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Ciampini2008.pdf
Available online at www.sciencedirect.com
Wear 264 (2008) 671–678
Characterization of vibratory finishing using the Almen system
D. Ciampini a , M. Papini b,a , J.K. Spelt a,b,∗
a
Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ont. M5S 3G8, Canada
b Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, Ont. M5B 2K3, Canada
Received 9 August 2006; received in revised form 4 April 2007; accepted 4 June 2007
Available online 19 July 2007
Abstract
The Almen system was adapted to a vibratory finishing process to characterize the effect of varying process parameters for the purposes of process
development and control. Saturation curves for two types of aluminum Almen strips were obtained by finishing at two distinct conditions using a
tub type vibratory finisher and steel spherical media. Comparison with the normal contact forces and effective impact velocities, measured for both
these conditions in a previous study [D. Ciampini, M. Papini, J.K. Spelt, Impact velocity measurement of media in a vibratory finisher, J. Mater.
Process. Technol., 183 (2007) 347–357], provided insight into the mechanics of the vibratory finishing process. An electromagnetic apparatus was
constructed to simulate the normal impacts in the vibratory finisher. It was found that surface-normal impacts at velocities comparable to the higher
range in the vibratory finisher produced Almen saturation curves similar to those created in the vibratory finisher. This provided support for the
modeling approximation of treating all contact events in a vibratory finisher as effective surface-normal impacts, and the accuracy of the effective
impact velocity measurement employed in [D. Ciampini, M. Papini, J.K. Spelt, Impact velocity measurement of media in a vibratory finisher, J.
Mater. Process. Technol., 183 (2007) 347–357].
© 2007 Elsevier B.V. All rights reserved.
Keywords: Almen; Vibratory; Finishing; Shot-peening; Impact
1. Introduction
Vibratory finishing is a widely used industrial process to modify the properties and microtopography of metal, ceramic, and
plastic parts. It is used to polish metal and plastic components,
to smooth the sharp edges of steel parts that have been cast or
stamped, to harden and texture metals, and to clean surfaces by
removing rust and other contaminants. Current industrial practice relies largely on experience and experimentation to optimize
the process for new parts and materials. Several authors [2–4]
have proposed correlations to predict erosion rates in vibratory
finishing.
In a typical configuration, a vibrating container fluidizes a bed
of granular media creating a circulating bulk flow. A workpiece
entrained in this flow is subject to the impacts of the vibrating
media. The impact velocity of the vibrating media will control
∗ Corresponding author at: Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ont. M5S 3G8,
Canada. Tel.: +1 416 978 5435; fax: +1 416 978 7753.
E-mail address: [email protected] (J.K. Spelt).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.wear.2007.06.002
the impact force and hence the degree of plastic deformation and
erosion of the workpiece surface.
The Almen system is well established [5] as an effective
standardized (i.e. SAE J442) means of characterizing the shot
peening process. Standardized metal strips are clamped to a rigid
support and subject to the shot peening stream. On release of the
clamps, residual stresses created by plastic deformation cause
the strips to curve. The degree of curvature and its rate of change
are related to the process parameters. The “Almen intensity” is
the ultimate strip arc height reached after a certain saturation
time defined such that doubling the exposure time causes only
a 10% increase in arc height [5]. The Almen system has the
advantage that it combines the effects of all the process parameters into one measurement allowing for the characterization
and determination of the repeatability of the process. These process parameters include shot density, radius, velocity, impact
frequency, impact coverage, elastic modulus, and yield stress, if
the shot deforms plastically during impact.
Baghbanan et al. [6] measured Almen strip curvature in a
vibratory finisher as a function of lubrication, finishing duration,
aluminum alloy, and finisher type. They measured the hardness
672
D. Ciampini et al. / Wear 264 (2008) 671–678
Fig. 1. Cross-section of the vibratory finisher showing direction of media circulation and resulting slope of the free surface. Dimensions in mm [6].
of finished workpieces as a function of depth from the finished
surface (Fig. 1). As the finishing time increased, the location
of the maximum hardness moved from the surface to a depth
of approximately 20 ␮m. The Almen strip curvatures for strips
made of AA1100-O, AA6061-T6, and Cu10100, were found
to increase consistently as function of time. It was also found
that at any given time, Almen strips finished in the tub had
more curvature than those finished in the bowl since the tub
was more energetic. Baghbanan et al. [6] also measured Almen
strip curvature in both the wet and dry finishing conditions. The
curvatures were always greater in the wet condition than the dry.
This was unexpected, since it had been shown that the dry condition corresponded to higher surface-normal contact forces. It
was hypothesized that, in the wet condition, water could find its
way between the Almen strip holding fixture and the Almen strip
itself, causing an increase in the effective stiffness and thus an
increase in the contact forces relative to the dry condition. This
was supported by demonstrating that the curvature of an Almen
strip increased when a layer of grease was sandwiched between
the strip and the holding fixture. Consequently, Baghbanan et al.
[6] recognized the need to provide a consistent boundary condition during finishing and suggested using a hot melt adhesive
to attach Almen strips to the holding fixture during the finishing
operation. These issues are more significant in vibratory finishing than in shot peening because of the much lower impact
velocities.
A number of investigators [7,8] measured contact forces in
a vibratory finisher. Yabuki et al. [8] concluded that tangential impact forces are approximately 10 times less than normal
forces. However, the measured contact forces were a function of
the compliance of the sensors employed and hence were not necessarily representative of contact conditions against an arbitrary
workpiece.
Ciampini et al. [1] related the normal component of the
measured contact forces in a vibratory finisher to the impact
velocity using a piezo-electric transducer. This method provided
a measure of the energy availability in the contacts that was
independent of the sensor compliance. Consistent with Yabuki
et al., contacts could be classified as being either impact or nonimpact events, depending on their duration. Both were expressed
in terms of an ‘effective’ normal impact velocity. Although the
piezo-electric sensor measured accurate impact velocities, it was
of interest to explore further the potential of the Almen system
to provide a simpler characterization of the impact conditions
in vibratory finishing. The objective was to develop a tool to aid
vibratory finishing process development and quality control.
In the present study, aluminum Almen strips were finished
using the same vibratory finisher and media as in [1], providing saturation curves at two of the contact conditions that were
characterized previously in terms of effective impact velocity
distributions. This provided a means of relating the impact conditions to the observed changes in Almen strip curvature as
a function of finishing time. The Almen strips were designed
specifically for vibratory finishing applications, and a novel
vacuum Almen strip attachment system was used to provide
a constant boundary condition for the Almen strips during finishing. The deflection of the Almen strips was measured using
a novel gauge that applied a negligible force to the strips
to provide an accurate measurement of the deflection. The
relationship between normal impact velocity and Almen strip
curvature saturation curves was also determined independently
using an apparatus that simulated the impacts in the vibratory
finisher.
2. Apparatus
2.1. Almen strips
Almen strips 76.2 mm long and 19.05 mm wide were
machined from aluminum 3003-H14 sheet, ensuring that they
remained flat. Two sheet thicknesses were used: 635 ␮m
(0.025 ) nominal thickness strips with an actual mean thickness
of 617 ␮m with a standard deviation of 3.84 ␮m, and 813 ␮m
(0.032 ) nominal thickness strips having an actual mean thickness of 779 ␮m with a standard deviation of 9.76 ␮m. The mean
and standard deviations of Almen strip thickness were based
on ten strips with three measurements per strip. The 617 ␮m
(0.025 ) thickness was chosen to be less than that found in shot
peening Almen strip specifications in order to accentuate the
arc heights resulting from the much lower impact velocities in
vibratory finishing.
Previous investigators have measured a small amount of
anisotropy in aluminum sheet properties as a function of orientation relative to the rolling direction. Wu et al. [9] found
that the yield stress of AA3104-H19 and AA5182-0 aluminum
in the cross-rolling direction was approximately 3.5% less than
in the rolling direction. Very similar results were recorded for
AA5019A-O by Choi et al. [10], who demonstrated that annealing can make the crystallographic textures nearly isotropic and
produce a slight reduction in yield stress anisotropy. In the
present work, the Almen strips were cut so that the sheet rolling
direction was normal to the long edge.
D. Ciampini et al. / Wear 264 (2008) 671–678
673
The Almen strips were annealed at 413 ◦ C for 1.5 h to Otemper in order to relieve all the residual stresses in the samples
from the manufacturing and machining processes. Comparisons
with unannealed strips showed that such pre-existing residual
stress can influence significantly the magnitude and variability
of the measured arc heights.
2.2. Vibratory finisher and media
The present measurements were conducted in the same tub
finisher used in Ref. [3] (Burr Bench 2016, Brandon Industries,
TX), having a U-shaped cross-section with a urethane chamber
liner and inner dimensions of 400 mm (x-direction) × 210 mm
(y-direction) × 400 mm (z-direction) (Fig. 1).
The media used were carbon steel balls (ABCO, Abbott
Ball Company Inc., West Hartford, Connecticut, USA) having
an average diameter and mass of 6.3 mm (standard deviation,
S.D. = ±0.005 mm) and 0.29 g (S.D. = ±0.02 g).
2.3. Vacuum almen strip holder
Almen strips were finished in a fixed position using a vacuum holder as shown in Figs. 2 and 3. This ensured that the
strip remained flat, providing a constant boundary condition,
and hence impact forces, as residual stresses accumulated during
finishing.
The vacuum holder consisted of a hollow aluminium cylinder with a polished, recessed rectangle (20.6 mm × 76.7 mm ×
0.5 mm) milled into the flat face and containing fourteen 0.5 mm
diameter holes through to the vacuum chamber. Placing an
Almen strip in the recess and evacuating the cylinder to approximately 3 kPa held the strip securely during finishing. To ensure
repeatable flow conditions over the surface, the long direction of
the Almen strip was always kept horizontal as shown in Fig. 3.
The holder was attached to a stiff supporting column connected
to an adjacent wall.
Fig. 2. Schematic of Almen strip vacuum fixture immersed in tub finisher.
Fig. 3. Photograph of Almen strip vacuum holder.
The curvature of the Almen strips may be a function of the
compliance of the holder and its support. By being very stiff, the
present vacuum holder and its supports provided a well defined,
reproducible boundary condition. A vacuum holder that was free
to flow with the media would have the advantage of providing an
average response for the tub, but it would introduce the complication of a compliance and relative impact velocity that varied
with its shape (drag coefficient), size, and mass.
2.4. Arc-height gauge
Traditional Almen gauges use a dial gauge to measure the
deflection of the finished Almen strips (the “arc height”) at their
mid-span over a span of 38 mm (1.5 ) while the strip rests on ball
supports. The force exerted by a gauge would create a significant
error with the relatively thin, flexible aluminum Almen strips
used in the present experiments. It has been estimated that typical
Almen gauges exert tip forces between 0.2 and 1.5 N during
typical usage [11]. Loads of this magnitude would result in midspan deflections (or arc-heights) of between 40 and 300 ␮m for
a 617 ␮m thickness AA3003 Almen strip over a 63.5 mm (2.5 )
span, and between 20 and 150 ␮m for a similar 779 ␮m thick
strip. Given that the actual maximum deflection, measured using
the optical system described below, was 760–335 ␮m, for the
617 and 779 ␮m thick strips, respectively, it is clear that the
forces exerted by a typical contact dial gauges are too high for the
presently utilized thin strips. Furthermore, the error is a function
of the material and thickness of the strip, thereby adding further
complications when trying to compare results from different
types of Almen strip. Consequently, an alternative approach was
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D. Ciampini et al. / Wear 264 (2008) 671–678
Fig. 4. Schematic of arc-height gauge.
developed using the apparatus shown in Fig. 4. The Almen strip
rested on four steel ball bearings over a span of 63.5 mm.
This larger span increased the measured deflection at the midspan and thereby the accuracy. With a strip in place, a microscope
was used to view the back-lit tip of the micrometer probe as it
was lowered to the Almen strip surface and stopped just as it
made contact with a negligible load. Repeated measurements of
arc height made in this manner were always within ±4 ␮m of
the mean measurement.
2.5. Vibratory finishing simulator
In order to investigate the hypothesis that only the surfacenormal component of the impact velocity had an appreciable
effect on the Almen strip curvature in the vibratory finisher; an
apparatus was constructed to finish an Almen strip with only
normal impacts of a specified velocity. This was accomplished
by using an electromagnet to pick up then drop the steel media
repeatedly onto the Almen strip surface while it was secured
using the vacuum holder. The vacuum holder and support beam
shown in Fig. 2 were rotated 90◦ such that the Almen strip
surface faced upward (Fig. 5).
A cage (25.4 mm × 82.55 mm) was constructed around the
perimeter of the strip to contain the steel media while permitting the balls to make contact with every portion of the Almen
strip surface. It was found that 33 steel balls provided enough
free space to create randomized impact locations while finishing
reasonably quickly. The electromagnet was placed on the upper
rim of the cage so that its flat surface was parallel to, and at a
fixed height above the Almen strip.
A function generator supplying a square wave at 2 Hz was
used to control a relay (Fig. 5) that cycled the electromagnet on
and off. The frequency of 2 Hz was chosen to allow the balls
enough time between drops to randomize their position and settle before they were picked up again. The diode in the circuit
protected the contacts of the relay.
The constant impact speed was determined by the ball drop
height (i.e. the height of the cage). Several cages of varying
height were constructed to provide different maximum impact
velocities. The range of possible impact velocities using the different cage heights was 0.1–0.37 m/s because of limitations of
the electromagnet’s strength.
The speed at which a ball dropped from the electromagnet
once it was turned off was measured using a high speed photographic apparatus. Several overlapping exposures of the ball
were taken as it fell from the electromagnet, to determine if the
impact speed was being affected by lingering magnetism of the
magnet or magnetization of the balls. Fig. 6 demonstrates that
there was no appreciable difference between the measured and
predicted velocities. Nevertheless, to eliminate ball magnetization as a possible source of error, a new set of 33 balls was used
for every new strip that was finished.
Although the balls struck every part of a strip, impacts tended
to occur more frequently in two parallel lengthwise rows due to
an uneven distribution of the magnetic field strength. This was
evident mostly in the appearance of the surface during the early
stages of finishing and became much less noticeable for longer
times. Since this effect was the same for all strips, it did not
influence comparisons among strips.
3. Results and discussion
3.1. Vibratory finisher saturation curves
Almen strips were finished at two of the vibratory finishing conditions that were characterized in [1] using an impact
velocity probe. The first condition (91Sa) utilized 91 kg of steel
Fig. 5. Schematic of vibratory finisher simulator.
D. Ciampini et al. / Wear 264 (2008) 671–678
675
flux (ETotal ) were all greater for the 91Sa condition compared to
the 112Sb condition. favg was calculated by dividing the number
of contacts found in a force signal by the signal duration (T).
ETotal was evaluated by summing the effective kinetic energy of
each contact found in a force signal and dividing by the sensor
contact area, Asensor , and the signal duration, T, i.e.
2
mavg N
i=1 vi
ETotal =
(1)
2TAsensor
Fig. 6. Velocity of dropping ball as a function displacement from its initial position: measured – open symbols representing four repeat experiments; predicted
assuming gravitational acceleration – filled symbols.
Fig. 7. Plan view of tub showing finishing locations “a” and “b”. Arrows indicate
orientation of exposed surface normal of the Almen strip. High wall (HW) and
low wall (LW) as shown in Fig. 1. Depth of finishing was 210 mm above bottom
of tub in all cases.
media at location and orientation “a”, and the second (112Sb)
utilized 112 kg of steel media at position and orientation “b”,
shown in Fig. 7. Ciampini et al. [1] found that the 91Sa condition
corresponded to a more aggressive finishing condition than the
112Sb condition. This is evident in Table 1 which shows that the
recorded average contact frequency (favg ), maximum (vmax ) and
average (vavg ) effective impact velocities and the total energy
where N is the total number of contacts that occurred during T,
and mavg and vi are the average media mass and effective impact
velocity of the ith contact, respectively.
Three types of Almen strips were finished at these two finishing conditions to generate the six saturation curves shown in
Fig. 8. A new strip was used to determine the arc height for each
finishing duration; i.e., strips were not finished again after their
arc height was measured. Each data point in Fig. 8 corresponds
to the average of five finished samples, and the error bars show
the minimum and maximum arc heights. Significant variability
was observed in the data from the vibratory finisher; the average
standard deviation was 44 ␮m for each set of five strips finished
at the same condition. Table 2 gives the average and standard
deviation for the five samples at each condition. The scatter was
due to the randomness of the contact conditions, the variability of
the Almen strip thickness, small changes in the strip positioning
during finishing, and arc height measurement error.
Fig. 8 shows that, for each of the three types of Almen
strips, the arc height at any given finishing duration was always
higher for the 91Sa condition than the 112Sb. This was expected,
because the 91Sa condition generated much higher effective
impact velocities as well as a higher contact frequency (Table 1).
Almen strips at 91Sa thus experienced more plastic deformation
per unit time than those at 112Sb.
It is apparent from Fig. 8 that the thin (617 ␮m) O-temper
strips were the most sensitive to either finishing condition, showing the greatest change in arc height. This made them the better
probes for characterizing the current vibratory finishing conditions. The arc heights of the thick (779 ␮m) O-temper and the
thin (617 ␮m) H14-temper were similar. The thin H14-temper
strips deflected to a lesser extent during the maximum finishing
duration than the thin O-temper strips because they had a higher
yield stress, 151 and 41 MPa, respectively.
The arc height of each Almen strip approached a saturation
value as finishing time increased (Fig. 8). In the case of the thin
O-temper sheet, the arc height began to decrease after approximately 120 min at the 91Sa condition. A related finite element
modeling study [12] has indicated that the compressive residual radial stress distribution is driven below the neutral axis by
repeated impacts leading to a decrease in curvature. After a sin-
Table 1
Finishing impact conditions measured using the impact velocity probe of [1] at locations “a” and “b” of Fig. 7
Condition
Average contact
frequency favg (Hz)
Average effective impact
velocity vavg (m/s)
Max. effective impact
velocity vmax (m/s)
Total energy flux ETotal
(W/m2 )
91Sa
112Sb
462
224
0.014
0.011
0.378
0.163
12.8
3.13
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D. Ciampini et al. / Wear 264 (2008) 671–678
Fig. 8. Saturation curves produced using two finishing conditions and three types of Almen strip (617 ␮m thick 3003-O and 3003-H14; 779 ␮m thick 3003-O). Fig.
(b) shows an expanded view of the data of Fig. (a) at low finishing durations.
Table 2
Average arc height and standard deviation (in brackets) of each set of five Almen strips finished in the vibratory finisher at the conditions in Fig. 8
Finishing duration (min)
0
1
2
5
15
60
120
Al 3003-O 617 ␮m
Al 3003-H14 617 ␮m
Al 3003-O 779 ␮m
91Sa
112Sb
91Sa
112Sb
91Sa
112Sb
0 (57)
347 (32)
378 (34)
501 (38)
585 (56)
714 (46)
643 (29)
0 (38)
103 (34)
121 (57)
105 (36)
311 (54)
316 (73)
390 (67)
0 (63)
181 (67)
111 (56)
160 (43)
212 (74)
273 (46)
333 (61)
0 (59)
53 (62)
59 (38)
91 (33)
122 (57)
200 (40)
243 (65)
0 (4)
117 (29)
150 (65)
192 (26)
206 (23)
275 (42)
295 (40)
0 (13)
75 (20)
76 (16)
88 (21)
155 (41)
193 (30)
254 (43)
gle 0.225 m/s impact the plastic depth was predicted to be about
half of the strip thickness, however, after one hundred overlapping impacts, this depth has extended to more than 75% of the
strip thickness, resulting in a decrease in the internal bending
moment. It is to be expected that this would have occurred earliest at the most aggressive finishing condition (91Sa) and with the
thin O-temper Almen strip which had the lowest yield strength.
Almen saturation curves created from shot peening data are
usually fit to exponential functions (Eq. (2)) in order to determine
the Almen intensity [13].
H(T ) = A(1 − exp(BT C )) + DT
(2)
where A, B, C, and D are parameters for the curve fit. Eq. (2) also
provides a good fit for saturation curves created by the vibratory
finishing of Almen strips (Fig. 9).
The arc height H, and finishing duration T, that satisfy Eq.
(3) are called the “Almen intensity” and “saturation time”,
Fig. 9. Exponential curve fits (Eq. (2)) through vibratory finisher saturations curves of the O-temper strips.
D. Ciampini et al. / Wear 264 (2008) 671–678
677
Table 3
“Almen intensity” and “saturation time” obtained using Eqs. (2) and (3) with values of parameters A and B given in the table and with C = 1 and D = 0
Condition
A (␮m)
B
Almen intensity (␮m)
Saturation time (min)
AA3003-O 617 ␮m 91Sa
AA3003-O 617 ␮m 112Sb
AA3003-O 779 ␮m 91Sa
AA3003-O 779 ␮m 112Sb
AA3003-H14 617 ␮m 91Sa
AA3003-H14 617 ␮m 112Sb
627
354
253
217
266
219
0.533
0.122
0.442
0.117
0.329
0.078
564
278
178
147
179
192
4
19
5
20
7
29
respectively [5].
H(2T ) − H(T ) = 0.1 · H(T )
(3)
There is no solution to Eq. (3) for the 617 ␮m thick Almen
strip finished under the 91Sa condition because of the maximum
observed in the arc height (Fig. 9). However, when the data are
approximated by Eq. (2) using C = 1 and D = 0, a solution can be
found. Table 3 lists the Almen intensities and saturation times
defined in this way for the data of Fig. 9 and for the AA3003-H14
strips. These parameters reflect the magnitude and frequency of
the impact forces in the vibratory finisher at locations “a” and
“b” with the two different media loads (91 and 112 kg).
3.2. Vibratory finishing simulator saturation curves
The 617 ␮m O-temper strips were finished in the vibratory
finishing simulator set to produce impact velocities of 0.18, 0.22,
and 0.33 m/s, representative of the higher end of the effective
velocity distributions measured for the 91Sa and 112Sb conditions. In addition, the 779 ␮m O-temper strip was finished
at 0.22 m/s. Note that only the first impact of each drop cycle
occurred at one of these target velocities; secondary impacts
due to the bouncing of the media had much lower velocities. For
example, a preliminary dynamic finite element analysis showed
that a 0.22 m/s initial impacts had a second bounce impact
velocity between 0.075 and 0.15 m/s, depending on the surface
hardness. Assuming a typical aluminum coefficient of restitution of 0.5, the impact velocity after the first bounce will be half
of the initial velocity.
The saturation curves created by the simulator (Fig. 10) are
similar to those resulting from the vibratory finisher (Fig. 8),
reaching comparable maximum arc heights; i.e., 648 and 714 ␮m
for the simulator and finisher, respectively. The principal differences between the curves from the simulator and those from the
vibratory finisher are that curvature relaxation is more common
in the simulator and the finishing durations are much longer than
in the finisher. Fig. 10 shows that the maximum curvature in the
simulator was seen at approximately 300–500 min at 0.22 m/s,
or 2.5–4 times later than in the vibratory finisher. At 0.33 m/s,
the curvature reached a maximum even more quickly in the simulator at; i.e., between 150 and 300 min or 1.25–2.5 times later
than in the vibratory finisher. In general, the time to reach the
maximum Almen curvature decreased as the impact velocity
increased.
As mentioned previously, the strip curvature begins to
decrease at some point corresponding to the movement of the
plastic zone below the neutral axis of the strip as a result of the
cumulative deformation from many impacts. A related finite element modeling study [12] has confirmed this hypothesis. Higher
impact velocities create larger individual plastic zones, thereby
reducing the number of impacts required to reach this point
where the curvature begins to decrease. The longer finishing
times seen in the simulator were expected since the operating
frequency of the vibratory finisher was approximately 46 Hz
compared with only 2 Hz in the simulator. Assuming that both
the simulator and vibratory finisher had a close-packed arrangement of media and created the same impact velocity distribution,
the simulator would have been expected to finish approximately
23 times slower. In reality, the media in the vibratory finisher
was more closely packed than in the simulator, and therefore the
vibratory finisher caused more impacts each cycle. Taking this
into account, the simulator would have been expected to finish
28 times slower than the finisher. However, these packing and
frequency effects were counteracted somewhat by the higher
impact velocities in the simulator which were representative of
maximum values in the finisher (Table 1).
The similarity of the Almen curves in the simulator and the
vibratory finisher supports the hypothesis that the normal media
velocity component is dominant [1]. It also indicates that, to a
first order approximation, arc height is controlled by the largest
impact velocities in vibratory finishing, with the more numerous
Fig. 10. Vibratory finisher simulator saturation curves for 617 ␮m strips (open
symbols) and 779 ␮m strips (filled symbols). Impact velocities of 0.33 m/s (circles), 0.22 m/s (squares), and 0.18 m/s (triangles).
678
D. Ciampini et al. / Wear 264 (2008) 671–678
lower velocities playing a relatively small role. This is consistent
with the approach taken in the analytical shot peening curvature
model of Hill [14].
Almen saturation curves in vibratory finishing may be used
to show how changes in a process affect the finishing rate or the
aggressiveness of the process.
4. Conclusion
Acknowledgment
The Almen system was adapted to vibratory finishing to
quantify the effect of varying process parameters on the aggressiveness of the process. Saturation curves at two distinct
operating conditions in a tub type vibratory finisher with steel
spherical media were similar in form to those seen in shotpeening.
More aggressive finishing conditions, characterized by
greater impact velocities, caused more plastic deformation and
resulted in more Almen strip deflection in less finishing time.
This supported the role of the Almen strip as a probe to characterize the aggressiveness of a vibratory finishing system in either
process development or control.
Certain elements of the usual shot peening Almen system
were modified to reflect the much lower impact velocities found
in most vibratory finishers. Relatively thin AA3003 O-temper
Almen strips (617 ␮m) displayed the greatest change in arc
height, making them more sensitive probes than the thicker
(779 ␮m) Almen strips. However, these thinner strips showed
no saturation point in the arc height and instead the arc height
decreased after reaching a maximum when subjected to the most
aggressive finishing conditions encountered in the present study.
The time to reach this maximum may be used as a characteristic of the impact conditions within a vibratory finishing system.
Nevertheless, the generation of a traditional Almen curve displaying a well-defined saturation requires the selection of a strip
having sufficient thickness given the impact conditions within
the finisher under study.
An apparatus was constructed that showed that normal
impacts alone produce saturation curves similar to those created
in the vibratory finisher when impact velocities are comparable
in magnitude. This provided support for normal impacts as the
dominant mechanism of finishing.
The research was supported by the Natural Sciences and
Engineering Research Council of Canada.
References
[1] D. Ciampini, M. Papini, J.K. Spelt, Impact velocity measurement of
media in a vibratory finisher, J. Mater. Process. Technol. 183 (2007) 347–
357.
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