Comparative Evaluation of Tensile Properties and ... · been made to evaluate the tensile and...

Comparative Evaluation of Tensile Properties and

Microstructural Behaviour of Friction Stir Welded Butt and

Lap Joints of AA2014-T6 Aluminum Alloy

1C. Rajendran, 2K. Srinivasan, 3V. Balasubramanian, 4H. Balaji, 5P. Selvaraj

1Assistant Professor, Department of Mechanical Engineering,

Sri Krishna College of Engineering and Technology, Coimbatore, India-641008 2Assistant Professor, 3Professor

Centre for Materials Joining and Research, Annamalai University, India-608002 4 Scientist D, 5Scientist-F

Aeronautical Development Agency, Bangalore, India

Email: [email protected]

.

Abstract Age hardening aluminum alloys such as 2XXX and 7XXX series are

suitable for parts and structures requiring high strength to weight

ratio and are commonly used in aircraft fuselage and wing skins. The

structures are conventionally joined by rivets. It is difficult to join

these aluminum alloys especially 2XXX series by traditional joining

process due to break up the temperature of Al2O3 which usually

results in solidification cracking, burns through and porosity. Hence to

overcome such problems solid-state welding technique is chosen.

Friction Stir Welding (FSW) is one such promising process, which can

be effectively applied to weld these alloys for aircraft application with

butt joint configuration. Hence, in this investigation, an effort has

International Journal of Pure and Applied MathematicsVolume 119 No. 12 2018, 2079-2091ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

2079

been made to evaluate the tensile and microstructural characteristics

of lap and butt joint of 3 mm thick plate welded by FSW, at constant

tool shoulder diameter (D) to plate thickness (T) ratio of 3 and tool tilt

angle of 2˚. It is observed that the butt joint fabricated with a D/T ratio

of 3 exhibited superior tensile and microstructural properties

compared to the lap joint fabricated with D/T ratio of 3. The width of

the thermo-mechanically affected (TMAZ) region in the lap joint was

comparatively higher than the butt joint. Hence the lap joints yielded

inferior tensile properties than the butt joint due to the enhanced grain

coarsening produced in the TMAZ region of the lap joint. In addition,

the hook formation on the AS and RS of the lap joint was bending

downwards, which is different from the ideal condition.

Keywords: Friction Stir Welding, Al-Cu alloys, Microstructure, Tensile

properties

1. Introduction

The precipitation hardening aluminum alloys such as 2XXX, 6XXX and 7XXX series

are ideal for aircraft structural applications. Most aircraft and aerospace sheets

involve butt and lap joint welds. Conventional joints are produced using rivets,

fusion welding limited to GMAW and GTAW [1]. It is very challenging to weld

aluminum due to the high temperature involved in breaking up of the oxide film.

The friction stir welding (FSW), a new solid-state welding, offers several benefits

over fusion welding processes. Some of the benefits are low heat input which

eliminates the melting and solidification process thus resulting in good dimensional

stability, improved mechanical properties, lesser weld defects and low internal

stress [2]. Hence, the use of FSW in place of riveting for joining Al alloys in lap

configuration could help reduce the weight and overall cost with enhanced

mechanical properties and manufacturing complexities. The FSW process

development for lap (LJ) and butt joints(BJ) are quite different from each other. The

removal of the Al2O3 at the interface of lap joint is very difficult than BJ [3]. In

general, the FSW tool penetrates the abutting faces in a BJ; whereas, in LJ, a non-

consumable rotating tool is made to pierce into lower sheet up to certain plunge

depth. The original joint line with severe plastic deformation (OJLwSPD) [4] kissing

bond [5] and interface [6] of the weld bends upward or downward with respect to the

movement of material around the pin resulting in the hook formation [7]. The hook

formation in an ideal LJ joint is a downward bending hook on the AS side and an

upward bending hook on the RS which are more beneficial for load carrying capacity

[8]. Cederqvist and Reynolds [9] investigated the parameters affecting the

properties of friction stir LJ between Al alloy 2024-T3 and 7075-T6. A joint

International Journal of Pure and Applied Mathematics Special Issue

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efficiency of 78% was reported in LJ which is much higher than what could be

achieved with riveting and electrical resistance spot welding. Several researchers

have highlighted the significance of process parameters on mechanical and

metallurgical properties. In the current study, comparative evaluation of friction

stir butt and lap joints, in terms of (1) microstructural behaviour and (2) Tensile

properties have been made.

2. Experimental work

The rolled sheets of 3 mm thickness, made of aluminum alloy AL clad 2014-T6,

were utilized in this work. The chemical proportion and mechanical properties of

the base metal are listed in Tables 1 and 2 respectively. The dimensions of BJ and

LJ were 150 mm x150 mm x 6 mm. The sides of the plates were machined using a

milling machine, and butt and lap joints were prepared by conducting FSW normal

to the rolling direction using CNC controlled FSW machine. The FSW tool was

prepared of super HSS with threaded taper cylindrical pin of length 2.8 mm for BJ

and 5.75mm for LJ with a concave profile head of 3o on 9 mm and 18 mm tool

shoulders. The tool details are listed in Table 3. The position of tool was kept at 2°

for each condition. Two unequal axial forces were attained by controlling the pierce

depth of welding tool since both the specimens had different thickness. In order to

calculate the efficiency of the LJ, plain sheet specimen (100 mm long, 10 mm wide

and 3 mm thick) was taken from the base material in T6 condition and loaded in

tension to failure. The tension to failure load of the base metal in T6 condition was

measured. The process parameters used in this study are presented in Table 4. The

metallographic observation was carried out by optical microscopy (OM) and

Scanning Electron Microscopy (SEM). The samples for OM were milled, polished

and etched using Keller’s etchant for macro and microstructure. The microhardness

was noted using microhardness tester applying 50 N force for 15 sec. In order to

obtain fracture location of the butt and lap joints, room temperature tensile tests

were carried out according to ASTM-E8 M-04 and ANSI/AWS/SAE/D8.9-97.

3. Results and Discussion 3.1 Macrostructure

Optical macrographs showing the cross-section of the FSW joints at different

configurations (type of joint) of BJ and LJ are shown in Fig.2 and Fig.3, it can be

observed that both the welded joints are sound and defect free at constant D/T ratio

of 3 and tool tilt angle of 2°. All joints showed elliptical weld nugget [13] with wider

nugget zone(NZ) at the top than the bottom. The top surface had direct contact with


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the shoulder, therefore, experienced more frictional heating and plastic flow which

resulted in wider weld zone than the bottom surface.

Table 1 Chemical composition (wt. %) of base metal

Si Fe Cu Mn Mg Zn Cr Ti Al

0.874 0.135 4.815 0.813 0.734 0.063 0.005 0.011 92.45

Table 2 Mechanical properties of base metal

Material

Yield

stress

( MPa)

Ultimate

tensile

stress (MPa)

Elongation

50 mm

gauge

length

(%)

Micro

hardness

50 N, 15 sec

(VHN)

Shear load

(kN)

AA2014-T6 431 455 10 163 17

Table 3 Geometry of used FSW tool

Joint

configuration

Pin

description

Pin diameter Pin

length

(mm)

Taper

angle

in

pin

diameter

(o)

Thread

pitch

(mm) Major

diameter

(mm)

Minor

diameter

(mm)

Butt joint Threaded

taper pin 3.0 2.5 2.75 5.19 0.75

Lap joint Threaded

taper pin 6.0 5.0 5.75 4.96 0.75

Table 4 Process parameters of butt and lap joint

Joint

configuration

Tool

rotational

speed (rpm)

Welding

speed

(mm/min)

Tool

Shoulder

diameter

(mm)

Tool tilt angle

(deg)

Butt joint 1400 50 9.0 2.0

Lap Joint 700 50 18.0 2.0

The bottom surface which was in contact with the backing plate extracted heat from

the bottom area of the joint which in turn contracted the lower portion of the weld

nugget.


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ig.1 Schematic diagram of the FSW tool used in this investigation

Fig.2 butt joint configuration and tensile

specimen extraction

Fig.3 Lap joint configuration and tensile

specimen extraction

D- Tool shoulder diameter (mm)

d1 - Major diameter of pin (mm)

d2 - Minor diameter of pin (mm)

L- Length of pin (mm)


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a. Fabricated Butt joint tools b. Fabricated lap joint tools

c. Fabricated butt joints d. Fabricated lap joints

e. Tensile specimen(Before testing) f. Lap shear specimen(Before testing)

g. Tensile specimen (After testing) h. Lap shear specimen(After testing)

a. Fractured butt joint b. Fractured lap joint


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Fig.4 Photograph of FSW butt and lap joint

In case of FSW joints of AA2014-T6, the dimension of each joint of weld nugget

closely matched with the dimension of tool ie. Shoulder diameter at the top surface

(10.1 and 20.3 mm) and pin diameter at the bottom (2.6 and 5.65mm) of both joints.

NZ at the top surface was wider than at the bottom surface, as the upper surface

was in contact with the tool shoulder. In FSLW a visible bright contrast line seen

along the original sheet interface in Fig. 5(b) is the al clad layer. The interface line

enters into the SZ and bends toward the bottom on the AS, whereas in RS the line

terminates at the TMAZ/SZ interface. In this investigation, the material flow in and

around the pin is seen in the macrostructure showed maximum width in SZ bottom

than the top. The reason for this problem may be the restriction of material flow

around the pin caused by the al clad layer between two sheets thus pushing the

material back to the SZ bottom, so the size was bigger.

3.2 Microstructure

The formation of NZ is due to the collective effect of thermal and mechanical

stresses caused by stirring action of the tool and axial force. TMAZ and HAZ in joint

are shown in Fig. 5. The micrographs of the center of weld NZ for both the joints are

shown in Fig. 5(c) and 5(d). All weld nuggets invariably showed fine grains

produced by severe plastic distortion and high temperature resulting in dynamic

recrystallization. Due to the revolution and movement of FSW tool during welding,

the coarser grains are converted into fine grain structure in the NZ. Fewer

strengthening precipitates of CuAl2 were observed in NZ as broken down and

uniformly distributed by the stirring tool. The LJ showed three distinct

microstructural regions SZ, TMAZ and HAZ. Very fine and recrystallized grains

were observed in the SZ of both the butt and lap joint Fig. 5(c) and 5(d), towards the

welded bottom distinct onion ring pattern, were seen in both the welds.The TMAZ

in both the case showed severely deformed un recrystallized grains, no significant

grain coarsening in the HAZ was observed in any of the welds. Within the region

covered by tool shoulder, two distinct bond region were observed in lap welds, such

as partially bonded region and fully bonded region. The partially bonded region has

been described some of few researchers such as an OJLwSPD [4], kissing bond line

[5] and Interface [6]. The upper and lower sheets closely mate with each other. They

are separated by a thin layer beginning somewhere near the tool shoulder mark on

either side of the weld and extend inwards, ending either at the TMAZ/SZ interface


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or SZ. In this weld, the partially bonded region was observed to extend into

TMAZ/SZ Fig. 5(b).

a. Macrostructure- butt joint b. Macrostructure-lap joint

c. Stir zone(Butt joint) d. Stir zone (lap joint)

e. Interface SZ/TMAZ- AS f. Interface SZ/TMAZ-AS

g. Interface SZ/TMAZ-RS h. Interface SZ/TMAZ-RS

50µm

50µm

50µm

50µm 50µm

50µm


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i. Thermo mechanical affected zone j. Thermo mechanical affected zone

Fig.5 Microstructure of various regions of butt and lap joints

a. Hook formation on RS b. Hook formation on AS

Fig.6 Microstructure of hook formation on lap joint

a. Base metal

b. FSW Butt joint

b. lap joint

Fig.7 Fractographs of base metal, FSW butt and Lap joint

50µm 50µm

50µm 50µm


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The second one was in the fully bonded region, the region within SZ, where the

upper and lower sheet gets metallurgically bonded to each other with no discernible

original interface line in the weld. The width of the fully bonded region is an

important consideration in lap joint. It was found that the width of bond in FSLW

was less than the butt joint. When the thread rotates in the favorable direction it

can result in such a strong downward metal flow [10-12]. The reason for lower lap

shear strength of AA2014-T6 aluminum alloy is due to different relative speeds of

plastic material on AS and on RS which results in different structures [14]. It was

found on the AS; the speed gradient is greater than the RS. Microstructure changes

rapidly and there was lack of necessary transition. In friction stir BJ, Fig.5 (e)

shows the microstructure in the weld nugget-TMAZ on the AS, the grain in the SZ

is finer than in the TMAZ. It can be seen from Fig.5 (e) that microstructures change

smoothly from SZ to TMAZ because of sufficient plastic material flow due to little

speed gradient. The grains are smaller and microstructure is even. The size of grain

changes gradually from SZ to TMAZ and this will not affect the tensile properties

very much. But an obvious boundary can be seen between SZ and TMAZ.

3.3 Tensile Properties

Table 5 shows tensile test results of the joints with different weld configurations.

Experimental results showed that FSW butt joint had significant efficiency than LJ.

The efficiency of BJ and LJ was inferior to the base material. Tensile strength,

hardness, and elongation were lower than the base material, the tool rotational

speed of 1400 rpm, welding speed of 50mm/min, D/T ratio of 3.0 and tilt angle 2.5

yielded the maximum tensile strength of 339 MPa. The increase in tool rotational

speed and welding speed with constant D/T ratio of 3.0 and tool tilt angle 2.5, was

found to reduce the ultimate tensile strength due to high heat input and improper

material flow resulting in a defect which was observed either on advancing side or

retreating side because of the grain coarsening occurring in the TMAZ/SZ interface

region. The process parameters influenced the heat input per unit length of the

weld, which controlled the degree of softness and flows ability of the plasticized

material. At low speed of tool, the amount of heat supplied to the deforming

material in weld area was greater and therefore wider softened region around the

stirring tool lead to more homogeneity of the NZ, which resulted in the inferior

ultimate tensile strength of welded joint having higher heat input per unit length

achieved at higher welding speed. The lower heat input per unit length of the weld,

resulting from reduced stirring of material and flow in weld area resulted in the

poor ultimate tensile strength of friction stir welded joint of AA2014-T6.


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For LJ, the average failure load measured on the base metal in T6 condition was 17

kN. This value was used to calculate the joint efficiency of weld specimens under lap

shear loading using the following formula, which was proposed by Cederqvist and

Reynolds [9]:

Weld failure load (kN)

Joint Efficiency [FSLW] = ------------------------------- X100% (1)

Base metal failure load (kN)

The lap shear testing conducted on the joint in alloy AA2014 sheet produced using

FSW is given in Table 5, the joint efficiency also included in Table 5. The efficiency

of LJ made with threaded tapper cylindrical tool (52%) was less than that of butt

joint (78%). The butt joint configuration efficiency thus confirmed its superiority

overlap joint. Examination of the fractured LJ specimen showed that the weld made

with the threaded taper cylindrical pin with the originating fracture at the

SZ/TMAZ interface on AS, and both the weld fractured on the top sheet. The lap

shear specimen was loaded on advancing side in the upper sheet and on the RS in

the lower sheet, since the hook is bent downward on both the side in the weld, since

the upper sheet is indeed the critical location for failure, this explains why the lap

joint fractured on the AS in the upper sheet. Fractographs of the transverse tensile

and lap sheared samples are presented in Fig.7, fracture surface is invariably

characterized by dimples of varying size and shape, which an indication of ductile

failure, welds produced in FSW butt joint exhibited finer and deeper dimple and

rupture features, as compared to the shallow and coarse dimples in the case of laps

joint. The average hardness in the SZ on butt and lap joints were found to be 135

HV and 115 HV respectively.

4 Conclusion

1. The best quality of weld was obtained at tool rotational speed of 1400 rpm,

welding speed of 50 mm/min, D/T of 3.0 and tool tilt angle of 2o for butt joint, and

tool rotational speed of 700 rpm, welding speed of 50 mm/min, D/T ratio of 3.0 and

tool tilt angle of 2o for lap joint of AA2014-T6 aluminum alloy.

2. The tensile properties of the butt joint and lap joint are lower than that of the

base material, and a maximum joint efficiency attained in terms of TS and TSFL is

78% and 52% respectively.


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3. In both the cases, crack propagated along the interface between nugget zones

(NZ) and TMAZ, due to the dissimilarity of grains observed in the microstructure of

the interface.

Acknowledgement

The authors are indebted to Aeronautical Development Agency (ADA), Bangalore, for

financial support through project FSED 83.07.03.

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