The development of high strength brazing technique for Ti-6Al-4V using TiZrCuNi amorphous filler

Yongjuan Jinga*, DiyaoSua, XishanYuea, T.B. Brittonb, Jun Jiangc

a, Aeronautical Key Laboratory for Welding and Joining Technology, Beijing 100024, PR China

b, Department of Materials, Imperial College London,ExhibitionRoad,London, SW7 2AZ, UK

c, Department of Mechanical Engineering, Imperial College London,ExhibitionRoad,London, SW7 2AZ, UK

Abstract: The brazing joint of the Ti-6Al-4V alloy was produced with a designed brazing

filler alloy and the optimized brazing temperature which is lower than the β-phase

transformation of the matrix. The strength and the ductility of brazing joined Ti-6Al-4V

samples were evaluated by conventional tensile tests with a DIC 2D-strain field

measurement. The Widmanstätten microstructure with no voids or cracks or intermetallic

compounds was found throughout the joint with a width of β-lamellar as ~1µm. Due to the

fine acicular α-Widmanstätten and β-lamellar, and the uniformly diffused filler elements

throughout the entire joint, the strength of the joint was as much as the matrix. In addition,

the hardness test results agreed well with the tensile strength tests. All fractures occurred in

the matrix rather than the brazing joints. Furthermore, the maximum local tensile strain was

measured as 20% in the matrix, while under the same stress, the brazing joint only reached

6.3% tensile plastic strain. Thus, the mechanical properties of the joint with the associated

microstructure demonstrated that a successful brazing filler alloy has been developed for the

Ti-6Al-4V alloy.

Keywords: brazing; joint strength; microstructure; Ti-6Al-4V alloy

1 Introduction

Ti-6Al-4V alloys have increasing importance due to structural demands for aerospace

and medical applications, as seen in the review[1]. In the joining of Titanium alloys, brazing

technology has been recognized as a promising joining approach. Until now, over 100

brazing filler metals (BFMs) have been developed and tested to meet industrial needs,

especially the joint strength. The most common BFMs are Ti-based BFMs [2].

It is noted that the joint strength was sensitive to the intermetallic, which was easily

formed in the joint due to the high chemical activity of the Ti element. The investigation by

*E-mail adress:jingyongjuan1982@126.com

Shiueet al.[3]showed that the presence of both Ti2Ni and interfacial Ti3Al phases in the joint

deteriorates the shear strength of the joint. In addition, the amount of Ti2Ni could be

decreased by increasing the brazing temperature and/or time due to the diffusion of Ni from

the BFMs (Ti-15Cu-25Ni and Ti-15Cu-15Ni) into the Ti-6Al-4V matrix during brazing. This

phenomenon was testified [4,5]with the other kind of Ti-based BFMs. Therefore, optimizing

the brazing process was one of the ways to solve this problem.

Even though the Ti-6Al-4V brazed joint strength can catch up to? the base metal by

optimizing the brazing process, as reported by Ganjehaet al.[6], the maintenance of the

mechanical properties for the matrix cannot be ignored. With reference to Leyenset al.[7] , the

β-transus temperature (Tβ) of Ti-6Al-4V was 1268 K, and it was suggested that for joining α

+ β titanium alloys, the brazing temperature should usually be about 38-66 K below the T β in

order to avoid the mechanical property impairment of the base materials caused by the phase

transformation and grain coarsening.

Moreover, the ductility of the joints is also one of the key mechanical properties related

to the structure integrity of joined parts, which has not been reported until now. Due to the

thin brazing joint (~400µm in width [8,9]), the Digital Image Correlation (DIC) technique

might be a suitable method to test the local strain of the brazing samples instead of the

conventional strain measurement methods such as strain gauges.

Earlier work by Jing et al.[10]has made some progress in balancing the interface

microstructure and maintaining the mechanical properties of the matrix by the composition

design on Ti-based BFMs for the Ti-6Al-4V honeycomb structure. With the earlier designed

Ti-18Zr-15Cu-10Ni wt.% BFMs, the current investigation aims to establish the relationship

between the developed microstructure of the joint and its associated mechanical properties for

Ti-6Al-4V.

2 Experimental processA braze joined Ti-6Al-4V alloy plate of 100mm thickness was used as the base material.

All tested specimens (ASTM E8, 2013) were cut from it as shown in Fig.1. The brazing filler

composition and processing procedure are listed in Table 1. Vacuum brazing was performed

in a vacuum of 5×10−5 mbar. The melting behaviors of the filler foil were examined by a

differential thermal analyzer (DTA, Shimadzu DTG-60H) from room temperature up to 960 ℃ at a heating rate of 20℃/min. The designed filler was measured to have a narrow melting

temperature range of 840-860 ℃, which is suitable for brazing.

2

Composition/wt.% Melting point/℃ Brazing process

Ti-18Zr-15Cu-10Ni 840-860 930℃/3600s

Table 1 Chemical composition of the filler metal and the brazing process

Fig.1 schematically showing the processes of sample

Mechanical tests including a tensile test and micro-hardness at room temperature were

carried out to evaluate the mechanical properties of the joint. The tensile tests were carried

out using a MTS tensile tester (MTS-810, U.S.A.) at a constant speed of 0.1 mm/min.

DIC is an optical 2D-strain field analysis method by tracing the movement of regions of

interest, which consists of random black and white patterns by spraying ~3µm sized printer

toner particles on the free surface. It enables the evaluation of the strain field across the

brazing line. High resolution digital images (2016x2016 pixels) for the DIC strain

measurement were captured using a Nikon D5500 camera. Thus, DIC could be used to study

the local strain fields on brazing joined samples.

Standard metallographic preparations of grinding and polishing were conducted. The

samples were finally etched by Kroll’s reagent (3 ml HF + 6 ml HNO 3 + 100 mlH2O) to

reveal microstructures in SEM (JEOL JXA 8200). The composite analysis and element

distribution were evaluated by using energy dispersive X-ray spectroscopy (EDX).

Furthermore, local crystal orientations and phase distribution near the joint were revealed by

the electron backscattered diffraction (EBSD) technique.

3 Results

3.1 Tensile strength of brazed samples

For joining α + β Titanium alloys, such as Ti-6Al-4V with a β-transus temperature (Tβ)

of 1268 K (995°C), the brazing temperature should usually be about 38-66 K (responding to

929-957°C), the brazing temperature should be lower than the Tβ in order to avoid the

mechanical property impairment of the base materials caused by the phase transformation and

3

grain coarsening. In this study, the brazing temperature was 930 ℃.

The tensile property of the Ti-6Al-4V matrix before brazing was shown in Fig.2. The

results show variations in the ultimate tensile strength from 900 to 990 MPa with 943 MPa on

average, and the tensile elongation was from 12 to 19 %. The scattering of tensile properties

is likely due to the presence of texture e.g. the strong anisotropic plasticity of Ti alloys. The

micro-texture has been discovered in Ti-6Al-4V alloy for industry application. After brazing,

the average tensile strength and the tensile elongation of the matrix was 920 MPa and 13 %.

Moreover, there was no obvious course of the duplex microstructure in the matrix.

Fig.2 The tensile stress-strain curve of the matrix at room temperature

The ultimate tensile strength values of three brazed samples of Ti-6Al-4V were 940MPa,

888 MPa and 889MPa respectively with 905MPa on average, [which is comparably strong

compared to the matrix. All fractures occurred in the matrix. The variation of the tested

results might be due to the variation of the strength in the matrix.

The ductility of the brazed joint is measured by the DIC technique. The strain map of the

gauge region of the brazed sample at 880MPa is shown in Fig3. The strain evolution of the

other two samples was the same as this one. The development of strain in the gauge region

through the tensile test was captured as a function of time, as seen in Fig.3(a).Repetition of

reference to fig3 The entire strain development can be divided into 4 stages.

At the beginning, the strain in the samples is relatively uniform (in light blue color, stage

I of Fig.3(a)). The joint endured the same strain as the matrix, as shown in Fig.3(b)-I and it

was almost 0.6%. Therefore, stage I is the elastic process. With an increase of tensile stress, a

higher strain appeared at the top half of the region (strain range as 5-10%, Fig.3(b)-II and III).

It was obvious that the accumulated strain at the joint is much lower than that of the matrix.

4

Thus, the plastic deformation occurred at stage II. A large plastic deformation occurred in the

upper half matrix of the sample. The site of strain localization (necking) is at the middle of

the upper half sample, which is away from the joint (in red color, stage IV), which

corresponded to the fracture location as shown in Fig.3d. The sample fractured at the necking

site with a nominal local strain of 20%, which was measured just before the occurrence of the

fracture (Fig.3(b)-IV). The local strain at the joints was found to be lower than that of the

samples. The maximum of the local strain of the joint was 6.3%, as shown in Fig.3c.

In addition, micro-hardness tests were conducted to check and compare the properties

between the joint and matrix. The results are illustrated in Fig.4. An excellent agreement was

reached compared with the tensile strength tests. It was found that after brazing, the average

hardness of the brazing joint (326HV) is higher than the matrix (301HV). The hardness

variation along the brazing line is small. These results confirm that the strength of the brazing

joints is considerably higher than the matrix.

(a)

(b)

5

(c) (d)

Fig.3 Ductility of brazing joined Ti-6Al-4V samples at room temperature

(a) Strain map of the brazed sample during tensile test; (b) the I, II, III and IV stages of

the strain evolution; (c) strain evolution of the brazed joint during tensile; (d) fracture of the

brazing samples;

Fig.4 the micro hardness of the brazing joined Ti-6Al-4V samples

3.2 Interface microstructure

The brazing joint was successfully formed in the examined samples as shown in an

enlarged brazing area in Fig.5a and b. With the developed brazing process, the Widmanstätten

microstructure was found throughout the entire joint section due to the eutectoid reaction.

The β-lamellar width was ~1µm in width and 37µm in length. No voids or cracks along the

joint were found. Furthermore, no intermetallic compounds were observed in the joint, which

have been commonly observed in the brazed joint for Ti-based alloys, as shown in table2.

Because the acicular phases are found to be harder than the equiaxed phases, developing

an acicular α-Widmanstätten microstructure improves the mechanical properties. Besides, as

seen in Fig.5c and d, no obvious microstructure variation was presented in the Ti-6Al-4V

6

matrix through the developed brazing process. The duplex microstructure is dominating.

Meanwhile, phase and crystal orientation maps were provided by the EBSD technique.

(a) (b)

(c) (d)

Fig.5 the microstructure of the ductility of brazing joined Ti-6Al-4V samples (SEM)

(a) The microstructure of the joint; (b) The magnification of the center in the joint;(c) and

(d) the microstructure of the matrix before and after brazing

Interface microstructure Matrix Filler metal composition ref.

acicular Widmanstätten microstructure with no intermettalics Ti-6Al-4V Ti-18Zr-15Cu-10Ni,wt% [this

study]Zr2Cu, Ti2Cu and (Ti,Zr)2Ni intermetallic compounds Ti-6Al-4V Ti-26.8Zr-13Ni-13.9Cu,wt.% [11]ZrO2/TiO+TiO2+Cu2Ti4O+Ni2Ti4O/α-Ti+(Ti,Zr)2(Cu,Ni)eutectic/(Ti,Zr)2(Cu,Ni)/acicular Widmanstäten

Ti-6Al-4V and Zirconia Ti47-Zr28-Cu14-Ni11,at.% [3]

a fine lamellareutectic joint microstructure consisting of α-Ti and ɣ-[Ti(Zr)]zCu (tetragonal MoSie-type ) phase Ti-6AI-4V 25Ti-25Zr-50Cu,wt.% [12]

Cu/Ni[Ti(Zr)]2 and Lave(Cu/Ni)2[Ti,(Zr)] Pure titanium(Ti-CP) Ti-27Zr-14Cu-13Ni,wt% [13]

Cu/Ni[Ti(Zr)]2 and Lave(Cu/Ni)2[Ti,(Zr)] Ti-6Al-4V Ti-27Zr-14Cu-13Ni,wt% [13]

a continuously segregated phase regarded as a [Ti,(Zr)]2(Cu,Ni) intermetallic phase;the acicular Ti-rich grains

Ti-6Al-4V 62.7Zr-11.0Ti-13.2Cu-9.8Ni-3.3Be,wt% [8]

three distinctive phases( a Ti-rich phase alloyed with low Cu, Ni,Zr contents, a Cu-Ni rich Ti phase and a Cu-Ni-Zr rich Ti phase)

Ti-6Al-4V and SP-700 Ti-20Zr-20Cu-20Ni,wt% [14]

Widmanstättenstructure consisting mainly of α-Ti, β-Ti and Ti-Zr-rich phase and a small amount of brittle intermetallics Ti-6Al-4V

Ti50-Zr27-Cu8-Ni4-Co3-Fe2-Al3-Sn2-Si1,at.% [15]

1Cr18Ni9Ti/Ag-rich/CuTi2/Ti-Cu-rich/β-Ti/TC4Ti-6Al-4V to stainless steel (1Cr18Ni9Ti)

AgCuTi filler [16]

Table 2 a summary of published results for brazing Ti-based alloys

7

Matrix

Matrix

Joint

Joint

Tensile direction

Interface

Matrix

Interface

Matrix

In the joint most of the grains had a hard and brittle hcp crystal structure (93 vol.%)

and the rest had a relatively soft and ductile bcc structure (as seen in Fig 6a). It is found that

the microstructure of the interface was homogeneous. In addition, the relatively large grains

with [110](bcc) (in green color, as shown in Fig.6b) observed separated in the joint

corresponded to [10-10](hcp), which is the hard orientation to the tensile direction.

(a) (b)

Fig.6 the microstructure of the brazing joined Ti-6Al-4V samples (EBSD)(a) Phases distribution of the joint ;(b) Grain orientation of the joint and the matrix

3.4 Elements distribution of the joint

Fig.7 demonstrates the composition of the elements across the joint measured by the

EDX technique. Al and V diffused from the matrix into the joint, while the Zr, Cu and Ni

elements diffused from the joint into the matrix. Compared with the composition of the filler

alloy(Ti-18Zr-15Cu-10Ni wt.%), the composition of the joint (Ti-(2.3~6)Al-(2.3~3)V-

(2.6~5.5)Zr-(2~4)Cu-(2~3)Ni wt.%) is significantly different from the original filler

composition.

8

Fig.7 the element distribution in the brazing joined Ti-6Al-4V samples

4 DiscussionThe BFMs for Titanium alloys can be divided into two groups. One is named Ti-based

filler metal with no Zr element. The other is named Zr-based filler metal with a Zr content

more than 50%. Both of them usually cause the formation of brittle intermetallic in joints (as

listed in Tab.2), which weakens the joint mechanical properties. In this study, TiZr-based

BFMs were designed to balance the brazing temperature, the microstructure and the strength

of the joint.

The literature results (e.g. the composition of the filler metal, the brazing process, and

the tensile strength of the brazing samples) for brazing the Ti-based alloy (especially the Ti-

6Al-4V) are summarized in Fig.8. In Fig.8, the area can be divided into four zones by the line

of brazing temperature as 960℃ and the line of strength of brazing samples as 800MPa. It is

better to make the brazing process in zone I without intermetallic in the joint. In addition, the

strain at the joints has not been published. Compared with the literature results, as shown in

Table 2 and Fig.8, the microstructure and the tensile strength achieved here is better, which

was carried out below the β-phase transformation temperature to maintain the microstructure

of the matrix.

9

Fig.8 The literature for brazing Ti-based alloys

(Some of the tensile strength was based on the rule that the shear strength is 0.577 of

maximum tensile strength by Dieter (2001))

The reduction of Zr content within the Zr-based filler material at a certain extent was

found to retain the melting point of the brazing filler material in our earlier research [10].It

remains in the lower brazing temperature below the β phase transition temperature.

Meanwhile, based on the Ti-Zr phase diagram, Ti and Zr elements are infinitely mutual

soluble with each other. Due to the size difference between Zr and Ti, the increase of Zr

might increase the solid solution hardening. However, the more Zr content present in the

filler metal, the more likely the segregation of elements as Ti,Zr,Cu,Ni occurs. This tendency

was prone to form the intermetallic compounds, as shown in table 2. For example, Pang et al

[15] increased the Zr content to design their novel multicomponent

Ti50Zr27Cu8Ni4Co3Fe2Al3Sn2Si1 (at.%) amorphous BFM which offered improved shear

strength of joints. However, intermetallic compounds were still formed during the brazing

process. This resulted in lower shear strength and ductility compared to our study. Thus,

appropriate control of Zr content is critical for determining the joint strength. The balance of

the solid strengthening, the brazing temperature and the formation of intermetallic

compounds is achieved in this study.

5 ConclusionsIn this study, the designed Ti-based BFMs as Ti-18Zr-15Cu-10Ni wt.% were applied to

balance the brazing temperature, the microstructure and the strength of the joint. The

employed brazing process was 930 °C for 3600s. The strength and the ductility of brazing

joined Ti-6Al-4V samples were evaluated by conventional tensile tests with a DIC 2D-strain

10

1-this study;2-[12];3-[8];4-[3];5-[13];6-[14];7-[13];8-[3];9-[16]

field measurement. Results were confirmed with micro-hardness tests. Microstructures and

phases were analyzed by SEM and EBSD. The following conclusions can be drawn:

1) The fine and interlocking acicular Widmanstätten microstructure (with β lamellar ~ 1um in

width) was found throughout the entire joint section due to the eutectoid reaction. The filler

elements diffused into the matrix with relatively uniform distribution in the joints.

2) Due to the fine acicular Widmanstätten microstructure, the grain with hard orientation and

the solid solution strengthening effects, the ultimate strength of the brazed samples showed

similar strength to the one without brazing joints.

3) No fracture was found at the brazing joints. All the fractures occurred in the matrix with

local tensile stain up to 20%. The localized tensile plastic strain at the braze joint is ~6.3%.

4) Composition of the joint can be described as Ti-(2.3~6)Al-(2.3~3)V-(2.6~5.5)Zr-(2~4)Cu-

(2~3)Ni wt.%, which is significantly different to that of the original filler, both in elements

and content, due to the elements’ diffusion during brazing.

Acknowledgements

The authors would like to appreciate Mr.YongLi, Mr.WeiXiong, Mr.ChongZhao,

Ms.BoChen, Pro.Jianguo Lin and Pro.ShushengShi. Significant support was also received

from the Chinese Scholarship Council of the Ministry of Education and the AVIC Centre for

Structural Design and Manufacture at Imperial College London, which is funded by the

Aviation Industry Corporation of China (AVIC).

References

[1] Shapiro A.E., FlomY.A.. Brazing of titanium at temperatures below 800 °C: review and prospective application. Proceedings of 8th International Conference in Brazing.2007[2] Shapiro A., RabinkinA.. State of the art of titanium-based brazing filler metals. Welding J., 2003. 83 (10). pp: 36–43. [3] Shiue R. K., Wu S. K., Chen Y. T., et al.. 2008. Infrared brazing of Ti50Al50 and Ti-6Al-4V using two Ti- based filler metal metals. Intermetallics. 16(9).pp:1083-1089.[4]JingYongjuan, Li Xiaohong, Yue Xishan. Research and analysis of processing parameter for brazing honeycomb sandwich construction in titanium alloy. Aeronautical Manufacturing Technology, 2012.13.pp:137-139 (in chinese) [5] Elrefaey A., TillmannW.. Effect of brazing parameters on microstructure and mechanical properties of titanium joints. J Mater Process Tech., 2009. 209(10)pp: 4842-4849. [6]Ganjeh E., Sarkhosh H., Bajgholi M.E., et al.. Increasing Ti-6Al-4V brazed joint strength equal to the base metal by Ti and Zr amorphous filler metal alloys. Materials characterization. 2012.71.pp:31-40.

11

[7] Leyens C., Peters M., 2013. Titanium and Titanium Alloys. Wiley-VCH, WeinheimFrick W.R.. 1991. Brazing Handbook. American Welding Society, Miami, Florida [8] Lee M.K., Lee J.G.. 2013. Mechanical and corrosion properties of Ti–6Al–4V alloy joints brazed with a low-melting-point 62.7Zr–11.0Ti–13.2Cu–9.8Ni–3.3Be amorphous filler metal. Materials characterization. 81.pp:19-27. [9] Rabinkin A., Liebermann H., Pounds S., et al.. 1991. Amorphous TiZr - basemetglas® brazing filler metals. Scripta Metallurgic.. 25(2).pp: 399-404. [10]Jing Yongjuan, Yue XiSHan, Gao XingQiang, et al.. 2016. The influence of Zr content on the performance of TiZrCuNi brazing filler. Materials Science & Engineering A. 678 .pp:190-196. [11]Liu Y., Hu J., Zhang Y., GuoZ.. 2013. Interface Microstructure of the Brazed Zirconia and Ti-6Al-4V Using Ti-based Amorphous Filler metal. Science of Sintering., 45.pp:313-321. [12]Botstein O.. Brazing of titanium-based alloys with amorphous 25wt.%Ti-25wt.%Zr-50wt.%Cu filler Metal. Materials Science and Engineering, A.1994. 188.pp: 305-315. [13]Ganjeh E., SarkhoshH.. Microstructural, mechanical and fractographical study of titanium-CP and Ti–6Al–4V similar brazing with Ti-based filler. Materials Science & Engineering A. 2013. 559:119-129.[14]Chang C.T., Wu Z.Y., Shiue R.K., Chang C.S.. 2007. Infrared brazing Ti–6Al–4V and SP-700 alloys using the Ti–20Zr–20Cu–20Ni braze alloy .Materials Letters. 61: 842-845. [15]Pang Shujie, Sun Lulu, XiongHuaping, et al. 2016.A multicomponent TiZr-based amorphous brazing filler metal for high-strength joining of titanium alloy. ScriptaMaterialia., 117.pp:55-59[16]Yue X., He P., Feng J.C., Zhang J.H., et al.. 2008. Microstructure and interfacial reactions of vacuum brazing titanium alloy to stainless steel using an AgCuTi filler metal. Materials characterization. 59(12).pp: 1721-1727.

12