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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5326_Библиотеки_им_академика_М_И_Перельмана
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Cryogenic machining and burnishing of magnesium alloys to improve in vivo corrosion resistance 117
80%
Cryogenic burnishing
70%
60%
50%
40%
30%
Percentage
20%
10%
0%
0.4 0.6 0.8 1 1 1.5 2 2.5 5 15 25 35
Dry burnishing
Grain size (μm)
Before burnishing
Figure 5.10 Distribution of average longitudinal axis grain size before and after burnishing
under dry and cryogenic conditions (Pu et al., 2012b).
reported in Section 5.3.1 that the increased plowing effects induced by large edge
radius cutting tools resulted in the remarkable increase in the intensity of the basal
plane. The deformation of the workpiece induced by the plowing effects in machining
and by burnishing is somewhat similar to what occurs in rolling. It has been reported
frequently that the rolled surfaces of AZ31 Mg alloy often exhibit strong basal textures
(Chang et al., 2003; Sakai, Hashimoto, Hamada, & Utsunomiya, 2011). Therefore, it is
expected that a burnished surface would become more basal textured, which should
then result in better corrosion resistance (Song et al., 2010).
Figure 5.12 shows residual stresses generated on the ground and burnished
AZ31B Mg surfaces at a depth of 25 mm from the surface (the smallest achievable
depth due to limits in the penetration depth of X-ray in this material). The
(a)
(1010)
(0002)
(1011)
(b)
Intensity (a.u.)
(c)
31 33 35 37
2 theta (degree)
Figure 5.11 Evolution of crystallographic orientations: (a) before burnishing and (b) after dry
and (c) cryogenic burnishing (Pu et al., 2012b).
118 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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40
30
20
10
-10
Residual stress (MPa)
-20
-30
Figure 5.12 Residual stress at 25 mm from the surface of AZ31B Mg disc before burnishing and
after burnishing under dry and cryogenic conditions (Pu et al., 2012b).
Circumferential
Axial
0
Initial Dry Cryogenic
Axial
Circumferential
circumferential residual stresses were claimed to be more critical than the axial ones
for in-service performance of a disc (Umbrello, Outeiro, M ’ saoubi, Jayal, & Jawahir,
2010). Although the axial residual stresses in the burnished samples are more
compressive than in the ground sample, the circumferential residual stresses become
less compressive or more tensile after burnishing compared with their state before
burnishing. This is different from what has been reported in most literature reports,
where burnishing normally induces more compressive residual stresses (Denkena &
Lucas, 2007; Salahshoor & Guo, 2011; Scheel et al., 2010; Zinn & Scholtes, 1999).
The contradictory findings from this study and the literature data may be caused by
the different roller settings. The roller used in this study was fixed, while in most
literature examples it was allowed to rotate.
5.4 Corrosion performance of machined and
burnished samples
It was shown in the previous section that the surface integrity of AZ31B Mg alloy was
remarkably changed after cryogenic machining and burnishing. These changes may
improve or reduce the corrosion resistance of the processed materials.
To evaluate the corrosion performance, the processed samples were immersed in
two different solutions. One solution was 5 wt% NaCl solution and the corrosion
test was conducted at room temperature. The other solution used was SBF, whose
composition is shown in Table 5.2 (Song & Song, 2007). The pH value of the SBF
was adjusted to 7.4. The solution was held in an incubator to maintain the temperature
at 37 1
the test in the SBF. After immersion, the samples were taken out of the solutions and
the corrosion products were removed by chromic acid (200 g/L CrO
AgNO
C.
The immersion time was 85 h for the test in 5 wt% NaCl solution and 10 days for
and 10 g/L
3
). After washing with distilled water and drying, photos were taken by a camera
3
Cryogenic machining and burnishing of magnesium alloys to improve in vivo corrosion resistance 119
Table 5.2 Composition of simulated body fluid (SBF)
Name Concentration [g/L]
NaCl 8
KCl 0.4
CaCl
2
NaHCO
3
C
6H12O6
MgSO
.7H2O 0.2
4
KH
Na
2
O 0.1
2PO4.H2
HPO4.7H2O 0.06
0.14
0.35
1
to show the overview of the corroded surface. A Zygo NewView 7300 measurement
system was used to measure the corrosion depth. A 2 mm 2 mm three-dimensional
(3D) topographic picture of the severely corroded area was recorded for each condition; the depth of the deepest pits was obtained by the corresponding line profiles.
5.4.1 Corrosion performance of machined samples
Figure 5.13 shows the photos of AZ31B Mg samples, machined under different cool-
ing conditions and cutting edge radii after the corrosion test in 5 wt% NaCl for 85 h
(Pu, 2012). Differences in corrosion performance were found between specimens
made using different machining conditions. The white or light gray portions on the
photos in Figure 5.13 represent the original surface, while the dark portions are the places where large amounts of Mg were lost due to corrosion. It is obvious that dry
machining using a 30-mm edge radius tool led to the worst corrosion performance.
With the same edge radius, the application of liquid nitrogen during machining significantly improved the corrosion performance. However, when the cutting edge radius
was increased to 70 mm, the differences of the corroded surface were limited between
the dry and cryogenic conditions. Under the same dry cutting conditions, increasing
the cutting edge radius from 30 mmto70mm remarkably improved the corrosion resis-
tance of the AZ31B Mg alloy.
The differences in corrosion performance correlate well with the thickness of the
“featureless layer” shown in Figure 5.4. The sample machined using a 30-mm tool un-
der dry condition, which exhibited the worst corrosion, has no featureless layer. All
other samples have the featureless layer of different thickness (8e15 mm). It could
be concluded that the featureless layer, which consisted of nanocrystalline grain structures, promoted the formation of a passive film and improved the corrosion resistance
of AZ31B Mg alloy. This result agrees with the finding that grain refinement improves
the corrosion resistance of Mg alloys (Alvarez-Lopez et al., 2010; Birbilis et al., 2010;
Wang et al., 2007).
120 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Figure 5.13 Photos of machined AZ31B Mg samples under different cooling conditions and
cutting edge radii after corrosion test in 5 wt% NaCl for 85 h (V ¼ 100 m/min, f ¼ 0.1 mm/rev)
(Pu, 2012).
Figures 5.14 and 5.15 show the 3D topographic pictures of the severely corroded
area, as marked by a red square in Figure 5.13, and the corresponding line profiles.
While the total corroded area is much smaller on the sample machined under cryogenic
condition (Figure 5.13) using the 30-mm edge radius tools, the difference of maximum
depths of the corrosion pits are very small: 175 and 198 mm for cryogenic and dry conditions, respectively (Figure 5.14). As shown in Figure 5.15, the application of liquid
nitrogen does not lead to notable differences in the total corroded area. However, cryogenic machining results in about 183% increase in the maximum depth of the corrosion
pits (130 mmvs71mm).
The results obtai ned when using both 30-mm and 70-mm edge radius tools show that
the application of liquid nitrogen does not lead to a notable reduction in the maximum
corrosion pit depth, and it even increases significantly when the large edge radius tool
is used. This could be explained by the microstructures of the machined samples in
Figure 5.4; large amounts of deformation twins, which were reported to increase the
corrosion rate of AZ31B Mg alloy (Aung & Zhou, 2010), formed below the featureless
layers when liquid nitrogen was used. Therefore, after the 5 wt% NaCl solution penetrates the featureless layer, the twinning areas in the samples after cryogenic
machining are more easily corroded the initial microstructure with no twinning, and
this leads to deeper corrosion pits.
Figure 5.16 shows the photos of AZ31B Mg samples machined under different cool-
ing conditions and cutting edge radii after corrosion test in SBF for 10 days. Similar to
the corrosion performance in 5 wt% NaCl, dry machining using the 30-mm cutting edge
radius tool led to the poorest corrosion resistance. This finding further proves that the
Cryogenic machining and burnishing of magnesium alloys to improve in vivo corrosion resistance 121
50
0
0 0.5 1 1.5 2
–50
–100
–150
–200
Corrosion pit depth (µm)Corrosion pit depth (µm)
–250
Line profile (mm)
(a) Dry, 30 µm
50
0
0 0.5 1 1.5 2
–50
–100
–150
–200
Line profile (mm)
(b) Cryogenic, 30 µm
Figure 5.14 Zygo three-dimensional (3D) topographic pictures of the corroded area as marked
by a red square in Figure 5.13 and corresponding line profiles under different cutting conditions:
a) dry, 30 mm and b) cryogenic, 30 mm (V = 100 m/min, f = 0.1 mm/rev) (Pu, 2012).
featureless layers formed on the other three samples shown in Figure 5.4 acted as protective coatings and prevented the matrix from further corrosion.
When using 30-mm edge radius tools, the maximum depth of the corrosion pits on the
machined samples under cryogenic cooling was reduced significantly from 167 mmto
73 mm when compared with dry machined specimens shown in Figure 5.17. This finding
is different from the results obtained in 5 wt% NaCl solution, which suggests that the
deformation twins might not reduce the corrosion resistance of AZ31B Mg alloy in
SBF. Even greater reductions in the maximum depth of corrosion pits were found on
samples machined using 70-mm edge radius tools. Almost no corrosion damage can
be found on the sample machined using cryogenic conditions with the large edge radius
tool. This finding agrees with the expectation because the surface integrity induced by
cryogenic machining with the 70-mm edge radius tool was the best in terms of nanograin
structures, strongest basal texture, and large and deep compressive residual stresses.
5.4.2 Corrosion performance of burnished samples
As shown in Figure 5.18, dramatic differences in surface morphologies can be found
between the ground and burnished surfaces after the immersion test of 200 h in 5 wt%
NaCl; the differences between burnished samples produced using dry and cryogenic
122 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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20
0
0 0.5 1 1.5
–20
–40
–60
–80
Corrosion pit depth (µm)Corrosion pit depth (µm)Corrosion pit depth (µm)
20
0
0
–20
–40
–60
–80
Line profile (mm)
12
Line profile (mm)
(a) Dry, 70 µm
20
0
0
–20
–40
–60
–80
–100
–120
–140
20
0
0
–20
–40
–60
–80
–100
–120
–140
Corrosion pit depth (µm)
12
Line profile (mm)
12
Line profile (mm)
(b) Cryogenic, 70 µm
Figure 5.15 Zygo three-dimensional (3D) topographic pictures of the corroded area as marked
by a red square in Figure 5.13 and corresponding line profiles under different cutting conditions:
a) dry, 70 mm and b) cryogenic, 70 mm (V = 100 m/min, f = 0.1 mm/rev) (Pu, 2012).
2
Cryogenic machining and burnishing of magnesium alloys to improve in vivo corrosion resistance 123
Figure 5.16 Photos of machined AZ31B Mg samples under different cooling conditions and
cutting edge radii after corrosion test in simulated body fluid (SBF) for 10 days (V ¼ 100 m/min,
f ¼ 0.1 mm/rev) (Pu, 2012).
conditions are much smaller for the same test duration than the difference between
ground and burnished surfaces. The preburnished ground samples were successively
ground using up to 4000 grit SiC paper in order to have similar surface roughness
as the burnished samples. This eliminates the possible influence of surface roughness
on corrosion resistance (Song & Xu, 2010).
The ground surface after the immersion test is very rough; large and deep pits are
visible over the entire surface. On the dry and cryogenic-burnished surfaces, there are
only some small pits in the surfaces and the corroded areas of the burnished samples
are smaller than those in the ground samples. This suggests that much less Mg was
corroded from the burnished surfaces than from the ground ones.
To investigate the depth of the corrosion pits, topographic maps of the severely
corroded portion of each surface (marked as square boxes in Figure 5.18) were obtained
using the Zygo NewV iew 6000 measurement system, as shown in Figure 5.19 (aec).
The depth profiles along the dotted lines shown in Figure 5.19 (aec) are presented in
Figure 5.19 (d). The profiles show that the corrosion pits on the ground samples are
very deep; the maximum measured depth is 512 mm while the pits on the burnished surfaces are less than 200 mm deep. Figure 5.19 also shows that the corrosion on the burnished samples is more uniform than that on the ground samples.
Typical cross -sectional microstructures of ground and cryogenic-burnished samples
after a 30-h immersion test are shown in Figure 5.20. There are significant differences in
corrosion morphology between the samples prepared by grinding and those made by
cryogenic burnishing. Wide and deep corrosion pits are present on the ground sample.
The grain boundaries are clearly visible and do not show any barrier effects to corrosion
development. The surface after cryogenic burnishing is much smoother and corrosion
124 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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20
012
–30
–80
–130
Corrosion pit depth (µm)
–180
(a) Dry, 30 µm
Corrosion pit depth (µm)
–100
20
–20
–40
–60
–80
0
0
Line profile (mm)
1
Line profile (mm)
2
(b) Cryogenic, 30 µm
20
0
012
–20
–40
–60
–80
Corrosion pit depth (µm)
–100
Line profile (mm)
(c) Dry, 70 µm
20
0
012
–20
–40
–60
–80
Corrosion pit depth (µm)
–100
Line profile (mm)
(d) Cryogenic, 70 µm
Figure 5.17 Zygo three-dimensional (3D) topographic pictures of the corroded area as marked
by a red square in Figure 5.16 and corresponding line profiles under different cutting conditions:
a) dry, 30 mm, b) cryogenic, 30 mm, c) dry, 70 mm, and d) cryogenic, 70 mm (V = 100 m/min,
f = 0.1 mm/rev) (Pu, 2012).
Cryogenic machining and burnishing of magnesium alloys to improve in vivo corrosion resistance 125
Figure 5.18 Surface morphology of AZ31B Mg samples processed by different treatments after
immersion in 5 wt% NaCl solution for 200 h (details in the square boxes are shown in
Figure 5.19)(Pu et al., 2012b).
occurred more uniformly, creating small and shallow pits. The grain boundaries are not
visible at this scale, and the microstructure is similar to the one shown in Figure 5.9(c),
which suggests that corrosion occurred within the grain refined layer after 30 h of immersion. The uniformity in corrosion may be due to the similarity in grain size and the
electrochemical homogenization (Orlov, Ralston, Birbilis, & Estrin, 2011) induced by
the burnishing process. Different from the ground sample, a layer of corrosion products
adhered to the cryogenically burnished surface, as shown in Figure 5.20 (b). Apparently, the uniform corrosion products better protect the substrate from corrosion.
This finding is directly supported by the literature showing that a higher density of grain
boundaries promoted better mechanical adhesion through an oxide pegging mechanism
(Balakrishnan, Lee, Kim, & Panigrahi, 2008; Tao & Li, 2006).
A corrosion test of the burnished samples in SBF will be conducted in the future.
Based on the similar trend of machined samples subje cted to immersion tests in 5
wt% NaCl solution and SBF, it is expected that the burnished samples will exhibit
improved corrosion resistance than the unburnished ones.
5.5 Finite element modeling of grain size changes
in cryogenic machining
It was shown in the previous section that both cryogenic machining and cryogenic
burnishing significantly changed the surface integrity of the AZ31B Mg alloy, which
led to remarkable changes in its corrosion resistance. To achieve the desired biodegradation rate (which is controllable and predictable) on the Mg-based implants, it is
important to use the correct processing conditions. Instead of extensive trial-and-
126 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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(a)
(b)
(c)
0.000
0.000
mm
mm
1.973
1.973
0.000
0.000
mm
mm
mm
+346.6
μm
-381.5
1.971
+118.6
μm
-100.9
1.973
+230.5
μm
-174.3
1.971
0.000
1.973
Grinding
Dry burnishing
Cryogenic burnishing
(d)
0.000
00.511.52
0
-100
-200
-300
-400
-500
Corrosion pit depth (μm)
-600
mm
Line profile (mm)
Figure 5.19 Topographic maps of the square boxes in Figure 5.18 obtained by Zygo: (a)
ground; (b) dry- and (c) cryogenic-burnished samples; and (d) corrosion pit depth profiles along
the dotted lines (Pu et al., 2012b).
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