SAITO et al.:ACCUMULATIVE ROLL-BONDING 581 Table 1.Roll-bonding conditions Material Heating Roll diameter(mm) Roll speed (m/min) Mean strain rate (/s) A1(1100 473K×5min )59 0 A-Mg(083) 473K×5min 310 4 IF steel 773K×10min 310 46 conditions.The heating and other conditions are gauge width was 5 mm.The cross-head speed was listed in Table 1.Well-bonded bulk materials were 0.5mm/min so that the initial strain rate was successfully obtained.However,excessively high 8.3×10-4/s total reduction,i.e.repetition times,sometimes resulted in edge cracks or center fracture as shown 4.RESULTS in Figs 2(b)and (c).It may be due to tensile stress caused by lateral spreading near the edges.The lat- Optical micrographs of ARB processed IF steel eral spreading cannot be neglected when the aspect are shown in Fig.3.In the case of two-cycle pro- ratio (width/thickness)is less than 10 [6].In order cessed material [Fig.3(c)],the interface introduced to avoid propagation of edge cracks in following in the second cycle is seen clearly.It is difficult to cycles,both edges of the roll-bonded strip were find the interfaces of the first pass at a quarter of trimmed by shearing.The leading and trailing ends the thickness.This meant that the subsequent roll- of strips were cropped.These edge cracks may not ing sufficiently improves the bonding of interfaces occur in the case of industrial materials with high introduced in a previous cycle.The severely sheared aspect ratio. structure can be observed just below the surface. The longitudinal cross sections normal to the After five cycles,the whole thickness is covered by transverse direction were observed by an optical very thin and elongated grains and it is very diffi- microscope.Transmission electron microscopy cult to observe individual grains as shown in (TEM)studies were also conducted using a Fig.3(d).Figure 4 shows TEM micrographs of sev- HITACHI H-800 microscope operated at 200 kV. eral-cycle ARB processed materials.The associated For this purpose,thin foils parallel to the rolling selected area diffraction (SAD)patterns taken from plane were prepared by twin-jet polishing.The the center of the field by use of an aperture (1.8 um mechanical properties of initial and several-cycle in diameter)are also shown in the figure.The struc- processed strips were measured by tensile test at ture is of a granular type with equiaxed grains.The ambient temperature by an Instron-type testing ma- grain sizes are less than 0.5 um.The SAD patterns chine.Tensile-test specimens were spark-machined have numerous reflections along circles.Such pat- so that the tensile direction was parallel to the roll- terns indicate that large misorientations exis ing direction.The gauge length was 10 mm and the between individual grains.Therefore,it is clear that (a)Initial (b)1 cycle (c)2 cycles (d)5 cycles 300um Fig.3.Longitudinal cross section of initial and ARB processed IF steel strips.conditions. The heating and other conditions are listed in Table 1. Well-bonded bulk materials were successfully obtained. However, excessively high total reduction, i.e. repetition times, sometimes resulted in edge cracks or center fracture as shown in Figs 2(b) and (c). It may be due to tensile stress caused by lateral spreading near the edges. The lat￾eral spreading cannot be neglected when the aspect ratio (width/thickness) is less than 10 [6]. In order to avoid propagation of edge cracks in following cycles, both edges of the roll-bonded strip were trimmed by shearing. The leading and trailing ends of strips were cropped. These edge cracks may not occur in the case of industrial materials with high aspect ratio. The longitudinal cross sections normal to the transverse direction were observed by an optical microscope. Transmission electron microscopy (TEM) studies were also conducted using a HITACHI H-800 microscope operated at 200 kV. For this purpose, thin foils parallel to the rolling plane were prepared by twin-jet polishing. The mechanical properties of initial and several-cycle processed strips were measured by tensile test at ambient temperature by an Instron-type testing ma￾chine. Tensile-test specimens were spark-machined so that the tensile direction was parallel to the roll￾ing direction. The gauge length was 10 mm and the gauge width was 5 mm. The cross-head speed was 0.5 mm/min so that the initial strain rate was 8.310ÿ4 /s. 4. RESULTS Optical micrographs of ARB processed IF steel are shown in Fig. 3. In the case of two-cycle pro￾cessed material [Fig. 3(c)], the interface introduced in the second cycle is seen clearly. It is dicult to ®nd the interfaces of the ®rst pass at a quarter of the thickness. This meant that the subsequent roll￾ing suciently improves the bonding of interfaces introduced in a previous cycle. The severely sheared structure can be observed just below the surface. After ®ve cycles, the whole thickness is covered by very thin and elongated grains and it is very di- cult to observe individual grains as shown in Fig. 3(d). Figure 4 shows TEM micrographs of sev￾eral-cycle ARB processed materials. The associated selected area di€raction (SAD) patterns taken from the center of the ®eld by use of an aperture (1.8 mm in diameter) are also shown in the ®gure. The struc￾ture is of a granular type with equiaxed grains. The grain sizes are less than 0.5 mm. The SAD patterns have numerous re¯ections along circles. Such pat￾terns indicate that large misorientations exist between individual grains. Therefore, it is clear that Table 1. Roll-bonding conditions Material Heating Roll diameter (mm) Roll speed (m/min) Mean strain rate (/s) Al (1100) 473 K 5 min 255 10 12 Al±Mg (5083) 473 K 5 min 310 43 46 IF steel 773 K 10 min 310 43 46 Fig. 3. Longitudinal cross section of initial and ARB processed IF steel strips. SAITO et al.: ACCUMULATIVE ROLL-BONDING 581