microstructure and mechanical properties of...

METAL 2007 22. – 24. 5. 2007 Hradec nad Moravicí

1

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF SPRAY

DEPOSITED Ni-BASED SUPERALLOYS

MI Guofa1, WANG Hongwei

2, TIAN Shifan

3, LI Zhou

3

(1. School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo,

454001, China;

2. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin

150001, China;

3. Institute of Aeronautical Materials, Beijing 100095, China)

Abstract: Three kinds of superalloys are prepared by spray deposited process. The analysis

results of microstructures and mechanical properties indicate that the spray deposited

preforms have higher integral densification and the oxygen content in each kind of superalloy

is very lower. The microstructures are consisted of fine no dendritic equi-axed grain. The

spray deposited superalloys has good ductility. The forging experiment illustrates that even

though the once deformation of spray deposited GH742 alloy more than 60%, the crack can

not be found. Also, the mechanical properties of spray deposited superalloys are increased

significantly.

Keywords: spray deposition, Ni-based superalloys, microstructure, mechanical properties

1 Introduction

With the rapid development of space navigation, the demands of mechanical properties

of high temperature structure materials for dynamical devices of aviation and spaceflight

became more and more higher [1-4]

. Conventional production process for high temperature

structure materials applied as high temperature structure parts is casting-forging process. Even

through vacuum arc melting or electroslag re-melting, the ingots still have coarsen grain,

serious segregation and inhomogenous microstructure and mechanical properties that make

the thermally process form become more difficult and the application of some alloys was

restricted. The spray deposition process is an advanced process that forms the ingots nearing

practical part shape by rapid solidification. Spray deposition can overcome the inherent

defects of traditional forming process and the ingots formed by spray deposition have new

microstructure and special mechanical properties [5-8]

.

In this paper three kinds of superalloy ingots were produced using spray deposition

process and the microstructure and mechanical properties were analyzed and tested

respectively.

2 Experimental procedures

2.1 Compositions and melting of master alloy

Table 1 show the chemical compositions used as experimental alloys. The master alloys

were prepared by using double vacuum induction melting.


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Table 1 Chemical composition of experimental alloys, wt.%

alloy C Co Cr Mo Al Ti V Nb B Ni

K417 0.14 14.8 9.02 3.3 5.32 4.40 0.73 －－ 62.29

GH742 0.06 9.95 14.7 5.11 2.87 2.66 － 2.65 － 62.00

IC6 0.01 －－ 14.00 7.80 －－－ 0.05 78.15

2.2 Experimental equipment

The experimental equipment is mainly consisted of vacuum, air feed, cooling, melting,

heat preservation, atomization, deposition, power and dirt collector, and control systems.

2.3 Determination of the optimum process parameters

Because the goal of spray deposition is to gain needed quality and shape, the quality and

shape of spray deposited ingots are the main guidelines to value the spray deposition process

parameters. The quality of spray deposited ingots is valued by analyzing gas content, testing

the density and observing the microstructure. The optimum spray deposition process

parameters are shown in Table 2. Fig.1 shows the spray deposited ingots based on the

optimum spray deposition process parameters.

Table 2 Optimum spray deposition process parameters

Atomization N2

Atomization pressure 1.5~2.5 MPa

Mass rate of atomization gas 1.75~2.8×10-2kg/s

Mass rate of atomization metal 1.5~2.0×10-1kg/s

Pouring temperature 1450~1600℃

Deposition distance 3.4~4.0×10-1m

Rotation speed of deposition device 1.5~1.8×102rpm

Descending speed of deposition device 4.0~6.0×10-4m/s

Bias distance of deposition device 2.0~5.0×10-2m

Sloping angle of deposition device 15~30°


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2.4 Analyzing and testing methods

The density of spray deposition ingots is measured by draining water. The observations

of microstructure are carried on optical microscope (OM), scanning electron microscope

(SEM) and transmission electron microscope (TEM). The ambient and elevated temperature

mechanical properties are tested on electron tensile instrument.

3 Experimental results and discussions

3.1 Gas content analysis of spray deposited preforms

Table 3 gives the analysis results of gas contents in spray deposited preforms. The results

demonstrate that there is scarce H content in preforms. Also the O content in spray deposited

preforms is deficient because the molten metal was solidified with very short time and they

were protected under high pure inert gases during their atomization process. However,

compared with H and O content, the N content of spray deposited preforms is higher due to

the N2 was used as atomization gas.

Table 3 Gas content in spray deposited superalloys, ppm

Alloy N H O

K417 200 1 14

GH742 320 － 15

IC6 80 2 18

3.2 Density of spray deposited preform

The measured results of density of deposited pre-forms indicate that the density of edge

and center of deposited pre-form can reach up to 90% and 98% of alloy’s theory density

respectively. The density of deposited pre-forms after HIP is uniform and can reach up to the

alloy’s theory density. Table 4 shows the density changing of the center of deposited pre-form

of spray deposited GH742 alloy before and after HIP.

b) a)

Fig.1. Spray deposited ingots

a) no manufacturing b) after manufacturing


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Table 4 Density changing of the center of preform of spray deposited

GH742 alloy before and after HIP, %

State 201 202 203 204 205 206 207 208

Before HIP 99.3 98.8 99.1 98.7 99.1 99.4 98.9 98.5

After HIP 99.9 99.9 100 99.9 99.9 100 99.9 99.5

3.3 Microstructure of deposited preform

Fig.2 shows the typical microstructure of spray deposited superalloys. The deposited

preforms have fine microstructure, little micro-porosity, and no macro metallurgy defects can

be seen. The grains are fine and uniform, and their average diameter is 10~40μm. By

observing under SEM and TEM, we can found that the strengthening phase γ′was refined and

the size less than half of the size of as cast. The second phases were uniformly distributed and

the volume fraction was remarkably decreased.

3.4 Mechanical properties of deposited preform

Table 5~7 show the mechanical properties of three kinds of spray deposited superalloys.

Compared with the as cast, the tensile strength, ductility, creep rupture and impact toughness

Fig.2. Microstructure of Ni –based superalloys

a) SD, K417 (b), (c) SD, GH742 (d) cast with slow cooling GH742

a) b)

c) d)

70µm 70µm

70µm 70µm


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of spray deposited superalloys are improved evidently. Furthermore, the mechanical

properties of spray deposited superalloys can be adjusted in a larger range by HIP and

following heat treatment. The hot deformation experiment reveals that on the conditions of

hot deformation temperature 1080~1120℃ and strain rate•

ε =3.2×10-2

~3.2×10-4

l/s, the spray

deposited K417 and GH742 superalloys have well ductility. Also, the forging experiment

illustrates that even though the once deformation of spray deposited GH742 alloy more than

60%, the crack can not be found.

Table 5 Tensile properties of spray deposited K417 alloy

Table 6 Mechanical properties of spray deposited GH742 alloy

Tensile

No.pref. Conditions UTS,

MPa

YS,

MPa

EL,

%

RA,

%

Stress rupture

650℃/834Mpa

τ, h

Impact

AK

KJ/㎡

E01 SD 1356 898 26.3 36.6 164:50 47.0

E02 HIP 1316 786.5 26.6 31.9 97:32 —

E03 F+HT1 1392.5 947 25.8 32.9 — 54.2

E04 F+HT2 1468 1063.5 22.2 22.9 175 62.7

Specification 1210 755 13 14 50 24

(1) HT1-1140℃ solution treated; (2) HT1-1080℃ solution treated

No. of test Conditions Ttest, ℃ UTS, MPa YS, MPa EL, % RA, %

24 SD 20 1306 904 18.0 15.8

28 SD 20 1393 927 21.2 20.4

12 HIP+HT 20 1373 799 33.2 29.2

21 HIP+HT 20 1368 810 32.4 29.9

16 HIP+HT 700 1053 — 23.0 23.1

23 HIP+HT 700 1041 — 23.2 32.0

As cast

[9]

As cast[9]

20

700

990

1000

765

774

11.5

13.0

19.0

20.0


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Table 7 Mechanical properties of spray deposited IC6 alloy

(1) 1150℃, 150Mpa, 2h; (2) 1180℃, 150Mpa, 3h; (3) directional solidification

3.5 Microstructure formation of deposited preform

The microstructures of three kinds of superalloys show that despite they have different

compositions, they have the similar no dendritic equi-axed grain microstructure and the grains

size are about 10~40µm. However, the volume fraction and distribution of micro-porosity in

these three kinds of superalloys are different. These results demonstrated that during spray

deposition process the formation of no dendritic equi-axed grain is the necessary results and it

is the inherent characteristics of spray deposited alloys [10, 11]

. The formation of no dendritic

equi-axed grains experiences two continuous quick quenching processes. The first quenching

process was carried out during molten atomization and spray. Atomized molten droplets with

different size experience a short time rapid solidification and form fine dendrites and finer

microstructure. The second quick quenching occurs in a short time after the molten droplets

impact on the deposition surface. In this process, the molten droplets are deformed and broken

up. The residual liquid solidified rapidly with two modes, one is dendritic growth, and another

is to form equi-axed grain depending on the broken fine dendrites. Some dendrites that were

impacted and broken up may re-melt because the residual liquid solidifies and charges crystal

latent energy. So, the microstructure will be consisted of finer dendrites and equi-axed grains

after the second quick quenching. Following the second quick quenching, the solidified spray

deposited superalloys will experience a slower cooling process and the finer dendrites

agglomerate and grow up [10, 11]

. In the meantime, some second phase particles dispersedly

precipitated.

4 Conclusions

(1) Spray deposited superalloys are integral densification and have lower oxygen

content. The spray deposited superalloys have uniform composite and the thermal

deformation is improved.

(2) The micro-porosity can be eliminated and make the alloy complete densification

by HIP treating preform.

(3) Compared with as cast, the mechanical properties of spray deposited superalloys

are increased significantly.

R.T. 650℃

State UTS,

MPa

YS,

MPa

EL,

%

RA,

%

UTS,

MPa

YS,

MPa

EL,

%

RA,

%

SD 1240 829 23.5 13.0 1041 842 — —

SD+HIP(1)

1369 784 26.9 21.1 1316 884 — —

SD+HIP(2)

1367 741 25.3 19.4 1240 874 9.1 8.5

DS(3)

1170 780 14.95 20.4 — — — —


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