A New Low Cost β Type High Strength Titanium Alloy with Lower Alloying Percentage for Spring Applications

A New Low-Cost

¢

-Type High-Strength Titanium Alloy

with Lower Alloying Percentage for Spring Applications

Xiaoli Zhao

1,+

, Shuchen Sun

2

, Lei Wang

1

, Yandong Liu

1

, Jidong He

2

and Ganfeng Tu

2 1_{Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy,} Northeastern University, Shenyang, Liaoning 110819, China

2_{School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning 110819, China}

Spring applications require materials that possess a low Young’s modulus and high strength, in addition to having a lower alloying percentage for reduced costs. In this study, a new low-cost¢-type titanium alloy with a lower alloying percentage than conventional¢-type alloys was developed for spring applications using [Al]eqand [Mo]eq. The alloy is located in the near-¢region and its¢transus temperature is

about 1023 K (750°C). This alloy could be cold rolled to a reduction ratio of 86.7%without any intermediate heat treatment. The solutionized alloy exhibits an ultimate strength of 925 MPa, a 0.2%proof stress of 910 MPa, an elongation of 18%, and a Young’s modulus of 78 GPa; the comprehensive characteristics are better than those of conventionally used alloys (Ti-15-3 and¢C) for spring applications.

[doi:10.2320/matertrans.M2014174]

(Received May 12, 2014; Accepted June 11, 2014; Published July 18, 2014)

Keywords: titanium alloy, high strength, low alloying percentage, low cost, spring applications

1. Introduction

Titanium alloys possess high specific strength, corrosion

resistance, thermal resistance, and compatibility; conse-quently, they are widely used in various industries such as

automobile, aircraft, shipping, and oil exploration.1)_Springs

are used by about all aircraft manufacturers and finds usage

in a limited capacity in applications ranging from the

horologe to automobiles.2­4) _{Beta-type titanium alloys are}

ideal materials for spring applications given their high strength, low modulus, low density, and excellent corrosion resistance. The low modulus and density in conjunction with the high strength enable the realization of springs that are only half the volume of steel springs (only half the number of

coils are required) with weight reductions of up to 70%

relative to the steel springs.5,6) In addition, titanium alloys

offer lifecycle cost advantages for spring applications, in light of the fact that they have higher corrosion resistance, as evidenced by previously used piano wire steel springs that required constant maintenance due to corrosion. Typical airframe applications include up-lock and down-lock springs as part of the landing gear, door counter-balance springs,

flight control springs to ensure speed limit compliance, feel

and centering springs for the yoke, pedal returns (brakes), and

hydraulic return springs.7,8)_{The extent of usage of springs in}

aircrafts seems to be limited only by the alloy used for the

springs. It is reported that Ti-8V-6Cr-4Mo-3Al-4Zr (¢C)9)

wires are used for the up-lock and down-lock springs for

Bombardier regional jets, while Cessna jetfleets employ

Ti-15V-3Cr-3Al-3Sn (Ti-15-3) clock-type springs10)to assist in

the deployment/retraction of the doors/fold-out stairs, with

both the alloys being beta-type alloys. However, a disadvant-age of the beta-type titanium alloy system is the greater possibility of segregation of elements occurring during melting. Reducing the alloying element content is one method to avoid such a problem, and doing so can also help

lower costs. In this study, a new low-cost ¢-type

high-strength titanium alloy with a low alloying percentage for spring applications was developed, and its microstructures and mechanical properties were examined.

2. Experimental Procedures

2.1 Materials design

Aluminum is a well-known solute-strengthening element for titanium alloys; however, the total content of Al should

be less than 7% in order to obtain ductility. Vanadium and

molybdenum are the alloying elements that offer increased strength, improved thermal stability, and excellent ductility. Iron and chromium have a lower density than Mo but a stronger beta-stabilizing effect, and are therefore the main alloying elements for high-strength metastable beta-type titanium alloys. Silicon is effective in improving thermal

resistance when its content is lower than 0.25%. Zirconium is

always added to titanium alloys for solid solution strengthen-ing effects, and for improvstrengthen-ing their thermal resistance and

welding performance.11)

The new alloy in this study was designed considering Al

equivalence (Aleq) and Mo equivalence (Moeq) conditions, as

shown in eqs. (1) and (2)12)

½Aleq¼%Alþ%Sn=3þ%Zr=6þ%Si=0:25 ð1Þ

½Moeq¼%Moþ%Ta=4:5þ%Nb=3:6þ%W=2:25

þ%V=1:5þ%Cr=0:63þ%Mn=0:65

þ%Fe=0:35þ%Co=0:7þ%Ni=0:9 ð2Þ

The% in the eqs. (1) and (2) refer to mass%.

It has been reported that the Moeqof metastable beta-type

titanium alloys is more than 11%, and if alpha-stabilizers

coexist in the alloys, it is important to increase the Moeqto

obtain the ¢ phase after water quenching.10) _{With the}

objective of lowering the alloying percentage and thereby the costs, the alloy Ti-3Al-4V-6Cr-4Mo-0.8Fe-0.4Zr-0.01Si

with a nominal [Al]eq=3.1, [Mo]eq=18.5 was developed;

this alloy is hereafter referred to as Ti-18.

+_{Corresponding author, E-mail: zhaoxl@smm.neu.edu.cn}

2.2 Materials

The alloy (80 g) was fabricated in the form of ingots by arc melting with a non-consumable tungsten electrode under a high-purity argon atmosphere. The ingots were then homogenized at 1373 K for 21.6 ks and hot-forged at 1273 K (both processes were performed in an argon atmosphere), after which they were quenched in ice water and the air-cooled, respectively. Cold rolling was carried out

to evaluate the cold workability of the alloy. The ¢ transus

temperature was determined by differential scanning calo-rimetry (DSC) and optical microscopy (OM). The chemical composition of the alloy was determined by conventional chemical and gas analyses: the metallic elements were determined using inductively-coupled plasma optical emis-sion spectrometry (ICP-OES); Si was determined using the

SiO2 gravimetric method; C was determined by the infrared

absorption method after combustion; O was determined using the He carrier fusion infrared absorption method; and N was determined using the He carrier fusion thermal conductivity method. The chemical composition is shown in Table 1.

2.3 Methods

The phase constitution of the designed alloy was identified

by X-ray diffraction (XRD) analysis using Cu-K_¡radiation

at a diffraction angle (2ª) range of 30­90°, an accelerating

voltage of 40 kV, and a current of 40 mA. The micro-structures were observed by OM. The specimens for OM were mechanically polished using SiC waterproof emery

paper of up to #4000 grit and a final level with a

colloidal silica suspension. They were then etched in a Kroll solution.

The mechanical properties of the prepared alloys were

evaluated by Young’s modulus measurements and tensile

tests. Rectangular specimens of dimensions 40 mm

(length)©10 mm (width)©1.5 mm (thickness) were used

for Young’s modulus measurements. Specimens for tensile

tests were of thickness 1.5 mm, width 3 mm, and gage length 13 mm. Specimens for both tests were polished after they were cut by wire electric discharge machining from the hot-rolled and solutionized plates of the alloy. The longitudinal direction of these plates was parallel to the rolling direction.

Young’s modulus (E) was measured at room temperature by

the free resonance vibration method. Uniaxial tensile tests

were performed at a crosshead speed of 8.33©10¹6ms¹1at

room temperature in air using an Instron-type machine. The

ultimate tensile strength (·B), yield strength at 0.2% offset

(0.2%proof stress) (·0.2), and elongation (¤) at fracture were

correspondingly determined. For determine each property, three specimens were used to minimize the errors caused by the measurement process and possible chemical inhomoge-neity. The error bars refer to the standard deviation of the triplicate results.

3. Results and Discussions

The [Mo]eq-[Al]eq map was drawn using 95 alloys with

different [Mo]eqand [Al]eqby Zhang.10) The distribution of

the alloys in the [Mo]eq-[Al]eqmap is shown in Fig. 1. The

newly designed alloy is in near-¢phase region in that map. A

comparison of the properties of similar alloys is presented in Table 2. The new alloy, Ti-18, exhibits lower density and alloying percentage than the other alloys compared in the table.

Figure 2 shows the optical microstructures and XRD

patterns of the alloy after hot forging. The peaks of¢,¡, and

½phases can all be observed in the XRD profiles, indicating

that the alloy consisted of the ¡and½ phases in addition to

the¢matrix. The designed alloy is a near-¢alloy, the¡and½

phases in the alloy are precipitated during air-cooling after hot forging. In the OM images, the clear quasi-equiaxial

grains are the¢phase, and the acicular structure observed in

the grains is the slip bands caused by hot forging. Probably as

a result of their small size, the¡and½phases are not visible

in the OM images.

The tensile properties of the alloy after hot forging are

listed in Table 3. The ultimate strength and the 0.2% proof

stress are both more than 1200 MPa and difference between them is quite small, indicating that there was negligible work

hardening. However, the elongation is less than 1%; this

small elongation can be attributed to the defects and dislocations formed during hot forging.

In order to obtain excellent tensile properties, the heat

treatment temperature needs to be optimized. The¢ transus

temperature is an important parameter to determine the heat treatment temperature, and can be determined through calculations, DSC, and OM observations.

The following expression is used for the calculation of

[image:2.595.43.549.85.125.2]

nominal¢transus temperature.

Table 1 Nominal (N) and experimental (E) chemical compositions of the Ti-18 alloy (mass%).

Al V Cr Mo Fe Zr Si C N O Ti [Mo]eq [Al]eq

N 3 4 6 4 0.8 0.4 0.01 ® ® ® Bal. 18.5 3.1

E 2.58 3.82 5.89 3.96 0.75 0.40 0.01 0.023 0.013 0.157 Bal. 18.0 2.7

0 2 4 6 8 10 12 14 16 18 20

0 1 2 3 4 5 6

[Al]

eq

[Mo]eq 1

2, 3 4

5 8

9 10

12 (Ti-18)

11(βC)

α′(α′′) β+ω

β

(Ti-15-3)6,7

Fig. 1 Distribution of alloys in the [Mo]eq-[Al]eq map. I:¡ and near-¡

[image:2.595.315.542.103.329.2]

T ¼885_{Cþ½X T½X ð3Þ}

where [X] is the percentage of the alloy element andT[X]is

the effect of the alloying element on the ¢ transus

temper-ature, examples of which are shown in Table 4.

Inserting the values of the various parameters in expression

(3), the calculated¢transus temperature is 1064 K. The DSC

results (Fig. 3) show the ¢ transus temperature to be about

1023 K (750°C); the OM images and the XRD patterns

(Fig. 4) confirm the DSC results. The peaks of the¡,½, and¢

phases are identified for the Ti-18 alloy subjected to heat

treatment at 973 K. The peaks of the¡phase become weaker

[image:3.595.44.548.85.253.2]

with increasing the heat treatment temperature, and after heat Table 2 Properties of the alloys in the [Mo]eq-[Al]eqmap.9)

Alloy Composition

(mass%) [Al]eq [Mo]eq

Density (Calculated)

Alloying percentage

1 Transage129 Ti-11.5V-2Al-2Sn-11Zr 4.5 8.2 4.77 26.50

2 Ti-16-2.5 Ti-16V-2.5Al 2.5 11.4 4.62 18.50

3 Beta III Ti-11.5Mo-6Zr-4.5Sn 2.5 11.5 4.97 22.00

4 Ti-15-5-3 Ti-15Mo-5Zr-3Al 3.8 15.0 4.90 23.00

5 Ti-15-5 Ti-15Mo-5Zr 0.8 15.0 5.00 20.00

6 Ti-15-3 Ti-15V-3Cr-3Al-3Sn 4.0 15.7 4.68 24.00

7 Beta 21S Ti-15Mo-3Al-2.7Nb-0.25Si 4.0 15.8 4.87 20.95

8 BT35 Ti-15V-3Cr-3Al-3Sn-1Zr-1Mo 4.2 16.7 4.72 26.00

9 Ti-8823 Ti-8Mo-8V-2Fe-3Al 3.0 17.7 4.77 21.00

10 BT32 Ti-8V-8Mo-1.5Cr-1.5Fe-3Al 3.0 19.2 4.78 22.00

11 ¢C Ti-8V-6Cr-4Mo-3Al-4Zr 3.7 19.7 4.78 25.00

12 Ti-18 (E) Ti-3Al-4V-6Cr-4Mo-0.8Fe-0.4Zr-0.01Si 2.7 18.0 4.70 17.51

90°

80°

70°

60°

50°

40°

110

Intensity

,

I

(arb

. unit)

Diffraction angle, 2θ

102 201 103 112 220 220

100

β α ω

30°

(b)

500 μm (a)

[image:3.595.56.535.86.429.2]

Fig. 2 Optical microstructure (a) and XRD profiles (b) of Ti-18 alloy after hot forging.

Table 3 Tensile properties of the hot-forged Ti-18 alloy.

·B/MPa ·0.2/MPa ¤(%)

[image:3.595.323.529.456.597.2]

Hot forged 1218«43 1214«45 0.87«0.12

Table 4 Effects of various alloying elements on the¢transus temperature.9)

Classification Alloying element

Alloying percentage/%

The effect on the¢ transus temperature

¡

Al 0­2.0 +14.5°C/1.0%

2.0­7.0 +23.0°C/1.0% 7.0­11.0 +15.5°C/1.0% 11.0­20.0 +10.0°C/1.0%

N 0­0.5 +5.5°C/0.01%

O 0­1.0 +2.0°C/0.01%

C 0­0.15 +2.0°C/0.01%

0.15­1.00 +0.15°C/0.01%

¢

Mo 0­5.0 ¹5.5°C/1.0%

5.0­30.0 ¹10.0°C/1.0%

V 0­10.0 ¹14.0°C/1.0%

10.0­18.0 ¹10.0°C/1.0%

Fe 0­15.0 ¹16.5°C/1.0%

Cr 0­7.0 ¹15.5°C/1.0%

7.0­15.0 ¹12.0°C/1.0%

Si 0­0.45 ¹1.0°C/0.1%

0.45­1.0 ¹3.5°C/0.1%

Neutral Zr 0­10.0 ¹2.0°C/1.0%

0 20 40 60 80 100 120 140 0

200 400 600 800 1000

Temperture DSC

Time, t/min.

T

emperature,

T

/

°

C

DSC/mW

-25 -20 -15 -10 -5 0 5 10 15

[image:3.595.45.290.471.501.2] [image:3.595.45.291.542.785.2]

treatment at 1273 K, the alloy are mainly composed of the¢

phase (the peaks of the½phase are attributed to the a thermal

½ formed during quenching) and is comprised of equiaxed

grains of size 45 µm. It is thus evident that the ¢ transus

temperature is lower than 1273 K. The grain size usually increases with increasing heat-treatment temperature, and the strength, which is inversely proportional to the grain size according to the Hall-Petch relationship, therefore decreases.

Refinement of the alloy grain size is thus beneficial for

strength improvement. The temperature of 1273 K, which is

greater than the ¢ transus temperature of the alloy and at

which the recrystallization finishes resulting in relatively

smaller grains, is therefore an optimum temperature for solution treatment.

The as-solutionized specimens heat-treated at 1273 K for 3.6 ks were cold-rolled to evaluate their cold workability. Figure 5 shows the images before and after cold rolling to a

reduction ratio of 86.7% without any intermediate heat

treatment. The fact that such a large reduction ratio could be achieved proves that the alloy exhibits excellent cold workability, which could lead to lower production costs.

The mechanical properties were measured using the solutionized plates (the cold-rolled plates are solutionized at 1273 K for 3.6 ks followed by ice-water quenching). The

Young’s modulus and tensile properties of the solutionized

alloy are listed in Table 5. The ultimate strength of the alloy

is 925 MPa, the 0.2% proof stress is 910 MPa, and the

Rolling Direction

10 mm 10 mm

(a) (b)

[image:4.595.100.498.69.373.2]

Fig. 5 Images of Ti-18 alloy before (a) and after (b) cold rolling.

Table 5 Young’s modulus and tensile properties of solutionized Ti-18 alloy.

·B/MPa ·0.2/MPa ¤(%) E/GPa

Solutionized 925«4 910«8 17.9«3.7 77.97«0.35

200

μ

m

200

μ

m

200

μ

m

(a)

(c)

(b)

(c) (b)

112

103

110

002

101

100

β α ω

Intensity

,

I

(arb

. unit)

110

200

211

220 220 (a)

90°

80°

70°

60°

50°

40°

Diffraction angle, 2θ

30°

[image:4.595.134.462.420.543.2] [image:4.595.299.548.612.638.2]

elongation is 18%; all indicating a better balance between strength and elongation in comparison to the hot-forged alloy. The decrease in strength and increase in elongation are attributed to the fewer defects and dislocations and the

disappearance of the ¡ phase after solutionizing. A

comparison of the specific strengths and alloying percentages

of the Ti-18 alloy and other such industrial alloys is presented in Fig. 6. This comparison indicates that the Ti-18 alloy exhibits excellent properties for spring application in light

of its high specific strength and low alloying percentage

compared to the alloys Ti-15-3 and ¢C. As the Ti-18 is a

near-¢ alloy, it is possible to further improve its tensile

properties via ageing treatment by controlling the size and

volume of precipitates (isothermal ½ and ¡ phases). Also,

adding Si to the alloy can be an effective way to improve

the alloy’s thermal resistance, which affords the possibility

of using the alloy for high-temperature applications. The thermal resistance of the Ti-18 alloy and the effect of ageing on its mechanical properties needs to be further investigated.

4. Conclusions

In this study, a new low-cost¢-type high-strength titanium

alloy with a lower alloying percentage is developed for spring applications, and the following conclusions are drawn:

(1) The new Ti-18 alloy is located in the near-¢region, and

its¢transus temperature is about 1023 K (750°C).

(2) The alloy could be cold rolled to a reduction ratio of

86.7% without any intermediate heat treatment.

(3) The solutionized alloy exhibits an ultimate strength

of 925 MPa, a 0.2% proof stress of 910 MPa, an

elongation of 18%, and a Young’s modulus of 78 GPa;

all of which make the alloy more competitive compared

to conventionally used alloys (Ti-15-3 and ¢C) for

spring applications.

Acknowledgments

This work was partly supported by China Postdoctoral Science Foundation (Grant No. 2013M530933), the

Funda-mental Research Funds for the Central Universities

(Grant No. N120302002), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant

No. 20130042120043), and the Scientific Research

Founda-tion for the Returned Overseas Chinese Scholars, State Education Ministry.

REFERENCES

1) Y. Q. Zhao, Y. N. Chen, X. M. Zhang, W. D. Zeng and L. Wang:Phase Transformation and Heat Treatment of Titanium Alloys, (Central South University Press, Changsha, 2011) pp. 2­5.

2) S. R. Seagle, R. Bajoraitis and C. F. Pepka:Mater. Sci. Tech.3(1987) 97­100.

3) R. R. Boyer: Proc. 1993 Annual TMS Meeting, ed. by D. Eylon, R. R. Boyer and D. A. Koss, (The Materials Society, Warrendale, Pennsylvania, 1993) pp. 335­346.

4) R. Boyer:JOM46(1994) 20­23.

5) A. M. Sherman and S. R. Seagle: Proc. Annual Meeting Mater. Soc., ed. by R. R. Boyer and H. W. Rosenberg, (Metallurgical Society of AIME, Warrendale, Pennsylvania, 1984) pp. 281­293.

6) Titanium Cooperation of Japan: Titanium and Its Applications, (Translated by L. Z. Zhou, Metallurgical Industry Press, Beijing, 2008) pp. 31­33.

7) R. R. Boyer and R. D. Briggs:J. Mater. Eng. Perform.14(2005) 680­ 684.

8) G. Lütjering and J. C. Williams:Titanium, (2nd Edition), (Translated by L. Ting, X. Y. Yang, and S. M. Fang, Metallurgical Industry Press, Beijing, 2011) pp. 283­287.

9) X. Huang, Z. T. Zhu and H. H. Wang:Advanced Aeronautical Titanium Alloys and Applications, (National Defense Industry Press, Beijing, 2012) pp. 226­236.

10) X. Zhang, Q. J. Wang and W. Mo: Physical Metallurgy and Heat Treatment of Titanium, (Metallurgical Industry Press, Beijing, 2009) pp. 67­115.

11) X. Cheng and C. Ye: Fundamental Research and Applications of Titanium in Aerospace and Biomedical Areas, (Science Press, Beijing, 2012) p. 28.

12) W. Mo: Titanium, (Metallurgical Industry Press, Beijing, 2008) pp. 291­294.

0 50 100 150 200 250

Allo

y

ing percentage (%)

The various alloys

Specif

ic strength,

σs

/N·m·g

-1

Specific strength Alloying percentage

0 5 10 15 20 25 30

(T

i-18)

(

β

C)

12 11 10 9 8 7 6 5 4 3 2 1

(T

i-15-3)

[image:5.595.67.272.71.216.2]