Alumina powders of 15~m size and 2~m size were processed by SLS using PMMA and a
copolymer. The 2~m powders were agglomerated and mixed with the polymer powder before
being processed by SLS. SLS bend strength specimens were made with parts built along
different orientations. The variation of the strength with incident energy density and with
orientation was studied.Mechanical Engineerin

Barlow, J.W.

Kamatchi Subramanian, P.

Marcus, H.L.

Vail, N.K.

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Anisotropy in Alumina Processed by SLS.P. Kamatchi Subramanian, N.K. Vail, lW. Barlow and H.L. MarcusCenter for Materials Science and Engineering,The University of Texas at Austin.AbstractAlumina powders of 15~m size and 2~m size were processed by SLS using PMMA and acopolymer. The 2~m powders were agglomerated and mixed with the polymer powder beforebeing processed by SLS. SLS bend strength specimens were made with parts built alongdifferent orientations. The variation of the strength with incident energy density and withorientation was studiedIntroductionSelective Laser Sintering (SLS) of ceramics is very challenging because of the highmelting points of the ceramics. In SLS one of the operating parameters, the energy density isdefined as the amount of laser energy input per unit area1 in each layer. In the case of aluminato form lines of sintered material energy densities of the order of 1000 callcm2 is required. Evenat this high energy densities the parts formed do not hold together because of the huge thermalgradients. One solution is to use a low melting temperature second phase material which willmelt under the laser and bond the particles together. This second phase may be inorganic2,3,4 ororganic5,6. This paper describes some of the SLS experiments of alumina with organic binder.This paper focuses on the variation of properties of the green shapes with processing parametersincluding the orientation of scanning in the SLS process.ExperimentsTwo kinds of alumina powder were investigated in this study. They were i) 2~m powderfrom Golden Technologies and ii) 15~m powder from Norton Corp. SLS of 2~m as receivedpowder was unsuccessful using a polymer binder even at compositions of 35 Vol % polymer. Inan attempt to overcome this problem it was decided to agglomerate the particles. Through aseries of experiments of heating to various temperatures for varied lengths of time it was decidedthat 1400e for 3hrs would give a desirable agglomeration. The 2~m powders were heated in akiln to 1400e in 12 hrs at a constant heating rate and held at 1400e for 3hrs. The powder wasalso cooled down in about 12hrs. This produced agglomerates of an average size of 1OO~m. Theflow diagram is shown in Fig.l.The organic binders used were i) PMMA and ii) a copolymer, Poly[methylmethacrylate-co-butyl methacrylate] (80 mol%MMA/20 mol%BMA). The preparation of these polymers isexplained elsewhere7. The PMMA had a melt index of 31.5g/10 min and the copolymer had amelt index of 11.6g/10min as 111easured following ASTM D1238 at 200e with an extrusionpressure of 0.52 MPa (75psi).3302JJ.m alumina powder 88 Polymer emul810n PMMAreceived or CopolymerAgglomerationSpray drying emulsion toI) Heat gradually to 14000 obtain PMMA powder orII) Hold at 14000 for 3hrs Copolymer powderIII) Gradual CooIdownMix agglomeratesand polymer powder~SLSFigure 1. Processing of 2Jl1D Alumina powderii i1BuildDirectionFigure 2. Orientations used in building the bend strength specimens. i:build alongthickness, scanning along width; ii: build along thickness scan along length; iii: buildalong width with scanning along length; iv: build along width, scan along thickness331The 15Jlm alumina powders were spray dried with the copolymer in a Anhydro spraydrier with inlet temperature of 200C and outlet temperature of 80C. The agglomerates of 2Jlmalumina were mixed with the polymer powder obtained by spray drying the polymer emulsion.Two mixtures were made, i) the agglomerates of 2Jlm alumina mixed with the spray-driedPMMA powder and ii)the agglomerates of 2Jlm alumina mixed with the spray-dried copolymerpowder. The spray-dried and mixed powders were then SLS processed.The layer thickness was kept constant at 175Jlm. The laser scanning speed and laserscan line spacing were varied along with the power of the laser to change the incident laserenergy density. The scan line spacing was varied between 75Jlm and 125 Jlm and the scan speedwas varied from 30cm/s to 150 cm/s. The laser power was varied between 6 and 14W. Theenergy density was varied between 2 cals/cm2 and 8 cals/cm2. Three point bend strengthspecimens 0.076mxO.025mxO.00625m (3 I x1 I xO.25") were made by SLS. The specimens weremade in 4 orientations i) built along thickness with scanning parallel to width ii) built alongthickness with scanning parallel to length, iii) built along width with scanning parallel tothickness and iv)built along width with scanning parallel to length as shown in Fig.2.Results and DiscussionSLS of parts from both the spray-dried and mixed powders produced strong green parts.For the case of the spray-dried 15Jlm powder built along thickness with scanning parallel towidth, condition (i), the variation of density with incident energy density is as shown in Fig.3.The density varies little with incident energy density. The variation of bend strength of the greenparts with incident energy density is shown in Fig.4. It may be seen that the green strengthincreases initially with energy density because of better melting and wetting of the alumina bythe molten polymer. At still higher energy densities the strength decreases due to decompositionof the polymer.Scanning along the length while building along thickness, condition (ii) produces a lowergreen strength at lower energy densities than those scanned along width and built alongthickness, condition (i). This anisotropy is due to the region scanned by the laser having longertime for heat transfer between two consecutive scans for condition (ii). At higher energydensities Le., around 6cals/cm2 the strength of the samples made by scanning along length has ahigher strength due to degradation of polymer for condition (i). Even in the case of scanningalong length we have a decrease in strength at higher energy densities. But this decrease occursat still higher energy densities.With increase in energy density, higher temperatures are reached in the sample beingmade. As temperature increases well above the glass transition temperature, the degradation ofthe polymer increases. Temperatures in the decomposition regime are attained at lower energydensities in the case of samples with shorter scan lines because the time between successive scanoverlays is smaller. In the case of samples made with longer scan lines the temperatures in thedecomposition regime are attained only at a higher energy density because there is some time forthe heat to be dissociated between successive scan overlays.In the case of parts built along width with scanning parallel to thickness, condition (iv),the green strength increases with energy density at low energy densities, Fig.5. At higher energy33242.79~.......fI'j=<UQ36.67 .;~~1.41.31.21.11.5,..-----,.-----,--------,-----,-1----, 45.85j!i coqdition i ! I____._ ~._ _ _.__i.--_._ __.__._.._ _.._..l. ._ _ i _ ..__ _j ! 1 I1 j I iIii I·--i-o-+---+-_·-··..·······/'·····_··_·_·_--i I 8' ,j j I!808~ 0 j 01 0--_....-t·_..__....__....··_·-,·__......·_......- .._·r---·_..·_.._..1··-......__....._ .._.I I i Ii l ! I'I I ! I. : ! II I I I---.--•.---.-.-- ----t..----.-.-.- - --+--- -- --.- , --.--__.._., I ' II ! j , Ii I I iI ! ! jI : ! :ii I1.01.'---'--'--'--'-2±-'--1..-...L-'--3~--'---'---'--4-7-'---L--JL.-..L..--;;-5 .....L..--I---J-..L.......J630.55Energy density (callcm2 )Figure 3. Green density of samples after SLS from 151lJ11 alumina coated with copolymer..0 ! I Il-Icondition il- II-~ IA..UI-~ I Il- Il- I i,., I --.U ~I- 10 u j lSI condition..I II0 10 I- I9 I- ()0010 I lSI....,'" i.U 0 I I- III- II- 00 lSI ,lSI'" i ;.U~lSI I~ 0 lSI I~ f1 lSIA..U- ~ I 0- lSI~ I f- 100-I!! 0- I00I I I I I I I I I I ...L60.10014018022030026020. 1 3.25 5.5 7.75 10Energy density (cal/cm2)Pigme 4. Bend strength of samples from lSJ.lI11 alumina coated with copolymer built alongthickness. 3336234 5Energy density (cal/cm2)501250- condition iv... IfIjfQ. 200 0 condition iii-- I.c... 0 0c.l= 0 0 §~ 150 0...... 0('J.)100 ~t:IFigure 5. Strength of samples from 15J.l.m alwnina coated with copolymer built alongwidth.~.- 775U)a.""'"".c 550... 0 condition iiC) 0C § 0CD 0a- D 0...08 8 0\en 325 0D§O condition ii_~__-.i~,_ .._.;_~_,....L_J1001 3.25 5.5 7.7J 10Energy density (c a IIc )Figure 6. Strength of samples from agglomerated 2J.Un alumina mixed with PMMA builtalong thickness. 334densities the strength decreases for reasons stated before. Once again it may be seen that whenthe scan lines are longer the green strength obtained at lower energy densities is lower. Againthe peak in the strength vs. energy density plot occurs at a higher energy density for the partsmade with longer scanlines.SLS of agglomerated 2J1m alumina mixed with 30 V% PMMA.Specimens for bend-strength were made from this mixture in the four orientationsdescribed above. For specimens built along thickness, conditions (i) and (ii), Fig.6 it may beseen that the maximum strength is higher than that obtained for the 151lm alumina spray-driedwith copolymer. This is due to the fact that the alumina agglomerates are much larger than 151lmand hence have a lower surface area to be wet by the polymer. The polymer content is alsoslightly higher. The variation of green strength with energy density follows similar trend as inthe previous case with the strength increasing with energy density at lower energy densities. Thepeak in the strength vs. energy density plots occurs at a higher value of the energy density for theparts made with longer scan lines.When specimens were built along the width at low energy densities the parts made withshorter scan lines, condition (iv) had higher green strength, Fig.? A slight increase in densitywith increase in energy density may also be observed, Fig.8.SLS of the mixture ofagglomerates of2J1m alumina with 20 V% copolymer.Specimens for bend strength measurement were made from this mixture by building onlyalong the thickness direction. Three different scanning methods were employed. They were i)scanning along width, ii) scanning along length and iii) scanning alternate layers in length andwidth. The density increases slightly with increase in energy density, Fig.9. Scanning along thelength produces samples of lower density compared to the other two modes at lower energydensities. In this case also it may be seen (Fig. 10) that the strengths are lower for scanning alongthe length at low energy densities, as described earlier. The case of mixed (alternate) scanningresults in samples of strength intermediate between the two other modes. This is to be expectedsince alternate layers will have higher and lower strengths corresponding to scanning in the othertwo modes.SummarySuccessful SLS was done on 21lm alumina particles by agglomerating the powders andmixing with polymer powder prior to Laser Sintering. Samples were built in differentorientations in the SLS process. In all cases the samples that were built using shorter scan lineshad a higher strength at lower energy densities. For both longer and shorter scan lines the greenstrength increases initially with increase in energy density, attains a peak and then startsdecreasing with further increase in energy density due to polymer degradation. The peak in thecase of samples built with longer scan lines occurs at a higher value of the energy density due tolower temperatures reached under those conditions.3355condition iv4 ~0,-....eu~3 0 0 0~ 0.......,/..c 0 00 § 0...... 00CI) 2c00 8 0<» condition...a-lii.....rJ:J 01o1 23456Ener2Y Density (cal/cm2)7Figure 7. Strength of samples from agglomerated 2Jllll alumina mixed with PMMA builtalong width.1234567Power density (cal/cm2)Figure 8. Density of samples as SLS for mixture of 2J.Ut1 alumina mixed with PMMA.1.61.51.41.31.21.1•••III~...fI)=~Q•41.49 ~~'---I-.&.-.L--L.-J.....!--I.-A.--J...-L--J...-L--J...-L--L.-.L.-L-..L.-J..-L..-J..-L..-J..-L......l-J......l...-..l-....L.-L......I....l-...L.J......J 35.11833632.24101.801.701.60.......(.)ib 1.50'-'".~1.40CI'.l5Q1.301.201.101I I !• I.-----t--...--..-.-.11 ..------·-t---·---·-..-·-....·-..Width I------L---..-----......--..-.------~.-------.---I L 10-----+---.-9-- -- -----l~~ ---..----.~! aOOOo Iog·:1 .Eta . 00'. q§J i..........._ ~. , _ _ 0_ , _ l..ength .CV IJB I Iio i =to... ....__._ j : .11~------ -r=1 j iI I 1I i3.25 5.5 7.75Energy Density (caVcm2)52.7547.6237.37Figure 9. Density of samples from agglomerated 2J.U11 alumina mixed with copolymer builtalong thickness.5---.~ 4~ Width Length~'-"/ Mixed/.c..., 3c.>== 0~ ~~~L......,fI)2 ~og BO~"0==~= () 01 S~ B 0BB00 2 4 6 8 10Energy Density (cal/cm2)Figure 10. Strength of samples from agglomerated 2J.U11 alumina mixed with copolymerbuilt along thickness.337AcknowledgmentsFinancial support of this research by DARPA/ONR grant N00014-92-J-1394 is acknowledged.References1. J.C. Nelson, Ph.D. Dissertation, The University of Texas at Austin, 1993.2. U. Lakshminarayan and H. L. Marcus, Proceedings of the Solid Freeform Fabricationsymposium, Ed. H.L. Marcus, J.J. Beaman, D.L. Bourell, J.W. Barlow and R.H. Crawford, 1991,p.2053. U. Lakshminarayan and H. L. Marcus, Proceedings of the Solid Freeform Fabricationsymposium, Ed. H.L. Marcus, J.J. Beaman, D.L. Bourell, J.W. Barlow and R.H. Crawford,1992,p.444. P.K. Subramanian, G. Zong and H.L. Marcus, ibid. p. 63N.K. Vail and J.W. Barlow, ibid. p. 1246.P.K. Subramanian, G. Zong, N.K. Vail, J.W. Barlow and H.L. Marcus, Proceedings of theSolid Freeform Fabrication symposium, Ed. H.L. Marcus, J.J. Beaman, D.L. Bourell, J.W.Barlow and R.H. Crawford, 1993, p. 3507. N.K. Vail, J.W. Barlow, J.J. Beaman, H.L. Marcus and D.L. Bourell, Journal of AppliedPolymer Science Vo1.52, 1994, p. 789338

Anisotropy in Alumina Processed by SLS.

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