ON LIQUATION AND LIQUID PHASE OXIDATION DURING LINEAR FRICTION WELDING OF NICKEL-BASE IN 738 AND CMSX 486 SUPERALLOYS

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1 ON LIQUATION AND LIQUID PHASE OXIDATION DURING LINEAR FRICTION WELDING OF NICKEL-BASE IN 738 AND CMSX 486 SUPERALLOYS M. Y. Amegdzie 1, O. T. Ol 1, O.A. Ojo 1, P. Wnjr 2, nd M.C. Chturvedi 1 1 Deprtment of Mechnicl nd Mnufcturing Engineering, University of Mnito, Winnipeg, Mnito, R3T 5V6, Cnd 2 Ntionl Reserch Council Cnd, Aerospce Mnufcturing Technology Centre, 5145 Ave Decelles Montrél, Quéec, H3T 2B2 Cnd Keywords: friction welding, superlloys, liqution, oxidtion, compressive stress. Astrct A systemtic study of the microstructurl response of conventionlly cst IN 738 nd single crystl CMSX 486 superlloys during liner friction welding ws performed. The results reveled tht, side from significnt liqution of vrious phses tht occurred during joining, continuous oxide films formed on the weld lines in oth mterils. The weld line oxides consisted of luminum-rich nd hfnium-se oxides in IN 738 nd CMSX 486 superlloys, respectively, which cn e deleterious to the mechnicl properties of the weld joint. Gleele thermo-mechnicl simultion showed tht the oxides were produced y the oxidtion of some liquid exposed to tmospheric oxygen t the mting surfces of the work pieces. A prcticle method of eliminting liquid phse oxidtion during liner friction welding of nickel-se superlloys is discussed. [8]. Crck-free welding of mterils y LFW ws previously ssumed to e result of n exclusively solid stte joining process. However, recent studies y the present uthors hve shown tht intergrnulr liqution, cused y the liqution of vrious phses, does occur during LFW [8, 9]. Therefore, prevention of weld crcking during LFW is not due to preclusion of liqution during joining. Crck-free welding y LFW cn e relted to rpid re-solidifiction of the liquid, which is ided y the imposed compressive forging stress during welding [8]. Although LFW is ecoming incresingly ttrctive for joining nickel-se superlloys, microstructurl developments in liner friction welded mterils re yet to e dequtely understood. The ojective of this present work ws to perform systemtic study of the microstructurl response of conventionlly cst IN 738 nd single crystl CMSX 486 nickel-se superlloys during LFW, which will e useful for optimizing the welding process. Introduction Precipittion strengthened conventionlly cst nickel-se superlloys, such s IN 738, re used in the mnufcturing of ero nd lnd-sed gs turine engine components due to their high temperture strength nd remrkle hot corrosion resistnce. Higher operting temperture demnds on ero engine components hve lso led to n extensive use of single crystl (SX) nickel se superlloys, such s CMSX 486, which exhiit superior elevted temperture performnce in comprison to their conventionlly cst counterprts. Friction of new turine components, s well s repir of service-dmged prts, mde of conventionlly cst nd SX superlloys usully require joining y different welding techniques. However, IN 738, like other precipittion strengthened nickel se superlloys, is very difficult to weld due to its high susceptiility to het-ffected zone (HAZ) crcking during fusion welding processes [1, 2]. HAZ crcking in IN 738 is ttriuted to the liqution of vrious phses in the lloy, susequent wetting of grin oundries y the liquid nd decohesion long one of the solid-liquid interfces due to oncooling tensile stresses [2, 3]. Similrly, weldility studies performed on fusion welded CMSX 486 superlloy showed tht the lloy is highly susceptile to fusion zone crcking [4]. Fusion zone crcking hs lso een oserved in other SX nickel-se superlloys [5-7]. A recent trend in welding reserch involves the use of friction welding processes, such s liner friction welding (LFW), to produce crck-free welds in difficult-to-weld mterils. In LFW, the het generted y reciprocting liner motion of two work pieces ginst ech other is utilized in plsticizing nd susequently joining them under the influence of n xil compressive forging force during the terminl stge of the process Experimentl Procedures Tle I shows the chemicl compositions of the s-received IN 738 nd CMSX 486 superlloys. Cst IN 738 ws received in the form of pltes hving dimensions of 240 mm 60 mm 15 mm. Welding coupons with dimensions of 17 mm 12.8 mm 11.1 Tle I. Chemicl compositions of s-received IN 738 nd CMSX 486 superlloys. Element IN 738 CMSX 486 Cr Co Mo W Al Ti Zr Hf Re C N Fe T B Ni Bl Bl 587

mm were mchined from the cst pltes y using Hnsvedt model DS-2 trveling wire electro-dischrge mchine (EDM) nd were given pre-weld stndrd solution het tretment (SHT) t 1120 o C for 2 hours, followed y

Welding coupons of 17 mm in length were produced from these rods y sectioning the s-received mteril trnsverse to the solidifiction direction [001] y using wire EDM.

2 mm were mchined from the cst pltes y using Hnsvedt model DS-2 trveling wire electro-dischrge mchine (EDM) nd were given pre-weld stndrd solution het tretment (SHT) t 1120 o C for 2 hours, followed y ir cooling. CMSX 486 ws received in form of rods hving dimensions of 200 mm in length nd 12 mm in dimeter. Welding coupons of 17 mm in length were produced from these rods y sectioning the s-received mteril trnsverse to the solidifiction direction [001] y using wire EDM. These were then ground to produce coupons with width nd thickness of 11 mm ech. IN 738 nd CMSX 486 coupons were welded seprtely y using LFW process development system (PDS) t the Aerospce Mnufcturing Technology Centre of the Ntionl Reserch Council (NRC), Cnd. In this work, the oscilltion frequency nd mplitude were kept constnt ut the forging pressure ws incresed from normlized vlue of P to 2.5P nd 4P for selected IN 738 coupons. Gleele thermo-mechnicl simultions were lso crried out y using Gleele 1500-D Thermo-Mechnicl Simultion System in order to study nd understnd microstructurl developments during joining. During Gleele simultion, IN 738 nd CMSX 486 mterils were heted t rpid rte of 150 o C per second to tempertures rnging from 1150 o C nd 1350 o C. Selected IN 738 nd CMSX 486 Gleele smples were held t pek tempertures of 1270 o C nd 1275 o C, respectively, under compressive lod tht ws pplied to chieve out 25% length reduction in 2.5 seconds, followed y ir cooling. The pre-weld, welded nd Gleele-simulted specimens were sectioned, prepred y stndrd metllogrphic techniques for microstructurl exmintion nd etched electrolyticlly in 12 ml H 3 PO ml HNO ml H 2 SO 4 solution t 6 volts for 5 seconds. Microstructures of the specimens were nlyzed y ZEISS Axiovert 25 inverted reflected-light opticl microscope equipped with CLEMEX vision 3.0 imge nlyzer (Clemex Technologies Inc., Longueil, Cnd), JEOL JSM 5900 scnning electron microscope (SEM) equipped with n Oxford (Oxford Instruments, Oxford, United Kingdom) ultrthin window energy-dispersive spectrometer (EDS) nd n Inc nlyzing softwre, nd CAMECA SX 100 Electron Proe Micronlyzer. Primry γ Secondry γ MC Cride Figure 1. SEM microgrphs of solution-het-treted (SHT) IN 738 showing () Primry nd secondry γ precipittes, () MC cride nd γ - γ eutectic phses. Results nd Discussion Microstructures of Pre-Weld nd Liner Friction Welded Mterils The microstructure of pre-weld solution het treted (SHT) IN 738 superlloy in Figure 1 shows imodl distriution of the strengthening γ precipittes consisting of regulr corse primry γ precipittes nd fine sphericl secondry γ precipittes. MC crides nd γ-γ eutectics, which usully form during csting of the lloy, were found to persist in the SHT IN 738 mteril. The pre-weld CMSX 486 superlloy hd directionlly solidified columnr dendritic microstructure consisting of regulr distriution of ordered γ precipittes with n ogdodiclly diced cue shpe (cluster of cues [10]), s shown in Figure 2. MC crides nd γ-γ eutectics were lso distriuted long the interdendritic regions of the CMSX 486 superlloy. In ddition to the MC crides nd γ-γ eutectics, creful SEM study reveled the occurrence of different eutectic-like hfnium-rich nickelse phse hed of some γ-γ eutectics in the interdendritic regions of CMSX 486 superlloy (Figure 2). Hfnium selectively prtitions into interdendritic liquid during solidifiction of nickelse superlloys, including CMSX 486 superlloy. Selective prtitioning of hfnium ws reported to result in the formtion of nickel-hfnium intermetllic phses y eutectic-type terminl solidifiction rection in front of γ-γ eutectics in nickel-se superlloys [11, 12]. An overview of the weld joints in oth IN 738 nd CMSX 486 superlloys showed distinct microstructurl regions cross the welds, which included the weld zone (WZ) tht formed t the interfce etween the two work pieces nd the thermomechniclly ffected zone (TMAZ) tht formed djcent to the WZ (Figure 3). A creful SEM nlysis of liner friction welds in oth mterils reveled tht liqution of vrious phses occurred during joining. This liqution of phses occurred due to nonequilirium eutectic-type rection of second phse prticles with the mtrix during the rpid therml cycling of LFW, through phenomenon known s constitutionl liqution which ws first proposed y Pepe nd Svge [13]. Figure 4 is SEM microgrph of the TMAZ of IN 738 superlloy, showing evidence of significnt liqution in the welded mteril. The residul liquid 588

γ Precipittes MC Cride c Nickel-Hfnium Intermetllic Figure 2.

in the lloy during joining hd trnsformed into γ-γ eutectic products, s shown in the microgrph.

Opticl microgrphs showing n overview of the liner friction welded joint in () IN 738 nd () CMSX 486 superlloys, which were welded with forging pressure of P.

chemiclly ppers to e sed on Ni 5 Hf intermetllic (Figure 4C) [12, 14].

intermetllic phse to form γ-ni 5 Hf in CMSX 486 superlloy y liqution during LFW hs een provided elsewhere [8, 9].

3 γ Precipittes MC Cride c Nickel-Hfnium Intermetllic Figure 2. SEM microgrph of CMSX 486 superlloy showing () γ precipittes, () MC cride nd γ - γ eutectic phses nd (c) Nickel-Hfnium intermetllic phse. in the lloy during joining hd trnsformed into γ-γ eutectic products, s shown in the microgrph. Figure 4 shows resolidified γ-γ eutectic products tht formed s result of liqution in the TMAZ of the liner friction welded CMSX 486 superlloy. Figure 3. Opticl microgrphs showing n overview of the liner friction welded joint in () IN 738 nd () CMSX 486 superlloys, which were welded with forging pressure of P. Additionlly, the interdendritic regions, which contined the nickel-hfnium intermetllic phse, liquted nd resulted in newly formed fine eutectic product consisting of mjor phse tht chemiclly ppers to e sed on Ni 5 Hf intermetllic (Figure 4C) [12, 14]. A detil discussion on liqution of vrious phses, including constitutionl liqution of the strengthening γ phse in IN 738 nd CMSX 486 superlloys, nd incipient melting of nickel-hfnium intermetllic phse to form γ-ni 5 Hf in CMSX 486 superlloy y liqution during LFW hs een provided elsewhere [8, 9]. Liqution occurrence in the HAZ during conventionl fusion welding processes hs een known to result in liqution crcking when on-cooling tensile stresses exceed the locl strength t solidliquid interfces. However, liqution crcking ws not oserved during LFW of IN 738 nd CMSX 486 superlloys, which cn e ttriuted to the imposed compressive stress during the forging stge of the joining process nd its effect in producing strininduced rpid solidifiction [8, 9]. Notwithstnding the strininduced rpid solidifiction of the liquted phses, continuous oxide phses were oserved long the weld lines of oth mterils. The morphology nd SEM-EDS spectr of these oxide phses re presented in Figure 5. Electron proe micronlysis 589

Hf-se oxide c c Incipient Melting Al-rich oxide Figure 4.

superlloy () CMSX 486 superlloy, nd (c) incipient melting of nickel-hfnium intermetllic

d (EPMA) of these oxide phses reveled tht hfnium-se oxide formed on the weld line of CMSX

lyers tht formed on Gleele specimens tht were processed under tmospheric condition ws

4 Hf-se oxide c c Incipient Melting Al-rich oxide Figure 4. SEM imges showing re-solidified products in the TMAZ of liner friction welded () IN 738 superlloy () CMSX 486 superlloy, nd (c) incipient melting of nickel-hfnium intermetllic phse in CMSX 486 superlloy. d (EPMA) of these oxide phses reveled tht hfnium-se oxide formed on the weld line of CMSX 486 superlloy, while the oxide on the weld line of IN 738 ws essentilly rich in luminum (Tle II). In order to understnd the formtion of these continuous oxide films, the nture of oxide lyers tht formed on Gleele specimens tht were processed under tmospheric condition ws crefully studied nd this is discussed next. Figure 5. () SEM imge nd () EDS spectrum of Hf-se oxide on the weld line of CMSX 486 superlloy, nd (c) SEM imge nd (d) EDS spectrum of Al-rich oxide on the weld line of IN 738 superlloy. The mterils were welded with forging pressure of P during LFW. 590

Tle II. Chemicl compositions of continuous hfnium-se nd luminum-rich oxide films on the weld lines of CMSX 486 nd IN 738 superlloys, respectively, determined y EPMA-WDS.

09 0.22 39.03 12.82 0.06 Figure 6. SEM microgrphs of () Gleele-simulted CMSX 486 mteril rpidly heted to 1275 o C nd held for 2.

5 Tle II. Chemicl compositions of continuous hfnium-se nd luminum-rich oxide films on the weld lines of CMSX 486 nd IN 738 superlloys, respectively, determined y EPMA-WDS. Element Cr Co Mo W Al Ti Zr Hf Re N O T Ni Smple Bkelite Oxide Comp. Wt% (CMSX 486) Oxide Comp. Wt% (IN 738) Oxide Oxide Figure 6. SEM microgrphs of () Gleele-simulted CMSX 486 mteril rpidly heted to 1275 o C nd held for 2.5 s () Gleelesimulted IN 738 mteril rpidly heted to 1230 o C nd held for 2.5 s Smple Bkelite Weld Line Oxidtion during LFW of IN 738 nd CMSX 486 Superlloys. The formtion of the hfnium-se nd luminum-rich oxides on the weld lines of CMSX 486 nd IN 738, respectively, during LFW, ws studied y using Gleele simultion. Two min types of oxide scles, sed on morphology nd chemicl composition, were oserved on the Gleele-simulted CMSX 486 nd IN 738 specimens. The type 1 oxide scle in CMSX 486 specimen consisted of nickel-sed oxide outer lyer nd etween this top lyer nd the sustrte lloy ws mixture of different oxide phses tht were rich in luminum, chromium nd nickel. Figure 6 shows n tomic-numer-sed ckscttered electron microgrph of the morphology of the type 1 oxide in CMSX 486 superlloy. Similrly, the type 1 oxide, consisting of mixture of different oxide phses nd nickel-se outer lyer, ws oserved in Gleele-simulted IN 738 superlloy (Figure 6). The type 1 oxide scle ws oserved on the surfces of ll Gleele simulted specimens. Type 1 oxide scle ws lso oserved on CMSX-486 nd IN 738 smples tht were heted to 1300 o C nd 1200 o C, respectively, in n ordinry lortory furnce nd held for 5 minutes in ir. In contrst to the multi-phse oxides of the type 1 scle with different lyers, the second type of oxide scle, type 2, essentilly consisted of one lyer of the oxide phse, which ws hfniumse in CMSX 486 nd luminum-rich in IN 738. The type 2 oxide scle ws only oserved in those Gleele-simulted CMSX 486 nd IN 738 specimens tht experienced significnt melting during rpid heting to the high pek tempertures. Exmples of these types of oxides in CMSX 486 nd IN 738 superlloys were oserved in 1275 o C nd 1270 o C treted specimens, respectively, where ppliction of compressive lod on the Gleele-simulted specimens resulted in squeezing out of interdendritic liquid phse (Figures 7). In oth lloys, the surfces of the expelled liquid exposed to the tmosphere were susequently oxidized to produce the type 2 oxide scle efore the liquid ecme solidified (Figure 8). Similrly, t 1350 o C, where significnt ulk melting occurred in CMSX 486 superlloy, type 2 oxide scle ws oserved on the exposed surfce of melted regions, which re-solidified with completely new dendritic microstructure (Figure 9). A similr hfnium-se oxide, suggested to e sed on HfO 2 nd with metllic composition tht is consistent with tht of the type 2 oxide scle oserved in this present work, hs een reported to form y oxidtion of hfnium-rich liquid during csting of hfnium-ering nickel-se superlloy [11]. The LFW technology is generlly considered s self-clening process, where the oxides tht presumly formed in the solidstte re roken down nd extruded out of the joint region during the forging stge of joining [15]. However, the results of this present work reveled tht the type 1 oxide scle ws not oserved long the weld line of the liner friction welded CMSX 486 nd IN 738 superlloys. The continuous hfnium-se nd luminum-rich oxide films tht formed long the weld lines were similr in morphology nd composition to the type 2 oxide scles tht were exclusively ssocited with liqution in Gleelesimulted specimens, suggesting tht the oxides t the weld line of the liner friction welded mterils formed y liquid phse rection. As discussed erlier, liqution does occur during LFW. Some of the liquid exposed to the tmosphere t the mting surfces of the work-pieces ppered to hve rected with oxygen 591

Gleele-Simulted Smple Resolidified Liquid Droplet Hf - se oxide Gleele-Simulted Smple Resolidified Liquid Droplet c Figure 7.

5 s, with 27 percent strin pek temperture, nd () Gleele-simulted IN 738 mteril rpidly heted to 1270 o C nd held for 2.

Al - rich oxide to produce the continuous hfnium-se nd luminum-rich oxide films long the weld lines of CMSX 486 nd IN 738 superlloys, respectively.

Consequently, the oserved formtion of continuous oxide films long the weld lines of the liner friction welded CMSX-486 nd IN 738 superlloys could e detrimentl to the mechnicl integrity of the

6 Gleele-Simulted Smple Resolidified Liquid Droplet Hf - se oxide Gleele-Simulted Smple Resolidified Liquid Droplet c Figure 7. Opticl microgrphs of () Gleele-simulted CMSX 486 mteril rpidly heted to 1275 o C nd held for 2.5 s, with 27 percent strin pek temperture, nd () Gleele-simulted IN 738 mteril rpidly heted to 1270 o C nd held for 2.5 s, with 20 percent strin t pek temperture, showing re-solidified expelled liquid droplets. Al - rich oxide to produce the continuous hfnium-se nd luminum-rich oxide films long the weld lines of CMSX 486 nd IN 738 superlloys, respectively. Formtion of continuous oxide films hs een reported to e detrimentl to the mechnicl properties of nickelse superlloys [11, 16]. Consequently, the oserved formtion of continuous oxide films long the weld lines of the liner friction welded CMSX-486 nd IN 738 superlloys could e detrimentl to the mechnicl integrity of the weld joint. Any fctor tht cn reduce or eliminte the formtion of continuous oxide films would improve the reliility of LFW for the joining of nickel-se superlloys. A prcticle method of eliminting liquid phse oxidtion during LFW is discussed next. d Elimintion of Liquid Phse Oxidtion during LFW Microstructurl developments during LFW of mterils re strongly dependent on the compressive stress tht is usully pplied during the terminl forging stge of the welding process. Figure 8. () SEM microgrph nd () EDS spectrum of Hf-se oxide in Gleele-simulted CMSX 486 superlloy, nd (c) SEM microgrph nd (d) EDS spectrum of Al-rich oxide in Gleelesimulted IN 738 superlloy. 592

The pplied stress induces compressive strin in the weld joint such tht the highest strin is experienced t regions closest to the weld line nd decreses with distnce wy from the weld line.

is eyond 300 μm wy from the weld line [8]. The WZ tht formed t the interfce etween the two welded work pieces ppered essentilly free from such eutectic products.

Accordingly, more liqution is expected to form within the WZ nd result in higher mount of re-solidifiction eutectic products compred to the TMAZ, which is wht is normlly oserved in mterils welded y

This nomlous microstructurl ehviour cn e relted to the concept of strin-induced rpid solidifiction during LFW.

7 The pplied stress induces compressive strin in the weld joint such tht the highest strin is experienced t regions closest to the weld line nd decreses with distnce wy from the weld line. A recent study reveled tht re-solidifiction eutectic products tht formed y non-equilirium solidifiction of residul liquid during cooling from welding pek tempertures were limited to the TMAZ, which is eyond 300 μm wy from the weld line [8]. The WZ tht formed t the interfce etween the two welded work pieces ppered essentilly free from such eutectic products. This is counterintuitive since temperture increses from the TMAZ towrds the WZ, with the highest temperture existing long the weld line tht seprtes the two welded work pieces. Accordingly, more liqution is expected to form within the WZ nd result in higher mount of re-solidifiction eutectic products compred to the TMAZ, which is wht is normlly oserved in mterils welded y conventionl fusion welding processes. However, this ws not the cse in the liner friction welded mterils. This nomlous microstructurl ehviour cn e relted to the concept of strin-induced rpid solidifiction during LFW. The liquid produced in the WZ during LFW cn e rpidly solidified due to the high strin experienced in this region during the forging stge, such tht no residul liquid remins to e trnsformed into the eutectic products during cooling from the welding tempertures. The TMAZ tht experienced mrginl compressive strin during forging contined the re-solidifiction eutectic products. A detiled discussion on strin-induced rpid solidifiction during LFW hs een reported elsewhere [8, 9]. However, despite strininduced rpid solidifiction in the WZ during joining, continuous liquid phse oxide formed y the rection of some of the exposed liquid with tmospheric oxygen, s discussed erlier. One possile method of removing liquid phse oxides during LFW is y incresing the compressive forging pressure, which cn id fster re-solidifiction of the liquid. This possiility ws investigted in this present work. SEM microgrphs of weld lines of liner friction welded IN 738 superlloy tht were welded with Residul Oxide Hfnium se oxide Oxide-free Weld Line Figure 9. SEM microgrphs of Gleele-simulted CMSX 486 mteril rpidly heted to 1350 o C nd held for 0.5 s, showing () hfnium-se oxide (ckscttered electron imge) nd () new dendritic structure. Figure 10. SEM microgrphs of liner friction welded IN 738 mterils, showing () residul oxide on the weld line of the mteril welded with 2.5 P () oxide-free weld line in the mteril welded with 4 P. 593

8 forging pressures of 2.5P nd 4P re presented in Figures 10 nd 10, respectively. Incresing the forging pressure from P to 2.5P resulted in significnt reduction in oxidtion, while the oxides were completely eliminted in the mteril welded with forging pressure of 4P. The incresed forging pressure produced n enhnced rpid re-solidifiction of the liquid, which resulted in welds tht were free of oxides. Therefore, n importnt enefit of incresing the forging pressure during LFW is reduction in the thick nd continuous weld line oxide film produced y liquid phse oxidtion, which cn result in improved mechnicl properties of liner friction welded mterils. Summry nd Conclusions 1. Significnt weld line oxidtion occurred during LFW of nickelse CMSX 486 nd IN 738 superlloys, which cn e deleterious to the mechnicl properties of the weld joints. 2. The study indictes tht the oxides tht formed on the weld lines were produced y the oxidtion of some liquid exposed to tmospheric oxygen t the mting surfces of the work pieces, contrry to the generl ssumption of exclusively solid-stte oxidtion during LFW. 3. Optimiztion of the compressive stress tht ws pplied during the terminl forging phse of the LFW process produced n oxidefree weld line y iding fster re-solidifiction of the liquid t the mting surfces. Acknowledgement The uthors cknowledge the finncil support received from NSERC. The technicl ssistnce of M. Guérin nd E. Dlgrd for LFW of CMSX 486 nd IN 738 is lso gretly pprecited. References 1. M. Prger nd C. S. Shir, Welding of Precipittion-hrdening Nickel-Bse Alloys, Weld. Res. Council Bull., 128 (1968), A.T. Egewnde et l., Improvement in Lser Weldility of Inconel 738 Superlloy through Microstructurl Modifiction, Mter. Trns. A, 40 (2009), E.A. Bonifz nd N.L. Richrds, Stress-Strin Evolution in Cst IN-738 Superlloy Single Fusion Welds, Interntionl Journl of Applied Mechnics, 2 (2010), Specilty Mterils, Oct , 2004, ASM Interntionl, Mterils Prk, OH, (2005), J.M. Vitek et l., Anlysis of Stry Grin Formtion in Single-Crystl Nickel-Bsed Superlloy Welds, Superlloys 2004, TMS, Wrrendle, PA, (2006), O.T. Ol et l., Anlysis of Microstructurl Chnges Induced y Liner Friction Welding in Nickel-Bse Superlloy, Metll. Mter. Trns. A, 42 (2011) O.T. Ol et l., A Study of Liner Friction Weld Microstructure in Single Crystl CMSX-486 Superlloy, Metll. Mter. Trns. A, 43 (2012), J. H. Westrook, "Precipittion of Ni 3 Al from Nickel Solid Solution s Ogdodiclly Diced Cues," Zeitschrift für Kristllogrphie, 110 (1958), Z. Yunrong nd L. Chenggong, Skin Effect of Hf-Rich Melts nd Some Aspects in its Usge for Hf-Contining Cst Nickel Superlloys, Superlloys 1988, TMS, Wrrendle, PA, (1998), A. Bldn, Electron Microproe Investigtion of Lower Melting Regions in the As-Cst Structure of DS200+Hf Single Crystl, J. Mter. Sci., 25, (1990), J.J. Pepe nd W.F. Svge, Effects of Constitutionl Liqution in 18-Ni Mrging Steel Weldments, Weld J., 46, 1967, 411s-422s. 14. W. Tillmnn nd E. Lugscheider, "Friction highperformnce joints y solid-stte diffusion onding - A theoreticl nd experimentl pproch," Interntionl Conference on the Joining of Mterils JOM-7, The Europen Institute for the Joining of Mterils, Helsingor-Denmrk, 76 (1995), pp M. E. Nunn, Aero-Engine Improvements through Liner Friction Welding, 1st Interntionl Conference on Innovtion nd Integrtion in Aerospce Sciences. Queen's University, Belfst, Northern Irelnd, UK, T.M. Simpson et l., HIP Bonding of Multiple Alloys for Advnced Disk Applictions, Advnced Technologies for Superlloy Affordility, TMS, Wrrendle, PA, 2000, pp Y.L. Wng et l., Weld Metl Crcking in Lser Bem Welded Single Crystl Nickel Bse Superlloys, Mter. Sci. Technol., 25 (2009), A. Hirose et l., Microstructure nd Mechnicl Property of Lser Welds of Single Crystl Nickel Bse Superlloy CMSX-4, Mter. Sci. Forum, (2003), S. Ktym nd M. Skmoto, Weldility of Ni-Bse Single Crystl Superlloys, ASM Conf. Proc. Joining of nd 594

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