Impact of cooled compressed air and high-speed cutting on the drilling of hybrid composite-metal stacks

doi:10.21203/rs.3.rs-2089844/v1

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Research Article

Impact of cooled compressed air and high-speed cutting on the drilling of hybrid composite-metal stacks

https://doi.org/10.21203/rs.3.rs-2089844/v1

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published 16 Feb, 2023

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Hybrid composite-metal stacks (HCMS) are used to manufacture and maintain aircraft combining metal alloys and composite materials. Drilling stands out due to its high use for installing rivets in fuselages. This work sought to identify the influence of cooled compressed air and high-speed cutting (HSC) on drilling multi-material joints with different configurations. Among the factors observed are the composite type (carbon- or glass-fiber-reinforced polymer), joint type with 2024 aluminum alloy (simple or composed), presence or absence of cooled air, cutting speed (40 and 220 m/min), and feed rate (0.02 and 0.08 mm/rev) in HCMS drilling. Adjusted delamination factor, hole wall roughness, hole roundness, and thrust force were evaluated. The combined effect between cutting speed and cooled air did not significantly impact the defects generated, making it possible to use HSC without harming the joint. Cooled air showed a tendency to decrease defects in HCMS drilling.

Drilling hybrid composite-metal stacks

Cooled compressed air

High-speed cutting

Delamination

Hole quality

Drilling hybrid composite-metal stacks (HCMS) is challenging due to the fiber delamination, tool damage [1], and matrix damage due to thermal gradients from the machining process [2]. In addition, it is challenging to select cutting parameters to guarantee machining quality and surface integrity for all constituents [1][2][3]. The most common problems during the drilling process of hybrid materials are damage on the contact surface of the composite resulting from its interaction with formed chips from machining aluminum (continuous chips) or titanium (saw-tooth chips). Furthermore, the thrust forces generated during drilling may be able to separate the composite from the metal, contributing to the accumulation of metallic chips and fiber fragments at the metal-composite interface, in addition to the adhesion of metallic material along the entire cutting edge of the tool [1][2][4].

Delamination defects on fiber-reinforced polymers (FRP) directly affect the quality of the assembly of structures and decrease the mechanical performance of the parts, especially when subjected to cyclic loads (fatigue) and can be identified in the entrance and exit region of the cutting tool [5][6]. Delamination at the entrance can be considered an outcome of tip angle contact with the upper layers; at the exit, it is caused by the compressive effect exerted by the drill tip on the upper layers of laminate [7][8][9][10]. Different authors have concluded that the feed rate is the input parameter of the FRP drilling process that has the most influence on the delamination factor [7][11][12][13] and thrust force [11][14]. Other authors have verified the influence of the drill (diameter, geometry, and wear) on delamination and thrust force [1][5][6][10][15]. Additionally, the dimensional deviation is mainly affected by the drill point angle (σ) [5][6][7].

Crown burrs on metallic phases are produced using drills with a lower σ angle, as they cause greater force and plastic deformation in the central region of the hole. The larger point angle evenly distributes the efforts on the periphery of the holes, producing a total or partial fracture in the area, making the generation of smaller and uniform burrs (more easily removed). If this rupture is partial, hat burr occurs [14][16][17]. The evaluation of burrs generated between the input and output surfaces of the plates is essential to verify the occurrence of crushing, that is, the burrs formation at the holes interfaces because of the performance of the fiber-reinforced polymer (FRP) as support (backplate). During the drilling of the metallic phase, chips are generated, failures occur in the tool, and burrs are produced at the hole exit. In addition, there is challenging to select cutting parameters and optimized tools possible to ensure a high quality of the drill hole in dimensional terms and the integrity of all constituent materials [18][19]. Among the consequences of burr formation on the entrance and exit surfaces, plastic deformations depend on the material's ductility and surface heating. These are sources of dimensional errors, interferences, misalignments, and initial crack points [20][21][22].

Because of the low thermal conductivity of FRP composites, the hardness and abrasiveness characteristics of the reinforcement material cause greater friction by the tool/part contact, resulting in the tool heating (it dissipates around 90% of the heat generated in the machining) and, by, therefore, larger tool wear of the same. The impact on polymer matrix degradation, which occurs between 150°C and 250°C, reduces its thermal resistance [23][24][25]. Thus, it is essential to reduce the temperature in the cutting region. A commonly used device is the Ranque-Hilsch vortex tube, which uses only compressed air separated into two streams of different temperatures: hot air and cold air. Physically, the vortex tube consists of a cylindrical tube into which high-pressure gas enters radially, tangential to the inner face of this tube. Secondary gases leave the vortex tube in axial and opposite or coincident directions, depending on the type of construction used, with reverse temperatures [26][27].

Although some studies pointed out that cutting speed (v_c) did not influence the holes' quality, distinct research mentioned that a higher v_c, combined with a low feed rate (f), significantly improved this quality, both in micro geometric and macro geometric terms. This effect occurs due to the minor influence of the low f, including lower vibration generation and lower thrust forces. Thus, high-speed cutting (HSC) emerges as a solution. Using HSC allows greater productivity during the process, and the combination with cooled compressed air can bring multiple benefits to the hole quality when applied in multi-material joints [28][29].

Therefore, this work aims to identify the effects of cooled air combined with high-speed drilling on the through-hole process in multi-material joints formed by 2024 aluminum alloy and glass- or carbon-fiber-reinforced polymer (GFRP or CFRP).

Figure 1 shows the analyzes performed and the different experimental combinations, and Table 1 presents the factors of interest considered for the experimental setup. The specimens were distributed in the following configuration and respective joint thickness:

Table 1

Input parameters considered for the study.
Parameter	Low Level (-1)	High Level (+ 1)
Cutting speed (v_c)	40 m/min	220 m/min
Feed rate (f)	0.02 mm/rev	0.08 mm/rev
Composite type (ct)	GFRP (1.5 ± 0.1 mm)	CFRP (2.1 ± 0.1 mm)
Joint type (jt)	Simple (SJ)	Composed (CJ)
Tool type (tt)	85C (σ = 118°)	86C (σ = 130°)
Cooled air (ca)	Yes (Y)	No (N)

CP1: GFRP + Al 2024 (5.6 ± 0.2 mm);
CP2: CFRP + Al 2024 (6.2 ± 0.2 mm);
CP3: Al 2024 + GFRP + Al 2024 (9.7 ± 0.4 mm);
CP4: Al 2024 + CFRP + Al 2024 (10.3 ± 0.4 mm).

The plates were joined using Loctite adhesive based on ethyl-cyanoacrylate to avoid slipping during positioning and subsequent displacement on the holder. After adjustment in the centering holder, the joints are fitted using the screw fixing system of plate support. Posteriorly each drilling, the set is moved by 15 mm to perform the new drilling. The fastening system has slots so as not to obstruct the generation of burrs and the flow of chips and drilling residues.

The Design of Experiments (DOE) included two replicates (R1 and R2) and a full fractional factorial (2^k − 1), resulting in 32 holes for each replicate. Table 2 exhibits the input parameter combinations for each drilled hole generated. This experiment used 4-factor and 5-factor interactions for the experimental error term. Combinations of five factors were applied to determine the fractioning of the investigation. The confidence level adopted was 95%. Factors and interactions with P-value < 0.05 were considered significant. The experiments were randomized to reduce and minimize the noise levels associated with the investigation. Three holes were performed with each condition tested. The evaluation of the drilled-joints properties with these conditions was carried out to assess the impact of different levels of defects on the mechanical properties.

Table 2

Combinations of input parameters.
Drilled Holes		v_c (m/min)	f (mm/rev)	ct	jt	tt	ca
R1	R2	v_c (m/min)	f (mm/rev)	ct	jt	tt	ca
1	33	220	0.08	CFRP	SJ	85C	N
2	34	40	0.08	GFRP	CJ	85C	Y
3	35	220	0.02	GFRP	SJ	85C	N
4	36	220	0.08	CFRP	CJ	86C	N
5	37	220	0.08	GFRP	SJ	85C	Y
6	38	40	0.02	CFRP	SJ	86C	Y
7	39	220	0.08	GFRP	CJ	86C	Y
8	40	40	0.08	CFRP	CJ	86C	Y
9	41	40	0.02	GFRP	SJ	85C	Y
10	42	40	0.02	GFRP	SJ	86C	N
11	43	220	0.08	CFRP	SJ	86C	Y
12	44	220	0.02	CFRP	CJ	85C	N
13	45	220	0.02	CFRP	CJ	86C	Y
14	46	220	0.08	GFRP	SJ	86C	N
15	47	40	0.02	CFRP	SJ	85C	N
16	48	40	0.08	CFRP	SJ	86C	N
17	49	40	0.08	GFRP	SJ	86C	Y
18	50	220	0.02	CFRP	SJ	85C	Y
19	51	220	0.02	GFRP	CJ	85C	Y
20	52	40	0.02	GFRP	CJ	86C	Y
21	53	40	0.08	CFRP	CJ	85C	N
22	54	40	0.08	GFRP	CJ	86C	N
23	55	220	0.02	GFRP	SJ	86C	Y
24	56	40	0.08	GFRP	SJ	85C	N
25	57	40	0.08	CFRP	SJ	85C	S
26	58	40	0.02	CFRP	CJ	85C	Y
27	59	220	0.02	GFRP	CJ	86C	N
28	60	40	0.02	CFRP	CJ	86C	N
29	61	220	0.02	CFRP	SJ	86C	N
30	62	40	0.02	GFRP	CJ	85C	N
31	63	220	0.08	GFRP	CJ	85C	N
32	64	220	0.08	CFRP	CJ	85C	Y

The tests were carried out in the DMG MORI DMU 60 eVo linear universal machining center with a maximum rotation of 18,000 rpm. The runout was measured at each tool change and remained below 1.3µm. The data acquisition of thrust force was developed using the Kistler^® 9272 stationary piezoelectric dynamometer with a load capacity of 20 kN and a torsional moment of 200 N⋅m (Fig. 2b). The acquired force signals were conditioned by the Kistler^® 5070A charge amplifier and processed via Measurement Computing^® PCIM-DAS 1602/16 data acquisition board. A specimen fixing system in the dynamometer was specially developed to distribute the cooled compressed air at the entrance and exit of the hole (Fig. 2a). Sandvik-Precorp uncoated twist drills (85C with σ = 118°; 86C with σ = 130°) with split point (cross point) sharpening, helix angle λ = 27°, 6-mm diameter, and 110-mm length were used. The Eurotools Turbo Air FTA-12-MC vortex tube has a body length of 250 mm. According to the manufacturer, at 6 bar (600 kPa), the vortex tube generates a 22.5-m³/h flow rate and − 10°C air exit temperature. During the experiment, temperature and pressure checks were carried out before drilling each hole. However, the temperatures were kept between 1.8°C and 2.2°C. The pressure remained stable at 6 bar. It was waiting for 120 s between drilling the holes for temperature stabilization. The process temperature measurement was performed using a portable thermometer (Fig. 2c).

The Hommel Tester T8000 profilometer comprises the TkL300 probe with a tip radius of 5 µm and an angle of 90° with a reading speed of 0.15mm/s. The hole wall roughness measurement covered only the metallic phase (aluminum) and used a 0.8-mm sampling length (l_e). The evaluation length considered practical was 3⋅l_e (2.4 mm), plus 1.6 mm for the signal filtering [30] due to the metal's low thickness (4.1 mm). The roughness parameter considered was the average roughness (R_a).

The measurement of the hole roundness deviation was performed on the Mitutoyo Crysta-Plus M7106 coordinate measuring machine (CMM) using the GEOPAK V2.4 R14 software; in this case, were considered eight measuring points distributed along the drilling circumference. The roundness analyses were accomplished at the FRP and metallic phases. The roundness value was considered at the bottom sheet metal in the composed joints compared with the upper sheet in the simple joints. Jointly, scanning electron microscopy (SEM) images of the holes were performed at the Carl Zeiss AG EVO^® 50 equipment to identify fractures in the fibers, analyze the surface aspect of the matrix of composites and examine surface irregularities of holes in the Al 2024 alloy. In addition, elemental chemical microanalysis with energy dispersive spectroscopy (EDS) was performed to characterize the residues observed in the tools.

The contrast radiography was the non-destructive technique chosen to assess the extent of damage in the peripheral zone of the hole. Thus, it was employed the AnalaR NORMATOR diiodomethane contrast, the Kodak 2100 Intraoral X-ray equipment associated with the Kodak RVG 5100 digital radiography system, and the acquisition by the Kodak Dental Imaging Software. The exposure time for the tests was 0.16 s. Finally, the images were processed using the ImageJ public domain software. The use of contrasting liquid is commonly necessary because not all defects are visible on radiography, such as delamination, which does not change the composition or quantity of the material that the X-ray will traverse. The contrast must have a radiopaque character. The parameters of exposure time in the liquid solution and exposure time to X-rays contribute to the final result. The adjusted delamination factor (F_da), defined by Eq. 1 [9], was a result of the contrast present in the voids observed on the surface of the holes, where F_d is the ratio between the maximum diameter (D_max) of the laminated area and the nominal diameter of the tool (D_o). The composition of defects observed at the hole entrance, exit, and internally was observed by radiography [31][32].

$${F_{da}}={F_d}+\left( {\frac{{{A_d}}}{{{A_{max}} - {A_o}}}} \right) \cdot \left( {F_{d}^{2} - {F_d}} \right)$$

The analysis of the results focused on evaluating the different impacts of the factors on the output variables. They were divided into thrust force (F_t), delamination, hole wall roughness, and hole roundness. In addition, the tools used in the experiments were evaluated for possible wear. Lastly, an analysis of the cost associated with the process was performed.

3.1 Thrust force (F_t)

Table 3 shows the P-values obtained by the variance analysis (ANOVA) performed for thrust force (F_t). The main effects (f, tt, ca, jt, and v_c) were significant for a 95% confidence interval. Among the interaction effects, the significant factors were jt x ca, ct x ca, v_c x tt, and ct x tt. It is worth noting that R² = 99.1% was observed. The further factors and their interactions were not influential for a significance level α < 0.05. Figure 3 exhibits the main effects for F_t. Feed rate (f) was an expected significant factor over F_t, which several researchers indicate as the leading influence [33][34]. For the analysis of the thrust force (F_t), the lowest value obtained for the drilling condition with the lowest feed rate value (f = 0.02 mm/rev) is observed. The joint type (simple jt or composed jt) was greatly influenced by the significant increase in the specific cutting force because of the two layers of metal presented [35]. Due to the different drill tip angles (118º and 130°), the tool ends up contributing to a greater contact area during the drilling process, which would have influenced F_t [1][5][6][10][15], i.e., low thrust force values with 85C tool. The cooled compressed air (ca) contributes to F_t in helping to remove chips from the flutes, generating fewer obstacles in the tool feed during drilling. The reduction in thrust force (F_t) with lower feed rate values combined with higher cutting speeds was observed by Phadnis et al. [36]. The cutting speed (v_c) effect of decreasing F_t was probably due to the lower adhesion of aluminum on the chisel and the main cutting edges. With low v_c, the adhesion of aluminum promotes the generation of a built-up edge (BUE) and reduces the tool's cutting capacity, contributing to chipping and possible tool failure. In HSC, the BUE was not identified [37]. In the interaction jt x ca, differences in specific cutting force (K_f) are observed, combined with the characteristic of facilitating the chip removal and the matrix and fiber residues that tend to obstruct tool movement [38][39]. The interaction ct x ca can contribute to composite delamination, making it less susceptible to failures [21][40][41][42]. In contrast, v_c x tt is associated with different drill tip geometries, which affect the thrust force (F_t) [23][43]. Finally, the interaction ct x tt is influenced by diverse critical delamination forces (F_cr) that can influence and contribute to the higher axial loads [3][19]. According to Kumar and Sing [43], the thrust force and delamination are closely tangled; an increase in the thrust force (F_t) induces an increase in the delamination factor.

Table 3

P-values for main and interaction effects by ANOVA of F_t.
Main Effects	P-value	Interaction Effects	P-value
feed rate (f)	< 0.001	joint type (jt) x cooled air (ca)	0.002
tool type (tt)	< 0.001	composite type (ct) x cooled air (ca)	0.017
cooled air (ca)	< 0.001	cutting speed (v_c) x tool type (tt)	0.002
joint type (jt)	< 0.001	composite type (ct) x tool type (tt)	0.032
cutting speed (v_c)	0.002
R² = 99.1%

3.2 Adjusted delamination factor

Table 4 exhibits the P-values generated by ANOVA of the adjusted delamination factor (F_da). The main effects (P-value < 0.05) significantly influent over the adjusted delamination factor (F_da) were joint type (jt), composite type (ct), and cutting speed (v_c). In this case, the determination coefficient was R² = 77.93%. Figure 4 exhibits the main effects on F_da. Considering the composite type, the F_da observed in GFRP is significantly lower than the F_da in CFRP. This behavior was expected because different levels of delamination defects were obtained for each composite. The strength associated with the type of fiber used and the differences in the critical delamination force (F_cr) associated with the composite are distinct and may be related to damage (major and minor). Considering the Mode I energy release rate [8], the estimated F_cr is 63.26 N for GFRP [7] and 94.68 N for CFRP [9]. Another significant main effect, the highest cutting speed (220 m/min) promotes a higher F_da than the lowest (40 m/min). This effect can be associated with the friction generated in the cutting zone resulting in regions with overheating (surface burns) that are unwanted for the material depending on the proximity to the vitrification temperature of the composite matrix, which reduces its durability and stability [9][36][44][45]. According to joint types, in the composed joints, the F_da is significantly higher compared to simple joints. This difference can be associated with a burr formation at the exit of the metallic material positioned at the composite entrance, which exerts a compressive load on the FRP, increasing the delamination. The influences on F_da associated with the tool type (85C and 86C) were not significant for the main effect. Then, regardless of the drill type used, both statistically promote the same level of damage by delamination. The feed rate (f) and cooled compressed air did also not demonstrate significant effects; however, higher f (0.08 mm/rev) and drilling with cooled air tends to reduce F_da.

Table 4

P-values for main and interaction effects by ANOVA of F_da.
Main Effects	P-value	Interaction Effects	P-value
cutting speed (v_c)	< 0.001	feed rate (f) x tool type (tt)	0.004
composite type (ct)	< 0.001	feed rate (f) x composite type (ct)	0.008
joint type (jt)	< 0.001	composite type (ct) x tool type (tt)	0.009
		composite type (ct) x joint type (jt)	0.041
R² = 77.8%

Some combinations proved to be significant for the interactions of the two factors. The interaction f x tt showed that using the 85C drill, the increase in F_da occurs with the highest feed rate. When using the 86C drill, the F_da decreases with f = 0.08mm/rev. However, an inversion occurs with the lowest feed rate (f = 0.02mm/rev), i.e., lower F_da using the 85C drill and higher F_da with the 86C drill. These variations can be attributed to the differences in the drill tip angles (118° and 130°), as they cause distinct shapes of defects, as well as contrasts in F_t generated by different feed rates, as shown by diverse researchers [5][6][7][34][43][46]. The damage associated with the interaction between feed rate (f) and joint type (jt) was significant. For drilling the simple joint, there was an increase in F_da with increasing f from 0.02 to 0.08 mm/rev; on the other hand, F_da decreased with the growth of feed rate in the composed joint machining. However, the drilling of the composed joint with f = 0,02 mm/rev generated the highest F_da value, possibly due to chip flow and adhesion of aluminum on the cutting edges [47][48]. The increase in f during the drilling of CFRP promoted a rise in F_da. However, the increase in f reduced F_da when GFRP machining. In addition, the highest F_da value occurred with f = 0.08 mm/rev in CFRP drilling. This characteristic may be associated with the composite susceptibility in the contact between the tool and FRP, contributing to lower fiber and matrix pullouts of the GFRP compared to CFRP [23][24][25]. This behavior can demonstrate the lower incidence of this damage on GFRP due to its lower critical delamination force (F_cr) than CFRP. For GFRP, the limit is reached with minor load values, promoted by the lowest feed rate. When f increases, the thrust force increases, and delaminations tend to propagate more quickly, increasing F_da [29][34][49].

Considering the interaction between composite type and tool type, when drilling GFRP with the 85C drill, there was a lower F_da than with the 86C drill; however, a lower F_da was generated in cutting CFRP using the 86C drill rather than the 85C. This characteristic can be associated with the drill tip angle, making it more susceptible to defects [41][43]. For the reasons mentioned earlier, the highest F_da occurred during the machining CFRP in the composite joint due to higher F_cr; on the other hand, the lowest F_da occurred when drilling GFRP in the simple joint. The interaction between cutting speed and cooled air showed that, with lower v_c (40 m/min), there is a tendency for lower F_da. However, the F_da increased for the highest v_c (220 m/min), and the cooled air had no influence. This situation demonstrates the weak effect of v_c when combined with cooled air over the delamination, allowing the use of higher cutting speeds without harming the HCMS. Although not significant, there was a tendency to decrease the F_da with the presence of cooled air in all interactions. This reduction can be related to the thermal effects caused in the composite matrix, which may have improved its integrity and the hole quality at low temperatures since it prevented the fiber pullouts and contributed to lower delamination [23][24][25][49][50][51].

The cooled air-assisted drilling provided a more regular cutting operation when compared with machining without cooled air. The cooled air probably impacted the decrease of the tool/composite contact temperature resulting in better stability of the FRP during the cutting process. The absence of cooled air increased hole wall roughness and delamination, as observed in some papers [21][49]. In this case, the heat removal effectiveness in drilling (tool, material, and chip) reduces the high-temperature effects on the matrix and fibers [52]. Moreover, delamination gradually diminishes with the decrease in temperature [23][24][25][49][50][51][53]. The effects on the delamination reduction in CFRP drilling with cooled air are in line with those obtained by Abish et al. [40] and Joshi et al. [42]. Hoffmann et al. [50] also observed that the drilling process with cooled compressed air facilitates fiber breakage, as they reduce their resilience at low temperatures. These ruptures avoided the pullout phenomenon, generating lower roughness values (fiber pullout causes high cavities on the surface) but increasing delamination. Although these effects have also been observed in this work, they were not significant.

The defects generated by drilling HCMS are attributed to aluminum burrs at the exit of the joint's upper plate due to increased compressive load on the FRP. The plastic deformation caused by the burr formation harms the composite [9][34]. As a result, defects in the hole region increase. In many situations, the effects are caused by unwanted burrs, as indicated [14][17]. Figure 5 shows the SEM images of holes 43 (simple joint with 86C drill) and 64 (composed joint with 85C drill) generated in the cooled air-assisted drilling with v_c = 220 m/min and f = 0.08 mm/rev. For hole 43 (F_da = 1.12), no apparent surface defects on CFRP were observed in SEM analysis (Fig. 5a). The hole contour region (Fig. 5b) and the fiber cut (Fig. 5c) are regular. When analyzing hole 64 (F_da = 1.24), multiple defects were observed around the surface of the hole, generating superficial marks on the CFRP (Fig. 5d) in the contour of the hole (Fig. 5e) and the cutting fibers and matrix (Fig. 5f). Thus, the origins of possible failures were characterized by the fibers rupture and matrix degradation, which originated in the burrs in the composed joints [48]. Although the 86C drill tended to reduce F_da, this comparison was selected because the tool type (tt) was not significant for a 95% confidence interval.

Figure 6 exhibits the SEM images of hole 03 exit (GFRP with cooled air) and hole 50 exit (CFRP without cooled air) after drilling single joints using v_c = 220 m/min, f = 0.02 mm/rev, and 85C tool. By the overview of hole 50 (Fig. 6d) with F_da = 1.11, the detail of the fibers' shape on the circumference (Fig. 6e) was confirmed by the regular cut of the CFRP fibers (Fig. 6f). On the other hand, when compared with hole 03 (GFRP with F_da = 1.13), hole 50 displays some threads, irregular cutting (Fig. 6a), fibers' shape in the circumference (Fig. 6b), and matrix degradation (Fig. 6c), indicating the differences between the composites.

Besides the differences in fiber characteristics, as expected by the different critical delamination forces associated with each material. This fiber characteristic is attributed to the high contact of the drill tip with the joint surface, which impairs the integrity of the composite and results in higher fragmentation and thermal degradation. The delamination results were similar to those of other researchers [8][12][42][44]. Although the cooled air tended to reduce F_da, this comparison was selected because it was not significant considering P-value < 0.05.

3.3 Hole wall roughness

Table 5 presents the P-value of the main and combined effects obtained by ANOVA of average roughness (R_a), with a coefficient of determination (R²) of 78.3%. The cutting speed (v_c), feed rate (f), and tool type were significant; a lower v_c, a higher f, and 86C drill (σ = 130°) are associated with the lowest R_a values (Fig. 7). Since the analysis was in the metallic phase (aluminum), the composite type (ct) and the joint type (jt) did not influence the hole wall roughness. The v_c x f interaction was significant and increased the average roughness when growing v_c with lower and higher feed rates. However, R_a has a lower increase in drilling with f = 0.08 mm/rev than with f = 0.02 mm/rev. For the interaction v_c and tool type, R_a increased with v_c = 220 m/min regardless of the drill type. This same tool characteristic was observed by other researchers and was associated with tool wear through the adhesion (due to aluminum ductility) and abrasion (due to fibers of composite) mechanisms [1][5][6][10][29].

Table 5

P-value for the main and combined effects for average roughness.
Main Effects	P-value	Interaction Effects	P-value
cutting speed (v_c)	< 0.001	cutting speed (v_c) x feed rate (f)	< 0.001
feed rate (f)	< 0.001	cutting speed (v_c) x tool type (tt)	0.002
tool type (tt)	0.008
R² = 78,3%

Figure 8 shows the surface characteristics analyzed by SEM of hole 59 (v_c = 220 m/min and Al/GFRP/Al joint) and hole 10 (v_c = 40 m/min and GFRP/Al joint) at the entrance on the bottom aluminum plate when drilling with 86C tool and f = 0.02 mm/rev. Since de joint type was not significant, the evaluation of holes generated with different cutting speeds was possible. When analyzing the detailed surface aspect of holes 59 and 10, noticeable differences in surface quality are observed. The surface patterns can be compared qualitatively between hole 59 (R_a = 3.06 µm) (larger grooves and irregular points) and hole 10 (R_a =1.59 µm) (surface more cohesive and regular), demonstrating the R_a values obtained in these conditions. The results agree with other works [29][48]. They justified this relationship with the presence of superficial microchipping and burns on the tools and the facility of aluminum flow [40][54].

3.4 Hole roundness deviation

Table 6 exhibits the P-values generated by ANOVA of hole roundness deviation (t). Feed rate (f) and cutting speed (v_c) were considered significant factors for the metallic phase (R² = 47.9%). At the same time, joint type (jt), composite type (ct), and the combination jt x ct significantly influenced the t deviation in the composite phase (R² = 60.4%). The large residues could be affected the coefficient of determination (R²) in the roundness measurement. However, it was decided to leave them present in the analyses, which may contribute to the increase in R².

Table 6

P-value for the main and combined effects for roundness on metallic and composite phases.
Metallic Phase (t_metal)		Composite Phase (t_comp)
Effects	P-value	Effects	P-value
cutting speed (v_c)	0.005	joint type (jt)	< 0.001
feed rate (f)	0.038	composite type (ct)	0.011
		jt x ct	< 0.001
R² = 47,9%		R² = 60,4%

Figure 9 regarding the roundness deviations on sheet metal, there was a tendency to increase the t_metal when used higher v_c and a decrease with higher f. However, these deviations did not exceed 25 µm. This association of effects is supported by the work of further researchers [18][34][49]. The other main factors (joint type, tool type, composite type, and cooled air) were not considered significant for metal roundness, i.e., they do not impact positively or negatively.

The composite type (CFRP promoted an increase in the t_comp), and the joint type (composed joint induced a rise in this deviation) impacted the roundness of the composite (Fig. 10). The input factors v_c, f, tool type, and cooled air were not significant for the 95% confidence interval. However, with the highest v_c, higher deviations in roundness were observed but did not exceed 55 µm. Cooled air at different cutting speeds did not show a significant difference. Nevertheless, considering the lowest f, differences in the t_comp were observed with cooled air. Gaitonde et al. [12] observed an increase in roundness deviation with this situation. Hence, the most considerable deviations from roundness were observed with v_c = 220 m/min and f = 0.02 mm/rev, possibly due to the composite matrix damage caused by the higher friction associated with this combination, as demonstrated by other authors [28][55]. Giasin and Soberanis [53] observed reduced roughness values with lower temperatures in the process at different v_c, associated with higher stability (rigidity) on the composite matrix. In addition, the contraction effect on the composite fibers after the drill passed through the material contributed to the increase in roundness at lower f. Because of a longer contact time between drill and FRP, a bending occurred during the tool path through the fibers, causing an elastic deformation similar to buckling in this region. When the return to the initial position after shear failure ensued, the fiber promotes a tightening around the drill, resulting in smaller hole diameters than the tool size [49][55]. For the combined effects, roundness deviation increased when using the composed joint. There was a tendency to move the upper plate after the drill had passed, creating greater instability and contributing to the buckling effects in the composite drilling; this was more relevant when the layers were overlapped, as in the case. For simple joints, these effects were not observed.

3.5 Tool state analysis

The analysis assesses whether tool wear was (comparing the drills before and after HCMS drilling) and wear types observed (abrasion, adhesion, chipping, and others). Figure 11 shows the abrasive wear on the main cutting edge of the 85C drill. Another feature observed was the material adhesion on the chisel edge. EDS identified metallic residues, with aluminum being the predominant element (as expected). This adhesion compromises the tool cutting performance [47][48]. Figure 12 exhibits some metallic adhesions (residual aluminum confirmed by EDS) on the main cutting edge and the chisel edge of the 86C drill, as expected. However, no wear is observed, which could compromise the drilling process [45]. Due to the drilling direction from FRP to aluminum, the residual presence of composite elements was not observed on 85C and 86C tools [56].

3.6 Evaluation tests

In the assessment study, fixed factors were defined to evaluate the results. This limitation was established due to the small number of specimens and materials available. The basic parameters selected were: tool type (85C), joint type (simple), composite type (GFRP), and lower feed rate (f = 0.02mm/rev). The evaluation tests include the following variables for analysis: v_c = 220 m/min without cooled air (OTM 1), v_c = 220 m/min with cooled air (OTM 2), v_c = 40 m/min with cooled air (OTM 3), and v_c = 40 m/min without cooled air (OTM 4). For each condition, three holes were drilled, i.e., holes 1 to 3 (OTM 1), holes 4 to 6 (OTM 2), holes 7 to 9 (OTM 3), and holes 10 to 12 (OTM 4). The average of the values was considered for the generation of the comparative graphs. The response variables considered were maximum in the Al phase thrust force (F_t) during drilling GFRP/Al, adjusted delamination factor (F_da) at the GFRP hole, hole wall roughness (R_a), and roundness deviation (t_metal) at the Al 2024 hole. Table 7 displays the results measured after drilling with the evaluated parameters.

Table 7

Drilling parameters and output variables.
GFRP/Al drilling				GFRP		Al 2024
Test	v_c (m/min)	cooled air	F_t (N)	F_t,comp (N)	F_da	R_a (µm)	t_metal (µm)
OTM 1	220	No	99.3	25.39	1.103	3.52	16.3
OTM 2	220	Yes	106.4	27.02	1.093	4.16	20.4
OTM 3	40	Yes	104.8	36.78	1.055	1.53	12.5
OTM 4	40	No	93.8	35.53	1.094	1.10	10.5

The feed force (F_t) remained uniform during the tests, with few differences even using different v_c due to the constant lower feed rate (f = 0.02mm/rev). This factor contributes significantly to the higher F_t values generated in HCMS drilling [6][57]. However, for both the high v_c (220 m/min) and the low v_c (40 m/min), F_t was higher with the presence of cooled air (OTM 2 and OTM 3). This effect can be attributed to compressed air producing axial and radial loads during the drilling process [40]. This lower temperature influence on Al 2024 enhances the machining difficulty, increasing the cutting forces associated with the process. This association was made by other researchers [46][58].

Under all optimization conditions, the thrust force generated during the Al phase drilling is greater than the F_t developed in the GFRP phase. In the absence of cooled air, the growth in Ft was evident when the cutting tool crossed from the GFRP phase to the Al phase: the increase was 391% in OTM 1 (220 m/min) e 264% in OTM 4 (40 m/min). In addition, higher v_c provided lower F_t values. This characteristic can be attributed to the decrease in the adhesion of the metallic material on the cutting edges. As a result, the cutting capacity of the tool is reduced [37].

OTM 1 promoted the highest values of adjusted delamination factor (F_da = 1.103) obtained with higher v_c (220 m/min) without cooled air. On the other hand, lower values were observed (F_da = 1.093) with cooled air for the same v_c (OTM 2). For OTM 3 and OTM 4, relevant differences are observed for F_da (1.055 and 1.094) with the presence and absence of cooled compressed air, respectively, with lower v_c (40 m/min). In general, applying cooled air for both v_c positively impacted the F_da reduction. These results are in accord with distinct authors [40][41][42]. In addition, due to the few differences in delamination observed in the process with both cutting speeds, it is feasible to use higher v_c without harming the composite material in fiber cutting and preserving the polymer matrix. This aspect is evidenced when holes associated with OTM 1 and OTM 4 are compared.

Regarding the hole wall roughness recorded, higher R_a values were observed with higher v_c, as expected (OTM 1 and OTM 2). The presence of compressed cooled air did not show a significant difference, despite presenting the highest R_a value with cooled air-assisted drilling (OTM 2) compared with drilling without cooled air (OTM 1) for high v_c (220 m/min). The same occurs with low v_c (40 m/min) when drilling with and without cooled air (OTM 3 and OTM 4). Despite contributing to positive thermal effects in the evaluation of the composite phase, the presence of cooled air does not significantly influence the surface characteristics of the metallic phase, as seen in the ANOVA analyses in previous experiments [4]. Other researchers have observed increased roughness values, even applying lubricating/cooling techniques [59].

For the hole roundness measured, higher t_metal values were observed with high v_c, as expected and indicated in the previous analyses. The highest roundness value (OTM 2) was observed when cooled air-assisted drilling compared to this deviation formed without cooled air (OTM 1). However, the presence or absence of cooled air was not significant. These variations could be attributed to process temperature, tool wear, lack of rigidity of the fastening system, among others [2][60].

To define the best drilling condition in the GFRP/Al joint, a higher weight (2) was considered for delamination in the GFRP phase and lower weights (1) for thrust force, hole wall roughness, and roundness deviation in the Al phase. Thus, despite obtaining the highest values of F_t (104.8 N) and F_t,comp (36.78 N), the best drilling condition was defined as the OTM 3 (v_c = 40 m/min with cooled air). OTM 3 provided the lowest adjusted delamination factor (F_da = 1.055), the second lowest average roughness (R_a = 1.53 µm), and the second lowest hole roundness deviation (t_metal = 12.5 µm). The reduction of these values directly contributes to decreasing defects expected for composites and good levels of finish, which provide better fixation and adjustments [1][2][3][5][6].

To evidence it, Fig. 13 shows the differences in the fracture of the GFRP fibers for high and low cutting speeds in comparing the hole exit with the presence and absence of cooled compressed air. Considering OTM 1 (Fig. 13a), regular cuts and minor delamination on the hole's surface are observed in hole 3 (v_c = 220 m/min). Meanwhile, hole 10 (v_c = 40m/min) by OTM 4 shows regular fiber cuts and preservation of the polymer matrix. This situation points to low significant differences for both v_c in the drilling process of the GFRP/Al simple joint. Regarding cooled air application (Fig. 13b), for OTM 2, regular cuts and smaller delamination are observed on the surface of hole 5 (higher v_c) and hole 7 (lower v_c) with OTM 3 displays steady fiber cuts and preservation of the polymer matrix. Again, no significant differences are evident for the different cutting speeds used.

This work investigated the effects of cooled compressed air associated with high and low cutting speeds on drilling hybrid composite-metal stacks (HCMS). The main conclusions are pointed out as follows.

The different composite types (glass- or carbon-fiber-reinforced polymer) were significant for the defects observed in the process.
The joint type (simple or composite) strongly influenced the delamination of composite materials. Composed joints (Al/FRP/Al) significantly enhance roundness deviation and delamination.
The cutting speed (v_c) effect proved significant for the adjusted delamination factor (F_da). However, its combination with cooled air did not significantly differ, allowing the highest v_c (220 m/min) to be used without impairing the HCMS drilling. Although not significant, the effect of cooled air can be considered adequate for not heating the composite matrix, preserving its strength, conserving the fibers, and contributing to lower F_da.
The low influence of v_c on roundness deviations contributes to obtaining an IT7 dimensional tolerance with higher v_c, enabling their implementation in processes with this limitation and improving productivity.
The hole wall roughness is dependent on v_c in the Al phase. It recommends using a lower v_c (40 m/min) when this factor becomes essential.
High and low feed rates (0.02 and 0.08 mm/rev) had little influence on the hole quality.
Lastly, the combined response variables show that cooled air-assisted drilling with the 85C tool (σ = 118°), v_c = 40 m/min, and f = 0.02 mm/rev produces the best hole quality in GFRP/Al simple joint.

Acknowledgments

The authors thank the Sandvik Coromant for donating drills, the GEA-EESC-USP for the CFRP and GFRP plates, and the Embraer for the Al 2024 plate. The authors also thank the CIDEM-ISEP and INEGI-FEUP (Portugal) for the availability of equipment and systems, the CMM-UFRGS for valuable aid to image processing (MEV/EDS), and the Micromazza Co. for roundness error tests.

Statements

Ethical Approval: The manuscript is original and has not been submitted for publication elsewhere (partially or in full). Also, the manuscript has not been submitted to more than one publication for simultaneous consideration.
Consent to Participate: Not applicable (this research did not involve human subjects).
Consent to Publish: Both authors consent to the manuscript's publication in the IJAMT, should the article be accepted by the Editor-in-chief upon completion of the refereeing process.
Authors Contributions: Each author contributed to the research presented in this manuscript, approved the contents now presented, and agreed to the compliance with ethical standards.
Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Competing Interests: The authors also declare that they have no competing financial interests or personal relationships that could have influenced this research.
Availability of Data and Materials: The manuscript has no associated data in a data repository.

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Impact of cooled compressed air and high-speed cutting on the drilling of hybrid composite-metal stacks

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Abstract

Figures

1 Introduction

2 Materials And Methods

3 Results And Discussions

3.1 Thrust force (F_t)

3.2 Adjusted delamination factor

3.3 Hole wall roughness

3.4 Hole roundness deviation

3.5 Tool state analysis

3.6 Evaluation tests

4 Conclusions

Declarations

References

Status:

Journal Publication

Version 1

Impact of cooled compressed air and high-speed cutting on the drilling of hybrid composite-metal stacks

Status:

Journal Publication

Version 1

Abstract

Figures

1 Introduction

2 Materials And Methods

3 Results And Discussions

3.1 Thrust force (Ft)

3.2 Adjusted delamination factor

3.3 Hole wall roughness

3.4 Hole roundness deviation

3.5 Tool state analysis

3.6 Evaluation tests

4 Conclusions

Declarations

References

Status:

Journal Publication

Version 1

3.1 Thrust force (F_t)