CONTRIBUTION OF INDIVIDUAL HARDENING PROCESS PARAMETERS ON MECHANICAL PROPERTIES OF 6061 ALUMINUM ALLOY - TAGUCHI APPROACH

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1 May st 3 rd 04, rno, zech Republc, EU ONTRIUTION OF INIVIUL HRENING PROE PRMETERS ON MEHNIL PROPERTIES OF 606 LUMINUM LLOY - TGUHI PPROH l M. HT, hmad FLHTI, nette NNINGER epartment of Mechancal and Industral Engneerng, Unversty of Trpol, Trpol, Lbya, epartment of Mechancal and Industral Engneerng, Unversty of Technology, Venna, ustra, EU bstract The commercal age hardenng l-mg-s (606-type) alloys fnd a wde range of applcatons as structural materal n automotve ndustral, due to the favorable combnatons of good mechancal propertes, formablty and excellent corroson resstance. However, the precptaton hardenng of the alloy s sluggsh and ts acceleraton would be benefcal n mprovng the alloy propertes. There are several ways to accelerate the precptaton process, fast quench and/or deformatons (stretchng) are among those processng technques n producng excess vacances, whch may enhance the transformaton rates of precptatng strengthenng phases n the alloy. The ndvdual effects of hardenng process parameters on agng response and mechancal propertes of l-mg-s (606-type) are carred out by employng Taguch approach. The L9 orthogonal array (O) s selected wth 4-parameters, namely, quenchng meda, delay tme before stretchng, stretchng steps and delay tme after stretchng, and 3-levels for each parameter to determne the most nfluental parameter and to optmze mechancal propertes when the alloy s artfcally aged. The obtaned results show that the combned mechansm of precptaton hardenng and stran hardenng n the optmal levels mprove the alloy agng response and thus the mechancal propertes, for ths set of experments. Keywords: lumnum lloys, age hardenng, stretch process, Taguch Method.. INTROUTION The expandng demand of the world market place of usng lghtweght materals, are the drvng force to mprove exstng lght alloys and to develop new lght alloys wth good mechancal propertes and servce performance to meet the ncreasng needs of these alloys. The heat treated hardenable l-mg-s (606-type) alloy s among those alloys ganng promnence by automotve and aerospace ndustres. For the l-mg-s alloys, the precptaton sequence [, ] can be represented by αsss solute-clusters GP zones β"- needles β'-rods β (MgS phase), and the hardenng effect s due to the formaton of β"- needles and/or β'-rods shaped. Several studes were reported n the lteratures to mprove the agng response of ths alloy. Hatab et. al. [3] nvestgated the effect of beryllum addton on age hardenng behavor of the l-mg-s (6063-type) alloy. They found that the addton of beryllum enhanced transformaton rates and reduced agng tmes to reach the peak aged condton. heng-yu hou et al. [4] studed the effect of heat treatments on 606alumnum alloy deformed by cross-channel extruson (E), and they reported that ths method enhanced the mechancal propertes of the alloy. The development of hgh strength l-mg-s alloy through cold rollng and agng has been reported by Nranjan et al. [5]. They reported that the combnaton of hgh strength and ductlty could be mproved by severe plastc deformaton followed by anneal/agng treatment. Jaafar et. al [6] nvestgated the effects of composton on mechancal propertes and mcrostructural development of dlute 6000 seres alloys. They found a correlaton between the Mg and S contents on the mechancal propertes and mcrostructures after agng treatment, and reported that an ncrease of solute contents of MgS and excess S resulted n an ncrease n hardness and tensle propertes of the alloys. The tensle propertes of 606 alloy n dfferent desgnated precptaton hardenng and cold workng condtons were nvestgated by Mrzakhan et al. [7]. They reported that combnaton of work hardenng and

2 May st 3 rd 04, rno, zech Republc, EU precptaton hardenng mproved mechancal propertes and negatve or postve effect of pre-agng manly depend on the amount of cold work. Taguch [] desgn of experment (OE) makes the use of orthogonal arrays to mnmze the number of parameters combnatons that are requred to test the parameter effects and can be performed at a lowered cost and tme wth results comparable to a full factoral experment. The am of ths study s to nvestgate the nfluence and contrbuton of ndvdual hardenng process parameters on mechancal propertes when the alloys aged at 60 o for 5 hours of the commercal 606 alumnum alloy usng Taguch OE.. EXPERIMENTL N ESIGN.. Materals The as-receved specmens are n the form of standard tensle specmens made from commercal 606-T6 alumnum alloy (chemcal composton, refer to reference [9]) wth thckness of ~.60mm by ~.6mm wdth by 50mm gage length. The specmens were soluton heat treated, quenched, stretched and then artfcally aged as shown n Fg.. ll specmens were deformed (stretched) and tenson tested usng Electromechancal testng machne Zwck (type-z050), wth load cell of 50 kn capacty; coupled wth extensometer type ; wth controlled stran rate of /s. The as-receved alloys n T6 condton are tested to determne the tensle propertes for comparson purposes.. esgn of experments Fg. Process dagram for 606 alumnum alloys Taguch desgn of experments [] can be used to optmze a complcated age hardenng process that has several varables. Hence, the quenchng meda (Qm), delay tme (natural agng) before stretch (t), stretch steps (Sp) and delay tme after stretch (t) are consdered to affect the precptaton process and thus the mechancal propertes of the alloy. standard Taguch experment of L9 (3 4 ) orthogonal arrays wth four parameters and three levels for each parameter s selected to study the effect of hardenng process parameter combnatons and to determne the contrbuton of each parameter on mechancal propertes of the alloy. The nteractons between the process parameters are neglected. Tables and show the hardenng process parameters and ther levels, and expermental layout respectvely. fter selectng the approprate O, the next steps n Taguch OE are to run the experments and then to evaluate the results of experments by applyng statstcal analyss (sgnal -to- nose rato and analyss of varance) n order to determne whch parameter are nfluental the qualty response.

3 May st 3 rd 04, rno, zech Republc, EU Table Hardenng process parameters and ther levels for 606 alumnum alloy. Parameters Level Level Level 3 Quenchng meda, Qm elay tme before stretch, t, mn Stretch steps, Sp Sngle (3%) Twce (.5% each) Trple (% each) elay tme after stretch,, t, mn Table Expermental layout usng L9 (3 4 ) orthogonal arrays Experment Quenchng elay tme Stretch steps elay tme Number meda before stretch after stretch Sngle (3%) Twce (.5% each) 3 Trple (% each) 4 Twce (.5% each) 5 Trple (% each) 6 Sngle (3%) 7 Trple (% each) Sngle (3%) 9 Twce (.5% each) Taguch [] has developed the sgnal-to-nose (S/N) rato as a response of the experment, whch s a measure of the amount of varaton present wthn a tral when nose factors are presented. Three categores may be used to perform analyss of S/N ratos as response: lower-s-better (L), nomnal-s-better (N), and hgher-s-better (H). In ths study, strength propertes (YS - yeld stress, TS - tensle stress, and FS - fracture stress) treated as qualtes response, and hence the S/N rato for the hgher-s-better characterstc s used for maxmzng the alloy propertes and can be calculated [] usng equaton : ( S / N ) () r H. 0log[ / y ] r where r s the number of measurements n the tral, and y s the th measured value of propertes n a row. The optmal level can be determned from the largest value of the calculated S/N rato among all other levels of the parameters. nalyss of Varance (NOV) s used to determne the nfluence and contrbuton of each paramter on the obtaned results. The calculaton of NOV [] can be obtaned base on the total sum of the squares (T) from the total mean of S/N ratos, and gven by equaton (): m m ( S / N ) m T ( S / N ) () where m s the total number of experments, and (S/N) s the th calculated value n a row. The sum of squares due to each parameter (p) s gven by equaton (3): j q (( S / N) ) m j p ( S / N) q m (3)

4 May st 3 rd 04, rno, zech Republc, EU where the subscrpt (p) represents one of the experment parameters (,, or ), j th level number of the specfc parameter, q s the repetton of each level of the parameter. The percentage (P) of the contrbuton of each parameter to the results s obtaned usng equaton (4): ( ) p P x00 (4) ( ) T fter obtanng the optmal levels of control parameters, the confrmaton test s the fnal step n Taguch desgn of experment method, whch s performed to verfy the mprovement of the qualty characterstc usng the optmal levels of parameters. If the confrmaton test equaton do not predct results of varous combnatons of control parameters, then a new Taguch OE s requred. The predcted (S/N)p rato usng the optmal levels of the process parameters can be determned usng equaton (5): n ( S / N) ( S / N) [(( S / N) ) ( S / N) ] (5) predct mean opt mean 3. RESULTS N ISUION 3. nalyss of sgnal-to-nose (S/N) rato Table 3 shows the expermental results for tensle propertes and the calculated S/N ratos accordng to equaton (). Fgs. and 3 show the stress-stran curves for runnng experments number and the ntal condton respectvely. The stress-stran curves are typcal for the other runnng experments. It s obvously from the obtaned results that experment number dsplays the better strength propertes among the nne runnng experments wth optmal levels of water quenchng, delay tme before stretchng (), sngle stretch of 3%, delay tme after stretchng () and then artfcal agng at 60 o for 5 hours, whch gves 307 yeld strength, 33 tensle strength and 303 fracture strength. The alloy hardenng s attrbuted to formaton of vacances clusters (water quenchng); solute clusters (delay tme before deformaton); dslocaton densty (deformaton); vacances or solute clusters and/ or substructure change (delay tme after deformaton); all these concentraton of defects assst n formaton of fne and hgher densty of coherent β"- needles precptatng strengthenng phases durng artfcal agng at 60 o for 5hrs. Table 3 Expermental results and calculated S/N ratos for 606 alumnum alloys Experment Number verage YS # 0.% S/N ratos verage TS # S/N ratos verage FS # S/N ratos # verage of three tenson tests; YS: Yeld Strength; TS: Tensle Strength; FS: Fracture Strength

5 May st 3 rd 04, rno, zech Republc, EU 3. nalyss of varance - NOV The strength propertes are analyzed usng equatons ( to 4) of the NOV method and the obtaned results for yeld strength, tensle strength and fracture strength are gven n Tables 4, 5 and 6 respectvely. Table 4 NOV results for yeld strength of 606 alumnum alloys Parameters egree of Freedom F Sum of Squares ontrbuton Percentage P Quenchng meda, Qm % elay tme before stretch, t % Stretch steps, Sp % elay tme after stretch,, t % Total % Table 5 NOV results for tensle strength of 606 alumnum alloys Parameters egree of Freedom F Sum of Squares ontrbuton Percentage P Quenchng meda, Qm % elay tme before stretch, t % Stretch steps, Sp % elay tme after stretch,, t % Total %

6 May st 3 rd 04, rno, zech Republc, EU Table 6 NOV results for fracture strength of 606 alumnum alloys Parameters egree of Freedom F Sum of Squares ontrbuton Percentage P Quenchng meda, Qm % elay tme before stretch, t % Stretch steps, Sp % elay tme after stretch,, t % Total.05 00% The NOV results show that the delay tme (natural agng) before deformaton s the most sgnfcant parameter; followed by delay tme after deformaton; followed by quenchng meda and the least nfluence parameter s stretchng step wth contrbutons shown n Tables 4, 5, and Experment Verfcaton The predcted strength propertes are determned by usng equaton (5). The results of confrmaton test are shown n Table 7. comparson between predcted propertes usng equaton (5) and actual experment are consstent and n a good agreement wth experment number showng n Table, and t can be used to determne the strength of alloy at any other combnatons of process parameters for ths range of experments. Further nvestgatons by transmsson electron mcroscopy (TEM) and dfferental scannng calormetry (S) are needed to understand n clear way the mechansms between stran hardenng and precptaton hardenng on mechancal propertes of runnng experment No.. Table 7 Results of verfcaton test for optmal hardenng process parameters for strength propertes ondtons Level rtfcal agng, hrs Yeld strength, Tensle strength, Fracture strength, Predcton Experment * Intal s receved T **Improvement 9 Not affected * Intal condton, the specmens are soluton heat treated at 535 o, water quenched, 0 mnutes delayed, and then artfcal aged at 60 o for 5 hrs. ** Improvement, the obtaned results of experment number s compared to ntal condton specmens. 4. ONLUSIONS The present study s demonstrated the applcaton of Taguch desgn of experment method for optmzng mechancal propertes to effcently establsh process parameters for the commercal 606 alumnum alloys. The obtaned mechancal propertes of the alloy by runnng experment No. (aged at 60 o for 5hrs) shows an ncrease ~9 n yeld strength, ~ n tensle strength, and no affect on fracture strength when compared wth ntal condton alloy aged at 60 o for 5hrs. The delay tme before (natural agng) deformaton s the most contrbuted parameter, followed by delay tme after deformaton, quenchng meda, and the least contrbuted parameter s the stretchng steps n ths set of the experments. The predcton

7 May st 3 rd 04, rno, zech Republc, EU equaton can be used to predct the strength of the alloy for any other combnatons of process parameters n ths set of experments. REFERENES [] Thomas, G. The agng characterstcs of alumnum alloys, Journal of nsttute of metals, vol.90, 96-6, 57 p. [] utta, I., llen, S.M., calormetrc study of precptaton n commercal alumnum alloy 606, journal of materals scence letters, vol. 0, 99, 33 p. [3] Hatab,., Youdels, W.V., Precptaton rates studes n l-0.7%mg0.4%s alloy, equvalence of hardness and resstvty technques, Recent Metallurgcal dvances n Lght Metals Industres, S.MacEwen and T.P. Glardeau, Eds, The Metallurgcal Socety of IM, Vancouver,.., anada, ugust, 995, 453 p. [4] heng-yu hou, he-we Hsu, Sheng-Long Lee, Kuan-Wen Wang, Jng-he Ln, Effects of heat treatments on 606 alumnum alloy deformed by cross-channel extruson, Journal of materals processng technology, 0, 00, p. [5] Nranjan, V.L., Har Kumar, K.., Subramanya Sarma, V., evelopment of hgh strength l-mg-s 606 alloy through cold rollng and ageng, Materals scence and engneerng 55, 009, 69 p. [6] za Jaafar, zm Rahmat, Ismal Zanol, Zuhalawat Hussan, Effects of composton on the mechancal propertes and mcrostructural development of dlute 6000 seres alloys, Journal of appled scences, (), 0, 775 p. [7] ahman Mrzakhan, Mostafa Mansournejad, Tensle propertes of 606 n dfferent desgnated precptaton hardenng and cold workng, Proceda engneerng 0, 0, 36 p. [] Ross, P.J., Taguch Technques for Qualty Engneerng, McGraw-Hll, New York, 9. [9] SM Specalty Handbook, lumnum and lumnum lloys, J.R avs, ed., SM, Oho, US, 993.