Materials Transactions, Vol. 53, No. 2 (2012) pp. 417 to 424
© 2011 The Japan Institute of Metals
Detectability of Holes in SiC Particulate-Reinforced Al­Si Alloy Composite
by Means of Ultrasonic Measurement
Hiroshi Kato1, Shota Otsuka1,+ and Haruo Kurita2
1
Course of Mechanical Engineering, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
Tajima Light Metals Co. Ltd, Hanyu 348-0064, Japan
2
For the development of nondestructive detection method of casting defects in SiC particulate-reinforced Al­Si alloy composite (the MMC
castings), plates of the alloy composite and the Al­Si alloy were cast, and holes of different diameters were introduced at different depths. Then
the ultrasonic measurement was carried out in water with the longitudinal waves of 5 and 10 MHz in frequency by the normal incidence, and also
with the surface acoustic wave (SAW) of 5 MHz in frequency. The hole of 2 and 1 mm in diameter or larger was detected at a depth of 2 to
10 mm below the surface of the MMC castings and the Al­Si alloy castings, respectively, by the normal incidence. And the hole of 2 mm in
diameter or larger was detected at a depth less than 1 mm below the surface of the MMC castings by the SAW measurement. The reflection
intensity from the hole in the MMC castings took a far lower value than the DGS curve. The DGS curve was modified in consideration of
scattering of the ultrasonic wave by cast structures and casting defects, and took a similar tendency with the reflection intensity form the hole.
Then, plates of MMC castings were subjected to X-ray radiography and ultrasonic measurement, and the depth of pores and shrinkage cavities of
more than 2 mm in size were estimated by the ultrasonic measurement successfully. [doi:10.2320/matertrans.M2011190]
(Received June 22, 2011; Accepted November 2, 2011; Published December 14, 2011)
Keywords: nondestructive testing, ultrasonic testing, castings, casting defects, aluminum alloy composite, silicon carbide particle
1.
Introduction
Recently, ceramic particulate-reinforced aluminum alloy
composites have been developed, and are used in machine
components. Since machines are subjected to static and
oscillating loads, lots of studies have been carried out on
mechanical properties of aluminum alloy composites. Kumai
et al.1) reviewed the effect of the particulate on the fatigue
property of the aluminum alloy composite, and explained that
the distribution of the reinforcement improves the rigidity of
the composite, and hence the fatigue property. However,
when the cohesion of the reinforcement to the matrix is weak,
it acts as the initiation site of the fatigue crack. Li et al.2)
carried out the fatigue testing of the Al­9 mass% Si alloy
containing SiC particles, and observed debonding of SiC
particles from the matrix and the fatigue crack initiated at
voids on the specimen surface.
Since the viscosity of molten alloy composites is higher
than that of monolithic alloys, lots of voids are left in the
composite castings. Casting defects act as the stress raiser to
reduce mechanical properties of castings. Especially, when
the casting defects exist beneath the surface, the strength of
the castings is largely reduced. Moreover, many of casting
products are machined in the finishing process, and casting
defects near the surface are exposed after machining to
reduce the quality of the products. Therefore, it has been
important to detect near-surface defects in the castings to
improve the reliability and the yield ratio of the casting
products.
Many studies have been carried out on the nondestructive
testing of cast products including the ultrasonic testing,3­12)
the X-ray radiography,13) and the eddy current testing.14)
Many of them are concerned with evaluation of the porosity
ratio in the castings.3­11,14) For detection of an individual
defect, the X-ray radiography has been used in many cases,
+
Graduate Student, Saitama University
but the depth of the casting defect is not known by the X-ray
radiography. For estimation of the depth of the casting defect,
the ultrasonic measurement is effective. In the ultrasonic
measurements, the phased array technique has been largely
developed, and its application fields have been widely
expanded, including the foundry engineering. However, in
the foundries, especially in foundries of small-lot production,
there are still cases that the current ultrasonic measurement
with the single probe is utilized for detection of casting
defects. Also, the detectability of the phased array technique
is dependent on each sensor installed. From these points,
it is meaningful to clarify the detectability of defects in the
casting products with the ultrasonic measurement of the
single probe.
In the ultrasonic measurement, a relation between the
reflection intensity of the ultrasonic wave from the flaw and
the flaw depth is obtained as the Distance-Gain-Size (DGS)
diagram.15) The DGS diagram is effective for estimation of
the flaw size in wrought products, such as rolled plates, but
is not applicable to casting products, especially the alloy
composite, because they contain lots of scattering sources,
such as coarse crystal grains, distributed crystallized particle,
and lots of casting defects. These scatter the ultrasonic wave,
and the reflection intensity from the casting defect is reduced
from one predicted by the DGS diagram. Therefore, it is
crucial to verify a detection limit of the casting defect, and to
clarify difference between the reflection intensity from the
casting defect and the DGS diagram.
In the present work, aiming to improve the detection
technique of casting defects in the foundry, the detectability
of pores and shrinkage cavities (hereafter referred to as the
cavities) existing near the surface of the aluminum alloy
composite castings containing SiC particles was examined
by the ultrasonic measurement with a single probe. The
specimens containing artificial holes of different diameters at
different depths were subjected to the ultrasonic measurement, and the results were compared with the DGS diagram.
418
H. Kato, S. Otsuka and H. Kurita
(a)
(b)
Table 1 Specimens with holes for ultrasonic measurement.
Specimen
Weight
Mold
Material
MMC-A
MMC-B
MMC-C
AlSi-B2
Pressurizer
MMC
Specimen
Gate
Sprue
Riser
Typical MMC castings.
Then, the ultrasonic measurement was also carried out with
the alloy composite to verify the detectability of the natural
casting defects.
2.
Semi-spherical
Conical
Al­Si alloy
Conical
Diameter, d/mm Depth, z/mm
2
4, 6, 8, 10
1, 2, 4, 6
5
2
2, 3, 4, 5
2, 4
0.5, 0.75, 1.0
2
4, 6, 8, 10
1, 2, 4, 6
5
Heater
Fig. 1 Hybrid low-pressure casting system of MMC castings with sand
mold. (a) Low pressure casting system. (b) Schematic representation of
casting system.
Fig. 2
Tip shape
Conical
MMC-D
AlSi-A2
Hole
Experimental Procedure
2.1 Preparation of specimen
Plate specimens of about 11 mm in thickness were cast in
the sand mold with the low-pressure casting apparatus, as
shown in Fig. 1, to obtain castings containing smaller amount
of porosities and cavities. The molten Al­Si alloy (the
nominal composition of Al­9 mass% Si­0.45 mass% Mg­
0.45 mass% Mn) containing SiC particles of about 20 µm
in diameter by about 30 vol% was poured into the selfhardening mold at 1023 K, and then solidified. Hereafter,
the Al­Si alloy composite castings will be referred to as the
MMC castings. As shown in Fig. 2, the molten metal was
pushed up in the sprue at the center, and poured into the plate
specimens at both sides. From the castings, the rectangular
plates were cut out, and surfaces were machined by the
milling machine. Then holes of different diameters were
introduced in the plates at different depths, as summarized in
Table 1. As shown in the table, the specimens MMC-A
and MMC-B were fabricated to examine holes existing at
relatively deep positions 4­10 mm below the specimen
surface, the specimen MMC-C was fabricated to examine
holes existing at intermediate depths of 2­5 mm below the
specimen surface, and the specimen MMC-D was fabricated
to examine holes existing near the specimen surface 0.5­
1 mm below the specimen surface. The tip of the hole was
conical, except the hole in the specimen MMC-C with the
semi-spherical tip. For comparison, the Al­Si alloy castings
with the same chemical composition as the matrix of the
MMC castings were produced by the gravity casting by
pouring into the self-hardening mold at 988 K. Then, holes
were introduced in the Al­Si alloy plates with the same
arrangement as the MMC-A and MMC-B.
For the observation of cast structures, the gate of the
castings was sectioned, polished, etched, and then subjected
to observation through the optical microscope. The cavity
size was measured with microphotographs (magnification of
50) to obtain the probability distribution of the cavity size. In
the measurement, the minimum cavity size was 20 µm. Some
of plates of MMC castings without a hole were subjected to
the X-ray radiography to detect inherent casting defects.
2.2 Ultrasonic measurement
The ultrasonic measurement was carried out on the surface
of the specimen opposite to the surface into which the holes
were introduced. Hereafter, the surface of the ultrasonic
measurement will be referred to as the specimen surface.
2.2.1 Normal incidence measurement
When the tip of the hole existed at a depth of 2 to 10 mm
below the specimen surface, the normal incidence measurement was carried out. The specimen was placed in the water
bath. Then the probe was set at a height of 10 mm above the
specimen surface, and the longitudinal wave was irradiated
normal to the specimen surface, below which the hole
was introduced. In the measurement, probes of 5 MHz
(the vibrator diameter of 10 mm) and 10 MHz (8 mm) in
frequency were used for detection of the hole.
2.2.2 SAW measurement
When the tip of the hole existed at a position less than
1 mm below the surface, the hole was not detected by the
normal incidence measurement, and hence the surface
acoustic wave (SAW) measurement was carried out for
detection of the hole. As shown in Fig. 3, the specimen was
placed in the water bath, and the probe (the vibrator diameter
of 10 mm) of 5 MHz in frequency was moved with an interval
of 1 mm at a height of about 17 mm above the specimen
surface at an angle of 30° from the specimen normal. By
irradiating the longitudinal wave on the specimen surface, the
SAW was generated and propagated on the specimen surface,
and reflected from the hole. In the figure, the distance X
Detectability of Holes in SiC Particulate-Reinforced Al­Si Alloy Composite by Means of Ultrasonic Measurement
3.
X
Water
30°
17 mm
SAW
z
Specimen
Hole
Intensity, I / V
Fig. 3 Generation and propagation of surface acoustic wave (SAW) on
specimen surface by irradiating longitudinal wave at an angle of 30
degree.
0.2
Range of analysis
S
B
0.1
0
-0.1
-0.2
0
1
2
3
Time, t / µs
4
5
Fig. 4 Typical profile of ultrasonic wave obtained by normal incidence of
longitudinal wave. S and B indicate surface and bottom reflections,
respectively.
indicates the distance between the irradiation center of the
longitudinal wave and the center position of the hole on the
specimen surface. It is known that the SAW propagates on
the specimen surface to a depth of about 1.5 times the
wavelength.17) The SAW of 5 MHz in frequency has a
wavelength of about 0.6 mm, and hence it was expected that
casting defects existing less than 0.9 mm in depth was
detected by the SAW measurement.
2.3 Ultrasonic wave analysis
Since casting products contain the coarse cast structure and
casting defects, the ultrasonic wave propagating in the
castings is largely scattered by these scattering sources to
generate considerable amount of noise. To distinguish the
reflection signal from the noise, the noise level was defined as
follows.
The ultrasonic wave was measured at ten points on the
specimen, and oscillating waveforms were obtained, as
shown in Fig. 4. In the figure, S and B indicate the reflections
from the specimen and bottom surfaces, respectively. Then
the
square of the wave intensity I, IRMS =
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
Proot mean
ð Ii2 Þ=N , was calculated in a range between a position
at which the surface reflection diminishes and a position
just before the bottom reflection. Since the value IRMS is
equivalent to the standard deviation of the noise, 1.95 © IRMS
can be treated as the 95% confidence limit of the noise, and
hereafter referred to as the noise level. When the intensity of
the ultrasonic signal exceeded this noise level, it was judged
the signal be the reflection from the hole.
419
Results and Discussion
3.1 Cast structure
Figure 5 shows typical cast structures observed at the gate
of the MMC and Al­Si alloy castings. In the figures, the
left and right sides show micrographs of lower and higher
magnification, respectively. Although SiC particles seem to
be uniformly distributed in the higher magnification micrograph, in the lower magnification micrograph, they are
slightly segregated. By comparing the microstructure with
that in the Al­Si alloy castings, it was thought that SiC
particles existed between dendrite arms, as was reported by
Li et al.,2) Wang and Zhan18) and Rohatgi et al.19) Dark spots
in the MMC castings were thought to be cavities.
In the present work, the secondary arm spacing in the Al­
Si alloy castings was measured to be about 30 µm. Kumar
et al.20) cast the Al­7Si­0.5Cu­0.45Mg alloy in the sand
mold with a copper chill, and measured the secondary arm
spacing to be 32 µm at a height of 11 mm from the chill face,
which value was coincident with that in the present Al­Si
alloy castings. In the present work, however, the secondary
arm spacing in the MMC casting was not measured.
Klimowicz21) reported the secondary arm spacing in the
A359 alloy containing SiC particles by 20 vol% to be 59 µm,
and Sukumaran et al.22) reported that the secondary arm
spacing of about 30 µm in the 2124 alloy containing SiC
particles by 10 vol% cast in the permanent mold. Since the
MMC and Al­Si alloy were cast in the sand mold in the
present work, the secondary arm spacing in the MMC
castings was thought to take a similar value to that in the
Al­Si alloy castings.
In Fig. 6, probability distributions of the cavity size in
MMC and Al­Si alloy castings are shown. The total volume
fraction of the cavity was 2.38 and 0.89% in MMC and Al­Si
alloy castings, respectively, and larger cavities were observed
in the MMC castings.
3.2
Change in reflection intensity from hole with hole
depth and diameter
The ultrasonic measurement was carried out with the
MMC and Al­Si alloy castings with holes of 2 mm in
diameter introduced at a depth of 2­10 mm. A typical profile
of the ultrasonic wave measured with the MMC castings is
shown in Fig. 7, in which a reflection from the hole is
indicated by an arrow. Figure 8 shows the change in the
reflection intensity from the hole with the hole depth. The
reflection intensity largely scattered, but the average intensity
exceeded the noise level for MMC and Al­Si alloy castings.
And, the reflection intensity in the MMC castings was
slightly lower than that in the Al­Si alloy castings. This
difference is due to distributed SiC particles and larger
amount of cavities, as shown in Fig. 6, in the MMC castings.
And, in both castings, the reflection intensity from the hole
tended to decrease with increasing hole depth.
Then, the ultrasonic measurement was carried out for
specimens with holes of different diameters introduced at a
depth of 5 mm, by using the longitudinal wave of 5 MHz in
frequency. A relation between the reflection intensity from
the hole and the hole diameter is shown in Fig. 9. In the
MMC castings, the reflection intensity increased with
420
H. Kato, S. Otsuka and H. Kurita
400 µm
100 µm
(a) MMC castings
400 µm
100 µm
(b) Al-Si alloy castings
Fig. 5
Typical cast structures in MMC and Al­Si alloy castings.
MMC
Al−Si alloy
0.4
0.1
0.3
0.1
0
0.02 0.1 0.18 0.25 0.33 0.41 0.49 0.57 0.65 0.72
Size, d / mm
Fig. 6 Probability distribution of cavity size in MMC and Al­Si alloy
castings.
Intensity, I / V
Noise level
0.05
0.2
Intensity, I / V
Probability
0.5
(a) MMC castings
0
0.1
Noise level
0.05
(b) Al-Si alloy castings
0.2
0
MMC
0.1
0
0
2
4
6
8
Hole depth, z / mm
10
Fig. 8 Change in reflection intensity from deep hole of 2 mm in diameter
with depth measured with a longitudinal wave of 5 MHz in frequency.
-0.1
z = 6 mm
-0.2
0
1
2
3
4
Time, t / µs
5
Fig. 7 Typical reflection echo from relatively deep hole at a depth of 6 mm
in MMC castings.
increasing hole diameter, but in the Al­Si alloy castings, the
reflection intensity increased with increasing hole diameter
up to 5 mm, and then decreased. The decrease in the
reflection intensity in the Al­Si alloy castings was due to
existence of casting defects beneath the measurement
position. In the Al­Si alloy castings, the reflection intensity
from the hole of 1 mm in diameter or larger exceeded the
noise level, which shows that the hole of 1 mm or larger is
detectable in the Al­Si alloy castings. On the other hand, in
the MMC castings, the reflection intensity exceeded the noise
level for the hole of 2 mm in diameter or larger.
The reflection intensity from the hole at a depth of 4 mm
largely scattered, because the reflection from the hole
Detectability of Holes in SiC Particulate-Reinforced Al­Si Alloy Composite by Means of Ultrasonic Measurement
0.3
0.4
z (mm)
1.0
0.75
0.5
(a) d = 2 mm
(a) MMC castings
0.3
0.2
421
Intensity, I / V
Intensity, I / V
0.2
0.1
Noise level
0
(b) Al-Si alloy castings
0.2
0.1
Noise level
0
d (mm)
2.0
4.0
(b) z = 0.75 mm
0.3
0.2
0.1
0
0
2
0
4
6
8
Hole diameter, d / mm
Fig. 9 Effect of hole diameter on reflection intensity from deep hole at
5 mm in depth measured with a longitudinal wave of 5 MHz in frequency.
Table 2
Material
Velocity of longitudinal wave and near field length.
Velocity of longitudinal wave,
VL/ms¹1
Near field length, L0/mm
5 MHz*
10 MHz*
1483
(by Ref. 16))
42.1
53.9
MMC
7730
(Measurement)
7.9
10.3
Al­Si alloy
6690
(Measurement)
9.2
11.9
/V
Intensity, I
5
0
Distance, X / mm
Water
0
-0.2
*Frequency of longitudinal wave.
(b) X = 6 mm
0.2
10
Fig. 11 Change in SAW reflection intensity from near-surface hole with
distance X from hole.
(a) X = 12 mm
0.2
Noise level
0.1
Noise level
0
reported two maxima in the reflection intensity from the hole
at positions some mm before the hole.
-0.2
0
5
10
Time, t / µs
15
Fig. 10 Typical SAW distributions for near-surface hole of 2 mm in
diameter at a depth of 0.75 mm.
overlapped with the trail of the surface reflection. Then, by
using the longitudinal wave of 10 MHz in frequency, the
ultrasonic measurement was carried out for MMC castings
with holes of 2 mm in diameter at intermediate depths of
2­5 mm.
For holes at a depth of 1 mm or less, the SAW measurement was carried out. Typical SAW profiles are shown in
Fig. 10. The reflection from the hole (indicated by the arrow)
overlapped with the largely extended surface reflection.
Therefore, the reflection from the hole was identified by
moving the probe slightly back and forth. The relation
between the SAW intensity and the probe position is shown
in Fig. 11. The SAW reflection intensity showed two local
maxima at 2­3 mm and 5­8 mm before the hole. The
positions at the local maximum intensity was independent
of the hole diameter and the hole depth. Kawada and Kato23)
carried out the SAW measurement with aluminum alloy diecast plates with holes near the specimen surface, and also
3.3
Relation between reflection from hole and DGS
diagram
The change in the reflection intensity from the disc-shaped
flaw with the flaw depth is known as the DGS diagram.15)
When the flaw exists in the far field, the reflection intensity
from the flaw monotonically decreases with increasing depth,
and in the near field, the reflection intensity largely fluctuates.
Here, the length of the near field L0 is given by15)
L0 ¼ ðd2 ­ 2 Þ=4­ ;
ð1Þ
where d is the diameter of the flaw, and ­ is the wavelength of
the ultrasonic wave. The velocity of the longitudinal wave
and the near field length are summarized in Table 2.
For holes at a depth of 2­8 mm in the MMC castings, the
change in the reflection intensity from the hole with the hole
depth was compared with the DGS diagram. Since the
present ultrasonic measurement was carried out by the
immersion method, the hole depth z (the distance between the
specimen surface and the hole) was corrected in the depth
z* (the distance between the vibrator and the hole), in
consideration of the water path zw (the distance between the
probe and the specimen surface) as follows,
z ¼ z þ zw ðVw =VMMC Þ;
ð2Þ
422
H. Kato, S. Otsuka and H. Kurita
MMC Hole tip S
f(MHz) Round Cone
-15 10
0.25
5
0.2
Reflection intensity (dB)
Reflection intensity (dB)
-10
DGS S = 0.25
-20
S = 0.2
-25
Mod-DGS
-30
-35
0.5
1
Normalized depth, z*/L 0
where Vw and VMMC are the sound velocity in water and the
MMC castings, respectively. The change in the reflection
intensity from the hole with the hole depth is shown in
Fig. 12, in comparison with DGS curves of the same size
ratio (S: a ratio of the flaw diameter d to the diameter of the
vibrator d0). The reflection intensity from the hole is the
average of the reflection intensities measured for three holes,
and normalized by the intensity of the surface reflection. As
shown in the figure, the reflection intensity from the hole was
far lower than the DGS curve. Also, the reflection intensity
took a different tendency from the DGS curve; the DGS
curve increased with increasing depth in the near field, but
the reflection intensity from the hole was constant or slightly
decreased.
The ultrasonic wave propagating in the castings is largely
scattered by material structures and casting defects, and even
in the near field, the wave intensity is decreased with
increasing depth. Here, p(z*), the reflection intensity at a
depth z*, is formulated as follows,
ð3Þ
where ¥(z*) is the reflection intensity given by the DGS
curve, and ¡T is the attenuation coefficient at a depth z, and is
given by,
¡T ¼ ¡structure þ ¡defect ;
MMC
ð4Þ
where ¡structure and ¡defect are the scattering factors depending
on material structures and casting defects, respectively. In the
present work, material structures, such as the grain size
distribution, were not obtained, and hence a value of ¡T was
estimated to fit the DGS curve to the measured reflection
intensity from the hole. In Fig. 12, modified DGS curves are
shown with ¡T of 2.15 and 1.70 dB/mm for 5 and 10 MHz,
respectively.
Figure 13 shows the relation between the reflection
intensity from the hole at a depth of 5 mm in the MMC
castings and the hole diameter, in comparison with the DGS
curve. The reflection intensity increased with increasing hole
size S, as was observed in the DGS curve. But, the reflection
intensity from the hole showed a tendency to converge with
a value of ¹30 to ¹35 dB. This tendency of convergence is
thought to be related with the radial distribution of the sound
pressure in the propagating wave, but no detailed discussion
was done.
DGS
z*/L 0 = 0.68
-10
0.88
-20
-30
f (MHz) Exp. z*/L0
10
0.68
5
0.88
-40
-50
1.5
Fig. 12 Comparison of reflection intensity from hole with DGS curve.
pðz Þ ¼ ðz Þ expð¡T zÞ;
0
0
0.2
0.4
0.6
S (=d/d 0)
0.8
Fig. 13 Change in reflection intensity from hole with hole diameter in
comparison with DGS curves.
40 mm
Fig. 14 Typical X-ray image of casting defects in MMC plate-2.
3.4 Detection of casting defects in MMC castings
In the preceding section, the hole with the minimum
diameter of 2 mm was detected in the MMC castings at a
depth of 2­10 mm below the specimen surface. In this
section, the depth of the inherent casting defects in the MMC
castings was estimated with the X-ray radiography and the
ultrasonic measurement.
First, the X-ray radiography was carried out with MMC
plates of about 22 mm in thickness. In Fig. 14, the typical Xray image of the MMC plate-2 is shown, in which some dark
spots are observed. Then, the ultrasonic measurement was
carried out at positions of the dark spots by irradiating the
longitudinal wave on the as-cast surface. Figure 15 shows
typical ultrasonic wave profiles measured at the dark spot-3
in MMC plate-4. In the figures, the echo seemed to be the
reflection from the casting defect was observed behind the
surface reflection, as indicated by the arrow. Figure 16 shows
a cross section of MMC plate-4. In the cross section, the
cavity of about 2.2 mm in size was observed at a depth of
about 4 mm, which depth was in good agreement with the
estimated depth of about 5 mm with the ultrasonic measurement. The results of the ultrasonic measurement are summarized in Table 3. In the table, the depth of the cavity
determined at the cross-section and the cavity size estimated
from the X-ray image are also included. As shown in the
table, multiple reflections were observed in the ultrasonic
wave profile, and one of them was coincident with the observed cavity. Other reflections were thought to be reflections
from casting defects existed out of the cross section.
Detectability of Holes in SiC Particulate-Reinforced Al­Si Alloy Composite by Means of Ultrasonic Measurement
Intensity, I / V
2
S
(a) f = 5 MHz
B
0
-2
0.4
(b) f = 10 MHz
0
Plate-4, Spot-3
-0.4
2
4
6
8
Time, t / µs
10
Fig. 15 Typical reflection at a position of dark spot-3 in MMC plate-4.
S and B indicate surface and bottom reflections, respectively.
4 mm
Plate-4
Dark spot 3
423
(1) The hole existed 2­8 mm below the specimen surface
was measured with the longitudinal wave of 5 and
10 MHz in frequency, and the hole existed at 0.5­1 mm
below the specimen surface was detected by the surface
acoustic wave (SAW) of 5 MHz in frequency. The
detection limit of the hole diameter was 2 and 1 mm at
a depth of 5 mm for MMC castings and Al­Si alloy
castings, respectively.
(2) The reflection intensity from the hole in the MMC
castings was far lower than the DGS curves, and tended
to decrease with increasing hole depth even in the near
field. In consideration of scattering by coarse crystal
grains, dispersed particles and casting defects, the DGS
curve was modified. Then, the reflection intensity from
the hole changed with a hole depth with similar
tendencies as the modified DGS curve.
(3) By the cooperative usage of the X-ray radiography and
the ultrasonic measurement, the position and depth of
the cavity of more than 2 mm in size was estimated in
the MMC castings.
Acknowledgements
5mm
Fig. 16 Cavity observed at a position of dark spot-3 in MMC plate-4.
Table 3 Depth of cavity in MMC castings estimated from ultrasonic
measurement and observation at cross section.
Specimen
Plate-1
Plate-2
Plate-3
Plate-4
Dark
spot
Depth of
reflection*1,
zUT/mm
1
6
2
6
Depth of
cavity*3,
zOBS/mm
Cavity
size*4,
d/mm
12.0
*2
1
4, 7, 17 (5)
2
4, 7
1
4, 7
2
4, 7, 11 (10)
3
4, 7
4.8
1
4, 7
5.6
2
6, 8
3.2, 4.8
3
5, 8 (10)
15
6.4
4.8
4.0, 6.4
11
4
6.4
4.0
*1
Depth of reflections from cavity estimated from the ultrasonic wave
profiles of 5 and 10 MHz in frequencies.
*2
A (B) means that the reflection at a depth of A mm was only detected
by the ultrasonic measurement of B MHz in frequency.
*3
Depth of cavity observed in the cross section.
*4
Cavity size estimated from the X-ray image.
4.
The present work was carried out under the financial
support of Small and Medium Enterprise Agency, the
Ministry of Economy, Trade and Industry, Japan. The
photomicrographs were courtesy of Mr. H. Inoue, Saitama
Industrial Technology Center (SAITEC), Japan. The authors
thank Mr. M. Matsuura, Tajima Light Metals, Inc., and Mr. Y.
Nagai, SAITEC, for the cooperative work with them. Thanks
are also for the technical help of Mr. Y. Kawada, Technical
Support Division, Research Management Bureau, Saitama
University, and for the useful discussion with Dr. K.
Kageyama, Associate Professor, Saitama University.
Conclusions
The ultrasonic measurement was carried out with the Al­Si
alloy composite castings containing SiC particulate by
30 vol% (MMC castings) and the Al­Si alloy castings to
detect holes introduced in the castings, and the results were
compared with the DGS diagram. Then, the following results
were obtained.
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