Volume 15 - Casting Part 13 pot

Additional information on the applications and processing of investment cast nickel-base heat-resistant alloys can be found in the articles "Classification of Processes and Flow Chart of

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In addition to creep strength and corrosion resistance, two other properties stability, and resistance to thermal are important considerations in the selection of nickel-base heat-resistant casting alloys Thermal-fatigue resistance is partially controlled by composition, but it is also significantly affected by grain-boundary area and alignment relative to applied stresses The crystallographic orientation of grains also influences thermal stresses because the modulus of elasticity, which directly influences thermal stresses, varies with grain orientation The stability of property values is directly influenced by metallurgical stability; any microstructural changes that take place during long-term exposure at high temperatures under stress cause attendant changes in properties For example, if the γ' phase coarsens, strength decreases Also, potentially deleterious topologically close-packed (tcp) secondary phases, such as σ, Laves, and , may form Coarsening of γ' can be controlled to some degree by adjusting alloy additions Formation of tcp phases is

fatigue controlled by adjusting the composition of the matrix to minimize the electron vacancy number, Nv A high Nv indicates a

tendency toward the formation of tcp phases In general, an Nv value below 2.4 indicates minimal formation of deleterious phases; however, this relationship varies with base-alloy composition The metallurgical structures of both cast and

wrought heat-resistant alloys are discussed in greater detail in Metallography and Microstructures, Volume 9 of ASM Handbook, formerly 9th Edition Metals Handbook

Alloys 713C and 713LC are closely related investment casting alloys used principally for low-pressure turbine airfoils

in gas turbines Intended for operation at intermediate temperatures from 790 to 870 °C (1450 to 1600 °F), these alloys are generally used in uncooled airfoil designs

Alloy 738X is an investment casting alloy similar in strength to Alloy 713C and Udimet 700 but with outstanding sulfidation resistance It is used principally for latter-stage turbine airfoils and for hot-corrosion-prone applications such

as industrial and marine engines

Udimet 700, although primarily a wrought alloy, is also used in investment cast high-pressure turbine blades In cast form, it is similar in strength to Alloy 713C but offers better hot-corrosion resistance It is designed for operation at intermediate temperatures from 730 to 900 °C (1350 to 1650 °F) A stability-controlled version of U-700 is known as René 77

Alloy 100 is designed for use at metal temperatures up to about 980 °C (1800 °F) in cooled and uncooled airfoils A stability-controlled version of Alloy 100 is known as René 100

B-1900, to which 1% Hf is usually added to improve ductility and thermal-fatigue resistance, is designed for use at metal temperatures up to about 980 °C (1800 °F) in cooled and uncooled airfoils

René 80 offers excellent corrosion resistance in sulfur-bearing environments It is designed for use at metal temperatures

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Fig 3 Various radial and axial turbine wheels made from Mar-M-247 alloy Courtesy of Howmet Corporation,

Whitehall Casting Division

DS MAR-M 200 + Hf is produced by directional solidification (see discussion below) and is designed for metal temperatures of about 1010 to 1040 °C (1850 to 1900 °F) It is used in cooled airfoils

Other alloys (such as Udimet 500) are occasionally used in turbine airfoil applications, and Alloy 718 has been cast into large static structures for gas turbines Additional information on the applications and processing of investment cast nickel-base heat-resistant alloys can be found in the articles "Classification of Processes and Flow Chart of Foundry Operations" and "Investment Casting" in this Volume

Alloys for directional and single-crystal solidification possess high elevated-temperature strengths

Directionally solidified turbine blades have high strength in the direction of principal stress (the longitudinal direction) because grain boundaries are aligned parallel to this direction Thus, the effect of grain boundaries on properties is minimized

Single-crystal alloys have no grain boundaries and therefore require no grain-boundary strengthening elements For this reason, they can be solution heat treated at higher temperatures than conventional alloys, precipitating a greater amount of the γ' strengthening phase The lack of grain boundaries also enhances the corrosion resistance of these materials Table 2 lists several DS/SC alloy compositions A turbine vane made from CM-247-LC DS alloy is shown in Fig 4 Properties and performance of DS/SC alloys are detailed in Ref 1, 2, 3, and 4

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Fig 4 Directionally solidified turbine vane made from CM-247-LC alloy Courtesy of Thyssen Guss AG

References cited in this section

1 K Harris, G.L Erickson, and R.E Schwer, "Development of the Single-Crystal Alloys CM SX-2 and CM SX-3 for Advanced Technology Turbine Engines," Technical Paper 83-GT-244, American Society of Mechanical Engineers

2 K Harris, G.L Erickson, and R.E Schwer, "Directionally Solidified DS CM 247 LC Optimized Mechanical Properties Resulting From Extensive γ' Solutioning," Paper presented at the Gas Turbine Conference and Exhibit, Houston, TX, March 1985

3 K Harris, G.L Erickson, R.E Schwer, J Wortmann, and D Froschhammer, "Development of Low-Density Single-Crystal Superalloy CMSX-6," Technical Paper, Cannon-Muskegon Corporation

4 K Harris, G.L Erickson, and R.E Schwer, "CMSX Single Crystal, CM DS & Integral Wheel Alloys Properties and Performance," Paper presented at the Cost 50/501 Conference, High Temperature Alloys for Gas Turbines and Other Applications, Liège, Oct 1986

Melting Practice

Electric induction furnaces have become the mainstay of the foundry industry for small heat sizes, especially when a

number of different alloys are produced They are also the least expensive of the major furnace types to install The foundry industry uses these furnaces in sizes ranging from 9 kg to 18 Mg (20 lb to 20 tons); however, most electric induction furnaces are in the 25 to 1350 kg (50 to 3000 lb) range

Figure 5 shows a cross section of an induction furnace The furnace shell rests on trunnions, which tilt the furnace during tapping A copper coil surrounds the furnace lining and the charge materials inside The metal charge is melted by its resistance to the current induced by a magnetic field when current flows through the coil More detailed information on induction furnaces can be found in the article "Melting Furnaces: Induction Furnaces" in this Volume

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Fig 5 Cross section of a coreless electric induction furnace

Vacuum Melting Nickel-base alloys containing more than about 0.2% of the reactive elements aluminum, titanium,

and zirconium are not suitable for melting and casting in oxidizing environments such as air At the higher alloying levels, these elements readily oxidize, resulting in gross inclusions, oxide laps, and poor composition control Consequently, such alloys generally require inert gas injection or vacuum melting and casting methods Extralow gas contents, which can be obtained by vacuum melting, are also required for certain nickel-base alloys Vacuum melting processes, which are described in the article "Vacuum Melting and Remelting Processes" are always used for directional solidification and single-crystal casting alloys

Metal Treatment

Argon Oxygen Decarburization (AOD) Some foundries have recently installed AOD units to achieve some of the

results that vacuum melting can produce The AOD unit looks very much like a Bessemer converter with tuyeres in the lower side-walls for the injection of argon or nitrogen and oxygen These units must be charged with molten metal from

an arc or induction furnace About 20%, but usually less, cold charge consisting of solid virgin material can be added to

an AOD unit The continuous injection of gases causes a violent stirring action and intimate mixing of slag and metal, which can lower sulfur values to below 0.005% The gas contents (hydrogen, nitrogen, and oxygen) may be even lower than those of vacuum induction melted alloys More information on AOD processing is available in the section "Argon Oxygen Decarburization" of the article "Degassing Processes (Converter Metallurgy)" in this Volume

Electroslag remelting furnaces represent another type of equipment that may see some use in the high-alloy

foundry in the next decade Electroslag remelting machines have been used for many years by the wrought steel companies to produce refined ingots In the Soviet Union, electroslag remelting is being used to cast shapes, and the technology is being evaluated in the United States as well The process works by taking an ingot (which becomes the electrode), remelting it in stages under molten slag to refine it, and then resolidifying the metal in a water-cooled mold See the section "Electroslag Remelting (ESR)" in the article "Vacuum Melting and Remelting Processes" in this Volume

Plasma Refining Steadily increasing requirements for alloy cleanliness have led producers to adopt several novel

refining technologies and process routes, many involving increased use of the ladle as a refining vessel Such procedures

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require longer holding times in the ladle, which necessitate either increased superheats in the furnace or external heating

in the ladle to avoid early solidification Higher superheat, in addition to requiring excessive energy expenditure, can contribute to the problem of melt contamination The preferred solution is to supply heat to the ladle, maintaining the alloy at minimal superheat during refining This can be accomplished by the transferred arc plasma torch, with the added benefit of enhanced refining reactions that aid in the production of clean metal with low levels of residual elements In this work, experiments have been carried out in an induction furnace equipped with a gas-stabilized graphite electrode to investigate the control of oxygen and induction levels and the enhancement of desulfurization afforded by the transferred arc plasma See the section "Plasma Heating and Degassing" in the article "Degassing Processes (Ladle Metallurgy)" in this Volume

Foundry Practice

Foundry practice for nickel-base alloys is for the most part similar to that used for cast stainless steels (see the article

"High-Alloy Steels" in this Volume) Specific aspects of foundry practice discussed here include pouring, gating and risering, cleaning, welding, and heat treatment of conventional corrosion-resistant nickel-base alloy castings The processing of investment cast and DS/SC alloys is reviewed in the articles "Investment Casting" and "Directional and Monocrystal Solidification", respectively, in this Volume

Pouring Practice

Three types of ladles are used for pouring nickel-base castings: bottom pour, teapot, and lip pour Ladle capacity normally ranges from 45 kg to 36 Mg (100 lb to 40 tons), although ladles having much larger capacities are available

The bottom-pour ladle has an opening in the bottom that is fitted with a refractory nozzle (Fig 6) A stopper rod,

suspended inside the ladle, pulls the stopper head up from its seat in the nozzle, allowing the molten alloy to flow from the ladle When the stopper head is returned to the position shown in Fig 6, the flow is cut off The position of the stopper head is controlled manually by the slide-and-rack mechanism shown at the left in Fig 6

Bottom pouring is preferred for pouring large castings from large ladles, because it is difficult to tip

a large ladle and still control the stream of molten steel Also, the bottom-pour ladle delivers cleaner metal to the mold Inclusions, pieces of ladle lining, and slag float to the top of the ladle; thus, bottom pouring greatly reduces the risk of passing nonmetallic particles into the mold cavity

On the other hand, it is impractical to pour molten metal into small molds from a large bottom-pour ladle The pressure head created by the metal remaining in the ladle delivers the molten metal too fast Also, the time required to fill a small mold is short, thus requiring that a large bottom-pour ladle be opened and closed many times in order to empty it This may cause the ladle to leak, although special nozzles have been developed to minimize leakage Despite the fact that the size of bottom-pour ladles could be scaled down for pouring smaller castings, this is unnecessary because of the almost equal ability of the teapot ladle to deliver clean metal

The teapot ladle incorporates a ceramic wall, or

baffle, that separates the bowl of the ladle from the spout The baffle extends almost four-fifths of the distance to the bottom of the ladle (Fig 7) As the ladle is tipped, hot metal flows from the bottom of the ladle up the spout and over the lip Because the metal is taken from near the bottom of the ladle, it is

Fig 6 Typical bottom-pour ladle used to pour large castings

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free of slag and pieces of eroded refractory The teapot design is feasible in various sizes, generally covering the entire range of casting sizes that are below the minimum size for which the bottom-pour ladle is used

Lip-pour ladles resemble the teapot type but have no baffles

to hold back the slag Because the hot metal is not taken from the bottom of the ladle, this type of ladle pours a more contaminated metal and is seldom used to pour high-alloy castings Nevertheless, it is widely used as a tapping ladle (at the melting furnace) and as a transfer ladle to feed smaller ladles of the teapot type

Pouring Time Ideally, the optimum pouring time for a given

casting would be determined by the weight and shape of the casting, the temperature and solidification characteristics of the molten metal, and the heat transfer and thermal stability characteristics of the mold However, most foundries are required to pour may different castings from one heat or even from one ladle Therefore, rather than attempting to control pouring time directly, foundries control the speed with which molten steel enters the mold cavity This control is achieved through the design of the gating system

Gating Systems

An effective gating system for pouring nickel-base alloys, as well as other metals, into green sand molds is one that fills the mold as rapidly as possible without developing pronounced turbulence It is essential that the mold be filled rapidly to minimize temperature variations within the metal; this results in optimized control of solidification

Turbulent metal flow is harmful because it breaks up the metal stream, exposing more surface area to air and forming metallic oxides The oxides can rise to the top of the mold cavity, resulting in a rough surface of inclusions in the casting

In addition, turbulent flow erodes the mold material These eroded particles also float to the top of the mold cavity

Preferred Metal Flow According to preferred practice, the pourer directs the metal stream toward the pouring cup at

the top of the mold, controlling the pouring rate to keep the cup full of molten metal throughout the pouring cycle The opening in the bottom of the cup is directly over the sprue, or downgate, which is tapered at the bottom, thus reducing the diameter of the stream of descending metal The taper prevents the stream from pulling away from the walls and drawing air into the gating system The descending metal impinges on the sprue well at the bottom of the sprue, and the direction

of flow changes from vertical to horizontal, with the metal flowing along runners to gates (ingates), and then to the main body of the casting A gating system that incorporates these features is shown in Fig 8

Fig 7 Typical teapot ladle used to pour small- to

medium-size castings

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Fig 8 Gating system for good metal flow

Gating system design largely determines the manner in which molten metal is fed into the mold, as well as the rate of

feeding The number of gates influences the distribution of the flow between gates A good design has even distribution between gates both initially and while the mold is filling The distribution of flow in the gating system affects the type of flow that occurs in the main body of the casting A large difference in the flow between gates creates a swirl of metal in the mold about a vertical axis, in addition to that occurring about a horizontal axis

The gating system shown in Fig 8 is an example of a so-called finger-type parting line system, in which the fingers feed metal to the casting just above the parting line Other major types of gating systems used in alloy foundries include the bottom gate, which feeds metal to the bottom of the casting, and the step gate, which feeds metal through a number of stepped gates, one above another In the system shown in Fig 8, the ratio of the cross-sectional area of the choke of the sprue to that of all of the runners emanating from the sprue basin and to all of the gates is 1:4:4 As shown in Fig 8, the runner area decreases progressively by an amount equal to the area of each gate it passes This practice ensures that, once the system is filled with metal, it remains full during the pouring cycle and feeds equally to each gate

Furthermore, the gates are located in the cope, while the runner, which extends beyond the last gate, is located in the drag Extension of the runner serves as a trap for the first, and usually the most contaminated, metal to enter the system The entire runner must fill before the metal will rise to the level of the gates Thus, each gate begins feeding at the same time The runners and gates are curved wherever a change in direction occurs This streamlining reduces turbulence in the metal stream and minimizes mold erosion

In contrast to the ratio of the system shown in Fig 8 (1:4:4), if the total cross-sectional area of the gates is less than that of the runners (1:2:1, for example), the result is a pressurized system The metal squirts into the mold cavity and flows turbulently over the mold bottom, which can cause roughening of the bottom surfaces

Conversely, if the total cross-sectional area of the gates is significantly greater than that of the runners (1:2:3, for example), the gating system will be incompletely filled, and flow from the gates will be uneven This condition increases the likelihood of mold erosion When this type of system is required, complicated additions to gating systems are used, including whirlgates, horn gates, strainer cores, tangential gates, and slit gates However, any addition to the gating system usually increases the cost of the casting because all gating must be removed More detailed information on gating practice can be found in the article "Gating Design" in this Volume

Mold Erosion In addition to the contribution of gating design to a reduction in mold erosion, further reduction can be

achieved by making the gating system out of tile, which is superior to green sand in erosion resistance However, the use

of tile is generally limited to gating systems for large castings, where the quantity and speed of molten metal passing through the gating system would seriously erode green sand and where precise control of the flow rate is less critical Thus, gating systems for smaller castings are rammed in sand, usually with a semicircular or rectangular cross section for the gates and runners

Risers

Molten nickel-base alloys contract approximately 0.9% per 55 °C (100 °F) as they cool from the pouring temperature to the solidification temperature They then undergo solidification contraction of 3% during freezing, and finally the solidified metal contracts 2.2% during cooling to room temperature Therefore, when casting nickel alloys, an ample supply of molten metal must be available from risers (reservoirs) to compensate for the volume decrease, or shrinkage cavities will develop in the locations that solidify last

Because feed from the riser occurs by gravity, risers are usually located at the top of the casting Riser forms are placed

on the pattern and molded into the cope half of the mold The riser cavity is usually open to the top of the mold, although blind risers are sometimes used

Riser Size and Shape As described in the article "Riser Design" in this Volume, formulas based on surface area,

volume, and freezing time of the casting are used to determine riser size Most risers are cylindrical in shape, with their heights approximately equal to their diameters This configuration provides a low ratio of surface area to volume, which prolongs the time the metal in the riser remains liquid

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Placement of a riser, in conjunction with its size, determines its effectiveness The thicker sections of a casting act as

reservoirs for feeding the thinner sections, which solidify first Thus, risers are placed over thick sections that cannot be fed by other areas of the casting Demonstrating this principle, the gear blank casting shown in Fig 9 is provided with a large riser over the central hub and six smaller risers spaced equally around the rim of the gear to ensure adequate feeding Metal enters the mold at the two gates, which are 180° apart

Feeding Distance Castings of uniform thickness

present a different problem Studies have established the feeding distances of a riser for various rectangular shapes in both the horizontal and vertical planes, with and without an end effect (that is, the extra cooling provided by the sand cover of an end surface)

For a uniform section, the maximum feeding distance can be extended by adding a taper The progressively thicker section solidifies in a progressively longer time, so that a favorable temperature gradient is established from the end of the section to the riser A tapered pad of exothermic material placed in the mold along the length of the casting will also produce a favorable temperature gradient

Welding

Cast Nickel Alloy CZ-100 can be readily repair welded or joined to other castings or to wrought forms by using any of

the usual welding processes with suitable nickel rod and wire Joints or cavities must be carefully prepared for welding because small amounts of sulfur or lead cause weld embrittlement

Nickel-Copper Alloys The weldability of the nickel-copper alloys decreases with increasing silicon content, but is

adequate up to at least 1.5% Si Niobium can enhance weldability, particularly when small amounts of low-melting residuals are present Careful raw material selection and proper foundry practice, however, have largely eliminated any difference in weldability between niobium-containing and niobium-free grades

The higher-silicon compositions (≥3.5% Si) are not considered weldable They can be brazed or soldered, as can the lower-silicon grades

Nickel-Chromium-Iron Alloys The CY-40 castings can be repair welded or fabrication welded to matching wrought

alloys by any of the usual welding processes Rod and wire of matching nickel-chromium contents are available Postweld heat treatment is not required after repair welding or fabrication, because the heat-affected zone is not sensitized by the weld heat

Nickel-Chromium-Molybdenum Alloys Alloys CW-12MW and CW-7M can be welded by any of the usual

welding processes, using wire or rod of matching composition For optimum weldability, carbon content should be as low

as practicable The usual practice is to solution treat and quench after repair welding Heat treatment after welding is generally necessary because these alloys are subject to sensitization in the heat-affected zone and because intermetallic precipitates may form in the heat-affected zone

Nickel-Molybdenum Alloys Alloys N-12MV and N-7M can be welded by using any of the usual welding processes

with wire or rod of matching composition Postweld heat treatment is usually performed because these alloys are subject

to the precipitation of intermetallic compounds in the heat-affected zone

Heat Treatment

Cast nickel (alloy CZ-100) is used in the as-cast condition Some other alloys are also used as-cast, but most require

some type of thermal treatment to develop optimum properties

Fig 9 Gating and feeding system used to cast gear blanks

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Nickel-copper alloys are used in the as-cast condition Homogenization at 815 to 925 °C (1500 to 1700 °F) may,

under some conditions, improve corrosion resistance slightly, but in most corrosive conditions, alloy performance is not affected by the minor segregation present in the as-cast alloy

At about 3.5% Si, silicon begins to have an age-hardening effect The resultant combination of aging and the formation of hard silicides when the silicon content exceeds about 3.8% can cause considerable difficulty in machining Softening is accomplished by a solution heat treatment, which consists of heating to 900 °C (1650 °F), holding at temperature for 1 h per 25 mm (1 in.) of section thickness, and oil quenching Maximum softening is obtained by oil quenching from 900 °C (1650 °F), but such treatment is likely to result in quench cracks in castings with complex shapes or varying section thickness

In the solution heat treatment of complicated or varying-section castings, it is advisable to charge them into a furnace below 315 °C (600 °F) and heat to 900 °C (1650 °F) at a rate that limits the maximum temperature difference within the casting to about 56 °C (100 °F) After being soaked, castings should be transferred to a furnace held at 730 °C (1350 °F), allowed to equalize in temperature, and then oil quenched Alternatively, the furnace can be rapidly cooled to 730 °C (1350 °F), the casting temperature can be equalized, and the castings can be quenched in oil Solution heat treated castings are age hardened by placing them in a furnace held at 315 °C (600 °F), heating uniformly to 595 °C (1100 °F), holding at 595 °C (1100 °F) for 4 to 6 h, and air or furnace cooling

Nickel-Chromium-Iron Alloys Alloy CY-40 is used in the as-cast condition because it is insensitive to the

intergranular attack encountered in as-cast or sensitized stainless steels A modified cast nickel-chromium-iron alloy for nuclear applications with 0.12% C (max) is usually solution treated as an additional precaution

Sensitization in the heat-affected zone is not a problem with CY-40 Unless residual stresses pose a problem, postweld heat treatment is therefore not required

Nickel-Chromium-Molybdenum Alloys The high chromium and molybdenum contents of CW-12MW and CW-7M

result in the precipitation of carbides and intermetallic compounds in the as-cast condition, which can be detrimental to corrosion resistance, ductility, and weldability These alloys should therefore be solution treated at a temperature of 1175

to 1230 °C (2150 to 2250 °F) and water or spray quenched

Nickel-Molybdenum Alloys Slow cooling in the mold is detrimental to the corrosion resistance, ductility, and

weldability of N-12MV and N-7M These alloys should therefore be solution treated at a minimum temperature of 1175

°C (2150 °F) and water quenched

Specific Applications

Corrosion-resistant nickel-base castings are primarily used in fluid-handling systems with matching wrought alloys; they are also commonly used for pump and valve components or for applications with crevices and velocity effects requiring a superior material in a wrought stainless system Because of their relatively high cost, nickel-base alloys are usually selected only for severe service conditions in which maintenance of product purity is of great importance and for which less costly stainless steels or other alternative materials are inadequate Detailed information on the corrosion resistance of

nickel-base alloys in aqueous media is available in the article "Corrosion of Nickel-Base Alloys" in Corrosion, Volume

13 of ASM Handbook, formerly 9th Edition Metals Handbook

In the application of heat-resistant alloys, considerations include:

• Resistance to corrosion (oxidation) at elevated temperatures

• Stability (resistance to warping, cracking, or thermal fatigue)

• Creep strength (resistance to plastic flow)

Numerous applications of cast heat-resistant nickel-base alloys were discussed earlier in this article Information on the high-temperature corrosion resistance of these alloys is available in the articles "Fundamentals of Corrosion in Gases,"

"General Corrosion" (see the section "High-Temperature Corrosion"), and "Corrosion of Metal Processing Equipment"

(see the section "Corrosion of Heat-Treating Furnace Accessories") in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

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Cast Nickel Nickel castings are most commonly used in the manufacture of caustic soda and in processing with caustic

(see the section "Corrosion by Alkalies and Hypochlorite" in the article "Corrosion in the Chemical Processing Industry"

in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook) As the temperature and caustic

soda concentration increase, austenitic stainless steels are useful only up to a point The copper and chromium-iron alloys take over as useful alloys under these conditions, while cast nickel is selected for the higher caustic concentrations, including fused anhydrous soda Minor amounts of such elements as oxygen and sulfur can have profound effects on the corrosion rate of nickel in caustic Detailed corrosion data should therefore be consulted before making a final alloy selection

nickel-Nickel-Copper Alloys The principal advantages of the Ni-30Cu alloys are high strength and toughness, coupled with

excellent resistance to mineral acids, organic acids, salt solution, food acids, strong alkalies, and marine environments The most common applications for nickel-copper castings are in the manufacture of, and processing with, hydrofluoric acid and the handling of salt water, neutral and alkaline salt solutions, and reducing acids

Nickel-chromium-iron alloys are commonly used under oxidizing conditions to handle high-temperature corrosives

or corrosive vapors where stainless steels might be subject to intergranular attack or stress-corrosion cracking In recent years, the CY-40-type alloy has found large-scale application in handling hot boiler feedwater in nuclear plants because

of a greater margin of safety over stainless steels More information on this application is available in the article

"Corrosion in the Nuclear Power Industry" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Nickel-chromium-molybdenum alloys are probably the most common materials for upgrading a system in which

service conditions are too demanding for either standard or special stainless steels because of severe combinations of acids and elevated temperatures These cast alloys can be used in conjunction with similar wrought materials, or they can serve to upgrade pump and valve components in a wrought stainless steel system

Nickel-molybdenum alloys have specialized application areas, primarily in the handling of hydrochloric acid at all

temperatures and concentrations Applications should not be based on upgrading in areas where stainless steels are inadequate, because the nickel-molybdenum alloys are unsuitable for handling most oxidizing solutions for which stainless steels are used

Alloys for directional and single-crystal solidification are used as blades for aircraft and some land-based

turbines (Fig 1 and 4) Under elevated temperatures, they have very high strength in the direction of primary stress

References

1 K Harris, G.L Erickson, and R.E Schwer, "Development of the Single-Crystal Alloys CM SX-2 and CM SX-3 for Advanced Technology Turbine Engines," Technical Paper 83-GT-244, American Society of Mechanical Engineers

2 K Harris, G.L Erickson, and R.E Schwer, "Directionally Solidified DS CM 247 LC Optimized Mechanical Properties Resulting From Extensive γ' Solutioning," Paper presented at the Gas Turbine Conference and Exhibit, Houston, TX, March 1985

3 K Harris, G.L Erickson, R.E Schwer, J Wortmann, and D Froschhammer, "Development of Low-Density Single-Crystal Superalloy CMSX-6," Technical Paper, Cannon-Muskegon Corporation

4 K Harris, G.L Erickson, and R.E Schwer, "CMSX Single Crystal, CM DS & Integral Wheel Alloys Properties and Performance," Paper presented at the Cost 50/501 Conference, High Temperature Alloys for Gas Turbines and Other Applications, Liège, Oct 1986

Selected References

• W.J Jackson, Ed., Steel Castings Design Properties and Applications, Steel Castings Research and Trade

Association, 1983

• J.D Redmond, Selecting Second-Generation Duplex Stainless Steels, Chem Eng., Oct 1986 and Nov 1986

• Steel Castings Handbook, Supplement 8, High Alloy Data Sheets, Corrosion Series, Steel Founders' Society

of America, 1981

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• Steel Castings Handbook, Supplement 9, High Alloy Data Sheets, Heat Series, Steel Founders' Society of

America, 1981

• P.F Wieser, Ed., Steel Castings Handbook, 5th ed., Steel Founders' Society of America, 1980

Titanium and Titanium Alloys

Jeremy R Newman, Titech International Inc.; Daniel Eylon, University of Dayton; John K Thorne, Precision Castparts Corporation

Introduction

SINCE THE INTRODUCTION OF TITANIUM and titanium alloys in the early 1950s, these materials have in a relatively short time become one of the backbone materials for the aerospace, energy, and chemical industries (Ref 1) The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material for many critical applications Today, titanium alloys are used for static and rotating gas turbine engine components Some of the most critical and highly stressed civilian and military airframe parts are made of these alloys

Net Shape Technology Development The use of titanium has expanded in recent years from applications in

nuclear power plants to food processing plants, from oil refinery heat exchangers to marine components and medical prostheses (Ref 2) However, the high cost of titanium alloy components may limit their use to applications for which lower-cost alloys, such as aluminum and stainless steels, cannot be used The relatively high cost is often the result of the intrinsic raw material cost of the metal, fabricating costs, and, usually most important, the metal removal costs incurred in obtaining the desired end-shape As a result, in recent years a substantial effort has been focused on the development of net shape or near-net shape technologies to make titanium alloy components more competitive (Ref 3) These titanium net shape technologies include powder metallurgy (PM), superplastic forming (SPF), precision forging, and precision casting Precision casting is by far the most fully developed and the most widely used net shape technology

Casting Industry Growth The annual shipment of titanium castings increased by 240% between 1978 and 1986 (Fig

1) and titanium casting is the fastest growing segment of titanium technology

Even at current levels (approaching 450 Mg, or 1 × 106 lb, annually), castings still represent less than 2% of total titanium mill product shipments This is in sharp contrast

to the ferrous and aluminum industries, where foundry output is 9% (Ref 5) and 14% (Ref 6) of total output, respectively This suggests that the growth trend of titanium castings will continue as users become more aware of industry capability, suitability of cast components

in a wide variety of applications, and the net shape cost advantages

Properties Comparable to Wrought The term

castings often connotes products with properties generally inferior to wrought products This is not true with titanium cast parts They are generally comparable to wrought products in all respects and quite often superior Properties associated with crack propagation and creep resistance can

be superior to those of wrought products As a result, titanium castings can be reliably substituted for forged and machined parts in many demanding applications (Ref 7, 8) This is due to several unique properties of titanium alloys One is the α+ β-to-β allotropic phase transformation at a temperature range of 705 to 1040 °C (1300 to 1900 °F), which is well below the solidification temperature of the alloys As a result, the cast dendritic β structure is wiped out during the solid state cooling stage, leading to an α+ β platelet structure (Fig 2a), which is also typical of β processed wrought alloy Further, the convenient allotropic transformation temperature range of most titanium alloys enables the as-cast microstructure to be improved by means of postcast cooling rate changes and subsequent heat treatment

Fig 1 Growth of 240% in United States titanium

casting shipments from 1978 to 1986 Source: Ref 4

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Fig 2 Comparison of the microstructures of (a) as-cast versus (b) cast + HIP Ti-6Al-4V alloys illustrating lack

of porosity in (b) Grain boundary α (B) and α plate colonies (C) are common to both alloys; β grains (A), gas (D), and shrinkage voids (E) are present only in the as-cast alloy

Reactivity Another unique property is the high reactivity of titanium at elevated temperatures, leading to an ease of

diffusion bonding As a result, hot isostatic pressing (HIP) of titanium castings yields components with no subsurface porosity At the HIP temperature range (820 to 980 °C, or 1500 to 1800 °F) titanium dissolves any microconstituents deposited upon internal pore surfaces, leading to complete healing of casting porosity as the pores are collapsed during the pressure and heat cycle Both the elimination of casting porosity and the promotion of a favorable microstructure improve mechanical properties However, the high reactivity of titanium, especially in the molten state, presents a special challenge to the foundry Special, and sometimes relatively expensive, methods of melting (Ref 9), moldmaking, and surface cleaning (Ref 7, 8) may be required to maintain metal integrity Additional information on HIP of castings may be found in the article "Hot Isostatic Pressing of Castings" in this Volume

References

1 H.B Bomberger, F.H Froes, and P.H Morton, Titanium A Historical Perspective, in Titanium Technology:

Present Status and Future Trends, F.H Froes, D Eylon, and H.B Bomberger, Ed., Titanium Development

Association, 1985, p 3-17

2 Titanium for Energy and Industrial Applications, D Eylon, Ed., The Metallurgical Society, 1981, p 1-403

3 Titanium Net Shape Technologies, F.H Froes and D Eylon, Ed., The Metallurgical Society, 1984, p 1-299

4 "Titanium 1986, Statistical Review 1978-1986," Annual Report of the Titanium Development Association,

1987

5 American Foundrymen's Society, private conversation, 1987

6 Aluminum Association, private conversation, 1987

7 D Eylon, F.H Froes, and R.W Gardiner, Developments in Titanium Alloy Casting Technology, J Met., Vol

35 (No 2), Feb 1983, p 35-47; also, in Titanium Technology: Present Status and Future Trends, F.H Froes,

D Eylon, and H.B Bomberger, Ed., Titanium Development Association, 1985, p 35-47

8 D Eylon and F.H Froes, "Titanium Casting A Review," in Titanium Net Shape Technologies, F H Froes

and D Eylon, Ed., The Metallurgical Society, 1984, p 155-178

9 H.B Bomberger and F.H Froes, The Melting of Titanium, J Met., Vol 36 (No 12), Dec 1984, p 39-47; also,

in Titanium Technology: Present Status and Future Trends, F.H Froes, D Eylon, and H.B Bomberger, Ed.,

Titanium Development Association, 1985, p 25-33

Historical Perspective of Casting Technology

Although titanium is the fourth most abundant structural metal in the earth's crust (0.4 to 0.6 wt%) (Ref 9), it has emerged only recently as a technical metal This is the result of the high reactivity of titanium, which requires complex methods and high energy input to win the metal from the oxide ores The required energy per ton is 1.7 times that of aluminum and

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16 times that of steel (Ref 10) From 1930 to 1947, metallic titanium extracted from the ore as a powder or sponge form was processed into useful shapes by P/M methods to circumvent the high reactivity in the molten form (Ref 11)

Melting Methods The melting of small quantities of titanium was first experimented with in 1948 using methods such

as resistance heating, induction heating, and tungsten arc melting (Ref 12, 13) However, these methods never developed into industrial processes The development during the early 1950s of the cold crucible, consumable-electrode vacuum arc melting process, "skull melting," by the U.S Bureau of Mines (Ref 13, 14) made it possible to melt large quantities of contamination-free titanium into ingots or net shapes Additional information on numerous melting methods is available

in the articles "Melting Furnaces: Electric Arc Furnaces," "Melting Furnaces: Induction Furnaces," "Melting Furnaces: Reverberatory Furnaces and Crucible Furnaces," "Melting Furnaces: Cupolas," and "Vacuum Melting and Remelting Processes" in this Volume

First Castings Shape casting of titanium was first demonstrated in the United States in 1954 at the U.S Bureau of

Mines using machined high-density graphite molds (Ref 13, 15) The rammed graphite process developed later, also by the U.S Bureau of Mines (Ref 16), led to the production of complex shapes This process, and its derivations, are used today to produce parts for marine and chemical-plant components such as the pump and valve components shown in Fig 3(a) Some aerospace components such as the aircraft brake torque tubes, landing arrestor hook, and optic housing shown

in Fig 3(b) have also been produced by this method

Fig 3 Typical titanium parts produced by the rammed graphite process (a) Pump and valve components for

marine and chemical-processing applications (b) Brake torque tubes, landing arrestor hook, and optic housing components used in aerospace applications

9 H.B Bomberger and F.H Froes, The Melting of Titanium, J Met., Vol 36 (No 12), Dec 1984, p 39-47; also, in Titanium Technology: Present Status and Future Trends, F.H Froes, D Eylon, and H.B

Bomberger, Ed., Titanium Development Association, 1985, p 25-33

10 E.W Collings, Physical Metallurgy of Titanium Alloys, American Society for Metals, 1984

11 "Titanium: Past, Present and Future," NMAR-392, National Materials Advisory Board, National Academy Press, 1983; also, PB83-171132, National Technical Information Service

12 W.J Kroll, C.T Anderson, and H.L Gilbert, A New Graphite Resistor Vacuum Furnace and Its Application

in Melting Zirconium, Trans AIME, Vol 175, 1948, p 766-773

13 R.A Beahl, F.W Wood, J.O Borg, and H.L Gilbert, "Production of Titanium Castings," Report 5265, U.S Bureau of Mines, Aug 1956, p 42

14 A.R Beall, J.O Borg, and F.W Wood, "A Study of Consumable Electrode Arc Melting," Report 5144, U.S Bureau of Mines, 1955

15 R.A Beahl, F.W Wood, and A.H Robertson, Large Titanium Castings Produced Successfully, J Met., Vol

7 (No 7), July 1955, p 801-804

16 S.L Ausmus and R.A Beahl, "Expendable Casting Molds for Reactive Metals," Report 6509, U.S Bureau

of Mines, 1964, p 44

Molding Methods

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Rammed Graphite Molding The traditional rammed graphite molding process uses powdered graphite mixed with

organic binders (see the article "Rammed Graphite Molds" in this Volume) Patterns typically are made of wood The mold material is pneumatically rammed around the pattern and cured at high temperature in a reducing atmosphere to convert the organic binders to pure carbon The molding process and the tooling are essentially the same as for cope and drag sand molding in ferrous and nonferrous foundries In the 1970s, derivations of rammed graphite mold materials were developed using components of more traditional sand foundries along with inorganic binders This resulted in more dimensionally stable and less costly molds that were capable of containing molten titanium without undue metal/mold reaction

Lost Wax Investment Molding The principal technology that allowed the proliferation of titanium alloy castings in

the aerospace industry was the investment casting method, introduced in the mid-1960s (see the article "Investment Casting" in this Volume) This method, used at the dawn of the metallurgical age for casting copper and bronze tools and ornaments, was later adapted to enable production of high-quality steel and nickel base cast parts The adaptation of this method to titanium casting technology required the development of ceramic slurry materials with minimum reaction with the extremely reactive molten titanium

Proprietary lost wax ceramic shell systems have been developed by the several foundries engaged in titanium casting manufacture Of necessity, these shell systems must be relatively inert to molten titanium and cannot be made with the conventional foundry ceramics used in the ferrous and nonferrous industries Usually, the face coats are made with special refractory oxides and appropriate binders After the initial face coat ceramic is applied to the wax pattern, more traditional refractory systems are used to add shell strength from repeated backup ceramic coatings Regardless of face coat composition, some metal/mold reaction inevitably occurs from titanium reduction of the ceramic oxides The oxygen-rich surface of the casting stabilizes the α phase, usually forming a metallographically distinct α case layer on the cast surface, which may be removed later by means of chemical milling

Foundry practice focuses on methods to control both the extent of the metal/mold reaction and the subsequent diffusion of reaction products inward from the cast surface Diffusion of reaction products into the cast surface is time-at-temperature dependent Depth of surface contamination can vary from nil on very thin sections to more than 1.5 mm (0.06 in.) on heavy sections On critical aerospace structures, the brittle α case is removed by chemical milling The depth of surface contamination must be taken into consideration in the initial wax pattern tool design Hence, the wax pattern and casting are made slightly oversize, and final dimensions are achieved through careful chemical milling Metal superheat, mold

temperature and thermal conductivity, g force (if centrifugally cast), and rapid postcast heat removal are other key factors

in producing a satisfactory product These parameters are interrelated, that is, a high g force centrifugal pour into cold

molds may achieve the same relative fluidity as a static pour into heated molds

Other Molding Systems. The combination of graphite powder, stucco, and organic binders has also been used as a

shell system for the investment casting of titanium After dewax, the shell is fired in a reducing atmosphere to remove or pyrolyze the binders before casting This technology has not been promoted as much as the use of refractory oxide shell systems and is presently primarily of historic interest

In addition to the rammed graphite and investment molding methods, a poured ceramic mold has also been used to produce large parts that require good dimensional accuracy This method, developed in the late 1970s, was used to a limited extent for several years

Semipermanent, reusable molds, frequently made from machined graphite, have been used successfully since the earliest U.S Bureau of Mines work, but only on relatively simple-shaped parts that allow metal volumetric shrinkage to occur without restriction The method is economical only when reasonably high volumes are required, that is, thousands of parts, because of the high cost of the solid mold material

Currently, a titanium sand casting technique based on conventional foundry mold-making making practices is under development at the U.S Bureau of Mines (Ref 17) Because the mold materials are less costly and the cast part is easier to remove from the sand mold than from other methods of titanium casting, this development could lower production costs However, surface quality problems are restricting the use of this method thus far

Foundries and Capacities Table 1 summarizes the use and capacities of the various titanium casting practices by a

number of foundries in several countries

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Table 1 Status and capacity of titanium foundries in the United States, Japan, and Western Europe in 1987

Approximate maximum envelope size Maximum

pour weight

Rammed graphite Investment casting Foundry

Melt stock Use of

postcast HIP

Howmet Corp (MI and

Often

VMC (Japan) 180 400 1270 diam × 50 diam × Research and development Billet and Seldom

Reference cited in this section

17 R.K Koch and J.M Burrus, "Bezonite-Bonded Rammed Olivine and Zircon Molds for Titanium Casting," Report 8587, U.S Bureau of Mines, 1981

Alloys

All production titanium castings to date are based on traditional wrought product compositions As such, the Ti-6Al-4V alloy dominates structural casting applications This alloy similarly has dominated wrought industry production since its introduction in the early 1950s, becoming the benchmark alloy against which others are compared However, other wrought alloys have been developed, for special applications, with better room-temperature or elevated-temperature

Trang 16

strength, creep, or fracture toughness characteristics than those of Ti-6Al-4V These same alloys are also being cast when net shape casting technology is the most economical method of manufacture As with Ti-6Al-4V, other cast titanium alloys have properties generally comparable to those of their wrought counterparts

Chemistry and Demand Table 2 lists the most prevalent casting alloy chemistries and the most unique attribute of

each in comparison with Ti-6Al-4V, plus current approximate market share

Table 2 Comparison of cast titanium alloys

Nominal composition, wt %

relative usage

of castings O N H Al Fe V Cr Sn Mo Zr

Special properties (a)

Ti-6Al-4V 90% 0.18 0.015 0.006 6 0.13 4

General purpose

Ti-6Al-4V ELI 2% 0.11 0.010 0.006 6 0.10 4

Cryogenic toughness

Ti-3Al-8V-6Cr-4Zr-4Mo <1% 0.10 0.015 0.006 3.5 0.2 8.5 6 4 4 Strength

Ti-15V-3Al-3Cr-3Sn <1% 0.11 0.015 0.006 3 0.2 15 3 3

Strength

(a) Superior, relative to Ti-6Al-4V

Typical Properties Table 3 is a summary of room-temperature tensile properties for various alloys These properties,

which are typical, vary depending on microstructure as influenced by foundry parameters such as solidification rate and any postcast HIP and heat treatments

Table 3 Typical room-temperature tensile properties of titanium alloy castings (bars machined from castings)

Specification minimums are less than these typical properties

Trang 17

Yield strength Ultimate strength Alloy

MPa ksi MPa ksi

Elongation,

%

Reduction of area, %

Commercially pure (Grade 2) 448 65 552 80 18 32

(a) Solution-treated and aged (STA) heat treatments may be varied to produce alternate properties

Specifications Industry-wide specifications, listed in Table 4 for reference, give more detail on mechanical property

guarantees and process control features In addition, most major aerospace companies have comparable specifications

MIL Handbook V, Aerospace Design Specifications does not presently include titanium alloy castings, but it is expected

that such information will be incorporated in the near future As with wrought products, commercially pure titanium castings are used almost entirely in corrosion service Commercially pure titanium pumps and valves are the principal components made using titanium casting technology for the corrosion resistance field, and are used extensively in chemical and petrochemical plants

Table 4 Standard industry specifications applicable to titanium castings

MIL-T-81915 Titanium and titanium alloy castings, investment

AMS-4985A Titanium alloy castings, investment or rammed graphite

AMS-4991 Titanium alloy castings, investment

ASTM B 367 Titanium and titanium alloy castings

MIL-STD-2175 Castings, classification and inspection of

MIL-STD-271 Nondestructive testing requirements for metals

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MIL-STD-453 Inspection, radiographic

MIL-Q-9858 Quality program requirement

MIL-I-6866B Inspection, penetrant method of

MIL-H-81200 Heat treatment of titanium and titanium alloys

ASTM E 155 Reference radiographs for inspection of aluminum and magnesium castings

ASTM E 192 Reference radiographs, investment steel castings

ASTM E 186 Reference radiographs, steel castings 50 to 102 mm (2 to 4 in.)

ASTM E 446 Reference radiographs, steel castings up to 50 mm (2 in.)

ASTM E 120 Standard methods for chemical analysis of titanium and titanium alloys

ASTM E 8 Methods of tension testing of metallic materials

AMS-2249B Chemical-check analysis limits for titanium and titanium alloys

AMS-4954 Titanium alloy welding wire Ti-6Al-4V

AMS-4956 Titanium alloy welding wire Ti-6Al-4V, extra low interstitial

Newer Alloys As aircraft engine manufacturers seek to use cast titanium at higher operating temperatures,

Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-2Sn-4Zr-6Mo are being specified more frequently Other titanium alloys for service to 595 °C (1100

°F) are being developed as castings Extra low interstitial grade Ti-6Al-4V has been used for critical cryogenic space shuttle service where fracture toughness is an important design criteria The most recent alloys to receive attention in the casting industry are the metastable β alloys Ti-3Al-8V-6Cr-4Zr-4Mo (Beta C) and Ti-15V-3Al-3Cr-3Sn (Ti 15-3) Originally developed as a highly cold-formable and subsequently age-hardened sheet material, these alloys are highly castable and are readily heat treated to an 1170 MPa (170 ksi) strength level, making them serious candidates for the replacement of high-strength precipitation-hardened stainless steels such as 17-4PH The full density advantage of titanium of about 40% is preserved because strength levels are comparable in both materials Titanium-aluminide castings are being developed for application in the compressor sections of aircraft gas turbine engines subjected to the highest temperatures Compositions based upon both the α2 (Ti3Al) and γ(TiAl) ordered phases have been cast experimentally, with the former being closer to limited-production status The low ductility of these alloys at room temperature has been the major producibility challenge It is anticipated that the service potential for titanium aluminides in the 595 to 925 °C (1100 to 1700 °F) temperature range will eventually be realized The difficulty in machining shapes in these brittle alloys may increase the advantage of net shape methods such as castings

Because Ti-6Al-4V dominates the industry, much more metallurgical work has been accomplished with this alloy and is discussed below

Microstructure of Ti-6Al-4V

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Cast Microstructure To understand the relatively high mechanical property levels of titanium alloy castings and the

many improvements made in recent years, it is necessary to understand the microstructures of castings and their influence

on the mechanical behavior of titanium The phase transformation from β to α+ β leads to the elimination of the dendritic cast structure The existence of such dendrites is evident in the surface morphology of shrinkage pores (Fig 4) The phase transformation, which in the alloy Ti-6Al-4V is typically initiated at 995 °C (1825 °F), results in the microstructural features shown in Fig 2(a) This microstructure, which will be discussed in detail, is very similar to a β-processed wrought microstructure and has similar properties Thus, in the study and development of titanium alloy castings, it is possible to draw much information from conventional ingot metallurgy

Hot isostatic pressing is now becoming almost a standard practice for all titanium

cast parts produced for the aerospace industry (Table 1) As a result, cast + HIP microstructure also needs to be considered (Fig 2b) Because the HIP temperature is typically well below the βt temperature, the as-cast (Fig 2a) and the cast + HIP microstructures look very much alike, except for the lack of porosity in the latter

As-Cast and Cast + HIP Microstructures Because most castings for demanding

applications are produced with Ti-6Al-4V alloy (Table 2), only microstructures of this α+ β alloy will be reviewed here

Beta Grain Size. Beta grains (A, in Fig 2a) develop during the solid state cooling stage between the solidus/liquidus temperature and the βt temperature As a result, large sections, which cool slower, show larger β grains The size range of the β grains is 0.5

to 5 mm (0.02 to 0.2 in.) As will be further discussed, large β grains may lead to large

α plate colonies This is beneficial for fracture toughness, creep resistance, and fatigue crack propagation resistance (Ref 18, 19) and detrimental for low- and high-cycle fatigue strength (Ref 20, 21)

Grain Boundary α This α phase (B, in Fig 2a) is formed along the β grain boundaries when cast material is cooled through the α+ β phase field (in Ti-6Al-4V this

is typically from 995 °C, or 1825 °F, down to room temperature) This phase is typically plate shaped and represents the largest α plates in the cast structure The length of these plates can equal the β grain radius This has been found to be very detrimental to fatigue crack initiation at room temperature (Ref 22, 23) and at elevated temperatures (Ref 23, 24) Many postcast thermal treatments eliminate this phase to improve fatigue life

Alpha Plate Colonies. Alpha platelets (C, in Fig 2a) are the transformation products of the β phase when cooled below the βt temperature The hexagonal close-packed (hcp) orientation of these plates is related to the parent body-centered cubic (bcc) β phase orientation through one of the 12 possible variants of the Burgers relationship (Ref 25, 26):

{110}β P (0001)α

<111>β P <1120>α

When cooling rates are relatively slow, such as in thick-section castings, many adjacent α platelets transform into the same Burgers variant and form a colony of similarly aligned and crystallographically oriented platelets The large colonies (like those marked C in Fig 2a) may be associated with early fatigue crack initiation (Ref 21), the result of heterogeneous basal slip across the plates (Ref 27) At the same time, the large colony structure is beneficial for fatigue crack propagation resistance (Ref 28, 29) Because α platelet colonies cannot grow larger than the β grains, titanium castings with large β grains typically have large colonies The α platelets are typically 1 to 3 μm (40 to 120 μin.) in thickness and 20 to 100 μm (800 to 4000 μin.) in length (Ref 30, 31), and the typical colony size in castings is 50 to 500

Fig 4 Dendritic structures

present in the surface

morphology of an as-cast

titanium component

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Modification of Microstructure Virtually all titanium castings produced commercially today are supplied in the

annealed condition However, much microstructural modification development work has recently been done, and it can be expected that solution-treated and aged or other postcast thermal processing will eventually become specified on cast parts requiring certain property enhancement The following section reviews several of the developmental procedures and their results

Modification of microstructure is one of the most versatile tools available in metallurgy for improving mechanical properties of alloys This is commonly achieved through a combination of cold or hot working followed by heat treatment Net shapes such as castings or P/M products cannot be worked, which limits the options for controlling microstructures A substantial amount of work has been done in recent years to improve the microstructures of titanium alloy net shape products, with an emphasis on Ti-6Al-4V material Most treatment schemes can be successfully applied to both cast parts (Ref 8) and P/M compacts (Ref 32, 33) In the case of titanium alloy castings, the main goal has been to eliminate the grain boundary α phase, the large α plate colonies, and the individual α plates This is accomplished either

by solution treatments or by a temporary alloying with hydrogen In some cases, the hydrogen and solution treatments are combined The details of these methods, including the appropriate references, are listed in Table 5 The typical resulting microstructures of the α-β solution treatment (ABST), βsolution treatment (BST), broken-up structure (BUS), and high-temperature hydrogenation (HTH) methods are shown in Fig 5(a), 5(b), 5(c), and 5(d), respectively As can be seen from the photomicrographs, these treatments are successful in eliminating the large α plate colonies and the grain boundary phase As discussed below, a substantial improvement of both tensile and fatigue properties is achieved with these processes

Table 5 Methods for modifying the microstructure of α+ β titanium alloy net shape products

Hydrogenation temperature

Intermediate treatment

Dehydrogenation temperature

Method (a) Typical

solution

treatment

Typical annealing or aging treatment

Applied

to product forms

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(a) Most data apply to Ti-6Al-4V βt temperature approximately 995 °C (1825 °F)

(b) GFC, gas fan cooled

(c) RT, room temperature

Fig 5 Photomicrographs of microstructures resulting from a variety of hydrogen and solution heat treatments

used to eliminate large α plate colonies and grain boundary α phase in titanium alloys (a) ABST (b) BST (c) BUS (d) HTH

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18 G.R Yoder, L.A Cooley, and T.W Crooker, "Fatigue Crack Propagation Resistance of Beta-Annealed

Ti6Al-4V Alloys of Differing Interstitial Oxygen Content," Metall Trans A, Vol 9A, 1978, p 1413-1420

19 R.R Boyer and R Bajoraitis, "Standardization of Ti6Al-4V Processing Conditions," AFML-TR-78-131, Air Force Materials Laboratory, Boeing Commercial Airplane Company, Sept 1978

20 D Eylon, T.L Bartel, and M.E Rosenblum, High Temperature Low Cycle Fatigue of Beta-Annealed

Titanium Alloy, Metall.Trans A, Vol 11A, 1980, p 1361-1367

21 D Eylon and J.A Hall, Fatigue Behavior of Beta-Processed Titanium Alloy IMI-685, Metall Trans A, Vol

8A, 1977, p 981-990

22 D Eylon, Fatigue Crack Initiation in Hot Isostatically Pressed Ti6Al-4V Castings, J Mat Sci., Vol 14,

1979, p 1914-1920

23 D Eylon and W.R Kerr, The Fractographic and Metallographic Morphology of Fatigue Initiation Sites, in

Fractography in Failure Analysis, STP 645, American Society for Testing and Materials, 1978, p 235-248

24 D Eylon and M.E Rosenblum, Effects of Dwell on High Temperature Low Cycle Fatigue of a Titanium

Alloy, Metall Trans A, Vol 13A, 1982, p 322-324

25 W.G Burgers, Physics, Vol 1, 1934, p 561-586

26 J.C Williams, Kinetics and Phase Transformation, in Titanium Science and Technology, Vol 3, R.I Jaffee

and H.M Burte, Ed., Plenum Press, 1973, p 1433-1494

27 D Schechtman and D Eylon, On the Unstable Shear in Fatigued Beta-Annealed Ti-11 and IMI-685 Alloys,

Metall Trans A, Vol 9A, 1978, p 1273-1279

28 G.R Yoder and D Eylon, On the Effect of Colony Size on Fatigue Crack Growth in Widmanstätten

Structure Alpha + Beta Alloys, Metall Trans A, Vol 10A, 1979, p 1808-1810

29 D Eylon and P.J Bania, Fatigue Cracking Characteristics of Beta-Annealed Large Colony Ti-11 Alloy,

30 R.J Smickley and L.P Bednarz, Processing and Mechanical Properties of Investment Cast Ti6Al-4V ELI

Alloy for Surgical Implants: A Progress Report, in Titanium Alloys in Surgical Implants, STP 796, H.A

Luckey and F Kubli, Ed., American Society for Testing and Materials, 1983, p 16-32

31 R.J Smickley, Heat Treatment Response of HIP'd Cast Ti6Al-4V, in the Proceedings of the WesTech

Conference, ASM INTERNATIONAL and Society of Manufacturing Engineers, 1981

32 F.H Froes, D Eylon, G.E Eichelman, and H.M Burte, Developments in Titanium Powder Metallurgy, J

Met., Vol 32 (No 2), 1980, p 47-54

33 F.H Froes and D Eylon, Powder Metallurgy of Titanium Alloys A Review, in Titanium, Science and

Technology, Vol 1, G Lutjering, U Zwicker, and W Bunk, Ed., DGM, 1985, p 267-286; also, in Powder Metall Int., Vol 17 (No 4), 1985, p 163-167 and continued in Vol 17 (No 5), 1985, p 235-238; also, in Titanium Technology: Present Status and Future Trends, F.H Froes, D Eylon, and H.B Bomberger, Ed.,

34 D Eylon and F.H Froes, Method for Refining Microstructures of Cast Titanium Articles, U.S Patent 4,482,398, Nov 1984

35 D Eylon and F.H Froes, Method for Refining Microstructures of Prealloyed Powder Metallurgy Titanium Articles, U.S Patent 4,534,808, Aug 1985

36 D Eylon and F.H Froes, Method for Refining Microstructure of Blended Elemental Powder Metallurgy Titanium Articles, U.S Patent 4,536,234, Aug 1985

37 D Eylon, F.H Froes, and L Levin, Effect of Hot Isostatic Pressing and Heat Treatment on Fatigue

Properties of Ti6Al-4V Castings, in Titanium, Science and Technology, Vol 1, G Lutjering, U Zwicker,

and W Bunk, Ed., 1985, p 179-186

38 D.L Ruckle and P.P Millan, Method for Heat Treating Cast Titanium Articles to Improve Their

Trang 23

Mechanical Properties, U.S Patent 4,631,092, Dec 1986

39 D Eylon, W.J Barice, and F.H Froes, Microstructure Modification of Ti6Al-4V Castings, in Overcoming

Material Boundaries, Vol 17, Society for the Advancement of Material and Process Engineering, 1985, p

585-595

40 W.R Kerr, P.R Smith, M.E Rosenblum, F.J Gurney, Y.R Mahajan, and L.R Bidwell, Hydrogen as an

Alloying Element in Titanium (Hydrofac), in Titanium '80, Science and Technology, H Kimura and O

Izumi, Ed., The Metallurgical Society, 1980, p 2477-2486

41 R.G Vogt, F.H Froes, D Eylon, and L Levin, Thermo-Chemical Treatment (TCT) of Titanium Alloy Net

Shapes, in Titanium Net Shape Technologies, F.H Froes and D Eylon, Ed., The Metallurgical Society,

45 C.F Yolton, D Eylon, and F.H Froes, High Temperature Thermo-Chemical Treatment (TCT) of Titanium

With Hydrogen, in the Proceedings of the Fall Meeting, The Metallurgical Society, 1986, p 42

Mechanical Properties of Ti-6Al-4V

Oxygen Influence Figure 6 is a frequency distribution of tensile properties from separately cast test bars representing

hot isostatic pressed and annealed Ti-6Al-4V castings Note that oxygen, a carefully controlled alloy addition, is in the 0.16 to 0.20% range, which is common for many aerospace specifications

Fig 6 Frequency distribution of tensile properties of hot isostatically pressed and annealed Ti-6Al-4V casting

test bars Percent frequency is plotted versus (a) 0.2% offset yield strength, (b) ultimate tensile strength, (c) percent reduction of area, and (d) percent elongation Alloy composition is 0.16 to 0.20% O2; sample size is

Trang 24

500 heats Source: Ref 46

Some specifications allow a 0.25% maximum oxygen content The resultant properties with oxygen in the 0.20 to 0.25% range are typically about 69 to 83 MPa (10 to 12 ksi) higher than those shown in Fig 6 with slightly lower ductility levels In this case, it is possible to guarantee 827 MPa (120 ksi) yield strength and 896 MPa (130 ksi) ultimate tensile strength levels with 6% minimum elongation This strength level is the same minimum guarantee for wrought-annealed Ti-6Al-4V

Microstructure Influence Because the microstructure of titanium alloy cast parts is very similar to β processed

wrought or ingot metallurgy (I/M) material, many properties of hot isostatic pressed castings, such as tensile strength, fracture toughness, fatigue crack propagation, and creep, are at the same levels as forged and machined parts Tensile strength and fracture toughness properties of cast, cast + HIP, and cast + HIP + heat-treated material (Table 5) are compared in Table 6 to wrought β-annealed data To provide a complete review, properties of castings treated by many of the methods listed in Table 5 are also included At the present time, fracture toughness data are available for only a few of the conditions As can be seen, some of the treated conditions present properties in excess of I/M β-annealed material However, it should be noted that tests were done on relatively small cast coupons Properties of actual cast parts, especially large components, could be somewhat lower, the result of coarser grain structure or slower quench rates Of special interest are the hydrogen-treated conditions (such as thermo-chemical treatment, or TCT; constitutional solution treatment, or CST; and HTH, in Table 5) that result in very high tensile strength (as high as 1124 MPa, or 163 ksi) with tensile elongation as high as 8%

Table 6 Tensile properties and fracture toughness of Ti-6Al-4V cast coupons compared to typical wrought β-annealed material

Yield strength

Ultimate tensile strength

KIc

Material condition (a)

MPa ksi MPa ksi

Trang 25

Typical wrought β-annealed 860 125 955 139 9 21 83 91 18, 19

(a) All conditions (except as-cast) are cast plus HIP

(b) See Table 5 for process details

Fatigue and Fatigue Crack Growth Rate The fatigue crack growth rate (FCGR) behavior of cast Ti-6Al-4V is

also, as expected, very similar to that of β-processed wrought Ti-6Al-4V (Ref 50, 51, 52) This is demonstrated in Fig 7

in which the scatterband of FCGR of cast and cast-HIP alloy is compared to β-processed I/M (Ref 18, 53)

Fig 7 Scatterband comparison of FCGR behavior of wrought βannealed 4V to cast and cast HIP

Ti-6Al-4V

The scatterbands of smooth axial fatigue results of cast, cast-HIP (Ref 15, 47, 48, 54, 55, 56, 57), and wrought Ti-6Al-4V are shown in Fig 8 This figure clearly indicates that the HIP process results in a substantially improved fatigue life well into the wrought-annealed region The fatigue properties of aerospace quality castings have always been an important issue, because in most other alloy systems this is the property that is most degraded when compared to wrought products However, because of the complete closure and healing of gas (D, in Fig 2a) and shrinkage (E, in Fig 2a) pores by HIP and the β-annealed microstructure, it is possible to get fatigue life comparable to wrought material in premium investment cast and hot isostatic pressed parts As indicated previously (Table 5), substantial work has been done in recent years to modify the microstructure of cast parts to produce fatigue properties either equivalent or superior to the best wrought-annealed products Figure 9 compares the smooth fatigue life of Ti-6Al-4V treated by ABST, BST, BUS, CST, Garrett treatment (GTEC), and HTH (Table 5) to wrought material scatterband As can be seen, all of these treatments were successful in improving fatigue life above wrought levels The hydrogen treatments (CST and HTH) resulted in the highest improvement in fatigue strength However, it should be noted that wrought products subjected to the same treatments result in comparable improvements in fatigue strength

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Fig 8 Comparison of smooth axial fatigue rate in cast and wrought Ti-6Al-4V at room temperature with R =

+0.1

Fig 9 Comparison of Ti-6Al-4V investment castings subjected to various thermal and hydrogen treatments

Smooth axial fatigue measured at room temperature R = +0.1; frequency = 5 Hz using triangular wave form

7 (No 7), July 1955, p 801-804

19 R.R Boyer and R Bajoraitis, "Standardization of Ti6Al-4V Processing Conditions," AFML-TR-78-131,

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Air Force Materials Laboratory, Boeing Commercial Airplane Company, Sept 1978

and W Bunk, Ed., 1985, p 179-186

38 D.L Ruckle and P.P Millan, Method for Heat Treating Cast Titanium Articles to Improve Their Mechanical Properties, U.S Patent 4,631,092, Dec 1986

585-595

1984, p 145-154

44 R.J Smickley and L.E Dardi, Microstructure Refinement of Cast Titanium, U.S Patent 4,505,764, March

1985

46 Titech International Inc., unpublished research

47 F.C Teifke, N.H Marshall, D Eylon, and F.H Froes, Effect of Processing on Fatigue Life of Ti6Al-4V

Castings, in Advanced Processing Methods for Titanium, D Hasson, Ed., The Metallurgical Society, 1982,

p 147-159

48 R.R Wright, J.K Thorne, and R.J Smickley, Howmet Turbine Components Corporation, Ti-Cast Division, private communication, 1982; also, Technical Bulletin TB 1660, Howmet Corporation

49 L Levin, R.G Vogt, D Eylon, and F.H Froes, Fatigue Resistance Improvement of Ti6Al-4V by

Thermo-Chemical Treatment, in Titanium, Science and Technology, Vol 4, G Lutjering, U Zwicker, and W Bunk,

Ed., 1985, p 2107-2114

50 L.J Maidment and H Paweltz, An Evaluation of Vacuum Centrifuged Titanium Castings for Helicopter

Components, in Titanium '80, Science and Technology, H Kimura and O Izumi, Ed., The Metallurgical

53 D Eylon, P.R Smith, S.W Schwenker, and F.H Froes, Status of Titanium Powder Metallurgy, in

Industrial Applications of Titanium and Zirconium: Third Conference, STP 830, R.T Webster and C.S

Young, Ed., American Society for Testing and Materials, 1984, p 48-65

54 J.K Kura, "Titanium Casting Today," MCIC-73-16, Metals and Ceramics Information Center, Dec 1973

55 J.R Humphrey, Report IR-162, REM Metals Corporation, Nov 1973

56 M.J Wynne, Report TN-4301, British Aircraft Corporation, Nov 1972

57 H.D Hanes, D.A Seifert, and C.R Watts, Hot Isostatic Processing, Battelle Press, 1979, p 55

Casting Design

The best casting design is usually achieved by means of a thorough review by the manufacturer and user when the component is still in the preliminary design stage (see the article "Casting Design" in this Volume) Additional features may be incorporated to reduce machining cost, and components may be integrated to eliminate later fabrication Specifications and tolerances may be reviewed vis-a-vis foundry capabilities, producibility, and pattern tool concepts to achieve the most practical and cost-effective design (see the articles "Dimensional Tolerances and Allowances" and

"Patterns and Patternmaking" in this Volume) When minimum cast part weight is critical, such as in aerospace components, the capability of the foundry to produce varying wall thicknesses, for example, may be beneficial Often, cast features that cannot be economically duplicated by any other method may be readily produced

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Titanium castings present the designer with few differences in design criteria, compared with other metals Ideal designs

do not contain isolated heavy sections or uniform heavy walls of large area so that centerline shrinkage cavities and regions with a coarse microstructure may be avoided From a practical sense, however, ideal tapered walls to promote directional solidification are not usually a reality The advent of hot isostatic pressing to heal internal as-cast shrinkage cavities has offered the designer much more freedom; however, there still is a practical limit to the size of internal cavity that can be healed through hot isostatic pressing without contributing significant surface or structural deformation due to the collapse of internal pores

The lost wax investment process provides more design freedom for the foundry to properly feed a casting than does the traditional sand or rammed graphite approach It is normal practice to add a gate and riser to hot isostatic pressed investment castings to achieve reasonably good internal x-ray quality so that hot isostatic pressing will not cause extensive surface or structural deformation

The usual required minimum practical wall thickness for investment castings is 2.0 mm (0.080 in.); however, sections as small as 1.1 mm (0.045 in.) are routinely made Even thinner walls may be achieved by chemical milling beyond that required for α case removal; however, as-cast wall variation is not improved and becomes a larger percentage of the resultant wall thickness Sand or rammed graphite molded castings have a usual minimum wall thickness of 4.75 mm (0.187 in.), although 3.0 mm (0.12 in.) is not unreasonable for short sections

Fillet radii should be as generous as possible to minimize the occurrence of hot tears While 0.76 mm (0.030 in.) radii are produced, the preferred minimum is 3.0 mm (0.12 in.) A rule of thumb is that a fillet radius should be 0.5 times the sum

of the thicknesses of the two adjoining walls

With proper tool design, zero draft walls are possible To promote directional solidification, a 3° included draft angle may

be preferred Hot isostatic pressing will close any centerline shrinkage cavities in zero draft walls, making it unnecessary

to provide draft Draft requirements are also dependent upon foundry practice, with rammed graphite tooling usually requiring draft, and investment casting typically not requiring draft

Tolerances Typically, the major area of concern is true position of a thin-section surface with respect to a datum

Surface areas of approximately 129 cm2 (20 in.2) or greater in sections of less than approximately 5.08 mm (0.200 in.) thickness are susceptible to distortion, depending on adjoining sections The high strength of titanium compared with aluminum and low elastic modulus compared with steel present challenges in straightening and in maintaining extremely tight, true positions General tolerance band capabilities for linear dimensions are shown in Table 7

Table 7 General linear and diametrical tolerance guideline for titanium castings

Size Total tolerance band (a)

25 to <102 1 to <4 0.76 mm (0.030 in.) or 1.0%, whichever is greater 1.52 mm (0.060 in.)

102 to <305 4 to <12 1.02 mm (0.040 in.) or 0.7%, whichever is greater 1.78 mm (0.070 in.) or 1.0%, whichever is greater

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in.) (±0.035 in.) mm (±0.050 in.)

(a) Improved tolerances may be possible depending on the specific foundry capabilities and overall part-specific requirements

Hot sizing fixtures have been increasingly used to help control critical casting dimensions This technique typically involves the use of steel fixtures to "creep" the casting into final tolerances in an anneal or stress relief heat treatment by the weight of the steel or the use of differential thermal expansion of the steel relative to the titanium

Standard casting industry thickness tolerances of ±0.76 mm (±0.030 in.) for rammed graphite and ±0.25 mm (±0.010 in.) for investment cast walls are more difficult to maintain with titanium primarily because of the influence of chemical milling As mentioned earlier, for critical applications it is necessary to mill all surfaces chemically to remove the residue

α case This operation is subject to variation because of part geometry and bath variables, and because it is usually manually controlled Standard industry surface finishes are shown in Table 8

Table 8 Surface finish of titanium castings

RMS equivalent

Process NAS 823

surface comparator

Melting and Pouring Practice

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Vacuum Consumable Electrode The dominant, almost universal, method of melting titanium is with a consumable

titanium electrode lowered into water-cooled copper crucibles while confined in a vacuum chamber This skull melting (see the section "Vacuum Arc Skull Melting and Casting" in the article "Vacuum Melting and Remelting Processes" in this Volume) technique prevents the highly reactive liquid titanium from dissolving the crucible because it is contained in

a solid "skull" frozen against the water-cooled crucible wall When an adequate melt quantity has been obtained, the residual electrode is quickly retracted, and the crucible is tilted for pouring into the molds A "skull" of solid titanium remains in the crucible for reuse in a subsequent pour or for later removal

Superheating The consumable electrode practice affords little opportunity for superheating the molten pool because of

the cooling effect of the water-cooled crucible Because of limited superheating, it is common either to pour castings centrifugally, forcing the metal into the mold cavity, or to pour statically into preheated molds to obtain adequate fluidity Postcast cooling takes place in a vacuum or in an inert gas atmosphere until the molds can be safely removed to air without oxidation of the titanium

Electrode Composition Consumable titanium electrodes are either ingot metallurgy forged billet, consolidated revert

wrought material, selected foundry returns, or a combination of all of these Casting specifications or user requirements can dictate the composition of revert materials used in electrode construction Figure 10 shows a typical centrifugal casting furnace arrangement

to enhance surface finish and control hydrogen pickup Hydrogen pickup is more likely the greater the β phase content of the alloy and is also influenced by etch rate and bath temperature Subsequent vacuum anneals may be used to remove hydrogen picked up in chemical milling The general objective is to remove the entire as-cast surface uniformly to the extent of maximum α case depth, and to retain the dimensional integrity of the part

Hot Isostatic Pressing

Hot isostatic pressing may be used to ensure complete elimination of internal gas (D, in Fig 2a) and shrinkage (E, in Fig 2a) porosity The cast part is chemically cleaned and typically subjected to argon pressure of 103 MPa (15 ksi) at 900 to

955 °C (1650 to 1750 °F) for a 2 h hold time (Ti-6Al-4V alloy) for void closure and diffusion bonding This practice has been shown to reduce the scatterband of fatigue property test results and significantly improve fatigue life (Fig 8) HIP temperature may coarsen the microstructure, causing a slight debit in tensile strength, but the benefits of HIP normally exceed this slight decrease in strength, and the practice is widely used for aerospace titanium alloy cast parts

Fig 10 A centrifugal vacuum casting furnace

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Heat Treatment

Conventional heat treatment of titanium castings is for stress relief anneal after any weld repair The Ti-6Al-4V alloy is typically heat treated at 730 to 845 °C (1350 to 1550 °F) This is done in a vacuum to ensure removal of any hydrogen pickup from chemical milling and to protect the titanium from oxidation As with HIP, castings must be chemically clean prior to heat treatment if diffusion of surface contaminants is to be avoided Alternate heat treatments for property improvement, such as solution-treating and aging of Ti-6Al-4V alloy castings, are available Numerous other heat treatments are in various stages of development, as discussed in an earlier section (Table 5)

Final Evaluation and Certification

Titanium castings are produced to numerous quality specifications Typically, these require some type of x-ray and dye penetrant inspection in addition to dimensional checks using layout equipment, dimensional inspection fixtures, and coordinate measuring machines Metallurgical certifications may include HIP and heat treatment run certifications, and chemistry, tensile properties, and microstructural examination of representative coupons for absence of surface contamination

In the absence of universally accepted x-ray standards, it is common practice to use steel or aluminum reference radiographs (Table 4) Because internal discontinuities in titanium do not necessarily appear the same as they do in other metals, it is necessary to have an expert evaluation of radiographs for proper interpretation Currently, an industry task force is working on the development of radiographic standards for titanium castings through the American Society for Testing and Materials (ASTM)

Product Applications

The titanium castings industry is relatively young by most foundry standards The earliest commercial applications, in the 1960s, were for use in corrosion-resistant service in pump and valve components These applications continue to dominate the rammed graphite production method; however, in more recent years, some users have justified the expense

of lost wax investment tooling for some commercial corrosion-resistant casting applications (see Fig 11)

Aerospace use of rammed-graphite-type castings became a production reality in the early 1970s for aircraft brake torque tubes, missile wings, and hot gas nozzles As the more precise investment casting technology developed and the commercial use of HIP became a reality in the mid-1970s, titanium casting applications quickly expanded into critical airframe (Fig 12) and gas turbine engine (Fig 13) components The first applications were primarily in Ti-6Al-4V, the workhorse alloy for wrought aerospace products, and castings were often substituted for forgings, with the addition of some net shape features; this trend continues With continuing experience in manufacturing and specifying titanium castings, applications expanded from relatively simple less-critical components for military engines and airframes

to large, complex structural shapes for both military and commercial engine and airframe programs Today, titanium cast parts are routinely produced for critical structural applications such as space shuttle attachment fittings, complex airframe structures, engine mounts, compressor cases and frames of many types, missile bodies and wings, and hydraulic housings (Fig 14) Quality and dimensional capabilities continue to be advanced Titanium castings are used for framework for very sensitive optical equipment due

to their stiffness and the compatibility of the coefficient of thermal expansion of titanium with that of glass (Fig 15) Applications evolving for engine airfoil shapes (Fig 16) include individual vanes and integral vane rings for stators, as well as a few rotating parts that would otherwise be made from wrought product Growth will continue as users seek to take advantage of the flexibility of design inherent in the investment casting process and the improvement in economics

of net and near-net shapes

Fig 11 Investment cast titanium components for use in

corrosive environments

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Fig 12 Investment cast titanium airframe parts

Fig 13 Typical investment cast titanium components used for gas turbine applications

Fig 14 Titanium hydraulic housings produced by the investment casting process

Fig 15 Titanium housings for military optical applications produced by the investment casting process

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Fig 16 Investment cast titanium engine

airfoil components

In spite of the wide acceptance of titanium castings for airframe applications, growth has been somewhat hindered because of the lack of an industry-wide data base to establish casting factors accurately, relative to wrought material Such standards are now being established with probable elimination of design casting factors (Ref 58) Foundry size capabilities are expanding to allow the manufacture of larger airframe and static gas turbine engine structures Widespread routine use of aerospace titanium castings is anticipated as the titanium foundry industry matches with well-established quality and product standards, and user understanding and confidence continue to be gained from satisfactory product performance

Concurrent with the above trend, investment cast titanium is increasingly being specified for medical prostheses because

of its inertness to body fluids, elastic modulus approaching that of bone, and the net shape design flexibility of the casting process Custom-designed knee and hip implant components (Fig 17) are routinely produced in volume Some of these are subsequently coated with a diffusion bonded porous surface to facilitate bone ingrowth or an eventual fixation of the metal implant with the patient's bone structure

Fig 17 Titanium knee and hip implant prostheses manufactured by the investment casting process

58 R.J Tisler, Fatigue and Fracture Characteristics of Ti6Al-4V HIP'ed Investment Castings, in the

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Proceedings of the International Conference on Titanium, Titanium Development Association, Oct 1986, p

23-41

References

1 H.B Bomberger, F.H Froes, and P.H Morton, Titanium A Historical Perspective, in Titanium

Technology: Present Status and Future Trends, F.H Froes, D Eylon, and H.B Bomberger, Ed., Titanium

Development Association, 1985, p 3-17

2 Titanium for Energy and Industrial Applications, D Eylon, Ed., The Metallurgical Society, 1981, p 1-403

3 Titanium Net Shape Technologies, F.H Froes and D Eylon, Ed., The Metallurgical Society, 1984, p 1-299

4 "Titanium 1986, Statistical Review 1978-1986," Annual Report of the Titanium Development Association,

1987

5 American Foundrymen's Society, private conversation, 1987

6 Aluminum Association, private conversation, 1987

7 D Eylon, F.H Froes, and R.W Gardiner, Developments in Titanium Alloy Casting Technology, J Met., Vol 35 (No 2), Feb 1983, p 35-47; also, in Titanium Technology: Present Status and Future Trends, F.H

Froes, D Eylon, and H.B Bomberger, Ed., Titanium Development Association, 1985, p 35-47

9 H.B Bomberger and F.H Froes, The Melting of Titanium, J Met., Vol 36 (No 12), Dec 1984, p 39-47; also, in Titanium Technology: Present Status and Future Trends, F.H Froes, D Eylon, and H.B

Bomberger, Ed., Titanium Development Association, 1985, p 25-33

10 E.W Collings, Physical Metallurgy of Titanium Alloys, American Society for Metals, 1984

11 "Titanium: Past, Present and Future," NMAR-392, National Materials Advisory Board, National Academy Press, 1983; also, PB83-171132, National Technical Information Service

12 W.J Kroll, C.T Anderson, and H.L Gilbert, A New Graphite Resistor Vacuum Furnace and Its

Application in Melting Zirconium, Trans AIME, Vol 175, 1948, p 766-773

13 R.A Beahl, F.W Wood, J.O Borg, and H.L Gilbert, "Production of Titanium Castings," Report 5265, U.S Bureau of Mines, Aug 1956, p 42

14 A.R Beall, J.O Borg, and F.W Wood, "A Study of Consumable Electrode Arc Melting," Report 5144, U.S Bureau of Mines, 1955

19 R.R Boyer and R Bajoraitis, "Standardization of Ti6Al-4V Processing Conditions," AFML-TR-78-131, Air Force Materials Laboratory, Boeing Commercial Airplane Company, Sept 1978

20 D Eylon, T.L Bartel, and M.E Rosenblum, High Temperature Low Cycle Fatigue of Beta-Annealed

Titanium Alloy, Metall.Trans A, Vol 11A, 1980, p 1361-1367

21 D Eylon and J.A Hall, Fatigue Behavior of Beta-Processed Titanium Alloy IMI-685, Metall Trans A, Vol

8A, 1977, p 981-990

22 D Eylon, Fatigue Crack Initiation in Hot Isostatically Pressed Ti6Al-4V Castings, J Mat Sci., Vol 14,

1979, p 1914-1920

23 D Eylon and W.R Kerr, The Fractographic and Metallographic Morphology of Fatigue Initiation Sites, in

Fractography in Failure Analysis, STP 645, American Society for Testing and Materials, 1978, p 235-248

24 D Eylon and M.E Rosenblum, Effects of Dwell on High Temperature Low Cycle Fatigue of a Titanium

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