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Materials Science and Engineering A 493 (2008) 130140 Scale up and application of equal-channel angular extrusion for the electronics and Stephane Ferrasse a, , V.M. a , Susan E. Euclid A, Whitmor form Abstract yond up; are pursued of ys in those to medium-to-hea ys, interplay K 1. niques have been the focus of intense research because they can produce metallic materials with submicrometer grain sizes ranging from 50 to 500 nm 1,2. One promising SPD method is equal-channel angular extrusion (ECAE) 3. It can pro- duce intense has ture, materials deformation literature, discussed cations continue A there commercialization well penetrate pro turing or design, (ii) provide superior product performance and (iii) answer an unmet need. One example involves the first ECAE products with submicrometer or micrometer grain sizes for high purity Al, Cu and Ti sputtering targets used in the fabrication 0921-5093/$ doi: bulk pieces of submicrocrystalline materials induced by plastic straining by simple shear. Till now, research made steady progress on the characterization of the tex- structure and mechanical properties of submicrocrystalline and the effect of main ECAE parameters and post- annealing 429. However, despite the abundant problems of engineering and commercialization were only recently 3032 and very few practical appli- are reported. The overwhelming majority of researchers to work with small long cylindrical or square billets. few attempts to scale up the billet size are known 3235 but is no report of successful commercialization. This paper reviews the efforts in die design, scale up and of ECAE for flat billets conducted at Honey- Corresponding author. Tel.: +1 509 2522118; fax: +1 509 2528743. E-mail address: Stephane.F (S. Ferrasse). of logic and memory components. Two other examples concern medium and heavily alloyed Al materials used in aerospace and transportation. Special attention is paid to the effects of ECAE on the structures and properties of single phase Cu and, espe- cially, Al when the amount of alloying composition increases from a very low level (as in sputtering targets) to a higher level (as in commercial alloys for aerospace). It is argued that new mechanisms and, therefore, additional opportunities for appli- cations arise as the alloying level increases because of the new interplay between plastic deformation and phase transformation during a thermo-mechanical treatment. 2. Process scale up and design Honeywells focus has been, historically, the ECAE of flat products, which was first introduced in Ref. 38. In that case (Fig. 1), a typical billet shape is characterized by thickness a, see front matter 2007 Elsevier B.V. All rights reserved. 10.1016/j.msea.2007.04.133 Janine Kardokus a Honeywell Electronic Materials, 15128 b EPM, 11228 Lemen Rd-Suite Received 9 February 2007; received in revised Two areas are critical to promote equal-channel angular extrusion be (ii) development of new submicrometer-grained products. Both goals ECAE for the production of sputtering targets from single phase allo reported in the literature. Other described applications are targeted vily alloyed Al materials used in aerospace. In these allo between plastic deformation and precipitation mechanisms. 2007 Elsevier B.V. All rights reserved. eywords: ECAE; Submicrocrystalline materials; Flat products; Sputtering; Fatigue; Introduction For the past 10 years, severe plastic deformation (SPD) tech- aerospace industries Segal b , Frank Alford a , Strothers a Avenue, Spokane, WA 99216, USA e Lake, MI 48198, USA 12 April 2007; accepted 25 April 2007 the stage of a laboratory curiosity: (i) tool/processing design and scale at Honeywell. The first case is the successful commercialization the electronic industry. Blank dimensions are significantly larger than the increase of tensile strength, high-cycle fatigue and toughness in the optimal properties can be reached with better understanding of the Toughness 36,37. Selected examples show that this technology can a market in one or more of the following ways: (i) vide an overall cost reduction versus the standard manufac- S. Ferrasse et al. / Materials Science and Engineering A 493 (2008) 130140 131 width b and length c with c, b greatermuch a 30,3840. Usually, dimen- sions c and b are equal to allow the use of the same tool for multi-pass processing (with 90 rotation between passes). The processing characteristics of one pass ECAE for flat and long billets are similar. However, usually, for flat billets, the axis of the permissible 90 billet rotation is perpendicular to the extrusion axis (axis Z in Fig. 1) whereas, for long products, it is parallel to the extrusion axis. During scale up, two considerations come into play: (i) tool design, and (ii) optimization of ECAE deformation mode. 2.1. Tool design From a production perspective, the major drivers for tool design include safety, cost and productivity. 2.1.1. Safety and cost The biggest issue is the potential breakage/buckling of the punch if conventional low cost tool steels are used. For a given material, the punch pressure p 1 must be significantly less than the 30 where shear of and sho sures friction choice nificant terms entrance channel are not needed for flat products. This is because a lessmuch b for flat products whereas a = b for long products. There- fore p 1 and n are lower for flat products and formulae (2) and (3) can be approximately reduced to p 1 2k 1 + mc a (4) ho atomically 2.1.2. ejection. is The for able hydraulic 2.2. ECAE. 2.2.1. ble mostly nel the the sharp tions e friction is e zone angle ditions simple problem of si the sis o 2.2.2. sequence of the yield strength of the punch material. The punch pressure is p 1 2k = p 2k + mF 2A (1) p is the pressure at the exit of first channel, k is the material flow stress, m is the plastic friction coefficient, F is the area stationary die walls and A is the billet cross-sectional area. For the tool itself, the maximum pressures on the punch p 1 channel wall n act at the end of the entrance channel. As wn in 30, for a low friction case (m < 0.25) p 1 2k 1 + m(cb + ca) ba (2) n 2k m(cb + ca) ba (3) Therefore, the preferable ways for reducing die/punch pres- are (i) to limit the ratio c/a < 610 and (ii) to minimize in both channels. Two corresponding strategies are the of effective lubricants and movable channel walls. A sig- advantage of flat ECAE billets versus long billets in of equipment and design is that movable walls along the Fig. 1. Principle of the ECAE technique for flat billets. n 2k mc a (5) A movable bottom wall at the exit channel is recommended wever for both flat and long products because lubricant is removed along the bottom of exit channel. Productivity The two important factors are processing speed and billet For reasonably ductile materials, the processing speed not a limiting factor and may be sufficiently high (510 mm/s). billet ejection presents a more complex problem, especially long cylindrical billets. In the case of flat billets, a mov- bottom wall of the exit channel operated by an additional cylinder provides an effective and simple solution. Optimization of ECAE There are two levels of optimization for single and multi-pass Single pass A level of simple shear straining should be as high as possi- for an effective refinement of microstructures 11. This is controlled by the conditions of friction and the chan- geometry which has in turn two critical parameters: (i) angle 2 between the two channels and (ii) the shape of channel intersection. Usually, channels are performed with (no radius) or round corner intersections. Slip line solu- 18,30,41 and finite element modeling 43 reveal the xistence of a fan-like deformation zone in cases of noticeable and/or round corner channels. In such cases, simple shear redistributed along three different directions 41. Moreover ven for frictionless conditions and sharp corners, a dead metal exists at the channel corner for 2 < 90. Therefore, tool 2 = 90, sharp corner channels and near frictionless con- are the optimum characteristics to realize the effective shear of = 2 along one direction. The most important is the elimination of the friction along the bottom wall the outlet channel where high compressive pressure and inten- ve slip act simultaneously. With the movable bottom wall, fan angle can be minimized as shown by slip line analy- 30,41. The Honeywell dies operate under those conditions wing to advanced die design and lubricants. Multi-pass processing The two major parameters are the deformation route (a of billet rotation after each pass) and the total number passes (accumulated strains). For flat billets, the definition of four fundamental routes A, B (or B A ), C and D (or Bc) 38 132 S. Ferrasse et al. / Materials Science and Engineering A 493 (2008) 130140 Fig. 2. Production ECAE die with 4000 tonnes press capacity. remains described 2.3. scale-up first standard occasionally capacity weekly let Cu processed 34,35 has a mass of 6.7 kg whereas the mass of the most typical 10 mm 10 mm 60 mm Al billet used for research is 0.016 kg. There is no report of a scale-up attempt of the ECAE process for Cu. Importantly, the effects of ECAE on microstructures, texture and properties have been verified at the various industrial scales as will be shown in the Section 2. In the authors view, the expe- rience attained on the production floor demonstrates that ECAE is scalable and opens up the era of its industrialization. 3. ECAE of sputtering targets ECAE is particularly interesting for high-purity materials because grain refinement is the only available mechanism that effectively enhances strength and retains good ductility (Hall- Petch hardening) whereas the other hardening mechanisms are either ineffective (precipitation and solution hardening) or detrimental to ductility (dislocation hardening). For specific materials and crystal structures, ECAE can also activate and con- trol texture hardening. This approach remains valid for doped or low-alloyed materials such as high-purity Cu, Ti and Al materials with or without doping and low alloying used in the manufacture of sputtering targets. In this section, we use abbre- viations of the electronic industry where 6N and 5N5 purity means 99.9999% and 99.9995% purity, respectively. 3.1. main v starting ture heat cipitates uniformity for ticular or similar to long billets except for the axis of rotation as earlier. Scale-up efforts Based on the above considerations, Honeywell started the efforts of ECAE in 1997 with the construction of the production die. Today, several large-scale die sets for a few billet sizes are in normal operation for Al, Cu and, , pure Ti using presses with 1000 and 4000 tonnes (Fig. 2). Most of these dies have been in use on a basis for 6 years. The mass of the largest ECAE bil- is 32.7 kg for Al alloys 36 and, most recently, 110 kg for and Cu alloys. As a comparison, the largest reported ECAE Al billet obtained with a die channel angle of 105 Fig. 3. EBSD of ECAE processed 6N Cu with a grain size of 5H9262m: (a) grain size Microstructures of targets after ECAE Multi-pass ECAE of high-purity materials results in a few effects: (i) development of either submicrocrystalline or ery fine (usually <20H9262m) microstructures independently of the grain size; (ii) enhanced structure uniformity; (iii) tex- control via the number of passes, route and post-processing treatment 39; (iv) elimination of large phases and pre- by solution heat treatment before ECAE. Grain size, and absence of large particles are the most influential sputtering performance. The critical factor for choosing par- structure is the thermal stability during target fabrication service. Here are some examples: and texture map; (b) distribution of boundary misorientation angles. S. Ferrasse et al. / Materials Science and Engineering A 493 (2008) 130140 133 Fig. 4. Grain size evolution as a function of accumulated strains for ECAE or rolling alone of 5N5 (99.9995%) Al and 5N5 (99.9995%) Al + 30 ppm Si. (i) For high-purity materials with low melting temperatures (Tm 1H9262m) grain size as a function of annealing temperature + 0.5% Sn. For 5N5 Al + 30 ppm Si, only the ECAE case is displayed. particular, simple shear is the most effective mode for struc- ture refinement for a given strain level as shown in Fig. 4. For example identical structures for 5N5 Al were detected after two passes of ECAE (accumulated strains 2.3) and after rolling reduction 99% (accumulated strain 4.8). A remarkable feature of such structures is the enhanced thermal stability. The possible contributing factors are the (1 h) for ECAE six pass route D or rolling alone of 5N5 Al, 6N Cu and 6N 134 S. Ferrasse et al. / Materials Science and Engineering A 493 (2008) 130140 Fig. as ECAE (iii) Fig. grain 3.2. details (ii) impro mance; collimation 3.3. strength 5N5 con 2 YS, it result doping ( for talline 6. Evolution of the recrystallization temperature (after 1 h heat treatment) a function of the amount and nature of a few dopants/alloying elements for 6N Cu. equiaxial grain morphology, low mobility of twin bound- aries, structure uniformity and near random texture (Fig. 3). Fig. 5 compares the evolution of the grain size versus the annealing time for both ECAE and standard 5N5 Al, 6N Cu 37 and Cu alloys. For example, for ECAE 6N Cu, full static recrystallization occurs at 225 C for 1 h and results in a uniform grain size of 58H9262m, which grows only slightly to 15H9262m after additional annealing at 300 C, 1 h. The structure remains uniform without abnormal grains. In comparison, the grain size of 6N Cu after standard pro- cessing (85% rolling) increases from 35 up to 65H9262m after annealing at 225 C, 1 h and 300 C, 1 h, respectively. (ii) For high purity Al and Cu, doping (defined here as up to 2000 ppm of a foreign element) is a powerful technique to refine further the fine micrometer ECAE grain sizes and/or improve the thermal stability of both the fine microme- ter and submicrometer ECAE microstructures to elevated temperatures. A notable example is 5N5 Al doped with 2030 ppm Si. The size of ultra fine grains decreases from 60 to 25H9262m and is far smaller than the as rolled structure after a similar strain level (Fig. 4). The simple shear defor- mation mode of ECAE and non monotonic loading path of route D(Bc) are believed to play a critical role in this remarkable difference in grain size between the as ECAE and as rolled structures 41,42. Fig. 6 displays the dramatic influence of the nature and quantity of dopants on temper- atures of static recrystallization after six ECAE passes via route D for submicrocrystalline 6N Cu. A near logarith- mic dependence is obtained. In particular, Ag, Sn and Ti have such a strong influence that a doping level is enough to produce submicron-grained structures that are stable for sputtering. In pure Al and Cu with a sufficient amount of doping or alloying components, submicrocrystalline structures are stable for sputtering applications during a target life. An example of a submicrometer-grained structure in ECAE processed Al0.5Cu alloy is shown in Fig. 7 36,37. Trans- mission electron microscopy (TEM) reveals a uniform and materials tar to bonded als design 7. TEM of microstructure of monolithic ECAE Al0.5Cu target with 0.5H9262m size. equiaxed submicrometer grain size of 0.30.5H9262m(Fig. 7) that corresponds to a refinement factor of 100 compared to conventional processes. Very fine dispersions (less than 50 nm) of second phase material are present. Sputtering performance ECAE targets exhibit superior sputtering performance (for see Refs. 36,37) that includes: (i) reduction of arcing; low level of particles and splat defects on the wafer; (iii) ved film thickness uniformity and consistent film perfor- (iv) improved step coverage due to the superior beam of the submicron-grained structures. Mechanical properties and target design Fig. 8 shows data on yield strength (YS) and ultimate tensile (UTS) for ECAE processed 6N Cu and doped 6N Cu, Al0.5Cu and 4N5 Ni at room temperature. Compared to ventional processing YS and UTS is from 4 to 10 times and to 3 times higher, respectively. The effect is most significant on which is a critical property for target applications because governs the onset of permanent plastic deformation and may in target warping during sputtering. In the case of 6N Cu, has a noticeable strengthening effect in addition to ECAE Fig. 8). The tensile elongation also remains high: above 20% submicrocrystalline Al0.5Cu and 3540% for submicrocrys- 6N Cu. The high strength of pure submicron-grained permits the use of a monolithic design, where the entire get is a mono-block (Fig. 9). This is a unique design compared that of traditional targets, which consists of a target material or soldered to a backing plate made from strong materi- like Al 6061 or CuCr. The main advantages of a monolithic are An increased target lifetime up to 50% in comparison with diffusion bonded designs because sputtering is no longer lim- ited by the diffusion bond line 36,37. A direct consequence is the increase in throughput (number of processed wafers per target) and lifetime of other chamber components and the reduction of downtime. S. Ferrasse et al. / Materials Science and Engineering A 493 (2008) 130140 135 Fig. 8. UTS and YS for the submicrocrystalline (ECAE) and conventional sputtering target microstructures of 5N5 Al0.5Cu, 6N Cu, 6N Cu0.15Ag, 6N Cu0.2Sn and 4N5 Ni. Simplified manufacturing by elimination of the costly, multi- step and risky diffusion bonding operation. Due to the high ductility, deformation by conventional means (rolling, draw- 4. soluble potentially and mechanical more allo strengthening be developed for heat-treatable alloys. For a medium level of alloying, precipitation hardening is usually as powerful as grain refinement and the goal is to optimize processing to combine both ECAE nents allo material ing such ECAE components. 4.1. 4.1.1. ied: Fig. 2738 393.7 ing) can be performed after ECAE to obtain the final products. Recent developments of ECAE Al and Cu targets are the hollow cathode magnetron (HCM) target. These targets require forming an ECAE blank into a complex cup-like shape with a final diameter of about 393.7 mm, a height of 381 mm and a thickness of 12.725.4 mm. ECAE of Al alloys for aerospace and transportation As alloying goes up, the number of second phases (either or insoluble) increases, which results in two other available strengthening mechanisms: (i) solution (ii) precipitation hardening. The effects of ECAE thermo- processing on microstructure and properties become varied and more difficult to predict. For non-heat-treatable ys, grain refinement during ECAE remains the dominant mechanism 2,12. More interesting cases can 9. (a) Flat 300 mm monolithic ECAE Al0.5Cu target with AMAT design and kWh (+52% life increase); (b) non-flat and non-sputtered 300 mm monolithic mm 25.4 mm thickness 381 mm height. these effects 13,2024. One example described below is of Al 2618 alloy, which is used in turbocharger compo- for the aerospace and transportation industries. For heavy ying, the effect of microstructure refinement by ECAE on the strength can become minor compared to other harden- mechanisms. Nonetheless, other important characteristics as toughness 2529 can be greatly enhanced by using as shown below for a spray-cast Al alloy for landing gear ECAE of Al 2618 for turbocharger components Processing Three cases of the pre-ECAE material conditions were stud- (I) Solutionizing at 529 C, 24 h with immediate water quenching to dissolve all soluble phases. overall dimensions diameter 523.8 mm 25.4 mm thickness sputtered up to ECAE 6N Cu with HCM Novellus design and overall dimensions diameter 136 S. Ferrasse et al. / Materials Science and Engineering A 493 (2008) 130140 Table 1 Mechanical properties of A2618 after ECAE