materials
ArticleResearch on the Properties and Low Cycle Fatigue of
Sc-Modified AA2519-T62 FSW Joint
Robert Kosturek * , Lucjan Śnieżek , Janusz Torzewski , Tomasz Śl˛ezak ,Marcin Wachowski and Ireneusz Szachogłuchowicz
Faculty of Mechanical Engineering, Military University of Technology, 2 gen. S. Kaliskiego St., 00-908 Warsaw,
Poland; lucjan.sniezek@wat.edu.pl (L.Ś.); janusz.torzewski@wat.edu.pl (J.T.); tomasz.slezak@wat.edu.pl (T.Ś.);
marcin.wachowski@wat.edu.pl (M.W.); ireneusz.szachogluchowicz@wat.edu.pl (I.S.)* Correspondence: robert.kosturek@wat.edu.pl; Tel.: +48-261-839-245
Received: 19 October 2020; Accepted: 16 November 2020; Published: 19 November 2020


Abstract: The aim of this research was to examine the mechanical and fatigue properties of friction stir
welded Sc-modified 5 mm thick AA2519-T62 extrusion. The joint was obtained using the following
parameters: 800 rpm tool rotation speed, 100 mm/min tool traverse speed, 17 kN axial, and MX Trifluteas a tool. The investigation has involved microstructure observations, microhardness distribution
analysis, tensile test with digital image correlation technique, observations of the fracture surface,measurements of residual stresses, low cycle fatigue testing, and fractography. It was stated that theobtained weld is defect-free and has joint efficiency of 83%. The failure in the tensile test occurredat the boundary of the thermo-mechanically affected zone and stir zone on the advancing side of
the weld. The residual stress measurements have revealed that the highest values of longitudinal
stress are localized at the distance of 10 mm from the joint line with their values of 124 MPa (theretreating side) and 159 MPa (the advancing side). The results of low cycle fatigue testing haveallowed establishing of the values of the cyclic strength coefficient (k0 = 504.37 MPa) and cyclic strain
hardening exponent (n0 = 0.0068) as well as the factors of the Manson–Coffin–Basquin equation:
the fatigue strength coefficient σ0 f = 462.4 MPa, the fatigue strength exponent b = −0.066, the fatigue
ductility coefficient ε0 f = 0.4212, and the fatigue ductility exponent c = −0.911.
Keywords: aluminum; friction stir welding; microstructure; mechanical properties; residual stresses;
fatigue; fracture

1. Introduction
Friction stir welding (FSW) is a very efficient technology in the production of aluminum alloy
joints, as it provides a number of advantages over traditional fusion welding such as the lower
temperature of the joining process [1–3]. This particular factor is crucial when it comes to welding of
high-strength aluminum alloys, which are mainly precipitated-hardening materials (2XXX and 7XXX
series) and their specific strength is an effect of thermally unstable precipitates (e.g., Guinier–Preston
zones, θ0 phase) [4–7]. Considering the production of efficient welds of these materials, the losses
in hardening must be taken into account due to the thermal affection of the joining process on theworkpiece. In this paper, AA2519-T62 armor grade aluminum is taken under investigation in termsof its friction stir-welded joint properties. This alloy contains 5.3–6.4% copper and is subjected tothe precipitation hardening process, and thus, acquires high specific strength, which makes it a very
desirable material in terms of aerospace, automotive, and military applications [8–10]. It is noteworthy
that the investigated alloy is a modification of AA2519 characterized by the addition of scandium andzirconium, which improves its mechanical properties and resistance to elevated temperature in theform of higher recrystallization temperature [9,11–13]. Although friction stir welding is a suitable
Materials 2020, 13, 5226; doi:10.3390/ma13225226

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technology for joining AA2519, it still significantly influences its microstructure and (at the sametime) mechanical properties. The changes in the alloy properties include grain refinement in the stirzone, dissolution and coarsening of strengthening phase, and generation of residual stresses [2,14,15].
In recent years, friction stir welded joints of high-strength aluminum alloys (including AA2519) were
the subject of several studies, focused mostly on some microstructural aspects of welds, their basic
mechanical properties, heat distribution, and optimization of welding parameters (also using artificial
intelligence) [16–21]. Despite major progress in the mentioned fields of study, the issue of the mostcomplex mechanical properties of FSW joints received disproportionately less attention, and the aspects
providing their structural integrity (corrosion, fatigue, dynamic load) are still a gap in the current stateof knowledge, especially for less popular alloys such as AA2519. All changes caused by the FSW process
generate discontinuity in the welded workpieces: differences in the grain size, in the distributionof the strengthening phase precipitates (for heat-treated aluminum alloys) and the residual stresses,
and the evolution of the surface geometry and roughness, that result in local stress concentration
during load-carrying operating of a welded element [22–29]. In these terms, several studies have been
undertaken to investigate the fatigue properties of 2XXX aluminum alloy FSW joints in the past few
years. Xu et al. investigated the low cycle fatigue (LCF) properties of 2219-T62 alloy under various FSW
parameters and cooling conditions (air/water) and reported that fatigue life decreased with increasing
welding speed (60 to 200 mm/min) and that it was independent of the tool rotation speed (300 to1000 rpm) [30]. The same study showed that the joints obtained by water-cooled FSW have higher stressamplitudes and fatigue lives. Sun et al. performed research on fatigue modeling and life prediction of
AA2219-T6 FSW joints; focusing on the microstructural aspects of the joints supported by the results of
micro tensile tests the authors proposed a model with good accuracy [31]. Vuherer et al. analyzed
fatigue behavior of AA2024 T-351 aluminum alloy sheets joined by FSW with various parameters
and based on the results they described the relationship between welding velocity and basic fatigue
properties focusing on the issue of heat input and coarse of microstructure [32]. Based on our own
research, we can state that post-weld heat treatment decreases the fatigue strength of AA2519 FSW
joints significantly [33] and the optimal welding parameters for welding of AA2519-T62 lie within
the range of 600–800 rpm tool rotation speed and 100 mm/min welding velocity, giving relatively
high joint efficiency (around 80%) and good fatigue properties [34]. Considering the issue of highcomplexity, which is evident in the behavior of FSW joints during cyclic loading in the low cycle
fatigue regime, a number of factors have to be taken under examination, susceptible to the highplastic strain amplitude (e.g., heat-affected zone, low-hardness zone, stir zone/thermo-mechanicallyaffected zone interface) [34–37]. LCF is important from the point of view of estimating the durability ofstructural elements at the design stage of the structure, during its operation, as well as in the analysisof the assessment of the structure life (and the possibility of its extension). The aim of this paper is toinvestigate the LCF properties of 5 mm-thick AA2519-T62 FSW joints together with basic mechanicalproperties, microstructure analysis, and residual stresses measurements.
2. Material and Experimental Procedures
The investigated material was AA2519-T62 extrusion with dimensions of 5 × 80 × 250 mm.
The precipitation hardening process was performed by solution treatment (530 ◦ C/2 h + cooling incold water) and subsequent artificial aging (165 ◦ C/10 h). The chemical composition and mechanicalproperties of the alloy are presented in Tables 1 and 2, respectively.
Table 1. Chemical composition of AA2519 (% weight) [34].
Fe

Si

Cu

Zn

Ti

Mn

Mg

Ni

Zr

Sc

V

Al

0.11

0.08

6.32

0.05

0.08

0.17

0.33

0.02

0.19

0.16

0.10

Base

Materials 2020, 13, 5226

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Table 2. Mechanical properties of AA2519-T62 [34].

Materials 2020, 13, x FOR PEER REVIEW

Young Modulus (E)78 GPaYield Strength (Re0.2 )Tensile Strength (Rm )Table 2. Mechanical properties of AA2519-T62 [34].
312 MPa469 MPa

3 of 18

Elongation (A)19%Young Modulus (E) Yield Strength (Re0.2) Tensile Strength (Rm) Elongation (A)78 GPa312 MPa469 MPa
19%
Prior to the joining process, the workpieces to be welded were ground and cleaned with isopropanol.
The FSW was conducted with an ESAB FSW Legio 4UT (ESAB, Gothenburg, Sweden) machine with
Prior to the joining process, the workpieces to be welded were ground and cleaned withthe applied axial force of 17 kN. The tool rotation speed and tool traverse speed were equal toisopropanol. The FSW was conducted with an ESAB FSW Legio 4UT (ESAB, Gothenburg, Sweden)
800machine
rpm andwith
100themm/min,typeTheof tool
wasspeed
an MX
Triflute(ESAB,speed
Gothenburg,
appliedrespectively.
axial force ofThe
17 kN.
tool usedrotationandtool traverse
were◦ . These parameters were selected based on the previous
Sweden)
and
the
tilt
angle
was
set
to
2
equal to 800 rpm and 100 mm/min, respectively. The type of tool used was an MX Triflute (ESAB,
study
performedby authors
[34].
joint
was sectioned
perpendicular
the welding
Gothenburg,
Sweden)and thetilt The
angleobtained
was set to
2°. Theseparameters
were selectedto
basedon thedirection.
The
metallographic
preparation
involved
cutting ajoint
samplethe joint,
mounting ittoin theresin,
previous
study
performed byauthors [34].
The obtained
wasfrom
sectioned
perpendicular
grindingitwithabrasive
papers
of
80,
320,
500,
800,
1200,
2400,
and
4000
gradations,andpolishingwelding direction. The metallographic preparation involved cutting a sample from the joint,
withdiamondpastes3 and1 µm).
The ofpreparedetchedKellermounting
it inresin,(gradations
grinding it ofwithabrasive
papers80, 320, sample
500, 800,was
1200,
2400, withand 4000
reagent(20 mL
Hpolishing63%
HNO3pastes+ 1 mL
38% HClof+3 andone 1drop40%HF) foraboutwas
10 s.
gradations,andwithdiamond(gradationsμm). ofThe
preparedsample2 O + 5 mL
with Kellerobservations
reagent (20 mL
H2Obeen+ 5 mL
63% HNO
1 mL
38% HClLEXT+ oneOLS
drop4100of 40%HF) for
Theetchedmicrostructure
haveconducted
on3 +theOlympusdigitallight
about 10 s.
The microstructure
observations
have microhardness
been conducted(Struers
on the Olympus
LEXTDenmark)OLS 4100 of
microscope(Olympus,
Tokyo, Japan).
The Vickers
Copenhagen,lightmeasured
microscopeon(Olympus,
Tokyo,
Japan). The
microhardness
Copenhagen,thedigitalweld wasits polishedcross-section
byVickers
applying
0.98 N load (Struersin accordancewith theDenmark)
of
the
weld
was
measuredon
its
polishedcross-section
byapplying
0.98
N
load
in
EN ISO 6507 standard. The distribution of microhardness was obtained for the top, middle, and bottom
accordance
with
the
EN
ISO
6507
standard.
The
distributionofmicrohardnesswasobtainedfortheparts of the cross-section of the weld: 0.8, 2.5, and 4.4 mm from the face of the joint, respectively.
middle,and bottom
parts ofofthecross-section
the weld:by
0.8,
2.5, and4.4 mmfrom theto
faceof
Thetop,basic
mechanicalproperties
thejoint were of
examined
tensiletesting
accordingASTM
thejoint,
respectively.
The
basic
mechanicalproperties
of
the
joint
were
examinedbytensiletesting
standard E8/E8M–13a [38]. Tensile tests were carried out on an Instron 8802 MTL universal testingaccording to ASTM standard E8/E8M–13a [38]. Tensile tests were carried out on an Instron 8802 MTL
machine (Instron, Norwood, MA, USA) with WaveMatrix computer software (Instron, Norwood,universal testing machine (Instron, Norwood, MA, USA) with WaveMatrix computer software
MA, USA). Additionally, the tensile test was supported by digital image correlation (DIC) in order to(Instron, Norwood, MA, USA). Additionally, the tensile test was supported by digital image
examine the local strain in the welded joint zone. For DIC, the Dantec Q-400 optical system (Dantec
correlation (DIC) in order to examine the local strain in the welded joint zone. For DIC, the Dantec
Dynamics GmbH, Ulm, Germany) was used, and the obtained data were processed with Istra4D
Q-400 optical system (Dantec Dynamics GmbH, Ulm, Germany) was used, and the obtained data
4.4.7
software(version
4.4.7). The
testing
was4.4.7).
conducted
on antesting
Instron8802
servohydraulicwereprocessedwith Istra4D
4.4.7fatigue
software(version
The fatigue
wasconducted
on anfatiguetestingsystemaccordingto
ASTM
E606/E606M
standard.
To
measurethevaluestrain during
Instron 8802 servohydraulic fatigue testing system according to ASTM E606/E606M ofstandard.
To
testinga 2520-603
dynamic
extensometer(Instron,
Norwood,USA) with(Instron,a gaugeNorwood,
length of 25
mmmeasurethe valueof strain
during testinga 2520-603
dynamicMA,
extensometer
MA,
was
used.
The
fatiguetestsofwere
carriedout onfivevariousof totalout
strainamplitude:
0.35%,
USA)witha gaugelength25 mm
was used.
The
fatiguetestslevels
were carriedon fivevarious levels
0.4%,
0.5%,
0.6%,
and
0.8%
with
strain
ratio
R
=
0.1.
For
each
level,
three
sampleswereexamined.
of total strain amplitude: 0.35%, 0.4%, 0.5%, 0.6%, and 0.8% with strain ratio R = 0.1. For each level,
Thethreefracturesurfaces
tensile andfatiguesampleswereanalyzed
the scanningmicroscope
samples
wereofexamined.
The
fracturesurfacesof tensile
andon
fatiguesamples electron
were analyzedon(SEM)
Jeol JSM-6610(Jeol,
Tokyo, Japan).
The JSM-6610
samples fortensileandfatiguetestingwere
the scanningelectronmicroscope(SEM) Jeol
(Jeol,
Tokyo,
Japan).
The
samples
forpreparedtensileandfatiguetesting
were prepared
with
with
the
geometrypresented
in Figure
1.the geometry presented in Figure 1.

Figure1. 1.
Theand (b)(b)lowlowcycle
cyclefatigue
fatiguetesting.
testing.
FigureThescheme
schemeofofsample
sampleforfor (a)(a) tensile
tensile test andMaterials 2020, 13, 5226

4 of 18

Materials 2020, 13, x FOR PEER REVIEW

4 of 18The
methodwhich
is describedin
The values
values ofofresidual
residualstresses
stresseswere
wereestablishedestablishedbybythe
thehole-drillinghole-drilling
methodwhich
is described
ASTM
E
837
in
detail[39].
Becauseofthethickness
of
the
plate
(5
mm),
the
blind-hole
procedure
wasin ASTM E 837 in detail [39]. Because of the thickness of the plate (5 mm), the blind-hole procedureused.
In thisthe hole
drilledat the depth
2 mm ofandthe released
arestrains
measured
was used.
Inapproach,this approach,theisholeis drilled
at theofdepth
2 mm
and thestrains
releasedarebythe
specially
designedrosette.
The
accurate
location
of
the
hole
in
the
centerpointoftherosettewasmeasured by the specially designed rosette. The accurate location of the hole in the center pointofensuredbywas
using
the VPG
System RS-200 milling
the rosette
ensuredby Micro-Measurement
using the VPG Micro-Measurement
System guideRS-200(Micro-Measurements,
milling guide (MicroRaleigh,NC, USA),
whichNC,is presented
in Figure
2a. A constant
value
used
Measurements,
Raleigh,USA), which
is presented
in Figure2a.ofAYoung’s
constantmodulus
value ofwas
Young’sduringtheanalysis,whichequals78
GPa.
The
measurements
were
made
in
10
pointslocatedatmodulus was used during the analysis, which equals 78 GPa. The measurements were made in 10
different
distancesof the joint
axis: two
joint
axis,
two
thejoint
weldline,two
andatsix
inweldthe vicinityofpoints locatedat different
distancesof inthethe
joint
axis:
two
inattheaxis,
theline, andthejoint,
bothattheadvancingside
andretreating
side.
The
schemaofmeasuringpoint
locations
issix in the vicinity of the joint, both at the advancing side and retreating side. The schema of measuring
presented
inFigure2b.
Threemeasuring
pointshad
tobeshiftedfromothersduringmounting
inpoint locations is presented in Figure 2b. Three measuring points had to be shifted from others duringorder
to makethe possibilityof possibility
proper installation
rosettes. of rosettes.
mounting
in orderto make theof properofinstallation

Figure 2.2. The
Theview
viewofofa measuring
stand
RS-200systemwiththe tensometer
bridgeTravelerESAMa measuring
standwithwith
RS-200systemwith thetensometer
bridge ESAM
Traveler
Plusthe(a)locationand the locationof numberedmeasuring
pointson theplatewith friction
stir welding
Plus(a) andof numberedmeasuring
points on
the platewithfrictionstir welding
(FSW)
(FSW)
joint (b).
joint
(b).

Strains revealed during drilling and measured at the individual
individual gauges of the rosetterosette enabled
enableddetermining ofofthe
thevalues
valuesofprinciple
stressesσ
andσ
togetherwith
theirdirections.
of principle stresses σmax andmaxσmin together
min with their directions. Obtained
Obtained
resultsused tothe
calculatethevalues longitudinalof stresses, longitudinal
σ11 , andresults wereused wereto calculate
values ofstresses,
σ11, and transverse
σ22, transverse
connected
σ,connected
with
the
FSW
joint.
Theerrorofcomputedvaluesofresidualstresseswasestimated
at
with
the
FSW
joint.
Theerrorofcomputedvaluesofresidualstresseswasestimatedat±(8–10)MPa
22±(8–10)MPa inthe wholewhich isAll
acceptable.
All testsinmentioned
in the experimental
partin the whole
range,
whichrange,is acceptable.
tests mentioned
the experimental
part were carriedwereout 4theweeks
afterprocess
the joining
processin orderthe
to stabilize
theofpropertiesof the weld [2].
out 4 carried
weeks after
joiningin orderto stabilize
properties
the weld [2].
3.
Results andand Discussion
Discussion
3. Results
3.1. Macroscopic Observations
3.1. Macroscopic Observations
The weld face of the obtained joint is presented in Figure 3. For FSW joints, two sides of the
The weld face of the obtained joint is presented in Figure 3. For FSW joints, two sides of the weld
weld can be distinguished: the retreating side (the direction of tool rotation is opposite to the welding
can be distinguished: the retreating side (the direction of tool rotation is opposite to the welding
direction) and the advancing side (the direction of tool rotation is concordant with the welding
direction) and the advancing side (the direction of tool rotation is concordant with the welding
direction) [2]. The weld face is characterized by a typical, regular weld track, formed by the tool
direction) [2]. The weld face is characterized by a typical, regular weld track, formed by the tool
shoulder. As can be observed, no visible defects are present and the flash is localized mainly on theshoulder. As can be observed, no visible defects are present and the flash is localized mainly on theretreating side of the weld.
retreating side of the weld.

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13, 5226
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55 of
of 18
18
5 of 18

Figure 3. The weld face.
Figure 3. The weld face.

The light microscopy image of the macroscopic view of the joint cross-section is presented in
The4.lightimage ofthe macroscopic
view of used
the joint
is presentedin
FigureLackmicroscopyof visible defects
confirms
that parameters
are cross-section
appropriate for
friction stir
Figure4.4.ofLackofalloy.
visibledefects
confirms
parametersusedappropriate
for friction
stirweldingof
Lackof
visibledefects
that isparameters
used
are welding
appropriate
forfrictionstirwelding
this
The
width
ofconfirms
the thatstir zone
about
8.5aremm.
The
process
has
reducedthisalloy.
The
width
of
the
stir
zone
is
about
8.5
mm.
The
welding
process
has
reducedthethicknessofweldingof
thisalloy.
The
width
of
the
stir
zone
is
about
8.5
mm.
The
welding
process
has
reducedthe thickness of the workpiece from 5 mm to 4.5 mm in the center of the weld. Generally, in FSW joint
thefrommmsubzone
to 4.5frommminthedistinguished:centertheinweld.
Generally,
FSWGenerally,
joint the twoandthicknessthe 5workpiece
5 bemm
to 4.5 ofmm
thecenterof theinweld.
in zones
FSW
joint
the workpiecetwo zonesof
and
one
can
thermo-mechanicallyaffectedzone(TMAZ)
one
subzonecanbedistinguished:
thermo-mechanicallyaffectedzone(TMAZ)
formedduetoeffects
of
the
two
zones
and
one
subzonecanbedistinguished:
thermo-mechanicallyaffectedzone(TMAZ)
formed due to effects of temperature and severe plastic deformation, stir zone (SZ), a specifictemperatureandsevereplastic
deformation,
stir
zone
(SZ),
a
specific
subregionofTMAZ
localizedinformeddue
to effects
of temperature
andpartsevereplasticcharacterized
deformation,bystir(SZ),ofaultrafine,specific
subregionof TMAZ
localizedin the centralof a joint,
the zonepresence
thecentralpartofajoint,
characterizedbythepresenceofultrafine,
dynamicallyrecrystallized
subregionofTMAZ
localizedinthecentralpartofajoint,
characterizedbythepresenceofultrafine,
dynamically recrystallized grains, and heat-affected zone (HAZ) affected only by the heat grains,of theandheat-affectedzone (HAZ)
affected
only
by of
theonionheatof (HAZ)
theprocess.
Theonlyobservations
revealed
dynamicallyrecrystallized
grains,andpresenceheat-affectedzone
affectedby theheatof theprocess.
The observations
revealedtheringpatterns
in thestir
zone,
visiblemainlythepresenceof onionring
patterns
in thestir zone,
visiblemainlyat theadvancing
side
of
the
weld.
process.
The observations
revealedthepresence
ofin
onion
ring
patterns
inof
the
stir
zone,
visiblemainlyat theadvancingside oftheweld. The
differences
the macrostructure
theTMAZ/SZ
boundaries
The
differences
insidetheofmacrostructureofthe been
TMAZ/SZ
at theadvancingandboundaries
retreating
atadvancing
the weld.sides
The differencesinthe boundaries
macrostructureof the
TMAZ/SZ
at thetheadvancingandretreating
have
taken
under investigation
duringmicrostructure
sides
have
been
taken
under
investigation
duringmicrostructure
analysis.
attheadvancingandretreating
sides
have
been
taken
under
investigation
duringmicrostructure
analysis.
analysis.

zones.
Figure 4. Macrostructure of the joint cross-section with distinguished zones.
Figure 4. Macrostructure of the joint cross-section with distinguished zones.

3.2.
Microstructure Analysis
3.2. Microstructure
Analysis andand Microhardness
Microhardness Measurements
Measurements
3.2. Microstructure
AnalysisandMicrohardness
Measurements
Theadvancing
side
of of
thethejoint
is characterizedbyThe microstructure
microstructureof
ofthe
theTMAZ/SZ
TMAZ/SZinterface
interfaceatatthetheadvancing
side
joint
is characterized
the
clear
borderbetweendynamicallyrecrystallized
grains
of
the
stir
zone
and
elongatedgrainsoftheThe
microstructureoftheTMAZ/SZ
interfaceattheadvancing
side
of
the
joint
is
characterized
by the clear border between dynamically recrystallized grains of the stir zone and elongated grains
thermo-mechanicallyaffectedaffected
zone, which
the flowof flowtheduringthestirringbythethermo-mechanically
clear border between
dynamicallyrecrystallized
grainsof material
thestir materialzone
andelongatedgrainsof thezone,visualize
which visualize
theof theduringthe process
stirring(Figure5a).
Concurrently,attheretreating
side,
the
borderbetween
SZ
and
TMAZ
is
of
transitionofthethermo-mechanicallyaffected
zone,
which
visualize
the
flow
of
the
material
duringthestirring
process (Figure 5a). Concurrently, at the retreating side, the border between SZ and TMAZ is ofnature(Figure5b).
The
microstructureof TMAZ
in TMAZ
close
to between
SZ can tobeSZ
identified
as bands
process(Figure5a).
Concurrently,
at theretreatingside,proximity
theborder
and
TMAZ
is of
transition
nature(Figure5b). The microstructureofin close
proximity
SZ
can beidentifiedofrecoveredgrainsirregularly
separatedbyrecrystallized
grains.
These
differences
betweeneachtransition
nature(Figure5b).
The
microstructure
of
TMAZ
in
close
proximity
to
SZ
can
be
identified
as bands of recovered grains irregularly separated by recrystallized grains. These differences between
side
have
sourcein the irregularly
unsymmetrical
distributionof the temperatureduringthe
FSWtheprocess.
asbands
recoveredgrainsseparatedbydistribution
recrystallized
These
differences
betweeneach
sideoftheir
have
their sourcein the unsymmetricalof grains.
the temperatureduringFSW
Studiesrevealedthat
theretreatingside
ischaracterizedbyhighereffectofthetemperature[40,41].
eachsideStudies
have theirsourcetheretreating
unsymmetricalthetemperature
the
FSW
process.
revealed
thatintheside is distribution
characterizedofby
highereffect of during
the temperature
Duringthe generated
heat
influences
theisplastically
deformed
microstructure
promotingprocess.
Studies welding,revealed
thatgenerated
theretreating
characterizedby higher
effect oftheby
temperature[40,41]. welding,
Duringtheheatsideinfluences
the plastically
deformed
microstructurebyheat-activatedphenomena
such
as
grain
recovery
andrecrystallization.
As
can
be
seen,
the
[40,41].
Duringwelding,thegeneratedheat
influencestheplastically
deformed
microstructurebypromoting heat-activated phenomena such as grain recovery and recrystallization. As candynamic
be seen,recoveryhas
arecovery
predominant
whilesuchfine,asdynamicallyrecrystallized
grainsare localized
mainlypromotingheat-activatedgrain
recoveryand recrystallization.
As canbeseen,
the dynamic
hasphenomenaarole
predominant
role
while
fine,
dynamicallyrecrystallized
grainsarebetweenlarge,
deformedgrains
in
the
form
of
long
bands
(Figure5b).
Thissuggeststhat
thegrainsthedynamic
recoveryhasapredominant
role
while
fine,
dynamicallyrecrystallized
grainsarelocalized mainly between large, deformed grains in the form of long bands (Figure 5b). This suggests
which
undergodynamic
recrystallization
arecompressed
andintensely
elongatedby5b).
theThislargergrainslocalized
mainlybetweenlarge,dynamic
deformedgrainsin the form
long bands(Figuresuggeststhat thegrainswhich
undergorecrystallization
are ofcompressed
andintensely
elongatedbyinthestirring
processandthequantityofheat
allowsthemtorebuild
theirseverely
deformedthatthegrainswhich
undergodynamic
recrystallization
arecompressed
andintensely
elongatedbythe larger grains in the stirring process and the quantity of heat allows them to rebuild their severely
microstructures.
Thishasandnot beenobserved
theallows
advancing(Figuretheir
5a).
thelarger grains
inthephenomenon
stirring
the hasquantity
of at
heat
them
to
rebuild
deformed
microstructures.
Thisprocess
phenomenon
not beenobserved
atthesideadvancingsideseverely(Figuredeformed
microstructures. This phenomenon has not been observed at the advancing side (Figure
5a).
5a).

Materials 2020, 13, x FOR PEER REVIEW
Materials 2020, 13, 5226

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6 of 18

Materials 2020, 13, x FOR PEER REVIEW

6 of 18

Figure 5. Microstructure of thermo-mechanically affected zone/stir zone (TMAZ/SZ) interface at theadvancing side (a) and at the retreating side (b).
thermo-mechanically affected zone/stir zone (TMAZ/SZ) interface at the
Figure 5. Microstructure of thermo-mechanicallyretreating side
side (b).
(b).
advancing side (a) and at the retreating

The microhardness distribution of the join is presented in Figure 6. The obtained microhardness
distribution
allows observationof a typical“W”-shaped
hardnesscurve.
In thecase ofmicrohardness
the advancing
The microhardness
distributionof the join
is presented
in Figure6. Theobtainedside,
thelowestvalue
ofmicrohardness(108
HV0.1)
wasreportedatthedistanceof
6 mmon thedistribution allows observation of a typical “W”-shaped hardness
advancinghardness curve.
curve. In the case of thebottomof valuethejoint.
Simultaneously,ontheretreatingside,atthe
significant
of weldedside,thelowestofof
microhardness(108
HV0.1)
waswasreported
theat
distanceof softening
6 mm
thebottomthepartlowestvalue
microhardness(108
HV0.1)
reported
the distance
of on
6 mm
on thematerialisreported.
Thelowhardnesszone
islocalizedatthedistance
of
8
to
10
mm
from
joint
part
of thejoint.
Simultaneously,on the retreatingside, the significant
softening of
welded materialisbottompartof thejoint. Simultaneously,on the retreatingside, the significant
softeningof thewelded
line,
where
the
material
washeatedwithout
being
strain-hardenedbyplastic
deformation.
Generally,
reported.
Thelowhardnesszone
islocalizedatthedistance
of
8
to
10
mm
from
the
joint
line,
where
the
material is reported. The low hardness zone is localized at the distance of 8 to 10 mm from the joint
it canwhere
bewas
statedthat without
thewasreduction
microhardness
theretreatingsideof thematerial
heatedbeing instrain-hardened
bypredominantly
plastic deformation.
it canGenerally,
be statedline,
thematerial
heatedwithout
being strain-hardened
byaffects
plasticGenerally,
deformation.
joint(about25%),
whichconfirms
thehighereffectofheat
duringwelding
in
this
area.
Due
to
that
in themicrohardness
the retreating
sidetheof retreating
the joint (about
25%),
it canthe
bereduction
stated that
reduction inpredominantly
microhardnessaffects
predominantly
affectsside ofthedynamic
recrystallizationin theofstir
zone
leading
to theformationof welding
fine-grain
whichconfirms
the higher
heat
duringwelding
in
area.
Due
to
dynamic
recrystallizationinjoint(about25%),
which effectconfirms
thehighereffectof thisheat
duringinmicrostructure,
this area. Due thetomicrohardness
slightly
increases
compared
to
TMAZ
and
HAZ.
As
can
be
seen,
the
highestincreases
valuetheof
thestir zone
leading to the informationfine-grain
theof
microhardness
slightly
dynamic
recrystallization
the stir ofzone
leadingmicrostructure,to the formation
fine-grain microstructure,
microhardness
in
the
stir
zone
is
obtained
for
the
upper
part
of
this
zone,
where
the
effectofthetoolcomparedto TMAZ
andincreases
HAZ. Ascomparedcan be seen,
the highest
value Asof microhardness
the stirvalue
zone ofismicrohardness
slightly
to TMAZand HAZ.
can be seen, theinhighest
shoulderseverelyrefinedthegrains.
obtainedfor theinupper
partzoneof thiszone, where
the oftool
shoulderseverely
thethegrains.
microhardness
the stir
is obtained
for the effectupperofpartthiszone, where
therefined
effect oftool
shoulder severely refined the grains.

FigureFigure 6.
6. Microhardness
Microhardness profile
profile ofof thethe welded
welded joint
joint cross-section.
cross-section.

3.3. Tensile Test Results
Figure 6. Microhardness profile of the welded joint cross-section.
3.3. Tensile Test Results
The obtained tensile curves for AA2519-T62 base material and its friction stir welded joint are
3.3. Tensile
Test Results
The obtained
tensile curves for AA2519-T62 base material and its friction stir welded joint arepresented in Figure 7. The welding process has caused a significant reduction in elongation to failure
presented
inFigure7. Thecurves
welding process
has caused amaterial
significant
reduction
instirelongation
tofailureobtained tensile
AA2519-T62
andweldedjoint
areof theThematerial—fromthe value offor
19%
to 7%. Thebase
tensile strengthof itsthefriction
tested joint
was establishedofthematerial—from
the
value
of
19%
to
7%.
The
tensilestrengthofthetestedjoint
wasestablishedpresented
in Figure
The welding
has causedComparing
a significantthis
reductionin elongation
to failureas389 MPa,
which 7.
represents
83%process
joint efficiency.
value withthe resultsof theas the389material—from
MPa, which represents
83%
joint
efficiency.
Comparing
this
value
with
the
resultsof the
of
the
value
of
19%
to
7%.
The
tensilestrengthofthetestedjoint
wasestablishedstudies [16,18], it is very high for high-strength aluminum alloy of 2XXX series, which can be partly
studies[16,18],
it
is
very
high
for
high-strength
aluminum
alloy
of
2XXX
series,
which
can
be
partlyas 389 MPa, which represents 83% joint efficiency. Comparing this value with the results ofthestudies [16,18], it is very high for high-strength aluminum alloy of 2XXX series, which can be partlyMaterials 2020, 13, 5226
Materials
Materials2020,
2020,13,
13,xxFOR
FORPEERPEERREVIEWREVIEW

7 of 18
7 7ofof1818

explained
explained
by
the
high
thicknessofofthetheworkpiece(5(5mm),
mm),
which
often
results
ininthethehighestvaluesofof
explainedby
bythe
thehighhighthickness
thicknessof
theworkpiece
workpiece(5mm),whichwhichoften
oftenresults
resultsin
thehighest
highestvalues
valuesof
jointefficiency
[2].
joint
jointefficiency
efficiency[2].
[2].

Figure7.7.Engineering
tensilestress–strain
curvesofofbasebasematerial
andFSW
joint.
Figure7.
Engineeringtensile
tensilestress–strainstress–straincurves
curvesof
basematerialmaterialandandFSW
FSWjoint.
joint.
FigureThe
The
digitalimage
correlation
techniqueprovidesmore
detailsabout
weldedspecimenbehavior
Thedigital
digitalimage
imagecorrelation
correlationtechnique
techniqueprovides
providesmore
moredetails
detailsabout
aboutwelded
weldedspecimen
specimenbehavior
behavior
duringthetensiletest[42,43].
The
obtainedstrainmaps
resulting
from
the
DIC
measurement
duringthetensiletest[42,43].
resulting
from
the
DIC
measurementfor
fiveduring the tensile test [42,43]. The obtained strain maps resulting from the DIC measurementfor
forfivefivepointsin
Figure8.8.8.
pointsmarkedonthecurve(Figure7)7)arearepresented
inin
Figurepointsmarked
markedononthethecurve
curve(Figure(Figure7)
arepresented
presented
FigureFigure 8. Strain distribution maps of the tensile sample corresponding to the points marked in Figure 7.
FigureFigure8.8.Strain
Straindistribution
distributionmaps
mapsofofthe
thetensile
tensilesample
samplecorresponding
correspondingtotothe
thepoints
pointsmarked
markedininFigure
FigureWeld faces marked with white lines.
7.7.Weld
Weldfaces
facesmarked
markedwith
withwhite
whitelines.
lines.

The
Thevisualization
visualizationofofstrainstraindistribution
distributionallows
allowsobserving
observingofofits
itsuneven
unevennature.
nature.The
TheSZ
SZand
andTMAZ
TMAZ
arearecharacterized
characterizedby
bythe
thehighest
higheststrain,strain,localized
localizedmainly
mainlyininthe
thecentral
centralpartpartofofthe
theSZ
SZand
andadvancing
advancingside
sideofofTMAZ.
TMAZ.Of
Ofnotenoteisisthe
thefact
factthat
thatdespite
despitethe
theretreating
retreatingsite
sitethetheTMAZ
TMAZhashasreceived
receivedaahigher
higheramountamountofofheat
heatduring
duringthethewelding
weldingprocess;process;its
itsbehavior
behaviorininthe
thetensile
tensiletest
testreveals
revealsthe
thespreadspreadofofthe
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8 of 18

The visualization of strain distribution allows observing of its uneven nature. The SZ and TMAZ
are characterized by the highest strain, localized mainly in the central part of the SZ and advancing sideof TMAZ. Of note is the fact that despite the retreating site the TMAZ has received a higher amount ofMaterials 2020, 13, x FOR PEER REVIEW
8 of 18
heat during the welding process; its behavior in the tensile test reveals the spread of the strain without
significant
concentration
the specific area.
failurein theoccurred
theoccurred
TMAZ/SZ
strain without
significantin
concentrationin theThespecificarea.
Thetensile
failuretestin thetensileintest
ininterfaceontheadvancing
side
of
the
weld.
The
fracturesurfaceofthespecimenwassubjected
tothe TMAZ/SZ interface on the advancing side of the weld. The fracture surface of the specimen wasscanning
microscope(Figure 9). (Figure 9).
subjectedelectron
to scanningelectronobservations
microscope observations

Figure 9.
9. Fracturetensile sample.
sample.
FigureFracture surface
surface ofof FSW
FSW tensile

The observations have revealed that decohesion took place on the boundary between SZ and
TMAZ. This area was identified in the microstructure analysis part (Figure 5a) as consisting ofseverely deformed, elongated grains of TMAZ adjoining the dynamically recrystallized grains
grains ofof SZ.
SZ.
differences in the microstructure
microstructure promote
promote decohesion
decohesion andand it can explain the better coherency ofSuch differences
SZ/TMAZinterface
interfaceon
onthe
theretreating
retreating
side,
where
boundary
betweenis clearnot clear
the SZ/TMAZ
side,
where
thetheboundary
betweeneacheachzonezoneis notandand TMAZ
has partly
recrystallized
microstructure.
TMAZ
has partly
recrystallized
microstructure.
3.4.
Residual StressStress Measurements
Measurements
3.4. ResidualIn
in inthetheFSW
joint
the thefollowing
equations
werewereusedused[44],
In orderorder totocomputecomputethe
theresidual
residualstresses
stressesFSW
joint
following
equations
Equations(1)–(3):[44], Equations(1)–(3):qσ= ε + ε−(1)1 ε − ε 2+ ε + ε − 2ε 2 (MPa)3−∙(ε3 − ε1 ) + (ε3 + ε1 − 2ε2 ) (MPa)σmax = 1 ∙
(1)
4·A
4·B
q
ε − ε
=
+
+ ε + ε − 2ε
(MPa)
σ
(2)
∙ ε3
∙ 1
ε1 +
+
σmin =(ε3 − ε1 )2 + (ε3 + ε1 − 2ε2 ) 2 (MPa)
(2)
4·A
4·B
ε − 2ε + ε1
α = arctg(3)
ε − 2ε2 + ε31
α =2 arctg 1 ε − ε(3)2
ε3 − ε1
where: σmax and σmin—the principle stresses; ε1, ε2, and ε3—the strains measured at the gauges ofwhere: σmax and σmin —the principle stresses; ε1 , ε2 , and ε3 —the strains measured at the gauges ofrosette no. 1, 2 and 3, respectively (Figure 2b); A and B—the coefficients dependent on material
rosette no. 1, 2 and 3, respectively (Figure 2b); A and B—the coefficients dependent on material
properties, type of rosette, and dimensions of drilled hole; and α—the angle between gauge no. 1 andproperties, type of rosette, and dimensions of drilled hole; and α—the angle between gauge no. 1 andthe direction of the nearest principle stress.
the direction of the nearest principle stress.
The obtained results are presented in graphical form in Figure 10. Red arrows indicate the
The obtained results are presented in graphical form in Figure 10. Red arrows indicate thedirections of tension stresses and blue arrows indicate the directions of compression stresses.
directions of tension stresses and blue arrows indicate the directions of compression stresses.

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Materials 2020, 13, x FOR PEER REVIEW

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Figure10. Thedirections
andvaluesofofdetermined
determined residual
in in
thetheFSW
jointjoint
and andits vicinity;
Figure10. Thedirections
andvaluesresidualstresses
stressesFSW
its vicinity;
σ11 and σ22 are the longitudinal and transverse stresses, respectively.
σ11 and σ22 are the longitudinal and transverse stresses, respectively.
Figure 10. The directions and values of determined residual stresses in the FSW joint and its vicinity;

Results obtained by hole-drilling method were used to compute the values of longitudinal
Results
obtainedby longitudinalhole-drillingwere
usedrespectively.
to compute the values of longitudinal stresses
σ11 and
σ22 are theandmethodtransverse
stresses,stresses σ11 and transverse stresses σ22. Nevertheless, in most cases, the values were obtained directly
σ11 and transverse stresses σ22 . Nevertheless, in most cases, the values were obtained directly during
during the measurements because the directions of the main gauges (tensometers nos. 1 and 3) were
Results obtained
hole-drilling
methodwereused (tensometers
to compute thevaluesthe measurements
becausebythe
directions of
the maingaugesnos.
1 andof3)longitudinal
were convergent
convergent with the directions of searched stresses. The Equation (4) was used in other cases.
stressesσ
11 and transverse stresses σ22. Nevertheless, in most cases, the values were obtained directlywith the directions of searched stresses. The Equation (4) was used in other cases.
σ
σ main gaugesσ− σ(tensometers nos. 1 and 3) wereduring the measurements because
the+directionsof the(4)=+∙ cos2ασ
22 σminconvergent with the directionsof searchedstresses.
Equationwas(maxused
σmax(minσmax) + σmin(max) The(min) −(4)) in other cases.

+σ· cos 2α(22) =
where α is anσ11
angle
betweenthe directionsof searchedstressσ− 2σσ11(22) and known principle stress
2+ σ
(4)
=
+
∙
cos2α
σ
σmax(min). The results are presented in Figure
11.
2
2

(4)

where α is an angle between the directions of searched stress σ11(22) and known principle stresswhere α is an angle between the directions of searched stress σ11(22) and known principle stress
σmax(min) . The results are presented in Figure 11.
σmax(min). The results are presented in Figure 11.

Figure 11. The distribution of longitudinal and transverse stresses in the FSW joint and its vicinity.

The results shown in Figure 11 indicate that tension stresses are present in the joint and its
Figure
11. Thedistribution
longitudinal andand transverse
stresses
in in
thetheFSW
jointandandits vicinity.
Figure11.
The
distribution
ofoflongitudinaltransverse
stressesFSW
joint
vicinity.
nearestvicinity.
Greater valuesof stresses arein a longitudinal
direction
than its
transverse;
additionally, they are greater on the advancing side than the retreating side. As the retreating side is
The results
shown
in Figure
11 indicate
tensionstressesare present
in the
joint
and
The
results
shown
in Figure
11 indicate
thatthattensionstressesarein theandits itsnearestcharacterizedby higher
heat input,
it promotes
stress relaxation
andpresent
can explain
thejoint
lower
value
of
nearestvicinity.
Greatervaluesofstressesareinalongitudinal
directionthantransverse;
vicinity.
Greatervaluesofstressesareinalongitudinal
directionthantransverse;
additionally,
they
residual stress in this zone of the weld [40,41]. The highest stresses are at the weld line on the are
additionally, they are greater on the advancing side than the retreating side. As the retreating side isgreateron the advancing
sideThethan
the retreatingside. reachAs theretreating
is characterizedadvancingside (159 MPa).
stressesin a joint centerof 33–77
MPa sideand aresimilar in all bycharacterized by higher heat input, it promotes stress relaxation and can explain the lower value ofhigherheat
input,
itpromotes
relaxation
andcanexplainvalue
ofresidual
directions.
At thedistance of stressapproximately
10 mm
from
the
weld thejoint,lower
the stresses
havevery lowstressresidual stress in this zone of the weld [40,41]. The highest stresses are at the weld line on thevalues,near
to zero.
It [40,41].
should beThe
notedthat at stresses
these points
thedirection
oflineprinciple
stressesis thein this
zone
of
the
weld
highest
are
at
the
weld
on
the
advancing
advancing side (159 MPa). The stresses in a joint center reach of 33–77 MPa and are similar in all sidemostrotated
of
all.
It
is
probablycausedbythematerialflowduringthestirring
process
resultingin(159 MPa).
TheAt
stressesin a joint
center reach10ofmm
33–77
MPa
similarin all havedirections.
At thedirections.
the distanceof approximately
fromthe andweldarejoint,
the stresses
very low
distanceof approximately
10 mmbefromthethatweld
joint,points
the stresses
have very
low values,
near
to zero.
values,near to zero. It should
noted
at thesethe directionof principle
stressesis themostberotatedall. Itatistheseprobablycausedby the material
flow during
the stirring
process
resultingin It isIt should
notedofthat
pointsthe directionof principle
stressesis the mostrotatedof all.

probably caused by the material flow during the stirring process resulting in the plastic deformation

Materials 2020, 13, x FOR PEER REVIEW

10 of 18

the plastic deformation of less heated areas in the surrounding area. With increasing distance10fromof 18
the joint, the stresses change to compressive which are significantly greater in a longitudinal direction
contrary to transverse. The obtained values of residual stresses and their nature of changes are similar
Materials 2020, 13, x FOR PEER REVIEW
10 of 18ofareas
in thepublications
surrounding[45–50].
area. With
distancestresses
from thethe stresses
to less
thoseheatedavailable
in other
In all increasingcases, the residual
arejoint,
the highest
at thechangetocompressive
which
are
significantly
greaterinalongitudinal
direction
contrarytotransverse.
weldtheline
and lower
at theofcenterof joints.
valuesof stresses
decreaseandchangeplastic
deformation
less heatedareasThein thesurroundingarea.significantly
With increasing
distance
fromThevaluesofintoresidual
andtheir natureof changes
are insimilarto thoseavailable
inthejoint, thestresseschangetostresses
compressive
which
are
significantly
greaterlongitudinal
direction
fromobtained
tensionstresses
compression.
Moreover,themaximum
value
ofa measured
residualstresscontrary
to transverse.
ofofresidual
stresses
theirnaturechangesotherpublications[45–50].
In
all cases,
the residual
stresses
areandthehighestatoftheweld are
line
and
lower
equalsapproximately
50%Theof obtained
theyieldvalues
stresspaternal
material
AA2519-T62.
It should
besimilar
noted
that
those
available
inThe
other
publications
In all cases,theresidualstresses
arefromthehighesttheatthetocenterof joints.using
valuesof stresses
andchangetensionstressesduringthe welding
eitherTIGor[45–50].
lasersignificantly
methodthedecrease
residualstressescanevenreachatthe
yield
weld
line
and
lower
at
the
centerofjoints.
The
valuesofstressessignificantly
decreaseandchangeinto
compression.
Moreover, the maximum value of measured residual stress equals approximately
stress[2,3,51].
from tension stresses into compression. Moreover, the maximum value of measured residual stress
50% ofthe yield stress of paternal material AA2519-T62. It should be noted that during the welding
equals approximately 50% of the yield stress of paternal material AA2519-T62. It should be noted that
3.5. Low
Cycle
Fatigue
Properties
Observations
using
eitherTIG
or lasermethodand
theFracture
residualSurface
stressescan even reach the yield stress [2,3,51].
Materials 2020, 13, 5226

during the welding using either TIG or laser method the residual stresses can even reach the yieldstress[2,3,51]. of stress and plastic strain amplitudes with the number of cycles are shown in
The variations

3.5. Low Cycle Fatigue Properties and Fracture Surface Observations
Figure 12a,b respectively. Regardless of the strain amplitude, three periods of fatigue life can be
3.5. Low
Cycle Fatigue
Properties
Fracture
Surface
Observationswith the number of cycles are shownThe
variations
stressandandplastic
strainamplitudes
distinguished:cyclicofhardening,
cyclicstabilization,and final rapid drop in the value of stressin
Figure12a,brespectively.
Regardlessofthestrainamplitude,
three
periods
fatiguelife
Thevariationsofstressandplastic
strainamplitudes
withthe numberof basecyclesof
areshown
in canamplitude until failure, which corresponds to the cyclic
propertiesof thematerial
[52].
The
be
distinguished:cyclichardening,
cyclicstabilization,
and
final
rapid
drop
in
the
value
of
stressFigure12a,brespectively.
Regardlessofthestrainamplitude,
three
periodsoffatiguelife
canbelength of each period differs depending on the used strain amplitude. Especially, for the highest
distinguished:cyclicwhichhardening,stabilization,
and
final
rapid
inmaterial
the stabilization
value
of Thestressamplitude
until failure,corresponds
to theitcyclic
propertiesof thedrop
base
[52].
lengthvaluesof strain
amplitude(0.5,
0.6, cyclic
and, 0.8%),
is difficult
to identify
the cyclic
periodamplitude
until
failure,which
correspondstothecyclicproperties
of
the
base
material
[52].
The
of
each
differs
dependingon the useduntil
strainamplitude.
Especially, forthe basehighestvaluestheof
andtheperiod
samples
undergocyclic hardening
rapid
failure. Comparedto thematerial,
lengthof each(0.5,
perioddiffers
dependingon the usedstrain amplitude.
Especially,
for thehigheststrainamplitude
0.6,
and,0.8%),
itisdifficulttoidentify
thecyclicstabilization
periodandtheobtained values of stabilized stress amplitude (Figure 12a) are lower for about 25% and also thevaluesof straincyclic
amplitude(0.5, 0.6,
and,rapid0.8%),failure.
it is difficult
to identify
stabilization
periodsamples
undergohardening
until
to thethecyclic
base
material,
theobtained
reductionof samples
fatigue life
was reported[52]. Atuntil
the rapid
sameComparedtime, theregisteredplastic
amplitudes
and theundergocyclic hardening
failure.
Comparedto thebasestrainmaterial,thevaluesofstabilized
stressamplitude(Figure
12a)
are
lower
for
about
25%
and
also
the
reductionof theobtained
AA2519-T62
joint arestressmuchamplitude
higher (Figure
differencescanbe andpartlyvaluesFSWof stabilized(Figure12b).
12a)Theseare lower
for about25%
alsoexplained
the
of
life
was
reported
[52].
At
the
same
time,
the
registeredplastic
strainamplitudesof thebyfatiguethelower
ductilityoftheobtained
joint
compared
to
the
base
material(Figure7).
reduction of fatigue life was reported [52]. At the same time, the registered plastic strain amplitudes
AA2519-T62
FSW jointFSW
are muchhigher12b). These
differences
can canbe partly
explainedby theof the AA2519-T62
joint aremuch(Figure
higher (Figure12b). These
differences
be partly
explained
lowerbyductilityoftheobtained
joint
compared
to
the
base
material(Figure7).
the lower ductility of the obtained joint compared to the base material (Figure 7).

Figure12. Variation
Variationof stress
stressamplitude(a)andplastic
strainamplitude
amplitude
(b)
with
the
numberof cycles.
cycles.
Figure12.
ofamplitude
strainamplitudewithnumberofFigure12. Variationof stress
amplitude(a)(a)andand plastic
plastic strain(b)(b)
withthe thenumberof cycles.

The hysteresis
hysteresis
stress–strain
loops
ofofexaminedexamined
weldedjoint
joint
for
variouslevelsof strainstrainamplitude
The
stress–strain
loops
joint
for
variouslevelsofamplitude
The
hysteresis
stress–strainloopsofexamined welded
weldedforvariouslevelsof strain
amplitude
are presented
presented
in Figure
Figure
13a–e
and
the
stabilized
loopsarearecompared
comparedin
Figure13f.
are presented
in Figure
13a–eandthestabilizedstabilizedloops
loops
inin
Figure13f.13f.
are
in
13a–e
and
the
are
compared
FigureFigure 13. Cont.

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13,xxFOR
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11

Figure13.
13.Stress–strain
Stress–strainloops
loopsofofFSW
FSWjoint
jointtested
testedwith
withεε=0.35%(a),(a),εε===0.4%
0.4%(b),(b),εεε===0.5%
0.5%(c),(c),εεε==
=0.6%
0.6%
Figure==0.35%
loops
of
FSW
joint
0.35%
0.4%
(b),
0.6%
=0.8%
0.8%(e),(e),andandthethecomparison
comparisonof
ofmid-life
mid-lifeloops
loops(f).
(f).
(d),(d),εε==Theobtained
obtainedloops
loopsreflectreflectthe
the fatigue
fatigue properties
propertiesobserved
observedin
inFigure
Figure 12a,b.
12a,b. In
firstcycles
cyclesofofInthethefirst
The
fatiguelife,
cyclichardeningoccurs,
which
is
much
more
visibleinthesamplestestedwiththestrainfatigue life, cyclic hardening occurs, which is much more visible in the samples tested with the strain
amplitudesof
of0.5,
0.5, 0.6,
0.6, andand 0.8%
0.8% (Figure(Figure13c–e).
samestrainstrainamplitudes
amplitudesarecharacterizedby
by aa(Figure 13c–e).
13c–e). The
The sameare characterized
amplitudes
noticeabledissipation
dissipationof
ofstrainstrainenergy
energymanifested
manifestedin
inaarelatively
highwidth
widthofofthe
relativelyhighthemid-life
mid-lifehysteresis
hysteresis
noticeable
loops.The
Thedissipation,
dissipation,caused
causedpredominantly
predominantlyby
bythe
theplastic
plasticdeformation
deformationleading
leadingto
tothe
thedevelopment
development
loops.
of microcracks
microcracks[53],[53], increases
increasessignificantly
significantlytogether
amplitude(Figure(Figure13f).
13f).
together with
with thethe applied
applied strainstrainamplitude
amplitude(Figure13f).
of
The parametersparameters ofof thethe stabilizedstabilized
loops allowed
allowed establishingestablishing ofof thethe relationshiprelationship ofof stress
stress amplitude
amplitude
stabilized loops
amplitudeTheversus plastic
plastic strainstrain amplitude,
amplitude,
presented
inFigure14.
amplitude,presented
presentedin
inFigure
Figure14.
14.
versusFigure14.
Stressamplitude
amplitudeversus
versusplastic
plasticstrainstrainamplitude
amplitudeof
ofFSW
FSWjoint
jointstabilizedstabilizedhysteresis
hysteresis
loops
in
Figure14.
14.Stress
Stressamplitude
versusplastic
strainamplitude
of
FSW
joint
stabilized
hysteresisloops
loopsinin
Figurelog-log
coordinates.
log-logcoordinates.
coordinates.
log-log

Theobtained
obtainedcurve
curvecancanbebedescribed
describedby
bythe
thefollowing
followingequation
equation[54]:
[54]:
The
σσ ==kk εε

(5)
(5)

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The obtained curve can be described by the following equation [54]:σa = k0 (εp ) n0(5)where:
σa —stressamplitude(MPa), εp —plastic strain amplitude [mm/mm], k0 —cyclic
strength
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2020, 13, x FOR
PEER REVIEW
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18
0
coefficient (MPa), and n —cyclic strain hardening exponent. The values of k’ and n’ have been taken
where:—stress
amplitude(MPa),—plastic
strain14:
amplitude [mm/mm], k′—cyclic strength
directly
fromσathefunction
describing
theεpplot
in Figurecoefficient (MPa), and n′—cyclic strain hardening exponent. The values of k’ and n’ have been taken
directly from the function describing thein Figureσ plot= 504.37(ε 14:)0.068ap

σ = 504.37 ε

.

(6)

(6)

The obtained value of the cyclic strength coefficient (504.37 MPa) is almost three times lower thanThe obtained value of the cyclic strength coefficient (504.37 MPa) is almost three times lower
for thebase material (1518.1 MPa) [52]. Although, the decrease of cyclic strength coefficient is expected
than for the base material (1518.1 MPa) [52]. Although, the decrease of cyclic strength coefficient isfor welded
joints,in thisjoints,
case,inthe
reported
is significant
and theandobtainedvalue value
is similartoexpectedfor welded
thiscase, thedrop
reported
drop is significant
the obtainedisthe values
by research[31,36,55–57].
In the next step,thenext
parameters
the stabilized
similarreported
to the values
reportedby research [31,36,55–57].
In thestep, the ofparametersof theloops
werestabilized
used for establishing
the forplots
of elastic andplastic
vs. numberof reversals,
loops were usedestablishing
the plotsof strainelastic amplitudes
and plastic strain
amplitudes
vs.
numberreversals,presented
in ofFigure15. presented in Figure 15.

FigureNumberhalf-cyclereversals
reversals vs.
in in
log-log
coordinates.
Figure15. 15.
Numberof ofhalf-cycle
vs. strainstrainamplitude
amplitude
log-log
coordinates.

The Manson–Coffin–Basquin
equationisisdescribed
described byformula[54]:
The Manson–Coffin–Basquin
equation
bythe
thefollowing
following
formula[54]:σε = ε + ε = σ0 2N + ε 2N
ε = ε + ε = Ef (2N )b + ε0 (2N )c
e

p

E

f

f

f

(7)

(7)

where: σ′f—fatigue strength coefficient (MPa), E—Young modulus (MPa); b—fatigue strength
exponent;
ε′f—fatigue
ductility
coefficient;c—fatigue
ductility
exponent.
The valueof Young
where: σ0 f —fatigue
strengthcoefficient(MPa),and
E—Young
modulus(MPa);
b—fatigue
strengthexponent;
modulusoftheweldedjoint
wasestablishedinthetensile
test
and
it
is
equal
to
68
GPa.
The
0
ε f —fatigue ductility coefficient; and c—fatigue ductility exponent. The value of Young specificmodulus ofvalues have been taken from the functions describing the plots in Figure 15:

the welded joint was established in the tensile test and it is equal to 68 GPa. The specific values have
462.4.
been taken from the functions
describing
the plots
15:
(8)
ε =
2N in. Figure+ 0.42122N
ε=ε +68,000462.4 it is very
In terms of the plastic
close
the parameters
set up for base material (8)ε = εestrain+ εpcomponent,=(2Nf )−0.066+to0.4212(2Nf )−0.911
68,deviates
000 from[52], while the elastic strain component
it noticeably.
The tested samples tend to fail in the HAZ on the retreating side of the joint and for the lowest
In terms of the plastic strain component, it is very close to the parameters set up for basevalue of strain amplitude (0.35%) and in the case of the higher values of amplitude the failurematerial[52], while
elastic strain
component
deviatesfrom itnoticeably.
occurredat thetheboundaryof TMAZ/SZ
on theadvancingsideof the weld. This zone wasThe
tested samples
tend to fail inanalysis
the HAZ
on(Figurethe retreatingof thejointofandfor inthethelowestcharacterized
in the microstructure
part5a) and itsideis alsoa placefailurevaluetensileof strain
and in thecase observations,of the higherthe
valuesamplitude thefailurewere
occurredtestamplitude(Figure 9). (0.35%)
For the fracturesurfacethreeofrepresentative
samplesand tested
at the amplitudesof 0.35%, 0.5%,
0.8%.
The fracturesurfacethe first samplein theat theselectedboundaryof TMAZ/SZ
on the advancing
sideandof theweld.
This zone
wasofcharacterizedispresented
inFigure16a–c.
microstructure analysis part (Figure 5a) and it is also a place of failure in the tensile test (Figure 9).

Materials 2020, 13, 5226

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For the fracture surface observations, the three representative samples were selected and tested at
the amplitudes of 0.35%, 0.5%, and 0.8%. The fracture surface of the first sample is presented in
Figure16a–c.
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Figure 16. SEM images of fatigue surfaces of a specimen tested at a strain amplitude of 0.35%: (a)Figure16. SEM images of fatigue surfaces of a specimen tested at a strain amplitude of 0.35%: (a) overall
overallview
the fracturesurface,(b) ofmagnified viewof theinitiationzone,(c)(a)
FigureSEM ofimagesof(b)fatigue
surfaces
at acrackstrainamplitudeof and
0.35%:
view of the16.fracturesurface,magnifiedview aofspecimen
the crack tested
initiation
zone,
and
(c) propagation
regionpropagation
regionnear
theinitiation
site.
The
yellowarrowsindicatethesecondarycracks.
overall view of the fracture surface, (b) magnified view of the crack initiation zone, and (c)near
the initiation site. The yellow arrows indicate the secondary cracks.
propagation region near the initiation site. The yellow arrows indicate the secondary cracks.
The fracture surface has a typical structure of fatigue fracture, consisting of an initiation site,
The fracture surface has a typical structure of fatigue fracture, consisting of an initiation site,
fatiguecrack propagation
region,and rapid
fracture
area (Figure16a). consisting
The initiation
crack
The fracturesurface has
a typical
structureof fatigue
fracture,of of
anfatigue
site,fatiguecrack
propagation
region,of
and
rapid
fracture
area the(Figure16a).
Theofinitiation
ofinitiation
fatiguecrack
inin
localized
in
the
top
cornertheinvestigatedsample,surfaceisfreevisibledefects,smooth,fatigue crack propagation region, and rapid fracture area (Figure 16a). The initiation of fatigue crack
localized
in
the
top
corneroftheinvestigatedsample,thesurfaceisfreeofvisibledefects,smooth,and contains river-like patterns (Figure 16b). The fatigue crack propagation area is characterized byin localized in the top corner of the investigated sample, the surface is free of visible defects, smooth,and contains
river-likepatterns(Figure16b).
The fatigue
crack propagationarea
is characterized
by thethe presenceof fatigue
striations
withnoticeableparticipationof secondary
cracks(marked withand contains river-like patterns (Figure 16b). The fatigue crack propagation area is characterized byyellowarrows) striations
indicating witha localnoticeable
intensificationof materialofdecohesion
The observed
presenceof fatigue
participation
secondary(Figure
cracks16c).
(markedwith yellow
the presence of fatigue striations with noticeable participation of secondary cracks (marked withfracturedsurfaceis of mixed
ductile andcharacter.
The fractureof theobserved
sample tested
arrows)indicating
a localintensificationof brittle
material
decohesion(Figuresurface16c). The
fracturedyellow arrows) indicating a local intensification of material decohesion (Figure 16c). The observedwiththestrainamplitude
of
0.5%
is
presented
inFigure17a–c.
surface is of mixed ductile and brittle character. The fracture surface of the sample tested with thefractured surface is of mixed ductile and brittle character. The fracture surface of the sample tested
strainof 0.5% is presented
in Figure in
17a–c.
withamplitude
the strain amplitudeof 0.5% is presented
Figure 17a–c.

Figure 17. Cont.

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FigureSEM
imagesof fatigue
surfacesa specimen
testedat a strain
amplitudeof 0.5%:(a)Figure17.17.
SEM
imagesof fatigue
surfacesof aofspecimentestedat a strain
amplitudeof 0.5%:(a) overall
overallviewofthefracturesurface,(b)bottompartofthefracturesurface,and(c)fatiguestriations.
Figure
SEM images
fatigue
surfaces
specimentested andat a(c)strainamplitudeof 0.5%:(a)viewof the17.
fracturesurface,of(b)bottompart of ofthea fracturesurface,fatiguestriations.
The yellow
The yellow
indicate surface,thesecondarycracks.
overallviewarrows
ofthe
thesecondary
fracture(b) bottom
part of the fracture surface, and (c) fatigue striations.
arrowsindicatecracks.
The yellow arrows indicate the secondary cracks.

Thesurfaceisismore
more similar
similar tosampleandandno areas
typicalfor fatigue
fracturefracture
haveThe
surfacetothethetensile
tensilesampleno areas
typicalfor fatigue
been
distinguishedundermacroscopic
observation.
TheThesurfacecontains
part
ofhavethetheThe
surface is moresimilarto the tensile
sample andnoobserved
areas
typicalfor fatigue
fracture
have
been
distinguishedundermacroscopic
observation.
observed
surfacecontains
partof
SZ/TMAZ
interfacewitha visible
texturedemonstrating
plasticflow
inthestirring
process(Figurebeen
distinguishedundermacroscopic
observation.
The
observed
surfacecontains
partof theSZ/TMAZ
interfacewitha visible
texturedemonstrating
plastic
flowin thestirring
process(Figure
17a).
17a).
At
this
interface,itispossibletoidentify
vertical“cliffs”
suggestingthegrain-boundary
SZ/TMAZ
interfacewithavisibletexturedemonstrating
plasticflow
inthestirring
process(FigureAt this interface, it is possible to identify vertical “cliffs” suggesting the grain-boundary separation
separation(Figure17b).
theto higher
the fatigue
striations
and of
17a).
At thisinterface,it However,ispossible
identifymagnification
vertical “cliffs”
suggesting
the andgrain-boundary(Figure17b).
However,thehighermagnification
revealed
the revealed
fatiguestriations
occurrence
occurrenceofnumerous,very
deep
secondarycracks(Figure
17c).
Not
only
the
specific
separation(Figure17b). However,higherthe fatigue striations
andbutnumerous,
very
deep secondary
cracksthe(Figure17c).magnification
Not only the revealed
specific microstructure(Figure 5a)
microstructure5a) but
the secondaryhigh valuescracks
of residual
stresses11) canoccurrenceof (Figure
numerous,veryalso
deep(Figure17c). (Figure
Not only
the promote
specificalso the high values of residual stresses (Figure 11) can promote decohesion in this area. The near-surface
decohesion in thisarea. 5a)Thebutnear-surface
stresses
directly
influence
the initiationa fatigue
crack.
microstructure(Figurealso the highvaluesof residual
stresses(Figureof11)
can promotestresses directly influence the initiation of a fatigue crack. This phenomenon is of great importance inThis phenomenon
is of great
importance inthe casesof relatively
lowstrainanddecohesionin this area.
The near-surface
stresses
directly
influence
theplastic
initiation
ofamplitudes,a fatigue crack.
the cases of relatively low plastic strain amplitudes, and for the large plastic deformation, the residual
for thelarge plasticisdeformation,
the residual
tend
to relax.
Thisphenomenonof great importancein thestresses
cases ofrelatively
low plastic strain amplitudes, andstresses tend to relax.
Nolargenoticeable
presence of secondary
cracksbetweenthefatigue striations was reported. The
for theplastic deformation,
the residual
stressestend torelax.
No
noticeable
presenceofsecondarycracksbetweenthefatiguestriations
was 18a–c.
reported.
fractureof thesampleoftested
with thestrainamplitude
0.8%
ispresented
in FigureNo surface
noticeable
presencesecondarycracksbetweenthe offatiguestriationswasreported.
The
Thefracture
fracturesurfacesampletestedwithstrainamplitudeof 0.8%is presented
in Figure
18a–c.
surfaceofofthethesampletestedwiththethestrainamplitudeof 0.8%is presented
in Figure
18a–c.

Figure 18. SEM images of fatigue surfaces of a specimen tested at a strain amplitude of 0.8%: (a)overallview
ofimagesthefracturesurface,(b) upper
partof thetested
SZ testedsurface,(c)magnifiedof
SZ
FigureSEM
imagesof fatigue
surfacesa specimen
atand
a strainamplitudeof 0.8%:(a)Figure18.18.
SEM
of fatigue
surfacesof aofspecimen
at a strain
amplitudeof view
0.8%:(a)theoverallsurface.
Theyellowarrowsindicatethesecondarycracks.
overallviewofthefracturesurface,
(b)
upper
part
of
the
SZ
surface,and(c)magnified
view
of
the
SZ
view of the fracture surface, (b) upper part of the SZ surface, and (c) magnified view of the SZ surface.
yellowarrowstheindicate
the secondary
Thesurface.
yellowThearrowsindicatesecondarycracks. cracks.

Materials 2020, 13, 5226

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The character of failure is similar to the quasi-static fracture (Figure 9) with high participation of
the SZ surface (Figure 18a) and no reported presence of fatigue striations (Figure 18a). In the upper
part of the SZ interface the occurrence of secondary cracks was registered (Figure 18b), probably thedelamination of compressed, elongated grains of the TMAZ, which have been identified in this zone
during the microstructure analysis (Figure 5a). The higher magnification of the SZ surface revealed thetypical, fine dimple structure (Figure 18c) with very small precipitates (below 1 µm).
4. Conclusions
The performed research on the mechanical properties and low cycle fatigue of friction stir welded
Sc-modified AA2519-T62 allowed drawing the following conclusions:
1.

2.

3.

4.

The microstructure analysis of the obtained weld has revealed the differences in the boundaries
between SZ and TMAZ on each side of the joint. On the advancing side, the severely deformed
and elongated grains of TMAZ are adjoined to the dynamically recrystallized grains of SZ,
while on the retreating side the boundary between SZ and TMAZ is not clear and TMAZ ischaracterized by partly recrystallized microstructure.
The failure in the tensile test has occurred in the boundary of TMAZ/SZ on the advancing side of
the weld with the registered value of 83% joint efficiency. The changes in strain distribution in
the FSW joint zone on the local scale were successfully assessed by the DIC system.
The residual stress measurements have revealed that the highest values of longitudinal stress are
localized at the distance of 10 mm from the joint line with their values of 124 MPa (the retreatingside) and 159 MPa (the advancing side).
The cyclic hardening of AA2519-T62 FSW joints during the low-cycle fatigue process wererevealed by a reduction of the width of hysteresis loops during the test, with the simultaneous
increase in the range of stress values.

Author Contributions: Conceptualization: R.K. and L.Ś.; Data curation: J.T.; Formal analysis: R.K.; Investigation:
R.K., J.T. and T.Ś.; Methodology: R.K., L.Ś., J.T., T.Ś., M.W. and I.S.; Project administration: L.Ś.; Supervision: L.Ś.;
Visualization: R.K., T.Ś., M.W. and I.S.; Writing—original draft: R.K.; Writing—review and editing: L.Ś. and J.T.
All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Polish Ministry of National Defence, grant number: PBG/13-998.
Conflicts of Interest: The authors declare no conflict of interest.

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4.
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6.
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