Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal 24-28 July 2016

Editors J.F. Silva Gomes and S.A. Meguid

Publ. INEGI/FEUP (2016)

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PAPER REF: 6364

IMPROVEMENT ON MECHANICAL PROPERTIES OF NiTi SHAPE

MEMORY ALLOY WELDED BUTT JOINTS BY GAS TUNGSTEN

ARC WELDING PROCESS

Raphael Melo1,2(*)

, Matheus Oliveira2, Pedro Vidal

3, Theophilo Maciel

3, Carlos de Araújo

3

1Unidade Acadêmica de Indústria (UNIND), IFPB, Cajazeiras, Brazil 2Programa de Pós-Graduação em Ciência e Engenharia de Materiais, UFCG, Campina Grande, Brazil 3Departmento de Engenharia Mecânica, UFCG, Brazil (*)Email: raphael.melo@ifpb.edu.br

ABSTRACT

This study aimed in improving the mechanical properties on welding nickel-titanium shape

memory alloy (SMA) by Gas Tungsten Arc Welding process (GTAW). The effects on

mechanical properties of welding parameters and post-weld heat treatment are studied.

Welded joints achieved ultimate tensile strength of 760MPa and 23% strain for optimized

parameters.

Keywords: Shape memory alloys; gas tungsten arc welding; mechanical properties.

INTRODUCTION

Shape memory alloys (SMA) are a unique class of materials which exhibit phase

transformations induced by temperature and/or stress variations. These materials which

includes NiTi alloys present both shape memory effect (SME) and pseudo-elasticity, two

important functional properties for engineering applications [1-3]. Use of a material for

industrial purpose may be limited unless processing technologies are developed or union

techniques of the material to itself are improved. Given the ubiquitous use of NiTi among all

the commercially SMAs, researchers over the past two decades have focused on the use of

reliable joining techniques, like welding, to connect NiTi to itself [4-5].

There’s a great challenge, however, on joining similar NiTi joints due the formation of

intermetallic compounds which leads to hot cracking associated to interdendritic

microstructure; precipitation of deleterious phases in the heat affected zone and columnar

brittle structure on the weld bead, resulting on severe strength reduction [6-9]. These

drawbacks are responsible for NiTi SMA limiting applications in multiple areas of interests.

The main welding process to join NiTi is the Laser Beam Welding (LBW) for its excel in

high precision and localized heat input resulting on narrow heat affected zone (HAZ), [4,10].

Despites its benefits regarding welded joints the LBW it’s a costly process which restricts

widely its applicability. Arc welding process like the Gas Tungsten Arc Welding (GTAW) is

commonly utilized for industrial applications. It’s well known GTAW results in high integrity

joints for steel, aluminum, copper and further alloys. One could expect the same for NiTi

SMA. Although, according to [11-13] the GTAW welding process negatively impacts directly

on mechanical properties of NiTi joints due to an extended HAZ. Since their report very few

researchers published GTAW of NiTi joints. So, this work focuses on unifying NiTi butt

joints by GTAW with desirable mechanical properties. For this the NiTi butt joints was

investigated by stress-strain curve and SEM analysis.

Symposium_18: Structural and Multidisciplinary Optimization

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RESULTS AND CONCLUSIONS

Thin sheets of NiTi (50.5 at.% Ni) were welded by GTAW process after vast study on

welding parameters optimizations and counting on previous experience on welding from the

authors. Optimized parameters included: average current, current nature and post weld heat

treatment (600 °C for 1 hour). Welding conditions are presented on Table 1.

Table 1 - Welding conditions used for GTAW NiTi butt joints.

Average welding current Current nature Post-weld heat treatment

C1 20 Continuous No

C2 26 Continuous No

C3 20 Pulsed No

C4 26 Pulsed No

C5 20 Continuous Yes

C6 26 Continuous Yes

C7 20 Pulsed Yes

C8 26 Pulsed Yes

The joints were capable of achieving maximum ultimate tensile strength (UTS) of 760MPa

and 23% strain for as welded conditions and 680MPa and 25% strain after post-welded heat

treatment for optimized welding parameters (Fig.1). This result compares and also exceeds

those of literature [5, 11, 14-17] for LBW NiTi SMA joints regarding joint mechanical

integrity as can be seen on Table 2. Analyzing as-welded conditions C3 and C4 presented

higher UTS. Average current (20 to 26A) did not influence significantly on mechanical

properties for NiTi joints. Current nature, however, direct influenced UTS and strain achieved

for welded samples.

Further investigation revealed influence of post-weld heat treatment on tensile plateau

reduction as can be seen for conditions C5, C6, C7 and C8 when comparing with conditions

without heat treatment (C1 to C4). Reduction from 400MPa to 300MPa for C7 and C8 is

mainly driven by phase changes [18, 19] provided by heat treatment. While greater plateau

reduction, from 400MPa to 250MPa for C5 and C6 are mainly due current nature. C5 and C6

were welded using continuous current. Samples C7 and C8 were welded using pulsed current

which promoted grain refinement [20]. Thus, a larger force needs to be induced in order to re-

orient the high amount of twinned martensites formed in refined grains of C7 and C8

contrasting those of C5 and C6 texture.

Fig. 1 - Strain-stress curve for welded NiTi joints.

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Table 2 - Mechanical properties of NiTi joints showing max ultimate tensile strength (UTS) and

strain achieved.

Welding

process Remarks Weld condition

UTS

(MPa)

Strain

(%) Reference

LBW Sheets Average power = 850W 520 7 Falvo et al., 2005

LBW Wire Average power = 100W 620 8 Gugel et al., 2008

LBW Wire Average power = 1000W 835 16 Mirshekari et al., 2013

LBW Wire Welding current = 119A 325 5 Song et al., 2008

LBW Sheets Average power = 1300W 600 11 Zhao et al., 2010

LBW Sheets Average power = 600W 300 8 Khan et al., 2008

GTAW Sheets

Average current = 20A,

Pulsed current, without

PWHT

760 23 This work

Fracture surfaces of tensile samples SEM micrographs are presented in Figure 2. A first look

of surface main aspect (a) indicates cleavage failure, transgranular, since fracture cracks pass

through grains, presenting grained or faceted texture, as a result of reorientation changes of

cleavage planes from one grain to another. Although after deeper examination (b) it can be

noticed presence of dimples suggesting a composite fracture, resembling those of ductile

behaviors. According to Toribio et al. [21] the surface aspect indicated in Fig.2 (c) is named

Tearing Topography Surface. Chan et al. [10] and Mirsheraki et al. [14] associated these

dimples presence to an intense plastic deformation and ductile failure mode. All samples

exhibited similar results concerning fracture surface aspects.

Fig. 2 - SEM micrographs showing fracture surface (a) 70x magnification, (b) 400x magnification, (c)

4000x magnification.

NiTi butt joints were successfully welded using arc welding by gas tungsten arc welding.

Process parameters such as average current, current nature and post weld heat treatment were

evaluated and optimized. All samples presented desirable mechanical properties. Ultimate

tensile strength of 760MPa and strain of 23% was achieved.

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ACKNOWLEDGMENTS

The authors would like to thank the CNPq (Process N. 552199/2011-7 and 503082/2011-2)

and CAPES in Brazil for financial aid which permitted this work’s results.

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