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  • Severe plastic deformation (SPD) processes for metals

    A. Azushima (1)a,*, R. Kopp (1)b, A. Korhonen (1)c, D.Y. Yang (1)d, F. Micari (1)e, G.D. Lahoti (1)f,P. Groche (2)g, J. Yanagimoto (2)h, N. Tsuji i, A. Rosochowski j, A. Yanagida a

    a Department of Mechanical Engineering, Graduate School of Engineering, Yokohama National University, Yokohama, Japanb Institute of Metal Forming, RWTH Aachen University, Aachen, Germanyc Department of Materials Science and Engineering, Helsinki University of Technology, Espoo, Finlandd Department of Mechanical Engineering, KAIST, Deajeon, Republic of Koreae Department of Manufacturing and Management Engineering, University of Palermo, Palermo, Italyf Timken Research, The Timken Company, Canton, OH, USAg Institute for Production Engineering and Forming Machines, University of Technology, Darmstadt, Germanyh Institute of industrial Science, The University of Tokyo, Tokyo, Japani Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, Osaka, Japanj Department of Design, Manufacture and Engineering Management, University of Strathclyde, Glasgow, United Kingdom

    CIRP Annals - Manufacturing Technology 57 (2008) 716735

    A R T I C L E I N F O

    Keywords:

    Forming

    Metal

    Strain

    A B S T R A C T

    Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very

    large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal. The

    objective of the SPD processes for creating ultra-fine grained metal is to produce lightweight parts by

    using high strength metal for the safety and reliability of micro-parts and for environmental harmony. In

    this keynote paper, the fabrication process of equal channel angular pressing (ECAP), accumulative roll-

    bonding (ARB), high pressure torsion (HPT), and others are introduced, and the properties of metals

    processed by the SPD processes are shown. Moreover, the combined processes developed recently are

    also explained. Finally, the applications of the ultra-fine grained (UFG) metals are discussed.

    2008 CIRP.

    Contents lists available at ScienceDirect

    CIRP Annals - Manufacturing Technology

    journal homepage: http://ees.elsevier.com/cirp/default.asp

    1. Introduction

    Processes with severe plastic deformation (SPD) may be definedas metal forming processes in which an ultra-large plastic strain isintroduced into a bulk metal in order to create ultra-fine grainedmetals [17]. The main objective of a SPD process is to producehigh strength and lightweight parts with environmental harmony.

    In the conventional metal forming processes such as rolling,forging and extrusion, the imposed plastic strain is generally lessthan about 2.0. When multi-pass rolling, drawing and extrusionare carried out up to a plastic strain of greater than 2.0, thethickness and the diameter become very thin and are not suitableto be used for structural parts. In order to impose an extremelylarge strain on the bulk metal without changing the shape, manySPD processes have been developed.

    Various SPD processes such as equal channel angular pressing(ECAP) [811], accumulative roll-bonding (ARB) [1214], highpressure torsion (HPT) [15,16], repetitive corrugation and straigh-tening (RCS) [17], cyclic extrusion compression (CEC) [18], torsionextrusion [19], severe torsion straining (STS) [20], cyclic closed-dieforging (CCDF) [21], super short multi-pass rolling (SSMR) [22]have been developed.

    The major SPD processes are summarized in Table 1 withschematic configurations and the attainable plastic strain. ECAP,ARB and HPT processes are well-investigated for producing ultra-

    * Corresponding author.

    0007-8506/$ see front matter 2008 CIRP.doi:10.1016/j.cirp.2008.09.005

    fine grained metals. It is known that the metals produced by theseprocesses have very small average grain sizes of less than 1 mm,with grain boundaries of mostly high angle mis-orientation.

    The ultra-fine grained metals created by the SPD processesexhibit high strength [2325], and thus they may be used as ultra-high strength metals with environmental harmony. The yieldstress of polycrystalline metals is related to the grain diameter d bythe following HallPetch equation:

    sY s0 Ad1=2 (1)

    where s0 is the friction stress and A is a constant.Eq. (1) means that the yield stress increases with decreasing

    square root of the grain size. The decrease of grain size leads to ahigher tensile strength without reducing the toughness, whichdiffers from other strengthening methods such as heat treatment.

    The relationship between proof stress and grain size of pureiron is shown in Fig. 1 [6]. The proof stress changes inversely withthe square root of the grain size, following the HallPetchrelationship. It is seen that the proof stress of the ultra-finegrained irons, with sub-micrometer grains, is five times greaterthan commercially pure iron. Thus, the conventional structuralmetals with ultra-fine grains are lighter due to their high strength.Since pure iron does not contain harmful elements, it is in harmonywith a clean environment. Moreover, the improvements of thesuperplasticity, corrosion and fatigue properties of metalsprocessed by SPD are expected. On the other hand, the ultra-finegrained metals are available only for micro-parts [26,27].

    http://www.sciencedirect.com/science/journal/00078506http://dx.doi.org/10.1016/j.cirp.2008.09.005

  • Table 1Summary of major SPD processes

    Process name Schematic representation Equivalent plastic strain

    Equal channel angular extrusion (ECAE) (Segal, 1977) e n 2ffiffi3p cot

    High-pressure torsion (HPT) (Valiev et al., 1989) e grffiffi3p , gr n 2prt

    Accumulative roll-bonding (ARB) (Saito, Tsuji, Utsunomiya, Sakai, 1998) e n 2ffiffi3p ln t0t

    A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716735 717

    In Fig. 2 [28], the mechanical properties of a wire specimenmade by SPD is plotted against the ratio of wire diameter D to thegrain size d, D/d. The proof stress decreases with decreasing D/dwhen D/d is less than 100. In particular, when D/d is less than 5, theproof stress decreases abruptly with decreasing D/d. From theseobservations, the ratio of D/d must be greater than 100 in order toguarantee the safety and the reliability of metals for micro-parts.

    This paper reviews the severe plastic deformation processesto create metals with ultra-fine grains. In the following, thefabrications of the SPD processes are shown in Section 2. Then, the

    Fig. 2. Material behavior during forming processes of micro-parts of a wirespecimen with diameter to grain size D/d [28].

    Fig. 1. Relationship between proof stress and grain size of pure iron [6].

    properties of metals processed by SPD processes are shownin Section 3, the combined processes developed recently areexplained in Section 4, and the applications of the ultra-finegrained metals are discussed in Section 5.

    2. SPD processes

    2.1. Equal channel angular press (ECAP) process

    2.1.1. Conventional ECAP processes

    Fig. 3 shows the schematic representation of side extrusionprocesses, which are a kind of double axis extrusion or sideextrusion [29]. Fig. 3(d and e) indicates the process in which pureshear deformation can be repeatedly imposed on materials so thatan intense plastic strain is produced with the materials withoutany change in the cross-sectional dimensions of the workpiece.These processes are named as ECAE (Equal channel angularextrusion) or ECAP.

    Segal [8,30] proposed this process in 1977 in order to create anultra-fine grained material. Although ECAP is generally applied tosolid metals, it may also be used for consolidation of metallicpowder. Kudo and coworkers [31] employed repetitive sideextrusion with back pressure to consolidate a pure aluminumpowder. In the 1990s, developments of ultra-fine grained materialswere carried out with this method by Valiev et al. [9,10,32], Horitaand coworkers [3345] and Azushima et al. [4648] and others[4951].

    The schematic representation of the ECAE process is shown inFig. 4. The specimen is side extruded through the sheardeformation zone with the dead zone in the outer corner of thechannel. When the workpiece is side extruded through thechannel, the total strain is

    e 1ffiffiffi3p 2cot f

    2c

    2

    c cosec f

    2c

    2

    (2)

    Fig. 3. Schematic illustration of side extrusion process, which are a kind of doubleaxis extrusion or side extrusion [29].

  • Fig. 4. Schematic representation of ECAE process.

    Fig. 5. Fundamental process of metal flow during ECAP. (a) The deformation of acubic element on a single pass [33]. (b) Shearing characteristics for four different

    processing routes [36].

    A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716735718

    where F is the angle of intersection of two channels and C is theangle subtended by the arc of curvature at the point of intersection.When F = 908 and C = 08, the total strain from the above equationis e = 1.15. After n passes, it becomes n e.

    Fig. 5 shows the fundamental process of metal flow during ECAP[6]. The channel is bent through an angle equal to 908 and thespecimen is inserted within the channel and it can be pressedthrough the die using a punch. There are four basic processingroutes in ECAP. In route A, the specimen is pressed withoutrotation, in route BA the specimen is rotated by 908 in an alternatedirection between consecutive passes, in route BC the specimen isrotated 908 counterclockwise between each pass, and in route C thespecimen is rotated by 1808 between passes.

    From these macroscopic distortions shown in Fig. 5, theinfluence of the processing route o