Abstract
Drilling carbon fiber reinforced polymer （CFRP） composites is liable to generate serious defects including burrs， delamination， fiber pullouts and matrix cracking because of their inherent anisotropy in mechanical properties. Therefore， studies on drilling quality during composites processing is necessary. The thrust force of different material drill bits in composites drilling process was measured by the dynamometer and the surface quality of the hole wall was observed by scanning electron microscope （SEM）， moreover， the tool wear and its effects on the hole wall quality were also taken into account.
Keywords
Carbon fiber reinforced polymer （CFRP） composites have been extensively used in aircraft and aerospace due to their advantages in mechanical properties and structural functionalities. The demands for CFRP composites are considerably rising because of their superior properties， such as high specific strength and stiffness， superior fracture toughness， excellent corrosion resistance and satisfactory durability. To obtain desired geometrical shapes and tolerances for a final component， secondary machining operations， such as trimming， milling and drilling， are necessar
However， the machinability of CFRP composites is extremely poor during drilling process owing to their inherent anisotropy/inhomogeneity， limited plastic deformation and abrasive characteristic
In recent years， characteristics of drilling force variations in terms of various tools and drilling parameters including spindle speed and feed rate have been widely investigated and published in Refs.［3-4， 20-26］. It is shown that the feed rate dominates the increase of thrust force， and different drill geometries may cause a variation in the thrust force evolution and hence influence the value of the maximum force in the composite drilling process. It is also concluded that the drilling parameters influence the tool wear behaviors and lifetime of drill bits during the composites drilling.
However， the influences of tool materials on tool wear and drilling quality is rarely reported. In this paper， effects of various materials that are extensively utilized in drill bits of tools on tool wear and hole quality are studied. Thrust force during drilling process is surveyed and hole wall surface is observed under scanning electron microscope （SEM）. Wear conditions and damage modes of holes drilled by different tools are investigated. Moreover， merits and demerits of different tools are also analyzed.
Since the high strength and extensibility， T800 CFRP composites are widely applied to the aircraft. In this study， the composite used was a carbon/epoxy with lamination sequence ［45/0/-45/90］3s， which was provided by Commercial Aircraft Corporation of China and applied to C919. The nominal ply thickness was 0.188 mm， yielding a nominal laminate thickness of 4.512 mm. Moreover， the dimension of the workpiece is 300 mm×200 mm， and three pieces of such laminated workpiece panels are used in the experiment. Different materials including HSS （high speed steel）， YG8， Y330 （without coating） and Y330 （with 0.002 mm diamond coating） of twist drills are utilized in the whole drilling process， and their chemical composition is shown in Tables
Indeed， thrust force is one of the most significant factors which have influence on drilling quality in drilling process. Thus， Kistler piezoelectric crystal dynamometer， Kistler charge amplifier and oscilloscope are accepted for measuring thrust force， and the drilling experiment is carried out on VMC-850 CNC machine tool with the maximum spindle speed of 10 000 r/min， as shown in
Fig.1 Experimental setup
To test the drilling quality， two main parameters including thrust force and surface integrity of holes are investigated in this work.
In the drilling process， the thrust force collected by the dynamometer is amplified by the charge amplifier and collected by the oscilloscope. Force measuring software DEWE soft 6.5 is utilized to obtain the force curve during the drilling process and the mean force in the stable cutting state is calculated. In order to eliminate the error caused by tool wear， the mean thrust force of first hole drilled by a new tool is taken as the objects under every different drill parameter， as demonstrated in
Fig.2 Thrust force under different feed rates and spindle speeds
It is can be seen that the thrust force of all type bits increases with the feed rate rising. Particularly， this phenomenon is the most obvious for the HSS tool. Moreover， under different feed rate levels， the thrust force of HSS bit is always much higher than that of any other bits. This is due to that the Rockwell hardness of ordinary high speed steel is 63 HRC， which is almost as much as that of T800 CFRP composite （53—65HRC）. Conversely， the thrust force of other tools which are fabricated of hard alloy steel is much lower than that of HSS tool and less affected by the feed rate.
Typically， the hole wall quality is an important factor which directly affects the fatigue behaviors of the composite joints and structures. However， due to the difficult processing of T800 CFRP composite， various damage modes， such as matrix cracking， delamination layer bending， and fiber debonding， often occur during drilling. In order to investigate the internal damage of the hole wall surfaces， specimens processed by different tools are examined by scanning electron microscope （SEM）， as demonstrated in
Fig.3 Hole wall surface damages of workpieces drilled by different tools under SEM at 6 900 r/min spindle speed and 0.01 mm/r feed speed
In the drilling process， the flank face， chisel edge， rake face， and edge of the tool are the main wear regions， as shown in
Fig.4 Main wear regions of the tool
Fig.5 Some holes with flaws
Fig.6 Wear of different tools under microscope
Fig.7 Thrust force of four tools after drilling different numbers of holes
Thrust force of the four tools rises sharply with the increase of numbers of drilling holes， especially HSS tools. However， thrust force of Y330 （with diamond coating） increases the most slowly and it even becomes lower than that of Y330 tool after drilling 20 holes and begins to be almost the same as that of YG8 after drilling 50 holes. Through
According to
Through
Likewise， tool wear can see the similar phenomenon that lifetime of YG8 drill bit is a little lager than that of the Y330 tool. According to
Effects of various tool materials on drilling quality and tool wear during drilling process are investigated. Dynamometer are utilized to measure thrust force and SEM are used to observe the hole wall surfaces， moreover， tool wear conditions are also surveyed by microscope. According to the results obtained in this paper， the following conclusions can be seen.
（1） Thrust force of high speed steel tool is much higher than that of hard alloy steel tools， which contributes to the more serious damages at the hole wall surfaces. Among the hard alloy steel tools， YG8 drill bit has the lower thrust force than that of Y330 drill bit during composites drilling.
（2） The main damage modes of hole wall surfaces drilled by hard alloy steel tools are layer bending and matrix cracking， while ordinary speed steel tools will lead to more serious damages， such as delamination， fiber debonding， fiber kinking， and fiber fracture.
（3） Durability of the HSS tool is extremely poor in composites drilling. Moreover， Y330 （with diamond coating） tool has the longest lifetime， which is twice as much as that of Y330 tools. This is due to that the coating can protect the drill bits thereby improving their durability. Interestingly， YG8 tools exhibit higher durability than Y330 tools， which may be a complex process that demands more studies.
Contributions Statement
Mr. ZHENG Guo designed the study， completed the experiments and wrote the manuscript. Prof. CAO Zengqiang contributed to the design， background and discussion of the study. All authors commented on the manuscript draft and approved the submission.
Conflict of Interest
The authors declare no competing interests.
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