of multiple pulses,defocus and pulse sequence on the ablation characteristics of holes were analyzed.The removal mechanism of In718 superalloy by nanosecond laser was clarified.Then,an experimental study was carried out on the contour roundabout filling laser machining method,and the influence of laser process parameters(scanning speed,repetition frequency,defocus,processing times)on the hole was studied.Through single factor analysis,the optimal combination of laser process parameters was obtained as follows: the power is 100 W,the scanning speed is20mm/s,the frequency is 100 k Hz,the processing times is 3,the linear filling spacing is20?m,the positive defocus is 200?m,and the minimum taper of the hole is 0.125.In order to eliminate the defects of microcracks,heat-affected zone and recasting layer caused by nanosecond laser machining micro-holes in air,the dynamic liquidassisted laser machining was proposed.The dynamic water-assisted laser machining micro-holes and dynamic chemical liquid-assisted laser machining micro-holes were studied respectively.First,the dynamic water-assisted laser percussion drilling method was used to study the influence of laser power,processing times and other process parameters on the hole depth and morphology,and compared with holes of laser drilling in the air.The results show that dynamic water flow Auxiliary laser micro-hole processing technology can effectively reduce the HAZ and improve the quality and efficiency of micro-hole processing.Then the dynamic chemical liquid assisted laser processing technology was explored,and the chemical solution that did not react with the base material of In718 superalloy at room temperature but reacted with it at high temperature was selected.The optimal solution configuration was a mixture of 2 mol/L HCl and 4mol/L Na NO3.The effect of the solution jet flow rate and offset distance on the hole of dynamic chemical liquid assisted laser drilling is studied.The results show that the solution jet flow rate and offset distance will affect the impact removal effect of the dynamic liquid film and the cooling effect,which has a great influence on the inlet diameter and outlet diameter of hole.Scanning electron microscopy(SEM)and confocal laser microscopy(LSCM)were used to detect the morphology of the micro-holes that laser machining under the conditions of air,dynamic water flow and dynamic chemical solution.It was found that the micro-holes processed by dynamic chemical solution assisted laser had no recasting stacking and no recasting particle defects,but the microholes processed by this method are unconventional cylindrical micro-holes with large taper.Finally,in order to improve the taper of the dynamic chemical liquid-assisted laser processing of the micro-holes,the dynamic chemical liquid-assisted laser processing has been optimized in two steps: the first step is to use the laser to process the pre-holes with a small taper in the air.In the second step,a pre-configured chemical liquid is introduced to form a dynamic liquid film to cover the processing area.The temperature of the chemical liquid is increased under the laser irradiation,which accelerates the reaction speed of the chemical liquid and the recast layer in the micropores,and realizes the controllable modification of the micropores by the laser.The influence of process optimization parameters(diameter of repairing circle,laser power,repetition frequency,defocusing amount)on the quality of micro-holes is studied.The results show that when the laser power is 120 W,the repetition frequency is 50 k Hz,and the negative defocus amount is 400?m,the optimized processing is carried out by tapping method.After optimized processing,the surface of the micro-hole is smooth,and there is no accumulation of recast in the inner wall of the hole,the hole shape is good,and the minimum taper is 0.123.In this paper,the mechanism,influence parameters and processing technology of micro-hole machining of IN718 superalloy by dynamic liquid assisted laser were studied.Finally,high-quality micro-hole machining with no crack,no recasting layer,small HAZ and small taper was realized,which provided important theoretical support and experimental basis for the realization of high-quality film hole machining by nanosecond laser.It is of great theoretical significance and practical value to improve the overall manufacturing technology level of China's aero-engine. |
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