FATIGUE STRENGTH OF 7050 T7451 ALUMINUM ALLOY: COATING
EFFECTS
Herman Jacobus Cornelis Voorwald, José André Marin de Camargo, Maria Odila Hilário Cioffi
Fatigue and Aeronautic Materials Research Group, Department of Materials and Technology
State University os São Paulo – UNESP. Av. Ariberto Pereira da Cunha, 333 CEP 13516-410
Guaratinguetá/SP-Brazil
[email protected]
Abstract
One of the most interesting environmentally safer and cleaner alternatives for the replacement of hard chrome plating or
anodizing is tungsten carbide thermal spray coating, applied by the high velocity oxy fuel (HVOF) process. However, it was
observed that residual stresses originated from these coatings reduce the fatigue strength of a component. The objective of
this research is to evaluate a tungsten carbide cobalt (WC-Co) coating applied by the high velocity oxy-fuel (HVOF) process,
used to replace anodizing. Anodic films were grown on 7050 T7451 aluminum alloy by sulfuric acid anodizing, chromic acid
anodizing and hard anodizing. The influence on axial fatigue strength of anodic films grown on the aluminum alloy surface is to
degrade the stress life performance of the base material. Three groups of specimens were prepared and tested in axial fatigue
to obtain S-N curves: base material, base material coated by HVOF and base material shot peened and coated.
Experimental results revealed increase in the fatigue strength of Al 7050 T7451 alloy associated with the WC 17% Co coating.
On the other hand, a reduction in fatigue life occurred in the shot peened and coated condition. Scanning electron microscopy
technique and optical microscopy were used to observe crack origin sites, thickness and coating/substrate adhesion.
Keywords: fatigue, aluminum alloy, WC-Co coatings, HVOF.
Introduction
Aluminum alloys have been the best choice for commercial aircraft with a much more rapid insertion due to lower
manufacturing costs, low replacing risk and the use of an existing production infrastructure. The 7xxx series are predominating
alloys when strength is the primary requirement [1,2]. Wear and corrosion control of several mechanical components is
accomplished by surface treatments of chrome plating on steel or anodizing of aluminum. The sealed anodic layer on Al 7050
confers protective efficiency against corrosion to the alloy [3]. Tungsten carbide (WC) coating applied by the high velocity oxy
fuel (HVOF) process on AISI 4340 steel, results in higher axial fatigue strength when compared to hard chromium
electroplated [4].
A direct relation between the increase in compressive residual stress in the WC-Co coating and the increase in fatigue life of
coated 6061 aluminum was also observed [5]. Considering that fatigue crack nucleation is a surface phenomenon associated
to the residual stress state on surface and subsurface, compressive residual stresses in the surface layers may increase
fatigue performance [6, 7].
A well known and used surface impact treatment to improve the fatigue strength of components subjected to variable
amplitude loading, is shot peening [8]. The present study evaluates a tungsten carbide cobalt (WC-Co) coating applied by the
high velocity oxy-fuel (HVOF) process, used to replace anodizing. S-N curves for axial fatigue tests were obtained for base
material 7050 T7451 aluminum alloy and base material with anodic films grown by sulfuric axial anodizing, chromic acid
anodizing and hard anodizing. To compare with previous experimental results, three groups of specimens were prepared and
tested in axial fatigue to obtain S-N curves: base material, base material WC – 17% Co coated by HVOF and base material
shot peened and WC – 17% Co coated by HVOF. The behavior of compressive residual stress field and the crack initiation
points of fatigued specimens were also evaluated.
Experimental
The base material used in this investigation was the Al 7050 T7451 alloy. It is a high copper and zinc aluminum alloy, with the
following chemical composition (in wt %): 6,06 Zn; 2,19 Cµ; 1,90 Mg; 0,15 Zn; 0,10Mn; 0,04 Cr; 0,12 Si; 0,14 Fe; 0,06 Ti.
Tensile tests were conducted according to ASTM E-8M standard procedure. Mechanical properties of the alloy are: elastic
modulus 65 GPa, yield strength 429 MPa, ultimate tensile strength 502 MPa and elongation 10%. Specimens were prepared
-3
from a 19x10 m thick 7050 T7451 rolled plate base material with the rolling axis parallel to the loading axis.
Aluminum alloy specimens were grinder machining, which represent surface roughness equal to 0,89± 0,32 µm.
2.1 Tungsten Carbide Coating
The tungsten carbide thermal spray coating applied by HVOF system used WC powder with 17% Co, resulting in thickness
equal to 150 µm. The tungsten carbide thermal spray coated specimens were blasted with aluminum oxide mesh 90 to
enhance adhesion.
2.2 Shot Peening
S-N curves were obtained for the aluminum alloy treated with shot peening intensity of 0,013N (30 psi), using glass
shot (∅ 0,4 mm) with coverage of 120%, carried out on an air-blast machine according to standard MIL – 13165.
2.3 Residual Stress Measurement
The compressive residual stress field induced by shot peening, anodizing and tungsten carbide thermal spray coating, was
determined by X-ray diffraction method.
A full description of the method is given by Gurova et al [9]. To determine the through thickness residual stress distributions in
the coating, thin layers of specimens were removed by electrolytic polishing with a nonacid solution.
2.4 Sulfuric Acid Anodizing
2
The sulfuric acid anodizing was carried out with 180 g/l to 240 g/l H2SO4, at 20ºC to 22ºC, with a current density from 1,1 A/dm
2
to 1,7 A/dm . Sealing was performed, with sodium dichromate, at 90ºC to 96º C.
2.5 Chromic Acid Anodizing
The chromic acid anodizing was carried out with 40g/l chromium trioxide, at 39ºC to 40ºC, with a current density from 0,3
2
2
A/dm to 1 A/dm . Sealing was performed, with sodium dichromate, at 90ºC to 96º C.
2.6 Hard Anodizing
2
The hard anodizing was carried out with 180 g/l to 350 g/l H2SO4, at –3ºC to 3ºC, with a current density from 2 to 5 A/dm .
Sealing was performed, with sodium dichromate, at 90ºC to 96º C.
2.7 Fatigue tests
The fatigue experimental program was performed on axial fatigue test specimens prepared according to standard ASTM E
466. For axial fatigue tests, a sinusoidal load of 50 Hz frequency and load ratio (R) equal to 0,1 was applied throughout this
study.
Six groups of fatigue specimens were prepared to obtain S-N curves for axial fatigue tests:
•
Smooth specimens of base metal (Al 7050 T7451 alloy);
•
Specimens of base metal with anodic films grown by sulfuric acid anodizing;
•
Specimens of base metal with anodic films grown by chromic acid anodizing;
•
Specimens of base metal with anodic films grown by hard anodizing;
•
Specimens of base metal with WC – 17% Co by HVOF process, 150 µm thick;
•
Specimens of base metal, shot peened and WC –17%Co by HVOF process, 150 µm thick;
•
Specimens of base metal, shot peened and WC –17% Co coated by HVOF process.
Typical dimensions of the specimen used for the axial fatigue tests are shown in Figure 1.
Ø162,5
100
15
30
10
40
15
30
4
Figure 1. Typical dimensions of axial fatigue specimens.
Results
The results of the axial fatigue tests for the base material and specimens with WC –17% Co thermal spray coated, are
shown in figure 2.
Figure 2. Axial fatigue life of base material, WC – 17% Co thermal spray- coated specimens and shot peened WC – 17% Co
thermal spray- coated specimens on Al 7050 T7451 alloy substrate.
It is possible to note that the influence of WC –17% Co thermal spray coating was to improve the fatigue strength of Al 7050 T7451 alloy.
According to Voorwald et al [4], a reduction in fatigue life of coated material with WC –17 % Co occurred, in comparison to base material.
The decrease is more significant in high cycle than in low cycle fatigue tests.
Internal residual stresses were obtained for Al 7050 T7451 alloy specimens WC – 17% Co spray coated by HVOF and results
are indicated in table 1.
Table 1. Internal residual stresses. Shot peened and WC –17% Co thermal spray coated.
Specimen
1
2
3
Depth (mm)
Interface
0,05
0,15
Stress (MPa)
+130
+170
+100
Despite the tensile internal residual stress at coating/substrate interface induced by HVOF process, experimental results show better
performance in the axial fatigue strength of spray coated aluminum alloy in comparison to base metal. Figure 3 shows the fracture surface
of a base metal specimen.
a)
b)
Figure 3. Fracture surface from a base metal specimen. σ = 381 MPa . a) 15 x magnification; b) 100 x magnification
Figure 4 illustrates a case in which the crack originated from the surface. It is interesting to observe in figure 4a, which represents
the fracture surface of a specimen WC – 17% Co thermal spray coated and tested at 381 MPa, multiple fatigue crack sources around
specimen fracture surface. Analysis of figure 4b indicates fatigue crack nucleation and propagation from interface coating/substrate
throughout base metal.
Figure 4. Fracture surface of specimen WC-17% Co thermal spray coated. σ = 430 MPa.500x magnification.
The influence on the axial fatigue strength of anodic films grown on 7050 T7451 aluminum alloy by sulfuric acid anodizing, chromic
acid anodizing and hard anodizing is to degrade the stress fatigue life performance of the base material as represented in figure 5. It can
be noted that the strongest effect of anodizing on the axial fatigue strength of the base material, is associated to sulphuric acid and hard
anodizing.
Figure 5. Axial fatigue S-N curves. Chromic, sulphuric and hard anodizing.
In order to understand the detrimental effect of anodic surface coating, residual internal stresses are indicated in table 2.
Table 2. Internal residual stresses. Anodic surface coating.
Condition
Depth (mm)
Stress (MPa)
0
0,05
0,09
0,13
0,25
0,35
+ 170
+ 180
+ 80
+ 100
+ 110
+ 90
Chromic Anodizing
0
0,06
0,10
0,16
0,22
0,30
0
- 80
- 60
- 50
+ 20
+ 60
Sulphuric Anodizing
0
0,05
0,11
0,17
0,25
0,35
0
+120
+ 60
0
+ 20
+ 60
Hard Anodizing
0
0,05
0,15
0,20
0,28
0,35
+ 80
+ 110
+ 140
+ 50
- 30
- 40
Machined
Based on experimental results shown in tables 1 and 2 it should be noted that tensile residual stresses are present in both studies; WC –
17% Co thermal spray coating and anodization on Al 7050 T7451 alloy. Cracks sources originated by sulphuric anodic coating are shown
in figure 6
Figure 6. Fatigue fracture surface. Sulphuric anodizing.
Considering the residual stress profile for both conditions studied: WC –17% Co thermal spray coating and anodization on Al
7050 T7451 alloy and crack sources noticed at coating/substrate interface, the axial fatigue strength contradictory results are
partly associated to the role of surface modifications produced by the thermal spray process, in special increased strain
hardening near surface.
Conclusions
1. The WC –17% Co thermal spray coating improved the axial fatigue strength of Al 7050 T7451 alloy.
2. Tensile residual stresses at interface coating/substrate (+ 130MPa) which increased to + 170 MPa at 0,05 mm from
interface, were induced by the WC –17% thermal spray coating process.
3. A reduction in the axial fatigue life of shot peened coated specimens in comparison to Al 7050 T7451 WC –17% Co
thermal spray coated, was observed. Tensile residual internal stresses were obtained at interface coating/substrate (+
190MPa and inside base material ( + 130 MPa at 0,10 MPa and + 100 MPa at 0,15mm).
4. The influence on axial fatigue strength of anodic films grown on 7050 T7451 aluminum alloy by sulphuric acid anodizing,
chromic acid anodizing and hard anodizing is to degrade the stress-life fatigue performance of base material. The sulphuric
acid and hard anodizing showed the strongest effect. Tensile residual stresses are present as a consequence of the
anodization process.
Acknowledgements
The authors express their acknowledgements to CNPq, process 304155/2006-3.
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