Transcript
Seminar Report on
Future Trends in Investment Casting
1
CERTIFICATE
This is to certify that the seminar report entitled “ Future Trends in Investment Casting ” submitted submitted by Mr. xxxx, in partial fulfillment of the requirements for the award of the degree of BACHELOR OF TECHNOLO! in Manu"a#turing Engineering is a bona fide seminar
work carried out by him under my guidance. In my opinion the work fulfills the requirements for which it is being submitted.
Place:
$r. xxxx
ate:
!ssociate Professor "orge ept.
2
CERTIFICATE
This is to certify that the seminar report entitled “ Future Trends in Investment Casting ” submitted submitted by Mr. xxxx, in partial fulfillment of the requirements for the award of the degree of BACHELOR OF TECHNOLO! in Manu"a#turing Engineering is a bona fide seminar
work carried out by him under my guidance. In my opinion the work fulfills the requirements for which it is being submitted.
Place:
$r. xxxx
ate:
!ssociate Professor "orge ept.
2
AC%NO&LE$EMENT
I sincer sincerely ely acknowled acknowledge ge the help and guidance guidance I recei# recei#ed ed from from Mr. Mano' %umar without which it would ha#e been difficult to complete this seminar work. $is constant encouragement and words of moti#ation ha#e been a source of inspiration for me. I e%press my sincere gratitude to him. I also e%press my gratitude to the seminar e#aluation committee for pro#iding me an opportunity to present this work.
%%%%% &. Tech Tech '(anufacturing )ngg.*
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Ta()e o" Contents Cover *age
+
Certi"i#ate
,
A#-no)edgement
/
Ta()e o" Contents
012
+. Introdu#tion
314
+.+. Histor5
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,. Mo Moderni6ing o" an An#ient Art
71+8
,.+. Modern $emands o" a Casting *ro#ess
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,.,. T9e Beginnings o" Investment Casting
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,./. &9at Materia)s are used in Investment Casting
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,.0. Examp)es o" Investment Casting
+8
/. In Investment Casting Mar-ets
++1+,
3.1. .1.
Expa Expand ndiing Mar Mark kets ets 11
3.2. 3.2.
The The Outl Outloo ook k for Inve Invest stme ment nt Cas Casti ting ng 11
3.3. 3.3.
The The Aut Autom omot otiv ive e Mar Mark ket 12
3.. 3..
Threat Threats s fro from m Alte Altern rnati ative ve Mater Material ials s 12
0. In Investment Casting *ro#ess
+/1,2
0.+. Overvie 0.,. *attern Materia)s
+/ +/
0.,.+. *attern &axes
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0.,.,. *)asti#s 0.,./. Ot9er *attern Materia)s 0./. *rodu#tion o" *atterns 0./.+. *attern $ies 0.0. *attern Assem()5 0.0.+. $esign o" *attern Tree or C)uster 0.2. *rodu#tion o" Cerami# S9e)) Mo)ds 0.2.+. Re"ra#tories 0.2.,. Binders 0.2./. Ot9er Cerami# S9e)) Constituents 0.2.0. S)urr5 *reparation 0.2.2. *attern Tree or C)uster *reparation
+2 +2 +2 +2 +: +: +3 +3 +7 +7 ,8 ,8
0.2.:. Coating and $r5ing 0.:. Cerami# Cores 0.3. Remova) o" *attern 0.4. Mo)d Firing and Burnout 0.7. Me)ting and Casting 7.+. Me)ting E;uipment 7.,. Casting Met9ods 0.+8. *ost1#asting Operations 0.++. Comparison o" Sand #ast and Investment Cast
,8 ,+ ,+ ,, ,/ ,/ ,/ ,0 ,:
2. $rivers "or Te#9no)og5
,3108
2.+. Environment 2.+.+ S9e)) Mou)ds 2.+., A))o5s 2.,. Engine E""i#ien#5 2.,.+ Titanium A))o5s 2.,., Supera))o5s 2.,./ Interna) Aero"oi) Coo)ing 2./. Costs 2.0. *ro#ess $eve)opment 2.0.+ Furna#e 2.0., $ire#t *atterns
Eva)uation 2.0.0 *ro#ess simu)ation 2.2. Future In")uen#es on t9e Investment Casting Industr5
:. Re"eren#es
,3 ,3 ,4 ,7 ,7 /8 /+ /, /0 /0 /4 /7 /7 08
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+. Introdu#tion Investment #asting is an industrial process based on and also called lost-wax casting , one of the oldest known metal+forming techniques. "rom ,--- years ago, when beeswa% formed the pattern, to todays high+technology wa%es, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, #ersatility and integrity in a #ariety of metals and high+performance alloys /0-1. +.+.
Histor5?
The history of lost+wa% casting dates back thousands of years. Its earliest use was for idols, ornaments and 2ewellery, using natural beeswa% for patterns, clay for the moulds and manually operated bellows for stoking furnaces. )%amples ha#e been found across the world in Pakistan3s $arappan 4i#ili5ation '6--76--- &4* idols, )gypt3s tombs of Tutankhamen '088870869 &4*, (esopotamia, !5tec and (ayan (e%ico, and the &enin ci#ili5ation in !frica where the process produced detailed artwork of copper, bron5e and gold. The oldest known e%amples of this technique are the ob2ects disco#ered in the 4a#e of the Treasure 'ahal (ishmar* hoard in southern Israel, and which belong to the 4halcolithic period '9--+8-- &4)*. 4onser#ati#e 4arbon 09 estimates date the items to c. 8;-- &4), making them more than ;-- years old /0-1. The earliest known te%t that describes the in#estment casting process was written around 00-!.. by Theophilus Presbyter, a monk who described #arious manufacturing processes, including the recipe for parchment. This book was used by sculptor and goldsmith &ene#ento 4ellini '0--70;0*, who detailed in his autobiography the in#estment casting process he used for the Perseus with the $ead of (edusa sculpture that stands in the =;. Its use was accelerated by ?illiam $. Taggart of 4hicago, whose 0=-; paper described his de#elopment of a technique. $e also formulated a wa% pattern compound of e%cellent properties, de#eloped an in#estment material, and in#ented an air+ pressure casting machine /001.
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In the 0=9-s, ?orld ?ar II increased the demand for precision net shape manufacturing and speciali5ed alloys that could not be shaped by traditional methods, or that required too much machining. Industry turned to in#estment casting. !fter the war, its use spread into many commercial and industrial applications that used comple% metal parts. The adoption of 2et propulsion for military and then for ci#ilian aircraft that stimulated the transformation of the ancient craft of lost wa% casting into one of the foremost techniques of modern industry /;1. In#estment casting e%panded greatly worldwide during the 0=>-s, in particular to meet growing demands for aircraft engine and airframe parts. Today, for e%ample in#estment casting is a leading part of the foundry industry, with in#estment castings now accounting for 0@ by #alue of all cast metal production in the AB /81.
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Fig. + Some Histori#a) t9ings made (5 Lost &ax *ro#ess @+8 @++
%
,. Moderni6ing o" an An#ient Art 4i#ili5ation at the start of the 60st century is dependent on the gas turbine engine for air transportation and a significant proportion of the worlds power generation. This has been the case for the past - years and is likely to remain the position for the ne%t - years unless unforeseen de#elopments, prompted by en#ironmental concerns, disco#er an alternati#e and economic source of power generation. It is not possible to o#er emphasi5e the synergy between the in#estment casting industry and the de#elopment of the gas turbine engine. The efficiency of these engines is directly related to the turbine operating temperature which in turn is controlled by both the aero+thermal design and material capability. $igh temperature materials by their #ery nature are difficult and almost impossible to form by mechanical working and their use is entirely due to the ability to manufacture comple% e%ternal and internal geometries by the in#estment casting process. $owe#er, such technology comes at a high financial cost with the latest single crystal alloys costing well in e%cess of C0-- D lb. This cost is a consequence of the use of increasingly scarce metallic elements such as rhenium and the uncertainty of supply of more abundant metals such as nickel. Eince it is unlikely that alloy de#elopment will produce less e%pensi#e materials, it is the responsibility of the in#estment caster to impro#e and de#elop the process to preser#e raw materials and reduce o#erall manufacturing costs. )n#ironmental concerns and fuel costs gi#e added pressure to de#elop the manufacturing capability to optimi5e the turbine aero+thermal efficiency with comple% and accurate geometries and to reduce the engine weight with thin section components /81. ,.+. Modern $emands o" a Casting *ro#ess
!gainst this background, attention turned to lost wa% casting to produce accurately shaped blades. In meeting this challenge, fi#e key problems had to be sol#ed: F F F F F
4astings had to be reproducible within close dimensional limits 4astings had to be produced in high melting point alloys There had to be high standards of metallurgical quality 4osts had to be lower than for alternati#e techniques. 4astings should ha#e less carbon foot print /;1.
,.,. T9e Beginnings o" Investment Casting
It was the successful solution of these problems that laid the foundation for the modern in#estment casting industry. Gne buoyant niche market relies on special techniques 'including hot isostatic pressing after casting* to produce components with fatigue strengths equal to forgings. 4astings can now be made for applications with oscillating stress /;1. ,./. &9at Materia)s are used in Investment Casting
1&
In#estment casting is used for a wide range of applications. Emall parts form the bulk of production, but #ery large components can also be made commercially. ickel and cobalt+base super alloys account for -@ of total output by #alue, steels account for 8@, aluminium accounts for about 0-@, and copper and titanium alloys make up large part of the remaining @ /;1. ,.0. Examp)es o" Investment Casting
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Fig. , )%amples of castings made by In#estment 4asting Process /1
/. Investment Casting Mar-ets !lthough the gas turbine industry is the ma2or customer for the in#estment casting industry it is by no means the only one. The medical and automoti#e industry account for a significant #olume of output, for e%ample the world market for turbochargers in 6--; was 6- million wheels. The 4!)" '4ommittee of !ssociations of )uropean "oundries* recei#e returns from member foundries to enable the 4!)" to prepare market statistics. "or 6--; the output in castings by #alue are shown. Internationally the total world output in 6--; was C0-.6 &illion of which 8;@ was from the AE! and 88@ from !sian countries 'ref 0*. !lthough the gas turbine market represents the largest customer in )urope and the AE!, the market in !sia has a substantial customer base for commercial 'e.g. pumps and #al#es* and automoti#e parts /81. Ta()e + )uropean In#estment "oundry Ehipments 'year ending 6--;* /61
3.1.
Expanding Markets 12
In#estment casting markets increased steadily o#er 9- years to the mid 0=>-s, when demands for new aircraft boosted both the sales of in#estment castings and industry capacity. The market continued e%panding up to 0==-, when annual worldwide turno#er reached about H8,--- million. G#er the ne%t few years, aerospace and defense demands dropped and output decreased, in some countries by as much as 0+6-@. Eince 0==9, howe#er, general commercial demand has picked up and with recent aircraft orders turno#er is approaching its pre#ious high. &ased on data published in early 0==, annual world turno#er is estimated as H6,>-- million. orth !merica 'essentially the AE* accounts for about half, ?estern )urope a quarter, and the Pacific Jim countries 6-@. ?ithin ?estern )urope, the AB remains the biggest national producer with output of H8-- million, followed by "rance 'H0> million* and Kermany 'H08 million*. &ritain is home to about - of the 06 or so western )uropean in#estment casting foundries, employing roughly ,-- people in an industry which leads the )uropean community /;1.
3.2.
The Outlook for Investment Casting
In#estment casting is reco#ering lost ground. 4ommercial business is up, land+based power generation demand is buoyant and the aircraft market is increasingly acti#e. Krowth can be e%pected until the end of the century and beyond if the industry can promote itself well enough /;1.
3.3.
The Automotive Market
)uropean in#estment foundries should look to opening up the automoti#e market to in#estment castings. Jealistic pricing of standard parts, howe#er, can only come with sufficient automation to deal cost effecti#ely with demand. &ut foundries will only in#est in such equipment when orders are secured. Industry competition + particularly price competition + is intense and likely to get tougher. The process ser#es an international market in which there are growing imports from one region to another /;1.
3..
Threats from Alternative Materials
Gther materials and processes 'especially other precision casting processes* pose threats. Euperalloys ha#e remarkable temperature capabilities through optimi5ation of composition, ad#anced processing techniques and comple% internal cooling, but further de#elopments are limited by the melting points of nickel and its alloys. To an industry so reliant on cast nickel+base superalloys, aluminide intermetallics, o%ide dispersion strengthened materials and engineering ceramics are a serious threat /;1.
13
0. Investment Casting *ro#ess 0.+.
Overvie?
1
Fig. / &asic In#estment 4asting Process G#er#iew /1 0.,. *attern Materia)s
Pattern materials currently in use are wa%es, and plastics, while other pattern materials are used sometimes, and for specific applications. ?a%es, blended and de#eloped with different compositions, are more commonly used, while use of plastic patterns, generally polystyrene, may sometimes be required, to produce thin+ walled, comple% +shaped castings, such as in aerospace integrally cast turbine wheels and no55les /01. 0.,.+. *attern &axes
?a%es are mostly the preferred material for patterns, and are normally used, modified and blended with additi#e materials such as plastics, resins, fillers, antio%idants, and dyes, in order to impro#e their properties. Paraffins and microcrystalline wa%es are the most widely used wa%es, and are often used in combination, because their properties tend to be complementary. Paraffin wa%es are a#ailable in many controlled grades, with melting points ranging from 6 to > L4 '06 to 0 L"*. They are readily a#ailable in different grades, ha#e low cost, high lubricity and 1!
low melt #iscosity. Their usage is, howe#er, limited because of high shrinkage and brittleness. (icrocrystalline wa%es tend to be highly plastic and pro#ide toughness to wa% blends. !#ailable in both hard, non+tacky grades as well as soft, adhesi#e grades, they ha#e higher melting points, and are often used in combination with paraffin. Gther wa%es used include: 4andelilla, a #egetable wa%, which is moderately hard and slightly tacky. 4arnauba wa% is a #egetable wa% with higher melting point, low coefficient of thermal e%pansion, and is #ery hard, non+tacky and brittle. &eeswa% is a natural wa%, widely used for modeling, and in pattern blends, pro#ides properties similar to microcrystalline wa%es. ?a%es, in general, are moderately priced, and can easily be blended to suit different requirements. ?a%es ha#e low melting points and low melt #iscosities, which make them easy to blend, in2ect, assemble into tree+ or cluster+assemblies, and melt out without cracking the thin ceramic shell molds. 4.2.1.1. Additives to Pattern Waxes ?a%es with their many useful properties are, howe#er, deficient in two practically important !reas: 'a* Etrength and rigidity especially required to make fragile patternsM and 'b* imensional control, especially in limiting surface ca#itation due to solidification shrinkage, during and after pattern in2ection. !dditi#es are made to wa%es to cause impro#ements needed in these two deficient areas. The strength and toughness of wa%es are impro#ed by the addition, in required #olumes, of plastics such as polyethylene, nylon, ethyl cellulose, ethylene #inyl acetate and ethylene #inyl acrylate. Eolidification shrinkage causing surface ca#itation in wa%es, is reduced to some e%tent by adding plastics, but is reduced to a greater e%tent by adding resins and fillers. Jesins suitable for this are: coal tar resins, #arious rosin deri#ati#es, hydrocarbon resins from petroleum and tree+ deri#ed resins such as dammar, &urgundy Pitch, and the terpene resins.
"illers are powdered solid materials, and are used more selecti#ely in wa%es than resins. This leads to the description of pattern wa%es as being either filled or unfilled. "illers ha#e higher melting point and are insoluble in the base wa%, thereby contributing to reduced solidification shrinkage of the mi%ture, in proportion to the amount used. "illers that ha#e been de#eloped and used in pattern wa%es include: spherical polystyrene, hollow carbon microspheres, and spherical particles of thermosetting plastic. 4.2.1.2. Factors for Pattern Wax Selection Process factors while selecting and formulating wa% pattern materials, that must be addressed are listed below, grouped with the material properties required or to be considered: In2ection: "ree5ing range, softening point, ability to duplicate de tail, setup time. Jemo#al, handling, and assembly: Etrength, hardness, rigidity, impact resistance, weldability. imensional control: Eolidification shrinkage, thermal e%pansion, ca#itation tendency. Ehell mold making: Etrength, wettability, and resistance to binders and sol#ents. ewa%ing and burnout: Eoftening point, #iscosity, thermal e%pansion, and ash content. (iscellaneous: !#ailability, cost, ease of recycling, to%icity, and en#ironmental factors.
1"
0.,.,. *)asti#s
Plastic is the most widely used pattern material, ne%t to wa%. Polystyrene is usually used, because it is economical, #ery stable, can be molded at high production rates on automatic equipment, and has high resistance to handling damage, e#en in e%tremely thin sections. Ase of polystyrene is howe#er limited, because of its tendency to cause shell mold cracking during pattern remo#al, and it requires more e%pensi#e tooling and in2ection equipment than for wa%. $owe#er, the most important application for polystyrene is for delicate airfoils, used in composite wa%+plastic integral rotor and no55le patterns, assembled using wa% for the rest of the assembly /01. 0.,./. Ot9er *attern Materia)s
"oamed Polystyrene has long been used for gating system components. It is also used as patterns with thin ceramic shell molds in a separate casting process kn own as Jeplicast Process. Area+based patterns, de#eloped in )urope, ha#e properties similar to plasticsM they are #ery hard, strong and require high+pressure in2ection machines. Area patterns ha#e an ad#antage o#er plastics: they can easily be remo#ed, without stressing the ceramic shell, by simply dissol#ing in water, or an aqueous solution. 0./. *rodu#tion o" *atterns
Patterns are usually produced by in2ecting pattern material in to metal dies, made with one or more ca#ities of the desired shape, in each die. ifferent equipments, with different operating parameters, ha#e been de#eloped to suit different pattern materials. ?a% patterns are in2ected at lower temperatures, '00- to 0;- L"*, and pressures, '9- to 0-- psi*, in split dies using specially designed equipment. The wa% in2ection equipment ranges from simple pneumatic units, to comple% hydraulic machines, which can accommodate large dies, and at high in2ection pressures. Polystyrene patterns are in2ected at higher temperatures, '8- to -- L"*, and pressures, '9 to 6ksi*, in hydraulic machines, normally equipped with water cooled platens that carry the die hal#es /01. 0./.+. *attern $ies
Narious pattern tooling options are a#ailable for wa%es because of their low melting point and good fluidity. (any die materials are used, including: rubber, plastic, plaster, metal+filled plastic, soft lead+bismuth tin alloys, aluminum, brass, bron5e, beryllium copper, steel or a combination of these. The selection is based on considerations of cost, tool life, deli#ery time, pattern quality, and production efficacy in a#ailable patternmaking equipment. Plastic patterns usually require steel or beryllium copper tooling. Pattern dies made b y machining 1#
use 44 'computer numerical controlled* machine tools and electric discharge machining. !lternati#ely, cast tooling made in aluminum, steel or beryllium copper is also used effecti#ely. ?a% can be cast against a master model to produce a pattern, which is then used to make an in#estment cast ca#ity for this type of cast tooling /01. 0.0. *attern Assem()5
Patterns for in#estment casting produced in dies are prepared for assembly in different ways. -@ Elurries are prepared by adding refractory powder to binder liquid, using agitation to break up agglomerates, remo#e any air entrainment. Etirring is continued until #iscosity falls to its final le#el before the slurry is put to use. 4ontinued stirring is also required in production to keep the powder from settling out of suspension. )ither rotating tanks with baffles or propeller mi%ers are used for this purpose. ontrol )rocedures for slurries #ary considerably among foundries. The most pre#alent controls are the measurement of the initial ingredients, slurry temperature, density, p$ and #iscosity. Niscosity is measured with a o.9 or ahn cup, or a &rookfield type rotating #iscometer. Properties of the finished ceramic shells that are monitored include: weight, modulus of rupture 'green and fired*, and permeability /01. 0.2.2. *attern Tree or C)uster *reparation
&efore dipping, pattern trees or clusters are usually cleaned to remo#e in2ection lubricant, loose pieces of wa%, or dirt. 4leaning is accomplished by rinsing the pattern clusters in solution of wetting agent, or a suitable sol#ent that does not attack the wa%. The trees, or clusters, are usually allowed to return to room temperature and dry, before dipping. 0.2.:. Coating and $r5ing
ipping, draining, and stuccoing of clusters are carried out manually, robotically, or mechanically. "oundries are increasingly using robots in order to heighten producti#ity, to process larger parts and clusters, and to produce more uniform coatings. ?hen robots are introduced, they are often programmed to reproduce actions of skilled operators. edicated mechanical equipment can sometimes operate faster, especially with standardi5ed clusters. (ost dipping is done in air, but dipping under #acuum has been found, in some limited applications, #ery effecti#e for coating narrow passageways and for eliminating air bubbles. The cleaned wa% cluster is dipped into the prime slurry and rotated. It is then withdrawn and drained o#er the slurry tank with suitable manipulation to produce a uniform coating. e%t the stucco particles are applied by placing the cluster in a stream of particles falling from an o#erhead screen in a rainfall sander, or by plunging the cluster into a fluidi5ed bed of the particles. In the fluidi5ed bed, the particles beha#e as a boiling liquid, because of the action of pressuri5ed air passing through a porous plate in the bottom of the bed. Kenerally, prime slurries contain finer refractory powder, are used at a higher #iscosity, and are stuccoed with finer particles than the backup coats. These characteristics pro#ide a smooth surfaced mold, capable of resisting metal penetration. &ackup coats are formulated to coat readily o#er the prime coats 'which may be somewhat porous and absorbent*, to pro#ide high 22
strength, and to build up the required thickness with a minimum number of coats. The number of coats required is related to the si5e of the clusters and the metal weight to be poured. It may range from for small clusters, to 0 or more for large ones. "or most applications, the number ranges from to =. &etween coats, the slurries are hardened by drying or gelling. !ir drying at room temperature with circulating air of controlled temperature and humidity is the most common method. rying is usually carried out on open racks or con#eyors, but cabinets or tunnels are sometimes used. rying is complicated by the high thermal e%pansion and contraction characteristics of wa%es. If drying is too rapid, the chilling effect causes the pattern to contract, while the coating is still wet and unbonded. Then, as the coating is de#eloping strength and e#en shrinking, the wa% begins to e%pand, as the drying rate declines and it regains temperature. This can actually crack the coating. Therefore, to pre#ent this, relati#e humidity is normally kept abo#e 9-@, usually at a recommended #alue of -@ /01.
0.:. Cerami# Cores
4eramic cores are widely used in in#estment castings to produce internal passageways in castingsM and cores are either self+formed or preformed. a. Self-formed cores are produced during the mold building, with the wa% patterns already ha#ing corresponding openings. (etal pull cores in pattern tooling are used for simple shapes, while soluble cores for other shapes are made and placed in the pattern tooling, and the pattern in2ected around them. The soluble core is then dissol#ed out in a solution that does not affect the wa% pattern, such as an aqueous acid. 4itric acid is used commonly. (. Preformed cores are required when self+formed cores can not be used, and are produced by a number of ceramic forming processes. Eimple tubes and rods are commonly e%truded from silica glass. (any cores are made by in2ection molding of fine ceramic powder with a suitable organic binder into steel dies and sub2ecting the cores to a two stage heat treatment. In the second stage, the core is sintered to its final strength and dimensions. Preformed cores are normally used by placing in pattern die and in2ecting wa% around them. Eince cores e%pand differently than the shell molds, due to their differences in composition, cores must be pro#ided with slip 2oints in the mold. 0.3. Remova) o" *attern
Pattern remo#al is the operation that sub2ects the shell mold to the most stresses, since the thermal e%pansion of wa%es are many times those of refractories used for molds. ?hen the mold is heated to liquefy the wa%, the e%pansion differential leads to enormous pressure that is capable of cracking, or e#en destroying the mold. In practice, this problem is effecti#ely circum#ented by heating the mold e%tremely rapidly from the outside in. This causes the surface layers of wa% to melt #ery quickly, before the rest of the pattern can heat up appreciably. This molten wa% layer either melts out of the mold or soaks into it, thus pro#iding the space to accommodate the e%pansion as the remainder of the wa% is heated. (elt+out tips are sometimes pro#ided, or holes are drilled in the shell to relie#e wa% pressure. )#en with these techniques, the shell is sub2ect to high stress. To get the shell as strong as possible, it should be thoroughly dried before dewa%ing. Ehells are sub2ect to 0 to 9> h of e%tended drying after the last coat, sometimes enhanced by the 23
application of #acuum or e%tremely low humidity. Two methods ha#e been de#eloped to implement the surface melting concept: autocla#e dewa%ing and high temperature flash dewa%ing. Autoclave dewaxing is the most widely used method. Eaturated steam is used in a 2acketed #essel, with a steam accumulator to ensure rapid pressuri5ation. !utocla#es are equipped with a sliding tray to accommodate a number of molds, a fast acting door with a safety lock, and an automatic wa% drain #al#e. Gperating pressures of appro%imately - to 6- kPa '>- to =- psig* are reached in 9 to ; s. (olds are dewa%ed in appro%imately 0 min. or less. ?a% reco#ery is good. Polystyrene patterns cannot be melted out in the autocla#e, but require flash dewa%ing. Flas" dewaxing is carried out by inserting the shell into a hot furnace at >;- to 0-= L4 '0-to 6--- L"*. The furnace is equipped with an open bottom so that wa% can fall out of the furnace as soon as it melts. Eome of the wa% begins to burn as it falls, and e#en though it is quickly e%tinguished, there is greater potential for deterioration than with an autocla#e. e#ertheless, wa% from this operation can be reclaimed satisfactorily. "lash dewa%ing furnaces must be equipped with an afterburner in the flue or some other means to pre#ent atmospheric pollution. Polystyrene patterns are readily burned out in flash dewa%ing. $owe#er, polystyrene can cause e%tensi#e mold cracking, unless it is embedded in wa% in the pattern 'as in integral no55le patterns*, or unless the polystyrene patterns are #ery small. *ot li%uid dewaxing has found some use among smaller companies seeking to minimi5e capital in#estment. $ot wa% at 0;; L4 '8- L"* is often used as the medium, while other liquids can also be used. 4ycles are longer than for autocla#e and flash dewa%ing, and there is potential fire ha5ard /01. 0.4. Mo)d Firing and Burnout
4eramic shell materials are fired to remo#e moisture 'free and chemically combined*, to burn off residual pattern material and any organics used in the shell slurry, to sinter the ceramic, and to preheat the mold to the temperature required for casting. In some cases, these are accomplished in a single firing. Gther times, preheating is performed in a second heating, after the mold is cooled down, inspected and repaired if necessary. 4racked molds can be repaired with ceramic slurry or special cements. (any molds are wrapped with a ceramic+fiber blanket at this time to minimi5e the temperature drop that occurs between the preheat furnace and the casting operation, or to pro#ide better feeding by insulating selected areas of the mold. Kas fired furnaces are used for mold firing and preheating, e%cept for molds for directional solidification processes, which are preheated in the casting furnace with induction or resistance heating. &atch and continuous pusher+type furnaces are most common, but some rotary furnaces are also in use. &urnout furnaces operate with temperatures between >;- and 0-= L4 '0-- and 6--- L"*, and ha#e some 0-@ e%cess air pro#ided to ensure complete combustion of organic materials. Preheat temperatures #ary depending on part configuration and the alloy to be cast. 4ommon ranges are: 0- to 9- L4 '8-- to 0--- L"* for aluminum alloys, 96 to >;- L4 '>-- to 0-- L"* for many copper base alloys, and >;- to 0-= L4 '0-- to 6--- L"* for steels and superalloys. (olds for the directional solidification process are preheated abo#e the liquidus temperature of the alloy being cast /01. 2
0.7. Me)ting and Casting
ifferent types of equipment are currently in use for melting, and support different casting methods adopted. 0.7.+. Me)ting E;uipment
oreless t#)e +nduction furnaces are used with capacities ranging from 0 to ;- lb., with normal melting rates of 8 lbDmin. They are usually tilting models, and can be employed for melting in air, inert atmosphere or #acuum. They are e%tensi#ely used for melting steel, iron, cobalt and nickel alloys, and sometimes copper and aluminum alloys. as- fired crucile furnaces are used for aluminum and copper alloy castings, while electrical resistance furnaces are sometimes preferred for aluminum casting, since they help reduce hydrogen porosity. The crucibles typically used are magnesia, alumina and 5irconia, which are made by slip casting, thi%otropic casting, dry pressing, or isostatic pressing. (agnesium alloys can be melted in gas+ fired furnaces using low+ carbon steel crucibles /01. 0.7.,. Casting Met9ods
&oth air and #acuum casting methods are used in in#estment casting. There is some use of rammed graphite molds in #acuum arc furnaces for casting titanium. (ost castings are gra#ity poured. Air casting is used for many in#estment+cast alloys, including aluminum, magnesium, copper, gold, sil#er, platinum, all types of steel, ductile iron, most cobalt alloys, and nickel+base alloys that do not contain reacti#e elements. inc alloys, gray iron and malleable iron are usually not in#estment cast for economic reasons. acuum casting pro#ides cleaner metals with superior properties and is used for alloys that can not be cast in air, such as the U+strengthened nickel base alloys, some cobalt alloys, titanium and the refractory metals. &atch and semicontinuous interlock furnaces are normally used. ! ma2or ad#antage of in#estment casting is its ability to cast #ery thin walls, due to the use of hot mold. This ad#antage is further enhanced by specific casting methods, such as #acuum+assist casting, pressuri5ed casting, centrifugal casting and countergra#ity casting. In vacuum-assist casting, the mold is placed inside an open chamber, which is then sealed with a plate and gaskets, lea#ing only the mold opening e%posed to the atmosphere. ! partial #acuum is drawn within the chamber and around the mold. The metal is poured into the e%posed mold opening, and the #acuum ser#es to e#acuate air through the porous mold wall and to create a pressure differential on the molten metal, both of which help to fill delicate detail and thin sections. In )ressuri/ed casting, rollo#er furnaces are pressuri5ed for the same purpose. The hot mold is clamped to the furnace+top with its opening in register with the furnace opening, and the furnace is quickly in#erted to dump the metal into the mold, while pressure is applied using compressed air or inert gas. 2!
entrifugal casting uses the centrifugal forces generated by rotating the mold to propel the metal and to facilitate filling. Nacuum arc skull furnaces discharge titanium alloy at a temperature 2ust abo#e its melting point, and the centrifugal casting is usually needed to ensure good filling. ental and 2ewelry casting use centrifugal casting to fill thin sections and fine detail. ountergravit# casting assists in filling thin sections, by applying a differential pressure between molten metal and the mold. This technique de#eloped for o#er 8- years, works effecti#ely in air or under #acuum, for air melted and #acuum melted alloys, to produce castings in aluminum and nonferrous alloys, many types of steels and superalloys, in weights from a few grams to 6- kg '99 lb*. ountergravit# $ow-Pressure Air 0$A Process has the preheated shell mold, with an e%tended sprue, placed in a chamber abo#e the melt surface of an air melted alloy. The sprue is lowered to below the melt surface, #acuum applied to mold chamber to cause controlled filling of the mold. The #acuum is released after castings and in+gates solidify, causing molten metal in the central sprue to return to the melt crucible, for use in the ne%t cycle. &esides substantial sa#ings in alloy usage and impro#ed gating efficiency, the other benefits from the process include impro#ed casting quality with reduced dross and slag inclusions. ountergravit# $ow-Pressure acuum 0$ Process is similar to 4 and are now implemented, foundries are required to ensure that the chemical substances used in their process are registered with J)!4$. These regulations do not apply to castings made outside the ).A. In recent years there ha#e been many efforts to find a use for mould scrap after cast. In certain )uropean countries it is difficult to dispose of 5ircon containing moulds because of the radioacti#ity concerns, also some countries do no permit the disposal of shells in land fill sites which contain hea#y metals such as cobalt aluminate used for grain refining. Ee#eral attempts to obtain research grants from the ).A. to in#estigate uses for recycled mould scrap ha#e been made but without success. $owe#er, the industry continues to look for alternati#e uses and with the increased disposal costs it is likely that a solution will e#entually be found /81. 2.+., A))o5s The price of superalloys for gas turbines and turbochargers depends on a#ailability of raw materials and market conditionsM prices for nickel, cobalt, tantalum, molybdenum ha#e been #olatile in recent years and rhenium, which is an essential metal for single crystal alloys, is currently o#er C9--- Dkg. There is no natural ore for rhenium and the price is likely to continue to rise since there is an increasing demand for single crystal alloys. Eince 6--- the price for rhenium has undergone an eight fold increase. The quality of single crystal castings is dependent on the quality of the master heat. Antil recently #irgin alloy has been the preferred master heat condition. The increasing cost of master heat has forced the foundries into using recycled material as mi%ed #irginDre#ert as -D9- blends. "or E casting, 0--@ re#ert is currently in foundry trials. The cost of scrap superalloy on the open market is appro%imately ;@ of #irgin, 3&
therefore foundries rely where#er possible on returning scrap to be remelted and refined as toll melt alloy. Eingle crystal material is sensiti#e to nitrogen le#el in the master heat and it is essential that the re#erted alloy 'or blend heats* do not contain unacceptable le#els of nitrogen 'Vppm* /81.
Fig. 3 Eingle 4rystal with high angle boundaries in a casting with high nitrogen content /81 2.,. Engine E""i#ien#5
Turbine efficiency is go#erned by the gas temperature and aerothermal efficiency of the design. Turbine entry temperatures are typically around 09--4 but are e%pected to increase to o#er 0;---4 in the future. Turbine blades are also e%pected to last for around 8--- flights which represents around 8-,--- hours for a long haul flight. "or materials to sur#i#e for this length of time in an o%idi5ing and corroding atmosphere at a temperature in e%cess of the melting point requires the blade to be substantially cooled. This is achie#ed by internal cooling and by creating a film of cooling air around the surface of the aerofoil. !round ;-@ of the cooling is from internal cooling with air taken from the engine compressor. It follows that to impro#e the engine and fuel efficiency it is necessary to run the turbine at the highest temperature possible, this leads to both alloy and cooling design de#elopment. ?eight is also a #ery important aspect of aeroengine design and this leads to the de#elopment of lightweight high temperature materials and thin section components /81. 2.,.+ Titanium A))o5s
Titanium as Ti D9 has been cast in production for o#er 6- years and current applications include fan casings of o#er 6 metres diameter. Titanium is highly reacti#e in the molten state and the casting process uses inert shell systems based on 5irconia or yttria. To a#oid contact with refractory ceramic crucibles the alloy is melted using direct or cold crucible methods. Eince titanium castings ha#e an o%ide layer at the surface 'alpha layer* from una#oidable shell reaction, it is necessary to remo#e this layer by chemical etching. This process can also be used to reduce the section thickness and by careful masking techniques the casting sections can be selecti#ely reduced in thickness. The quest for lighter engines has led to the de#elopment of the titanium aluminide intermetallicsM for e%ample the T& alloys 'Ti, 9!l, +0-b, -.64, -.6& atomic @*. These alloys together with their processing 'moulding, melting, casting, and finishing* are currently under de#elopment funded by the ).A. I(PJ)EE pro2ect. Ti!l alloys ha#e about @ density of Ti alloys and they can be used up to ;-L4. This material is normally thermo+ mechanically processed howe#er the cost is high. This pro2ect is aimed at de#eloping Ti!l casting alloys but to pro#ide the cast Ti!l with mechanical properties which are equi#alent those of thermo+mechanically processed /81. 31
Fig. 4 )%ample of titanium compressor casing with 0mm wall section /81. 2.,., Supera))o5s
The history of superalloy de#elopment is well documented and the paper by 4hester Eims gi#es an e%cellent re#iew up to the introduction of single crystal. Jecent de#elopments ha#e included the de#elopment of the so called second generation superalloys containing 8@ rhenium in the 0==-s. uring the 0==-s the importance of sulphur on o%idation resistance was in#estigated. These in#estigations found that sulphur at the relati#ely low le#el of 6ppm was sufficient to disrupt the protecti#e aluminium o%ide layer on superalloys. "urther work showed that yttrium additions to the alloy were effecti#e in pre#enting sulphur migrating to the surface by forming yttrium o%ysulphide. !t the 6ppm sulphur le#el it was necessary to ha#e at least 6ppm of yttrium to effecti#ely contain the sulphur. Anfortunately there are a number of problems with yttrium, the element forms a low melting point eutectic with nickel and any le#el o#er -ppm can result in the formation of incipient melting. The other main problem is the containment of yttrium in the casting, yttrium will dissol#e in the shell and core and it is necessary to make additions of up to --ppm of yttrium in the melt to retain 6ppm in the casting. The control of this process is therefore #ery difficult and necessitates the use of low silica shells and alumina cores. The problem of containing adequate le#els of yttrium has been significantly helped with the de#elopment of #ery low sulphur master heat. The process for making #ery low sulphur heats has been greatly impro#ed and it is now possible to produce master heats with W-.ppm sulphur. ?ith this le#el of sulphur it is only necessary to retain yttrium at a 0-+0ppm at which le#el it is possible to cast using con#entional ceramics. uring the 0==-s, 8rd generation superalloys were de#eloped, these were characteri5ed by ha#ing Je content of @. These alloys were de#eloped for their high temperature creep properties but had the disad#antage of being #ery difficult to solution heat treat. Eingle crystal superalloys owe their high temperature properties to their ability to be solution heat treated to homogeni5e the structure and refine the second phase by sophisticated ageing heat treatments. The heat treatment difficulty with 8rd generation EX alloys and to increase further the temperature capability has led to the de#elopment of alloys containing platinum group metals, in particular ruthenium which had the ad#antage of increasing strength, 32
impro#ing heat treatability and reducing the tendency to form undesirable T4P phases. These alloys are now generally referred to as 9th generation single crystal alloys /81. 2.,./ Interna) Aero"oi) Coo)ing
!s mentioned pre#iously, ad#anced gas turbines operating with a gas entry temperature in e%cess of 09-4 require cooling of at least --4 to bring the aerofoil temperature down to a le#el in which the blade can sur#i#e 8-,--- hours. ;-@ of this cooling is obtained by passing cold compressor air through comple% passages in the aerofoil section. The ceramic cores used to produce these passages must be compatible with the casting process: + They must be supported during the wa% in2ection process so as to maintain the wall section in the wa% pattern and not to be displaced during the dewa% operation. + They must not react with the alloy, + They must be chemically remo#ed with a caustic solution + They must be dimensionally compatible with the shell mould + They must not distort during the casting process The location of a core in the wa% impression die has also e#ol#ed o#er many years, originally the core was printed at both ends and located in the die and held firmly by the prints. In recent years the established practice is to use si% point nesting where the core is allowed to fairly align within the die using location points which also ser#e as inspection points. Pins are placed in the die to locate the core on the nesting points, alternati#ely plastic chaplets placed on the surface of the core can be used for location. "or single crystal casting the core chemistry is designed to pre#en t sintering at the casting Temperature 'e.g. 0--4*. ! core which sinters will ha#e too much strength at the solidification temperature and cause e%cessi#e stress to be retained by the casting with the result that the casting will recrystalli5e when solution heat treated. !n added issue with single crystal cores is that the cores are fired during manufacture at a lower temperature 'e.g. 00---4* than the mould temperature at which metal is cast. The core therefore has a relati#ely low strength during the phase of the process where the mould temperature is raised to the casting temperature. To pre#ent unacceptable core mo#ement during this phase, cores are supported in the mould with platinum pins. These pins instantly dissol#e in the alloy when the metal is poured. "uture aerofoil designs will in#ol#e the concept of wall cooling in which cooling air is passed through the aerofoil wall. These designs are confidential but will require inno#ati#e core technology to create the necessary casting configuration to achie#e o#er >-- degrees of cooling /01.
33
Fig. 7 Turbine rotor blade core /81
Fig. +8 o55le Kuide Nane 4ore /81
2./. Costs
Production costs are continually under re#iew, in particular the automoti#e industry is constantly impro#ing its cost base. Turbochargers represent the largest market for the in#estment casting industry and alloy costs and conser#ation are at the forefront of cost reduction. The Trucast process is widely used by the industry, whereby the centre sprue of the tree is directly reused for the alloy melting stock. Gpportunities for the re+use of shell material is frequently under consideration by research Grgani5ation, unfortunately, efforts to secure ).A. funding to carry out e%tensi#e in#estigations ha#e not been successful to date. 1
2.0./ Non $estru#tive Testing>Eva)uation
It is likely that the industry can look forward to many significant ad#ances in T and dimensional inspection. Jegarding T, micro focus %+ray is well established and used to detect core position and micro cracks in both cores and castings. igital %+ray is becoming increasingly established although concerns regarding standards ha#e yet to be totally resol#ed. ! possible future de#elopment is the concept of eyes off technology to detect and sentence defects using ad#anced #ision technology. Trials are underway to use this technology to detect "PI indications and automatically sentence the part as either re2ect, sal#ageable or acceptable. imensional inspection using laser technology to measure geometries has been used e%tensi#ely for re#erse engineering and is now becoming increasingly used for con#entional measurements /81. 2.0.0 *ro#ess simu)ation
Process modeling to simulate mould fill and solidification has become an established aid for (anufacture. "ully integrated systems which use process modeling to assist with wa% impression and core tooling design, gating system design and the definition of casting parameters ha#e yet to be fully implemented but remain a distinct possibility for the future. igital technology and computing power are fore#er increasing and the future for the industry will be totally influenced by the de#elopment of these technologies /81. 4onsiderable de#elopment efforts ha#e been made to pro#ide many solidification simulation models of #alue in in#estment casting production. 4urrently, alternati#e computer simulation software systems are a#ailable applying heat transfer models, based either on finite element or finite difference methods. These are being utili5ed on the shop floor in many larger foundries, especially in the design of gating and feeding systems, to determine effect of solidification 1
conditions on alloy microstructure, and for accurate predictions of tooling dimensions. The use of simulation models plays a ma2or role in the de#elopment of in#estment casting process for gas turbine blades, specified with equia%ed grains, E 'directionally solidified* columnar grains, or with single crystal, in many super alloys. !dditionally, rapid ad#ancements in the solidification software show continual impro#ement in the ability to predict accurately many grain defects that can occur in the production of directionally solidified, E, or single crystal components /01.
2.2. Future In")uen#es on t9e Investment Casting Industr5
In the financial in#estment industry it is said about those who predict the future, that there are those who dont know and those who dont know they dont know. The same could be said about the in#estment casting industry but it may be possible to be a little more certain than the financial Industry. $ere, therefore, are a few possible predictions: F
F F F F F
F
Jaw materials will become increasingly e%pensi#e and in some cases the earths limited resources will restrict their use without effecti#e recycling. This will be increasingly true of metallic elements such as rhenium. Ehortages of non metallic raw materials will cause the industry to in#estigate alternati#e materials $ealth and safety regulations will place increasing demands on manufacturing and may restrict the use of materials currently in common use. )ngineering demands will place greater emphasis on dimensional control and geometric comple%ity. 4ost consciousness will increase demands for less human in#ol#ement and greater automation. Increasing computing power and knowledge base systems such as neural networks will beincreasingly important to the industry. It is possible to en#isage a future foundry where the process is defined by knowledge systems and controlled by e%pert systems cognisant with defect detection and correction. The factory would be self learning and self correcting with limited human interaction. In the meantime the in#estment casting industry will continue to grow and remain one of the essential strategic industries for the world /81.
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