EVALUATION ON THE FATIGUE STRENGHT OF TI-6AL-4V WITH WC 10CO4CR DEPOSITED BY HVOF
a
Midori Yoshikawa Pitanga Costa, aMaria Odila Hilário Cioffi, aHerman Jacobus Cornelis Voorwald
a
Fatigue and Aeronautic Materials Research Group, Department of Materials and Technology
State University of São Paulo – UNESP. Av. Ariberto Pereira da Cunha, 333 CEP 13516-410
Guaratinguetá/SP-Brazil
[email protected]
ABSTRACT
Titanium alloys is an attractive material for application in landing gears due to characteristics as it high corrosion resistance
and high strength/weight ratio. In components subjected to sliding wear it is usual improve titanium tribological properties by
surface coatings. High velocity oxygen fuel thermally sprayed coatings (HVOF) with excellent wear resistance properties have
been used in aerospace, oil and paper industries. Considering security design of components subjected to cyclical load,
fatigue strength of Ti-6Al-4V coated was evaluated. Recent researches show that WC-10Co-4Cr has superior fatigue
performance than chromium electroplated AISI 4340 steel [1]. The aim of this paper is to investigate the morphology of WC10%Co-4%Cr deposited by HVOF in Ti-6Al-4V substrate by scanning electron microscopy and its influence on the fatigue
behavior of Ti-6Al-4V coated.
Introduction
Fatigue is the main parameter to design components subjected to cyclic loading, as landing gears. Some parts of this
component operate under severe conditions, which require high strength materials as AISI 4340 and Al 7000 series. The
development of titanium alloys were motivated by the capability of high temperature application. Despite having 56% of the
steel density, the higher strength/weight ratio allows smaller components with the same level of strength [2-4].
The Ti-6Al-4V alloy represents 50 % of all the worldwide titanium production, reaches levels of tensile strength from 900MPa
that can be increased by heat treatment. Titanium has an oxide surface film due to its reactivity with oxygen, resulting in high
corrosion resistance in aggressive environment. On the other hand, the application in components subjected to wear is limited
by the low resistance to plastic shear and the high friction coefficient. Therefore, modifications in surface treatments have been
studied to improve the tribological properties of titanium [4-8].
Thermal sprayed coatings are a recent technology that allow wear and corrosion resistance for structural components, with an
environmental clean process. HVOF has the ability of spray semi-molten particles with lower porosity and low decomposition
of WC, which produce higher wear and corrosion resistance coatings when compared with other spraying process [9-10].
Latest papers related that in dry conditions, AISI 1040 coated by WC-10%Co-4%Cr has superior wear resistance than hard
chrome electroplated and lower friction coefficient [11]. For AISI 4340 substrate coated by WC-10%Co-4%Cr and WC-17%Co,
results evidence a fatigue strength about 49% of base material yield strength and the same alloy coated with hard chrome
show a fatigue strength 34% of base material yield strength, in axial fatigue experiments [1]. In rotating bending fatigue, fatigue
strength of Ti-6Al-4V coated by WC-12%Co is 20% from the tensile strength and a reduction of 70% is noted when this result
is compared with fatigue strength of Ti-6Al-4V base material [12].
The purpose of this paper is to evaluate the morphology aspects of Ti-6Al-4V coated by WC-10%Co-4%Cr and the influence of
the coating on Ti-6Al-4V base material axial fatigue strength.
Experimental
The chemical composition of Ti-6Al-4V used was Ti-6Al-4V: 6.29% Al, 4.95% V, 88.76% Ti. Mechanical properties of this alloy
are: 380 HV300g and ultimate tensile strength of 1270 MPa, in the annealed condition.
Axial fatigue tests according to ASTM 466 were conducted using a sinusoidal load of frequency 20 Hz and ratio R = 0,1, at
7
Ø10±0,05
Ø5,1±0,03
room temperature, considering, as fatigue strength, when specimens were fractured or 10 load cycles. Two groups of fatigue
specimens were prepared, according to Figure 1 to obtain S-N curves for axial fatigue tests:
Specimens of base metal;
Specimens of base metal coated with WC-10%Co-4%Cr;
47 ±1,0
30±1,0
35 ±1,0
35 ±1,0
100±1,0
Figure 1. Axial fatigue testing specimens
Thermal spray coated specimens were blasted with mesh 90 aluminum oxide to enhance adhesion. The tungsten carbide
thermal spray coating applied by HVOF system used WC powder with 10% Co-4% Cr and result in thickness equal to 160 µm,
according to Embraer-Liebherr standard NEDE E40-045.
Fracture planes of fatigue specimens and surface polished specimens were examined using a scanning electron microscope
model JEOL JSM 5310 in order to identify the crack initiation points.
Results
Figure 2 shows S-N curves from axial fatigue tests of Ti-6Al-4V base material and Ti-6Al-4V coated with WC-10%Co-4%Cr.
Results indicate that the fatigue limit of base material is about 75% of the ultimate strength. From fatigue data is possible to
observe a reduction in the fatigue strength of Ti-6Al-4V coated with WC-10%Co-4%Cr around 98%.
For 965 MPa, the fatigue behavior changes from high cycle regime in base material, about 122000 cycles, to low cycle regime
for the coated alloy, about 4000 cycles. Figure 3 shows the fracture surface for this stress level; it is visible a change in the
fatigue crack behavior. The presence of coating at 965 MPa results in a fracture surface similar to that obtained in a tensile
test. The reduction in fatigue life is associated with coated porosity, presence of micro cracks and pre-treatment with aluminum
oxide.
Metal Base
MB with WC-10Co-4Cr
1100
1000
900
Stress (MPa)
800
700
600
500
400
300
200
10000
100000
1000000
1E7
Cycles
Figure 2. SN curve for Ti-6Al-4V coated with WC-10%Co-4%Cr
(a)
(b)
Figure 3. a) Base Material 965 MPa 15X; b) Base Material with WC-10%Co-4%Cr 965 MPa 15X
A polished surface of metal base coated with WC-10%Co-4%Cr was observed accordant to scheme at figure 4.
a
d,e
b
c
Figure 4. Surface polished of Ti-6Al-4V with WC-10%Co-4%Cr
It can be seen from figure 5a that WC-10%Co-4%Cr present an uniform carbide distribution, a very dense morphology and
lower porosity than other spray process, as observed by G. D.Toma et al [10] and E. Celik et al [13]. However, the presence of
porous can reduce fatigue life.
Figure 5b was captured from a region where occurred a delimitation of the coating during the cutting process, this image
confirm the strong adhesion between the substrate and the coating. Figure 5c indicate the surface roughness due the
aluminum oxide blasted, process that enhanced coating adhesion.
Figure 5d and 5e shows a crack in the coating with around 60 µm of extension. T. Ogawa et al [12] affirm that cracks starts in
the substrate due to the stress concentration of a crack in the coating, when the coating is harder than the substrate and has
high bonding strength. In this research, the hardness of WC-10%Co-4%Cr was around 1900 HV300g and the substrate, 380
HV300g. From figure 6 it is possible see that cracks initiate in the interface coating/substrate.
Energy Dispersive Spectroscopy (EDS) analysis with samples only blasted with aluminum oxide results in a chemical
composition of 9,84% Al, 4,16% V and 86,01% Ti; indicating that after blasted Ti-6Al-4V samples retain aluminium oxide
particles, which act as stress concentration too.
a
b
c
d
e
Figure 5. Surface polished of Ti-6Al-4V with WC-10%Co-4%Cr, 2000X, 3500X and 5000X
(a) 15X
(b) 150X
Figure 6. Base Material with WC-10%Co-4%Cr 450 MPa
Conclusion
1.
WC-10%Co-4%Cr HVOF coating presents porosity, micro cracks and decreased the axial fatigue strength of Ti-6Al4V alloy in 98%.
2.
Porosity and cracks inside the coating acting as stress concentration due hardness difference between substrate and
coating, fatigue cracks nucleation at coating/substrate interface are observed.
3.
The pre-treatment with aluminum oxide to enhance adhesion, impregnate Ti-6Al-4V creating stress concentrations
too.
References
1.
Voorwald, H.J.C., Souza, R.C., Pigatin, W.L. and Cioffi, M.O.H. “Evaluation of WC–17Co and WC–10Co–4Cr thermal
spray
coatings
by
HVOF
on
the
fatigue
and
corrosion
strength
of
AISI
4340
steel”
Surf. and Coat. Tech., 190, 155-164(2005).
2. Torres, M. A. S. and Voorwald, H. J. C. “An evaluation of shot peening, residual stress and stress relaxation on the fatigue
life of AISI 4340 steel”. Inter. J. of Fatigue, 24, 877-866 (2002).
3. Barter, S. A., Clayton, J. Q., Clark, G. “Aspects of fatigue affecting the design and maintenance of modern military aircraft.
Inter. J. of Fatigue”, 15, 325-332 (1993).
4. Boyer, R. R. “An overview on the use of titanium in the aerospace industry”. Mat. Sc. & Eng. A, 213, 103-114, (1996).
5. Williams, J. C. and Starke, E. A. Jr. “Progress in structural materials for aerospace systems”
Acta Mat., 51, 5775-5799 (2003).
6. Straffelini, G. and Molinari, A. “Dry sliding wear of Ti–6Al–4V alloy as influenced by the counterface and sliding conditions”
Wear, 236, 328-338 (1999).
7. Sundaram, V.S. “Diamond like carbon film as a protective coating for high strength steel and titanium alloy“
Surf. and Coat. Tech., 201, 2707-2711 (2006).
8. Nolan, D., Huang, S.W., Leskovsek, V. and Braun, S. “Sliding wear of titanium nitride thin films deposited on Ti–6Al–4V
alloy by PVD and plasma nitriding processes” Surf. and Coat. Tech., 200, 5698-5705 (2006).
9. Wielage,B., Wank, A., Pokhmurska, H., Grund, T., Rupprecht, C., Reisel, G. and Friesen, E. “Development and trends in
HVOF spraying technology”. Surf. and Coat. Tech., 201, 2032-2037 (2006).
10. Toma, D., Brandl, W., Marginean, G. “Wear and corrosion behaviour of thermally sprayed cermet coatings” Surf. and
Coat. Tech., 138, 149-158(2001).
11. Bolelli, G, Cannillo, V., Lusvarghi, L. and Riccò, S. “Mechanical and tribological properties of electrolytic hard chrome and
HVOF-sprayed coatings” Surf. and Coat.Tech., 200, 2995-3009 (2006).
12. Ogawa, T., Tokaji, K., Ejima, T., Kobayashi, Y., Harada, Y. “Evaluation of fatigue strengths of WC cermet-and 13Cr steelsprayed materials”. Mat. Sci.Res.Inter., 4, 12-18 (1998).
13. Celik, E., Culha, O., Uyulgan, B., Ak Azem, N.F., Ozdemir I. and Turk, A. “Assessment of microstructural and mechanical
properties of HVOF sprayed WC-based cermet coatings for a roller cylinder” Surf. and Coat. Tech, 200, 4320-4328
(2006).