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Aluminium sandwich
Composites: Part B 43 (2012) 1480–1488
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Influence of machining parameters and new nano-coated tool on drilling performance of CFRP/Aluminium sandwich Redouane Zitoune a,⇑, Vijayan Krishnaraj d, Belkacem Sofiane Almabouacif a, Francis Collombet a, Michal Sima b, Alain Jolin c a
Institut Clément Ader, ‘‘INSA, UPS, Mines Albi, ISAE’’, Université de Toulouse, 133 c Avenue de Rangueil, 31077 Toulouse, cedex 04, France SHM, sro. Prumyslova, 3, CZ - 78701 Sumperk, Czech Republic LATECIS, Avenue Pierre Georges Latécoère, 31570 Ste Foy D Aigrefeuille, France d PSG College of Technology, Coimbatore, 641 004 , India b c
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 2 July 2011 Accepted 30 August 2011 Available online 5 September 2011 Keywords: A. Hybrid Material B. Wear C. Drilling E. Nano-composite coating
a b s t r a c t Drilling and fastening of hybrid materials in one-shot operation reduces cycle time of assembly of aerospace structures. One of the most common problems encountered in automatic drilling and riveting of multimaterial is that the continuous chips curl up on the body of the tool. Drilling of carbon fiber reinforced plastic (CFRP) is manageable, but when the minute drill hits the aluminium (Al) or titanium (Ti), the hot and continuous chips produced during machining considerably damage the CFRP hole. This study aims to solve this problem by employing nano-coated drills on multimaterial made of CFRP and aluminium alloy. The influence of cutting parameters on the quality of the holes, chip formation and tool wear were also analyzed. Two types of tungsten carbide drills were used for the present study, one with nano-coating and the other, without nano coating. The experimental results indicated that the shape and the size of the chips are strongly influenced by feed rate. The thrust force generated during drilling of the composite plate with coated drills was 10–15% lesser when compared to that generated during drilling with uncoated drills; similarly, the thrust force in the aluminium alloy was 50% lesser with coated drills when compared to thrust force generated without coated drills. Thus, the use of nano-coated drills significantly reduced the surface roughness and thrust force when compared with uncoated tools. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The use of composite materials based on carbon fibers in the aeronautical field is growing. In a commercial aircraft like Airbus A380 or the Boeing 787, the composite panels are arranged in the form of a sandwich-type stacking with Carbon/Carbon, Carbon/Aluminium, Carbon/Titanium etc. [1]. Drilling and fastening of these hybrid materials in one-shot operation allows reducing time for manufacturers. For that LATECIS a French company developed an OPERA system. Ideally the OPERA system is expected to handle high production rates (1 attachment mounted approximately every 12 s). The automation of these tasks must also enable greater mounting precision, improved ergonomics, health and safety of the operators, particularly for the new hybrid materials like composite/metal or composite/composite assemblies. Due to the different mechanical properties of materials constituting the hybrid panels, their machinability remains an open problem and can prevent the automation of these tasks. Within the framework
⇑ Corresponding author. E-mail address: [email protected] (R. Zitoune). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.08.054
of this project, our research group and LATECIS company working on the feasibility of machining with the OPERA system. In the first part of this introduction, various works of literature on problems related to the machining of composite materials are reported. In the second part, the major issues related to the machining of aluminium are highlighted. The final section of this introduction highlights the issues related to machining of sandwich structures such as Carbon/Aluminium, Carbon/Titanium and its solutions are proposed.
1.1. CFRP machining Machining of long fiber composite materials using standard twist drill (used for the machining of metallic materials) revealed damages that could affect the life of bolted or riveted joints. For economic reasons, the carbide twist drill with two lips is used extensively in the case of drilling of composites. In literature, the classification of defects associated with the drilling of composite is carried out according to their position of occurrence: at the hole entry – by debonding the laminate or on the wall of the hole surface – by tearing fibers and degradation of the resin and at the exit
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side of the hole – by the delamination of the last layer – (mainly due to the thrust force of the drill) [2]. In aviation, these defects are considered to be responsible for rejecting almost 60% of the composite parts caused by drilling, in addition to other contributing factors such as variation in hole diameter, higher surface roughness, and roundness are also the cause for rejection [1]. As revealed in literature [3–5], the defect on the surface wall of the hole is affected by the interaction between the cutting edges of the drill and fibers orientation of the material machined. The results indicate that the machining quality is affected partly by the choice of cutting depth and between the angle (h) of the fiber orientation and the direction of cutting speed. For values of h between 45° and 90°, important damages were observed on the machined parts. In this case, these damages are mainly related to the rupture of fibers caused by bending and shearing of the carbon fibers [3–5]. Drilling of CFRP composite plates at low feed rate and high spindle speed, causes damages in the form of carbonized resin at on the hole surface wall. These damages occur mainly due to the low thermal conductivity of composites. Such machining conditions could accelerate tool wear as well. This can be explain by the fact that, the CFRP is a highly abrasive material and during cutting tool edge chipping and an excessive abrasive wear may happen [6]. In the work of Faraz et al. [6], the authors have shown that, during drilling of CFRP at high speeds with WC drill, the dominant wear mechanism is edge micro-chipping and abrasive wear by the hard fractured graphite fibers and carbide grains. The fracture of the carbide grains is linked to the fact that, the carbon fibers can attack the cobalt binder and accelerate wear and fracture of the tool. To reduce the wear problems, composite can be either drilled with a coated tool or by using a high feed and high spindle speed. However, using high feed causes delamination at the exit side of the hole. Several studies have been done to analyze delamination both at the entry and the exit points of the hole. A number of work in drilling of composites show that the defect is influenced by the choice of the machining parameters [7], the geometry of the cutting tool tip [8–11], the nature of its material [12–14], as well as the composite plate manufacturing process and the prepreg form (unidirectional or woven) [15,16]. Campos Rubio et al. [17] have shown that, during drilling with high spindle speed (40,000 rpm), the increasing of the feed rate does not cause the increasing of the damage size. Based on the work of Erik et al. [18], delamination at the exit of a hole has a significant effect on the fatigue life of the final structure. This delamination under the effect of a loading/unloading propagates in the form of a crack until failure. In other work of Eriksen [19], the author has shown that, the mechanical strength proprieties of the composite parts were found to be independent of the surface roughness. Today, in the industrial field, the roughness is one of the criterion, used to validate the quality of machining of composite materials. 1.2. Aluminium machining Unlike composites, the material removal during machining of aluminium is mainly done by shearing the material. In the case of drilling of aluminium and its alloys, the main problem is the adhesion of aluminium on the main cutting edges (BUE), on the rake face and on the flutes of the drill (BUL). This bonding is responsible for premature wear of the cutting tool, for the poor surface finish of the hole and the variations in the diameter of the hole. Several authors showed that, with low cutting speeds (up to 25 m/min), bonding of aluminium occurs at the rake face and at the main cutting edges [20–22]. One way to overcome problems related to the machining of aluminium and its alloys is to increase cutting speeds. However the machining with very high cutting speeds (Eg. 300 m/min) causes a significant increase in
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the cutting temperature (above 300 °C) [22]. At this temperature a chemical reaction between aluminium and cobalt occurs to form micro welding of aluminium on the cutting edges of the tool by diffusion. In the experimental works of [23,24] the authors during the dry turning of aluminium alloy have shown that, in the first time of machining, BUL is caused by thermo-mechanical mechanisms. Once the BUL is formed, the initial cutting conditions change thus enabling the BUE formation through mechanical adhesion. However, during continuous machining, the BUE continues to grow until a critical thickness is reached, and once it reaches the point, it is plastically extended over the BUL due to the action of the mechanical forces. 1.3. Multi-materials machining Due to the different mechanical properties of constituent materials in sandwich panels, it is difficult to ensure proper diameter tolerances during drilling for assembly. Also, during dry drilling, we encounter the problem of chip removal and the phenomenon of bonding of aluminium and titanium on the cutting edges of the tool as well as the burr formation at the exit side of the hole. The cutting tests performed on graphite bismaleimide (Gr/Bi) and titanium (Ti) stacks showed the presence of degradation of the resin at the interface. Due to the low thermal conductivity of titanium during drilling, removal of heat generated by the interaction tool/part/chips is largely by the cutting tool and chips. The contact of these with the matrix of the composite leads to thermal degradation of the resin due to high temperature titanium (Ti) chips and the cutting tool [25]. The work of Kim and Ramulu [26,27] have shown that to achieve holes with acceptable quality during the drilling of a material type Gr/Bi–Ti using a tungsten carbide tool, it is necessary to drill with low cutting speed and low feed rate. However, for drilling with HSS-Co, it is more preferable to select low-cutting speed and high feed rate. Experimental works of [28] have shown that drill using minimum quantity lubrication (MQL) can reduce the adhesion of aluminium on the chip grooves. The works reviewed [25,26] indicates that, at low spindle speed and low feed rate ensures less surface roughness in aluminium. However, while drilling at low speed and feed generates continuous chips which can damage the surface of the composite hole during their chip removal. In addition, these chips rotate with the drill body and damage the composite when it is stacked at the top of the aluminium. While automating the process of drilling and riveting sandwich panels using a CNC system, discontinuous chips are desirable. Further vacuum pipe is facilitated in automating drilling and riveting machines. Increase in feed rate during drilling aluminium breaks the chip, which leads to higher values of surface roughness of both materials [29,30]. For example when the feed rate is increased from 0.05 mm/rev to 0.15 mm/rev, the measured values of roughness in the holes of aluminium plate varies from 0.43 lm to 0.98 lm. The roughness values measured in the carbon/epoxy ranged from 3.3 lm to 6.93 lm. The variations in CFRP are mainly because of high feed rates and also because of continuous chips passing through the hole of CFRP. The industrial need is to drill down to the last layer of the multi-material in a single operation so that the components need not be disassembled, deburred and then joined again. The deburring operations consume almost 30% of the cost of certain structural parts machining [31]. This paper reports a full experimental design planned for drilling a CFRP/Aluminium stack to ensure discontinuous chips while using an automatic (OPERA) system developed by LATECIS Industries (France), by providing an acceptable surface finish (Ra 6 3 lm in the composite and Ra 61 lm in the aluminium). The influence of the new nano-coated tool (Coated in SHM Industries, Czech), and the cutting parameters (speed and feed rate of the tool) on the machining quality, as well as the thrust force responsible for
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the delamination on the composite material are investigated. The variation of thrust force versus number holes drilled is discussed.
2. Experimental setup The sandwich plates studied are composed of a carbon/epoxy and aluminium. The composite plate is made of unidirectional prepregs, with 16 plies. The stacking sequence of the CFRP is quasiisotropic with a thickness of 4.25 mm. The aluminium alloy used in this study is extensively used in aviation industry have referenced as Al 2024. The percentage of alloying elements is as follows: Al 93.5% Si 0.5%, Cu 3.8–4.9%, Mg 1.2–1.8%, 0.1% Cr. Drilling is done
(a)
CFRP/AL plate fixed on the dynamometer OPERA system
(b)
on a CNC machine developed by LATECIS company (Toulouse, France) under the research project OPERA (Automated drilling and riveting of aircraft structures) is shown in Fig. 1a. The acquisition of cutting forces is carried out using a four-component Kistler dynamometer as shown in Fig. 1b. The dynamometer is connected to a Kistler charge amplifier type 5019. The output of the amplifier is transformed into a cutting force through a computer that stores the force signals versus cutting time. The sandwich panel to be drilled is clamped on a dedicated support (see Fig. 1c). On the latter, a hole of 18 mm is machined to allow the drill bit and to prevent the bending of the sandwich plate. The tools used in this study are micro grain carbide grade K20 diameter 6 mm with a point angle of 132°. Some of these tools are coated with a nano composite coating type ncCrAlN/a-Si3N4 (Tripple Alwin) made by the SHM a Czech tooling company. This type of coating is present in a nanocrystalline form. It is formed by single crystals of nanometer size (10 nm) which exhibits a high degree of hardness. Fig. 2 shows the different layers of the coating used as well as theirs thicknesses. The total thickness of the coating is about 2.32 lm (measured on flat substrate). The surface roughness (Ra) of the hole was measured by surface roughness tester (Mitutoya SJ 500) with a sampling length (cut-off) of 0.8 mm. For CFRP the length of measurement through the hole was 3.2 mm (0.8 4 = 3.2 mm) and for aluminium the length of measurement was 2.4 mm (0.8 3 = 2.4 mm). The drilling tests performed are based on full factorial experimental design using three spindle speeds and three feed rates (see Table 1). The machining conditions used are listed in Table 1.
3. Results and analysis
CN frame
3.1. Chip shape analysis
Dynamometer
The macroscopic analysis of chip formation indicated that the shape and size of the chip in the drilling of CFRP are not influenced by the choice of cutting parameters. On the other hand, while drilling on aluminium, the shape and size of chips are strongly influenced by the feed rate of cutting tool, regardless of the type of drill used (with or without coating). Fig. 3 shows the influence of feed rate on the shape and size of aluminium chips after drilling with coated tools. It was noted that drilling with a low feed rate (f = 0.05 mm/rev) produces continuous chips when the spindle speed (1050–2750 rpm) was increased no influence on the shape and size of the chips was observed. The experiments conduced
(c) Adhesion layer - TiN
Drill Core layer - AlTiN
CFRP/Al Camera
Fastening system of the rivet 2.32 µm
Fig. 1. (a) Experimental setup for drilling carried out on OPERA system, (b) fixing the dynamometer on the frame of the NC machine and (c) front view of the assembly.
Top layer – nanocomposite CrAlSiN
Fig. 2. Characteristic of the nc-CrAlN/a-Si3N4 (Tripple Alwin) nano-coating.
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CNC machine with 10 kW spindle power Thickness 4.25 mm Thickness of 3 mm Grade K20, U 6 mm diameter, 136° point angle nc-CrAlN/a-Si3N4 Feed: 0.05, 0.1 and 0.15 mm/rev
the machining quality is better (small Ra) [29]. However, since after every machined hole, the operator has to remove the chips attached to the body of the tool thus machining time increases. If the chips are curled (entangled) on the body of the drill, two problems may occur. On one hand, automatic drilling and riveting cannot be done; on the other hand, we increase the probability of damaging the hole entry of the composite as well as that of the wall. The visual analysis of the state of holes achieved with the cutting parameters used has revealed the presence of damage at the holes entry. Also no burr was observed at the exit side of holes drilled in aluminium. Though with higher feed rates (from 0.1 mm/rev to 0.15 mm/rev) chips are broken, which increase the thrust force as well as the surface roughness [29] of the sandwich structure.
3.2. Thrust force analysis Fig. 4a shows the evolution of the thrust force (Fz) measured during the drilling of aluminium and the composite as a function of the feed rate after drilling with coated drill and uncoated drill. It can be noted that each point of the Fig. 4 and Table 2 represent an average value of six tests. Also, the thrust force is directly proportional to feed rate for both CFRP and Al materials. Furthermore, the thrust force recorded during drilling of aluminium was found to be two to three times higher than these recorded during drilling of the composite material. For example while drilling with a spin-
800 700
Thrust Force (N)
600 500 400 Al- tool without coating Al-tool with coating CFRP- tool without coating CFRP- tool with coating
300 200 100 0 0.04
0.06
0.08
0.1
0.12
0.14
0.16
Feed rate (mm/rev)
(a) 800 700
Thrust force (N)
600 500 Al - tool without coating Al - tool with coating CFRP - tool without coating CFRP - tool with coating
400 300 200
Fig. 3. Influence of the machining parameters on the form of aluminium chips after drilling with coated drill. (a) N = 2750 rpm, f = 0.05 mm/rev, (b) N = 2750 rpm, f = 0.1 mm/rev and (c) N = 2750 rpm, f = 0.15 mm/rev.
100 0 1000
1300
1600
1900
2200
2500
2800
Spindle speed (rpm)
by Brinksmeier and Janssen [28] showed that the there is no difference in the use of coated tools when compared to uncoated tools on the CFRP damage caused by the erosion phenomena between the sharp chips and the CFRP. When the chips are continuous,
(b) Fig. 4. Influence of cutting parameters and the coating of tool on thrust forces in CFRP and aluminium. (a) Influence of feed rate at a constant spindle speed 2750 rpm and (b) influence the spindle speed at a constant feed rate 0.15 mm/rev.
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Al - tool with coating Al - tool without coating
7 6
Ra (um)
dle speed of 2750 rpm and a feed rate of 0.1 mm/rev using an uncoated drill, the thrust force generated increased from 142 N in the CFRP to 485 N in aluminium. This difference on the forces can be explained by the difference in specific cutting pressures generated between the drill and the workpiece materials [32]. With the same machining parameters and with coated tool it can be observed that a significant reduction in thrust is recorded during drilling of aluminium. This reduction is less important in the case of the composite. In this case, it was observed that, the thrust forces recorded during drilling of composites with uncoated tool is 20–25% larger than those recorded during drilling with a coated tool. This difference reached a value of 47% when drilling the aluminium part and can be attributed to the fact that, the coating tools largely reduced the friction between the body of the drill and the machined surface as well as the friction between the chips and the flutes of the cutting tool (rake face). Fig. 4b shows the evolution of thrust with respect to spindle speed for a constant feed rate of 0.15 mm/rev for both types of drills studied. It was observed that the spindle speed has little influence on the thrust force (Fz). A slight decrease in thrust force in the aluminium (around 5%) was observed during drilling with uncoated tool. This could be because of increase in drilling temperature with the increase in the spindle speed. With respect to the drilling of CFRP with uncoated tool, the reduction of the thrust force was observed to be around 10%. In this case it can be assumed that with uncoated tool the temperature generated during machining is high whereas in a nano-coated tool, the heat generation is less because nano-coating reduces the friction between tool and the chip, and between tool and machined surface of the hole. Further, nano-coating also improves the thermal conductivity of the tool.
5 4 3 2 1 0 0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.14
0.16
feed rate (mm/rev)
(a) 9 8 CFRP - tool with coating CFRP - tool without Coating
7 6
Ra (um)
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5 4 3 2 1 0 0.04
0.06
0.08
0.1
0.12
feed rate (mm/rev)
(b)
3.3. Surface roughness analysis
Fig. 5. Evolution of the surface roughness versus the feed rates (a) in the aluminium and (b) in the CFRP.
Ramulu et al. [33] studied how the fiber orientation influenced the quality of the machined surfaces. According to König and Graß [34], measurement of surface roughness in FRP is less dependable than in metals, because protruding fiber tips may lead to incorrect results or at least to large variations of the reading. Additional errors may result from hooking of the fibers to the stylus. Conventional machining of fiber-reinforced composites is difficult due to diverse fiber and matrix properties, fiber orientation, inhomogeneous nature of the material, and the presence of high volume fraction (volume of fiber over total volume) of hard abrasive fiber in the matrix. Most of the studies on FRP machining shows that minimizing the surface roughness was very difficult and is to be controlled. In composite machined surface, the result of the roughness depends mainly on the stylus path with respect to fiber direction since the main direction of fibers may change from layer to layer. For this reason, the roughness has been measured several
times and averaged. The average surface roughness (Ra), which is mostly used in industries, was taken for this study. Fig. 5 shows the evolution of the mean roughness (Ra) as a function of feed rate at a constant spindle speed of 2750 rpm for the two types of drills studied during drilling of aluminium. In Fig. 5a, each point represents an average value of six tests. It can be seen that the increase of feed rate leads to a significant increase in the value of the roughness, regardless of the type of drill used. For example, while the feed rate is increased from 0.05 mm/rev to 0.15 mm/rev, the mean roughness also raises from 0.43 lm to 0.94 lm for an uncoated drill. The experimental work conducted by [35] shown that, the use of coatings (TiAlN/AlN, TiAlN, MoS2) did not seem to affect the surface roughness of the hole produced of aluminium part. However, in this study we show that, the use of a nano-coating tool leads to an improved surface finish (in the
Table 2 Thrust forces versus machining parameters for coating tools and uncoating tools. Tool without coating
Tool with coating
f (mm/rev)
N (rpm)
Fz (N)–CFRP
Fz (N)–Al
f (mm/rev)
N (rpm)
Fz (N)–CFRP
Fz (N)–Al
0.05 0.1 0.15 0.05 0.1 0.15 0.05 0.1 0.15
1050 1050 1050 2020 2020 2020 2750 2750 2750
123.6 159.2 204.4 112 170.33 195.6 107.5 142.4 180.4
240 445.2 684.4 269.6 472 670.4 285.2 485.6 658.4
0.05 0.1 0.15 0.05 0.1 0.15 0.05 0.1 0.15
1050 1050 1050 2020 2020 2020 2750 2750 2750
89 126.5 168 88 124 165 79.4 114 149
184 331 493 184 329 494.2 195.2 331.5 503
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(a)
(a) Cutting edges
BUE phenomena
Clearance face
Clearance face
Cutting edge
(b)
50 µm
BUE phenomena on the principal cutting edge
(b) Clearance face
Cutting edge
Clearance face
50 µm
(c)
Fig. 6. SEM of the cutting edges of drills before machining (a) uncoated tool and (b) coated tool.
CFRP and Al). While drilling aluminium, the values of the Ra measured raises from 0.35 lm to 0.68 lm, when the feed rate increases from 0.05 mm/rev to 0.15 mm/rev. A similar tendency is found when measuring the roughness in the CFRP holes. However, values of the roughness measured in the CFRP holes are significantly higher than those measured in holes made of aluminium (Fig. 5b). This can be attributed to the heterogeneous nature of composite materials and also to the effects of carbon fiber orientation relative to the direction of cutting speed. Earlier studies have shown that the Ra increase in the composites is related to the fibers orientation at 45° compared to the cutting speed direction [3–5]. The SEM analysis of coated and uncoated drills before machining operation shows that the coated tools have a better surface finish when compared to the surface of uncoated drills (see Fig. 6). This difference is mainly because of the polishing of tools especially before coating (PVD) for better bonding of nanocrystalline layer. Because of this, the drilling with coated drills improves the surface quality of aluminium and composite. The SEM analysis of cutting tools after machining shows the presence of a layer of aluminium on the cutting edges (BUE) and the rake face as well as the flutes of the tool (BUL) (Fig. 7). At higher magnifications, it can be observed that the interface between aluminium and the tool cutting edge occurs purely due to mechanical adhesion, and there is no reaction such as diffusion or oxidation
Crack
Fig. 7. SEM pictures showing the phenomenon of adhesion of aluminium on the cutting edges of an uncoated tool after drilling with f = 0.05 mm/rev and N = 1050 rpm. With: (a) overview and (b and c) successive zooms of the surrounded area on the figure (a).
(See Fig. 7c). Similar conclusions can be arrived at while drilling with coated tools (Fig. 8). Although the coating used on drills can significantly improve the status of the surface, it does not prevent the phenomenon of adhesion of aluminium on the cutting edges. An experimental analysis revealed that higher spindle speed reduces the amount of aluminium bonding on coated and uncoated drills. However in some cases, drilling at a spindle speed of 2750 rpm increases the
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(a)
600
BUE and BUL phenomena
Thurst force (N)
500
Cutting edges
400 CFRP - tool without coating CFRP - tool with coating
300
200
Clearance face 100
0 5
20
40
60
70
Number of holes
(b)
(a) BUE phenomena
600 Al - tool without coating Al - tool with coating
Thurst force (N)
500
Clearance face
400
300
200
100
(c)
0 5
20
40
60
70
Number of holes
Aluminium
(b) Fig. 9. Influence of the number of holes drilled on the thrust force. (a) Thrust forces measured in the CFRP and (b) thrust forces measured in the aluminium.
Clearance face
Principal cutting edge
Fig. 8. SEM pictures showing the phenomenon of adhesion of aluminium on the cutting edges of a coated tool after drilling at a feed rate of 0.05 mm/rev and a spindle speed of 1050 rpm with (a) overview, (b) and (c) successive zooms.
onset of delamination on composite and can thus prove to be counterproductive. 3.4. Influence of wear on thrust forces In order to study the effect of tool wear and its influence on the thrust forces and also the quality of machining, drilling tests were carried out with a spindle speed of 2020 rpm and a feed rate of 0.1 mm/rev. Fig. 9 shows the influence of the number of holes on the thrust force in the composite with coated tool. It was observed that after 70 number of holes drilled the thrust force increased to 72% (115–198 N). In the cases of uncoated tool, this increase is more important and is around 92% (from 142 N to 278 N). This increase can be attributed to higher wear in the uncoated tool. With respect to the evolution of thrust force in the aluminium, it can be
seen that the latter is less susceptible to wear. From the first hole to the last hole (70th hole), an increase of about 11% has been recorded regardless of the type of tool. This can be explained by the fact that during the drilling of a CFRP main wear caused by abrasive nature of the carbon fibers on cutting edges. This type of wear can modify the radius of the tool edge and has no or very little influence on the forces during drilling of a metallic material as this falls on the dead zone area [36]. During drilling of aluminium or isotropic materials the principal observed wear is the flank wear. As the thrust force variation in the aluminium is very less between the first and the last hole it can be assumed that the flank wear is negligible. The SEM photographs performed on the coated tool after the 70th hole show no presence of wear on the flank face, which further confirms our analysis (Fig. 11). From Fig. 11, it observed that the quantity of aluminium adhered compared to the beginning of machining is approximately the same. More number of holes is to be drilled in aluminium in order to observe flank wear. Fig. 10b shows the influence of the number of the hole on the roughness measured in the aluminium and composite for the two types of tools used. It can be seen that the roughness obtained in aluminium with an uncoated tool is always greater than 1 lm after the 20th hole. However, machining with a coated tool, helps to ensure that the overall average roughness measured in aluminium remains stable up to 55 number of holes (see Fig. 10a). Regarding the roughness measured in the composite, (Fig. 10b) this difference
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(a) Zone 2 or chisel edge
Zone 1 or Corner
(b) Aluminium
(c)
Aluminium
Fig. 10. Influence of the number of holes drilled on the surface roughness. (a) Ra measured in the Al holes and (b) Ra measured in the CFRP holes.
can be explained by the phenomenon of wear on the main cutting edge induced by the abrasive carbon fibers that led to an increase both in the thrust forces and in temperature.
4. Conclusions In the framework of drilling of sandwich panels made of CFRP/ aluminium, the experimental study shows that the shape and size of the chips are strongly influenced by the selection of the feed rate of the cutting tool. To ensure drilling and riveting in an automated assembly, the chips should be broken into segments and should not be attached to the body of the drill. Moreover no delamination should occur at the entry/exit of the holes drilled in CFRP composite structure. From the analysis it can be found that drilling at a feed rate of 0.1 mm/rev and a spindle speed of 2020 rpm with nano-composites coated tool gives broken chips and better results compared to uncoated drills. In this case, drilling with nano-composite coated drill remarkably reduces the surface roughness of the holes drilled in aluminium and composite (more than 40%). Drilling using nano-coated drills reduces the thrust force during drilling in aluminium and composite. The reduction in thrust force generated in drilling of aluminium is 47% when compared uncoated drills, where as it is only 20–25% when drilling composites. The surface roughness of the holes machined in the sandwich structure is better with coated drills when compared to those machined with uncoated drills. The surface roughness measured in
Fig. 11. SEM observation of the coated tool after 70 holes drilled. (a) Point of tool observation, (b) detail of zone 1 unrounded on (a) and (c) detail of zone 2 unrounded on (a).
CFRP is 30% less when compared to the surface roughness of the holes drilled using uncoated tools. Presently, dry drilling is one of the most important applications of nano coated drills. The wear tests carried out on both the coated and uncoated drills have shown that the quality of holes machined with coated tool is better when compared to uncoated tool [37,38]. Also, the increase of the thrust force in the composite with the number of holes is more important when uncoated tool was used when compared to coated tool. Finally, using the nano-coated drill with OPERA system, drilling and assembly in one shot operation is made be possible, by getting discontinuous chips in aluminium and surface roughness Ra 63 lm in composites. References [1] Krishnaraj V, Zitoune R, Collombet F. Comprehensive Review On Drilling Of Multimaterial Stacks. Int J Mach Form Technol 2010; 2(3/4): p. 171–200, ISSN: 1947-4369.
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[2] Koënig W, Wulf Ch, Grass H, Willerscheid H. Machining of fibre reinforced plastics. Ann CIRP 1985;345:37–47. [3] Zitoune R, Collombet F, Piquet R, Lachaud F, Pasquet P. Experiment-calculation comparison of the cutting conditions representative of the long fibre composite drilling phase. Compos Sci Technol 2005;65:455–66. [4] Arola D, Ramulu M, Wang H. Chip Formation in orthogonal trimming of graphite/epoxy composites. Composites: Part A 1996;27A:121–33. [5] Wang DH, Ramulu M, Arola D. Orthogonal cutting mechanisms of graphite/ epoxy composite, Part I. unidirectional laminate. Int J Mach Tools 1995;35(1):1623–38. [6] Faraz A, Biermann D, Weinert K. Cutting edge rounding: An innovative tool wear criterion in drilling CFRP composite laminates. Int J Mach Tools 2009;49:1185–96. [7] Dharan CKH, Won MS. Machining parameters for an intelligent machining system for composite laminates. Int J Mach Tools Manuf 2000;40:415–26. [8] Krishnaraj V, 2006. Study of drilling tool geometry while machining of glass fibre reinforced plastic. Ph.D Dissertation, Anna University, Chennai, India. [9] Singh I, Bhatnagar N, Viswanath P. Drilling of uni-directional glass fiber reinforced plastics- experimental & finite element study. J Mater Design 2008;29:546–53. [10] Campos Rubio J, Abrao AM, Faria PE, Esteves Correia A, Paulo Davim J. Effect of high speed in the drilling of glass fiber reinforced plastic: evaluation of the delamination factor. Int J Mach Tool Manuf 2008;48:715–20. [11] Durão Luís Miguel P, Gonçalves Daniel JS, Tavares João Manuel RS, de Albuquerque Victor Hugo C, Aguiar Vieira A, Torres Marques A. Drilling tool geometry evaluation for reinforced composite laminates. Compos Struct 2010;92:1545–50. [12] Langella A, Nele L, Maio A. A torque and thrust prediction model for drilling of composite materials. Composites: Part A 2005;36:83–93. [13] Antoniomaria DL, Alfonso P, Francesco V. Tool wear in drilling thermoset and thermoplastic matrix composites. Eng Syst Design Anal 1996;75:41–6. [14] Davim JP, Reis P. Study of delamination in drilling carbon fiber reinforced plastics (CFRP) using design experiments. Compos Struct 2003;59(4):481–7. [15] Zitoune R, Collombet F. Experimental and analytical study of the influence of HexFitÒ glass fiber composite manufacturing process on delamination during drilling. Int J Mach Machinab Mater 2008;3(3/4):326–42. [16] Islam Shyha, Sein Leung Soo, David Aspinwall, Sam Bradley. Effect of laminate configuration and feed rate on cutting performance when drilling holes in carbon fibre reinforced plastic composites. J Mater Process Technol 2010;210:1023–34. [17] Campos Rubio J, Abrao AM, Faria PE, Esteves Correia A, Davim JP. Effects of high speed in the drilling of glass fibre reinforced plastic: Evaluation of the delamination factor. Int J Mach Tools Manuf 2008;48:715–20. [18] Erik P, Ingvar E, Leif Z. Effects of hole machining defects on strength and fatigue life of composite laminates. Composite A 1997;31:141–51. [19] Eriksen E. The influence of surface roughness on the mechanical strength properties of machined short-fibre-reinforced thermoplastics. Compos Sci Technol 2000;60:107–13.
[20] Chamberlain B. Metals Handbook. ASM; 1998. [21] List G, Nouari M, Géhin D, Gomez S, Manaud P, Le Petitcorps Y, et al. Wear behaviour of cemented carbide tools in dry machining of aluminium alloy. Wear 2005;259:1177–89. [22] Nouari M, List G, Girot F, Géhin G. Effect of machining parameters and coating on wear mechanisms in dry drilling of aluminium alloys. Int J Mach Tools Manuf 2005;45:1436–42. [23] Carrilero MS, Bienvenido R, Sanchez JM, Alvarez M, Gonzalez A, Marcos M. A SEM AND EDS insight into the BUL and BUE differences in the turning processes of AA2024 Al-Cu alloy. Int J Mach Tools Manuf 2002;42:215–20. [24] Sanchez JM, Rubio E, Alvarez M, Sebastien MA, Marcos M. Microstructural characterisation of material adhered over cutting tool in the dry machining of aerospace aluminium alloys. J Mater Process Technol 2005;164-665:911–8. [25] Ramulu M, Branson T, Kim D. A study on the drilling of composite and titanium stacks. Compos Struct 2001;54:67–77. [26] Kim D, Ramulu M. Drilling process optimization for graphite/bismaleimidetitanium alloy stacks. Compos Struct 2004;63(1):101–14. [27] Kim D, Ramulu M. Machinability of titanium/graphite hybrid composites in drilling. Trans NAMRI/SME 2005;33:445–52. [28] Brinksmeier E, Janssen R. Drilling of multi-layer composite materials consisting of carbon fiber reinforced plastics (CFRP), titanium and aluminum alloys. Annals CIRP 2002;51(1):87–90. [29] Zitoune R, Krishnaraj V, Collombet F. Study of drilling of composite material and aluminium stack. Compos Struct 2010;92(5):1246–55. [30] Krishnaraj V, Zitoune R, Collombet F. Investigations on drilling of multimaterial and analysis by ANN. Key Eng Mater 2010;443:347–52. [31] Miguel A, Joel G, Corinne RW, Shantanu T, Athulan V, David D. Strategies for burr minimization and cleanability in aerospace and automotive manufacturing. SAE transactions, vol. 114, no. 1, 2005, ISSN 0096-736X. [32] Abrao AM, Faria PE, Campos Rubio JC, Reis P, Paulo Davim J. Drilling of fiber reinforced plastics: review. J Mater Process Technol 2005;186:1–7. [33] Ramulu M, Werna CW, Garbinia JL. Effect of fibre direction on surface roughness measurements of machined graphite/epoxy composite. Compos Manuf 1993;4(1):39–51. [34] König W, Graß P. Quality definition and assessment in drilling of fibre reinforced thermosets. CIRP Annals Manuf Technol 1989;38(1):119–24. [35] Kalidas, Richard ED, Kapoor Shiv G. Experimental investigation of the effect of drill coating on hole quality under dry and wet drilling conditions. Surf Coating Technol 2011;148:117–28. [36] Karpat Y, Özel T. Analytical and thermal modeling of high-speed machining with chamfered tools. J Manuf Sci Eng (ASME) 2008;130:1–15. [37] Yaoa SH, Sub YL, Kaoc WH, Chengb KW, Sud CT. Nanolayer CrNx/WNy coatings used on micro drills for machining of printed circuit boards. J Mater Process Technol 2010;210(4):660–8. [38] Veprek S, Maritza JG. Veprek-Heijman, industrial applications of superhard nanocomposite coatings. Surf Coat Technol 2008;202:5063–73.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Influence of machining parameters and new nano-coated tool on drilling performance of CFRP/Aluminium sandwich Redouane Zitoune a,⇑, Vijayan Krishnaraj d, Belkacem Sofiane Almabouacif a, Francis Collombet a, Michal Sima b, Alain Jolin c a
Institut Clément Ader, ‘‘INSA, UPS, Mines Albi, ISAE’’, Université de Toulouse, 133 c Avenue de Rangueil, 31077 Toulouse, cedex 04, France SHM, sro. Prumyslova, 3, CZ - 78701 Sumperk, Czech Republic LATECIS, Avenue Pierre Georges Latécoère, 31570 Ste Foy D Aigrefeuille, France d PSG College of Technology, Coimbatore, 641 004 , India b c
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 2 July 2011 Accepted 30 August 2011 Available online 5 September 2011 Keywords: A. Hybrid Material B. Wear C. Drilling E. Nano-composite coating
a b s t r a c t Drilling and fastening of hybrid materials in one-shot operation reduces cycle time of assembly of aerospace structures. One of the most common problems encountered in automatic drilling and riveting of multimaterial is that the continuous chips curl up on the body of the tool. Drilling of carbon fiber reinforced plastic (CFRP) is manageable, but when the minute drill hits the aluminium (Al) or titanium (Ti), the hot and continuous chips produced during machining considerably damage the CFRP hole. This study aims to solve this problem by employing nano-coated drills on multimaterial made of CFRP and aluminium alloy. The influence of cutting parameters on the quality of the holes, chip formation and tool wear were also analyzed. Two types of tungsten carbide drills were used for the present study, one with nano-coating and the other, without nano coating. The experimental results indicated that the shape and the size of the chips are strongly influenced by feed rate. The thrust force generated during drilling of the composite plate with coated drills was 10–15% lesser when compared to that generated during drilling with uncoated drills; similarly, the thrust force in the aluminium alloy was 50% lesser with coated drills when compared to thrust force generated without coated drills. Thus, the use of nano-coated drills significantly reduced the surface roughness and thrust force when compared with uncoated tools. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The use of composite materials based on carbon fibers in the aeronautical field is growing. In a commercial aircraft like Airbus A380 or the Boeing 787, the composite panels are arranged in the form of a sandwich-type stacking with Carbon/Carbon, Carbon/Aluminium, Carbon/Titanium etc. [1]. Drilling and fastening of these hybrid materials in one-shot operation allows reducing time for manufacturers. For that LATECIS a French company developed an OPERA system. Ideally the OPERA system is expected to handle high production rates (1 attachment mounted approximately every 12 s). The automation of these tasks must also enable greater mounting precision, improved ergonomics, health and safety of the operators, particularly for the new hybrid materials like composite/metal or composite/composite assemblies. Due to the different mechanical properties of materials constituting the hybrid panels, their machinability remains an open problem and can prevent the automation of these tasks. Within the framework
⇑ Corresponding author. E-mail address: [email protected] (R. Zitoune). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.08.054
of this project, our research group and LATECIS company working on the feasibility of machining with the OPERA system. In the first part of this introduction, various works of literature on problems related to the machining of composite materials are reported. In the second part, the major issues related to the machining of aluminium are highlighted. The final section of this introduction highlights the issues related to machining of sandwich structures such as Carbon/Aluminium, Carbon/Titanium and its solutions are proposed.
1.1. CFRP machining Machining of long fiber composite materials using standard twist drill (used for the machining of metallic materials) revealed damages that could affect the life of bolted or riveted joints. For economic reasons, the carbide twist drill with two lips is used extensively in the case of drilling of composites. In literature, the classification of defects associated with the drilling of composite is carried out according to their position of occurrence: at the hole entry – by debonding the laminate or on the wall of the hole surface – by tearing fibers and degradation of the resin and at the exit
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side of the hole – by the delamination of the last layer – (mainly due to the thrust force of the drill) [2]. In aviation, these defects are considered to be responsible for rejecting almost 60% of the composite parts caused by drilling, in addition to other contributing factors such as variation in hole diameter, higher surface roughness, and roundness are also the cause for rejection [1]. As revealed in literature [3–5], the defect on the surface wall of the hole is affected by the interaction between the cutting edges of the drill and fibers orientation of the material machined. The results indicate that the machining quality is affected partly by the choice of cutting depth and between the angle (h) of the fiber orientation and the direction of cutting speed. For values of h between 45° and 90°, important damages were observed on the machined parts. In this case, these damages are mainly related to the rupture of fibers caused by bending and shearing of the carbon fibers [3–5]. Drilling of CFRP composite plates at low feed rate and high spindle speed, causes damages in the form of carbonized resin at on the hole surface wall. These damages occur mainly due to the low thermal conductivity of composites. Such machining conditions could accelerate tool wear as well. This can be explain by the fact that, the CFRP is a highly abrasive material and during cutting tool edge chipping and an excessive abrasive wear may happen [6]. In the work of Faraz et al. [6], the authors have shown that, during drilling of CFRP at high speeds with WC drill, the dominant wear mechanism is edge micro-chipping and abrasive wear by the hard fractured graphite fibers and carbide grains. The fracture of the carbide grains is linked to the fact that, the carbon fibers can attack the cobalt binder and accelerate wear and fracture of the tool. To reduce the wear problems, composite can be either drilled with a coated tool or by using a high feed and high spindle speed. However, using high feed causes delamination at the exit side of the hole. Several studies have been done to analyze delamination both at the entry and the exit points of the hole. A number of work in drilling of composites show that the defect is influenced by the choice of the machining parameters [7], the geometry of the cutting tool tip [8–11], the nature of its material [12–14], as well as the composite plate manufacturing process and the prepreg form (unidirectional or woven) [15,16]. Campos Rubio et al. [17] have shown that, during drilling with high spindle speed (40,000 rpm), the increasing of the feed rate does not cause the increasing of the damage size. Based on the work of Erik et al. [18], delamination at the exit of a hole has a significant effect on the fatigue life of the final structure. This delamination under the effect of a loading/unloading propagates in the form of a crack until failure. In other work of Eriksen [19], the author has shown that, the mechanical strength proprieties of the composite parts were found to be independent of the surface roughness. Today, in the industrial field, the roughness is one of the criterion, used to validate the quality of machining of composite materials. 1.2. Aluminium machining Unlike composites, the material removal during machining of aluminium is mainly done by shearing the material. In the case of drilling of aluminium and its alloys, the main problem is the adhesion of aluminium on the main cutting edges (BUE), on the rake face and on the flutes of the drill (BUL). This bonding is responsible for premature wear of the cutting tool, for the poor surface finish of the hole and the variations in the diameter of the hole. Several authors showed that, with low cutting speeds (up to 25 m/min), bonding of aluminium occurs at the rake face and at the main cutting edges [20–22]. One way to overcome problems related to the machining of aluminium and its alloys is to increase cutting speeds. However the machining with very high cutting speeds (Eg. 300 m/min) causes a significant increase in
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the cutting temperature (above 300 °C) [22]. At this temperature a chemical reaction between aluminium and cobalt occurs to form micro welding of aluminium on the cutting edges of the tool by diffusion. In the experimental works of [23,24] the authors during the dry turning of aluminium alloy have shown that, in the first time of machining, BUL is caused by thermo-mechanical mechanisms. Once the BUL is formed, the initial cutting conditions change thus enabling the BUE formation through mechanical adhesion. However, during continuous machining, the BUE continues to grow until a critical thickness is reached, and once it reaches the point, it is plastically extended over the BUL due to the action of the mechanical forces. 1.3. Multi-materials machining Due to the different mechanical properties of constituent materials in sandwich panels, it is difficult to ensure proper diameter tolerances during drilling for assembly. Also, during dry drilling, we encounter the problem of chip removal and the phenomenon of bonding of aluminium and titanium on the cutting edges of the tool as well as the burr formation at the exit side of the hole. The cutting tests performed on graphite bismaleimide (Gr/Bi) and titanium (Ti) stacks showed the presence of degradation of the resin at the interface. Due to the low thermal conductivity of titanium during drilling, removal of heat generated by the interaction tool/part/chips is largely by the cutting tool and chips. The contact of these with the matrix of the composite leads to thermal degradation of the resin due to high temperature titanium (Ti) chips and the cutting tool [25]. The work of Kim and Ramulu [26,27] have shown that to achieve holes with acceptable quality during the drilling of a material type Gr/Bi–Ti using a tungsten carbide tool, it is necessary to drill with low cutting speed and low feed rate. However, for drilling with HSS-Co, it is more preferable to select low-cutting speed and high feed rate. Experimental works of [28] have shown that drill using minimum quantity lubrication (MQL) can reduce the adhesion of aluminium on the chip grooves. The works reviewed [25,26] indicates that, at low spindle speed and low feed rate ensures less surface roughness in aluminium. However, while drilling at low speed and feed generates continuous chips which can damage the surface of the composite hole during their chip removal. In addition, these chips rotate with the drill body and damage the composite when it is stacked at the top of the aluminium. While automating the process of drilling and riveting sandwich panels using a CNC system, discontinuous chips are desirable. Further vacuum pipe is facilitated in automating drilling and riveting machines. Increase in feed rate during drilling aluminium breaks the chip, which leads to higher values of surface roughness of both materials [29,30]. For example when the feed rate is increased from 0.05 mm/rev to 0.15 mm/rev, the measured values of roughness in the holes of aluminium plate varies from 0.43 lm to 0.98 lm. The roughness values measured in the carbon/epoxy ranged from 3.3 lm to 6.93 lm. The variations in CFRP are mainly because of high feed rates and also because of continuous chips passing through the hole of CFRP. The industrial need is to drill down to the last layer of the multi-material in a single operation so that the components need not be disassembled, deburred and then joined again. The deburring operations consume almost 30% of the cost of certain structural parts machining [31]. This paper reports a full experimental design planned for drilling a CFRP/Aluminium stack to ensure discontinuous chips while using an automatic (OPERA) system developed by LATECIS Industries (France), by providing an acceptable surface finish (Ra 6 3 lm in the composite and Ra 61 lm in the aluminium). The influence of the new nano-coated tool (Coated in SHM Industries, Czech), and the cutting parameters (speed and feed rate of the tool) on the machining quality, as well as the thrust force responsible for
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the delamination on the composite material are investigated. The variation of thrust force versus number holes drilled is discussed.
2. Experimental setup The sandwich plates studied are composed of a carbon/epoxy and aluminium. The composite plate is made of unidirectional prepregs, with 16 plies. The stacking sequence of the CFRP is quasiisotropic with a thickness of 4.25 mm. The aluminium alloy used in this study is extensively used in aviation industry have referenced as Al 2024. The percentage of alloying elements is as follows: Al 93.5% Si 0.5%, Cu 3.8–4.9%, Mg 1.2–1.8%, 0.1% Cr. Drilling is done
(a)
CFRP/AL plate fixed on the dynamometer OPERA system
(b)
on a CNC machine developed by LATECIS company (Toulouse, France) under the research project OPERA (Automated drilling and riveting of aircraft structures) is shown in Fig. 1a. The acquisition of cutting forces is carried out using a four-component Kistler dynamometer as shown in Fig. 1b. The dynamometer is connected to a Kistler charge amplifier type 5019. The output of the amplifier is transformed into a cutting force through a computer that stores the force signals versus cutting time. The sandwich panel to be drilled is clamped on a dedicated support (see Fig. 1c). On the latter, a hole of 18 mm is machined to allow the drill bit and to prevent the bending of the sandwich plate. The tools used in this study are micro grain carbide grade K20 diameter 6 mm with a point angle of 132°. Some of these tools are coated with a nano composite coating type ncCrAlN/a-Si3N4 (Tripple Alwin) made by the SHM a Czech tooling company. This type of coating is present in a nanocrystalline form. It is formed by single crystals of nanometer size (10 nm) which exhibits a high degree of hardness. Fig. 2 shows the different layers of the coating used as well as theirs thicknesses. The total thickness of the coating is about 2.32 lm (measured on flat substrate). The surface roughness (Ra) of the hole was measured by surface roughness tester (Mitutoya SJ 500) with a sampling length (cut-off) of 0.8 mm. For CFRP the length of measurement through the hole was 3.2 mm (0.8 4 = 3.2 mm) and for aluminium the length of measurement was 2.4 mm (0.8 3 = 2.4 mm). The drilling tests performed are based on full factorial experimental design using three spindle speeds and three feed rates (see Table 1). The machining conditions used are listed in Table 1.
3. Results and analysis
CN frame
3.1. Chip shape analysis
Dynamometer
The macroscopic analysis of chip formation indicated that the shape and size of the chip in the drilling of CFRP are not influenced by the choice of cutting parameters. On the other hand, while drilling on aluminium, the shape and size of chips are strongly influenced by the feed rate of cutting tool, regardless of the type of drill used (with or without coating). Fig. 3 shows the influence of feed rate on the shape and size of aluminium chips after drilling with coated tools. It was noted that drilling with a low feed rate (f = 0.05 mm/rev) produces continuous chips when the spindle speed (1050–2750 rpm) was increased no influence on the shape and size of the chips was observed. The experiments conduced
(c) Adhesion layer - TiN
Drill Core layer - AlTiN
CFRP/Al Camera
Fastening system of the rivet 2.32 µm
Fig. 1. (a) Experimental setup for drilling carried out on OPERA system, (b) fixing the dynamometer on the frame of the NC machine and (c) front view of the assembly.
Top layer – nanocomposite CrAlSiN
Fig. 2. Characteristic of the nc-CrAlN/a-Si3N4 (Tripple Alwin) nano-coating.
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R. Zitoune et al. / Composites: Part B 43 (2012) 1480–1488 Table 1 Machining conditions used during the drilling tests drilling of sandwich plates. Machine CFRP composite plate with Vf = 58% Al 2024 aluminium Cutting tool carbide Coating Spindle speed (rpm) 1050, 2020 and 2750
CNC machine with 10 kW spindle power Thickness 4.25 mm Thickness of 3 mm Grade K20, U 6 mm diameter, 136° point angle nc-CrAlN/a-Si3N4 Feed: 0.05, 0.1 and 0.15 mm/rev
the machining quality is better (small Ra) [29]. However, since after every machined hole, the operator has to remove the chips attached to the body of the tool thus machining time increases. If the chips are curled (entangled) on the body of the drill, two problems may occur. On one hand, automatic drilling and riveting cannot be done; on the other hand, we increase the probability of damaging the hole entry of the composite as well as that of the wall. The visual analysis of the state of holes achieved with the cutting parameters used has revealed the presence of damage at the holes entry. Also no burr was observed at the exit side of holes drilled in aluminium. Though with higher feed rates (from 0.1 mm/rev to 0.15 mm/rev) chips are broken, which increase the thrust force as well as the surface roughness [29] of the sandwich structure.
3.2. Thrust force analysis Fig. 4a shows the evolution of the thrust force (Fz) measured during the drilling of aluminium and the composite as a function of the feed rate after drilling with coated drill and uncoated drill. It can be noted that each point of the Fig. 4 and Table 2 represent an average value of six tests. Also, the thrust force is directly proportional to feed rate for both CFRP and Al materials. Furthermore, the thrust force recorded during drilling of aluminium was found to be two to three times higher than these recorded during drilling of the composite material. For example while drilling with a spin-
800 700
Thrust Force (N)
600 500 400 Al- tool without coating Al-tool with coating CFRP- tool without coating CFRP- tool with coating
300 200 100 0 0.04
0.06
0.08
0.1
0.12
0.14
0.16
Feed rate (mm/rev)
(a) 800 700
Thrust force (N)
600 500 Al - tool without coating Al - tool with coating CFRP - tool without coating CFRP - tool with coating
400 300 200
Fig. 3. Influence of the machining parameters on the form of aluminium chips after drilling with coated drill. (a) N = 2750 rpm, f = 0.05 mm/rev, (b) N = 2750 rpm, f = 0.1 mm/rev and (c) N = 2750 rpm, f = 0.15 mm/rev.
100 0 1000
1300
1600
1900
2200
2500
2800
Spindle speed (rpm)
by Brinksmeier and Janssen [28] showed that the there is no difference in the use of coated tools when compared to uncoated tools on the CFRP damage caused by the erosion phenomena between the sharp chips and the CFRP. When the chips are continuous,
(b) Fig. 4. Influence of cutting parameters and the coating of tool on thrust forces in CFRP and aluminium. (a) Influence of feed rate at a constant spindle speed 2750 rpm and (b) influence the spindle speed at a constant feed rate 0.15 mm/rev.
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Al - tool with coating Al - tool without coating
7 6
Ra (um)
dle speed of 2750 rpm and a feed rate of 0.1 mm/rev using an uncoated drill, the thrust force generated increased from 142 N in the CFRP to 485 N in aluminium. This difference on the forces can be explained by the difference in specific cutting pressures generated between the drill and the workpiece materials [32]. With the same machining parameters and with coated tool it can be observed that a significant reduction in thrust is recorded during drilling of aluminium. This reduction is less important in the case of the composite. In this case, it was observed that, the thrust forces recorded during drilling of composites with uncoated tool is 20–25% larger than those recorded during drilling with a coated tool. This difference reached a value of 47% when drilling the aluminium part and can be attributed to the fact that, the coating tools largely reduced the friction between the body of the drill and the machined surface as well as the friction between the chips and the flutes of the cutting tool (rake face). Fig. 4b shows the evolution of thrust with respect to spindle speed for a constant feed rate of 0.15 mm/rev for both types of drills studied. It was observed that the spindle speed has little influence on the thrust force (Fz). A slight decrease in thrust force in the aluminium (around 5%) was observed during drilling with uncoated tool. This could be because of increase in drilling temperature with the increase in the spindle speed. With respect to the drilling of CFRP with uncoated tool, the reduction of the thrust force was observed to be around 10%. In this case it can be assumed that with uncoated tool the temperature generated during machining is high whereas in a nano-coated tool, the heat generation is less because nano-coating reduces the friction between tool and the chip, and between tool and machined surface of the hole. Further, nano-coating also improves the thermal conductivity of the tool.
5 4 3 2 1 0 0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.14
0.16
feed rate (mm/rev)
(a) 9 8 CFRP - tool with coating CFRP - tool without Coating
7 6
Ra (um)
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5 4 3 2 1 0 0.04
0.06
0.08
0.1
0.12
feed rate (mm/rev)
(b)
3.3. Surface roughness analysis
Fig. 5. Evolution of the surface roughness versus the feed rates (a) in the aluminium and (b) in the CFRP.
Ramulu et al. [33] studied how the fiber orientation influenced the quality of the machined surfaces. According to König and Graß [34], measurement of surface roughness in FRP is less dependable than in metals, because protruding fiber tips may lead to incorrect results or at least to large variations of the reading. Additional errors may result from hooking of the fibers to the stylus. Conventional machining of fiber-reinforced composites is difficult due to diverse fiber and matrix properties, fiber orientation, inhomogeneous nature of the material, and the presence of high volume fraction (volume of fiber over total volume) of hard abrasive fiber in the matrix. Most of the studies on FRP machining shows that minimizing the surface roughness was very difficult and is to be controlled. In composite machined surface, the result of the roughness depends mainly on the stylus path with respect to fiber direction since the main direction of fibers may change from layer to layer. For this reason, the roughness has been measured several
times and averaged. The average surface roughness (Ra), which is mostly used in industries, was taken for this study. Fig. 5 shows the evolution of the mean roughness (Ra) as a function of feed rate at a constant spindle speed of 2750 rpm for the two types of drills studied during drilling of aluminium. In Fig. 5a, each point represents an average value of six tests. It can be seen that the increase of feed rate leads to a significant increase in the value of the roughness, regardless of the type of drill used. For example, while the feed rate is increased from 0.05 mm/rev to 0.15 mm/rev, the mean roughness also raises from 0.43 lm to 0.94 lm for an uncoated drill. The experimental work conducted by [35] shown that, the use of coatings (TiAlN/AlN, TiAlN, MoS2) did not seem to affect the surface roughness of the hole produced of aluminium part. However, in this study we show that, the use of a nano-coating tool leads to an improved surface finish (in the
Table 2 Thrust forces versus machining parameters for coating tools and uncoating tools. Tool without coating
Tool with coating
f (mm/rev)
N (rpm)
Fz (N)–CFRP
Fz (N)–Al
f (mm/rev)
N (rpm)
Fz (N)–CFRP
Fz (N)–Al
0.05 0.1 0.15 0.05 0.1 0.15 0.05 0.1 0.15
1050 1050 1050 2020 2020 2020 2750 2750 2750
123.6 159.2 204.4 112 170.33 195.6 107.5 142.4 180.4
240 445.2 684.4 269.6 472 670.4 285.2 485.6 658.4
0.05 0.1 0.15 0.05 0.1 0.15 0.05 0.1 0.15
1050 1050 1050 2020 2020 2020 2750 2750 2750
89 126.5 168 88 124 165 79.4 114 149
184 331 493 184 329 494.2 195.2 331.5 503
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(a)
(a) Cutting edges
BUE phenomena
Clearance face
Clearance face
Cutting edge
(b)
50 µm
BUE phenomena on the principal cutting edge
(b) Clearance face
Cutting edge
Clearance face
50 µm
(c)
Fig. 6. SEM of the cutting edges of drills before machining (a) uncoated tool and (b) coated tool.
CFRP and Al). While drilling aluminium, the values of the Ra measured raises from 0.35 lm to 0.68 lm, when the feed rate increases from 0.05 mm/rev to 0.15 mm/rev. A similar tendency is found when measuring the roughness in the CFRP holes. However, values of the roughness measured in the CFRP holes are significantly higher than those measured in holes made of aluminium (Fig. 5b). This can be attributed to the heterogeneous nature of composite materials and also to the effects of carbon fiber orientation relative to the direction of cutting speed. Earlier studies have shown that the Ra increase in the composites is related to the fibers orientation at 45° compared to the cutting speed direction [3–5]. The SEM analysis of coated and uncoated drills before machining operation shows that the coated tools have a better surface finish when compared to the surface of uncoated drills (see Fig. 6). This difference is mainly because of the polishing of tools especially before coating (PVD) for better bonding of nanocrystalline layer. Because of this, the drilling with coated drills improves the surface quality of aluminium and composite. The SEM analysis of cutting tools after machining shows the presence of a layer of aluminium on the cutting edges (BUE) and the rake face as well as the flutes of the tool (BUL) (Fig. 7). At higher magnifications, it can be observed that the interface between aluminium and the tool cutting edge occurs purely due to mechanical adhesion, and there is no reaction such as diffusion or oxidation
Crack
Fig. 7. SEM pictures showing the phenomenon of adhesion of aluminium on the cutting edges of an uncoated tool after drilling with f = 0.05 mm/rev and N = 1050 rpm. With: (a) overview and (b and c) successive zooms of the surrounded area on the figure (a).
(See Fig. 7c). Similar conclusions can be arrived at while drilling with coated tools (Fig. 8). Although the coating used on drills can significantly improve the status of the surface, it does not prevent the phenomenon of adhesion of aluminium on the cutting edges. An experimental analysis revealed that higher spindle speed reduces the amount of aluminium bonding on coated and uncoated drills. However in some cases, drilling at a spindle speed of 2750 rpm increases the
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(a)
600
BUE and BUL phenomena
Thurst force (N)
500
Cutting edges
400 CFRP - tool without coating CFRP - tool with coating
300
200
Clearance face 100
0 5
20
40
60
70
Number of holes
(b)
(a) BUE phenomena
600 Al - tool without coating Al - tool with coating
Thurst force (N)
500
Clearance face
400
300
200
100
(c)
0 5
20
40
60
70
Number of holes
Aluminium
(b) Fig. 9. Influence of the number of holes drilled on the thrust force. (a) Thrust forces measured in the CFRP and (b) thrust forces measured in the aluminium.
Clearance face
Principal cutting edge
Fig. 8. SEM pictures showing the phenomenon of adhesion of aluminium on the cutting edges of a coated tool after drilling at a feed rate of 0.05 mm/rev and a spindle speed of 1050 rpm with (a) overview, (b) and (c) successive zooms.
onset of delamination on composite and can thus prove to be counterproductive. 3.4. Influence of wear on thrust forces In order to study the effect of tool wear and its influence on the thrust forces and also the quality of machining, drilling tests were carried out with a spindle speed of 2020 rpm and a feed rate of 0.1 mm/rev. Fig. 9 shows the influence of the number of holes on the thrust force in the composite with coated tool. It was observed that after 70 number of holes drilled the thrust force increased to 72% (115–198 N). In the cases of uncoated tool, this increase is more important and is around 92% (from 142 N to 278 N). This increase can be attributed to higher wear in the uncoated tool. With respect to the evolution of thrust force in the aluminium, it can be
seen that the latter is less susceptible to wear. From the first hole to the last hole (70th hole), an increase of about 11% has been recorded regardless of the type of tool. This can be explained by the fact that during the drilling of a CFRP main wear caused by abrasive nature of the carbon fibers on cutting edges. This type of wear can modify the radius of the tool edge and has no or very little influence on the forces during drilling of a metallic material as this falls on the dead zone area [36]. During drilling of aluminium or isotropic materials the principal observed wear is the flank wear. As the thrust force variation in the aluminium is very less between the first and the last hole it can be assumed that the flank wear is negligible. The SEM photographs performed on the coated tool after the 70th hole show no presence of wear on the flank face, which further confirms our analysis (Fig. 11). From Fig. 11, it observed that the quantity of aluminium adhered compared to the beginning of machining is approximately the same. More number of holes is to be drilled in aluminium in order to observe flank wear. Fig. 10b shows the influence of the number of the hole on the roughness measured in the aluminium and composite for the two types of tools used. It can be seen that the roughness obtained in aluminium with an uncoated tool is always greater than 1 lm after the 20th hole. However, machining with a coated tool, helps to ensure that the overall average roughness measured in aluminium remains stable up to 55 number of holes (see Fig. 10a). Regarding the roughness measured in the composite, (Fig. 10b) this difference
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(a) Zone 2 or chisel edge
Zone 1 or Corner
(b) Aluminium
(c)
Aluminium
Fig. 10. Influence of the number of holes drilled on the surface roughness. (a) Ra measured in the Al holes and (b) Ra measured in the CFRP holes.
can be explained by the phenomenon of wear on the main cutting edge induced by the abrasive carbon fibers that led to an increase both in the thrust forces and in temperature.
4. Conclusions In the framework of drilling of sandwich panels made of CFRP/ aluminium, the experimental study shows that the shape and size of the chips are strongly influenced by the selection of the feed rate of the cutting tool. To ensure drilling and riveting in an automated assembly, the chips should be broken into segments and should not be attached to the body of the drill. Moreover no delamination should occur at the entry/exit of the holes drilled in CFRP composite structure. From the analysis it can be found that drilling at a feed rate of 0.1 mm/rev and a spindle speed of 2020 rpm with nano-composites coated tool gives broken chips and better results compared to uncoated drills. In this case, drilling with nano-composite coated drill remarkably reduces the surface roughness of the holes drilled in aluminium and composite (more than 40%). Drilling using nano-coated drills reduces the thrust force during drilling in aluminium and composite. The reduction in thrust force generated in drilling of aluminium is 47% when compared uncoated drills, where as it is only 20–25% when drilling composites. The surface roughness of the holes machined in the sandwich structure is better with coated drills when compared to those machined with uncoated drills. The surface roughness measured in
Fig. 11. SEM observation of the coated tool after 70 holes drilled. (a) Point of tool observation, (b) detail of zone 1 unrounded on (a) and (c) detail of zone 2 unrounded on (a).
CFRP is 30% less when compared to the surface roughness of the holes drilled using uncoated tools. Presently, dry drilling is one of the most important applications of nano coated drills. The wear tests carried out on both the coated and uncoated drills have shown that the quality of holes machined with coated tool is better when compared to uncoated tool [37,38]. Also, the increase of the thrust force in the composite with the number of holes is more important when uncoated tool was used when compared to coated tool. Finally, using the nano-coated drill with OPERA system, drilling and assembly in one shot operation is made be possible, by getting discontinuous chips in aluminium and surface roughness Ra 63 lm in composites. References [1] Krishnaraj V, Zitoune R, Collombet F. Comprehensive Review On Drilling Of Multimaterial Stacks. Int J Mach Form Technol 2010; 2(3/4): p. 171–200, ISSN: 1947-4369.
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