Influence of Welding Processes on Tensile Properties, Microstructure, and Hardness of Friction Stir Welded AZ31B Magnesium Alloy

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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

mailto:[email protected]







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




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




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




ISSN 2277-5056 | © 2014 Bonfring

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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Documents

Influence of Welding Processes on Tensile Properties, Microstructure, and Hardness of Friction Stir Welded AZ31B Magnesium Alloy