The Drilling Defects Machanism, Evaluation Method And Process Optimization On Carbon Fiber Reinforced Polymer

Carbon Fiber Reinforced Polymer(CFRP),which is of high specific strength,high specific stiffness,low thermal expansion coefficient,good anti-fatigue ability,good anti-corrosion ability and performance designability,has been widely used in aerospace,national defense and automobile industries.Especially with the development of CFRP manufacturing and machining technology,it has been applied more and more in the manufacturing of aircraft structure and even become the main structure material of aircraft.In the aircraft manufacturing,the final assembly is usually completed by hole connection,therefore drilling is the busiest machining procedure in CFRP assembly process due to enormous amount of assembly holes.As CFRP consists of resin matrix and carbon fiber,the characteristics of resin matrix are low strength,low hardness and strong viscoplasticity while the characteristics of carbon fiber are high strength and hardness,CFRP is highly anisotropic,the interlaminar strength is lower than interlayer strength,which caused the disparity in material removing.The material is hard to be removed along the high strength direction,and is incline to have defects along the low strength direction.Regarding tool material,tool morphology and CFRP's anisotropy,it is meaningful and beneficial to promote the application of CFRP in aerospace industry by doing research on the mechanism,the evaluation method of drilling defect and process optimization of CFRP drilling.Firstly,the cutting mechanism of CFRP was analyzed under different fiber orientation angle,and the drilling experiments were done with four common drills in CFRP drilling,which were CVD diamond coated brad spur drill,CVD diamond coated multi-facet drill,twist drill and dagger drill respectively.Then based on the SEM morphology of holes with different fiber orientation angles,the characteristics of hole surface and drilling defects of different drills were analyzed.Besides,the mechanism of burr,splintering and delamination for CFRP drilling was discussed.According to the theory of plates and shells,the model of drilling delamination was established and the relationship between critical delamination thrust force,load distribution,hole diameter and strength of CFRP material were analyzed.Furthermore,the critical delamination thrust force for different drills was obtained.From the delamination model,it was found that delamination was more inclined to appear if the drilling load was more concentrated to drill point,and the dagger drill was the most difficult to induce drilling delamination while the brad spur drill was most likely to cause drilling delamination.Secondly,the objective to analyze the drilling defects mechanisms is to find out the effective way to control them,but there should be a set of defect evaluation system in advance which could evaluate the drilling defects directly and correctly.Therefore,this paper provided the evaluation methods for burr,splintering and delamination respectively,and introduced the comprehensive evaluation method which could evaluate the overall drilling defects finally.Based on the optical microscope imaging and C scanning image of SAM,how the drilling defects were affected by drills and drilling parameters were studied,and the optimized drilling parameters range was provided to minimize drilling defects for each drill.It was found that multi-facet drill could provide the best machining quality with highest drilling efficiency.Thirdly,the machining technologies for small holes on CFRP material were studied.In the research for small holes machining,the relationships between drilling force and drilling parameters were analyzed based on the experimental results and data fitting results.Combined with the drilling temperature and defects mechanism,the drilling parameters for small hole machining were optimized and the drilling experiments were conducted to valid the optimized technology by analysis of process capability on diameter and roughness.Finally,in the research for large holes machining technology,the thrust force for each procedure and hole morphology were analyzed in machining process,and the optimized machining sequence were obtained.Besides,the drilling experiment was conducted to research the stability of cutting tools,and the relationships between thrust force,material removal rate and machined hole numbers were acquired by data fitting,based on which the guideline for tool changing scheme was established.In addition,the validation drilling experiment for optimized machining technology was conducted and the rationality of hole-making process was verified by analyzing the process capability on diameter and roughness.Based on process optimization,the design method of integrated drilling-reaming tool was induced,which provided the analytic relationship between working allowance and critical delamination thrust force.This method provided the guideline for choosing the working allowance,which could make the thrust force be lower than the critical value for delamination and thus help to achieve the objective of improving machining quality and efficiency for large CFRP holes.