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4 SULZER TECHNICAL REVIEW 1/2008 4228

SUBRAMANIAM RANGASWAMY

DIENTJE FORTUNA

SULZER METCO

Nickel-base braze filler metals are widely

used for brazing high-temperature plate-type

heat exchangers and metallic catalytic

converters in automotive applications.

The recent emission/gas regulations and

the development of efficient exhaust-gas

recirculation (EGR) coolers by the auto-

motive industry increased the demands on

the braze filler metals, which need to have

higher resistance to hot oxidation and

corrosive atmospheres. Sulzer Metco

introduced a new family of boron-free braze

filler metal compositions containing high

amounts of chromium. These new filler

metals offer technical and economical

advantages in fabricating flat-plate type

heat exchangers and metallic catalytic

converters.

Corrosion-ResistantBraze Joints

Novel, High-Chromium Containing Braze Filler Metals

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Many of the conventional

braze filler metals used for

automotive applications, such as,

heat exchangers, catalytic convert-

ers or exhaust-gas coolers, have

technical limitations (Fig.1). BNi-2

braze filler metal, e.g., contains a

relatively high amount of boron as

an alloying element. During the

brazing process, the boron diffus-

es into the thin stainless-steel sheet

metal and subsequently reduces

its strength. For this reason, boron-

free filler metals are recommend-

ed for brazing many stainless-steel

heat exchangers. Another conven-tional filler metal, BNi-5, does not

contain any significant amount of

boron, but it has a relatively high

melting point. The high tempera-

ture necessary for its processing

induces grain growth in the stain-

less-steel base metals during the

brazing cycle, which may degrade

the strength of the sheet metal af-

ter brazing is completed.

Whether boron-free or not, many

of the conventional braze filler

metals contain a high percentage

of nickel. Extreme volatility in the

price of nickel has significantly af-

fected the cost and affordability of

these materials.

New Materials Developed

Sulzer Metco directed significant

effort towards developing new

braze filler metals to overcome

some of the technical and commer-

cial limitations of conventional

nickel-base filler metals. The ma-

terial experts of the division devel-oped 3 new compositions, expand-

ing to the proven Amdry product

line:

Nichrome-base Amdry 105

Inconel-base Amdry 108

High-chrome, stainless-steel-

base Amdry 805

All of the compositions contain

high amounts of chromium (at

least 23% of weight) and employ

3 Amdry 108 electrolytically etched. The material contains

15% (weight) iron in the chemical formulation.

2 Amdry 105 electrolytically etched. The material

contains no iron in its composition.

1 Automotive parts such as heat exchangers have to withstand hot

and corrosive gases and liquids, which imposes stringent requirements

also on the braze materials used. Sulzer Metco introduced a set of

improved braze materials for such applications.controlled amounts of silicon and

phosphorus alloying elements to

depress the melting points.

High Content of Chromium

Metallographic examinations of all

the filler metals were performed

on solid buttons prepared by melt-

ing the powder in a ceramic cru-

cible in a vacuum furnace. Optical

micrographs revealed 3 distinct

phases in each of the alloys: a ma-

trix phase, a primary precipitated

phase, and a eutectic-like struc-

ture. The in-depth metallurgical

analysis confirmed that all phases,

including the matrix, contain sig-nificant amounts of chromium.

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Amdry 105 contains a nickel-

chrome silicon-rich matrix and 2

major phasesnickel-chrome sili-

cide and a chrome-nickel silicide

(Fig. 2). Amdry 108, which con-

tains 15 weight percent iron in the

chemical formulation, has a simi-

lar microstructurea nickel-based

matrix with significant amounts of

Cr and Si, a major phase of NiCrFesilicide and a eutectic structure

comprised of a silicide and a

CrNiP phase (Fig.3). Amdry 805

appears somewhat different

(Fig. 4). It contains a FeCr-rich

gamma matrix, a major phase

comprised of a FeCrNiP composi-

tion and at least 2 more minor

phases which appear to be a com-

bination of a silicide and a

phosphorus-rich phase (Fig.5).

Application Test

In addition to the metallurgical

phase analyses, Sulzer Metco con-

ducted tests to evaluate the suit-

ability of these filler metals for

brazing applications:

Measurement of solidus and

liquidus temperatures by

differential thermal analysis(DTA)

Vacuum furnace brazing of a

T-joint and metallography of

the joints

Corrosion tests in several

aqueous solutions

High-temperature oxidation

tests at 815C (1500F)

The liquidus temperature is the

temperature above which no crys-

tals are present in a material and it

is homogenously liquid, whereas

the solidus temperature quantifies

the point at which a material is

completely solid. Between these

temperatures, both solid and liq-

uid phases exist and the materialbehaves like a slurry.

The temperature differential be-

tween the solidus and liquidus in

the new alloys is on the order

of 2030 C as compared with

3055C in conventional alloys

such as BNi-2 and BNi-5. Such a

narrow gap is an indication of the

high flowability of the alloy at

brazing temperatures. This flowa-

bility enables tight joints with thehighest strengths.

Brazing on Many Materials

The new filler metals were applied

to a stainless steel T-joint with a

0.0508mm (0.002 inch) gap and

vacuum-furnace brazed (Fig. 6).

The joints showed that all the

alloys had excellent flow proper-

ties and filled the gaps with clean

fillets. Similar microstructures

were obtained for all the new filler

metals. It may be noted that the

vacuum brazing atmosphere must

be carefully controlled for these al-

loys. Specifically, these alloys con-

tain high amounts of oxygen-sen-

sitive elements such as chromium

and silicon. Also, they do not con-

tain fluxing elements, such as

boron, that would inherently tol-erate some oxygen partial pres-

sures in the vacuum furnace. Braz-

ing temperatures 50C (90F)

above the liquidus will ensure ex-

cellent melting and flow of the

filler metal into the joint gaps. Sim-

ilar experiments have demonstrat-

ed that the new filler metals braze

well on several ferrous base met-

als, nickel-alloy base metals and,

in some cases, even copper base

metals.

Corrosion-Resistant

In order to confirm the suitability

of the new materials for exacting

automotive applications, SulzerMetco studied the resistance of

these new alloys to several types

of corrosive media. Separate

brazed coupons were immersed in

a 10% solution of sulfuric acid, a

10% solution of hydrochloric acid,

and a saturated salt solution. The

coupons were immersed at room

temperature for 150 hours and ex-

amined under the microscope for

pitting, etching, or other possiblecorrosive damage. Metallographic

examinations did not reveal any

corrosive attack on the braze-joint

samples after the long test dura-

tion. They were also examined for

stability and strength before and

after the immersion test.

High-temperature oxidation re-

sistance was measured by expos-

ing melt buttons to a temperature

of 815C (1500F) for 24 hours and

measuring for weight gain. The re-

sults clearly indicate there was no

significant weight change com-

pared with the pre-oxidation

weight of the test samples.

In all cases, the brazed samples

of Amdry 105, Amdry 108, and

Amdry 805 exhibited excellent

stability and minimal degrada-

tion. It is believed that the pres-ence of at least 23% chromium and

4 Micrograph of electrolytically etched Amdry 805.

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5 The phase analysis of Amdry 805 showed a matrix (1, 2, 3),

a major phase (4, 5, 6), and secondary phases (7, 8, 9).

Contact

Sulzer Metco (US) Inc.

Dientje Fortuna

1972 Meijer Drive

Troy, MI 48084

USA

Phone +1 248 288 3113

Fax +1 248 288 4162deni.fortuna@sulzer.com

6% silicon in these alloys enhances

their oxidation and corrosion re-

sistance.

Reduced Material Costs

The 3 new alloysAmdry 105,

Amdry 108, and Amdry 805be-

long to a new family of boron-free,

high-chromium containing filler

metals that use relatively small

amounts of silicon and phospho-

rus as melt depressants. Braze tests

show that these alloys have melt-

ing points within the range of con-

ventional nickel braze filler metals

and are capable of producing ex-cellent braze joints that can with-

stand the corrosive and oxidation

conditions expected in modern

high-temperature applications like

heat exchangers and catalytic con-

verters.

Among these alloys, Amdry 105, a

nickel-base filler metal, was devel-

oped as a benchmark. Subsequent

alloys (Amdry 108 and Amdry

805) were developed to maintainthe high performance of Amdry

105, but to reduce the cost of the

filler metal. This goal was

achieved by replacing a portion of

nickel with iron. Based on the sig-

nificantly lower cost of iron com-

pared with nickel, the raw materi-

al costs for Amdry 108 and Amdry

805 will be 30% and 70% less than

those of conventional BNi-2 and

BNi-5 alloys.

Customized for

Easy Application

Based on the technical perform-

ance and cost advantages, these

new alloys are especially suited for

brazing in automotive applica-tions. These new filler metals offer

technical and commercial advan-

tages particularly in the manufac-

ture of brazed stainless-steel heat

exchangers and metallic catalytic

converters. Amdry 105, Amdry

108, and Amdry 805 can be pur-

chased as powder, paste, custom-

sized tape, and customized pre-

forms for ease of application and

improved reproducibility that re-duces production costs.

6 A typical microstructure of the

examined braze joints.