influence of rpm and feed rate on the micro structure and tensile properties of friction stir spot...

8/8/2019 Influence of Rpm and Feed Rate on the Micro Structure and Tensile Properties of Friction Stir Spot Welded AHSS

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Influence of rpm and feed rate on the microstructure and tensile properties of Friction

Stir Spot welded AHSS

R. Sarkar1

, Dr. T.K. Pal1

and Dr. M. Shome2

1Welding Technology Centre, Dept of Metallurgical and Material Engg, Jadavpur University, Kolkata, India

2Materials Joining and Characterization, R & D, Tata Steel, Jamshedpur, India

Introduction

Energy saving and environmental preservation are important issues that must be resolved. Since reducing the

weight of the vehicles is one of the counter measures against them, the use of advanced high strength steel (AHSS) in

place of mild steel has been increasing in fabrication of vehicles. The ultra light steel auto body (ULSAB) project has

shown that car body mass can be reduced by 25% by using AHSS. Furthermore, recent enhancements in US

automobile rollover standards require vehicle designers to meet more stringent safety requirements. Achieving strict

safety requirements without adding weight necessitates increased utilisation of materials with high – strength – to –

weight ratios in structural component design and fabrication. Such escalation in the use of AHSS only emphasizes the

importance of providing viable joining techniques that both preserve and protect the unique microstructural balance

required to maintain their desirable mixture of mechanical properties. Electric resistance spot welding (RSW),

typically employed in automotive industry in joining steel has yet to prove a completely effective joining method forAHSS. This is mainly due to the fact that microstructural changes during RSW dramatically affect mechanical

properties by transforming the base metal microstructure derived from extreme post weld thermal gradient [8].

Friction Stir Spot Welding (FSSW) is a new process that recently has received considerable attention from the

automotive and other industries. A novel variant of the “linear” friction stir welding (FSW) process, FSSW creates a

spot, lap weld without bulk melting. FSSW has proven to be cost effective and productive means for joining light

weight structural alloys such as aluminium. Similar in concept and appearance to its predecessor RSW, FSSW avoids

the severe heating and cooling cycles induced during the resistance method. Furthermore, additional advantages and

success of solid state approach has made FSSW the attractive method for spot welding of AHSS.

In FSSW, a cylindrical rotating tool with a protruding pin plunges at a specific rate into the overlapping sheets

to a predetermined depth. It is then retracted at a rapid rate either immediately or after a dwell period. The frictional

heat generated softens the metal and the rotating pin causes material flow in both circumferential and axial directions.

The forging pressure applied by the tool shoulder results in the formation of an annular, solid state bond around the

pin. The retraction of the pin leaves a characteristic exit hole.

The four main FSSW parameters are tool rotational speed, plunge rate, plunge depth and dwell time (time

prior to retraction of the tool). Although the feasibility of joining AHSS using FSSW has been recently considered,

information on how these parameters affect the weld quality is limited. While working with FSSW of 1.6 mm thick

DP 600 steel sheet using constant rpm (1500) and varying weld time from 1.6 to 3.2 s through changes in plunge rate,

Feng et al. [7] reported that solid state metallurgical bond was produced with welding time in the range of 2 -3 s. The

bond strength however increased with increasing weld time through improvement in bonding ligament width.

Furthermore, bonding region located in TMAZ, exhibited similar microstructure and hardness as in the base metal.

Hovanski et al. [8] successfully lap joined hot stamped boron steel via FSSW using PCBN tool. Variation in welding

parameters such as rotational velocity (800 – 2000) and total weld cycle time (1.9 – 10.5 s) i.e. plunging velocity and

dwell time lead to a variation in lap shear tensile strength from 6 -12 MPa. Increased dwell yielded between 40 -90%direct increase in lap shear strength for all plunge rates and the effect of rotational speed on weld strength is dependent

on plunging conditions. The original martensitic base metal microstructure with the exception of a thin region of

ferrite originating from the interface of the centre of the nugget is mostly retained. A comparative study between RSW

and FSSW of 1.2 mm thick Zn coated DP 600 AHSS revealed [9] that the microstructure of the HAZ is similar in both

RSW and FSSW and martensite is observed in both fusion zone and stirred zone with difference in morphology only.

However, TMAZ consists of a mixture of lath martensite, bainte and ferrite. Furthermore, in both processes failure

load increased with bonded area and energy input into the weld. Kinya Aota and Kenji Ikeuchi [10] studied the effect

of plunge depth and dwell time on microstructure of failure load of FSSW low carbon steel (0.5 mm thick) using a

tool without probe. It was observed that failure load increased with the tool plunge depth and failure mode changed

from interface rupture to plug rupture at a plunge depth greater than 0.16 mm. In terms of dwell time, failure load

increased with dwell time and was almost completely unchanged over 0.4s at a plunge depth of 0.14 mm under plug

rupture conditions. The type of microstructure of the joint such as ferrite + bainite, fine ferrite and ferrite + pearlitewas observed depending on the distance from the axis of the rotating tool. Though earlier works studied feasibility on

FSSW of AHSS and report on characterization of microstructural and mechanical properties of welds, the

microstructural characterization was specific to welds made with only one or two sets of process parameters. The



present investigation aims to study detailed microstructural evolution and corresponding properties with variation of

process parameters such as feed rate and rpm in FSSW of AHSS.

Experimental

The FSSW was performed on two overlapping 1.6mm thick DP 590 steel, the chemical composition of which is

presented in Table 1.

Table 1: Base metal chemistry in Wt%

C Mn Si S P

0.09 0.9 0.35 0.009 0.012

The visual schematic of the FSSW process is illustrated in Fig. 1. All welds were produced using a RM Series

Friction Stir Welder Model RM1A-0.7. Capabilities of this particular machine include tool rotational speed of up to

3000 rpm, an axial load of 67 kN and plunge rates from 0.1 to 1000 mm/min. The axial load and the torque values

were measured using a load cell, which was coupled with a DAQ system so that the axial force, torque and penetration

depth values were recorded simultaneously during each spot welding operation. All the welds were made under

displacement control mode – the tool was plunged into the material to a predetermined depth of 1.8 mm under varying

processing parameters such as rpm and feed rate as summarised in Table 2. The plunge depth was set to this value

after several trials. The rpm was varied from 400 to 1600 rpm for a constant feed rate of 2 mm/min. In one case of

1600 rpm, the feed rate was increased to 10 mm/min to examine its effect.

A polycrystalline cubic boron nitride (PCBN) tool was used in the present investigation, a material that has

been recommended for friction stir welding of steel. The tool used in this study had a convex scrolled shoulder with a

protruding pin. Since the machine and the tool were exposed to high temperatures during FSSW, a water cooled tool

holder was used. No shielding gas was used during welding. Tensile-shear testing were performed for all the welding

conditions to evaluate the mechanical strength of the joints. Since no specification exists for FSSW tensile shear

specimens sample geometry for RSW was used as per BS 1140 : 1993. Tensile shear tests were performed in

INSTRON under a crosshead speed of 0.5 mm/min. Both macro and micro-graphs of the joints were taken. Samples

for optical metallography representing a cross-section of the FSSW joint were polished and etched with 2 % nital.

Microhardness was taken along the cross section including the upper, centre and bottom portions of the sample using

Leco microhardness testing machine with 100 g load and a holding time of 13 s.

Table 2. Process Windows for FSSW

RPM Feed Rate

(mm/min)

Weld

Time (s)

400 2 72

800 2 72

1200 2 72

1600 2 72

1600 10 20

Fig. 1: Visual schematic of three step FSSW process ( after Hovanski et al. )



Results

Microstructural Characterization

The macrophotograph of a typic

different zones of FSSW are shown sche

different zones are also mentioned.

The microstructure of base met

within a softer ferritic matrix as displaye

The microstructure in the Stirred

As we move downwards from the surfacmicrostructural variation observed is con

the surface [2]. With increasing rpm, the

The TMAZ microstructure consi

move towards higher rpm (1200,1600)

4(h),(k)].

For rpm 1200 and 1600, as w

increasing amount of bainite / acicular

from Fig.s 4(b),(e), (h),(k) the grain size

Like arc welding, the microstruc

local thermal cycle experienced during

coarsened region (CGHAZ) surrounding

an intercritical region (ICHAZ) encompconsisting primarily of polygonal ferrite

the HAZ increases [Fig. 4(l)]. The micr

base metal is shown in Fig. 7.

The microstructures of the SZ,

Fig.s 5(a),(b) and (c). These microstructu

rate [Fig.s 4(j),(k),(l)]. With increasing r

the respective areas decrease.

Fig. 2: (a) Macro photograph of a typi

friction stir spot weld

Fig. 2: (b) Macrostructure of a cross s

1 – SZ, 2 – TMAZ, 3 – HAZ,

4

5

al friction stir spot weld is shown in Fig. 2(a).

matically in Fig. 2(b), and the positions of the p

l shows typical dual phase containing islands

in Fig. 3.

Zone (SZ) as shown in Fig.s 4(a),(d),(g),(j) co

to between SZ and TMAZ, the grain size progrsistent with the development of strain gradients

grain size of the SZ increases [Fig. 4(j)].

sts of blocky ferrite at lower rpm (400,800) as s

, the structure shows increasing amount of ba

move from TMAZ towards HAZ, the micro

errite and decreasing amount of ferrite [Fig.s 6

in TMAZ increases with increasing rpm.

tures of the different sub zones of the HAZ of FS

elding. With increasing distance from the TMA

the TMAZ, a grain refined region (FGHAZ) en

ssing FGHAZ. The HAZ in general has a finerand some pearlite [Fig.s 4(c),(f),(i),(l)]. With inc

structural transition from HAZ to base metal fe

MAZ and HAZ of the specimen welded with h

res are considerably finer than the ones welded

pm, the areas of the TMAZ and HAZ increase w

al Fig. 3: Microstruc

ection of the weld specimen with the different

4 – Top portion of TMAZ, 5 – Bottom portion

1

2

he approximate locations of

hoto micrographs taken from

f hard martensite embedded

sists of fine grains of ferrite.

ssively becomes larger. Thiss a function of distance from

own in Fig.s 4(b),(e). As we

inite / acicular ferrite [Fig.s

tructure is characterized by

a),(b)]. Again it can be seen

SW develop according to the

Z, the HAZ exhibited a grain

compassing the CGHAZ and

tructure than the base metal,reasing rpm, the grain size of

turing FGHAZ, ICHAZ and

igher feed rate are shown in

ith same rpm but lower feed

hile with increasing feed rate

ure of the base metal

zones :

of TMAZ

3



(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)

Fig. 4: (a)SZ (b) TMAZ (c) HAZ of 400 rpm weld, (d)SZ (e)TMAZ (f) HAZ of 800 rpm weld, (g)SZ (h)

TMAZ (i) HAZ of 1200 rpm weld, (j)SZ (k) TMAZ (l) HAZ of 1600 rpm weld. The feed rate for all these

samples was 2 mm/min. The locations of the different zones corresponding to the microphotographs are shown

in Fig. 2(b).

SZ TMAZ HAZ



(a) Fig. 5: (a) SZ (b) TMAZ (c)HAZ of

Fig. 6:(a) Top of the TMAZ (Positio

for the weld made with 1200 rpm

Fig. 7: Microstructure of the FGHAZ

(b) (c)

eld made with feed rate 10 mm/min. The rp

4 in Fig. 2(b)) (b) Bottom of the TMA

(bottom left), BM (top right) and ICHAZ (mid

for the weld was 1600.

(Position 5 in Fig. 2(b))

dle)



140

160

180

200

220

240

0 5000 10000 15000

Vi

c

k

e

r

s

H

a

r

d

n

e

s

s

(

H

V )

Distance ( Microns )

400 rpm 800 rpm

1200 rpm 1600 rpm

150

170

190

210

230

0 5000 10000 15000

V

i

c

k

e

r

s

H

a

r

d

n

e

s

s

(

H

V )

Distance (microns)

2 mm/min 10 mm/min

Microhardness CharacterizationThe micro-hardness plots for different welded samples corresponding to 400, 800, 1200 and 1600 rpm are

shown in Fig. 8(a). The feed rate for all these samples were kept constant at 2 mm/min. The micro hardness profiles

show a central higher hardness region which corresponds to the TMAZ. It can be observed that micro-hardness of the

TMAZ and HAZ increase with increasing rpm up to 1200 rpm. For 1600 rpm some softening is observed in TMAZ.

The average base metal hardness is about 160 -170 HV. The maximum hardness of the TMAZ was seen in the weld

made with 1200 rpm. The maximum hardness of the TMAZ reached 186 HV, 196 HV, 232 HV and 219 HV for

welds made with 400, 800, 1200 and 1600 rpm respectively. The largest softening in the HAZ (155 HV) was

observed at the 800 rpm weld. However, it is to be mentioned here that such softening is relatively insignificantcompared to the base metal micro-hardness level (160 -170 HV).

The micro-hardness plots for samples weld with different feed rates at a given rpm (1600) are shown in Fig.

8(b). The TMAZ region with higher feed rate weld showed marginally higher hardness (>210 HV) than with lower

feed rate (<210 HV) except few scattered values of higher hardness recorded for the lower feed rate weld. In general,

micro-hardness of the HAZ is found to increase with increasing feed rate.

(a) (b)

Fig.8 :(a) Microhardness profiles for specimens weld with different rpm

(b) Microhardness profiles for specimens weld with different feed rate.

Mechanical PropertiesThe tensile testing data are presented in load vs extension curves for samples weld with different process

parameters as shown in Fig 9. The breaking loads for different process parameters are presented in Fig.10. It is to be

noted that breaking load data are average of 3 samples weld with the same parameters.



18.666

20.55

23.08633

24.31157

15

17

19

21

23

25

27

0 400 800 1200 1600 2000

L

o

a

d

(

K

N )

RPM

24.31157

21.83037

21.5

22

22.5

23

23.5

24

24.5

0 2 4 6 8 10 12

L

o

a

d

(

K

N )

Feed Rate (mm/min)

0

5000

10000

15000

20000

25000

30000

0 2 4 6

L

o

a

d

(

N

)

Extension ( mm )

400 rpm 1200 rpm 1600 rpm

0

5000

10000

15000

20000

25000

30000

0 2 4 6

L

o

a

d

(

N

)

Extension ( mm )

2 mm/min 10 mm/min

(a) (b)

Fig. 9: Load vs Extension curves for (a) different rpm (b) different feed rate

(a) (b)

Fig.10: Breaking Loads for samples weld with (a) different rpm (b) different feed rate

As can be seen from Fig. 9 and 10, the maximum load and the elongation increases with increasing rpm anddecreases with increasing feed rate. Increasing rpm produces small increase in breaking loads, but significantly

improves the elongation. Again, increasing feed rate slightly decreases the breaking load, but produces a significant

decrease in elongation.

Discussion

Microstructure

With increasing rpm, heat input and peak temperature ( Tp ) increases [1,9,11]. On the other hand, with

increasing feed rate, the heat input and Tp decrease [1]. This explains the fact that coarser microstructure of the

different zones are observed with increasing rpm, while a finer structure is observed with increasing feed rate.

Fine micro structure in SZ is produced due to intimate contact with the tool during stirring. Evidentlydeformation and temperature experienced is maximum in this region which triggers dynamic recrystallisation followed

by rapid cooling with the withdrawal of the tool. The temperature reached in SZ is well above AC 3 temperature in the

range of 1100 – 1200OC [1,2,8]. The fine ferritic structure of SZ [Fig.s 4(a),(d),(g),(j)] in spite of high temperature



exposure is probably derived from the dynamic strain induced transformation (i.e. transformation during deformation)

from austenite to ferrite [3].

The TMAZ region experiences high Tp along with some ‘soaking’. Obviously, the degree of austenite grain

coarsening depends upon Tp which increases with increasing rpm. Microstructural evidence indicates that the Tp in

TMAZ reaches well into the austenite phase field, allowing appreciable grain growth [Fig.s 4(b),(e), (h),(k)]. The

final microstructure of the TMAZ, however, depends on the combined effects of strain, strain rate, temperature and

cooling rate. Higher rpm resulting in higher Tp and higher cooling rate [1], attributes predominantly bainitic

structure[Fig. 4(k)]. However, the variation in the amount of bainite from top to bottom in TMAZ can be explained by

the ‘mechanical stabilisation of austenite’ [4,6]. When an externally applied stress exceeds the yield strength of

austenite, it is possible that the transformation of austenite to bainite is retarded. This is because displacive

transformations occur by the advance of glissile interfaces, which can be hindered or rendered sessile on encountering

defects such as dislocations or grain boundaries. Such defects act as obstacles to the migration of the interface into

austenite, similar to the effects which lead to work hardening when the passage of slip dislocation is obstructed. If the

density of the dislocations is increased by deforming the austenite plastically, growth of the ferrite plates will be

limited. The final fraction of bainite may then become smaller in the deformed austenite than in undeformed austenite.

If phase transformation immediately follows deformation, ferrite is nucleated intragranularly at the places with highest

dislocation density, thus resulting in grain refinement [3] as seen in SZ. Under these circumstances, ferrite nucleates

on the unrecovered dislocation substructures in the austenite grains. However, if there is a delay between deformation

and phase transformation, recovery alleviates the substructure from becoming the preferred site for ferrite nucleation.

At the top region of the TMAZ, the strain experienced due to stirring motion is considerably higher than the bottomregion, due to the intimate contact of work piece and tool shoulder resulting in a structure consisting primarily of

ferrite with some amount of bainite [Fig6(a)]. The bottom portion experiences lower strains and also increased

undercooling; it is closer to the backing plate which acts as heat sink and hence shows higher amount of bainite [Fig.

6(b)]. There is also a significant delay between deformation and phase transformation in this region.

The HAZ experiences considerably lower Tp than the TMAZ and hence show a finer grain size. Material in

the CGHAZ experienced the highest temperature in the HAZ. Microstructural evidence suggests that Tp was well

above the effective A3 temperature, thus allowing some austenite grain growth. Microstructural results indicate that

the temperature in the FGHAZ fell just above the effective A3 temperature. The decomposition of austenite to ferrite

and pearlite on cooling promoted a finer grain size in this region. The ICHAZ was characterized by a bimodal

distribution of ferrite grain sizes[2,7]. The ICHAZ was exposed to Tp in the two phase (ferrite + austenite) region

between the effective A1 and A3 temperatures. The resultant microstructure is a network of fine grained microstructuresurrounding the large ferritic grains [Fig. 8].

In case of samples welded with higher feed rate show finer structure of the different zones [Fig.s 5(a),(b),(c)],

due to lower Tp (less interaction time between tool and work piece) and higher cooling rate [1].

MicrohardnessMicrohardness of the TMAZ increase with increasing rpm due to the transition from blocky ferrite to bainitic /

acicular ferritic structures found with higher rpm. At 1600 rpm, the microhardness decreases which may be due to

excessive coarsening of grains.

The higher hardness at the edges of the TMAZ suggests that the cooling rate was greatest at those positions.

This is probably because of the geometry of the FSSW process: the temperature of the entire TMAZ region is

expected to be high, therefore, the highest cooling rates will be at the edges of the TMAZ, corresponding to higher

hardness [5]. The micro hardness of the SZ could not be recorded effectively because of the very small width of the

SZ, but it is expected to be high due to very fine grain size [Fig.s 4(a),(d),(g),(j)].

Micro-hardness of the HAZ is found to increase with increasing rpm as well as feed rate. This can be

attributed to finer structures resulting from higher cooling rate experienced with increasing rpm as well as with

increasing feed rate [1].

Mechanical PropertiesLike RSW, breaking loads of the joints increase with increasing nugget diameter which is directly correlated

with heat input. In FSSW, heat input increases with increasing rpm; but decreases with increasing feed rate.

Consequently with increasing rpm, weld area (nugget diameter) increases which in turn increases the maximum load

and also the elongation. With increasing nugget diameter, the total crack path surrounding the periphery of the nugget

increases leading to increased elongation. Similarly, with increasing feed rate the breaking load as well as elongation

decrease due to decrease in weld area. Thus, it can be observed that the weld time plays an important factor in

determining the strength of the joints, as with decreasing weld time (increasing feed rate) the joint strength and

elongation decrease.



Conclusion

In the present investigation, the evolution of metallurgical and mechanical properties of FSSW with

variation in rpm and feed rate has been studied and the following conclusions may be drawn:

1. Welding parameters such as rpm and feed rate exert significant variation in microstructures of DP 590 steel.

2. Among the different zones in FSSW joints, alteration of microstructure is more pronounced in TMAZ.

3.

The difference in microstructure in different zones is a result of strain, strain rate and temperature gradientexisting in this region.

4. Welds made with high rpm (1200,1600) and low feed rate (2 mm/min) show significant improvement in

metallurgical and microstructural properties. However, for improving productivity, if high feed rates are

required to be used, rpm must be kept high for quality welds.

5. Increasing rpm produces a small increase in breaking load, but significantly increases the elongation. On the

other hand, increasing feed rate produces a small decrease in breaking load but significantly decreases

elongation. Thus a optimum combination of the two factors is required for obtaining desired mechanical

properties.

AcknowledgementThe authors would like to thank Ministry of Steel and Tata Steel for their financial support for this research

project.

References

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The Welding Journal. Jan.2003

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