Production of structurally sound parts by any rapid prototype technique is essential, because
fully functional features are necessary where application testing is required. In the present work,
a powder blend of A1203/AI (3:1 by weight) was mixed with ammonium dihydrogen phosphate
and subjected to selective laser sintering (SLS) using a C02 laser. An attempt has been made
to increase the powder bed density by introducing vibration to the part cylinder. These SLS
processed preforms were then subj ected to a secondary heat treatment in a hydrogen
atmosphere and to hot isostatic pressing. Densification behavior of these Al20 3/Al composite
preforms is discussed.Mechanical Engineerin

Bourell, D.L.

Marcus, H.L.

Rao, T. Srinvasa

English

Texas ScholarWorks

DENSIFICATION BEHAVIOR OF SLS PROCESSED AI203/Al COMPOSITET. Srinivasa Rao+ ; D.L. Bourell* ; H.L. Marcus**+ Regional Engineering College, Trichy-620 015, India* Center for Materials Science and Engineering, TheUniversity of Texas at Austin, Austin, TX 78712** Institute of Materials Science, The University ofConnecticut, Connecticut 06269ABSTRACTProduction of structurally sound parts by any rapid prototype technique is essential, becausefully functional features are necessary where application testing is required. In the present work,a powder blend of A1203/AI (3:1 by weight) was mixed with ammonium dihydrogen phosphateand subjected to selective laser sintering (SLS) using a C02 laser. An attempt has been madeto increase the powder bed density by introducing vibration to the part cylinder. These SLSprocessed preforms were then subj ected to a secondary heat treatment in a hydrogenatmosphere and to hot isostatic pressing. Densification behavior of these Al20 3/Al compositepreforms is discussed.INTRODUCTIONRapid technological developments are driving increased global competition for compresseddesign and manufacture of new products. In recent years, a number of solid freeformfabrication (SFF) techniques have been developed to produce rapid prototype solid objects.These SFF techniques reduce time and cost of production substantially compared toconventional methods of fabrication of prototype solid parts. However, parts produced bymany of these SFF techniques exhibit inferior mechanical properties relative to conventionallyproduced counterparts. The selective laser sintering (SLS) process, one of the SFF techniques,has commercial feasibility for the production of solid parts using polymeric and wax materials.Presently, the SLS process is in the developmental stage relative to production of metallurgicallysound and fully functional parts from metals, ceramics and composite materials [1-6].Functional features in the prototype solid part are essential where applications testing isrequired. Successful processing of these materials, however, depends on process parametersdescribed elsewhere [7]. The aim of the present investigation is to study densificationbehavior of an A1203/AI composite system under the following conditions: a) introduction ofvibration to the part cylinder to increase powder bed density before SLS, b) secondary heattreatment of the SLS processed preforms in hydrogen and c) hot isostatic pressing (HIP) of theSLS processed preforms.EXPERIMENTAL DETAILSElectronics grade calcined alumina powder and aluminum powder used in the present studywere uniformly mixed in the weight ratio of 3:1, respectively. To this powder blend, 20% by374weight ammonium dihydrogen phosphate, NH4H 2P04 was added as a binder. Thecharacteristics of these powders are highlighted in the T~ble. After proper blending, thecomposite mix was subjected to SLS using a C02 laser. The process parameters maintainedduring SLS were as follows:Incident PowerBeam Diameter=14W=0.5 mmScan Speed = 6.35 cm/sScan Line Spacing = 5 mils (125 !lm)TABLE: Characteristics of the powders used in this studyMaterial Average particle Shape Purity Sourcesize (!lm) (%)Alumina 15 Flake 99.55 Norton CompanyAluminum 20 Spherical 99.00 Johnson Matthey CatalogAmmoniumCompanydihydrogen ( 20 Flake 99.95 Johnson Matthey Catalogphosphate* Company* received in crystalline form and ground to (20!lm sizeTo study the effect of powder bed density on the SLS processed preforms, the powder beddensity was varied before SLS. A stainless steel disc weighing 300 gm was placed on thepowder bed surface and with an electric engraver on top of it, and the powder bed was vibratedfor a period of 0 to 30 seconds to generate a wide range of powder bed density. To remove thebinder from the composite system and to study the densification behavior, these SLS processedpreforms (with an average relative density of 46%) were subjected to a secondary heat treatmentin a hydrogen atmosphere at two different temperatures, 800T and 850°C. Soaking time wasvaried from 2 to 10 hours and the preforms were furnace cooled. Volumetric shrinkage wascalculated by measuring the change in dimensions of the preforms after the heat treatment. Anattempt was also made to study the densification behavior of the SLS processed preforms duringHIP. Initially, SLS processed preforms were encapsulated in pyrex glass before HIP. Thetemperature, pressure and duration of HIP were 940±10°C, 10,000 psi and 60 minutes. Densitywas calculated by dividing the mass of the samples with nominal dimensions of 10.0mm X10.0mm X 3.0mm (± 0.3mm variation) by their true volume determined from linear dimensionsand relative density was calculated by dividing the actual density of the SLS processed preformsby the theoretical density of the composite system.RESULTS AND DISCUSSIONIt is known that once the powder bed is tapped or vibrated, there is a change in packingconfiguration of the powder bed from loose random packing towards close random packing. Thisresults in improvement in the packing density. An attempt was made to study the densificationbehavior of the SLS processed preforms with the powder bed density. Figure I shows the effectof vibration to the Al20 3/Al + NH4H2P04 composite powder bed. Curve a represents thepowder bed and Curve b represents the SLS processed preform. The relative density of thepowder mix in the loose random configuration is 35% and increases up to a maximum of 42% asvibration time is increased. No further improvement in packing density was observed beyond37525 seconds of vibration time. When the powder bed was subjected to after vibration,improvement in the relative density of the SLS preform was seen. indicatesthere is a possibility for improving the initial density of the SLS processed preform by as muchas 27% by vibrating the powder bed. Figures 2a and 2b are scanning electron micrographs ofthe SLS processed A1203/A1 composite preform with 46% relative density. demonstratethe wettability of the ammonium dihydrogen phosphate. Figure 2a represents a lowregion and figure 2b represents a high density region. In both micrographs, it is thatAl203/Al particles are completely wet and covered by the ammonium dihydrogen phosphate.Despite good wetting, part overall density remained less than 50%.Though ammonium dihydrogen phosphate was used as a binder, like most its presencein the system reduces high-temperature properties of the composite. To remove phase fromthe composite system, SLS processed preforms were subjected to a heat treatment a hydrogenatmosphere. After the heat treatment, in spite of volumetric shrinkage of the processedpreforms, the overall density of the preforms was not improved, and in some cases,actually decreased. This is attributed to the loss of mass due to the evolution of NH3 andduring the heat treatment. Figure 3 shows the effect of the heat treatment temperature and timeon the percent volumetric shrinkage. Curve a represents 800°C and Curve b represents 850°C.Volumetric shrinkage increases with increasing temperature and holding time. This increasethe volumetric shrinkage is higher at initial stages of holding time and tends toward saturationafter 8 hrs holding time. Figure 4 is a scanning electron micrograph (cross section) of the heattreated (850°C for 8 hrs )SLS processed preform. The preform has extended interconnectedporosity, and the glassy phase enveloping the A1203/Al particles as shown in figure 2b isremoved. However, an earlier study of this binder in SLS [8] indicated that a ofremains in the system.Figure 5 is a scanning electron micrograph of a HIP SLS processed preform with 76% theoreticaldensity. In spite of the high temperature and pressure used during HIP, full density could not beachieved for this system, and interconnected porosity is still present. Generation of ammoniaand water vapor in the pores may have contributed to incomplete closure of pores.CONCLUSIONSThe salient features of the present study are: a) improvement in initial density of theprocessed preforms up to 27% is observed by increasing the powder bed density via vibration ofthe part cylinder; b) by heat treating the SLS processed preforms, the glassy phasecomposite system could be removed but with concomitant loss of density ; and c) maximumrelative density that could be achieved for this system by HIP is approximately 76%.ACKNOWLEDGMENTSOne of the authors (TSR) would like to thank the Department of Science and Technology,Government of India and Board of Directors of the Regional Engineering College, Trichy-620015, India and Center for Materials Science and Engineering (The University of Texas at Austin)for sponsoring his sabbatical leave under a BOYSCAST Fellowship.376REFERENCES1. D.L. Bouren, H.L. Marcus, lW. Barlow and J.l Beaman,"Selective Laser Sintering Metalsand Ceramics", Int.lPowder Met., 28 (4), 1992, pp. 369-381.2. H.L. Marcus and D.L. Bouren, "Solid Freeform Fabrication Finds New Applications",Adv. Mater. & Proc., 144 (4), 1993, pp. 28-35.3. Wendy L. Weiss and D.L. Bouren, "Selective Laser Sintering of Intermetanics", Met.Trans., Vol. 24 A, 1993, pp. 757-759.4. M.K. Agarwala, D.L. Bouren, II Beaman, H.L. Marcus, and lW. Barlow, "Direct SelectiveLaser Sintering of Metals", The RapidPrototyping Journal, 1#1, 1995, pp. 26-36.5. M.K. Agarwala, D.L. Bouren, lJ. Beaman, H.L. Marcus, and lW. Barlow, "Post-Processingof Selective Laser Sintered Metal Parts", The Rapid Prototyping Journal, 1#2, 1995, pp. 36-44.6. lChristian Nelson, Neal K. Vail, Joel W. Barlow, Joseph J. Beaman, David L. Bouren, andHarris L. Marcus, "Selective Laser Sintering™ of Polymer-Coated Silicon Carbide Powders",Industrial and Engineering Chemistry Research, 34, 1995, pp. 1641-1651.7. lC. Nelson and J.W. Barlow, "Relating Operating Parameters Between SLS Machineswhich have Different Scanner Geometries and Laser Spot Sizes", Proc. of SFF Symposium,Ed. by J.J. Beaman, H.L. Marcus, D.L. Bouren, and lW. Barlow, The University of Texas atAustin, Austin, Texas, 1992, pp. 228-236.8. Uday Lakshminarayan and H.L. Marcus, "Microstructural and Mechanical Properties ofAl203/P205 and Al203IB203 Composites Fabricated by Selective Laser Sintering", ibid.,1991, pp 205-212.37750 .------------------------,4846444240383634 ..........L-.&.....I--L......L.-r...-L..-l-..L-.L............L.-L....l.....I-..l..-I.--'-..&.-L-I--L-..I--.L-L....L.-JI.....J,.-'--........o 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32Vibration time (sec)Fig.l Effect ofvibration on percent relative density of the compositea) powder bed and b) SLS processed preformFig. 2a SEM micrograph of the A1203/AI composite at a low density region(30 seconds vibration, 46% relative density)378Fig.2b SEM micrograph of the A1203/AI composite at a high density region(30 seconds vibration, 46% relative density)10(2J~ 8~~=......-..cl 6f'-l~......-.......(2Jsa 4:=-~>'J( 20L...---L_--I-_-L..._.........._L---J.._--'-_...L-._...a..-.......lL..---L_.......o 2 4 6 8 10 12Holding time(hrs)Fig.3 Effect ofpost-SLS heat treatment temperature and time on percent volumetricshrinkage; a) 800°C and b) 850°C, Hydrogen atmosphere379FigA SEM micrograph of the heat treated SLS processed preform(30 seconds vibration, 46% relative density, HT at 850°C for8 hr, Hydrogen atmosphere)Fig.5 SEM micrograph of the HIP SLS processed preform(30 seconds vibration, 46% relative density, HIP at 940±10°Cfor 60 minutes)380

Densification Behavior of SLS Processed Al2O3/Al Composite

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Densification Behavior of SLS Processed Al2O3/Al Composite

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