1. Introduction
Carbon fibre-reinforced polymer (CFRP) has gained wide acceptance in different engineering applications including aircrafts, automobiles and consumer goods, due to its light weight and good strength [
1,
2]. CFRP is a class of polymer matrix composite (PMC) consisting of two distinct constituents—carbon fibre and polymer matrix [
3,
4]. Due to the presence of the carbon fibres, CFRPs show anisotropy. Hence, its tensile modulus, electrical and thermal conductivities are greater along the fibre direction than those perpendicular to the fibre axis [
5]. Polymer matrix material acts as a binder that is able to bind the fibres together and protects the fibre from environmental damage [
6]. Although CFRP products are usually cured into desired shapes, they often require hole drilling for assembly and fastening [
7]. Drilling CFRP is different from drilling metals in many aspects [
6]. Traditional mechanical drilling of CFRP suffers from various types of damage, including delamination, matrix cracks, burrs, thermal damage, etc. [
6,
8]. This can result in a high rejection rate of components [
9]. Standard tungsten carbide coated drilling tools may only last 20 holes in CFRP dry drilling processes [
10]. In the United States alone, the total tooling cost to produce holes in CFRP was estimated to be over 300 million dollars per year [
11]. Therefore, if the tool life can be extended, not only can significant savings be gained in the hole production process in CFRP, but also considerable energy could be saved and CO
2 emissions reduced. Recent research has proposed methods for reducing drilling defects [
12,
13,
14,
15,
16,
17]. However, high quality and high efficiency drilling of CFRP is hampered by excessive tool wear due to the abrasive nature of the carbon fibre [
18].
Nonconventional drilling processes include abrasive waterjet (AWJ), laser, and electrical discharge machining (EDM). AWJ technology is able to cut a wide range of advanced materials including CFRP. However, the AWJ drilling process requires a high-pressure water jet, which may result in delamination, fibre pull-out, moisture absorption and particle embedment on drilled CFRP workpieces [
6,
18,
19]. More importantly, the jet diameter in AWJ process is usually in a range of 0.5–2.5 mm. Hence, this process may not be suitable to producing small features [
20]. EDM utilizes spark discharge between electrode and workpiece to remove undesired material via melting and vaporization [
21]. Low machining efficiency and high tool (electrode) wear limit the application of EDM for cutting or drilling CFRP [
22].
Laser drilling is free of tool wear, has a higher degree of flexibility, and is non-contact [
18,
20]. However, challenges still exist in laser drilling of CFRP. The main challenge is to minimise the heat affected zone (HAZ), taper control and obtain high drilling efficiency. Laser drilling of CFRP composite has been investigated extensively in the last 30 years [
7,
23,
24,
25,
26,
27,
28,
29]. Infrared wavelength continuous wave or short pulse (e.g., nanosecond pulses, or micro-second pulses) laser drilling of CFRP is dominated by a photon-thermal process to disintegrate or vaporise the resin and carbon fibre. This process often causes thermal damage such as heat affect zone (HAZ), which is considered as a main obstacle in laser drilling/machining of CFRP [
6]. The presence of HAZ is mainly due to the huge difference in thermal properties between carbon fibre and the polymer matrix [
25]. More importantly, epoxy resin, as a matrix material in CFRP, shows low absorbability at the wavelength of about 1 μm for high power fibre lasers. During the laser-workpiece interaction period, epoxy resin is degraded more quickly than the carbon fibre under the same amount of laser energy. However, carbon fibre has a good thermal conductivity. As a result, a great amount of thermal energy is conducted along the fibre direction to the surrounding resin, overheating/degradation and fibre debonding [
22,
30,
31]. The size of HAZ is mainly described by the length of extruding fibre and defined by the zone where matrix material loses or reduces its ability to transfer load. It has been suggested that the extension of HAZ reaches a certain limit when fibre temperature is equal to the resin decomposition temperature [
6,
32]. Li et al. [
23] used a diode-pumped solid state ultraviolet (UV) laser with nanosecond pulses to drill CFRP. They introduced a novel drilling method by utilizing multiple parallel rings to improve material removal rate. A minimum HAZ size of 50 μm was achieved. Salama et al. [
25] used a high power picosecond laser to drill a CFRP sheet of 6 mm in thickness with multiple-ring drilling strategy. A minimum HAZ of 25 μm at the entrance side was reported. They suggested the use of lower laser power and higher scanning speed to reduce the extension of HAZ, due to less material–beam interaction time. Salama et al. [
24] also stated that this parallel ring method would increase heat accumulation between each ring. However, the temperature may not be high enough to vaporise the carbon fibre. Instead, epoxy resin would be degraded and lose its ability to hold the fibre. Thus, chopped fibre chips can escape from the machining area. More importantly, the kerf width would be widened due to multiple rings or parallel lines which allow the debris and vapour to be ejected from the machining area more easily. Hence, machining efficiency was increased compared with single-ring drilling.
Li et al. [
33] compared the drilling with single and multi-ring drilling strategy by using a high power fibre laser. They found that the machining time was significantly reduced by using multiple rings, multi-passes strategy compared with that of single-ring drilling. Li et al. [
18] drilled CFRP with a fibre laser with adjustable pulse duration. Parallel ring drilling strategy was also adopted in their experiment. They found that a shorter pulse duration would reduce the size of HAZ due to less laser beam interaction time. A longer pulse duration would lead to carbonization which is due to matrix material being affected by heat. Thus, a shorter pulse duration was suggested. Apart from HAZ, hole wall tapering is another feature associated with laser drilling/machining of CFRP. Due to Gaussian distribution of the laser beam and the use of a round shaped beam, the laser energy is mostly concentrated on the centre, and decreases towards the beam edge. Additionally, as the material ejection from the top entry erodes the side walls, a tapered hole is often formed [
6,
34].
Climate change, global warming, and ozone depletion have drawn wide attention to investigating more energy efficient manufacturing processes [
35]. Hence, it is crucial to investigate energy efficiency in manufacturing processes to achieve low carbon production and effective use of energy [
36].
Despite a large amount of research in drilling CFRP composites, few studies have been conducted previously to investigate the effect of drilling process on energy consumption and CO2 emission.
In this investigation, a new approach of stepped laser pulse parameter multiple-ring drilling of CFRP is introduced (i.e., laser pulse parameters are different for different rings) and the energy consumption, carbon emission, drilling time, and hole quality are evaluated. The key novelty of the research is the development and characterization of a new drilling method that has shown significant reduction in energy consumption and significant improvement in drilling efficiency compared with the previously reported laser drilling with parallel rings under constant drilling parameters and it is the first study of its kind in laser drilling of CFRP.
4. Discussion
This research has shown that by introducing stepped parameter parallel drilling laser drilling method, significant benefit can be gained in terms of drilling efficiency and energy consumption reduction, without sacrificing the drilling quality compared with the previously reported parallel ring laser drilling with constant drilling parameters. The temperature profiles in laser parallel ring processing of CFRP has been previously studied, e.g., Li et al. [
33]. The key findings from the previous temperature profile study are the rapid damage and disintegration of the resin by the high temperature of the highly conducting carbon fibre and direct laser irradiation. The parallel ring drilling can more effectively prevent heat from the inner drilling rings from expanding through the cut channels of the outer ring to the parent material, and carbon fibres can be removed more effectively by multiple rings by chopping them into short fibres rather than vaporizing them all. The key difference is that, in the present study using a stepped parameter parallel ring approach, the outer ring can be produced using a lower energy input to limit the heat affected zone, while preventing the heat from inner rings from reaching the parent material. A higher energy input was used for the inner rings to remove the material more rapidly. It should be noted that in this laser drilling process, the hole drilling was realized by only cutting out a central disk, rather than vaporizing the entire material in the hole centre, thus the energy consumption can be significantly reduced.
In conventional mechanical drilling, although much less energy is used during the drilling process, hole delamination damage and tool wear would occur, which is dependent on the type of drilling tools and material size and applications. Thus, the analysis of energy consumption for the replacement of drilling tools (depending on how the tools are made, what tool and coating materials are used) and loss of damaged product (say, a CFRP panel with x% of failed holes) in specific applications would be much more complex, thus beyond the scope of the current study. More detailed bench marking against conventional drilling will be studied in the near future using “from cradle to grave” full life cycle analysis.
The key problem of mechanical drilling is the fibre delamination in the hole exit, as shown in
Figure 17, drilled using the same drill bit as shown in
Table 5, in addition to the significant tool wear. A good drilling tool can typically drill 20–30 holes in CFRP, depending on the material thickness. Replacement of these tools would result in significant energy consumption and cost.
The current research is the first step in understanding of energy efficiency in laser drilling holes in CFRP with a workpiece thickness of 2 mm. In practical applications, such as the manufacture of modern aircraft structures in Boeing 787 and Airbus A350, a significant proportion (50–60% by weight) of aircraft panels are made of CFRP of sheets of 5–75 mm in thickness, and many thousands of fastening holes are needed in these CFRP panels. Tool wear and quality control are critical considerations. In order to reduce damage to the CFRP, very high-cost special tools are needed and tool wear would lead to significant cost and overall energy consumptions not just to the aircraft manufacturers but also to the supply chains.
A rapid laser pre-scribing process, followed by standard mechanical drilling, is designed to chop off the continuous fibre at the hole exit periphery. Then, mechanical drilling is carried out from the opposite side to avoid hole exit fibre delamination. This is illustrated in
Figure 18 together with the effect of drilling to demonstrate that fibre delamination can be avoided without a backing plate in mechanical drilling. The mechanical drilling took about 2 s and the laser pre-scribing took about 2 s. If both are carried out simultaneously, then there would be no time delay to the mechanical drilling process. This, however, does not solve the problem of tool wear. A more detailed study of this novel process will be presented elsewhere.
Laser drilling alone, while taking longer than mechanical drilling with the present set up, eliminates the problem of tool wear and fibre delamination at the hole exit associated with mechanical drilling. With the availability of high-power picosecond and nanosecond lasers, both drilling time and quality can be expected to be further improved.