790 Y.Estrin,A.Vinogradov/Acta Materialia 61 (2013)782-817 modification of the CONFORM method for processing of sheets or strips [123,124]. 250um 0☐255 Continuous ECAP for sheet manufacturing was dis- cussed by Lapovok et al.[125,126].For the Al alloys tested it was established that just a single ECAP pass was suffi- cient to obtain a significant reduction of normal and in- plane anisotropy.A variant of the process is continuous equal-channel angular drawing [125,126]. Repetitive corrugating and straightening (RCS)has an obvious advantage of providing a simple modification of rolling to enable continuous SPD processing,as illustrated graphically in Table 1q [74,75]. Incremental ECAP (I-ECAP).Rosochowski and co- authors extended general knowledge of incremental metal forming operations,such as rolling,swaging or rotary forging,and adapted it to ECAP by modifying it for pro- cessing of long billets.This process was dubbed incremen- tal ECAP (I-ECAP)[127].The basic version of I-ECAP is shown in Table Ir.The deformation mode is simple shear. and it is uniform within the marked zone similarly to the Fig.3.Steel preform with 100 um porosity infiltrated with Al (seen in red) shear deformation in "classical"ECAP.Separation of the as revealed by energy dispersive spectroscopy.A90 ECAP-like"knee"is feeding and deformation stages reduces or eliminates fric- seen in the bottom left part of the picture.After Ref.[132](reprinted with permission). tion during feeding;this substantially reduces the feeding force and enables processing of very long or continuous billets. Concluding this section we would like to note that a great Continuous manufacturing of bolts by ECAP.The group variety of SPD techniques are now available.Their com- of Prof.Y.-T.Im at KAIST in Korea developed a method mon features are the high hydrostatic pressure and the tool [128.129]which overcomes the discrete character of ECAP geometry permitting multipass operation to achieve ultra- by integrating what they call"spring-loaded ECAp"in a high strains.Differences are mainly related to the deforma- continuous bolt manufacturing process (Table It). tion mode,the work-piece shape.the efficacy with respect to AA6016 bolts produced using this technology were shown the strain imposed per pass and the load involved.All these to be superior to those manufactured in a conventional way factors affect,to a varying extent,the resultant microstruc- in terms of their tensile strength and fatigue strength. ture,the properties of the product and the upscaling capac- Continuous high-pressure torsion.An advanced version ity of the technique used.These aspects of SPD processing of the HPT technique was proposed by Edalati and Horita will be addressed in the next sections. [130],who demonstrated its viability as a method to pro- A great advantage of the SPD techniques is that they are duce sheets 0.6-mm thick and 3 mm wide,which possess based on a "top-down"approach involving grain refine- UFG structure,in a continuous fashion,cf.Table Is. ment through "breaking down"the microstructure of the While for most structural applications upscaling of the bulk to the submicron scale.SPD processing is thus free SPD technologies is required,there may be niche applica- from problems of excessive residual porosity and contami- tions where downscaling would be desirable.The feasibility nation,which are common in nanostructured materials of such downscaling was demonstrated for the ECAP pro- manufactured in a"bottom-up"fashion,e.g.by consolida- cess [131].Miniaturized dies with channel diameters in the tion of nanopowders.Furthermore,no health hazards millimeter range were used to deform Al specimens and potentially associated with handling of nanopowders are achieve grain refinement in a single pass. involved in SPD processing. An SPD-like process of an entirely different type was As will be seen below,perhaps the most important dis- proposed by Estrin et al.[132].In this "solid-state infiltra- advantage of SPD is that the efficiency of grain refinement tion"method,solid aluminium was forced to fill a porous drops with strain [134].A way to overcome the problem by steel preform under high pressure in much the same way suppressing dynamic recovery,e.g.by using SPD process- vias are filled with metal in fabrication of metallic intercon- ing at cryogenic temperatures,was suggested in Ref.[81]. nects by the force-fill process in microelectronics [1331.The However,the microstructures obtained in this way retain random paths taken by the plastically flowing Al are pretty a large volume fraction of low-angle boundaries,giving rise tortuous and involve numerous kinks.Some of them may to considerable thermal instability and coarsening of the be similar to those seen in ECAP channels and induce ultrafine microstructure produced. ECAP-like localized shear zones and ensuing grain refine- Given the rapid progress in the field,we are confident ment.Penetration of Al into the porous steel preform is that new processes with higher throughput,upscaling illustrated in Fig.3. capacity and greater cost-effectiveness will emerge,meetingmodification of the CONFORM method for processing of sheets or strips [123,124]. Continuous ECAP for sheet manufacturing was dis￾cussed by Lapovok et al. [125,126]. For the Al alloys tested it was established that just a single ECAP pass was suffi- cient to obtain a significant reduction of normal and in￾plane anisotropy. A variant of the process is continuous equal-channel angular drawing [125,126]. Repetitive corrugating and straightening (RCS) has an obvious advantage of providing a simple modification of rolling to enable continuous SPD processing, as illustrated graphically in Table 1q [74,75]. Incremental ECAP (I-ECAP). Rosochowski and co￾authors extended general knowledge of incremental metal forming operations, such as rolling, swaging or rotary forging, and adapted it to ECAP by modifying it for pro￾cessing of long billets. This process was dubbed incremen￾tal ECAP (I-ECAP) [127]. The basic version of I-ECAP is shown in Table 1r. The deformation mode is simple shear, and it is uniform within the marked zone similarly to the shear deformation in “classical” ECAP. Separation of the feeding and deformation stages reduces or eliminates fric￾tion during feeding; this substantially reduces the feeding force and enables processing of very long or continuous billets. Continuous manufacturing of bolts by ECAP. The group of Prof. Y.-T. Im at KAIST in Korea developed a method [128,129] which overcomes the discrete character of ECAP by integrating what they call “spring-loaded ECAP” in a continuous bolt manufacturing process (Table 1t). AA6016 bolts produced using this technology were shown to be superior to those manufactured in a conventional way in terms of their tensile strength and fatigue strength. Continuous high-pressure torsion. An advanced version of the HPT technique was proposed by Edalati and Horita [130], who demonstrated its viability as a method to pro￾duce sheets 0.6-mm thick and 3 mm wide, which possess UFG structure, in a continuous fashion, cf. Table 1s. While for most structural applications upscaling of the SPD technologies is required, there may be niche applica￾tions where downscaling would be desirable. The feasibility of such downscaling was demonstrated for the ECAP pro￾cess [131]. Miniaturized dies with channel diameters in the millimeter range were used to deform Al specimens and achieve grain refinement in a single pass. An SPD-like process of an entirely different type was proposed by Estrin et al. [132]. In this “solid-state infiltra￾tion” method, solid aluminium was forced to fill a porous steel preform under high pressure in much the same way vias are filled with metal in fabrication of metallic intercon￾nects by the force-fill process in microelectronics [133]. The random paths taken by the plastically flowing Al are pretty tortuous and involve numerous kinks. Some of them may be similar to those seen in ECAP channels and induce ECAP-like localized shear zones and ensuing grain refine￾ment. Penetration of Al into the porous steel preform is illustrated in Fig. 3. Concluding this section we would like to note that a great variety of SPD techniques are now available. Their com￾mon features are the high hydrostatic pressure and the tool geometry permitting multipass operation to achieve ultra￾high strains. Differences are mainly related to the deforma￾tion mode, the work-piece shape, the efficacy with respect to the strain imposed per pass and the load involved. All these factors affect, to a varying extent, the resultant microstruc￾ture, the properties of the product and the upscaling capac￾ity of the technique used. These aspects of SPD processing will be addressed in the next sections. A great advantage of the SPD techniques is that they are based on a “top-down” approach involving grain refine￾ment through “breaking down” the microstructure of the bulk to the submicron scale. SPD processing is thus free from problems of excessive residual porosity and contami￾nation, which are common in nanostructured materials manufactured in a “bottom-up” fashion, e.g. by consolida￾tion of nanopowders. Furthermore, no health hazards potentially associated with handling of nanopowders are involved in SPD processing. As will be seen below, perhaps the most important dis￾advantage of SPD is that the efficiency of grain refinement drops with strain [134]. A way to overcome the problem by suppressing dynamic recovery, e.g. by using SPD process￾ing at cryogenic temperatures, was suggested in Ref. [81]. However, the microstructures obtained in this way retain a large volume fraction of low-angle boundaries, giving rise to considerable thermal instability and coarsening of the ultrafine microstructure produced. Given the rapid progress in the field, we are confident that new processes with higher throughput, upscaling capacity and greater cost-effectiveness will emerge, meeting Fig. 3. Steel preform with 100 lm porosity infiltrated with Al (seen in red) as revealed by energy dispersive spectroscopy. A 90 ECAP-like “knee” is seen in the bottom left part of the picture. After Ref. [132] (reprinted with permission). 790 Y. Estrin, A. Vinogradov / Acta Materialia 61 (2013) 782–817