Turbo Machinery Dynamics Part 13 pptx

Nickel-based alloys are used for aeroengine blades with a protective nickel-aluminidediffusion coating for oxidation resistance at high temperatures during operation.. Temperature is mea

stresses at the same temperature load, making the segmenting network finer and individualcracks to become wider under cyclic loading

The amplitude and frequency of the acoustic signals during the heating phase decreasewith the number of cycles In-plane tensile as well as compressive stresses are present inthe coating, depending on the depth Parallel cracks occur because the coatings are applied

in layers, and can grow because tensile stresses are present at their end The formation ofdelamination cracks depends on the oven conditioning to a much greater degree than is thecase with vertical and parallel cracks Few delamination cracks occur on specimens thatare not conditioned, even when subjected to a large number of cycles Aging by cycliclaser irradiation does lead to compaction of the ceramic coating, but the relatively coolerbond coat results in only a slight growth in the oxide coating Consequently, the cycliclaser aging promotes the growth of delamination cracks to a lesser extent than a preced-ing oven aging

Nickel-based alloys are used for aeroengine blades with a protective nickel-aluminidediffusion coating for oxidation resistance at high temperatures during operation Theeffect of the presence of the coating on the operational life of single crystal superalloyblades is an issue of concern, in large measure due to the oxidation of the coating’s con-stituents Under combined mechanical and thermal load conditions that mimic the strain-temperature behavior at critical locations in blades, the presence of an aluminide coating

on SRR99 results in substantial life reduction at 0.7 percent mechanical strain (Bressers

et al., 1996)

Corroboration of the difference in life between the noncoated and Ni-aluminide coatedsamples is provided by Johnson et al (1997) Cylindrical bars of the Ni-based superalloySRR99 with the long axis oriented 10° of the <001> direction are used in the evaluation Thecoating has two layers The outer layer is a polycrystalline B2-NiAl (3–6 µm grain size) + g ′

of thickness 23 ± 2.5 µm The inner subcoat diffusion zone (17.5 ± 2.5 µm thick) is followed

by a continuous layer of g′ (2 to 3 µm thick) The coated and uncoated samples are subjected

to 0.7 percent mechanical strain and temperature cycle varying between 300 and 1050°C Digitized light microscope images of the coating surface are obtained during the testswith a video camera Incremental polishing of the images allows the characterization of thecoating and of the cracks as a function of depth over selected areas Images are then digi-tized for measurement to obtain density, spatial characteristics, and number of cracks.Energy dispersive spectroscopy is used to chemically define the surface

The cracks in the uncoated sample generally initiate at or near the surface following theformation and concentration of the oxides that may be described as a discontinuous oxida-tion fatigue process Some cracks emanate from the top and base of the spikes, but the event

is generally difficult to detect After the test sequence, the cracks grow enough in size topermit identification and correlation with the earlier-detected indications A simplifiedsequence of events for the crack initiation procedure may be sketched by

The fraction of the oxidized area to the local regional area may be used as a gauge fordamage initiation and growth studies Both oxide spikes and cracks behave as rigid inclu-sions at the sample surface under compression Since a strain gradient is formed betweenthe inclusion and the alloy, both features contribute to discontinuous oxidation process onthe surface Figure 11.20 shows the experimental data, where contributing factors to theoxidation process are expressed by the Avrami type rate equation

where K= 3.27 × 10−11and n = 2.6 Values of n in this range indicate that the initiation rate

Casting defectOxidation fatigue→Oxide spikeTensile failure→Cracks

FIGURE 11.22 Surface event initiation rate (Johnson et al., 1997).

FIGURE 11.23 Uncoated sample with oxide layer removed (left), oxide concentration area (right) (Johnson et al., 1997).

464

FIGURE 11.21 Time sequence of surface changes in coated sample (Johnson et al., 1997).

is decreasing, and the overall transformation is mostly two-dimensional Parameter K defines

the character of the mixed surface damage process under different conditions

A time sequence of images of the surface for the coated sample is shown in Fig 11.21

A large number of bright particles are noticed around 1000 cycles, and may correspond tooxidation products of coarsened coating constituents Few cracks are detected before 2000

cycles Accumulation of initiation events of surface features larger than 50 mm for the

uncoated and coated test specimens are shown in Fig 11.22 Larger concentration of dation features may be considered a precursor to cracking Initiation rate in the coated sam-ple surface changes considerably between 2000 and 7000 cycles, and tends to saturate wellbefore the end of the test

oxi-The impact of the coating on fatigue life is most noticeable in the growth rate ofcracks until eventual failure Growth rates are similar for the coated and uncoated testpieces up to 5000 cycles, when the rate accelerates in the coated sample In the coatedpiece, failure is encountered after 9469 cycles, while the uncoated piece is good for21,820 cycles This observation may be explained by the coalescence of crack events,leading to rapid extension of the major crack In the posttest examination, surface oxide

is removed to expose oxidation areas and cracks Figure 11.23 shows the front surface at

a depth of 80 µm and a section through an oxidation region A central nickel oxide areaforms to create concentric layers of oxidation products of Al, Cr, and Ta In the coated mate-rial, three main layers of differing damage and microstructure are identifiable At 24 µmdepth, damage is chiefly in the form of oxidized cavities in the coating, composed mainly of

Al rich and lesser extent of Ni compounds At 37 µm depth, cracking predominates at theboundaries of Al and Ni oxides The presence of coarse grain boundary particles sug-gests the coating may offer greater resistance to creep As the subcoating is pene-trated, some cracks indicate association with substrate solidification defects At 65 µm

depth the continuous g′ layer is passed, but the microstructure does not fully matchthat of the bulk portion Etching revealed that the Ni and Al oxides in the coating areclosely twined

11.11 FIBER-REINFORCED CERAMICS

FOR COMBUSTOR LINER

Improved turbine efficiency can be achieved through higher temperatures at the inlet,and hence the emphasis on effective cooling and high-temperature resistant materialsfor the components of the hot path Carbon- and silicon-carbide-based materials offersuch a potential

But the advantageous qualities of monolithic materials in structural applications, such

as Young’s modulus, thermal conductivity, and oxidation resistance, are partly offset bytheir relatively low fracture toughness Additives and particulate reinforcements have beentried as toughening agents Reinforcement with SiC or C continuous fibers increase thefracture toughness by one order of magnitude and fracture energy by two orders of magni-tude (Helmer, Petrelik, and Kromp, 1995) The hostile stress and temperature environmentrestricts the choice of toughening phases because of chemical incompatibility and thermalexpansion coefficient mismatch with the matrix

MTU of Munich, Germany has researched the composite materials in an experimentalinvestigation (Filsinger et al., 1997) The materials are selected from a group of C, SiC, andfiber-reinforced glasses Under an oxidizing atmosphere, fiber degradation may be expected

in the two-directional composites Assuming a minimum tensile strength of 150 MPa to besufficient for reliable operation of the component, the durability in a 1000°C environment

would be only 8 min for the C/C material and 6 min for the C/SiC material This clearlyshows the need for effective external oxidation protection The coatings influence themechanical properties of the base material, and hence the tests are conducted for coatedspecimens

A cross-sectional view of the ceramic can form of combustor is shown in Fig 11.24.Combustor walls are made of an inner layer of a hot-gas resistant composite, a middle layer

of a flexible oxide fiber, and an outer metal casing The flexible insulation in the radialdirection and the spring-supported swirler in the axial direction allow for an almost unhin-dered thermal expansion of the ceramic flame tube The insulation is intended to keep thefiber-reinforced material at an approximately uniform temperature and lower thermal stresslevel Temperature is measured by thermocouples placed along the outer ceramic wall.Flanges on the outside of the combustion chamber provide access to the flame tube for mea-suring the radial temperature distribution with Pt-Rh-Pt thermocouples

The combustion chamber is assembled in a Klockner-Humboldt T216 gas turbinewith a power output of 74 kW at 50,000 rpm The nominal pressure ratio is 2.8, air-massflow is 0.9 kg/s, and turbine inlet temperature is 810°C The combustor dome is made ofsintered silicon carbide Since large holes are required for dilution at the end of the flametube, the dome is separated from the flame tube by a nickel alloy spacer The flame tube

is 210 mm long and has an inner diameter of 144 mm The thickness of the ceramic wall

is 3 mm

Pressure, temperature, speed, and power are recorded during engine operation Walltemperatures determine the thermal loading of the composite materials Figure 11.25 dis-plays typical axial temperature distributions in the ceramic wall at different operatingconditions Peak temperature is 1050°C, occurring between the dilution holes in the mid-dle of the flame tube at nominal speed, with all four flame tubes displaying similar values

Thermocouple

FIGURE 11.24 Ceramic flame tube construction (Filsinger et al., 1997).

Accumulated test time is limited to 10 h Starting with operation at low values the thermalload is gradually intensified to a peak of 87 percent maximum load.

Between the tests, the flame tubes are inspected visually for recording morphologicalchanges by macrophotography Hot gas profiles are measured for all operating conditions.All tubes withstood the thermal load under the oxidizing atmosphere without severe dam-age One flame tube showed no damage on the inner surface, but on the outside a small chip

of the chemical vapor deposition SiC coating is separated This may have resulted from amismatch between the localized thermal growth The thermal load causes considerable dis-coloration, and may be the result of a layer of SiO2arising from the oxidation process Thethin amorphous glass layer reflects different wavelengths of the incoming light, depending

on the thickness, and the surface appears with a rainbow of colors (Fig 11.26)

The test program is extended for the promising SiC/SiC composite combustion ber, and is in operation for 90 h without indications of any damage The unit has gone

FIGURE 11.25 Temperature distribution on outer surface of flame tube (Filsinger

et al., 1997).

FIGURE 11.26 Flame tube after 10 h of operation (Filsinger et al., 1997).

through a number of start/stop cycles that have the potential for developing critical loadsbecause of the high temperature gradients

A regenerative twin spool ceramic gas turbine design aims to achieve thermal efficiency

of 42 percent at turbine inlet temperature of 1350°C Developed by Kawasaki HeavyIndustries (Takehara et al., 1996), the lower pollutant emission and multifuel capability gasturbines are to be used in cogeneration systems Some unique features include simple-shaped ceramic components and stress-free structures using ceramic springs and rings.Figure 11.27 provides a cross-sectional layout of the engine

Compressor impeller

Combustor liner

Gas generator rotor

Gas generator nozzle

GGT nozzle wave ring

PT nozzle wave ring

FIGURE 11.28 Stress free support system (Takehara

et al., 1996).

A single can combustor and a high-pressure ratio recuperator are conventionallydesigned A ceramic gas generator and power turbine nozzles and scroll are supported inthe metal engine casing by elastic ceramic parts Piston rings, also made of ceramic, areused for inner and outer seals Wave rings are designed to absorb thermal expansion anddynamic displacements, illustrated in Fig 11.28 The seals and rings are made of Si3N4.Nozzle assemblies are produced by binding segments with SiC fibers, which are convertedinto fiber-reinforced ceramic in the form of a monolithic ring The nozzles are capable ofwithstanding elevated temperatures, provide adequate stress characteristics, and can bereadily installed within the engine.

A single-stage impeller provides compression ratio of 8:1 and flow rate of 0.9 kg/s with

an adiabatic efficiency of almost 80 percent A channeled diffuser provides for adjustment

of inlet angle to the impeller’s discharge angle A schematic drawing of the combustor isshown in Fig 11.29 The ceramic liner is supported by coil springs to absorb relative ther-mal growth between the liner and the metal case The combustor has a bypass line with avalve to control the ratio of air and fuel Endurance testing at this stage of development ofthe turbine records 19 cycles with 94 accumulated hours

11.12 CERAMIC COMPONENTS

IN MS9001 ENGINE

The advisability of implementing ceramic components in a utility-sized turbine in mercial service for power generation has been assessed by General Electric Company inconjunction with Tokyo Electric Power Company The program calls for evaluating theperformance of the ceramic combustion transition piece, stage 1 bucket, nozzle and shroud,and stage 2 bucket and nozzle (Grondahl and Tuschiya, 1998) A recent productionMS9001FA gas turbine in a single-shaft advanced combined cycle mode of operation isspecified as the baseline for the comparison

com-Primary performance evaluation study is conducted at a constant NOxemission level

of 25 ppm in the exhaust from the turbine Since the emissions are directly related to the

FIGURE 11.29 Combustor arrangement (Takehara et al., 1996).

combustion reaction zone temperature, the turbine inlet temperature (at the exit plane of thetransition piece) is held constant in the analyses Baseline compressor airflow is also main-tained constant But the pressure drop in the combustion system and cooling air extractionfrom the compressor discharge are decreased with ceramic components, causing increasedairflow through the combustor This results in reduced fuel-to-air ratio, lower flame tem-perature, and less NOx Hence, fuel flow and firing temperature are increased as necessary

to maintain the level of temperature at the turbine inlet

The materials considered in the study are limited to monolithic ceramics Monolithicsilicon-nitride, SN-88, is the primary candidate for all the components, with its physicalproperties shown in Table 11.3, and a fracture map of the material strength capability pro-vided in Fig 11.30 The peak application temperature of the ceramic is limited from oxi-dation considerations to 1315°C

The ceramic transition piece is placed in an outer metal casing, and has provision forimpingement cooling The metal shell supports the pressure difference between theinside and the outside of the system, and minimizes the leakage at the ceramic liner tiles

TABLE 11.3 Properties of SN-88, Sintered Silicon Nitride

Temperature (°C)

The attachment of the ceramic segments permits exchange of heat radiation between theouter shell and the ceramic The exit seal leakage area is similar to a conventional design The stage 1 shroud is cooled by air from the compressor discharge, and is subject to max-imum gas-path temperature below the lower-limit oxidation temperature limit of 1204°C forSN-88 The stage 1 nozzle uses film cooling from a number of holes in the airfoil and side-walls to efficiently reduce heat transfer to the metal by reducing the film temperature at thesurface (Tsuchiya et al., 1995) The vanes also require impingement cooling The vanes havesidewall thickness between 5 and 6 mm The stage-2 nozzle uses cooling air from the com-pressor 13th stage The first- and second-stage buckets are convectively cooled by airextracted from the 17th stage of the compressor The bucket design is described by Terama

et al (1994), together with a discussion of the associated development effort

The gross combined cycle efficiency and improvement in the output relative to the line engine are shown in Fig 11.31, using the minimum oxidation limit values for theceramic as shown in Fig 11.30 The results include benefits from the increased fuel flowand the firing temperature needed to maintain turbine inlet temperature and NOxemissionsconstant Maximum gain is obtained from the first-stage nozzle vane and bucket, mostlybecause of the reduced cooling airflow with the ceramic design and the consequent increase

base-in the flow through the combustor Mabase-intabase-inbase-ing the fuel-to-air ratio for constant NOxalsoresults in the large increment in the firing temperature

FIGURE 11.31 Cumulative performance benefits with ceramic components (Grondahl

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p 136, 1969

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Fleischer, R L., The Strengthening of Metals, p 93, Reinhold, New York, 1964.

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ASM, Park, Ohio, p 41, 1986

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of American Ceramics Society 78:133–136, 1995.

Hughes, S E., and Anderson, R E., Technical Report # AFML-TR-79-4146, U.S Contract # 76-C-5136, 1978

F33615-Immarigeon, J P A., “The role of microstructure in the modeling of plastic flow in P/M super alloys

at forging temperatures and strain rate,” Advisory Group for Aerospace Research and Development,Neuilly-sur-Seine, France, AGARD-LS-137, 4–1, 1984

Johnson, P K., Arana, M., Ostolaza, K M., and Bressers, J., “Crack initiation in a coated and uncoatednickel-base super alloy under TMF conditions,” ASME Paper # 97-GT-236, New York, 1997 Kempster, A., and Czech, N., “Protection against oxidation of internal coating passages in turbineblades and vanes,” presented at Power Gen Conference, 1998

Klarstrom, D L., Super Alloys 1980, ASM, Metals Park, Ohio, p 131, 1980.

Kool, G A., Agema, K S., and Van Buijtenen, J P., “Operational experience with internal coatings in

aero and industrial gas turbine airfoils,” Proceedings of the ASME Turbo Expo, The Netherlands,

Paper # GT-2002-30591, New York, 2002

Koul, A K., Immarigeon, J P., Dainty, R V., and Patnaik, P C., “Degradation of high performance aero

engine turbine blades,” Proceedings of the ASM Materials Congress, Pittsburgh, Pa., pp 69–74, 1993.

Lackey, W J., Report # ORNL/TM-8959, 1984

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Diego, p 271, 1984

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153, 1983

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super alloy turbine blades,” in Reichmann et al (eds.), Superalloys Book, TMS AIME, Warrendale,

N.J., pp 815–824, 1988

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creep and fracture in ceramics,” Materials Science and Engineering, A233(1–2):167–175, 1997.

Rettig, U., Bast, U., Steiner, D., and Oechsner, M., “Characterization of fatigue mechanisms of mal barrier coatings by a novel laser based test,” ASME Paper # 98-GT-336, New York, 1998

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MANUFACTURING METHODS

12.1 INTRODUCTION

The evolution of modern aircraft and industrial gas turbine engines has coincided with theevolution of superalloys and means to cast, cut, machine, and join them into finished prod-ucts The toughness required of the materials for withstanding exceptionally high temper-atures at stress levels approaching their elastic limits translates into difficult conditions forcutting, forming, machining, and joining the parts Dimensional instabilities arise fromresidual stresses and metallurgical alterations introduced by the manufacturing processes.The primary and secondary procedures lead to changes in the surface layer such as plasticdeformation, which affect the surface integrity and stability of the dimensions

A nearly net shape can be obtained from many different casting methods of superalloys.Alloying of wrought superalloys must be restricted to preserve their hot workability char-acteristics Cast superalloy compositions are not so confined, and alloys with much greaterstrengths consistent with restraints are possible Mechanical properties such as creep andrupture are maximized by the casting and heat-treating processes Ductility and fatigueproperties in the castings are generally not as good as their wrought counterparts, but refin-ing the grain size alleviates many of the defects associated with casting Production of hol-low airfoils for turbine blades and vanes with an intricate system for cooling passages isaided by the “lost-wax” or the investment casting process The shapes are developed whenthe mold slurry flows around the wax pattern defining the part shape Use of preformedceramic cores adds to the capability to produce hollow airfoils

High-performance aerospace components are also produced economically using thepowder metallurgy technology Powder-based alloys are used when cast or wrought com-ponents cannot meet the requirements of the application Failure in conventionally cast andwrought parts often arises from segregation, resulting in inconsistent and reduced thermo-mechanical response The powder-based process is then employed when cast or wroughtcomponents are not suitable Some intrinsic attributes of powder materials make themappropriate for turbine components A high rate of solidification results in smaller inter-metallic particles and reduces the spacing between the particles, and is a feature that can-not be duplicated in casting High levels of mechanical properties can be realized with theforgeable microstructures Their unique structure has the ability to provide for special envi-ronments, such as strengthening from oxide dispersion Broader application of powder met-allurgy is hampered by performance limitations and cost The alloys are sensitive tocontamination, especially in highly stressed parts that may be critical from fracture andfatigue considerations Thermomechanical processing is essential for neutralizing the ensu-ing defects The powders are required to have a very fine size

The magnitude of the stress and metallurgical alterations during machining depend onmachining parameters such as feed, speed, depth of cut, cutting tool material, part geometry,

CHAPTER 12

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and cutting fluid Normal industry process calls for relieving the thermal stresses to reducethe effects on dimensional instability at the machining stage But there is no stress-relieving cycle, for example for the Inconel 718 alloy, other than solutioning method thatcan change the mechanical properties and hardness Also, only a limited aging process ispermitted for this material to avoid degradation This metallurgical restriction compels con-trol of the residual stresses by substantially altering the machining parameters.

Welding plays a major role in the fabrication of parts for aircraft engines and industrialgas turbines The procedure permits economical fabrication of subcomponents withoutadding weight or cost Welded joints do not suffer from problems related to deterioratedservice capabilities Cracks and fissures can develop in the welds and represent a majordrawback of the process The welding may also cause reduction in the material properties

in the region, mostly in the form of reduced ductility because the structure of the solidifiedweld material is segregated and is less ductile than an equivalent wrought structure Thisaspect is also responsible for deteriorated oxidation resistance When elements with a high

vacancy of electrons segregate, s and other embrittling phases may precipitate during

welding and even after placement in operational service Thus, the alloys must be ually checked for degradation in properties as a consequence of welding Reinforcement ofthe weld in the form of over- and underbeads must be eliminated when fatigue is identified

individ-as the primary mode of failure

Curvic couplings provide a precise method for connecting, centering, and improving theload-carrying capacity of turbine shafts and disks Their design calls for a high level ofaccuracy in the positioning and indexing of the teeth on the disk face The concave and con-vex tooth profiles are generated by using the outside and inside surfaces of a cylindricalgrinder, with a conical machining surface of the grinder

Diffusion and overlay protective coatings are used for gas turbine components to enablethem to withstand a severe environment The corrosion and oxidation resistance provided

by the coatings extend the component’s operating life Diffusion coats provide a surface

enriched with aluminum, chromium, or silicon In earlier production the electron beam physical vapor deposition (EB-PVD) was used, but because of the high capital cost in set- ting up a commercial plant, plasma-spraying systems are preferred Air plasma spraying

(APS) is a widely accepted procedure, in particular the argon-shrouded and the vacuummethods An inert gas or a vacuum allows application of the low-oxygen-content coating.Another recent innovation is the high-velocity oxygen fuel system in open air, and has beenestablished for production of coatings with a low-oxide content, low porosity, and highbonding strength MCrAlY coatings may be applied with this procedure in open air and stillachieve near chamber quality, mostly because of the higher particle velocity when com-pared with other thermal spraying systems Thicker coats can also be applied with this tech-nique for improving the residual stress Cracking and delamination of the sprayed coat ismostly a consequence of the residual stresses

12.2 CENTRIFUGALLY SPUN ALLOY

STEEL CASTING

As the name implies, the essential features of the centrifugal casting process consist of jecting molten metal to centrifugal pressure created in a rapidly rotating mold in such amanner that the metal is directed to assume the shape of the mold All extraneous non-metallic material, being less dense than steel, is retained at the surface of the bore togetherwith the microporosity formed by the directional solidification The inner surface is thenremoved by machining to provide a sound and more homogeneous casting

sub-The cleanliness and convenience of this process is attractive for aircraft engine andindustrial gas turbine manufacturers (Nixon, 1987) Production of vertical and horizontaldie castings ranging from 6- to 60-in diameter has been achieved Over the past severalyears, centrifugal castings have been perfected by casting into refractory molds to givediameters up to 120 in and weighing 10,000 lb.

Two distinct forms of centrifugal casting machines are employed, one with the axis ofrotation oriented horizontally, the other being vertical (Fig 12.1) The suitability of a pre-ferred direction is based on the shape of the casting When the diameter exceeds the length,the direction is generally vertical, and is horizontal if the length is greater than the diame-ter In the former process the die spins at a low rate when compared with the horizontalmethod, and the metal flows through an open top lid into an inclined injector, which trans-fers the melt on to the wall of the die at a tangent in the direction of rotation In the hori-zontal method the die rotates on twin rolls, and the molten metal is introduced through anopen end plate at one end of the die and moves along the die to give a casting of uniformthickness along its length

The values for a number of parameters must be carefully selected to obtain the sions required for the finished part In the horizontal method of production the outside

FIGURE 12.1 Centrifugal spinning operation for castings.

diameter controls the metal die The outside diameter is dictated by the finished diameter,with an allowance for surface roughness contraction and the thickness of the refractorycoating inside the die The inner diameter requires an allowance for the unsound material

to be removed in the bore, a peculiarity of this process From these dimensions the tity of metal required to produce the casting is computed Accurate digital scales locatedabove the ladle ensure the exact amount of metal is poured into the die

quan-Production by the vertical method can be obtained with metal die or refractory shapedmolds Metallic molds are simpler in configuration, but to produce a shaped outer diame-ter calls for a corresponding cavity in the die As an example, stiffening rings may belocated on the outer surface But this adds to the cost of the tooling, and can be warrantedonly when the production run is large Conventional sand castings, on the other hand, incurthe cost of making patterns, molding, and drying Large castings with flanges at the top andbottom ends and on the split line are more conveniently produced by this process Theprocess also has the added advantage of giving a good finish on the outer face Typical splitouter casings are shown in Figs 12.2 to 12.5

The shaped outer diameter on a centrifugal casting can save considerable amount oftime during machining and grinding Centrifugally spun castings also offer some distinct

FIGURE 12.3 Centrifugal spun casing with shaped outer cast

pro-file (Nixon, 1987).

FIGURE 12.2 Measurements on a split outer casing (Nixon, 1987).

advantages over the nonrotating, or static, process Progressive solidification occurs whenthe die is spinning Commencing at the inner wall, whether metallic or refractory, the solid-ification progresses radially from the outer edge toward the axis of rotation Foreign bod-ies and lighter inclusions are directed toward the bore initially by the centrifugal force, andthen by progressive solidification of the metal Columnar grains grow continually from theouter wall to the bore to give an unacceptable layer in the inner narrow zone, which is sub-sequently machined away With the conventional sand casting method, cooling proceedsfrom two faces, accompanied by the formation of columnar grains meeting toward the cen-ter of the wall of the casting All the unsoundness arising from shrinkage, tears, and inclu-sions remains in the region, and is carried into the finished component In general, a metaldie centrifugal casting meets ASTM E446 Class I radiography criteria, and a refractoryshape centrifugal casting falls in Class II, but in many circumstances the latter can qualify

to be in Class I

As with all fabrication procedures, the centrifugal casting requires good control on theshop floor Control starts with the metal to be melted The charge for the melt is selectivelycontrolled with both quality and economy in mind The castings generally undergo exten-sive machining operation, which at the end of the operation must yield high percentagereturns in turnings After chipping and degreasing, the turnings are returned to the furnace forreuse in another melt Prior to this, it is essential to segregate the materials to avoid contami-nation In the preparation of the die a mandatory requirement is to avoid lapped surfaces,

FIGURE 12.4 Centrifugal spun casting after final machining

(Nixon, 1987).

FIGURE 12.5 Compressor casing: as cast (left); machined,

assembled, bladed (right) (Nixon, 1987).

cracks, and hot tears This calls for the dies to be sprayed with a refractory slurry after aninitial preheat to 400°C in a stove The slurry aids in the flow of the metal along the surface

of the die and in preventing fusion with the die Refractory molds also similarly need theright mix of refractories and accurate control of the pattern and during molding Pouring ofprecise quantity of the metal into the casting is important If the quantity exceeds therequirements, the excess amount must be machined to the detriment of the cost of produc-tion Insufficient metal into the die, on the other hand, produces an unsound casing, sincethere is not enough material at the bore to satisfy the feeding requirements

Once the casting is extracted from the die, it is stamped to identify the cast number,material, and machine number on which it is cast The identifying numbers remain on thepart throughout its life and into the customer’s records There’s also an accompanying cardwith the casting details information on heat treatment, cast and machined size and weight,dies used, spinning speed, and other related details Emphasis on the correct spinning speed

is placed because the castings tend to be unsound when the centrifugal force is insufficient,causing deleterious inclusions to accumulate at the bore

Inspection segregates scrap castings before any work is performed on them Finalinspection of gas turbine engine components is routinely done by the red dye procedure,although the fluorescent dye line is more sensitive The dye line is in the shape of a horse-shoe, is split into seven stations, and can accommodate castings up to 120-in diameter.Defects are noted and compared with the requirements laid down by the industry, which inthe case of aircraft engines are near perfection A high proportion of castings is subjected

to radiographic examination, particularly for aircraft engines

12.3 INVESTMENT CASTINGS

The investment casting process has retained most of its features over the centuries in themaking of items of jewelry, with innovations introduced in applications for gas turbinecomponents made of superalloys A precise replica of the part is first produced in wax or aplastic polymer Compensation due to shrinkage of the component dimensions during thesequence of processing must be provided (Sims, Stoloff, and Hagel, 1987)

Where cooling channels are parts of the design, a preformed ceramic core of the same figuration is placed inside the cavity around which pattern material is injected For smallercomponents, the pattern may be duplicated and assembled into a cluster, with the cavities heldtogether and connected by a system of ducts through which liquid metal flows into individ-ual cavities The assembly of connected patterns is then dipped in a slurry of aqueous ceramicmaterial The fragile shell pattern is coated with a granular form of ceramic material to pro-vide a semblance of rigidity, and the steps may be repeated for this purpose The mold is thendried to eliminate all moisture content, followed by melting of the wax The mold is fired in

con-a furncon-ace for further strengthening Prior to ccon-asting, the configurcon-ation is wrcon-apped in con-an lation blanket to reduce loss of heat and to obtain a controlled rate of solidification After thecasting is cooled, the shell and the core are separated from the metal parts mechanically andchemically, and also split from the cluster at the connecting ducts The parts then go through

insu-inspections, heat treatment, or densification by the hot isostatic press (HIP) method.

The pattern for the preparation of the mold must precisely duplicate all the finer features

of the component to be cast, and hence a complex configuration such as a turbine airfoilwill require a special pattern The pattern must also have stable dimensions, have a smoothsurface, and permit its easy removal from the ceramic shell Urea-based compounds, poly-styrene, and synthetic wax combined with various resins are generally used for making pat-terns Strength, compatibility with ceramic shells and cores, little expansion or contraction,and economy are some factors in the selection of the material Patterns made from wax may

be produced by the low-temperature liquid injection process, while plastic patterns are made

by the injection molding method The patterns thus obtained often need to be reformed because

of thin walls by making small adjustments

Internal cooling passages and other features in hollow components are created withcores made of silica-based or alumina ceramics The core is fixed inside the wax patternduring the injection according to engineering requirements Room for relative thermalgrowth between the shell and the core restricts attachment between the two at a single point.Molten metal is poured into the cavity formed after the wax is melted around the core Theouter shell and the inner core are then separated from the casting

The core must have adequate strength to withstand the pressures of the flowing wax andmolten metal, be chemically resistant to the melt at a high temperature and be refractory toretain its configuration during the full cycle Larger and less complex ceramic cores are made

by the injection molding method The ceramic is mixed with a thermoplastic material, heated

in the barrel of the injection-molding machine, and forced into the core-shaped cavity of thedie The binder material hardens over a short time period, the machine ram retracts and the part

is removed from the die The binder separates during baking at a low temperature, and the core

is sintered The transfer molding method uses a thermosetting form of binding material andyields a more durable green core, so more complex shapes can be handled by this procedure.The core is subjected to a thermal shock, and must not crack or deform during the cast-ing The core is removed from the casting either mechanically or chemically Cores made

of materials with a high silica content enjoy the advantage of leaching by bases such assodium hydroxide and potassium hydroxide Acids may also be used if the alloy is notaffected To avoid the bases from attacking the intergranular region, the container material,caustic chemistry, and process parameters must be strictly controlled Larger cores may beseparated by air blasting with sand or glass beads

With adequate precautions to ensure the core does not shift or warp, cast parts can havewall thickness as low as 0.015 in and holes of 0.020-in diameter The length of the core mayexceed 15 in (Fig 12.6)

The shell of the mold is exposed to mechanical and thermal loads during the casting ation, but must not be excessively durable to make it difficult to separate the shell and cause

FIGURE 12.6 Ceramic core and cut section of turbine blade with cooling passages (Sims, Stoloff, and Hagel, 1987).

fracture of the casting as the metal freezes and shrinks Mismatch in the thermal growthbetween the metal and the ceramic shell during solidification and cooling can affect the qual-ity The mold shell is subjected to elevated temperatures in processes calling for directionalsolidification of the metal for a period of time, and must avoid consequent distortion.

A ceramic slurry made of a finely grained refractory material and a binder of silica and

a dry refractory grain are used to make the mold shell The slurry and the dry grain are nately applied to the wax pattern The assembly is first dipped in the slurry than immersed

alter-in a fluidized bed of the particles The balter-inder then cures by chemical reaction for furtherapplications The rate at which drying occurs must be controlled to avoid distortion A typ-ical shell mold may be coated 5 to 10 times to develop thickness and strength The ceramicslurry is made of a finer grain then the subsequent particles

Beside grain configuration, microstructural features and freedom from inclusions, asound casting requires controlled solidification, with sufficient time to permit the moltenmetal to flow into the geometry Increased melt and mold temperatures reduce the rate ofsolidification, and this in turn improves the quality of the casting Localized hot spotswhere the metal impinges on the shell walls result in porosity on the surface, and must beavoided by redirecting the flow The melt enters the turbine airfoil castings through the rootattachment Blades with a tip shroud have a separate gating to achieve the right form at thejunction of the airfoil and the shroud Larger turbine blades may also need a gate for localfeeding, but may not be advisable due to its impact on the surface finish and possible alter-ation of the airfoil’s dimensions Microshrinkage of the castings cannot be avoided as thematerial solidifies, but if it can be restricted to the region around the centerline of the partthen the HIP process may be able to eliminate it

A vacuum avoids the risk of oxidation of the reactive elements during the casting formost superalloys Some cobalt base alloys are cast in induction or indirect arc furnaces in thepresence of air Zirconia crucibles lined with silica are generally used with the preweighedcharge, introduced through a one-way device The temperature measured during the melt-ing is above that of the liquidus, and is critical in obtaining the proper grain size and orien-tation The melt is then poured at a controlled rate into the preheated mold and transferred

to the evacuated main furnace

Directionally solidified single or polycrystal castings are produced in special ment, where the mold is kept at a higher temperature than the liquidus of the alloy to becast The mold, open at the bottom, is placed on a chilling plate to obtain the proper ther-mal gradient, and is then retracted from the heater at a controlled rate The procedure results

equip-in the formation of a contequip-inuous graequip-in

A finer size of the grain produced by a relatively rapid rate of solidification results inimprovement in the tensile, fatigue, and creep properties at medium temperatures On theother hand, high-temperature rupture performance calls for slower solidification, and cool-ing rates aid in coarsening the grain size In turbine blades the airfoil operating at high tem-peratures require a coarse grain, but the heavier dovetail section are less rupture dependentand need a fine-grain microstructure The problem may be tackled by adding a gate at theedges of the airfoil for the melt to enter, and deliberately set up hot spots in the region toslow the solidification rate This practice may also be followed to delete the formation ofcolumnar grains at the edges

Casting defects appear in the form of inclusions, hot tears, and peculiar microstructuralfeatures, and are of special interest if they cannot be identified by nondestructive methods(Fig 12.7) Nonmetallic inclusions are easier to identify on thinner sections, and may becaused during the manufacture of the alloy or during casting Inadequate level of vacuum dur-ing remelting and casting and improper operating practice at the furnace can cause dross toadhere to the crucible wall prior to pouring Certain alloys such as INCO 718 require filteringthrough reticulated ceramic foam to alleviate the problem Hot tears appear when high tem-peratures in the casting lead to plastic strains, causing the just solidified material to split in