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Fabrication of Ni-Al Intermetallic Compounds on the Al Casting alloy by SHS Process G.S. Cho*, K.R. Lee*, K.H. Choe*, K.W. Lee* and A. Ikenaga** *Advanced Material R/D Center, KITECH, 994-32 Dongchun-dong, Yeonsu-gu, Incheon, 406-130, Korea * Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan

Abstract Combustion synthesis with the advantages of time and energy savings has been recognized as an attractive alternative to the conventional methods. In order to improve the surface properties of Al casting components, Ni-Al intermetallic compounds are fabricated by the SHS(Self-propagating High temperature Synthesis) process using the reaction heat of the elemental powders. Three kinds of nickel aluminides, Ni3Al, NiAl and NiAl3 were produced by the emission heat from the Al molten metal and a coating layer of intermetallic phase was simultaneously formed on Al casting alloy surface. Microstructure and phase formation behavior of Ni-Al based intermetallic compounds synthesized by combustion reaction were investigated in terms of thermal and phase analysis using scanning electron microscope(SEM), energy dispersive x-ray spectrometer (EDS), Electron Probe Micro Analyzer (EPMA) and X-ray diffractometer(XRD). The microstructures of nickel aluminides were varied by casting temperature, mixing condition and mixing ratio of elemental powders. The reaction layer formed between Al alloy and the powder mixtures of nickel aluminide was observed to be different depending on process conditions such as green density of elemental powders, molten metal temperature and chemical composition. The differences of junction behavior with pouring casting alloys were compared and the schematic model of bonding reaction was defined. Key words Ni-Al intermetallic compounds, combustion reaction, casting process, Al alloy, reaction layer.

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Introduction Nickel aluminide intermetallic compounds are regarded as promising candidates for the development of the next generation of high-performance high-temperature structural materials[1,2]. In the Ni-Al systems, there are three typical intermetallic compounds, NiAl3, NiAl and Ni3Al. Lately, considerable interest has been focused on the reactive synthesis of these materials. In reactive processing, two intimately mixed metallic reactants A and B react to form an intermetallic product AxBy. If the reaction is highly exothermic, the process is called self-propagating high temperature synthesis(SHS)[3,4]. By utilizing the SHS reaction, it is expected that near net shaped compound can be obtained from the elemental powder. If Ni-Al intermetallic compound is formed on the surface of casting alloy, it is anticipated that its casting surface will be improved and show good wear resistance. In the previous work[5,6], Ni-Al intermetallic compounds were formed onto the solid substrate by hot pressing method for joining intermetallic compound and for surface modification via solid-state process. The solid-state joining process needs large equipment and high cost to apply it on the commercial components. In the present work, by setting the mixture of elemental Ni and Al powders in a casting mold, the powder mixture reacted to form Ni-Al intermetallic compound by SHS reaction ignited by the heat of molten casting metal and simultaneously bonded with the Al casting alloy. By the presence of hard intermetallic compound on Al casting surface, wear property will be remarkably improved. By applying the combustion synthesis to the dissimilar bonding, it is expected that joining can be easily achieved under easier bonding condition, i.e., lower bonding temperature or shorter bonding time. We attempted to investigate in-situ joining method for surface modification via the formation of Ni-Al intermetallic compound on the Al casting alloy. Experimental Table 1 shows the chemical composition of Al casting alloy. The sizes of Ni, Al and Si elemental powders with 99.9% purity were 3, 3 and 10µm respectively, which were mixed in the composition of Ni - 75at.%(Al-0, 4, 8, 12at.% Si), Ni-50at.%(Al - 0, 4, 8, 12at.% Si), and Ni - 25at.%(Al-0, 4, 8, 12at.% Si). Composition of these mixtures is shown in Table 2. These mixtures were then cold-pressed with a load of 700MPa for 60s into disc-shaped compact of 20mm in diameter and 3-5mm in thickness. The relative density of the compact was in the range of 82-92%. These green compacts were set in a sand mold and molten Al alloy was poured at 700, 800 and 900�. The schematic drawing of sand mold and green compact for in-situ joining is shown in Fig.1. The molten pure Al was poured at 750� to compare the differences of junction behavior with various casting alloys. The Al casting alloy and pure Al bonded with the Ni-Al intermetallic compound was sectioned and observed by optical microscopy and scanning electron microscopy (SEM). The composition of the intermetallics in the coated layer and reaction layer formed in the interface were

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identified by energy dispersive spectroscopy (EDS), Electron Probe Micro Analyzer (EPMA) and X-ray diffraction (XRD) analysis. The effect of Si addition on the reactivity of the powder compact was examined by differential thermal analysis(DTA) at the heating rate of 20K/min under Ar flow atmosphere. Results and Discussion Fig.2 shows the as-cast cross sections of Al alloy bonded with different composition of Ni-Al based compacts poured at 800�. Bonding interface of Ni-75at.%Al and Ni-50at.%Al compact is not clearly found between the reacted Ni-Al compact and Al casting alloy. The formed Ni-Al intermetallic compounds are mixed with Al casting alloy and swelled off during the SHS reaction. It is considered that the liquid Al casting alloy is penetrated into and mixed with the Ni-Al intermetallic compound during the combustion synthesis. But Ni-25at.%Al compact corresponding with the equilibrium phase of Ni3Al showed a curved linear interface and well bonded with the Al casting alloy. Fig.3 shows the as-cast macrostructures of Ni-25at.%(Al-0at.%Si) compact with different pouring temperatures. When the pouring temperature increased to 900�, the bonded coating layer was fractured at the inside of Al casting alloy. Also the bonded interfaces formed at lower pouring temperatures were curved. It was probably due to the difference of thermal expansion between Ni-Al intermetallic compound and Al alloy. The two kinds of intermetallic compounds form in Ni-75at%(Al-0at%Si) and Ni-50at%(Al-0at%Si) compacts[7]. The NiAl3 and Ni2Al3 intermetallic compounds were formed by the SHS combustion synthesis ignited from the heat of Al casting alloy. Ni2Al3 phase was surrounded by the NiAl3 intermetallic compound Fig. 4 shows typical XRD patterns of intermetallic compounds formed in Ni-75at.%(Al-0at.%Si), Ni-50.at%(Al-0at.%Si) and Ni-25at.%(Al-0at.%Si) compacts. For the sample with a composition of Ni-75at.%Al, diffraction peaks of Al and NiAl3 were mainly detected but Si peak from the Al casting alloy was also recognized due to the mixed compacts as shown in Fig.2. The phase of Ni2Al3 was detected at the composition of Ni-50at.%Al compact. There was no peak of unreacted Ni in the XRD patterns, which means that the Ni particle is fully reacted with Al and consumed by Ni-Al intermetallic compound formation. Only the Ni3Al phase was formed at the composition of Ni-25at.%Al synthesized compact. It was considered that the Al peak of Ni-50at.%Al and Ni-75at.%Al specimens came from the melted Al powders and mixed Al casting alloy. Fig.5 shows the optical microstructures and SEM image showing the bonded interface between Ni-25at.%Al intermetallic layer and Al casting alloy. Pores in the upper part of Ni-25at.% coating layer were observed near the bonded interface. The reaction synthesized materials often contain significant amounts of porosity. This porosity can arise from

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various sources such as intrinsic pores, solidification shrinkage and volume change[1]. It is considered that pores remained near the bonded interface are under a much lower compression stress state than the lower part of the reacted compact due to the residual thermal stress. Fig.6 shows the results of EPMA mapping around the bonded interface of Ni-25at.%Al compact. Si element is enriched near the bonded interface, which means that liquid Al alloy near the interface solidified in the end as Al-Si eutectic structure. It is considered that the exothermic heat of Ni-Al intermetallic formation and pore distribution near the interface decreases the solidification rate of liquid Al alloy near the Ni-25at.%Al compact. Also the thermal residual stress will be applied to the bonded interface during cooling, the weak Al-Si eutectic structure near the bonded interface will be cracked and fractured. Fig.7 shows microstructure of the reaction layer formed between the Ni-25at.%Al compact and Al casting alloy. NiAl3 intermetallic reaction layer of 25㎛ thickness was formed between Ni-25at%Al compact and Al alloy. The NiAl3 phase will be immediately formed through interdiffusion of the liquid Al and solid Ni elemental powder as the Al casting alloy is poured into the sand mold. Also the crack line was observed in the Al-Si eutectic layer near the bonded interface. Fig. 8 shows the cross section of the interface between the Ni-25at%Al compact and pure Al. The Ni particles diffused into the pure Al during the reaction and formed a diffusion layer about 700㎛ in thickness near the interface, which means that liquid pure Al near the interface solidified in the end as NiAl3-Al eutectic structure. In this NiAl3-Al eutectic layer, some cracks and fractures were observed near the interface by the thermal residual stress. Fig.9 shows the bonded interface between the Ni-25at%Al compact and pure Al. The NiAl3 and Ni2Al3 reaction layers were sequentially formed at the interface with 10㎛ in thickness respectively. Ni3Al intermetallic compound was observed in the compact because the molten Al was reacted with the Ni particle in the compact. But the ratio of Ni particle was higher at inner part of the compact than surface, so NiAl3, Ni2Al3 and Ni3Al intermetallic compounds were formed sequentially. Fig.10 shows the schematic modeling of the junction behavior at the interface between Ni-25at.%Al compact and casting alloys when the combustion synthesis started at the surface of the compact and the reaction was completed. The ignition is started when the Al powder melted in the compact enough to accumulate the heat from the molten Al alloy. This reaction heat was propagating into the inner part of the compact and also remelting the Al alloy were cooling after pouring into the sand mold

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near the interface. So, in this remelting zone, Al-Si and NiAl3-Al eutectic layer was formed due to the liquid Ni-Al intermetallic compounds of the compact interdiffused with the casting alloy near the interface. Moreover, these weak eutectic layers near the bonded interface will be cracked and fractured. Conclusions 1. Initial disc-shape of Ni-75at%Al and Ni-50at%Al compacts was lost. However, initial disc-shape of the Ni-25at%Al compact was hold.

2. NiAl3 and Ni2Al3 intermetallic compounds were observed in the Ni-75at%Al and Ni-50at%Al compacts, respectively. And Ni3Al intermetallic compound was formed in the Ni-25at.%Al compact and NiAl3 intermetallic reaction layer with about 20 ㎛ in thickness was observed at the bonded interface.

3. Al-Si eutectic layer was formed after completed solidification in Al alloy near the interface and some cracks were observed at this Al-Si eutectic zone.

4. Ni is diffused into the pure Al and formed NiAl3 intermetallic layer. The NiAl3 and Ni2Al3 reaction layers are sequentially formed at the interface. References 1. K. Morsi: Review reaction synthesis processing of Ni-Al

intermetallic materials, , Materials Sci. and Eng., A299, 2004, pp1-15.

2. N.S.Stoloff, C.T.Liu and S.C.Deevi: Emerging applications of intermetallics, Intermetallics, Vol.8, Issues 9-11, 2000, pp.1313-1320

3. Z. A. Munir: Ceram.Bull., 67, 1988, pp.342 4. A. G. Merzhanov: Int.Chem.Eng., 20, 1980, pp.150 5. T.Kimata, K.Uenishi, A.Ikenaga and KF.Kobayashi: Mater.Trans.,

44, 2003, pp.407 6. T.Kimata, K.Uenishi, A.Ikenaga and KF.Kobayashi: Intermetallics,

11 (2003), p.947-952 7. G.S. Cho: in press Tables Table 1 Chemical composition of Al casting alloy (mass%) Si Mg Fe Cu Cr Al

7.22 0.428 0.168 0.013 0.035 Bal. Table 2 Compositions of Ni-Al-Si mixtures

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Chemical composition (at.%) Sample name

Ni Al Si Ni-75at.(Al-12at.%Si) 25 66 9

Ni-50at.(Al-12at.%Si) 50 44 6

Ni-25at.(Al-12at.%Si) 75 22 3

Figures

Fig. 1. Schematic illustration of sand mold and green compacts.

Fig. 2 As-cast cross sections of the synthesized products with different composition of (a) Ni-75at.%(Al-4at.%Si), (b) Ni-50at.%(Al-8at.%Si) and (c) Ni-25at.%(Al-8at.%Si).

Fig. 3 As-cast cross sections of Ni-25at.%(Al-0at.%Si) compacts with different pouring temperatures of (a) 700�, (b) 800�, (c) 900�.

Sand MoldNi Al based compact

Cavity

Runner

Sprue

10m(c) (a) (b)

10mm(a) (b) (c)

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Fig. 4. X-ray diffraction patterns of combustion synthesized compacts with different composition of (a) Ni-75at.%Al, (b) Ni-50at.%Al, (c) Ni-25at.%Al

Fig. 5. Bonded interface of Ni-25at.%Al compact showing (a) optical microstructure, (b) SEM image of interface and (c) SEM image of inner part of the reacted Ni-25at%Al compact.

Ni-25at%Al

Al Alloy

Interfa

1mm

(a) (b)

(c)

20 30 40 50 60 70 80 90

(a)

(b)

(c)

Intensity (arb.unit)

2θ, degree

Ni3Al Ni2Al3

NiAl3 AlSi

(NiSi)x

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Fig. 6 EPMA mapping showing the bonded interface on Ni-25at.%Al compact

Fig. 7 Reaction layer formed between the Ni-25at.%Al compact and Al casting alloy.

Image Al

Ni Si

CCrraacckk

Crack line

Ni-25at%Al compact

Al-Si eutectic layer

NiAl3 layer

Al alloy

NiAl3

AlNiAl3Si

Ni3Al

50 ㎛

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Fig. 8 The cross section of the interface between the Ni-25at.%Al compact and pure Al (a) optical microstructure and (b) SEM image of Ni diffusion layer

Fig.9. The bonded interface between the Ni-25at.%Al compact and pure Al.

A

NiAl

Ni3Al20 ㎛

Ni2Al

NiAl3-Al(Ni) eutectic

1mm

compact

Pure Al Ni diffusion

layer

(a)

100 ㎛

NiAl3

NiAl3-Al(Ni) eutectic

Cracks

(b)

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Fig.10. Schematic Modeling of the junction behavior at the interface between Ni-25at.%Al compact and casting alloys when (a) the combustion synthesis started at the surface of the compact and (b) the reaction was completed

Ignition

Molten Al alloy CCoooolliinngg

Remelting area

Solidifying of the casting alloy

Propagating of the combustion synthesis in the compact

((aa))

Ni-25at.%Al Compact

Reaction heat

NNii33AAll IInntteerrmmeettaalllliicc

CCoommpplleetteedd SSoolliiddiiffiiccaattiioonn

((bb))

EEuutteeccttiicc llaayyeerr -- AA 335566 :: AAll--SSii eeuutteeccttiicc -- AAll ppuurree :: NNiiAAll33--AAll((NNii)) eeuutteeccttiicc

AA 335566 :: NNiiAAll33 iinntteerrmmeettaalllliicc AAll ppuurree :: NNiiAAll33 aanndd NNii22AAll33 iinntteerrmmeettaalllliiccss

Some CrackAl alloy

Ni-25at.%Al Compact