Optimization of drilling parameters on delamination and burr formation in drilling of neat CFRP and hybrid CFRP nano-composites

Carbon fibre-reinforced polymer (CFRP) composites have exceptional mechanical advantages such as high specific strength and stiffness, lightweight, and high damping capacity, making them very attractive for aircraft, aerospace, automotive, marine, and sporting applications. However, various defects such as delamination, burr formation, and surface roughness are observed during the drilling of CFRP composites, which are influenced by various drilling process parameters. In this work, the drilling quality of uni-directional CFRP composites. and the hybrid Al2O3 and hybrid SiC nano-composites are investigated experimentally using different types of drills such as step drill, core drill, and twist drill, as there is a limited study done on the comparative analysis of the impact of the above drills on the delamination factor and burr area on the above CFRP and hybrid nano-composites. The design of the experiment table was developed using response surface methodology (RSM) for input process parameters of spindle speed, feed, drill diameter, and drill type. The output surface characteristics (delamination factor and burr area) of the hole were measured quantitatively using the stereo zoom optical microscope. The main effect plots, contour plots, and analysis of variance (ANOVA) were used to examine the effect of spindle speed, feed, drill diameter, and drill type on exit delamination and burr formation. The analysis of main effect plots, contour plots, and analysis of variance showcased the optimum process parameters, such as a high spindle speed of 5500 rpm, low feed of 0.01 mm/rev, and drill diameter of 4 mm. The step drill demonstrated the least damage mechanism among drill geometries, followed by the twist and core drills. The minimum drilling damage was observed for the Al2O3 hybrid nano-composite compared to the neat CFRP composites.


Introduction
Carbon fibre-reinforced polymer (CFRP) composites have been the most advantageous material among fibrereinforced polymers as they are highly used in aerospace, automobiles, and marine applications [1,2].In recent decades metals have been replaced by CFRP composites.The unique advantage of using CFRP composites is their high strength-to-weight ratio as compared to metals [3].The performance of these CFRP composites can be further improved by adding inorganic fillers into the epoxy.Recently, various inorganic fillers have been used by researchers to improve the performance of these composites.Some of the inorganic fillers are as follows: aluminum oxide (Al 2 O 3 ), silicon carbide (SiC), silicon Oxide (SiO 2 ), titanium dioxide (TiO 2 ), and calcium carbonate (CaCO 3 ) [4][5][6][7].The inorganic fillers added are either micro or nano-sized particles.The varying size of these inorganic fillers also affects the variation in properties.The researchers have found that blending epoxy with nano-size fillers has provided better results than adding micro-size fillers [8].The nano-fillers have a high surface area in comparison to the micro fillers; as a result, the better blending of nano-size fillers epoxy with epoxy is possible, and the mechanical properties also improve due to the high cross-linking of nano-size fillers with epoxy resin [9].In the study of Mohanty et al tensile strength and modulus significantly improved by the addition of Al 2 O 3 nanoparticles into hybrid carbon and glass fibre-reinforced composites [10].
Similarly, Priyadarshi et al investigated the mechanical properties such as flexural, impact, and microhardness strength of Al 2 O 3 filled in jute epoxy composite at different conditions.They observed an improved impact strength of 1.902 Joules, increased flexural strength of 72.94 MPa, and maximum hardness of 29.9 Vickers hardness number [11].Also, while analyzing the wear behavior of carbon nanotubes (CNT) in glass epoxy composites, Venkateshan et al [12] found that the wear rate decreased with the increasing percentage of CNT particles in glass epoxy composites.The growing demand for CFRP composites has spurred research into optimizing their manufacturing processes, with drilling being a critical area of investigation.
Drilling in CFRP composites is a fundamental process for fastening and assembling components in aerospace applications, as 60% comprise composites.These components' structural integrity and functionality are closely tied to the quality of holes drilled in CFRP composites [13].However, the unique properties of CFRP materials, such as their anisotropic nature, abrasiveness, and sensitivity to heat, pose substantial challenges to conventional drilling methods [14].As a result, researchers and engineers have been actively exploring innovative drilling techniques to overcome these challenges and ensure the integrity of drilled holes in CFRP composites.After drilling, defects like microcracking, debonding, fibre pull out (burr formation), and delamination occurs at the drilled hole's entry and exit [15,16].The quantitative measurement of these drillinginduced damages is obtained by digital image processing using an optical microscope, C-scan, x-ray, and laserbased imaging techniques [17,18].
During the drilling operation, the drill initiates a peeling force that results in the separation of laminate layers at the surface of the composite.This separation is induced by fracture due to mode III, although the drill's vibrations compromise the efficiency of fiber tearing shear.Simultaneously, fibers are pulled as a crack opening occurs in the laminate, which is attributed to mode I failure.Peel-up delamination manifests as a consequence of the combined failures in modes I and III [19].Similarly, in push-out delamination, when the tool advances to the lowest layers of the composite, the unbroken plies below the drill easily delaminate and bend in reaction to the composite's decreasing thickness.Push-out delamination results from the combined failures in modes I and II.However, as push-out delamination lacks sufficient backing force to offset the thrust force applied by the tool to the composite, it is more hazardous than peel-up delamination [20,21].
Another surface damage observed during the drilling process of CFRP composites is the uncut fibers obtained around the machined edges, contributing to the formation of burrs [22].The presence of these burrs necessitates additional machining costs for their removal, thereby increasing the overall machining expenses [23,24].Several experimental investigations have been performed to overcome these drilling-induced damages.To mitigate the burr area around the cutting edges, Xu et al [25] proposed that increasing the feed rate significantly impacts the burr area.A similar conclusion was achieved at the entry point of the hole by Heisel and Pfeifroth during the dry drilling of CFRP [26].Davim and Reis investigated the drilling-induced damage in CFRP composites by implementing a combined approach of the Taguchi technique and analysis of variance (ANOVA).They concluded that implementing the above techniques can establish a correlation between cutting speed and feed rate with the delamination factor [27].In a similar investigation of the drilling of CFRP composites, Chen proposed that the delamination damage can be reduced by selecting proper tool geometry and cutting parameters [28].Among the different approaches, response surface methodology (RSM) has been a very simple and easy technique for correlating the machining parameters with the output variables.Palanikumar implemented the RSM technique to predict the surface roughness in the drilling of GFRP composites [29].Similarly, Davim and Palanikumar studied the tool wear with the same RSM approach while machining GFRP composites for predicting the tool wear [30].
From the above literature, it is comprehended that the drilling-induced damages in CFRP composites, such as delamination and burr formation, can be reduced by selecting appropriate machining parameters and tool geometry.Therefore, in the present research, the drilling investigation of CFRP, Al 2 O 3 , and SiC hybrid nanocomposites are investigated at various cutting parameters (spindle speed, feed, drill diameter, and drill type), and their respective output variables (delamination factor and burr area) are measured.RSM is used to develop a design for experiments with different machining parameters.The main effects plots, contour plots, analysis of variance (ANOVA), and optimization plots for optimizing the machining parameters are also drawn by utilizing the experimental results.
Also, the authors in their previous work have reported their experimental results on the enhancement of mechanical properties at different nano-filler (Al 2 O 3 and SiC) loadings, such as tensile, flexural, interlaminar shear strength, impact, and hardness of the neat CFRP compared to hybrid nano-composites [4,5].The maximum mechanical properties were noted at 1.75 wt% filler loading in the case of Al 2 O 3 hybrid nanocomposites and at 1.25 wt% filler loading for SiC hybrid nano-composites.

Materials and methodology
For the present work, the uni-directional CFRP material is used as a reinforcement, and bisphenol-A epoxy resin and amine-based hardener are used as a polymer matrix.Two types of inorganic nanofillers, Al 2 O 3 and SiC, were added at different filler loadings to prepare hybrid nano-composites.The uniform dispersion of nanofillers was achieved by sonication and magnetic stirring methods.The neat CFRP and hybrid nano-composites were fabricated using hand lay-up followed by a compression molding approach.The thickness of the fabricated composites obtained was 6 ± 0.2 mm.Drilling was performed using a computer numerical control vertical machining center for neat CFRP composite, 1.75 wt% Al 2 O 3 hybrid nano-composite and 1.25 wt% SiC hybrid nano-composite.The drilling was performed at room temperature conditions.The experimental setup for drilling is represented in figure 1.
The different machining parameters chosen for the study are represented in table 1.The following machining parameters were used to develop an experiment design with a statistical tool response surface methodology (RSM) using Minitab V15 software.The composite strips of size 250 × 25 mm were cut using an abrasive water jet cutting machine.Three types of drill geometry were selected: twist drill, step drill, and core drill, as represented in figure 2. A total of 180 holes were drilled, with 60 holes drilled for each composite type.The output variables measured were delamination factor and burr formation.All 60 holes for each type of composite were analyzed to determine the damage characteristics caused during drilling.To obtain the magnified image of the damage caused during drilling, the exit surface area of the hole was analyzed using the Nikon Stereo Zoom Microscope (SMZ745T series), as shown in figure 3. To calculate the delamination factor and burr area the following equations (1) and (2) were used [1,26].
D max is the maximum diameter (mm), D nom is the nominal hole diameter (mm), A nom is the nominal hole area in mm 2 , and A free is the burr-free area in mm 2 .The image captured from the microscope was then imported to Image-J software to measure the output variables quantitatively by setting the appropriate threshold frequency.The main effect plots and optimization plots were drawn using the same statistical software response surface methodology from the results obtained.

Delamination analysis
The drilling-induced delamination causes issues during the assembly of composite structures.Hence, delamination during drilling has to be prevented by selecting appropriate machining parameters.Table 2 represents the experimental delamination factor for hybrid nano-composites and neat CFRP composites.The minimum delamination factor was noted for Al 2 O 3 hybrid nano-composites (F d = 1.406), followed by SiC hybrid nano-composites (F d = 1.410), and the maximum value was observed for neat CFRP composite (F d = 1.421) respectively.This clearly states that adding nanofillers in CFRP composites shows better drilling performance in reducing the delamination factor.This indicates that the nanofillers act as a lubricant to distribute the heat generated at the tool-workpiece interface.Also, previous studies show that adding nanofillers enhances the thermal conductivity of polymer composites [31,32].Spindle speed, feed, drill diameter, and drill type significantly influence CFRP composites' drilling.The main effects plot is drawn to represent the effect of the above parameters on the delamination factor, as shown in figure 4.
From figure 4, it is observed that for all the composites, the delamination factor decreases with increasing spindle speed and increases with increasing feed and drill diameter for both hybrid and neat composites.The decrease in delamination during the drilling of composites can be attributed to the escalation in spindle speed, which concurrently amplifies frictional forces and heat generation at the cutting tool and composite interface.The increased spindle speed facilitates a more effective conversion of mechanical energy into heat.Consequently, the elevated temperature at the tool-workpiece interface softens the composite's matrix material.Notably, the matrix material typically possesses a lower softening or melting point when compared to the reinforcing fibers.This thermal softening phenomenon enhances the matrix's pliability, significantly reducing its susceptibility to delamination throughout the drilling process.As a result, the delamination decreases [33].
Similarly, the delamination increases with increasing feed and drill diameter.A higher feed force generated by the drill tool causes interlayer crack growth in the composites, further advancing delamination [34].
In the case of drill diameter, the delamination increases with increasing drill diameter as the 8 mm drill has a higher chisel edge length, one of the primary factors for higher delamination compared to the chisel edge length of 6 and 4 mm drill diameter.This is mainly because when the feed rises, the chisel edge impact on the fibers increases.Furthermore, because there is insufficient time for the cutting process and chip formation, the drill punches through the laminate, further increasing the delamination to a diameter of 8 mm drill [35].From the earlier work of the authors, it was proven that an optimal quantity of 1.75 wt% Al 2 O 3 and 1.25 wt% SiC nanofiller added to CFRP composites enhances interlaminar shear strength, tensile strength, toughness, and flexural resistance as compared to a neat CFRP composite.This further showcases that the addition of nanofiller to the matrix decreases stiffness between laminas, resulting in lower delamination [4,5].The minimum F d value was obtained for the 4 mm drill diameter, higher spindle speed (5500 rpm), and lower feed (0.01 mm rev -1 ).Among the drill types, the delamination obtained was minimum for the step drill, followed by the twist and core drill.
Step drill consists of a primary drill with a smaller diameter than a secondary drill with a larger diameter.For instance, considering the 4 mm drill diameter, the primary drill diameter is 2 mm, and the secondary drill diameter is 4 mm.As the primary drill has a lower chisel edge and lower diameter, the tool entering the composite drills a pilot hole initially, and then the secondary drill enters where the heat generated due to the tool and workpiece interaction is reduced.Hence, the damage caused is less than that caused by twist and core drills [36].In previous studies, the researchers obtained reduced delamination and thrust force by performing pilot hole drilling for the CFRP composites [37][38][39].

ANOVA analysis for delamination factor
The results of the ANOVA for the delamination factor are represented in table 3. The analysis was carried out for a significance level of α = 0.05 for a confidence level of 95%.From the ANOVA table, it is evident that all the process parameters have a significant impact on the delamination factor.The order of significant influence increases from feed, spindle speed, drill diameter, and drill type.Amongst all the parameters, drill type and drill diameter are the most crucial parameters affecting the delamination factor for both hybrid nano-composites and CFRP composites.The drill type shows 84.41, 84.26 and 84.21% contribution for delamination factor for Al 2 O 3 , SiC, and neat composites, respectively.The contribution of drill diameter for delamination was 10.53, 10.59 and 10.71% for hybrid and neat composites, respectively.Similarly, the contribution of spindle speed was 0.98, 1.14 and 1.12% respectively.However, the feed has less impact on the delamination factor as the contribution was less than 1%, that is, 0.32, 0.38, and 0.42%, respectively.

Contour plot analysis for delamination factor
In figure 5(a), the minimum delamination in Al 2 O 3 hybrid nano-composite during drilling is achieved with a spindle speed exceeding 3500 rpm, a feed of 0.01 mm rev -1 , and a drill diameter of 4 mm.Similarly, in figure 5(b), minimizing delamination in SiC hybrid nano-composite involves drilling at a spindle speed of 5500 rpm, a feed below 0.025 mm rev -1 , and a 4 mm drill diameter.Figure 5(c) illustrates that delamination decreases when drilling with a spindle speed above 3500 rpm, a feed below 0.02 mm rev -1 , and a 4 mm drill diameter.

Burr formation analysis
Burr formation during the drilling of composites impacts indirectly as they do not reduce the strength of the composite.However, they may have a potential risk of causing further cracks and delamination in the future.From table 2. the minimum burr area noted is as follows, for Al 2 O 3 hybrid nano-composites, the burr area obtained was 1.575 mm 2 , followed by SiC hybrid nano-composites with a burr area of 1.861 mm 2 , and the maximum burr area was observed for neat CFRP composite with 2.496 mm 2 respectively.As explained in the above section, nanofillers reduce the burr area more effectively than the composite without nanofillers.Both delamination and burr signify the surface characteristics of drilling-induced damage.A similar trend was noted in the main effect plots of burr formation, as represented in figure 6.
From the main effects plot (figure 6), it can be observed that the burr area decreases with increasing spindle speed and increasing drill diameter.And similar to the delamination damage, the burr formation is minimum while drilling with a step drill, followed by a twist and core drill.Compared to spindle speed, drill diameter and drill type feed has less influence on the burr area, as observed in the main effects plots and ANOVA analysis.As the feed range selected is very low in 0.01 to 0.03 mm rev -1 , the influence on the burr area is less [40].Therefore, the machining parameters recommended for minimum burr area are spindle speed 5500 rpm, 4 mm drill diameter, and step drill.

ANOVA analysis for burr area
Similar to the delamination factor, the ANOVA analysis was performed for the burr area.From table 4, all the process parameters have a significant influence on burr area for different types of composites.The drill type and   drill diameter contribution towards burr area are 90.83,93.53, and 91.04%, and for drill diameter, 7.71, 5.58, and 5.97%, respectively, for Al 2 O 3 , SiC, and neat CFRP composites.Similarly, the contribution of spindle speed and feed was found to be less than 1%.

Contour plot analysis for burr area
Contour plots in figure 7 illustrate the impact of various drilling parameters on the burr area.When working with Al 2 O 3 hybrid nano-composite (figure 7(a)), minimizing burr defects involves using a 4 mm drill diameter, spindle speeds exceeding 5000 rpm, and a feed below 0.02 mm rev -1 .Similarly, for SiC hybrid nano-composite (figure 7(b)), optimal results are achieved with a 4 mm drill diameter, spindle speeds above 4500 rpm, and a feed below 0.03 mm rev -1 .In the case of neat composite (figure 7(c)), reducing burr defects requires a spindle speed of 4500 rpm, a feed under 0.03 mm rev -1 , and a 4 mm drill diameter.

Optimization of machining parameters
The optimization techniques have played a significant role in selecting various drilling process parameters.Hence, optimizing the process parameters is essential in the drilling of composites.By optimizing the process parameters, the researchers have noted improved quality of drilled holes and increased tool life.The desirability function approach of response surface methodology was used to optimize the machining parameters.The desirability function transforms the output variable (i.e., delamination factor, burr area) into a dimensionless variable called desirability index di as represented in equation (3).The desirability index cascades in the close interval range of (0, 1).A higher value of the desirability index for output variables indicates a higher contribution to the product performance by the specific output variable.The desirability index d i is a function of the (output variables) y i .An individual desirability function assigns a number between 0 and 1; 0 is undesirable, and 1 is a desirable or ideal output variable value [41].In equation (3), y represents the output variable, y min denotes the minimum value, and y target signifies the target values.The variable q denotes the weight, which can take either low values (0 < q < 1) or high values (q > 1), depending on the desired effect.A value of one creates a linear ramp function among the low, target, and high values.Increasing the weight (q) can progressively shift the outcome towards the desired objective.The comprehensive product performance evaluation is accomplished by computing the geometric mean of all desirability indices to establish an aggregate (global or composite) desirability index, denoted as D, and m  7. The error variation of less than 3% was obtained from the optimization plots for the experimental findings performed when compared to the predicted values during the confirmation test.

Figure 2 .
Figure 2. Different drill geometries used for the present study (a) twist drill, (b) step drill, (c) Core drill.

Figure 3 .
Figure 3. Optical microscope image of delamination and burr.

Table 1 .
Machining parameters for drilling.

Table 2 .
Experimental results of delamination factor and burr area for neat CFRP and hybrid nano-composites.

Table 3 .
ANOVA table for delamination factor.

Table 4 .
ANOVA table for burr area.

Table 5 .
Confirmation test performed on the optimum process parameters for delamination factor and burr area.Mater.Res.Express 11 (2024) 035006 S M Shahabaz et al 6.The minimum delamination factor (F d = 1.406) and burr area (A b = 1.575 mm 2 ) was noted for Al 2 O 3 hybrid nano-composites.