porosity of ni3al-based alloys prepared by gravity and...

23. - 25. 5. 2012, Brno, Czech Republic, EU

POROSITY OF Ni3Al-BASED ALLOYS PREPARED BY GRAVITY AND CENTRIFUGAL

CASTING

Martin POHLUDKAa, Tomáš ČEGANa, Jitka MALCHARCZIKOVÁa, David KAŇÁKb,

René FRIDRICHb

aVŠB – Technical University of Ostrava, 17. listopadu 15/2172, 708 33 Ostrava Poruba, Czech Republic

bVÚHŽ a.s., 739 51 Dobrá 240, Czech Republic

[email protected], [email protected], [email protected], [email protected],

[email protected]

Abstract

Five alloys of the chemical composition of Ni-12.67Al (hm. %) were prepared by induction melting followed

by casting and centrifugal casting. The alloys were melted and cast under different conditions for the effect of

preparation conditions on porosity and pore morphology to examine. Melting and casting of the charges in air

led to preparation of the alloys with high porosity. These alloys were full of large cavities and shrinks which

had irregular shapes. On the contrary, the alloys melted in vacuum and cast in argon had a low porosity.

Their pores were not so large and had not so irregular shapes.

Keywords

gravity and centrifugal casting, Ni-12.67Al, porosity

1. INTRODUCTION

Ni3Al is a nickel aluminide used for preparation of special materials which resist high temperatures. This

resistance is caused by anomalous deformation behaviour which Ni3Al exhibits. The principle of anomaly is

that yield stress increases with temperature up to 800 °C. Unfortunately, polycrystalline Ni3Al is too brittle at

room temperature. The brittleness may be partly reduced by alloying with boron [1].

Ni3Al is prepared by many methods including gravity and centrifugal casting. After solidification, the casting is

often full of pores, cavities and shrinks. Image analysis belongs to the methods intended for statistical and

morphological description of the pores. The pore morphology may be estimated from the functional

dependence of elongation factor FE [-] on circularity factor FC [-]:

(1)

(2)

Dmin [m] and Dmax [m] are the minimal and the maximal distance of parallel tangets at opposing pore border,

A [m2] is a pore area and P [m] is a pore perimeter [2].

2. EXPERIMENT

Five alloys of the chemical composition of Ni-12.67Al (hm. %) were prepared by two methods including

induction melting followed by casting and horizontal centrifugal casting. Charges were made from piece

nickel with a purity of 99.94 % and piece aluminium with a purity of 99.70 %. Induction melting and casting

was carried out in a furnace LEYBOLD of the type IS3/1. Four castings (samples Nos. 1a-1d) were prepared

under different conditions by this method. Only the one casting (sample No. 2) was made by centrifugal

casting in a furnace KPS. The conditions of melting and casting are listed in Tab. 1.

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Tab. 1 Preparation conditions of Ni-12.67Al alloys

Sample No. Melting Casting Mould Shape of casting

1a vacuum argon graphitic short rods

1b air and argon argon graphitic short rods

1c air air steel with coating1)

short rods

1d vacuum argon steel short rods

2 air and argon air steel with coating1)

ring

1) The coating was formed by a layer of oxide ceramic (Al2O3) whose thickness was 1.5 mm.

The castings prepared by induction melting followed by casting had a shape of short rods. Length of the rod

was 100 mm and diameter was 10 mm (Fig. 1). The casting made by horizontal centrifugal casting was a

ring with outer and inner diameter of 113 and 90 mm and with length of 30 mm (Fig. 2).

Fig. 1 Casting of Ni-12.67Al alloy prepared by

induction melting followed by casting

Fig. 2 Casting of Ni-12.67Al alloy prepared by

horizontal centrifugal casting

Chemical composition of the samples was checked by the OES method with a help of spectrometer

SPECTROMAXx (Tab. 2). In addition, contents of oxygen, nitrogen and hydrogen were determined using

two analysers, namely LECO TC-436 and LECO RH600. Oxygen and nitrogen analysis of the sample No. 1d

was not successful. Results of the remaining analyses are given in Tab. 2.

Tab. 2 Results of chemical composition analysis and gas content determination

Sample No. Chemical composition Gas content

Ni

[hm. %]

Al

[hm. %]

O

[ppm]

N

[ppm]

H

[ppm]

1a 87.50 12.40 67 4 10

1b 87.40 12.40 162 12 12

1c 87.40 12.42 170 13 13

1d 87.30 12.58 – – 9

2 87.59 12.31 336 12 33

Finally, metallographic samples were prepared from transversal and longitudinal sections of the castings.

The sections were used for microstructure documentation and statistical and morphological evaluation of

casting porosity.

3. RESULTS

3.1 Microstructure

Microstructure of the samples was documented by optical light microscope OLYMPUS GX51 equipped with

digital camera OLYMPUS DP12. Scanning electron microscope QUANTA FEG 450 with a probe EDAX

APOLLO X was used for chemical analysis of phases.


Despite different methods and conditions of preparation, final microstructure of all castings was same.

Microstructure was formed by grains oriented in direction from mould walls to casting axis. The grains, made

of Ni3Al phase, were coarse and contained dendrites of so-called mesh (Fig. 3). The mesh consisted of

channels of (Ni) phase and small islands of Ni3Al phase (Fig. 4) [3].

Fig. 3 Microstructure of the sample No. 2

prepared by horizontal centrifugal casting

Fig. 4 Dendritic arm of the sample No. 2

3.2 Porosity

Pore distribution on surfaces of the samples was different. An increase of pore concentration in the axis of

the rods was a common feature. The samples melted and cast in air contained also shrinks and cracks (Fig.

5). Area with the maximum concentration of pores in the casting of the ring was in the inner wall of the ring.

Two types of pores were observed in this case – the small pores in the centre of grains and the large ones at

grain boundaries (Fig. 6).

Fig. 5 The crack and shrinks in the axis

of the sample No. 1c

Fig. 6 Pores and cavities in structure

of the sample No. 2

Porosity of the metallographic samples prepared by induction melting followed by casting and centrifugal

casting was determined by the same microscope as microstructure. Ten photographs of non-etched surface

of each sample were obtained at two hundred times magnification. Direction of photography was different.

The rods were photographed from casting edge to the opposite edge. The ring was photographed from the

outer wall to the inner wall. The photographs were evaluated by computer program analySIS auto. Analyzed

parameters are given in Introduction. The results of porosity P [%] with the number of identified pores n [-]

are listed in Tab. 3.

Coarse grain of Ni3Al phase

Small grain of Ni3Al phase

Channel of (Ni) phase


Tab. 3 Porosity of the samples of Ni-12.67Al alloy prepared by induction and centrifugal casting

Method Melting Casting Sample No. n P

[-] [%]

induction

melting

followed by

casting

vacuum argon 1a 2416 0.06 0.06

air and argon argon 1b 1738 0.03 0.02

air air 1c 1001 0.07 0.10

vacuum argon 1d 547 0.02 0.01

centrifugal

casting air and argon air 2 7152 1.69 3.14

It is not easy to draw conclusions from the Tab. 3. The castings contained large pores, i.e. cavities and

shrinks, which distorted porosity values. This is proved by the standard deviations which are greater than the

arithmetic means. It is better to plot a functional dependence of porosity change depending on sample

surface and to draw the conclusions from it. Fig. 7 shows the curve. Sample surface is represented by the

direction of porosity measurement which is described in the preceding paragraph. There are two conclusions

resulted from the graph. Porosity of the sample No. 2 increases in direction to the inner wall of the ring and

porosity of the samples Nos. 1a-1d is the maximum in axis of the rods.

Fig. 7 Porosity change of the sample No. 2

(principal axis) and the samples Nos. 1a-1d

(secondary axis) on the sample surfaces

Fig. 8 Pore morphology in the areas with the

maximum and the minimum porosity (medians)

Fig. 8 shows a dependence of medians of shape factors for pores in the areas with the maximum and the

minimum porosity (Fig. 7). The areas with the minimal porosity mainly contained small pores. These pores

were not so elliptic and had smoother surface than the pores from the areas with the maximum porosity.

Morphology of pores from the areas with the maximum porosity was affected by presence of large cavities

and shrinks which had irregular shapes.

The total number of identified pores in all castings was 12,854. Pore sizes, characterized by a diameter,

were various. The casting of the ring contained the pores whose diameter was from 1.5 to 280.0 m whereas

the pores in the castings of the rods had a diameter from 1.5 to 35.0 m1. According to frequency of

occurrence, these pore ranges might be divided in two same groups. The first group was occupied by small

pores whose diameter lay in a range from 1.5 to 11.5 m. These pores were mostly situated in the grain

centres and their frequency was high. The second group was formed by pores which were larger than 12

m. Most of them lay along grain boundaries. The frequency of these pores is low. Distribution of small pores

1 Lower limit of pore diameter range is given by a resolving power of used microscope objective.

minimum

maximum

minimum

maximum

minimum

maximum

0,00

0,12

0,24

0,36

0,48

0,00

2,00

4,00

6,00

8,00

1 2 3 4 5 6 7 8 9 10

Po

rosi

ty, P

[%

]

Po

rosit

y,

P [

%]

Distance, x [mm]

2 1a 1b 1c 1d

Dmin = 3.61 m

Dmax = 4.01 m

Dmin = 3.52 m Dmax = 7.63 m

Dmin = 3.15 m

Dmax = 2.49 m

Dmin = 1.93 m

Dmax = 4.35 m

Dmin = 2.73 m

Dmax = 3.15 m

0,38

0,46

0,54

0,62

0,70

0,78

0,58 0,66 0,74 0,82 0,90 0,98

Elo

ng

ati

on

Facto

r, F

E [

-]

Circularity Factor, FC [-]

2 1a 1b 1c 1d ellipse


was log-normal with the maximal frequency of pores with a diameter of 2.5 m. Distribution of large pores

was disordered hence it was not evaluated.

Functional dependence of shape factors on pore diameter confirmed these conclusions (Fig. 9 and 10).

Small pores belonged to the range from 1.5 to 11.5 m and had a wide morphology of shapes. Very elliptic

pores coexisted with round pores and pores with smooth surface with the ones with rough surface.

Morphology of pores larger than 12 m was limited.

Fig. 9 Pore elongation for the pores

whose diameter is from 0 to 15 m

Fig. 10 Pore circularity for the pores

whose diameter is from 0 to 15 m

Morphology of all pores may be estimated from a functional dependence of elongation factor on circularity

factor (Fig. 11). In a bubble graph, the pore size is demonstrated by bubble width. Values of elongation

factors of the samples Nos. 1a-1d are figured on secondary axis because these samples did not contain as

large pores as the sample No. 2. Ellipse line is an important element of the graph. Pores in the castings may

maximally reach such values of both shape factors which correspond coordinates of the line. Pore becomes

more circular when elongation factor increases in the direction from the ellipse line. But pore surface

becomes rougher when circularity factor decreases in the direction from the ellipse line.

Fig. 11 Morphology of all pores in the sample

No. 2 (principal axis) and the samples

Nos. 1a-1d (secondary axis)

Fig. 12 Morphology of pores in the sample No. 2

and the samples Nos. 1a-1d (medians)

Nevertheless, graphic dependence of shape factors is not easy to survey because shows many pores with

wide morphology which overlap. For this reason, it is better to use only medians of shape factors (Fig. 12).

0,0

0,2

0,4

0,6

0,8

1,0

0 3 6 9 12 15

Elo

ng

ati

on

Facto

r, F

E [

-]

Pore Diameter, D [m]

2 1a 1b 1c 1d

0,0

0,2

0,4

0,6

0,8

1,0

0 3 6 9 12 15C

ircu

lari

ty F

acto

r, F

C [

-]

Pore Diameter, D [m]

2 1a 1b 1c 1d

0,0

0,2

0,4

0,6

0,8

1,0

0,0

0,2

0,4

0,6

0,8

1,0

0,0 0,2 0,4 0,6 0,8 1,0

Elo

ng

ati

on

Facto

r, F

E [

-]

Elo

ng

ati

on

Facto

r, F

E [

-]



D = 2.94 m D = 2.49 m

D = 2.36 m

D = 3.15 m

D = 3.52 m

0,51

0,57

0,62

0,68

0,73

0,66 0,74 0,82 0,90 0,98

Elo

ng

ati

on

Facto

r, F

E [

-]




Now, general conclusions may be drawn from this graph. Pores of the samples No. 2 and No. 1c, which

overlap each other, had the worst morphology. These pores were more elliptic and their surface was rougher

than the pores in remaining castings. It was probably caused by melting and casting in air. On the contrary,

the castings melted in vacuum and cast in argon (the samples No. 1d and 1a) had the most circular pores

with the smoothest surface. These results consent to the results of gas determination (Tab. 2).

4. CONCLUSION

Five alloys of Ni-12.67Al composition (hm. %) were prepared by induction melting followed by casting and

centrifugal casting under different conditions. Analysis of chemical composition has confirmed that

microstructure of all castings was same. Coarse grains of Ni3Al phase were oriented in the direction from

mould walls to casting axis. There were dendrites of mesh in the centres of the grains. The mesh was formed

by the channels of (Ni) phase which surrounded the small grains of Ni3Al phase.

Conditions of alloy preparation strongly affected porosity and pore morphology. Pore distribution on surface

of transversal and longitudinal sections of alloys was not same. There was a higher pore concentration in the

centre of the sections of rods and in the area of inner wall of ring.

Statistical evaluation of alloy porosity has confirmed a presence of small pores whose diameter was from 1.5

to 11.5 m and large pores whose diameter was larger than 12 m. The small pores were mainly in the

centres of the grains whereas the large ones together with cavities and shrinks were along grain boundaries.

Alloys melted and cast in air had the worst pore morphology. These alloys contained elongated pores with a

rough surface. On the contrary, the alloys melted in vacuum and cast in argon were full of circular pores with

a smooth surface.

For the next work, it would be better to melt Ni3Al-based alloys in vacuum and cast them in a protective

atmosphere. If it is not possible, the alloys must be properly heat treated for decreasing of porosity.

ACKNOWLEDGEMENT

The presented results were obtained within the frame of solution of the research project

TA 01011128 “Research and development of centrifugal casting technology of the

Ni-based intermetallic compounds” and the project CZ.1.05/2.1.00/01.0040

“Regional materials science and technology centre”.

LITERATURE

[1] MASAHASHI, N. Physical and mechanical properties in Ni3Al with and without boron. Materials Science and

Engineering: A, Volume 223, Issues 1-2, 1997, Pages 42-53

[2] MARCU PUSCAS, T. et al. Image analysis investigation of the effect of the process variables on the porosity of

sintered chromium steels. Materials Characterization, Volume 50, Issue 1, 2003, Pages 1-10

[3] KURSA, M. et al. Microstructural analysis and mechanical properties of polycrystalline Ni-rich Ni3Al alloy prepared

by directional solidification. Kovové materiály, Ročník 46, Číslo 6, 2008, Strany 351-359

porosity of ni3al-based alloys prepared by gravity and...

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