THE INFLUENCE OF THERMOMECHANICAL TREATMENT AND CHEMICAL COMPOSITION ON RECRYSTALLIZATION OF Al-Mg ALLOYS

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1 Assoition of Metllurgil Engineers of Seri Sientifi pper AMES UDC: =20 THE INFLUENCE OF THERMOMECHANICAL TREATMENT AND CHEMICAL COMPOSITION ON RECRYSTALLIZATION OF Al-Mg ALLOYS LJ.RADOVIĆ 1,, M.NIKAČEVIĆ 1,, M.POPOVIĆ 2,, E.ROMHANJI 2,d 1 Militry Tehnil Institute, R.Resnović 1, Belgrde, Seri, 2 Fulty of Tehnology nd Metllurgy, Krnegijev 4, Belgrde, Seri nenrd@ptt.yu, nik@ptt.yu, miljn@tmf.g..yu, d endre@tmf.g..yu ABSTRACT Rerystlliztion ehviour of AlMg3, AlMg4.5Mn nd AlMg6Mn lloy pltes sujeted to different thermomehnil tretments (TMTs), hs een studied y mens of optil mirosopy nd hrdness mesurements. Applied two-stge nneling t 565 C/6h C/4h simulted homogeniztion nd heterogeniztion proesses, respetively, nd ws followed y old rolling with 20, 40 nd %. Finl nneling ws performed for 3h t 250, 280 nd 350 C, with slow nd rpid heting rtes (0.6 nd 25 C/min). Inrese of rolling redution nd temperture, either individully or in omintion, promotes rerystlliztion during finl nneling. The rerystlliztion proess ws found non-simply relted to the hemil omposition, ut rther omplex rerystlliztion ehviour of these lloys is ttriuted to the effet of the size nd distriution of the seond phse prtiles formed during the thermomehnil tretment. Key words: Al-Mg lloys, hemil omposition, old deformtion, reovery, heting rte, rerystlliztion INTRODUCTION The ontrol of mirostruture development during thermomehnil proessing is essentilly importnt in the prodution of AlMg lloy sheet mteril. It is well known tht the hemil omposition nd proessing prmeters ffet the development of the mirostruture in these lloys, determining their mehnil properties nd formility [1-9]. Rerystlliztion hrteristis nd resulting grin sizes in Al lloy re losely relted to the mount nd distriution of seond phse prtiles in the mtrix estlished during preeding TMTs. [10-15]. At lrger pplied strins in prtile ontining lloys the inrese of the stored energy due to prtiles inreses the driving pressure for rerystlliztion. In ddition, more prtiles eome tive sites for nuletion (prtile stimulted nuletion-psn), whih lso enhnes the rerystlliztion [16]. On the other hnd, prtiulrly smll nd losely sped prtiles my exert signifint pinning effet on grin oundries nd retrd rerystlliztion. Therefore, the overll rerystlliztion ehviour of the prtile ontining mteril depends on differene etween the driving fore, due to inrese in stored energy, nd retrding fore, due to prtiles pinning effet [16-19].

2 2 MJoM METALURGIJA - JOURNAL OF METALLURGY The ojetive of this pper ws to ssess the influene of TMTs nd the ontent of Mg nd Mn on rerystlliztion proess nd rerystllized grin size, AlMg3, AlMg4.5Mn nd AlMg6Mn lloys. EXPERIMENTAL Three different Al-Mg lloys, produed y IMPOL-SEVAL Aluminium Rolling Mill AlMg3 (3.1 Mg, 0.03 Mn, 0.31 Fe, 0.09 Si); AlMg4.5Mn (4.1 Mg, 0.54 Mn, 0.36 Fe, 0.12 Si), AlMg6Mn (5.95 Mg, 0.57 Mn, 0.41 Fe, 0.16 Si) were tested in this work. The s reeived pltes were 7.4, 15 nd 8 mm thik in the s-reeived hot rolled ondition. The s-reeived pltes were nneled t 565 C for 6h with heting rte of 35 C/h, thn furne ooled to 450 C, held for 4h, nd finlly furne ooled to the room temperture. Following this proess the smples were old rolled y 20, 40 nd % redutions. Susequent nneling ws pplied t 250, 280 nd 350 C for 3h with rpid heting -RH (25 C/min) nd slow heting -SH (0.6 C/min). In oth ses ir-ooling ws pplied. To revel the grin struture, fter eletro-polishing, the smples were ethed in Brker s solution (25ml 40%HBF 4, 1000ml distilled wter). The rerystlliztion ehviour during finl nneling ws ssessed y oth optil metllogrphy nd hrdness mesurement. The verge grin size ws determined y the liner interept method. Hrdness ws mesured y the Vikers hrdness tester (HV5). RESULTS AND DISCUSSION The two-stge nneling ws pplied in order to develop s muh s possile homogenous mirostruture of the tested lloys. During the first step of nneling simulting homogeniztion, the dissolution of the intermetli prtiles t 565 C tkes ple s it ws found in previous ppers [13,14,20]. During susequent nneling t 450 C preipittion nd prtile orsening should tke ple, leding to inresed moility of grin oundries nd the development of rther homogenous grin struture [14,17,21]. Aordingly, this stge of nneling tretment n e treted s n heterogeniztion proess. Mirostruture nd prtiles distriution of the tested lloys fter two-stge nneling re shown in Figs.1 nd 2, respetively. In the AlMg3 lloy, the low Mn ontent enled the high moility of grin oundries nd the norml grin growth is the most expressed. In omprison with the AlMg3 lloy (Fig.1) the grin growth in the AlMg4.5Mn ws impeded (Fig.1). The reson for this n e the higher ontent of the seond phse prtiles, mostly due to the presene of MnAl 6 fine prtiles [10,11,22], nd lso due to the higher ontent of Mg-sed seond phse prtiles [13], whih lso n pin the grin oundries nd slow down their migrtion [11,15,17]. A higher Mg ontent nd lso the ddition of Mn in the AlMg4.5Mn lloy ontriute to the higher density of the seond phse prtiles ompred with AlMg3 lloy (Figs.2 nd ). In AlMg6Mn lloy, despite of signifint orsening of the prtiles (Fig.2), the seondry rerystlliztion ws impeded nd homogenous struture with equixed grins with smooth oundry ws otined. This ehviour is ttriuted to the lrge mount of smll dispersoids preipitted during slow ooling fter heterogeniztion nneling, whih effetively pinned the grin oundries.

3 THE INFLUENCE OF THERMOMECHANICAL TREATMENT AND 261 Fig. 1. Grin struture fter two-stge nneling of the AlMg3 (), AlMg4.5Mn () nd AlMg6Mn () lloy sheets Fig. 2. Prtile distriution in the tested lloys fter two-stge nneling: () AlMg3, () AlMg4.5Mn nd () AlMg6Mn The results given in the Tle 1 summrize the rerystlliztion ehviour of the previously old rolled Al-Mg lloys fter finl nneling. Optil mirosopy showed

4 262 MJoM METALURGIJA - JOURNAL OF METALLURGY tht the inrese of rolling redution or nneling temperture promote rerystlliztion. A higher rolling strin introdues higher stored energy thus promoting rerystlliztion (strin indued nuletion) [17, 19]. The effet of nneling temperture is equivlent to the effet of strin. The higher temperture filittes the nuletion nd grin growth [17, 19]. This results in the lrger numer of the rerystllized grins in the mirostrutures. Tle 1. Rerystlliztion ehviour of exmined Al-Mg lloys fter nneling for 3 hours t, C Alloy r (%) SH RH SH RH SH RH AlMg3 AlMg4.5Mn AlMg6Mn reovered strutures - prtilly rerystllized strutures - fully rerystllized strutures. SH - slow heting RH - rpid heting The results of Tle 1 lso implite strong influene of lloying dditions on the rerystlliztion ehviour. In the AlMg4.5Mn smples the rerystlliztion temperture ws found lower ompred to the AlMg3 lloy. However, in the AlMg6Mn smples the onsiderly higher ontent of lloying elements inreses the rerystlliztion temperture. Inrese in the volume frtion nd the size of seond phse prtiles in AlMg4,5Mn (Fig. 2) ws ssumed to e fvorle for the dislotion umultion, reting high dislotion density round prtiles. Also, these prtiles served s grin nuletion sites during stti rerystlliztion (PSN) [16]. It hs een estlished tht hrd, non-deformle prtiles lrger thn ritil size n serve s sites for rerystlliztion. For most Al lloys the ritil prtile size is round 1-2 µm [16]. On the ses of detiled metllogrphi exmintion, it ws estimted tht there ws n inrese in the mount of seond phse prtiles lrger thn 1 µm (Fig. 4) in AlMg4.5Mn. This vlue is in greement with some erlier pulished results [3, 8, 11-13, 15]. The prtiles inrese the dislotion density nd stored energy of deformtion, providing the driving fore for rerystlliztion. However, nother spet of present prtiles is the Zener drg pressure, whih sometimes leds to the opposite effet [8, 17, 18], i.e. prtiles of ll sizes my exert signifint pinning effet on moving oundries. This effet is more expressed if prtiles re smll nd prtiulrly losely sped [16-18]. Mirostruture development in AlMg6Mn lloy fter nneling t different tempertures is shown in Fig. 3. In this lloy the ddition of Mg lowers the solid soluility of Mn in Al [1,6] leding to preipittion of higher volume frtion of Mn-ering prtiles, whih hs een reported erlier [8, 12, 15].

5 THE INFLUENCE OF THERMOMECHANICAL TREATMENT AND 263 Additionlly, previous two-stge nneling signifintly ltered preipitte distriution resulting in highly orser prtiles in AlMg6Mn lloy ompred with prtiles in AlMg4.5Mn lloy (Figs. 4 nd ). At the sme time slight inrese in the numer of very fine prtiles ould e produed [4, 8, 11]. Fig. 3. Mirostrutures in AlMg6Mn lloy fter % redution nd nneling t: () 250 C/3h, () 280 C/3h nd () 350 C/3h; rpid heting Fig. 4. Prtiles in the tested lloys fter two-stge nneling: () AlMg4,5Mn nd () AlMg6Mn The prtiles orsening ppers to minimize ny enefiil effet noted in the AlMg4.5Mn lloy retrding stti rerystlliztion in AlMg6Mn lloy. The orsening leds to smller numer of prtiles nd thus the numer of potentil nulei dereses. However, the orse intermetli prtiles were roken down during rolling, nd this tendeny signifintly rises with inresing strin. This hs een reported erlier in the non-het tretle lloys, espeilly in Mn-ontining lloys [23]. Despite eing nuleted erlier, the new grins in the viinity of prtiles re smller in omprison to the other, proly euse of the lrger numer of nulei t prtiles (Fig.5).

6 264 MJoM METALURGIJA - JOURNAL OF METALLURGY Fig. 5. A non-uniform grin development in AlMg6Mn While AlMg3 lloy exhiits prtil rerystlliztion within 3 hours t 280 C, AlMg4.5Mn lloy ws rerystllized ompletely under the sme onditions (Fig.6). However, in AlMg6Mn lloy (Fig.6) new rerystllized grins my e seen round lrge prtiles, ut the frtion of rerystllized struture is smller ompred to AlMg3 lloy. Results shown in Fig.7 indite tht inresing the old rolling redution prior to nneling redued the rerystllized grin size. The results lso suggest tht the ddition of Mg up to 4.5% nd pproximtely 0.5% Mn redued grin size. However, fter higher redution (%), the influene of lloying dditions on grin size ws signifintly redued. Thus, the finl grin size seems to e predominntly influened y the rolling redution. In the AlMg3 lloy t low strin, the low prtiles ontent llowed the grin growth. The ddition of Mg nd prtile forming Mn, produed lrger mount of seond phse prtiles nd the preferene for more nuletion sites. This leds signifint redution in the finl grin size, prtiulrly under the higher strin [16]. Fig. 6. Mirostrutures of () AlMg3, () AlMg4.5Mn nd () AlMg6Mn fter 40% old rolling followed y nneling t 280 C for 3 hours; rpid heting

7 THE INFLUENCE OF THERMOMECHANICAL TREATMENT AND 265 The heting rte ws found to influene the onset of the rerystlliztion (Fig. 7). It seems tht slow heting rte promotes the reovery tht ours efore rerystlliztion, so the driving pressure is redued nd rerystlliztion is retrded. This n e lerly notied in the lloy without Mn, i.e. the AlMg3 lloy. In ontrst, the rpid heting suppresses reovery, promotes rerystlliztion nd indues the strt of rerystlliztion t lower temperture, whih re in ordne with previously pulished results [6,17,19]. Also, the more potentil nuletion sites tivted during rpid heting ould influene the rerystllized grin size. This ws onfirmed y grin size mesurements nd illustrted in Fig 7, for AlMg6Mn lloy C/3h, RH 70 AlMg6Mn 350 C/3h Slow heting r=20% Rpid heting r=40% 50 r=% Mg, % r (%) 50 Fig. 7. The influene of () Mg ontent nd () old rolling redution nd heting rte, on rerystllized grin sizes d v, µm d v, µm HV r, % HV r, % HV r, % Fig. 8. The influene of old rolling redution on hrdness fter nneling for 3 hours t: C, C, C, slow heting-losed symols; rpid heting opened symols; () AlMg3, () AlMg4.5Mn nd () AlMg6Mn

8 266 MJoM METALURGIJA - JOURNAL OF METALLURGY The vritions in the hrdness (Fig. 8) in Al-Mg lloys re in greement with mirostruture hrteristis. The onset of the rerystlliztion, oserved y metllogrphy, ws followed y softening. The hrdness rises with inresing strin in reovered struture (AlMg6Mn t 250 C-Fig.8) or in prtilly rerystllized struture when the frtion of rerystllized struture is smll (AlMg3 %+250 C-Fig.8). When rerystlliztion strts the hrdness ontinuously dereses with inrese of the rerystllized grins frtion. In ompletely rerystllized struture (AlMg4.5Mn lloy %+350 C-Fig. 8 nd Tle 1.) hrdness slightly inreses with inresing strin s result of grin size redution (Fig. 7). SUMMARY The influene of the hemil omposition nd proessing onditions on struture development in AlMg3, AlMg4.5Mn nd AlMg6Mn lloy sheets were investigted. The lloying dditions hve strong influene on the prtile size nd their distriution produed y two-stge nneling (565 C/6h+450 C/4h), nd the rerystlliztion ehviour fter susequent nneling. The pplied two-stge nneling ws found to e followed with n norml grin growth in AlMg3 nd AlMg4.5Mn lloys, while in AlMg6Mn it ws suppressed. The inrese of the Mg ontent from 3 to 4,5%, oupled with ddition of pproximtely 0,5% Mn, dereses oth the rerystlliztion tempertures nd the rerystllized grin size fter finl nneling. However, in the AlMg6Mn lloy in respone to the inrese in the Mg ontent, the rerystlliztion temperture rises nd the grin size slightly inreses. The inrese of the rolling redution prior to finl nneling dereses the rerystlliztion tempertures in AlMg3 nd AlMg4.5Mn lloys. On the other hnd in the AlMg6Mn lloy the rerystlliztion temperture seems to e independent of the pplied redutions. The rerystllized grin size is redued with inresing redutions in ll investigted lloys. The heting rte hs influene on the onset of the rerystlliztion nd rerystllized grin size. It ws ssumed tht the slow heting rte promotes the reovery tht ours efore rerystlliztion, reduing the driving pressure nd retrding rerystlliztion. In ontrst, the rpid heting suppresses reovery nd promotes rerystlliztion, reduing the rerystllized grin size. REFERENCES [1] ASM Hndook, Vol. 2, Metls Prk, Ohio, [2] C.Johnson nd D.Lloyd, Mt.Si.Forum, Vols , 2000, [3] Wei Wen, Yumin Zho, J.G.Morris, Mt.Si.Eng. A 392, 2005, [4] S.A.Court et l., Mt.Si.Eng. A , 2001, pp [5] J.Lui nd J.G.Morris, Mt.Si.Eng. A385, 2004, [6] R.E.Snders Jr. et ll, Mterils Forum, Vol. 28, ed. J.F.Nie et ll., 2004.

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