A. Simchi / Materials Science <strong>and</strong> Engineering A 428 (2006) 148–158 157<strong>laser</strong> beam [32]. The formation <strong>of</strong> oxide layer on the surface<strong>of</strong> powder particles significantly increases the absorption rate<strong>of</strong> CO 2 <strong>laser</strong> radiation [33]. This changes the temperature-timehistory <strong>of</strong> <strong>sintering</strong> <strong>and</strong> increases the melt volume, allowingsurface tension become more dominant. Another concern is theliquid <strong>metal</strong> surface tension, which influences the wetting anglebetween the solid <strong>and</strong> the liquid phases that can disrupt bondingbetween rastered lines <strong>and</strong> individual layers [10,34]. Therefore,the amount <strong>of</strong> oxygen present during the heating, melting, <strong>and</strong>fusion <strong>of</strong> <strong>metal</strong> <strong>powders</strong> in the <strong>laser</strong> <strong>sintering</strong> process shouldinfluence the densification <strong>and</strong> the attendant microstructural features.As it has been addressed in the previous publication [30],the effect <strong>of</strong> oxygen can be considered in the densification coefficient,K. Anyway, except the role <strong>of</strong> oxygen that can be takeninto account by using the densification coefficient, the effect <strong>of</strong>protecting gas is not very pronounced <strong>and</strong> does not impact the<strong>sintering</strong> rate considerably.5. ConclusionsThe effect <strong>of</strong> processing parameters on the densification <strong>of</strong><strong>metal</strong> <strong>powders</strong> in the <strong>laser</strong> <strong>sintering</strong> process was investigated.Based on the empirical <strong>sintering</strong> rate data, a relationship wasestablished between the densification <strong>of</strong> <strong>metal</strong> <strong>powders</strong> duringDMLS <strong>and</strong> the energy delivered to the powder bed bythe <strong>laser</strong> beam. This relationship has been shown to be usefulfor <strong>metal</strong>s with congruent melting point or alloys, which fullmelting/solidification approach is feasible mechanism <strong>of</strong> rapidparticle bonding in DMLS process. The following conclusionscan be afforded.1. The <strong>laser</strong> <strong>sintering</strong> can be considered as “high power densityshortinteraction time” process. The delivered energy heatsup the exposed powder particles rapidly beyond the meltingtemperature. The particle bonding is then performed <strong>and</strong> the<strong>kinetics</strong> <strong>of</strong> densification depends on the working temperature.Consequently, the parameters involved in determining themethod <strong>of</strong> energy delivered to the powder medium control the<strong>sintering</strong> rate. Meantime, the evaporation <strong>of</strong> exposed powderin the <strong>laser</strong> <strong>sintering</strong> process may occur, particularly at anintensive <strong>laser</strong> energy input.2. It was found that as the <strong>laser</strong> energy input increases (higher<strong>laser</strong> power; lower scan rate; lower scan line spacing; lowerlayer thickness) better densification is achieved. Nevertheless,there is a saturation level, in which, even at very intensive<strong>laser</strong> energy full density cannot be obtained.3. When melting/solidification approach is the mechanism <strong>of</strong>densification, the rate changes in void fraction <strong>of</strong> powderbed in DMLS process obeys the first order kinetic law:∂ε/∂t = −k ′ ε. The <strong>sintering</strong> rate (k ′ ) was found to be a function<strong>of</strong> the <strong>laser</strong> energy input. Therefore, the sintered density<strong>of</strong> <strong>metal</strong> <strong>powders</strong> in DMLS process should be an exponentialfunction <strong>of</strong> the <strong>laser</strong> energy input.4. Besides the fabrication parameters, the powder propertiesstrongly influence the densification <strong>kinetics</strong>. Finer particlesprovide lager surface area to absorb more <strong>laser</strong> energy, leadingto a higher <strong>sintering</strong> rate. The chemistry <strong>and</strong> the shape<strong>of</strong> the particles also affect the densification in DMLS process.To take into account the material characteristics in the<strong>sintering</strong> model, a densification coefficient (K) was defined<strong>and</strong> used. It was shown that this coefficient is related to thepowder characteristics (chemical composition, particle size,particle size distribution, oxygen content, etc.) <strong>and</strong> the <strong>sintering</strong>rate increases as the K value decreases.5. The results showed that <strong>laser</strong> scanning strategy <strong>and</strong> <strong>sintering</strong>atmosphere influence the densification. Although the development<strong>of</strong> thermal stresses highly depends on the scanningstrategy, the effect <strong>of</strong> scan vector length on the sintereddensity was found to be marginal. If the oxygen potential<strong>of</strong> the <strong>sintering</strong> atmosphere is kept constant, the role <strong>of</strong>protective atmosphere on the densification is also not verypronounced.AcknowledgementsThe <strong>laser</strong>-sintered specimens were prepared at Fraunh<strong>of</strong>erInstitute for Manufacturing <strong>and</strong> Advanced Materials (IFAM),Bremen, Germany. The help <strong>of</strong> Dr. F. Petzoldt <strong>and</strong> Dr. H. Pohlis gratefully acknowledged. The grant <strong>of</strong> the Office <strong>of</strong> VicePresident for Research <strong>and</strong> Technology, Sharif University <strong>of</strong>Technology, is sincerely appreciated.References[1] J. Haenninen, DMLS Moves from Rapid Tooling to Rapid Manufacturing,Metal Powder Report, September 2001, pp. 24–29.[2] R. Jiang, W. Wang, J.G. Conley, in: Proceedings <strong>of</strong> the Solid FreeformFabrication Symposium on Application <strong>of</strong> FFF in the Metal CastingIndustry, Compiled by D.L. Bourell, J.J. Beaman, R.H. Crawford, H.L.Marcus, J.W. Barlow, The University <strong>of</strong> Texas at Austin, TX, 1999, p.767.[3] T. Wohlers, Rapid Prototyping <strong>and</strong> Tooling Worldwide: Stalled Growth,Countless Benefits, Vast Confusion, CATIA Solution Magazine, January/February2000.[4] P. Fischer, V. Romano, H.P. Weber, N.P. Karapatis, E. Boillat, R. Glardon,Acta Mater. 51 (2003) 1651–1662.[5] J.D. Williams, C.R. Deckard, Rapid Prototyping J. 4 (2) (1998) 90–100.[6] Y. Zhang, A. Faghri, Int. J. Heat Mass Transfer 42 (1999) 775–788.[7] S. Kolossov, E. Boillat, R. Glardon, P. Fischer, M. Locher, Int. J.Machine Tools Manufact. 44 (2004) 117–123.[8] K. Dai, L. Shaw, Acta Mater. 52 (2004) 69–80.[9] H. Chung, S. Das, Int. J. Heat Mass Transfer 47 (2004) 4153–4164.[10] D.L. Bourell, H.L. Marcus, J.W. Barlow, J.J. Beaman, Int. J. PowderMetall. 28 (4) (1992) 369–381.[11] J.C. Nelson, Selective <strong>laser</strong> <strong>sintering</strong>: a definition <strong>of</strong> the process <strong>and</strong>an empirical <strong>sintering</strong> model, Ph.D. dissertation, University <strong>of</strong> Texas,Austin, TX, 1993.[12] M.M. Sun, Physical modeling <strong>of</strong> the selective <strong>laser</strong> <strong>sintering</strong>, Ph.D.dissertation, University <strong>of</strong> Texas, Austin, TX, 1991.[13] G. Bugeda, M. Cervera, G. Lombera, Rapid Prototyping J. 5 (1) (1999)21–26.[14] A. Simchi, F. Petzoldt, H. Pohl, J. Mater. Process. Technol. 141 (3)(2003) 319–328.[15] Y. Kizaki, H. Azuma, S. Yamazaki, H. Sugimoto, S. Takagi, Jpn. J.Appl. Phys. 32 (1A) (1993) 205–212.[16] J.P. Kruth, L. Froyen, J. Van Vaerenbergh, P. Mercelis, M. Rombouts,B. Lauwers, J. Mater. Process. Technol. 149 (2004) 319–328.[17] A. Simchi, <strong>Direct</strong> Laser Sintering <strong>of</strong> Iron Base Powders, Presented inFraunh<strong>of</strong>er Institute IFAM, Bremen, Germany, March 2001.[18] A. Simchi, H. Pohl, Mater. Sci. Eng. 359A (1–2) (2003) 119–128.
158 A. Simchi / Materials Science <strong>and</strong> Engineering A 428 (2006) 148–158[19] S. Das, in: Proceedings <strong>of</strong> the Solid Freeform Fabrication Symposiumon some Physical Aspects <strong>of</strong> Control in <strong>Direct</strong> Selective Laser Sintering<strong>of</strong> Metals—Part III, Compiled by D.L. Bourell, J.J. Beaman, R.H.Crawford, H.L. Marcus, J.W. Barlow, The University <strong>of</strong> Texas at Austin,2001, pp. 102–109.[20] R.M. German, Liquid Phase Sintering, Plenum Press, NY, 1985.[21] M. Rombouts, L. Froyen, J.-P. Kruth, J.V. Vaerenberg, P. Mercelis, in:Proceedings <strong>of</strong> the Powder Metallurgy World Congress on Production<strong>and</strong> Properties <strong>of</strong> Dense Iron Based Parts Produced by Laser Meltingwith Plasma Formation, Euro PM2004, Compiled by H. Danninger, R.Ratzi, vol. 5, Vienna, 2004, pp. 115–121.[22] A. Simchi, H. Pohl, Mater. Sci. Eng. A383 (2004) 191–199.[23] S. Akhtar, C.S. Wright, M. Youseffi, C. Hauser, T.H.C. Childs, C.M.Taylor, in: Proceedings <strong>of</strong> the European Powder Metallurgy Conferenceon <strong>Direct</strong> Selective Laser Sintering <strong>of</strong> Tool Steel Powders to High Density,Euro PM2003, EPMA, Schrewsbury, vol. 3, Valencia, 2003, pp.379–384.[24] C.S. Wright, S.P. Akhtar, M. Youseffi, C. Hauser, THC Childs, J. Xie,P. Fox, in: Proceedings <strong>of</strong> the Powder Metallurgy World Congress onLaser Re-melting <strong>of</strong> Prealloyed Steel Powders to High Density, EuroPM2004, Compiled by H. Danninger, R. Ratzi, vol. 5, Vienna, 2004,pp. 109–114.[25] A.J. Pinkerton, L. Li, Thin Solid Film 453–454 (2004) 600–605.[26] W. O’Nill, C.J. Sutcliffe, R. Morgan, K.K. B. Hon, in: Proceedings <strong>of</strong> theSolid Freeform Fabrication Symposium on Investigation <strong>of</strong> Short PulseNd:YAG Laser Interaction with Stainless Steel Powder Beds, Compiledby D.L. Bourell, J.J. Beaman, R.H. Crawford, H.L. Marcus, J.W. Barlow,The University <strong>of</strong> Texas at Austin, 1998, pp. 147–159.[27] H.J. Niu, I.T.H. Chang, Scripta Mater. 41 (1) (1999) 25–30.[28] X.C. Li, J. Stampfl, F.B. Prinz, Mater. Sci. Eng. A282 (2000) 86–90.[29] A.H. Nickel, D.M. Barnett, G. Link, F.B. Prinz, in: Proceedings <strong>of</strong> theSolid Freeform Fabrication Symposium on Residual Stresses in LayeredManufacturing, Compiled by D.L. Bourell, J.J. Beaman, R.H. Crawford,H.L. Marcus, J.W. Barlow, The University <strong>of</strong> Texas at Austin,1999.[30] C. Hauser, T.H.C. Childs, K.W. Dalgarno, R.B. Eane, in: Proceedings<strong>of</strong> the Solid Freeform Fabrication Symposium on Atmospheric Controlduring <strong>Direct</strong> Selective Laser Sintering <strong>of</strong> Stainless Steel 314S Powder,Compiled by D.L. Bourell, J.J. Beaman, R.H. Crawford, H.L. Marcus,J.W. Barlow, The University <strong>of</strong> Texas at Austin, 1999, pp. 265–272.[31] N. Tolochko, S.E. Mozzharrov, N.V. Sobolenko, Yu.V. Khlopkov, I.A.Yadroitsev, V.B. Mikhailov, J. Adv. Mater. 2 (2) (1995) 151–157.[32] A. Simchi, Metal. Mater. Transact. 35B (2004) 937–948.[33] A. Simchi, H. Asgharzadeh, Mater. Sci. Technol. 20 (11) (2004) 1462.[34] H. Asgharzadeh, A. Simchi, Mater. Sci. Eng. A 403 (1–2) (2006)290–298.
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