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Available online at w.sc iencedi rec t .commcteNoe D IRCOT . Transa ctions ofNonferrous MetalsSociety of China@

Trans. Nonferrous Met. SOC.China 16(2006) 376-381www.csu.edu.cnlysxbi

Effects of magnesium content on phase constituents of Al-Mg-Si-Cu alloysLIU Hong($d Z)',ZHAO Gang(& Wq)', LIU Chun-ming(3118s8)2,ZUO Liang(& @)*

1. School of M echanical Engineering, Shandong Institute of Light Industry, Ji'nan 25010 0, China ;2. School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China

Ab stra ct: B y means of scm nin g electron microscopy(SEM ), energy dispers ive spectrum(EDS), X-ray dii%-actometry(XRD) a ndmetallographic analysis, the effects of variation of magnesium content on phase constituents of Al-Mg-Si-Cu alloys were investigated.The results indicate that the constituents formed during casting alloys are main All,9CuM&,lSi3,3, A ~ ( M I I F ~ ) ~ S ~ ~and MgzSi, whilepure Si is only present in the alloy containing lower magnesium content. Increasing Mg content leads to increasing the amount ofMg2Si, but decreasing the amount of A11.9C~M&.lSi3.3a n d A L + ( M ~ I F ~ ) ~ S ~ ~ .During the following homogenization process,A11.9C~M&.ISi3.3is completely dissolved, A4(M nFe)3S iz and pure Si remain unchanged. A fter rolling and final heat treatment, theconstituents in the alloys change no longer.Ke y words: Al-Mg-Si-Cu alloys; phase constituent; magnesium content; A11 ,9C ~M &.l Si3 ,3;AL+(MIIF~)~S~*;Mg2Si

1 IntroductionThe heat-treatable A1-Mg-Si-Cu(6000 series) alloysused as automotive body sheet materials are beingbrought to practical use more and more in North

American and Europe, and becoming optimal materialscaused by mass reduction of automotive instead of steelsheets[l-51. Magnesium, as a major alloying element inAl-Mg-Si-Cu alloys, has been widely investigated. Theprevious work included the influences of magnesiumcontent on precipitation behaviour, precipitation kineticsand peak-hardness of Mg2Si hardening phase formedduring aging[6-81. In addition, it was also reported thatincrement in m agnesium content had action to soften ingeffect[9] produced during pre-aging treatment, andpaint-bake response[ 10, 111. How ever, up to date, littleattention has been given to the influence of magnesiumcontent on phase constituents of Al-Mg-Si-Cu alloys.In this work the authors aim to investigate theinfluence rule of variation of magnesium content, whilekeeping the contents of Si, Cu and Mn constant, onconstituents characteristics, in terms of type, amount andmorphology, through various treatments includingas-cast, homogenization, rolling, solution treatment andaging, so as to effectively control the change of consti-tuents by controlling the proper proportion between Mg

and Si, and establish foundation for enhancing theproperties of alloys.2 Experimental

The alloys studied in this work were prepared usinghigh purity aluminium(99.9%Al), electrolysis copper,industrial pure magnesium, and intermediate alloys ofA1-9.5%Si and Al-9.0%Mn(mass fraction, %). These rawmaterials were melted in an electric resistance cruciblefurnace, and then cast into ingots (220 mm X 120 mm X30 mm ) in the copper mould with cooling water. Thechemical composition of the alloys prepared is given inTable 1.Table 1 Chemical compositions of experimental alloys(mass!?action, %)AlloysN o. Mg Si Cu Mn Fe A1

1 0.59 1.12 0.72 0.32 0.08 Bal.2 0.80 1 . 1 1 0.67 0.32 0.03 Bal.3 1.19 1.06 0.65 0.27 0.09 Bal.4 1.70 1.08 0.67 0.29 0.04 Bal.Two-stage homogenization treatment of the ingotswas performed at 470 "C for 5 h and at 540 'C for 16 h.Then the ingots were hot-rolled and cold-rolled to thin

Foundation item: Project(2002AA331050) supported by Hi-tech Research and Development Program of China; project(O208) supported by Science andTechnology Research of Ministry ofEducation ofChinaCorresponding author: LIU Hong; Tel: + 86-53 1-88523981 ; E-mail: hongshuox in@163.com

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LIU Hong, et al/Trans. Nonferrous Met. S OC .China 16(2006) 311sheets of 1.2 mm in thickness. Specimens to be analysedin the as-cast and homogenization states were cut fromthe top of ingots, respectively; other specimens, to beanalysed, cut from cold-rolled sheets were solutiontreated at 540 "C for 30 min and aged at 170 "C for 30min.The specimens, without chemical etching, wereexamined using an LEICA MPS30 optical microscopeand SSX-550 SEM. The compositional analysis ofvarious phases was performed by DX-4 energyspectrometer coupled with SEM. The XRD equippedwith the C u K , radiation was also used for further identi-fication of the phases.3 Results and discussion3.1 As-cast microstructu reThe as-cast alloys contain a substantial amount ofconstituents. They form dendritic network structure.These constituents present in an irregular shape such asplates and particles, and increasing magnesium contentincreases the amount of the constituents markedly, asshown in Fig. 1.

Fig.2 shows the SEM micrographs of the consti-tuent shapes of the as-cast alloys and energy spectrumcurves of the typical phases. Table 2 lists the EDSanalysis results of the positions marked in those SEMmicrographs. It is visible that the main constituentsidentified can be divided into four groups: AICuMgSi,AIMnFeSi, MgSi and Si. The results of the XRD

measurement as shown in Fig.3 (as-cast) further confirmthat these four groups of constituents are A11.9Cu-Mg4.1Si3.3,A14Mn3Si2,MgzSi and pure Si phase.

The results of EDS analysis and XRD measurementprove that AlCuMgSi phase with the mole ratio of C d S iin a range of 0.22 and 0.35 is A11.9CuMg4.1Si3.3,i.e.Q-phase[l2]. Its shape is granular, shown as point 1 inFigs.Z(a)-(d). In comparison with the XRD patterns ofthese alloys, it can be s een that with the increment of themagnesium content, there is no significant variation forthe (1 11) and (21 1) peaks of AlI,9CuMg4,1Si3.3phase. Inaddition, a slight increase of the (101) peak, which isoverlapped with (111) peak of Mg2Si phase, isconsidered to be the contribution fro m Mg2S i. However,weakening of the (401) peak till disappearance impliesthat increasing the magnesium content decreases theamount of A11.9CuMg4.1Si3.3phase.EDS analysis shows the mole ratio of Mg/Si forMgSi phase is close to 2, indicating that the MgSi phaseis Mg2Si, which is further confirmed by the XRDanalysis. This phase presents in an irregular plate-like inblack marked as point 2 in Figs.2(a)-(d). The SEMresults as shown in Fig.2 also display that increasing themagnesium content obviously increases the amount ofMg2Si phase. When the m agnesium content reaches1.7% (Alloy 4), the Mg2Si phase presents a feature ofeutectic marked as point 5 in Fig.2(d).

Because the crystallizing point of Mg2Si is higherthan that of Q-phase[l3], as a result, for alloyscon ta in ing the same con ten t o f S i , the more the M g

Fig.1 As-cast microstructures of alloys: (a) Alloy 1; (b) Alloy 2; (c) Alloy 3; (d) Alloy 4

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( e )

LIU Hong, et al/Trans. Nonferrous Met. SOC.China 16(2006)

A1 I -AICuMgSi

I 3 5 7 9ElkeV

1 3 5 7 9ElkeV

2 - MgzSiMg

3 5 7 9ElkeV

4- Si

I 3 5 7 9ElkeV

Fig.2 SEM photographs of as-cast microstructures and EDS analyses of phases in alloys: (a) Alloy 1; (b) Alloy(d) Alloy 4; (e)-(h) EDS analysis

2; (c) Alloy 3;

content, the more the amount of MglSi formed ahead,implying that more Si is consumed, i.e. Si content in thealloy is decreased, resulting in the reduction o f theamount of AlI.9CuMg4.1Si3.3phases containing Si.

For AlMnFeSi phase in the alloys, the characteristic

peak of AlMnSi phase without Fe content is obtained bythe XRD measurement, but EDS analysis indicates thatthe phase contains AlMnFeSi. Since in the AlMnFeSiphase the content of Fe is low while that of M n is high, itis believed that AlMnSi phase is formed during casting,

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LIU Hong, et aVTrans. Nonferrous Met. S OC.China 16(2006)

I

37 9

1

) 30 40 50 60 70 80204")

D 30 40 50 60 70 80204 O )

Fig3 XR D patterns of Alloy l(a), Alloy 2(b), Alloy 3(c) and Alloy 4(d) in as-cast state( 1) and after ho mog enization(2): 0 AI,.&u-Mp4,,Si3.,; 0 AI4Mn3Si2(Al4(MnFe)3Si2);0 Mg2Si;V A155MnzoSi25(A155(MnFe)2~Si25);W SiTable 2 ED S analysis results of constituents of as-cast alloys(mo1e fraction,%)

Alloy Measured position Al c u Mg Si Mn Fe x (Mn+Fe)/x(Si) x (Cu)/x (Si)1 63.1 5.0 9.6 22.3 0.223 72.6 11.1 12.1 4.2 1.471 58.3 5.3 18.9 17.4 0.303 75.2 10.2 11.5 3.1 1.431 62.3 5.7 9.9 22.1 0.263 71.8 11.2 12.0 5.0 1.521 61.6 9.4 1.8 27.2 0.353 74.3 10.3 9.7 5.7 1.49

1234

the formation of Al(MnFe)Si becomes probable due tothe replacement of some Mn by Fe, which is similar tothe discussion of Refs.[l4, 151. The mole ratio of(Mn+Fe)/Si gained by EDS analysis is between 1.43 and1.52, which is close to the value o f 1.5, namely, the moleratio of M d S i for ALM n3Si2 phase, therefore, it isbelieved that the phase formed in Alloy 1 to Alloy 4 isA & ( M ~ # e ) ~ s i ~ .The shape of the phase is thin-platemarked as point 3 in Figs.2(a)-(d). Moreover, it is clearthat the amount of the phase decreases with the increase

Point 4 marked in Fig.Z(a) exhibits a sphericalshape phase with dark-grey colour. Combined resultsfiom EDS and XRD indicate that this phase is pure Si.The pure Si phase only exists in A lloy 1 containing lowmagnesium. This might be due to the low magnesiumwhich is not enough to form compounds with Si,resulting in part of Si independently presented in the A1matrix.3.2 Homogenization-in magnesium content in comparison with the SEMmicrographs. Fig.4 shows SEM micrographs of the alloys afterhomo genization. It can be seen that the dend ritic network

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380 LIU Hong, et al/Trans. Nonferrous Met. SOC.China 16(2006)

Fig.4 SE M micrographs ofAlloy 1(a), Alloy 2(b), Alloy 3(c) and Alloy 4(d) after homogenizationformed by the constituents is eliminated by breaking upinto irregular pieces. SEM/EDS analyses show thatQ-phase (AlI.9CuM&.1Si3.3)is completely dissolved, andthe other phases like Al(MnFe)Si, pure Si and Mg2S i stillremain.

EDS analysis of the composition of the non-solutionconstituents in different morphologies, such as blockyand fine plates of light-grey in co lour (as sho wn as points2 and 3 in Fig.4) reveals that the mole ratio of(Mn+Fe)/Si is between 1.35-1.5, there is no evidentchange compared with the ratio of as-cast Al(MnFe)Si.Further results from XR D patterns shown in Fig.3(as-homogenized) confirm that such phases are stillALMn3Si2 (i.e. A14(MnFe)3Si2). Since the peaks ofA14Mn3Si2 phase in hom ogenization are higher thanthose of the as-cast phase, and the peaks of AISs(Mn-Fe)20Si25also present, it implies that the precipitation ofA14(MnFe)3Si2and A155(MnFe)20Si2stakes place duringhomogenization. On the one hand, this is becausemanganese is supersaturated in the matrix of aluminum,and on the other hand, because Q phase dissolves, incompany with the release of Si, and provides thecondition for the precipitation of Al(MnFe)Si asdispersion phases.In addition, EDS analysis of the black,irregular-shaped and non-solution constituent marked aspoint 4 in Fig.4 and XRD detection indicate that thisphase is Mg2Si, however, which presents in differentmorphology from that in the as-cast state, and distributesmostly in the form of blocky shape. It is more obvious

for Alloy 4 that the eutectic feature of Mg2S i phase in theas-cast condition disappears. Otherwise, pure Si remainsunchanged, shown as point 1 in Fig.4(a).3.3 Final heat-treatmentThe cast ingots after homogenization weremachined into the sheets by hot-rolling and cold-rolling,and aged at 170 "C for 30 min after solution treatmentat 540 'C for 30 min. The constituents in sheets arecompletely broken up, and present in a form of particles.Compared with the as-homogenized constituents, thetypes of constituents after rolling and final heattreatments remain unchanged.

4 Conclusions1) The main phases in the as-cast structure ofAL(0.6-1.8)Mg-1.O Si-0.7Cu-0.3Mn (mass fraction, %)

alloys are A11.9CuMg4.1Si3.3,A b ( M t ~ F e ) ~ s i ~and Mg2Si,while pure Si is only present in the alloy containinglower magnesium content. During the followinghomogenization process, A11.9CuM&.lSi3.3is completelyd isso lved . A b( M r~ Fe )~ s i ~and pure Si remain unchanged.

2) In the as-cast structure, A11.9CuM&.ISi3,3phasepresents in the form of part icles; A L ( M I I F ~ ) ~ S ~ ~phase isof thin-plate and M g2Si phase is of irregular, fine blockin black, but exhibits in eutectic feature in the alloyscontaining high magnesium content; pure Si phase iscircular one with black colour. In the as-homogenizedstructure, all insoluble phases are obviously refined,

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LIU Hong, et aVTrans. Nonferrous Met. SOC.China 16(2006) 381while their morphology basically remains unchangedexcept that the eutectic feature of as-cast M g2Si phasedisappears.

3) Increasing Mg content increases the amount ofMg2Si, but decreases the amount of A lI.&hMg4.1Si3.3andA&(MIIF~)~S~* .4) After rolling and doing final heat treatment, theconstituents in the alloys are not changed.References

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1895-1902.HIRTH S M, MARSHALL G J. Effects of Si on the aging behaviourand formability of aluminium alloys based on AA6016 materials[J].Marshall Science and Engineering A, 2001,319-321: 452-456.GUPTA A K, LLOYD D J, COURT S A. Precipitation hardening inAl-Mg-Si alloys with and w ithout excess Si[J]. Materials Science andEngineeringA, 2001,316(1-2): 11-17.GUPTA A K, MAROIS P H, LLOYD D J. Study of the precipitationkinetics in a 6000 series automotive sheet material[J]. MaterialsScience Forum, 1996,217 -222: 801-808.ZHUANG L, HAAN R D, BOTTEMA J, LAHAYE C T W,SMET PD. Improvement in bake hardening response of Al-Si-Mg alloys[J].Materials Science Forum, 2000, 331-337: 1309-1314.KLEMER S , HENKEL C, SCHULZ P, RANSHOFEN,UGGOWITZER P J, ZURICH. Paint bake response of aluminiumalloy6016[J]. Aluminium, 2001, 77(3): 185-189.VAUMOUSSE D, CEREZO A. An atom probe study of tine scalestructure in AlMgSi(Cu) alloys[J]. Materials Science Forum, 2002,396402,693-698.CHEN X 4 LANGLAIS J. Solidification behavior of AA6111automotive alloy[J]. Materials Science Forum, 2002, 33 1-337:2 15-222.LIU Hong, LIU Yan-hua, ZHAO Gang, LIU C hun-ming, ZUO Liang.Effects of Mn on con stituents of AI-Mg-Si-Cu alloys[J]. The ChineseJournal ofNonferrous Metals, 2004, 14(1I) : 1906-1911.(in Chinese)LIU Hong, ZHAO Gang, LIU Chun-ming, ZUO Liang. Phaseconstituents of some kinds of 6000 series aluminium alloys forautomotive body sheets[J]. Journal of Northeastern University(Natural Science), 2005, 26(1 I) : 1070-1073.(in Chinese)

(Edited by YUAN Sai-qian)