Optimization of Surface Roughness in Selective Laser Sintered Stainless Steel Parts

Transcription

1 Internatonal Journal of ChemTech Research CODEN (USA): IJCRGG ISSN : Vol.6, No.5, pp , Aug-Sept 04 Optmzaton of Surface Roughness n Selectve Laser Sntered Stanless Steel Parts I.Vay arasu *, K.Chockalngam, C.Kalasanathan and M.Svabharathy Dept. of Mechancal Engneerng, Sethu Insttute of Technology, Pulloor, Karapatt- 66 5, Taml Nadu, Inda. Dept. of Mechancal Engneerng, Thagaraar College of Engneerng, Madura-65 05, Taml Nadu, Inda. Dept. of Physcs, Sethu Insttute of Technology, Pulloor, Karapatt- 66 5, Taml Nadu, Inda. *Corres.author: arasu_@yahoo.com, Abstract: Decreasng surface roughness (RS) durng selectve laser snterng process mproves qualty and functonalty of the metal parts. Ths paper emphaszed thoroughly the effect of process parameters lkely laser power, orentaton and scan spacng on surface roughness. Taguch s desgn of experments approach L9 orthogonal array (OA) was selected and optmum level of parameters was chosen by lower-the-better sgnal-tonose (S/N) rato for ths nvestgaton. Also Analyss of varance (ANOVA) was utlzed and found that the scan spacng was the most mportant parameter n fnalzng upward-facng surface roughness and developed the emprcal model. Fnally an optmum level of parameters was used n confrmaton test and confrmed that the predcted values n both methods e., regresson model and the equaton of predcted S/N rato were close enough themselves and also n expermental values. Keywords: Infltraton,lasers,optmzaton,roughness, snterng, taguch, steels, prototypng. Introducton The selectve laser snterng (SLS) and selectve laser meltng (SLM) are the wdely used rapd prototypng (RP) processes for makng functonal parts n metal. The prmary challenge of ths process s to choose the correct workng parameter to acheve the preferred shape, sze, strength, mcrostructure, qualty and hence mprove the functonalty of the product [-]. SLS & SLM are such an addtve layerng processes utlzng CO laser and powder materal (polymers, metals, ceramcs and compostes) to buld a -D part wthout molds or support [4]. SLS s manly used for the fabrcaton of functonal metal parts n small batches and enables the producton of relatvely large obects compared to other RP processes. The producton speed of ths technology s also very hgh. But SLM whch uses hgh powered laser and t leads to hgh temperature causes an ncrease n spatter generaton as well as metal vaporzaton durng the process [5]. Hence SLS has been chosen for ths study. Today there are lot of rapd prototypng technologes commercally avalable n the feld of tool makng, medcal and aerospace applcaton and also t has been analyzed the qualty of the surface tends to decrease [6, 7]. The surface roughness s a man drawback n the SLS process as t can affect the accuracy, post processng cost and functonalty of the parts. The detals on the mnmum surface roughness of stanless steel functonal parts n SLS by optmzng the process parameters usng Taguch method fulfll the ndustry need.

2 I.Vay arasu et al /Int.J. ChemTech Res.04,6(5),pp Several studes have demonstrated that the part surface roughness and mechancal propertes depend on the type of RP technology, the base materal, laser type, and layer thckness, buld strategy, and post-processng [8-0]. Thus the surface roughness s an mportant qualty obectve of parts obtaned n RP technologes. Many papers revealed that the Taguch technque has been used for optmzaton of surface roughness n varous RP process and machnng operatons [-4]. The dfferent post processng methods lke shot peenng and cold sostatc pressng of components reportng hardness and roughness have also been dscussed [5,6]. The man obectve of ths study s to mnmze the surface roughness of stanless steel metal parts n SLS by optmzng the process parameters usng Taguch method. Among the many parameters whch nfluence the requred surface qualty of SLS parts the laser power determnes the temperature gradent, Orentaton causes star steppng effect and scan spacng [8] are the most essental parameters. The parameter levels were fnalzed based on the potental of SLS machne and the prelmnary experment done. Taguch s desgn of experments (DOE) approach L9 OA was used for ths nvestgaton. Taguch technques [7, 8], such as OA, S/N rato, and ANOVA have been generally appled to optmze the varous process. Then ANOVA was used and consequently developed the emprcal model. Fnally, confrmaton experment at an optmum level of parameters are confrmed that the predcted values n both methods were close enough themselves and also n expermental values. Experments The dfferent parameters, desgnaton and values at all levels are fnalzed and descrbed n Table.A wedge shaped samples were modeled n Desgn software as shown n Fg. s used to fnd the roughness characterstcs. Ths fgure clearly shows the buld orentaton (Φ) and drecton of surface roughness measurement.a mean dameter of µm LaserForm ST-00 powder materal was utlzed n ths expermental work. The partcles were coated wth a mxture of thermoplastcs (phenolc resn). The buld orentaton and postons were fnalzed and fxed accordngly n the software (Fg.). Consequently the slced fles n layers of 0.0mm thckness were repared and transferred to SLS system. The SLS green part fabrcated was then subected to bronze nfltraton. The fnal composton obtaned after nfltraton s 40% bronze and 60% 40 stanless steel. Typcal samples were bult n the DTM sterstaton 500 plus SLS system and SURFCOM 0A roughness tester measured the surface roughness. The photography of the manufactured samples are shown n Fg.. Table. Parameters, Desgnaton and levels for experments Parameter () Desgnaton Unt Level Level Level Laser power Orentaton Scan spacng P O SS [Watts] [Degree] [mm] The L9 orthogonal array and mean surface roughness value for the parameters and ther levels are llustrated n Table. In ths study the nteracton among the parameters was not consdered. Table. Expermental results of L9 orthogonal array for Surface Roughness Expermental run Laser power [Watts] (58) (58) (58) (60) (60) (60) (6) (6) (6) Orentaton [Degree] ( 0) (45) (90) ( 0) (45) (90) ( 0) (45) (90) Scan spacng [mm] (0.08) (0.09) (0.0) (0.09) (0.0) (0.08) (0.0) (0.08) (0.09) Mean Surface Roughness [Mcrons]

3 I.Vay arasu et al /Int.J. ChemTech Res.04,6(5),pp Fg. Wedge Part model for surface roughness characterstcs Fg. Preparatons of samples for fabrcaton n machne software Fg. Fabrcated samples by SLS process. Result Analyss and Dscusson After the experment results have been collected, the Taguch s parameter approach wth lower-thebetter S/N rato was used to fnd the optmum process parameter []. Sgnal-to-nose rato n n db s defned as Lower the better ( n n = -0log y ) 0 n () = Where, n s the number of recurrence (now n = ) and y k s the measured value of surface roughness of k th tral (here y k =RS ). Snce n s equal to, Eq. reduces to n = -0 log 0 ( ) y k Table provdes the calculated values of S/N rato, average S/N rato and rank for all parameters and ts levels. To acheve the mnmum surface roughness of the stanless steel parts produced by SLS process the lower S/N rato value and correspondng level s selected as the optmum level as mentoned below. The Fg. 4-6 shows the man effect of parameter (P, O and SS) on S/N ratos at all levels and lower values of S/N ratos are consdered as the optmum levels.

4 I.Vay arasu et al /Int.J. ChemTech Res.04,6(5),pp Power : 60 Watts (Level, S/N: 6.99), Orentaton : 0 Degree (Level, S/N: 7.5) Scan spacng : 0.08 mm (Level, S/N: 6.8). Next step, the ANOVA was appled to dentfy the parameter whch nfluences the surface roughness. Obvously the maxmum percentage of contrbuton s the most nfluence parameter to the surface roughness. Now the ANOVA table 4 was done based on the standard procedure [9] and dentfed the sgnfcant parameter. Table. S/N Rato of each parameter and level for Surface Roughness Parameter P O SS Level Expermental run Mean Surface Roughness RS (S/N ) rato n Average(S/N) rato n ave (maxmummnmum value) Rank Over all mean surface roughness = 7.66 Over all mean S/N rato value = Bold letters ndcate the optmum levels.

5 I.Vay arasu et al /Int.J. ChemTech Res.04,6(5),pp Fg.4 Effect of power on S/N rato Fg.5 Effect of orentaton on S/N rato Fg. 6 - Effect of scan spacng on S/N rato Table.4 ANOVA for Surface Roughness Parameter P O SS Error Total Sum of Squares SS Degrees of freedom DOF 8 Sgnfcant at 97.5% confdence level Mean Sum of Squares MSS F statstc F (0.05),,. tabulated Percentage of contrbuton (P RS ) Due to varous reasons the uncertantes of estmated surface roughness are unavodable. The estmated surface roughness [8] s Ra ± Ra, where Ra=t α/, DF V e () The calculated Ra value s equal to 0.4 µm (α =0.05) arrved by takng the values from Table 4. The establshment of a mathematcal model gves more useful nformaton of roughness on process parameters. Ths model equaton provdes the surface roughness value well n advance for the parameter fxed by the desgner or RP machne user before fabrcatng the parts. The model s derved from an orthogonal polynomal method wth orthogonal array data. The followng Eq. s proposed to establsh a mathematcal model between surface roughness (response varable) and process parameters [9]. RV β P = β + () 0 g [ + ] + = β P Where, RV: response varable (RS: surface roughness) The regresson Eq. for the process parameters whch s nfluencng the surface roughness (response varable) s wrtten as -

6 I.Vay arasu et al /Int.J. ChemTech Res.04,6(5),pp (4) Smlarly Table 5 provdes the calculated values of coeffcents for all the parameters. The results are as follows. β P = 4.8/6 = 0.806; β O =.0/6 = 0.68; β SS = 4.96/6 = β P = 5.9/8 =0.98; β O = -.5/8 =-0.866; β SS = -.50/8 = β0= 68.90/9 = By substtutng the parameters ndcated above n Eq.4, the process model for surface roughness (RS) s derved and gven n Eq. 5 below. RS = P P O.558 O O SS SS (5) Table.5 Values of constant and coeffcents RS Coded value of the levels Orthogonal contrast for lnear term C Orthogonal contrast for non-lnear term C ) p (β (β ) (β p ) o ) o (β ) ss (β ( β ) ss P O SS P O SS P O SS Confrmaton Test In the present research to valdate the model equaton derved, the confrmaton experment and [S/N] predcted methods were adopted at an optmum level of parameters. Table 6 shows the varaton between experment and predcted values n both methods of surface roughness at an optmum settng. The average mnmum roughness of part attaned n confrmaton experment at an optmum parameter was 6.08 µm. The followng equaton s used to calculate the predcted S/N rato by utlzng the optmal process parameters []. S [ N ] predcted S =[ ] N m + n = ( [ N S S ] -[ ] ) N In the current study by substtutng the optmal values from Table n the above Eq. 6 and get the predcted S/N rato as -5.4dB and thus Eq. become as shown below = -0log0 y m Then the value of predcted surface roughness s calculated as 5.85 mcrons. Obvously from the Table 6, the predcted values n both methods were close enough themselves and to the expermental value at an optmum settngs. Thus the optmum parameters and ther levels are A, B and C of laser power 60 watts, orentaton 0 and scan spacng 0.08 mm. (6)

7 I.Vay arasu et al /Int.J. ChemTech Res.04,6(5),pp Table.6 Comparson between expermental and predcted values for Surface Roughness Settngs P O SS Experment value E.V Regresson value From Eq. 5 RS Predcted S/N rato Value [S/N] predcted OptmalSettng ± ±0.4 Concluson The process parameters that nfluence the surface roughness of SLS stanless steel part has been analyzed n ths paper successfully. The detals on the mnmum surface roughness of stanless steel metal parts n SLS by optmzng the process parameters usng Taguch method has been done n ths research to avod addtonal fnshng operaton. A process Engneer can use ths research to conclude the parameter values well n advance before manufacturng the parts. From the results the followng ponts also concluded. ) Among all the three parameters consdered for the analyss, the scan spacng was the mportant contrbutng factor affectng the upward-facng surface roughness. ) The confrmaton experment was done at an optmum level of parameters and confrmed that the estmated values n both methods were close enough themselves (regresson equaton value 5.80±0.4 µm) (Equaton of S/N value 5.85±0.4 µm) and also n expermental values. (6.08 µm). ) It s found that the optmal process parameter for upward-facng surface roughness of the SLS metal parts are power of 60 watts, orentaton 0 o and scan spacng 0.08 mm. Acknowledgement The authors would lke to thank PSG TIFAC CORE n Product Desgn, Combatore, Taml Nadu, Inda for ther support of ths work. References. Sharant Sngha, S.Vshal Sharmaa, Ansh Sachdevaa, Mater. Manuf. Process 7(6) (0) Sharant Sngha, S.Vshal Sharmaa, Ansh Sachdevaa, Mater. Manuf. Process 8() (0) Uday Lakshmnarayan, H.L.Marcus, Mater. Manuf. Process 9(5) (994) N.Raghunath, P.M Pandey, Int. J. Mach. Tools & Manuf. 47(6)(007) Kamran Mumtaz, Nel Hopknson, Rapd Prototypng Journal 5()(009) Z.L.Lu, A.F.Zhang, Z.Q.Tong, X.H.Yang, D.C. L, B.H. Lu, Mater. Manuf. Process 6(7) (0) I.Palcc, M.Balazc, M.Mlfelner,B.Buchmester,Mater. Manuf. Process 4(7-8) (009) P.B.Bacchewar,S.K. Snghal,P.M. Pandey, Proc. of Insttute of Mech. Engneers.part B: J. Engg. Manuf. (007) K.M.Patel, P.M.Pandey, P.Venkateswara Rao, Mater. Manuf. Process 4(6) (009) M.V.Kuznetsov, I.V. Shshkovsky, G.Yu.Morozov,I.P.Parkn,Mater. Manuf. Process (6) (008) R.Antha, S.Arunachalam,P. Radhakrshnan, J. Mater. Process. Technol. 8 (00) J. Kechagas, Rapd Prototypng Journal () (007) 7-.. Munsh chhabra, Rupnder sngh, Rapd prototypng Journal 8(6) (0) C.H.Len, Y.H.Guu, Mater. Manuf. Process (8) (008) B.Sustarsc,S.Dolnsek,M.Jenko,M,V. Leskovsek,Mater. Manuf. Process 4 (7-8) (009) Y.Y. Du,Y.S. Sh,Q.S. We, Mater. Manuf. Process 5()(00) P.J.Ross, Taguch Technques for Qualty Engneerng, nd edton., Mc-Graw-Hll, New York, D.C.Montgomery, Desgn and Analyss of Experments, rd ed., John wley & sons Inc. New York, K.Chockalngam, N.Jawahar, U.Chandrasekar, K.N.Ramanathan,J. Mater. Process. Technol. 08(008) *****