Refined Solidification Structure and Improved Formability of A356 ...

Materials Transactions, Vol. 45, No. 2 (2004) pp. 403 to 406 #2004 The Japan Institute of Metals

Refined Solidification Structure and Improved Formability of A356 Aluminum Alloy Plate Produced using a High-Speed Twin-Roll Strip Caster*1 Kenta Suzuki1 , Shinji Kumai1 , Yuichi Saito1 *2 , Akikazu Sato1 and Toshio Haga2 1 2

Department of Materials Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502, Japan Department of Mechanical Engineering, Osaka Institute of Technology, Osaka 535-8585, Japan

An A356 aluminum alloy was cast into a 2 mm thick plate by a strip caster equipped with a pair of water-cooled rolls made of pure copper. The twin-roll cast plate exhibited fairly refined primary aluminum dendrite- and eutectic solidified structure. The estimated maximum cooling rate at the near-surface region of the plate is 4000 K/s. These values are significantly higher than the cooling rate for the permanent mold cast (2 K/s). The cast plate was cold-rolled and annealed into a 0.5 mm thick sheet. The resultant sheet was subjected to the 180 bending test. No cracking occurred in the twin-roll cast product. Improved formability is considered to be mainly due to the homogeneous distribution of fine Si particles. (Received November 11, 2003; Accepted January 15, 2004) Keywords: twin roll caster, strip casting, rapid solidification, A356 aluminum alloy, bending test

1.

Introduction

Strip casting is a method for producing a plate directly from an alloy melt.1,2) In the past strip casting generated great enthusiasm, with the idea of avoiding investment in hotrolling mills; however, cast strip could not match the quality of strip produced by conventional hot-rolling. Recently, strip casting has gained new impetus. The incentive for recycling is particularly strong in the case of aluminum. For example, used beverage can (UBC) recycling is much more efficient when UBC can be transformed into canstock in decentralized locations, in which the strip casting method is available.3) However, scrap remelting tends to downgrade the alloys. Generally, rapid solidification can provide characteristic features in microstructure. In view of these benefits, fabrication of alloy strips at a high cooling rate is considered to be one of the answers to alleviating the detrimental effect of impurities. One of the present authors has developed several twin-roll strip casters4,5) while focusing on achievement of rapid cooling. The present study aims to demonstrate the importance of fine solidification structure of the cast strip produced by twin-roll casters. 2.

Experimental Procedure

The material selected in the present study is A356 aluminum alloy. Figure 1 shows two types of twin-roll casters used in the present study; (1) a melt drag twin-roll caster (MDTRC),4) and (2) a hydrostatic press twin-roll caster (HPTRC).5) The MDTRC (Fig. 1(a)) is a horizontal-type caster. The melt in the nozzle is dragged onto the rotating lower roll. About eighty percent of the solidification toward the thickness direction of the strip takes place on the lower roll. The semi*1This Paper was Presented at the Autumn Meeting of the Japan Institute of

Metals, held in Sapporo, on October 13, 2003. Student, Tokyo Institute of Technology

*2Graduate

Spring (load)

(a)

Water-cooled pure copper roll Nozzle

Melt

Strip

Crucible

(b)

Melt Slope Water-cooled pure copper roll

Nozzle

Spring (load)

Strip Fig. 1 Two types of twin-roll casters used in the present study. (a) melt drag twin-roll caster, (b) hydrostatic press twin-roll caster.

solid layer at the upper surface is cooled by the contact with the upper roll. In contrast, the HPTRC (Fig. 1(b)) is a vertical-type twin-roll caster. A casting nozzle is mounted on the roll. The width of the nozzle controls the solidification length. Both roll casters are equipped with a pair of watercooled pure copper rolls, and no lubricant is employed on the roll surface. The rolls have a diameter of 300 mm and a face

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K. Suzuki, S. Kumai, Y. Saito, A. Sato and T. Haga

width of 100 mm. The alloy melt was melted at 898 K. The cast strip was fabricated at a speed of 20 m/min for the MDTRC, and 90 m/min for the HPTRC. The width and thickness of the resultant strips were 35 mm and 2.5 mm for the MDTRC, and 100 mm and 2 mm for the HPTRC. For comparison, the melt was also cast into a conventional booktype permanent mold (20 mm thickness). The twin-roll cast plate and the machined permanent mold cast plate (2 mm thickness) were homogenized at 803 K for 14.4 ks, cold rolled into a 0.5 mm thick sheet and then annealed at 723 K for 14.4 ks. Bend specimens (30 mm  10 mm) were taken from the sheet such that the bend axis is either parallel or vertical to the rolling direction. The 180 bending test was conducted under a bending radius of 0.5 mm. Optical microstructural observation was carried out for both as-polished and anodized specimens. SEM observation was also performed at the sheet surface after bending 180 . 3.

(a)

(b)

Results and Discussion

50µm

3.1

Refined solidification structure of the twin-roll cast strips Figure 2 shows optical micrographs of the transverse cross

Fig. 3 Solidified structure of the cast plate by hydrostatic press twin-roll caster. (a) mid-thickness and (b) near-surface.

section of the cast plate fabricated by MDTRC. A difference in the solidified structure is observed at the regions. As shown in Fig. 2(c), a fine dendrite structure (DAS: 3.5 mm) was obtained near the bottom surface. In the mid-thickness region (Fig. 2(b)), equiaxed grains and a condensed eutectic solidification region were observed (DAS: 6.5 mm). Two types of solidified structure (relatively coarse floating dendritic crystals and a fine dendrite structure) were observed at the near top surface region (DAS: 14.2 mm and 3.6 mm). Figure 3 shows solidified structure of the cast plate by the HPTRC. In this case, a microstructural difference is observed only between the surface and the mid-thickness. These regions exhibit fairly refined primary dendrite and eutectic solidified structure. DAS is 2.6 mm for the near-surface region (Fig. 3(b)) and 4.5 mm the mid-thickness region (Fig. 3(a)). Area fraction of the eutectic solidification is larger in the mid-thickness region. In contrast, the permanent mold cast plate has a DAS of 32.2 mm.

(a)

(b)

3.2

Estimation of cooling rate from the solidified structure The relationship between DAS and cooling rate for A356 alloys had been investigated previously, and is shown in Fig. 4.6) The relationship can be used to estimate cooling rates of the cast strip from the obtained DAS values. The estimated cooling rate at the near-surface region of the HPTRC product is 4000 K/s, and that for the mid-thickness region is 500 K/s. For the MDTRC strip, the estimated maximum cooling rates is 2000 K/s at both the near bottom and the near top surface region. These values are considerably larger than the cooling rate for the permanent mold cast (2 K/s).

(c)

50µm

3.3 Fig. 2 Solidified structure of the cast plate by melt drag twin-roll caster. (a) near top surface, (b) mid-thickness and (c) near bottom surface.

Microstructural change caused by cold-rolling and annealing Surface structure of the cast plate is shown in Fig. 5. Quite

Refined Solidification Structure of A356 Aluminum Alloy Plate

mold cast product, Si particles are segregated, being dispersed along the original dendrite branches, even after cold rolling and annealing.

A356 Permanent mold cast

100

405

2K/s DAS, d / µm

3.4 32.2 µm 14.2 µm 10

6.5 µm 3.6 µm 3.5 µm

Twin-roll cast

4.5 µm 2.6 µm

20K/s

300K/s

2000K/s

500K/s

4000K/s

1 0.1

1

10

100

1000

10000

Cooling rate, V / K s-1

Fig. 4 Relationship between DAS and cooling rates for A356 alloy.

a large difference is observed between the top surface and the bottom surface of the MDTRC plate. These cast plates were subjected to cold-rolling followed by annealing. Figure 6 shows optical micrographs of the sheet surface for the twin-roll cast and the permanent mold cast products. All samples exhibit recrystallized grain structure. Fine and homogeneous distribution of Si particles is observed for both twin-roll cast products. We should mention that the large microstructural difference between the top surface and bottom surface of the MDTRC product was completely diminished in the rolled sheet. For the permanent

4.

(c)

Conclusions

A twin-roll cast plate of about 2 mm thickness exhibited fairly refined primary aluminum dendrite- and eutectic solidified structure. From the solidified structure the maximum cooling rate is estimated at 4000 K/s in the nearsurface region of the HPTRC plate. Refined solidification structure was beneficial for formability of the downstream product. No cracking occurred in the 180 bending test. Improved formability is considered to be mainly due to the homogeneous distribution of fine Si particles.

Melt drag twin-roll cast (top surface)

Permanent mold cast (a)

Formability of rolled sheet of the twin-roll cast products Each sheet was subjected to the 180 bending test. Figure 7 shows SEM images of the cross section of the specimen after bending test. Cracking occurred only in the permanent mold cast sheet that the bend axis is vertical to the rolling direction. No cracking occurred for twin-roll cast sheets, which are both parallel and vertical to the rolling direction. Improved formability is considered to be mainly due to the homogeneous distribution of fine Si particles, shown in Fig. 6. We should mention that such a microstructure originates from the initial rapidly solidified structure produced by the twin-roll casting method.

Melt drag twin-roll cast (bottom surface) (e)

Hydrostatic press twin-roll cast (g) Longitudinal direction

50µm

(b)

(d)

(f)

(h)

50µm

Fig. 5 Surface structure of the cast plate (as-polished structure in the upper column and anodized structure in the lower column). (a), (b) permanent mold cast, (c)  (f) melt drag twin-roll cast and (g), (h) hydrostatic press twin-roll cast.

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K. Suzuki, S. Kumai, Y. Saito, A. Sato and T. Haga

Melt drag twin-roll cast (top surface)

Permanent mold cast (a)

(c)

Melt drag twin-roll cast (bottom surface)

Hydrostatic press twin-roll cast (g)

(e)

Rolling direction

50µm

(b)

(f)

(d)

(h)

50µm

Fig. 6 Optical micrographs of the sheet surface (as-polished structure in the upper column and anodized structure in the lower column). (a), (b) permanent mold cast, (c)(f) melt drag twin-roll cast and (g), (h) hydrostatic press twin-roll cast.

Acknowledgements

(a)

This work has been supported by RISTEX of JST (Japan Science and Technology Agency) and Light Metal Education Foundation. One of the authors (Kenta Suzuki) is pleased to thank Japan Aluminum Association for its financial support. REFERENCES (b)

1) A. Saxena and Y. Sahai: Mater. Trans. 43 (2002) 206–213. 2) A. Saxena and Y. Sahai: Mater. Trans. 43 (2002) 214–221. 3) D. Altenpohl: ALUMINUM: TECHNOLOGY, APPLICATIONS, AND ENVIRONMENT, (The Aluminum Association Inc. and TMS, 1998) pp.86–93. 4) T. Haga, T. Nishiyama and S. Suzuki: J. Mater. Proc. Technol. 133 (2003) 103–107. 5) T. Haga, K. Takahashi, M. Ikawa and H. Watari: J. Mater. Proc. Technol. 140 (2003) 610–615. 6) M. C. Flemings: Solidification Processing, (McGraw-Hill, 1974) p. 150.

(c)

100 µ m

Fig. 7 SEM images of the cross section of the specimen during bending. (a) permanent mold cast, (b) melt drag twin-roll cast and (c) hydrostatic press twin-roll cast.