244 P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250 Fig. 1. (a) Instron 1136; (b) zirconia disk compaction die set; (c) section view of the tooling.has a ﬁnal dimension of 90 mm after uniaxial compaction and relatively slow speed. Finally, the base plate was replaced by an ejection ring,sintering with minimum distortion. and the green part was pushed out. The die wall was lubricated with mineral oil, which was recommended by the powder vendor. Based on the shrinkage ratios from the preliminary experiments, the dimen-2. Experimental and simulation procedure sions of the die for the ring component was determined as shown in Fig. 2. A previous attempt with a one-piece die design similar to the disk shaped part2.1. Experimental setup and procedures as shown in Fig. 1 but with a diameter of 114 mm resulted in green parts with cracks due to excessive friction during ejection process, which caused a large Two types of experimental setup have been designed and fabricated for this density gradient in the green compact. Therefore, a split die design was adoptedstudy. To acquire a ﬁnal part dimension of 90 mm, green part dimensions need to reduce the friction during ejection as shown in Fig. 2. The Container wasto be larger than 110 mm, which is rather large for a part made by uniaxial designed, so it may be split into two halves to facilitate the ejection process. Thecompaction. Since the ﬁnal target part dimensions were larger than those of two symmetric pieces were assembled by eight screws and two aligning pins.a typical ceramic powder compaction and sintering, a preliminary experiment The core rod was designed to produce a ring shaped part, which was assembledwas performed to compact a cylindrical part with a diameter of 46.4 mm to by two screws and an aligning pin. All the tooling components were made frominvestigate appropriate range of process parameters. Based on the results from A2 tool steel, which were heat treated to the hardness of HRC 55 and werethis preliminary experiment, a die for the larger ring shaped part was designed precision ground.and fabricated. The detailed experimental setup for the compaction and sintering The experiment cycle requires assembly and disassembly of the die com-of both sizes are described below. ponents. First, the Dividable Container is assembled. Next, core rod and base A commercial zirconia powder (YSZ, Inframat Advanced Materials, LLC) plate are placed in the container, and then 380 g of zirconia powder is pouredstabilized by 3 mol% Y2 O3 was used for all the experiments conducted in this into the container. The Punch is placed on top of the powder, which is guided bystudy. The particles have a mean particle size of 0.5 m. the core rod and the container during the compaction. A constant punch speed is The setup for the preliminary experiment is shown in Fig. 1, which includes retained by Instron 1136 material testing system. After holding the punch at thea punch, a base plate, and a die. The components were made by precision milling ﬁnal position until the load is stabilized, it is released at a relatively lower speed.and turning process from A2 tool steel. 50 g of zirconia powder was poured into After the compaction, the Dividable Container is disassembled, and the basethe assembled die/punch set. The setup was then placed in the Instron 1136 plate is detached from the core rod to eject the compact. Finally, the ejected partmaterial testing system for compaction, which had a single acting upper punch. is sintered in an oven. Three sintering procedures (Fig. 3) were used in this study.The compaction process was achieved by a constant punch speed. The punch Procedure II has a higher sintering temperature than Procedure I. Procedure IIIwas held at the ﬁnal position until the force stabilized and was released at a has a very slow heating speed than those of Procedure I and II. Fig. 2. (a) Zirconia ring compaction die set; (b) section view of the tooling.
P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250 245 Table 1 Material properties of zirconia powder  Full density Initial density Particle size β α 6.08 (g/cm3 ) 1.885 (g/cm3 ) 0.53 m (Nominal) 54.3◦ 0.03 E ν d R 206 GPa 0.31 1.53 MPa 0.835 Fig. 3. Illustration of the sintering procedures.2.2. Material modeling for compaction simulation In order to simulate the zirconia powder compaction process, a ﬁnite elementanalysis (FEA) model was developed utilizing the material property data from Fig. 5. Densiﬁcation behavior of the zirconia powder .previous study in literature . It is necessary to recognize the major physicalphenomena that occur during the compaction of ceramic particles. The com-paction process can be divided into three main distinctive stages. In the early deformation. If the stress state is such that Eq. (1) is satisﬁed, the material failsstages of compaction, particles are rearranged (we will refer to this as stage 0 in shearing. At high hydrostatic pressures, the yield surface is described by acompaction). As compaction force is further exerted onto the powders, the rel- cap surface, Fc :ative density (RD, which is deﬁned as the ratio of the density of the compact tothe full density of the material) increases, and compaction is accommodated by R×q 2elastic deformation of the particles (stage 1 compaction). At higher pressure, the Fc (q, p) = (p − pa )2 +compact structure will breakdown with a small amount of particle rearrangement 1 + α − α/cos(β)(stage 2 compaction). −R × (d + pa × tan(β) = 0 (2) In this study, we employed the modiﬁed Drucker–Prager/Cap (DPC) model,which has been widely used in powder metallurgy and ceramic industry (Fig. 4). R is a material parameter that controls the shape of the cap. pa is anIt is a phenomenological model that has been adapted from soil mechanics. The evolution parameter that represents the volumetric inelastic strain driven hard-model is attractive in compaction modeling because it contains features that ening/softening, which is related to hydrostatic compression yield stress (pb ).are in accordance with the physical response of particulate compacts [12,15]. The parameters pa and R may be obtained from compaction experiments. TheThe DPC model at low hydrostatic pressure is a shear failure model, similar to parameter α does not have a physical meaning, but ensures a smooth transitionthose used in granular ﬂow, which reﬂects the dependence of the strength on between the cap and the shear failure regions for numerical robustness. Typi-the conﬁning pressure. This enables the model to predict the strength in tension cally, a small value (α = 0.01–0.05) is used to avoid the situation of α = 0, whichto be smaller than the strength in compression, a concept which is common for will form a sharp corner at the intersection of Fc and FS . This may lead torocks, brittle materials, and pressed powder compacts. In its simplest form, it numerical problems . The geometric representation of the complete yieldis represented by a straight line in the p–q plane, which is also known as the locus is represented in the p–q plane as a limiting curve F(q, p, RD) = 0 in Fig. 4.Mohr–oulomb shear failure line, FS : This form is consistent with the DPC model implemented in the ﬁnite element package ABAQUS, which was used for this study.FS (q, p) = q − d − p × tan(β) = 0 (1) The FEA results were compared with the medium size compaction exper- iments (Ø = 46.4 mm). The material properties used for the simulation arewhere d and β are cohesion and internal friction angle, respectively. If the stress summarized in Table 1. These values were adopted from the experimental workstate is such that the corresponding Mises equivalent stress (q) and hydrostatic of Kim et al.  (3 mol% Y2 O3 stabilized zirconia powder, HSY-3.0, Daiichi-pressure (p) result in a value of F(q, p) < 0, then the stress causes only elastic Kigenso Kagaku Kogyo Co. Ltd., Japan). Fig. 5 shows the densiﬁcation behavior of this powder. Considering the geometric symmetry of the process, only an axisymmet- ric section of the compact was simulated using the commercial FEA software, ABAQUS v6.5. The tooling was represented by rigid elements, whereas the material mesh for the powder consisted of an array of 4-node bilinear axisym- metric quadrilateral elements with reduced integration (CAX4R). 3. Results and discussions 3.1. Compaction of ceramic disk (Ø = 46.4 mm) 3.1.1. Compaction results and discussion As summarized in Table 2, different process parameter levels Fig. 4. The Drucker–Prager/Cap (DPC) model . were investigated. The green part quality was evaluated in terms
246 P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250Table 2Experimental conditions for the compaction (Ø 46.4 mm disk)Case Pressure (MPa) Pressing speed (mm/s) Releasing speed (mm/s) Ejection speed (mm/s) Green Part 1 92.30 0.085 0.004 0.008 Horizontal crack 2 134.5 0.042 0.021 Manual Horizontal crack 3 118.7 0.042 0.008 Manual Horizontal crack 4 65.93 0.042 0.008 2.117 Crack free 5 (3 repeats) 65.93 0.042 0.008 4.233 Crack free 6 (3 repeats) 52.74 0.042 0.008 4.223 Crack freeof the occurrence of cracks. It was found that the most important tion as shown in Fig. 7. In general, the simulation results agreeprocess parameter was the compaction pressure. The effects of well with the experiment. The underestimation of the load atpressing, releasing and ejection speed on cracking were rela- the initial loading stage is most likely due to the inaccuracy oftively small. As long as the compaction pressure was less than the material modeling at low densities. As explained in , theor around 65.9 MPa, cracks did not occur. Typically, a crack material parameters for DPC model at low densities are usu-is initiated by the existence of a sharp density gradient. When ally not obtainable from the material testing experiment: thethe compaction force is reduced, the density gradient of the lower the density, the more measurement noise in the experi-green part also decreases correspondingly. Therefore, a smaller ment. In addition, the powders used in the experiment (Inframatcompression pressure helps to avoid cracks . Fig. 6 shows a Advanced Materials, LLC) and simulation (Daiichi-Kigensosuccessful and an unsuccessful case from the compaction: one Kagaku Kogyo Co. Ltd.) were from different sources, which mayhas a horizontal crack (case #2), and the other is free of crack have contributed to the different loading characteristic, although(case #5). the powders were the same grade. The green part height from the simulation (11.866 mm) agreed well with that of the actual3.1.2. FEA results and discussion part (11.557 mm), and the ﬁnal density of the green part from The loading curve obtained from the experiment case #5 from the simulation (2.554 g/cm3 , at the top surface) also matched theTable 2 was compared with the loading curve from the simula- actual part density (2.575 g/cm3 ). Furthermore, the model was utilized to study the relationship between the density distribution and the crack formation. Thus, a crack-free case (case #5) and a case with cracks (case #2) were simulated. Fig. 8 shows the relative density distribution of the simulated parts after ejection. Following observations have been made from Fig. 8. 1. The highest density in a crack-free case (case #5) occurred at the upper corner of the compact, which agreed with the common practice in the sense that the upper corner of the part experiences the highest compaction pressure . On Fig. 6. Compacted part: (a) Case #2; (b) Case #5. Fig. 7. Loading curve comparison (Case #5).
P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250 247 Fig. 8. Density distribution after ejection: (a) Case #5; (b) Case #2. the other hand, the highest density for the case #2 (which Fig. 9. Density distribution after ejection (ring compaction with a pressure of had cracks in the experiment) occurred below the upper 40.49 MPa). surface.2. Case #2 has a sharper density gradient near the upper corner The simulation results indicated the migration of the loca- region, and the corresponding relative density curves exhibit tion of the highest density region of a compact from the top a sharp distribution. This indicates a sudden change of density surface to below the surface as the pressure was increased. As in a localized area, while the relative density curves of case the pressure increased, cracks developed under the top surface #5 are smoother and corresponds to a more uniform density as shown in the actual part (Fig. 6a case #2). The location of the distribution. crack corresponds to the highest density area in Fig. 8b. WhenTable 3Zirconia ring compaction experiment conditionsCase Pressure (MPa) Green part before ejection Lubrication of the base plate surface Separation between the compact and lower portion of core rod and the base plateRing 1 40.49 Crack free Mineral oil A blade was used for the separation, which resulted in a bad compact surface conditionRing 2 40.49 Crack free Mineral oilRing 3 40.49 Crack free WaxRing 4 40.49 Crack free Aluminum foil Easy separation, but the foil was embedded into the compact, elimination of the foil resulted in a very bad compact surface condition and cracks, Fig. 9 a)Ring 5 40.49 Crack free Coolube 5500 metalworking ﬂuid Failed, cracksRing 6 28.92 Crack free Water based graphite particle Successful lubricant (Lubrodal F705 ALX)Ring 7 57.84 Crack free Oil based graphite particle lubricant Successful, Fig. 9 b) (Lubrodal Hykogeen Conc HI)Ring 8 40.49 Crack free Oil based graphite particle lubricant Successful (Lubrodal Hykogeen Conc HI)Ring 9 52.05 Crack free Oil based graphite particle lubricant Successful (Lubrodal Hykogeen Conc HI)Ring 10 58.3 Crack free Oil based graphite particle lubricant Successful (Lubrodal Hykogeen Conc HI)Ring 11 63.62 Crack free Oil based Graphite Particle lubricant Successful (Lubrodal Hykogeen Conc HI)Ring 12 72.88 Crack free Oil based graphite particle lubricant Successful (Lubrodal Hykogeen Conc HI)
248 P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250the high-density region is located on the top surface, a densitygradient is created from top to bottom as indicated in Fig. 8,case #5. In contrast, as the pressure increased, the high-densityregion shifted under the top surface, and the density gradient wascreated such that it caused tensile internal stress at the locationas indicated in Fig. 8, case #2. It is speculated that the cracksinitiate from these highly dense regions to relieve the internalstress built up from the density gradient.3.2. Compaction and sintering of a large scale ceramicring (Ø = 114 mm)3.2.1. Compaction simulation results and analysis The simulation tool was used to detect potential cracks thatcan form in the ring shaped part prior to designing of an exper-imental setup. The resulting density distribution of the ejectedpart at a compaction pressure of 40.49 MPa is shown in Fig. 9.The relative density distribution was smooth, and no potentialcrack locations could be identiﬁed. This was proved later by theexperiment having the same condition (Case Ring 1 in Table 3).Higher densities were found at the top surface where the movingpunch contacted the powder. The density distribution at the ringinner perimeter is very close to that at the ring outer perimeter.The design change from the disk shape to the ring shape alsoreduced the required load due to the decreased contact area.3.2.2. Compaction experiment results and discussion Pressing speed and releasing speed of the punch also affectthe crack formation of the compacted parts. A too high punchspeed will result in a higher density at the contacting surface sus-ceptible to cracks. Also, a too high releasing speed will discharge Fig. 10. Green parts: (a) unsuccessful case (Ring 4); (b) successful casethe internal pressure too quickly and lead to cracks. Thus, based (Ring 7).on the process parameters used in Section 3.1, and a few trial-and-errors from both simulations and experiments with the new 3.2.3. Sintering experiment results and analysiscompaction die set, the pressing speed and releasing speed was Two biggest challenges encountered during the sintering areselected to be 0.042 and 0.002 mm/s, respectively. As shown in cracks and distortions. Cracks are mostly due to the non-uniformTable 3, the pressure for experiments was selected in the range density distribution induced from the compaction process andof values used in the simulation. The parts showed no cracks the temperature gradient . The distortions that are commonlyafter the compaction; however, a strong bond was formed at observed in cylindrical parts are of a conical shape, which hasthe interface of the powder compact-base plate and the powdercompact-lower portion of the core rod. Hence, the green partswere frequently damaged during the separation process. In order to successfully detach the powder compact from thedie, various lubrication and separation methods were evaluatedas summarized in Table 3. Mineral oil, which was suggestedby the vendor, only seemed to be effective for smaller parts asseen in previous experiments. As the part size became largerand interface area increased for the ring shape part, all thecompacts failed during ejection (Ring 1 and Ring 2). The wax(Ring 3), aluminum foil (Ring 4), and metal working lubricant(Ring 5) also failed to maintain the part intactness while ejectingas shown in Fig. 10a. However, as demonstrated in cases Ring 6through Ring 12, the graphite particle based lubrication greatlyimproved the separation performance as shown in Fig. 10b. Forthe unsuccessful attempts, a mechanical press was used to ejectthe part, whereas the compact could be taken out by hands whengraphite lubrication was used. Fig. 11. Schematic view of the part orientations in the oven.
P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250 249Table 4Sintering experiment conditions and resultsCase Compaction pressure (MPa) Sintering procedure Orientation of the part in the oven CracksRing 1 40.49 Procedure I Bottom up YesRing 6 28.92 Procedure II Sideways YesRing 7 57.84 Procedure III Bottom down NoRing 9 52.05 Procedure III Bottom down NoRing 10 58.30 Procedure III Bottom down NoRing 11 63.62 Procedure III Bottom down NoRing 12 72.88 Procedure III Bottom down NoTable 5Shrinkage and the dimensional differences between top and bottom after sinteringCase Shrinkage in diameter Height after sintering (mm) Diameter difference (mm) Conical taper Top BottomRing 1 0.271 0.282 31.43 1.32 7.22E–02Ring 6 0.292 0.283 32.36 1.04 4.20E–02Ring 7 0.258 0.263 27.78 0.5 1.80E–02Ring 9 0.267 0.278 31.84 0.9 2.83E–02Ring 10 0.256 0.266 30.18 0.83 2.75E–02Ring 11 0.254 0.259 29.93 0.62 2.07E–02Ring 12 0.249 0.254 29.86 0.41 1.37E–02been quantiﬁed by the amount of change in diameter over a unit (Ring 6 was orientated sideways in the oven so that the conicallength (conical taper) in this work. shape was purely due to shrinkage anisotropy). The other is the Three sintering procedures (Fig. 3) and various part orienta- friction drag introduced by the support substrate, which restrictstions (Fig. 11) in the oven were evaluated, and the results are the shrinkage of the bottom compared with the unrestricted topsummarized in Table 4. It was observed that the sintering curve portion (Fig. 12c).had a signiﬁcant effect on the crack formation. The parts sin- Table 5 shows detailed information regarding the shrinkagetered using sintering Procedure III (Ring 7–Ring 12) are free of and the dimensional differences between top and bottom aftercracks while other parts which used sintering Procedure I and II sintering. Case Ring 1 in Fig. 12a demonstrates the combinedhad cracks (Ring 1 and Ring 6). A slower sintering procedure effect of friction drag and shrinkage anisotropy. Since the bottomhelped to prevent cracks by minimizing the temperature gradient of the green compact was placed facing upwards, the distortion. from the density gradient and friction drag will multiply. Mea- According to the classic sintering theory , there are two sured conical taper in this conﬁguration is the largest, and thecontributors to the conical shape. The ﬁrst is the non-uniform result is conﬁrmed by the observation. Therefore, to reduce thedensity distribution of the green part (shrinkage anisotropy). distortion by offsetting the distortion caused by the density gra-Since the bottom of the green part has a lower density com- dient and the friction drag, the bottom of the green part waspared with the top (Fig. 9), the bottom shrinks more than the placed facing down on the substrate. As conﬁrmed by the con-top does and results in a conical shape as shown in Fig. 12b ical taper measurement (Ring 7–Ring 12), the measured taper signiﬁcantly decreased. In addition, the effect of compaction force on the distortion can be observed from Ring 9 through Ring 12. A higher compaction force produced a denser green part, which resulted in less shrinkage during the sintering pro- cess, and therefore helps to reduce conical shape. 4. Conclusions In this study, a large-scale ceramic part (Ø114 mm) was suc- cessfully compacted and sintered using uniaxial die compaction technique. The effects of die design, compaction pressure, lubri- cation, sintering procedure, and part orientation in the oven on the P/M part quality were investigated, and the preferred process conditions were discussed. Furthermore, a FEA tool was utilized to predict the location of a crack for a disk shaped part. On theFig. 12. Illustration of the effect of shrinkage anisotropy and friction drag: (a) basis of the quantitative and qualitative analysis made herein,Ring 12; (b) Ring 6; (c) Ring 1. the following conclusions could be drawn: (1) A compaction
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