Y.Estrin,A.Vinogradov/Acta Materialia 61 (2013)782-817 789 recently with the aim of processing samples other than sim- diate annealing and/or post-ECAP processing by conven- ple rod or disk stock and/or enabling a higher throughput. tional rolling,drawing or extrusion.The advantageous Some of them are illustrated in Table 1.A list of these tech- effect of post-processing was confirmed by many research- niques (which is admittedly not exhaustive)includes: ers who combined different post-ECAP techniques to fur- ther enhance strength [103-105],modify texture [106]or repetitive side extrusion [66; improve ductility by subsequent annealing [107-109]. ·rotary die ECAP[67 Finally,new integrated processing schemes.which adopt .parallel channel ECAP [68]: features of different processes and combine them in a sin- hydrostatic extrusion [69-71]and hydrostatic extrusion gle-step integrated processing workflow [110-112],have combined with torsion [72]: recently been developed,cf.Table 11.The use of the inte- repetitive corrugating and straightening(RCS)for pro- grated semicontinuous processing techniques may be a cessing of sheets or plates [73-751 promising way of overcoming obstacles to uptake of SPD constrained groove pressing [76]; techniques by industry. cyclic extrusion-compression(CEC)[77]; Among the recent developments of SPD methods one cyclic closed-die forging (CCDF)[78]: can recognize a trend to target thin products,particularly cone-cone method(CCM)[79]; thin-walled tubes,and produce grain refinement by fric- cryogenic rolling [80,81]; tion-induced shear.One of the work-piece dimensions used asymmetric rolling (ASR)[82]; in such processes,namely the thickness,is much smaller continuous frictional angular extrusion(CFAE)[83,84]; than the other two dimensions.The cone-cone method friction stir processing(FSP)[85,86]; [79,113]and high-pressure tube twisting [96]are in that cat- super short interval multi-pass rolling(SSMR)[87,88]; egory,as is a modified tube-twisting technique suggested in severe torsion straining(STS)[89,90]; Ref.[114].Depending on the wall thickness,grain refine- torsion extrusion [91]; ment can be achieved throughout the tube wall thickness ECAP with rotation tooling in which the conventional or only within near-surface regions of the wall.This fixed die is replaced by rotating tools [92]; method was also applied for producing bimetallic Al-Cu reversed shear spinning [92]; tubes with ultrafine grain size(as small as about 140 nm transverse rolling [92]; near the interface of the two metals)[115]. non-equal channel angular pressing (NECAP)for plate- In a similar vein,Umemoto [116-118]made a point that shaped billets [93]; conventional metal processing techniques,such as shot tube channel pressing [94]; peening,drilling and wear [119],can be used as an effective ·KOBO forming[95l: way to create UFG structure and concomitant strengthen- high-pressure tube twisting (HPTT)for thin-walled ing in near-surface regions of metals and alloys. tubes [96]; cyclic expansion-extrusion CEE-a modified CEC pro- 2.3.Continuous SPD techniques and post-processing cess [97]: simple shear extrusion [98,99]; Many of the SPD methods presented above involve a vortex extrusion [100]; large number of discrete steps and are not labour and cost helical rolling [101]; efficient.Furthermore,they suffer from the inability to deli- .high-pressure sliding [102]. ver sufficiently large work-pieces as required for industry- scale applications.A number of approaches to SPD pro- From this list alone one can see that there are really no cessing seek to alleviate these disadvantages.In what fol- bounds to the imagination and resourcefulness of SPD pro- lows,we touch upon them briefly. cess designers.and more and more new SPD techniques Continuous forming (CONFORM),Table Im,is a well- have been emerging recently.From a purist's viewpoint, known process,the principles of which were first formu- not all of them would qualify to be termed "nanoSPD" lated by Etherington [120]with the aim of improving the processing according to the definition in Ref.[6],but most efficacy of materials recycling.They were later adapted of these techniques are cognate with the principal pro- by Segal and co-workers to continuous ECAP of bulk cesses-ECAP.HPT.TE or ARB-they derive from and materials [37].These principles were implemented by Raab bear some semblance with.Most of these processes use et al.in a rig for production of Al and Ti rods [121].In this shear deformation in conjunction with hydrostatic pressure process,the rod is placed in a groove within a rotating to produce large strains.The potential benefits of these shaft and is driven forward by frictional forces and then "derivative"techniques include simplified tool design, extruded through an outlet cannel of the die similarly to lower loads,reduced material loss,the possibility to pro- ECAP.A modification of this process was proposed by Sai- cess larger work-pieces,automated handling and/or poten- to et al.[122]as continuous shearing,Table 1o,for process- tial continuous operation. ing of sheets or strips. It is broadly recognized that strength and ductility may Continuous confined strip shearing (C2S2),sometimes greatly benefit from a combination of ECAP with interme- referred to as ECA-rolling process,Table 1p,is arecently with the aim of processing samples other than sim￾ple rod or disk stock and/or enabling a higher throughput. Some of them are illustrated in Table 1. A list of these tech￾niques (which is admittedly not exhaustive) includes:  repetitive side extrusion [66];  rotary die ECAP [67];  parallel channel ECAP [68];  hydrostatic extrusion [69–71] and hydrostatic extrusion combined with torsion [72];  repetitive corrugating and straightening (RCS) for pro￾cessing of sheets or plates [73–75];  constrained groove pressing [76];  cyclic extrusion–compression (CEC) [77];  cyclic closed-die forging (CCDF) [78];  cone–cone method (CCM) [79];  cryogenic rolling [80,81];  asymmetric rolling (ASR) [82];  continuous frictional angular extrusion (CFAE) [83,84];  friction stir processing (FSP) [85,86];  super short interval multi-pass rolling (SSMR) [87,88];  severe torsion straining (STS) [89,90];  torsion extrusion [91];  ECAP with rotation tooling in which the conventional fixed die is replaced by rotating tools [92];  reversed shear spinning [92];  transverse rolling [92];  non-equal channel angular pressing (NECAP) for plate￾shaped billets [93];  tube channel pressing [94];  KOBO forming [95];  high-pressure tube twisting (HPTT) for thin-walled tubes [96];  cyclic expansion–extrusion CEE—a modified CEC pro￾cess [97];  simple shear extrusion [98,99];  vortex extrusion [100];  helical rolling [101];  high-pressure sliding [102]. From this list alone one can see that there are really no bounds to the imagination and resourcefulness of SPD pro￾cess designers, and more and more new SPD techniques have been emerging recently. From a purist’s viewpoint, not all of them would qualify to be termed “nanoSPD” processing according to the definition in Ref. [6], but most of these techniques are cognate with the principal pro￾cesses—ECAP, HPT, TE or ARB—they derive from and bear some semblance with. Most of these processes use shear deformation in conjunction with hydrostatic pressure to produce large strains. The potential benefits of these “derivative” techniques include simplified tool design, lower loads, reduced material loss, the possibility to pro￾cess larger work-pieces, automated handling and/or poten￾tial continuous operation. It is broadly recognized that strength and ductility may greatly benefit from a combination of ECAP with interme￾diate annealing and/or post-ECAP processing by conven￾tional rolling, drawing or extrusion. The advantageous effect of post-processing was confirmed by many research￾ers who combined different post-ECAP techniques to fur￾ther enhance strength [103–105], modify texture [106] or improve ductility by subsequent annealing [107–109]. Finally, new integrated processing schemes, which adopt features of different processes and combine them in a sin￾gle-step integrated processing workflow [110–112], have recently been developed, cf. Table 1l. The use of the inte￾grated semicontinuous processing techniques may be a promising way of overcoming obstacles to uptake of SPD techniques by industry. Among the recent developments of SPD methods one can recognize a trend to target thin products, particularly thin-walled tubes, and produce grain refinement by fric￾tion-induced shear. One of the work-piece dimensions used in such processes, namely the thickness, is much smaller than the other two dimensions. The cone–cone method [79,113] and high-pressure tube twisting [96] are in that cat￾egory, as is a modified tube-twisting technique suggested in Ref. [114]. Depending on the wall thickness, grain refine￾ment can be achieved throughout the tube wall thickness or only within near-surface regions of the wall. This method was also applied for producing bimetallic Al–Cu tubes with ultrafine grain size (as small as about 140 nm near the interface of the two metals) [115]. In a similar vein, Umemoto [116–118] made a point that conventional metal processing techniques, such as shot peening, drilling and wear [119], can be used as an effective way to create UFG structure and concomitant strengthen￾ing in near-surface regions of metals and alloys. 2.3. Continuous SPD techniques and post-processing Many of the SPD methods presented above involve a large number of discrete steps and are not labour and cost efficient. Furthermore, they suffer from the inability to deli￾ver sufficiently large work-pieces as required for industry￾scale applications. A number of approaches to SPD pro￾cessing seek to alleviate these disadvantages. In what fol￾lows, we touch upon them briefly. Continuous forming (CONFORM), Table 1m, is a well￾known process, the principles of which were first formu￾lated by Etherington [120] with the aim of improving the efficacy of materials recycling. They were later adapted by Segal and co-workers to continuous ECAP of bulk materials [37]. These principles were implemented by Raab et al. in a rig for production of Al and Ti rods [121]. In this process, the rod is placed in a groove within a rotating shaft and is driven forward by frictional forces and then extruded through an outlet cannel of the die similarly to ECAP. A modification of this process was proposed by Sai￾to et al. [122] as continuous shearing, Table 1o, for process￾ing of sheets or strips. Continuous confined strip shearing (C2S2), sometimes referred to as ECA-rolling process, Table 1p, is a Y. Estrin, A. Vinogradov / Acta Materialia 61 (2013) 782–817 789