Friction Stir Welding of Ti-6Al-4V alloy

Leo Paola1, a, Cerri Emanuela2,b 1 Innovation Eng. Dept.University of Salento73100, Lecce, Italy

2 Innovation Eng. Dept.University of Salento73100, Lecce, Italy

apaola.leo@unisalento.it,

bemanuela.cerri@unisalento.it

Keywords: Friction Stir Welding, Ti-6Al-4V, microstructure, microhardness

Abstract. Titanium (Ti) and its alloys are used extensively in aerospace industry where there is a

critical need for material with high strength-to–weight ratio and high elevate temperature

properties. Friction stir welding (FSW) is a new solid state welding process in which a cylindrical–

shouldered tool with an extended pin is rotated and gradually plunged into the joint between the

workpieces to be welded. The material is frictionally heated to a temperature at which it becomes

more plastic but no melting of the blanks to be welded is reached therefore the presence of defects

typically observed in and close to the welding seam is strongly reduced. The final result is the

improvement of the mechanical performances of the welded joints even in some materials with poor

fusion weldability. In this paper the authors analyze the microstructure of FSW joints of Ti-6Al-4V

processed at the same travel speed (50 mm/min) and at different rotation speed (300-500rpm). The

microstructure of base material (BM) is not homogenous. It is characterized by distorted α/ β

lamellar microstructure together with smashed zone of fragmented β layer and β retained grain

boundary phase. The BM has been welded in the as received state, without any previous heat

treatment. The microstructure of the transverse section of joints is not homogeneous. Close to the

top of weld cross sections a much finer microstructure than the initial condition has been observed

while in the center of the joints the microstructure is mixed and less refined.

Introduction

The industrial importance of Ti-6Al-4V alloy is well known. It provides an exceptional good

balance of strength, ductility, fatigue and fracture properties together with good corrosion resistance

and good metallurgical stability. It is currently used in a wide range of low and high temperature

applications such as blades or discs for aircraft turbines, steam turbine blades , marine components

and chemical pump components. In all these applications the parts are brought to the final shape by

high temperature processing [1].

The FSW process uses an inert rotating mandrel and a force on the mandrel normal to the plane of

the sheets to generate the frictional heat. The heat and the stirring action of the mandrel create a

bond between the two sheets without melting the base metal. As matter of fact, the use of a solid

state welding process limits the insurgence of defects, due to the presence of gas in melting bath,

and avoids the negative effects of materials metallurgical transformation strictly connected with the

change of phase. Finally the reduced thermal flux-with respect to traditional fusion welding

operation- leads to smaller residual stress values in the joint and, consequently, in limited distorsion

in final product [2]. Moreover a refined microstructure is obtained in the welded zone that usually

leads to increases of strength/hardness (according to Hall-Petch relationship) with respect to base

material (BM). The higher melting temperature and the higher strength of titanium alloys poses a

greater challenge for successfully developing a FSW process [1].

In order to improve FSW process it is imperative to understand the deformation mechanism and the

microstructural evolution that accompanies deformation. By applying a thermo mechanical process

(TMP) in the α+β region of Ti-6Al-4V, phase transformation, recrystallization, recovery and work

hardening may occur simultaneously. Several studies [4-6] indicate that a refinement of

microstructure for Ti-6Al-4V can be obtained during hot deformation and even at deformation

Materials Science Forum Vols. 783-786 (2014) pp 574-579© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.783-786.574

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: 79.16.89.132, Salento University, Lecce, Italy-26/04/14,12:30:00)

temperature between 600°C and 800° both in the α and β phase [4-6]. Moreover, if the

thermomechanical process induces large strains in a sample (for example ECAP, multi-step

isothermal forging, FSW, hydrostatic extrusion) a microstructure characterized by

submicrocrystallite ( d<1 µm) can be developed at T lower than those commonly used in the

conventional manufacture of semi finished products ( for example at 550°C [3]).

In this paper the authors analyze the microstructure developed in FSW joints processed at different

rotational speeds.

Experimental procedures

Ti-6Al-4V sheets 100mm x 200mm and 3mm thick with composition shown in Table 1 have been

used in this study.

Table1: Composition of materials (weight %)

The sheets were welded together at rotating speed of 300 and 500 rpm. Fixed advancing speed equal

to 50 mm/min, tilt angle equal to 2° and tool shoulder plunge depth of 0.2mm were considered for

all the welds. A tungsten carbide tool with 16mm shoulder and 30°conical pin 2.6mm in height and

5mm in major diameter was utilized. Both the backplate and the tool were cooled by 2l/min flow of

water.

The process temperatures were measured by two thermocouples placed between the two sheets, at

1.5mm from the bottom of the joints, at a distance of 5mm and 40mm, respectively, from the edge

of the sheets to be welded. Through the thermocouple placed at 40mm from the sheets edge,

temperatures have been measured at the center of the weld till the tool destroys the thermocouple

itself.

The base material (BM) and the welded cross-sections ( respectively BM and WCS) have been cut

by electrical discharge machining and prepared by standard metallographic procedure.

Heat treatments were performed on BM at the interested temperatures during the FSW process in

order to isolate the effect of heat cycle from the stirring effect on microstructural evolution. The

microstructure of heat treated base material cross sections (HTBM) have been analized by optical

and electron microscopy, microhardness and electrical conductivity measurements.

The microstructural analysis has been performed by optical microscope (OM) Nikon Epiphot 200

and by scanning electron microscopy (SEM JEOL JSM-6480 LV, SEM FIB ZEISS 1540, IXRF

System SphinX 130) . BM, WCS and HTBM have been mechanically polished and etched by Kroll

reagent. The SEM observations for the weld zone have been performed along the axes of the joints

and close to the Thermo Mechanically Affected zone (TMAZ).

Vickers microhardness (0,5/15s) measurements were performed into the welded cross sections at

0,7mm from top surface of joints. The distance between two points has been chosen equal to 1mm.

Results

Due to thermo-mechanical process (TMP) the microstructure of the BM is not homogenous,

quite fine and similar to the mill-annealed microstructure [1]. It is characterized by distorted prior β

grains (average size 3,7±0,4 µm), distorted α/β lamellae inside prior β grains [1], zones of smashed

β layers (Fig.1) [5,7]. The average thickness of the α lamellae (HCP crystal structure) is equal to 0,3

±0,08 µm, the average length is equal to 1,6 ± 0,5 µm so an aspect ratio (length/width) equal to 8 is

obtained. The α lamellae are separated by β layer (BCC crystal structure) thick 0,2± 0,05 µm.

Along grain boundaries, the retained β phase (BCC crystal structure) is about 11%±4 volume

fraction with average thickness of 1,33±0,8µm. Some smaller areas of recrystallized α grains are

visible too. In SEM micrographs, the dark and white regions represent the α and β phase

Ti Al V Fe C N H O

89,2 6,1 4 0,3 0,1 0,05 0,01 0,2

Materials Science Forum Vols. 783-786 575

respectively while the relationship between contrast and phase is opposite in OM. The average

Vickers hardness of BM is equal to 326±6 and the electrical conductivity to 1,07±0,001 IACS.

Figure 1: BM microstructure (SEM 4000X)

The highest values of temperature recorded at 300 and 500 rpm have been respectively 590 ° C and

700 ° C. In order to isolate the effect of heat cycle from the stirring effect of the tool on

microstructure of the welded joints, the BM was exposed to high (830°C) and low temperature

(590°C) for 30, 60, 90, 150s and air cooled. In Fig.2 is shown the microstructure after 150s of

exposure at 590°C (Fig.2a) and 830°C (Fig.2b). The heat treated microstructure looks more or less

equiaxed and dissolution of intragranular β phase can be observed respect to BM (Fig.1). Along

grain boundaries, the retained β phase is equal to 13% ± 4. After 150s of exposure at 590°C and

830°C the microhardness average value of HTBM is increased (respectively 347 ±4 and 351 ± 5)

respect to BM while electrical conductivity is almost constant (1,07±0,001 and 1,06 ± 0,001 IACS).

Figure 2: HTBM microstructure (SEM 4000X) after 150s at 590°C (a) and 830°C (b)

The macrostructures of both FSW joints are characterized by two zones, named zone 1 or stirred

zone and zone 2 as shown in Fig.3a. Into the zone 2 a refined microstructure respect to BM has been

observed. The side of zone 1 closest to the refined area exhibits elongated grains typical of TMAZ.

As the distance from zone 1 increases the grains appear less deformed and the microstructure

become more similar to that observed in BM (Fig.3b). As the temperature increases the width of

TMAZ strongly decreases (Fig.3c).

The analysis of microstructure has been developed in three different areas of WCS. Along the axes

of the joint at 0,7mm (point A, Fig.4) and 1,4 mm (point C, Fig.4) from the top surface. The

average grain size of the joint processed at 300rpm evaluated at point A is equal to 1,2±0,3 µm.

Along the axes of WCS the microstructure is still mixed, characterized by few α lamellae and by

equiaxed grains. The fraction of equiaxed grains decreases and their average size increases with

distance from the top surface. The average thickness of α lamellae is similar in A and C but their

a b

576 THERMEC 2013

length and their fraction increases with distance from the top surface. With rising rotation speed,

the average grain size increases.

The average microhardness of WCS at 0,7 mm from top surface is 368±5 at 300 rpm and 365±4 at

500 rpm.

Figure 3: Macrostructure of weld cross section (a), higher magnification of interface zone 2/zone1

microstructure of joint processed at 300 rpm (b) and 500 rpm (c)

Discussion

The microstructure of BM is mixed and quite fine. The average width of α lamellae is equal to 0,2

±0,08 µm and the average grain size of prior β grains is quite fine equal to 3,7±0,4 µm.

After heat treatment at 590°C and 830°C a strong dissolution of smashed β phase is observed and

the aspect of microstructure is more or less equiaxed even if some areas characterized by α/β

lamellae can still be observed. The fraction of intergranular β phase does not vary appreciably

probably due to the brief time of the heat treatment. After 150s of exposure at 590°C and 830°C

the microhardness average value of HTBM is respectively 347±4 and 351± 5 respect to BM. The

increase of hardness could be due to dissolution of the softer intergranular β phase [1, 4-6].

In this study the maximum values of T induced by FSW at half of thickness along the longitudinal

axes are respectively 590°C at 300 rpm and 730°C at 500 rpm. Of course, close to top surface of

joints the value of T is higher [2,11] and the authors do not know exactly which is the maximum T

that has been reached. Zhang et al [8] and Zhou et al [9, 10] have discussed the microstructures in

Ti-6Al-4V FSW joints. In those papers, the maximum temperature in stirred zone was assumed

higher than β transus (1040°C) or close to its value. So, they explained the refined microstructure

found in the stirred zone as induced by dynamic recrystallization (DRX). In addition, due to

exposure to T close to β transus, they did not observe the TMAZ [8] because masked by phase-

trasformation [8] that can also occur. In fusion welding process the microstructure closest to the

fusion zone differs notably from that of BM experiencing temperatures exceeding β transus [1].

Moreover as the distance from fusion zone increases, and therefore the maximum T decreases, the

microstructure becomes closer and closer to that of BM [1].

In the FSW joints analyzed in this study, a microstructure refinement has been observed in the

stirred zone (the average grain size at point A (Fig.4) is equal to 1,2±0,3 µm whereas in BM it is

equal to 3,7±0,4 µm). But, unlike the above studies [8,1], both the TMAZ and a microstructure

a

b c

Materials Science Forum Vols. 783-786 577

similar to BM into the zone 1 of Fig.2 a,b. have been observed, suggesting that possibly the highest

process temperature could be lower than β transus. Nevertheless, a refinement of microstructure for

Ti-6Al-4V can be obtained even at deformation temperature between 500°C and 800° both in the α

and β phase by globularization process [4-6] above all if large strain and/or a change in strain path

occur during deformation [6], as during FSW.

Particularly, FSW induces both tension/compression strains during the deformation paths around the

tool and the highest strain/strain rate (intense shearing) on the material closest to the rotating tool

[11]. As a consequence, in the point A of Fig.4a the processed material has been interested by high

T [2] togheter with high strain /strain rate and with change in strain path [11] leading to an

homogeneous refinement of the microstructure respect to BM (Fig.4b). Moreover, during FSW, the

absolute values of T, strain and strain rate were found to be smaller as distance from the shoulder in

thickness direction increases [2,11] so justifying a less efficient refinement process. In fact, with

increasing the distance from the top surface of joint (point C in Fig.4a, Fig.4d), the equiaxed grains

fraction decreases, the average grain size increases and an higher fraction of deformed/ unrefined

lamellae have been observed respect to the point A (Fig4d).

Due to both dissolution of smashed β phase, refinement of microstructure and strain hardening

induced by the process, the average value of microhardness is higher in WCS (368±5 at 300 rpm

and 365±4 at 500 rpm) respect to BM (326±6). As the process temperature increases because of

higher rotation speed, a lower value of microhardness occurs because of the improved recovery

and/or coarsening of microstructure.

Figure 4: Schematization of the zones in which microstructure has been analyzed (a), BM

microstructure (b), microstructure observed in point A (c) and C (d) points of joint processed at 300

rpm.

Conclusions

The main conclusions are in the following:

1) The microstructure of BM is quite fine and mixed. It is characterized by distorted prior β grains,

distorted α/β lamellae inside prior β grains, zones of smashed β layers.

a

d

b

c

578 THERMEC 2013

2) After heat treatment at 590°C and 830°C a strong dissolution of smashed β phase is observed.

The fraction of intergranular β phase does not vary appreciably. The microhardness average value of

HTBM increases respect to BM possibly due to dissolution of the softer intergranular β phase.

3) The FSW joints are characterized by a zone of homogeneous and fine equiaxed microstructure

along the axis of WCS and close to the top surface of joint. The TMAZ is clearly observed in the

joint processed at 300rpm, suggesting that possibly the highest process temperature could be lower

than β transus.

4) Along the WCS axis, as the distance from the shoulder in thickness direction increases, the

equiaxed grains fraction decreases, the average grains size increases and a higher fraction of

deformed/unrefined lamellae have been observed due to lower efficiency of the refinement process.

5) Due to both dissolution of smashed β phase, refined microstructure and strain hardening induced

by the process, the average value of microhardness is higher in WCS respect to BM and slightly

decreases with increasing rotational speed.

References

[1] G. Lutjering, J. C. Williams, “Titanium”, Springer 2nd edition, New York, 2007

[2] G. Buffa, L. Fratini, F. Micari, “Mechanical and microstructural properties prediction by

artificial neural networks in FSW processes of dual phase titanium alloys”, Journal of

Manufacturing Processes 14 (2012) 289-296.

[3] S.V. Zherebtsov, G.A. Salishchev, R.M. Galeyev, O.R. Valiakhmetov, S. Yu. Mironov, S.L.

Semiatin, “ Production of submicrocrystalline structure in large-scale Ti-6Al-4V billet by warm

severe deformation processing”, Scripta Materialia 51 (2004) 1147-1151.

[4] Chan Hee Park, Young Gun Ko, Jin-Woo Park, Chong Soo Lee, “Enhanced superplasticity

utilizing dynamic globularization of Ti-6Al-4V alloy”, Materials Science and Engineering A 496

(2008) 150-158

[5] S. Zherebtsov, M. Murzinova, G. Salishechev, S.L. Semiatin, “ Spheroidization of the lamellar

microstructure in Ti-6Al-4V alloy during deformation and annealing”, Acta Materialia 59 (2011)

4138-4150.

[6] S. Mironov, M. Murzinova, S. Zherebtsov, G.A. Salishchev, S.L. Semiatin, “Microstructural

evolution during warm working of Ti-6Al-4V with a colony-α microstructure” Acta Materialia 57

(2007) 2470-2481.

[7] Lijia He, A. Dehghan-Manshadi, R.J. Dippenaar, “The evolution of microstructure of Ti-6Al-

4V alloy during concurrent hot deformation and phase transformation”, Materials Science and

Engineering A 549 (2012) 163-167

[8] Yu Zhang, Yutaka S. Sato, Hiroyuki Kokawa, Seung Hwan C. Park, Santoshi Hirano,

“Microstructural characteristics and mechanical properties of Ti-6Al-4V friction stir welds”,

Materials Science and Engineering A 485 (2008) 448-455.

[9] L. Zhou, H.J. Liu, P. Liu and Q.W. Liu, “The stir zone microstructure and its formation

mechanism in Ti6Al4V friction stir welds”, Scripta Mater, Vol. 61 (2009), pp.596-599.

[10] L. Zhou, H.J. Liu, Q.W. Liu “Effect of rotation speed on microstructure and mechanical

properties of Ti–6Al–4V friction stir welded joints”, Materials and Design, Vol.31 (2010), pp.

2631-2636

[11] A.Arora, Z.Zhang, A.De and T Debroy, “ Strain and strain rates during Friction stir welding”,

Scripta Materialia61, (2009), 863-866.

Materials Science Forum Vols. 783-786 579