Experimental investigation of the effective parameters and performance of different types of structural tools in drilling CFRP/AL alloy stacks

Abstract

In the modern aerospace industry, the application of hybrid multilayers (CFRP)/aluminum7075 stacks to manufacture various parts of modern aircraft fuselages is increasing in order to improve the quality of the structure. This study investigates the effective parameters when drilling carbon fiber composite (CFRP) and aluminum alloy 7075 stacks with different tool geometries/materials to compare dry machining with the cooling method and investigate the effect of cutting input variables of the drilling operation on machining results. With ANOVA analysis, the most influential parameter for the drilling operation of this kind of multilayer stack was determined as the feed rate. According to the experimental results, the surface roughness of the cooling system drilling was 38% lower than the average roughness of the other two tools (B and C). Moreover, the tool’s geometry and the cooling conditions (friction) have a greater influence on the drilling performance than the spindle speed and feed rate.

Keywords

drilling CFRP/Aluminium7075 stacks cooling system delamination thrust force ANOVA

Introduction

In recent years, composite materials made of carbon fiber (CFRP) and laminated aluminium are used widely in aircraft wings and tails. In the aerospace industry, lightweight parts and improved mechanical properties play an essential role and are of paramount importance. A CFRP/Al hybrid offers the benefits of both materials while minimizing their limitations. As an example, CFRP has high specific stiffness, good corrosion resistance, and excellent fatigue strength. The prevailing airframe manufacturing processes require the production of large numbers of rivets or bolt holes on the body materials. To ensure hole quality and boost productivity simultaneously, the two parts are drilled individually and then assembled instead of drilling and assembling separately.¹ Hybrid materials can be used for fuselage or wing box assemblies, motor cowlings, fairings, wing panels, etc. Within the next decade, they will become more widely used.² In the following paragraphs, the machining literature for composites is reviewed first, followed by aluminium and then multilateral machining and its significance.

Drilling composite/metal stacks is challenging as the machinability characteristics of each are different. As a consequence, various hole defects often occur in the assembly process, severely reducing accuracy.³ The quality of holes directly affects the flight performance and durability of the aircraft, as statistics show that 60% of the rejections occur because of defective holes.⁴ CFRPs are relatively non-machinable due to their abrasive character, causing tools to wear, and having the potential to experience drilling-induced damage, mainly delamination, due to their nature. A workpiece damaged during machining can affect the component’s service life.⁵ Aluminum alloys have low elastic modulus and melting points that make them prone to adhesion on cutting edges, rake faces, and flutes of drills. Furthermore, high-temperature aluminum chips passing through the CFRP layers in CFRP/aluminum alloy stacks can deteriorate hole quality.⁶ Several researchers have indicated that most of these problems arise from choosing non-optimized process parameters and tool structures^7,8 illustrated that the use of double cutting angle tools improved self-centering ability, reduced fracture of a tool, and increased drilling tolerances in multilayers stacks.⁹ Studied cutting response in the drilling operation of CFRP/Aluminum stacks using different cutting tools, including TiAlN coated DLC tools. Their results revealed that the hole diameter of entry is larger than exit and that thrust force and delamination factor have a positive correlation.¹⁰ investigated the drilling of CFRP plates of different thicknesses and found that chip congestion during drilling caused high tool wear and increased drilling output force.¹¹ It was found that both thrust force and torque in the drilling operation of CFRP/Aluminium stacks will increase with feed rate but decrease with spindle speed. Furthermore, when drilling Aluminium compared to CFRP, the thrust force and torque are doubled at 0.05 mm/rev but tripled at 0.1 mm/rev and 0.15 mm/rev. This is due to the high impact of the fibers and the cutting tools with lower effective clearance angles, which create friction between the combined CFRP/aluminum plates.¹² examined the impact of point angles on drilling CFRP and concluded that a lower point angle would lower the feed force.¹³ The effect of flute number and step bit geometry on the machining of CFRP/Ti6Al4V stacks was examined. According to the results, the step-drill bit with three flutes contributed to slow wear, smaller cutting output forces, and reduced damage to the hole.

Several studies have been done to analyse the effect of cooling and lubrication systems on drilling output parameters. The cutting temperature has a high effect on the quality of the holes and tool wear in the production of holes. Several factors have an undeniable influence on this hybrid stack drilling such as the stack sequences, spindle speeds, feed rates, cooling conditions, and tool cutting edge structure influence.¹⁴ A lack of cooling in dry machining usually leads to high process temperatures, thus causing thermal damage to the workpiece. Combined with the lack of effective flushing, high temperatures can increase the risk of metallic chip accumulation in the chip flute.¹⁵ an investigation of CFRP/aluminum stack drilling under a variety of cooling conditions was carried out. When compared to drilling in dry conditions, essentially the hole quality aspects are improved with MQL system, which has an efficiency of the same quality as a flood coolant system.¹⁶ According to the study, cryogenic drilling of CFRP/Ti stacks reduced thrust, torque, and hole diameter error, but burr height was higher than under wet conditions.

In order to understand CFRP/Al7075 cutting physics and improve drilling technology, considerable effort has been conducted. However, no detailed studies have been conducted on the drilling of CFRP/Al7075 stacks using different drill structures and cutting environments, optimum conditions for hole quality and still even many problems still exist such as hole quality, including the poor hole surface roughness, delamination of composite, and tool wear. A particular focus of this study was the comparison of several proposed tools in different investigations, which have different mechanisms and also the highest efficiency in terms of hole quality generation and machining forces generated. In addition, the influence of drilling machining input cutting variables such as tool geometry, spindle speed, and feed rate on machining results such as thrust force, delamination, and surface roughness is investigated. It allows for accurate predictions and scientific understanding of hole quality generation, in terms of delamination and fiber pull-out, and thrust force for the drilling operation of this multilayer stack.

Workpiece material and cutting tools

In this study, the workpiece specimens used are multilayer stacks constructed by carbon/epoxy which carbon fiber reinforced polymer on top of aluminum 7075 alloy was hand layup. The composite material will delaminate less with this configuration. The aluminum alloy used in this study is extensively used in the Industrial aerospace have referenced as Al 7075. Composite is made of carbon fiber reinforced plastic (CFRP), with h = 0.2 mm for each layer and 0^o, 90^o for the layer sequence to achieve a quasi-isotropic structure. Stack workpiece was a rectangular plate with a length of 170 mm, a width of 100 mm, and a total thickness of 14 mm (7 mm thickness for the CFRP board and 7 mm thickness for the Aluminium7075 sheet). Table 1 Presents the fundamental mechanical and physical characteristics of both materials. Drilling experiments have been carried out on CFRP/Al stack using three 6 mm diameter drills, as shown in Figure 1. Selection of this diameter was according to the aircraft’s industrial requirements. The first drill, designated drill (A), is a TiAlN-coated solid carbide drill with a helix angle of 30ᵒ and a point angle of 140ᵒ. It features an internal coolant supply designed for higher feed applications and metal/composite stacked materials, as well as an optimized flute design for maximum chip evacuation. A second drill, designated drill (B), is a cobalt end mill designed for machining difficult CFRP (carbon fiber reinforced plastic). The last drill, designated drill (C), is a two-stepped drill whose geometries have been studied. Using step drills has the potential to improve surface quality, tool wear, and diameter tolerance when drilling CFRP/Al stacks.¹⁷

Table 1.

Mechanical properties of CFRP/Al7075 materials.

CFRP		Al7075
Tensile strength	3800 Mpa	Tensile strength	503 MPa
Young modulus	62 Gpa	Young modulus	73 GPa
Shear strength	75 Mpa	Poissionʾs ratio	0.33
Glass transition	170ᵒC	Ult strength	572 MPa

Experimental setup

As part of the experiment plan, we used three factors at three levels. Levels refer to the values taken by the factors. The experimental setup in Figure 2 Shows CFRP/Al stacks fixed on the top of a dynamometer on the table of a vertical milling machine. The choosing of appropriate input machining parameters is critical to achieving high-quality results. We selected three ranges of feed rate and spindle speeds to suit the requirements of CFRP and aluminum. The summary of experimental conditions can be found in Table 2. A JOHNFORD VMC 600 three-axis vertical milling machining center with a 5.6 k_W spindle drive was used for the drilling tests. During machining, the thrust force was measured by a Kistler 9255 piezoelectric dynamometer. Drilling forces signals were converted to voltage signals via a multi-channel charge amplifier (Kistler 2111E type) connected to a (Dewe43-A) data acquisition device. The surface roughness (R_a) of the hole wall was obtained by surface roughness tester (Marsurf PS10) with a scan length of 0.5 mm.

Table 2.

Experimental parameters.

Cutting parameter	Level 1	Level 2	Level 3
Spindle speed (rpm)	1000	2000	3000
Feed rate (mm/rev)	0.02	0.05	0.08
Tool diameter (mm)	6	6	6

The spindle system of the machining center does not have an internal cooling function. To use the tool (A) with an internal cooling supply, a tool holder was designed in which the flow to the outer surfaces of the drill is directed from the outside of the spindle by the high pressure of a pump, as shown in Figure 3. The designed holder consists of two parts, a shaft rod, and a shell. The function of the shaft rod is to transmit the rotary motion from the spindle to the tool and an application of the shell is to hold the fluid transmission connections between the pump and the drilling tool. In machining processes, flood coolant is probably the most commonly used method to control heat and chips and provide lubrication. Its main advantages are its versatility in numerous cutting operations and its ability to dissipate heat and remove crisps from the cutting zone.

Results and discussion

Effect of cutting variables on thrust force

Thrust force is one of the most important response variables studied in the present study. In addition, the most critical type of damage, this is because the drilling-induced delamination is directly connected with the machining thrust forces. It is well established that thrust force is affected by several aspects, such as tool coating, tool geometry, workpiece material, lubrication-cooling technique, cutting parameters, etc. Studies have been conducted on the effects of drilling process input variables and cooling conditions on drilling forces. An abrupt change in machining forces during a drilling operation can negatively affect the drill bit performance and the quality of the hole, especially in the composite layers. The differences in the forces can be explained by the difference in specific cutting pressure generated between the workpiece material and the drill.¹⁸ The aim of the (ANOVA) statistical analysis is used to determine which variable during drilling most affects the drilling performance. (ANOVA) made the percentage contribution of each input parameter using equations (1)–(4), and using the software spss24:

p = \frac{S S A}{S S T + S S B + S S A} * 100

(1)

Each individual (SSA or SSB) is divided by the total (SS) and multiplied by 100 to obtain the percent contribution.

S S A = \sum_{j = 1}^{b} n_{j} {(\bar{y_{j}} - \bar{y})}^{2}

(2)

S S B = \sum_{i = 1}^{a} n_{i} {(\bar{y_{i}} - \bar{y})}^{2}

(3)

S S T = \sum_{i = 1}^{a} \sum_{j = 1}^{n} {(y_{i j} - \bar{y})}^{2}

(4)

ANOVA predicts the optimal combinations of process parameters. According to ANOVA analysis for the machining force of CFRP by tools (A), (B) (Table 3), illustrated that the feed rate (54%) made the largest contribution compared to spindle speed (23%). Feed rate has a high effect on the thrust force of aluminum (20%), while the effect of spindle speed on the thrust force is (14%) which indicates the thrust force in the composite layers is more controllable compared to the aluminum layer. In Figure 4 We show thrust force as a function of feed rate after machining aluminum and composite separately, under the dry conditions with a drill (B) and an internal cooling system with a drill (A). According to the results, the thrust force that measured in the Aluminum7075 layer is at least three times higher than the thrust force of the CFRP layer. Various chip removal mechanisms and different mechanical properties of each phase can be the main reason for this phenomenon. It should be noted that the magnitude of specific cutting energy for both stacked phases is much smaller under the cooling conditions of the tool (A). Regarding the parametric effects of machining, in both cutting environments, linear increases in machining forces can be seen with growing up the proportion of feed rate in each material phase. This shows that the feed rate factor significantly influences the force generated during the machining of these kinds of stacks. The phenomenon occurs due to the enlarged cross-sectional area of un-deformed chips as feed rates rise, while spindle speed generally reduces thrust forces. In both machining environments, increasing the proportion of spindle speed certainly reduces thrust force values, regardless of the composite and aluminum 7075 materials. This occurs due to the softening effect of increased speed on the workpiece sample, which results in a reduction of cutting resistance, allowing the more softened chips to be removed. The maximum and minimum values of thrust forces were determined during the drilling operation with tool (B) spindle speed 1000 rpm and

f

= 0.08 mm/rev and tool (A) 3000 rpm and

f

= 0.02 mm/rev, respectively. This was due to the cutting edge structure and point angle of tool B (end mill) causing the poor evacuation of the continuous Al chips which increased the possibility of chips clogging the flutes during the aluminum phase machining. The difference of thrust force generated between tools (A) and (B) reaching 91.96% during drilling at the aluminum phase and this was possible because of the cooling system which extremely reduced the chips abrasion between drill flutes and the machined surface. This occurs because the frictional forces acting on the composite hole wall are reduced, as demonstrated by the following analysis of the surface macrography of CFRP holes. To increase drilling efficiency, it is suggested to use a cooling system at a higher spindle speed, but the feed rate must not exceed 0.08 mm/rev for the purpose of prevent chip congestion in drill flutes.

Table 3.

ANOVA for thrust force of CFRP and Al.

Source of variance	Sum of squares	Df	Variance	Test F	F	P*
Thrust force-CFRP
F.R (mm/rev)	5629.468	2	2814.734	15.776	0.000	54.16
S.S (rpm)	2444.630	2	1222.315	6.851	0.009	23.52
Error	2319.380	13	178.414	‐	‐	22.31
Total	52656.413	18	‐	‐	‐	‐
Thrust force-Al
F.R (mm/rev)	53805.180	2	26902.590	15.776	0.170	45.84
S.S (rpm)	37140.226	2	18570.213	6.851	0.280	31.64
Error	171710.003	13	13208.462	‐	‐	22.50
Total	1454314.829	18	‐	‐	‐	‐

The drilling operation of hybrid Composite/Aluminum stacks, the different properties of both materials, and the two-step configuration of the drill causes the drilling process to go through a series of unstable stages. The forces are reflected by the signals, and this sequence must be comprehended for the purpose of correlating force data with cutting phenomena.¹³ the different stages of drilling operation with a tow-step drill bit are described comprehensively. For the stepped geometry tool (c), thrust forces are identified at five stages over drilling time, as indicated in Figure 5. The corresponding simplified design for this drilling process can be seen. Assuming a one-shot operation, it is possible to divide the whole composite fiber carbon/Al stack drilling operation into five stages:

• Stage 1: The first phase starts when the cutting edge contacts the CFRP for the first time and continues until the first cutting edge is completely engaged. The thrust force is also steadily increased from zero.

• Stage 2: In this stage, both cutting edges are involved in the machining of the CFRP/Al interface and the first cutting edge of the tool passes from the complete machining of the CFRP phase to the machining of aluminum phase. The moment the tool touches the aluminum plate for the first time, an increase in cutting force can be seen. After the first cutting edge of the tool has penetrated 3 mm into the aluminum and the shear force has been recorded, the second cutting edge of the tool enters the composite phase. During this stage, the highest cutting forces will be experienced.

• Stage 3: Describes a phase where the drill bit progressively exits the aluminum plate, whilst 3 mm of the second step continues in machining CFRP. As a result, parameters of the first cutting edge of tool in aluminum will decrease to zero whereas the second cutting edge will remain at the CFRP.

• Stage 4: this stage indicated a stable machining whereat the drill bit flute of the second step are in the drilling process of aluminum.

• Stage 5: indicates the period of time in which the major cutting edge gradually leaves the aluminum plate. Thrust force will decrease until the completion of the drilling operation.

In Figure 6. The thrust force generated during the drilling of two layers was illustrated. Thrust force evolution results based on feed rate in three different spindle speeds show that the feed rate has a positive effect on the magnitude of the thrust force. It is because of the increased chip thickness associated with high feed rates, which increases the deformation forces of the chip, especially in the Al7075 phase. Moreover, step geometry thrust force is lower than that of the twist tool because of the smaller effective diameter, declining by 23% in the first step and 71% in the second step compared to the twist tool. This special geometry eliminates part of the damage generated in the first section by removing the material from the second section, it was also reported by.¹⁹

Hole wall roughness

All surface roughness measurements were performed separately for the two materials (CFRP phase and Al phase). The roughness of composite machined surfaces is determined by the stylus path with respect to fiber direction since the fiber direction can change between layers.²⁰ Roughness measurements in FRP are less reliable than those in metal, due to protruding fiber tips can affect the results or make them very variable in reading. Due to this reason, the roughness value was measured more than once and the average was taken. Machining fiber-reinforced composites are difficult due to the different properties of the fibers and the matrix, as well as the inhomogeneity of the matrix and the high volume fraction of hard abrasive fibers. Most studies on fiber reinforced polymer machining show that minimizing surface roughness is very challenging. The maximum acceptable roughness in aerospace is currently 1.6 μm for aluminum and 3.2 μm for CFRP.²¹ In the aluminum phase, the roughness increases slightly but remains within acceptable limits. Analyzing the variance (ANOVA) for the surface roughness of CFRP by tools (A), (B), (C) in (Table 4), it is found that spindle speed has a contribution of 25% followed by feed rate of 4%. The result shows that the cutting variables have a very small effect on the variance of surface roughness in the CFRP phase. It can be concluded that the surface roughness or delamination in the composite layer depends on another variable of the drilling factors, e.g. the flute geometry of the cutting tool.

Table 4.

ANOVA for surface roughness of CFRP and Al.

Source of variance	Sum of squares	Df	Variance	Test F	F	P*
Surface roughness-CFRP
F.R (mm/rev)	7.351	2	3.676	2.218	0.133	16.29
S.S (rpm)	1.311	2	0.655	0.395	0.678	2.90
Error	36.461	22	1.657	‐	‐	80.80
Total	221.861	27	‐	‐	‐	‐
Surface roughness-Al
F.R (mm/rev)	0.088	2	0.044	0.654	0.530	4.16
S.S (rpm)	0.544	2	0.272	4.046	0.032	25.77
Error	1.479	22	0.067	‐	‐	70.06
Total	15.117	27	‐	‐	‐	‐

Figure 7 Shows the evolution of Ra as a function of feed rate and spindle speed for Al and CFRP layers separately by tools (A) (B) (C). According to the results, the aluminum phase has much lower surface roughness than the carbon fiber composite phase. This may be due to the isotropic nature of the composites. In addition, the effect of machining variables on the surface roughness of the aluminum layer is less than that of the composite layer. The results dedicate that low surface roughness was measured for the first holes. While, as the number of holes increases and the feed rate approaches 0.08 mm/rev, an increase in surface roughness is observed. This is mainly due to the more significant impact of the fibers, which is associated with a higher feed rate when drilling the CFRP layer. The epoxy shear strength is reduced when drilling at high speeds due to the increase of temperature in the drill bit cutting edge. Increasing the number of drilling operations at high spindle speeds will result in chip congestion and reduce free edge angles at the cutting flutes of the tool. When the sharpness of the tool cutting edge becomes too low, the drill can no longer cut through the carbon fibers, and these fibers are pulled out in the cutting direction which will causes damage to the hole wall surfaces. Based on the results, tool (A) with an internal cooling system, the Ra values of CFRP phase (0.99 - 2.5 μm) and Al phase (0.46 - 0.68 μm) are comparable to the and partially below the required Ra criterion of 3.2 μm. Furthermore, the results showed that at high spindle speeds of 2000 and 3000 rpm, the surface roughness decreases with increasing feed rate. This phenomenon is caused by the interal cooling system designed in this research to reduce and control the heat in the cutting area. The high pressure of the coolant breaks up chips and pushes them up through the flutes and out of the hole. However, in this study, we show that a solid carbide tool coated TiAlN with an internal cooling system is able to produce more smooth surfaces on both aluminum and CFRP layers.

Delamination damage analysis

A critical aspect of improving the service performance of composite components is the assessment of delamination during drilling. Composite delamination occurs when there is an interlaminar debonding between two adjacent layers and is considered the most crucial damage since it negatively impacts composite plates assembly performance and fatigue life.²² Damage around the holes was measured with a Mitutoyo TM-500 microscope, as shown in Figure 8. We measured the delamination factor value (F_d) after measuring the maximum diameter (D_max) around each hole in the damage zone. A damage zone’s maximum diameter (D_max) is compared with the hole’s diameter (D) to calculate this factor. The equation below can be used to compute delamination factor (F_d):

F_{d} = \frac{D_{\max}}{D}

(5)

A damaged hole has a maximum diameter of (D_max) and a diameter of (D) in micrometre. An analysis of variance (ANOVA) of the CFRP laminate data is performed, with the objective of analysing the influence of each tool on the delamination factor (F_d).

A significance level of 5% is used for this analysis (i.e., a level of confidence of 95%). It’s found that except tool (c) with F_α = p > 0/05 (p = 0/0023) (p = 0/008) shown in the Table 5, other tools didn’t reach the level of significance. We can observe that the fiber delamination generated by the tool (A) and cooling system has the lowest reaction value to the cutting variable. In contrast, the delamination has increased with the increase in feed rate and the number of holes, especially for drill (B). This phenomenon may be due to the geometrical difference between the two types of drills and the friction conditions. Figure 9 shows the parametric effects on the delamination factor (F_d) caused by drilling according to different drills. In general, delamination increases with feed rate in all cutting environments, which is in agreement with the findings of²³ and it is related to delamination damage formation mechanisms. The thrust force of a drilling operation typically causes delamination damage, which leads to interlaminar debonding between adjacent plies of a composite, resulting in push-out delamination around the hole. Delamination damage becomes more severe as thrust force increases. This phenomenon also explains the reason for the delamination trend results concerning the machining parameters in the cooling conditions by the tool (A) is much lower than the results obtained in the dry drilling conditions. In the composite phase, when the feed rate reaches the maximum value of 0.08 mm/rev, the delamination coefficient generated by the step drill and milling cutter gets worse because the thrust force attains the top level in the composite phase. This phenomenon can be seen clearly in the next section (hole edge quality) Figure 10 that delamination occurs along a radial direction and extends into a large region around the delamination. Considering the delamination damage that can be generated and the results, tool (A) TiAlN-coated solid carbide drill with cooling conditions is suitable for drilling CFRP/7075 stacks.

Table 5.

ANOVA for delamination factor of CFRP (tool C).

Source of variance	Sum of squares	Df	Variance	Test F	F	P*
F.R (mm/rev)	0.006	2	0.003	11.274	0.023	22.127
S.S (rpm)	0.011	2	0.006	20.564	0.08	41.127
Error	0.001	4	0.000	‐	‐	‐
Total	11.140	9	‐	‐	‐	‐

Figure 9 shows the parametric effects on the delamination factor (F_d) caused by drilling according to different drills. In general, delamination increases with feed rate in all cutting environments, which is in agreement with the findings of²³ and it is related to delamination damage formation mechanisms. The thrust force of a drilling operation typically causes delamination damage, which leads to interlaminar debonding between adjacent plies of a composite, resulting in push-out delamination around the hole. Delamination damage becomes more severe as thrust force increases. This phenomenon also explains the reason for the delamination trend results concerning the machining parameters in the cooling conditions by the tool (A) is much lower than the results obtained in the dry drilling conditions. In the composite phase, when the feed rate reaches the maximum value of 0.08 mm/rev, the delamination coefficient generated by the step drill and milling cutter gets worse because the thrust force attains the top level in the composite phase. This phenomenon can be seen clearly in the next section (hole edge quality) Figure 10 that delamination occurs along a radial direction and extends into a large region around the delamination. Considering the delamination damage that can be generated and the results, tool (A) TiAlN-coated solid carbide drill with cooling conditions is suitable for drilling CFRP/7075 stacks.

Hole edge quality

An important factor in determining the accuracy of the assembly is the quality of the hole surface. In the drilling process, the quality of the machined hole wall is a direct reflection of the friction between the tool, the chip, and the hole wall. The results showed that the CFRP entry was more susceptible to damage propagation. Therefore, the hole edge quality evaluations were performed only on this side to observe the extent of the defect at the hole entrance and Hole quality is evaluated based on fiber pull-outs and delamination at the hole entry and interface between CFRP and Al. The macrography of the drilled CFRP hole wall (Because of the large number of images in this part, only the variable spindle speed = 3000 rpm was examined) under dry (Tools B and C) and cooling conditions (Tool A) are shown in Figure 10. As the figure depicts, there are many defects on the hole walls edge and at the hole exit of CFRP (i.e., interface between Al and CFRP) under dry cutting conditions like delamination and fiber pull out, especially by tool B, this phenomenon could be owing to the structure and cutting angle of the end mill flute which by increasing the number of holes, congestion of aluminum chips on the cutting edge occurs easily, and the scratching effect of the aluminum chips causes intense plowing force and rubbing between the drill-bit and surfaces of the CFRP phase. In dry machining with an increasing number of drilling, thrust forces rise steadily, possibly because cutting edges lose sharpness. A change in tool wear also results in a change in the mechanism of material removal. During drilling operation with the interior cooling condition by the tool (A), the CFRP hole walls are smoother, and the surface grooves and cavities are significantly reduced. The results show that the use of an internal cooling system reduces the friction coefficient between chip, drill bit, and the hole wall during the drilling of CFRP/Al7075, resulting in improved hole quality.

Conclusions

This paper investigates the drilling characteristics of CFRP/Al hybrid stacks. An investigation has been conducted on machinability under dry and cooling conditions with different tool geometries/materials in drilling CFRP/Al stacks, and their results are compared. Additionally, the effect of input cutting variables on thrust force, surface roughness, hole edge quality when drilling multilayer composites/aluminum was performed. Main results achieved can be summarized as follows:

• Tool geometry and cooling conditions (friction) have the highest impact on machining results in comparison to spindle speed and feed rate.

• Based on the results, it was found that drilling with tool (A) at a point angle of 130° with internal cooling system at spindle speed of 3000 (rpm) and a feed rate of 0.08 (mm/rev) achieves a smoother surface quality and a lower thrust force compared to other tools.

• A model was developed to analyze hole wall roughness, and the model’s results were consistent with experimental findings. The roughness of the hole wall meets industry needs when drilling CFRP/Al stacks with the drill (A). CFRP walls can be controlled with a roughness value less than 2 μm, while Al walls can be controlled all within a roughness of 1 μm.

• The study also highlighted in most experiments under different conditions, the aluminum layer of the workpiece is not affected by input cutting variables and has less roughness compared to the composite phase.

• There were grooves and surface cavities, as well as microcracks, on the hole walls of CFRP layers caused by resin loss and degradation under high temperatures. In fact, these machining defects are greatly reduced by the cooling process due to the lower cutting heat.

• The surface roughness of holes drilled with the tool (A) and enteral cooling system designed decreased from an average of two other tools to 38.20%. Internal cooling systems during drilling operation can remove heat more effectively at the cutting zone and increase tool life. They can also be used at higher feed rates and spindle speeds.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors received financial support from Babol Noshirvani university of technology as grant No BNUT/370119/1401 for this research.

ORCID iD

Mohammad Ghasemian

Appendix

References

Wang

Yin

Wang

, et al. Edge chipping mechanism and failure time prediction on carbide cemented tool during drilling of CFRP/Ti stack. Int J Adv Manufacturing Tech 2017; 91(9–12): 3015–3024.

Kuo

Soo

Aspinwall

, et al. Development of single step drilling technology for multilayer metallic-composite stacks using uncoated and PVD coated carbide tools. J Manufacturing Process 2018; 31: 286–300.

Wang

Chen

, et al. Drilling temperature and hole quality in drilling of CFRP/aluminum stacks using diamond coated drill. Int J Precision Eng Manufacturing 2015; 16(8): 1689–1697.

Haddad

Zitoune

Bougherara

, et al. Study of trimming damages of CFRP structures in function of the machining processes and their impact on the mechanical behavior. Composites B: Eng 2014; 57: 136–143.

Zitoune

Krishnaraj

Sofiane Almabouacif

, et al. Influence of machining parameters and new nano-coated tool on drilling performance of CFRP/Aluminium sandwich. Composites Part B: Eng 2012; 43(3): 1480–1488.

Caggiano

. Machining of Fibre Reinforced Plastic Composite Materials. Materials. 2018;11(3):442.

Fleischer

Teti

Lanza

Mativenga

Möhring

Caggiano

. Composite materials parts manufacturing. CIRP Ann 2018; 67(2): 603–626.

Kuo

Soo

Aspinwall

, et al. Tool wear and hole quality when single-shot drilling of metallic-composite stacks with diamond-coated tools. Proc Inst Mech Eng B: J Eng Manufacture 2014; 228(10): 1314–1322.

D’Orazio

El Mehtedi

Forcellese

, et al. Tool wear and hole quality in drilling of CFRP/AA7075 stacks with DLC and nanocomposite TiAlN coated tools. J Manufacturing Process 2017; 30: 582–592.

10.

Fernandes

Cook

. Drilling of carbon composites using a one shot drill bit. Part I: five stage representation of drilling and factors affecting maximum force and torque. Int J Machine Tools Manufacture 2006; 46(1): 70–75.

11.

Zitoune

Krishnaraj

Collombet

. Study of drilling of composite material and aluminium stack. Compos Structures 2010; 92(5): 1246–1255.

12.

Heisel

Pfeifroth

. Influence of point angle on drill hole quality and machining forces when drilling CFRP. Proced CIRP 2012; 1: 471–476.

13.

Alonso

Calamaz

Girot

, et al. Influence of flute number and stepped bit geometry when drilling CFRP/Ti6Al4V stacks. J Manufacturing Process 2019; 39: 356–370.

14.

Wei

Ming

, et al. Effect of drilling parameters and tool geometry on drilling performance in drilling carbon fiber–reinforced plastic/titanium alloy stacks. Adv Mech Eng 2016; 8(9): 168781401667028.

15.

Meshreki

Damir

Sadek

, et al. Investigation of drilling of CFRP-aluminum stacks under different cooling Modes. Advanced Manufacturing 2016; 2.

16.

Impero

Dix

Squillace

, et al. A comparison between wet and cryogenic drilling of CFRP/Ti stacks. Mater Manufacturing Process 2018; 33(12): 1354–1360.

17.

Brinksmeier

Janssen

. Drilling of multi-layer composite materials consisting of carbon fiber reinforced plastics (CFRP), titanium and aluminum alloys. CIRP Ann 2002; 51(1): 87–90.

18.

Abrão

Faria

Rubio

, et al. Drilling of fiber reinforced plastics: a review. J Mater Process Tech 2007; 186(1–3): 1–7.

19.

Marques

Durão

Magalhães

, et al. Delamination analysis of carbon fibre reinforced laminates: evaluation of a special step drill. Composites Sci Tech 2009; 69(14): 2376–2382.

20.

König

Graß

. Quality definition and assessment in drilling of fibre reinforced thermosets. CIRP Ann 1989; 38(1): 119–124.

21.

Montoya

Calamaz

Gehin

, et al. Evaluation of the performance of coated and uncoated carbide tools in drilling thick CFRP/aluminium alloy stacks. Int J Adv Manufacturing Tech 2013; 68(9–12): 2111–2120.

22.

Geng

Liu

Shao

, et al. Delamination formation, evaluation and suppression during drilling of composite laminates: a review. Compos Structures 2019; 216: 168–186.

23.

Sorrentino

Turchetta

Bellini

. A new method to reduce delaminations during drilling of FRP laminates by feed rate control. Compos Structures 2018; 186: 154–164.