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New characterization methods for powder die fill process for producing powder metallurgical components
Powder Technology 232 (2012) 7–17
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
New characterization methods for powder die fill process for producing powder metallurgical components D. Aole, M.K. Jain ⁎, M. Bruhis Department of Mechanical Engineering, McMaster University, Hamilton, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 20 December 2011 Received in revised form 2 August 2012 Accepted 4 August 2012 Available online 11 August 2012 Keywords: Die filling Perforated plate Colored powder Surface porosity Resin-bonding Gamma-ray densitometry
a b s t r a c t Powder die fill (PDF) process prior to compaction affects spatial density of the final part and its in-service performance. Inhomogeneous density leads to part distortion (or warpage) during sintering, resulting in inability to get the desired dimensional tolerances and mechanical properties in the final product. Therefore, to improve quality of the final part, it is important to eliminate several causes of non-uniformity during the powder die filling stage. This experimental study relates to some new approaches toward characterization of PDF process, using an experimental apparatus with transparent shoe and die sections. In this paper, the influence of a newly designed perforated plate (PP), incorporated at the bottom of the feed shoe on PDF is studied. The effect of PP on powder redistribution in the feed shoe, powder transfer and resulting powder spatial density uniformity, and porosity within a thin ring-shaped die is compared with the powder flow and segregation behavior in standard shoe (SS) filling. Various colored salt powder experiments using arrangements in layers and columns within the feed shoe as well as a commercial iron power are considered in an attempt to analyze the subsequent powder redistribution within the shoe, during shoe motion and final powder packing in the die. It is determined that the colored salt is a promising medium for simulating the flow characteristics and segregation of commercial iron powder macroscopically. Further, critical shoe speed for powder delivery into the die-cavity using a perforated plate and a standard shoe are studied. Continuous imaging of the transparent shoe and die wall sections with an on-line high speed CCD camera are also used to record powder flow and to obtain resulting porosity from quantitative pixel-based analysis of the post-fill final images. The effectiveness and limitations of the perforated plate in achieving a more homogenous powder fill and reduced porosity in the die is explained with the help of both colored powder experiments and the surface porosity data from the transparent die face section. Finally, a successful attempt is made to bind the loose iron powder using a polymer resin blend into the shape of a ring within the die. This ring shaped component is then analyzed for its spatial porosity distribution, and thereby its spatial density distribution, using gamma ray densitometry. © 2012 Elsevier B.V. All rights reserved.
1. Introduction and literature review 1.1. Powder flow characteristics Powder metallurgy process includes the preparation of powder blend, which is then transferred into the die using a feed shoe and compacted using rigid punches. The final part is ejected from the diecavity and subject to post-compaction processing such as sintering, sizing and heat treatment. In powder die fill (PDF) operation, a powderfilled feed shoe translates in a reciprocating motion over the die-cavity depositing the powder into the die in one or more passes. At the end of this stage, the volume occupied by the powder in the cavity is constant. The shoe motion results in intense shear deformation as blocks of loose
⁎ Corresponding author. E-mail address: [email protected] (M.K. Jain). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2012.08.002
powder slide over each other. PDF process can be looked upon in terms of powder flow from feed shoe into the die and powder packing and segregation inside the die cavity during filling. In general, the filling process involves a complex interaction of process variables (for example, shoe speed and shoe design), part variables (die design), die wall friction and powder physical characteristics such as particle size and shape. Any non-uniformity at the PDF stage is carried over to the subsequent stages of powder compaction and sintering and could lead to defects in the final part that can precipitate failure during service. Non-uniform PDF also affects the tool deflection and tool life during compaction. Powder flowability is defined as the ability of the powder to flow under a range of process conditions. Flowabiltiy is not an inherent material property, but is a combination of physical properties of the particulate matter and the design of mechanical components that make up the handling and feeding system. Hopper and feed shoe are perhaps the two most critical components of the powder delivery
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system. A nonuniform powder delivery from the hopper to the feed shoe results in variation in the head pressure, and together with the flow rate and other flow characteristics translates into variation in component spatial density (i.e., density variation from one region of the part to another) and part dimensions. The friction between the shoe and die walls and powder particles restrict the movement of the powder; thus, contributing to the spatial density variation in the part. The flow property of the powder is often described in terms of the angle of repose. This angle, measured in a standard test, forms between the free surface of the powder and the horizontal base when the powder, stored in a funnel, is dropped from a specified height. Typically, the powder with superior flow rate makes a low angle of repose. The flow properties have been also related to the cohesion and internal friction between the particles. Powder material does not flow freely through dies with sharp corners, long and thin sections, and where crosssections have large variations, resulting in lower density of the part after pressing [10]. Cocks and co-workers stated that during gravity PDF different types of flows occur when shoe moves over the die cavity [Cocks et al., 2003a and [1]. Nose flow occurs when the loose powder in the shoe slides over each other, during the initial acceleration of the shoe, and then the top layer which is a free surface cascades into the die creating a nose-shaped profile. In nose flow, the larger particles from the top free surface of the powder inside the shoe flow more rapidly in the die-cavity, whereas the fine particles flow with lower velocities due to strong frictional forces. [3] also noted that nose flow promotes faster evacuation of air, and thus, a higher powder flow rate is achieved. It was further noted that smaller amount of powder in the shoe, lower shoe speed, and larger die opening promote nose flow. As the shoe moves further over the die cavity, particles from the bottom layer of the shoe starts filling the die, which is referred to as bulk flow. This flow dominates when the shoe speed is high. In bulk flow, the bottom particles flow later and slowly from the bottom due to the combined effect of friction between the base and the powder mass, and cohesion forces. Kim proposed a promising approach to PDF process by utilizing a filter (or a perforated plate) at the bottom of the feed shoe to increase uniformity of the die fill [11]. The presence of a filter at the bottom of the shoe breaks the bulk powder flow into a vertically downward free flow. Kim utilized two types of filters in the form of different arrays of holes in a plate (linear and staggered) as well as different sizes of the feed shoe versus the volume of the holes in the filter for controlling the flow of powder in the die. In the present work, similar to the work of Kim, a perforated plate was designed and utilized in some of the PDF experiments to further assess the powder flow in the shoe and the die and resulting spatial density.
depending on particle size and shape [13]. It was found that by using a vibrating die radial segregation can be reduced as the fines get leveled off [8]. The vertical segregation was found to be dependent on the frequency and amplitude of vibration. [5] studied the avalanche to characterize the fluidity and cohesion of the powders, which affects the final distribution of particles in the die during the filling process. It was reported that the powder material behaves differently in static and dynamic modes. The avalanche occurs when the balance between cohesion of particles and gravity breaks. They described that in the pre-avalanche stage cohesion forces maintain the particle arrangement stability. At the time of avalanche, segregation of powder layers and distance travelled by the particles in the die determines the final packing density of powder in the die fill condition. Also, the slope of powder following the avalanche correlates well with the quantity of powder delivered into the die. 1.3. PDF parameters and spatial density measurement in the die-fill condition During PDF, the kinetic energy of moving particles can be partially transferred into the die cavity, where it causes rearrangement of the particles and compaction. The increase of shoe speed results in an increase of kinetic energy of particles and results in increased densification in the die [4]. Also, the shoe kinematics affects the evacuation of the air out of the die. At higher shoe speeds, due to less filling time, air gets entrapped in the powder body. As a result, density distribution inside the die becomes uneven. In the present work, the effect of shoe speed on powder flow characteristics in the shoe and spatial density distribution in the die has been studied. Although the die geometry also influences the flow rate and flow pattern during die filling, only one ring- shaped die geometry has been considered. A crucial step in evaluating the PDF process is to measure the density distribution of powder particles in the die fill condition without shifting or physical disturbance to them once they are in the die. There are no accurate and reliable methods known at present to quantify the PDF density in the loose powder state. Recently, [12] investigated the initial density distribution after filling by sintering the loose powder and by examining the die in filled condition with X-ray tomography technique. However, moving the whole die in the filled condition for testing and sintering the powder is not very convenient procedure to get the accurate density distribution data. Thus, attempt has been made in the present work to develop a more reliable method for measuring the density distribution in die-fill condition without affecting the loose powder arrangement in the die. 2. Experimental procedures 2.1. Experimental PDF set-up
1.2. Power segregation and avalanche Different segregation mechanisms and characteristics during PDF process were initially identified by [14], namely, trajectory, air flow, rolling, angle of repose, embedding, impact, push away, displacement, percolation, fluidization, agglomeration, and concentration drive displacement. Segregation pattern during the powder flow can influence the final particle packing in the die. In a granular mixture differing in size and shape, the large particles with lower density or round shape were found to lie closer to the top of the flowing layer. This leads to longer travel time of the particles before they come to rest. However, the smaller particles were unable to roll easily over the large particle surface. In such cases, the larger particles were found at the bottom of the pile, whereas the smaller particles remained at the top. The powder flow from a shoe interacts with air during filling, the drag force of air lifts up the light particles leading to the segregation of the powder mass. In addition, the small and angular particles flow with low velocity due to stronger frictional force during filling. Therefore, different trajectories of powder flow are observed
A test-rig was developed to study the PDF process in a ring-shaped die. The die was mounted on a large base plate and a square shaped feed shoe was located above the die and attached to a double action pneumatic actuator with a solenoid valve for easy reciprocating motion. The entire test set-up was mounted on a rigid frame as shown in Fig. 1. The front die and shoe wall faces were fitted with a rectangular piece of Perspex to act as observation windows to record the powder flow characteristics and post-avalanche angle at the end of filling. A small conical hopper with an outlet diameter of 50.8 mm was also attached to the front of the shoe for some of the experiments to provide continuous powder supply to the shoe during the PDF process. The extra mass in the small hopper was utilized to study the influence of increased powder level in the front portion of the shoe and its effect on powder flow. The geometric details of the feed shoe and the die are shown in Fig. 2. A perforated plate (PP) insert was placed at the bottom of the die in some experiments to modify the flow of powder in the die cavity (Fig. 3). This plate, fabricated from aluminum, was designed to have a grid of straight holes (i.e., perpendicular to the plane of the plate)
D. Aole et al. / Powder Technology 232 (2012) 7–17
Die-cavity
Hopper
Feed Shoe
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Actuator Cylinder
Base Plate
High-speed camera
Transparent die face
Actuator rod Solenoid Valve
Lights
Fig. 1. Various components of PDF experimental set-up.
with a diameter and hole center-to-center spacing of 4.06 mm and 8 mm respectively. The standard shoe, i.e., without an insert, and shoe with the perforated plate insert are referred to as SS and SPP respectively in the remainder of this paper. 2.2. Powder flow analysis using colored powder bed Powders used in the present studies were (i) salt (NaCl) powder, colored using food coloring, and (ii) commercially available ATOMET 1001 water atomized iron powder without lubricant. Also, a mixture of ATOMET 1001 and polyethylene powders was used in powder heating experiments to bind the iron powder in the loose or un-compacted, state in the die. The salt, iron, and polyethylene powder particles were of sizes 50–750 μm, 45–300 μm, and 100–750 μm respectively. The salt powder is non-cohesive in nature and iron powder is cohesive. The PDF experiments were first performed using tri-layer colored salt (TLCS) and tri-column colored salt (TCCS) powder beds inside the feed shoe (see Fig. 4a,b). For these tri-layer patterns, different colors of salt powder were poured one by one manually and leveled with a brush to get three uniform 12 mm thick layers in a 190.5 mm wide shoe (i.e., along x dimension of the shoe). In TCCS case, two removable thin walls (i.e., metal strips) were temporarily placed inside the shoe to separate the different color powder columns. The three different colored salt powders were then poured separately in to three equal size columns (i.e., each of thickness 12 mm and width 63.5 mm) and leveled to the same height inside the shoe. The interest in the TCCS powder bed tests was to understand the effective area of powder delivery across the length of the shoe. For colored iron powder bed, iron powder was slightly oxidized in an electric furnace at 180 °C for 2 h to get the light brown color (middle layer, Fig. 4c). The flow regimes noted from these experiments with the
Separating plate Feed Shoe Table frame
mixture of grey (original) and light brown colored layers were then correlated with the results obtained from the colored salt experiments. The PDF processes using SS and SPP were analyzed at different shoe speeds and compared with each other. For this purpose, the powder flow from the shoe into the die cavity was continuously recorded with the help of a Canon SX110 digital camera with 9 megapixel resolutions. The parameters such as shoe speed and powder level were varied to note the flow patterns and final particle packing in the die. The postavalanche angle (i.e., static angle of repose) inside the shoe at the end of the filling cycle was also compared for SS and SPP fill conditions. 2.3. Procedure for PDF rate determination The filling rate of powder into the die (or PDF rate) is the ratio of total mass of powder filled into the die cavity to the filling time [1–3]. During the filling stage, the mass transferred into the die after a forward pass of the shoe was measured with an electronic weighing balance. The filling time was determined by measuring the total time from the point the shoe filled with powder starts discharging powder into the diecavity till the shoe leaves the die completely, during the forward pass. The filling rate study was performed using SS and SPP, with and without the hopper. The experiment was repeated three times for each test condition to obtain an average filling rate data. 2.4. Determination of surface porosity from pixel-based image analysis system High resolution images were recorded from the transparent die face (TDF) section using a high-speed camera with up to 500 frames/s (Schneider CCD-1300BG) during the PDF process. These images were analyzed for surface porosity using image analysis software from Nikon Instruments (NIS-Elements). The analysis was based on the assumption that all pixels brighter than a threshold value were part of the grain space, and those darker than the threshold value were considered to be part of the pore space. The deposited powder images at the end of the filling process were analyzed at 6 specific locations
Bottom Rails
Die-cavity
Fig. 2. Top view of the die cavity and feed shoe.
Fig. 3. Perforated plates used to observe the powder flow.
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Fig. 4. Initial powder bed arrangements inside the standard shoe: (a) TLCS, (b) TCCS, and (c) colored iron powder layers.
on the TDF as shown in Fig. 5a. A typical image utilized for porosity analysis is shown in Fig. 5b. In order to measure the repeatability of porosity distribution for two different filling methods (SS and SPP), images were recorded from 8 different filling tests for each test condition. The porosity measurements were rather sensitive to the light conditions during image capture. Therefore, uniform lighting was maintained in all of the experiments to get consistent results.
radiations were counted, using a multichannel analyzer in pulse height analysis mode. The count rate time was set to 60 s to measure the accuracy of porosity fraction, εgcalculated using the following formula: h i ln Nðεg ¼0Þ =Nεg i εg ¼ h ln N ðεg ¼0Þ =Nðεg ¼1Þ
2.5. Resin bonding technique for filling density determination For ex-situ density measurement from the powder body in the die after PDF process, iron particles were bonded together in the die by using a thermoplastic resin. The aim was to develop a quantitative experimental method to determine the density distribution at different positions within the powder body as a function of process parameters. For this purpose, a blend of the iron powder (ATOMET 1001) with 10% high density polyethylene (HDPE) fine powder was made in a ball mill using ceramic balls of different diameters as a mixing media. By mixing the iron powder with HDPE powder resin, the sintering temperature of the filled power was reduced to the melting temperature (195 °C) of the HDPE powder. This powder blend was then utilized in the PDF experimental set-up. The steel die was then carefully moved from the PDF test frame to a furnace for bonding. The temperature of the furnace was set to 195 °C with a heating rate of 5.5°/min. The sample was held at 195 °C for 90 min and furnace cooled to room temperature. During the powder heating process, the liquid resin filled the inter-particle space between iron powder particles to create a strong bond. In addition, the heating rate was slow so that liquid resin did not push the iron particles. Subsequently, the samples were cut from both SS filling and SPP samples using a saw cutter. The cut pieces of the resin-bonded samples were utilized for bulk density measurements using gamma ray densitometry. Gamma ray densitometry set-up consisted of an americium-241 gamma source (gamma ray energy of 60 keV), a sodium iodide scintillating device and a multichannel analyzer. The basic principle of gamma ray densitometry technique is the exponential decrease in the intensity of collimated beam of gamma rays as it passes through the matter. The gamma rays were passed through the material, and from the other end, the transmitted
Region 1
Region 2
T1
T2
B1
B2
(a)
Region 3
where, N(εg = 0) and N(εg = 1)are the two extreme values corresponding to count rates of a zero porosity and unit porosity respectively and Nεg is the porosity count to be measured for the test sample. The full-energy peak, which corresponded to the un-scattered gamma rays, was compared with the output from the test samples, to obtain the porosity fraction of the test sample based on instruction provided in the gamma ray densitometry laboratory manual. 3. Results and discussion 3.1. Powder flow patterns Three major types of powder flow from the moving shoe are readily apparent in Fig. 6(a, b). During die filling with the standard shoe (SS), powder body moved toward the back wall of the shoe, forming a nose shaped profile inside the shoe. The particles from the top surface of the powder bed (colored in green) cascaded rapidly into the die creating a powder contact free region between the diewall and the front stream of powder. This flow is commonly referred to as nose flow (Fig. 6a). As the shoe traversed over the die cavity, particles from the bottom layer of the powder body (colored in blue) detached and fell into the die-cavity. This flow is commonly referred to as bulk flow of powder (Fig. 6a). The observed nose and bulk flows confirm the powder flow types mentioned by [3]. Powder distribution was uneven resulting in nonuniform packing of powder in left and right sides of the die cavity. During the die filling using SPP, powder flow occurred in small segments through the holes in the perforated plate into the die-cavity, as shown in Fig. 6b. This type of flow is referred as standpipe flow in this
Porosity (black space)
T3 Iron particles (grey space)
B3
(b)
Fig. 5. Photographs showing, (a) chosen point locations on TDF for porosity analysis, and (b) a typical image acquired for porosity analysis from TDF.
D. Aole et al. / Powder Technology 232 (2012) 7–17
Nose flow
Bulk flow
(a)
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Stand-pipe flow
(b)
Fig. 6. Types of flow in PDF process using a) SS and b) SPP.
work. The benefit of adding a perforated plate was that the powder mass in the holes of perforated plate held equal amount of powder; thus, the packing pressure was more uniform on the leading and trailing side of the die cavity. 3.2. Die filling with TLCS powder bed In this sub-section, results from feed shoe with tri-layer colored salt (TLCS) powder bed are presented. The effect of shoe velocity on the powder flow for a standard shoe (SS) are shown in Fig. 7(a–d), and for a shoe with perforated plate (SPP) are given in Fig. 7(e, f). The initial arrangement of colored powder body was shown earlier in Fig. 4a. At a shoe speed of 170 mm/s (Fig. 7a), all three layers of the powder body slightly curl-up against the rear wall of the shoe. As a result, a gap is created in the front of the shoe, which grows as the shoe translates. A
post-avalanche angle of 38° forms in the front portion of the powder bed at the end of the forward shoe cycle. This suggests that the major deposition is taking place from the front of the shoe. The final particle packing shows that top layer of the powder mass deposits into the bottom of the die cavity first. However, particles from the middle layer (colored in white) segregate and form a thin band in the mid-section of the die cavity. The top region of the die cavity is filled by the bottom layer of the powder mass in the shoe, as the shoe moves slowly over the die giving enough time to the bottom layer particles to detach from the bulk (Fig. 7b). Therefore, both nose and bulk flows contribute to the filling process at a shoe speed of 170 mm/s. The mixing of the mid-layer (white) particles with the top (green) and bottom (blue) layer particles inside the die occurs due to final particle rearrangement in the die (Fig. 7b). However, at higher shoe velocity of 385 mm/s (Fig. 7c), the powder layer mixture in the shoe deforms to a greater extent and rises
38o
(a)
(b)
(c)
(d)
(e)
(f)
31o Large gap
20o
Fig. 7. Powder flow patterns of TLCS powder bed, (a) image of SS at the end of the forward pass, 170 mm/s, (b) image of the TDF at the end of the forward and reverse pass, 170 mm/s, (c, d) are same as (a, b) but for 385 mm/s, and (e, f) are same as (c, d) but for SPP and for 385 mm/s.
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considerably toward the back shoe-wall (Fig. 7c). The particles are accelerated to a higher velocity from the top surface; thus, tip of the nose moves quickly over the die and rest of the filling takes place from the bottom layer. A more severe rotational flow is observed during the shoe motion causing rearrangement of particles. The top and middle layer particles deliver the majority of the powder to the die. However, a thin layer of particles from the bottom layer (blue) in the shoe also makes up the top of the die-cavity compared to the shoe speed of 170 mm/s (Fig. 7d). The bottom layer particles have limited time to separate from the cohesive mass of the powder; thus, nose flow dominates in the filling process and the post-avalanche angle is reduced at 385 mm/s compared to 170 mm/s (Fig. 7a and c). For SPP filling Fig. 7 (e, f), the powder profile in the shoe and the final particle packing in the die were found to be similar at shoe speeds of 170 mm/s and 385 mm/s. Therefore, only the segregation results at 385 mm/s are presented in this paper. The powder profile in the shoe at the end of the forward pass did not show significant rearrangement of powder layers during shoe motion, which means that the motion of powder is confined to the bottom of the shoe and in the vicinity of the perforated plate. A post-avalanche slope with an angle of 20° was obtained (Fig. 7e). This angle is significantly smaller in comparison to the angle obtained with SS filling (Fig. 7c) because the powder filling takes place mainly from the bottom layer. Fig. 7f shows that, at first, the bottom layer particles are delivered from the front stream of the powder mass into the right side of the die-cavity. Further, as the shoe translates over the die, the middle layer (white) particles flow through the front holes in the perforated plate and deposit near the middle portion of the die-cavity. Toward the end of forward stroke, front portion holes in the perforated plate deliver the particles from the top layer of the shoe (orange) into the die. The final powder packing and end powder profile in the shoe illustrates that mainly front stream of the powder mass (i.e., leading side of the shoe) fills powder into the cavity, and the powder mass near the rear end of the shoe pours significantly less amount into the die. Due to the change in the degree of aeration in the powder mass during the avalanche, the segregation and the packing patterns are significantly different in SS and SPP fill processes. 3.3. Die filling with TCCS powder bed Fig. 8 below describes the flow behavior of powder during die filling with a tri-column colored sand (TCCS) powder using SS. The
initial arrangement of colored powder body was shown earlier shown in Fig. 4b. At shoe speed of 170 mm/s, all three columns in the shoe have displaced toward the rear shoe wall (Fig. 8a) but the final packing arrangement in the die (Fig. 8b) shows that the entire filling is taking place from the front column particles (orange, Fig. 8b). The other two columns (white and blue) in the shoe do not accelerate to a greater extent, and further move back toward the shoe wall. A postavalanche angle of 35° in front portion of the shoe can be noted. The difference in the post-avalanche angle for TLCS and TCCS powder bed using SS is because of the initial arrangement of powder in the shoe. When the salt powder is poured in the shoe with a temporary wall, the powder bed containing different size particles segregate under gravity and as a result, smaller particles sink to the bottom of the shoe and larger particles remain at the top. Thus, the flow patterns of the powder vary somewhat during the filling process. At a shoe speed of 385 mm/s (Fig. 8c and d), it can be seen that all the three columns move to the rear of the shoe forming a sharp nose shaped profile in the shoe (Fig. 8c). This makes the middle column particles (white) to avalanche down the slope into the left-side of the die-cavity, at the end of the forward filling pass. A smaller post-avalanche angle of 27° is obtained at a shoe speed of 385 mm/s. This means the powder shows better flowability at higher shoe velocity. The final particle packing inside the die-cavity shows particles from left and middle columns of the powder mixture only (i.e., orange and white particles only, Fig. 8d). The particles from the right column (blue) do not contribute to the filling because by the time they start to cascade down the slope, the shoe comes to rest. Fig. 9 describes the powder flow behavior during die filling with a TCCS powder bed using SPP at 2 different shoe speeds (170 mm/s and 385 mm/s). At a shoe velocity of 170 mm/s, tri-column mixture delivers the powder entirely from the left column (Fig. 9a, b). Thus, the final powder packing in the die shows only front column particles (orange, Fig. 9b). The powder profile in the shoe at the end of the forward pass shows a depression in the left column (orange), but the other two columns (white and blue) move up and curl back near the rear shoe wall due to initial acceleration of shoe (Fig. 9a). A post-avalanche angle of 20° is obtained, which is the same as that obtained earlier for the case of TLCS (Fig. 7e). At 385 mm/s, again, the powder columns still displace only slightly (Fig. 9c) but a lower post-avalanche angle of 10° is obtained. The left column particles (orange) still fill major portion of the die-cavity, but now the middle column of the powder in the shoe (blue) fills the top region of the die cavity (Fig. 9d). This confirms that the front
35o
(a)
(b)
(c)
(d)
27o
Fig. 8. Powder flow patterns of TLCS powder bed, (a) image of SS at the end of the forward pass, 170 mm/s, (b) image of the TDF at the end of the forward and reverse pass, 170 mm/s, (c, d) are same as (a, b) but for 385 mm/s.
D. Aole et al. / Powder Technology 232 (2012) 7–17
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20o
(a)
(b)
(c)
(d)
10o
Fig. 9. Powder flow patterns of TCCS powder bed with SPP, (a) image of shoe at the end of the forward pass, 170 mm/s, (b) image of the TDF at the end of the forward and reverse pass, 170 mm/s, (c) image of shoe at the end of the forward pass, 385 mm/s, and (d) image of the TDF at the end of the forward and reverse pass, 385 mm/s.
shoe and die. Also, another layer of sand powder with a different color was added to the powder bed making it a 4 layer system. The powder in the hopper was given the same color as the top layer. Fig. 10 shows the images of the shoe and the TDF for this condition using both SS and SPP at a shoe velocity of 385 mm/s. For SS (Fig. 10a–c), the increased mass exerts more pressure on the front stream of powder. As a result, the top layer (blue) flows with more intensity and fills about three-fourth portion of the die while the remaining space is filled by second layer (white) from the top (Fig. 10c). The difference in the packing pressure in front and rear side of the shoe makes the front stream deposit more powder into the die than the rest of the shoe. However, as the rate of flow of material from the hopper to the shoe is less than the rate from shoe to the die, some depression in the front of the shoe is still observed
stream of powder moves quickly over the die cavity decreasing the filling rate of powder, as the particles from the rear portion of the shoe take time to detach from the bulk. It is clear that, with the use of a perforated plate, the shoe velocity shows very little effect on the rearrangement of powder inside the shoe after the first pass, whereas the powder profile inside the SS changes significantly with the shoe velocity. 3.4. Die filling with increased powder level in the shoe As stated earlier, a hopper filled with powder was attached to the front of the shoe in some of the experiments to observe the effect of increased powder level at that location on powder flow behavior in the
Hopper
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 10. Powder flow patterns for a 4-layer colored sand system with a hopper, (a) image of SS at start of the forward pass, 385 mm/s, (b) image of SS at end of the forward pass, 385 mm/s, (c) image of TDF at the end of the forward and reverse pass, 385 mm/s, (d) image of SPP at start of the forward pass, 385 mm/s, (e) image of SPP at end of the forward pass, 385 mm/s, and (f) image of TDF at the end of the forward and reverse pass, 385 mm/s.
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(Fig. 10b). The nose flow still dominates as no filling is contributed from the bottom layer. On the other hand, for SPP case (Fig. 10d–f), the powder inside the hopper (colored black) drops in the shoe and compresses the powder layers in front of the shoe in the downward direction resulting in a stepped powder profile in the shoe (Fig. 10e). As a result, more particles from the bottom two layers (orange and white) separate quickly and deposit into the die-cavity (Fig. 10f). The bottom layer particles take up the back side of the die while the white layer particles deposit near the front. In summary, segregation of powder into the die occurs from all the three layers during die filling with standard powder level (with three layers, Fig. 7f) whereas, with an increased level of powder material, only bottom two layers contribute to the filling. 3.5. Colored iron powder filling The next set of experiments were performed on commercial iron powder to see if certain characteristics of salt as described in the previous section are also observed for the iron powder in spite of the differences in the particle size and density. Fig. 11(a–d) shows the powder profile in the shoe at the end of the filling process using SS and SPP at shoe speeds of 170 mm/s and 385 mm/s. For SS with a shoe speed of 170 mm/s (Fig. 10a), a rather large post-avalanche angle of 35° is obtained indicating that the top layer particles slide down into the cavity first and the powder bed moves toward the back of the shoe-wall (Fig. 11a). This is similar to the observations made earlier for sand (Fig. 7a). For SS, with a shoe speed of 385 mm/s the entire powder mass has moved toward the rear shoe wall, and a large space is created between the left shoe wall and the front stream of powder. A post-avalanche angle of 32° which is slightly smaller than the angle obtained at 170 mm/s. Again, this reduction is similar to the case of sand PDF process (see Fig. 7a, c). For iron powder PDF process with perforated plate at a shoe speed of 170 mm/s (Fig. 11c), the particles in the bottom layer detach from the powder mass through the holes in the perforated plate and drop first in the die cavity. The powder profile inside the shoe shows some packing toward the back of the shoe and a stepped profile in the front (also observed for sand). However, at 385 mm/s, the powder profile in the shoe shows that complete bottom layer flows into the diecavity (Fig. 11d). In addition, for the iron powder, the powder profile in the shoe resulted in a small post-avalanche angle of only 5°, and
35o
the rest of the layers were intact. In this respect, the results obtained for iron powder are different from the colored salt when a perforated plate is used in the PDF process. During salt powder PDF process at 385 mm/s, all the three layers flow inside the die cavity through the front holes in the perforated plate (Fig. 7e, f), whereas, for the iron powder, the entire bottom layer passes through the holes and only a few particles from the middle layer and possibly none from the top layer flow into the die. It is to be noted that the amount of powder poured inside the die, and the particle packing toward the back of the shoe does differ for sand and iron powder PDF processes due to the differences in the particle density and cohesiveness. This is consistent with the findings reported by [6,7]. They explained that during die filling with a cohesive powder, the cohesive forces prevent the relative movements of particles so that the change of the shape of powder bed in the shoe is limited at the early stage. However, the powder flow process is smooth during die filling with a non-cohesive powder. In general, larger particles produce less friction compared to the smaller ones and spherical particles result in lower friction compared to nonspherical ones. In this research, salt particles are larger in size than iron particles (150–748 μm versus 45–300 μm), and salt powder density is also small. Therefore, loose packing and less mixing of different layers for salt powder is observed in the die at the end of filling stage; whereas, for iron powder the layers are more mixed in the die (and partially in the shoe powder bed) possibly due to higher friction and cohesiveness. In spite of these differences, the results indicate that the salt with its better coloring attributes, and resulting image quality during the die filling experiments, offers an efficient medium to analyze the macroscopic flow behavior of the PM powders during the PDF process. As mentioned earlier, images of the iron PDF process were also recorded using a high-speed camera with a close view of the die cavity to observe other qualitative features of powder flow. Some portion of powder falls over the die-core when the shoe starts filling the cavity. This powder over the die-core drops into the die in a direction perpendicular to the shoe motion (i.e., cross-flow occurs) when the shoe further moves toward the center of the die. For SS, the flow of the powder appears to occur in two directions, i.e., in the direction of the shoe motion as well as perpendicular to it and over the die-core (Fig. 12a). Also, during PDF process with SS, turbulent flow of powder was observed resulting in fine particle dust cloud over the avalanche in the die (Fig. 12b) whereas with SPP a far less intense flow and negligible dust cloud was generated (Fig. 12c). High
32o
(a) Steps in the
20
(b)
5o
o
powder bed
(c)
(d)
Fig. 11. Powder flow patterns in tri-layered iron powder bed at the end of forward pass, (a) image of SS, 170 mm/s, (b) image of SS, 385 mm/s, (c) image of SPP, 170 mm/s, and (d) image of SPP at the end of the forward pass, 385 mm/s.
D. Aole et al. / Powder Technology 232 (2012) 7–17
15
Flow from a die-core
Higher velocity particles
Flow from perforated plate holes
(a)
(b)
(c)
Fig. 12. High speed camera images of powder flow in the die, (a) cross-flow of powder, SS, 170 mm/s, (b) avalanche with dust cloud, SS, 170 mm/s, and (c) avalanche with minimal dust cloud, SPP, 170 mm/s.
speed images resulted in 28° and 24° dynamic angles of repose for SS and SPP fillings respectively. These angles are slightly higher than the static angle of repose (i.e., avalanche angle) implying that the powder behaved differently in static and dynamic modes. Also, due to more uniform and controlled flow from the perforated plate, the dynamic angle of repose in SPP filling was slightly lower than the SS filling. 3.6. Filling rate as a function of shoe speed Average filling rate of iron powder were measured as a function of shoe speed for SS and SPP fillings following a test procedure described earlier. The filling rate increased non-linearly with shoe speeds ranging from 120 mm/s up to 385 mm/s, for SS filling (Fig. 13). The highest filling rate was achieved at 385 mm/s and the die was almost filled in one pass at this shoe speed. Thus, the critical shoe speed with SS is 385 mm/s above which the filling rate will start decreasing. However, for SPP case, the filling rate increased initially and reached at peak at 170 mm/s and then decreased with increasing shoe speed. More than one pass was required to fill the die above the shoe speed of 170 mm/s. Thus, the critical shoe speed for die filling with SPP was significantly lower than with SS. The reason is that, at higher shoe speeds with SPP, the trailing side (right-side of the die cavity) remained partially filled, as
165
3.7. Surface porosity results As stated earlier, surface porosity data at 6 specific locations was obtained from the high resolution TDF images using a quantitative image analyzer system (see earlier Section 2.4 and Fig. 5a). For PDF process using SS, results showed differences in porosity distribution in the top and bottom regions of the die. The top locations T1 and T3)
SS 60
SPP
Top
50
% Porosity
Filling rate, (g/s)
190
the powder passed very slowly through the holes on the rear side of the shoe and the front stream quickly moved over the die. The amount of powder that passed through the holes depended critically on the holediameter and hole-spacing. The filling rate results with SS filling are in good agreement with the results quoted by [9] for a ring cavity using square shoe. They found incomplete filling of the circular cavity at speeds of 200 mm/s and 500 mm/s, with a single pass of the shoe. They also quoted that the amount of discharged powder was higher for the lower shoe speed. With the addition of a hopper to the feed shoe, the results were slightly improved for SS filling but remained largely unmodified for SPP. In this case, bottom layer delivered more powder than the top layers in the die. Since, powder interlocking at the shoe bottom increases with an increase in the powder level; the particles are more constrained in flowing through the holes into the die. However, for SS, the hopper placed on the front of the standard shoe forces powder to flow more intensely into the die (nose flow) increasing the filling rate.
140 115
Bottom
40 30 20 10 0
90 120
170
240
385
Shoe speed, mm/s Fig. 13. Plot of average filling rate of iron powder as a function of shoe speed.
1
2
3
Point location Fig. 14. A comparison of percentage porosity data between top and bottom locations within the die from TDF images using SS.
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Shoe motion
exhibited higher porosity compared to the bottom locations B1 and B3, whereas top location T2 showed slightly less porosity compared to B2 (Fig. 14). In general, the porosity data from TDF showed nonuniform distribution. Similar results for SPP indicate a more uniform porosity at the above specified locations on the die face, a very desirable characteristic from the technological perspective (Fig. 15). By comparing SS and SPP porosity data, it can be seen that the percentage surface porosity for T1, B2 and T2 locations is decreased with the use of SPP, whereas contrary is observed for locations T3, B3, and B1. Therefore, the results indicate that there is no significant benefit in terms of a reduced ‘net’ porosity with the use of a perforated plate.
Six different samples were cut from the resin-bonded ring shaped part retrieved from the die for Gamma ray densitometer experiments to obtain porosity percent data. These samples are marked L3, L2, L1 for the left side and R1, R2, and R3 for the right side of the ring part in Fig. 16. The porosity percent in the different sections are presented in the graph at the bottom of Fig. 16. It is to be noted that the numbers 1, 2, 3, 4, 5, and 6 along x-axis in the graph denote L3, L2, L1, R1, R2, and R3 samples respectively. As evident, the average porosity percentage in the left region is about 8% higher than in the right region for a SS filled sample. However, for SPP filling, the average porosity percentage in the left region is only 0.6% higher than in the right region. The porosity distribution in the sections L1 and R1 closer to the central axis (sections parallel to the shoe motion) are more uniform in SPP sample than in SS; due to uniform flow of powder form the perforated plate holes. However, porosity distribution is slightly uneven and higher in the sections L2, L3, R2, and R3 (perpendicular to the shoe motion) for SPP filled samples. This is consistent with the results reported by [4] for a ring-shaped die. Also, [12] have studied 3D density distribution in a narrow ring cavity using X-ray tomography where similar results have been reported. The regions L1 and R1 show less porosity as compared to those away from it (such as L3 and R3) due to incomplete filling in the regions perpendicular to the shoe motion. It is noted that, at the beginning of the die fill stage, predominant nose flow results in faster evacuation of air from the die. However, as the filling proceeds and the shoe moves to the center of the die-cavity, bulk flow of powder starts from the bottom of the shoe. As the standard shoe starts from rest the powder mass inside the shoe moves toward the back of the shoe-wall, which results in close packing of the particles in the rear portion of the shoe. Thus, the packing pressure applied by the powder mass in the shoe over filled die-cavity is greater on the right-side side than left-side of the die-cavity. As described earlier, the phenomenon of two-directional powder flow is more prominent in SS than SPP. Further, in the filling process with SPP, powder level inside the shoe changes slightly during shoe motion, and maintains uniform packing weight on the left and right sides of the die-cavity. Consequently, for the case of SPP, the porosity at locations L1 and R1 is more uniform than for SS. The results of Gamma
% Porosity
60 50
Top
40
Bottom
30 20 10 0 1
2
3
Point location Fig. 15. A comparison of percentage porosity data between top and bottom locations within the die from TDF images using SPP.
% Porosity
3.8. Gamma ray densitometry results from resin-bonded iron sample
Sections Fig. 16. Porosity distribution in a resin-bonded iron sample.
ray densitometry suggest that the current design of perforated plate makes the powder distribution in the die-cavity more uniform, and consistent with the earlier TDF data of Fig. 15. Gamma ray densitometry is an efficient way of measuring the voluminous density in a thick PM sample. Gamma ray spread in all directions, thus sample cutting can be minimized or eliminated, which will reduce the loss of particles from the surface, whereas in X-ray tomography the part has to be thin sectioned. From this study, it is established that gamma ray densitometry is an efficient way of measuring the voluminous density in a thick PM sample. Gamma ray spread in all directions, thus sample cutting can be minimized or eliminated, which will reduce the loss of particles from the surface, whereas in X-ray tomography the part has to be thin sectioned. 4. Conclusions The usefulness of incorporating a perforated plate at the bottom of the feed shoe to improve the density uniformity in the filling condition is successfully assessed. It is noted that a stand-pipe flow exhibits controlled flow, and reduces turbulence of powder flow during filling; thus, uniform distribution of powder is obtained. Another benefit of adding a perforated plate is that the powder mass in the holes of perforated plate holds equal amount of powder; thus, the packing pressure is uniform on the leading and trailing side of the die-cavity. Further, the filling rate experiments demonstrated that a low filling rate is obtained at high shoe speed with the current design of perforated plate. Thus the critical shoe speed with SPP filling is lower than SS filling. The reason is that the flow rate of powder from the front portion of SPP is higher than the rear portion as observed from the high speed camera images. Eventually, new designs of the perforated plate by changing the hole-diameter, orientation of the holes or by changing the thickness of the plate need to be developed to extract more benefits from the perforated plate. The colored salt powder experiments have revealed various aspects of powder flow and segregation that are not readily apparent with regular commercial iron powder. Although the size and morphology of the salt particles is different from iron particles, still it is found that the salt powder profiles in the shoe and sequence of segregation of layers and flow patterns closely mimic the iron powder flow patterns. Therefore, use of colored powder as a layered medium within the feed
D. Aole et al. / Powder Technology 232 (2012) 7–17
shoe makes the PDF investigation more insightful and informative. The effective area of powder discharge at different shoe velocities for both SS and SPP is also clearly identified using colored salt powder. It is also concluded that the avalanche characteristics of a powder alter with the use of a perforate plate at the bottom of the shoe as well as by changing the initial arrangement of powder bed inside the shoe (i.e., tri-layer or tri-column). The major limitation of salt powder is its hygroscopic nature but this was overcome by drying the salt powder with a fan prior to PDF experiments. With the pixel based analysis of the images of transparent die face (TDF) sections; it is possible to acquire spatial surface porosity data. Therefore, a rapid non-destructive and quantitative measure of the spatial fill density is possible. The results demonstrate the positive effect of perforated plate addition to the bottom of the shoe in terms of the powder distribution and flow control. The resin bonding method for measuring the die-fill density at the end of die filling was satisfactorily utilized for binding iron powder particles in the loose un-compacted state within the die at low temperature. This test has uniquely used a polymer resin which has the advantage of binding the die fill powder at low temperature without significant displacement or redistribution of particles during the bonding process. Voluminous porosity measurements in the PDF state from different sections of a ring shaped resin bonded sample could be successfully made using gamma ray densitometry. This method could be potentially applied to optimize PDF in industrial parts. Acknowledgments The authors wish to express their special thanks to Mr. Roger Lawcock of Gates Canada Limited for his encouragement and advice during this research.
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References [1] C.Y. Wu, A.C.F. Cocks, O.T. Gillia, Die filling and powder transfer, International Journal of Powder Metallurgy 39 (2003) 51–64 (No. 4). [2] C.Y. Wu, A.C.F. Cocks, O.T. Gillia, A. Thompson, Experimental and numerical investigations of powder transfer, Powder Technology 138 (2003) 216–228. [3] C.Y. Wu, L. Dihoru, A.C.F. Cocks, The flow of powder into simple and stepped dies, Powder Technology 134 (2003) 24–39. [4] E. Hjortsberg, B.S. Bergquist, Filling induced density variation in metal powder, Powder Metallurgy 45 (2) (2002) 146–153. [5] F. Lavoie, L. Cartilier, R. Thibert, New methods of characterizing avalanche behavior to determine powder flow, Pharmaceutical Research 19 (6) (2002) 887–893. [6] Y. Guo, C.-Y. Wu, C. Thornton, The effects of air and particle density difference on segregation of powder mixtures during die filling, Chemical Engineering Science 66 (2011) 661–673. [7] Y. Guo, C.-Y. Wu, K.D. Kafui, C. Thornton, 3D DEM/CFD analysis of size-induced segregation during die filling, Powder Technology 206 (2011) 177–188. [8] L.R. Lawrence, J.K. Beddow, Powder segregation during die filling, Powder Technology 2 (1968) 125–130. [9] C. Bierwisch, T. Kraft, H. Reidel, M. Moseler, Three-dimensional discrete element models for the granular statics and dynamics of the powder in gravity filling, Journal of the Mechanics and Physics of Solids 57 (2009) 10–31. [10] J.K. Prescott, R.A. Barnum, On powder flowability, Pharmaceutical Technology (2000) 60–85. [11] M. Kim, Improved density distribution in powder metallurgy parts with filtering method, Advanced Materials Research 26–28 (2007) 445–448. [12] S.F. Burch, A.C.F. Cocks, J.M. Prado, J.H. Tweed, Die fill and powder transfer, Modelling of Powder Die Compaction Engineering Materials and Processes (2008) 131–150, doi:10.1007/978-1-84628-099-3_9 (Die filling and compaction , Chapter 9). [13] Y. Guo, K.D. Kafui, C.-Y. Wu, C. Thornton, J.P.K. Seville, A coupled DEM/CFD analysis of the effect of air on powder flow during die filling, AICHE Journal 55 (1) (2009) 49–62. [14] S. De Silva, A. Dyroy, G.G. Enstad, Segregation mechanisms and their quantification using segregation testers, in: A.D. Rosato (Ed.), Proceedings of IUTAM Symposium on Segregation in Granular Flows, Kluwer Academic Publishers, Boston, MA, USA, 1999, pp. 11–29.
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
New characterization methods for powder die fill process for producing powder metallurgical components D. Aole, M.K. Jain ⁎, M. Bruhis Department of Mechanical Engineering, McMaster University, Hamilton, Ontario, Canada
a r t i c l e
i n f o
Article history: Received 20 December 2011 Received in revised form 2 August 2012 Accepted 4 August 2012 Available online 11 August 2012 Keywords: Die filling Perforated plate Colored powder Surface porosity Resin-bonding Gamma-ray densitometry
a b s t r a c t Powder die fill (PDF) process prior to compaction affects spatial density of the final part and its in-service performance. Inhomogeneous density leads to part distortion (or warpage) during sintering, resulting in inability to get the desired dimensional tolerances and mechanical properties in the final product. Therefore, to improve quality of the final part, it is important to eliminate several causes of non-uniformity during the powder die filling stage. This experimental study relates to some new approaches toward characterization of PDF process, using an experimental apparatus with transparent shoe and die sections. In this paper, the influence of a newly designed perforated plate (PP), incorporated at the bottom of the feed shoe on PDF is studied. The effect of PP on powder redistribution in the feed shoe, powder transfer and resulting powder spatial density uniformity, and porosity within a thin ring-shaped die is compared with the powder flow and segregation behavior in standard shoe (SS) filling. Various colored salt powder experiments using arrangements in layers and columns within the feed shoe as well as a commercial iron power are considered in an attempt to analyze the subsequent powder redistribution within the shoe, during shoe motion and final powder packing in the die. It is determined that the colored salt is a promising medium for simulating the flow characteristics and segregation of commercial iron powder macroscopically. Further, critical shoe speed for powder delivery into the die-cavity using a perforated plate and a standard shoe are studied. Continuous imaging of the transparent shoe and die wall sections with an on-line high speed CCD camera are also used to record powder flow and to obtain resulting porosity from quantitative pixel-based analysis of the post-fill final images. The effectiveness and limitations of the perforated plate in achieving a more homogenous powder fill and reduced porosity in the die is explained with the help of both colored powder experiments and the surface porosity data from the transparent die face section. Finally, a successful attempt is made to bind the loose iron powder using a polymer resin blend into the shape of a ring within the die. This ring shaped component is then analyzed for its spatial porosity distribution, and thereby its spatial density distribution, using gamma ray densitometry. © 2012 Elsevier B.V. All rights reserved.
1. Introduction and literature review 1.1. Powder flow characteristics Powder metallurgy process includes the preparation of powder blend, which is then transferred into the die using a feed shoe and compacted using rigid punches. The final part is ejected from the diecavity and subject to post-compaction processing such as sintering, sizing and heat treatment. In powder die fill (PDF) operation, a powderfilled feed shoe translates in a reciprocating motion over the die-cavity depositing the powder into the die in one or more passes. At the end of this stage, the volume occupied by the powder in the cavity is constant. The shoe motion results in intense shear deformation as blocks of loose
⁎ Corresponding author. E-mail address: [email protected] (M.K. Jain). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2012.08.002
powder slide over each other. PDF process can be looked upon in terms of powder flow from feed shoe into the die and powder packing and segregation inside the die cavity during filling. In general, the filling process involves a complex interaction of process variables (for example, shoe speed and shoe design), part variables (die design), die wall friction and powder physical characteristics such as particle size and shape. Any non-uniformity at the PDF stage is carried over to the subsequent stages of powder compaction and sintering and could lead to defects in the final part that can precipitate failure during service. Non-uniform PDF also affects the tool deflection and tool life during compaction. Powder flowability is defined as the ability of the powder to flow under a range of process conditions. Flowabiltiy is not an inherent material property, but is a combination of physical properties of the particulate matter and the design of mechanical components that make up the handling and feeding system. Hopper and feed shoe are perhaps the two most critical components of the powder delivery
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system. A nonuniform powder delivery from the hopper to the feed shoe results in variation in the head pressure, and together with the flow rate and other flow characteristics translates into variation in component spatial density (i.e., density variation from one region of the part to another) and part dimensions. The friction between the shoe and die walls and powder particles restrict the movement of the powder; thus, contributing to the spatial density variation in the part. The flow property of the powder is often described in terms of the angle of repose. This angle, measured in a standard test, forms between the free surface of the powder and the horizontal base when the powder, stored in a funnel, is dropped from a specified height. Typically, the powder with superior flow rate makes a low angle of repose. The flow properties have been also related to the cohesion and internal friction between the particles. Powder material does not flow freely through dies with sharp corners, long and thin sections, and where crosssections have large variations, resulting in lower density of the part after pressing [10]. Cocks and co-workers stated that during gravity PDF different types of flows occur when shoe moves over the die cavity [Cocks et al., 2003a and [1]. Nose flow occurs when the loose powder in the shoe slides over each other, during the initial acceleration of the shoe, and then the top layer which is a free surface cascades into the die creating a nose-shaped profile. In nose flow, the larger particles from the top free surface of the powder inside the shoe flow more rapidly in the die-cavity, whereas the fine particles flow with lower velocities due to strong frictional forces. [3] also noted that nose flow promotes faster evacuation of air, and thus, a higher powder flow rate is achieved. It was further noted that smaller amount of powder in the shoe, lower shoe speed, and larger die opening promote nose flow. As the shoe moves further over the die cavity, particles from the bottom layer of the shoe starts filling the die, which is referred to as bulk flow. This flow dominates when the shoe speed is high. In bulk flow, the bottom particles flow later and slowly from the bottom due to the combined effect of friction between the base and the powder mass, and cohesion forces. Kim proposed a promising approach to PDF process by utilizing a filter (or a perforated plate) at the bottom of the feed shoe to increase uniformity of the die fill [11]. The presence of a filter at the bottom of the shoe breaks the bulk powder flow into a vertically downward free flow. Kim utilized two types of filters in the form of different arrays of holes in a plate (linear and staggered) as well as different sizes of the feed shoe versus the volume of the holes in the filter for controlling the flow of powder in the die. In the present work, similar to the work of Kim, a perforated plate was designed and utilized in some of the PDF experiments to further assess the powder flow in the shoe and the die and resulting spatial density.
depending on particle size and shape [13]. It was found that by using a vibrating die radial segregation can be reduced as the fines get leveled off [8]. The vertical segregation was found to be dependent on the frequency and amplitude of vibration. [5] studied the avalanche to characterize the fluidity and cohesion of the powders, which affects the final distribution of particles in the die during the filling process. It was reported that the powder material behaves differently in static and dynamic modes. The avalanche occurs when the balance between cohesion of particles and gravity breaks. They described that in the pre-avalanche stage cohesion forces maintain the particle arrangement stability. At the time of avalanche, segregation of powder layers and distance travelled by the particles in the die determines the final packing density of powder in the die fill condition. Also, the slope of powder following the avalanche correlates well with the quantity of powder delivered into the die. 1.3. PDF parameters and spatial density measurement in the die-fill condition During PDF, the kinetic energy of moving particles can be partially transferred into the die cavity, where it causes rearrangement of the particles and compaction. The increase of shoe speed results in an increase of kinetic energy of particles and results in increased densification in the die [4]. Also, the shoe kinematics affects the evacuation of the air out of the die. At higher shoe speeds, due to less filling time, air gets entrapped in the powder body. As a result, density distribution inside the die becomes uneven. In the present work, the effect of shoe speed on powder flow characteristics in the shoe and spatial density distribution in the die has been studied. Although the die geometry also influences the flow rate and flow pattern during die filling, only one ring- shaped die geometry has been considered. A crucial step in evaluating the PDF process is to measure the density distribution of powder particles in the die fill condition without shifting or physical disturbance to them once they are in the die. There are no accurate and reliable methods known at present to quantify the PDF density in the loose powder state. Recently, [12] investigated the initial density distribution after filling by sintering the loose powder and by examining the die in filled condition with X-ray tomography technique. However, moving the whole die in the filled condition for testing and sintering the powder is not very convenient procedure to get the accurate density distribution data. Thus, attempt has been made in the present work to develop a more reliable method for measuring the density distribution in die-fill condition without affecting the loose powder arrangement in the die. 2. Experimental procedures 2.1. Experimental PDF set-up
1.2. Power segregation and avalanche Different segregation mechanisms and characteristics during PDF process were initially identified by [14], namely, trajectory, air flow, rolling, angle of repose, embedding, impact, push away, displacement, percolation, fluidization, agglomeration, and concentration drive displacement. Segregation pattern during the powder flow can influence the final particle packing in the die. In a granular mixture differing in size and shape, the large particles with lower density or round shape were found to lie closer to the top of the flowing layer. This leads to longer travel time of the particles before they come to rest. However, the smaller particles were unable to roll easily over the large particle surface. In such cases, the larger particles were found at the bottom of the pile, whereas the smaller particles remained at the top. The powder flow from a shoe interacts with air during filling, the drag force of air lifts up the light particles leading to the segregation of the powder mass. In addition, the small and angular particles flow with low velocity due to stronger frictional force during filling. Therefore, different trajectories of powder flow are observed
A test-rig was developed to study the PDF process in a ring-shaped die. The die was mounted on a large base plate and a square shaped feed shoe was located above the die and attached to a double action pneumatic actuator with a solenoid valve for easy reciprocating motion. The entire test set-up was mounted on a rigid frame as shown in Fig. 1. The front die and shoe wall faces were fitted with a rectangular piece of Perspex to act as observation windows to record the powder flow characteristics and post-avalanche angle at the end of filling. A small conical hopper with an outlet diameter of 50.8 mm was also attached to the front of the shoe for some of the experiments to provide continuous powder supply to the shoe during the PDF process. The extra mass in the small hopper was utilized to study the influence of increased powder level in the front portion of the shoe and its effect on powder flow. The geometric details of the feed shoe and the die are shown in Fig. 2. A perforated plate (PP) insert was placed at the bottom of the die in some experiments to modify the flow of powder in the die cavity (Fig. 3). This plate, fabricated from aluminum, was designed to have a grid of straight holes (i.e., perpendicular to the plane of the plate)
D. Aole et al. / Powder Technology 232 (2012) 7–17
Die-cavity
Hopper
Feed Shoe
9
Actuator Cylinder
Base Plate
High-speed camera
Transparent die face
Actuator rod Solenoid Valve
Lights
Fig. 1. Various components of PDF experimental set-up.
with a diameter and hole center-to-center spacing of 4.06 mm and 8 mm respectively. The standard shoe, i.e., without an insert, and shoe with the perforated plate insert are referred to as SS and SPP respectively in the remainder of this paper. 2.2. Powder flow analysis using colored powder bed Powders used in the present studies were (i) salt (NaCl) powder, colored using food coloring, and (ii) commercially available ATOMET 1001 water atomized iron powder without lubricant. Also, a mixture of ATOMET 1001 and polyethylene powders was used in powder heating experiments to bind the iron powder in the loose or un-compacted, state in the die. The salt, iron, and polyethylene powder particles were of sizes 50–750 μm, 45–300 μm, and 100–750 μm respectively. The salt powder is non-cohesive in nature and iron powder is cohesive. The PDF experiments were first performed using tri-layer colored salt (TLCS) and tri-column colored salt (TCCS) powder beds inside the feed shoe (see Fig. 4a,b). For these tri-layer patterns, different colors of salt powder were poured one by one manually and leveled with a brush to get three uniform 12 mm thick layers in a 190.5 mm wide shoe (i.e., along x dimension of the shoe). In TCCS case, two removable thin walls (i.e., metal strips) were temporarily placed inside the shoe to separate the different color powder columns. The three different colored salt powders were then poured separately in to three equal size columns (i.e., each of thickness 12 mm and width 63.5 mm) and leveled to the same height inside the shoe. The interest in the TCCS powder bed tests was to understand the effective area of powder delivery across the length of the shoe. For colored iron powder bed, iron powder was slightly oxidized in an electric furnace at 180 °C for 2 h to get the light brown color (middle layer, Fig. 4c). The flow regimes noted from these experiments with the
Separating plate Feed Shoe Table frame
mixture of grey (original) and light brown colored layers were then correlated with the results obtained from the colored salt experiments. The PDF processes using SS and SPP were analyzed at different shoe speeds and compared with each other. For this purpose, the powder flow from the shoe into the die cavity was continuously recorded with the help of a Canon SX110 digital camera with 9 megapixel resolutions. The parameters such as shoe speed and powder level were varied to note the flow patterns and final particle packing in the die. The postavalanche angle (i.e., static angle of repose) inside the shoe at the end of the filling cycle was also compared for SS and SPP fill conditions. 2.3. Procedure for PDF rate determination The filling rate of powder into the die (or PDF rate) is the ratio of total mass of powder filled into the die cavity to the filling time [1–3]. During the filling stage, the mass transferred into the die after a forward pass of the shoe was measured with an electronic weighing balance. The filling time was determined by measuring the total time from the point the shoe filled with powder starts discharging powder into the diecavity till the shoe leaves the die completely, during the forward pass. The filling rate study was performed using SS and SPP, with and without the hopper. The experiment was repeated three times for each test condition to obtain an average filling rate data. 2.4. Determination of surface porosity from pixel-based image analysis system High resolution images were recorded from the transparent die face (TDF) section using a high-speed camera with up to 500 frames/s (Schneider CCD-1300BG) during the PDF process. These images were analyzed for surface porosity using image analysis software from Nikon Instruments (NIS-Elements). The analysis was based on the assumption that all pixels brighter than a threshold value were part of the grain space, and those darker than the threshold value were considered to be part of the pore space. The deposited powder images at the end of the filling process were analyzed at 6 specific locations
Bottom Rails
Die-cavity
Fig. 2. Top view of the die cavity and feed shoe.
Fig. 3. Perforated plates used to observe the powder flow.
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Fig. 4. Initial powder bed arrangements inside the standard shoe: (a) TLCS, (b) TCCS, and (c) colored iron powder layers.
on the TDF as shown in Fig. 5a. A typical image utilized for porosity analysis is shown in Fig. 5b. In order to measure the repeatability of porosity distribution for two different filling methods (SS and SPP), images were recorded from 8 different filling tests for each test condition. The porosity measurements were rather sensitive to the light conditions during image capture. Therefore, uniform lighting was maintained in all of the experiments to get consistent results.
radiations were counted, using a multichannel analyzer in pulse height analysis mode. The count rate time was set to 60 s to measure the accuracy of porosity fraction, εgcalculated using the following formula: h i ln Nðεg ¼0Þ =Nεg i εg ¼ h ln N ðεg ¼0Þ =Nðεg ¼1Þ
2.5. Resin bonding technique for filling density determination For ex-situ density measurement from the powder body in the die after PDF process, iron particles were bonded together in the die by using a thermoplastic resin. The aim was to develop a quantitative experimental method to determine the density distribution at different positions within the powder body as a function of process parameters. For this purpose, a blend of the iron powder (ATOMET 1001) with 10% high density polyethylene (HDPE) fine powder was made in a ball mill using ceramic balls of different diameters as a mixing media. By mixing the iron powder with HDPE powder resin, the sintering temperature of the filled power was reduced to the melting temperature (195 °C) of the HDPE powder. This powder blend was then utilized in the PDF experimental set-up. The steel die was then carefully moved from the PDF test frame to a furnace for bonding. The temperature of the furnace was set to 195 °C with a heating rate of 5.5°/min. The sample was held at 195 °C for 90 min and furnace cooled to room temperature. During the powder heating process, the liquid resin filled the inter-particle space between iron powder particles to create a strong bond. In addition, the heating rate was slow so that liquid resin did not push the iron particles. Subsequently, the samples were cut from both SS filling and SPP samples using a saw cutter. The cut pieces of the resin-bonded samples were utilized for bulk density measurements using gamma ray densitometry. Gamma ray densitometry set-up consisted of an americium-241 gamma source (gamma ray energy of 60 keV), a sodium iodide scintillating device and a multichannel analyzer. The basic principle of gamma ray densitometry technique is the exponential decrease in the intensity of collimated beam of gamma rays as it passes through the matter. The gamma rays were passed through the material, and from the other end, the transmitted
Region 1
Region 2
T1
T2
B1
B2
(a)
Region 3
where, N(εg = 0) and N(εg = 1)are the two extreme values corresponding to count rates of a zero porosity and unit porosity respectively and Nεg is the porosity count to be measured for the test sample. The full-energy peak, which corresponded to the un-scattered gamma rays, was compared with the output from the test samples, to obtain the porosity fraction of the test sample based on instruction provided in the gamma ray densitometry laboratory manual. 3. Results and discussion 3.1. Powder flow patterns Three major types of powder flow from the moving shoe are readily apparent in Fig. 6(a, b). During die filling with the standard shoe (SS), powder body moved toward the back wall of the shoe, forming a nose shaped profile inside the shoe. The particles from the top surface of the powder bed (colored in green) cascaded rapidly into the die creating a powder contact free region between the diewall and the front stream of powder. This flow is commonly referred to as nose flow (Fig. 6a). As the shoe traversed over the die cavity, particles from the bottom layer of the powder body (colored in blue) detached and fell into the die-cavity. This flow is commonly referred to as bulk flow of powder (Fig. 6a). The observed nose and bulk flows confirm the powder flow types mentioned by [3]. Powder distribution was uneven resulting in nonuniform packing of powder in left and right sides of the die cavity. During the die filling using SPP, powder flow occurred in small segments through the holes in the perforated plate into the die-cavity, as shown in Fig. 6b. This type of flow is referred as standpipe flow in this
Porosity (black space)
T3 Iron particles (grey space)
B3
(b)
Fig. 5. Photographs showing, (a) chosen point locations on TDF for porosity analysis, and (b) a typical image acquired for porosity analysis from TDF.
D. Aole et al. / Powder Technology 232 (2012) 7–17
Nose flow
Bulk flow
(a)
11
Stand-pipe flow
(b)
Fig. 6. Types of flow in PDF process using a) SS and b) SPP.
work. The benefit of adding a perforated plate was that the powder mass in the holes of perforated plate held equal amount of powder; thus, the packing pressure was more uniform on the leading and trailing side of the die cavity. 3.2. Die filling with TLCS powder bed In this sub-section, results from feed shoe with tri-layer colored salt (TLCS) powder bed are presented. The effect of shoe velocity on the powder flow for a standard shoe (SS) are shown in Fig. 7(a–d), and for a shoe with perforated plate (SPP) are given in Fig. 7(e, f). The initial arrangement of colored powder body was shown earlier in Fig. 4a. At a shoe speed of 170 mm/s (Fig. 7a), all three layers of the powder body slightly curl-up against the rear wall of the shoe. As a result, a gap is created in the front of the shoe, which grows as the shoe translates. A
post-avalanche angle of 38° forms in the front portion of the powder bed at the end of the forward shoe cycle. This suggests that the major deposition is taking place from the front of the shoe. The final particle packing shows that top layer of the powder mass deposits into the bottom of the die cavity first. However, particles from the middle layer (colored in white) segregate and form a thin band in the mid-section of the die cavity. The top region of the die cavity is filled by the bottom layer of the powder mass in the shoe, as the shoe moves slowly over the die giving enough time to the bottom layer particles to detach from the bulk (Fig. 7b). Therefore, both nose and bulk flows contribute to the filling process at a shoe speed of 170 mm/s. The mixing of the mid-layer (white) particles with the top (green) and bottom (blue) layer particles inside the die occurs due to final particle rearrangement in the die (Fig. 7b). However, at higher shoe velocity of 385 mm/s (Fig. 7c), the powder layer mixture in the shoe deforms to a greater extent and rises
38o
(a)
(b)
(c)
(d)
(e)
(f)
31o Large gap
20o
Fig. 7. Powder flow patterns of TLCS powder bed, (a) image of SS at the end of the forward pass, 170 mm/s, (b) image of the TDF at the end of the forward and reverse pass, 170 mm/s, (c, d) are same as (a, b) but for 385 mm/s, and (e, f) are same as (c, d) but for SPP and for 385 mm/s.
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considerably toward the back shoe-wall (Fig. 7c). The particles are accelerated to a higher velocity from the top surface; thus, tip of the nose moves quickly over the die and rest of the filling takes place from the bottom layer. A more severe rotational flow is observed during the shoe motion causing rearrangement of particles. The top and middle layer particles deliver the majority of the powder to the die. However, a thin layer of particles from the bottom layer (blue) in the shoe also makes up the top of the die-cavity compared to the shoe speed of 170 mm/s (Fig. 7d). The bottom layer particles have limited time to separate from the cohesive mass of the powder; thus, nose flow dominates in the filling process and the post-avalanche angle is reduced at 385 mm/s compared to 170 mm/s (Fig. 7a and c). For SPP filling Fig. 7 (e, f), the powder profile in the shoe and the final particle packing in the die were found to be similar at shoe speeds of 170 mm/s and 385 mm/s. Therefore, only the segregation results at 385 mm/s are presented in this paper. The powder profile in the shoe at the end of the forward pass did not show significant rearrangement of powder layers during shoe motion, which means that the motion of powder is confined to the bottom of the shoe and in the vicinity of the perforated plate. A post-avalanche slope with an angle of 20° was obtained (Fig. 7e). This angle is significantly smaller in comparison to the angle obtained with SS filling (Fig. 7c) because the powder filling takes place mainly from the bottom layer. Fig. 7f shows that, at first, the bottom layer particles are delivered from the front stream of the powder mass into the right side of the die-cavity. Further, as the shoe translates over the die, the middle layer (white) particles flow through the front holes in the perforated plate and deposit near the middle portion of the die-cavity. Toward the end of forward stroke, front portion holes in the perforated plate deliver the particles from the top layer of the shoe (orange) into the die. The final powder packing and end powder profile in the shoe illustrates that mainly front stream of the powder mass (i.e., leading side of the shoe) fills powder into the cavity, and the powder mass near the rear end of the shoe pours significantly less amount into the die. Due to the change in the degree of aeration in the powder mass during the avalanche, the segregation and the packing patterns are significantly different in SS and SPP fill processes. 3.3. Die filling with TCCS powder bed Fig. 8 below describes the flow behavior of powder during die filling with a tri-column colored sand (TCCS) powder using SS. The
initial arrangement of colored powder body was shown earlier shown in Fig. 4b. At shoe speed of 170 mm/s, all three columns in the shoe have displaced toward the rear shoe wall (Fig. 8a) but the final packing arrangement in the die (Fig. 8b) shows that the entire filling is taking place from the front column particles (orange, Fig. 8b). The other two columns (white and blue) in the shoe do not accelerate to a greater extent, and further move back toward the shoe wall. A postavalanche angle of 35° in front portion of the shoe can be noted. The difference in the post-avalanche angle for TLCS and TCCS powder bed using SS is because of the initial arrangement of powder in the shoe. When the salt powder is poured in the shoe with a temporary wall, the powder bed containing different size particles segregate under gravity and as a result, smaller particles sink to the bottom of the shoe and larger particles remain at the top. Thus, the flow patterns of the powder vary somewhat during the filling process. At a shoe speed of 385 mm/s (Fig. 8c and d), it can be seen that all the three columns move to the rear of the shoe forming a sharp nose shaped profile in the shoe (Fig. 8c). This makes the middle column particles (white) to avalanche down the slope into the left-side of the die-cavity, at the end of the forward filling pass. A smaller post-avalanche angle of 27° is obtained at a shoe speed of 385 mm/s. This means the powder shows better flowability at higher shoe velocity. The final particle packing inside the die-cavity shows particles from left and middle columns of the powder mixture only (i.e., orange and white particles only, Fig. 8d). The particles from the right column (blue) do not contribute to the filling because by the time they start to cascade down the slope, the shoe comes to rest. Fig. 9 describes the powder flow behavior during die filling with a TCCS powder bed using SPP at 2 different shoe speeds (170 mm/s and 385 mm/s). At a shoe velocity of 170 mm/s, tri-column mixture delivers the powder entirely from the left column (Fig. 9a, b). Thus, the final powder packing in the die shows only front column particles (orange, Fig. 9b). The powder profile in the shoe at the end of the forward pass shows a depression in the left column (orange), but the other two columns (white and blue) move up and curl back near the rear shoe wall due to initial acceleration of shoe (Fig. 9a). A post-avalanche angle of 20° is obtained, which is the same as that obtained earlier for the case of TLCS (Fig. 7e). At 385 mm/s, again, the powder columns still displace only slightly (Fig. 9c) but a lower post-avalanche angle of 10° is obtained. The left column particles (orange) still fill major portion of the die-cavity, but now the middle column of the powder in the shoe (blue) fills the top region of the die cavity (Fig. 9d). This confirms that the front
35o
(a)
(b)
(c)
(d)
27o
Fig. 8. Powder flow patterns of TLCS powder bed, (a) image of SS at the end of the forward pass, 170 mm/s, (b) image of the TDF at the end of the forward and reverse pass, 170 mm/s, (c, d) are same as (a, b) but for 385 mm/s.
D. Aole et al. / Powder Technology 232 (2012) 7–17
13
20o
(a)
(b)
(c)
(d)
10o
Fig. 9. Powder flow patterns of TCCS powder bed with SPP, (a) image of shoe at the end of the forward pass, 170 mm/s, (b) image of the TDF at the end of the forward and reverse pass, 170 mm/s, (c) image of shoe at the end of the forward pass, 385 mm/s, and (d) image of the TDF at the end of the forward and reverse pass, 385 mm/s.
shoe and die. Also, another layer of sand powder with a different color was added to the powder bed making it a 4 layer system. The powder in the hopper was given the same color as the top layer. Fig. 10 shows the images of the shoe and the TDF for this condition using both SS and SPP at a shoe velocity of 385 mm/s. For SS (Fig. 10a–c), the increased mass exerts more pressure on the front stream of powder. As a result, the top layer (blue) flows with more intensity and fills about three-fourth portion of the die while the remaining space is filled by second layer (white) from the top (Fig. 10c). The difference in the packing pressure in front and rear side of the shoe makes the front stream deposit more powder into the die than the rest of the shoe. However, as the rate of flow of material from the hopper to the shoe is less than the rate from shoe to the die, some depression in the front of the shoe is still observed
stream of powder moves quickly over the die cavity decreasing the filling rate of powder, as the particles from the rear portion of the shoe take time to detach from the bulk. It is clear that, with the use of a perforated plate, the shoe velocity shows very little effect on the rearrangement of powder inside the shoe after the first pass, whereas the powder profile inside the SS changes significantly with the shoe velocity. 3.4. Die filling with increased powder level in the shoe As stated earlier, a hopper filled with powder was attached to the front of the shoe in some of the experiments to observe the effect of increased powder level at that location on powder flow behavior in the
Hopper
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 10. Powder flow patterns for a 4-layer colored sand system with a hopper, (a) image of SS at start of the forward pass, 385 mm/s, (b) image of SS at end of the forward pass, 385 mm/s, (c) image of TDF at the end of the forward and reverse pass, 385 mm/s, (d) image of SPP at start of the forward pass, 385 mm/s, (e) image of SPP at end of the forward pass, 385 mm/s, and (f) image of TDF at the end of the forward and reverse pass, 385 mm/s.
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(Fig. 10b). The nose flow still dominates as no filling is contributed from the bottom layer. On the other hand, for SPP case (Fig. 10d–f), the powder inside the hopper (colored black) drops in the shoe and compresses the powder layers in front of the shoe in the downward direction resulting in a stepped powder profile in the shoe (Fig. 10e). As a result, more particles from the bottom two layers (orange and white) separate quickly and deposit into the die-cavity (Fig. 10f). The bottom layer particles take up the back side of the die while the white layer particles deposit near the front. In summary, segregation of powder into the die occurs from all the three layers during die filling with standard powder level (with three layers, Fig. 7f) whereas, with an increased level of powder material, only bottom two layers contribute to the filling. 3.5. Colored iron powder filling The next set of experiments were performed on commercial iron powder to see if certain characteristics of salt as described in the previous section are also observed for the iron powder in spite of the differences in the particle size and density. Fig. 11(a–d) shows the powder profile in the shoe at the end of the filling process using SS and SPP at shoe speeds of 170 mm/s and 385 mm/s. For SS with a shoe speed of 170 mm/s (Fig. 10a), a rather large post-avalanche angle of 35° is obtained indicating that the top layer particles slide down into the cavity first and the powder bed moves toward the back of the shoe-wall (Fig. 11a). This is similar to the observations made earlier for sand (Fig. 7a). For SS, with a shoe speed of 385 mm/s the entire powder mass has moved toward the rear shoe wall, and a large space is created between the left shoe wall and the front stream of powder. A post-avalanche angle of 32° which is slightly smaller than the angle obtained at 170 mm/s. Again, this reduction is similar to the case of sand PDF process (see Fig. 7a, c). For iron powder PDF process with perforated plate at a shoe speed of 170 mm/s (Fig. 11c), the particles in the bottom layer detach from the powder mass through the holes in the perforated plate and drop first in the die cavity. The powder profile inside the shoe shows some packing toward the back of the shoe and a stepped profile in the front (also observed for sand). However, at 385 mm/s, the powder profile in the shoe shows that complete bottom layer flows into the diecavity (Fig. 11d). In addition, for the iron powder, the powder profile in the shoe resulted in a small post-avalanche angle of only 5°, and
35o
the rest of the layers were intact. In this respect, the results obtained for iron powder are different from the colored salt when a perforated plate is used in the PDF process. During salt powder PDF process at 385 mm/s, all the three layers flow inside the die cavity through the front holes in the perforated plate (Fig. 7e, f), whereas, for the iron powder, the entire bottom layer passes through the holes and only a few particles from the middle layer and possibly none from the top layer flow into the die. It is to be noted that the amount of powder poured inside the die, and the particle packing toward the back of the shoe does differ for sand and iron powder PDF processes due to the differences in the particle density and cohesiveness. This is consistent with the findings reported by [6,7]. They explained that during die filling with a cohesive powder, the cohesive forces prevent the relative movements of particles so that the change of the shape of powder bed in the shoe is limited at the early stage. However, the powder flow process is smooth during die filling with a non-cohesive powder. In general, larger particles produce less friction compared to the smaller ones and spherical particles result in lower friction compared to nonspherical ones. In this research, salt particles are larger in size than iron particles (150–748 μm versus 45–300 μm), and salt powder density is also small. Therefore, loose packing and less mixing of different layers for salt powder is observed in the die at the end of filling stage; whereas, for iron powder the layers are more mixed in the die (and partially in the shoe powder bed) possibly due to higher friction and cohesiveness. In spite of these differences, the results indicate that the salt with its better coloring attributes, and resulting image quality during the die filling experiments, offers an efficient medium to analyze the macroscopic flow behavior of the PM powders during the PDF process. As mentioned earlier, images of the iron PDF process were also recorded using a high-speed camera with a close view of the die cavity to observe other qualitative features of powder flow. Some portion of powder falls over the die-core when the shoe starts filling the cavity. This powder over the die-core drops into the die in a direction perpendicular to the shoe motion (i.e., cross-flow occurs) when the shoe further moves toward the center of the die. For SS, the flow of the powder appears to occur in two directions, i.e., in the direction of the shoe motion as well as perpendicular to it and over the die-core (Fig. 12a). Also, during PDF process with SS, turbulent flow of powder was observed resulting in fine particle dust cloud over the avalanche in the die (Fig. 12b) whereas with SPP a far less intense flow and negligible dust cloud was generated (Fig. 12c). High
32o
(a) Steps in the
20
(b)
5o
o
powder bed
(c)
(d)
Fig. 11. Powder flow patterns in tri-layered iron powder bed at the end of forward pass, (a) image of SS, 170 mm/s, (b) image of SS, 385 mm/s, (c) image of SPP, 170 mm/s, and (d) image of SPP at the end of the forward pass, 385 mm/s.
D. Aole et al. / Powder Technology 232 (2012) 7–17
15
Flow from a die-core
Higher velocity particles
Flow from perforated plate holes
(a)
(b)
(c)
Fig. 12. High speed camera images of powder flow in the die, (a) cross-flow of powder, SS, 170 mm/s, (b) avalanche with dust cloud, SS, 170 mm/s, and (c) avalanche with minimal dust cloud, SPP, 170 mm/s.
speed images resulted in 28° and 24° dynamic angles of repose for SS and SPP fillings respectively. These angles are slightly higher than the static angle of repose (i.e., avalanche angle) implying that the powder behaved differently in static and dynamic modes. Also, due to more uniform and controlled flow from the perforated plate, the dynamic angle of repose in SPP filling was slightly lower than the SS filling. 3.6. Filling rate as a function of shoe speed Average filling rate of iron powder were measured as a function of shoe speed for SS and SPP fillings following a test procedure described earlier. The filling rate increased non-linearly with shoe speeds ranging from 120 mm/s up to 385 mm/s, for SS filling (Fig. 13). The highest filling rate was achieved at 385 mm/s and the die was almost filled in one pass at this shoe speed. Thus, the critical shoe speed with SS is 385 mm/s above which the filling rate will start decreasing. However, for SPP case, the filling rate increased initially and reached at peak at 170 mm/s and then decreased with increasing shoe speed. More than one pass was required to fill the die above the shoe speed of 170 mm/s. Thus, the critical shoe speed for die filling with SPP was significantly lower than with SS. The reason is that, at higher shoe speeds with SPP, the trailing side (right-side of the die cavity) remained partially filled, as
165
3.7. Surface porosity results As stated earlier, surface porosity data at 6 specific locations was obtained from the high resolution TDF images using a quantitative image analyzer system (see earlier Section 2.4 and Fig. 5a). For PDF process using SS, results showed differences in porosity distribution in the top and bottom regions of the die. The top locations T1 and T3)
SS 60
SPP
Top
50
% Porosity
Filling rate, (g/s)
190
the powder passed very slowly through the holes on the rear side of the shoe and the front stream quickly moved over the die. The amount of powder that passed through the holes depended critically on the holediameter and hole-spacing. The filling rate results with SS filling are in good agreement with the results quoted by [9] for a ring cavity using square shoe. They found incomplete filling of the circular cavity at speeds of 200 mm/s and 500 mm/s, with a single pass of the shoe. They also quoted that the amount of discharged powder was higher for the lower shoe speed. With the addition of a hopper to the feed shoe, the results were slightly improved for SS filling but remained largely unmodified for SPP. In this case, bottom layer delivered more powder than the top layers in the die. Since, powder interlocking at the shoe bottom increases with an increase in the powder level; the particles are more constrained in flowing through the holes into the die. However, for SS, the hopper placed on the front of the standard shoe forces powder to flow more intensely into the die (nose flow) increasing the filling rate.
140 115
Bottom
40 30 20 10 0
90 120
170
240
385
Shoe speed, mm/s Fig. 13. Plot of average filling rate of iron powder as a function of shoe speed.
1
2
3
Point location Fig. 14. A comparison of percentage porosity data between top and bottom locations within the die from TDF images using SS.
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D. Aole et al. / Powder Technology 232 (2012) 7–17
Shoe motion
exhibited higher porosity compared to the bottom locations B1 and B3, whereas top location T2 showed slightly less porosity compared to B2 (Fig. 14). In general, the porosity data from TDF showed nonuniform distribution. Similar results for SPP indicate a more uniform porosity at the above specified locations on the die face, a very desirable characteristic from the technological perspective (Fig. 15). By comparing SS and SPP porosity data, it can be seen that the percentage surface porosity for T1, B2 and T2 locations is decreased with the use of SPP, whereas contrary is observed for locations T3, B3, and B1. Therefore, the results indicate that there is no significant benefit in terms of a reduced ‘net’ porosity with the use of a perforated plate.
Six different samples were cut from the resin-bonded ring shaped part retrieved from the die for Gamma ray densitometer experiments to obtain porosity percent data. These samples are marked L3, L2, L1 for the left side and R1, R2, and R3 for the right side of the ring part in Fig. 16. The porosity percent in the different sections are presented in the graph at the bottom of Fig. 16. It is to be noted that the numbers 1, 2, 3, 4, 5, and 6 along x-axis in the graph denote L3, L2, L1, R1, R2, and R3 samples respectively. As evident, the average porosity percentage in the left region is about 8% higher than in the right region for a SS filled sample. However, for SPP filling, the average porosity percentage in the left region is only 0.6% higher than in the right region. The porosity distribution in the sections L1 and R1 closer to the central axis (sections parallel to the shoe motion) are more uniform in SPP sample than in SS; due to uniform flow of powder form the perforated plate holes. However, porosity distribution is slightly uneven and higher in the sections L2, L3, R2, and R3 (perpendicular to the shoe motion) for SPP filled samples. This is consistent with the results reported by [4] for a ring-shaped die. Also, [12] have studied 3D density distribution in a narrow ring cavity using X-ray tomography where similar results have been reported. The regions L1 and R1 show less porosity as compared to those away from it (such as L3 and R3) due to incomplete filling in the regions perpendicular to the shoe motion. It is noted that, at the beginning of the die fill stage, predominant nose flow results in faster evacuation of air from the die. However, as the filling proceeds and the shoe moves to the center of the die-cavity, bulk flow of powder starts from the bottom of the shoe. As the standard shoe starts from rest the powder mass inside the shoe moves toward the back of the shoe-wall, which results in close packing of the particles in the rear portion of the shoe. Thus, the packing pressure applied by the powder mass in the shoe over filled die-cavity is greater on the right-side side than left-side of the die-cavity. As described earlier, the phenomenon of two-directional powder flow is more prominent in SS than SPP. Further, in the filling process with SPP, powder level inside the shoe changes slightly during shoe motion, and maintains uniform packing weight on the left and right sides of the die-cavity. Consequently, for the case of SPP, the porosity at locations L1 and R1 is more uniform than for SS. The results of Gamma
% Porosity
60 50
Top
40
Bottom
30 20 10 0 1
2
3
Point location Fig. 15. A comparison of percentage porosity data between top and bottom locations within the die from TDF images using SPP.
% Porosity
3.8. Gamma ray densitometry results from resin-bonded iron sample
Sections Fig. 16. Porosity distribution in a resin-bonded iron sample.
ray densitometry suggest that the current design of perforated plate makes the powder distribution in the die-cavity more uniform, and consistent with the earlier TDF data of Fig. 15. Gamma ray densitometry is an efficient way of measuring the voluminous density in a thick PM sample. Gamma ray spread in all directions, thus sample cutting can be minimized or eliminated, which will reduce the loss of particles from the surface, whereas in X-ray tomography the part has to be thin sectioned. From this study, it is established that gamma ray densitometry is an efficient way of measuring the voluminous density in a thick PM sample. Gamma ray spread in all directions, thus sample cutting can be minimized or eliminated, which will reduce the loss of particles from the surface, whereas in X-ray tomography the part has to be thin sectioned. 4. Conclusions The usefulness of incorporating a perforated plate at the bottom of the feed shoe to improve the density uniformity in the filling condition is successfully assessed. It is noted that a stand-pipe flow exhibits controlled flow, and reduces turbulence of powder flow during filling; thus, uniform distribution of powder is obtained. Another benefit of adding a perforated plate is that the powder mass in the holes of perforated plate holds equal amount of powder; thus, the packing pressure is uniform on the leading and trailing side of the die-cavity. Further, the filling rate experiments demonstrated that a low filling rate is obtained at high shoe speed with the current design of perforated plate. Thus the critical shoe speed with SPP filling is lower than SS filling. The reason is that the flow rate of powder from the front portion of SPP is higher than the rear portion as observed from the high speed camera images. Eventually, new designs of the perforated plate by changing the hole-diameter, orientation of the holes or by changing the thickness of the plate need to be developed to extract more benefits from the perforated plate. The colored salt powder experiments have revealed various aspects of powder flow and segregation that are not readily apparent with regular commercial iron powder. Although the size and morphology of the salt particles is different from iron particles, still it is found that the salt powder profiles in the shoe and sequence of segregation of layers and flow patterns closely mimic the iron powder flow patterns. Therefore, use of colored powder as a layered medium within the feed
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shoe makes the PDF investigation more insightful and informative. The effective area of powder discharge at different shoe velocities for both SS and SPP is also clearly identified using colored salt powder. It is also concluded that the avalanche characteristics of a powder alter with the use of a perforate plate at the bottom of the shoe as well as by changing the initial arrangement of powder bed inside the shoe (i.e., tri-layer or tri-column). The major limitation of salt powder is its hygroscopic nature but this was overcome by drying the salt powder with a fan prior to PDF experiments. With the pixel based analysis of the images of transparent die face (TDF) sections; it is possible to acquire spatial surface porosity data. Therefore, a rapid non-destructive and quantitative measure of the spatial fill density is possible. The results demonstrate the positive effect of perforated plate addition to the bottom of the shoe in terms of the powder distribution and flow control. The resin bonding method for measuring the die-fill density at the end of die filling was satisfactorily utilized for binding iron powder particles in the loose un-compacted state within the die at low temperature. This test has uniquely used a polymer resin which has the advantage of binding the die fill powder at low temperature without significant displacement or redistribution of particles during the bonding process. Voluminous porosity measurements in the PDF state from different sections of a ring shaped resin bonded sample could be successfully made using gamma ray densitometry. This method could be potentially applied to optimize PDF in industrial parts. Acknowledgments The authors wish to express their special thanks to Mr. Roger Lawcock of Gates Canada Limited for his encouragement and advice during this research.
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