Turbine Airfoil Manufacturing Technology

CONTRACT INFORMATION

Contract Number DE-AC05-84OR21400

Contractor PCC Airfoils, Inc.25201 Chagrin Boulevard, Suite #290Beachwood, OH 44122216-766-6253216-766-6217

Contractor Project Manager Charles S. Kortovich

Principal Investigators Craig HayesPeter O’Neill

DOE Project Manager B. (Rad) Radhakrishnan

Period of Performance February 20, 1995 to June 30, 1999

Schedule and Milestones Program was completed June 30, 1999

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OBJECTIVESOBJECTIVES

The program objective was to definemanufacturing methods that will allow singlecrystal (SX) technology to be applied to complex-cored airfoil components for power generationapplications. A number of specific technicalissues formed the task structure of the programand included the following.

§ Alloy melt practice to reduce sulfur content inalloys.

§ Modification/Improvement of SX castingprocess.

§ Core materials and design.

§ Grain orientation control.

The specific objectives for these tasks arelisted as follows:

Alloy Melt Practice

Establish a process to reduce sulfur incastings to a sufficiently low level to promote theadhesion of a protective oxide scale.

SX Casting Process

Define manufacturing methods that will allowSX technology to be applied to complex-coredand solid airfoils for land-based turbineapplications.

Core Materials

Provide ceramic cores for the SX castingprocess for the complex-cored component, whichcan control wall thickness to tight requirementsover long spans.

Grain Orientation Control

Provide data to enable decisions to be madeconcerning the establishment of grain limit defectcriteria to reduce the risks associated withliberalizing the criteria.

BACKGROUND INFORMATIONBACKGROUND INFORMATION

The efficiency and effectiveness of the gasturbine engine are directly related to the turbineinlet temperatures. The ability to increase thesetemperatures has occurred as a result ofimprovements in materials, design, andprocessing techniques. A number of technicaladvances have allowed the designs andinnovations to be applied on a high volume, cost-effective scale in the aircraft gas turbine market.

Examples of these advances are:

§ Vacuum melting for the nickel-basesuperalloys.

§ Ceramic technology that produces complex,dimensionally reliable, stable cores that can bereadily removed.

§ Casting methods and furnace designs thatallow both directionally solidified (DS) andsingle crystal (SX) product to be produced inaircraft airfoils at yields approaching 90%.

§ Advanced nickel-base alloy compositions,which utilize hafnium, rhenium, and in somecases, small quantities of yttrium to improvethe required DS and SX properties.

§ Coatings that protect the airfoils fromoxidation and provide thermal barriers.

Although land-based turbines can make use ofthe technology developed for aircraft engines,some very real challenges and differences are

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present. Land-base turbines operate underdifferent duty cycles. Therefore, designs havedifferent creep and low-cycle fatigue criteria. Inaddition, alloy compositions are often aimed atgreater sulfidation resistance (higher chromiumcontent).

The major limiting factor, however, indirectly transferring aircraft engine technology toland-base designs is increased size and weight.The largest SX complex-cored part for a militaryengine is approximately 10 inches in length. DSparts in this range are routinely produced at highyields. Both of these applications have relativelysmall root sections and pour weights compared toindustrial gas turbine applications.

A need exists to expand the capability of thecomplex-cored airfoil technology to larger sizesso that higher turbine inlet temperatures can beattained in land-base hardware in a cost-effectivemanner. The Department of Energy hasrecognized this need as part of the planning effortfor the Advanced Turbine Systems (ATS)Program to develop advanced gas turbines forpower generation in utility and industrialapplications.

In response to this need, the Turbine AirfoilManufacturing Technology Program wasconducted by PCC Airfoils, Inc./General ElectricPower Systems Group which envisioned usingthe available methods for producing SX airfoilsand scaling them up to much larger land-basecomponents.

PROJECT DESCRIPTIONPROJECT DESCRIPTION

The Program comprised five tasks whichincluded a planning task (Task 1) and four tasks(2-5) of technical effort. The program wascompleted in June 1999.

Task 1 - Program Plan

Program planning involved the creation of awork breakdown structure and program scheduleto allow close coordination between programelements and provide a clear blueprint forprogram review and management. To include thedesire for extended time environmental testing,additional casting trials to optimize the singlecrystal casting process for large-cored airfoils,and a final decision relative to the secondconfiguration chosen to demonstrate the singlecrystal process, the Program Plan and schedulewere revised accordingly.

Task 2 - Alloy Melt Practice

Alloy Melt Practice activity is shown inFigure 1. Four uncoated conditions were studied -a baseline composition with no desulfurization, adesulfurization condition using the PCC alloydesulfurization method, and both of theseconditions to which hydrogen anneals wereapplied. Two coated conditions were alsoevaluated in the testing including the baselinematerial without desulfurization and the meltdesulfurized material. No hydrogen anneals wereapplied to these materials and both were diffusionaluminide-coated. Each of these conditions weretested on specimens in GE Power Systems cyclicoxidation and corrosion test rigs.

Figure 1. Work Breakdown Structure forTask 2 Alloy Melt Practice

Ingot preparation included making thevacuum induction melted (VIM) master metal forcasting test pin configurations for rig testing. Aningot of standard N5 as well as a desulfurizedingot were produced. The desulfurization was

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accomplished in the melting operation with thesulfur aim being less than 0.5 parts per million(ppm).

Mold preparation employed molds configuredsuch that 24 pins each 0.180 inches in diameterby 6 inches long were produced and these pinswere then cropped to the desired length for theGE tests. The cast pins were solution heat treatedto homogenize the structure and one half the pinswere given a hydrogen heat treatment to furtherreduce sulfur levels.

Rig testing including oxidation and hotcorrosion tests were conducted as per the testmatrix shown in Table 1.

Table 1Test Plan to Evaluate Desulfurization

Task 3 - SX Casting Process

SX Casting Process activity is shown inFigure 2 and included a series of iterative castingtrials for cored buckets and nozzles. The coredbucket, a GE Power Systems Prototype Bucket,featured complex cooling and was approximately15 inches long while the nozzle (anotherprototype) was approximately 10 inches long.

Efforts for the bucket encompassed a total ofnine mold trials. Thermal simulation modelingwas conducted in conjunction with the trials to

establish the effects of important castingvariables. The initial trial addressed moldstructural integrity including survivability duringthe casting process. Subsequent trials addressedprocess optimization with variables beingselected as a result of earlier trial activity. Thefinal trial addressed process verification toprovide statistical significance to the existing dataas well as determine the possible extent ofprocess variability associated with castingprocedures.

Efforts for the nozzle encompassed one moldtrial. The activity was directed towards moldstructural integrity and process refinement.

The castings resulting from the trials weresubjected to thorough evaluations including NDT,chemistry, microstructural characterizations, andselected mechanical property testing.

Figure 2. Work Breakdown Structure for Task 3Modification/Improvement of SX Casting Process

Task 4 - Core Materials

Core Material activity is shown in Figure 3.Core body characterization involved anevaluation of a number of silica-based corecompositions available for application to the SXcasting process. Cores were processed andevaluated for a number of characteristicsincluding shrinkage, modulus of rupture, stability,and porosity. The characterization data on thevarious core bodies was analyzed and adownselection was made for the initial castingtrials. As part of the casting trials, evaluations ofcore performance were conducted including

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resistance to core/metal reactions, stability,additional shrinkage during the casting processand core removal kinetics. Assessments of thiscore performance were made for the selection ofcores required for the subsequent casting trials.

Optimized core processing included themanufacture of cores for the subsequent castingtrials and was evaluated for the samecharacteristics established during core bodycharacterization.

Figure 3. Work breakdown Structure forTask 4 – Core Materials and Design

Task 5 - Grain Orientation Control

Grain Orientation Control activity is shown inFigure 4 and included tensile, stress rupture andLCF testing of cast specimens containing defects.Defects included high and low angle boundariesand freckles. Specimens with no defects providedbaseline information. The LCF test plan is shownin Table 2.

Figure 4. Work breakdown Structure forTask 5 - Grain Orientation Control

Table 2Test Plan to Evaluate Grain Orientation Control

Note1) θ -refers to a mismatch angle of LAB or HAB.2) Numbers represent quantity of tests.

RESULTSRESULTSTask 2 - Alloy Melt Practice

Specimen Preparation. An ingot of standardN5 master metal alloy was obtained to serve asthe baseline and its sulfur level was 2.1 ppm. Twodesulfurized ingots were produced using thepractice of reducing sulfur during the meltingoperation.

Molds were prepared following proceduresestablished for the production of molds used forSX castings. Casting involved remelting the alloyingots and pouring the molten metal into themolds to produce the SX pins for GE rig testing.

A 6-pin candelabra configuration was castwith each pin within the candelabra solidifiedfrom the same single crystal starter geometry.

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Each mold was comprised of four suchcandelabras with pins being cropped to thedesired length to accommodate the GE burner rig.

Heat treatments including homogenizationcycles and hydrogen desulfurization cycles wereapplied after the casting operation. All of the pinsreceived the standard N5 heat treatment to homo-genize the structure. Hydrogen heat treatmentswere applied to one half of the pin specimensintended for rig testing. Both the standard N5alloy and the desulfurized alloy were similarlyheat treated. Sulfur levels were measured formaterials of the various conditions making up thetest matrix shown in Table 1.

Table 3 shown below lists the variousmaterial conditions included as part of thisevaluation. The table also lists the sulfur levelsfor the conditions as measured by glow-dischargemass spectrometry. Note that two types ofspecimens are listed for the desulfurizedconditions, B with a sulfur level of 0.8 ppm andB2 with a sulfur level of 0.4 ppm. Bothconditions were included in order to evaluatedesulfurized heats with a range of sulfur content.

Table 3. Test Material Used in Oxidation Tests

Material Condition Sulfur content (ppm)

A - N5 Baseline (Standard) 2.1

AH - Standard + H2 Anneal 0.7

B - Desulfurized 0.8

B2 - Desulfurized 0.4

BH - Desulfurized + H2 Anneal 0.3

Burner Rig Testing. Burner rig testinginvolved conditions shown in the testing plan ofTable 1.

Each rig consisted of a tube inside a tubefurnace, with an atomizing fuel nozzle at one endand an exhaust at the other. Atomized fuel wascombined with air in the combustion chamber and

burned, resulting in hot gases which flowed past adouble beam fixture suspending test samples inthe hot gas stream. Up to 12 pin or disk samplescan be housed in a sample fixture.

Upon completion of the designated exposuretimes, the pins were examined metallographicallyand the thickness of the remaining metal and thedepth of internal attack were measured as anindication of resistance to the environment.

Uncoated Oxidation Results. The N5(without yttrium) oxidation mechanism has beenestablished as the result of previous studiesconducted at GE Power Systems. During thecourse of the oxidation, a thin alumina-rich oxidescale forms at the specimen surface. The adjacentsurface layer becomes depleted of gamma-primeas a function of the loss of aluminum. In addition,aluminum nitrides form primarily within thedepleted zone, further lowering the amount ofaluminum available to participate in the gamma-prime formation reaction. The metallographicexamination of the test pins was conducted withthis oxidation mechanism in mind.

For all of the material conditions, a surfacedepleted zone was observed the depth of whichvaried considerably as a function of the materialcondition. The zone was characterized by theabsence of gamma-prime particles and wassomewhat depleted in chromium and aluminum.The extent of this loss of chromium andaluminum (thought to occur as a function ofsurface scale formation/spallation duringenvironmental exposure) is considered anindication of resistance to environmental attack.A relatively thin depleted zone is consideredmore desirable than a thick depleted zone. Al-richnitrides were also observed in the depleted zone.

Upon completion of the environmental tests,the oxidation pins were examinedmetallograhically for the extent of surface

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depleted zone. Measurements were made of themaximum depth of penetration and the results ofall tests (both uncoated and coated) are showngraphically in Figure 5.

Figure 5 – Burner rig oxidation test results

The results are normalized relative to thedeepest attack observed for Baseline N5(Condition A) exposed 1900°F/2000 hours. Thismeasurement is thus reported as “1” and all othermeasurements are reported relative to it. Theseplots of maximum penetration enable directcomparisons to be made between the variousconditions for both uncoated and coated material.The data indicated there was scatter andvariability in maximum depth of penetrationobserved for specific temperatures/times. Becauseof this scatter situation the absolute amount ofattack measured in inches is not as meaningful asthe overall relative comparison of trends betweenmaterials. On this relative basis then, the results

are consistent with a general trend that reducingsulfur to levels below 1 ppm improves oxidationresistance for all materials/test conditions.

The results for the 1900° tests indicate littlesignificant improvement in oxidation resistance atthe 2000 hour test interval. At 4000 hours andlonger, the beneficial effect of desulfurizationbecomes more apparent with the maximum depthof penetration only approximately 50% of thatseen for the baseline material. Data wereunavailable for several conditions at 8000 hoursbut the limited testing which was conductedsupports the general observations of the benefitsof desulfurization. In addition, there appears to belittle significant benefit by applying a hydrogenheat treatment to melt desulfurized material. Thebeneficial effect of desulfurization can be seen interms of a thinner surface depletion layer. Theseresults suggest improved resistance to 1900°Foxidation for all conditions where the sulfur levelis below 1 ppm.

The results for the 2000°F tests weresomewhat different in that a significant benefitwas discerned immediately at the shorter 2000hour exposure time. The beneficial effect becamemore significant at longer times with maximumdepths of penetration less than 50% of that seenfor the baseline material. Unfortunately, baselinematerial was not available for the 12,000 hour testto enable comparisons to be made. Comparisonswith hydrogen heat treated baseline, however,indicated the same types of trends. Typicalmicrostructures of oxidation pins exposed at2000°F/12,000 hours are shown in Figure 6.

Again, the beneficial effect can be manifestedby a thinner surface layer. Similar to the 1900°Fresults, the 2000°F results suggest improvedoxidation resistance for all conditions where thesulfur level is below 1 ppm.

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Figure 6 – Typical microstructures of uncoated N5oxidation pins exposed 2000°° F/12000 hours. 200X

Coated Oxidation Results. Both Baseline N5(Condition A) and melt desulfurized (ConditionB) material were subjected to 2000°F oxidationtesting out to 8000 hours exposure. For coatedmaterials surface attack consists offormation/spallation of oxide layers on the outersurfaces of the coating as well as general oroverall reduction in coating thickness. This ismeasured by preparation of transversemetallurgical mounts to reveal the test pin incross section. Measurements of the minimumdiameter of the cross section (including thecoating layer) unaffected by the attack are thenmade and compared to the original total diameterof the untested pin. The specimen diameter is thusreduced as a function of either the presence ofoxide layers (which are not included in themeasurement) or their absence due to spallationduring the test.

The results of the measurements of the coatedoxidation specimens are shown graphically inFigure 5. Again, these results are normalizedrelative to the deepest attack observed foruncoated Baseline N5 (Condition A) exposed1900°F/2000 hours. These results for the coatedtest pins confirm the beneficial effects ofdesulfurization observed for uncoated materialand are consistent with the previous trend foruncoated material in that melt desulfurizationimproves oxidation resistance at all exposuretimes. The benefit of melt desulfurization isevident after only 2000 hours of exposure and ismanifested in terms of less overall surface attack.This pattern of improvement in oxidationresistance continues to the longer exposure timesand increases in magnitude. At the 8000 hourexposure, for example the depth of attack fordesulfurized material is only approximately 30%that of the Baseline N5. Typical microstructuresof coated oxidation pins exposed at 2000°F/4000and 8000 hours are shown in Figure 7. Thebeneficial effect of desulfurization can be seen interms of a thinner surface depletion layer.

Viewing these results with a differentperspective it can be seen that employingdesulfurized material in combination with coatingfurther enhances the effectiveness of the coatingitself in providing improved environmentalresistance particularly at the longer exposuretime. With a more adherent protective aluminaformed on the surface of the nickel aluminide,significantly less spallation/reformation of thesurface oxide occurs resulting in a more stablestructure relative to gamma' decomposition.

Uncoated Corrosion Results. Plannedexposures were to reach a 4000 hour maximumbut most of the specimens were pulled from thecorrosion rigs in shorter times because of thecondition of the specimens. The longest timesranged from 3600 - 3800 hours exposure. The

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Figure 7 – Typical microstructures of aluminide-coatedN5 oxidation pins exposed for 4000/8000 hours.

specimens exhibited a moderate to heavycorrosion attack. Metallographic evaluationswere completed and revealed considerable scatterin the attack exhibited by the specimens. Withinthis scatter in the data, some samples showedsignificant amounts of attack in only a fewhundred hours and others showed relatively littleattack after 1000 - 2000 hours exposure. Thisscatter was common to all sample groups tested,including the non-desulfurized baseline alloy. Arelationship, however, was observed betweenmaximum depth of observed penetration attackand material condition and this is shown inFigure 8. Similar to the oxidation situation,desulfurization appears to have a beneficialeffect upon corrosion resistance as well.

In spite of this relationship, however, thedegree of scatter indicated it was not possible todistinguish between the different conditions giventhe severity of the test for this alloy system. Itwould appear that sulfur does not have any

appreciable effect on the (poor) hot corrosionresistance of this alloy. This situation tends tomake generalizing the effects of sulfur oncorrosion somewhat difficult for these aggressivecorrosion tests. At this point, therefore, theconclusion is that sulfur level has little influenceon the hot corrosion resistance of N5 alloy underthese particular, and fairly aggressive conditions.

Figure 8 – 1700°° F Corrosion Results(Uncoated Specimens)

Summary

Oxidation testing of uncoated and aluminide-coated N5 (without yttrium) test pins at 1900°Fand 2000°F has shown that reducing sulfur tolevels below 1 ppm results in significantlyimproved oxidation resistance compared tobaseline material (sulfur level of 2.1 ppm).Further, the method of achieving the reducedsulfur was not critical to achieving theimprovement in oxidation resistance. Whetherdesulfurization was accomplished by metal meltpractice or by a hydrogen annealing heattreatment did not appear to be significantprocessing variables. Finally, it was noted that

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superimposing a hydrogen anneal onto meltpractice desulfurization did not result in asignificant increment of benefit. Since hydrogenanneals are time-consuming and expensive, theresults suggest that melt desulfurization is a moreeconomical method to accomplish the desiredsulfur reductions.

Hot-corrosion testing of uncoated N5 (withoutyttrium) test pins at 1700°F indicated no apparenttrend with alloy sulfur content. There was a largeamount of scatter in the data, with some samplesshowing significant amounts of attack in only afew hundred hours and others showing relativelylittle attack after 1000-4000 hours exposure. Atthis point, the results suggest that sulfur level haslittle influence on the 1700°F hot corrosionresistance of the N5 alloy under these particularconditions.

Task 3 - SX Casting Process

SX casting activity focused on several coredbucket configurations including the GE 7EC,6FA, and 7FA Stage 1 Buckets and the 7H Stage1 Nozzle. These are all complex-cooledconfigurations ranging in size fromapproximately 10-15 inches which exhibit thegeneral types of features (complex internal cores,overall length and width and complexity of airfoilcontours) that anticipate the types of designsplanned for the ATS. These were thus consideredto be appropriate configurations to expand thecapability range for complex-cored SX airfoilsfor land-based turbine systems.

Casting Trials. Each mold was constructed toyield 1-7 castings depending upon the specificconfiguration. The castings were oriented tip-down so that solidification initiated in the thinnercross section and proceeded upward into thethicker shank/root mass. Ceramic rods wereselectively positioned to offer support tominimize/avoid mold cracking. The castings were

processed through the production routingincluding the various NDT characterizations.

7EC Stage 1 Prototype Bucket

The series of five casting trials involved withthe 7EC Prototype Bucket configurationidentified acceptable mold temperature, metaltemperature and withdrawal rates. The castingparameters resulted in no randomly-nucleatedequiaxed grains nucleating from the solidificationprocess. This indicated the casting process itselfoffered reasonable consistency. Additionally,with these parameters and optimized bafflingconfigurations, freckles were eliminated from thecasting surfaces. Localized grain defects havebeen observed, however, and platform cornersand overhangs appear to be particularly sensitiveto nucleation in these areas. It was establishedthat grain feeds are preferred to insulationwrapping to eliminate separately nucleated grainsin these areas but optimization, however, isrequired to establish exact placement of the feeds.One other type of grain problem was noted andthat was isolated nucleation in the grain selectorramp areas. Using alternative types of supportrods in the ramp area reduced the number of thesenucleations but they were not completelyeliminated. Finally, the trials have resulted incastings free of FPI or X-Ray detectable defectssuch as dross or porosity. All of this supports thebelief that the SX process can be successfullyscaled-up to produce 7EC Stage 1 Bucket typesof configurations.

It was assessed, however, that because ofcertain limitations, other configurations would bemore suitable to complete the casting processwork. These limitations were associated with thefact that the 7EC tooling was prototype in nature.As a prototype, the core die tooling was never afinalized tool. As a consequence, satisfactory coreproduction was somewhat limited and at timessubject to core ejection difficulties. In addition,there were trailing edge core break and core

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stability concerns and it would be difficult tofirmly establish dimensional success. Since hardproduction tooling became available for two otherconfigurations and because of the 7EClimitations, a decision was made to terminateactivity with this configuration. The twoalternative configurations are the GE 6FA and7FA Stage 1 Buckets. The 6FA (approximately10 inches long) and the 7FA (approximately 14inches long) are both representative of the classof large land-based turbine componentsanticipated for ATS applications.

6FA Stage 1 Bucket

Casting activity for the 6FA Prototype Bucketwas designated as Trial 6 and involved a total offour molds. Two of the molds were configured toyield 7 castings while the other two were to yield5 castings for a total of 24 casting for this trial. Inaddition to cluster size, the other variableincluded in this trial was baffle design in which acluster of each size was produced with both thestandard baffle configuration and a modifiedfinger design minimizing the average distancebetween the mold and the baffle. Four of thecastings were short-cast, the result of incompletefill, in that metal ran out of a crack in the mold.The results of the grain evaluation indicated fivecastings would be acceptable as per currentaircraft grain reject criteria. This represented anoverall yield of 25%. Three of the defect-freecasting came from a 7-piece mold made with thestandard baffle. The remaining castings exhibiteddifferent types of surface grain defects with anumber exhibiting more than one type of defect.

Confirming the previous results for the 7ECconfiguration, there were no instances of root-face freckle chain formation. As a generalizedobservation for this castings, most of the grainswere nucleated or located not on shank or airfoilsurfaces, but on convex platforms, grain feed barsand airfoil trailing edges. Ramp nucleation (twoinstances) was much less prevalent in Trial 6 than

in the two previous trials for the 7ECconfiguration. It is believed that alternate ceramicmold support rods reduced flash at mold/rodjunctions and carefully controlled moldpreparation/drying conditions reduced flash atramp surfaces, both of which contributed toreduced grain nucleation at these sites. It wasunusual to note that for the first time slivers wereobserved as part of defect occurrence. These arerelatively short and narrow grains which havenucleated on the casting surface generally inshank areas. The name derives from thelong/narrow shape which results when thenucleated grain is crowded out by the growth ofthe main crystal which nucleated in the starter.

7FA Stage 1 Bucket

The series of three casting trials involved withthe 7FA Prototype Bucket configurationidentified casting parameters and moldconstruction details which produced an overallgrain yield of 23%. The production of defect-freecastings supports the conclusion that the SXprocess can be successfully scaled-up to producethese types of large configurations for land-basedturbine applications.

Employment of the improved LCS shellsystem reduced the incidence of mold crackingand metal run-out. Optimized baffleconfigurations eliminated freckle grain defectsfrom root surfaces and combined feed bar andmold wrap positioning schemes eliminated grainnucleation at platform corners, overhangs andfeed areas. Additionally, the trials have resultedin castings free of FPI or X-Ray detectabledefects such as dross or microshrinkage.

It is believed that improved grain yields canbe achieved by elimination of the isolated casesof columnar grain initiating in either the ramparea or the core support windows at the castingtips. The key to improved yields here involvesminimizing the numbers of flash sites (by

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eliminating mold cracking) which havepreviously been shown to reduce grain nucleationin these areas. Macrophotos of one of the defect-free castings are shown in Figure 9.

Figure 9 – Photos of defect-free 7FA Stage 1Bucket casting.

7HA Stage 1 Nozzle

All activity for this 10-inch 1ong cored partwas focused on Trial 1 – Mold StructuralIntegrity. Because of the size and geometry of the7H Stage 1 Nozzle, the Trial 1 molds wereconfigured to yield a single casting per mold. Atotal of four molds were constructed for Trial 1.For these molds, the nozzle wax patterns werepositioned and gated such that the SX starterwould feed into the inner and outer shrouds at theleading edge corners.

The results for the four Trial 1 moldshighlighted the fact that significant problemswere associated with producing this nozzlecomponent. The most significant problems wererelated to mold structural integrity in that of theour molds, only one survived the mold-build

process for casting. The other three all exhibitedserious evidence of handling damage. It wasassessed that the extent of this handling damagewas such that localized patching, replacement orrepair could not salvage the molds. It was clearthat a more robust mold design relative to theshell thickness and mold support rods wereneeded to withstand the stresses generated duringthe mold-dipping/slurry-draining movementsexperienced in mold build.

The mold that survived assembly and buildwas cast successfully without any run-out. Grainetch revealed the presence of a number ofequiaxed grains in the outer shroud area nearestthe leading edge and the fillet between theleading edge and the outer shroud. It wasreasoned that this nucleation took place relativelylate in the solidification process because theequiaxed grains did not subsequently grow in acolumnar manner along the body of the airfoilportion of the nozzle. These results highlight theneed to incorporate grain feeds in this area toavoid this spurious nucleation.

Summary

The Task 3 SX Casting Process activitydemonstrated that SX casting technology can besuccessfully applied to complex-cored airfoilcastings for land-based power generationapplications. Casting efforts were focused onseveral cored bucket configurations, including theGE 7EC, 6FA, and 7FA Stage 1 Buckets and the7H Stage 1 Nozzle. Their size (approximately 10-15 inches long) and complexity were consideredappropriate to demonstrate expanding thecapability range for complex-cored SX airfoilsfor land-based turbine systems.

The cored buckets employed tooling beingused to produce DS-columnar grainedcomponents. As such, features had to be added tothe tooling to enable SX versions to be produced.

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Thermal simulation modeling was used to helpdetermine the feasibility of developing a soundcasting process for these large components. First,three-dimensional models of the envisionedmolds were constructed and simulations werethen conducted of the casting processes. Theresults highlighted certain areas in the castingconfigurations where grain nucleations couldoccur. This formed the framework for theplacement of grain feed bars and mold insulatingwrap to eliminate these nucleations.

Through an iterative series of casting trialswhich incorporated modeling results as well asthe results from previous trials, an acceptablecombination of mold temperature, metaltemperature and withdrawal rates was establishedfor the cored bucket castings. These castingparameters resulted in no randomly nucleatedequiaxed grains nucleating from the solidificationprocess. Localized grain defects occurring ingenerally the same regions of the castings wereobserved throughout the trials. The successfulincorporation of furnace and mold configurationfeatures eliminated many of these grain defects.Baffle configurations eliminated freckle-chaingrains on root surfaces and optimized grain feedbars/mold insulation wrapping eliminated grainnucleations on platform corners, overhangs andfeed areas. Overall grain yields of 25% and 23%were achieved for the 6FA and 7FA coredbuckets, respectively. Isolated grain nucleationswere observed, however, in either the ramp areasor the core support windows at the casting tips.The key to improved grain yields in theseinstances involves minimizing the numbers ofmetal flash sites (these result when molten metalpenetrates very small cracks in the mold surfacelayers) which can act as grain nucleation sites.

Employment of the improved LCS shellsystem reduced the incidence of mold crackingand metal run-out. Additionally, the trialsconsistently resulted in castings free of FPI or

X-Ray detectable defects such as dross ormicroshrinkage.

Mechanical property tests, including tensileand stress rupture characterizations wereconducted on thin-sheet specimens machinedfrom 7EC buckets and compared to data availablein the literature for N5 SX alloy. The tensileresults indicated lower strength and ductilityvalues compared to published data.Microshrinkage and carbide formations presenton the fracture surfaces may have contributed tothis situation. The fact that the cross sectionalarea of the sheet specimens was approximately1/9 that of test bars usually used for aircraftcomponent testing may also have had an effecthere. The stress rupture results, on the other hand,exhibited equivalent or superior performancecompared to published data. This suggests thatstress rupture performance was not as sensitive tofactors such as microshrinkage, carbideformations, and specimen configuration as thetensile properties.

The casting results for the 7H Stage 1 Nozzlehighlighted the need to optimize mold buildprocedures to eliminate mold cracking and metalrun-out. The grain results highlighted therequirement to optimize grain feed in order toeliminate instances of spurious grain nucleationin shroud areas.

Task 4 - Core Materials & Design

The Task 4 Core Material activitydemonstrated that a number of silica-based corebodies are available to produce complexserpentine cores for SX castings intended for usein land-base turbine engine applications. The coreefforts were focused on evaluating both low andhigh pressure core manufacturing processes forthe production of cores for the GE 7EC, 6FA, and7FA Stage 1 Bucket configurations. The coreconfigurations and complexity were consideredappropriate to demonstrate extending the

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capability range of cores for land-based turbinesystem components.

An iterative series of casting trials wereconducted in which the configurations wereproduced employing a number of silica-basedcore bodies including the PCC SRI 200-SXA lowpressure silica core body, the LED ICCP lowpressure silica/zircon core body and the CertechP-36 high pressure silica/zircon/alumina corebody. The core evaluation results for all of thebucket casting trials are listed in Table 4.Included in the table are the configurations cast,the trial number, the core body used and summarycomments relative to core performance. In allcases, the cores were able to be leached usingstandard caustic core removal cycles. In all cases,there was no evidence of unusual core/metalinteraction products. Finally, in all cases the corestability assessments indicated results comparableto the production measurements of columnar-grain castings currently being produced with thetooling. The findings all indicated that toolrework would not be needed to producedimensionally acceptable castings.

Table 4 – Core Performance Summary

(1)Cores removed with standard caustic leach cycles with no unusualcore/metal reaction products observed.

The major issue addressed during this taskinvolved core break and this was found to be afunction of the specific core die used. For the7EC castings, prototype core die tooling was usedinitially and this resulted in considerable corebreak as revealed by FPI and X-Ray inspection

operations. Selective die modifications and corestrengthening rods were able to significantlyreduce the incidents of core break. For the 6FAand 7FA castings, hard production-ready core dietooling was available which also resulted insignificantly reduced incidents of core break. Itwas thus assessed that core break was not somuch a function of the particular type of corebody used but, in fact, more a function of theavailable core die tooling.

Task 5 - Grain Orientation Control

The Task 5 Grain Orientation Control activitywas focused on mechanical property testing ofRene N5 specimens exhibiting a range of defectsincluding grain boundary mismatch angle andfreckles. Testing included tensile, stress rupture,and LCF tests conducted at various temperaturesand conditions.

Tensile tests conducted at room temperatureand 1400°F and stress rupture tests at 1600°F/80ksi and 1800°F/40 ksi were conducted onspecimens machined from double-seed slabcastings exhibiting grain boundary mismatchangles ranging from approximately 5-45 degrees.The results of these tests are listed in Table 5 andTable 6. Tensile results indicated strengthproperties were sensitive to mismatch,particularly at the higher angles with reductionsof 30% at mismatch angles above 20 degrees.Room temperature ductilities exhibited littledegradation with mismatch while the 1400°Fductilities were only slightly degraded atmismatch angles above 20 degrees. The stressrupture lives at both test temperatures wereextremely sensitive to mismatch in that 90%reductions were observed at mismatch anglesgreater than 8 degrees.

LCF testing also indicated a sensitivity tomismatch angle. At higher test temperatures, forexample, a 35% life reduction was observed byincreasing mismatch angle from 11 degrees to 21

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degrees, and 65% reduction going from 21degrees, and 65% reduction going from 21degrees to 31 degrees (Figure 10). Based onlimited data, LCF life for 0.06 inch-sized frecklesappeared to be comparable to that for nominallydefect-free material.

Table 5 – Double-Seed Test Slab Tensile Test Results

Table 6 – Double-Seed Test Slab Stress Rupture Results

In overall summary, the results suggestedsome potential for relaxation of freckle defectrejection criteria, but it is believed more testing isrequired to characterize a wider range of (1)temperature response, (2) freckle-sizedistributions, and (3) the effects of freckles onother mechanical properties such as tensile, stressrupture and high cycle fatigue (HCF) response.There appeared, however, to be little potential forrelaxation of LAB/HAB rejection criteria.Significant stress rupture life reductions were

observed at mismatch angles above 8 degrees andsignificant LCF life reductions were observed atmismatch angles above 11 degrees.

SUMMARY/CONCLUSIONSSUMMARY/CONCLUSIONS

The Turbine Airfoil ManufacturingTechnology program was conducted to allowaircraft single crystal (SX) casting technology tobe applied to land-based power generationapplications. The approach involved scale-up ofavailable manufacturing procedures to producelarge-size airfoil components. A number oftechnical issues were addressed including alloymelt practice to reduce sulfur in nickel-basesuperalloys, modifications/improvements of theSX process, core materials and grain orientationcontrol. The objectives/conclusions relative to thespecific issues included the following:

Figure 10 LCF life vs. mismatch angle of LAB/HABspecimens at Level 2-test temperature.

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Alloy Melt Practice – Reduce sulfur in N5 Alloy (withoutyttrium).

§ Desulfurization can be accomplished duringalloy melting operations resulting in sulfurlevels below 1.0 - 0.5 ppm in superalloy N5.

§ Desulfurization to sulfur levels below 1 ppmresults in enhanced resistance to hightemperature oxidation (1900/2000°F) for bothcoated and uncoated superalloy N5.

§ The method of desulfurization is not critical toachieving improvement in oxidationresistance. Melt desulfurization and hydrogen-annealing treatments offer equivalentimprovements in oxidation resistance.

§ Superimposing a hydrogen anneal onto meltdesulfurization did not result in any significantadditional benefits.

§ Desulfurization had little positive influence on1700°F hot corrosion resistance.

SX Casting Process – Define methods for casting coredSX airfoils for land-based power generation applications.

§ Aircraft SX casting technology can be appliedto large-sized airfoil components.

§ Castings can be produced free of dross andmicroshrinkage defects.

§ Grain defects are generally confined tospecific locations including the SX starterramps and casting tips.

§ Eliminating mold cracks (which act to producemetal flash/fin sites) can minimize grainnucleations at SX starter ramps and castingtips.

Core Materials – Provide stable cores to produce largeSX airfoil castings.

§ Silica cores are available to produce large SXairfoil castings

§ Silica cores can be removed with standardcaustic leach cycles.

§ No unusual core/metal reaction products areformed during casting operations.

§ Satisfactory core dimensional stability hasbeen demonstrated.

BENEFITSBENEFITS

The anticipated program benefits include thedefinition of cost-effective methods to producelarge SX airfoil castings for land-base powergeneration applications. These achievements willconfirm the ability to use the extensive andcompetent industrial base, which has been usedfor military and commercial aircraft applicationsfor the expanding markets available to land-baseturbines.

An additional benefit, which has beenidentified, includes the demonstration of a newlow-cost process developed to remove the sulfurfrom high temperature superalloys. The presenceof sulfur as a contaminant in the metal causes theprotective oxide (which normally forms duringservice) to flake/spall off, resulting in increasedexposure to extreme conditions for the basemetal. Reducing the sulfur content significantlydecreases damage to the protective outer layer.Currently, the sulfur is removed by hydrogenannealing of the blades after the casting process.Through the melt desulfurization processdemonstrated in the Turbine AirfoilManufacturing Technology program, sulfurcontents have been reduced to levels comparableto those achieved through heat treatment atsignificantly reduced costs. As a consequence of

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this, oxidation resistance for the melt-desulfurized condition are also comparable tothose for hydrogen-treated material. This type ofbenefit will result in lower equipment cost forindustry and electrical cost for consumers. Theimprovement in oxidation resistance was alsoobserved for coated specimens. This enhancedenvironmental resistance also represents apotential benefit to the aircraft industry, whichhas traditionally been the focal point for advancesin gas turbine engine technology.