*3.4. Driven Force in CFRP, UNS A97075 and CFRP/UNS A97075 Stacks*

Figure 15a shows the values of the thrust force with regard to the height of the tool in CFRP plate, while Figure 15b shows the results obtained in UNS A97075 plate. In the first case (CFRP), the force when the tool is fully embedded in the material was increased by the number of holes drilled, from approximately 40 N to 100 N. This might be due to the rounding of the cutting edge by the abrasive action of the fibre. The force level remained significantly lower than the aluminium alloy with the same number of holes drilled. In this second case, the force remains almost constant after 25 holes. Adhesion wear does not appear to be significant.

Evolution of the values of thrust forces regarding to the feed rate of the tool in the stack is shown in Figure 16. The axial load values were increased in both materials with the number of holes.

**Figure 15.** (**a**) Axial force with respect to the time applied to the drilling of CFRP plate; (**b**) axial force with respect to the time applied to the drilling of UNS A97075 plate.

**Figure 16.** Axial force respecting the time applied to the drilling of CFRP/UNS A97075 stack.

A potential model was used to represent tool wear, as suggested by different studies [4,7,23]:

$$Fz = KN^m \tag{1}$$

where *Fz* is the axial force, *K* is a constant depending on the geometry of the tool and the properties of the material being machined, *N* is the number of holes, and *m* measures the degree of wear of the tool.

The axial force values and their models for the same number of holes are shown in Figure 17. The mean axial drilling forces in CFRP separately go from 39.44 N to 106.72 N, with deviations from 7.5 N to 12.78 N. The mean axial drilling forces in Al separately go from 136.96 N to 141.44 N, with deviations from 8.12 N to 11.36 N. The mean axial drilling forces in CFRP in stack go from 44.94 N to 111.15 N, with deviations from 7.26 N to 11.44 N. The mean axial drilling forces in Al stack drilling

range from 148.95 N to 197 N, with deviations from 7.2 N to 11.59 N. The force values during stack machining were similar to those for CFRP plank separately, but were highly increased by the number of holes in comparison to the Al plank. This might be caused by the increased friction due to the adhesion of Al and CF, which resulted in an elevation of the process temperature. This increased the affinity between the tool material and the aluminium and intensified the plasticity of the alloy, favouring the generation of BUE [7–9]. The wear of the drill bit seems to be more influenced by the adhesion wear of the Al than by the abrasion of the CFRP.

**Figure 17.** Axial load versus number of holes for CFRP and Al alloy plates alone and CFRP/UNS A97075 stack.

### *3.5. Hole Quality in Drilling: Diameter*

Figure 18 shows the evolution of the hole diameter with the number of holes measured on both materials when machined separately or in a stack. The mean diameters in the separate CFRP drilling range from 7.95 to 7.94 mm, with deviations from 0.001 to 0.002 mm N. The mean diameters in the separate Al drilling range from 7.98 to 7.99 mm, with deviations from 0.002 to 0.014 mm. The average diameters in the drilling of CFRP in stack go from 7.93 to 7.97 mm, with deviations from 0.001 to 0.003 mm. The average diameters in the Al stack drilling range from 7.95 to 7.97 mm, with deviations from 0.001 to 0.012 mm. There were discrepancies in the diameter between the CFRP and UNS A97075 plates, with the lowest and most stable values being the CFRP ones. The general trend of CFRP drilling is that the diameter decreases with the number of holes, which can be attributed to the progressive loss of material in the tool due to the abrasion effect of CF, while the diameter of the hole in the plate of UNS A97075 is the opposite, attributed to the adhesion effect of the cut material produced.

When machining the stack, there was a variation in the diameter of the holes compared to machining the same material separately. The CFRP inlet could cause a deterioration in the hole in this material in the evacuation of Al that can be attributed mainly to the abrasion on the surface of the hole caused by the rotation of the aluminium chip next to the drill bit. On the contrary, in UNS A97075 the variation in the diameter of the holes was reduced, possibly as a result of the negative synergies produced by the CFRP and the UNS A97075 on the tool. The discrepancy between the diameters could also be related to the different temperatures developed during drilling. As a result, thermal deformations in the planks may vary and lead to different diameters.

**Figure 18.** Evolution of hole diameter with number of holes.

#### **4. Conclusions**

This paper reports a comparison of the wear mechanisms produced during the dry drilling of CFRP and UNS A97075 planks separately and when machined as stacks.

Tool wear combines the effects of CFRP and UNS A97075 alloy machining. In CFRP drilling, the fundamental wear is the abrasion by impact and/or drag of the carbon fibre particles removed during the process. However, adhesion is detected by incipient fusion of the epoxy matrix, which in turn incorporates CF particles. This makes chip evacuation difficult, especially when combined with drilling of the metal alloy.

In the drilling of alloy UNS A97075, the wear mechanism includes different stages. The secondary adhesion of the aluminium alloy on the tool is the main wear mechanism detected. In the first stage, the aluminium matrix is melted and welded onto the tool surface, forming a pure Al layer. This creates the primary BUL, which facilitates the mechanical adhesion of the alloy, mainly in the areas of the edge and making up a BUE that grows to a critical size, from which it begins to extrude. This leads to a secondary BUL, deposited as a second layer over the primary BUL.

The interaction of CFRP and Al materials when drilled together is reflected by the comparison of wear when drilled separately. Bond wear seems to predominate over abrasion of the tool surface. The BUE generated on the edge of the tool contains CF fragments that seem to facilitate the adhesion of aluminium and reduce the diameter differences in the drilling of materials.

**Author Contributions:** J.S., M.B. and S.R.F.-V. developed drilling tests and data treatment. S.F.-V., M.B. and S.R.F.-V. analysed the influence of the parameters involved. S.F.-V. collaborated in preparing figures and tables and S.R.F.-V. and S.F.-V. wrote the paper.

**Funding:** This research was funded by the Spanish Government via the Ministry of Economy, Industry and Competitiveness, the European Union (FEDER/FSE) and the Andalusian Government (PAIDI), project MINERVA.

**Acknowledgments:** The authors acknowledge the financial support for the work. A special mention for a passionate engineer with a tireless capacity for work, whose academic contributions have left a legacy and shown us the way forward due to his determination to continue progressing and developing not just engineering professionals, but good people overall. His great research contributions and his memory are a stimulus for us to try to be at his level, day by day, without disappointing him. Mariano Marcos-Barcena, in memoriam.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

1. Cirillo, P.; Marino, A.; Natale, C.; Di Marino, E.; Chiacchio, P.; De Maria, G. A low-cost and flexible solution for one-shot cooperative robotic drilling of aeronautic stack materials. *IFAC-PapersOnLine* **2017**, *50*, 4602–4609. [CrossRef]


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