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 Gaier [18], 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 transformation [Girot,
2018]. 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.