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Precision casting of Ti-15V-3Cr-3Al-3Sn

alloy setting *Nan Hai, Liu Changkui, Huang Dong, Zhao Jiaqi, Zhao Hongxia

(Beijing Institute of Aeronautical Materials, Beijing100095, P. R. China)

No metal rises so swiftly to match the preeminence oftitanium alloy in aerospace industry [1]. The low density

and high strength make titanium alloy an excellent candidate forairplane frame components. Lighter weight can improve theperformance of aircraft and reduce fuel consumption [2-6].Nowadays, medium strength titanium alloy, such as Ti-6Al-4V,is widely used in aerospace industry. With the development ofnew generation aircraft, it is important to develop high strengthtitanium alloy components. Researchers in United States, Japan,and Britain obtained significant achievements in Ti-15V-3Cr-3Al-3Sn alloy research and applications [7-10].

In present work, precision casting of Ti-15V-3Cr-3Al-3Sn alloysetting, with a 1.5 mm thin wall arc-shaped face, was producedby Vacuum Arc Melting (VAM) and centrifugal investmentcasting in a laboratory scale. The preparation of investmentceramic mold, melting process, pouring system design andmechanical properties were studied.

1 Experiment procedures

1.1 Preparation of Ti-15V-3Cr-3Al-3Sn alloy

ingot

The pressed Ti-15V-3Cr-3Al-3Sn alloy electrode was melted byvacuum self-consumable-electrode arc furnace. The preliminaryingot was re-melted to increase composition homogeneity. Inorder to prevent oxidation and increase alloy purity, the vacuumwas set below 0.1 Pa before melting. After melting, specimenswere taken from ingot melted for composition analysis.

1.2 Preparation of investment ceramic mold

Since Ti-15V-3Cr-3Al-3Sn alloy setting has an arc-shaped face

with 1.5 mm thin wall and � 10 mm legs, it is very difficult tobe manufactured and has very low material efficiency if preparedby machining. Precision casting process is very suitable toproduce complicated component such as setting. Figure 1 showsthe setting wax pattern.

Ceramic shell is the key in titanium investment casting. Chineseresearchers have mastered main titanium casting shelltechnologies such as graphite shell, metal surface layered ceramicshell, and oxide surface layered ceramic shell.

In this study yttrium oxide surface layered ceramic shell wasused. This kind of oxide ceramic shell has high strength and lowreactivity. It is suitable for gravity casting and centrifugal casting.Figure 2 shows the baked oxide ceramic shell.

Male, born in 1968, Ph. D. Research areas: titanium alloy precision

casting technology, mechanism of titanium alloy solidification, and

its heat treatment.

E-mail: hai.nan@biam.ac.cn

Received: 2007-08-14; Accepted: 2007-12-02

*Nan Hai

Abstract: In this research, Ti-15V-3Cr-3Al-3Sn alloy ingots were prepared using ceramic mold and centrifugal

casting. The Ti-15V-3Cr-3Al-3Sn setting casting, for aeronautic engine, with 1.5 mm in thickness was manufactured.

The alloy melting process, precision casting process, and problems in casting application were discussed. Effects of

Hot Isostatic Pressing and heat treatment on the mechanical properties and microstructure of the Ti-15V-3Cr-3Al-

3Sn alloy were studied.

Key words: centrifugal pouring; precision casting process; Hot Isostatic Pressing

CLC number: TG 146.2+3 Document Code: A Article ID: 1672-6421(2008)01-012-04

Fig. 2 The baked oxide ceramic shell

Fig. 1 The setting wax pattern

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1.3 Melting and pouring process

In this research, self-consumable-electrode vacuum arc skullfurnace was used to melt and pour the Ti-15V-3Cr-3Al-3Snsetting castings. The ceramic shell was fixed on the centrifugalplate. Since skull furnace offers low melting superheat and thesetting has 1.5 mm thin wall, it is very difficult to fill shell mouldcompletely by gravity pouring. Centrifugal pouring can greatlyincrease the liquid metal filling ability. The melting and pouringparameters were given in Table 1. Post cast cooling took placein the furnace until the mould was safely removed withoutoxidizing the casting.

and mechanical properties. HIP process was as follows:

950�/110 MPa/2 h (1)

Since cast Ti-15V-3Cr-3Al-3Sn alloy has low elongation, heat

treatment (HT) was applied to increase its ductility and tensilestrength. The HT process was as follows:

Solution treatment:

700� J 800�/0.5 h J 1.5 h (2)

Aging treatment:

500�J600�/8 h J 15 h (3)

2 Results and discussion

2.1 Composition of Ti-15V-3Cr-3Al-3Sn alloy

ingot and casting

The analyzed chemical compositions of Ti-15V-3Cr-3Al-3Sningot and its casting are shown in Table 2 and Table 3,respectively. From Table 2 and Table 3 it can be seen that theanalyzed results at different position of the re-melted ingot andthe casting can satisfy the specified composition. Thus, it can beconcluded that the melting and casting processes were acceptable.

Table 1 Melting and pouring parameters

Vacuum

Pa

1J3 10000 J15000 18 J25 300

Melting current

A

Melting voltage

V

Rotating speed

r/min

1.4 Hot Isostatic Pressing (HIP) and heat

treatment of cast Ti-15V-3Cr-3Al-3Sn Alloy

HIP was applied for castings and specimens in order to reduceshrinkage voids, and to increase their compactness, reliability

Table 2 Chemical composition of the Ti-15V-3Cr-3Al-3Sn re-melted ingot, wt-%

Nominal composition

V Cr Al Sn O N

Analyzed

composition

Top of ingot

Middle of ingot

Bottom of ingot

14.0 J16.0

14.68

14.58

14.80

2.5J 3.5

2.86

2.79

2.91

2.5J 3.5

2.99

2.88

2.86

2.5J 3.5

2.95

2.80

2.61

0.15

0.12

0.14

0.13

0.05

0.022

0.029

0.033

Table 3 Chemical composition of the Ti-15V-3Cr-3Al-3Sn casting, wt-%

Nominal

Analyzed

V Cr Al Sn O NFe C H

14.0 J16.0

14.68

2.5J 3.5

2.86

2.5J 3.5

2.99

2.5J 3.5

2.95

0.25

0.07

0.15

0.13

0.05

0.020

0.05

0.021

0.015

0.002

2.2 Centrifugal casting of Ti-15V-3Cr-3Al-3Sn

alloy setting

The ability of the skull furnace to superheat the molten metal isvery limited because of the cooling effect of the water-cooledcrucible. Therefore, the fluidity of liquid Ti-15V-3Cr-3Al-3Snalloy was low. In this research, bottom pouring and centrifugalcasting were used to enforce liquid alloy to fill the shell rapidly,completely, and steadily. Based on the ceramic shell’s strengthand casting structure, rotate plate speed of the skull furnace wasset at 300 r/min. In order to further increase the mould fillingspeed, open pouring system was selected. Experimental resultshowed that the casting was formed successfully.

Since as-cast Ti-15V-3Cr-3Al-3Sn alloy is brittle at roomtemperature and its castings often crack when being knockedout of the shells, high pressure water jet cleaner was used toknock the shell. This can reduce greatly the impact on castings,and thus reduce the cracks in Ti-15V-3Cr-3Al-3Sn settings aswell.

Using investment centrifugal casting process, the Ti-15V-3Cr-3Al-3Sn setting was successfully cast. Surface contamination

layer on the casting was easily eliminated by chemical millingand sand blasting. The surface roughness was below 6.3 µm.The wall thickness varied from 1.4 J 1.6 mm within the tolerance.The contact gap between the arc-shaped face and master profiletemplate was below 0.2 mm. All results could meet therequirements of Ti-15V-3Cr-3Al-3Sn setting casting drawing.The investment casting process has achieved near net-shapecasting level. Figure 3 shows the investment cast Ti-15V-3Cr-3Al-3Sn settings.

Fig. 3 Investment cast Ti-15V-3Cr-3Al-3Sn alloy settings

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Table 4 Mechanical properties of cast

Ti-15V-3Cr-3Al-3Sn alloy at room temperature

Specimen

No.

1

2

3

4

5

1222

1193

1233

996

982

1.4

1.6

1.6

1.2

1.0

HIP+HT

HIP+HT

HIP+HT

HIP

HIP

Ultimate tensile strength

MPa

Elongation

%State

Fig. 6 Microstructure of cast Ti-15V-3Cr-3Al-3Snalloy after HIP+HT

Fig. 4 Microstructure of cast Ti-15V-3Cr-3Al-3Sn alloy

Fig. 5 Microstructure of cast Ti-15V-3Cr-3Al-3Sn

alloy after HIP

2.3 HIP and HT of cast Ti-15V-3Cr-3Al-3Sn alloy

In order to improve the properties of as-cast Ti-15V-3Cr-3Al-3Sn setting casting, HIP and HT were used. Mechanical propertiesafter HIP and HT were shown in Table 4. Microstructure of theas-cast, HIPed and HIPed+HTed samples were shown in Fig.4,Fig. 5 and Fig. 6, respectively.

As-cast Ti-15V-3Cr-3Al-3Sn alloy has an elongation near tozero at ambient temperature. Table 4 shows that elongation atambient temperature was increased to 1.4 % J 1.6% and ultimatetensile strength was increased greatly from 982 MPa to 1193MPa after HIP and HT. Because the ductility was over 1% afterHIP and HT, castings and specimens can be successfullymachined by drilling, finishing and cutting without any cracks.

At as-cast state, because of the high cooling rate, themicrostructure of cast Ti-15V-3Cr-3Al-3Sn is fine, as shown inFig. 4(a). Figure 4(a) shows that there were some micro-voids inthe casting. These micro-voids lead to low tensile strength andlow elongation. After HIP, coarse microstructure was obtained,as shown in Fig. 5(a). Micro-voids disappeared and some needle-like alpha phase appeared along the boundary and also in thebeta grains, as shown in Fig. 5(b). The elongation slightlyincreased after HIP. After HIP and HT, the microstructure of Ti-15V-3Cr-3Al-3Sn alloy was coarser than those after HIP only,as shown in Fig. 6(a). Figure 6(b) shows that significant amountsof fine-needle-like alpha phases appeared in beta matrix afterHT. From the alpha phases strengthening effect, cast Ti-15V-3Cr-3Al-3Sn obtained a high tensile strength of over 1,100 MPa.By selecting suitable solution treatment and aging processingparameters, the cast Ti-15V-3Cr-3Al-3Sn can have high tensilestrength and optimal elongation.

3 Conclusions

(1) Cast Ti-15V-3Cr-3Al-3Sn ingot could be produced byVacuum Arc Melting process, and its composition washomogeneous.

(2) With investment centrifugal casting process, cast Ti-15V-3Cr-3Al-3Sn setting with 1.5mm thin wall was successfullymanufactured. The contact gap between the arc shaped face andmaster profile template was below 0.2 mm. The cast Ti-15V-3Cr-3Al-3Sn setting has achieved near net-shape casting level.

(3) After HIP and HT, the mechanical properties of the casting

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increased greatly; its ultimate tensile strength exceeded 1,100 MPa.

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