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Materials Science and Engineering A 493 2008 130 140 Scale up and application of equal channel angular extrusion for the electronics and aerospace industries Stephane Ferrassea V M Segalb Frank Alforda Janine Kardokusa Susan Strothersa aHoneywell Electronic Materials 15128 E Euclid Avenue Spokane WA 99216 USA bEPM 11228 Lemen Rd Suite A Whitmore Lake MI 48198 USA Received 9 February 2007 received in revised form 12 April 2007 accepted 25 April 2007 Abstract Two areas are critical to promote equal channel angular extrusion beyond the stage of a laboratory curiosity i tool processing design and scale up ii developmentofnewsubmicrometer grainedproducts BothgoalsarepursuedatHoneywell Thefi rstcaseisthesuccessfulcommercialization of ECAE for the production of sputtering targets from single phase alloys in the electronic industry Blank dimensions are signifi cantly larger than those reported in the literature Other described applications are targeted to the increase of tensile strength high cycle fatigue and toughness in medium to heavily alloyed Al materials used in aerospace In these alloys the optimal properties can be reached with better understanding of the interplay between plastic deformation and precipitation mechanisms 2007 Elsevier B V All rights reserved Keywords ECAE Submicrocrystalline materials Flat products Sputtering Fatigue Toughness 1 Introduction For the past 10 years severe plastic deformation SPD tech niques have been the focus of intense research because they can produce metallic materials with submicrometer grain sizes ranging from 50 to 500nm 1 2 One promising SPD method is equal channel angular extrusion ECAE 3 It can pro duce bulk pieces of submicrocrystalline materials induced by intense plastic straining by simple shear Till now research has made steady progress on the characterization of the tex ture structureandmechanicalpropertiesofsubmicrocrystalline materials and the effect of main ECAE parameters and post deformation annealing 4 29 However despite the abundant literature problems of engineering and commercialization were discussed only recently 30 32 and very few practical appli cations are reported The overwhelming majority of researchers continue to work with small long cylindrical or square billets A few attempts to scale up the billet size are known 32 35 but there is no report of successful commercialization This paper reviews the efforts in die design scale up and commercialization of ECAE for fl at billets conducted at Honey Corresponding author Tel 1 509 2522118 fax 1 509 2528743 E mail address Stephane Ferrasse S Ferrasse well 36 37 Selected examples show that this technology can penetrate a market in one or more of the following ways i provide an overall cost reduction versus the standard manufac turing or design ii provide superior product performance and iii answeranunmetneed Oneexampleinvolvesthefi rstECAE productswithsubmicrometerormicrometergrainsizesforhigh purity Al Cu and Ti sputtering targets used in the fabrication 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 Honeywell s focus has been historically the ECAE of fl at products which was fi rst introduced in Ref 38 In that case Fig 1 a typical billet shape is characterized by thickness a 0921 5093 see front matter 2007 Elsevier B V All rights reserved doi 10 1016 j msea 2007 04 133 李瑜 S Ferrasse et al Materials Science and Engineering A 493 2008 130 140131 width b and length c with c b a 30 38 40 Usually dimen sions c and b are equal to allow the use of the same tool for multi passprocessing with 90 rotationbetweenpasses The processing characteristics of one pass ECAE for fl at and long billetsaresimilar However usually forfl atbillets theaxisofthe permissible 90 billetrotationisperpendiculartotheextrusion axis axis Z in Fig 1 whereas for long products it is parallel to theextrusionaxis Duringscaleup twoconsiderationscomeinto play i tooldesign and ii optimizationofECAEdeformation 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 p1 must be signifi cantly less than the yield strength of the punch material The punch pressure is 30 p1 2k p 2k mF 2A 1 wherepisthepressureattheexitoffi rstchannel kisthematerial shearfl owstress mistheplasticfrictioncoeffi cient Fisthearea of stationary die walls and A is the billet cross sectional area For the tool itself the maximum pressures on the punch p1 and channel wall nact at the end of the entrance channel As shown in 30 for a low friction case m 0 25 p1 2k 1 m cb ca ba 2 n 2k m cb ca ba 3 Therefore the preferable ways for reducing die punch pres sures are i to limit the ratio c a 6 10 and ii to minimize friction in both channels Two corresponding strategies are the choice of effective lubricants and movable channel walls A sig nifi cant advantage of fl at ECAE billets versus long billets in terms of equipment and design is that movable walls along the Fig 1 Principle of the ECAE technique for fl at billets entrancechannelarenotneededforfl atproducts Thisisbecause a b for fl at products whereas a b for long products There fore p1and n are lower for fl at products and formulae 2 and 3 can be approximately reduced to p1 2k 1 mc a 4 n 2k mc a 5 A movable bottom wall at the exit channel is recommended however for both fl at and long products because lubricant is atomically removed along the bottom of exit channel 2 1 2 Productivity The two important factors are processing speed and billet ejection For reasonably ductile materials the processing speed isnotalimitingfactorandmaybesuffi cientlyhigh 5 10mm s The billet ejection presents a more complex problem especially for long cylindrical billets In the case of fl at billets a mov able bottom wall of the exit channel operated by an additional hydraulic cylinder provides an effective and simple solution 2 2 Optimization of ECAE Therearetwolevelsofoptimizationforsingleandmulti pass ECAE 2 2 1 Single pass A level of simple shear straining should be as high as possi ble for an effective refi nement of microstructures 11 This is mostly controlled by the conditions of friction and the chan nel geometry which has in turn two critical parameters i the angle 2 between the two channels and ii the shape of the channel intersection Usually channels are performed with sharp no radius or round corner intersections Slip line solu tions 18 30 41 and fi nite element modeling 43 reveal the existence of a fan like deformation zone in cases of noticeable frictionand orroundcornerchannels Insuchcases simpleshear is redistributed along three different directions 41 Moreover even for frictionless conditions and sharp corners a dead metal zone exists at the channel corner for 2 90 Therefore tool angle 2 90 sharp corner channels and near frictionless con ditions are the optimum characteristics to realize the effective simple shear of 2 along one direction The most important problem is the elimination of the friction along the bottom wall oftheoutletchannelwherehighcompressivepressureandinten sive slip act simultaneously With the movable bottom wall the fan angle can be minimized as shown by slip line analy sis 30 41 The Honeywell dies operate under those conditions owing to advanced die design and lubricants 2 2 2 Multi pass processing The two major parameters are the deformation route a sequence of billet rotation after each pass and the total number of passes accumulated strains For fl at billets the defi nition of the four fundamental routes A B or BA C and D or Bc 38 132S Ferrasse et al Materials Science and Engineering A 493 2008 130 140 Fig 2 Production ECAE die with 4000tonnes press capacity remains similar to long billets except for the axis of rotation as described earlier 2 3 Scale up efforts Based on the above considerations Honeywell started the scale up efforts of ECAE in 1997 with the construction of the fi rst production die Today several large scale die sets for a few standard billet sizes are in normal operation for Al Cu and occasionally pure Ti using presses with 1000 and 4000tonnes capacity Fig 2 Most of these dies have been in use on a weekly basis for 6 years The mass of the largest ECAE bil let is 32 7kg for Al alloys 36 and most recently 110kg for Cu and Cu alloys As a comparison the largest reported ECAE processed Al billet obtained with a die channel angle of 105 34 35 hasamassof6 7kgwhereasthemassofthemosttypical 10mm 10mm 60mmAlbilletusedforresearchis0 016kg Thereisnoreportofascale upattemptoftheECAEprocessfor Cu Importantly theeffectsofECAEonmicrostructures texture and properties have been verifi ed at the various industrial scales as will be shown in the Section 2 In the authors view the expe rience attained on the production fl oor 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 refi nement 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 specifi c materialsandcrystalstructures ECAEcanalsoactivateandcon 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 Microstructures of targets after ECAE Multi pass ECAE of high purity materials results in a few main effects i development of either submicrocrystalline or veryfi ne usually 20 m microstructuresindependentlyofthe starting grain size ii enhanced structure uniformity iii tex ture control via the number of passes route and post processing heat treatment 39 iv elimination of large phases and pre cipitates by solution heat treatment before ECAE Grain size uniformityandabsenceoflargeparticlesarethemostinfl uential for sputtering performance The critical factor for choosing par ticular structure is the thermal stability during target fabrication or service Here are some examples Fig 3 EBSD of ECAE processed 6N Cu with a grain size of 5 m a grain size and texture map b distribution of boundary misorientation angles S Ferrasse et al Materials Science and Engineering A 493 2008 130 140133 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 30ppm Si i For high purity materials with low melting temperatures Tm1 m grain size as a function of annealing temperature 1h for ECAE six pass route D or rolling alone of 5N5 Al 6N Cu and 6N Cu 0 5 Sn For 5N5 Al 30ppm Si only the ECAE case is displayed 134S Ferrasse et al Materials Science and Engineering A 493 2008 130 140 Fig 6 Evolution of the recrystallization temperature after 1h heat treatment as a function of the amount and nature of a few dopants alloying elements for ECAE 6N Cu equiaxial grain morphology low mobility of twin bound aries structureuniformityandnearrandomtexture 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 1h and resultsinauniformgrainsizeof5 8 m whichgrowsonly slightly to 15 m after additional annealing at 300 C 1h 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 65 m after annealing at 225 C 1h and 300 C 1h respectively ii For high purity Al and Cu doping defi ned here as up to 2000ppm of a foreign element is a powerful technique to refi ne further the fi ne micrometer ECAE grain sizes and or improve the thermal stability of both the fi ne microme ter and submicrometer ECAE microstructures to elevated temperatures A notable example is 5N5 Al doped with 20 30ppm Si The size of ultra fi ne grains decreases from 60 to 25 m 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 andasrolledstructures 41 42 Fig 6displaysthedramatic infl uence 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 infl uence that a doping level is enough to produce submicron grained structures that are stable for sputtering iii In pure Al and Cu with a suffi cient amount of doping oralloyingcomponents submicrocrystallinestructuresare 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 Fig 7 TEMofmicrostructureofmonolithicECAEAl0 5Cutargetwith0 5 m grain size equiaxed submicrometer grain size of 0 3 0 5 m Fig 7 that corresponds to a refi nement factor of 100 compared to conventional processes Very fi ne dispersions less than 50nm of second phase material are present 3 2 Sputtering performance ECAE targets exhibit superior sputtering performance for details see Refs 36 37 that includes i reduction of arcing ii low level of particles and splat defects on the wafer iii improved fi lm thickness uniformity and consistent fi lm perfor mance iv improved step coverage due to the superior beam collimation of the submicron grained structures 3 3 Mechanical properties and target design Fig 8 shows data on yield strength YS and ultimate tensile strength UTS for ECAE processed 6N Cu and doped 6N Cu 5N5 Al0 5Cu and 4N5 Ni at room temperature Compared to conventional processing YS and UTS is from 4 to 10 times and 2to3timeshigher respectively Theeffectismostsignifi canton YS which is a critical property for target applications because it governs the onset of permanent plastic deformation and may result in target warping during sputtering In the case of 6N Cu dopinghasanoticeablestrengtheningeffectinadditiontoECAE Fig 8 The tensile elongation also remains high above 20 forsubmicrocrystallineAl0 5Cuand35 40 forsubmicrocrys talline 6N Cu The high strength of pure submicron grained materialspermitstheuseofamonolithicdesign wheretheentire targetisamono block Fig 9 Thisisauniquedesigncompared to that of traditional targets which consists of a target material bonded or soldered to a backing plate made from strong materi als like Al 6061 or CuCr The main advantages of a monolithic design are An increased target lifetime up to 50 in comparison with diffusionbondeddesignsbecausesputteringisnolongerlim 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 130 140135 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 Simplifi ed manufacturing by elimination of the costly multi step and risky diffusion bonding operation Due to the high ductility deformation by conventional means rolling draw ing canbeperformedafterECAEtoobtainthefi nalproducts Recent developments of ECAE Al and Cu targets are the hollow cathode magnetron HCM target These targets requireforminganECAEblankintoacomplexcup likeshape with a fi nal diameter of about 393 7mm a height of 381mm and a thickness of 12 7 25 4mm 4 ECAE of Al alloys for aerospace and transportation As alloying goes up the number of second phases either soluble or insoluble increases which results in two other potentially available strengthening mechanisms i solution and ii precipitation hardening The effects of ECAE thermo mechanicalprocessingonmicrostructureandpropertiesbecome morevariedandmorediffi culttopredict Fornon heat treatable alloys grain refi nement during ECAE remains the dominant strengthening mechanism 2 12 More interesting cases can be developed for heat treatable alloys For a medium level of alloying precipitation hardening is usually as powerful as grain refi nement and the goal is to optimize processing to combine both these effects 13 20 24 One example described below is ECAE of Al 2618 alloy which is used in turbocharger compo nents for the aerospace and transportation industries For heavy alloying theeffectofmicrostructurerefi nementbyECAEonthe material strength can become minor compared to other harden ing mechanisms Nonetheless other important characteristics such as toughness 25 29 can be greatly enhanced by using ECAE as shown below for a spray cast Al alloy for landing gear components 4 1 ECAE of Al 2618 for turbocharger components 4 1 1 Processing