Selective Laser Sintering of Alumina-Boron Oxide Composites. Abstract. Introduction

Transcription

1 Selective Lser Sintering of Alumin-Boron Oxide Composites Insup Lee, A. Mnthirm nd H. Mrcus Center for Mterils Science nd Engineering, ETC The University of Texs t Austin Austin, TX Abstrct The selection of n optimum composite system for selective lser sintering (SLS) is bsed on mterils properties such s the melting point nd the wettbility between the components in the composite powder. The lumin-boron oxide composite system is ttrctive for SLS becuse the presence of the low melting component B23 (melting point 45 C) cn enhnce sintering. A better wetting of solid lumin powder by molten boron oxide cn lso id densifiction process. The lumin-boron oxide conlposite system hs been investigted by SLS nd selective lser rective sintering (SLRS). The role of boron oxide content s binder, lser power density, nd secondry het tretment on the microstructure nd mechnicl properties is discussed. Introduction Solid freeform fbriction (SFF) techniques hve recently been developed to overcome some of the brriers of conventionl mnufcturing techniques, such s difficulties in tooling complexshped cermic prts nd long production time in fbricting prototypes [1]. Selective lser sintering (SLS) is form of SFF technique nd employs focused lser bem which is controlled by CAD dt bse to selectively scn the powder bed surfce nd bind the loose powder [2]. The two-phse powder pproch to SLS, which involves binding high temperture cermics such s lumin nd silicon crbide with low melting inorgnic binder, is promising technology to fbricte cermic composite prts [3,4,5]. The selection of n optimum mterils system for this pproch depends on mterils properties such s the melting point of the binder mteril nd interprticle wetting between the components in the composite powder [6]. It is known tht the role of boron oxide in borosilicte glsses is to reduce the therml expnsion coefficient nd to improve workbility by decresing the viscosity [7]. The low viscosity of boron oxide is ttributed to its linked ring structure, since there is high probbility tht the bonds between rings re more susceptible to filure thn the bonds within the rings [8]. The selection of the system Ah3-B23 for SLS provides n dvntge in densifiction. Boron oxide hs low melting point (45 C) nd the liquid generted due to the locl melting of B2 3 powder during lser bem irrdition cn id the sintering process. In ddition, the molten boron oxide completely envelops the neighboring solid lumin prticles due to its low viscosity nd better wetting. Mterils nd Experimentl Procedure High purity, electronic grde 15)lm, luminum oxide powder provided by Lnxide Corportion nd 6 mesh 99% boron oxide powder from Johnson Mthey re the strting mterils. Fig. 1 shows the overll steps ssocited with SLS process nd mteril chrcteriztion. The initil boron oxide powder of 25)lm ws ground by Szegyri ttritor system nd sieved to less thn 75)lm. Alumin nd boron oxide powder blends in vrious rtios by weight were bked out in vcuum oven t 12 C for 3 hours. Pre-therml tretment cused the powder blends to form wek powder ckes, which were subsequently broken nd sieved. Bked-out powder blends were immeditely sintered in SLS system of the University oftexs t Austin. Test specimens with 339

2 A12) (Corundum) Lnxide 15 j.1 m B2) (Amorphous) 25 j.1 m (s rcccivcd) < 75 j.1 m (fter grinding & sieving) Powder Mixing nd Bking out t 12 C for 3 hours in Vcuum Oven Wek Powder Cke AI23 (Corundum) & Monoclinic HB2 Grinding nd Sieving Selective Lser Sintering I Testing/Anlysis I Post Therml Processing (9 C, 6 hours) A123 & 2A123.B23 Testing/Anlysis Figure 1. Flow chn for selective lser sintering of lumin-boron oxide composites dimension of 1"x3"xO.25" were fbricted in n inert nitrogen environment using the opertionl prmeters listed in Tble 1. Tble I. SLS opertionl prmeters Scn Spcing (mils) 5 Lyer Thickness (mils) 8 Scn Speed (inch/sec) The strength of green nd smples fired t vrious tempertures for 6 hours ws determined by 3-point bend test using Instron constnt displcement rte mchine. Density ws obtined by direct mesurement of dimension nd mss. Identifiction ofphses nd microstructurl evolution t every step of processing ws crried out by x-ry diffrction nlysis nd SEM. Results nd Discussion Fig. 2 shows the phse trnsformtion of the initil boron oxide upon rection with tmospheric wter. The s received boron oxide is morphous. It rects with moisture in ir nd trnsforms into boric cid (H3B3) very quickly. As result, without bking out before lser sintering, the strting powder blend for lser sintering is mixture of lumin, morphous boron oxide nd boric cid. Becuse boric cid hs much lower melting point (l7 C) thn boron oxide (45 C), it cuses wek bed cke through the whole powder bed during SLS process with bed heting 34

3 ,, morphous 82:3 ~T"T'T'TTI ii, I ' I I i I I, iii' iii ii, 1' I I, I ', I, I' i, If' iii i, II, i I I ' '8 28 Je l/ () s received (b) fter exposure to the tmosphere Orthorhombic HB(h + morphous B2:3 1., J, J Monoclinic HB(h j! -l I "1, '8 (c) fter bking out in vcuum oven t 12 C for 13 hours (d) fter bking out in vcuum oven t 12 C for 3 hours Figure 2. X-ry diffrction nlysis showing phse trnsformtion of the initil boron oxide 341

4 round 8 C. The cke prlyzes the powder delivery nd leveling system. Without employing bed heting system, it ws possible to void the bed cking. However, curling of the previously sintered lyers tkes plce, mking powder leveling difficult due to the displcement of those lyers. In order to overcome bed cking nd curling of sintered lyers, the boric cid in the initil powder blend must be removed. Dehydrtion of boric cid gives different forms of metboric cid HBOz or boron oxide BzJ depending on the temperture. Tble II shows the physicl properties of vrious forms of metboric cid HBOz [7]. TbleII Physicl properties ofcrystlline metboric cid HBOz CN* of B Density (glcm 3 ) Orthorhombic Monoclinic 3 nd Cubic * CN refers to coordintion number Melting Point COC) Creful dehydrtion of boric cid t 12 C in rough vcuum oven for 3 hours nd slow quenching fter bke-out yields monoclinic metboric cid HBOz whose melting point is 21 C. Orthorhombic HBOz cn be generted fter 13 hours of het tretment under the sme conditions. It is found tht monoclinic HBOz voids the bed cking phenomenon with powder bed heting t 8 C nd reduces the curling problem significntly during SLS process. Fig. 3 shows the effect of the binder content on bend strength of both green nd fired (t 9 C for 6 hours) composites. The bend strength of the composites increses s the binder content increses since ll the cermic interprticle bonds tht provide the strength of the composites originte from the meltingofthe binder nd the cotingofcermic (lumin) prticles during lser bem irrdition. The effect of lser power density on bend strength is lso shown in Fig. 3. Power density is defined s lser power / (scn spcing x scn speed) [9]. At lower power density, the green composites show poor strength due to insufficient melting nd flowing of the binder compred to t higher power density where molten binder cots the cermic prticles completely s reveled by SEM. Similrly, the bend strength of f!fed prts fbricted with 25 nd 2 wt. % boron oxide powder blends is proportionl to the power density. Becuse t higher power density, the lser bem loclly melts the binder sufficiently, the binder flows nd encpsultes more cermic prticles thn t lower power density s reveled by SEM. As result, the green nd fired density increses s the power density increses. Therefore, t higher power density, the fired composites show higher strength due to higher density (FigA). However, bend strength of fired test brs mde with 15 wt.% boron oxide is independent of power density. For this system, it ws required tht the process be done t higher bed temperture (1 C) nd t higher power density in order to melt the smll mount of the binder completely nd bind cermic prticles together. Consequently, there might be threshold power density to melt the whole binder thoroughly nd fbricte green prts which cn keep the shpe. Above this vlue, the strength of the fired prts will lmost be independent of power density since there is no more dditionl binder which will cot the cermic prticles. Fig. 5 illustrtes the effectoffmng temperture on the mechnicl properties ofprts fbricted with 25 wt.% boron oxide powder blends. At intermedite firing tempertures t round 8 11 C, the test brs show higher bend strength due to the formtion of luminum borte (2Ah:3 Bz:3) t the surfce of the lumin prticles by the rection of lumin nd boron oxide t round 8 C. According to X-ry diffrction nlysis (Fig. 6), the mount of 2Ah 3 B2 3 increses reltive to tht of lumin s the fmng temperture increses up to 11 C. At 12 C, 9Ah3 2B23 phse is formed by the rection: 2 (2Ah3 B23) +5A Ah 3 2B 2 3 [1]. Below 7 C, it ws found tht there ws no substntil rection. The bend strength of smples fired t 13, nd 15 C decreses due to the decomposition of the compound 342

5 -- 2 f-- 25 I I I ' -.-V:J c.. '-' - -..c f-- - OJ) I: <lj s f-- rj:j "C I: <lj _-DO = 5 " wt.% B wt.% B 2 3 I I I Power Density (Cl!cm 2 ) - () green prts U) c.. en - 25 wt.% "'-'.c..., 1 c: (1) - D D 2 wt.% , CO (J) 5 <> 15 wt. f<, " c: <> <> <> <> <> <> <> (1) <> m Power Density (Cl/cm 2 ) (b) fter fired t 9 C for 6 hours Figure 3. Bend strength of test coupons fbricted with Lnxide 15 /lid lumin-boron oxide powder blends 343

6 c.8 Green Density o Fired Density Power Density (Cl/cm 2 ) 7 () Effect of lser power density on the density 15 i I I i I I I I i I I I I I I I I I Fired Density (g/cm 3 ) 1.15 (b) Effect of fired density on the bend strength Figure 4. Mechnicl properties of green smples nd smples, fired t 9 C for 6 hours, mde from lumin-25 wt.% boron oxide powder blend 344

7 .- 15.c 1..., ') c:: Q) :r..., en 5 "'C c:: Q) m Temperture (OC) - en E (.) ') , >-.-(f) c:: Q).95.9 C.85 D () Bend strength o o D G.D. D F.D. D D Temperture (OC) (b) Density Figure 5. Effect of firing temperture on the mechnicl properties of smples mde from lumin-25 wt.% boron oxide powder blend 345

8 I.!i!i "k!i 1!i t!i I~ j I~ A- i, A l e () before SLS (c) fired t 8 C e e I. l'... 9 J 41 (b) fired t 7 C e ea '. (d) fired t 11 C!i Monoclinic HB(h (e) fired t 12 C Figure 6. X-ry diffrction nlysis of lumin-25 wt. % composites fired t vrious tempertures for 6 hours 346

9 9A123 2B23 [11]. Below 13 C, secondry het tretments increse the density, wheres bove 13QoC, fired density is lower thn the green density due to the increse in porosity ssocited with the decomposition of the 9A123 2B2o, phse (Fig. 5(b». Summry It ws demonstrted tht both composite body AI23-2AI2} B2o, nd monolithic mteril 9Aho, 2B2} could be successfully fbricted by selective lser sintering nd rective sintering of the mteril system Ah3-B2o,. The bend strength of the composite body increses s the binder content increses. At higher lser power density, the composites show higher bend strength due to higher density. At intermedite firing tempertures round 8-11 C, test brs show higher bend strength due to the formtion of luminum borte (2AI23 B23 ) t the surfce of the luminprticles by the rection ofluminnd boron oxide t round 8 C. Acknowledgments Finncil support from DARPA-ONR grnt NOOI4-92-J-1394 is grtefully cknowledged. References 1. Mrcus, H.L., Bemn, J.J., Brlow, J.W. nd Bourell, D.L. (199, April) From computer to component in 15 minutes: The integrted mnufcture of three dimensionl objects. Journl of Metls, pp Bourell, D.L., Mrcus, H.L., Brlow, J.W. nd Bemn, J.J. (1992). Selective lser sintering of metls nd cermics. The Interntionl Journl of Powder Metllurgy, 2.8., Lkshminryn, U., Mrcus, H.L., (1992, August). An experimentl study of reltionship between microstructure nd mechnicl properties of cermic composites fbricted by selective lser sintering. In Mrcus, H.L., Bemn, J.1., Brlow, J.W., Bourell, D.L. nd Crwford, R.H. (Eds. ), Proceedings of the Solid freefonn fbriction symposium, (pp ). Austin, Texs: University of Texs Press. 4. Subrmnnin, P.Kmtchi, ZOng, Guisheng nd l\1rcus, H.L.(1992, August). Selective Lser Sintering of Cermic Composites. In Mrcus, H.L., Bemn, J.1., Brlow, J.W., Bourell, D.L. nd Crwford, R.H. (Eds. ), Proceedings of the Solid freeform fbriction symposium, (pp ). Austin, Texs: University of Texs Press. 5. Lkshminryn, U. (1992, My). Ph.D. Disserttion, University of Texs, Austin. 6. Germn, R. M. (1984). Powder met.:!llurgy science. Princeton, New Jersey: Metl Powder Industries Federtion. 7. Greenwood, N. N., Ernshw, A. (1984). Chemistry of the elements. Oxford: Pergmon Press. 8. Doremus, R. H. (1973). Glss Science. New York. John Wiley & Sons, Inc. 9. Nelson, J.C., Vil, N.K., Brlow, J.W. (1993, August). Lser Sintering Model of Composite Mterils. In Mrcus, H.L., Bemn, J.J., Brlow, J.W., Bourell, D.L. nd Crwford, R.H. (Eds. ), Proceedings of the Solid freefonn fbriction symposium, (pp ). Austin, Texs: University of Texs Press. 1. Ry, Sib P. (1992). Preprtion nd Chrcteriztion of Aluminum Borte. J. Am. Cerm. Soc., '12., Rymon-Lipinski, T., Hennicke, H.W., Linenber, W. (1985). Decomposition of Aluminum oxide-boron oxide system t high tempertures. Kermmische Zeitschrift, 37(9),