| Laser drilling offers advantages such as high efficiency,high precision,no mechanical stress and no tool loss,and has become one of the most popular method among the micro drilling techniques.Nowadays,laser drilling has been used in many industries like aerospace sectors,marine,medical instruments,chemical and automotive industries,etc.However,certain defects such as recast layers,oxide layers,heat affected zones and microcracks are generated during laser drilling.In order to further improve the quality and efficiency of laser drilling for aero-engine turbine blades with high-precision,high-quality,and high-efficiency,the process and mechanism of laser drilling on nickel superalloy GH4037 using water-based ultrasonic or magnetic assistance were carried out based on the finite element numerical simulation of laser percussion drilling and circular laser hole-cutting.This research provides theoretical and technical support for the processing of microholes in various industrial applications.The main work are as follows:1.Three-dimensional finite element models of laser percussion drilling and circular laser hole-cutting were established using the element birth and death method.Based on ANSYS parameter design language(APDL),special finite element analysis programs for laser percussion drilling and circular laser hole-cutting were compiled.The change of temperature field in the laser drilling and the temperature change of different nodes near the microhole entrance and exit were analyzed.The predicted through hole geometry characteristics were validated with their measured results.It was shown that in laser percussion drilling,the temperature decreased with the increase of the distance from the hole sidewall.Under the laser pulses irradiation,the temperature of nodes 1-4 around the hole entrance increased and decreased instantaneously,the minimum temperature of the model rose continuously,the temperature at the location near the laser beam was the highest,and the microhole changed from blind hole to through hole.In circular laser hole-cutting,the temperature fields on the workpiece transiently altered with the rotation of the laser beam.Under the combined effect of the uniform circular motion of the laser beam and the periodic irradiation induced by the successive laser pulses,the local temperature on the thermal cycle curve for nodes 11-14 around the hole entrance and 17-20 around the hole exit rose obviously with transient multi-peak temperature fluctuations when the rotational laser beam moved near to nodes 11-14 and 17-20.The simulated results agreed well with the experimental measurements with an acceptable prediction accuracy.2.Combined with water assisted method and ultrasonic assisted method,a waterbased ultrasonic assisted millisecond laser percussion drilling method was proposed.The effects of air,water assistance and water-based ultrasonic assistance on millisecond single pulse laser drilling were compared and analyzed.It was found that both water assistance and water-based ultrasonic assistance could improve the quality and efficiency of millisecond single pulse laser drilling,and the microhole quality was the highest using water-based ultrasonic assistance.The influence of ultrasonic power on blind hole and through hole drilling with water-based ultrasonic assistance was carried out.The research shown that with the increase of ultrasonic power,the roundness of hole increased,the inner surface roughness of hole decreased.For the blind hole,the cross-sectional area of the blind hole increased and the material removal increased with the increase of ultrasonic power;the thickness of the recast layer near the bottom of the blind hole decreased,the thickness of the recast layer near the middle of the blind hole first increased and then decreased,and the distribution of the recast layer on the hole sidewall was more uniform.For the through hole,the uniformity of recast layer on the hole sidewall was better than that of the blind hole,and the thickness difference between the middle and the bottom of the through hole was small.The thickness of the recast layer on the hole sidewall decreased with the increase of ultrasonic power.3.The mechanism of water-based ultrasonic assisted millisecond laser percussion drilling was revealed by high-speed camera shooting the changes of the transient melt ejection,molten material spattering and vapor-plasma plume in the process of waterbased ultrasonic assisted millisecond laser percussion drilling.It was found that the molten material removal was enhanced with water-based ultrasonic assistance,which leaded to the improvement of the millisecond laser percussion drilling efficiency,more spatter was observed above the workpiece and the material removal was increased with ultrasonic assistance,the time required for the workpiece to be drilled through decreased with the increase of ultrasonic power.On the other hand,the quality of millisecond laser percussion drilling was improved,which was mainly manifested in the reduction of molten residue near the hole entrance and exit,the increase of roundness of the hole,the increase of the cross-section perpendicularity of the hole,the decrease of the thickness of the recast layer of the hole,and the more uniform distribution of the recast layer.It was because ultrasonic vibration,liquid motion caused by bubbles,and the increase of vapor and plasma recoil pressure could promote the removal of molten material.In addition,the propagation of ultrasound in water would produce cavitation,which further promoted the removal of molten material.4.Combined with water assisted method,magnetic assisted method and galvo scanning system,a water-based magnetic assisted femtosecond laser layered ring trepanning method was proposed,and the mechanism of water-based magnetic assisted femtosecond laser layered ring trepanning was revealed.The results shown that compared with the magnetic assistance and water assistance,the quality and efficiency of water-based magnetic assisted femtosecond laser layered ring trepanning were the highest,the hole taper angle was the smallest,and the hole sidewall roughness was the lowest.The higher the magnetic intensity was,the higher the quality of microhole was.The main reason was that both the transverse magnetic field and water medium could improve the quality and efficiency of laser drilling.In the transverse magnetic field,the shielding effect of plasma on laser was weakened,and the removal of material on the hole sidewall was more uniform.In addition,the impact of plasma on the hole sidewall was enhanced.As a result,the efficiency and quality of femtosecond laser layered ring trepanning was improved.In the range of 0-200 m T magnetic intensity,the greater the magnetic intensity was,the more obvious the effect of magnetic assistance was.However,the effect of magnetic assistance in improving the quality and efficiency of femtosecond laser layered ring trepanning was not obvious.The reason was that the lifetime of plasma was generally very short under irradiation of femtosecond laser.When the water entered the microhole,the material removal efficiency and the quality of femtosecond laser layered ring trepanning would be improved due to the liquid movement caused by the bubbles,an increase of the plasma recoil pressure in water,and the weakened plasma shielding effect. |
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