Crystal preferential orientation of Al-1.0%Si alloy in

continuous unidirectional solidification process

ZHANG Hong(张 鸿)¹, WANG Zi-dong(王自东)¹,

XIE Jian-xin(谢建新)¹, CHEN Ya-jun(陈亚军)², WANG Ying-wei(王瑛伟)¹

1. School of Materials Science and Engineering, University of Science and Technology Beijing,

Beijing 100083, China;

2. School of Mechanical Engineering, Tsinghua University, Beijing 100084, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Al-1.0%Si alloy rods containing unidirectional microstructures were fabricated by continuous unidirectional solidification (CUS) process. The crystal growth orientation evolution of Al-1.0%Si alloy in CUS process was studied. The microstructure was observed and the orientation results examined by electron back scattering diffraction(EBSD) indicate that at the beginning of CUS process the crystals grow along a certain preferred orientation, 〈100〉direction, but there are also other growth directions deflecting from this preferred orientation. It is found that as directional solidification continues, the crystals with high growth speed remain and those with low growth speed vanish. The crystal preferred orientation tends to be 〈100〉direction in competition growth process. Namely, the 〈100〉fiber texture of Al-1.0%Si alloy rod becomes stronger in the stable directional solidification establishing process. When the solidification parameters remain stable, the crystal growth orientation also obtains in steady state. Once the technological parameters fluctuate, the stray crystals deviating from the preferred orientation appear. After the process of competition growth the crystals regain strong preferred orientation. The orientation distribution function(ODF) results also testify the above conclusions.

Key words: Al-Si alloy; preferential orientation; continuous unidirectional solidification

1 Introduction

Al-Si alloy wires are widely used as bonding wires in integrated electrical circuits because of their excellent conductivity and anti-erosion properties[1]. Since the aluminum-silicon rods are materials for Al-Si wires, their properties directly influence the performance of ultimate bonding wires. It’s necessary to investigate the fabrication technology of such rods.

The continuous unidirectional solidification(CUS) process is an effective method for aluminum silicon rods containing unidirectional microstructures[2-4]. The difference between this technique and conventional casting technique lies in the mold. The mold for the former technique is heated, while that for the latter is cooled. So in the CUS process nucleation on the wall of the mold can be avoided[5-7]. Rods with excellent axial properties and high surface quality can be fabricated. There have been few reports on crystal growth of aluminum-silicon alloys in CUS process. In this study, the aluminum-1.0% silicon rods were produced by vacuum melting and CUS process with argon shield. This kind of rod contains unidirectional columnar crystals or single crystal with transverse crystal borders eliminated. The crystal orientations of the rods have important influence on their properties. So the crystal preferential orientations in CUS process were examined.

2 Experimental

Fabrication experiment of Al-1.0%Si alloy rods were conducted on the CUS equipment. In the experiment the alloy was melted in vacuum furnace with vacuum degree changing between 6.6 Pa and 20 Pa and molten temperature ranging from 680 ℃ to 710 ℃. After the alloy was held at molten temperature for about half an hour the rod was cast by withdrawal equipment. In the casting process the technological parameters were set as follows: the temperature of the molten alloy varying in range of 660-680 ℃, cooling water volume of 1 000 L/h, cooling distance ranging among 10-20 mm and casting velocity rising from 2.42 mm/min to 12.09 mm/min gradually.

The orientation measurements were performed at longitudinal sections of the Al-1%Si alloy rods. The textures were measured by electronic back scattering diffraction(EBSD) method. The ODFs were presented as plots of constant f₂ sections with isointensity contours in Euler space defined by the Euler angles f₁, Φ, and f₂ [8-10].

3 Results and discussion

3.1 Crystal orientations at beginning of CUS process

At the beginning of solidification the microstructure of Al-Si alloy rod (sample 1) is shown in Fig.1. The orientation results of crystals 1-4 in Fig.1 are shown in
Table 1. The heat flow transfer direction is. The orientations of crystals 1 and 2 are both. And the angle between  and  is 27.9?.Because this angle is rather big and the growth velocity is rather slow. The growth orientation of crystal 4 is [100], which is opposite to heat flow transfer direction, and is preferential growth direction. The orientation of crystal 3 is , which deviates from  with 12.6?. So the growth velocity of crystal 3 is between crystal 1(2) and crystal 4.

Fig.1 Microstructure of rod at beginning of CUS process

Since this deflection angle is small, crystal 3 can grow with crystal 4 at a certain stage together. In the process of crystal growth crystals with slow growth speed eliminate.

At the beginning of solidification or at the time of technological parameters changing polycrystal comes into being in the CUS progress. And such crystals grow forward and evolve into axial columnar crystals. Under the effect of columnar crystal elimination preferential growth mechanism and crystal combination mechanism single crystal Al-Si alloy can be obtained (sample 2), as shown in Fig.2. And its crystal orientation (〈100〉)  is listed in Table 2.

Fig.2 Single crystal of rod at stage of stable growth of CUS

Table 1 Orientation data of crystals of sample 1

Table 2 Orientation data of crystals of sample 1 (crystals 3 and 4)

3.2 Crystal orientation at stable stage of CUS process

When the withdrawal velocity increases from 3.0 mm/min to 3.4 mm/min columnar polycrystal generates again (see sample 3). Fig.3 shows the microstructure of sample 3. The orientations of the crystals are shown in Table 3.

Fig.3 Microstructure of rod with changing technological parameters

This demonstrates that when the withdrawal velocity increases, the crystals with different orientations, such as crystals 1 ([120]), 2 (), and 3, originate from the initial〈100〉single crystal. Thus the polycrystal forms again, as shown in Fig.3. In succedent growing process, the crystals adjacent to〈100〉orientation preferentially grow, and crystals deviate from 〈100〉orientation retards. At the same time new crystals adjust their orientations and tend to concentrate to〈100〉direction. And single crystal can be obtained at last, as shown in Fig.4. The crystal orientation data are listed in Table 4.

3.3 ODF plot analysis

Fig.5 shows the ODFs of sample 3. According to the plots of f₂ equaling 15?,  texture has maximum intensity (16.0), which is represented by inner curves in the ODF figure. The intensity of  component (8.0) is next to that of  texture, as indicated by the adjacent plots. Thus  is the main orientation of sample 3. While〈100〉fiber texture is minor orientation with intensity of 2.0. The  orientation deviates from [100] fiber texture about 15?, which demonstrates that the main growth orientation deflects from heat flow transfer direction a rather small degree. The above results reveal that at the beginning or change of technological parameters of CUS process polycrystal with different orientations generates.

Fig.4 Microstructure of final single crystal

The ODFs of sample 4 in stable growth stage is illustrated in Fig.6. It is clear from the figure that the sample exhibits a very strong 〈100〉texture. The intensity of〈100〉fiber texture obtains 32.0, which rises greatly compared with that of sample 3. This means in competition growth course〈100〉direction is the preferential orientation of Al-1.0% Si alloy rod.

Table 3 Orientation data of crystals of sample 3

Table 4 Orientation data of crystals of sample 4

Fig.5 ODF plots of sample 3

Fig.6 ODF plots of sample 4

4 Conclusions

1) Polycrystal generates at the beginning of CUS process.

2) When the solidification parameters remain stable, the crystal growth orientation also obtains steady state. Once the technological parameters fluctuate, the stray crystals deviated from the preferred orientation appears. After a process of competition growth the crystals regain strong preferred orientation.

3) The orientations deviating from〈100〉fiber texture eliminate in competition growth process because of the preferential growth under the conditions of CUS caused by crystal growth anisotropy. The crystals of Al-1.0% Si alloy rod tend to grow along the〈100〉fiber texture and the strong〈100〉texture is obtained finally.

References

[1] TIAN Chun-xia. Materials of conducting silks and their development using in electronics packaging [J]. Chinese Metal of Rare Metals, 2003, 27(6): 782-787. (in Chinese)

[2] ZHANG Hong, XIE Jian-xin, WANG Zi-dong. Fabrication of pure copper rods containing continuous columnar crystals by continuous unidirectional solidification technology [J]. Journal of University of Science and Technology Beijing, 2004, 11(3): 240-244.

[3] XIE Jian-xin, WANG Zi-dong, WU Chun-jing, HU Han-qi, LI Jing-yuan, CHANG Guo-wei. Fabrication technology for ultra-high strength steel wire strengthened by unidirectional fibrous crystals [J]. Journal of University of Science and Technology Beijing, 1998, 20(6): 556-559. (in Chinese)

[4] XIE Jian-xin, ZHANG Hong, WANG Zi-dong. Processing technologies for high property metal materials by precise microstructure control [A]. Proceedings of the 1st Materials Science and Technology Conference Paper Collections by Chinese Scholars Studying or ever Studied in Japan in 21 Century [C]. Beijing: Metallurgy Industry Publishing Company, 2000: 109-117. (in Chinese)

[5] SODA H, MCLEAN A, SHEN J, XIA Q, MOTOYASU G, KOROTKIN M, YAN K. Development of net-shape cast aluminium-yttrium alloy wires and their solidification structures [J]. Journal of Materials Science, 1997, 32: 1841-1847.

[6] SODA H, MCLEAN A, WANG Z, MOTOYASU G. Pilot-scale casting of single-crystal copper wires by the ohno continuous casting process [J]. Journal of Materials Science, 1995, 30: 5438-5448.

[7] SODA H, MOTOYASU G, MCLEAN A, OHNO A. Alloy casting by the horizontal ohno continuous casting system [J]. Cast Metals, 1993, 16(2): 76-86.

[8] LIU W C, ZHAI T, MORRIS J G. Texture evolution of continuous cast and direct chill cast AA 3003 aluminum alloys during cold rolling [J]. Scripta Materialia, 2004, 51: 83-88.

[9] LIU W C, MORRIS J G. Texture evolution of polycrystalline AA 5128 aluminum alloy with an initial {001}〈100〉 texture during rolling [J]. Scripta Materiallya, 2002, 47: 487-492.

[10] LI D S, GARMESTANI H, SCHOENFELD S. Evolution of crystal orientation distribution coefficients during plastic deformation [J]. Scripta Materialia, 2003, 49: 867-872.

(Edited by LI Xiang-qun)   

Foundation item: Project(2002AA336070) supported by the Hi-Tech Research and Development Program of China

Corresponding author: ZHANG Hong; Tel: +86-10-62333152; E-mail: zhanghongyf@sohu.com