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8/11/2019 Influence of Welding Processes on Tensile Properties, Microstructure, and Hardness of Friction Stir Welded AZ31B M…
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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 4, No. 2, May 2014 96
ISSN 2277-5056 | © 2014 Bonfring
Abstract--- Friction Stir Welding (FSW) was performed on
5-mm thick plates, machined from rolled AZ31B Magnesium
alloy.The microstructure and defects formation were
investigated by optical microscope. The mechanical properties
were determined by tensile and hardness tests. Frictional heat
and plastic flow during friction stir welding create the fine
recrystallized grain (Stir Zone, SZ) and the elongated andrecovered grain (Thermo-Mechanical Affected Zone, TMAZ)
in the weld zone. Heat affected zone (HAZ), which can be
identified only by hardness test due to no difference in grain
structure compared with the base metal, is formed beside the
weld zone. In this study, the effect of rotational speed on
microstructure , hardness and mechanical properties of
Friction stir welded Mg AZ31B alloy have been investigated. Friction stir welding (FSW) is carried out at different
rotational speeds of 900 rpm, 1120 rpm, 1400 rpm and 1800
rpm with High speed steel (HSS) at a constant welding speed
of 40 mm/min, tilt angle of 2.50 and axial force of 5 KN. It isobserved that the joint fabricated using HSS tool material at a
rotational speed of 1400 rpm obtained higher mechanical
properties as compared to those of 900 rpm, 1120 rpm and
1800 rpm.
Keywords--- Friction Stir Welding, AZ31B Mg Alloy,
Mechanical Properties, Rotational Speed
I. I NTRODUCTION
RICTION Stir Welding (FSW) technique which was
invented by The Welding Institute (TWI) in 1991 [1].
On observing the advantages associated with FSW, mainly
grain refinement, the phenomenon has been extended to
processing of commercial alloys. Friction stir processing
(FSP) is a solid-state process in which a specially designed
rotating cylindrical tool, consisting of a pin and a shoulder, is
plunged into the sheet. The tool is then traversed in the desired
direction. The rubbing of the rotating shoulder generates heatwhich softens the material (below the melting temperature of
the sheet) and with the mechanical stirring caused by the pin,
S.Ugender, Ph.D Scholar, Mechanical Engineering Department, JNTU,
Hyderabad, India. E-mail: [email protected]
Dr.A. Kumar, Associate Professor, Mechanical Engineering Department, National Institute of Technology, Warangal, India. E mail:[email protected]
Dr.A. Somi Reddy, Professor, Mechanical Engineering Department,VITS, Karimnagar, India. E-mail:[email protected]
DOI : 10.9756/BIJIEMS.4826
the material within the processed zone undergoes intense
plastic deformation yielding a dynamically recrystallized fine
grain structure.
Despite the large number of studies that are being
conducted to advance FSP technology, the effects of FSP onvarious mechanical and micro structural properties are still in
need for further investigations. In addition, correlations
between FSP parameters, mechanical properties and micro
structural characteristics are not yet well understood. Accurate
correlations are needed for successful modelling and process
optimization. Most of the work that has been done in the fieldof friction stir processing focuses on aluminium alloys [2] – [5].
Magnesium is the lightest constructional metal on earth; it
is 35% lighter than aluminium, and 78% lighter than steel.Magnesium offers a great potential for weight reduction by
replacing steel and aluminium, if proper design considerations
are made. Until now most of the successfully produced
magnesium parts are cast-components, however significant
weight reduction cannot be achieved unless magnesium usageis expanded to cover other areas, mainly sheet metal forming.
The metal‟s inferior ductility at room temperature still hinders
its widespread uses. AZ31 magnesium alloy is commercially
available in sheet form, and offers good mechanical
properties. Unfortunately, the alloy exhibits very limitedductility accompanied by brittle-like behaviour at room
temperature. Recent results however indicated that it is possible to form AZ31 sheets at elevated temperatures under
certain conditions, and even achieve super plastic-like
behaviour [6] – [7]. The results also suggest that improved
ductility and formability can be achieved by refining and
homogenizing the grain structure of the sheet. FSP has the
potential to become an effective tool for microstructural
modification of sheet metals.
In this present investigation, effect of rotational speed (i.e.
900 rpm, 1120 rpm, 1400 rpm and 1800 rpm with HSSmechanical properties of friction stir welded of AZ31B
Magnesium alloy of the mechanical properties are evaluated.
II. EXPERIMENTAL
The AZ31B Mg alloy plates of 5 mm thickness were cut
into the required size (240 mm x 120 mm) by power hacksaw
cutting and milling. The joint was obtained by butting the two
plates and stirring them together with a rotating tool assembly
by using vertical milling machine. Schematic sketch of theweld joint and tool is as shown in Fig.1and Schematic sketch
of the weld joint and tool is as shown in Fig.2. Non-
consumable High Speed Steel (HSS) tool steel with flat
Influence of Welding Processes on Tensile
Properties, Microstructure, and Hardness of
Friction Stir Welded AZ31B Magnesium AlloyS. Ugender, Dr.A. Kumar and Dr.A. Somi Reddy
F
8/11/2019 Influence of Welding Processes on Tensile Properties, Microstructure, and Hardness of Friction Stir Welded AZ31B M…
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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 4, No. 2, May 2014 97
ISSN 2277-5056 | © 2014 Bonfring
shoulder is chosen as tool material to fabricate the joints,
because of its high strength at elevated temperature, thermal
fatigue resistance and low wear resistance. The diameter of the
shoulder and pin used were 18 mm, 6 mm respectively andlength of the pin is 4.8 mm. The butted plates were clamped
on a steel backing plate. The macrographs of VMM shown inFig.3. For various testing the required dimensions of the
specimens were cut from the region under the tool shoulder
(i.e. stir zone) by using wire EDM.The welding tool is tilted by 2.5 degree of angle with
reference to the welded plates and tool was rotated in the
clockwise direction. A constant axial force is applied for all
the joints. The FSW joints were fabricated with taper threaded
tool pin profile and found to be defect free welds. Specimens
for tensile testing were taken in transverse to the weld
direction and machined as per ASTM E8/E8M-11 standards.Tensile test was conducted using computer controlled
universal testing machine (Model: Autograph, Make:
Shimatzu) with a cross head speed of 0.5 mm/min. Specimens
for impact testing were taken in transverse to the weld
direction and machined as per ASTM A370 standards. The
charpy „V‟ notch impact test was conducted at roomtemperature using pendulum type impact testing machine. The
amount of energy absorbed in fracture was recorded and the
absorbed energy is defined as the impact toughness of thematerial. The Schematic sketch of tensile and impact
specimens and the schematic sketch of charpy specimen wereshown in Fig.4.and Fig.5. respectively. Specimens were cut at
the middle of the joints in transverse direction for conducting
micro hardness survey. Micro hardness test was carried out
using Vickers digital micro hardness tester (Model:
Autograph, Make: Shimatzu) with a 10 g load for 10 s
duration. The microhardness was measured at an interval of
0.15 mm across the WZ, Thermo-Mechanical Affected Zone
(TMAZ), and Heat-Affected Zone (HAZ) and (base metal)
BM. Defect free welds were obtained at all the conditions such
as tool rotation speed at 1400 rpm and weld speed at 40
mm/min. The microstructure at the weld zone of friction stir
welded joint at the condition of weld speed at 40 mm/min isobserved to be having finer grains than that of other weld
conditions due to dynamic recrystallization. The joints madewith tool rotation speed at 1400 rpm and weld speed at 40
mm/min resulted in good mechanical properties as compared
with other weld conditions due to sufficient heat generationand proper mixing of the material in the weld zone.
Table I: Chemical Composition (wt %) of base metal AZ31BMagnesium Alloy
Figure 1: The Schematic Diagram of AZ31B Mg Alloy Plates
used for FSW
Figure 2: The Schematic Diagram of Tool Geometry
Figure 3: The Macrographs of Vertical Milling Machine
Figure 4: The Schematic Diagram of the Tensile Specimen
Fig.5: The Schematic Sketch of Charpy Specimen
8/11/2019 Influence of Welding Processes on Tensile Properties, Microstructure, and Hardness of Friction Stir Welded AZ31B M…
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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 4, No. 2, May 2014 98
ISSN 2277-5056 | © 2014 Bonfring
Table II: FSW Process Parameters and Tool Nomenclature
Parameters Values
Rotational speed(rpm) 900,1120,1400, 1800
Tilt angle 2.5
Pin diameter(mm) 6
D/d Ratio of tool 3.0Tool ProfileWelding
Speed(mm/min)
Taper Thread40
III. R ESULTS AND DISCUSSIONS
A. Tensile Properties
The effect of tool rotational speed (i.e. HSS tool material)
on Mechanical properties such as tensile strength, yield
strength and % of elongation of Friction Stir Welded AZ31B
magnesium alloy joints are presented in Table 3. In FSW, tool
rotation speed results in stirring and mixing of material around
the rotating pin which in turn increase the temperature of the
metal. It appears to be the most significant process variablesince it is tends to influence the transitional velocity. It is
known that the maximum temperature observed to be a strong
function of tool rotation speed [14]. At lower rotational speed(900rpm), the ultimate tensile strength, yield strength and % of
elongation of FSW joints is lower. When the rotational speed
is increased from 900rpm, correspondingly the ultimate tensile
strength also increases and reaches a maximum at 1400 rpm
made of HSS tool material. If the rotational speed is increased
above 1120 rpm, the tensile strength of the joint decreased.
Higher tool rotational speed (1800 rpm) usually resulting in
higher heat input per unit length and slower cooling rate in the
FSW zone causes excessive grain growth, which subsequently
lead to lower tensile properties of the joints. A higherrotational speed also causes expensive release of stored
materials to the upper surface, which produces micro-voids in
the stir zone and this may be one of the reasons for lower
tensile properties of the joints, even at lower rotational speed(900 rpm) results in lower tensile properties which is due to
lack of stirring and lower heat input per unit length that leads
to insufficient plasticization. It is observed that the joint
fabricated at a tool rotational speed of 1400 rpm made of HSS
tool material exhibited higher tensile strength, yield strength
and % of elongation and this may be due to optimum heatgeneration which is sufficient to cause free flow of plasticized
material and adequate mechanical working [15].
Table III: Effect of Rotational Speed on Mechanical Propertiesof AZ31B Mg Alloy using HSS Tool
Figure 6: Effect of Rotational Speed Tensile Strength
Figure 7: Effect of Tool Rotational Speed Yield Strength
Figure 8: Effect of Rotational Speed on Percentage ofElongation
0
50
100
150
200
900 1120 1400 1800
T E N S I L
E S T R E N G T H
ROTATIONAL SPEED
HSS
HSS
0
20
40
60
80
100
120
140
160
900 1120 1400 1800
Y I E L D S
T R E N G T H
ROTATIONAL SPEED
HSSYS
HSSYS
0
1
2
3
4
5
6
900 1120 1400 1800
%
E
L
N
G
A
T
I
O
N
ROTATIONAL SPEED
HSSELG
HSSELG
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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 4, No. 2, May 2014 99
ISSN 2277-5056 | © 2014 Bonfring
B. Microstructure Studies
The optical micrographs taken at stir zone of FSW of all
the joints are displayed in Fig.9. (A-E).From the micrographs,
it is understood that there is in appreciable variation in average
grain diameter of weld region in AZ31B Magnesium alloy.
Due to FSW, the coarse grains of base metal are changed in to
fine grains in the stir zone. The joints fabricated with a
rotational speed of 1400 rpm with a constant welding speed of
40 mm/min and HSS tool contain finer grains in the weldregion compared to other joints. This is one of the reasons for
higher tensile properties of these joints compared to other
joints. From the micrographs, it is inferred that there is anappreciable variation in grain size across the welds; this is
because of in sufficient plastic flow and thermal exposure, It
has been observed during this work that the total impact
energy increased in the friction stir welding of (medium
strength) AZ31B Mg alloy for both temper conditions
especially at 1400 rpm and 40 mm/min with respect to the
base metal while rotation and transverse speed have little
effect on the impact value of (high strength) results were very
close to each other. Finally it is important to mention that the
relation between rotation speed, transverse speed and inputheat which affect on the impact value seems to be compoundand depend on the material properties being welded, Grains
are relatively smaller in the retreading side of SZ compared to
the advancing side, and this is caused by the greater strainingin this location. The similar observation was made by Pareek
et.al.[16]. in friction stir welding of AZ31B Magnesium
alloy. This may be another reason for failure along the SZ
region on the advancing side.
Figure 9: (a) Optical Microstructure as received Material (b)FSW Material at 900 rpm (c) at 1120rpm (d) 1400 rpm with
HSS Tool
C. Hardness
The hardness was measured across the weld in the nugget
zone using Vicker‟s microhardness testing machine, and the
values are presented in Table 3. The hardness of base metal
(unwelded parent metal) is 69 Hv. Vickers microhardness is
measuring along the mid thickness line of cross section of the joint. The joint fabricated with the rotational speed of 1120
rpm, welding speed of 40 mm/min, recorded higher hardness
(70Hv) in the stir zone, and this is also one of the reasons forsuperior tensile properties of these joints compared to other
joints. These are two main reasons for the improved hardness
of stir zone. Firstly, since the grain size of stir zone is muchfiner than that of base metal, grain refinement plays an
important role in material strengthening, secondly the small
particles of intermetallic compounds are also a benefit to
hardness improvement [17]. Higher tool rotational speed
resulted in higher heat generation and this lead to the
excessive release of stirred material to the upper surface which
results in lower hardness.
Figure 10: Effect of Rotational Speed on Hardness
D. Impact Toughness
Charpy impact toughness of FSW joint was evaluated and
presented in Table 3.The impact toughness of unwelded basemetal is 8J.However, the impact toughness of FSW joint with
notch placed at the SZ region and reached maximum 7 J at
1400 rpm, compared to the other rotational speeds. This may
be due to optimum heat generation which is sufficient to cause
free flow of plasticized material
0
20
40
60
80
-20 0 20
M i c r o h a r d n e s s ( H v
)
Distance from weld centre
(mm)
HSS90
0
HSS11
20
HSS14
00
HSS18
00
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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 4, No. 2, May 2014 100
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IV.
CONCLUSION
In this study, the effect of rotational speed on the
microstructural changes and the mechanical properties of
friction stir welding of Mg AZ31B alloy have been
investigated. It has been found that rotational speed has a
significant influence on grain refinement of material. The
optimum rotational speed which gives better mechanical properties of Mg AZ31B alloy is 1400 rpm. The micro
hardness of nugget zone is more compare to as-received Mg
alloy. At 1400 rpm tensile properties are yield strength,
ultimate strength and % of elongation are exhibited maximum
mechanical properties compared to those of other rotational
speeds.
ACKNOWLEDGMENT
The authors would like to thank the authorities of JNTU
Hyderabad, NIT Warangal and SR Engineering College,
Warangal, AP, India for providing the facilities to carry out
this work.
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