Fabrication of Large Scale Bars of Ti-6Al-4V ELI for Aerospace Application

Baoquan Fu1. 2, Pingxiang Zhang1. 2

, Wensheng Wang2, Xiaoyun Shi2

, Xiaomiao Cheng2,

Xianghong Liu1. 2, Yong Feng1. 2

1 > School of Materials Science and Engineering, Northwestern Polytechnical University, Xi' an 710072, China; 2 > Western Superconducting Technologies Co. ,Ltd,Xi'an 710018,China

This paper researched the production and mechanical properties of large scale Ti-6Al-4 V ELI bars. Three times vacuum arc melting were used to

fabricate big Ti-6Al-4 V ELI ingots. The chemical elements of impurity and aluminum. vanadium were analyzed from the top, middle and bottom

of ingot. The chemical elements of cross section also were analyzed. The difference of main and impurity elements are very small. To get the

homogeneous mechanical properties was very difficult to fabricate the large scale Ti-6Al-4 V ELI bars. A special forging technology was adopted

to improve the metallurgy microstructure for large scale bars afterwards. The macro and micro structures become finer and finer from the begin­ning to the end during forging. The very fine grain and very homogenous microstructure are observed in large scale bars from <j.200mm to

4'650mm. The mechanical properties and the damage tolerance properties indicate that the large scale bars could be used to make the important

parts of aircraft.

Keywords: Ti-6Al-4 V EL I , moderate strength , damage tolerance properties, vacuum arc melting ,chemical elements, macro and micro structure

1. Introduction

Ti-6Al-4 V (Ti-6-4) alloy is the most widely used material for aerospace application. Relatively high im­purity composition will increase the risk of metallurgy defect, decrease the mechanical properties and fatigue life!>. The US and Britain developed the new Ti-6Al-4 V ELI (Ti-6-4 E) alloy which has relatively low im­purity components recently 20 years2>. It is a kind of structure material with moderate strength and high damage tolerance properties. It can be used to design long life aircraft. It is necessary to produce the large scale bars of Ti-6-4 E for aircraft's beam and frame. In order to get good working and mechanical properties, it is very important to ensuring low chemical impurity and vacancy elements. At the same time, the melting and forging process become very difficult for its large scale size. The very homogenous chemical elements for big ingots and fine microstructure for large scale bars must be considered as the most important factors for aerospace application. This paper summarize the results of composition and large scale bars from 8 ton Ti-6-4 E ingots which were fabricated by Western Supercon­ducting Technologies Co. ,Ltd. (WST). The large scale bars of c/>200mm-c/>650mm were successfully fabrica­ted. The ultrasonic test shows that the bars micro­structure is very homogenous. The damage tolerance properties are also tested. The mechanical properties also proved this large scale bars can be used for the im­portant parts of aircraft.

2. Experiments

A c/>920mm ingots of Ti-6-4 E were prepared by triple vacuum arc melting. The chemical impurity and vacancy elements were analyzed from the top, middle and bottom of ingots. A very special forging technology was used to improving the metallurgy microstructure

for large scale bars afterwards. The macrostructure and microstructure were observed at every forging procedure.

The main chemical elements from different parts of ingot were analyzed by Chemical Laboratory of NIN. The mechanical properties, macrostructures and microstructures were analyzed by Xi' an Aviation En­gine Laboratory.

3. Results and Discussion

3. 1 The Chemical Composition for cl><120mm Ingots of Ti-6-4 E To evaluate the quality of ingot, two methods

were used to get the sample. Figure 1 showed the posi­tion of samples.

SI

(a) Longitude direction

S6

S7

S2 S3 S4

SS

S9

( b) Cross direction

Figure I. The sample positions of ingot

Fu boog uan

SS

The Figure 2 (a) shows chemical composition re­sults of main elements for c/>920mm ingot from the longi­tude direction. The cross direction results were showed in Figure 2 ( b). Figure 2 ( c) shows the chemical compo­sition results of oxygen element for c/>920mm ingot.

Fu boog uan The results indicated that the chemical composi­

tions of Al and V for Ti-6-4 E were very homogeneous from the top to bottom and the side to center. The

9. Aerospace Applications • 1985 •

difference of Al elements were only 0. 15 % , and 0. 1 % for V elements. The homogeneous level of impurity el­ement oxygen was very import for big sized ingot. The chemical compositions of oxygen element could be con­trolled at very narrow range about 0. 02%~0. 03 %. It performed that WST had the ability to produce the big sized ingot for large scale bars. The metallurgy defects were not found in this big sized ingot.

~ 0

7 ~ v 6

5

4 ~

3

2

0 2 3 4 5

( a ) The main elements composition results of longitude direction

7

6

5

4 ~ 3

2

I

0

-

2

-

3 4 5

l~¢1 I

- - -

6 7 8 9 ( b ) The main elements composition results of cross direction

0.14 Oxygen Max:O. 13%

0.12

0.10 CTI 3

0.08

0.06

0.04

0.02

0.00 2 3 4 5 6 7 8 9

( c ) The oxygen elements composition results of cross direction and longitude direction

Figure 2. The chemical composition results for 4'920mm ingot

3. 2 The Macro and Micro Structure for Different Sized Bars of Ti-6-4 E The macrostructure was very important for large

scale titanium alloy bars. The samples from each billet were cut by band saw machine. And the turner was used to working the good surface of sample. The mac­rostructure was appeared by acid erosion. The samples for microstructure got from each macrostructure sam­ple which were cut from the side to center by liner cut­ting machine.

Figure 3 shows the macrostructure of Ti-6-4 E for

different size bars. The macrostructure of all sized of

bars were blur grain. It indicates that the grain size will be very fine. At same time, the forging process c;ould solve the non-uniform macrostructure problems of cen­ter of big sized bars. Especially, the macrostructure of 'i>650mm bar was also very good. It was the largest Ti-6-4 E bars in the world.

Ca) <f>210mm ( b) <f>300mm

Cc ) <f>380mm ( d) H50mm

Ce) <f>650mm

Figure 3. The macrostructure of Ti-6-4 E for different size bars

Figure 4 shows the microstructure of Ti-6-4 E for different size of bars. From the side to center, all the microstructures were observed. The microstructure was very homogeneous from the side to center for dif­ferent size of bars. The microstructure was the typical a+~ two phase. It was very fine equiaxed structure. The grain was spheroidized. Especially for <j>650mm bar, the grain size was also very small. It was satisfied the requirements for aerospace application. It indicates that WST had already charged the forging technology for big sized bars.

3. 3 The Mechanical Properties for Different Sized Bars of Ti-6-4 E

The mechanical properties were tested by Xi' an Aviation Engine Laboratory. Three samples were test­

ed for each position.

1986 • Proceedings of the 12'h World Conference on Titanium

Figure 5 shows the mechanical properties for dif­ferent sized bars. The mechanical properties for each sized bar were almost uniform from the side to center and the longitudinal direction to cross direction. It indi­cates that very fine microstructure ( referenced by above results) could ensure the relatively good me­

chanical properties.

( a ) S ideof +210mmbar ( b ) Center of + 2 IOmm bar

( c ) Side of +300mm bar ( d ) Center of + 300mm bar

( e ) Sideof+450mmbar ( f) Center of +450mm bar

( g ) Side of +650mm bar ( h ) Center of + 650mm bar

Figure 4. The microstructure of Ti-6-4 E fo r different size bars

900

800

700

600

~ 500 2

400

300

200

100

0 --. ................. -"----

1/40 Center Side Cross l/40

Longitudinal

( a ) Strength properties of +21 Omm bar

800

~ 600

~

400

200

0

1000

800

600

200

0

800

600

400

200

0

- -- ~ -

I"

I/ 40 Center Side Cross I/ 40 Longitudinal

( b ) Strength properties of + 300mm bar

2 3 4 Longitudinal I/ 40

5 6 7 Cross I /40

( c ) Strength properties of +450mm bar

Side Middle Center Cross

( d ) Strength properties of +650mm bar

Figure S. T he mechanical properties of Ti-6-4

E for different size bars

3. 4 The Comparing Fatigue Properties between the Ti-6-4 E and Ti6Al4V(Ti-6-4) The crack growth rate was the very important fac­

tor for describing the fatigue properties of titanium al­

loy. In order to evaluate the advantage fatigue proper­ties of Ti-6-4 E alloy, the Ti-6-4 alloy was compared.

Figure 6 shows the figure of comparing fatigue proper­

ties between the Ti-6-4 E and Ti-6-4 alloy bars. The

da/ dN values were almost same with the different ~K

9. Aerospace Applications • 1987 •

value for Ti-6-4 E alloy. The da/dN values of Ti-6-4 al­loy bar were higher than Ti-6-4 alloy. It indicates that the fatigue properties of Ti-6-4 E alloy are better than Ti-6-4 alloy.

~ ...

0.00248

9. I I 882E-4

~ 3 .3 5463 E-4 u ....... ~ 1.2341 E-4

~ 4.53999E-5 ....... "' -0

I .67017E-5

6.14421 E-6

Dia.210 Dia.300 Dia.380 Dia.450 Dia.350 Dia.350

The Size of Bar (mm)

Figure 6. The figure of comparing fatigue properties

between the Ti-6-4 E and Ti-6-4 alloy bars

3. 5 The Results of Ultrasonic of Ti-6-4 E Alloy Bars The ultrasonic test used to detect the metallurgy

defects and evaluate the microstructure homogene­ous. The table 1 lists the ultrasonic test results for different sized bars of Ti-6-4 E. The results show that the noise of ultrasonic are very low. It indicates that the microstructure of Ti-6-4 bar is very fine and homogeneous.

Table I. the ultrasonic test results for different sized bars of Ti-6-4 E

Size( mm) Flat bottom hole Results

+210 2.0 -I6dB

4'300 3.2 -14dB

4'380 3. 2 -12dB

"'450 3. 2 -8dB

4'650 3. 2 -8dB

4. Conclusions

The ~920mm Ti-6-4 E ingot was fabricated by WST successfully. And the large scale Ti-6-4 E bars (such as. ~200, ~300, ~380, #50, ~650mm ) were pre­pared by special forging process. The chemical compo­sition of large ingot is very homogeneous. The macro and micro structure of large scale bars are very fine and uniform. The mechanical properties and the fatigue properties are satisfied with the requirements of avia­tion standard. The noises of ultrasonic test are also ver­y low. Comparing with the normal Ti-6-4, the Ti-6-4 E alloy is very perfect material for aerospace application.

Acknowledgements The authors wish to thank the Western Supercon­

ducting Technologies Co. , Ltd for founding and sup­port.

REFERENCFS 1) F. H. Froes.A Mitchell,0. N. Senkov,Published by TMS, War­

rendale, USA, 1998: 135-145.

2) B. Hussey,J. Wilson. Published by Chapman & Hall, London,

UK, 1998: 320-342.