**2. Materials and Methods**

This work focuses on the assessment of three distinct drill geometries with regard to their cutting performance of AA-CFRP hybrid laminates, through conventional single-step drilling operations. Roughness measurement and load monitoring were carried out for distinct sets of cutting parameters on each geometry. Delamination, which may account for 60% of the rejected parts [22] is thoroughly analysed and a critical load threshold was estimated for its occurrence, based on delamination modelling of AA-CFRP interface through asymmetric double cantilever beam experimental testing procedure. Previous knowledge of the critical loads associated with the drilling operation is highly convenient in the design and selection of appropriate tooling solutions for hole quality compliance.

Specimens were built from CFRP and AA, with a stacking configuration of three CFRP layers (two external, one internal) and two internal aluminium layers, as illustrated in Figure 1a. The CFRP layers consisted of four 0.13 mm thick prepreg plies for the internal CFRP layer and three of the same plies for the external CFRP layers. Each aluminium layer was composed of 0.2 mm thick AA 5754 sheet. The composite material was constituted by polyamide 6 (PA 6) thermoplastic matrix reinforced by uni-directional carbon fibre with a volume fraction of 48.5%. Layer adhesion was promoted using conventional pretreatment techniques such as degreasing and laser texturing of the metallic sheets. The

laminate was submitted to a hot plate press curing process at a temperature interval between 240 and 280 ◦C and a pressure of 2 to 6 bar. Fibre direction was kept the same in all composite layers (uni-directional). Rectangular-shaped plates (240 × 250 mm2) were manufactured, with a resulting thickness (post-curing) of 1.2mm, which were posteriorly cut into 40 × 225 mm2 strips. Table 1 exhibits the mechanical properties of each material, according to the respective datasheets [23,24]. The experimental tests were conducted in a DMG Mori DMU60eVo series machining centre (25 kW), equipped with a piezoeletric dynamometer (Kistler 9272) and a signal amplifier (Kistler 5070A), connected to a data aquisition system (Advantech USB4711). A clamping system was developed for fixation of the material strips to the load cell, enabling drilling operations with and without sacrificial back support. The laminate strip is secured in-between two circular plates (top and bottom, refer to Figure 1b,c) which have a centre hole (*φ* 36 mm), enabling the drilling operation and placement of a PTFE cylinder under the laminates for sacrificial back support (when used). A constant torque was applied on the bolts which hold the top plate against the material and lower plate. With regard to the cutting tools, diamond-coated (through Chemical Vapour Deposition, CVD) tungsten carbide drills with a diameter of 6 mm were employed. This drill material and coating configuration has been increasingly used in the hole-making of hybrid materials [25,26]. Moreover, three different drill geometries were tested: (i) a conventional drill geometry (herein referred to as CNV) with a 120◦ point angle, 30◦ helix angle, 20◦ rake angle and 10◦ clearance angle, refer to Figure 1d; (ii) a chip-breaking drill geometry (herein referred to as CBR) identical to CNV, with v-shaped grooves on the principal cutting edge periphery, refer to Figure 1e; (iii) a double-point angle tool (herein referred to as 2PA) also identical to the CNV with a 60◦ secondary point angle (2:1 ratio) and the same geometry as the previous tools, refer to Figure 1f.

**Table 1.** Mechanical properties of the AA and CFRP separate materials.


**Figure 1.** Experimental setup used in drilling operations: (**a**) Fibre metal laminate sequence and layer number of CFRP plies and AA sheets; (**b**) Clamping system and load cell assemblage with mounted FML strip; (**c**) Cross-sectional view scheme showing internal placement of back support; Drill tip geometry (detail B in **Figure 1c**) of conventional drill (**d**), chip-breaking drill (**e**) and double-point angle drill (**f**).

Despite the constant search for novel drill geometries capable of generating fewer defects, the conventional drill geometry (such as the CNV in the current study) still constitutes a widely employed solution, which in this work has been used for reference/control and comparison with other geometries. It is also relevant to note that their performance can sometimes match or exceed newer, more intricate geometries regarding drilled hole quality [27]. Diamond-coated double-point angle drills, such as the considered 2PA, can be effectively employed in hole drilling of CFRP materials given the consequent action of

lower cutting forces on the drill step (secondary cutting edge), that is mainly responsible for the final surface condition of the drilled hole [28]. Their overall good performance has motivated its study in fibre metal laminated hole drilling. With regard to chip-breaking features on drills (such as the considered CBR drill) the goal is to promote more efficient chip evacuation by creating grooves on the drill geometry (typically on principal cutting edge) capable of chip segmentation and width dividing, thus minimizing load and torque. Such concern is particularly relevant when drilling materials with thermoplastic resin (such as PA 6) which unlike the often employed thermoset resins (i.e., epoxy) promotes long chip morphology rather than fragmented chips. By dividing the chip width, a more convenient scenario of chip removal could be attained (i.e., more fragile resultant split chips, clogging minimization at flute). Moreover, in order to avoid excessive friction of the chip with the newly generated hole surface, the groove has been positioned at the cutting edge margin in order to act as a chip relief that tentatively minimizes delamination due to smaller chip-hole contact. Cutting parameters testing range was selected based on the literature and tool manufacturer indications for laminate materials. Table 2 illustrates the tested levels of each considered variable. The full combination of parameters was tested using a random order generated by the Response Surface Methodology (RSM) Design Expert 13 software. Moreover, the operative conditions' influence on cutting load, roughness and delamination was investigated through analysis of variance (ANOVA). In order to mitigate the occurrence of wear mechanisms, each drill performed a maximum of 20 holes.

**Table 2.** Variables and respective levels used for cutting parameter assessment in experimental drilling operation tests, considering a full factorial testing plan.


Delamination defects were observed through radiographic image analysis, using Satelec X-Mind X-Ray generator and a Kodak RVG 5100 digital sensor. For this, the samples were submitted to a diidomethane bath for a period of 30 min, which enables contrast creation between delaminated and non-delaminated zones. A fixed exposure time of 0.16 s and a radiographic contrast of 70 kVp were selected. The obtained X-ray images were post-processed (converted into binary maps) allowing for delamination assessment and quantification, using the the criteria shown in Equations (1)–(3), where: *Dmax* and *Amax* correspond to the maximum diameter of the delamination area and its area, respectively; *Ad* to the actual delamination area; *Dnom* and *Anom* to the nominal hole diameter and area, respectively. For the calculation of the delamination factors, a Matlab script capable of measuring *Amax* and *Dmax* from the previously generated binary maps was used. This method ensures control process repeatability and minimization of data analysis effort.

$$F\_d = D\_{\max} / D\_{nom} \tag{1}$$

$$F\_a = A\_d / A\_{nom} \tag{2}$$

Although the diameter-based (*Fd*) and area-based (*Fa*) delamination factors are the most employed criteria, they do not fully portray the drilled hole quality [11]. The diameterbased delamination factor (*Fd*) may account for the same delamination values (same maximum diameter around hole), for instance, in two very different scenarios: (i) whole delamination of a full annular section area or (ii) crack delamination of a crack (very small area). Similar interpretation errors can occur when considering an area-based delamination factor, given that (i) uniform damage and (ii) uniform damage with small cracks may result in the same area. In sum, whereas *Fd* accounts only for the delamination maximum extent in the radial direction, *Fa* cannot account for crack delamination, prone to occur in CFRP, as

only the area is used for its calculation. For this reason, an adjusted delamination factor *Fda*, proposed by Davim et al. [29] has been used. It tends to *F*<sup>2</sup> *<sup>d</sup>* values with uniformly distributed delamination and to *Fd* values when it is strongly directional, allowing for a more accurate estimate of delamination shape and its extension.

$$F\_{da} = F\_d + \frac{A\_d}{(A\_{max} - A\_{nom})}(F\_d^2 - F\_d) \tag{3}$$

With regard to roughness analysis, it was optically estimated using the 3D measurement system (Alicona Infinite Focus SL). A three-measurement average was calculated for each drilled hole.
