Of Processing Parameters In SLS Of Metal...Polymer Powders

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1 Of Prcessing Parameters In SLS Of Metal...Plymer Pwders B.Badrinarayan and J.W.Barlw Dept. fchemical Engineering, The University ftexas at Austin, Austin, Texas ,U.S.A. Abstract paper describes the effect f prcessing parameters in SLS f metal-plymer pwder mixtures. Test bars fr measurement f strength and density were made frm cpper-pmma pwder mixtures using SLS. The effect f Energy Density during laser prcessing, effect f vectr length, bed temperature, plymer melt index and initial binder cntent n part strength and density are discussed in this paper. The green part strengths and densities were fund t increase with Energy Density upt a value f 6. cavcm 2 and then they drp ff due t plymer degradatin. Parts made with lwer vectr lengths yielded higher strengths and densities than thse with higher vectr lengths. Parts prcessed with a lw melt index plymer binder (arund 6. gmllomin) shwed higher strengths than parts prcessed with higher melt index binders. High strength values were btained fr green parts made frm pwders that had a greater initial binder cntent. Bed temperature did nt have a very significant effect n part strength and densities. Intrductin Selective Sintering cmes under the class f Slid Freefrm Fabricatin Prcesses fr rapid manufacture parts directly frm a CAD mdel withut part specific tling r human interventin [1,2]. This technlgy has been develped at the University f Texas at Austin t prduce plymer parts as well as indirect metal and ceramic parts that have plymer as an intermediate binder. There are a variety f material and prcessing issues invlved in manufacture fgreen metal parts using an intermediate plymer binder, namely the plymer melt index, Energy density during laser prcessing f parts, vectr length fpart being fabricated, the bed temperature emplyed during prcessing and the cntent f plymer binder that is used fr green part fabricatin. This wrk is directed twards a better understanding f these varius issues which help develping a gd prcess cntrl system fr the SLS prcess and ensure quality repeatability the manufacturing prcess. Experimental Prcedure Cpper pwder, -325 mesh (me.an size f 5-1Jlm) which was used alng with a plymer binder in metal part fabricatin by SLS, was btained frm Reade Internatinal, Rumsn, New Jersey. PMMA and cplymers f MMA and BMA were used as intermediate binders fr the metal pwder in SLS prcessing. The synthesis f these plymer emulsins has been discussed in by Vail [3]. The plymer was btained in pwder frm by spray drying the emulsin plymer in an Anhydr Mdell Spray Drying Unit. All SLS prcessing experiments were carried ut by mixing the plymer pwder with the cpper pwder in the desired prprtin. SLS prcessing was carried ut in a nitrgen envirnment t keep the prcessing chamber inert. A layer thickness f.5 inches was used in all the SLS prcessing experiments. Three pint bend specimens (3." x 1." x.25") were made by SLS fr measurement f strength and density f green parts. 55

$\.&""".V JL FO. the strength f energy amunt f that the green t 6 scan vectrs fr a vectr length n green part system at vectr n callcm 2. Figure 3 tw vectr lengths. There is t.$

2 \.&""".V JL FO. the strength f energy amunt f that the green t 6 scan vectrs fr a vectr length n green part system at vectr n callcm 2. Figure 3 tw vectr lengths. There is t. 4 shws the plt same pwder system the different vectr case the green parts with lwer vectr... 1.""... ""'... starts t decmpse and s the density lwer Densities the green scan vectrs. Effect OfPlymer imprtant The Melt the laser As strength f the parts ""'..."'.. """'... Index n strength separately spray prcessed by SLS. 2J.1sec, Scan UBJ,..."""JL...,.FO. 16 t 56 grid strength f parts Increases high viscsity weight, green UVJLJL...VU by ASTM D1238, is an...nll"''i''lr'\i311'' as intermediate binder. UJL... J..."'" f the pwder under molec1jlar the plymer the the effect f Melt 31 and 4 were..." '\!h''i''lr'\i311'' """'...,... BJ''''''''...,... f 4 vl% and... -"". (SP) was set t (SS) was varied frm We can see that the Index results frm a ""...,... i'.'"... is prprtinal t mlecular This trend is in gd 56

agreement with the data f Vail [3] fr cplymer cated A-5 sda-lime glass pwder (cplymers 8/2-4-S, 8/2-6-S, 8/2-8-S). Vail als fund that when binder melt flws were substantially smaller (in the range f.

3 agreement with the data f Vail [3] fr cplymer cated A-5 sda-lime glass pwder (cplymers 8/2-4-S, 8/2-6-S, 8/2-8-S). Vail als fund that when binder melt flws were substantially smaller (in the range f.7 t.96 gms/iomin) the mdulus f rupture fr the green SLS bars were ften lwer than the green SLS bars made with binders that had higher Melt Indices. The effect f Melt Index n green density is shwn in Fig 6. The parts made frm all the three plymer binders have cmparable densities ranging frm 41 % t 46%. The higher the Melt Index the better the flw and therefre greater the sintering and the resulting green density. Hwever in SLS the effect f Melt Index n density is nt very prnunced as fr the mst part the density seems t be determined by the interlayer prsity that ccurs n layering pwder. Effect OfBed Temperature: In SLS prcessing the material directly under the laser beam is at a much higher temperature than the material that has already been expsed t the laser energy r the material unexpsed t the laser beam. Als, the surface temperature f the pwder under the laser is much higher than that f the material belw the surface. The temperature gradients in the laser heated material can cause differential vlume changes because the sintering rate is a functin f temperature, and hence residual stresses can build up in the material. The actual residual stress distributin in the sintered part depends n the temperature variatins in the pwder bed, the assciated cling rates, and the abilityf the material t stress relax. Plymers shw rapid stress relaxatin at temperatures larger than their glass transitin temperature. During SLS prcessing f pwders the part side pwder surface is usually heated t prevent curling f the part due t thermal stresses n laser irradiatin. The metal-plymer pwder systems the bed temperature is usually set clse t the glass transitin temperature f the plymer binder (abut 2_3 belw Tg). This helps in effective sintering and als remves curling. The effect f bed temperature n strength and density f green parts was studied fr a cppercplymer mixed pwder system. The cplymer cmpsitin was 4% by vlume. Three different bed temperatures, 85,75 and 65 C, were used. The laser pwer was fixed at 9.5 Watts, the step perid (SP) was set t 2Jlsec, the Scan Spacing (SCSP) was set at.5 inches, and the Step Size (SS) was varied frm 16 t 57 grid units. Three pint bend bars were made fr strength tests. Figure 7 shws the effect f bed temperature n part strength. It is seen that the green part strength increases with the bed temperature, but this increase is limited t the glass transitin temperature f the plymer. Ifthe bed temperature is kept abve the glass transitin temperature f the plymer the entire pwder bed cakes up and the SLS run wuld get abrted. Parts scanned at bed temperatures near the plymer Tg shwed minimal r n curl cmpared t thse scanned at lwer temperatures. Figure 8 shws the % density f the green parts with bed temperature at different Energy Densities. We fmd that bed temperature has n significanteffect n the green part density. Effect famunt f Plymer Binder In SLS: Anther factr that is t be cnsidered in SLS prcessing f metals with a plymer binder is the amunt f plymer t be used as binder. Lwer amunts f plymer will be desirable if the aim is t g t fully dense metal parts with lw shrinkages. If, hwever, the aim is t btain a lightly sintered metal r xidized metal prefrm with subsequent infiltratin by anther lwer melting metal r a liquid epxy resin then it is nt necessary t ensure a lw bindercntentin the green part. The effect fbinder vlume fractin n part strength is shwn in Figure 9. This curve suggests that high binder cntent might be desirable fr prducing green parts with gd strengths. As the amunt f plymer increases there is greater sintering and bnding with the metal pwder and hence there is an increase in strength fr the green cmpact. 57

4 Summary In this paper the effects f material prperties and prcessing issues have been explred. It is fund that energy densities arund 6. callcm 2 yield parts with gd green strengths. Green parts fabricated with lw Melt Index PMMA shwed greater strengths than thse prcessed with higher Melt Index PMMA. Parts made with lwer vectr lengths yielded greater strengths than thse made with higher vectr lengths. Hwever, it is imprtant t nte that t small a vectr length culd lead t plymer degradatin and hence lwer strength values. Bed Temperature des nt have a significant impact n strength values in the temperature ranges cnsidered in prcessing. Hwever it is necessary t keep the bed temperature clse t the glass transitin temperature (abut 2_3 belw Tg) t prevent warping f parts during SLS prcessing. Higher binder cntent in raw pwder leads t greater strengths n prcessing. The results btained in this wrk can be used as a guideline fr SLS prcessing fmetal parts with an intermediate plymer binder. Acknwledgment The authrs gratefully acknwledge supprt fr this wrk by DARPA / ONR Grant NOOO14-92-J References 1. Carl Deckard and Jseph Beaman, 15th Cnference n Prductin Research and Technlgy, University f Califrnia at Berkeley, p , (1989). 2. Harris L. Marcus, Jseph Beaman, Jel W. Barlw and David L. Burell, JOM, 42(2), P 8-1, (199). 3. N. K. Vail, " Preparatin and Characterizatin fmicrencapsulated, Finely Divided Ceramic Materials Fr Selective Laser Sintering," Ph.D. Dissertatin, Department fchemical Engineering, The University f Texas at Austin, Austin, Texas. 58

5 Cu-PMMA mixed pwder vl% plymer =4, m.l = Energy Density, ca1icm 2 8 Figure 1 : Effect fenergy Density On Green Part Strength 46r Cu-PMMA mixed pwder vl% plymer:::: 4, m.l :::: Energy Density, callcm 2 8 Figure 2 : Effect OfEnergy Density n Green Part Density 59

6 shrt vectr ( 1." ) 3 Q" 'v.! 25 2 lng vectr (3." Energy Density, cal/cm 2 6 Figure 3: Effect fyectr Length n Green Strength 48.5Or , shrt vectr(l.o") lsi lng vectr(3.") lsi lsi lsi 6 Energy Density, calicm 2 Figure 4 : Effect f Vectr Length n Green Density 6

7 1fJO(lr , (Cu-PMMA mixed pwder, vl % plymer =4)... M.1. =6... <J:).. Q;{ A- M.I. = 31 At.... M.I. = Energy Density, cal/cm 2 Figure 5 : Effect Of Plymer Melt Index On Green Strength 46.. Cu-PMMA mlxed pwder vl% plymer=4 45. A A c Energy Density, cal/cm 2 8 Figure 6 ; Effect OfPlymer Melt Index n Green Density 61

8 2, Cu- cplymermixed pwder 85 C (4vl% plymer, M.1. =35) CI 75 C CI + v.! (:l, CI ::E + 65 C 5C - l- I, I, I, I, I, " Energy Density, cal/cm 2 Figure 7 ; Effect fbed Temperature n Green Strength ,e- <I'.l , Cu-cplymer mixed pwder 45.5 (4vl% plymer, M.l. =35.) d g t-, 3 - I I I 456 Energy density, cal/cm 2-85 C A 75T -, I 65T 7 8 Figure 8 ; Effect f Bed Temperature n Green Density 62

9 , Cu-PMMA mixed pwder MJ. = 6, BT =12 C 4 vl% plymer c;f.).. c::;::l vl % plymer vl% plymer I.. I I I.. I I I I J I Energy Density, callcm 2 Figure 9 : Effect fplymer Cntent in Raw Pwder n Green Strength 63

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