World PM2016 - Shaping Manuscript refereed by Dipl-Ing Walter Rau (Dorst Technologies, Germany)
New Opportunities in Centrifugal Powder Compaction 1
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S. Riecker , T. Studnitzky , O. Andersen , B. Kieback (Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Branch Lab Dresden, Winterbergstraße 28, 01277 Dresden, Germany) 1
[email protected] [email protected] 3
[email protected] 4
[email protected] 2
Abstract. Centrifugal sedimentation processes such as the high speed centrifugal compaction process (HCP) are known to produce green bodies with high packing density and very few defects resulting in high quality components. However the producible geometries as well as the freedom of possible gradient designs are limited. At Fraunhofer IFAM Dresden a new sedimentation process route was developed that uses a continuous suspension feed to overcome the limitations of a batchwise sedimentation. The process allows to produce homogenous sediments in terms of porosity as well as mean particle size over the height of the sediment. The green parts are strengthened by a binder infiltration step and can be machined in green state in order to achieve desired shapes with structure’s dimensions down to 200 µm. Having a high green density of 68% and low organic content of 100.000 g. The green compacts produced by the centrifugal compaction are characterized by a high relative green density of 0.63-0.68 [13], [10], [14], high sintered density >0.99 [10], [15], and superior component properties such as high strength, crack resistance or hardness [16], [17]. The high acceleration forces are needed to accelerate the slow sedimentation process and are necessary for the use of small particles at a high particle volume content, which helps to minimize the particle size segregation effect that may occur during a batch sedimentation. Due to this segregation effect, the sedimentation of slurries is also used to fabricate functionally graded materials (FGM) [8], [18]. Most sedimentation processes are limited to small batches, slow sedimentation, and a narrow gradient adjustment. In this research, Fraunhofer IFAM focused on developing a sedimentation technology using centrifugal force with a constant particle flow in order to overcome the segregation effect of different particle sizes and material densities. By utilizing this technology, it is possible to achieve both homogenous sediments and particle size gradients as well as material gradients by controlling the particle flow. Subsequent to the sedimentation process a binder agent can be infiltrated into the sediment to strengthen it for green machining. Experimental procedure Experimental Setup The sedimentation experiments were carried out using a self-built centrifuge with a basket diameter of 280 mm and an inserted sediment ring with an outer diameter of 200 mm (Figure 1). The basked lid had an opening with a diameter of 120 mm, which was used for the drainage of supernatant fluid caused by the continuous suspension flow and to allow the spray nozzle to move into the basket
World PM2016 - Shaping during the sedimentation process. At the outer diameter of the sediment ring, a centrifugal acceleration up to 161 g could be provided by the rotation speed of the basket, which could be varied in the range of 0 Hz to 20 Hz. Suspension storage and metering was controlled by a computer dispensing system. The suspension was mixed with carrier fluid in a mixing unit for better dispersing properties in the fluid of the centrifuge basket. The spray nozzle, a rotary vaporizer, could be moved along the rotational axis of the basket to spread the suspension evenly over the height of the sediment ring. The centrifuge basket and sediment ring could be detached for demolding.
Figure 1: Experimental setup. 1 Carrier fluid, 2 Suspension, 3 Computer control, 4 Mixing unit, 5 Spray nozzle (rotary vaporizer), 6 Centrifuge basket with lid, 7 Sediment ring Experimental workflow For the sedimentation experiments, a 316L stainless steel powder, PF-3F from Epson Atmix Corporation with a D50 of 6.9 µm, was used. Due to its high amount of large irregular particles, the 316L powder is prone to segregate during sedimentation and suspension handling, so it represents a good choice for testing the experimental setup. The powder was dispersed in ion-exchanged water using polyglycolesters- and phosphonic acid ester-based dispersing additives up to a particle volume loading of 60 vol%. The suspension was then mixed for 1 min in a speed mixer (DAC 150 Hauschild Enigeering) under 1000 rpm. The centrifuge basket was prepared by filling it with the fluid and starting the rotation. While the nozzle was being brought into the basket, the carrier fluid was set at a constant flow of 200 ml/min to 300 ml/min, and while using a fixed particle mass flow between 50 g/min and 100 g/min, the suspension was mixed into the carrier fluid in order to achieve a particle volume filling from 4 vol% to 6 vol% in the spray suspension. This spray suspension was suspended onto the fluid’s surface in the basket with a uniform up and down movement of the nozzle. Subsequent to the sedimentation process, the centrifuge fluid was drained and the sediment was dried under rotation. A binding agent was applied on the sediment’s surface in order to infiltrate and stabilize the sediment. After stopping the rotation, the sediment was then demolded and dried.
Figure 2: Workflow of the centrifugal sedimentation process. Samples of the sediment were then either pre-sintered at 920°C or 1000°C or sintered at 1250°C under hydrogen atmosphere. To examine the cross section of the sediment for defects such as cracks, sedimentary layers, and voids, the samples were cut, polished, and investigated by optical microscopy. Porosity measurements were performed on the sintered samples and particle image analysis was done on cross section polished (CSP) green samples utilizing a laser confocal microscope to rate particle segregation and homogeneity over the height of the sediment. Further samples were infiltrated with the binder solution and machining experiments were performed with a 1600 rpm rotational speed, 125 - 160 mm/min traversing speed, and 0.5 mm feed to rate the surface
World PM2016 - Shaping and machinability of the infiltrated samples. The machined samples were then sintered at 1250°C under a hydrogen atmosphere. Results During the development of this process, multiple factors were found to have significant influence on the outcome. This paper describes two systematic effects that lead to sedimentary layers in the green body and shows some first manufacturing results of defect free green samples. The other parameters, such as drying time, varying rotation speed during the process or suspension feed, and suspension composition, are assumed to be good working parameters, and further disturbing effects, such as sedimentation in the dispensing system or impurity inclusions, are not considered in this paper. Sedimentary layer defects In powder suspensions with a wide particle size distribution, a distinct segregation tendency is present during the entire processing time. Whenever a settling of particles occurs, the larger particles tend to fall with enhanced settling speed compared to the smaller particles. Even at volume contents of up to 50 vol%, this segregation effect may occur during settling [14] depending on the radius distribution width of the powder. When only low inter-particle forces are present, as is the case in a dispersed powder suspension, the particles may segregate at low energy input such as vibration or shearmovement of the suspension, which is known as granular convection or the Brazil nut effect. In case of the centrifugal sedimentation process with continuous particle flow, it was found that the homogeneity of the spreading of particles over the sediments surface was a critical aspect in producing defect free sediments. A locally increased feed of particles leads to an increasing piling effect until the upper particles slide down (Self-organized criticality). During this motion, segregation occurs resulting in a local layer pattern with an angle of repose that depends on the powder characteristics and the process parameters (Figure 4). To avoid this effect, a moving nozzle was used to ensure a homogenous spread of particles. The nozzle could be moved along the rotational axis throughout the height of the sediment ring. In doing so, the particle flow at one sediment’s location is not continuous but pulsed. The different particle pulses can segregate along the sedimentation distance leading to a layered sediment structure ranging over the entire cross section of the sediment, which makes it prone to cracks during sintering. This effect was observed in the green parts and heat treated samples of the sediment. It increased with higher rotation speed of the centrifuge basket, slower movement of the nozzle, and shorter sedimentation distance.
Figure 3: Layer defects within the sediment due to piling (left) and a pulsed particle flow (right). Sediments Both the fixed and the moving nozzle setup could be used to produce homogenous sediments if the setup was designed to avoid the mentioned layering effects. Depending on the centrifuge setup, the powder material, and size distribution, it is possible to evaluate a process window where successive particle pulses will mix during sedimentation. In this way, the pulsed particle feed on the fluid surface results in a constant particle feed with varying intensity at the sediment’s surface, which leads to a homogenous packing. A simple process model calculation based on stokes’ law was used to approximate the overlap of successive particle pulses during settling. Porosity and particle size measurements were taken at the top, middle and bottom areas of the sediment, where bottom refers to the sediment’s outer region in the radial direction of the centrifuge
World PM2016 - Shaping basket. In addition, a cross section of the sediment was examined by optical microscopy. No cracks or sedimentary layers could be found in the cross section of the pre-sintered samples, and the porosity measurement as well as the particle size analysis (Figure 4) show constant results over the height of the green samples within the error range of the measurement. The mean equivalent diameter of the particle cross sections within the images validates the concept of using continuous suspension flow to generate gradient free sediments. If sedimentation can be prevented within the dispense system using a stable suspension, the particle feed and thus the particle packing in the sediment can be homogenous.
Figure 4: Porosity measurement of the pre-sintered sample (1000°C) and particle size measurement of the green body in the top, middle and bottom area of the sediment. The measurements show a constant porosity as well as mean particle size over the height of the sediment, which is illustrated by the microscopy images on the right. The green bodies produced by centrifugal sedimentation have a high packing density of about 68 %, and the pre-sintered samples (1000°C) display a homogenous microstructure with a porosity of about 16.5 % (Figure 5). The homogenous microstructure provides good sintering dynamics as can be seen in the samples sintered at 1250°C for only 5 minutes, which lead to a porosity of only 1.6 %. Looking at the concept of the centrifugal sedimentation process, it is possible to upgrade the setup with additive dispensing systems and a larger centrifuge basket leading to an output that is much higher. Arbitrary gradients of the green part can be adjustable by controlling the different suspension feeds. However the sediment’s geometry is depending on the geometry of the sediment ring that is feasible in the centrifuge setup.
Figure 5: Microscopy images and porosity of the pre-sintered samples and sintered sample show the local homogeneity of the particle packing and good sintering performance. Some bigger voids can be attributed to hollow particles. Green machining of sediments Machining experiments were performed on binder stabilized green samples, and the machinability was tested by line patterns with different linewidths from 100 µm to 500 µm and by drilled holes with a diameter of 2 mm. The material is soft to machine, and very few defects were produced during the tests. The surface roughness Rz was 18 µm, which means it is within the range of the biggest particles
World PM2016 - Shaping of the powder. Small line structures of 300 µm could be machined with an aspect ratio of h/w = 7 (Figure 6), and the minimum linewidth of the structures produced without defects in the tests was about 200 µm. Since the green structures have a low volume fraction of organic content of less than 1 wt.%, they can be thermally debinded and sintered at 1250°C without warpage or cracks.
Figure 6: Exemplary machining result (green state) of one of the binder strengthened samples (left) and line scan of the test structure with a 300 µm line and a line that is stepwise thickening towards the bottom starting at 150 µm. Conclusion Centrifugal powder compaction processes are known to produce green bodies with high packing density and good sintering dynamics. The segregation effect of different particle sizes can be used to produce functionally graded materials, but the adjustment possibilities are limited. Furthermore, the segregation effect can’t be easily suppressed when sedimentation processes are designed for batchwise production. A new centrifugal sedimentation process with a continuous particle flow has been developed which overcomes the segregation issues and produces green parts with a homogenous particle size distribution and density over the height of the green sample. Sedimentation experiments show the importance of a continuous particle feed that is homogenously spread over the surface of the sediment: A locally increased particle feed can cause piling and segregation during the particle slip, and a pulsed particle supply tends toward segregation over the sedimentation distance to produce sedimentary layers in the entire sediment. By choosing the right process window of rotation speed and nozzle movement, the sediments could be made to exhibit a homogenous mean particle size and porosity over the height of the sediment, and green samples formed through this process could be produced with a green density of 68 %. After binder infiltration, the green parts could be machined with a high accuracy of about 200 µm and a low surface roughness of 18 µm. Due to the low organic content of less than 1 wt.% in these green parts, the samples could be easily thermally debinded and sintered to high density. The process could be upgraded to multiple suspension dispense systems to achieve adjustable gradient structures and could be upscaled to high output. References [1]
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