Thixoforming of laminate made from semisolid cast strips

Thixoforming of laminate made from semisolid cast strips

Journal of Materials Processing Technology 157–158 (2004) 508–512 Thixoforming of laminate made from semisolid cast strips T. Hagaa,∗ , P. Kapranosb ...

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Journal of Materials Processing Technology 157–158 (2004) 508–512

Thixoforming of laminate made from semisolid cast strips T. Hagaa,∗ , P. Kapranosb b

a Department of Mechanical Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan Thixoforming Group, Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield S13JD, UK

Abstract In order to reduce the cost of the feedstock, a laminate, in the form of a slug, made from roll cast strips has been used as feedstock for thixoforming. The size of the laminate cube was 40 mm × 40 mm × 40 mm. A high-speed twin roll caster was used to cast A356 strip from which the laminate was manufactured. The strip thickness varied from 3 mm to 2 mm, when the strip was cast at the speed from 30 m/min to 90 m/min. The strip was cast from semisolid slurry that was produced using a cooling slope. The solid fraction of semisolid slurry was lower than 10%. The primary crystal of the strip become spheroidal when heated up to semisolid condition. The ability of the laminate to flow, in the semisolid condition, was similar to that of conventional feedstock for thixoforming. The mechanical properties of the thixoformed strip laminate were: tensile strength of 315, 253 MPa, 0.2% proof stress and 8% elongation. © 2004 Elsevier B.V. All rights reserved. Keywords: Twin roll caster; Strip caster; Thixoforming; Semisolid casting; Laminate

1. Introduction Thixoforming has many advantages over casting, for example, better mechanical properties, less energy intensive process, longer die-life, to name a few. However, special feedstock, usually in the form of slugs (ingot), is needed for thixoforming. The special slugs (feedstock) used in thixoforming, must have spheroidal primary crystals as opposed to the dendritic structures associated with castings, therefore, slugs that are not made specifically for thixoforming, i.e. not possessing the appropriate microstructures, cannot be used. A process involving electric stirring continuous casting usually casts the billets from which the slugs used in thixoforming are made. Because of the technology involved and because there are only few producers worldwide, the cost of these special continuous cast bars of thixoforming feedstock is high. In addition, there is a limit on the size of the bar that can be produced. On the other hand, the technique described here utilizes a twin roll caster that has the advantages of low running costs and rapid solidification. If the strip that is cast by the twin roll caster could be used as the feedstock for ∗

Corresponding author. Tel.: +81 6 6954 4888; fax: +81 6 6957 2134. E-mail address: [email protected] (T. Haga).

0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.07.111

thixoforming, the costs associated with this produces would be substantially reduced. Ingots, cast from semisolid slurry, exhibited microstructures that contained spheroidal primary crystals in a liquid matrix, when re-heated up to the semisolid condition. In this present study, semisolid strip casting, using a cooling slope, has been experimentally investigated as a possible production route of thixoformable feedstock and the microstructures of the resulting products were assessed in the semi-solid state. Thixoforming of laminate slugs assembled from semisolid cast strip has been tried, and the mechanical properties of the products assessed.

2. Experimental procedures A356 aluminum alloy was used. This casting alloy is commonly used for thixoforming applications. In order to produce the necessary spheroidal morphology in the feedstock, a vertical type twin roll caster was used as shown in Figs. 1 and 2 [1]. Fig. 1 shows in schematic flow-chart form the sequence of steps required to go from the roll casting to thixoforming. The vertical type twin roll caster is suitable for high-speed roll casting; its productivity increases as the roll speed increases. Therefore, as a direct consequence, the vertical type twin roll

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Fig. 3. A coil of the roll cast A356 strip.

Fig. 1. Process flow of the present method.

caster can reduce the cost of the strip produced. Roll speeds (casting speeds) used, were 30, 60 and 90 m/min. The casting speed of the conventional twin roll caster for aluminum alloy varies usually from 0.5 to 5 m/min (Figs. 3 and 4).

Fig. 2. Strip casting in operation.

The roll speed of the present study was higher than that of the conventional twin roll caster for the aluminum alloy. Steel rolls are used and lubricant is sprayed onto them to prevent sticking of the strip to the rolls at conventional roll caster. However, the lubricant sticks to the surface of the strip and therefore decreases the heat transfer between the roll and strip. Copper rolls were also used, without lubricant in the roll caster as part of the present study. The strip did not stick to the rolls, even without use of lubricant, due to the effect of the large thermal conductivity of copper rolls. Roll speed affected the strip thickness, the higher the roll speed, and the thinner the thickness. The strip thickness was between 2 and 3 mm. Strip was cast from molten metal and semisolid slurry [2]. A cooling slope was attached to the twin roll caster in order to produce the semisolid slurry [2]. The cooling slope is simple water-cooled plate made from mild steel. Pouring the melt onto the cooling slope where it becomes the semisolid slurry by virtue of its flow on the cooling slope produces semisolid slurry. The cooling slope has the strong advantage of low equipment cost and running cost. The mounting of the cooling slope on the twin roll caster can be done with-

Fig. 4. Surface of A356 roll cast strip.

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T. Haga, P. Kapranos / Journal of Materials Processing Technology 157–158 (2004) 508–512 Table 1 Experimental conditions Copper roll Roll speed Alloy Pouring temperature Cooling slope Heating temperature Laminate Ram speed Load Die cavity

Diameter 300 mm, width 100 mm 30, 60, 90 m/min A356 620 ◦ C Length 300 mm, width 100 mm 580 ◦ C 40 mm × 40 mm × 40 mm 500, 300, 1000 mm/s 100 kN 80 mm × 70 mm × 8 mm

dition, without any further cleaning. The cutting of the strip was carried out using shearing machine, and the edge was in the as-cut condition. There was a gap between the strips. Thixoforming was carried out using a vertical type computer controlled hydraulic thixoforming press. The laminate slugs were heated up to semisolid condition and thixoformed into a simple shape graphite die. Test pieces were cut from thixoformed products, and tensile tests were carried out. The experimental conditions are shown in Table 1.

3. Results

Fig. 5. Laminate assembled from strips: (a) welding surface; (b) nonwelding surface.

out modification of the twin roll caster. Solid fraction of the semisolid slurry was about from 3 to 10%. When the solid fraction is from 3 to 10%, the ability of the semisolid slurry to flow is almost same as that of molten metal and therefore the semisolid slurry does not stick to the nozzle. The strips cast from the melt and the semisolid slurry were further cold rolled at a reduction of 20%. The microstructure of the straininduced metal becomes globular when the metal is heated up to semisolid condition. The microstructures of both as-cast and cold rolled strips were investigated after being heated up to semisolid condition. Laminate was produced from strips as shown in Figs. 1 and 5. The as-cast and cold rolled strips were cut into rectangular plate. Welding was used in order to connect the strips and form a square slug for thixoforming. The size of the laminate was 40 mm × 40 mm × 40 mm cubic. The strip surface was in the as-cast or as rolled con-

The strip could be cast from the melt and the semisolid slurry. The morphology of the primary crystals of the strip after being heated up to the semisolid condition is shown in Table 2. The primary crystals of the strip cast from the melt did not become spherical when heated up to semisolid condition. In contrast, the primary crystals of the strip cast from the semisolid slurry did spheroidise when re-heated to the semisolid state. The cold rolling was very useful in assisting the primary crystal development into spheroids. The roll speed did not appear to have affected the amount of spheroidisation of the primary crystals. The microstructure of the ascast strip and the cubic after being heated up to semisolid condition is shown in Fig. 6. The microstructure of the strip is a duplex structure. In the strip cast from the molten metal, the microstructure around the center of the thickness appears to be equiaxed, but the rest appears to be of acicular nature. The microstructure of the strip cast from the semisolid slurry was also duplex. The microstructure at the center of Table 2 Spheroidicity of primary crystals in the semisolid condition

SS SS + CR MM MM + CR

30 m/min

60 m/min

90 m/min

Yes Yes No Yes

Yes Yes No Yes

Yes Yes No Yes

Yes: the morphology of the primary crystals of the strip became1 spherical after being heated up to the semisolid condition; no: the morphology of the primary crystals of the strip did not became spherical after being heated up to the semisolid condition; SS: casting from semisolid slurry; SS + CR: semisolid casting and cold rolling; MM: casting from molten metal; MM + CR: casting from molten metal and cold rolling, 30, 60, 90 m/min roll speeds.

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Fig. 6. Microstructure of as-cast strip and semisolid condition.

the cross section was a near spherical structure. The primary crystals of the strip cast from semisolid slurry were more spheroidal than the primary crystals of conventional slugs used for thixoforming. The morphology of the primary crystals of the semisolid cast strip was sufficiently spheroidal to allow them to be used as feedstock for thixoforming. There were no observable differences in the microstructure between the edge and center of the cross sectional thickness after be-

ing heated up to semisolid condition. The microstructure appeared quite uniform throughout the thickness direction. Strip cast at 90 m/min was used to assemble the laminate slugs to be thixoformed in the conventional manner. The effect of the direction of the laminate slug arrangement relative to the direction of the moving ram is shown in Table 3. The laminate shown in Figs. 1 and 5 was made up by welding strips in a horizontal arrangement. When the laminate slugs were made

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Table 3 Forming of semisolid laminated slugs

SS SS + CR MM MM + CR

Table 4 Mechanical properties (T6)

Horizontal

Vertical

Yes Yes No Yes

Yes Yes No Yes

For SS, SS + CR, MM, MM + CR refer to legend of Table 2. Yes: die cavity filled by slurry; No: die cavity not filled by slurry.

Tensile strength (MPa) 0.2% proof stress (MPa) Elongation (%)

315 253 8

not appear to affect the flow of the semisolid metal. Although an oxide film existed between the strips of the laminate, it appears that the ram speed did not affect the behavior of this film. Most of the oxide film was not broken, and did not flow into the cavity. Only the semisolid metal between the oxide films flowed into the cavity. Table 4 shows the mechanical properties of thixoformed ingot after T6 heat treatment. The laminate slugs were made from the as-cast strip cast from semisolid slurry. The tensile strength and 0.2% proof stress were better than the conventional thixoformed ingots. However, the elongation of the laminate was slightly lower than that of conventional thixoformed slugs. The primary crystals of the thixoformed ingot made from laminate were finer than the primary crystals of conventional thixoformed products. This may offer improved mechanical properties. The oxide film may have an effect on the elongation. Although most of the oxide film existing on the strip surfaces appears not to have flowed into the die cavity, some part of the oxide film may have been incorporated into the semisolid slurry. This oxide film may have contributed in the reduction in elongation recorded. 4. Conclusions

Fig. 7. Metal flew into the cavity.

from the as-cast strip cast from the melt, the metal did not fill the cavity of the die as shown in Fig. 7(a). Laminate slugs made up from strips, arranged in both horizontal and vertical directions, could not fill the cavity. The reason for this is that the primary crystals were not spherical and the ability of the metal to flow into the die was impaired. Laminate slugs assembled from the strips produced under other conditions did fill the die cavity as shown in Fig. 7(b). The primary crystals became near spherical, and the ability of the semisolid slurry to flow was much improved. Naturally, oxide film existed at the free surfaces of the strips. The direction of assembly of the laminations did not affect the flow of semisolid metal at the conditions used in this the present study. The ram speed did

The use of laminate slugs for thixoforming was devised and demonstrated. The primary crystals of the strip cast from semisolid slurry become near spherical when the strip is heated up to semisolid condition. The ability of the semisolid slurry to flow when produced from a laminate slug appears to be very similar to that of slugs used in conventional thixoforming. The mechanical properties of thixoformed products using the laminate slugs as starting feedstock are very close to the mechanical properties of conventional thixoformed products. References [1] T. Haga, Study on a high-speed twin roll caster for aluminum alloys, in: Proceedings of the International Conference on Advanced Materials and Processing Technology, 2001, pp. 191–197. [2] T. Haga, Semi-solid roll casting of aluminum alloy strip by melt drag twin roll caster, J. Mater. Process. Technol. (2001) 64–68.