inter diffusion involved in shs welding

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Interdiusion involved in SHS welding of SiC ceramic to itself and toNi-based superalloy

Shujie Li a,*, Huiping Duan a, Shen Liu a, Yonggang Zhang a, Zijiu Dang b,Yan Zhang b, Chengang Wu a

a Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, People's Republic of Chinab State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, People's Republic of China

Received 26 October 1999; accepted 24 January 2000

Abstract

In order to reach a better understanding of the mechanism of joining of SiC ceramic to itself and to Ni-based superalloy by

pressure-aided self-propagating high-temperature synthesis (SHS) welding process, microstructural study and energy dispersive X-

ray microanalysis (EDX) were carried out with the welded samples. The wettability between SiC ceramic and the liquid reaction

products of the ller is very good. A composition transition layer forms at the SiC/ller interface mainly due to the diusion of Ti

from the ller to the ceramic. The anity between Ti and SiC is better than that between Ni and SiC. The components of the Ni-

based superalloy remarkably diuse into the reaction products of the ller. The microstructure shows an excellent bonding at the

interfaces. 2000 Published by Elsevier Science Ltd.

Keywords: Diusion; Multilayer structure; Powder technology

1. Introduction

Ni-based superalloys are widely used to manufacture

the parts of aeronautical turbine serving at elevated

temperatures. The working temperature of these mate-

rials can reach 950C (type GH128) or higher. But, it is

still not high enough for the next new turbines. In order

to further increase the working temperature and de-

crease the weight of the turbines, new high temperature

structure materials are being expected and investigated.

Among them, SiC ceramic and SiC matrix composite are

promising ones. However, one problem arising with the

application of SiC is joining of the ceramic to metals,

particularly Ni-based superalloys. As indicated by

Ref. [1], the application of ceramic materials normally

depends on the joining of ceramics to metallic structures,

as we are living in a technological world based on the

application of metals.

Self-propagating high-temperature synthesis (SHS) of

powder compacts is a novel processing technique being

developed as a route for the production of engineering

ceramics and other advanced materials [2]. This process

seems to oer just the sort of innovative new method for

welding of ceramics and intermetallics. SHS oers many

attractive features such as energy eciency, limitation of

thermal disruption of heat-sensitive substrate micro-

structures, due to rapid and highly localized heat gen-

eration, easy availability of chemical compatibility

between the reaction products and substrates, as the

process being used to produce the joint is often the same

as the process that was or could have been used to

produce the substrates, the possibility to form a com-

posite ller incorporating reinforcing phases such as

particles, chopped bres or whiskers, and the possibility

to form functionally gradient material (FGM) joints

promising for overcoming mismatches between the

chemical composition, physical and mechanical prop-

erties of dissimilar materials [3,4]. Although SHS weld-

ing of ceramics to metals is currently being in the stage

of laboratory investigation, it has shown broad pros-

pects from both technical and economical points of

view. This paper will deal with the interdiusion and

microstructures relevant to SHS welding of SiC ceramic

to itself and to Ni-based superalloy.

International Journal of Refractory Metals & Hard Materials 18 (2000) 3337

* Corresponding author. Tel.: +86-10-82314972; fax: +86-10-

82316100.

E-mail address: [email protected] (S. Li).

0263-4368/00/$ - see front matter 2000 Published by Elsevier Science Ltd.

PII: S 0 2 6 3 - 4 3 6 8 ( 0 0 ) 0 0 0 0 8 - 1


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2. Experimental

The commercial recrystallized SiC ceramic with the

density of 2.65 gacm3, the porosity of 1516% and the

purity ofb 99 wt% was used as the starting ceramic tobe welded. The forged Ni-based superalloy (type

GH128) was used as the starting metallic material to be

welded. This alloy contains Cr(19.022.0 wt%), W(7.5

9.0 wt%), Mo(7.59.0 wt%), Ti(0.40.8 wt%), Al(0.40.8

wt%) and other elements with low content. The utilized

welding materials (llers) and their purities are as fol-

lows: Ti powder (99.23 wt%), Ni powder (99.7 wt%) and

Al powder (99.6 wt%). The particle size of the metallic

powders is 200 mesh. C powder was also used as acomponent of the welding materials. Its particle size is

0.11 lm.

The SiC ceramic and the Ni-based superalloy were

machined to cylindrical billets with the size /10 mm

5 mm. The surface for welding was polished. The SiCbillets were washed in NaOH solution with an ultrasonic

bath for 15 min, then washed in pure water. The Ni-

based superalloy billets were washed in acetone with the

same ultrasonic bath for 15 min too. Then the cleaned

billets were put in a desiccator.

The powders with designed composition were mixed

homogeneously and then cold compacted to form

compacts with the size /10 mm (11.5) mm, whichwere used as welding materials. The SiC ceramic billet,

the powder compact and the Ni-based superalloy billet

were loaded into a graphite die. Then, pressure-aided

SHS welding test was carried out by a thermo-me-chanical testing machine, type Gleeble 1500, as sche-

matically shown in Fig. 1. This is a completely

computer-controlled apparatus. The samples were

heated up by passing an electrical current, and the at-

tainable maximum temperature is 1500C. As SiC ce-

ramic is an electrical insulator, the electrical current was

transferred by the graphite die. The temperature of the

sample was measured by the thermocouple as shown in

Fig. 1 and controlled automatically.

In order to facilitate the interfacial reactions between

the SiC ceramic and the powder compact, it was de-

signed that the compact includes a reaction layer

neighboring with the ceramic, which contains sucient

active element. This layer should be as thin as possible

for preventing the welded samples from reducing of the

properties. As the Gleeble 1500 is not equipped with

ignition facilities, it is necessary that the compact also

includes an ignition layer, of which the ignition tem-

perature is lower than the attainable maximum tem-

perature of the machine. It should be noticed that the

presence of the reaction products of the ignition layer

should not cause the decrease of the properties of the

welded samples.

As indicated in Ref. [4], the degree of mismatch be-

tween the coecients of thermal expansion of the joint

materials and the ller should be smaller than about 15

25% in order to avoid joint failure. This should be fol-

lowed in designing the FGM llers. However, for the

sake of convenience to investigate the interfacial diu-

sion, interfacial reaction and microstructure, a simpliedFGM ller was studied in this paper, of which the

composition is listed in Table 1.

The temperature system tested is as follows:

Room temperature 35KaS

1273 Kkeep for 5 min

32KaS

673 K 3cooling in furnace

473 KX

The SHS welding test was conducted at constant pres-

sure 10 MPa in vacuum 35103 Torr. The micro-structure and composition of the welded area were

analyzed by scanning electron microscope (SEM) and

energy dispersive X-ray microanalysis system (EDX).The same process was utilized in welding SiC ceramic

to itself. The composition of the llers tested is presented

in Table 2. SEM and EDX analyses were also carried

out to investigate the microstructure and composition of

the welded interfaces.

3. Results and discussion

3.1. SHS welding of SiC ceramic to itself

3.1.1. Experimental results

The experimental results of SHS welding of SiC ce-

ramic to itself are presented in Table 2. (The sign `0'

denotes success of welding, `x' denotes failure.) This

table also shows the composition of the llers and the C

percentage of each combination. One can see that every

ller can reach a successful welding except ller No. SS-

6. It is noticed that the C content of this ller is quite

high. C may react precedently with the active compo-

nent Ti to form stable TiC, which consumes the active

component. Besides, the presence of TiC may decrease

the mobility of the remaining active components leading

to reducing the opportunity of contact between SiCFig. 1. Sketch map of SHS welding of SiC ceramic to Ni-based

superalloy.

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ceramic and the active components. This may be the

reason for the failure of welding.

3.1.2. Wettability

The SHS welding materials as shown in Table 2 will

form a great quantity of liquid phase during the thermal

explosive reaction. The wettability between the liquidand SiC ceramic is very important with regard to eec-

tive interfacial reactions, interdiusion and even a suc-

cessful welding.

Fig. 2 shows the microstructure at the interface be-

tween SiC ceramic and the reaction products of ller

No. SS-2. The black phase on the right is SiC ceramic. It

can be seen that the wettability between the liquid

formed during the thermal explosive reaction and SiC

ceramic is very good, which provides the possibility of

full contact, interdiusion, interfacial reactions and

formation of chemical bonding. Also, one can see that

many open pores exist at the surface of the ceramic. The

liquid can penetrate into the pores under the capillary

and external pressures, forming mechanical bonding

between the ceramic and the reaction products aftersolidication. This enhances the strength of joining at

the interface.

The experimental results indicate that for all the llers

except No. SS-6 listed in Table 2 the liquid formed

during SHS reaction can wet SiC ceramic and result in

mechanical bonding with SiC ceramic after solidica-

tion.

3.1.3. Interdiusion and interfacial reaction

The strength of joining at the interface between SiC

ceramic and the reaction products of the llers is

strongly aected by the interdiusion, the interfacial

reactions and the chemical bonding. As reported in Ref.

[5], all the active elements such as Ti, Ni and Al can

react with SiC ceramic.

The microstructure and the distribution of Si, Ti and

Ni at the interface and its neighboring areas between

SiC ceramic and the reaction products of ller No. SS-

3(Ni:Ti:C 3:2:0.5) are presented in Fig. 3. The blackphase on the left is SiC ceramic. There are no open pores

at the local surface of the ceramic. The reaction prod-

ucts are mainly composed of the grey Ti-rich phase and

the white Ni-rich phase. The distribution of Si, Ti and

Ni along the white line in the photograph shows gradualFig. 2. SEM micrograph of the interface of SiC ceramic/reaction

products of ller No. SS-2.

Table 2

Composition of llers (atomic ratio) and results of SHS welding of SiC ceramic to itself

No. of ller Ni Ti Al C Percentage of C (at.%) Result of welding

SS-1 7 2 0.5 5.26 0

SS-2 2 5 0.5 6.67 0

SS-3 3 2 0.5 9.09 0

SS-4 2 3 0.5 9.09 0

SS-5 7 2 0.5 5.26 0SS-6 0.5 2 1 28.57 SS-7 3 2 0.5 9.09 0

SS-8 2 3 0.5 9.09 0

Table 1

Composition of FGM welding materialsa

Reaction layer Second layer Ignition layer

Composition Ti Ti1C1 20Ni Ti1C1 40Ti1Ni1Weight of powders (g) 0.03 0.5 0.5

a Note: Ti1C1 20Ni denotes that the atomic ratio Ti X C 1 X 1, the weight of Ti powder plus C powder is 80 wt% of the total mixed powders, theweight of Ni powder is 20 wt% of the total.

S. Li et al. / International Journal of Refractory Metals & Hard Materials 18 (2000) 3337 35


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changes at the interface, which indicates the presence of

a composition transition area (i.e., reacted layer) at the

interface. From the gure, one can also see that the

position of the composition transition area shifts to SiC

ceramic. The results show that the composition transi-

tion area forms mainly due to the diusion of Ti from

the ller to the ceramic. This is in accordance with the

fact that the diusion coecient of SiC is very small as

this chemical compound is dominated by covalent bond.

In addition, it can be seen that the grey Ti-rich phase

forms at the interface, indicating the precedent concen-

tration of Ti at the interface during welding. Therefore,

it can be concluded that the anity between Ti and SiC

is better than that between Ni and SiC.

For all the llers except No. SS-6 listed in Table 2, the

interdiusion takes place during SHS welding as the

llers contain sucient active elements.

3.2. SHS welding of SiC ceramic to Ni-based superalloy

3.2.1. Microstructure of interface

The microstructure of interfaces including the in-

terface of Ni-based superalloy/reaction products, the

interfaces between the dierent layers of the reaction

products and the interface of reaction products/SiC

ceramic is presented in Fig. 4. The microstructure

shows that the bonding at the interfaces is excellent,

especially at the interface of Ni-based superalloy/re-

action products and the interface of reaction products/

SiC ceramic.

The chemical composition of the 4 points a,b,c,d as

shown in Fig. 4(a) was analyzed by EDX, and the results

are listed in Table 3. The results clearly prove that the

components of the Ni-based superalloy, Cr, W and Mo,

remarkably diuse into the reaction products of the

ller.

Fig. 4. SEM micrographs of the interfaces (a) Ni-based superalloy/

FGM ller; (b) between dierent layers of FGM ller; (c) FGM ller/

SiC ceramic.

Fig. 3. SEM micrographs of the interface of SiC ceramic/reaction

products of ller No. SS-3 and the distribution of element (a) Si, (b) Ti,

(c) Ni.

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3.2.2. Result of SHS welding

As shown in Fig. 4, excellent bonding exists at the

interfaces within the total welded area. This result in-

dicates that the joining of SiC ceramic to Ni-based su-

peralloy has been realized by SHS welding. However, as

stated in Section 2, for the sake of convenience to in-

vestigate the interdiusion, interfacial reaction and mi-

crostructure, a simplied FGM ller was utilized, which

is not sucient for overcoming the thermal stress caused

by the mismatch between the thermal expansion coe-

cients of the welded dissimilar materials, leading to the

generation of microcracks in the ceramic near the in-

terface of SiC ceramic/reaction products of the ller.

The experiments to overcome the thermal stress using

FGM ller consisting of multi-layer together with other

measures are well under way.

4. Conclusions

1. The liquid phase formed in SHS reaction of theNi+Ti+C (less than 9 at.%) series llers can wet

SiC ceramic. The liquid can penetrate into the open

pores of the ceramic, forming mechanical bonding

between the ceramic and the reaction products after

solidication.

2. The liquid phase formed in SHS reaction of the series

llers reacts with SiC ceramic at the interface. This

interfacial reaction is realized mainly by the diusion

of Ti from the ller to the ceramic. Successful SHS

welding of SiC ceramic to itself has been achieved uti-

lizing this series llers.

3. The anity between Ti and SiC ceramic is better than

that between Ni and SiC ceramic.

4. SiC ceramic has been joined to Ni-based superalloy

by SHS welding with the simplied FGM ller. The

bonding at the interfaces is excellent. Interdiusion

takes place between the ller and the welded materi-

als.

Acknowledgements

The authors gratefully acknowledge the nancial

support of the project by the Aeronautical Science

Foundation of China and the National Natural Science

Foundation of China (Grant No. 59881001).

References

[1] Nicholas, MG. Joining Structural Ceramics, In: Peteves SD,

editors, Designing Interfaces for Technological Applications:

CeramicCeramic, CeramicMetal Joining. Amsterdam: Elsevier,

1991, p. 49.

[2] Yi HC, Moore JJ. Review self-propagating high-temperature

(combustion) synthesis(SHS) of powder-compacted materials.

Journal of Materials Science 1990;25:115968.

[3] Orling TT, Messler RW, Jr., Fundamentals of the SHS joining

process. In: Carim AH, Schwartz DS, Silberglitt RS. editors,

Proceedings of the 1993 MRS Symposium on Joining and

Adhesion of Advanced Inorganic Materials, MRS, Pittsburgh,

1993;314:17783.

[4] Messler RW, Jr., Zurbuchen MA, Orling TT, Welding with self-

propagating high-temperature synthesis. Welding Journal Oct.

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[5] Rabin BH. A Review of Silicon Carbide/Metal Interactions with

Relevance to Silicon Carbide Joining, EGG-MS-9633, DE91

012768, EG&G Idaho Falls, ID 83415, 1991.

Table 3

Composition at the interface of Ni-based superalloy/reaction products

of the ller (at%)

Position Ti Cr Ni Mo W

a 0.48 19.00 62.51 9.01 9.00

b 51.86 10.89 5.02 17.12 15.12c 72.92 12.94 5.34 4.43 4.37

d 90.70 2.13 2.77 2.43 1.97

S. Li et al. / International Journal of Refractory Metals & Hard Materials 18 (2000) 3337 37

inter diffusion involved in shs welding

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