| Carbon Fiber Reinforced Polymer/Plastic(CFRP)and Titanium(Ti)stack structures have been widely adopted in various aircraft components,such as wing and central wing box,due to their superior physical and mechanical properties.To realize the reliable connection between the two kinds of materials,numerous high-precision holes are needed for the Ti/CFRP stack components in the assembly stage.Howe ver,drilling the Ti/CFRP stacks in single shot is a great challenge because of the different material and machining properties of the two kinds of materials.The existing drill structures hardly satisfy the high-quality drilling of CFRP and Ti in single shot,and various unacceptable drilling defects,such as CFRP delamination and Ti burrs,frequently occur in drilling Ti/CFRP stacks.Aiming at these problems,this paper investigated the fundamental mechanism and suppression method of damage,and further developed a novel drill bit structure to reduce damage in drilling Ti/CFRP stacks.The main research contents are as follows:Firstly,in order to reveal the relationship between drill bit structure,cutting parameter and drill loads,the drill geometrical characteristics were analyzed and cutting forces on the typical features of drill bit in drilling process were clarified.The cutting edges were discretized into finite elements,and the mathematical expressions of key angles of elementary cutting edge were obtained considering the effect of fiber orientation on cutting angle.Based on the contact form between drill structure and workpiece material,an orthogonal cutting model and an oblique cutting model were proposed to calculate the cutting forces of elementary cutting edge with the consideration of fiber cutting angles,flank face friction,and drilling parameters.Integrating the cutting forces of elementary cutting edge,a mechanical model for predicting thrust force and torque was developed in drilling Ti/CFRP stacks.A series of experiments were carried out to calibrate and validate the model.Based on the experimental results,the coefficient of specific energy was acquired,the effects of feed rate and spindle speed on specific energy was analyzed,and the model was also validated in different drilling parameters.And the maximum errors of the predicted axial force and torque were less than 10%and 15%,respectively.The research provided a theoretical basis for the rational control of thrust force and torque in drilling Ti/CFRP stacks.Secondly,in order to reveal the cutting and damage mechanisms at drill-exit area and the rebound mechanism of machined surface,the constraint state and initial damage of materials at the exit of CFRP were analyzed.A mechanical model with one side constraint and initial delamination for cutting single fiber was built.Meanwhile,considering the contact state between tool structure and material and the changing fiber orientation,a prediction model for spring back height of machined surface was established.The fracture and debonding laws between fiber and epoxy with initial delamination and spring back behavior were then solved analytically according to the developed two models.The effects of cutting depth on exit material removal,exit damage and rebound height of machined surface were analyzed as well.The results indicated that the cutting depth should be greater than the initial damage length and a small cutting depth is recommended to reduce the rebound height of machined surface.The results provide guidance for the suppression of exit damage and hole diameter error.Thirdly,prediction models for delamination and burr height at the exit were established respectively,and the investigation on the effects of step drill geometry on drilling induced delamination and burr of Ti/CFRP stacks was carried out.The contact relationship between cutting tool and workpiece material in different drilling stages was analyzed,and the location where delamination and burr frequently occur was determined.Based on the fracture mechanics,the critical delamination thrust force model and delamination extension length model at drill-exit in CFRP layer were established considering step drill geometry,cutting loads induced by step drill bit and the material fracture formation of end-loaded split.The relationship between the ratio of drill bit step diameter to the delamination length and the critical thrust force have been obtained.Furthermore,considering the geometry parameters of drill bit and drilling parameters,a critical burr height model for Ti alloy at the interface of stacks was established which assumed the remaining material as a symmetric cone with variable thickness,and the critical height of Ti burr was obtained.Based on the above prediction model of drilling loads,the relationship among the burr height,delamination length and the diameter of step-drill bit were clarified.The ratio of step-drill bit was improved,and the length of the CFRP exit delamination was reduced by 18%to 28%,the height of the Ti export burr was reduced by 55%to 80%.The research results provide a basis for the structural design of low damage drill bit in drilling Ti/CFRP stacks.Lastly,based on the fundamental study on the stacks material removal behavior and the relationship between the ratio of step-drill and critical defect condition,the damage suppression principle of "generating cutting" has been proposed.A novel multi-margin drill structure with specific step ratio was then invented which could remove materials micro by stages and reduce cutting depth of the cutting edge in both axial and radial directions in drilling process.Comparison experiments have been conducted and proved that the multi-margin structure is effective on reducing drill-exit delamination,CFRP burrs and Ti burrs in drilling Ti/CFRP stacks.Meanwhile,the hole diameters were also controlled under desired tolerance using the drill bit with multi-margin structure.It also verified the validity of the principle of"generating cutting" on damage reduction in drilling Ti/CFRP stacks and the effectiveness of multi-margin structure.Besides,aiming at the problem of unstable service life of such drill bit structure,considering the wear rules of feature structure in drilling common,alternating and independent materials,the decay mechanism of tool performance under the condition of continuous overall machining is obtained.And the drill bit with multi-margin structure is further optimized based on the geometric feature wear.Finally,the modified drill bit with multi-margins structure have been verified to be effective by conducting laboratory experiments and drilling tests in companies,which provided new paths for the low-damage drilling of Ti/CFRP stacks.Through the above researches,the variation of drilling loads with time in drilling Ti/CFRP stack structures has been obtained,and the relationship between thrust force and damage critical condition at the exit has been revealed.Furthermore,the generatingcutting theory of Ti/CFRP stack structures has been innovated and the drill bits with multi-margin structure have been developed.Through conducting experiments in aviation companies,the new theory and tool structure for drilling Ti/CFRP stacks have been verified.The researches in this paper have provided new methods for drilling Ti/CFRP stack structure in low-damage. |
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