Hyperworks learnig.pdf

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012)

212

Simulation of Peak Temperature & Flow Stresses during

Friction Stir Welding of AA7050-T7451 Aluminium

Alloy Using Hyperworks

K.D.Bhatt1, Bindu Pillai

2

1M.Tech.PG student, CIT, Changa, Affiliated to CHARUSAT, Changa

2Assistant Professor, Mechanical Engineering Department, CIT, Changa

[email protected]

[email protected]

Abstract— To overcome limitations of fusion welding of the

AA7050-T7451aluminum alloy friction stir welding (FSW) has

become a prominent process which uses a non-consumable

FSW tool to weld the two abutting plates of the workpiece.

The FSW produces a joint with advantages of high joint

strength, lower distortion and absence of metallurgical

defects. Process parameters such as tool rotational speed, tool

traverse speed and axial force and tool dimensions play an

important role in obtaining a specific temperature

distribution and subsequent flow stresses within the material

being welded. Friction stir welding of AA7050-T7451

aluminum alloy has been simulated to obtain the temperature

profiles & flow stresses using a recent FEA software called

HyperWorks.; the former controlling the microstruture and

in turn, mechanical properties and later, the flow of material

which depends up on the peak temperatures obtained during

FSW. A software based study has been carried out to avoid

the difficulty in measuring the temperatures directly and

explore the capabilities of the same to provide a basis for

further research work related to the said aluminum alloy.

Keywords: AA7050 Aluminum alloy, Flow stresses, Friction

stir welding, Peak temperature, Simulation.

I. INTRODUCTION

Friction stir welding (FSW) was originally invented &

developed by The Welding Institute (TWI) at UK in 1991 by

using Aluminum alloy [1-2]. The FSW involves the use of a

specially designed tool having a shoulder and a protruded pin

with a specific geometry. The tool, being non-consumable, is

rotated at certain rotational speed (rpm) and plunged into the

abutting edges of the two plates to be welded. After the due

rotations at the joining line (given time being called as ‘dwell

time’), the tool is traversed along this line with a certain

welding speed (mm/min) till the end of the plates (Figure - 1).

Thus, it is a solid-state joining technique in which no

melting of workpiece occurs and the required heat to join two

plates is produced by the tool in two fold: (i) friction between

tool shoulder & the workpiece-surface and (ii) heavy plastic

deformation of the workpieces. Thus, during the FSW, fine

equiaxed recrystallized grains result as the material undergoes

tremendous plastic deformation at very high temperature [3-6].

The fine microstructure obtained by FSW process produces

remarkable mechanical properties in welds. The joint is

eventually produced as a result of material movement around

the pin which is found to be very complex due to geometrical

features of the tool [7].

Figure – 1 Schematic of Friction Stir Welding [8]

The Aluminum alloys from 7XXX series are from the non-

weldable class of material having poor solidified

microstructure and porosity in the fusion zone. Also, there is

significant loss of mechanical properties during fusion

welding as compared to their base metals. Both these factors

lead to difficulty in joining these alloys by the conventional

methods of welding [8].



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As the FSW results in tremendous plastic deformation

around the tool and friction between tool and workpiece, the

temperature rise and its distribution in the weld zones become

responsible for establishing the microstructure of the weld that

includes grain size, grain boundary character, coarsening and

dissolution of precipitates and resulting mechanical properties

of the welds. It is, therefore, necessary to obtain information

regarding temperature distribution during FSW. Of course, the

direct measurements of temperatures within the stirred zone

are quite difficult because of tremendous plastic deformation

taking place during FSW due to rotation and translation of the

tool along the weld center-line. Despite, the attempts have

been made to estimate the maximum temperatures within the

stirred zone from the microstructure of the weld [3, 4, 9] or

record the same by embedding thermocouples in the region

nearer to rotating pin [ 10, 11-13].

Rhodes et al. [3] investigated for microstructural evolution

in AA7075-T651 during FSW that larger precipitates dissolve

and reprecipitate in the weld center. Therefore, they concluded

that maximum process temperatures are between about 400 to

4800C in the AA7075-T651. Murr and coworkers [4, 9]

indicated the non-dissolution of some of the precipitates and

suggested a temperature rise roughly to 4000C for AA6061

alloy during FSW. A study of microstructural evolution of

AA6063 during FSW was done using transmission electron

microscopy (TEM) by Sato et al. [11] and by comparison of

these with those obtained by simulated thermal cycles at

different peak temperatures they concluded that in the regions

of 0 to 8.5, 10.0, 12.5 and 15.0 mm away form weld center,

the temperatures were higher than 402, 353, 3020C and lower

than 2010C respectively. Tang et al. [12] made an attempt to

measure heat input and temperature distribution within friction

stir weld by embedding thermocouples in the region to be

welded for AA6061-T6 Aluminum alloy having thickness of

6.4 mm. They concluded that (i) Maximum peak temperature

was recorded at the weld center and it decreased with

increasing distance from the weld centre-line. (ii) At tool

rotation speed of 400 rpm and a traverse speed of 122

mm/min, a peak temperature of 4500C was observed at the

weld center one quarter from top surface. (iii) The temperature

distribution within stirred zone is relatively uniform. Tang et

al. [12] investigated further that increasing both tool rotation

rate and weld pressure result in an increase in the weld

temperature. Further, they [12] studied the effect of shoulder

on the temperature field and concluded that contact area &

vertical pressure between shoulder and workpiece are much

larger than those between the pin and the workpiece, also,

shoulder has higher linear velocity compared to small radiused

pin. Hashimoto et al. [14] reported that the peak temperature

in the weld zone increase with increasing the ratio of tool

rotation rate to traverse speed for FSW of AA2024-T6,

AA5083-O and AA7075-T6. A peak temperature of > 5500C

was reported in FSW of AA5083-O at a higher ratio of tool

rotation/traverse speed. Frigaad et al. [15] suggested that the

tool rotation rate and the shoulder radius are the main process

variables in FSW, and pressure P can not exceed the actual

flow stress of the material at the operating temperature if a

sound weld without depressions is to be produced. They

performed FSW of AA 6082-T6 and AA7108-T79 at constant

tool rotation rate of 1500 rpm and a constant welding force of

7 kN at three welding speeds of 300, 480 and 720 mm/min.

They [15] revealed that (i) peak temperature of above ~5000C

was recorded in the FSW zone, (ii) peak temperature

decreased with increasing traverse speeds from 300 to 720

mm/min. For a three dimensional thermal model based on

finite element analysis developed by Chao and Qi [16] and

Khandkar & Khan [17] showed reasonably good match

between the simulated temperature profiles and experimental

data for both butt and overlap FSW process. The effect of

FSW parameter on temperature was further examined by

Arbegast and Hartley [18]. They concluded that for a given

tool geometry and depth of penetration, the maximum

temperature was a strong function of the rotation rate (rpm)

while rate of heating was a strong function of the traverse

speed (mm/min). The maximum temperature observed during

FSW of various Aluminum alloys is found to be between

0.6Tm and 0.9Tm which is within the hot working range for

those Aluminum alloys, where Tm is melting point of

material. Ulysee [19] studied the impact of varying weld

parameters on temperature distribution in AA7050-T7451

plate. Khandkar et al. [20] introduced a more comprehensive

model of heat input based on the torque of the FSW tool that

they used to model temperature history of friction stir welded

Aluminum alloy AA6061-T651 plate. The prime objective of

the present paper is to simulate peak temperature and

distribution of flow stresses produced during the FSW of

AA7050-T7451 Aluminum alloy and to compare the same



214

with those of the researchers’ results. Use of a recent software

known as HyperWorks9.0 has been explored in the modeling

FSW process. It basically includes a specially designed

module for friction stir welding for modeling.

II. EXPERIMENTAL WORK

Simulations were performed by entering following

properties of the AA7050-T7451 Aluminum alloy as shown in

table – 1. The default tool material available in

HyperWorks9.0 database was used for simulating the FSW

using properties as shown in table – 2.

Table - 1: Physical & Thermal properties of AA7050-T7451

Density 2830 kg/m3

Melting Point 488 - 629°C

Modulus of Elasticity 7.17 x 1010 Pa

Poisons Ratio 0.33

Thermal Conductivity 155 W/m-K

Specific Heat 860 J/kg-K

Volumetric heat source 0.0 W/m3

Table - 2: Physical & Thermal properties of Default tool

Density 2260 kg/m3

Specific Heat 896 J/kg-K

Modulus of Elasticity 2.0 x 1010 Pa

Poisons Ratio 0.35

X – conductivity 198 W/m-K

Y – conductivity 198 W/m-K

Z – conductivity 198 W/m-K

The dimensions of plates of AA7050 alloy were selected

as 381 x 127.5 x 6.4 mm and FSW tool geometry was

selected with cylindrical pin having a shoulder diameter (D),

shoulder length (L), pin diameter (d), and pin length (l) as

shown in table – 3 below:

Table - 3: Dimensions of Default tool

Element D,

mm

L,

mm

d,

mm

l,

mm

Tool 20.3 70.2 7.1 5.76

Table – 4 indicates the FSW process parameters as tool

rotational speed (RS, rpm) and welding speed (WS, mm/min)

and tool tilt angle selected for the simulation in

HyperWorks9.0:

Table - 4: FSW parameters

III. RESULTS & DISCUSSIONS

The graphical results showing temperatures and flow stress

distributions obtained by running the simulations on

HyperWorks9.0 indicate the effects of varying welding

parameters particularly welding speed (WS). At the constant

value of 180 rpm (RS), the peak temperatures obtained at two

values of welding speeds of 51.0 mm/min and 76.2 mm/min

differ only by 100C from the value indicated in literature.

Figure – 2(a) and - 2(b) shows peak temperature values of

3400C and 360

0C for the said alloy at tow welding speeds of

51.0 mm/min and 76.2 mm/min respectively. The peak

temperatures are found as maximum at the tool pin center.

Sr. No. RS,

rpm

WS,

mm/min

Tool tilt angle

1 180 51.0 2.50

2 180 76.2 2.50



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2(a) At RS = 180 rpm, WS = 51.0 mm/min

2(b) At RS = 180 rpm, WS = 76.2 mm/min

Figure –2: Temperature Distributions at RS & WS shown at (a) & (b)

3(a) At RS = 180 rpm, WS = 51.0 mm/min

Also, the flow stress distribution is shown in figure – 3(a)

and – 3(b) for the same parameters selection. The results

show that at constant RS = 180 rpm the flow stresses are

lower for WS = 51.0 mm/min as compared to those at WS =

76.2 mm/min.

3(b) At RS = 180 rpm, WS = 76.2 mm/min

Figure – 3: Flow Stress Distributions at RS & WS shown at (a) & (b).

IV. CONCLUSION

It can be concluded that at constant tool rotational speed

(RS) and tool with the same geometry; variation in tool

traverse speed has prominent effects on temperature history &

flow stresses developed during FSW of AA7050-T7451

Aluminum alloy. The induced temperatures and flow stresses

are in good confirmation with the results obtained in

literatures. The flow stresses at lower peak temperature of

3400C are as high as 720 MPa but are as low as 680 MPa at

higher peak temperature of 3600C as the flow of material

becomes easier at higher temperatures. It is also observed that

at constant rotational speed the peak temperature has increased

by increasing the welding speed. These temperature profiles

govern viscosity at and ahead of the tool which affects the

flow of material and thus, contribute in establishing a

microstructure which in turn dictates the mechanical

properties of the joint produced. Simulations performed on

computer software opens the new horizon of modeling friction

stir welding process in virtual laboratory and help predict the

mechanical properties of FSW-joints.

REFERENCES

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Bampton C.C. Scripta Mater. 36 (1997) 69.

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welding: input torque based model, Science and Technology of

Welding and Joining 8 (3) (2003) 165–174.

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