Mechanical Properties of NiTi Shape Memory Alloys Welded Joints: A

Materials Science Forum ISSN: 1662-9752, Vol. 930, pp 526-531 doi:10.4028/www.scientific.net/MSF.930.526 © 2018 Trans Tech Publications, Switzerland

Online: 2018-09-14

Mechanical Properties of NiTi Shape Memory Alloys Welded Joints: A Comparative Study between the GTAW, PAW and LBW Processes Raphael Henrique Falcão de Melo1,2,a,*, Matheus José Cunha de Oliveira2,b, Pedro Ítalo Vidal de Oliveira3,c, Milton Sérgio Fernandes de Lima4,d, Theophilo Moura Maciel2,5,e, Carlos José de Araújo2,5,f 1

Department of Eletrocmechanics, Instituto Federal de Educação, Ciência e Tecnologia da Paraíba, BRAZIL

2

Department of Materials Engineering, Universidade Federal de Campina Grande, BRAZIL

3

Department of Petroleum Engineering, Universidade Federal de Campina Grande, BRAZIL 4

Photonics Division, Instituto de Estudos Avançados, BRAZIL

5

Department of Mechanical Engineering, Universidade Federal de Campina Grande, BRAZIL

a

[email protected], [email protected], [email protected], d [email protected], [email protected], [email protected]

Keywords: Welding, Shape Memory Alloys, Mechanical Properties, NiTi alloy

Abstract: In this study mechanical behavior of Nickel-Titanium (NiTi) shape memory alloy (SMA) thin sheets welded joints obtained by Gas Tungsten Arc Welding (GTAW), Plasma Arc Welding (PAW) and Laser Beam Welding (LBW) processes were evaluated. The effects on mechanical properties of welding parameters and post-weld heat treatment were verified by means of tensile tests and hardness measurements. NiTi welded joints achieved ultimate tensile strength of 760MPa and 23% strain in GTAW welding process, 550 MPa and 11% strain in LBW, 500 MPa and 8% strain in PAW welding process. These results can be considered compatible and even superior to those verified in the literature, mainly for the GTAW and PAW processes. Introduction Shape memory alloys (SMA) are a unique class of metals that present martensitic thermoelastic phase transformations induced by temperature and/or stress variations. These materials, which include NiTi alloys, present both shape memory effect (SME) and pseudoelasticity effect (PE), two functional properties of great importance for advanced engineering applications [1-3]. The use of these materials in industrial applications may be limited until effective mechanical/metallurgical processing techniques are developed or that joining techniques for these alloys, similar or dissimilar, are optimized and ensure satisfactory mechanical properties [4, 5]. Joining NiTi SMA is a technological challenge due to the formation of fragile intermetallic compounds, which leads to a marked loss of tenacity in the welded joints, due to solidification cracks associated with the dendritic microstructure of the weld metal. In addition, precipitation of deleterious phases in the thermally affected zone and in the weld metal can result in severe reduction of mechanical resistance [6-9]. These phenomena are responsible for limiting the applicability of NiTi SMA welded joints in multiple areas of interest. The main welding process used to joint these SMA is the laser beam welding (LBW), due to its excellent precision and focusing of the welding heat, resulting in a shorter heat affected zone (HAZ) [4-10]. Electric arc welding processes, such as the arc welding process with non-consumable tungsten electrode and gas shielding (GTAW) or plasma arc welding (PAW), are commonly used in industrial applications and are known to provide high quality welded joints and mechanical strength for most steels, aluminum, copper and their alloys. It would be possible to expect the same behavior for the NiTi SMA. However, according to several authors [7, 11-14] the GTAW and PAW welding processes negatively affects the mechanical properties of welded joints of NiTi alloys due to a large extension of HAZ. Therefore, this article aims to evaluate the mechanical and physical properties of 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 Trans Tech Publications, www.scientific.net. (#109380666-11/09/18,22:15:58)

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NiTi SMA welded joints by GTAW, PAW and LBW processes through microhardness measurements, tensile tests and phase transformation temperature. Materials and Methods NiTi SMA thin sheets were welded in this work by GTAW, PAW and LBW processes. PE NiTi thin sheets were Ni-rich (55,92%wt) with 0.8 mm thickness and are in austenitic phase at room temperature. Table 1 shows the welding parameters evaluated in this paper. The welds made by the GTAW and PAW processes were manual, while those of the LBW process were automatic. It was used an Yb fiber laser, model YLR – 2000 from IPG Photonics. The protective gas used was argon, with flow rate of 5L/min. The welds made by the LBW process did not have shielding gas. PostWeld Heat Treatment (PWHT) was another variable to analyze. It consisted in heating the joints at 500ºC for 120 minutes after welding. Table 1 – Welding conditions and related techniques used to obtain the joints.

LBW

PAW

GTAW

Processes

Condition G2 G3 G5 G8 P1 P4 P5 P8 L2 L3 L6 L7

Welding Current [A] 26 20 26 20 12 16 12 16 P [W] 550 450 550 450

Current Type CC PC CC PC CC PC CC PC Travel Speed [mm/min] 900 1500 900 1500

Post-Weld Heat Treatment Without Without With With Without Without With With Post-Weld Heat Treatment Without Without With With

The phase transformation temperatures were determined by variation of the electrical resistance as a function of temperature [2] with an average heating or cooling rate of 4°C/min cooling from 100°C to -60°C and then heating to 100°C again. The microhardness measurements were performed using a microindenter model FM-700 from Future Tech, applying a load of 50 gf for 15 seconds with a distance between the indentations of the order of 200 µm. Tensile tests were performed on an MTS 810 testing machine with a displacement rate of 0.05mm/min at room temperature. The fracture surfaces were observed by scanning electron microscopy (SEM) using a TESCAN microscope, VEGA 3 SBH model. Results and Discussion Fig.1 shows the percentage variation curves of the electrical resistance as a function of temperature (ERT), taking the temperature of 100°C as the austenitic reference state. The profile of the curves in the as welded condition (Fig. 1a) indicates, qualitatively, that independently of the welding process used the material will be in the austenitic state at room temperature (about 25°C). For all these conditions, it is only possible to identify a slight inflection of the ERT curve, indicating probably the temperature of beginning of the R phase transformation (Rs) from austenite. This behavior is similar to that of NiTi SMA alloys that have been subjected to cold or hot rolling, were phase transformation is blocked due to the characteristic hardening of this manufacturing process [2, 15]. The R phase is a transition state between the austenitic and martensitic phases of NiTi SMA characterized by a trigonal crystalline structure with rhombohedral distortion [16].

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(a)

(b)

Figure 1 – Variation of electrical resistance as a function of temperature (ERT) in NiTi joints. (a) As welded condition and (b) PWHT conditions. Contrarily, on the other hand, the ERT behavior for the welded conditions that were submitted to the post-weld heat treatment (PWHT), Fig 1b, indicates qualitatively, at room temperature, all welded joints are in a mixed state between the R phase and the austenitic phase. The BM and BM PWHT corresponds to unwelded NiTi SMA specimens. After the PWHT, it is possible to observe and determine the start and end temperatures of the R phase (Rs and Rf), the martensitic phase (Ms and Mf) and the austenitic phase (As and Af). This behavior is characteristic of NiTi SMA that have been subjected to heat treatments, since these thermal treatments initially relieves the residual stresses of the rolling and welding process, unlocking the phase transformation by relaxation of the crystalline lattice [15]. In addition, it is well known that this heat treatment favors the formation of Ni-rich precipitates in the NiTi matrix of the material, impoverishing it in Ni, causing a variation of Ti/Ni ratio [9], leading to an increase of phase transformation temperatures, since Ni is the stabilizing element of austenite at room temperature [17, 18]. Table 2 shows the phase transformation temperatures obtained by the application of tangents in the ERT curves [2] from Fig. 1. It can also be verified that the phase transformation temperatures varied according to the welding process. In [9], author welded a NiTi SMA by the LBW and CDW processes and found that the transformation temperatures were different from those of base metal (BM) and between them. Fig.2 show microhardness variation along weld of some NiTi joints. The average hardness in the weld metal (WM) was higher in the GTAW process than in the PAW and LBW processes. In addition, the GTAW and PAW processes relied on the use of addition metal filler rod during welding, unlike the autogenous LBW process. This directly influenced the observed microhardness profile. The welded joints that were submitted to the PWHT, Fig. 2b, presented less hardness than those that were not submitted to this operation. Qualitatively, the microhardness profiles observed in the GTAW and PAW processes agree with those observed by other authors [11, 19, 20], as well as those profiles observed in the LBW process [10, 21-24]. Fig. 3 shows the microstructure of G4, P8 and L6 welded conditions, respectively. It is possible to observe that with the increase of RS there is an increase of the black phase observed in the microstructure of the welded joints. Specifically, in the P4 welding condition, where there is the highest content of R phase and martensite and room temperature. Table 2 – Phase transformation temperatures of NiTi SMA welded joints for GTAW, PAW and LBW processes.

Rs Rf Ms Mf As Af

BM

G1

G4

P1

P4

L2

L3

-13,7

6,5 -

2,7 -

5 -

-3,6 -

12,5 -

14,3 -

BM G5 PWHT 38,4 28,7 28,8 25,2 -28,1 -10,2 -46,9 -20,4 21,4 29,6 28,8 36,9

G8

P5

P8

L6

L7

26 16,2 -8,9 -17 28,5 35,4

42,9 30,4 -6,5 -18 40,9 45,9

41,4 28,4 -7,6 -14,5 38 47

38,2 22,6 0,8 -7,5 34,8 41,6

34,9 22,6 1,9 -6,9 35,9 40,8


(a)

529

(b)

Figure 2 – Microhardness profiles in the cross section of NiTi welded joints. (a) As welded condition and (b) PWHT conditions.

Figure 3 – Microstructure in the cross section of NiTi welded joints. (a) G4, (b) P8 and (c) L6. In Fig. 4 it is possible to observe the stress-strain behavior of NiTi SMA welded joints. Among the welded joints that were not submitted to PWHT (Fig. 3a), only the joints welded by the GTAW process showed satisfactory mechanical resistance [21, 23, 25], reaching about a tensile strength of 740 MPa and 13,8% strain, higher than those observed by other authors [5, 7, 10, 11, 14, 20, 22, 24, 26-30]. After the PWHT, the joints welded by the PAW and LBW processes were able to withstand higher stress and strains, with the exception of welded joint L7 that was welded with a welding speed of 1500 mm/min, making the heat it impossible to fully penetrate in the weld zone besides facilitated the precipitation of brittle compounds in HAZ. In addition, the G8 welding condition was able to reach high ultimate tensile stress (~552 MPa) and supported large strains (~17.6 %), being therefore the best welding condition under study, as can be seen in Fig. 3b.

(b) (a) Figure 4 – Stress-strain behavior for NiTi SMA welded joints by GTAW, PAW and LBW processes. (a) As welded condition and (b) PWHT conditions.

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Conclusions The welded NiTi SMA joints were in the austenitic state in the as welded condition. After PWHT welded joints were in a mixed state between the R phase and the austenite phase, regardless of the welding process used. The joints welded by the GTAW process presented greater hardness and satisfactory mechanical strength comparable to joints welded by the LBW process (L6 condition). In addition, the PAW welding process proved to be quite promising, since the mechanical strength obtained by this process was similar to those observed by the LBW process in this study (P8 and L6 specimens). Therefore, the joints welded by the GTAW and PAW processes are extremely attractive, since they presented satisfactory mechanical strength at a specific low cost when compared to the LBW process. Acknowledgement The authors are grateful to CNPq for funding of the following projects: INCT of Intelligent Engineering Structures (grant number 574001/2008-5), Casadinho UFCG-UFRJ-ITA (grant number 552199/2011-7), Universal 14/2012 (grant number 474524/2012-4), CT-Aerospace 22/2013 (grant number 402082/2013-3) and PQ 1D (grant number 304658/2014-6). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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