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Experimental investigation of vibratory finishing of aluminum
Wear 243 (2000) 147–156
Experimental investigation of vibratory finishing of aluminum S. Wang a , R.S. Timsit b , J.K. Spelt a,∗ a
b
Department of Mechanical & Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ont., Canada M5S 3G8 Timron Scientific Consulting, 467 Woburn Avenue, Toronto, Ont., Canada M5M 1L6
Received 25 October 1999; received in revised form 15 May 2000; accepted 23 May 2000
Abstract The normal contact forces in a vibratory finishing machine were measured and compared with the resulting changes in surface roughness and hardness of two aluminum alloys, AA1100-O and AA6061-T6. The principal variables were the media size, degree of lubrication and the duration of the vibratory finishing. The changes in hardness and roughness were found to depend mostly on the lubrication condition, the media roughness, and the size of the media, since these influenced the interaction between the media and the workpiece, and hence the extent of plastic surface deformation per impact. The impact force parameters such as the average force, maximum force, and impulse, however, did not vary appreciably amongst the three media for dry and water-wet conditions. Thus, the differences observed in hardness and roughness were due to smaller scale differences in the impact contact conditions. On average, a sensing disk with a diameter approximately equal to that of the media was in contact with media for approximately 30% of the total finishing time. This was consistent with videotaped observations showing that the media was loosely-packed as it flowed past the workpiece, with relatively large gaps in the packing near the workpiece surface. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Vibratory finishing; Impact; Abrasive wear; Hardness; Roughness; Aluminum; Surface finish
1. Introduction Since at least the 1950s, vibratory finishing has been used widely in manufacturing to improve surface appearance and wear resistance, to polish and increase hardness, and to clean and dry surfaces. The process has been applied to metal, ceramic, and plastic parts employing a wide variety of media such as steel balls, rough ceramic spheres and angle-cut cylinders, and softer materials such as crushed corn cobs. A vibratory finishing system usually consists of a springmounted open chamber containing granular media, to which a vibratory motion generator is attached. The motion generator normally consists of one or two rotating shafts with eccentric weights. By adjustment of the eccentric weight or the speed of the drive system, the amplitude and frequency of the finisher can be controlled. The chamber of the finisher can be a round or oval bowl, or a long tub having a U-shaped cross-section. The vibrations are usually driven at 30 Hz with amplitude components of several millimeters in three dimensions. As a result, the media become fluidized and develop complex flow fields within the chamber. The ∗ Corresponding author. Tel.: +1-416-978-5435; fax: +1-416-9787753. E-mail address: [email protected] (J.K. Spelt).
parts to be finished are entrained by the flowing media and experience a slower relative velocity. The media interact with the part surface through a combination of normal impacts and sliding. Depending on the parameters of the process, this can produce a wide range of contact conditions encompassing varying degrees of rubbing, burnishing, ploughing, cutting and three-body abrasive wear. This makes vibratory finishing a versatile technique. There is very little scientific literature published on the vibratory finishing process. The mechanics of relatively deep, vibrationally fluidized bed have also received relatively little attention, although Fraas [1] provides a very useful discussion of the machinery requirements for generating certain types of flow fields. Vibrated beds are, however, well established as a means of drying granular materials since they enhance heat transfer [2,3]; however, related studies have usually tended to focus on relatively shallow beds of small particles, typically <1 mm diameter [4,5]. The objective of the present study was to measure the normal contact forces in a vibratory finishing machine and correlate them with the resulting changes in surface roughness and hardness of aluminum workpieces. The principal variables were the media size, degree of lubrication, finishing duration and aluminum alloy.
0043-1648/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 0 ) 0 0 4 3 7 - 3
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S. Wang et al. / Wear 243 (2000) 147–156 Table 1 Average roughness parameters of the fresh 7, 9, and 11 mm diameter mediaa Media (mm)
Ra (m)
Rku
Rsk
7 9 11
20±3.2 22±8.6 19±5.7
2.9±1.8 3.8±3.3 2.5±0.35
−0.32±0.97 −0.72±0.90 −0.18±0.32
±95% confidence intervals based on averages of single measurements on five spheres. Ra : average roughness, Rsk : skewness, Rku : kurtosis (measurement length: 300 m). a
Table 2 Average roughness parameters of 9 mm media after 20 and 40 h of usea
Fig. 1. Schematic of bowl-type vibratory finisher.
Time (h)
Ra (m)
Rku
Rsk
20 40
15.4±6.3 13.8±2.3
3.5±3.4 2.9±2.2
−0.43±0.54 −0.16±0.35
±95% confidence limits based on averages of single measurements on five spheres (measurement length: 300 m). a
2. Experiments 2.1. Vibratory finisher and media The vibratory finisher consisted of an annular polyethylene bowl with an outer diameter of 38 cm, an inner diameter of 15 cm, and a depth of 15 cm (Model 150S, Burr King Mfg. Co., Inc., Warsaw, USA) (Fig. 1). The bowl was attached to an electric motor that rotated an eccentric mass at 30 Hz, and the entire assembly was mounted on five coil springs. The resulting vibrations in the vertical (x-z) and horizontal (x-y) planes were videotaped and appeared to follow roughly elliptical paths with x, y, z amplitudes of approximately 0.7, 0.8 and 1.8 mm, respectively. Three types of moderately rough, approximately spherical ceramic media were used (Abrasive Finishing Inc., Chelsea, MI 48118, USA) with average diameters of 7.1 mm (S.D.=0.35 mm, number of spheres measured, N=20), 8.6 mm (S.D.=0.56 mm, N=20), and 11 mm (S.D.=
0.73 mm, N=20). The media were composed mainly of Si and Al, with lesser amounts of K, Ca, Ti and Fe. The bulk density of the static media as a granular material was approximately 1.4 g/cm3 . The initial roughness parameters of the fresh media were measured using an optical surface profilometer (Wyko, RST Plus) and are given in Table 1. At the 95% level of confidence, there was only a difference between skewness, Rsk , of the 9 and 11 mm media; all other parameters were essentially the same. In the experiments, the media were used in cycles consisting of 2.5 h dry, 2.5 h wet, and 2.5 h lubricated. The diameter decreased negligibly over this period by <0.4 mm. Fig. 2 shows that after approximately 20 h of use in the finisher, the average roughness, Ra , of the media decreased slowly. Table 2 shows the roughness parameters for 9 mm media after 20 and 40 h of use. At the 95% confidence level, a t-test showed that the kurtosis,
Fig. 2. Mean average roughness, Ra , of media as a function of total finishing time in cycles of 2.5 h dry, 2.5 h wet, and 2.5 h lubricated. Error bars indicate 95% confidence intervals based on single measurements on five spheres (measurement length: 300 m).
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Rku , remained essentially unchanged, but Ra decreased and Rsk became less negative as a result of particle wear. Prior to each experiment, the media were washed in tap water to remove fines. The media were then used in one of three conditions: dry, wet with water, and wet with a water–detergent solution (2% by volume detergent (Fantastik, DowBrands Inc.) in tap water). In the latter two conditions, after removal from the water or detergent solution, the soaked media were placed in the finisher with approximately 100 ml of excess liquid in the bottom of the bowl to keep the media wet during the experiments. 2.2. Workpieces A cylindrical geometry was selected for the prototypical workpiece. Three types of rotation were observed to occur simultaneously in the finisher when a cylinder was initially placed in the vibrating media with its axis horizontal: the cylinder rotated about its own axis; it moved radially in the finisher, submerging into the media at the inner wall and rising to the free surface at the outer wall; the cylinder also moved circumferentially around the bowl. The first two rotational motions always occurred at the same frequency. The resultant of these three components was a spiral around the annular bowl. An end-over-end tumbling motion could also be created if the cylinder were placed in the media such that its axis was not horizontal. The influence of this type of motion was not investigated. In order to establish the size and density of the cylindrical workpiece, six aluminum cylinders of different dimensions and density were tested in the finisher: lengths of 50 and 70 mm, and diameters of 26, 32 and 51 mm. The densities ranged from 0.6 to 1.1 g/cm3 . It was found that, over this range, the periods of motion did not change significantly. As a result, the workpiece was chosen to be 80 mm long, 68 mm in diameter with a density of 0.75 g/cm3 . Table 3 gives the measured periods of motion for this workpiece in the three media under three lubrication conditions. It is seen that the motion was largely unaffected by the size of the media, with the exception of the circumferential period when lubricated with detergent. The lubrication condition, however, had a large influence, with the three cylinder periods becoming much longer as the friction was reduced.
Fig. 3. Cross-section of cylindrical workpiece showing force sensor.
As explained below, aluminum workpieces of identical size and density were fitted to hold a contact force sensor and a small video camera. AA1100-O and AA6061-T6 0.8 mm thick sheets were cut into 50 mm diameter disks that were glued to the ends of two workpieces so that each alloy could be finished simultaneously, under identical conditions. The glue was a hot melt adhesive that was remelted at approximately 80◦ C after finishing periods of 5, 10, 20, 40 and 80 min, and the aluminum specimens were then rinsed in tap water and washed in methanol in an ultrasonic bath for 5–10 min. Hardness was measured using a Vickers indenter and roughness parameters were obtained using an optical surface profilometer (Wyko, RST Plus). In each case, six measurement points were selected on each disk surface. Several experiments were repeated to assess the reproducibility of the hardness and roughness measurements. 2.3. Contact forces The normal contact force between the media and the end of the cylindrical workpiece was measured in dry and wet conditions using a specially constructed load cell consisting of a clamped-clamped aluminum (AA6061-T6) beam, 50 mm long, 10 mm wide and 0.40 mm thick (Fig. 3). Four resistance strain gauges (4.0 mm wide, 3.0 mm gauge length)
Table 3 Motion periods of a cylindrical parta in different media and lubrication conditionsb Media diameter in mm (spherical)
7 9 11 a b
Results
Dry
Water-wet
Pr , Pa (s)
Pc (s)
Pr , Pa (s)
Pc (s)
Pr , Pa (s)
Pc (s)
Average Stdev Average Stdev Average Stdev
10 1 12 1 12 1
36 0.5 37 0.5 38 1
15 1 18 2 15 1.5
43 1 42 2 40 1
34 3 31 2 33 3
59 1 54 3 45 2
68 mm diameter×80 mm length. Pr : radial period, Pa : axial period, Pc : circumferential period (averages of 10 measurements).
Detergent-wet
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were mounted on the beam to create a full-bridge circuit (two on the upper surface, two on the lower surface, on each side of the beam midpoint) that gave a force resolution of 1% of the rated maximum load of 2 N. This arrangement of strain gauges eliminates the effect of surface shear forces, so that only surface-normal forces are recorded. Media contacted a 10 mm diameter aluminum disk that was connected to the sensor beam via a 3 mm long, 4 mm diameter stem glued to the beam centre. The disk was flush with the end face of the workpiece leaving a 1 mm gap around the disk. The strain gauge wires passed out of the opposite end of the workpiece cylinder and were then suspended above the bowl so that they remained vertical, directly above the workpiece. The drag of the wires in the media had a small effect on the motion of the workpiece; with dry 9 mm media, the radial and axial periods increased from 12 s without wires to 14 s with the wires, while the circumferential period increased from 37 to 46 s. The sensor was calibrated statically and dynamically on a shaker table (sensor calibration factor was 0.398 V/N). Up to 60 Hz, at accelerations of up to 20 m/s2 , the peak dynamic force produced by an attached 1 g mass was identical to that predicted using the static calibration factor; a similar result was obtained using a 4 g mass at a peak acceleration of 10 m/s2 . The inertial mass of the sensor was approximately 1 g, so that when placed in the finisher with the sensing disk shielded, the average maximum recorded force was approximately 0.038 N; this was used as the dynamic noise
threshold. When struck by a single piece of 9 mm diameter media (mass of 1 g) dropped from a height of 10 mm, the sensor response indicated a coefficient of restitution of 0.44, a natural frequency of 700 Hz and a damping time constant of 7.2 m s; i.e. the ringing of the sensor decayed to half its peak value in 5 m s. The spring constant of the sensor was approximately 5650 N/m, which was considerably less than that of the aluminum disks glued to the ends of the workpieces during finishing experiments. Since impact forces are a function of surface rigidity [6], the contact forces measured using this sensor can be used to assess the relative influence of media and lubrication, but should not be construed as those that resulted in the hardness and roughness changes of the present, more rigid, workpieces. Moreover, because the impact force changes so rapidly in the immediate vicinity of the maximum, very high digitizing frequencies would be required to reliably identify the maximum force. In the present experiments, the force sensor output was digitized at 500 Hz, which was high enough to record approximately 7 force values during a typical impact. Although this was sufficiently fast to accurately record the average contact forces and the percentage of contact time, it could not ensure that the absolute maximum force was resolved. The force signals from the sensor were highly variable, occurring in random bursts as a single particle moved (bounced) across the sensing disk. Fig. 4 illustrates this for 9 mm media under water-wet conditions. A Matlab program (The Math Works Inc.) was written to identify individual
Fig. 4. (a) Normal contact force in wet finishing with 9 mm media; (b) Fourier transform of signal in (a).
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Table 4 Statistics for normal contact forces of 7, 9, and 11 mm media in dry finishing for 8 s with sensor facing forwarda Media
Number of impacts in 8 s
Maximum impact force (N)
Impact force (N)
Impact duration (m s)
Impulse per impact 10−3 (N s)
Contact percentage (%)
7 mm 95%CI 9 mm 95%CI 11 mm 95%CI
130 ±52 130 ±50 136 ±43
0.18 ±0.03 0.21 ±0.05 0.29 ±0.07
0.086 ±0.01 0.10 ±0.02 0.12 ±0.03
20 ±2.3 19 ±3.8 18 ±3.5
1.6 ±0.5 2.0 ±0.8 2.1 ±0.8
32 ±13 31 ±11 29 ±11
a
Averages of 10 runs with 8 s for each run; 95% confidence intervals are listed.
impacts that were then analyzed for their maximum force, average force, impulse and impact duration. In order to obtain reliable average values of these quantities, it was necessary to average the results of 10 sets of force recordings each of 8 s duration (in comparison, the radial period was 10–12 s). 2.4. Videotaping relative media motion In a separate series of experiments, the relative motion of the media were videotaped through a 20 mm diameter glass window installed in the end face of one of the cylindrical workpieces. A small colour video camera (IK-SM40A, Toshiba), 42 mm long and 7 mm diameter, with a 410,000 pixel resolution was mounted in the workpiece beside a flexible fibre-optic cable (No. A08025.40, Subtechnique Inc., USA). The video and fibre-optic cables had approximately the same effect on the workpiece motion as did the force sensor wires, reported above. After recording an experiment, the videotape was replayed frame-by-frame to measure the speed of the media and the approximate percentage of time that media appeared to contact a 10 mm diameter area in the centre of the field of view. 3. Results and discussion 3.1. Normal contact forces and videotaped relative motion The force signal of Fig. 4 was typical, displaying a dominant Fourier transform peak at the finisher driving frequency,
30 Hz, and much smaller peaks at the first and second harmonics. The average impact parameters for the three media under dry and water-wet conditions are given in Tables 4 and 5, respectively. The data in these tables correspond to the sensor at the leading end of the cylindrical workpiece; i.e. facing forward as the workpiece spirals around the bowl. Comparable results were obtained with the sensor facing backwards, leading to the conclusion that impact wear conditions were relatively uniform against all of the workpiece surfaces. A few trials were conducted with the workpiece fixed to an external support with the sensor either facing the oncoming media flow around the bowl or facing the wake behind the workpiece. In the latter case, the contact percentage decreased to 18% while it increased to 36% when facing the flow. In both situations, the contact forces were greater than those measured when the workpiece was free to move with the media. It is noted in Tables 4 and 5 that the average number of impacts occurring in 8 s was relatively constant under all conditions. Thus, the observation in Table 3 that the workpiece moved more slowly under wet conditions, did not seem to result in more impacts per unit time. The average maximum impact force tended to increase with the media size, but did not show any consistent change with the degree of lubrication, dry or water-wet. It should be noted, however, that the maximum impact force is the least accurate of the measurements because of the resolution of the 500 Hz sampling frequency. The average impact force and the average impulse increased with media size under dry conditions, but when
Table 5 Statistics for normal contact forces of 7, 9, and 11 mm media in water-wet finishing for 8 s with sensor facing forwarda Media
Number of impacts in 8 s
Maximum impact force (N)
Impact force (N)
Impact duration (m s)
Impulse per impact 10−3 (N s)
Contact percentage (%)
7 mm 95%CI 9 mm 95%CI 11 mm 95%CI
126 ±53 115 ±55 110 ±75
0.20 ±0.05 0.20 ±0.04 0.24 ±0.06
0.10 ±0.02 0.10 ±0.02 0.11 ±0.03
21 ±3.4 20 ±2.1 17 ±2.6
1.8 ±0.60 1.8 ±0.50 1.7 ±0.60
31 ±14 29 ±12 23 ±15
a
Averages of 10 runs with 8 s for each run; 95% confidence intervals are listed.
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magnitudes will only be directly applicable to the vibratory finishing of surfaces that have a similar compliance. Using the videotapes, the average relative speed between the workpiece and the 9 mm media under dry, water-wet and detergent-wet conditions was found to be 11, 7 and 4 cm/s, respectively. Referring to Table 3, it is thus observed that as the amount of lubrication increases, both the absolute workpiece speed decreases and the speed of the media relative to the workpiece decreases. This implies that the lubrication condition has a greater relative effect on the speed of the media than on the speed of the workpiece. These observations suggest that the predominant driving force for the media flow and the entrainment of workpiece is friction between the wall of the bowl and the media. These frictional driving forces are then transmitted between the media and ultimately to the workpiece surface, and are larger than the frictional drag forces at each of these interfaces. Nevertheless, the measured differences in the relative speed of the media did not have a significant influence on the percentage contact time or the number of impacts per unit time.
Fig. 5. Effect of lubrication on Vickers hardness for AA1100-O (a) 7 mm, Ra =17.5±4.1 m (estimated media average roughness with 95% confidence interval), (b) 9 mm, Ra =15.4±6.3 m, and (c) 11 mm, Ra = 16.2±4.7 m. Error bars indicate 95% confidence intervals for six measurements on one workpiece. Dry: (×), water-wet: (䊊), detergent: (䊐).
the media were wet, these parameters did not change significantly with different media. The reason for this is not understood. Nevertheless, in comparing dry and wet conditions, the differences in the average impact force and the average impulse were relatively small (<16%). The average impact duration was similar in both wet and dry tests, and did not change significantly with media diameter. The average percentage of time that a particle was in contact with the 10 mm diameter sensing disk was also relatively constant amongst all of the test conditions. The value, approximately 30%, is surprisingly low, and illustrates that the media packing at the workpiece surfaces was rather sparse. This result was confirmed by the videotapes, which showed that the media vibratory flow seemed to display characteristics that were analogous to a two-phase flow of a liquid containing gas bubbles. The compliance of the sensor will affect measured impact parameters, and so these values have their greatest use in a comparing media and lubrication conditions. Their absolute
Fig. 6. Effect of lubrication on Vickers hardness for AA6061-T6 (a) 7 mm, Ra =17.5±4.1 m (estimated media average roughness with 95% confidence interval), (b) 9 mm, Ra =15.4±6.3 m, and (c) 11 mm, Ra = 16.2±4.7 m. Error bars indicate 95% confidence intervals for six measurements on one workpiece. Dry: (×), water-wet: (䊊), detergent: (䊐).
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3.2. Hardness changes Figs. 5 and 6 show that dependence of the hardness (kgf/mm2 ) of the two aluminum alloys changed with time in the finisher for different media and lubrication conditions. It is noted that because the media were not of identical roughness, the differences observed among 7, 9 and 11 mm diameter media cannot be attributed solely to media diameter. Several trends can be observed in the behaviour of both AA1100-O (Fig. 5) and AA6061-T6 (Fig. 6): Generally, the hardness increased with finishing time, although maxima were observed in several cases, particularly under dry conditions. A t-test showed that the AA1100-O hardness differences between 40 and 80 min under dry conditions with 9 and 11 mm media (Fig. 5b and c, respectively) were significant at the 95% confidence level. The local maximum seen in Fig. 6c under dry conditions was, however, not established since the t-test showed that the hardness difference between 10 and 20 min was not significant at the 95% level. The magnitude of the hardness changes after a given time usually followed the order: dry>water-wet>detergent-wet. Finishing under detergent-wet conditions usually made the
Fig. 7. Effect of 9 mm media roughness (Ra =15.4±6.3 m, Rsk = −0.43±0.54 (×); Ra =13.8±2.3 m, Rsk =−0.156±0.35 (䊊)) on Vickers hardness of AA1100-O: (a) dry, (b) water-wet, and (c) detergent-wet. Error bars indicate the 95% confidence intervals for six measurements on one workpiece.
153
hardness of the surfaces more uniform than under dry and water-wet conditions, as reflected by the smaller error bars in Figs. 5 and 6. For the AA1100-O alloy, 9 and 11 mm media (Fig. 5) produced clear maxima in hardness at 40 min under dry conditions. The effect of lubrication was much smaller with 7 mm media. After 80 min finishing, the hardness of the AA1100-O converged to approximately 42 kgf/mm2 for the three media and three lubrication conditions. The trends of Fig. 5 suggest that under the dry condition, this is probably not a steady state value and that some oscillation is to be expected at longer times. The AA6061-T6 alloy showed a lesser dependence on lubrication than AA1100-O. The dry 9 and 11 mm media produced local maxima in hardness after 20 and 10 min, respectively, with hardness subsequently increasing further. As with AA1100-O, the hardness of AA6061-T6 finished with 7 mm media displayed an increasing trend after 80 min. The influence of media roughness was investigated by repeating the AA1100-O finishing experiments with two types
Fig. 8. Effect of lubrication on average roughness, Ra , for AA1100-O (a) 7 mm, Ra =17.5±4.1 m (estimated media average roughness with 95% confidence interval), (b) 9 mm, Ra =15.4±6.3 m, and (c) 11 mm, Ra = 16.2±4.7 m. Error bars indicate 95% confidence intervals for six measurements on one workpiece. Dry (×); water-wet (䊊); detergent-wet: (䊐).
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dependent of media size, if it is assumed that the incident normal velocity was also independent of the media size. Under these conditions, the smaller media would have a higher rebound velocity and, hence, a higher coefficient of restitution. The larger media, impacting with greater kinetic energy, would have a smaller coefficient of restitution and do more plastic work. It is also hypothesized that the degree of lubrication would tend to decrease the surface damage per impact by distributing the contact stresses more uniformly and promoting sliding by reducing the friction coefficient, which in turn would decrease the depth of the plastic damage zone and be responsible for the observed smaller changes in hardness. Although the dominant velocity component is normal to the surface, the tangential energy loss of the media will increase with the friction coefficient (up to the point where sliding gives way to rolling at the point of rebound) [7]. 3.3. Roughness changes Figs. 8 and 9 show the changes in average roughness of AA1100-O and AA6061-T6 as a function of finishing time, media type and lubrication condition. As with
Fig. 9. Effect of lubrication on average roughness, Ra , for AA6061-T6 (a) 7 mm, Ra =17.5±4.1 m (estimated media average roughness with 95% confidence interval), (b) 9 mm, Ra =15.4±6.3 m, and (c) 11 mm, Ra =16.2±4.7 m. Error bars indicate 95% confidence intervals for six measurements on one workpiece. Dry (×); water-wet (䊊); detergent-wet (䊐).
of 9 mm media: one that had been used previously in the finisher for 20 h and one that had been used for 40 h (initial roughness parameters listed in Table 2). Fig. 7 shows that the newer media, which had a slightly larger Ra and a more negative Rsk , produced significantly greater hardness changes than did the older, smoother media under all lubrication conditions. It is believed that the increase in hardness with finishing time observed with both alloys, was a result of workhardening due to accumulated plastic, surface impact damage. Beyond a certain damage threshold, the surface layer became susceptible to brittle erosion, exposing a new surface and resulting in the maxima seen under dry conditions with 9 and 11 mm media. These trends were clearer with AA1100-O simply because it had an initial hardness that was approximately one-third that of AA6061-T6. For a given finishing time and lubrication condition, the larger media tended to produce slightly greater hardness changes because they would have generated more plastic surface deformation per impact. This is consistent with the impulse measurements being approximately constant, in-
Fig. 10. Effect of 9 mm media roughness ((×), Ra =15.4±6.3 m, Rsk =−0.43±0.54 (䊊), Ra =13.8±2.3 m, Rsk =−0.156±0.35) on average roughness, Ra , of AA1100-O: (a) dry, (b) water-wet, and (c) detergent-wet. Error bars indicate the 95% confidence intervals for six measurements on one workpiece.
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Fig. 11. Scanning electron micrographs of individual impact sites on AA1100-O after dry finishing for 10 s.
hardness, the roughness usually increased in the order dry>water-wet>detergent-wet, although Figs. 8a and 9a show that for 7 mm media, water-wet produced the greatest roughness changes; the reason for this is unknown. With AA6061-T6, the lubrication condition had a larger relative effect on roughness changes than it did on changes in hardness (Fig. 6); thus, the curves in Figs. 8 and 9 tend to be relatively more separated. Fig. 10 shows the effect of media roughness (20 h and 40 h media of Table 2) on the average roughness of AA1100-O using the same specimen that was used to generate the
hardness data of Fig. 7. As expected, the rougher media produced rougher AA1100-O surfaces, but there was a clear tendency for this effect to be smaller as the degree of lubrication increased; i.e. roughness changes were in the order: dry>water-wet>detergent-wet. The reproducibility of the roughness and hardness changes produced by the vibratory finishing process was assessed by immediately repeating the dry finishing of AA1100-O using 9 mm media. The average roughness and hardness values were measured after 40 and 80 min in the first and second experiments were indistinguishable at the
Fig. 12. Scanning electron micrographs of individual impact sites on AA1100-O after water-wet finishing for 10 s.
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95% confidence level. Specifically, the following were the mean average roughness and Vickers hardness values in each of the two experiments, together with the standard deviation (S.D.) and the number of measurements (N) on a single AA1100-O specimen: After 40 min., Ra1 =0.413 (S.D.=0.0316, N=7) Ra2 =0.456 (S.D.=0.0404, N=6); HV1 =33.0 (S.D.=2.08, N=5) HV2 =31.4 (S.D.=2.27, N=6); and after 80 min., Ra1 =0.360 (S.D.=0.0221, N=6) Ra2 =0.363 (S.D.=0.00866, N=6); HV1 =34.8 (S.D.=3.96, N=5) HV2 =31.6 (S.D.=2.92, N=6). In some cases, experiments with 9 mm media and AA1100-O alloy were interrupted after 10 s to take scanning electron micrographs of individual impact sites. Figs. 11 and 12 show representative impact sites under dry and water-wet conditions, respectively. The rolling lines seen on the surfaces had a depth of approximately 3 m, and the impact sites had a maximum depth of between 1 and 2 m. In general, impact sites were approximately 100 m in average lateral dimension, tending to be circular under dry conditions and elongated scratches under wet conditions. This is believed to be a result of the reduced friction between the media and the surface.
4. Conclusions Vibratory finishing can produce relatively large changes in both the hardness and roughness of aluminum alloys AA110-O and AA6061-T6. The changes in hardness appeared to be dependent mostly on the lubrication condition, since this influences the ratio of normal to tangential impulse and hence the extent of plastic surface deformation per impact. Media roughness also had a significant effect on the hardness of the aluminum alloys, while media diameter produced relatively smaller hardness changes. Hardness
displayed maxima as a function of time, suggesting that hardened surface layers were removed by impact erosion. The impact parameters such as the average force, maximum force, and impulse did not vary appreciably amongst the three media for dry and water-wet conditions. The impact conditions were also quite uniform in the vibratory finisher, being essentially the same on the leading and trailing ends of the cylindrical workpiece. On average, the 10 mm diameter sensing disk was in contact with media for approximately 30% of the total finishing time. This was consistent with the videotaped observations showing that the media was loosely-packed as it flowed past the workpiece, with relatively large gaps in the packing near the workpiece surface. Consequently, the time to uniformly finish parts was relatively long. References [1] A.P. Fraas, Design of machines for driving complex-mode vibration-fluidized beds, in: Proceedings of International Mechanical Engineering Congress & Exhibition, Atlanta, GA, Nov. 17–22, 1996, American Society of Mechanical Engineers. [2] B. Thomas, Y.A. Liu, O. Mason, A.M. Squires, Vibrated beds: new tools for heat transfer, Chem. Eng. Progr. (1988) 65–75. [3] Z. Pakowski, A.S. Mujumdar, C. Strumillo, Advances in drying, in: Drying 84, A.S. Mujumdar (Ed.), Vol. 3, Hemisphere, Washington, DC. 1984, pp. 245–306. [4] C.R. Wassgren, C.E. Brennan, M.L. Hunt, Vertical vibration of a deep bed of granular material in a container, J. Appl. Mech. 63 (1996) 712–719. [5] M.E. Fayed, L. Otten (Eds.), Handbook of Powder Science and Technology, 2nd Edition, Chapman & Hall, New York, 1997. [6] K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, 1985, pp. 363–365. [7] R.M. Brach, Impact dynamics with applications to solid particle erosion, Int. J. Impact Eng. 7 (1988) 37–53.
Experimental investigation of vibratory finishing of aluminum S. Wang a , R.S. Timsit b , J.K. Spelt a,∗ a
b
Department of Mechanical & Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ont., Canada M5S 3G8 Timron Scientific Consulting, 467 Woburn Avenue, Toronto, Ont., Canada M5M 1L6
Received 25 October 1999; received in revised form 15 May 2000; accepted 23 May 2000
Abstract The normal contact forces in a vibratory finishing machine were measured and compared with the resulting changes in surface roughness and hardness of two aluminum alloys, AA1100-O and AA6061-T6. The principal variables were the media size, degree of lubrication and the duration of the vibratory finishing. The changes in hardness and roughness were found to depend mostly on the lubrication condition, the media roughness, and the size of the media, since these influenced the interaction between the media and the workpiece, and hence the extent of plastic surface deformation per impact. The impact force parameters such as the average force, maximum force, and impulse, however, did not vary appreciably amongst the three media for dry and water-wet conditions. Thus, the differences observed in hardness and roughness were due to smaller scale differences in the impact contact conditions. On average, a sensing disk with a diameter approximately equal to that of the media was in contact with media for approximately 30% of the total finishing time. This was consistent with videotaped observations showing that the media was loosely-packed as it flowed past the workpiece, with relatively large gaps in the packing near the workpiece surface. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Vibratory finishing; Impact; Abrasive wear; Hardness; Roughness; Aluminum; Surface finish
1. Introduction Since at least the 1950s, vibratory finishing has been used widely in manufacturing to improve surface appearance and wear resistance, to polish and increase hardness, and to clean and dry surfaces. The process has been applied to metal, ceramic, and plastic parts employing a wide variety of media such as steel balls, rough ceramic spheres and angle-cut cylinders, and softer materials such as crushed corn cobs. A vibratory finishing system usually consists of a springmounted open chamber containing granular media, to which a vibratory motion generator is attached. The motion generator normally consists of one or two rotating shafts with eccentric weights. By adjustment of the eccentric weight or the speed of the drive system, the amplitude and frequency of the finisher can be controlled. The chamber of the finisher can be a round or oval bowl, or a long tub having a U-shaped cross-section. The vibrations are usually driven at 30 Hz with amplitude components of several millimeters in three dimensions. As a result, the media become fluidized and develop complex flow fields within the chamber. The ∗ Corresponding author. Tel.: +1-416-978-5435; fax: +1-416-9787753. E-mail address: [email protected] (J.K. Spelt).
parts to be finished are entrained by the flowing media and experience a slower relative velocity. The media interact with the part surface through a combination of normal impacts and sliding. Depending on the parameters of the process, this can produce a wide range of contact conditions encompassing varying degrees of rubbing, burnishing, ploughing, cutting and three-body abrasive wear. This makes vibratory finishing a versatile technique. There is very little scientific literature published on the vibratory finishing process. The mechanics of relatively deep, vibrationally fluidized bed have also received relatively little attention, although Fraas [1] provides a very useful discussion of the machinery requirements for generating certain types of flow fields. Vibrated beds are, however, well established as a means of drying granular materials since they enhance heat transfer [2,3]; however, related studies have usually tended to focus on relatively shallow beds of small particles, typically <1 mm diameter [4,5]. The objective of the present study was to measure the normal contact forces in a vibratory finishing machine and correlate them with the resulting changes in surface roughness and hardness of aluminum workpieces. The principal variables were the media size, degree of lubrication, finishing duration and aluminum alloy.
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S. Wang et al. / Wear 243 (2000) 147–156 Table 1 Average roughness parameters of the fresh 7, 9, and 11 mm diameter mediaa Media (mm)
Ra (m)
Rku
Rsk
7 9 11
20±3.2 22±8.6 19±5.7
2.9±1.8 3.8±3.3 2.5±0.35
−0.32±0.97 −0.72±0.90 −0.18±0.32
±95% confidence intervals based on averages of single measurements on five spheres. Ra : average roughness, Rsk : skewness, Rku : kurtosis (measurement length: 300 m). a
Table 2 Average roughness parameters of 9 mm media after 20 and 40 h of usea
Fig. 1. Schematic of bowl-type vibratory finisher.
Time (h)
Ra (m)
Rku
Rsk
20 40
15.4±6.3 13.8±2.3
3.5±3.4 2.9±2.2
−0.43±0.54 −0.16±0.35
±95% confidence limits based on averages of single measurements on five spheres (measurement length: 300 m). a
2. Experiments 2.1. Vibratory finisher and media The vibratory finisher consisted of an annular polyethylene bowl with an outer diameter of 38 cm, an inner diameter of 15 cm, and a depth of 15 cm (Model 150S, Burr King Mfg. Co., Inc., Warsaw, USA) (Fig. 1). The bowl was attached to an electric motor that rotated an eccentric mass at 30 Hz, and the entire assembly was mounted on five coil springs. The resulting vibrations in the vertical (x-z) and horizontal (x-y) planes were videotaped and appeared to follow roughly elliptical paths with x, y, z amplitudes of approximately 0.7, 0.8 and 1.8 mm, respectively. Three types of moderately rough, approximately spherical ceramic media were used (Abrasive Finishing Inc., Chelsea, MI 48118, USA) with average diameters of 7.1 mm (S.D.=0.35 mm, number of spheres measured, N=20), 8.6 mm (S.D.=0.56 mm, N=20), and 11 mm (S.D.=
0.73 mm, N=20). The media were composed mainly of Si and Al, with lesser amounts of K, Ca, Ti and Fe. The bulk density of the static media as a granular material was approximately 1.4 g/cm3 . The initial roughness parameters of the fresh media were measured using an optical surface profilometer (Wyko, RST Plus) and are given in Table 1. At the 95% level of confidence, there was only a difference between skewness, Rsk , of the 9 and 11 mm media; all other parameters were essentially the same. In the experiments, the media were used in cycles consisting of 2.5 h dry, 2.5 h wet, and 2.5 h lubricated. The diameter decreased negligibly over this period by <0.4 mm. Fig. 2 shows that after approximately 20 h of use in the finisher, the average roughness, Ra , of the media decreased slowly. Table 2 shows the roughness parameters for 9 mm media after 20 and 40 h of use. At the 95% confidence level, a t-test showed that the kurtosis,
Fig. 2. Mean average roughness, Ra , of media as a function of total finishing time in cycles of 2.5 h dry, 2.5 h wet, and 2.5 h lubricated. Error bars indicate 95% confidence intervals based on single measurements on five spheres (measurement length: 300 m).
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Rku , remained essentially unchanged, but Ra decreased and Rsk became less negative as a result of particle wear. Prior to each experiment, the media were washed in tap water to remove fines. The media were then used in one of three conditions: dry, wet with water, and wet with a water–detergent solution (2% by volume detergent (Fantastik, DowBrands Inc.) in tap water). In the latter two conditions, after removal from the water or detergent solution, the soaked media were placed in the finisher with approximately 100 ml of excess liquid in the bottom of the bowl to keep the media wet during the experiments. 2.2. Workpieces A cylindrical geometry was selected for the prototypical workpiece. Three types of rotation were observed to occur simultaneously in the finisher when a cylinder was initially placed in the vibrating media with its axis horizontal: the cylinder rotated about its own axis; it moved radially in the finisher, submerging into the media at the inner wall and rising to the free surface at the outer wall; the cylinder also moved circumferentially around the bowl. The first two rotational motions always occurred at the same frequency. The resultant of these three components was a spiral around the annular bowl. An end-over-end tumbling motion could also be created if the cylinder were placed in the media such that its axis was not horizontal. The influence of this type of motion was not investigated. In order to establish the size and density of the cylindrical workpiece, six aluminum cylinders of different dimensions and density were tested in the finisher: lengths of 50 and 70 mm, and diameters of 26, 32 and 51 mm. The densities ranged from 0.6 to 1.1 g/cm3 . It was found that, over this range, the periods of motion did not change significantly. As a result, the workpiece was chosen to be 80 mm long, 68 mm in diameter with a density of 0.75 g/cm3 . Table 3 gives the measured periods of motion for this workpiece in the three media under three lubrication conditions. It is seen that the motion was largely unaffected by the size of the media, with the exception of the circumferential period when lubricated with detergent. The lubrication condition, however, had a large influence, with the three cylinder periods becoming much longer as the friction was reduced.
Fig. 3. Cross-section of cylindrical workpiece showing force sensor.
As explained below, aluminum workpieces of identical size and density were fitted to hold a contact force sensor and a small video camera. AA1100-O and AA6061-T6 0.8 mm thick sheets were cut into 50 mm diameter disks that were glued to the ends of two workpieces so that each alloy could be finished simultaneously, under identical conditions. The glue was a hot melt adhesive that was remelted at approximately 80◦ C after finishing periods of 5, 10, 20, 40 and 80 min, and the aluminum specimens were then rinsed in tap water and washed in methanol in an ultrasonic bath for 5–10 min. Hardness was measured using a Vickers indenter and roughness parameters were obtained using an optical surface profilometer (Wyko, RST Plus). In each case, six measurement points were selected on each disk surface. Several experiments were repeated to assess the reproducibility of the hardness and roughness measurements. 2.3. Contact forces The normal contact force between the media and the end of the cylindrical workpiece was measured in dry and wet conditions using a specially constructed load cell consisting of a clamped-clamped aluminum (AA6061-T6) beam, 50 mm long, 10 mm wide and 0.40 mm thick (Fig. 3). Four resistance strain gauges (4.0 mm wide, 3.0 mm gauge length)
Table 3 Motion periods of a cylindrical parta in different media and lubrication conditionsb Media diameter in mm (spherical)
7 9 11 a b
Results
Dry
Water-wet
Pr , Pa (s)
Pc (s)
Pr , Pa (s)
Pc (s)
Pr , Pa (s)
Pc (s)
Average Stdev Average Stdev Average Stdev
10 1 12 1 12 1
36 0.5 37 0.5 38 1
15 1 18 2 15 1.5
43 1 42 2 40 1
34 3 31 2 33 3
59 1 54 3 45 2
68 mm diameter×80 mm length. Pr : radial period, Pa : axial period, Pc : circumferential period (averages of 10 measurements).
Detergent-wet
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were mounted on the beam to create a full-bridge circuit (two on the upper surface, two on the lower surface, on each side of the beam midpoint) that gave a force resolution of 1% of the rated maximum load of 2 N. This arrangement of strain gauges eliminates the effect of surface shear forces, so that only surface-normal forces are recorded. Media contacted a 10 mm diameter aluminum disk that was connected to the sensor beam via a 3 mm long, 4 mm diameter stem glued to the beam centre. The disk was flush with the end face of the workpiece leaving a 1 mm gap around the disk. The strain gauge wires passed out of the opposite end of the workpiece cylinder and were then suspended above the bowl so that they remained vertical, directly above the workpiece. The drag of the wires in the media had a small effect on the motion of the workpiece; with dry 9 mm media, the radial and axial periods increased from 12 s without wires to 14 s with the wires, while the circumferential period increased from 37 to 46 s. The sensor was calibrated statically and dynamically on a shaker table (sensor calibration factor was 0.398 V/N). Up to 60 Hz, at accelerations of up to 20 m/s2 , the peak dynamic force produced by an attached 1 g mass was identical to that predicted using the static calibration factor; a similar result was obtained using a 4 g mass at a peak acceleration of 10 m/s2 . The inertial mass of the sensor was approximately 1 g, so that when placed in the finisher with the sensing disk shielded, the average maximum recorded force was approximately 0.038 N; this was used as the dynamic noise
threshold. When struck by a single piece of 9 mm diameter media (mass of 1 g) dropped from a height of 10 mm, the sensor response indicated a coefficient of restitution of 0.44, a natural frequency of 700 Hz and a damping time constant of 7.2 m s; i.e. the ringing of the sensor decayed to half its peak value in 5 m s. The spring constant of the sensor was approximately 5650 N/m, which was considerably less than that of the aluminum disks glued to the ends of the workpieces during finishing experiments. Since impact forces are a function of surface rigidity [6], the contact forces measured using this sensor can be used to assess the relative influence of media and lubrication, but should not be construed as those that resulted in the hardness and roughness changes of the present, more rigid, workpieces. Moreover, because the impact force changes so rapidly in the immediate vicinity of the maximum, very high digitizing frequencies would be required to reliably identify the maximum force. In the present experiments, the force sensor output was digitized at 500 Hz, which was high enough to record approximately 7 force values during a typical impact. Although this was sufficiently fast to accurately record the average contact forces and the percentage of contact time, it could not ensure that the absolute maximum force was resolved. The force signals from the sensor were highly variable, occurring in random bursts as a single particle moved (bounced) across the sensing disk. Fig. 4 illustrates this for 9 mm media under water-wet conditions. A Matlab program (The Math Works Inc.) was written to identify individual
Fig. 4. (a) Normal contact force in wet finishing with 9 mm media; (b) Fourier transform of signal in (a).
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Table 4 Statistics for normal contact forces of 7, 9, and 11 mm media in dry finishing for 8 s with sensor facing forwarda Media
Number of impacts in 8 s
Maximum impact force (N)
Impact force (N)
Impact duration (m s)
Impulse per impact 10−3 (N s)
Contact percentage (%)
7 mm 95%CI 9 mm 95%CI 11 mm 95%CI
130 ±52 130 ±50 136 ±43
0.18 ±0.03 0.21 ±0.05 0.29 ±0.07
0.086 ±0.01 0.10 ±0.02 0.12 ±0.03
20 ±2.3 19 ±3.8 18 ±3.5
1.6 ±0.5 2.0 ±0.8 2.1 ±0.8
32 ±13 31 ±11 29 ±11
a
Averages of 10 runs with 8 s for each run; 95% confidence intervals are listed.
impacts that were then analyzed for their maximum force, average force, impulse and impact duration. In order to obtain reliable average values of these quantities, it was necessary to average the results of 10 sets of force recordings each of 8 s duration (in comparison, the radial period was 10–12 s). 2.4. Videotaping relative media motion In a separate series of experiments, the relative motion of the media were videotaped through a 20 mm diameter glass window installed in the end face of one of the cylindrical workpieces. A small colour video camera (IK-SM40A, Toshiba), 42 mm long and 7 mm diameter, with a 410,000 pixel resolution was mounted in the workpiece beside a flexible fibre-optic cable (No. A08025.40, Subtechnique Inc., USA). The video and fibre-optic cables had approximately the same effect on the workpiece motion as did the force sensor wires, reported above. After recording an experiment, the videotape was replayed frame-by-frame to measure the speed of the media and the approximate percentage of time that media appeared to contact a 10 mm diameter area in the centre of the field of view. 3. Results and discussion 3.1. Normal contact forces and videotaped relative motion The force signal of Fig. 4 was typical, displaying a dominant Fourier transform peak at the finisher driving frequency,
30 Hz, and much smaller peaks at the first and second harmonics. The average impact parameters for the three media under dry and water-wet conditions are given in Tables 4 and 5, respectively. The data in these tables correspond to the sensor at the leading end of the cylindrical workpiece; i.e. facing forward as the workpiece spirals around the bowl. Comparable results were obtained with the sensor facing backwards, leading to the conclusion that impact wear conditions were relatively uniform against all of the workpiece surfaces. A few trials were conducted with the workpiece fixed to an external support with the sensor either facing the oncoming media flow around the bowl or facing the wake behind the workpiece. In the latter case, the contact percentage decreased to 18% while it increased to 36% when facing the flow. In both situations, the contact forces were greater than those measured when the workpiece was free to move with the media. It is noted in Tables 4 and 5 that the average number of impacts occurring in 8 s was relatively constant under all conditions. Thus, the observation in Table 3 that the workpiece moved more slowly under wet conditions, did not seem to result in more impacts per unit time. The average maximum impact force tended to increase with the media size, but did not show any consistent change with the degree of lubrication, dry or water-wet. It should be noted, however, that the maximum impact force is the least accurate of the measurements because of the resolution of the 500 Hz sampling frequency. The average impact force and the average impulse increased with media size under dry conditions, but when
Table 5 Statistics for normal contact forces of 7, 9, and 11 mm media in water-wet finishing for 8 s with sensor facing forwarda Media
Number of impacts in 8 s
Maximum impact force (N)
Impact force (N)
Impact duration (m s)
Impulse per impact 10−3 (N s)
Contact percentage (%)
7 mm 95%CI 9 mm 95%CI 11 mm 95%CI
126 ±53 115 ±55 110 ±75
0.20 ±0.05 0.20 ±0.04 0.24 ±0.06
0.10 ±0.02 0.10 ±0.02 0.11 ±0.03
21 ±3.4 20 ±2.1 17 ±2.6
1.8 ±0.60 1.8 ±0.50 1.7 ±0.60
31 ±14 29 ±12 23 ±15
a
Averages of 10 runs with 8 s for each run; 95% confidence intervals are listed.
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magnitudes will only be directly applicable to the vibratory finishing of surfaces that have a similar compliance. Using the videotapes, the average relative speed between the workpiece and the 9 mm media under dry, water-wet and detergent-wet conditions was found to be 11, 7 and 4 cm/s, respectively. Referring to Table 3, it is thus observed that as the amount of lubrication increases, both the absolute workpiece speed decreases and the speed of the media relative to the workpiece decreases. This implies that the lubrication condition has a greater relative effect on the speed of the media than on the speed of the workpiece. These observations suggest that the predominant driving force for the media flow and the entrainment of workpiece is friction between the wall of the bowl and the media. These frictional driving forces are then transmitted between the media and ultimately to the workpiece surface, and are larger than the frictional drag forces at each of these interfaces. Nevertheless, the measured differences in the relative speed of the media did not have a significant influence on the percentage contact time or the number of impacts per unit time.
Fig. 5. Effect of lubrication on Vickers hardness for AA1100-O (a) 7 mm, Ra =17.5±4.1 m (estimated media average roughness with 95% confidence interval), (b) 9 mm, Ra =15.4±6.3 m, and (c) 11 mm, Ra = 16.2±4.7 m. Error bars indicate 95% confidence intervals for six measurements on one workpiece. Dry: (×), water-wet: (䊊), detergent: (䊐).
the media were wet, these parameters did not change significantly with different media. The reason for this is not understood. Nevertheless, in comparing dry and wet conditions, the differences in the average impact force and the average impulse were relatively small (<16%). The average impact duration was similar in both wet and dry tests, and did not change significantly with media diameter. The average percentage of time that a particle was in contact with the 10 mm diameter sensing disk was also relatively constant amongst all of the test conditions. The value, approximately 30%, is surprisingly low, and illustrates that the media packing at the workpiece surfaces was rather sparse. This result was confirmed by the videotapes, which showed that the media vibratory flow seemed to display characteristics that were analogous to a two-phase flow of a liquid containing gas bubbles. The compliance of the sensor will affect measured impact parameters, and so these values have their greatest use in a comparing media and lubrication conditions. Their absolute
Fig. 6. Effect of lubrication on Vickers hardness for AA6061-T6 (a) 7 mm, Ra =17.5±4.1 m (estimated media average roughness with 95% confidence interval), (b) 9 mm, Ra =15.4±6.3 m, and (c) 11 mm, Ra = 16.2±4.7 m. Error bars indicate 95% confidence intervals for six measurements on one workpiece. Dry: (×), water-wet: (䊊), detergent: (䊐).
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3.2. Hardness changes Figs. 5 and 6 show that dependence of the hardness (kgf/mm2 ) of the two aluminum alloys changed with time in the finisher for different media and lubrication conditions. It is noted that because the media were not of identical roughness, the differences observed among 7, 9 and 11 mm diameter media cannot be attributed solely to media diameter. Several trends can be observed in the behaviour of both AA1100-O (Fig. 5) and AA6061-T6 (Fig. 6): Generally, the hardness increased with finishing time, although maxima were observed in several cases, particularly under dry conditions. A t-test showed that the AA1100-O hardness differences between 40 and 80 min under dry conditions with 9 and 11 mm media (Fig. 5b and c, respectively) were significant at the 95% confidence level. The local maximum seen in Fig. 6c under dry conditions was, however, not established since the t-test showed that the hardness difference between 10 and 20 min was not significant at the 95% level. The magnitude of the hardness changes after a given time usually followed the order: dry>water-wet>detergent-wet. Finishing under detergent-wet conditions usually made the
Fig. 7. Effect of 9 mm media roughness (Ra =15.4±6.3 m, Rsk = −0.43±0.54 (×); Ra =13.8±2.3 m, Rsk =−0.156±0.35 (䊊)) on Vickers hardness of AA1100-O: (a) dry, (b) water-wet, and (c) detergent-wet. Error bars indicate the 95% confidence intervals for six measurements on one workpiece.
153
hardness of the surfaces more uniform than under dry and water-wet conditions, as reflected by the smaller error bars in Figs. 5 and 6. For the AA1100-O alloy, 9 and 11 mm media (Fig. 5) produced clear maxima in hardness at 40 min under dry conditions. The effect of lubrication was much smaller with 7 mm media. After 80 min finishing, the hardness of the AA1100-O converged to approximately 42 kgf/mm2 for the three media and three lubrication conditions. The trends of Fig. 5 suggest that under the dry condition, this is probably not a steady state value and that some oscillation is to be expected at longer times. The AA6061-T6 alloy showed a lesser dependence on lubrication than AA1100-O. The dry 9 and 11 mm media produced local maxima in hardness after 20 and 10 min, respectively, with hardness subsequently increasing further. As with AA1100-O, the hardness of AA6061-T6 finished with 7 mm media displayed an increasing trend after 80 min. The influence of media roughness was investigated by repeating the AA1100-O finishing experiments with two types
Fig. 8. Effect of lubrication on average roughness, Ra , for AA1100-O (a) 7 mm, Ra =17.5±4.1 m (estimated media average roughness with 95% confidence interval), (b) 9 mm, Ra =15.4±6.3 m, and (c) 11 mm, Ra = 16.2±4.7 m. Error bars indicate 95% confidence intervals for six measurements on one workpiece. Dry (×); water-wet (䊊); detergent-wet: (䊐).
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dependent of media size, if it is assumed that the incident normal velocity was also independent of the media size. Under these conditions, the smaller media would have a higher rebound velocity and, hence, a higher coefficient of restitution. The larger media, impacting with greater kinetic energy, would have a smaller coefficient of restitution and do more plastic work. It is also hypothesized that the degree of lubrication would tend to decrease the surface damage per impact by distributing the contact stresses more uniformly and promoting sliding by reducing the friction coefficient, which in turn would decrease the depth of the plastic damage zone and be responsible for the observed smaller changes in hardness. Although the dominant velocity component is normal to the surface, the tangential energy loss of the media will increase with the friction coefficient (up to the point where sliding gives way to rolling at the point of rebound) [7]. 3.3. Roughness changes Figs. 8 and 9 show the changes in average roughness of AA1100-O and AA6061-T6 as a function of finishing time, media type and lubrication condition. As with
Fig. 9. Effect of lubrication on average roughness, Ra , for AA6061-T6 (a) 7 mm, Ra =17.5±4.1 m (estimated media average roughness with 95% confidence interval), (b) 9 mm, Ra =15.4±6.3 m, and (c) 11 mm, Ra =16.2±4.7 m. Error bars indicate 95% confidence intervals for six measurements on one workpiece. Dry (×); water-wet (䊊); detergent-wet (䊐).
of 9 mm media: one that had been used previously in the finisher for 20 h and one that had been used for 40 h (initial roughness parameters listed in Table 2). Fig. 7 shows that the newer media, which had a slightly larger Ra and a more negative Rsk , produced significantly greater hardness changes than did the older, smoother media under all lubrication conditions. It is believed that the increase in hardness with finishing time observed with both alloys, was a result of workhardening due to accumulated plastic, surface impact damage. Beyond a certain damage threshold, the surface layer became susceptible to brittle erosion, exposing a new surface and resulting in the maxima seen under dry conditions with 9 and 11 mm media. These trends were clearer with AA1100-O simply because it had an initial hardness that was approximately one-third that of AA6061-T6. For a given finishing time and lubrication condition, the larger media tended to produce slightly greater hardness changes because they would have generated more plastic surface deformation per impact. This is consistent with the impulse measurements being approximately constant, in-
Fig. 10. Effect of 9 mm media roughness ((×), Ra =15.4±6.3 m, Rsk =−0.43±0.54 (䊊), Ra =13.8±2.3 m, Rsk =−0.156±0.35) on average roughness, Ra , of AA1100-O: (a) dry, (b) water-wet, and (c) detergent-wet. Error bars indicate the 95% confidence intervals for six measurements on one workpiece.
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Fig. 11. Scanning electron micrographs of individual impact sites on AA1100-O after dry finishing for 10 s.
hardness, the roughness usually increased in the order dry>water-wet>detergent-wet, although Figs. 8a and 9a show that for 7 mm media, water-wet produced the greatest roughness changes; the reason for this is unknown. With AA6061-T6, the lubrication condition had a larger relative effect on roughness changes than it did on changes in hardness (Fig. 6); thus, the curves in Figs. 8 and 9 tend to be relatively more separated. Fig. 10 shows the effect of media roughness (20 h and 40 h media of Table 2) on the average roughness of AA1100-O using the same specimen that was used to generate the
hardness data of Fig. 7. As expected, the rougher media produced rougher AA1100-O surfaces, but there was a clear tendency for this effect to be smaller as the degree of lubrication increased; i.e. roughness changes were in the order: dry>water-wet>detergent-wet. The reproducibility of the roughness and hardness changes produced by the vibratory finishing process was assessed by immediately repeating the dry finishing of AA1100-O using 9 mm media. The average roughness and hardness values were measured after 40 and 80 min in the first and second experiments were indistinguishable at the
Fig. 12. Scanning electron micrographs of individual impact sites on AA1100-O after water-wet finishing for 10 s.
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95% confidence level. Specifically, the following were the mean average roughness and Vickers hardness values in each of the two experiments, together with the standard deviation (S.D.) and the number of measurements (N) on a single AA1100-O specimen: After 40 min., Ra1 =0.413 (S.D.=0.0316, N=7) Ra2 =0.456 (S.D.=0.0404, N=6); HV1 =33.0 (S.D.=2.08, N=5) HV2 =31.4 (S.D.=2.27, N=6); and after 80 min., Ra1 =0.360 (S.D.=0.0221, N=6) Ra2 =0.363 (S.D.=0.00866, N=6); HV1 =34.8 (S.D.=3.96, N=5) HV2 =31.6 (S.D.=2.92, N=6). In some cases, experiments with 9 mm media and AA1100-O alloy were interrupted after 10 s to take scanning electron micrographs of individual impact sites. Figs. 11 and 12 show representative impact sites under dry and water-wet conditions, respectively. The rolling lines seen on the surfaces had a depth of approximately 3 m, and the impact sites had a maximum depth of between 1 and 2 m. In general, impact sites were approximately 100 m in average lateral dimension, tending to be circular under dry conditions and elongated scratches under wet conditions. This is believed to be a result of the reduced friction between the media and the surface.
4. Conclusions Vibratory finishing can produce relatively large changes in both the hardness and roughness of aluminum alloys AA110-O and AA6061-T6. The changes in hardness appeared to be dependent mostly on the lubrication condition, since this influences the ratio of normal to tangential impulse and hence the extent of plastic surface deformation per impact. Media roughness also had a significant effect on the hardness of the aluminum alloys, while media diameter produced relatively smaller hardness changes. Hardness
displayed maxima as a function of time, suggesting that hardened surface layers were removed by impact erosion. The impact parameters such as the average force, maximum force, and impulse did not vary appreciably amongst the three media for dry and water-wet conditions. The impact conditions were also quite uniform in the vibratory finisher, being essentially the same on the leading and trailing ends of the cylindrical workpiece. On average, the 10 mm diameter sensing disk was in contact with media for approximately 30% of the total finishing time. This was consistent with the videotaped observations showing that the media was loosely-packed as it flowed past the workpiece, with relatively large gaps in the packing near the workpiece surface. Consequently, the time to uniformly finish parts was relatively long. References [1] A.P. Fraas, Design of machines for driving complex-mode vibration-fluidized beds, in: Proceedings of International Mechanical Engineering Congress & Exhibition, Atlanta, GA, Nov. 17–22, 1996, American Society of Mechanical Engineers. [2] B. Thomas, Y.A. Liu, O. Mason, A.M. Squires, Vibrated beds: new tools for heat transfer, Chem. Eng. Progr. (1988) 65–75. [3] Z. Pakowski, A.S. Mujumdar, C. Strumillo, Advances in drying, in: Drying 84, A.S. Mujumdar (Ed.), Vol. 3, Hemisphere, Washington, DC. 1984, pp. 245–306. [4] C.R. Wassgren, C.E. Brennan, M.L. Hunt, Vertical vibration of a deep bed of granular material in a container, J. Appl. Mech. 63 (1996) 712–719. [5] M.E. Fayed, L. Otten (Eds.), Handbook of Powder Science and Technology, 2nd Edition, Chapman & Hall, New York, 1997. [6] K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, 1985, pp. 363–365. [7] R.M. Brach, Impact dynamics with applications to solid particle erosion, Int. J. Impact Eng. 7 (1988) 37–53.