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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threshold. Table 3 shows the data obtained from the analysis, clearly showing the advance of the delamination considering the diameter and the area of the hole (F da ) with both the increasing feed and cutting speed, being the first the one which contributes the most to surface defects in the multi-materials.

Table 3. Delamination factors of holes performed with different sets of parameters.

Hole number

Feed (mm/rev.)

Cutting speed (m/min.)

A nom

A del

A max

D nom

D max

F a

F d

F da

1 2 3 4 5 6

0.05

50

8.237

32.412 44.344 82.259 74.081 78.257 89.082

6.424 7.514

0.291 0.692 0.932 1.033 1.086 0.992

1.071 1.252 1.706 1.619 1.664 1.775

1.211 1.637 2.293 2.257 2.342 2.410

0.1

100 150 300 150 300

19.560 26.364 29.214 30.697 28.057

28.274

6

0.15 0.15

10.234

9.712 9.982

0.3 0.3

10.650

All these holes analysed had visible delamination on their surface, reason why their delamination factors are above the 1.200 acceptable limit and, thus, considered severe. Holes without visible delamination on their surface were also measured, but their delamination factors were all between 1.000 and 1.100, closer to the former, as there were registered almost no surface defects. Table 4 shows the surface roughness values comparison between the two drills in different holes of multi material type II, since it was where the burr was most proeminent. The holes with drill 1 were performed with a feed of 0.05 mm/rev. and cutting speed of 300 m/min., whereas the ones done by drill 2 had a feed of 0.2 mm/rev. and a cutting speed of 150 m/min. The results show that drill 2 is capable of performing holes with better quality, with lower roughnesses in the curved region of the hole s’ internal surface. Nonetheless, the roughnesses registered are still above the imposed limit, reason why this topic needs a deeper study, to overcome this problem.

Table 4. Surface roughness values obtained for holes with the same parameters using both drills.

Arithmetic Average Roughness - Ra

Average Maximum Height of the Profile - Rz

Drilling tool

Hole 1

Hole 2

Hole 3

Hole 1 50.163 50.042

Hole 2 130.757 82.264

Hole 3 101.981 61.937

Drill 1 (N-type) Drill 2 (W-type)

3.608 2.724

5.193 3.069

6.202 3.382

The large surface roughness variance between holes drilled with the same tool and parameters sustains the CFRP’s anisotropic nature and that each hole in the multi-material results in a different surface. Besides this, the combination of parameters also has its influence on the surface roughness as well as the tool geometry, reason why in a future work this will be taken into account and be properly compared, so as to evaluate the impact of parameters on the holes’ roughness , performed with the same drill. The number of holes analysed will also be higher, resulting in a more relevant analysis. 3.3. Chip morphologies and tool assessment During the drilling tests, two distinct chip morphologies were observed: in multi-material type II (Figure 7b), the upper aluminium layer exhibited uniform and long spiral chips, indicating an effective chip evacuation; however the lower aluminium layer showed tangled chips, the same as multi-material type I (Figure 7a), suggesting less efficient chip removal. There is an increased difficulty in evacuating chips as the drill goes deeper into the hole, as the CFRP middle layer’s relative softening and higher surface roughness contribute to chip entrapment and interrupt uniform spiral chip formation. On the other hand, Figure 7c shows CFRP particles adherence to the aluminium chip, demonstrating a good adhesion between the two materials on the laminate. When the CFRP chips were not adhered to the aluminium, these had the form of a powder, being very brittle.