EXPERIMENTS ON THE COMPRESSION BEHAVIOUR OF TI6AL4V FOR WIDE RANGES OF STRAIN RATE

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3^ème Conférence Internationale sur

le Soudage, le CND et l’Industrie des Matériaux et Alliages (IC-WNDT-MI’12) Oran du 26 au 28 Novembre 2012,

http://www.csc.dz/ic-wndt-mi12/index.php 35

EXPERIMENTS ON THE COMPRESSION BEHAVIOUR OF TI6AL4V FOR WIDE RANGES OF STRAIN RATE

A. Darsouni¹, D. Toualbia^{1, 2} and L. Darsouni¹

1Laboratory of Metallurgy and Material Engineering, Badji Mokhtar University, Annaba 23000, Algeria Email address: darsouniabdel@yahoo.fr

2University of Souk Ahras 41000 Souk Ahras Algeria Email address: toualbia_djamel@yahoo.fr

Abstract.

In this paper, the thermo-viscoplastic behaviour of Ti6Al4V under compression loading is analyzed.

Experiments using two different setups have been performed. Tests at low strain rates

1 1 p

1

4s 5.10 s

10

5 ^ ^   ^ ^

were conducted using hydraulic machine. Dynamic tests were carried out for strain rates in the range4564s^¹^p 5874s^¹ . For that task, it was used a modified Hopkinson bar which is based on direct-impact technique. For strain rate level higher than10² s^-1, the process of plastic deformation in most metals and alloys is assumed adiabatic [1]. The heat generated inside the material due to plastic deformation cannot be transmitted. The increase of temperature is dependent on the flow stress of the material. It has particular relevance in titanium alloys, which are characterized by their high flow stress level. In this work, special attention is focused on this phenomenon.

1 Introduction

Titanium alloys are considered among the most important metal alloys in engineering. They play a relevant role in several application fields, for example in aeronautical, chemical, marine and military industries [2-3]. Titanium alloys exhibit elevated strength-to-weight ratio, good fatigue performance, high toughness and considerable work-hardening.

In the present paper Ti6Al4V is tested under compression loading for a wide range of strain rate varying from 510^⁴s^¹^p5874s^¹. Adiabatic heating takes place at high strain rates, leading to thermal softening of the material. By application of the first principle of thermodynamics, thermal softening can be analytically estimated. Therefore, isothermal behaviour of material at high rate of deformation can be defined. It allows for comparison of the material behaviour at low and at high strain rates. Strain rate sensitivity of the material can be analyzed without thermal softening effect causing a stress decrease.

2 Experimental characterization of Ti6Al4V under compression loading

The geometry of the specimen used during experiments is based on the considerations reported in [4]. The sample has a diameter of Φ=5mm and its height is L0=3mm. A lubricant layer was applied on the contact surface specimen/support. It allows reducing eventual mechanical disturbances on the tests caused by friction effect. Thus, influence of friction during the tests may be neglected and the deformation field of the sample can be assumed homogeneous and one-dimensional.

2.1 Quasi-static behaviour of Ti6Al4V

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http://www.csc.dz/ic-wndt-mi12/index.php 36 The experimental results obtained from quasi-static tests are reported in Fig. 1. In this range of strain rate, 510^⁴s^¹^p 5.10^¹s^¹isothermal condition of deformation is assumed. The flow stress of the material is higher than 1 GPa, Fig. 1-a. The strain hardening rate is revealed as strongly dependent on the plastic strain level, Fig. 1-b. Moreover, it can be observed reduction of ductility with strain rate increase.

0 500 1000 1500 2000 2500

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4

0.0005 1/s 0.05 1/s 0.5 1/s

True stress,  (MPa)

True strain,  (-)

Dependence of strain hardening rate on plastic deformation

Material: Ti6Al4V

To = 293 K

(a)

0 2000 4000 6000 8000 1 10⁴

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4

0.0005 1/s 0.05 1/s 0.5 1/s

Strain hardening, d/d (MPa)

Material: Ti6Al4V

To = 293 K

True strain,  (-)

Dependence of strain hardening rate on plastic deformation

(b)

Figure. 1. a) True stress versus true strain at room temperature under quasi-static loading.

b) Strain hardeninig versus true strain under quasi-static loading.

In the following section are reported the results obtained from the tests performed at high strain rates

.

2.2 Dynamic behaviour of Ti6Al4V

The technique used to test the material at high strain rate is a modification of the classical Hopkinson bar configuration, Fig. 2-a. Therefore, the projectile impacts directly on the sample, Fig. 2- b. Such arrangement allows reaching larger plastic deformation and higher strain rates than classical configuration.

Figure. 2. Hopkinson bar technique; a- Classical configuration; b- Direct impact

During impact, an elastic wave is generated and transmitted to the Hopkinson bar, Fig. 2. Based on Fig. 2-b., the transmitted wave is registered using strain gauges. With knowledge of the transmitted

Projectile

Specimen Hopkinson bar

Transmitted wave propagation T

Projectile

Specimen Hopkinson bar

T

Hopkinson bar

I

R

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http://www.csc.dz/ic-wndt-mi12/index.php 37 wave _T(t), it is possible to define the stress , the strain  and the strain rate d dt to which the material is subjected during loading [5], Eqs.1-2.

 

^



^

 (t)dt L

t C _T

0

(1)

 

(t)

A E A

t _T

0







(2)

where C0 is the longitudinal velocity of the elastic wave in the Hopkinson bar, L0 is the height of the specimen , E is the young’s modulus of the Hopkinson bar, and A/A0 is the ratio of the transversal- areas of the bar and of the specimen. By combination of Eq. 1 and Eq. 2, it is possible to define the thermo-viscoplastic behaviour of material.

In Fig. 3. Are shown experimental data obtained from the dynamic tests. In comparison with the results obtained from the tests at low strain rates, the flow stress is considerable increased due to strain rate sensitivity. On contrary, strain hardening has been subjected to relevant decrease. Such behaviour is usually related to the thermal softening process. However, quantification of such phenomenon is not usually reported in the international literature. In the present paper is conducted an attempt to evaluate temperature increase effect on the material behaviour.

(a) (b)

(c)

Figure.3. Ti6Al4V true stress versus true strain at room temperature under dynamic loading in adiabatic and isothermal condition.

0 500 1000 1500 2000 2500 3000

0 0,05 0,1 0,15 0,2 0,25 0,3

TA6V

True strain 

Curve after test, isothermal

Curve after test, adiabatic

300 °K 4564 1/s

0 500 1000 1500 2000 2500 3000

0 0,05 0,1 0,15 0,2 0,25 0,3

TA6V

True strain 

300 °K 4648 1/s

0 500 1000 1500 2000 2500 3000

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35

TA6V

True strain 

300 °K 5874 1/s

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http://www.csc.dz/ic-wndt-mi12/index.php 38 The results obtained from experiments (adiabatic) are converted to isothermal condition following the procedure reported by Klepaczko [1]. The isothermal stress may be defined by Eq. 3.

  

^, ^,

^

^T ^T

^  

p^, p^,

^

^T0 ^T

^ 

0 p p adiab p

isoth        

  

⁽³⁾ Where _adiab is obtained from experiments and  is calculated by application of the first principle of thermodynamics, Eq. 4.

 

   ^ ^ 

^p

0

p 0 adiab p p

p 0 p

p max

d T T , C ,

T T ,

,     



 













 ^ ^ ^

(4) Where ρ is the density of the material, Cp is the specific heat at constant pressure and β is the Quinney- Taylor coefficient. The values of these parameters for Ti6Al4V are reported in Table 1.

Table 1. Some physical constants of Ti6Al4V ρ (kg/m

³

) C

_p

(J/kgK) β (-)

4430 564 0.9

Moreover,is the temperature sensitivity of the material. In a general way it is defined by Eq. 5.

) 5 ) (

T (

) (

p





 





In the present case we have assumed a constant value of



 1.4 MPa/Kas it is reported in [6]

for titanium alloys. Using such procedure it can be observed in Fig. 3., the relevance of thermal softening on the material behaviour. For an imposed strain level,



^p0.25, the stress decrease is about 15.63% for

 

^p

 4564 s

^¹and 14.18% for

 

^p

 5874 s

^¹_.

On the following curve, Fig. 4., the temperature increase Talong with plastic deformation is reported for different strain rate values. It is observed large temperature increase close to 

T



200 K

for the highest strain rate level considered.

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http://www.csc.dz/ic-wndt-mi12/index.php 39 Figure.4. Temperature increase at high strain rates during plastic deformation

With knowledge of the isothermal behaviour



_isoth

 

^p , it is possible to obtain the rate sensitivity of the material, Fig. 5., without thermal softening effect inducing a stress and strain rate sensitivity decrease.

Figure. 5. Strain rate sensitivity definition for an imposed strain level at room temperature

It is observed linear strain rate sensitivity up to5000s^¹. Beyond that point the rate sensitivity sharply increases. Such behaviour is related to viscous drag as reported in [7-9]. It is characteristic of many FCC and HFC alloys.

3 Conclusion

0 50 100 150 200 250

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35

5874 (K) 4648 (K) 4546 (K)

y = -2,1656 + 635,2x R= 0,99931 y = -1,2491 + 521,81x R= 0,99955

Temperature increase, T (K)

True strain, 

For 5874 1/s

For 4546 1/s

Slope T-

0 500 1000 1500 2000 2500 3000

-4 -3 -2 -1 0 1 2 3 4

Strain level = 0.05 Strain level = 0.10

True stress,  [MPa]

Logarithm of the strain rate, log (1/s).

TA6V

isothermal condition 300 °K

93.99 MPa/log(1/s)

3505.2 MPa/log(1/s)

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http://www.csc.dz/ic-wndt-mi12/index.php 40 In the present paper the behaviour of Ti6Al4V under compression loading is studied. Using two different experimental techniques a wide range of strain rates has been covered during the test. Due to high flow stress level exhibited by Ti6Al4V alloy, adiabatic heating at high strain rates plays a relevant role on the material behaviour. Thermal softening is analytically calculated by application of the first principle of thermodynamics. Its influence on the material behaviour is shown up. Obtaining the isothermal behaviour of the material at high strain rates allows for defining rate sensitivity of the material. Such procedure is useful for forthcoming modelling of the material behaviour.

Acknowledgement: Thanks Mr. Richard Bernier for his participation on the experiments.

This paper is dedicated to my PhD director and our friend, Professor Janusz Roman Klepaczko.

We were performing the last tests together in July, 2008. We miss you.

References

[1] J.R. Klepaczko, 2007. Introduction to experimental technique for materials testing at high strain rates. Al. Krakowska, 110/114, 02-256, pp237-256. Warsaw, POLAND.

[2] C. Mary, S. Fouvry, J.M. Martin, B. Bonnet, 2008. High temperature fretting wear of a Ti alloy/CuNiIn contact Surf&Coat Tech.doi:10.1016, pp 691-698.

[3] J.R. Meyer, H.B. Bomberge, F.H. Froes, 1984. Corrosion behavior and use of titanium and its alloys. J. Met. 36(10), pp 50-60.

[4] J.Z. Malinowski, J.R. Klepaczko, Z.L. Kowaleski 2007. Miniaturized compression test at very high strain rates by direct impact. Exp. Mech, doi 10.1007/s11340-006-9007-7.

[5] H. Kolsky, 1952. Stress waves in solids, Dover Publications Inc., New York.

[6] M. Jarkas, H. Elkhatib, R. Haje Chehade , B. Ghazi. 2008. Etude experimentale du comportement viscoplastique du titane pur Ti40 à differentes vitesses de déformation et températures. Lebanese Science journal, Vol.9, N°1, pp 123-129.

[7] A. Rusinek, J. A. Rodríguez-Martínez. Thermo-viscoplastic constitutive relation for aluminum alloys, modeling of negative strain rate sensitivity and viscous drag effects.

Materials and Design.

Submitted for publication.

[8] Regazzoni G., Kocks U. F., Follansbee P. S.1987. Dislocation kinetics at high strain rates.

Acta Metallurgica ; (35) 12, pp 2865-2875.

[9] Rodriguez-Martinez J.A., Rusinek A., Klepaczko J. R., Pęcherski R.B. 2008. Extension of RK constitutive relation to phase transformation phenomena. J Mater Design, doi:

10.1016/j. matdes. 09.043.

[10] Guo W.G., Nemmat-Nasser S. 2006. Flow stress of nitronic-50 stainless steel over a wide range of strain rates and temperatures. Mech. Mater, 38:1090-103.

EXPERIMENTS ON THE COMPRESSION BEHAVIOUR OF TI6AL4V FOR WIDE RANGES OF STRAIN RATE

EXPERIMENTS ON THE COMPRESSION BEHAVIOUR OF TI6AL4V FOR WIDE RANGES OF STRAIN RATE

1 Introduction

2 Experimental characterization of Ti6Al4V under compression loading

2.1 Quasi-static behaviour of Ti6Al4V

(a)

(b)

.

2.2 Dynamic behaviour of Ti6Al4V

 



(1)

 

(2)

(a) (b)

(c)

  



  



 

 

     

Table 1. Some physical constants of Ti6Al4V ρ (kg/m

) C

(J/kgK) β (-)

) 5 ) (

T (

) (

 

 4564 s

 

 5874 s

T

200 K

 

3 Conclusion

References

^

^  

^

^ 

   ^ ^ 