titanium – stainless steel brazing using...

Special Issue | October 2014 219

BARC NEWSLETTERFounder’s DayTITANIUM – STAINLESS STEEL BRAZING

USING AG-BASED ALLOYS

A. Laik and G.K. DeyMaterials Science Division

andA.A. Shirzadi

Materials Engineering, The Open University, Milton Keynes MK7 6AA, United Kingdom

Abstract

A detailed characterization of vacuum brazed joints of titanium and stainless steel type 304L, using three Ag-based

brazing alloys, Cusil, Ticusil and Silver-ABA has been carried out. The intermetallic compounds formed at the braze

zone due to interaction of the joining components with the brazing alloys were identified. The microstructural

evolution at the interfaces and the mechanisms of brazing in the three cases are compared.

This paper received the Best Poster, 2nd Prize (Materials Science) at the International Conference on Electron Microscopy & XXXIV Annual Meeting of EMSI, Kolkata, 3-5 July, 2013

Introduction

Both titanium and stainless steel (SS) are widely used

as engineering materials in industries due to their

excellent mechanical properties as well as corrosion

resistance. The extensive use of Ti and its alloys in

various sectors, for example, aerospace, transportation,

chemical, nuclear and power generation, requires them

to be joined to other materials for integration and

fabrication of various components [1, 2]. Of particular

interest to the nuclear industry, is the application of Ti

in fabricating the dissolvers of spent nuclear fuel used

in reprocessing plants. This requires it to be joined to

the piping of the plant, which is generally made of

stainless steel (SS) [3].

Conventional fusion welding has not been a good

choice since it needs to be performed in inert

atmosphere due to the reactive nature of Ti, and the

significant difference in physico-chemical properties of

the two materials that lead to chemical, mechanical and

structural inhomogeneities [7]. Although the progress

of research on diffusion bonding of SS to Ti with

suitable interlayers has led to some success, formation

of undesirable intermetallic compounds of various

combinations such as Ti–Fe, Ti–Cr, Ti–Ni could not be

completely eliminated [2, 9]. The formation of these

intermetallics produce residual stresses originating

from a mismatch in their thermal expansion [7, 10,

11]. Further, the method is expensive and not an

obvious choice for mass production [8]. In comparison

to these techniques, vacuum brazing offers two major

advantages: (a) low thermal residual stresses in the

joints, as a major part of the stresses generated due to

differential contraction of the two materials is relieved

by plastic deformation of the ductile braze alloys, and

(b) a low compressive stress is required for joining [7].

Dececco and Parks [12] suggested the use of silver

and its alloys to braze Ti and since then various Ag-

based filler alloys have been extensively used for the

brazing of Ti and its alloys, primarily due to good flow

characteristics of these filler alloys and high strength

and adequate ductility of these brazed joints.

The present study compares the brazing characteristics

of stainless steel/titanium brazing using different Ag-

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CONTENTS

220 Special Issue | October 2014

BARC NEWSLETTERFounder’s Daybased brazing alloys. The mechanisms of brazing and

microstructural evolution at the interfaces are also

determined.

Experimental Procedure

The base materials used in the experiments were

annealed plates of commercially pure grade-2 titanium

(ASTM B265) and austenitic stainless steel (SS 304L).

The nominal compositions of these materials are given

in Table 1. Specimens of Ti grade 2 and SS 304L, in

the form of cylindrical pieces with dimensions of 35

mm length and 14 mm diameter were vacuum brazed

using 50 mm thick foils of Ag-based alloys Cusil, Ticusil

and Silver-ABA at temperatures 835 oC, 920 oC and

930 oC, respectively. The nominal compositions of

these alloys are given in Table 2. The joining faces

of the pieces were ground with successive grades

of emery papers up to 1200 grit, followed by 1 mm

diamond polish. Prior to brazing, all the components

were ultrasonically cleaned with ethyl alcohol followed

by acetone. The brazing operation was carried out in a

furnace under vacuum of 5 X 10-5 mbar.

scanning electron microscope (SEM), energy dispersive

spectrometry (EDS), electron probe microanalyser

(EPMA) and transmission electron microscope (TEM).

Results

Brazing with Silver-ABA

A typical backscattered electron (BSE) image showing

the microstructure of the cross section of the BZ of

the SS-Ti joint, brazed at 930 oC for 15 minutes using

Silver-ABA alloy is shown in Fig. 1. It was noted that

the interfaces between the braze alloy and the base

materials, on either side, were free from pores, cracks,

or discontinuities. The entire BZ, may be divided into

four zones A-D, as marked in Fig. 1. X-ray maps of

elements Fe, Ni, Cr, Ag, Cu, Al, and Ti along with the

corresponding backscattered electron (BSE) image of a

typical region of the BZ is shown in Fig. 2.

On the SS-side, at the interface between SS and braze

alloy, a layer A of solid solution consisting of about 5

at. % of Al and 21 at. % of Cr in Fe along with 5.5 at.

Elements C H O Mn Si Ni Cr Fe TiTi Gr-2 0.009 0.008 0.12 --- --- 0.011 --- 0.09 Bal.SS 304L 0.02 --- --- 1.47 0.52 10.6 19.1 Bal. ---

Table 1: Nominal chemical composition of the base materials (in wt. %).

Table 2. Nominal chemical composition of the Ag-based brazing alloys used (in wt. %).

Elements Ag Cu Ti AlCusil 72 28 --- ---Ticusil 68.8 26.7 4.5 ---Silver-ABA 92.75 5 1.25 1

The joints was sectioned in the direction perpendicular

to the bond interface, using slow speed diamond blade

and the cross-sections were prepared for microscopic

examination using standard metallographic

techniques and were subsequently etched with a

solution of 5% HF, 40% HNO3 and 55% H2O, by

volume. The microstructure of the braze zone (BZ)

was characterized using an optical microscope, and a

Fig. 1: BSE micrograph of the Ti-SS joint brazed using Silver-ABA alloy at 930 oC. A: Fe(Al), B: unreacted layer, 1: (Ni,Fe)2TiAl, 2: Cu4Ti, 3: CuTi, 4: AgTi, 5: (Ag,Cu)Ti2, 6: a-Ti + CuTi2 eutectoid layer, 7: a-Ti Plates.


BARC NEWSLETTERFounder’s Day

% Ni. Layer B was identified as the unreacted braze

alloy, consisting mainly of Ag and Cu. Due to solid-

liquid interaction on the Ti-side of the BZ, parallel

layers of intermetallic compounds (IMC)

with planar morphology, collectively

marked as zone C, were formed.

These IMCs were identified as Cu4Ti,

CuTi, AgTi, and (Ag,Cu)Ti2 [13]. On

the Ti-side of the BZ, Cu, Ag, and Al

were found to diffuse into the Ti-base

material up to a depth of about 40

mm, labeled as zone D. In most of the

regions of this zone, a fine lamellar

two-phase microstructure consisting of

a-Ti and CuTi2, marked as 6, formed in

colonies. Fig. 3 shows the bright field,

dark field micrographs and SADP of the

eutectoid structure as acquired in TEM.

The lamellae measured almost 5 to 6 mm

in length, and the interlamellar spacing

was about 600 nm. The orientation

relationship between the phases is

(0001)a|| (0-13)CuTi2 and [2-1-10]a ||

[-100]CuTi2 [13]. The CuTi2 phase was found to be

internally twinned as evidenced in Fig. 3.

Fig. 2: X-ray maps of elements Fe, Ni, Cr, Ti, Al, Cu and Ag along with the BSE image of a region at the SS-Ti joint interface, showing the distribution and variation in the relative concentrations of these elements in the BZ.

Fig. 3: Bright field and dark field micrographs of the eutectoid structure, labeled 6 in Fig. 1, along with the selected area diffraction pattern and its key.


BARC NEWSLETTERFounder’s DayBrazing with Cusil

The microstructure of the interface between SS

and Ti in case of SS-Ti joints brazed at 835 oC for

10 min, using Cusil alloy. The basic scheme of the

microstructure was similar to that of the Silver-ABA

brazed joints. Fig. 4 shows a typical BSE image of the

interface. The formation of layers of IMCs is evident

from the micrograph. On the SS side, a layer marked

1 with composition Cu-37 % Ti-10% Fe was found to

form as continuous layer. On the Ti side, three binary

Cu-Ti IMCs, viz., Cu3Ti2, Cu4Ti3 and CuTi, as marked

as 3, 4 and 5 in Fig. 4, formed as contiguous layers.

Islands of bright phase, labeled 2 in Fig. 4, showed to

be almost pure Ag with about 9 % Cu and 1.5 % Ti.

This phase was hence identified to be solid solution

Ag(Cu), which remained unreacted in the process of

brazing [14]. The compositions of these phases were

determined using electron microprobe analysis.

Adjacent to the CuTi layer, on the Ti side, a 10 mm

thick two-phase layer formed. It was evident from

its lamellar morphology that it was a eutectoid layer,

formed due to phase transformation during cooling.

Microanalysis results showed that the layer contained

substantial amount of Cu. Therefore, during the

process of brazing, Cu had diffused into Ti to a depth

of about 10 mm. Being a b-stabiliser, it had promoted

the formation of a layer of b-Ti even at temperatures

lower than the a-b transus. Laik et al. [15] have

recently demonstrated the formation of such b-Ti layer

below the a-b transus in Cu-Ti diffusion couples. The

structure was resolved in FESEM and EDS analysis was

used to identify the structure as lamellar aggregate of

b-Ti and CuTi2.

Brazing with Ticusil

Fig. 5 shows a BSE image of of the interface between

SS and Ti in case of SS-Ti joints brazed at 920 oC for 10

min, using Ticusil alloy. Although the microstructure

seems quite identical to that of Cusil brazed joint, the

number of IMCs formed in the BZ is not as many. Apart

from the layer of Cu-37Ti-10Fe adjacent to SS, marked

as 1 in Fig. 5, the only other IMC layer that formed in

the BZ is CuTi (layer 3). Islands of unreacted Ag were

Fig. 4: BSE micrograph of the Ti-SS joint brazed using Cusil alloy at 835 oC. 1: Cu-37Ti-10Fe, 2: Ag(Cu), 3: Cu3Ti2, 4: Cu4Ti3, 5: CuTi, 6: a-Ti + CuTi2. The eutectoid layer 6 is shown at higher magnification in the inset.


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also found to form in this case, between the Fe rich

layer 1 and that of CuTi. The fraction of unreacted

alloy is quite substantial as compared to Cusil. Cu was

found to have diffused to a depth of about 50 mm

inside Ti, because of higher temperature of brazing.

A similar a-Ti + CuTi2 eutectoid layer formed during

cooling of the b-Ti(Cu) solid solution phase from the

brazing temperature. However, some elongated plates

of b-Ti formed on the Ti side of the eutectoid layer,

which can be attributed to partitioning of Cu to the

b-phase since the solubility of Cu in the b-phase in

room temperature is negligible. The interlammelar

spacing in the eutectoid aggregate in this case is finer

than that in Cusil braze joint [14].

Discussion and Summary

The microstructures of the SS/Ti joint interfaces

revealed that the interfaces between the braze alloy

and the base materials, on either side, and the braze

zones were free from pores, cracks, or discontinuities,

in all the three cases. It was also noted that in all

the three cases the BZ consisted of layered structure

due to the formation of intermetallic compounds by

reaction of the substrate materials and the braze alloy.

Although the basic microstructure of the BZ remained

identical, the nature of the intermetallic compounds

varied with the braze alloy. In the case of Silver-ABA

brazing the compounds were identified as (Ni,Fe)2TiAl,

Cu4Ti, CuTi, AgTi and (Ag,Cu)Ti2, and those in Cusil

brazing were Cu3Ti2, Cu4Ti3 and CuTi formed, while

during brazing with Ticusil primarily CuTi formed in

the reaction zone.

A part of the brazing alloy remained unreacted in

the BZ in all the cases. The fraction of the unreacted

alloy was quite substantial in Silver-ABA brazing, and

consisted of a major portion of the BZ. However, this

fraction in Cusil brazing was small and the BZ consisted

primarily of the intermetallic compounds. A significant

feature of the microstructure is the formation of

eutectoid layer consisting of lamellae of (Cu,Ag)Ti2 in

a-Ti matrix. The orientation relationship between the

two phases was determined using composite SADP

in TEM. The CuTi2 phase was found to be internally

twinned. The variation in the width of this eutectoid

layer can be attributed to the difference in the brazing

conditions in different cases.

Fig. 5: BSE micrograph of the Ti-SS joint brazed using Ticusil alloy at 920 oC. 1: Cu-37Ti-10Fe, 2: Ag(Cu), 3: CuTi , 4: a-Ti + CuTi2. The eutectoid layer 4 is shown at higher magnification in the inset.


BARC NEWSLETTERFounder’s DayAcknowledgements

The authors are thankful to Shri P. Mishra, Materials

Processing Division, for providing the Silver-ABA foils.

The authors would also like to thank Dr. R. Tewari for

the TEM analysis of the samples. The assistance of Shri

P. G. Adiga in conducting the brazing experiments is

gratefully acknowledged.

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