Download Mechanical behavior of friction stir welded joints of aeronautic

Mechanical behavior of friction stir welded joints of aeronautic aluminum alloy
AA2024-T351
Abstract
This investigation work focus in the analysis of Friction Stir weldability of the aluminum alloy AA2024-T351, a heat treated
aluminum alloy, typically used in the aeronautical construction, due to its good mechanical properties and corrosion resistance.
Therefore, FSW parameters have been developed and an evaluation of the influence of various characteristic defects of FSW in the
mechanical resistance, under static tensile and bending loads, and fatigue tests. The mechanical efficiency of welded joints with
various types of defects has been established and compared to the base material and welded joints without defects. It has been
shown that the most critical defects in FSW are the root defects, both kissing-bond and intermittent flaws.
Keywords
Friction Stir Welding; AA2024-T351; Welded joints defects; Fatigue
1) Introduction
FSW was invented by The Welding Institute in 1991, and since the first industrial application in 1995 that shows to be a
superior welding method in the joining of many engineering materials, with emphasis to aluminum alloys. The FSW process
uses a non-consumable rotating tool, consisting of a shoulder, generally normal to the axis of rotation and a smaller diameter
profiled pin, rotating at high speed which plunges into butting edges of the workpieces to be joined. The movement of the tool
over the work pieces results in friction and plastic deformation that generates heat. As the rotating tool translates along the
joint line, plasticized material is stirred and forged behind the trailing face of the pin where it consolidates to form a solid state
weld [1; 2].
The aluminum alloys, like 2XXX series are, extensively used in the aeronautic industry to produce airplane components
such as frame parts, bulkhead, longeron and so forth. The fastening and riveting method along with adhesives have
dominated this industry over the past decades, due to the low weldability of the most used aluminum alloys (2XXX and 7XXX
series). The elimination of fasteners and riveters would not just only reduce the manufacturing cost, but would also provide a
considerable weight reduction in the airplanes [3;4].
In the vast majority of engineering practice, most aluminum structures are welded and mainly subject to fatigue loading.
The fatigue leads to damage of structural parts even if the stress applied to the part is quite low. The main factors that
influence fatigue resistance of welded structures are type of loading, microstructure and weld defects. Although FSW joints
have better quality over fusion welding techniques, there are some typical defects that can be found in FS welds when the
used parameters are not correct , or when small variations of the correct parameters occurs [5].
The major aims of this investigation are to establish and assess the influence of this type of defects in the mechanical
resistance under static and dynamic loads and find correct parameters to weld the aluminum alloy AA2024-T351.
In order to know the relative importance of the 3 most typical defects in FSW, over the mechanical behavior of FSW
structures, 4 types of specimens were welded. Specimens without FSW typical defects, two kinds of root defects and one
internal defect were compared with base material specimens over static efforts of tensile and bending tests, and over
dynamic efforts for fatigue.
In the fatigue tests, S-N curves and metallographic analysis of fractures surfaces has been done. The joints
characterization under the different conditions was complemented with non-destructive control techniques, such as visual
analysis and x-rays, and destructive techniques such as metallographic analysis and hardness. The fracture surface
observation using SEM have been done to investigate the cause of fatigue failure.
FSW Typical Defects
The typical defects that used to occur in FSW butt joint are represented in Figure 1. The defects that are going be
studied are: 1) Lack of penetration (Kissing-bonds); 2) Root flaws (weak or intermittent linking); 3) Voids on the advancing
side.
Figure 1 - Typical defects on FSW butt joint
These defects result from, e.g., lack of penetration of the pin and non–uniform vertical force along the material
thickness, imperfect stir of materials during the process and inadequate surface preparation.
For each type of defect a code name was given. The characterization of the defect types is described in Table 1.
Defect Type Code
0
I
II
III
Description
Not defective
Root defect type I
(particles alignment)
Root defect type II
(kissing-bond)
Internal defects
(voids)
Table 1 - Establishment of the 4 different Defect Types included in the FSW joints produced
2) Experimental Details
2.1) Material
The base material used in the present study was rolled sheets of AA2024-T351 with 4mm thickness. The material has
been delivered in sheets with dimensions of 500mm x 250mm. All the welds were performed in a butt-joint arrangement. The
chemical composition and mechanical properties are presented in table 1.
Elements
Cr
Chemical
composition
Max
0.1
Tensile yield
strength
Cu
Fe
3.8 - 4.9
Max
0.5
Mg
Mn
1.2 - 1.8
0.3 - 0.9
Ultimate tensile
strength
324MPa
469MPa
Si
Ti
Zn
Al
Max
Max
Max
0.5
0.15
0.25
remain
Elongation at
break
20%
Table 2 - Chemical composition and mechanical properties of AA2024-T351
2.2) Welding conditions
The original rolled sheets have been divided in plates with dimensions of 250mm x 100mm, and were welded
parallel to the rolling direction in a conventional milling machine. For all the welds the rotation speed was 710rpm, travel
speed of 224mm.min-1 and a tilt angle of 0.5º. The used tool was modular dual tool allowing different combinations of
shoulders, pins and different pin lengths. The different tools, with the different pin lengths, used to do the welds are
schemed in Table 3.
Shoulder
Pin
FSW tool
assembled
Pin length
Defect type
produced
Tool 1
3.8mm
0
3.6mm
I
3.3mm
II
3.9mm
III
Tool 2
Table 3 – The different tools, with the different pin lengths, used to produce the 4 different Defect Types
For purposes of fast analyses in situ, and to minimize the possibility of deviations from the steady state, 50mm at the
beginning and about 30mm at the end of the welding plates, were removed.
From each welded plate, one tensile specimen, one bending specimen and four fatigue specimens were cut out.
2.3) Metallographic Analysis
The metallographic analysis allows to characterize the influence of the welding process in the microstructure in the
base material. Distribution of the precipitates in the different zones of the weld and the changes of grain size resulting from
the FSW process are investigated.
Cross sections of the welds perpendicular to the welding direction were prepared using a sequence of grit paper of
240, 320, 600, 800, 1000, 2400 and 4000 mesh, and then polished on a microcloth using a diamond suspension of 6µm
diamond. The specimens were etched using Keller´s reagent, and after were washed in distilled water. Finally the specimens
were observed using an Olympus CK40M optic microscope.
2.4) Fatigue Testing
The fatigue tests were carried out in a servo-hydraulic testing machine Instron, model 8874 equipped with a load
cell of 25kN. The weld was transverse to the stress axis in the S-N specimen. A sinusoidal load time function was used, with
the stress ratio R of 0.1. Oscillation was set between 4 – 14 Hz.
3) Results
3.1) Microstructure
In Figure 2 is possible to see the macrostructure of the FS weld joint. The microstructure of a joint welded by the
FSW process normally is divided in four distinct zones: a) the dynamically recrystallized zone, also known as nugget; b) the
thermo-mechanically affected zone (TMAZ); c) heat affected zone (HAZ); d) base material (BM).
Figure 2 – Macrograph of the cross section perpendicular to the welding direction of an etched Defect Type 0 specimen
The weld nugget (in the middle of Figure 3) is characterized by a fine grain structure due to dynamic
recrystallization. The TMAZ, in Figure 3, is the zone in the transition between the weld nugget and the HAZ. This zone is
characterized by a highly plastic deformed grain structure due to the FSW tool. In this zone the heat and the mechanical
deformation were not enough to dynamically recrystalize the grain structure, originating elongated grains around the weld
nugget. The HAZ, in Figure 4, extends approximately to the shoulder diameter for friction stir welds. The HAZ is similar to
heat affected zone resulting from conventional fusion welding processes and is characterized by no material flow and
relatively low temperatures as compared to the weld nugget during welding. Depending on the base material, its initial heat
treatment, and the proximity to the centerline, precipitate coarsening, precipitate dissolution, recovery, recrystallization or
grain growth can occur in the HAZ. Finally the BM shown in Figure 5 is the zone in which is contained the unaffected material
that is remote from the weld and although it may have experienced a thermal cycle from the process, this has little effect on
microstructure or mechanical properties.
Figure 3 – Microstructure of the TMAZ and weld nugget of the AA2024 – T351 welds
Figure 4 – Microstructure of the HAZ of the AA2024 – T351 welds
Figure 5 – Microstructure of the BM zone of the AA2024 – T351 welds
In the figure 6 it can be seen micrographs of all the Defect Types studied in this work. In the Defect Type I, Figure
6.a), is possible to see some oxide layers (in a darker tone), as it was expected. The Defect Type II micrographs, Figure 6.a)
and 6.b), show a weld nugget with a smaller height, not enclosing all the thickness of the plates, and are encircled by a
similar granular structure to the TMAZ, due to plastic deformation that was induced to the material under the pin. The height
of this defect has an average height of 0.4mm. In this Defect Type a zone is distinguished easily where the parts only are
strongly leaned, with the surface of interface slightly deformed and another zone slightly deformed, where although already
exists linking, the oxides have not been destroyed. The Defects Type III, Figure 6.d) and 6.e), appears in the advancing side,
under the weld nugget, next to the border with the TMAZ. This zone has a clear influence of the pin in the stirred material.
The nugget form is also different in this type of defect, because of the form of the pin that was different in this Defect type.
Figure 6 – Different types of defects studied: a) Defect Type I; b) Defect Type II; c) Defect Type II; d) Defect Type III; e) Defect Type III
3.2) Hardness
Vickers hardness tests were performed in order to obtain important information about the mechanical properties
along the welded zone. The measures were done with an indentation load of 0.5kg, in a specimen used for metallographic
analysis, along the center of the welding, and 0.5mm from the top and the base of the welded specimen. The results are
presented in Figure 7.
Figure 7 – Vickers hardness tests in the centre, top and base of the welds
From the results obtained the following comments can be done:
The BM hardness is higher in the base and top, with a medium value of 150HV, than in the centre with a
medium value of 140 HV, due to the higher deformation induced by the rolling process, being higher than
any welding zone;
The lowest hardness is located in the retreating side, in the TMAZ to HAZ interface, because it is the
hottest zone without dynamic recrystallization or grain deformation;
The highest values of hardness, in the weld nugget, are found in the base and top.
3.3) Tensile and Bending Tests
In order to analyze the results of tensile and bending tests, two analyses criteria’s were used for each one of the
tests. For the tensile tests EGRET (eficiência global de resistência estática à tracção) that means global efficiency to tensile
static resistance, and EGREF (eficiência global de resistência estática á flexão) which means global efficiency to bending
static resistance.
For the tensile tests:
Ei; σ 0,2i; σmáxi; Ai; Teni – Mechanical properties of the welded joint i (respectively Young Modulus, Yield
Strength, Ultimate strength, Elongation at break and Toughness );
EMB; σ 0,2MB; σmáxMB; AMB; TenMB – Mechanical properties of the BM;
CE; Cσ 0,2; Cσ máx; CA; CTen – Constants corresponding to the ponderation coeficients atributted to each
one of the five mechanical properties in the EGRET factor with the values of respectively 0.05, 0.28,0.33,
0.11 and 0.23 .
The results are schemed in Figure 8.a) .
For the bending tests:
Fi; εi; Ei – Mechanical properties of the welded joint I (respectively maximum load, dislocation at
maximum load and Energy absorbed at break);
FMB, εMB e EMB – Mechanical properties of the BM;
CF, Cε e CE – Constants corresponding to the ponderation coeficients atributted to each one of the five
mechanical properties in the EGREF factor with the values of respectively 0.25, 0.25 and 0.5 .
The results are schemed in Figure 8.b) .
Figure 8 – EGRET and EGREF factors for tensile and bending tests respectively
3.4) Fatigue
Figure 9 – Fatigue resistance for BM and the 4 defect type conditions. a) S-N curve; b) Number of cycles efficiency relatively to the BM
performance
From the analysis of the fatigue results, shown in Figure 9.a), is possible to see the good quality of the welds
without defects, with a fatigue behavior near from the one of the BM.
In the welded specimens with root defects, defect type I and defect type II, show a higher loss of fatigue resistance,
being the defect type II, the one whose has worst fatigue properties.
In the defect type III, the void defect, the gravity of this defect on the loss of properties over fatigue loading, is
lesser than the root defects as it can be seen by the S-N curve.
In the Figure 9.b) it is possible to see the efficiency in number of cycles relatively to the BM, and the performance
of all the type of welds has bad results in high stress amplitudes. In all the welds the results get better when the stress
amplitude became smaller. In the case of welds with defect type 0 the efficiency is higher than one in when the stress
amplitudes has values smaller than 70MPa.
3.5) Fractography
Analyzing the fracture zone of the welds, not all of them fractured in the same zone. The welds with defect type 0
fractured over the HAZ in the advancing side, the defect type I and II fractured in the weld root over the HAZ and the welds
with defect type III fractured in the HAZ, in the defect, over the advancing side.
The crack initiated, in the defective welds type 0, in multiple zones over the striates in the top of the weld, left by
the tool and propagated over the HAZ. In the defective welds type I and II, the crack initiated in the weld root and propagated
over the nugget. Finally, in the defective welds type III, the crack initiated over the defect in the advancing side and
propagated over two directions (root and top) in the HAZ.
In figure 10.a) is shown the typical cleavage plans in the vicinity of the top surface where the crack was initiated.
It’s possible to see to that there should be various nucleation sites over the fracture zone. In the Figure 10.b) is shown the
zone of ductile fracture with typical dimple characteristic. There are no second phase particles visible in this zone, because
the fracture happened in the HAZ, and in this zone might have happened precipitate coalescence due to high temperatures
reached.
Figure 10 – SEM fractography of defect type 0: a) Localization of cleavage plans in the vicinity of the striates; b) Zone of final rupture
In Figure 11.a) are shown the defect type I, and in Figure 12.a) the defect type II, in their crack initiation and
propagation zone. It is possible to see the higher length of the root defect type II. In Figures 11.b) and 12.b) is shown the
zone of ductile fracture.
In figure 13.a) is shown the nucleation site, which is the internal void. Figure 13.a) also shows the deformation
pattern in the vicinity of the internal void of the specimens with defect type III. The ductile fracture zone, shown in Figure
13.b) is similar to the one of defect type 0.
Figure 11 – SEM fractography of defect type I illustrating: a) Localization of cleavage plans in the vicinity of the defect; b) Zone of final
rupture
Figure 12 - SEM fractography of defect type II illustrating: a) Localization of cleavage plans in the vicinity of the defect; b) Zone of final
rupture
Figure 13 – SEM fractography of defect type III illustrating: a) Localization of cleavage plans in the vicinity of the defect; b) detail of a); c)
Zone of final rupture
4) Conclusions
The fatigue and mechanical properties of the friction stir welded joints, with and without defects and base material
AA2024-T351 alloy were investigated. The following conclusions can be drawn:
The several defects have a considerable effect in the mechanical properties of the welds, affecting the
performance of those, in tensile and bending tests. This effect was higher in root defects (defect type I
and II), than in defect type III. This defect seems not to play a major role in static tests;
The welds have hardness values smaller than the BM (135 HV – 155HV). The smaller value is found in
the interface between the TMAZ and HAZ (123HV) in the retreating side. This is the zone where the
specimens break in the tensile and bending tests, in welds with defect type 0 (no defects). The weld
nugget has values smaller than the BM but higher than the TMAZ and HAZ (130HV – 144HV);
Over fatigue loading all the defect types play a major role in the weld performance. The BM has the
better fatigue resistance, followed by defect type 0. The other types of defects lead to a decrease of
resistance.
In the end all the different material and defective conditions are ordered in decrescendo order of the level
obtained for all the mechanical properties that were studied:
1.
Base material
2.
Welded joints without defects
3.
Welded joints with internal defects (voids)
4.
Weld joints with root defect of particles and oxides alignment
5.
Welded joints with root defect (kissing-bond)
5) References
[1]
Vasco Godinho, Rudolf Zettler, Silvia Lomolino, Friction stir weld zone evaluation by image analysing techniques,
Outubro de 2003;
[2]
VILAÇA, P.; Fundamentos do Processos de Soldadura por Fricção Linear: Análise Experimental e Modelação
Analítica, PhD Thesis, IST, 2003;
[3]
Shusheng Di, Xinqi Yang, Guohong Luanb, Bo Jian; Comparative study on fatigue properties between AA2024-T4
friction stir welds and base materials; China;2006;
[4]
M. Ericsson, R. Sandstro¨m; Influence of welding speed on the fatigue of friction stir welds, and comparison with
MIG and TIG; Department of Materials Science and Engineering and Brinell Centre, Royal Institute of Technology
(KTH), Sweden; 2002;
[5]
Caizhi Zhou, Xinqi Yang, Guohong Luan; Effect of oxide array on the fatigue property of friction stir welds; School
of Materials Science and Engineering, Tianjin University, China; 2005;
[6]
Santos, Telmo; Vilaça, P; Quintino L. ; Freitas M. ; Advances in NDT techniques for friction stir welding; IDMEC,
IST, Technical University of Lisbon, Lisboa, Portugal;
[7]
Vilaça, P; Pépe, Nuno; Quintino L. ; Metallurgical and corrosion features of friction stir welding of AA5083-H111;
IST, Technical University of Lisbon, Lisboa, Portugal;