4.2. FCG behavior of welding joint
In current study, it was found that significant inhomogeneity of FCG
rate existed in different regions of 7085Sc FSWed joint (Fig. 11). As
widely known that FCG behavior is significantly associated with the
microstructure of alloys. For heat-treatable alloys, previous works have
proved that grain structure and hardening phases are the main
microstructure factors on FCG rate [31, 39, 40]. The BESD analysis
and TEM observation shown in Fig. 6 and Fig. 7 revealed that the grain
structures in BM and AS-HAZ were approximately similar, while the
morphology of hardening phases in these two regions were quite different
(Fig. 8 and Fig. 9). Thus, it can be reasonably concluded that the
difference of FCG behavior between BM and AS-HAZ was mainly induced by
hardening phases, while the effect of grain structure could be
negligible. As compared with BM/AS-HAZ, grain structure and hardening
phases in WNZ had more distinctive characteristic, therefore, the
effects of both two factors on the FCG behavior should be highlighted.
According to early researches [25, 31, 41, 42], hardening phases
played a noticeable role to intervene FCG process by interaction between
dislocations. During the FCG process, dislocation nucleation and
movement were promoted at the crack tip, undoubtedly, the interaction
between hardening phase and movable dislocation turn to be frequent in
this region. For coherent or semi-coherent phase, dislocation could
shear it and glide reversibly with in a persistent slip band during
cyclic loading. This kind of kinematical reversibility of cyclic slip
would lead to a more deflective, tortuous and bifurcate fatigue crack
path as well as inhomogeneous deformation on the crack tip, contributing
to enhancement of FCG resistance. On the contrary, dislocation tends to
bypass non-coherent phases rather than shear. In this situation,
irreversible deformation turns to be dominated and increased damage
accumulation on the crack tip, and finally degrade the FCG resistance.
It should be noted that whether in shear or bypass mechanism, the radius
(r) and the volume fraction
(f) of hardening phases are two important microstructural
parameters. Wen et al. [25] summarized and proposed succinct
equations to express the relationship as listed below:
shear mechanism:\(\frac{\text{da}}{\text{dN}}\propto\left(r\bullet f\right)^{-1/2}\)(1)
bypass mechanism:\(\frac{\text{da}}{\text{dN}}\propto{r\bullet f}^{-1/2}\) (2)
Obviously, the FCG rate is
negatively
related to r and f of shearable precipitates according to
Eq. (1), and the FCG rate is positively related to r of
non-shearable precipitates while negatively related to its faccording to Eq. (2). These relationships can be used to explain the FCG
inhomogeneity of 7085Sc FSWed joint.
As can be seen from TEM images, the dominated hardening phases in both
BM and AS-HAZ samples consisted mostly of coherent GPII and
semi-coherent πβ², indicating that shear mechanism was the main
interaction mode in these two regions during cyclic loading. However,
the
average size and volume fraction of precipitates in AS-HAZ are much
higher than that of BM. According to Eq. (1), the FCG resistant of
AS-HAZ should theoretically be higher than BM. This theoretical
deduction is entirely consistent with experimental data shown in Fig.
11. For WNZ sample, precipitates in matrix consisted of a considerably
amount of reprecipitated coherent phase (πβ² and GPII) as well as some
coarse non-coherent phase (π). According to
shear mechanism mentioned above,
high volume fraction of coherent phases would be beneficial to
enhancement of FCG resistance. The supporting evidences could be found
in Brayβs work demonstrating that
naturally aged Al-Cu-Mg alloys had
excellent fatigue properties [41]. According to Eq. (2),
bypass mechanism of π phases with
large r and low f in WNZ
also contributed to reduction of
FCG rate. In addition to the effects of precipitates, another
considerable factor on FCG rate of WNZ is the grain structure. It has
been known that due to small angle of preferential slip planes between
adjacent grains, fatigue crack tends to expand across multiple grains at
a time in alloy with high texture intensities, resulting low FCG
resistance [43, 44]. As compared with BM and AS-HAZ, WNZ possessed
recrystallized grains with smaller size and random crystal orientation
(Fig. 6 and 7), in this situation, a higher force was required for the
propagation of fatigue crack as the impeding effect caused by high angle
grain boundary. Therefore, the deflection, torsion and bifurcation of
fatigue crack occurred more frequently (Fig. 12), leading to a
significant decrease of FCG rate. Due to the combined effects of
precipitates and grain structures, FCG property of WNZ was better than
other welding regions as well as BM.
In addition, the influence of grains structure on FCG is associated with
the cyclic plastic zone (CPZ) at the crack tip [25]. The CPZ size
could be estimated by\(\delta_{f}=\frac{1}{8\pi}\left(\frac{K}{\sigma_{y}}\right)^{2}\),
here (Οy) accurately would be the strength of the
plastic zones near the fatigue crack tip. Obviously, the CPZ rises with
increasing of ΞK. When CPZ size is larger than a grain scale, the
effects of grain structure on the FCG would turn to be dominant
[45]. Because BM and AS-HAZ processed similar grain structure, thus,
the crack in BM and AS-HAZ samples showed roughly the same FCG rate at
high ΞK (>20 MPa m1/2).