Materials Transactions, Vol. 43, No. 1 (2002) pp. 24 to 29 c
2002 The Japan Institute of Metals

Continuous Semi-Solid Casting Process for Aluminum Alloy Billets Hakaru Nakato1 , Michio Oka2 , Seiji Itoyama3 , Masao Urata1 , Tatsuo Kawasaki1 , Ko-ichi Hashiguchi1 and Shinobu Okano4 1

Process & Products Engineering Division, Kawasaki Steel Techno-research Corp., Chiba 260-0835, Japan Chiba Works, Kawatetsu Machinery Co., Ltd., Chiba 260-0835, Japan 3 Research Laboratories, Kawasaki Steel Corp., Chiba 260-0835, Japan 4 Rheo-technology, Ltd., Chiba 260-0835, Japan 2

A new process for continuous semi-solid casting of billets of aluminum alloys was developed. Round aluminum alloy billets 75 mm and 150 mm in diameter are continuously cast in a semi-solid state by agitating the alloy in the agitating vessel with a mechanical screw and/or an electromagnetic stirrer. The solidification structure of the billets obtained by this process is a mixed structure of granular particles and a fine eutectic structure, except in a thin chill layer about 2 mm in thickness, which shows a dendrite structure. It was possible to use billets of AC4C alloy obtained by this process in the thixoforming process without surface conditioning. (Received September 17, 2001; Accepted November 19, 2001) Keywords: aluminium alloy, rheo-casting, semi-solid casting, continuous casting, thixoforming, round billet

1. Introduction

2. Experimental Procedure

Products manufactured by thixoforming of aluminum alloys cast from reheated semi-solid billets show good soundness and excellent mechanical properties. Since thixoforming was first proposed by Flemings et al.1) in 1972, many efforts have been made to realize a practical thixoforming process,2–9) and theoretical investigation10, 11) of this type of processing has also been reported. Bertrand and Patrick2) studied the development of a continuous semi-solid casting process for 7075 aluminum alloy using electromagnetic stirring in the mold. Idegomori et al.,5) reported the results of the application of thixotropic technology to automobile parts using billets cast with electromagnetic stirring in the mold. Products manufactured by this technology have been applied to outer rigger of the rear part of automobiles. Uetani et al.,6) developed a process for semi-solid cast 7075 aluminum alloy billets using a mechanical stirring method. The tensile strength of the product after T6-treatment was nearly equal to that of hot extruding products, but elongation was somewhat lower than with hot extruding. Although some studies have reported the production of semi-solid billets for use in thixoforming of aluminum alloys, the current production capacity is inadequate to supply the many different kinds of billets required by customers immediately. Hence, semi-solid materials are strongly desired for the thixoforming process for manufacturing sound products in the aluminum industry. Cooperative research by Rheo-technology, Ltd., Kawasaki Steel Techno-research Corp., Kawatetsu Machinery Co., Ltd., and two billet users successfully realized a new continuous semi-solid casting process for aluminum alloys. This paper presents an outline of the process and the operating results.

2.1 Experimental apparatus The appearance of the continuous semi-solid casting process for billets of aluminum alloys is shown in the photograph in Fig. 1. The main specifications of the process are given in Table 1. The process is characterized by two main parts, an

Fig. 1 Appearance of continuous semi-solid casting machine for aluminum alloy billets. Table 1 Main specifications of continuous semi-solid caster. Melting furnace

Capacity (kg)

200

Heater

Propane burner

Agitating vessel

Screw speed (min−1 )

Max. 800

Solid fraction

0–0.4

Diameter (mm)

75, 150

Length (m)

Max. 4

Pinch roll

Withdrawal speed (m/min)

0.05–2.0

equipment

Drive system

AC motor

Billet size

Continuous Semi-Solid Casting Process for Aluminum Alloy Billets

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Table 2 Experimental conditions. (150 mm round billet) Al alloys used

AC4C, ADC12

Agitating vessel

Material

Stainless steel

Mold

Material

Al alloy

Length (mm)

150

(75 mm round billet) Al alloys used

AC4C, AC1B

Agitating vessel

Material

Feed nozzle and break ring materials Mold

Fig. 2 Schematic diagram of major parts of continuous semi-solid casting process.

agitating vessel, which is used to produce semi-solid material (material with coexistent solid and liquid phases) and a cooling mold for producing the solidified shell (billet), which are independent parts of the process. As shown in Fig. 2, the process equipment comprises eight units: a melting furnace, a tundish or teeming gutter, a rotating shaft (screw), an electromagnetic coil, an agitating vessel, a feed nozzle and break ring, a cooling mold, and pinch rolls. A general description of this equipment is presented below. (1) Melting furnace Maximum capacity of 200 kg; the alloy is heated by a propane burner from outside the graphite crucible. (2) Tundish or teeming gutter The wall of this equipment is coated with a refractory mortar and is preheated from the top with a propane burner. The preheating system in the tundish or teeming gutter and the agitating screw comprises a pair of propane burners. (3), (4) Rotating shaft, Electromagnetic coil A mechanical screw installed in the vessel was also used and/or an electromagnetic stirrer was applied to the alloy in the vessel from outside the wall (75 mm round billet). (5) Agitating vessel The wall, which is made of stainless steel, is water-cooled. A mechanical screw installed in the vessel was used (150 mm round billet). A silica crucible was used and was preheated electrically by a spiral rod heater installed in the crucible. The mechanical screw installed in the vessel with a rotating shaft was also used and/or an electromagnetic stirrer was applied to the alloy in the vessel from outside the wall (75 mm round billet). (6) Feed nozzle and break ring A feed nozzle made of stainless steel was used with 150 mm round billets, and a feed nozzle made of graphite and break ring made of graphite or silica were used with 75 mm round billets. The feeding area between the agitating vessel and the mold was electrically heated using a heating coil from outside the feed nozzle and break ring to prevent temperature drop and maintain a constant temperature in these parts. (7) Mold

Conditions of electromagnetic stirrer

Silica

Silica, graphite Lining materials

Al alloy, graphite

Length (mm)

60, 100, 150

Intensity at mold center (T)

Max. 0.3

Applied current (A)

210, 430

Applied frequency (Hz)

5, 10

Two types of molds (75 mm and 150 mm in diameter) were used. The molds were made of an aluminum alloy with a water-cooling channel. The mold length was from 60 to 150 mm (100 mm in most cases). (8) Pinch roll equipment A 2-high pinch roll device driven by an AC motor was used. Cast billets were held by an oil hydraulic mechanism. 2.2 Experimental conditions The experimental conditions are listed in Table 2. In both campaigns in these experiments (i.e., 150 mm and 75 mm round billets), the main material used in casting was AC4C alloy. 3. Results and Discussion 3.1 Casting practice The casting conditions used in the experiment are listed in Table 3. Temperature control of the semi-solid aluminum alloy in the agitating vessel was a key factor in successful casting. Casting practice for 150 mm round billets Heat loss in the agitating vessel was mainly controlled by the heat loss through the water-cooled stainless steel wall. Therefore, the quantity of cooling water was strictly regulated to control the condition of heat flow from the bulk alloy to the cooling water. By applying a cooling water flow rate of less than 100 l/min from outside the stainless steel wall, mild cooling of the aluminum alloy in the agitating vessel was effectively accomplished, enabling stable casting of billets. The relationship between the motor torque of the rotating screw and the solid fraction of the melt is shown in Fig. 3. The solid fraction was calculated from the temperature in the semi-solid region measured 30 mm below the tip of the screw. At solid fractions of more than 0.3, motor torque increased, and when the solid faction exceeded 0.4, large torque fluctuations were also observed.

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H. Nakato et al. Table 3 Casting conditions used in experiments. (150 mm round billet)

Metal temperature in tundish (K) Teeming rate (kg/min)

953 (ADC12) 993–1013 (AC4C) 100–150

Holding time of metal in mold after pouring (s) Withdrawal speed (m/min)

Cooling water intensity in agitating vessel (l/min)

30 Initial stage

0.2–0.3

Steady state

0.2–0.5

20–200

Cooling water intensity in mold (l/min) Heating temperature of feeding nozzle (K) Agitating screw speed

(min−1 )

800 923–953 Initial stage

100

Steady stage

600

Setting torque limit of agitating screw (J)

167

Fig. 4 Relationship between temperature and solid fraction of AC4C alloy.

(75 mm round billet) Metal temperature in furnace (K) Preheating temperature of outer surface of crucible (K)

908–923 Upper

503–593

Middle

553–713

Lower

483–653

Preheating temperature of feed nozzle (K)

883–903

Preheating temperature of break ring (K)

868–893

Teeming rate (kg/min) Cooling water intensity in mold (l/min) Initial holding time of teemed alloy in mold (s) Agitating screw speed (min−1 )

Withdrawal speed (m/min)

150 100–450 15–25 Initial stage

10–100

Steady state

30–400

Initial stage 0.15–0.20 Steady state 0.15–0.40

Fig. 3 Relationship between motor torque of agitating screw and solid fraction calculated from temperature measured 30 mm below tip of screw.

The relationship between the solid fraction and the temperature of the AC4C alloy calculated thermodynamically by using “THERMOCALC” software is shown in Fig. 4. In Fig. 4, the experimental results obtained by S. Okano are also plotted. In this paper, the data presented by Okano were used to obtain the relationship between the solid fraction and temper-

Fig. 5 Appearance of cast billet of AC4C alloy.

ature. The slope of the relationship between the solid fraction and temperature of the AC4C alloy in the vessel is steeper with solid fractions of more than 0.6 than with smaller solid fractions. When the temperature drops less than 850 K, large fluctuation in the solid fraction arises from small fluctuations in temperature. In high solid fraction region exceeds 0.4, even small temperature deviations cause large fluctuations in the solid fraction, making stable casting impossible. Casting practice for 75 mm round billets Heat flow from the agitating vessel was mainly controlled by the heat loss through the rotating screw rod. Therefore, the temperature of the rotating screw rod was preheated strictly by controlling the preheating conditions. With AC4C alloy billets 75 mm in diameter, stable casting of sound billets with good surface quality was achieved by optimizing the casting conditions. An example of the surface appearance of the cast billet is shown in the photograph in Fig. 5. Billets of AC4C alloy obtained by the newly developed process could be cast successfully by thixoforming without surface conditioning. Two typical examples of the measured change in temperature in these experiments are shown in Fig. 6. In this figure, run No. 18 was successfully cast, but run No. 20 ended in failure. The temperature in the break ring installed between the feed nozzle and the mold was measured by a thermocou-

Continuous Semi-Solid Casting Process for Aluminum Alloy Billets

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respectively. The heat flux in the upper part of the mold was approximately 400×104 (W·m−2 ·K−1 ), whereas that in lower part was approximately 250 × 104 (W·m−2 ·K−1 ), with small fluctuations over time.

Fig. 6 Two typical examples of temperature change in experiments.

Fig. 7 Change over time in temperature at centerline of billets (run/mold length: run No. 35/60 mm, run No. 36/100 mm).

Fig. 8 Change in mold heat flux during continuous semi-solid casting of AC4C alloy.

ple embedded at a depth of 3 mm from the inner surface. In the failed run No. 20, large temperature fluctuations were observed. This indicated that the solidified shell was sticking to the surface of the break ring and then breaking away periodically. The change over time in the temperature at the centerline of an AC4C alloy billet is shown in Fig. 7. Although the temperature was substantially constant as long as the billet was in the mold, the temperature dropped rapidly at the end of the mold due to a cooling-water film flowing from the lower outlet of the mold. The change in heat flux in the mold during continuous semi-solid casting of AC4C alloy is shown in Fig. 8. The heat flux in the upper and lower parts of the mold was measured at points of 30 mm and 60 mm from the top of the mold,

3.2 Quality of billets 3.2.1 Solidification structure The solidification structure of an aluminum alloy (AC4C) billet obtained by this continuous semi-solid casting process is shown in Fig. 9. With 150 mm round billets, as shown in the upper part of Fig. 9, a mixed structure consisting of granular particles and a fine eutectic structure was observed, except in the chill layer which formed 2 to 3 mm from the surface and showed a dendritic structure. The average size of the granular particles was 75 µm, and the ratio of granular particles was in the range of 0.61 to 0.69 in the billet cross section. The observed values of 0.61 to 0.69 were much greater than the values of 0.2 to 0.4 which had been calculated from the temperature measured at 30 mm below the tip of the screw in the semi-solid region. This indicates that the primary granular structure grew in the mold during solidification. As shown in the lower part of Fig. 9, a mixed structure similar to that in the upper part of Fig. 9 was observed with 75 mm round billets except in the chill layer. The granular structure somewhat changed from the globular structure that existed in the initial period of casting. An example of a treering-like (coring structure) in the granular structure associated with the solidification process is shown in Fig. 10. The relationship between the solid fraction calculated from the temperature measured 30 mm below the tip of the screw in the semi-solid region and the solidification structure of the solidified billet is shown in Fig. 11. In the range of solid fractions from 0 to 0.2, rosette-type crystals formed. However, at solid fractions of more than 0.2, granular crystals formed. In the experiment, the effect of rotating method on the solidification structure in the billet was not clarified. 3.2.2 Mechanical properties of as-cast billets at elevated temperatures The high temperature deformation behavior of as-cast billets was examined using a hot forging test machine (“FORMASTER”). The size of the test pieces prepared from the as-cast billet was 8 mm in diameter and 12 mm in height. The effect of the test temperature on the maximum deformation resistance is shown in Fig. 12. The maximum deformation resistance depended on the billet size. At a test temperature of 773 K, the maximum deformation resistance of a billet with a diameter of 75 mm was greater than that of a billet with a diameter of 150 mm due to the difference in the solidified structure. Moreover, above 823 K, the maximum deformation resistance decreased abruptly as the test temperature increased, being approximately 30 MPa at 828 K, but falling to below 10 MPa at 838 K. Above 833 K, the maximum deformation resistance of billets cast in the semi-solid state was lower than that of conventional billets cast with a superheat in the molten material, and at 838 K, the difference in the maximum deformation resistance of the two types of billets became more pronounced.

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H. Nakato et al.

Fig. 9 Cross-sectional view of solidification structures of 150 mm round billet (upper part of figure; estimated solid fraction, 0.30 at point 30 mm below tip of screw) and of 75 mm round billet (lower part; estimated solid fraction, 0.14). Percentage values are the cross-sectional area ratio of primary granules.

Fig. 10 Coring structure formed in the granular structure associated with solidification.

4. Conclusion A new process for continuous semi-solid casting of billets of aluminum alloys was developed. The results obtained in experimental campaigns with two different sizes are summarized below. (1) Round billets of aluminum alloys with diameters of 75 mm and 150 mm were continuously cast in the semi-solid state by agitating the alloy in the agitating vessel with a mechanical screw and/or an electromagnetic stirrer.

(2) The solidification structure of the billets obtained by this process was a mixed structure consisting of granular particles and a fine eutectic structure, except in a thin chill layer (about 2 mm), which showed a dendrite structure. (3) With the 150 mm round billets, the average size of the granular particles was 75 µm, and the ratio of granular particles was in the range of 0.61 to 0.69 in the billet cross section. With the 75 mm round billets, when the solid fraction calculated from the temperature measured 30 mm below the tip of the screw was in the range of 0 to 0.2, a rosette-type structure formed. On the other hand, when the solid fraction exceeded 0.2, the granular crystal structure formed. (4) The maximum deformation resistance at high temperature depended on the billet size. At a test temperature of 773 K, the maximum deformation resistance of the 75 mmφ billets was greater than that of the 150 mmφ billets due to the difference in the solidified structure. Above 823 K, the maximum deformation resistance abruptly decreased as the test temperature increased. Deformation resistance was approximately 30 MPa at 828 K, but dropped to less than 10 MPa at 838 K. Above 833 K, the maximum deformation resistance of billets cast in the semi-solid state was lower than that of conventional billets cast with a superheat in the molten metal. At 838 K, the difference in the maximum deformation resistance of the two types of billets became more pronounced. (5) It was possible to use billets of AC4C alloy obtained by this process in the thixoforming process without surface conditioning.

Continuous Semi-Solid Casting Process for Aluminum Alloy Billets

Fig. 11

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Effect of solid fraction calculated from temperature measured 30 mm below tip of screw on solidification structure of billet.

3)

4)

5) 6) 7) 8) Fig. 12 Effect of test temperature on maximum deformation resistance(150D: 150 mm billet, die cast; 150S: 150 mm billet, semi-solid cast; 75D: 75 mm billet, die cast; 75S: 75 mm billet, semi-solid cast).

Acknowledgements The authors wish to express their sincere thanks to the Japan Small and Medium Enterprise Corp., which sponsored this work. The authors are also particularly grateful to members of the technical committee of the project for helpful discussions. REFERENCES 1) D. B. Spencer, R. Mehrabian and F. C. Flemings: Metall. Trans. 3 (1972) 1925–1930. 2) C. Bertrand and P. Patrick: Proc. of the 4th Int. Conference on

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