582 SAITO et al:ACCUMULATIVE ROLL-BONDING 5.DISCUSSIONS It is made clear that the proposed accumulative roll-bonding (ARB)process causes ultra-fine (sub- micron)grains and surprising strength.These effects are confirmed experimentally by three materials: aluminum(1100),Al-Mg alloy(5083)and Ti-added interstitial free steel. There are two possible additional mechanisms in (a)8-cycle processed 1100 the ARB process which differ from other high straining processes.The first possible mechanism is the effect of severe shear deformation just below the surface.It has been reported that severe shear de- formation is introduced by friction between the workpiece and the roll under dry conditions [8]. This shear deformation significantly increases the equivalent strain from the value calculated by equation (3)and promotes grain refinement. Moreover,the ARB process can introduce this (b)7-cycle processed 5083 severely deformed region into the interior of the material by repetition.The whole thickness of ma- terials may be severely strained after several cycles The other mechanism is the introduction of new interfaces.A large number of interfaces are intro- duced by several ARB cycles.These interfaces show a well-developed fiber structure.The oxide films on the surfaces,as well as inclusions,are dispersed uni- formly by repetition.These things contribute to the strength and may act as obstacles for grain growth (c)5-cycle processed IF steel In the case of 1100 aluminum,the variation in structure and mechanical properties in the ARB Fig.4.TEM micrographs and SAD patterns of ARB pro- process were investigated in Ref.[9].However,the cessed strips. general mechanism of the grain refinement during ARB is still unclear at this stage and requires further study an ultra-fine (sub-micron)grain structure with large The advantage of this process against other high misorientations,i.e.polycrystal,was formed. straining processes is its high productivity and the Mechanical properties of initial and ARB pro- feasibility of large-sized material production cessed materials are compared in Table 2.In the Although the experiments have been carried out case of aluminum 1100,the tensile strength of com- with narrow 20 mm wide materials in this study,it mercially available full-hardened material (temper is supposed that application to bulk materials such grade H18)is ~165 MPa [7].The tensile strength of as wide strips in a coil is not difficult.The process the ARB processed 1100(eight cycles)is 1.8 times does not require any special machines because the higher than that of the 1100-H18.The ARB pro- roll-bonding is widely adopted in clad metal cessed 5183 and IF steel also showed extremely production [10].The process can be readily industri- high strength,however,the elongation decreased alized. from 8 to 5%.On the other hand.the material still shows sufficient ductility,despite the fact that the materials were highly strained. 6.CONCLUSIONS Table 2.Mechanical properties of initial and ARB processed ma- A novel ultra-high straining process,the accumu- terials lative roll-bonding (ARB)process is proposed.The No. Tensile ARB process has successfully been applied to Material of cycles strength(MPa) Elongation (% aluminum(1100),Al-Mg alloy (5083)and Ti-added interstitial free steel.All three several-cycle ARB A1(1100) 0(initial） 84 processed materials have structures with sub-micron A11100m 8 30 A-Mg(083) 0(initial) 319 grains and show very high strength.The proposed A-Mg(5083) 351 ARB is a promising process for the manufacture of IF steel 0(initial) 274 IF steel 751 6 high-strength bulk materials at a high level of pro- ductivityan ultra-®ne (sub-micron) grain structure with large misorientations, i.e. polycrystal, was formed. Mechanical properties of initial and ARB pro￾cessed materials are compared in Table 2. In the case of aluminum 1100, the tensile strength of com￾mercially available full-hardened material (temper grade H18) is 0165 MPa [7]. The tensile strength of the ARB processed 1100 (eight cycles) is 1.8 times higher than that of the 1100-H18. The ARB pro￾cessed 5183 and IF steel also showed extremely high strength, however, the elongation decreased from 8 to 5%. On the other hand, the material still shows sucient ductility, despite the fact that the materials were highly strained. 5. DISCUSSIONS It is made clear that the proposed accumulative roll-bonding (ARB) process causes ultra-®ne (sub￾micron) grains and surprising strength. These e€ects are con®rmed experimentally by three materials: aluminum (1100), Al±Mg alloy (5083) and Ti-added interstitial free steel. There are two possible additional mechanisms in the ARB process which di€er from other high straining processes. The ®rst possible mechanism is the e€ect of severe shear deformation just below the surface. It has been reported that severe shear de￾formation is introduced by friction between the workpiece and the roll under dry conditions [8]. This shear deformation signi®cantly increases the equivalent strain from the value calculated by equation (3) and promotes grain re®nement. Moreover, the ARB process can introduce this severely deformed region into the interior of the material by repetition. The whole thickness of ma￾terials may be severely strained after several cycles. The other mechanism is the introduction of new interfaces. A large number of interfaces are intro￾duced by several ARB cycles. These interfaces show a well-developed ®ber structure. The oxide ®lms on the surfaces, as well as inclusions, are dispersed uni￾formly by repetition. These things contribute to the strength and may act as obstacles for grain growth. In the case of 1100 aluminum, the variation in structure and mechanical properties in the ARB process were investigated in Ref. [9]. However, the general mechanism of the grain re®nement during ARB is still unclear at this stage and requires further study. The advantage of this process against other high straining processes is its high productivity and the feasibility of large-sized material production. Although the experiments have been carried out with narrow 20 mm wide materials in this study, it is supposed that application to bulk materials such as wide strips in a coil is not dicult. The process does not require any special machines because the roll-bonding is widely adopted in clad metal production [10]. The process can be readily industri￾alized. 6. CONCLUSIONS A novel ultra-high straining process, the accumu￾lative roll-bonding (ARB) process is proposed. The ARB process has successfully been applied to aluminum (1100), Al±Mg alloy (5083) and Ti-added interstitial free steel. All three several-cycle ARB processed materials have structures with sub-micron grains and show very high strength. The proposed ARB is a promising process for the manufacture of high-strength bulk materials at a high level of pro￾ductivity. Fig. 4. TEM micrographs and SAD patterns of ARB pro￾cessed strips. Table 2. Mechanical properties of initial and ARB processed ma￾terials Material No. of cycles Tensile strength (MPa) Elongation (%) Al (1100) 0 (initial) 84 42 Al (1100) 8 304 8 Al±Mg (5083) 0 (initial) 319 25 Al±Mg (5083) 7 551 6 IF steel 0 (initial) 274 57 IF steel 5 751 6 582 SAITO et al.: ACCUMULATIVE ROLL-BONDING