AMM.592-594.43 (1)

8/12/2019 AMM.592-594.43 (1)

http://slidepdf.com/reader/full/amm592-59443-1 1/6

An Assessment on Friction Stir Welding of

High Melting Temperature Materials*Ramachandran K.K.1, a, Murugan N.2, Shashi Kumar S.2

1Department of Mechanical Engineering, Govt. Engineering College, Thrissur - 680009, Kerala,

India, Mobile: +9194464105122Department of Mechanical Engineering, Coimbatore Institute of Technology, Coimbatore -

641014, Tamil Nadu, India

[email protected]

Keywords: friction stir welding, high melting temperature material, FSW equipment, FSW toolmaterial

Abstract. Friction Stir Welding (FSW) as a joining technique with regard to low melting

temperature materials such as aluminum alloys has already been established and implemented in theindustry. But, with regard to high melting temperature metals and alloys the major issue still to get

successfully addressed is a pertinent tool material for a class of work materials and to get their

operating parameters optimized. This paper presents a detailed assessment on the FSW of high

melting temperature (HMT) materials, giving emphasis on the tool materials, tool geometry and

FSW equipment aspects based on the information gathered from experimental studies and research

publications.

Introduction

Friction Stir Welding (FSW) is a solid state joining process invented and patented by The

Welding Institute (TWI), Cambridge, England during 1991. This technique is initially developed forlow temperature non-ferrous materials such as aluminium and its alloys and already commercially

implemented in manufacturing industries. As for HMT materials such as steel, titanium and its

alloys, though there were research trials from the year 1998 onwards, a comprehensive success is yet

to be achieved. The main challenge is the tool material that can withstand the high temperature,

severe stirring and welding forces and corrosive environment at the weld zone [1]. Even at elevated

temperatures most of the HMT materials have very high yield strength when compared to

aluminium alloys. Figure 1 shows a comparison of the hot strength of a typical aluminium alloy and

steel. As steels, titanium etc. have much higher hardness and elevated-temperature properties, it is

important to select tool materials with good wear resistance and toughness at temperatures in excess

of 1000 0C or higher. Among the various FSW factors, the tool material, tool geometry and FSW

equipments are critical in the success of FSW of HMT materials. This paper make a precise

assessment on the above aspects of FSW of HMT materials based on the information gathered,

mostly from published research information available in the literature and also from experimental

studies.

Tool Material Aspects

Mishra et al. [1] reported that in FSW investigations of HMT material, the peak temperature

observed at the tool shoulder is over 1000 0C and that at the stirred zone is in excess of 1100 0C.

Therefore, it is important to select tool materials with good wear resistance and toughness at

temperatures in excess of 1000 0C or higher. Also, it is reported that most of the tool wear has

appeared to occur during the initial tool pin plunge stage and both rubbing wear and deformation of

the tool are reported as the origin of the changes in tool dimensions. In order to reduce the tool wear

and breakage, various measures such as pre-heating of the plunge region, pre-heating of the entire

work piece and introduction of heat sources just in front of the tool were recommended [2, 3, 4, 5].

Applied Mechanics and Materials Vols. 592-594 (2014) pp 43-47 Online available since 2014/Jul/15 at www.scientific.net © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.592-594.43

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 115.241.6.145-15/07/14,17:50:07)

http://www.scientific.net/

http://www.ttp.net/

http://www.ttp.net/

http://www.scientific.net/

8/12/2019 AMM.592-594.43 (1)


8/12/2019 AMM.592-594.43 (1)


Sintered Titanium Carbide (TiC), molybdenum based alloys and Ir (iridium) -10% Re tools have

also found used for FSW of titanium alloys and steels. But, the refractory metal, molybdenum and

its alloys are not reported as a very successful material over W and W alloys. Composite tools with

different combinations of pin, shoulder and shank materials were tried by many researchers. Tools

with W shoulder and WC pin, high speed steel (HSS) shoulder and WC pin, WC pin and shoulder

with HSS/tool steel shank etc. were successfully experimented.

The weld joint characteristics are affected by the tool wear, also. W and other alloying element

transfer to the stir zone from worn W based tools and boron and nitrogen pick-up from worn PCBN

tools are critical issues. Also, boron from the PCBN tool reacts with chromium in austenitic steels

to form borides leaving the weld material susceptible to corrosion and pitting. The use of PCBN

tool to weld commercially pure Ti observed severe tool wear and the debris from the tool reacted

with Ti to form TiB2. Therefore, the cause for defective or poor performing joints or variation in

properties at joint area may be tool wear also and while optimizing FSW parameters, tool wear

aspect should also be taken into account.

Tool Geometry Aspects

Along with other FSW parameters, the tool geometry, also, affects the flow of plasticized

material in the stir zone, the heat generation rate, torque requirement, welding force requirement and

the thermo-mechanical environment experienced by the tool. The important tool geometry parameters are; shoulder diameter, nature of shoulder surface and shape, pin geometry and size and

the nature of tool surface finish [7]. Table 1 gives some of the tool geometry and dimensions

experimented for various tool - work material combinations.

The diameter of the tool shoulder and nature of the shoulder surface are important aspects of

FSW tool design, because the heat generation and material flow are largely depends on these two

factors. There are studies in the literature with flat, flat with spiral, convex, convex with step spiral

and concave tool shoulders in relation to FSW of HMT materials. It was reported that the

microstructure, geometry and failure mode of a weld can be significantly altered with concave

shoulder and the FSW process stability found improved on convex shoulder with scrolls [12]. It was

suggested that when a convex scroll shoulder is used in constant axial force mode, it has the ability

to maintain constant plunge depth.The effect of tool pin shape and size in the FSW of HMT materials are reported little in the

literature. It is seen that the shape of the tool pin influences the flow of plasticized material and

affects weld properties. Various researchers reported better mechanical properties for FSW joints

0

20

40

60

80

100

120

140

160

180

J o i n t s t r e n g t h a s % o f b a s

e m e t a l s t r e n g t h

Base m aterial

0

30

60

90

120

150

180

J o i n t s t r e n g t h a s % o f b

a s e m e t a l s t r e n g t h

Base ma terial

Fig. 2 (a) Joint strength as a % of the base metalStrength with W based tools

(b) Joint strength as a % of the base metal

Strength with W based tools

Applied Mechanics and Materials Vols. 592-594 45

8/12/2019 AMM.592-594.43 (1)


made with tapered cylindrical (TC) pin profile and higher taper angle is beneficial in producing

better plastic flow conditions that are conducive to high strength welds. Also, as for the tool life is

concerned, the straight cylindrical (SC) pin is more susceptible to failure when compared to the TC

pin. Kumar et al. [13] reported that the tool pin foster a layer by layer material flow. Furthermore, it

has been suggested that the helical motion of a conical pin pushes the material downwards in the

front and upwards in the rear. The improved material flow results in more uniform properties of theweld joint across the work piece thickness. Pin profiles other than SC and TC are seldom found

experimented for HMT materials.

Equipment Aspects

Most of the FSW machines built for FSW of aluminium alloys have capacity in the range of 10 –

30 kN. At low loads, in addition to low heat generation, the inadequate forge pressure leads to poor

performing joints. Therefore, for FSW of HMT materials, the machine should have a capacity range

of 30 – 60 kN. As the tool strengths are only marginally higher than the alloys being welded, spindle

run-out and machine stiffness are significant factors which limit the tool life [6]. Manufactures of

PCBN tools recommends the permissible maximum spindle run-out as 0.01 mm. Many FSWmachines built for aluminium alloys have relatively high spindle run-out and low stiffness and

hence they cannot be used as such for FSW of HMT materials. Deflections under load can lead to

tool failure, particularly with PCBN tools. It is reported that for FSW of HMT materials the stiffness

of the machine should be such that the deflection should be less than 0.75 mm under a load of 45

kN. The tool plunge speed is very important as for FSW of HMT materials are concerned.

Therefore, the machine should have very fine axial pitch of the order of 0.5 – 5 mm/min and

provisions for very accurate control of the same.

As the weld zone temperature will be in the range of 900 to 1200 °C and also, the materials used

for the tool/tool shank have high thermal conductivity, in order to prevent damage to the spindle

bearings and to establish a consistent thermal environment for the tool, the machine should have provision for cooling of the tool holder/shank. During the initial stages of tool pin plunge the

shoulder and base metal. The frictional heating causes the temperature of the trapped metal to

Work

material

Thickness

(mm)

Tool

material

Tool geometry

ReferenceShoulder

dia. (mm )

Pin

shape

Pin dia

(mm)

AISI304L 3.2 W a lloy 19 ** ** Reynolds (‘03)

S35C 1.6 WC 12 C 4 Fujii et al. (‘06)

Plain C S 2.3 WC 12 C 4 Ueji et a l. (‘06)

S70C 1.6 WC 12 C 4 Cui et al. (‘07)Inconel 600 2 WC-Co 15 C 6 Song et a l. (‘09)

AISI1080 1.6 WC 12 C 4 Chung et al. (‘10)

AISI304 3 WC-Co 16 Tr 5 Meran et al. (‘10)

AISI430 3 WC-Co 20 Tr 5 .7 Burak et al. (‘12)SK4 2 WC 12 C 4 Khodir et al. (‘12)

AISI409M 4 W alloy 20 T D-8, d-5 Jafarzadgan (‘12)

DSS 1.5 WC 16 C 4 Esmailzadeh (‘13)

S45C 3.2 WC 15 C 6 Sun et al. (‘13)SUJ12 2.3 PCBN 14 T D-5.8, d-4 Sato et al. (‘07)

RTQ701 12.5 W-Re 25 Trf D-8, d-6 Barnes et al. (‘08)

AISI409 2 PCBN 36.8 C 5 .7 Cho et al. (‘12)

AISI316 2 PCBN 15 T D-5, d-3.5 Jeon et al. (‘13)ASS 2 Si3 N 4 15 C 6 Miyno et al. (‘11)AISI409 3 Si3 N 4 20 T D-5, d-3 Ahn et a l. (‘12)

Table 1 Tool material and geometry experimented for various HMT m aterials

C – cylindrical, T – tapered, Tr – triangular, Trf – Triflute, D – major diameter, d – minor diameter, ** not specified

46 Dynamics of Machines and Mechanisms, Industrial Research

8/12/2019 AMM.592-594.43 (1)


increase and get oxidized. This oxidized layer trapped between the tool shoulder and base metal acts

as a thermal insulator and also modifies the frictional characteristics which warrants setting of

higher axial loads. But, when the welding progresses the oxide layer gets displaced from underneath

the shoulder and a portion get distributed in the weld zone. This causes friction between the tool

shoulder and base metal to increase, resulting higher heat generation and tool shoulder may

penetrate into the base metal. Therefore, the use of shielding gas is very important to get uniformsound weld without oxide inclusions. Also, for avoiding tool shoulder plunge during welding and

for better stability, provision for providing tool tilt of the order of 1 -3 degrees is very important.

Conclusion

In this paper an assessment on FSW of high melting temperature materials based on experimental

studies and appraisal of published research works are presented. Three very important aspects of

FSW - the tool material, tool geometry and the FSW equipment are highlighted. Both refractory and

super abrasives tool materials can produce good joints in HMT materials and give reasonable tool

life, but tool fracture, tool deformation and tool wear are co-existing. WC and W based materials

such as W-La alloys are tougher and less costly. The new PCBN tool such as MS80, Q60 and Q70can give very good results but cost is a big factor to be addressed. As the tool strengths and

properties are only marginally higher than the material being joined, very accurate control of the

FSW parameters within a narrow range is absolutely necessary to produce good joints and to get

appreciable tool life. FSW machines built for low melting temperature materials and general

purpose machine such as milling machine may produce isolated successful welds but dedicated

robust built, precision machines only can produce consistent good quality welds with reasonable

tool life.

References

[1] R. S. Mishra and Z. Y. Ma: Mater. Sci. Eng. R Reports, vol. 50 (1–2) (2005), p. 1–78

[2] W.M. Thomas, P.L. Threadgill, E.D. Nicholas: Sci. Technol. Weld. Join., Vol. 4 ( 6) (1999), p.

365-372

[3] W. M. Thomas: Proc.First Int.Symposium on Friction Stir Welding, CA, USA, (1999)

[4] T.J. Lienert, W.L. Stellwag Jr., B.B. Grimmett, R.M. Warke: Weld. J., Vol. 82 (1)(2003), p. 44

- 51

[5] S. Mandal, K. Williamson: J. Mater. Process. Technol., Vol. 174 (2006), p. 190–194

[6] Sorensen, C. D., Nelson, T. W.: ASM Int., (2007), p. 111-121

[7] Leonhardt, T.: J. J. Mineral. Met. Mat. Soc., Vol. 61(7) (2009), p. 68-71

[8] Rai, R., De, a, Bhadeshia, H. K. D. H., Deb Roy, T.: Sci. Technol. Weld. Join., Vol. 16 (4)

(2011), p. 325 - 342

[9] Edwards and M. Ramulu: Sci. Technol. Weld. Join., Vol. 14, (7) (2009), p. 669 –680

[10] B. Thomson, Babu, S. S.: Weld. J., Vol. 89 (2010), p. 256-261

[11] Bentley, A. R., and Hogwood, M. C.: Tungsten and Tungsten Alloys Conference, (1992), p.

419–429

[12] L. Cederqvist, C. D. Sorensen, A. P. Reynolds, T. Oberg: Sci. Technol. Weld. Join., Vol. 14,

(2) (2009), p. 178–184

[13] K. Kumar and S. V. Kailas: Mater. Sci. Eng. A 485, (1–2), (2008), P. 367–374

Applied Mechanics and Materials Vols. 592-594 47

8/12/2019 AMM.592-594.43 (1)


Documents

AMM.592-594.43 (1)