Effect of drilling parameters on hole quality and delamination of hybrid GLARE laminate
Introduction
FML (Fiber Metal Laminate) is a hybrid material system that consists of thin metal sheets bonded into a laminate with intermediate thin fiber reinforced composite layers [1], [2], [3]. The aerospace industry has recently increased their use of FMLs due to the considerable weight reduction and consequent benefits for critical load-carrying locations in commercial aircraft, such as upper fuselage crown skin panels [2], [4], [5]. The only commercially available FML system is GLARE (GLass Aluminum-Reinforced Epoxy) material, which combines thin aluminum sheets with unidirectional glass fiber reinforced epoxy [6], [7]. It is produced for the upper fuselage skin structures of Airbus A380 at Stork Fokker in collaboration with Akzo and Alcoa [8], [9], [10]. Machining operations in GLARE structures can be carried out using conventional machinery with adaptations. Drilling is one of the most frequently used machining processes to produce holes for assembly, especially for mechanical fasteners, such as bolts and rivets. Machinability presents difficulties in relation to hole quality and integrity due to their different machining properties between a monolithic metal sheet and an anisotropic fiber-reinforced composite.
A large amount of studies has been done for the last several decades to characterize the influence of cutting parameters on the hole surface quality and the resulting machining loads (i.e., thrust force and torque) and understand the tool wear mechanisms in the drilling of fiber-reinforced composites [11], [12], [13], [14]. The uneven machining loads along the plies were typically observed when drilling of composite structures, resulting in the onset of delamination growth [15]. The delamination is a major problem associated with drilling of fiber-reinforced composites, which tends to reduce structural integrity of the composite structures. In addition, the abrasive nature of carbon and glass fibers in composites increases the flank wear of cutting tool, such as cutting edge rounding and/or tool bluntness [16]. Moreover, various damages such as fiber pullout, fiber fragmentation, matrix burning and cracking and sub-surface damages lead to increasing the complexity of the drilling processes in composites [16], [17].
A technique for hole drilling of multi-layered structures such as FML or composite-metal stacks (e.g., CFRP/Ti and CFRP/Al) has presented more challenges in the aerospace industry than the standard fiber-reinforced composites due to the coupled interaction between composite- and metal-phase cutting [18]. For the fiber-reinforced composite, the reinforcing fiber has an elastic-brittle behavior and poor thermal conductivity while the polymer matrix has relatively higher plastic deformation compared to the continuous reinforcing fiber [18], [19]. As for the metallic phase, the aluminum alloys have low elastic modulus and high chemical affinity for most tool materials at high cutting temperature [18]. In particular, the different elastic modulus between composites and metals results in different elastic deformation during drilling process, leading to varying dimensional tolerances along the drilled hole [20], [30]. Furthermore, the evacuated chips from the work-piece continuously rub against the internal surface wall of the hole throughout the thickness and possibly cause delamination [21]. A summary of research publications in the field of drilling of GLARE laminates and composite-metal stacks is presented in Table 1. The ultimate goal of these studies is to gain a better understanding of the quality of the hole surface such as hole-size/circularity [14], [22], [23], surface roughness [24], [25], burr formation [14], [22], [23], delamination [14], [21] and the resulting machining loads during drilling process [14], [22], [24], [25]. P. Tyczynski et al. [22] were among the earliest to perform a comparative study on the drilling process of GLARE laminate with three types of tool materials; high-speed steel (HSS), high-speed cobalt (HSS-Co) and carbide drill bits. They had shown that carbide drill outperformed the HSS and HSS-Co drills in terms of tool life and hole quality on each stacked material. Recently, the potential of drilling process on GLARE laminate has been extensively investigated by Giasin and coworkers [21], [23], [24], [25], [26]. They have shown that feed rate, panel thickness (combined with different fiber orientations) and lubrication condition have major influence on the quality of the hole surface [23], [25]. Two drilling parameters of feed rate and cutting speed had a significant impact on the thrust force, where increasing the feed rate (0.05–0.9 mm/rev) significantly increased the thrust forces, and increasing the cutting speed (1000–9000 RPM) reduced the thrust forces when drilling at constant feed rates [24]. Pawar et al. [14] looked into the impact of tool geometries and found that two flute and four facet drills outperformed 8 facet and 3 flute drill bits in terms of thrust forces and the quality of the hole surface. In particular, the use of high hardness carbide tools over HSS was recommended for better tool wear resistance and good hole quality due to the abrasive nature of glass fibers in GLARE laminate [22]. Nevertheless, a comparative study to establish the impact of process variability on the quality of the hole surface (surface roughness, hole size, roundness and burr formation) and delamination extent has not yet been reported in the open literature.
For aero-structural assembly, interference strengthening technology is a common method used to create a local state of stress that is beneficial for fatigue lifetime enhancement. In addition, the fatigue performance of a mechanically fastened joint is highly dependent on its hole surface quality as well as the presence of residual stress [31], [32]. GLARE laminate has low fracture interlaminar strength at the aluminum sheet-prepreg interface and poor stretch ratio, which make it difficult to apply the interference-fit technique [3], [23]. Botelho et al. [33] experimentally compared the interlaminar shear strength between GLARE and glass fiber/epoxy composite laminate by using the ASTM D 2344 short beam shear test, the apparent interlaminar shear strength results obtained for GLARE presented lower values (40.9 MPa) when compared to glass fiber/epoxy composite (69.3 MPa), due to the interface of aluminum and epoxy. It is also difficult to achieve high hole-tolerance levels due to different elastic moduli and coefficients of thermal expansion between constituent materials as discussed by Pawar et al. [14]. In particular, the delamination discrepancy is a specific failure mode introduced by the thrust force (Fz) when a drilling process is applied perpendicular to the GLARE laminate layer [14]. This discrepancy is sufficiently large to result in push-down delamination when a drill bit exits from the laminate as schematically shown in Fig. 1. It has been recognized as a limiting factor for the usage of GLARE laminate in the aircraft industry because it can lower the bearing strength and reduce the structural integrity of the material [34], [35].
The aim of this study is to examine the effect of processing variables on the cutting mechanism in a GLARE laminate drilling process. The effects of cutting variables (tool types, cutting speeds and feed rates) on thrust force, torque, push-down delamination and the interaction of variables in a drilling process are documented in detail. Thrust force and torque were measured using a two-component dynamometer. Hole qualities, such as hole size error, roundness and surface finish (i.e. surface roughness) and push-out delamination on the holes drilled, were analyzed and compared according to the different cutting variables. Finally, delamination size is quantitatively measured by ultrasonic inspection, and the results correlated with the thrust force.
Section snippets
Materials and specimen preparation
GLARE laminates were prepared by stacking alternating layers of 2024-T3 bare sheets (Alcoa Inc., USA) and unidirectional S2 glass fiber/epoxy prepregs (SK Chemical Industries, South Korea). The lay-up scheme of the GLARE laminate is GLARE3 4/3–0.4 where 4/3–0.4 represents four layers of aluminum sheet (0.4 mm nominal thickness per sheet) and three layers of glass fiber reinforced epoxy prepreg. Cross-sectional view of GLARE3 4/3–0.4 laminate is presented in Fig. 2. The aluminum substrates were
Thrust force (Fz) and torque (Γ) measurements
Representative diagrams of each type of thrust force (Fz) and torque (Γ) at 3000 RPM and 0.20 mm/rev are shown in Fig. 6. The drilling action in each layer of the GLARE laminate can be analyzed. The thrust and torque are observed for each material. In general, the four parts of the loading sequence are clearly distinguished; region 1 defines the period at when the chisel edge in the drill bit point pushes aside the outer aluminum sheet in the GLARE laminate as it penetrates into the hole. In
Conclusions
This study has investigated the drilling of GLARE laminates with conventional HSS-Co and cemented carbide tools at commonly utilized cutting parameters (feed rate and cutting speed). As a final conclusion, it can be stated that drilling GLARE laminate is possible at commonly utilized cutting parameters. Based on the utilized methodologies and experimental results, the following conclusions can be drawn.
- 1)
Overall, the interaction between tool hardness and feed rate was the most significant
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2023, Composite StructuresCitation Excerpt :Data relating to GLARE® machinability is less investigated in the literature. Nevertheless, some researchers have made interesting investigations dealing with damage that occurs during the drilling of this hybrid composite material [3–8]. It was reported in these experimental studies that the heterogeneity and the anisotropy of GLARE® constituents can lead to different types of damage in the machined generated surface such as fiber pull-out, matrix thermal failure, tearing, and debonding [9,10].
Comprehensive experimental investigation on drilling multi-material carbon fiber reinforced aluminum laminates
2022, Journal of King Saud University - Engineering SciencesCitation Excerpt :However, an increase in microhardness was reported (Giasin et al., 2016b). Replacement of HSS drills with WC tools lowered the surface roughness and delamination tendency in GLARE (Park et al., 2018). Hole drilling with TiN-coated drills helped decrease the surface roughness and burr size Giasin et al., 2019) and reduced the cylindricity errors and dimensional inaccuracy (Giasin et al., 2020).