Seminar Report onFuture Trends in Investment Casting 1 CERTIFICATE This is to certify that the seminar report entitled “Future Trends in Investment Casting” submitted by Mr. xxxx, in partial fulfillment of the requirements for the award of the degree of BACHELOR OF TECHNOLOGY in Manufacturing 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: Dr. xxxx Date: Associate Professor Forge Dept. 2 ACKNOWLEDGEMENT I sincerely acknowledge the help and guidance I received from Mr. Manoj Kumar without which it would have been difficult to complete this seminar work. His constant encouragement and words of motivation have been a source of inspiration for me. I express my sincere gratitude to him. I also express my gratitude to the seminar evaluation committee for providing me an opportunity to present this work. xxxxx B. Tech (Manufacturing Engg.) 3 Table of Contents Cover Page 1 Certificate 2 Acknowledgement 3 Table of Contents 4-5 1. Introduction 7-8 1.1. History 7 2. Modernizing of an Ancient Art 9-10 2.1. Modern Demands of a Casting Process 9 2.2. The Beginnings of Investment Casting 9 2.3. What Materials are used in Investment Casting 9 2.4. Examples of Investment Casting 10 3. Investment Casting Markets 11-12 3.1. Expanding Markets 11 3.2. The Outlook for Investment Casting 11 3.3. The Automotive Market 12 3.4. Threats from Alternative Materials 12 4. Investment Casting Process 13-25 4.1. Overview 4.2. Pattern Materials 13 13 4.2.1. Pattern Waxes 13 4 2. Melting and Casting 9.2.4. Engine Efficiency 5.1.3 Internal Aerofoil Cooling 5.1. Pattern Assembly 4.2. Plastics 4.5.4. Future Influences on the Investment Casting Industry 6. Binders 4.1.5. Design of Pattern Tree or Cluster 4. Environment 5.2.1 Furnace 5.2.4.1 Titanium Alloys 5.8. Production of Patterns 4.9.3.5. Other Pattern Materials 4. Costs 5.5.2 Direct Patterns (Rapid Prototyping) 5. Mold Firing and Burnout 4.1.6.2.2 Alloys 5.11.2.4.1 Shell Moulds 5. Drivers for Technology 27-40 5.6.4.4.3.4.5.5. Pattern Dies 4.5. References 27 27 28 29 29 30 31 32 34 34 38 39 39 40 41 5 . Post-casting Operations 4. Removal of Pattern 4. Casting Methods 4.3.7. Production of Ceramic Shell Molds 4.4 Process simulation 5.1. Melting Equipment 9. Refractories 4.10. Slurry Preparation 4.1.5.2.2.4.3 Non Destructive Testing/Evaluation 5.5. Coating and Drying 4. Other Ceramic Shell Constituents 4.4.2 Superalloys 5. Process Development 5.1. Pattern Tree or Cluster Preparation 15 15 15 15 16 16 17 17 19 19 20 20 4.3. Ceramic Cores 4. Comparison of Sand cast and Investment Cast 20 21 21 22 23 23 23 24 26 5.3. 6 . Introduction Investment casting is an industrial process based on and also called lost-wax casting. ornaments and jewellery.D. the castings allow the production of components with accuracy.000 years ago. Investment casting came into use as a modern industrial process in the late 19th century. and invented an airpressure casting machine [11]. From 5. as described by Barnabas Frederick Philbrook of Council Bluffs. Its earliest use was for idols. including the recipe for parchment. who detailed in his autobiography the investment casting process he used for the Perseus with the Head of Medusa sculpture that stands in the Loggia dei Lanzi in Florence. 1. Examples have been found across the world in Pakistan's Harappan Civilization (2500–2000 BC) idols. Mesopotamia. History: The history of lost-wax casting dates back thousands of years. Egypt's tombs of Tutankhamen (1333–1324 BC). a monk who described various manufacturing processes. when dentists began using it to make crowns and inlays. Italy [11]. 7 .1. and which belong to the Chalcolithic period (4500-3500 BCE). when beeswax formed the pattern. to today’s high-technology waxes. whose 1907 paper described his development of a technique. refractory materials and specialist alloys. Aztec and Mayan Mexico. Iowa in 1897. developed an investment material. versatility and integrity in a variety of metals and high-performance alloys [10]. This book was used by sculptor and goldsmith Benevento Cellini (1500–1571). repeatability. bronze and gold. one of the oldest known metal-forming techniques. Its use was accelerated by William H. using natural beeswax for patterns. 3700 BCE. by Theophilus Presbyter.1. Taggart of Chicago. and the Benin civilization in Africa where the process produced detailed artwork of copper. He also formulated a wax pattern compound of excellent properties. The oldest known examples of this technique are the objects discovered in the Cave of the Treasure (Nahal Mishmar) hoard in southern Israel. making them more than 5700 years old [10]. Conservative Carbon 14 estimates date the items to c. The earliest known text that describes the investment casting process was written around 1100 A. clay for the moulds and manually operated bellows for stoking furnaces. The adoption of jet propulsion for military and then for civilian aircraft that stimulated the transformation of the ancient craft of lost wax casting into one of the foremost techniques of modern industry [7].In the 1940s. for example investment casting is a leading part of the foundry industry. Investment casting expanded greatly worldwide during the 1980s. with investment castings now accounting for 15% by value of all cast metal production in the UK [3]. in particular to meet growing demands for aircraft engine and airframe parts. Today. World War II increased the demand for precision net shape manufacturing and specialized alloys that could not be shaped by traditional methods. After the war. Industry turned to investment casting. or that required too much machining. its use spread into many commercial and industrial applications that used complex metal parts. 8 . Fig. 1 Some Historical things made by Lost Wax Process [10] [11] 9 . prompted by environmental concerns. What Materials are used in Investment Casting 10 . discover an alternative and economic source of power generation.2. High temperature materials by their very nature are difficult and almost impossible to form by mechanical working and their use is entirely due to the ability to manufacture complex external and internal geometries by the investment casting process. Since it is unlikely that alloy development will produce less expensive materials. However. It is not possible to over emphasize the synergy between the investment casting industry and the development of the gas turbine engine.2. Modernizing of an Ancient Art Civilization at the start of the 21st century is dependent on the gas turbine engine for air transportation and a significant proportion of the world’s power generation. Castings can now be made for applications with oscillating stress [7]. 2.1. Castings should have less carbon foot print [7]. five key problems had to be solved: • • • • • Castings had to be reproducible within close dimensional limits Castings had to be produced in high melting point alloys There had to be high standards of metallurgical quality Costs had to be lower than for alternative techniques. such technology comes at a high financial cost with the latest single crystal alloys costing well in excess of $100 / lb.3. Environmental concerns and fuel costs give added pressure to develop the manufacturing capability to optimize the turbine aero-thermal efficiency with complex and accurate geometries and to reduce the engine weight with thin section components [3]. 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. Modern Demands of a Casting Process Against this background. attention turned to lost wax casting to produce accurately shaped blades. One buoyant niche market relies on special techniques (including hot isostatic pressing after casting) to produce components with fatigue strengths equal to forgings. 2. 2. In meeting this challenge. This has been the case for the past 50 years and is likely to remain the position for the next 50 years unless unforeseen developments. it is the responsibility of the investment caster to improve and develop the process to preserve raw materials and reduce overall manufacturing costs. The Beginnings of Investment Casting It was the successful solution of these problems that laid the foundation for the modern investment casting industry. 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. but very large components can also be made commercially. Small parts form the bulk of production. and copper and titanium alloys make up large part of the remaining 5% [7]. Examples of Investment Casting 11 . Nickel and cobalt-base super alloys account for 50% of total output by value.4. steels account for 35%. aluminium accounts for about 10%.Investment casting is used for a wide range of applications. 2. Table 1 European Investment Foundry Shipments (year ending 2007) [2] 3. the market in Asia has a substantial customer base for commercial (e. The CAEF (Committee of Associations of European Foundries) receive returns from member foundries to enable the CAEF to prepare market statistics. pumps and valves) and automotive parts [3].Fig. for example the world market for turbochargers in 2007 was 20 million wheels.g. Internationally the total world output in 2007 was $10. 2 Examples of castings made by Investment Casting Process [5] 3.1. The medical and automotive industry account for a significant volume of output. Investment Casting Markets Although the gas turbine industry is the major customer for the investment casting industry it is by no means the only one. Expanding Markets 12 .2 Billion of which 37% was from the USA and 33% from Asian countries (ref 1). For 2007 the output in castings by value are shown. Although the gas turbine market represents the largest customer in Europe and the USA. general commercial demand has picked up and with recent aircraft orders turnover is approaching its previous high. annual world turnover is estimated as £2.particularly price competition . advanced processing techniques and complex internal cooling. Commercial business is up. Based on data published in early 1996. land-based power generation demand is buoyant and the aircraft market is increasingly active. North America (essentially the US) accounts for about half.4. Within Western Europe. The market continued expanding up to 1990. The Outlook for Investment Casting Investment casting is recovering lost ground. The Automotive Market European investment foundries should look to opening up the automotive market to investment castings. aerospace and defense demands dropped and output decreased.is intense and likely to get tougher.800 million. Western Europe a quarter. Threats from Alternative Materials Other materials and processes (especially other precision casting processes) pose threats. however. But foundries will only invest in such equipment when orders are secured. Industry competition . Since 1994. 3.000 million. 3.500 people in an industry which leads the European community [7]. To an industry so reliant on cast nickel-base superalloys. Britain is home to about 50 of the 125 or so western European investment casting foundries. Over the next few years. can only come with sufficient automation to deal cost effectively with demand.2. the UK remains the biggest national producer with output of £300 million. but further developments are limited by the melting points of nickel and its alloys. however.3. The process serves an international market in which there are growing imports from one region to another [7]. Superalloys have remarkable temperature capabilities through optimization of composition. and the Pacific Rim countries 20%. employing roughly 5. in some countries by as much as 15-20%. when demands for new aircraft boosted both the sales of investment castings and industry capacity. oxide dispersion strengthened materials and engineering ceramics are a serious threat [7]. 3.Investment casting markets increased steadily over 40 years to the mid 1980s. aluminide intermetallics. followed by France (£185 million) and Germany (£135 million). Realistic pricing of standard parts. 13 . when annual worldwide turnover reached about £3. Growth can be expected until the end of the century and beyond if the industry can promote itself well enough [7]. 1. Investment Casting Process 4.4. Overview: 14 . because their properties tend to be complementary. and dyes. to produce thin. such as in aerospace integrally cast turbine wheels and nozzles [1]. generally polystyrene. high lubricity and 15 . and are often used in combination. blended and developed with different compositions.Fig. are more commonly used. 3 Basic Investment Casting Process Overview [5] 4. modified and blended with additive materials such as plastics. complex -shaped castings. They are readily available in different grades. fillers. 4. Pattern Waxes Waxes are mostly the preferred material for patterns. and are normally used. Pattern Materials Pattern materials currently in use are waxes. in order to improve their properties. have low cost. and for specific applications. resins.1. and plastics. Paraffin’s and microcrystalline waxes are the most widely used waxes. may sometimes be required. Waxes. with melting points ranging from 52 to 68 °C (126 to 156 °F). while other pattern materials are used sometimes.2. Paraffin waxes are available in many controlled grades.walled.2. while use of plastic patterns. antioxidants. widely used for modeling. in proportion to the amount used. Carnauba wax is a vegetable wax with higher melting point. weldability. 4. Factors for Pattern Wax Selection Process factors while selecting and formulating wax pattern materials. wettability. and resistance to binders and solvents. nylon. in required volumes. ability to duplicate detail. and are often used in combination with paraffin. inject. Microcrystalline waxes tend to be highly plastic and provide toughness to wax blends. and can easily be blended to suit different requirements. ethyl cellulose. a vegetable wax.2. which is moderately hard and slightly tacky.2. but is reduced to a greater extent by adding resins and fillers. The strength and toughness of waxes are improved by the addition. low coefficient of thermal expansion.low melt viscosity. adhesive grades. Beeswax is a natural wax. rigidity. Additives to Pattern Waxes Waxes with their many useful properties are.or cluster-assemblies. of plastics such as polyethylene. and environmental factors. and in pattern blends. Available in both hard. • Dimensional control: Solidification shrinkage. especially in limiting surface cavitation due to solidification shrinkage. cavitation tendency. Fillers that have been developed and used in pattern waxes include: spherical polystyrene. • Shell mold making: Strength. and are used more selectively in waxes than resins. hydrocarbon resins from petroleum and treederived resins such as dammar. Burgundy Pitch. that must be addressed are listed below.1. This leads to the description of pattern waxes as being either filled or unfilled. and melt out without cracking the thin ceramic shell molds. toxicity. thermal expansion. and spherical particles of thermosetting plastic. and the terpene resins. impact resistance. ethylene vinyl acetate and ethylene vinyl acrylate. hollow carbon microspheres. Additives are made to waxes to cause improvements needed in these two deficient areas. Their usage is. is reduced to some extent by adding plastics. however. however. cost. softening point. during and after pattern injection.2. • Removal. provides properties similar to microcrystalline waxes. they have higher melting points. various rosin derivatives. are moderately priced. Fillers are powdered solid materials. deficient in two practically important Areas: (a) Strength and rigidity especially required to make fragile patterns. and assembly: Strength. non-tacky and brittle. and (b) Dimensional control. limited because of high shrinkage and brittleness. ease of recycling.1. viscosity. Resins suitable for this are: coal tar resins. • Miscellaneous: Availability. • Dewaxing and burnout: Softening point. Waxes. thermal expansion. Other waxes used include: Candelilla. Fillers have higher melting point and are insoluble in the base wax. in general. and ash content. handling. thereby contributing to reduced solidification shrinkage of the mixture. Solidification shrinkage causing surface cavitation in waxes. setup time. 16 . non-tacky grades as well as soft.1. assemble into tree. which make them easy to blend. 4. and is very hard. hardness. grouped with the material properties required or to be considered: • Injection: Freezing range. Waxes have low melting points and low melt viscosities. Pattern dies made by machining 17 . used in composite wax-plastic integral rotor and nozzle patterns. plastic. which can accommodate large dies. even in extremely thin sections.2. (350 to 500 °F). because it is economical. have properties similar to plastics.2. and production efficacy in available patternmaking equipment. because of its tendency to cause shell mold cracking during pattern removal. to complex hydraulic machines. next to wax. can be molded at high production rates on automatic equipment. beryllium copper. Pattern Dies Various pattern tooling options are available for waxes because of their low melting point and good fluidity. aluminum. Polystyrene patterns are injected at higher temperatures. in hydraulic machines. Other Pattern Materials Foamed Polystyrene has long been used for gating system components. steel or a combination of these. and pressures. Different equipments. tool life. Plastic patterns usually require steel or beryllium copper tooling. by simply dissolving in water. have been developed to suit different pattern materials. brass. delivery time. The wax injection equipment ranges from simple pneumatic units. (40 to 1500 psi).3. (4 to 20 ksi). developed in Europe. pattern quality. with different operating parameters. Many die materials are used. However. Wax patterns are injected at lower temperatures. metal-filled plastic.4. 4. soft lead-bismuth tin alloys. and at high injection pressures. without stressing the ceramic shell. assembled using wax for the rest of the assembly [1]. 4. strong and require high-pressure injection machines. It is also used as patterns with thin ceramic shell molds in a separate casting process known as Replicast Process. Use of polystyrene is however limited. Urea patterns have an advantage over plastics: they can easily be removed. made with one or more cavities of the desired shape. including: rubber. in split dies using specially designed equipment. very stable. or an aqueous solution. normally equipped with water cooled platens that carry the die halves [1]. Urea-based patterns. plaster. the most important application for polystyrene is for delicate airfoils. in each die. and has high resistance to handling damage.1. The selection is based on considerations of cost.3. bronze. (110 to 170 °F). and pressures. Production of Patterns Patterns are usually produced by injecting pattern material in to metal dies. Polystyrene is usually used. Plastics Plastic is the most widely used pattern material.3. 4. they are very hard.2. and it requires more expensive tooling and injection equipment than for wax. steel or beryllium copper is also used effectively. and the parts are pressed together until bonded. Large patterns are set-up and are processed individually. only one of the two halves needs to be wet. 4. but some automation is currently being introduced in some investment casting foundries. For example. which is then used to make an investment cast cavity for this type of cast tooling [1]. while small to medium size patterns are usually assembled into clusters for economy in processing. Currently. Alternatively. polystyrene becomes very tacky when wet with solvent. especially for mock-up work. Joints must be strong. with only the wax being melted. Care also must be taken to avoid damaging patterns or splattering drops of molten wax over the patterns being assembled. Preformed ceramic pour cups are often used in place of wax pour cups. The joint is then smoothed over. Fixtures are essential to ensure accurate alignment in assembling patterns. Wax components are assembled by wax welding. The assembly of polystyrene to wax is done by welding. pattern clusters of aircraft turbine blades may range from 6 to 30 parts.4.use CNC (computer numerical controlled) machine tools and electric discharge machining. a robot is used to apply sealing compound in the assembly of patterns for different integrally cast nozzles. The plastic at the interface is softened with solvent. Polystyrene pattern segments are assembled by solvent welding.1. Design of Pattern Tree or Cluster The following preliminary requirements are considered essential: 18 . with each nozzle having. and completely sealed with no undercuts. from 52 to 120 airfoils apiece [1]. using hot iron or spatula. and patterns assembled with these to produce the wax-tree or pattern cluster. Wax can be cast against a master model to produce a pattern. and readily adheres to itself. A hot melt adhesive can be used instead of wax welding. However. Frequently. Most assembly is done manually. In one application. 4. cast tooling made in aluminum. Wax at the interface between two components is quickly melted. laser welding units have been developed to provide improvements in assembling of wax components. with skilled personnel. For small hardware parts. The capacity of injection machines and the cost of tooling are important considerations. Gating components. or a small gas flame. including pour cups. which are assembled into final form. patterns set in clusters may range from tens to hundreds.4. Most patterns are injected with the gates. Pattern Assembly Patterns for investment casting produced in dies are prepared for assembly in different ways. Most assembly and setup operations are performed manually. and the components are pressed together until the wax solidifies. gating and runner components forming trees or clusters are produced separately. However. large or complex parts are injected in segments. Standard extruded wax shapes are often used for gating. until the required shell thickness is achieved. Alumina is expensive.5. help to prevent it from cracking or pulling away. as well as. 4. It also contains the bonding agent. The design of the assembled pattern tree. then withdrawn from the slurry. Others adopt standardized trees. Yttria is used in prime coats for casting titanium. The coarse stucco particles serve to arrest further runoff of the slurry. Since the process is very flexible. stuccoing. in order to avoid the occurrence of loose particles on the shell mold surface [1]. which is crushed and screened to 19 . chemical gelling. with coarse ceramic ‘stucco’ particles embedded in its outer surface. since it can affect every aspect of the investment casting process.5. Silica is generally used in the form of fused silica (silica glass). or by sprinkling or ‘raining’ on it the stucco particles from above.1. alumina and various aluminum silicates are commonly used refractories for both slurry and stucco in making ceramic shell molds. including the smooth surface of the pattern. directionally solidified columnar. other factors are adjusted to maximize profitability.• Providing a tree or cluster design that is properly sized and mechanically strong enough to be handled through the process • Meeting all metallurgical requirements • Providing test specimens for chemical or mechanical testing. when required. Typical properties of refractories are listed in Table 2. Some foundries prefer cluster design tailored to each individual part to maximize parts per cluster and metal usage. or single crystal casting. to facilitate handling and processing. The fine ceramic layer forms the inner face of the mold. Refractories Silica. and build up shell thickness faster. and reproduces every detail. 4. such as graphite. When close control of grain is required. Production of Ceramic Shell Molds Investment shell molds are made by applying a series of ceramic coatings to the pattern tree assemblies or pattern clusters. The wet layer is immediately stuccoed with coarser ceramic particles. or a combination of these. and hardening are repeated a number of times. and to produce a uniform layer. provide keying or bonding between individual coating layers. often called a seal coat. This is accomplished by drying. Each coating consists of a fine ceramic layer. Each coating is allowed to harden or set before the next one is applied. circular clusters are often used to provide thermal uniformity during solidification in the casting process. and manipulated to drain off excess slurry. either by immersing it into a fluidized bed of the particles. and as such used selectively. in effectively meeting all the quality and metallurgical requirements of the final product [1]. The design of the pattern tree or cluster is however. Other refractories. The operations of coating. or clusters. zircon. such as for equiaxed. critical and important. or cluster critically impacts various stages of the investment casting process. The tree assembly or cluster is first dipped into a ceramic slurry bath. such as in directional solidification processes. Once these essentials are satisfied. The final coat. Fused silica is made by melting natural quartz sand and then solidifying it to form a glass. which provides strength to the structure. is left unstuccoed. foundries approach this goal in various ways. zirconia and Yttria have been used with reactive alloys. produced from bauxite ore by the Bayer process. its use is primarily confined to superalloy casting [1]. Its ready solubility in molten caustic solutions provides a means of chemically removing shell material from areas of castings that are difficult to clean by other methods. abrupt expansion at 573 °C (1063 °F) accompanying its α-to-β-phase transition. Use of zircon is generally limited with prime coats. 20 . its utility is limited because of its high coefficient of thermal expansion and by the high. Silica is sometimes used as naturally occurring quartz. Zircon occurs naturally as a sand. as it does not occur in sizes coarse enough for stuccoing backup coats. It is ground to powder for use in slurries. Its primary advantages are high refractoriness. resistance to wetting by molten metals. and it is ground to a powder for use in slurries.produce stucco particles. causing excessive cracking of shell mold. if the mold is not fired slowly [1]. Refractoriness and cost increase with alumina content. which is usually in the form of silica glass. Fired pellets are crushed or ground and carefully sized to produce a range of powder sizes for use in slurries. mullite (Al2O3.2SiO2) with some free silica. and used in this form as a stucco. often in conjunction with fused silica and aluminosilicates. and round particle shape. imparts thermal shock resistance to molds. Aluminum silicates are generally composed of stable compound. However. expense of which is very low. However. is more refractory than silica or mullite. (which contains 72% alumina) and free silica. They are made by calcining fireclays to produce different levels of mullite. and is less reactive toward many alloys. Alumina. The extremely low coefficient of thermal expansion of fused silica. and granular materials for use as stuccos. Table 2 Normal compositions and typical properties of common refractories for investment casting [1]. 10%.5. however.5 to 10% by weight of the slurry) for equiaxed superalloy castings.3. with its alcohol base. are inferior to colloidal silica in room temperature bonding properties [1]. Other Ceramic Shell Constituents Wetting Agents. 4. They have poor refractoriness. to promote wetting of the pattern or prior slurry coats. a reaction with water carried out in ethyl alcohol. sodium alkyl aryl sulfonates. since it is much more expensive. silicates. for longer times. in addition to the refractory and the binder. which subject the mold to high temperatures. Wetting agents are sometimes omitted from ethyl silicate alcohol slurries and from water based back-up slurries.4. which limits their application. These are effective in low concentrations of 0. Both these binders. Colloidal silica is most widely used. or grain refiners. Commonly used defoamers are aqueous silicone emulsions and liquid fatty alcohols such as noctyl alcohol. Liquid sodium silicate solutions are sometimes used where a very inexpensive binder is desired. such as colloidal alumina and colloidal zirconia binders. Other Constituents.5.3% by weight of the liquid.phenoxy polyethoxy ethanol. Ethyl silicate. rapidly declining. Where wetting agents are used. or octyl.03 to 0. Small additions of clay have been used to promote coating characteristics [1]. based on the liquid weight.2. Its main disadvantage is that its water base makes it slow drying. 21 . and poses fire and environmental hazards. Antifoam Compounds. especially in prime coats. Slurries generally contain wetting agents. Use of this binder is however. hydrolyzed ethyl silicate and sodium silicate. Ethyl silicate slurries are readily gelled after rapid dripping cycles. Nucleating agents. which are refractory cobalt compounds such as aluminates. dries much faster than colloidal silica. Wetting agents such as sodium alkyl sulfates. are generally used in amounts of 0. It is an excellent general purpose binder. Binders The commonly used binders include colloidal silica. using acid catalyst such as hydrochloric acid. It consists of a colloidal dispersion of spherical silica particles in water. Organic film formers are sometimes used to improve green strength. Other Binders: The operation of directional solidification and single crystal processes. Ethyl silicate which has no bonding properties is converted to ethyl silicate binder by hydrolysis. along with the introduction of more reactive superalloys. where close control of grain size is required.002 to 0. by exposure to an ammonia atmosphere. especially in inaccessible pockets or cores. and oxides are added to the prime slurry (in amounts from 0. has led to the development of more refractory binders. an antifoam compound is included to suppress foam formation and to permit air bubbles to escape. Viscosity is measured with a No. or by plunging the cluster into a fluidized bed of the particles. Backup coats are formulated to coat readily over the prime coats (which may be somewhat porous and absorbent). draining. before dipping. very effective for coating narrow passageways and for eliminating air bubbles. to provide high 22 .5. or mechanically. These characteristics provide a smooth surfaced mold. The most prevalent controls are the measurement of the initial ingredients. are used at a higher viscosity. or a Brookfield type rotating viscometer. are based on the particular refractory powder and the type of binder. Cleaning is accomplished by rinsing the pattern clusters in solution of wetting agent. In the fluidized bed.5. and permeability [1]. It is then withdrawn and drained over the slurry tank with suitable manipulation to produce a uniform coating. Slurry composition is generally in the following broad range: • Binder solids: 5 -10% • Liquid (from binder or added) : 15 – 30% • Refractory powder: 60-80% Slurries are prepared by adding refractory powder to binder liquid.4. pH and viscosity. and are stuccoed with finer particles than the backup coats. because of the action of pressurized air passing through a porous plate in the bottom of the bed.4 or 5 Zahn cup. Next the stucco particles are applied by placing the cluster in a stream of particles falling from an overhead screen in a rainfall sander. or dirt. Continued stirring is also required in production to keep the powder from settling out of suspension. capable of resisting metal penetration. and to produce more uniform coatings. Properties of the finished ceramic shells that are monitored include: weight. or a suitable solvent that does not attack the wax. The trees.6. 4.5. modulus of rupture (green and fired). but dipping under vacuum has been found. Most dipping is done in air. Foundries are increasingly using robots in order to heighten productivity. and stuccoing of clusters are carried out manually. they are often programmed to reproduce actions of skilled operators. Control procedures for slurries vary considerably among foundries. Coating and Drying Dipping. When robots are introduced. Generally. prime slurries contain finer refractory powder. or clusters. are usually allowed to return to room temperature and dry. The cleaned wax cluster is dipped into the prime slurry and rotated. Stirring is continued until viscosity falls to its final level before the slurry is put to use. Slurry Preparation Compositions of the slurry. 4. in some limited applications. Pattern Tree or Cluster Preparation Before dipping.5. which are usually proprietary. especially with standardized clusters. pattern trees or clusters are usually cleaned to remove injection lubricant. remove any air entrainment. slurry temperature. using agitation to break up agglomerates. loose pieces of wax. the particles behave as a boiling liquid. robotically.4. Dedicated mechanical equipment can sometimes operate faster. density. to process larger parts and clusters. Either rotating tanks with baffles or propeller mixers are used for this purpose. the chilling effect causes the pattern to contract. due to their differences in composition. Since cores expand differently than the shell molds. thus providing the space to accommodate the expansion as the remainder of the wax is heated. while the coating is still wet and unbonded. Many cores are made by injection molding of fine ceramic powder with a suitable organic binder into steel dies and subjecting the cores to a two stage heat treatment. or even destroying the mold. 4. a. To get the shell as strong as possible. Drying is complicated by the high thermal expansion and contraction characteristics of waxes. the shell is subject to high stress. with the wax patterns already having corresponding openings. Therefore. Drying is usually carried out on open racks or conveyors. Air drying at room temperature with circulating air of controlled temperature and humidity is the most common method. but cabinets or tunnels are sometimes used. the number ranges from 6 to 9. to prevent this. This molten wax layer either melts out of the mold or soaks into it. it should be thoroughly dried before dewaxing. or holes are drilled in the shell to relieve wax pressure. In practice. It may range from 5 for small clusters. This can actually crack the coating. Between coats. If drying is too rapid. cores must be provided with slip joints in the mold. Shells are subject to 16 to 48 h of extended drying after the last coat. Preformed cores are normally used by placing in pattern die and injecting wax around them. b. Then. as the drying rate declines and it regains temperature. to 15 or more for large ones. and to build up the required thickness with a minimum number of coats. the slurries are hardened by drying or gelling. Ceramic Cores Ceramic cores are widely used in investment castings to produce internal passageways in castings. Even with these techniques. since the thermal expansion of waxes are many times those of refractories used for molds. while soluble cores for other shapes are made and placed in the pattern tooling. the wax begins to expand.strength. this problem is effectively circumvented by heating the mold extremely rapidly from the outside in. The number of coats required is related to the size of the clusters and the metal weight to be poured. Citric acid is used commonly. usually at a recommended value of 50% [1]. the core is sintered to its final strength and dimensions. before the rest of the pattern can heat up appreciably. This causes the surface layers of wax to melt very quickly. Self-formed cores are produced during the mold building. Preformed cores are required when self-formed cores can not be used. Removal of Pattern Pattern removal is the operation that subjects the shell mold to the most stresses.6. Metal pull cores in pattern tooling are used for simple shapes. The soluble core is then dissolved out in a solution that does not affect the wax pattern. the expansion differential leads to enormous pressure that is capable of cracking. For most applications. and the pattern injected around them. Melt-out tips are sometimes provided.7. When the mold is heated to liquefy the wax. and cores are either self-formed or preformed. In the second stage. and are produced by a number of ceramic forming processes. as the coating is developing strength and even shrinking. Simple tubes and rods are commonly extruded from silica glass. sometimes enhanced by the 23 . such as an aqueous acid. relative humidity is normally kept above 40%. 4. wax from this operation can be reclaimed satisfactorily. Cycles are longer than for autoclave and flash dewaxing. but some rotary furnaces are also in use. and there is potential fire hazard [1]. with a steam accumulator to ensure rapid pressurization. Operating pressures of approximately 550 to 620 kPa (80 to 90 psig) are reached in 4 to 7 s. Two methods have been developed to implement the surface melting concept: autoclave dewaxing and high temperature flash dewaxing. 425 to 870 °C (800 to 1600 °F) for many copper base alloys. • Flash dewaxing is carried out by inserting the shell into a hot furnace at 870 to 1095 °C (1600 to 2000 °F). • Autoclave dewaxing is the most widely used method. Saturated steam is used in a jacketed vessel. unless it is embedded in wax in the pattern (as in integral nozzle patterns). Many molds are wrapped with a ceramic-fiber blanket at this time to minimize the temperature drop that occurs between the preheat furnace and the casting operation. except for molds for directional solidification processes. Molds for the directional solidification process are preheated above the liquidus temperature of the alloy being cast [1]. and an automatic wax drain valve. and 870 to 1095 °C (1600 to 2000 °F) for steels and superalloys. or to provide better feeding by insulating selected areas of the mold. or less. a fast acting door with a safety lock. Autoclaves are equipped with a sliding tray to accommodate a number of molds. Cracked molds can be repaired with ceramic slurry or special cements.8. Burnout furnaces operate with temperatures between 870 and 1095 °C (1600 and 2000 °F). while other liquids can also be used.application of vacuum or extremely low humidity. Wax recovery is good. • Hot liquid dewaxing has found some use among smaller companies seeking to minimize capital investment. and have some 10% excess air provided to ensure complete combustion of organic materials. to burn off residual pattern material and any organics used in the shell slurry. Mold Firing and Burnout Ceramic shell materials are fired to remove moisture (free and chemically combined). Batch and continuous pusher-type furnaces are most common. 4. The furnace is equipped with an open bottom so that wax can fall out of the furnace as soon as it melts. Polystyrene patterns cannot be melted out in the autoclave. which are preheated in the casting furnace with induction or resistance heating. after the mold is cooled down. and to preheat the mold to the temperature required for casting. inspected and repaired if necessary. Flash dewaxing furnaces must be equipped with an afterburner in the flue or some other means to prevent atmospheric pollution. Some of the wax begins to burn as it falls. these are accomplished in a single firing. Polystyrene patterns are readily burned out in flash dewaxing. to sinter the ceramic. preheating is performed in a second heating. Gas fired furnaces are used for mold firing and preheating. Hot wax at 177 °C (350 °F) is often used as the medium. Preheat temperatures vary depending on part configuration and the alloy to be cast. polystyrene can cause extensive mold cracking. Molds are dewaxed in approximately 15 min. and even though it is quickly extinguished. Other times. However. 24 . Nevertheless. In some cases. there is greater potential for deterioration than with an autoclave. or unless the polystyrene patterns are very small. but require flash dewaxing. Common ranges are: 150 to 540 °C (300 to 1000 °F) for aluminum alloys. Batch and semicontinuous interlock furnaces are normally used. and the vacuum serves to evacuate air through the porous mold wall and to create a pressure differential on the molten metal. all types of steel. Magnesium alloys can be melted in gasfired furnaces using low. or isostatic pressing. with normal melting rates of 3 lb/min. 25 . inert atmosphere or vacuum. and can be employed for melting in air. 4. both of which help to fill delicate detail and thin sections. • In vacuum-assist casting. Casting Methods Both air and vacuum casting methods are used in investment casting. Gas. and support different casting methods adopted. such as vacuum-assist casting. iron. • Air casting is used for many investment-cast alloys. They are usually tilting models. platinum. • In pressurized casting. titanium and the refractory metals. rollover furnaces are pressurized for the same purpose. dry pressing.9.9.. and nickel-base alloys that do not contain reactive elements. A major advantage of investment casting is its ability to cast very thin walls. while pressure is applied using compressed air or inert gas. ductile iron. This advantage is further enhanced by specific casting methods.2. pressurized casting.carbon steel crucibles [1]. thixotropic casting. leaving only the mold opening exposed to the atmosphere. magnesium. cobalt and nickel alloys.9. gold. including aluminum. There is some use of rammed graphite molds in vacuum arc furnaces for casting titanium. A partial vacuum is drawn within the chamber and around the mold. while electrical resistance furnaces are sometimes preferred for aluminum casting. Most castings are gravity poured. most cobalt alloys. the mold is placed inside an open chamber. some cobalt alloys. Melting and Casting Different types of equipment are currently in use for melting. alumina and zirconia. which is then sealed with a plate and gaskets. Melting Equipment Coreless type Induction furnaces are used with capacities ranging from 15 to 750 lb. • Vacuum casting provides cleaner metals with superior properties and is used for alloys that can not be cast in air. silver. centrifugal casting and countergravity casting. due to the use of hot mold. copper. and the furnace is quickly inverted to dump the metal into the mold.1.4. The crucibles typically used are magnesia. such as the γ’-strengthened nickel base alloys. since they help reduce hydrogen porosity. The hot mold is clamped to the furnace-top with its opening in register with the furnace opening. The metal is poured into the exposed mold opening.fired crucible furnaces are used for aluminum and copper alloy castings. Zinc alloys. gray iron and malleable iron are usually not investment cast for economic reasons. 4. which are made by slip casting. and sometimes copper and aluminum alloys. They are extensively used for melting steel. High-pressure water (6 to 10 ksi) is sometimes used instead of mechanical knock out. Dental and jewelry casting use centrifugal casting to fill thin sections and fine detail. usually shotblasting. Vacuum arc skull furnaces discharge titanium alloy at a temperature just above its melting point. works effectively in air or under vacuum. The vacuum is released after castings and in-gates solidify.10. Some shell material may spall off during cooling. a. If cores are to be removed in a molten caustic bath. • Countergravity Low-Pressure Air (CLA) Process has the preheated shell mold. Besides substantial savings in alloy usage and improved gating efficiency. The sprue is lowered to below the melt surface. causing molten metal in the central sprue to return to the melt crucible. the entire cluster can be hung in the bath. Postcasting Operations Post casting operations represent a significant portion. Alternative methods may be available for performing the same operation. • Countergravity casting assists in filling thin sections. 4. and the most efficient one should be selected. by applying a differential pressure between molten metal and the mold. and this is done before parts are removed from the cluster. often 40 to 60%. For example. it is often cost-effective to scrap early to avoid wasting finishing time. for air melted and vacuum melted alloys. of the cost of producing investment castings.• Centrifugal casting uses the centrifugal forces generated by rotating the mold to propel the metal and to facilitate filling. and the centrifugal casting is usually needed to ensure good filling. This technique developed for over 30 years. Some specifications require verification of alloy type. 26 . vacuum applied to mold chamber to cause controlled filling of the mold. Brittle alloys require special attention. • Countergravity Low-Pressure Vacuum (CLV) Process is similar to CLA process has the crucible in a vacuum chamber for vacuum melted alloys such as in nickel-base and cobalt-base superalloys. but a good portion usually remains on the casting and is knocked off with a vibrating pneumatic hammer or by hand. This is removed in a separate operation. even if this means including an extra inspection operation. with an extended sprue. the other benefits from the process include improved casting quality with reduced dross and slag inclusions. for use in the next cycle. Part of the prime coat sometimes remains adhered to the casting surface. and the remaining refractory can be removed along with the cores. and the bulk shell material may remain lodged in pockets or between parts. and large savings can be realized by specifying the most cost-efficient routing. placed in a chamber above the melt surface of an air melted alloy. Knockout. to produce castings in aluminum and nonferrous alloys. Clusters are hung on a spinner hanger inside a blasting cabinet. many types of steels and superalloys. or are placed on a blasting table. in weights from a few grams to 20 kg (44 lb). The actual sequence in which operations are performed can be important. especially for aluminum and other nonferrous alloy parts. A standard shop routing is provided for each part. Some brittle alloys can be readily tapped off with a mallet. c. Steel or iron grit or shot. coining and abrasive grinding are used to improve dimensional accuracy. Before they are heat treated. in applications such as: acid pickling to remove scale. Cutoff. and small hand grinders equipped with mounted stones. Abrasive Cleaning. and superalloys. Hot isostatic pressing (HIP) is being increasingly adopted to eliminate porosity. chemical milling to remove α-case on titanium. Broaching. Aluminum. using molten caustic bath (sodium hydroxide) at 900 to 1000 °F. Chemical finishing treatments are also used. and chemical treatment to apply a satin finish to aluminum or to polish stainless steel. and silica. e. aluminum. or in high-pressure autoclave. d. heated usually. f. Shear dies have also been used to remove castings from standardized clusters. ductile iron. Straightening of investment castings. Cores can be removed by abrasive or water blasting. magnesium.b. Core Removal. Following cutoff. either manually or using hydraulic presses. Torch cutting is sometimes used for gates that are inaccessible to the cutting wheel. Gates are to be properly notched for these two cutoff techniques. Other copper alloys. and superalloys are cut off with abrasive wheels operating at about 3500 rpm. the cores can be dissolved out. If blasting can not be used. or alumina sand are commonly used. Heat Treatment. Air or vacuum heat treatments are performed extensively as needed to meet property requirements.11. normally confined to selected areas requiring closer dimensions. passivation treatment for stainless steel. gate stubs are ground flush and smooth using abrasive wheels. using pneumatic and centrifugal blasting machines. Blast cleaning is used to remove scale resulting from core removal or heat treatment. and used selectively for steel. and some copper alloys are cut off with band saws. steel. Comparison of Sand cast and Investment Cast 27 . 4. and to improve properties especially for titanium. is performed when required. or a boiling solution of 20 to 30% sodium hydroxide or potassium hydroxide in an open pot. Other Post-casting operations. with suitable fixtures. single-crystal castings must be handled very carefully to avoid recrystallization during subsequent heat treatment. Machining is often performed on investment castings. Some steel and ductile iron parts can be cutoff after soaking parts in frozen liquid nitrogen (-320 °F). Fig.1. 4 Comparison of Cast and welded part with singe cast investment [9] Fig. Environment 28 . Drivers for Technology 5. 5 Comparison of Sand cast and Investment Cast Engine [5] 5. It is perhaps a little ironic that the current shell binder is silica sol since over the last 60 years the binder has changed from the original HES (with MgO gel accelerator). issues concerning the supply and use of shell mould materials: 29 .5. and still are. There was little choice other than to use silica sol as the replacement binder but there were three problems to overcome: (i) To prevent shell cracking during dewax (ii) To produce a shell with equivalent dimensional consistency to HES moulds (iii) To produce a shell which did not induce casting defects such as cracks. to HES (with NH3 accelerator) and back to silica sol.1 Shell Moulds The environmental protection act of 1990 for the control of air pollution was a major concern for UK foundries using the hydrolised ethyl silicate (HES) process since both the alcohol and ammonia used in the process were emitted to atmosphere. the subsequent warming of the shell after drying caused the wax to expand and crack the shell. The issue of dimensional equivalence has not been totally resolved and there will probably always be a small difference in dimensions when the process is changed from HES to silica sol. inclusions or surface finish issues These problems were eventually overcome by the development of polymer additions to give green strength and by controlled drying using a combination of humidity. The process developed was to ensure that the shell drying was controlled so as to prevent the harmful temperature excursions. airflow and temperature in specially constructed drying tunnels. An alternative process was necessary and both foundries and suppliers were engaged in extensive work to evaluate new systems. Fig. 6 The silica gel cycle [3] Since 2000 there have been. Early trials identified that shell drying caused the temperature of the wax pattern to drop by several degrees. to silica sol.1. These regulations were introduce in 2008 and are now implemented. domestic tiles and bathroom furniture. the industry has yet to replace zircon as the material of first choice. In recent years there have been many efforts to find a use for mould scrap after cast. Evaluation. It is also the subject of social dialogs with the foundry industry. and Restriction of Chemicals) do not directly require castings to register. alumina and zircon can cause quality problems as alternative sources of raw material are used. The increasing cost of master heat has forced the foundries into using recycled material as mixed virgin/revert as 60/40 blends.g. It is recognized that it would be impossible for the industry to survive without silica and therefore the control of dust and the personal protection for operators will become increasingly regulated. but suppliers of chemicals and refractory materials are required to register chemicals used by the foundries. Silica is widely used in the industry. The alternative sources of zircon available to the industry were not of a sufficient standard and the industry was required to examine alternative refractories. the industry continues to look for alternative uses and with the increased disposal costs it is likely that a solution will eventually be found [3]. The quality can be compromised if they are contaminated with such impurities as iron. also some countries do no permit the disposal of shells in land fill sites which contain ‘heavy metals’ such as cobalt aluminate used for grain refining. The REACH regulations (Registration. Authorization. Since 2000 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. is currently over $4000 /kg. in addition to its use as a shell binder it is also used as a refractory flour and stucco and is the main constituent in ceramic cores. These impurities can reduce the life and quality of the slurry. Several attempts to obtain research grants from the E. cobalt. to investigate uses for recycled mould scrap have been made but without success. Crystalline silica as cristobalite is a known carcinogen and has been subject to review by the European Carcinogens Directive. Until recently virgin alloy has been the preferred master heat condition. For DS casting.U. These regulations do not apply to castings made outside the E.(i) Periodic shortages in the supply of common refractory materials. The cost of scrap superalloy on the open market is approximately 75% of virgin. Also zircon may contain an unacceptable quantity of radioactive impurities which render it difficult to dispose of after use. alumino-silicates. tantalum. However. which is an essential metal for single crystal alloys.2 Alloys The price of superalloys for gas turbines and turbochargers depends on availability of raw materials and market conditions. molybdenum have been volatile in recent years and rhenium. prices for nickel.1. This work continues and although alternative materials such as mullite are used. sodium etc. foundries are required to ensure that the chemical substances used in their process are registered with REACH. 30 . Suppliers to the industry continue to investigate alternatives and expect to have environmentally friendly material available by 2010. e.U. 100% revert is currently in foundry trials. (ii) A zircon shortage in 2005-7 caused significant problems for the industry as supplies of zircon were acquired for alternative use. 5. 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. In certain European countries it is difficult to dispose of zircon containing moulds because of the radioactivity concerns. Engine Efficiency Turbine efficiency is governed by the gas temperature and aerothermal efficiency of the design.U. it is necessary to remove this layer by chemical etching. Turbine entry temperatures are typically around 14500C but are expected to increase to over 17000C in the future. 0.therefore foundries rely wherever possible on returning scrap to be remelted and refined as toll melt alloy. This is achieved by internal cooling and by creating a film of cooling air around the surface of the aerofoil. For materials to survive for this length of time in an oxidizing and corroding atmosphere at a temperature in excess of the melting point requires the blade to be substantially cooled. It follows that to improve the engine and fuel efficiency it is necessary to run the turbine at the highest temperature possible.2. Around 70% of the cooling is from internal cooling with air taken from the engine compressor. Single crystal material is sensitive to nitrogen level in the master heat and it is essential that the reverted alloy (or blend heats) do not contain unacceptable levels of nitrogen (>5ppm) [3]. 7 Single Crystal with high angle boundaries in a casting with high nitrogen content [3] 5. IMPRESS project. Titanium is highly reactive in the molten state and the casting process uses inert shell systems based on zirconia or yttria. This process can also be used to reduce the section thickness and by careful masking techniques the casting sections can be selectively reduced in thickness. this leads to both alloy and cooling design development. Turbine blades are also expected to last for around 3000 flights which represents around 30. 0. These alloys together with their processing (moulding. 5-10Nb. To avoid contact with refractory ceramic crucibles the alloy is melted using direct or cold crucible methods. and finishing) are currently under development funded by the E.2C. TiAl alloys have about 65% density of Ti alloys’ and they can be used up to 750°C. This material is normally thermomechanically processed however the cost is high.1 Titanium Alloys Titanium as Ti 6/4 has been cast in production for over 20 years and current applications include fan casings of over 2 metres diameter. Weight is also a very important aspect of aeroengine design and this leads to the development of lightweight high temperature materials and thin section components [3]. casting.2. 31 .2B atomic %).000 hours for a long haul flight. The quest for lighter engines has led to the development of the titanium aluminide intermetallics. 45Al. Fig. for example the TNB alloys (Ti. This project is aimed at developing TiAl casting alloys but to provide the cast TiAl with mechanical properties which are equivalent those of thermo-mechanically processed [3]. 5. melting. Since titanium castings have an oxide layer at the surface (alpha layer) from unavoidable shell reaction. these were characterized by having Re content of 6%. At the 2ppm sulphur level it was necessary to have at least 25ppm of yttrium to effectively contain the sulphur. Further work showed that yttrium additions to the alloy were effective in preventing sulphur migrating to the surface by forming yttrium oxysulphide.2 Superalloys The history of superalloy development is well documented and the paper by Chester Sims gives an excellent review up to the introduction of single crystal. yttrium will dissolve in the shell and core and it is necessary to make additions of up to 500ppm of yttrium in the melt to retain 25ppm in the casting. The heat treatment difficulty with 3rd generation SX alloys and to increase further the temperature capability has led to the development of alloys containing platinum group metals. in particular ruthenium which had the advantage of increasing strength. 3rd generation superalloys were developed. Unfortunately there are a number of problems with yttrium. The control of this process is therefore very difficult and necessitates the use of low silica shells and alumina cores. The process for making very low sulphur heats has been greatly improved and it is now possible to produce master heats with <0. the element forms a low melting point eutectic with nickel and any level over 50ppm can result in the formation of incipient melting. Single crystal superalloys owe their high temperature properties to their ability to be solution heat treated to homogenize the structure and refine the second phase by sophisticated ageing heat treatments. With this level of sulphur it is only necessary to retain yttrium at a 10-15ppm at which level it is possible to cast using conventional ceramics. 8 Example of titanium compressor casing with 1mm wall section [3].2.Fig. These investigations found that sulphur at the relatively low level of 2ppm was sufficient to disrupt the protective aluminium oxide layer on superalloys. Recent developments have included the development of the so called second generation superalloys containing 3% rhenium in the 1990s. 32 .5ppm sulphur. During the 1990s. The problem of containing adequate levels of yttrium has been significantly helped with the development of very low sulphur master heat. 5. These alloys were developed for their high temperature creep properties but had the disadvantage of being very difficult to solution heat treat. The other main problem is the containment of yttrium in the casting. During the 1990s the importance of sulphur on oxidation resistance was investigated. advanced gas turbines operating with a gas entry temperature in excess of 1450C require cooling of at least 600C to bring the aerofoil temperature down to a level in which the blade can survive 30. An added issue with single crystal cores is that the cores are fired during manufacture at a lower temperature (e.g. . alternatively plastic chaplets placed on the surface of the core can be used for location. To prevent unacceptable core movement during this phase. A core which sinters will have too much strength at the solidification temperature and cause excessive stress to be retained by the casting with the result that the casting will recrystallize when solution heat treated. 11000C) than the mould temperature at which metal is cast. 33 . 70% of this cooling is obtained by passing cold compressor air through complex passages in the aerofoil section. The core therefore has a relatively low strength during the phase of the process where the mould temperature is raised to the casting temperature. 5.They must be chemically removed with a caustic solution .improving heat treatability and reducing the tendency to form undesirable TCP phases.They must be dimensionally compatible with the shell mould .3 Internal Aerofoil Cooling As mentioned previously.g. For single crystal casting the core chemistry is designed to prevent sintering at the casting Temperature (e. These alloys are now generally referred to as 4th generation single crystal alloys [3].2. originally the core was printed at both ends and located in the die and held firmly by the prints. . These pins instantly dissolve in the alloy when the metal is poured. cores are supported in the mould with platinum pins.They must not distort during the casting process The location of a core in the wax impression die has also evolved over many years. These designs are confidential but will require innovative core technology to create the necessary casting configuration to achieve over 800 degrees of cooling [1]. The ceramic cores used to produce these passages must be compatible with the casting process: .000 hours.They must not react with the alloy. Pins are placed in the die to locate the core on the nesting points. Future aerofoil designs will involve the concept of wall cooling in which cooling air is passed through the aerofoil wall. 1500C). In recent years the established practice is to use six point nesting where the core is allowed to fairly align within the die using location points which also serve as inspection points.They must be supported during the wax injection process so as to maintain the wall section in the wax pattern and not to be displaced during the dewax operation. funding to carry out extensive investigations have not been successful to date. 34 . whereby the centre sprue of the ‘tree’ is directly reused for the alloy melting stock. have included the concept of automated wax assembly which is ideally suited for high volume commercial parts [3]. Automation is well established for shell moulding and many finishing operations. Low cost refractories are possible for commercial products but high value components such as those for aerospace and medical use are unlikely to change from the established refractories.3. Opportunities for the re-use of shell material is frequently under consideration by research Organization. 9 Turbine rotor blade core [3] Fig.Fig. Recent developments by Mueller Phipps Inc. Costs Production costs are continually under review. 10 Nozzle Guide Vane Core [3] 5. Turbochargers represent the largest market for the investment casting industry and alloy costs and conservation are at the forefront of cost reduction. Robotic handling significantly improves the process consistency and reduces labour costs. efforts to secure E. The ‘Trucast’ process is widely used by the industry.U. unfortunately. in particular the automotive industry is constantly improving its cost base. Fig. 11 Automated wax assembly and robotic handling [9] Fig. 12 Fully Automated Investment Casting [5] 35 . In recent times liquid metal cooling has been developed for very high gradient solidification. Process Development 5. Table 3 DS and Single Crystal Defects [3] Many of these defects can be reduced with good furnace technology. These include. 36 . These furnaces include a bath of liquid aluminium or tin in which the casting is slowly immersed immediately after casting. On the debit side. slivers. Single crystal castings owe their high strength to their ability to be fully heat treated and the absence of grain boundaries. Other furnace developments for DSX include the use of ‘donut’ chills in which blades are placed on a chill ring with a centre chill to increase the temperature gradient and combination furnaces which can be used for either directional solidification or equiax casting [3]. single crystals suffer from all the defects associated with equiax castings plus those associated with DS plus those specific to single crystals. This is led by the requirements for large Industrial gas turbine aero foils cast as DS or single crystals and cost reduction through improved cast quality and volume of parts per furnace cycle. recrystallised grains. freckles etc.4. A high thermal gradient is most beneficial since it will reduce the incidence of secondary grains and refine the microstructure by reducing the dendrite arm spacing and thus assisting the solution heat treatment.4.5.1 Furnace Most process development is related to furnace design. secondary grains. (b) Donut Chil DSX. (c) Liquid Metal Cooling DSX [3] Fig. 13 Furnace Developments for DSX solidification (a) Conventional DSX.(a) (b) (c) Fig. 14 Combination Furnace [3] 37 . CRIMSON Method Layout [4] Table 4 Thermal Efficiency of CRIMSON Method [4] 38 . 15 University of Birmingham.Fig. Fig. 16 Schematics of CLA and CLV Processes [1] Fig. 17 Different Pouring Processes [5] 39 . Of these the most popular in Europe is a wax impregnated SLS marketed as ‘Castform’. repeatability. Selective Laser Sintering(SLS).g. and the limited number of units required deviation from conventional techniques used in IC. Complexity of the objects made it impossible to use milling in its fabrication.2 Direct Patterns (Rapid Prototyping) Over 100. sockets of constant wall thickness where tubular structural members are designed to mate. it was decided to prototype these parts using a stereolithography apparatus (SLA-250). This represents $250M worth of cast products.000 patterns were produced in the USA by rapid prototyping in 2007 of which 50% were for production standard castings. The advantage of this approach is the rapid pace at which the parts could be created using the CAD/CAM technology and its support to the freeform designs. hollowness.Fig. Solidscape etc. Keeping this in mind. The process used here is an innovative approach because the SL patterns are used to create wax patterns instead of being used as a substitute for wax and 40 . Thermojet. however cost and time constraints in addition to the accuracy. The manufacturing requirement that prompted this research is the fabrication of a complex swing frame structure. The technology has matured over the last 10 years and there are a number of established processes e.4. The structure had unique but complex geometry with varying radii and angles. Stereolithography. as well as sizes exceeding work envelope. 18 Molten Metal Filtration Using Cores [5] 5. IC was selected as the method of fabrication. The SLA parts could then be used to create the wax patterns for investment casting. Rapid Prototyping has also been used for direct core manufacture and proprietary trials are underway to produce integrated shells direct from rapid prototyping [3]. subsequently burnt out. to determine effect of solidification 41 . Dimensional inspection using laser technology to measure geometries has been used extensively for reverse engineering and is now becoming increasingly used for conventional measurements [3]. This process ensures the reusability of the SL pattern for future requirements. micro focus x-ray is well established and used to detect core position and micro cracks in both cores and castings. which is the current trend in the technology. salvageable or acceptable. A possible future development is the concept of ‘eyes off’ technology to detect and sentence defects using advanced vision technology.3 Non Destructive Testing/Evaluation It is likely that the industry can look forward to many significant advances in NDT and dimensional inspection.4. especially in the design of gating and feeding systems. alternative computer simulation software systems are available applying heat transfer models. Table 5 Rapid Prototyping for Investment Casting Application [8] 5. and working off the same master pattern improves the repeatability apart from the overall accuracy the process provides [6]. Considerable development efforts have been made to provide many solidification simulation models of value in investment casting production. 5. Fully integrated systems which use process modeling to assist with wax impression and core tooling design. based either on finite element or finite difference methods.4 Process simulation Process modeling to simulate mould fill and solidification has become an established aid for Manufacture. Trials are underway to use this technology to detect FPI indications and automatically sentence the part as either reject. Regarding NDT.4. Digital technology and computing power are forever increasing and the future for the industry will be totally influenced by the development of these technologies [3]. gating system design and the definition of casting parameters have yet to be fully implemented but remain a distinct possibility for the future. Currently. These are being utilized on the shop floor in many larger foundries. Digital x-ray is becoming increasingly established although concerns regarding standards have yet to be totally resolved. or single crystal components [1]. in many super alloys. that there are those who don’t know and those who don’t know they don’t know.5. It is possible to envisage a future foundry where the process is defined by knowledge systems and controlled by expert systems cognisant with defect detection and correction. therefore. Cost consciousness will increase demands for less human involvement and greater automation. 42 . are a few possible predictions: • • • • • • • Raw materials will become increasingly expensive and in some cases the earth’s limited resources will restrict their use without effective recycling. This will be increasingly true of metallic elements such as rhenium. DS (directionally solidified) columnar grains. 5. Increasing computing power and knowledge base systems such as neural networks will beincreasingly important to the industry. Future Influences on the Investment Casting Industry In the financial investment industry it is said about those who predict the future. rapid advancements in the solidification software show continual improvement in the ability to predict accurately many grain defects that can occur in the production of directionally solidified. DS. The factory would be self learning and self correcting with limited human interaction.conditions on alloy microstructure. Shortages of non metallic raw materials will cause the industry to investigate alternative materials Health and safety regulations will place increasing demands on manufacturing and may restrict the use of materials currently in common use. The use of simulation models plays a major role in the development of investment casting process for gas turbine blades. Additionally. The same could be said about the investment casting industry but it may be possible to be a little more certain than the financial Industry. In the meantime the investment casting industry will continue to grow and remain one of the essential strategic industries for the world [3]. and for accurate predictions of tooling dimensions. Here. or with single crystal. Engineering demands will place greater emphasis on dimensional control and geometric complexity. specified with equiaxed grains. B.com/pdfs-wm/39309. Investment casting – Wikipedia (http://en.wikipedia. Xiaojun Dai.pdf) [2].org/loyola/rp/10_03.pdf) [3].ac. Sandia National Laboratories.htm) [7].engineersedge.uk/protem/components/pdfs/SusTEM2011/T3S1_02_Birmingham_XDai_SusTEM2011. Jamal Nayfeh.com/documents/waxmatters/Wax %20Matters%20Issue%2012.aspx?ArticleID=2104) [8].wtec. Review of world Investment Casting Markets. Atwood.com/downloads/ml29.A Text book of Science and Technology of Casting Processes (Chapter 2 Progress in Investment Castings) by Ram Prasad published by INTECH.Investment Casting Using Stereolithography: Case of Complex Objects by Yasser A. http://www. USA published in Engineers edge. (http://cdn. (http://www. Newcastle. Hosni. (http://research. (http://www.htm.com/watch? v=CjKMd-0Dgl8) [10]. Florida.History. http://www. David A Ford (http://www.ncl.org/wiki/Lost-wax_casting) 43 .Latest Trends in Investment Casting by Dr. Ravindran Sundaram University of Central Florida.htm.Reduction of Energy Consumption in Investment Casting Process by application of a New Casting Facility Mark Jolly.wtec.com/article. 12th World Conference on Investment Casting. Lost-wax casting – Wikipedia (http://en. The A to Z of Materials (http://www.org/loyola/rp/10_02. References: [1].com/investment_casting_article.com/downloads/tl6. Materials and The Future.Future Trends in Investment Casting – Drivers for Development.6.Ing.pdf) [4]. 2008.Milan Horáček Brno University of Technology (http://www. Binxu Zeng University of Birmingham presented at Sustainable Thermal Energy Management in the Process Industries meet (SusTEM2011). (http://www.org/loyola/rp/10_01. new Investment Casting Video (http://www.intechopen.azom.youtube.wikipedia. Orlando.Williams R.Rapid Prototyping in Investment Casting by Clinton L.RLM Industries Inc.investmentcastingwax. 25th-26th October 2011.investmentcastingwax.blayson.Investment Casting . Oct.pdf) [5].pdf) [6].htm) [9].wtec.org/wiki/Investment_casting) [11].