Process Analyses And Interface Damage Controling In Drilling Of Interface Region Of CFRP/Ti Stacks

As one of most important composite materials,carbon fiber reinforced plastic/polymer(CFRP)is widely used in aircraft structures,which reduces weight of aircrafe and improves the fatigue life.Weight ratio of composite has already become an index for aircraft performance.In assembly process,CFRP needs to be connected with aluminium alloy(Al)or titanium alloy(Ti),forming CFRP/Al or CFRP/Ti stacks.Holes for connection are drilled in one operation to obtain high coaxiality and to reach high efficiency.Both CFRP and titanium alloy are difficult-to-cut.They have different material property and machinability.Thus,problemes such as high cutting temperature,server tool wear,and poor quality of hole have been observed in drilling CFRP/Ti stacks.Fuselage and wing are deformable due to their big size and thin thickness.When to drill with robot automatic drilling system,damage is easily happened in the interface region,which leads to poor quality and low efficiency.In allusion to the problems mentioned above,some researches are carried out about analyse of drilling processs and suppression of interlayer damage in drilling low-stiffness CFRP/Ti stacks.The main works of this study are as follows:Firstly,a mechanistic model is developed to predict thrust force and torque varying with machining time in drilling interface region of CFRP/Ti stacks.Tool geometries and process parameters are analyzed during stacks drilling process.Drill edges are divided into small elements with constant cutting angles based on mathematical description of drill geometries.Thus,cutting angles and parameters of elementray edge are obtained.A transformation matrix is established to decompose the cutting forces in elementary coordination into drilling coordination.In the model,three-dimensional drilling forces distributed along cutting edges are expressed as the multiplication of transformation matrix,specific energies,and uncut area.Torque and thrust force with time in the interface region equale the integration of elementary drilling forces of both materials.Drilling experiments are carried out to calibrate coefficieces of specific energy and to study influence of radius distance,feed rate,and spindle speed on specific energy.Beginning value,ending value,and evolution route are selected as evaluation criterion to compare predicted curves of drilling forces with the experimental.Experimental data is used to validate the model in a wide range of spindle speed and feed rate.The reason for predictive error is also discussed.Secondly,a novel mechanistic model for CFRP/Ti stacks is developed to predict thrust force of worn drill bits characterized by cutting edge radius.Analyses of wear mechanism and wear type of drill bits in drilling CFRP/Ti stacks show that cutting edge radius is a suitable wear criterion for drill edges.Wear volume is expressed as function of wedge angle and radius distance whoes values are calculated based on Archard model and distribution load along drill edges.Shearing and ploughing forces acting on cutting elements are modeled and their components in axial direction are summed along the whole drill edges to obtain thrust force.Drilling experiments of CFRP/Ti stacks are preformed to mearsure profile of cutting edges.Influences of feed rate,cutting distance,and number of hole drilled on edge radius are studied.Extrapolation method is employed to obtain shearing and ploughing forces using for calibration of model.Thrust force curves of worn drill bits are comparied with the measured.Predicted errors of maximum thrust force for both material are calculated to evalue the presented model.Thirdly,a new mechanical model for prediction of thrust force is developed taking into account workpiece stiffness.Its effect on delamination of CFRP material is also investigated.Analyzing drilling process of low-stiffness workpiece indicates that deformation of workpiece results in an additional feed rate which is expressed as function of derivation of deflection.Then,model for predicting thrust force considering stiffness is established.The model is used to analyze interaction between thurst force and deformation.Drilling experimants of low-stiffness CFRP plates are carried out to investigate how stiffness affects thrust force curve and its maximum value.Predicted curves of thrust force are compared with the measured curves to validate the model.Reason for predictive error is also analyzed.In order to determine proper index for free delamination in drilling low-stiffness workpieces,influence rule of stiffness on delimanation is studied.Fourthly,a model for predicting burr size is developed taking into account the influence of release movement in drilling titanium alloys.Processes of burr formation and growth are analyzed,then method for calculating burr thickness is presented based on theory of slip line.Criteria value used to determining burr type and height is proposed to decide whether burr grows or not at each drilling revolution.Release movement changes actual feed rate between drill bit and workpiece.Average value of actual feed rate in exit stage is used in calculation of burr size.In order to study how stiffness affects acutual feed rate,experiments of drilling low-stiffness titanium plates are carried out.Influence of release movement on burr size and type is also studied.Experimental data is used for validation of the proposed model.Lastly,a model is developed for predicting interlayer gap leading to interface damage in through-hole drilling process of low-stiffness CFRP/Ti stacks.A method also is presented to eliminate interlayer gap with minimum deflection of the stacks.There are four specific points in drilling process,which divide the whole process into five stages.A mechanical model is developed to predict thrust force of multilayer stacks taking into account bending of each layer.Then,method to predict interlayer gap is proposed as function of drilling parameters.Experiments of drilling low-stiffness CFRP/Ti stacks are preformed.The measured data is used to discuess influence of experimental variables on thrust force and to valid the model.Forming processes of interface damage were investigated,as well as its type.Interlayer gap is calculated based the valided model.Correlation analysis were conducted between interlayer gap and interlayer damage.Foot pressure on stack's surface is an effective way to control interlayer damage.Model is proposed to determine critial pressure,and experiments are conducted to valid the critical value.