Drilling Damage Mechanism Of CFRP/Ti Stacks Based On Hole Wall Quality And Accuracy And Damage Suppression Technology

In modern aerospace industry,the use of hybrid carbon fiber reinforced polymer(CFRP)/titanium alloy(Ti)stacks has experienced an increasing trend due to their outstanding mechanical/physical properties and flexible structural functions.As the key load-bearing components in the aircraft structure,the CFRP/Ti stacks is often subjected to extreme mechanical-thermal coupled load,so their assembly hole requires high quality,precision and reliability.However,due to their anisotropic mechanical behaviors and poor machinability,drilling of hybrid CFRP/Ti stacks in one shot time still faces several challenges,such as,severe hole damages,poor hole accuracy and hard to evaluate the load-carrying properties.Therefore,drilling of hybrid CFRP/Ti stacks in one shot time with high quality still remains a key issue to be solved urgently.In view of the above problems,this paper makes an in-depth study and discussion on the damage generation,inhibition,bearing capacity of CFRP/Ti stacks,and the application of low damage drilling process.(1)Firstly,an experimental method for cutting of UD-CFRP laminate discs was proposed.The distribution and characteristics of surface defects in a continuum of 0-180~o fiber cutting angles during cutting of CFRP was studied.Based on the single fiber fracture morphology analysis,four kinds of basic fiber/matrix fracture forms in CFRP cutting were clarified:fiber/resin debonding,bending-induced fiber fracture,shear-induced fiber fracture and compression-induced fiber fracture.Six typical surface morphologies of the machined surface and their basic material fracture forms were summarized.Based on the analysis of the cutting force behavior,formation mechanism of the surface pit surface mechanism of the cavity defects was revealed:the occurrence and propagation of the fiber/matrix debonding and the overall bending fractures of the fibers.(2)The cutting process evolution of cutting edge-workpiece material assembly unit during drilling of CFRP/Ti stacks under CFRP?Ti and Ti?CFRP two different drilling sequences was studied comparatively.The cutting force and cutting temperature variation at different drilling stages(pure CFRP phase cutting,CFRP and titanium alloy mixed cutting and pure titanium alloy cutting)were analyzed.The main manifestations and distribution regularity of hole-wall surface damages in CFRP layer were found.The evolution mechanism of the mechanical and thermal damages in CFRP layer under the erosion action of continuous titanium alloy chips with high temperature was revealed.The mapping relationship between pore wall surface damage and hole dimensional accuracy of CFRP layer was clarified.(3)Based on the titanium alloy chip formation regularity,a method of suppressing the occurrence of the hole surface damages during drilling of CFRP/Ti stacks was put forward.Through the analysis of chip formation in the drilling process of titanium alloy,peck drilling process was employed to control the chip formation,and the active control of the chip morphology was realized.The relationship among the hole wall surface damages,the hole precision and the titanium alloy chip morphology was clarified.The results showed that reducing the length and thickness of titanium alloy chips could greatly reduce the erosion effect of titanium alloy chip on the hole-wall surface of CFRP layer,thus restraining the occurrences of hole-wall surface damages in CFRP layer and improving the hole precision of CFRP/Ti stacks.(4)The effect of cutting speed on the hole damages during drilling of CFRP laminates was studied.The results showed that with the increase of cutting speed,the drilling force was reduced to a certain extent,the push-out delamination was reduced and the hole precision was improved within a certain range.But the high cutting speed would significantly increase the cutting temperature,resulting in resin matrix softening,thereby accelerating the fiber/resin debonding and deterioration of the surface of the hole wall damages.(5)The bearing capacity of CFRP laminates members with pore hole damages was analyzed and evaluated by quasi-static tensile tests and tensile-tensile dynamic fatigue tests.The mechanism of static tensile/fatigue damages of CFRP laminated was revealed.The bearing capacity of CFRP members was analyzed and evaluated.Experimental results showed that the mechanical bearing capacity of the CFRP members could be reduced due to the hole surface damages.And the greater the hole damage,the lower the mechanical bearing capacity.By improving the hole quality of the CFRP laminates,better mechanical bearing performance could be obtained.(6)The basic principle of low-frequency vibration drilling process from active control chip formation in time domain was studied.The trajectory equation of cutting edge and the thickness model of instantaneous cutting layer were established.The intrinsic relationship between the parameters,such as amplitude and feed rate,and the regularity of chip formation was clarified.The active control of titanium alloy chip morphology was realized.A composite/alloy stacks automatic drilling process validation test platform was developed and a series of experiments were conducted.The experimental results showed that the low frequency vibration drilling process could greatly reduce the mechanical/thermal damages of the CFRP layer due to the erosion effect of the titanium alloy chips.By this way,the occurrence of the surface damages of CFRP layer could be effectively suppressed and the hole precision could be improved.