Joining NiAl using simultaneous combustion synthesis and pressure

Scripta METALLURGICA et MATERIALIA

Vol. 30, pp. 463-468, 1994 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

JOINING NiAI USING SIMULTANEOUS COMBUSTION SYNTHESIS AND PRESSURE

R.D. Torres and T.R. Strohaecker PPGEMM/UFRGS Porto Alegre, Brazil J.J. Moore and G.R. Edwards Department of Metallurgical and Materials Engineering Colorado School of Mines Golden, Colorado 80401 (Received September 13, 1993) (Revised November 9, 1993)

Introduction Nickel aluminide-based intermetallics are attractive in applications requiring high thermal stability, corrosion and oxidation resistance, and high-temperature mechanical properties. However, if components are to be produced using these intermetallics, efficient joining techniques need to be developed. This paper provides an initial investigation of the application of combustion (self propagating, high temperature) synthesis (SHS) as a means of joining NiAI intermetallic materials. Combustion synthesis [1-4] is a technique whereby an exothermic reaction mixture is used to synthesize the required product(s). If the reaction is sufficiently exothermic, it can be self sustaining once initiated at the ignition temperature, Tig. The heat generated by the reaction results in an increase in temperature to a maximum combustion temperature, Tc, which is usually less than the calculated adiabatic temperature, Tad, on account of heat losses from the reaction. Combustion synthesis reactions can be operated in two different modes of ignition, i.e. propagating and simultaneous combustion modes. In each case, the exothermic reactant mix, typically in powder form, is pressed in the required reaction stoichiometry and at a certain green density. In reaction mixtures which are less exothermic, such as the synthesis of an intermetallic compound from its elements, e.g. reaction (1), the simultaneous combustion mode is often used. In this case, the whole pellet is heated to above the ignition temperature of the reaction, whereupon the exothermic reaction is initiated in a near-simultaneous process. Ni + AI = NiAI

(1)

An enthalpy-temperature plot for the reactants and products for the combustion synthesis of NiAI using reaction (1) is presented in Figure 1, i.e, line Ni + AI (reactants) and line NiAI (product). The Ni-AI phase diagram is presented in Figure 2. The theoretically predicted Tad (NiAI), [5] associated with an ignition temperature, Tig, of 1000K for the combustion synthesis reaction (1) ignited in the simultaneous combustion mode is also schematically presented in Figure 1. The incorporation of a diluent or inert material into the reaction mix will decrease Tad and therefore, Tc, since this material will take heat out of the reaction enthalpy. Therefore, the addition of such diluents to the reactants provides a useful technique of controlling Tc. In this investigation, either AI203 or previously synthesized NiAI were added as diluents to the reactant mixture for reaction (1) in order 463 0956-716X/94 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.

464

JOINING

OF NiAI

Vol.

"C 2

. . . .

i . . . .

i

. . . .

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i

. . . .

i

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i

0

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A£%AI 4 0 ,.,¢0 dO 70 8 0

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30, No.

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o

Reactants

o.

Ni÷AI

~'7 . . . . . . . . . .

: : ;"

....,,/,"~F-,AI. r~p o

"~'

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Product

c

T

NiA,,mp.l ..,=.....= ...J=

~--*

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' ..j.*~

NiAI

~..=-~' 'o

_2

....

0

i ....

500

i ....

ddO

i , , l

, i l ,i l i i i

°C

i i , i ....

1000 1500 2000 2500 3000 3500 T.

0

K

FIG. 1. Enthalpy-temperature plot for the SHS reaction (1) Ni + AI = NiAI.

20

40

dO

~0

IO0

Wt.*/eAI

FIG. 2. Ni-AI phase diagram.

to control the combustion temperature and, therefore, the microstructure and properties of the joint material. The effect of adding either 10 wt.% AI203 or 10 wt.% of previously synthesized NiAI as diluents to the reactants, i.e. reactions (2) and (3), on the enthalpy-temperature plots are presented in Figure 3, i.e.

Ni + AI + xAI203 = NiAI + xAI203

(2)

or

Ni + AI + yNiAI = (1 + y)NiAI

(3)

Using a similar construction to that in Figure 1, it can be seen that the adiabatic temperature and therefore, the combustion temperature decreases on adding the NiAI or AI203 diluents. Since the thermochemical data for liquid NiAI is not available the H-T lines have been estimated and plotted as discontinuous lines in Figure 3. Examination of Figure 1 indicates a heat of reaction of approximately -33kJ/mole for an ignition temperature of 1000K, which provides a corresponding theoretical adiabatic temperature Tad(NiAI) of approximately 2166K for the extrapilated (estimated) data. A similar examination of the Ni-AI phase diagram indicates that the equiatomic NiAI exhibits a melting point of 1911K. Hence, conducting the combustion synthesis reaction (1) under adiabatic conditions will result in a liquid NiAI product. Using the same reaction conditions and incorporating 10% AI203 or 10% NiAI into the reactant mix, lowers the Tad to approximately 1980K, on the extrapolated (estimated) data lines.

Experimental Procedure The intermetallic material required to be joined was produced by a powder metallurgy, pressing and sintering process of NiAI powders reinforced with 2.8% by weight of AI203. This material was electric discharge machined (EDM) into 12.7mm (0.5 in) diameter discs.

4

Vol.

30, No. 4

JOINING

Reactants

,

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. ~

465

~: :

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E .x

-

[

Products

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./

~--A N i + A I A--= NiAt N i ÷ A I + 10wtX

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=

,

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i

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.

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.

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*--+ N i A l + 1 0 w t ~ AI20:) H Ni+Al÷10wtx NiAI H NiAI+ |OwHcNiAI

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2000

.

,

.

I

l

2500

i

.

.

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,

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T, K

FIG. 3.

Enthalpy-temperature plot of reactants (Ni + AI, Ni + AI + 10%NiAI, Ni + AI + 10%AI203) and product (NiAI, NiAI + 10%NiAI, NiAI + 10% AI203 respectively) for the combustion synthesis reactions (1), (2) and (3).

The joint material was produced from Ni and AI powders which were thoroughly mixed in a small ball mill with AI203 balls for two hours together with up to 25% NiAI or up to 12% AI~O3 powders used as diluents as represented by reactions (3) and (2) respectively. The NiAI powder used as a diluent was produced by crushing and grinding previously combustion synthesized NiAI material produced by reaction (1). The size ranges of both the reactant and diluent powders used in this investigation are listed in Table I. Table I. Powder particle sizes, melting and boiling points. Ni

AI

AI20~ (diluent)

NiAI (diluent)

Size (pm)

-44

-44

-105 + 44

-74 + 44

rapt (°C)

1453

660

2050

1638

bpt (°C)

2730

2470

The thoroughly mixed reactant powders were pressed into a 12.7mm diameter disc of approximately 5mm in height using an universal double acting press at room temperature and a pressure of 13.8 MPa. The green pellet or disc was dried in an oven for 1 hour at 110°C and subsequently positioned between the two NiAI-2.8% AI203 pieces to be joined and placed in the graphite die and press as indicated in Figure 4(a). The graphite die was rapidly heated by means of a tungsten

466

JOININGOF NiAI

Vol. 30, No. 4

electrical resistance coil surrounding the die as indicated in Figure 4(a). Typical heating rates of 7.5=C/s were used until the ignition temperature (typically 1075K) was reached. The time-temperature data were recorded on a chart using a W-W5%Re thermocouple positioned in the wall of the graphite die adjacent to the green disc or joint, Figure 4(b). Once the exothermic reaction was observed on the chart recorder, a load was immediately applied to the die and, therefore, the joint assembly. Various pressures were applied for times of up to 20 minutes and temperatures up to 1600°C. Once the required temperature had been reached, the power supply was turned off and the pressure was maintained on the die and joint assembly while it cooled to room temperature. The joints were examined and characterized using optical and scanning electron microscopy (SEM) interfaced to an energy dispersive spectroscopy (EDS) facility.

(a)

(b)

oP..~PHrrE RELT

. GREEN

PELI.~T

HEATING E

DIE [.....

~ 7 8 ~ (Tig) C PO wF..~ sUPPLY

Time(s)

FIG 4. (a) Schematic representation of the hot press system used to provide siumltaneous hot pressing with combustion synthesis to join the NiAI materials. (b) Typical time-temperature plot for the simultaneous combustion synthesis and hot pressing process used to join the intermetallic.

Experimental Results and Discussions The effects of AI20 a and NiAI used as diluents on Tc are given in Figures 5 and 6. It is apparent that AI20a is a more efficient diluent than NiAI in lowering Tc. An addition of 2wt% of AI20 a effectively reduced the Tc to 2100K with very little change in Tc up to 12wt% AI203. On the other hand, the NiAI diluent resulted in a linear but less dramatic decrease in Tc. The corresponding SEM photomicrographs of the NiAI-2.8wt% AI203 and NiAl-10wt% NiAI joints are presented in Figures 7 and 8. In each case presented in Figures 7 and 8, the pressure used on the joint, on observing the exothermic reaction on the chart recorder was 24.8MPa (3600psi). The NiAI-2.8wt%AI203 joint exhibited a uniformly dispersed AI20s phase, similar to that in the parent metal, with no obvious porosity. However, the NiAI-10%NiAI joint exhibited extensive porosity. The corresponding combustion temperatures were 2100K (1827°C) for the NiAI-2.8wt%A120z joint composition and 2350K (2077°C) for the NiAl-10wt%NiAI joint composition. Examination of Figures 1 and 2 indicates that a considerable amount of superheating of liquid NiAI (melting point 1638°C) will be present at 2077°C which could result in extensive solidification shrinkage and some AI vaporization from the reactant mixture. The partial pressure of AI at 2077°C was estimated to be 5 x 10"2 at.

Vol.

3000

30, No.

4

JOINING

OF NiAI

467

Combustion Telltl~rature (K)

8OO0

2600

Cc.nbmlttonTemDetstum(10

26Q0'

2000

2O00

1600

1800

1000

lOOO

800

6O0 r 2

i 4

t e

i 8

i 10

0

i 15

i 1@

% wt. of N 2 0 8

i 29

e/, wt. of NIAI

FIG. 5. The effect of wt% AI203 diluent on the combustion temperature for reaction

FIG. 6. The effect of wt% NiAI diluent on the combustion temperature for reaction (3).

(2). (a)

i 16

(b) JOINT

~

~

q

100 pm

|

! J

FIG. 7. The fracture surface through the joint (a) at low magnification and (b) at the interface with the base material when 2.8% of AI20 3 was used as diluent in reaction (2) for the simultaneous combustion synthesis and hot pressing (24.8MPa, maintained for 5 minutes at 1600°C) on observing the initiation of the exothermic reaction.The arrows on the edge of each photomicrograph indicate the interface between the 2.8% AI203-NiAI base material and the joint. The sample in Figure 7(a) was cut part way through using a diamond saw, and subsequently fractured by impact to reveal the fracture surface on the, left hand side of the photomicrograph. Note the large plates of AI20 ~ in Figure 7(b) in both base and joint materials.

(5.07kPa). This level of gaseous species could result in considerable porosity coupled with solidification shrinkage. It appears that it is important to maintain an appropriately low combustion temperature while the reaction system is under pressure in order to provide a sound solid-liquid reaction and interdiffusion, bonding and densiflcation of the joint. In this respect approximately 3% AI203 used as a diluent in reaction (2) is more efficient at lowering Tc than 10% NiAI.

468

(a)

JOINING OF NiAI

Vol. 30, No. 4

(b)

FIG. 8. The fracture surface through the joint (a) at low magnification and (b) near the interface with the base material when 10% NiAI is used as a diluent in reaction (3) for the simultaneous combustion synthesis and hot pressing (24.8MPa maintained for 5 minutes) on observing the initiation of the exothermic reaction.

Conclusions • • •

The simultaneous application of pressure and combustion synthesis has produced a microscopically sound joint in a NiAI-2.8wt%AI203 intermetallic composite material. Control of the combustion temperature while the reaction is under pressure appears to be an important process control parameter in producing sound joints. AI203 is a more effective diluent in decreasing Tc than NiAI in the Ni + AI = NiAI combustion synthesis reaction system. References

1. 2. 3. 4. 5.

H.C. Yi and J.J. Moore, J. Mater. Sci. 25 (1990) p. 1159. J. Subrahmanyam and M. Vijayakumar, J. Mater. Sci. 27(1992). Z.A. Munir and Anselmi-Tamburini, Mater, Sci. reports, Vol. 3, No. 7 & *, May 1989. K.A. Philpot, Z.A. Munir and J.B. Holt, J. Mater. Sci. 22 (1987), pp. 159169. J.J. Moore, "An Examination of the Thermochemistry of Combustion synthesis Reactions", to be pub. in Proceedings of Symposium on the Processing and Fabrication of Advanced Materials pub by TMS of AIME, Materials Week Conference, Pittsburgh, Oct. 1993. Acknowledgements

One of the authors, R.D. Torres, is most grateful for a scholarship from the Brazilian government that allowed him to study for a short period of time at Colorado School of Mines, Golden, Colorado, USA.