Numerical Simulation And Ablation Characteristics Of 2.5D C_f/SiC Composite Fiber Laser Micro-hole Processin

2.5-dimensional carbon fiber reinforced silicon carbide（2.5D C_f/SiC）ceramic matrix composites（CMC）are widely used in aerospace and automobile manufacturing for their excellent properties such as high temperature resistance,high specific strength,and corrosion resistance.There are a large number of micro-holes in these key components to improve the heat dissipation performance.However,the excellent characteristics of CMC make the drilling technology based on traditional cutting methods prone to tool wear and tool breakage,so it is difficult to realize the micro-hole machining of CMC.In recent years,laser processing has gradually become a key technology for processing micro-holes in difficult-to-machine materials in the aerospace field due to its non-contact,no tool wear,and high efficiency.However,the significant thermal effect existing in fiber laser processing will aggravate the initiation and propagation of cracks on the surface and inside of the material,which greatly reduces the service life of the component.Therefore,studying the ablation behavior and ablation mechanism of 2.5D C_f/SiC composites when interacting with lasers to achieve high-precision and low-damage micro-hole machining is of great significance for improving the reliability of CMC in key areas.The main contents are as follows:A 2.5D C_f/SiC composites fiber laser trepan drilling model was established.The establishment of the numerical model takes into account the characteristics of the light source,laser energy loss,laser path,and material characteristics,and realizes the thermal-mechanical coupling of the numerical model based on the theory of heat conduction and thermal deformation.The hole shape characteristics of the micro-holes under different parameters were obtained through the numerical model,and the hole-making experiment was established to verify the diameter and taper of the exit/entrance of the micro-holes,which proved the accuracy of the model.Combining the micro-hole temperature field,stress field,and strain field distribution in the simulation model results,it reveals the relationship between it and the ablation morphology and ablation damage of the material surface.In addition,through anisotropic material modeling,the temperature and stress load transfer behavior between the carbon fiber and the matrix is revealed.The hole shape,thermal damage,recast morphology,and particle morphology of the micro-hole entrance and exit are studied,and the ablation mechanism of transverse carbon fiber and needled-punched carbon fiber is revealed.Through a single factor2.5D C_f/SiC composites laser micro-hole machining experiment,the influence of processing parameters on the shape of the hole entrance and hole exit was quantitatively analyzed,and the ablation damage on the surface of the micro-hole was analyzed based on image recognition.In addition,the ablation morphology of the composite material at the entrance and exit of the micro-holes was observed through a scanning electron microscope（SEM）,and it was found that the transverse fiber is prone to needle-like,tapered,transverse fracture,longitudinal crack,and bending damage,and the needle-punched fiber bundles easily appear to interfacial debonding and extrusion fracture during the fiber laser drilling.More importantly,the results show that the layered accumulation,adherent morphology,thin shell,and bubble-like morphology are found to be the four main mechanisms of recasting layers.The"ablation evolution behavior"of fiber laser in 2.5D C_f/SiC composites micro-hole machining is proposed.The 0°fiber experiences the ablation evolution from filiform connection,flat ellipse,needle-like to convex structure.And the 90°fiber suffers the ablation evolution from cylindrical structure,lotus lead shape,bud-shape to closed shape.The core of the 90°fibers undergoes the topography structures from cylindrical protrusion,spherical protrusion to needle-like.The four new recast layer structures,named transverse strip,longitudinal strip,shell structure,and multi-layer structure are found and analyzed.Further,the mechanism analysis reveals that the recast layer contains both oxidized characteristics and highly carbonized characteristics.Finally,a 2.5D C_f/SiC composites laser drilling model was established.A comprehensive optimization method including Taguchi L₁₆（4⁵）orthogonal experiment,analysis of variance（ANOVA）,regression modeling,and multi-objective genetic algorithm（MOGA-Ⅱ）is proposed to reduce the taper and average heat affected zone thickness（HAZ_a）in fiber laser drilling.The key laser processing parameters（scanning speed,pulse frequency,pulse width,defocus amount,and energy percentage）as optimization parameters,and taper and average heat affected zone thickness（HAZ_a）as optimization targets.Through the ANOVA,the significance of the processing parameters to the response target is obtained.To comprehensively optimize the taper and HAZ_a,a high-order regression model for the taper and the HAZ_a is established,and the regression model is optimized through the MOGA-Ⅱ.Using optimized process parameters for processing,it is found that the taper is minimized from 2.603°to 0.333°,and the HAZ_a is minimized from 124.972μm to77.4μm.In addition,the new process of hydrofluoric acid to remove the micro-holes oxidation zone was verified.Therefore,the proposed comprehensive optimization method can realize high-quality fiber laser micro-hole processing of 2.5D C_f/SiC composites.