Dissimilar Joint Welding Through Friction Stir Techniques: Mechanical and Microstructural Properties of AA2198-T8 & AA2024-T3

Abstract

Friction stir welding is used to evaluate the metallurgical and material characteristics of AA2198-T8 and AA2024-T3 welds. One of the most often utilised high-strength aluminium alloys in the aerospace industry is AA2024-Tx. The latest generation of aluminium lithium alloys, AA2198-T8, just replaced AA2024-T3. Yet, sound weld methods with the highest mechanical qualities have yet to be adequately characterised. These two materials were selected and joined through two techniques. The first is a single side friction stir weld, while the second is a double side friction stir weld. This technique was used to create specimens by holding them side by side. After the stirring, the hybrid joint was created by taking spindle speeds of 580 RPM and travel speeds of 36, 76, 102, 146, and 216 mm/min into account. The tool angle and dwell duration evaluated in this study were 2o and 20 seconds, respectively. Fabricated samples were then sectioned in accordance with industry standards. Tests to determine factors like as tensile strength, impact load, and microstructures at various temperature zones. The results show that the maximum joint efficiency for a double-side friction stir welded connection at 102 mm/min was 96%.

Full text

Dissimilar Joint Welding Through Friction Stir
Techniques: Mechanical and Microstructural
Properties of AA2198-T8 & AA2024-T3
Ahmed Anwar Samir (  ahmed.anwarsu.edu.krd@gmail.com )
Salahaddin University - Erbil
Shawnim R Jalal
Mohammedtaher M Mulapeer
Research Article
Keywords: Al-Li Alloys, FSW, Mechanical Properties, Impact properties, Microstructure
Posted Date: March 22nd, 2023
DOI: https://doi.org/10.21203/rs.3.rs-2699145/v1
License:   This work is licensed under a Creative Commons Attribution 4.0 International License. 
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DISSIMILAR JOINT WELDING THROUGH FRICTION STIR TECHNIQUES:
MECHANICAL AND MICROSTRUCTURAL PROPERTIES OF AA2198-T8 &
AA2024-T3
Ahmed Samir A. Alemdar 1, *, Shawnim R. Jalal 2 and Mohammedtaher M. Mulapeer 3,
(1) Lecturer, Mechanical & Mechatronics Dept., College of Engineering, Salahaddin University – Erbil – IRAQ.
(2) Professor, Mechanical & Mechatronics Dept., College of Engineering, Salahaddin University – Erbil – IRAQ.
(3) Assistant Professor, Mechanical & Mechatronics Dept., College of Engineering, Salahaddin University – Erbil –
IRAQ.
Corresponding author e-mail: ahmed.anwarsu.edu.krd@gmail.com

DISSIMILAR JOINT WELDING THROUGH FRICTION STIR
TECHNIQUES: MECHANICAL AND MICROSTRUCTURAL
PROPERTIES OF AA2198-T8 & AA2024-T3
Abstract:
Friction stir welding is used to evaluate the metallurgical and material characteristics of
AA2198-T8 and AA2024-T3 welds. One of the most often utilised high-strength
aluminium alloys in the aerospace industry is AA2024-Tx. The latest generation of
aluminium lithium alloys, AA2198-T8, just replaced AA2024-T3. Yet, sound weld
methods with the highest mechanical qualities have yet to be adequately characterised.
These two materials were selected and joined through two techniques. The first is a
single side friction stir weld, while the second is a double side friction stir weld. This
technique was used to create specimens by holding them side by side. After the stirring,
the hybrid joint was created by taking spindle speeds of 580 RPM and travel speeds of
36, 76, 102, 146, and 216 mm/min into account. The tool angle and dwell duration
evaluated in this study were 2o and 20 seconds, respectively. Fabricated samples were
then sectioned in accordance with industry standards. Tests to determine factors like as
tensile strength, impact load, and microstructures at various temperature zones. The
results show that the maximum joint efficiency for a double-side friction stir welded
connection at 102 mm/min was 96%.
Keywords: Al-Li Alloys, FSW, Mechanical Properties, Impact properties,
Microstructure.
Introduction
Joining two flat, opposing surfaces together without melting the material,
friction stir welding use a single-use instrument to create a solid-state weld. Large
distortions and solidification cracking are two problems that plague traditional fusion
welding methods, and FSW is gaining popularity as a possible solution [1]. The impact
of tool geometry on aluminum-steel FSW is outlined in [2]. They mentioned that the
tool's shape was fine-tuned incrementally to get the best possible measurements. Solid-
state joining requires a far smaller amount of heat compared to fusion-based joining
techniques. In solid-state welding methods, the materials are not melted and then
resolidified [2]. The below figure 1 shows the FSW schematic representation during the
weld process.

Fig 1. Schematic Representation of FSW during the weld passage [3]
Currently, the most used method for connecting dissimilar metals is through
solid-state welding. For joining aluminium and other pliable materials, TWI's 1991
friction stir welding technology is widely employed. FSW has also been studied by
various researchers for its potential to be used in aluminium and steel welding [2]. This
study looks into the welding of two aerospace aluminium alloys, AA2198 and AA7475,
in both homogeneous and heterogeneous conditions. Using differential scanning
calorimetry (DSC), the strengthening of precipitation throughout the ageing of welded
specimens was monitored. The Falcon9 rocket, developed by SpaceX, is a two-stage
rocket that recently used this material for its exterior. The chemical and mechanical
characteristics of the base materials make dissimilar joining more difficult than welding
[5]. The importance of aluminium (Al) and copper (Cu) in industries such as aerospace,
transportation, and electrical power generation is well-known. There is a lot of research
into the Al/Cu bimetallic dissimilar junction because it can reduce weight and material
costs while increasing durability [6]. This study [7] analyses the effects of varying
friction stir welding (FSW) parameters on the development of microstructure and
mechanical properties at incompatible Al-Cu junctions. These microstructures formed
in a way that caused hardness to be unevenly distributed. An increase in hardness was
seen in particular along the Al/Cu junction. Strong tensile and bending characteristics
can be attributed to the excellent metallurgical connection between the Al and Cu. The
primary focus is on optimising the FSW parameters to generate welds that are defect-
free and have the best possible tensile and electrical properties given the material's
macro- and microstructural characteristics. Secondly, we want to examine the interfacial
and stir zone regions to better understand microstructure formation. SEM analysis of
Weld passage
Weld Tool
Heat Zone
Specimen

fractured tensile test specimens revealed no significant changes in material properties
(SEM). When comparing the microstructure of AA7075 and AA5083 BMs, When
compared side by side, it is clear that the former has a far finer grain structure, with an
average grain size of 40 m and no substructure at all. It was instead found that AA5083
BM has a deep substructure [9] and an average grain size of 25 m.
The feasibility of friction stir butt welding 10- and 16-millimeter-thick
AA7075T651 plates was explored. Welds devoid of flaws and with full penetration
were produced thanks to careful selection of process parameters. Both welds had
recrystallized grains in their nuggets, but the grains in the welds created on 10-mm-
thick plates were significantly finer. The heat-affected zone has a much lower hardness
than the surrounding material, as shown by a number of micro hardness tests [10]. Since
the two aluminium alloys have different properties, friction stir welding is a good way
to take advantage of both of them (FSW). Friction stir welded (FSW) joints fabricated
from the aluminium alloys AA6061 and AA5086 were examined for their
microstructural characteristics and tensile performance in [11]. Microstructures were
analysed and studied using optical and scanning electron microscopy. Micro hardness
testing was performed on several locations of the welded joints. Mechanical qualities
were shown to be affected by the joint configuration, with the two-sided welds
displaying weaker characteristics due to increased heat transfer during the second pass.
Changes in weld microstructure were reflected in the material's mechanical
characteristics, as evidenced by the fact that the softer weld nugget was connected with
extensive dynamic recovery, resulting in grains that were almost devoid of dislocations
[12].
Methodology & Materials
In the present work, the two materials were joined using FSW. The two Al alloys
containing lithium & copper alloys which are the major alloying elements in the AA
2198 & AA 2024, which are welded together. The tool's varied heights, from 3mm to
5.5mm, were crafted from AISI H13 tool steel. In Table 1 below, we can see the
chemical make-up of AA2198 and AA2024. The weld has been made for the specimen
1 using single sided weld (SS) and specimen 2 using double sided weld (DS) at different
travel speeds maintaining the spindle speed constant. The FSW of the specimens is
shown in the below figure 2.

Table 1. Comparison of the chemical make-up of BM AA2198-T8 and BM AA2024-T3
Element
Cu
Li
Fe
Mg
Mn
Si
Ti
Ag
Zn
AA 2198
3.4
1.02
0.08
0.75
0.3
0.05
0.07
0.5
0.3
AA 2024
3.83
-
0.45
1.84
0.75
0.5
0.12
0.5
0.23
Fig 2. FSW of the specimens
The FSW parameters are as follows describing the nature of work and
parameters considered while joining the two materials. The two main considerations are
spindle speed and travel speed and other considerations are as follows.
(1) Dwell time = 20 sec
(2) Tilt angle = 2o
(3) Spindle Speed = 580 rpm.
(4) Travel Speed = 36, 76, 102, 146 & 216 mm / min.
(5) Tool Material = AISI H13 steel grade.
Tool Design
After being heat treated, the FSW tool's hardness reached 66 HR. It was made out of
AISI H13 tool steel, which has been demonstrated to be useful for the FSW method
when working with aluminium alloys, and was subjected to hot working. Both of the
FSW design's pins have a concave shoulder and a tapered thread. Concurrently, two
examples of engaging dissimilar connections of the materials were presented, both of
which were bonded using friction stir welding and included pin and shoulder diameters

with varying thicknesses. As can be seen in the third diagram below, DS-FSW and SS-
FSW have switched places.
Fig 3. AISI H13 tools with different height of pins
The below images indicate the joining of two materials at different travel speeds
and rate of tool direction which imposes the heat to travel away from the specimen. The
below images also include the slag content for each and every specimen. The below
images indicate the specimens welded at different travel rates making the spindle speed
as constant which indicates the material deformation is less in single side weld (SS).
Where as in the single sided weld (SS), the material tends to slag more at lower speeds
and tends no material at higher speeds which the tool reaches high speed and tends to
slow down. This is shown in the below images.
Fig 4. DSW at 36 mm/min
Fig 5. DSW at 76 mm/min

Fig 6. DSW at 102 mm/min
Fig 7. DSW at 146 mm/min
Fig 8. DSW at 216 mm/min
Fig 9. SSW at 36 mm/min
Fig 10. SSW at 76 mm/min
Fig 11. SSW at 102 mm/min

Fig 12. SSW at 216 mm/min
Fig 13. Top view of Specimen fixed to
equipment
After the fabrication of the hybrid joint, mechanical testing such as tensile,
impact were conducted on the hybrid joint specimen. The welds obtained at hybrid joint
were further characterized using SEM for further investigation to determine the
behaviour of microstructure, pores on the surface of the weld, surface propagation, and
morphology. The following images show the mechanical testing and microstructural
characterization equipment’s used for testing and dimensions of the specimen used for
testing the specimen as per AWS standards. These are all 300 x 75 x 6 mm in size and
are crafted from either an aluminum-lithium alloy (AA2198-T8) or an aluminum-copper
alloy (AA2024-T3). The tensile test specimen is cut in the shape of 2D projection of
dumbbell and for impact specimen a rectangular body is fabricated and at the centre a v
notch is cut at an angle of 450 symmetry for the body.
Fig 14. Dimensions of Tensile test
specimen
Fig 15. Dimensions of Impact test specimen.

Fig 16. Charpy Test equipment
Tensile machine Model HUALONG 600KN

Fig 16. SEM & Spectro MaxX metal analyser machine
Results & Discussions
The results include that mechanical and microstructural characterization was
performed on the fabricated hybrid joint of AA2198 & AA2024 alloy material. The
maximum stress to total deformation produced in the body and the maximum impact
load of the constructed specimen were determined using mechanical testing including
tensile and impact. Surface-to-surface friction stir welded hybrid joints are shown in
Figures 17 and 18. To facilitate testing, the material is chopped according to the AWS
standard.

Fig 17. Single side weld specimens at different travel speeds
Fig 18. Double sided weld specimens at different travel speeds
Mechanical Characterization - Tensile Test Results
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min

Fig 19. S – S curve DS – FSW specimen at 580 RPM
Fig 20. S – S curve SS – FSW specimen at 580 RPM
The figure 19 shows the s – s curve for the double side (DS) FSW specimen at
580 RPM spindle speed. For DS FSW specimen, the maximum stress was found to be
315.2 MPa where the total elongation was 12.0 %. The minimum stress was found at
213.1 MPa and the elongation was found to have 5.6 %. The figure 20 shows the s – s
curve for the single side (SS) FSW specimen at 580 RPM spindle speed. For SS FSW
specimen, the maximum stress was found at 323 MPa where the total elongation of the

specimen was 5.8 %. The minimum stress was found at 109.2 KN where the total
elongation was found 4.2 %.
By comparing the two results of SS and DDS FSW specimen, SS FSW
specimen withstand to higher stress value when compared to DS FSW. It indicates as
welding is done on both sides the material becomes thin which tends the material to
deform quickly when load is applied. The below figures 21 & 22 indicates the
specimens broke after the specimens for both SS and DS FSW indicating ductile nature.
The SS FSW specimen tends to break hard during the working loads whereas the DS
FSW specimen tends to break at low loads when compared to the SS FSW working
loads.
Fig 21. Single side weld specimens after tensile test
36
mm/min
76
mm/min
102
mm/min
146
mm/min
216
mm/min

Fig 22. Double side weld specimens after tensile test
Impact Test Results
The results indicate a brief comparison between the SS FSW specimen and DS
FSW specimen. Below is a table that clearly describes the notch position of the heat
affected zone (HAZ) and the stir zone (SZ) of positive direction of SS FSW and
inverted DS FSW specimens as the travel speed changes and the spindle speed is held
constant. The travel speeds considered in the research are 36, 76, 102, 146 and 216
mm/min. As indicated in the table, as travel speed increases the HAZ decreases. This is
due to heat dissipation in the work piece during the weld to the outer atmosphere.
Maximum values for HAZ and SZ were found to be 10 and 14, respectively, at a
velocity of 102 mm/min when comparing the SS FSW result with the DS FSW result.
Table 2. Impact test results of SS FSW Vs DS FSW
SS-FSW
Travel Speed
Reversed DS-
FSW
Notch
Position
Notch Position
HAZ
SZ
HAZ
SZ
9
6
36 mm/min
9.5
10
7.5
8.5
76 mm/min
8.25
9.25
8
6.5
102 mm/min
10
14
6
7
146 mm/min
8
10.25
8
4.5
216 mm/min
7
9.75
36
mm/min
76
mm/min
102
mm/min
146
mm/min
216
mm/min

Fig 26. SS – FS welded v notch specimens
Fig 27. DSW – FS welded v notch specimens
The above figures 26 &27 represents the SS & DS FSW specimens cut to v
notch to perform the impact testing. The figures 28 & 29 represents the after impact
specimens for DS & SS FSW hybrid joints. When viewing at fig 25, 102 mm/min
shows a perfect bend with an s cut deformation in the specimen. And when comparing
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min

with the 216 mm/min, the specimen tends to break into half. This is because of the
higher travel speeds which instigates the tool deeply into the specimen making the
specimen brittle at the joint. At 102 mm/min, the bonding is strong and the material is
still in contact with the AA 2024.
Fig 28. DS – FSW Notch in Stir Zone (SZ)
Fig 29. DS – FSW Notch in HAZ
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min

Fig 30. SS – FSW Notch in Stir Zone (SZ)
Fig 31. SS – FSW Notch in HAZ
Metallurgical Characterization
Using an image analyzer coupled with optical microscope, metallography allows for the
observation of microstructural changes in welds. According to the findings, there are
three distinct regions in any given welded joint: the molten metal, the fusion zone, and
the heat-affected zone (HAZ). Corrosion resistance can be compromised, however, if
the welding method and fusion welding parameters are not strictly adhered to. There
may be grain coarsening in the heat-affected zone, liquation cracking in the weld zone,
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min

and solidification cracking in the weld zone. are all caused by the thermal cycling that
occurs during fusion welding (HAZ). The following displays the SEM images of SS &
DS FSW at different travel speeds and at different magnifications.
Fig 32. Microstructure of SS at 2 microns
Fig 33. Microstructure of SS at 5 microns
Fig 34. Microstructure of SS at 10 microns
Fig 35. Microstructure of SS at 20 microns
Fig 36. Microstructure of SS at 50 microns
Fig 37. Microstructure of SS at 100
microns
The microstructures indicate that the intermetallic bonding between the two
compounds is very strong indicating the flakes closely to each other. The photograph
clearly shows that the grains are columnar. The columnar structure is better suited for
plastic deformation, and AA2198 has excellent formability. The strength of AA2024
alloy is mostly attributable to precipitation hardening. The creation of Al-Cu
intermetallic particles, which reinforce the alloy, is the reason of the rise in strength.

SEM investigations on the AA2024 alloy were performed to learn more about it. These
photos show that new grains have formed and precipitates have emerged from the Al
matrix as a result of significant plastic deformation. As cast AA2024 SEM photos are
processed.
Fig 38. Microstructure of DS at 2 microns
Fig 39. Microstructure of DS at 5 microns
Fig 40. Microstructure of DS at 10
microns
Fig 41. Microstructure of DS at 20
microns
Fig 42. Microstructure of DS at 50
microns
Fig 43. Microstructure of DS at 100
microns
Conclusions
Mechanical and metallurgical properties of manufactured specimens of SS and
DS FSW were evaluated. Tensile test results indicated that For DS FSW specimen, the

maximum stress was found to be 315.2 MPa where the total elongation was 12.0 %. The
minimum stress was found at 213.1 MPa and the elongation was found to have 5.6 %.
For SS FSW specimen, the maximum stress was found at 323 MPa where the total
elongation of the specimen was 5.8 %. The minimum stress was found at 109.2 KN
where the total elongation was found 4.2 %.
By comparing the two results of SS and DDS FSW specimen, SS FSW
specimen withstand to higher stress value when compared to DS FSW. And elongation
of DS FSW is higher compared to SS weld. The impact results indicate that as the
position of weld changes the HAZ values of the DS weld is higher compared to SS
weld. The bonding behaviour of flake particles and their grain distribution at various
magnifications have also been studied from a metallurgical perspective. This states the
DS FSW specimen possess grain distribution is all over the weld compared to SS weld.
In the SS weld, the grains were distributed and collated at the ends making the
molecules large and hard.
Declarations:
Funding statement: The authors declared that no funding was received for this
Research and Publication.
Conflicts of Interest: The authors declare that they have no conflicts of
interest.
Authors Contributions: The data used to support the findings of this study are
included in the article. Should further data or information be required, these are
available from the corresponding author upon request.
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