Effect of interlayer thickness on microstructure and mechanicalproperties of as welded Ti6Al4V/Cu/NiTi joints

A. Shojaei Zoeram n, S.A.A. Akbari MousaviWelding Lab, School of Metallurgy and Materials, College of Engineering, University of Tehran, PO Box 11155-4563, Tehran, Iran

a r t i c l e i n f o

Article history:Received 4 May 2014Accepted 21 June 2014Available online 28 June 2014

Keywords:Shape memory materialsWeldingMicrostructureX-ray techniques

a b s t r a c t

Dissimilar welding of Nitinol shape memory alloy to Ti–6Al–4V with copper interlayer was performed bya pulsed Nd:YAG laser beam and effect of interlayer thickness on microstructure, chemical composition,mechanical properties and fracture behavior was investigated. The results indicated that the thickness ofinterlayer has a great influence on chemical composition and Ultimate Tensile Strength of joints. Withthe increase of interlayer thickness to 75 mm, tensile strength and elongation of the joint reached themaximum value of 300 MPa and 3.3%. Further increase in interlayer thickness resulted in deterioration ofmechanical properties because of formation of Ti–Cu IMCs and shrinkage cavities in weld metal. Thisshows that the thickness of the copper interlayer for dissimilar welding of these alloys needs to beoptimized.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

High resistance to corrosion, high strength-to-density ratio andbiocompatibility have turned Ti6Al4V alloy to an interesting alloy formany industries [1]. Moreover Nitinol is a shape memory alloy withunique properties such as super elasticity, high resistance to corro-sion and biocompatibility [2]. Both alloys contain large amounts oftitanium in their composition. The main problem associated withdissimilar welding of these alloys is the formation of brittle inter-metallic phases in weld metal due to high-temperature reactivity oftitanium with many elements of the periodic table. Precipitations ofIMCs in weld metal increases hardness of weld metal sharply andthis decreases the fracture toughness of dissimilar joints [3,4].According to Ti–Ni phase diagram IMCs between Ti and Ni are Ti2Ni,NiTi and Ni3Ti [5]. The Ti2Ni IMC forms even in welding of Ti-richNitinol plates and severely impair mechanical properties [6]. InTi6Al4V/NiTi joint Ti6Al4V provides more Ti therefore Ti2Ni phaseforms in weld metal [7]. Modification the chemical composition ofweld metal and increasing the solidification rate are among sug-gested solutions for restriction the formation of such brittle phases[8,9]. Copper has a good compatibility with NiTi and a ductilizingeffect on IMCs [4,10], therefore could be used as interlayer in order tomodification the chemical composition of weld metal. On the otherhand Laser and Electron Beam Welding methods have highersolidification rates and Pulsed Nd:YAG laser source due to more

welding parameters supplies more accurate energy [11]. Therefore apulsed Nd:YAG laser source was used in this study and effect ofcopper interlayer thickness on the joint properties was investigated.

2. Experimental procedures

The chemical composition of used Ti6Al4V was: Ti: Base, Al:6.5%, V: 3.7%, Fe: 0.18% in wt% and used Nitinol alloy contained45.62 wt%Ti. The laser welding operation was carried out with anSW-1 pulsed Nd:YAG laser machine with maximum averagepower of 100 W and wavelength of 1.064 mm. The samples weresymmetrical double-sided butt welded and welding parameterswere chosen so that depth of fusion in both sides be equal to 55%of sheet thickness. The welding process parameters were: voltage:425 V, defocusing distance: �0.5 mm, beam diameter: 0.4 mm,frequency: 7.7 Hz, pulse duration: 3 ms, welding speed: 1.70 mm/sand Average Peak Power: 1.47 W.

Etching process was performed by immersion of the samples inthe acid solution of HF: HNO3:H2O¼1:4:5 for 20 s. The OM andSEM was used to analysis the weld metal microstructure andfracture surface respectively. X-ray Diffraction analysis was carriedon the fracture surfaces of tensile samples to identify the formedphases in the weld metal. SEM equipped with an EDS used toevaluate the chemical composition of weld metal. Moreover, inorder to evaluate the effect of interlayer thickness on the mechan-ical properties of joints, tensile tests were employed on tensilesamples with length of 50 and Gage length of 20 mm, welds were

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.06.1410167-577X/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ98 216 111 4164; fax: þ98 218 800 6076.E-mail address: a.shojae@alumni.ut.ac.ir (A. Shojaei Zoeram).

Materials Letters 133 (2014) 5–8

located in the middle of tensile samples, tests carried out withstrain rate of 1 mm/min.

3. Results and discussion

Microstructure of weld metal: Fig. 1 shows the microstructuresof weld metals with different interlayer thicknesses. In this figure,copper-rich areas are easily recognizable because of their differentcolors. As can be seen microstructure of the weld metals withdifferent interlayer thicknesses are similar to Nitinol microstruc-ture in Nitinol/MZ interface. This makes detection of Nitinol/MZfusion boundary difficult. As depicted in Fig. 1, Microstructure ofCu-rich zones is not similar to pure copper microstructure. There-fore copper is dissolved in fusion metal during welding completelyand reacts with other elements.

Because of the impulse nature of pulsed laser welding Cu-richareas are dispersed in weld metal randomly and microstructure isnot identical through weld metal. As can be seen in Fig. 1, there issome Cu-rich areas in weld metal, which increases with the increaseof interlayer thickness. Further increase in interlayer thickness to

100 mm leads to formation of asymmetrical shrinkage cavities inCu-rich areas. These cavities are marked by arrows in Fig. 1e and f.The cause of formation of these cavities could be high thermalcontraction coefficient of copper compared to the base metals. Thisfeature of copper leads to rapid contraction of the copper-rich liquidin the final moments of solidification. Therefore lack of liquid in theinter-dendritic areas leads to formation of asymmetrical shrinkagecavities in Cu-rich areas. Therefore in areas which amount of copperis higher probability of formation of these cavities is increased. In thelower parts of weld metal due to imminution of width of weld metalcopper proportion is increased and probability of formation ofshrinkage cavities is increased.

EDS analyze results: Weld dimensions remain constant with thesame welding parameters (average maximal width of the weldswas about 700 mm). Therefore, increase of interlayer thicknessresults in the reduction of the base metals proportion in the weldmetal. Therefore the increase of interlayer thickness could reducethe amount of Titanium and Nickel in the weld metal andconsequently the probability of formation of Ti–Ni IMCs.

Average composition of fusion metals from EDS analysis isgiven in Table 1. As can be seen, increase of interlayer thickness

Fig. 1. Effect of interlayer thickness on microstructure of weld metal. (a) and (b) 50 mm. (c) and (d) 75 mm. (e) and (f) 100 mm.

A. Shojaei Zoeram, S.A.A. Akbari Mousavi / Materials Letters 133 (2014) 5–86

decreases the amount of Titanium and Nickel in weld metal.Therefore the probability of formations of Ti2Ni phase in weldmetal will be decreased.

XRD analysis results: The results of XRD analysis are shown inFig. 2. As can be seen the insert of copper interlayer cannot restrictthe formation of Ti2Ni brittle phase in the weld metal completely.The remarkable point in XRD patterns is the presence of peakscorresponding to Ti–Cu IMCs. As can be seen in Fig. 2, increase ofinterlayer thickness to 75 mm decreases intensity of peaks corre-sponding to Ti2Ni phase and further increase of interlayer thick-ness to 100 mm, increases the intensity of peaks corresponding toCu2Ti and Cu4Ti3. Therefore the insertion of copper interlayer leadsto formation of Ti–Cu IMCs in weld metal, but as it has been shownin previous works, these IMCs are not as brittle as Ti–Ni inter-metallic phases [1,3,7].

The fracture during tensile test occurred on Ti6Al4V/Cu inter-face, this justifies the appearance of peaks corresponding to the Tiand Cu rich phases.

Mechanical properties: The Ultimate Tensile Strength and elonga-tion percentage of the welded joint with 50, 75 and 100 interlayerthickness were 250 MPa, 2.1%, 300 MPa, 3.3% and 260 MPa, 2.3%respectively. According to results of EDS and XRD analysis, increaseof interlayer thickness reduces probability of formation of Ti2Niphase. Therefore increase of the interlayer thickness to 75 mm leadsto improvement of mechanical properties. On the other handaccording to Fig. 1c and the results of XRD analysis, formations ofshrinkage cavities and Cu-rich IMCs in weld metal can be reason fordeterioration of mechanical properties in effect of increase of theinterlayer thickness to 100 mm. Compared to mechanical propertiesof references materials: 1100 MPa, 14% for Ti6Al4V and 1000 MPa,13% for Nitinol, a sharp drop in the mechanical properties due towelding is observed. According to the results of XRD analysis,formation of the brittle phases such as Ti2Ni, Ti2Cu, Cu2Ti andCu4Ti3 can be responsible for the lower tensile strength andelongation of welded joints with copper interlayer compared tothe base metals.

As mentioned; fracture during tensile test occurred on Ti6Al4V/MZ interface. This zone is close to Ti6Al4V and is richer in Ti thanother parts of the weld, according to Ti–Ni and Ti–Cu phasediagrams [5], in Ti-rich areas (Ti6Al4V/MZ interface); free Gibbsenergy for Ti2Ni and Ti2Cu phases is lower. Therefore from thethermodynamic aspect probability of formation of Ti2Ni and Ti2Cuphases in Ti6Al4V/MZ interface is higher. Regarding to higherhardness of these phases [3,7,12] these areas could be an suitableinitiation site for initiation and propagation fracture. This justifiescrack propagation on Ti6Al4V/MZ interface.

Fracture surface: Fig. 3 shows fracture surface of tensile testspecimens. This figure shows the specimens have cleavage fracturesurfaces; which is a characteristic of brittle fracture. These resultsare in good agreement with the results of the tensile test, whichrepresented brittle joints with low elongations.

According to XRD patterns, weld metals with different inter-layer thicknesses contain various IMCs with different chemicalcomposition. This can be the reason for brittle fracture of all joints.

4. Conclusions

Different thickness of copper thin film was used as interlayerfor dissimilar welding of Ti–6Al–4V to Nitinol. The increase ofinterlayer thickness to 75 mm, increased Cu-rich areas in the weldmetal and further increase to 100 mm resulted in the formation ofshrinkage cavities in the weld metal. The increase of interlayerthickness to 75 mm, restricted precipitation of Ti2Ni phase andimproved mechanical properties to the maximum value (300 MPaand 3.3%). Further increasing the interlayer thickness deterioratedmechanical properties because of formation shrinkage cavitiesand Cu-rich IMCs in weld metal. High-temperature reactivityof Ti led to formation of Ti-rich IMCs in Ti6Al4V/MZ interface.

Table 1Result of EDS analysis of fusion metal (in at%).

Interlayer thickness [mm] Ti [at%] Ni [at%] Cu [at%] Ti/(NiþCu) ratio

50 51.72 33.25 15.03 1.0775 48.69 30.45 20.86 0.95

100 44.66 28.64 26.70 0.81

Fig. 2. X-ray diffraction patterns of fracture surfaces.

Fig. 3. Fracture surface of welded samples with different interlayer thicknesses (a) 50 mm. (b) 75 mm. (c) 100 mm.

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The brittleness of these phases is more than other formed phasesin weld metal. This turned Ti6Al4V/MZ interface to a brittle pathwhich is suitable for initiation and propagation of cracks. There-fore the joint fracture occurred in this interface. Formation ofTi2Cu, Ti2Ni and Cu rich IMCs in Ti–6Al–4V/Cu/NiTi joints can bethe origin of the brittle fracture of joints.

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