South Korea. Keywords: Mahalanobis distance, normal distribution, welding quality, fault weld, and online monitoring system

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1 Appled Mechacs ad Materals Ole: ISSN: , ols , pp do: / Tras Tech Publcatos, Swtzerlad PREDICTION WELDING QUALITY IN MULTI-PASS WELDING PROCESS USING MAHALANOBIS DISTANCE METHOD Reeal Rtesh Chad 1, Ill-Soo Km 1, J-Hye Lee 1, J-Su Km 2 1 Departmet of Mechacal Egeerg, Mokpo Natoal Uversty, 61, Dorm-r, Chugkye-myu, Mua-gu, Jeoam , South Korea E-mal: lsookm@mokpo.ac.kr 2 Korea Isttute of Scece ad Techology, Wolchul-dog, Buk-gu, Gwagju, South Korea Keywords: Mahalaobs dstace, ormal dstrbuto, weldg qualty, fault weld, ad ole motorg system Abstract: The weldg qualty mult-pass weldg s maly depedet o the pre-heatg from pervous pass or root-pass weldg. I ths study, a Mahalaobs Dstace ad ormal dstrbuto method s llustrated ad employed to determe whether weldg faults have occurred after each pass weldg ad also to quatfy weldg qualty percetage. To successfully accomplsh ths objectve, sets of mult-pass weldg expermet were performed wth dfferet weldg parameters each pass; the welded samples of SS400 steel flats adoptg the bead-o-plate techque were employed the expermet. The result of curret ad voltage for each pass s obtaed through the real tme metorg systems. I order to verfy the effect of the performace ad weld qualty of the dfferet weld-pass, Mahalaobs dstaces for voltage ad curret values were calculated ad used for qualtatve ad quattatve aalyss wth comparso to values obtaed from the root-pass as referece welds. The results of the expermet ad statstcal aalyss have demostrated that the weld faults after each weld pass s feasble. Itroducto The weldg qualty motorg ad quatfyg weldg qualty s extremely mportat to avod weldg defect ad faults o the maufactured product. The ablty to motor the weldg qualty s automatcally s mportat order to reduce producto cost ad to assure ad mprove weld [1-6]. Due to recet crease emphass o assurace weldg qualty, a real tme qualty motorg ad cotrollg weldg parameter s crtcally mportat to avod weldg faults o the workpece [5]. If weldmets used wthout determg weldg fault ad qualty could pose a crtcal rsk o the weldg product. There has bee myrad statstcal method used to determe the weldg ole faults; however there s very lmted lterature descrbg the weldg qualty o mult pass weldg s wth respect to the weldg parameter ad weldg codtos. I prevous researches, may studes have vestgated the weldg ole faults based o the weldg voltage ad curret. For stats, Shegqag et. al [1] used the cocept of Mahalaobs dstace method to perform the qualtatve ad quattes aalyss GMAW weldg fault ad compared wth the referece weld. The weldg voltage ad curret values were calculated ad used for qualtatve ad quattatve aalyss wth comparso to values obtaed from the referece welds. Furthermore, Adolfsso et. al. [3] used statstcal method based o the varace of the arc voltage to predct weld qualty. The method volved the automatc motorg of the weld qualty produced by robotzed short-arc weldg. I ths study, a fve pass GMA weldg expermet s performed o SS400 steel plate. The weldg curret ad arc voltage obtaed from the metorg system are aalyzed usg the Mahalaobs dstace (MD) ad ormal dstrbuto method to determe weldg fault ad quatfed weldg qualty. Durg weldg process the data obtaed cosst of trasformed curret All rghts reserved. No part of cotets of ths paper may be reproduced or trasmtted ay form or by ay meas wthout the wrtte permsso of Tras Tech Publcatos, (ID: , Pesylvaa State Uversty, Uversty Park, USA-12/05/16,00:07:11)

2 Appled Mechacs ad Materals ols ad voltage wth rate of data per secods. The fault ad good weldg s determed usg the cocetrato curret ad voltage durg weldg process. MD values are beg calculated through the obtaed data ad compared betwee the fault ad good weld. Developmet of Mahalaobs Dstace Algorthms The proposed statstcal method s to determe the weldg qualty based o Mahalaobs dstace ad ormal dstrbuto methods. Mahalaobs dstace (MD) s maly used effectvely to calculate the smlarty of two ukow samples that may have smlar spatal oretato but may be located far part [7-8]. Ths method s dfferet from Euclda dstace by meas: frst by matag the correlato betwee data, ad secod by depedecy from the measuremet scale. The calculato of MD s descrbed the feld of statstcs, ad s commoly used for multvarate outler detecto [9]. MD s superor to other multdmesoal dstace, because t volves a covarace matrx whch takes to accout the dstrbuto ad sze of each pot. The MD s defed as; T 1 MD x C x 1/ 2 (1) Where µ s the mea multvarate locato ad C s mea covarace matrx. For ormally dstrbuto data, the value of MD 2 s approxmately ch-square dstrbuto wth p degree of freedom. Accordg to the cocept of MD, multvarate outler ca smply be defed as observato havg a large MD. I other word, a loger MD space llustrates the exstece of a outler. I qualty cotrol, a loger MD elucdates worse qualty. Meawhle, the magtude of the compared values of MD s a dcator of a weldg faults whe large dstace betwee pots. The weldg qualty whether t fault or good could be determed by the observatos MD space of the multvarate outlers. I order to provde relable measures for recogto of outlers curret ad voltage for weldg qualty, equato 1 s beg trasformed by meas of robust procedure compled matlab software, the trasformed equato s defe below; M 2 2r 1 r d 2 (2) Where ad C dcate the ormalzed th value of weldg curret ad voltage. Frstly the mea ad stadard devatos for voltage ad curret C are calculate by followg equatos. The value of voltage ad curret are ormalzed usg equato below, C r C C * 1 (5) It s mportat to ormalze the value of r because t reles o the dstrbuto of the data ad dcates the relatoshps of the two parameters. Quatfcato of weldg Qualty The ormal dstrbuto s mportat cocept besde MD to quatfy the weldg qualty. The ormal dstrbuto s descrbed by the desty fucto; f x 2 e 2 C C ( x) (6) Accordg to geeral qualty cotrol (QC) ad the theory of the ormal dstrbuto, the threshold for cosderg a pot to belog to the dstrbuto s commoly set to ±3σ. That s, f results are wth the rage of ±3σ ths shows predcted results good ad outlers. C 2 (3) (4)

3 2690 Advaces Eergy Scece ad Techology MD values dstrbuto s assumed to obey ormal dstrbuto rule. However, MD values are always postve, therefore, the rage of MD s acceptable from 0 to ±3σ. Ths dcates that f the calculated MD from weldg voltage ad curret le the rage from 0 to ±3σ, weldg qualty s cosdered to be good weld. Coversely, f the values of MD le outsde ths rage weldg qualty regards as fault. Therefor the weldg qualty ca be quatfed as formula below; Q(%) x100% N (7) where s the umber of the quatfed data, ad N s the total umber of the data obtaed oe secod. Expermetal Procedures To perform GMA robotc weldg o SS400 steel whch s maly used structural requres specfcato o the maual weldg codto. The weldg detals based o stadard Weldg Specfcato Procedure (WPS) ad maual weldg codtos based o prelmary expermets to drve a stable bead area. The expermet parameters selected as show table 1. I each pass the weldg parameter such as arc voltage ad weldg curret are dfferet, but weldg speed ad the gap betwee two plates s the same both cases. Fg. 1 shows the expermet setup. Table 1; GMA weldg process parameters for fve dfferet weld. Pass Curret (A) oltage () Weldg Speed (cm/m) For quatfyg weldg qualty, the result data of arc voltage ad weldg curret from the ole motorg system were obtaed ad chaged from aalog to dgtal. Through the defed formulas above, mea ad stadard devatos, MD the sample data was aalyzed ad quatfed to determe the weldg qualty per secods of weldg for each pass. Fg. 1. Mult-pass GMA weldg expermet setup. Usg the calculated MD, ormal dstrbutos methods to quatfy weldg qualty s obtaed. There are three steps determg the weldg faults, frst the weld fault s determed by cocetrato of trasformed curret ad voltage whch s set as referece for qualty weld. Secod determe the behavor of MD wth respect to tme durg weldg process ad lastly determe weldg qualty percetage usg ormal dstrbuto.

4 Appled Mechacs ad Materals ols Results ad Dscusso The put varables that have the greatest mpact factor o weldablty are weldg curret, arc voltage, ad weldg codtos, whereas the output varables that occur o the plate are bead wdth ad bead wdth. Durg tal weldg state the weldg regarded as weldg referece because at the begg ad ed part of the weldg, the process s ustable ad weldg qualty s regarded as fault compared wth other part of weldg. (a) Good weld (b) Fault weld Fg. 2 Sample typcal trasformed weldg curret ad arc voltage. Fg. 2 shows typcal trasformed averaged arc voltage ad weldg curret graphs for sample of two weld pass. The data show Fg. 2(a) s obtaed from the sample weld whch shows that the dstrbuto of data s more cocetrated durg weldg process compared to the Fg. 2 (b) where data s more dstrbuted outward to each other. Ths prelmary aalyss determes fault weld ad good weld ad ths aalyss also set as a referece to determe the fault weldg qualty percetage. Accordg to MD cocept, a loger data space exstece betwee each other dcates the fault weld. Therefore, Fg. 2(a) s foud to be faultless weld, whereas Fg. 2(b) s foud to a fault weld. The start ad ed weldg curret ad arc voltage was also recorded o the ole motorg system, ad preset o trasformed curret verse trasformed voltage. The result shows that curret ad voltage are very low ad ustable durg start ad ed weldg process. Fg. 3 shows the quatfed weldg qualty percetage obtaed from MD aalyss of fve-pass weldg expermets. For all pass expermet the thresold values of the weldg qualty s defed by 95 %, whch s represeted dash les Fg. 3. Accordg to MD ormal dstrbuto ad sx sgma theory the thresold value should be below value of 99.9%. Ths meas that, f the quatfed weldg qualty obtaed from the expermet s above threshold value, the weldg qualty could be cosdered good weld qualty, stead f the value of the quatfed weldg quatfed weldg qualty are below threshold value, the weldg qualty s regraded as fault. Fg. 3(a) ad (b) shows the quatfed good weldg qualty for at frst pass ad secod pass respectvly, sce the weldg qualty percetage s above ad aroud threshold value. The weldg qualty at thrd ad forth pass s ot good, sce qualty worse wth tme. The worst weldg was quatfed at last pass, where the qualty percetage s below the threshold percetage. Ths clearly shows that weldg process parameter ad weldg codtos has a great fluece o weldg qualty such arc voltage, weldg curret. I mult-pass weldg, prevous heat put has also a large mpact o the qualty of the ext weld pass.

5 2692 Advaces Eergy Scece ad Techology (a) Frst weld pass (b) Secod weld pass (c) Thrd weld pass (d) Fourth weld pass (e) Ffth weld pass Fg.3. Quatfed weldg qualty wth respect of tme. Cocluso Ths study vestgates the applcato of statstcal method for quatfyg the weldg qualty ad determes the relatoshp betwee weldg process parameter, weldg codtos ad weldg qualty a mult-pass GMA weldg. The proposed method employs the MD ad ormal dstrbuto to evaluate the weldg qualty a fve pass weldg. The trasformed arc voltage ad weldg curret data obtaed from the expermet were aalyzed usg MD ad ormal dstrbuto. Typcal trasformed arc voltage ad weldg curret graphs descrbes as referece for determg weldg qualty percetage. The results obtaed clearly predct that weldg parameters has a large mpact o weldg qualty. The bead geometry also dcated the effect of weldg parameter o the weld jot qualty. Cotrollg the weldg parameters such as arc voltage, weldg curret ad weldg codtos such coolg tme betwee each pass could help cotrollg weldg qualty ad matag good weld qualty. These results also suggest that statstcal such as Mahalaobos dstace (MD) ad ormal dstrbuto method could be used to determe the ole weldg qualty to avod jot falure. Ackowledgmets: Ths research was facally supported by the Mstry of Kowledge Ecoomy (MKE), Korea Isttute for Advacemet of Techology (KIAT) ad Daekyug Leadg Idustry Offce through the Leadg Idustry Developmet for Ecoomc Rego

6 Appled Mechacs ad Materals ols Referece [1] Feg, S., O. Hroyuk, T. Hdeor, K. Yomzo, ad S.Hu, 2011: Iteratoal Joural of Precso Egeerg ad Maufacturg, olume, 12, 6. [2] Wu, C. S., J. Q. Gao ad J. K. Hu 2007: Iteratoal Joural of Measuremet Scece ad Techology.18. [3] Adolfsso, S., A. Bahram, G. Bolmsjo ad I. Claeso, 1999: weldg Joural. 78, 2. [4] Pa, L.K., C.C. Wag, S.L. We, H.F. Sher, 2007: J Mater Process Techol 182: [5] Pal, K., S. Bhattacharya, S. K. Pal, 2010: J Mater Process Techol 210: [6] L, X. ad S. W. Smpso, 2009: Scece ad Techology of weldg ad jog, 14, 2. [7] Taguch, G. ad R. Jugulum, 2002: The Mahalaobs Taguch strategy: A patter techology system, Joh Wley & Sos, New York. [8] Avshek, P. ad J. Mat, 2010: Expert System wth Applcatos, 37, 2. [9] Abbas, S. E., A. Aaghae ad M. Fazlal, 2011: Joural of Idustral Egeerg, 45. [10] Rusz, J., J. Bber ad M. Loptsberger, 2007: Iteratoal Joural of Advace Maufacturg Techology, 33, [11] Targ, Y. S., W. H. Yag, S. C. Juag, 2000: Iteratoal Joural Advace Maufacturg Techology, 16. [12] Smpso,S. W., 2008: Scece ad Techology of weldg ad Jog, 13, 1. [13] Lee, H. T., M. Wag, R. Maev, E. Maeva, 2003: Iteratoal Joural Advace Maufacturg Techology, 20, [14] Zhyog, L., W. Bao, D. Jgb, 2009: Joural Materal Process Techology, 209.

7 Advaces Eergy Scece ad Techology / Predcto Weldg Qualty Mult-Pass Weldg Process Usg Mahalaobs Dstace Method /