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Vol. 10
November Issue
Year 2009
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Science Update


in Vol. 10 - November Issue - Year 2009
Enhancing Fatigue Performance of the New Titanium Alloy TIMETAL-54M by Thermo-Mechanical Treatments and Mechanical Surface Treatments



Figure 1: Quasi-binary phase diagram of TIMETAL-54M








Figure 2: Examples of AC microstructures


Table 1: Tensile properties of the various microstructures


Figure 3: Micro-hardness-depth profiles (EQ)


Figure 4: Residual stress-depth profiles (EQ)








Figure 5: S-N curves (R = -1) in rotating beam loading

Introduction

TIMETAL-54M was developed by TIMET Company in Henderson, NV (USA) to provide a cost benefit for components that require extensive machining. Compared to the well known Ti-6Al-4V alloy, TIMETAL-54M has a lower Al content and contains slight additions of Mo and Fe that reduce the ?-transus temperature [1]. Therefore, the working temperatures necessary in (?+?) thermo-mechanical processing are also reduced. While the superiority in forming behavior and machine ability over Ti-6Al-4V is already documented in the literature [2] there is a lack of information if mechanical properties particularly, fatigue performance is as good as in Ti-6Al-4V. In addition to microstructural aspects in fatigue performance, the present paper gives a short overview to what extent the fatigue strength of this new titanium alloy can be enhanced by suitable mechanical surface treatments.

Thermo-Mechanical Processing, Microstructures and Tensile Properties

Material blanks were uni-directionally rolled in the (?+?) phase field and subsequently annealed at various temperatures to generate fully lamellar (?), duplex (D) and fully equiaxed (EQ) microstructures (Fig. 1).

The cooling rate from the various annealing temperatures was varied from water-quenching (WQ: 8000 K/min) to air cooling (AC: 400 K/min). All conditions were given a final heat treatment at 500 °C for 24 hours to age-harden the ?-phase by Ti3Al precipitates and the ?-phase by fine secondary ? particles.

As examples, the various AC microstructures are illustrated in Figure 2.

The tensile properties of the various microstructures comparing fully lamellar, duplex and fully equiaxed AC as well as WQ conditions are listed in Table 1.

Mechanical Surface Treatments and Fatigue Performance

Shot peening (SP) was performed using SCCW14 to full coverage at an Almen intensity of 0.20mmA which resulted in the best fatigue performance.
Ball-burnishing (BB) was done by means of a conventional lathe using a hydrostatic tool system with a Ø6mm hard metal ball and a rolling pressures of 300 bar which gave best fatigue results.

Fatigue tests were performed in rotating beam loading (R = -1) on hour-glass shaped specimens (4mm minimum diameter) at a frequency of 50 Hz in lab air. For comparison, an electrolytically polished condition (surface removal of roughly 100µm after machining) was taken as reference to which the results after mechanical surface treating will be compared.

Results and Discussion

Yield stress and tensile strength of fully lamellar microstructures strongly depend on cooling rate as shown in Table 1. This dependency becomes less pronounced in duplex microstructures as the ?p-volume fraction increases from 15 to 40%. Finally, the cooling rate from the recrystallization anneal completely vanishes in case of fully equiaxed microstructures. With an increase in ?p-volume fraction from 0 (fully lamellar) to 100% (fully equiaxed), the tensile ductility values clearly increase, this being caused by a reduction in slip length of moving dislocations. Similar results are reported on Ti-6Al-4V [3].

As shown in Figure 3 on the fully equiaxed microstructure, both SP- and BB-induced plastic deformations increase the surface layer hardness. While not much of a difference in the maximum hardness at the surface was found between BB and SP, BB results in much greater depths of plastic deformation.

Residual stress-depth profiles after SP and BB of the fully equiaxed microstructure are shown in Figure 4. Compressive residual stresses were observed with pronounced maxima below the surface. Again, the penetration depth of residual compressive stresses after BB is much higher than after SP.
The HCF performance of the various microstructures and surface conditions are shown in Figure 5.

The 107 cycles fatigue strengths of the electrolytically polished (EP) conditions slightly decrease from fully lamellar (Fig. 5a) over duplex (Fig. 5b) to the fully equiaxed microstructure (Fig. 5c).

Shot peening markedly improves the HCF strength values of these electropolished conditions in the various microstructures while ball-burnishing leads to the most marked enhancements. The latter result is thought to be related to the deeper penetration depths of work hardening (Fig. 3) and residual compressive stresses (Fig. 4). In addition, ball-burnishing results in surface roughness values which are only marginally higher than those of the electrolytically polished reference.

Summary

Among the various microstructures of the new titanium alloy TIMETAL 54M, the duplex microstructures result in the best overall combination of high yield stress, high tensile ductility and good HCF performance. While the fully lamellar microstructure is the best in HCF performance presumably, due to the absence of ?p phase, its poor ductility (Table 1) clearly limits the application of this microstructure in monotonic loading as well as under LCF conditions. Finally, the fully equiaxed microstructure can be of first choice for components of thicker section size since unlike the situation in duplex microstructures, yield stress and HCF strength of this lamellae-free microstructure do not suffer from lowering the cooling rate from the recrystallization anneal (Table 1). Comparing the fatigue performance of TIMETAL-54M with that of similar microstructures in Ti-6Al-4V [3], TIMETAL-54M is slightly superior, probably due to higher degrees of solid solution hardening in the ?-phase.

Both shot peening and ball-burnishing significantly enhance the HCF performance of the various microstructures similarly as previously found on Ti-6Al-4V [5]. The enhancement of HCF strengths in TIMETAL-54M relative to the electropolished baseline ranges from 5 to 12 % due to shot peening and from 13 to 21% in case of ball-burnishing.

Acknowledgements

Thanks are due to Dr. Yoji Kosaka of TIMET Company for providing the TIMETAL-54M alloy.

References

[1] V. Venkatesh, Y. Kosaka, J. Fnning, S. Nyakana:
Ti-2007 Science and Technology (M. Niinomi, S. Akijama, M. Hagiwara, M. Ikeda, K. Maruyama, eds.), The Japan Institute of Metals (2007) 713.

[2] Y. Kosaka, S. P. Fox, K. Faller:
Ti-2007 Science and Technology (M. Niinomi, S. Akijama, M. Hagiwara, M. Ikeda, K. Maruyama, eds.), The Japan Institute of Metals (2007) 1383.

[3] J. Mueller, H.J. Rack, L. Wagner:
Ti-2007 Science and Technology (M. Niinomi, S. Akijama, M. Hagiwara, M. Ikeda, K. Maruyama, eds.), The Japan Institute of Metals (2007) 383.

[4] L. Wagner:
Surface Performance of Titanium (J.K. Gregory, H.J. Rack, D. Eylon, eds.) TMS-AIME (1996) 199.




Author: Prof. Lothar Wagner, Dr.-Ing. Mansour Mhaede, Khaung Zay, M.E.
Institute of Materials Science and Engineering
Clausthal University of Technology
Agricolastrasse 6, 38678 Clausthal-Zellerfeld, Germany
Tel. +49.5323.72.2770, Fax +49.5323.72.2766
E-mail: lothar.wagner@tu-clausthal.de,
mansour.mhaede@tu-clausthal.de,
zay.khaung@tu-clausthal.de