PSI - Issue 53

R.D.F.S. Costa et al. / Procedia Structural Integrity 53 (2024) 376–385 Costa, R. D. F. S. / Structural Integrity Procedia 00 (2023) 000 – 000

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edge, are two common wear types in drilling CFRP and aluminium alloys, respectively, being also present in multi material drilling (Wang, Kwon, Sturtevant, Kim, & Lantrip, 2013). Several works have been carried out before in this field, drilling CFRP/Al multi-materials with different aluminium alloys. Authors adopt different approaches, combining various levels of feed and cutting speed, in order to evaluate the best set of parameters in determined conditions. Diamond coatings show good results in improving the performance of the tools in drilling fibre metal laminates (Kuo, Li, & Wang, 2017; Soo, Abdelhafeez, Li, Hood, & Lim, 2019), being a good alternative to uncoated drills. The objectives of this work are to compare the performance of two drills with different geometries when drilling CFRP/Al multi-materials with two configurations, regarding the thrust forces and torque generated, impact of the parameters, surface defects, roughness, chip morphologies and tool wear, side by side with a statistical approach. 2. Experimental procedures A type N geometry drill (referred as drill 1) and a type W geometry drill (referred as drill 2), present in Figure 1, were used for the experimental tests. Drill 1 can be identified as N-type drill (helix angles between 18º and 30º), providing a good balance between chip removal and stability during the drilling process. It has a 6 mm diameter, 120° point angle and 30° helix angle. On the other hand, drill 2 is a W-type drill (helix angles between 35º and 45º), specifically designed for applications which require higher chip removals and cutting rates, making them particularly effective in a wide range of materials. This has a 6.4 mm diameter, 140° point angle and 37° helix angle.

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Fig. 1. Drilling tool geometries used in the study and tip detail: (a, b) type N geometry (drill 1); (c, d) type W geometry (drill 2).

Both drills possess a diamond coating, produced through the hot-filament chemical vapor deposition (HFCVD) process. The coating has a 10,000 HV microhardness and a multilayer composition, with alternate microcrystalline and nanocrystalline diamond layers, whose excellent adhesion allied to its unique smoothness provides a more effective drilling of the multi-materials. A scanning electron microscopy (SEM) analysis was also performed to determine the thickness of the diamond coating, an important aspect to guarantee a very sharp cutting edge, enabling the better separation of the fibres during drilling. The coating has a thickness between 4.3 µm and 4.6 µm. Two different configurations of CFRP/Al2024 specimens were manufactured and tested, with dimensions of 200x200 mm and composed of three layers of 1.5 mm thickness each, totalizing a 4.5 mm thickness. The multi materials were divided in type I, CFRP/Al/CFRP, and type II, Al/CFRP/Al, being present in Figure 2. The aluminium plates were subjected to a surface treatment using sandpaper, to increase surface roughness with the aim of improving the adhesion between layers, followed by acetone to clean the surfaces and an adhesion promoter (Sika Aktivator 205) to enhance the bonding between the two different materials. The CFRP layers (twill weave 2/2) were stacked while simultaneously impregnating seven weaves with a thermosetting epoxy resin mixed with a catalyst to improve the adhesion between layers, after which the resulting stack was placed in the hot press (temperature of 80 ºC, pressure of 30 kPa). The resulting fibre volume fraction of composite layers is 79%.