4. Discussion

This study focuses on drilling processes corresponding to industrial applications, that is to say, using optimum tools and cutting parameters in order to obtain high surface quality in accordance with the industrial specification. For this study case, the hole roughness is not a major factor influencing the fatigue life of the drilled part. In accordance with Siebel and Gaier18, it seems that, below a given limit, which could be the aircraft specification here, the hole roughness does not significantly impact the fatigue behaviour. However, if the roughness reaches high values, its influence could be significant. This confirms the importance of the roughness specification for these aluminium alloys.
In contrast, the hole hardness has a significant positive influence on the fatigue life of the drilled part in this study. A variation in hardness can be related to several factors. It can be related to mechanical loading inducing the strain hardening of the hole subsurface, to thermal loading inducing material transformations and, for nanohardness, it can also be related to the residual stresses induced by thermomechanical loading. Regarding the impact of the thermal loading, the temperatures reached during an aluminium drilling operation do not seem to be high enough to induce a phase transformation1. Besides, an SEM analysis of the precipitation state of the hole subsurface, conducted as part of the study, showed no increase in the number or size of the precipitates, which could reflect precipitation hardening, for the case of axial drilling at D = 9.53 mm. Thus, it seems that the thermal loading involved during drilling of a 2024-T351 aluminium part has no significant influence on the hole surface integrity. Regarding the impact of the mechanical loading, the SEM-EBSD analysis showed a significant material depth affected by strain hardening for the axial drilling process at D=9.53 mm. Thus, it seems that, in our study case, the mechanical loading involved in drilling controls the hole surface integrity.
Therefore, the increase in hole hardness for the aluminium alloy studied is essentially related to the strain hardening of the hole subsurface, which is induced by the severe mechanical loading applied on the machined surface during the drilling process. The positive influence of the hole hardness on the fatigue life can then be explained by the increase in yield strength in the hole edge induced by the strain hardening, which slows damage generation (crack initiation) in this area.
HOCT results showed that a significant hole hardness induced by strain hardening is associated with a significant residual stress state of the part. Severe mechanical loading applied to the machined surface simultaneously generates high plastic strain and high residual stresses. The residual stresses can remain in the part because of the strain hardened material layer. Thus, it seems difficult to dissociate these two aspects. In order to study the impact of the residual stress on the fatigue strength in greater depth, it would be interesting to develop a numerical model capable of using the inverse method to determine, the residual stress state corresponding to the fatigue specimen deformation obtained with the HOCT.
For the aircraft industry, it would be interesting to identify a surface integrity characteristic that would be quite easy to evaluate, as an indicator of the fatigue strength, in order to be able to assess the fatigue life of aircraft structures easily. The correlation between the hole surface microhardness and the fatigue strength of the drilled part has been demonstrated in this study and the measurement of this surface integrity characteristic seems relatively accessible: it does not require surface preparation, the test time is short, and the cost of the device is moderate. Thus, the Vickers microhardness could be identified as an indicator of the fatigue strength for aluminium 2024-T351 drilled parts.