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Generation, Validation and Application of Knowledge Links in the Chain Called Innovative Processing

1Shigeki Yoshie, 2Genjiro Motoyasu, 2Atsumi Ohno, 3Hiroshi Soda and 3Alexander McLean

1 Osaka Fuji Corp., Amagasaki, Hyogo-ken, Japan.

2 Department of Mechanical Science, Chiba Institute of Technology, Narashino-shi, Chiba-ken, Japan. 3 Department of Materials Science and Engineering, University of Toronto, Canada.

Keywords: Casting, Heated mold, Continuous casting, Bismuth alloy, Cast wire

Introduction

Throughout a long and distinguished career, the research activities conducted by Professor Guthrie at the McGill Metals Processing Centre have been characterized by a tri-partite concept involving the Generation, Validation and Application of new knowledge. In efforts to characterize and improve the performance of an existing process, or in the quest to generate fundamental information as a basis for the development of new manufacturing routes, this knowledge triumvirate so elegantly demonstrated by Professor Guthrie provides a strong foundation for progress within the exciting field of innovative processing. In this paper the development and implementation of new knowledge is illustrated with reference to the Ohno Continuous Casting process, a heated mold system that permits the generation of single crystal materials or cast products with a unidirectionally solidified structure. In this process, the mold is heated above the solidification temperature of the alloy being cast and cooling occurs outside the mold. Solidification thus takes place at the mold exit, significantly reducing or eliminating friction between the cast product and the mold wall. This casting configuration permits the generation of net-shape or near-net-shape cast products with a high quality surface and controlled solidification structure which in turn can result in materials with significantly enhanced properties.

Generation of Knowledge A New Clue for Cast Structure Control

One of the important factors that influence the quality of cast products is the way heat is removed from liquid metals. Casting processes involve heat removal in various ways: slow cooling, rapid cooling, or directional cooling in which the liquid solidifies into shapes with certain properties. During cooling, the formation of grains of different size and length occur, influencing the mechanical properties of the cast products. In the 1960s and 70s, the solidification structure of cast products was extensively studied and it was concluded that the equiaxed grains, observed in the center region of the cast ingot, formed on the mold surface at the initial stage of solidification and were carried by convection to the center of the ingot rather than nucleated within the melt at a late stage of solidification(1).These studies were further confirmed by the direct in-situ observation of solidifying alloys in a glass ampoule(2). Fig. 1 shows the Sn-5%Bi alloy solidified by cooling one end of the ampoule. The casting consists of a columnar zone at the cooled end and an equiaxed zone in the rest of the ingot. In order to clarify how and when these crystals formed, the solidification phenomena occurring at the cooling end were directly observed using the microscope as shown in Fig. 2. As can be seen in Fig. 2, many granular shaped crystals floated up along

the glass mold wall and pushed their way to the hotter end of the ampoule, while it was observed that crystals formed on the glass wall grew in globular shapes and then separated away from the wall to the hotter end due to the convection occurring within the melt.

Figure 1. Macrostructure of Sn-5%Bi alloy, exhibiting columnar and equiaxed crystals. The alloy was solidified by forced air cooling from one end (2).

Figure 2. Direct observation of solidifying alloy and crystal separation from a glass wall (dark region), cooled by forced air(2). Only in the later stages of solidification did the crystals on the wall finally grow in dendrite form as shown in Fig. 3, resulting in the columnar zone (Fig. 1).

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Figure 3. Dendrite growth from the cooled wall in the later stages of solidification (2).

Validation of Knowledge Laboratory and Pilot Plant Studies

These fundamental studies on solidification structure described above led to the inception of new casting systems. If the formation of seed crystals is promoted using a cooling device during pouring before the melt enters the mold, cast products of grain refined structure will be the result. This led to a seed pouring or semi-solid casting process as indicated in Fig. 4(3). The formation of crystals can be halted by heating areas where the crystals would generate. In this way cast products with fewer crystals will be the result. This concept formed the basis of a heated mold continuous casting technique known as Ohno Continuous Casting (OCC)(4). The general concept of the OCC process, as illustrated schematically in Fig. 5, is that molten metal is introduced continuously into an externally heated mold and the temperature of the mold is held just above the solidification temperature of the metal to be cast, thus preventing the nucleation of crystals on the mold surface. Heat is extracted from the cast product by means of cooling water located near the mold exit, differentiating the direction of heat flow in the OCC process from that in the traditional process. The features of the OCC process include the ability to produce:

- single crystal or unidirectionally cast products(3,4,6-8);

- net or near-net-shape cast products(5,9,10);

- a clean surface with no witness marks(11,12);

- cast rods with fewer cavities and porosity defects(12);

- cast rods with good workability(4,13,14).

Some examples of products are shown in Fig. 6.

Figure 4. Schematic illustration of seed pouring process (3).

Figure 5. Schematic diagram of Ohno Continuous Casting process in contrast to the traditional continuous casting process (5).

Figure 6. Examples of products (a) as-cast aluminum wire 15 mm in diameter, showing a high quality surface, (b) cast aluminum plate, chemically etched to reveal the directional structure, (c) as-cast Sn-57%Bi eutectic alloy tube 3.5 mm O.D. and 2 mm I.D., and (d) a foil of Sn-Bi eutectic alloy 0.1 mm in thickness, rolled from a cast wire 2mm in diameter. Fig. 7 shows the first OCC equipment built within the solidification laboratory at Chiba Institute of Technology to test the concept of heated mold continuous casting. Since then, various OCC units were built and tested to confirm the concept. Over the years from the late 1960s to the 1980s, many of the

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students trained in solidification at Chiba Institute of Technology entered the Osaka Fuji Corporation thus establishing an important linkage between academia and industry. With this relationship in place, Mr. I. Oshima, President of Osaka Fuji Corp., established the OCC Research Centre, housing several pilot-scale facilities to evaluate the feasibility of the OCC Process for manufacturing industrial products. At the same time, several laboratory-scale units were built by the Company for Chiba Institute of Technology and the University of Toronto, which enabled strong University-Industry collaborative programs to be conducted based on investigations of the fundamental aspects of the process as well as the practical implications.

Figure 7. First experimental OCC equipment built at Chiba Institute of Technology and pilot-scale horizontal OCC facility built by the Osaka Fuji Corporation and located at Ortech International.

Figure 8. Cut-out view of horizontal OCC unit

Application of Knowledge Development of Alloy Wires

Cast bismuth wires Small diameter, bismuth alloy wires are required for applications such as thermal fuses and solders. However, since bismuth is brittle, friable, and expands upon solidification(15), it is difficult to produce bismuth and high-bismuth bearing alloy wires 1-3 mm in diameter by traditional continuous casting or even by extrusion. With the OCC process, external heat is applied to the mold, crystal nucleation on the mold surface is prevented, and as shown in Fig. 5, the wire solidifies at the mold exit. For the casting of fine wires, the solidification front is actually located outside the mold (9,16,17). As a result, problems associated with friction between the mold and wire surface, and the expansion of bismuth during freezing, are all eliminated. This permits the casting of small bismuth wires. In fact, single crystal bismuth wires 0.5-2 mm in diameter have been cast and, depending on the crystal

orientation, some of the wires were remarkably soft and ductile and were easily wound into a coil. Bismuth cleaves along the (111) plane and wires in which the cleavage plane lies closer to the wire axis exhibit remarkable ductility(6). Cast bismuth alloy wires Solder and thermal fuse alloys containing a high amount of bismuth can also be made into small diameter wires by the OCC process. Good microstructural control of cast products is essential for these alloys in order to achieve uniform chemical composition. With the controlled cast microstructure, mechanical properties can be enhanced significantly. Fig. 9 (a to c) shows examples of microstructures of the Bi-In-Sn, 77C-eutectic alloy solidified by slow cooling in the furnace, slow directional growth, and fast cooling by suctioning the melt into a small glass tube, 2 mm i.d. As expected, different microstructures were produced, depending on the solidification method. For example, the microstructure of Fig. 9(a), which was furnace cooled, exhibited a gravity-induced, highly segregated structure, containing massive primary bismuth in a block form (white), mottled tin dendrites (dark), and Bi-Sn eutectic cells with complex patterns (white) in a BiIn-Sn eutectic matrix (gray) (18). The microstructure in Fig. 9(b), which grew unidirectionally, shows a clear double binary eutectic structure in which the Bi-Sn eutectic cells (indicated by arrow) segregated along the BiIn-Sn dendrite cells. Fig. 9(c) also exhibited a non-uniform microstructure containing massive bismuth blocks (white phase) despite the fact that the sample was solidified quickly by suctioning the melt into a small glass tube. Since bismuth is brittle, these massive bismuth blocks observed in Fig. 9 (a) and (c) easily fracture under strain, causing a catastrophic failure of the materials. A lamellar structure of the BiIn phase, shown in Fig. 9(b), also readily fractures along the spine and arm or across the arm of the BiIn dendrite cell, causing trans-granular cracking (19). Examples of cracks due to strain in these phases are shown in Fig. 10.

Figure 9. Various microstructures of the Bi-In-Sn, 77C-eutectic alloy: (a) solidified within the furnace at a cooling rate of 1C/min showing gravity-induced segregation of bismuth phases (white), (b) solidified unidirectionally at a speed of approximately 2mm/min showing the segregation of complex bismuth structures (white) along the grain boundaries of the BiIn-Sn eutectic and (c) solidified by suctioning the melt into a glass tube, showing non-uniform structure (18).

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Figure 10. Cracks (indicated by arrows) (a) in massive bismuth block and (b) within BiIn-Sn eutectic cell of the Bi-In-Sn alloy (19). Fig. 11 shows an example of a cast Bi-In-Sn eutectic wire 2.4 mm in diameter produced by the OCC process at a speed of 79 mm/min(6). The microstructure of continuously cast wires is significantly different from those noted above. With the OCC process, free growing crystals in the liquid ahead of the solidification front do not exist owing to the externally heated mold. This eliminates the possibility of gravity related segregation. In addition, cast products quickly solidify by means of the cooling water located near the mold exit, resulting in a fine uniform microstructure compared to those of slowly cooled cast products(17). In the wire produced by the OCC process, the defects noted above are essentially eliminated. Bismuth exists as a small, discrete phase dispersed uniformly throughout the cross sectional area, and the BiIn dendrite cells, clearly visible in the microstructure of Fig 9(b), no longer exist. These differences in microstructure are reflected in differences in ductility. For example, as shown in Fig. 12, among the specimens strained in tension at an initial strain rate of 6.67x 10-3/s, the furnace-cooled eutectics fractured in brittle mode, while the OCC wires exhibited considerable ductility (19,20).

Figure 11. Cast wire 2.4 mm in diameter and microstructure (longitudinal cross section) of Bi-In-Sn, 77C-eutectic wire produced by the OCC process at 79 mm/min. showing the discrete bismuth phase (white) dispersed in the BiIn matrix (18).

Figure 12. Differences in ductility of the Bi-In-Sn eutectic alloy specimens produced by (a) furnace cooling and (b) the OCC process. The initial strain rate was 6.67x10-3/s (20). The uniform solidification structure exhibited in the Bi-In-Sn eutectic alloy wires generated by the OCC process as shown in Fig. 11 also applies to the microstructures of the Sn-30%Zn and the Sn-57%Bi alloys of hyper-eutectic and eutectic compositions respectively. The degree of uniformity in microstructure strongly influences the fracture behavior of cast wires(21). A significant workability was reported for the Sn-30%Zn alloy wires generated by the OCC process(22). Based on these results, a new manufacturing route using the OCC process for the production of fine solder and thermal-fuse wire materials was implemented. Near-net shape Sn-57%Bi eutectic alloy wires approximately 2 mm in diameter (Fig. 13) were produced with a fine unidirectionally solidified structure (Fig. 14). This product has enhanced ductility, permitting extended deformation processing and cannot be produced through traditional casting routes. The high-quality surface condition of cast wires may also play a significant role during deformation in addition to elongated crystals growing along the casting direction (22, 23). These cast wires were further processed to generate finer wires of various diameters (Fig. 15). Micro flux-cored solder wires were also produced by casting Sn-57%Bi eutectic alloy tubes using the OCC process. An example of the product is shown in Fig. 16.

Figure 13. Cast Sn-57%Bi eutectic wires 2 mm in diameter produced at a speed of approximately 300-350 mm/min.

(a)

(b)

(a)

(b)

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Figure 14. Microstructure of a cast Sn-Bi eutectic wire, exhibiting unidirectional structure.

Figure 15. Final Sn-57%Bi solder wire products 0.3, 0.5, and 0.7 mm in diameter drawn from cast wires 2 mm in diameter.

Figure 16. Cross-section of flux-cored, Sn-Bi eutectic solder wire 0.8 mm in diameter.

Closing Comment Throughout his long and distinguished career at McGill University, the activities of Professor Rod Guthrie have provided firm foundations for progress within the field of metallurgical processing. His pronounced influence for good on the careers of young scientists and engineers, many of whom now occupy leading positions within academia and industry, has been outstanding. In the final analysis, the pre-eminent aim of collaborative activities between our educational institutions and industrial organizations, must be to ensure the availability of men and women with a sound understanding of the fundamental

aspects and practical implications of their discipline and who are fully equipped with the essential attributes and communicative skills that will enable them to apply their knowledge, with wisdom and integrity, within this most challenging and satisfying field of activity, the science and technology of innovative processing. In all of our efforts aimed at innovative processing, an activity that encompasses generation, validation and application of knowledge, quality communications, university-industry collaborations and the training of people, let us resolve to emulate the highest standards of achievement and professionalism so well exemplified by our distinguished colleague and honoured friend, Professor Rod Guthrie.

Summary Based on the findings from studies carried out on the solidification structures and the origin of equi-axed crystals in cast products, a novel processing system, known as Ohno Continuous Casting (OCC), has been developed with the aid of collaborative projects between academia and industry. Using a heated mold concept, fundamental and practical studies have been undertaken within the laboratory, validated in pilot plant trials and implemented in production operations. These collaborative efforts have led to new processing routes for the generation of net or near-net shape products such as, small diameter rods, tubes, wires and cored materials suitable for niche markets that include micro solder and thermal fuse wire applications.

Acknowledgement The financial support provided by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

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(12) H. Yamazaki, A. Ohno, G. Motoyasu, T. Shimizu and H. Soda, J. Japan Inst Metals, 57 (1993) 190-194, (in Japanese). (13) A. Ohno, H. Soda, A. McLean and H. Yamazaki, Proc. Advanced Materials- Application of Mineral and Metallurgical Principles, (Society for Mining, Metallurgy and Exploration, Feb 26-Mar 1, Salt Lake City, USA,1990) pp. 161-168. (14) G. Motoyasu, T. Hosaka, K. Kanazawa and M. Uehara, Z.A. Daya, H. Soda, A. McLean and H. Yamazaki, Fourth Pacific Rim International Conference on Materials and Processing, (eds: S. Hanada et al.,The Jpn. Inst. of Metals, 2001) pp. 2391-2394. (15) L. Wang, Bismuth Metallurgy, (Metallur. Industry Press, Beijing China, 1991) p. 4. (16) H. Soda, A. McLean and G. Motoyasu, Int. J. Cast Metals Research, 18 (2005) 163-173. (17) S. Sengupta, H. Soda, A. McLean and J.W. Rutter, Metall. Mater. Trans. A, 31A (2000) 239-248. (18) S. Sengupta, H. Soda and A. McLean, J. Mater. Sci. Letters, 40 (2005) 2607-2610. (19) S. Sengupta, H. Soda and A. McLean, J. Mater. Sci., 37 (2002) 1747-1758. (20) S. Sengupta, Comparative study of Bi-In-Sn ternary eutectic cast wires, M.A. Sc. Thesis, University of Toronto, 1998. (21) Y. A. Kwon, Z. A. Daya, H. Soda, Z. Wang and A. McLean, Mater. Sci. Eng. A, A368 (2004) 323-331. (22) G. Motoyasu, T. Motegi and A. Ohno, J. Jpn. Int. of Metals, 51 (1987) 935-940 (in Japanese). (23) G. Motoyasu, H. Soda, A. McLean and S. Yoshie, Proc. of the 66th. World Foundry Congress, Casting Technology 5000 Years and Beyond, Vol. 1, (The Foundrymens Association of Turkey, Sept. 6-9, 2004, Istanbul, Turkey, 2004) pp. 385-395.