Materials Science Forum Vols. 449-452 (2004) pp. 605-608 online at http ://www.scien패c.nel iþ 2004 Trans Tech Publications, Switzerland

Rapid Grain Growth Of Hot Extruded AI-Zn-Mg-Cu-(SC) Alloy During Heat Treatment

Dong-Woo Suh1 , Sang-Yong Lee1, Jun-Yun Kang2 and Kyu Hwan Oh2

1 Materials Processing Department, Korea Institute of Machinery and Materials

66 Sangnam-dong , Changwon, Kyungnam , 641 -010, Korea

2 School of Materials Science and Engineering , Seoul National University

Shinrim-dong , Kwanak-gu , Seoul , 151-742, Korea

얀효I쁘으쁘흐: AI a lloy, Grain growth, Heat treatment

Abstract. Rapid grain growth and artificial aging characteristics during heat treatment is investigated for hot extruded AI-Zn-Mg-Cu-(Sc) alloys. Two AI-O.l wt%Sc alloys with different alloying element content are hot extruded to make T-shape bars at 380oC, and then the bars are solution treated for 2 hours at 4800 C followed by artificial aging for 24 hours at 120oC. Microstructural evolution ofthe hot extruded bar is analyzed with optical microscope and electron back scattered diffraction (EBSD) mapping. Two kind of extruded bar shows different grain growth behavior at surface region and different artificial aging characteristics. The interaction between the precipitates and the grain growth during the heat treatment is thought to be responsible for the different grain growth behavior.

Introduction

Hot extrusion is one of the most common manufacturing processes for Al alloys[l]. After the extrusion, post heat treatment is usually adopted to ensure the proper combination of the strength and toughness ofthe products. Since the energy minimization process such as recovery, recrystallization and grain growth as well as the diss이ution 없d precipitation of various compounds can proceed during the heat treatment, the microstructural change is likely to occur with the heat treatment. The microstructure is closely related with the mechanical properties of product, therefore its control during the heat treatment is a matter of concern. In this work, the rapid grain growth ofhot extruded Al alloy containing Sc during the heat treatment is studied. Recently, Al alloy containing Sc attracts great interests because Sc addition can increase the stren망h， thermal stability and the weldability of Al alloys[2J. Previously, present authors have reported the microstructural evolution of Al alloy containing Sc during the hot extrusion and heat treatment[3] , however, the rapid grain growth during the heat treatment was rather qualitatively investigated. A closer study on the rapid grain growth behavior is attempt in present work.

Exper imental Procedure

The chemical compositions of Al alloy used in present study are listed in Table 1. Billets of which diameter is 5 inch are continuously cast using mother alloy containing 2% of Sc. The billets are hot extruded to make T -shape bar at 3800 C after preheating of 1 hour. The extrusion ratio is 38: 1 and the extrusion speed is 0.0 17rnm1s. The shape and dimension of the extruded bar is shown in Fig. 1. The extruded bar is solution treated at 4800 C for 2 hours followed by artificial aging at 1200 C for 24 hours. The microstructural evolution during the hot extrusion and heat treatrnent was analyzed with the optical microscope and the electron back-scattered diffraction (EBSD) mapping. EBSD mapping also used for the analysis ofthe crystaIIographic detaiIs ofthe microstructures.

606 Designing , Processing and Properties of Advanced Engineering Materials

Results Fig. 1 Dimension ofthe extruded bar

Microstructures of as-extruded bar. The microstructure of billet at extrusion temperature is consists of equiaxed grains of which size are around 100μm with fine precipitates. Microstructural evolutions after hot extrusion 앙e shown in Fig. 2. Microstructural observation is carried out on a cross-section perpendic비ar to the extrusion axis. An EBSD mapping was used for the analysis ofthe upper surface region and the central region of the extruded bar. The solid line in Fig. 2 indicates the boundary of which misorientation angle exceeds 5 degree and it can be thought that this solid line represents the configuration of grain boundary. As shown in Fig. 2, fine equiaxed grains are found at upper surface region ofthe extruded bar, whilst severely elongated grains are observed at the central region of the extruded bar. The elongated grains at the central region of extruded bar represent that dyn없nic recovery is the main restoration process during the hot extrusion at the central region. On the other hand, the fine equiaxed grains near the surface suggest that recrystallization possibly proceed during the extrusion. lt is known that three types of dynamic recrystallization 없e likely to evolve such microstructure[4,5]. (i) discontinuous dynamic recrystallization, (ii) continuous dynamic recrystallization 뻐d (iii) geometric recrystallization. The discontinuous dynamic recrystallization is reported to be a limited restoration process in high stacking fault energy like AI alloys[6] . Considering the high extrusion ratio and the friction between the extruded material and die, it is thought that the continuous dynamic recrystallization or geometric dynamic recrystallization possibly proceed, because the continuous dynamic recrystallization or geometric dynamic recrystallization is Iikely to proceed under severely deformed condition.

(a) upper surface ofSl (b) center ofSl (c) upper surface ofS2 (d) center ofS2 Fig. 2 Microstructure of as-extruded bar (EBSD mapping)

Microstructural evolution during the heat treatment. Fig. 3 shows the microstructure of the extruded bar after the solution treatment and aging. For the central region of the extruded bar, the elongated grains are still found and the microstructure is similar to that of as-extruded bar shown in Fig. 2. For upper surface region, rapidly grown grains are usually evolved for Sl during the heat treatment. The microstructure of the upper surface region of S 1 consists of fine equiaxed grains and rapidly grown large grains as shown Fig. 3(a). However, the large grains are hardly found at upper surface region of of S2 during the heat treatment.

Materials Science Forum Vols. 449-452 607

(a) upper surface ofSl (b) center of S 1 (c) upper surface of S2 (d) center of S2 Fig. 3 Microstructure of T6 πeated bar

Crystallographic details. Fig. 4 represents the microstructural evolution of solution treated S 1 for ISmin, 30min 뻐d 60min at 480oC. lt can be clearly known that the large grains rapidly grow consuming the fme equiaxed grains during the solution σeatment. The inverse pole figures in Fig. 4 show the crystallographic orientation of fme equiaxed grains and rapidly grown large grains. During the solution treatment, the orientation of fine equiaxed grains along extrusion axis is mostly < 1 OO>+< 111 > which is well known annealing texture of drawn FCC materials[ 6] . The orientation of large grains along the extrusion 없is indicates somewhat different feature. The orientation along the extrusion direction is around <110> which is not usual texture for drawn FCC mate디외s. lt is interesting that few grains, which have <110> orientation along extrusion axis, are found in as extruded bar, but its growth rate is quite high during the heat treatment.

F

=二]

i

Extrusion direction (fme grains) Extrusion direction (large grains) Extrusion direction (large grains)

100 lnvene Pole (“‘u , ““‘ìn 11.8 0.0

T. direction (fine grains) T. direction (large grains) T. direction (large grains)

Fig. 4 Microstructure and inverse pole figure of S 1 (a) solution treated for ISmin, (b) solution treated for 30min 뻐d (c) solution treated for 60min

608 Designing , Processing and Properties of Advanced Engineering Materials

Discussion

It is reported that second phase particles and texture can be considered as main factors which lead to the rapid growth of a few grains as shown in Fig. 3(a)[6]. In present study, the rapidly grown grains at upper surface region usually have <110> orientation along the extrusion 없is， which is quite different from typical <100>+<111> fiber texture of drawn FCC material. Recently, Park[7] has reported that abnormally grown grains in Al wire have around <110> orientation along drawing direction. These results suggest that grain growth behavior during the heat treatment have some relation with crystallographic orientation of grains. That is, the boundaries of the grains which have <110> orientation along the extrusion axis is thought to have higher energy and mobility than those of the grains with textured orientation because within highly textured region the grain boundaries have a lower misorientation 뻐gle hence lower energy and mobility. However, considering the normal grain growth behavior of S2 during the heat treatment, of which texture is almost similar to that of S 1, it is thought th따 the texture is not the only inf1uencing factor on the rapid grain growth. The m에or

difference between S 1 and S2 is the amount of the alloying element. Since the 없nount of alloying elements of S 1 is higher than that of S2, the second phase particles probably have more remarkable effect on the grain growth behavior in S 1. The normal grain growth in S2 during the heat treatment imply that the difference in boundary mobility due to the texture development is not so enough to evolve the rapid grain growth, however, when the inf1uence of second phase particles becomes not negligible with the increase of alloying elements, the complex effect of texture and second phase particles may ev이ve the rapid grain growth. Further study on the interaction ofthe texture and second phase particles with the grain boundary mobility is in progress for more quantitative understanding of the rapid grain growth phenomenon during the heat treatment.

Summary

Rapid grain growth and artificial aging characteristics during heat treatment is studied for hot extruded AI-Zn-Mg-Cu-(Sc) alloys. For the extruded bar with higher alloying elements, SI , a few grains with <110> orientation along extrusion axis rapidly grow at surface region during the heat treatment, whilst, the microstructure of the extruded bar with lower alloying elements, S2, shows normal grain growth. The texture development and the second phase p없ticle are thought to be responsible for the different grain growth behaviors during the heat treatment.

References

[1] P.C.Varley: The Technolo용 01 Aluminum and lts Alloys (Newnes-Butterworths, London 1970)

[2] B.Lenczowski, R.Ra뼈， D.Wieser, G.Tempus, G .. Fisher, J.Becker, K.Folkers, R.Braun and G.L때ering: Aluminium, Vol.76 (2000), p.200

[3] D.W.S삐， S. Y.Lee, K.H.Lee, S.K.Lim and K.H.Oh: Proceedings of Intemational Conference on Advanced Materials Processing Technology 2003 (AMPT2003), Dublin, Ireland, Vo1.2 (2003) p.1626

[4] S.Gourdet and F.Montheillet: Mat. Sci.Eng., Vol.A283 (2000), p.274

[5] D.W.Suh, S.Torizuka, A.Ohrnori, T.lnoue and K.Nagai: ISIJ Intemational, Vol.42 (2002), p.432

[6] F.J . Humphreys and M.Hatherly: Recrystallizatiion and related annealing phenomena (Pergamon, Oxford 1995)

[7] H. Park and D.N.Lee: 1. Mat. Proc. Tech., Vol.1 13 (2001), p.551