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`International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 11, November 2017, pp. 528–534, Article ID: IJMET_08_11_056
Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=11
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication Scopus Indexed
INFLUENCE OF PROCESS PARAMETERS ON MECHANICAL PROPERTIES OF FRICTION
STIR WELDING OF AA 6061-T6 ALLOY
D.Raghavendra
Assistant professor in S R Engineering College
S.Sravya
Assistant professor in S R Engineering College
ABSTRACT:
This paper includes butt joining of 5 mm of AA 6061 plates. The present study
investigates the influence of welding parameters on microstructure and mechanical
properties of friction stir welded AA 6061-T6 Alloy was studied. An attempt is made
here to review the fundamental principle of this process its tensile strength and
examination of its metallurgical consequences. An improved milling machine is
fabricated for performing friction stir welding and its effectiveness in joining Al 6061-
T6 Alloy plates is demonstrated in the current work. The FSW Process has proved to
be very efficient and has immense potential for future application. Present
investigation is to find out the optimum mechanical properties of friction stir welding
of Al 6061-T6 alloy. In this present study an attempt has been made to study the effect
of tool rotational speed(RPM), welding speed(mm/min),tool geometry and D/d ratio
on FSW zone transformation in Aluminium alloy. For three different tools, rotational
speeds, and three different tool D/d ratios, one tool pin profile have been used to
fabricate the joints. Tensile properties, toughness and microstructure of the joint were
evaluated and correlated as received Al 6061-T6 alloys. The joints fabricated using
rotational speed of 900rpm, a welding speed of 60 mm/min, taper thread pin profile
and tool shoulder diameter of 18 mm, (D/d)=3.0 showed higher tensile properties
compared to other joints.
Keywords: Aluminium alloy, Friction Stir Welding, Microstructure, AA 6061,
Mechanical properties
Cite this Article: D.Raghavendra and S.Sravya, Influence of Process Parameters on
Mechanical Properties of Friction Stir Welding of Aa 6061-T6 Alloy, International
Journal of Mechanical Engineering and Technology 8(11), 2017, pp. 528–534.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=11
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D.Raghavendra and S.Sravya
http://www.iaeme.com/IJMET/index.asp 529 [email protected]
1. INTRODUCTION
In 1991 a new process for welding of aluminium alloys was invented by The Welding
Institute, Cambridge, UK. The process was duly named friction stir welding (FSW), and The
Welding Institute filed for world-wide patent protection.This welding process has made
possible to weld a number of aluminum alloys that were previously not recommended (2000
series & copper containing 7000 series aluminium alloys) for welding [3] Consistent with the
more conventional methods of friction welding, which have been practiced since the early
1950s, the weld is made in the solid phase, that is no melting. After the invention of process,
it has received attention and today most of the companies are using the technology in
production, particularly for joining aluminum alloys.
Friction-stir welding (FSW) is a solid-state joining process in which (the metal is not
melted) that uses a third body tool to join two flat surfaces. During the process the region
between the material and the FSW tool softend due to the heat is generated between the tool
and material. It then mechanically intermixes the two work pieces of metal at the place of the
join, then the softened metal (due to the elevated temperature) can be joined using mechanical
pressure which is applied by the movement of tool, much like joining clay, or dough. It is
primarily used on aluminum, and most often on extruded aluminum (non-heat treatable
alloys), and on structures which need superior weld strength without a post weld heat
treatment. The process of Friction Stir Welding process has been widely used in the industries
and aerospace many applications. It is used mainly because of many of its advantages over the
conventional welding techniques[5]. Friction stir welding joints exhibits better mechanical
properties compared to other welding process and they are free from blowholes or porosity
compared to the other conventionally welded materials. The schematic Diagram of Friction
Stir Welding process is shown in figure 1
Figure 1 Schematic Diagram of Friction Stir Welding process
Aluminium alloys have light weight and higher strength to weight ratio due to this reason
alluminium alloys find wide applications in aerospace, automobile industries, railway
vehicles, bridges, offshore structure topsides and high speed ships. Welding is the primary
joining method in all cases which has always shows a great challenge for designers and
technologists. The main problem is lots of difficulties are associated with this kind of joint
process, mainly related to the presence of a tenacious oxide layer, high thermal conductivity,
high coefficient of thermal expansion, solidification shrinkage and, above all, high solubility
of hydrogen, and other gases, in the molten state . Further problems occur when attention is
focused on heat-treatable alloys, since heat, provided by the welding process, is responsible
for the decay of mechanical properties, due to phase transformations and softening. AA 6061
aluminium alloy containing magnesium and silicons as its major alloying elements is the most
widely used medium strength aluminium alloy, and has gathered wide acceptance in the
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Influence of Process Parameters on Mechanical Properties of Friction Stir Welding of Aa 6061-T6
Alloy
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fabrication of light weight structure. 6061 is resistant to corrosion even when the surface is
abraded and more easily worked[2].
The tool serves three primary functions; the heating of the workpiece, the movement of
material to produce the joint, and the containment of the hot metal beneath the tool shoulder
[1]. The heat generated during the FSW process is often assumed to occur predominantly
under the shoulder; due to its greater surface and to be equal to the power required to
overcome the friction forces between the tool and the workpiece [3]. To an extent, with the
increase in the shoulder diameter the heat input into the welds increases 4]. To study the heat
input into the welds while varying the process parameters the three different shoulder
diameters are used.
2. EXPERIMENTAL PROCEDURE
The experimental study includes the butt joining of 5mm AA 6061 flat plates. The welding
process is carried out on a vertical milling machine. Tool is hold in a tool arbour. Special
welding jigs and fixtures are designed to hold on two plates of 250mm X 60mm X 5mm
thickness combinations of the tool rotational speed(RPM), welding speed(mm/min), tool
geometry and diameter of the tool shoulder to the diameter of the tool pin (D/d) FSW
experiment is performed on AA 6061 by keeping welding speed (40mm/min) constant and by
changing three rotational speeds(900,1120 and 1400) rpm’s and by using stainless steel tool
having screwed taper pin profile with three D/d ratios(ratio of diameter of tool shoulder to the
tool pin).The diameters of tool shoulder(D) were 18mm,21mm and 24mm and that of insert
pin diameter(d) and pin length(L) are 6mm and 4.8mm respectively[7].
A milling machine have spindle that is positioned vertically and a worktable that is
parallel to the ground. The vertical milling machine used to fabricate friction stir welding
joints was shown in figure 2 and the experimental tool set up is shown in figure 3
Figure 2 image of Vertical Milling Machine Figure 3 image of experimental tool set up
The materials used for the welding are similar AA 6061 alloy, and tool used is stainless
steel. AA 6061 plates before performing friction stir welding are shown in figure 4 and taper
screwed pin profile of stainless steel tools of three D/d ratio tools are shown in figure 5
The chemical composition of AA 6061is shown in fallowing table 1
Element Mg Si Cu Zn Ti Mn Cr Al
Amount(Wt%) 0.85 0.68 0.22 0.07 0.05 0.32 0.06 Balance
Table 1 Chemical composition of AA 6061 alloy
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D.Raghavendra and S.Sravya
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Figure 4 AA 6061 work piece material Figure 5 Stainless steel tools of screwed taper pin profile
The initial joint configuration was obtained by securing the plates in position using
mechanical clamps. The direction of welding is normal to the rolling direction and single pass
FSW used to fabricate the joints and the three weld joints are in figure 6
Figure 6 Image of AA 6061 after fsw
3. RESULTS AND DISCUSSION
3.1. Tensile test on welded joints
Tensile test, also known as tension testing, and it is a destructive testing method that measures
and provides valuable information of the strength of a particular material, part, sample, weld
or other items. is a traditional materials science test in which a sample is subjected to ultimate
failure by performing the increasing the load capacity on the sample. The tensile test can be
commonly used to predict how a material will react under other types of forces or loads and to
select a material for an application, for quality control. In this paper the properties that are
directly measured by using tensile test are ultimate tensile strength, percentage of elongation
and yield strength. From these measurements the following properties can also be determined:
Young's modulus, Poisson's ratio and strain-hardening characteristics[6]. A tensile specimen
has a standardized sample cross-section,it has two shoulders and a gauge in between the
shoulders. The tensile specimen is shown in figure 7
Figure 7 Tensile test Specimen
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Influence of Process Parameters on Mechanical Properties of Friction Stir Welding of Aa 6061-T6
Alloy
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3.2. Impact test on welded joint
In the impact test, the test specimen is provided by a V-notch. The amount of energy absorbed
by a material during fracture when a external force is applied was calculated. This absorbed
energy is a measure of a given material's notch toughness and acts as a tool to study
temperature-dependent ductile-brittle transition. Impact is a high force or shock applied on
two or more bodies collide over a short time period. Such a force or acceleration usually has a
greater effect than a lower force applied over a proportionally longer time period of time.The
impact specimen is shown in figure 8
Figure 8 Impact specimen
The results of tensile properties, yield strength, percentage of elongation and impact
strength are given in the table 2
S.NO Rotation
speed(RPM) D/d
ratio Ultimate Tensile
Strength(N/mm2) Yied
Strength(N/mm2)
PERCENTAG
E Of Elongation
Impact Sterngth
Weld
1 900 3 190.33 139.59 8.22 20
Weld
2 1120 3.5 169.75 138.63 8.56 20
Weld
3 1400 4 167.92 134.64 7.62 20
Table 2 A graph is drawn between weld joints and tensile strength is shown in figure 9[7]
Figure 9 weld jonts vs tensile strength
UTS(N/mm2)
weld1
weld2
weld3
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D.Raghavendra and S.Sravya
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3.3. Microstructure
The optical micrographs of all Aluminum 6061-T6 alloy (Exp.1-3) are shown in Fig.1.It show
the optical micrographs of the heat affected zone of all the samples. Significant grain
refinement can be noticed in the alloy on FSW in the first joint. Since the second phase
particles were not visible by optical microscopy. Density of precipitates has decreased as a
result of coarsening; Grain boundary precipitates have also coarsened. The effects of process
and tool parameters on macrostructure of the friction stir welded joints. It is generally known
that the fusion welding of aluminum alloys accompanied by the defects like porosity, slag
inclusion, solidification cracks, etc., deteriorates the weld quality and joint properties.
Generally, friction stir welded joints are joined in solid state due to the stirring action of tool
on the work. so joints are free from solidification related defects but FSW joints had defects
like pin hole, tunnel defect, piping defect, kissing bond, Zig-Zag line and cracks, etc., due to
bad consolidation of metal in the weld region and improper flow of metal and insufficient
consolidation of metal in the weld region.
The particles of Mg and Si were observed to be dispersed uniformly in the NZ for all the
conditions of composites made by FSP due to rotating tool gives sufficient heat generation
and a circumferential force to distribute the reinforcement particles to flow in wider area. It is
found that the sample made at 900rpm and D/d=3 shows severe plastic deformation and
frictional heating in the heat affected zone during FSW resulted in generation of a
recrystallized equiaxed microstructure. The microstructure specimen is shown in figure 10
After FSW, microstructural observations were carried out at the cross section of heat
affected zone zone(HAZ) of aluminum 6061-T6 alloy normal to the FSW direction,
mechanically polished and etched with Keller’s reagent (2 ml HF, 3 ml HCl, 20 ml HNO3 and
175 ml H2O) by employing microscope(OM)
Figure 10 Microstructure specimen
Microstructure obtained at heat affected zones at 100µm are shown in figure 11 to 13
Figure 11 AA 6061at 900rpm and D/d ratio 3 Figure 12 AA 6061at 1120rpm and D/d ratio 3.5
Figure 13 AA 6061at 1400rpm and D/d ratio 4
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Influence of Process Parameters on Mechanical Properties of Friction Stir Welding of Aa 6061-T6
Alloy
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4. CONCLUSION
Aluminium alloys 6061 and was successfully welded.The influence of rotational speed and
D/d ratio on mechanical properties of Aluminum alloy 6061-T6 joined via Friction Stir
Welding were investigated and the following conclusions are obtained.
At rotational speed 900 rpm, welding speed 60mm/min and D/d ratio 3,the weld joint 1
exhibits maximum tensile properties compared to other joints.
It is found that the weld joint made at 900rpm and D/d=3 shows severe plastic
deformation and frictional heating in the heat affected zone during FSW resulted in generation
of a recrystallized equiaxed microstructure.
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