ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 1 Forging temperature ranges for 12 nickel-base alloys

Closed-die forging of nickel-base alloys is generally done below the ' solvus temperature in order to avoid excessive

grain growth. Approximately 80% of the reduction is scheduled in the recrystallization temperature range, with the

remaining 20% done at lower temperatures to introduce a certain amount of warm work for improved mechanical

properties. Preheating of all tools and dies to about 260 °C (500 °F) is recommended to avoid chilling the metal during

working.

Forging Rate. A very rapid rate of forging often causes heat buildup (due to friction and deformation heating), a

nonuniform recrystallized grain size, and mechanical property variations. Susceptibility to free surface ruptures also

increases with forging rate (and forging temperature). Therefore, slow strain rates are typically used during the initial

closed-die reductions of such alloys as Astroloy (UNS N13017) and René 95 (Ni-14Cr-8Co-3.5Mo-3.5W-3.5Nb-3.5Al-

2.5Ti). With proper selection of starting stock and forging temperature, however, the forging rate is less critical. For

example, some Astroloy turbine components are currently hammer forged.

Forging Reduction. A sufficient amount of recrystallization is necessary in each of a series of closed-die forging

operations to achieve the desired grain size and to reduce the effects of the continuous grain-boundary or twin-boundary

carbide networks that develop during heating and cooling. This condition contributes more to mechanical-property and

other problems than any other single factor. Poor weldability, low-cycle fatigue, and stress rupture properties are

associated with continuous grain-boundary carbide networks. Heat treatment can do very little to correct this problem

without creating equally undesirable mechanical-property problems when higher solution treatment temperatures are

used. All portions of a part must receive some hot work after the final heating operation in order to achieve uniform

mechanical properties.

In open-die forging, a series of moderate reduction passes along the entire length of the forging is preferred. In working a

square section into a round, the piece should be worked down in the square form until it approaches the final size. It

should then be converted to an oversize octagon before finishing into the round. Billet corners that will be in contact with

dies should be chamfered rather than left square. The work should be lifted away from the dies occasionally to permit

relief of local cold areas.

Other Considerations. The precipitation-hardenable nickel alloys are subject to thermal cracking. Therefore, localized

heating is not recommended. The entire part should be heated to the forging temperature.

If any ruptures appear on the surface of the metal during hot working, they must be removed at once, either by hot

grinding or by cooling the work and cold overhauling. If the ruptures are not removed, they may extend into the body of

the part.

For sections equal to or larger than 405 mm (16 in.) square, precautions should be taken in heating precipitation-

hardenable alloys. They should be charged into a furnace at 870 °C (1600 °F) or colder and brought up to forging

temperature at a controlled rate of 40 °C (100 °F) per hour.

Reference cited in this section

A.J. DeRidder and R. Koch, in

MiCon 78: Optimization of Processing, Properties, and Service Performance

Through Microstructural Control, H. Abram et al.,

Ed., American Society for Testing and Materials, 1979, p

547

Forging of Nickel-Base Alloys

Revised by H.H. Ruble, Inco Alloys International and S.L. Semiatin, Battelle Columbus Division

Cooling After Forging

The rate of cooling after forging is not critical for alloys 200, 400, and 625. Alloys K-500 (UNS N05500) and 301 (UNS

N03301) should be water quenched from forging temperatures to avoid the excessive hardening and cracking that could

occur if they were cooled slowly through the age-hardening range and to maintain good response to subsequent aging.

Alloy 825 (UNS N08825) should be cooled at a rate equal to or faster than air cooling.

Alloys 800 and 600 are subject to carbide precipitation during heating in or slow cooling through the temperature range of

540 to 760 °C (1000 to 1400 °F). If sensitization is likely to prove disadvantageous in the end use, parts made of these

alloys should be water quenched or cooled rapidly in air.

The precipitation-hardenable alloys should, in general, be cooled in air after forging. Water quenching is not

recommended, because of the possibility of thermal cracking, which can occur during subsequent heating for further

forging or heat treating.

Forging of Nickel-Base Alloys

Revised by H.H. Ruble, Inco Alloys International and S.L. Semiatin, Battelle Columbus Division

Forging Practice for Specific Alloys

The following practices are used in the forging of nickel-base alloys. Variations from these procedures may be necessary

for some specialized applications (see the sections "Thermal-Mechanical Processing" and "Isothermal Forging" in this

article).

Alloy 200 should be charged to a hot furnace, withdrawn as soon as the desired temperature has been reached, and

worked rapidly. The recommended range of forging temperatures is 650 to 1230 °C (1200 to 2250 °F). Because the metal

stiffens rapidly when cooled to about 870 °C (1650 °F), all heavy work and hot bending should be done above that

temperature. High mechanical properties can be produced by working lightly below 650 °C (1200 °F). The best range for

hot bending is 870 to 1230 °C (1600 to 2250 °F).

Alloy 301. The optimum temperature range for the forging of alloy 301 is 1065 to 1230 °C (1900 to 2250 °F). Light

finishing work can be done down to 870 °C (1600 °F). Finer grain size is produced in forgings by using 1175 °C (2150

°F) for the final reheat temperature and by taking at least 30% reduction of area in the last forging operation.

After hot working, the alloy should be quenched from a temperature of 790 °C (1450 °F) or above. Quenching retains the

strain hardening imparted by the forging operation and produces better response to subsequent age hardening. Quenching

in water containing about 2 vol% alcohol results in less surface oxidation.

Material that must be cooled prior to subsequent hot working should also be quenched. Slow cooling may cause age

hardening, which sets up stresses in the workpiece that can cause cracking during subsequent reheating.

Alloy 400. The maximum heating temperature for forging alloy 400 is 1175 °C (2150 °F). Prolonged soaking at the

working temperature is detrimental. If a delay occurs during processing, the furnace temperature should be reduced to

1040 °C (1900 °F) and not brought to 1175 °C (2150 °F) until operations are resumed.

The recommended metal temperature for heavy reductions is 925 to 1175 °C (1700 to 2150 °F). Light reductions may be

taken at temperatures down to 650 °C (1200 °F). Working at the lower temperatures produces higher mechanical

properties and smaller grain size.

A controlled forging procedure is necessary to meet the requirements of some specifications for forged hot-finished parts.

Both the amount of reduction and the finishing temperature must be controlled in order to develop the desired properties.

One procedure for producing forgings to such specifications consists of taking a 30 to 35% reduction after the final

reheat. This is done as follows:

• Reheat

• Forge to a section having about 5% larger area than the final shape (take at least 25% reduction)

• Cool to 705 °C (1300 °F)

• Finish to size (5% reduction)

High-tensile forgings, as described in certain military specifications, also require a minimum of 30 to 35% reduction after

the last reheat. This is taken in the following manner:

• Reheat

• Forge to a section having an area about 25% larger than the final shape (take about 5% reduction)

• Cool to 705 °C (1300 °F)

• Finish to size (25% reduction)

Grain refinement is achieved by using a temperature of 1095 °C (2000 °F) for the final reheat and by increasing the

amount of reduction taken after the last reheat.

Alloy K-500. The maximum recommended heating temperature for the forging of alloy K-500 is 1150 °C (2100 °F).

Metal should be charged into a hot furnace and withdrawn when uniformly heated. Prolonged soaking at this temperature

is harmful. If a delay occurs such that the material would be subject to prolonged soaking, the temperature should be

reduced to or held at 1040 °C (1900 °F) until shortly before working is to begin, then brought to 1150 °C (2100 °F).

When the piece is uniformly heated, it should be withdrawn. In the event of a long delay, the work should be removed

from the furnace and water quenched.

The forging temperature range is 870 to 1150 °C (1600 to 2100 °F). Heavy work is best done between 1040 and 1150 °C

(1900 and 2100 °F), and working below 870 °C (1600 °F) is not recommended. To produce finer grain in forgings, 1095

°C (2000 °F) should be used for the final reheat temperature, and at least 30% reduction of area should be taken in the last

forging operation.

When forging has been completed or when it is necessary to allow alloy K-500 to cool before further hot working, it

should not be allowed to cool in air, but should be quenched from a temperature of 790 °C (1450 °F) or higher. If the

piece is allowed to cool slowly, it will age harden to some extent, and stress will be set up that may lead to thermal

splitting or tearing during subsequent reheating. In addition, quenched material has better response to age hardening

because more of the age-hardening constituent is retained in solution.

Alloy 600. The normal forging temperature range for alloy 600 is 870 to 1230 °C (1600 to 2250 °F). Heavy hot work

should be done in the range from 1040 to 1230 °C (1900 to 2250 °F). Light working can be continued down to 870 °C

(1600 °F). Generally, forging should not be done between 650 and 870 °C (1200 and 1600 °F) because of the low

ductility of the alloy in this temperature range. Judicious working at a temperature below 650 °C (1200 °F) will develop

higher tensile properties.

The rate of cooling after forging is not critical with respect to thermal cracking. However, alloy 600 is subject to carbide

precipitation in the range between 540 and 760 °C (1000 and 1400 °F), and if subsequent use dictates freedom from

sensitization, the part should be rapidly cooled through this temperature range.

Alloy 625 should be heated in a furnace held at 1175 °C (2150 °F) but no higher. The work should be brought as close to

this temperature as conditions permit. Forging is done from this temperature down to 1010 °C (1850 °F); below 1010 °C

(1850 °F) the metal is stiff and hard to move, and attempts to forge it may cause hammer splits at the colder areas. The

work should be returned to the furnace and reheated to 1175 °C (2150 °F) whenever its temperature drops below 1010 °C

(1850 °F). To guard against duplex grain structure, the work should be given uniform reductions. For open-die work, final

reductions of a minimum of 20% are recommended.

Alloy 718 is strong and offers considerable resistance to deformation during forging. The forces required for hot

deformation are somewhat higher than those employed for alloy X-750. Alloy 718 is forged in the range from 900 to 1120

°C (1650 to 2050 °F). In the last operation, the metal should be worked uniformly with a gradually decreasing

temperature, finishing with some light reduction below 955 °C (1750 °F). Figure 2 shows a forged and machined alloy

718 marine propeller blade. In heating for forging, the material should be brought up to temperature, allowed to soak a

short time to ensure uniformity, and withdrawn.

Fig. 2 Forged and machined alloy 718 marine propeller blade. Courtesy of Ladish Company

Alloy 718 should be given uniform reductions in order to avoid duplex grain structure. Final reductions of 20% minimum

should be used for open-die work, and 10% minimum for closed-die work. Parts should generally be air cooled from the

forging temperature, rather than water quenched.

Alloy 706 (UNS N09706) is similar to alloy 718, except that alloy 706 is more readily fabricated, particularly by

machining. Forging should be done using the same procedures and temperatures as those for alloy 718.

Alloy X-750. The forging range for alloy X-750 is 980 to 1205 °C (1800 to 2200 °F). Below 980 °C (1800 °F), the

metal is stiff and hard to move, and attempts to work it may cause splitting. All heavy forging should be done at about

1040 °C (1900 °F), and the metal should be reheated whenever it cools to below that temperature. Forgings can be

finished with some light reduction in the range between 980 and 1040 °C (1800 and 1900 °F).

As a general rule, alloy X-750 should be air cooled rather than liquid quenched from the forging temperature. Liquid

quenching can cause high residual stresses that may result in cracking during subsequent heating for further hot work or

for heat treatment. Parts with large cross sections and pieces with variable cross sections are especially susceptible to

thermal cracking during cooling. In very large cross sections, furnace cooling may be necessary to prevent thermal

cracking.

Alloy 800. Hot working of alloy 800 is started at 1205 °C (2200 °F) and heavy forging is done at temperatures down to

1010 °C (1850 °F). Light working can be accomplished down to 870 °C (1600 °F). No working should be done between

870 and 650 °C (1600 and 1200 °F). As with alloy 600, thermal cracking is not a problem, and workpieces should be

cooled rapidly through the range between 540 and 760 °C (1000 and 1400 °F) to ensure freedom from sensitization.

Alloy 825. The forging range for alloy 825 is 870 to 1175 °C (1600 to 2150 °F). It is imperative that some reduction be

accomplished in the range between 870 and 980 °C (1600 and 1800 °F) during final forging in order to ensure maximum

corrosion resistance.

Cooling after forging should be done at a rate equal to or faster than air cooling. Heavy sections may become sensitized

during cooling from the forging temperature and therefore be subject to intergranular corrosion in certain media. A

stabilizing anneal of 1 h at 940 °C (1725 °F) restores resistance to corrosion. If the forged piece is to be welded and used

in an environment that could cause intergranular corrosion, the piece should be given a stabilizing anneal to prevent

sensitization from the heat of welding, regardless of the cooling rate after forging.

Alloy 925. The hot-working characteristics of alloy 925 (UNS N09925) are similar to those of alloy 825 at temperatures

to 1095 °C (2000 °F). At higher temperatures, alloy 925 has lower ductility and higher strength. The forging range is 870

to 1175 °C (1600 to 2150 °F). For maximum corrosion resistance and highest mechanical properties after direct aging,

final hot working should be done in the range of 870 to 980 °C (1600 to 1800 °F).

Alloys 722 and 751 (UNS N07722 and N07751, respectively) are forged using the same procedures and temperatures

as those for alloy X-750.

Alloys 903, 907, and 909 (UNS N19903, N19907, and N19909, respectively) are best forged in three stages in order

to obtain the desired properties after aging. The initial breakdown of 40% minimum reduction should be performed at a

temperature of 1060 to 1120 °C (1940 to 2050 °F). For intermediate forging at a minimum of 25% reduction, these alloys

should be heated between 995 to 1050 °C (1825 and 1925 °F). The final heating for alloys 907 and 909 should be 980 to

1025 °C (1800 to 1875 °F) for a minimum reduction of 20% over a falling temperature range (finishing at 925 °C, or

1700 °F). The final heating for alloy 903 should be 870 °C (1600 °F) with a final forging reduction of 40% minimum.

Other nickel-base heat-resistant alloys are discussed in the article "Forging of Heat-Resistant Alloys" in this Volume.

Forging of Nickel-Base Alloys

Revised by H.H. Ruble, Inco Alloys International and S.L. Semiatin, Battelle Columbus Division

Thermal-Mechanical Processing (TMP)

Thermal-mechanical processing refers to the control of temperature and deformation during processing to enhance

specific properties. Special TMP sequences have been developed for a number of nickel-base alloys.

The design of TMP sequences relies on a knowledge of the melting and precipitation temperatures for the alloy of

interest. Table 3 lists these temperatures for several nickel-base alloys. Although nickel-base (as well as iron- and cobalt-

base) alloys form various carbides--for example, MC (M = titanium, niobium, etc.), M

C (M = molybdenum and/or

tungsten), or M

(M = chromium)--the primary precipitate of concern in the processing of such materials is the γ'-

strengthening precipitate. Gamma prime is an ordered face-centered cubic (fcc) compound in which aluminum and

titanium combine with nickel to form Ni

(Al, Ti). In nickel-iron alloys such as alloy 718, titanium, niobium, and to a

lesser extent, aluminum combine with nickel to form ordered fcc γ' or ordered body-centered tetragonal γ''. Nickel-iron

base alloys are also prone to the formation of other phases, such as hexagonal close-packed Ni

Ti (η), as in titanium-rich

alloy 901, or orthorhombic Ni

Nb (δ) in niobium-rich alloy 718.

Table 3 Critical melting and precipitation temperatures for several nickel-base alloys

First

melting

temperature

Precipitation

temperature

Alloy UNS No.

°C °F °C

°F

Alloy X N06002 1260

2300

760

1400

Alloy 718 N07718 1260

2300

845

1550

Waspaloy N07001 1230

2250

980

1800

Alloy 901 N09901 1200

2200

980

1800

Alloy X-750

N07750 1290

2350

955

1750

M-252 N07252 1200

2200

1010

1850

Alloy R-235

. . . 1260

2300

1040

1900

René 41 N07041 1230

2250

1065

1950

U500 N07500 1230

2250

1095

2000

U700 . . . 1230

2250

1120

2050

Astroloy N13017 1230

2250

1120

2050

Source: Ref 2

Early forging practice of nickel- and nickel-iron base alloys consisted of forging from and solution heat treating at

temperatures well in excess of the γ' solvus temperature. High-temperature solution treatment dissolved all of the γ',

annealed the matrix, and promoted grain growth (typical grain size ASTM 3 or coarser). This was followed by one or

more aging treatments that promoted controlled precipitation of γ' and carbide phases. Optimal creep and stress rupture

properties above 760 °C (1400 °F) were thus achieved. Later in the development of forging practice, it was found that

using preheat furnace temperatures slightly above the recrystallization temperature led to the development of finer grain

sizes (ASTM 5 to 6). Coupling this with modified heat-treating practices resulted in excellent combinations of tensile,

fatigue, and creep properties.

State-of-the art forging practices for nickel-base alloys rely on the following microstructural effects (Ref 3):

• Dynamic recrystallization is the most important softening mechanism during hot working

• Grain boundaries are preferred nucleation sites for recrystallization

• The rate of recrystallization decreases with the temperature and/or the extent of deformation

•

Precipitation that may occur during the recrystallization can inhibit the softening process.

Recrystallization cannot be completed until the precipitate coarsens to a relatively ineffective

morphology

Forging temperature is carefully controlled during the thermal-mechanical processing of nickel- and nickel-iron base

alloys to make use of the structure control effects of second phases such as γ'. Above the optimal forging temperature

range (Table 4), the structure control phase goes into solution and loses its effect. Below this range, extensive fine

precipitates are formed, and the alloy becomes too stiff to process. Several examples of specific TMP sequences are given

below.

Table 4 Structure control phases and working temperature ranges for various heat-resistant alloys

Working temperature range

Alloy UNS No.

Phases for structure control

°C

°F

Nickel-base alloys

Waspaloy N07001 γ' (Ni

(Al,Ti) 955-1025

1750-1875

Astroloy N13017 γ' (Ni

(Al,Ti) 1010-1120

1850-2050

IN-100 . . . γ' (Ni

(Al,Ti) 1040-1175

1900-2150

René 95 . . . γ' (Ni

(Al,Ti) 1025-1135

1875-2075

Nickel-iron-base alloys

901 N09901 η(Ni

Ti) 940-995

1725-1825

718 N07718 δ(Ni

Nb) 915-995

1675-1825

Pyromet CTX-1

. . . η(Ni

Ti), δ(Ni

Nb), or both 855-915 1575-1675

Waspaloy. A typical TMP treatment of nickel-base alloys is that used for Waspaloy (UNS N07001) to obtain good

tensile and creep properties. This consists of initial forging at 1120 °C (2050 °F) and finish forging below approximately

1010 °C (1850 °F) to produce a fine, equiaxed grain size of ASTM 5 to 6. Solution treatment is then done at 1010 °C

(1850 °F), and aging is conducted at 845 °C (1550 °F) for 4 h, followed by air cooling plus 760 °C (1400 °F) for 16 h and

then air cooling.

René 95. Initial forging of René 95 is done at a temperature between 1095 and 1140 °C (2000 and 2080 °F). Following

an in-process recrystallization anneal at 1175 °C (2150 °F), finish forging (reduction 40 to 50%) is then imposed below

the γ' solvus, typically at temperatures between 1080 and 1105 °C (1975 and 2025 °F). The large grains formed during

high-temperature recrystallization are elongated and surrounded by small recrystallized grains that form during finish

forging.

Alloy 901. The thermal-mechanical processing of alloy 901 is often done to produce a fine-grain structure that enhances

fatigue strength (Ref 5). This is accomplished by using the (Ni

Ti) phase, which is introduced in a Widmanstdätten form

at the beginning of processing by a heat treatment at 900 °C (1650 °F) for 8 h. Forging is then conducted at 955 °C (1750

°F), which is below the η solvus; the forging deformation is completed below the recrystallization temperature. A fine-

grain structure is generated by a subsequent recrystallization treatment below the η solvus. The needle-like η phase will

become spherical during forging and will restrict grain growth. Aging is then conducted according to standard procedures.

References cited in this section

T. Altan, F.W. Boulger, J.R. Becker, N. Akgerman, and H.J. Henning,

Forging Equipment, Materials, and

Practices, MCIC-HB-03, Metals and Ceramics Information Center, 1973

T.G. Byrer, Ed., Forging Handbook, Forging Industry Association and American Society for Metals, 1985

D.R. Muzyka, in

MiCon 78: Optimization of Processing, Properties, and Service Performance Through

Microstructural Control, H. Abrams et al., Ed., American Society for Testing and Materials, 1979, p 526

L.A. Jackman, in Proceedings of the Symposium on Properties of High Temperature Alloys,

The

Electrochemical Society, 1976, p 42

Forging of Nickel-Base Alloys

Revised by H.H. Ruble, Inco Alloys International and S.L. Semiatin, Battelle Columbus Division

Isothermal Forging

Nickel-base alloys that are hard to work or are typically used in the cast condition can be readily forged when in a

powder-consolidated form. The most common forging technique using powder preforms is isothermal forging; (see the

article "Isothermal and Hot-Die Forging" in this Volume). In this process, powder is produced by inert gas atomization

and is compacted into billet form by extrusion. The billets are fabricated below the γ' solvus temperature for alloys such

as IN-100 in order to maintain a fine grain size and a fine distribution of precipitates. In this condition, the material

exhibits superplastic properties that are characterized by large tensile elongations (during sheet forming) and good die-

filling capacity (during forging). Multiples of the extruded bar are then isothermally forged into a variety of complex

turbine engine and other high-temperature parts.

The key to successful isothermal forging of nickel-base alloys is the ability to develop a fine grain size before forging and

to maintain it during forging. With regard to the latter, a high volume percentage of second phase is useful in preventing

grain growth. Therefore, alloys such as IN-100, René 95, and Astroloy, which contain large amounts of γ', are readily

capable of developing the superplastic properties necessary in isothermal forging. In contrast, Waspaloy, which contains

less than 25 vol% γ' at isothermal forging temperatures, is only marginally superplastic. Nickel-iron base alloys such as

alloys 718 and 901 have even lower volume fractions of precipitate and are therefore even less frequently used in

isothermal forging.

As the term implies, isothermal forging consists of forging with the workpiece and the dies at the same temperature.

Because this temperature is often of the order of 980 to 1095 °C (1800 to 2000 °F), the dies are usually made of

molybdenum for elevated-temperature strength. The isothermal forging system must be operated in a vacuum or inert

atmosphere in order to protect such die materials from oxidation.

Compared to conventional forging, isothermal forging deformation rates are slow; hydraulic press speeds of

approximately 2.5 mm/min (0.1 in./min) are typical. However, the slower production rate is largely offset by the ability to

forge complex shapes to closer tolerances, which leads to less machining and substantial material savings. In addition, a

large amount of deformation is accomplished in one operation, pressures are low, and uniform microstructures are

achieved. For example, the as-forged weight of a finish-machined 68 kg (150 lb) Astroloy disk is about 110 kg (245 lb)

for a conventional forging versus 72 kg (160 lb) for the corresponding isothermal forging.

Forging of Nickel-Base Alloys

Revised by H.H. Ruble, Inco Alloys International and S.L. Semiatin, Battelle Columbus Division

References

A.J. DeRidder and R. Koch, in

MiCon 78: Optimization of Processing, Properties, and Service Performance

Through Microstructural Control, H. Abram et al.,

Ed., American Society for Testing and Materials, 1979, p

547

T. Altan, F.W. Boulger, J.R. Becker, N. Akgerman, and H.J. Henning,

Forging Equipment, Materials, and

Practices, MCIC-HB-03, Metals and Ceramics Information Center, 1973

T.G. Byrer, Ed., Forging Handbook, Forging Industry Association and American Society for Metals, 1985

D.R. Muzyka, in

MiCon 78: Optimization of Processing, Properties, and Service Performance Through

Microstructural Control, H. Abrams et al., Ed., American Society for Testing and Materials, 1979, p 526

L.A. Jackman, in Proceedings of the Symposium on Properties of High Temperature Alloys,

The

Electrochemical Society, 1976, p 42

Forging of Titanium Alloys

G.W. Kuhlman, Aluminum Company of America

Introduction

TITANIUM ALLOYS are forged into a variety of shapes and types of forgings, with a broad range of final part forging

design criteria based on the intended application. As a class of materials, titanium alloys are among the most difficult

metal alloys to forge, ranking behind only refractory metals and nickel/cobalt-base superalloys. Therefore, titanium alloy

forgings, particularly closed-die forgings, are typically produced to less highly refined final forging configurations than

are typical of aluminum alloys (although precision forgings in titanium alloys are produced to the same design and

tolerance criteria as aluminum alloys; see the section "Titanium Alloy Precision Forgings" in this article) and to

equivalent or more refined forging design sophistication than carbon or low-alloy steel forgings, because of reduced

oxidation or scaling tendencies in heating. Because of the high cost of titanium alloys in comparison to other commonly

forged materials, such as aluminum and alloy steels, final forging design criteria in titanium closed-die forgings are

typically balanced between producibility demands and cost considerations (particularly machining costs and overall metal

recovery).

In addition, the working history and forging parameters used in titanium alloy forging have a significant impact on the

final microstructure (and therefore the resultant mechanical properties) of the forged alloy--perhaps to a greater extent

than in any other commonly forged material. Therefore, the forging process in titanium alloys is used not only to create

cost-effective forging shapes but also, in combination with thermal treatments, to create unique and/or tailored

microstructures to achieve the desired final mechanical properties through thermomechanical processing (TMP)

techniques. For a given titanium alloy forging shape, the pressure requirements in forging vary over a large range,

depending primarily on the chemical composition of the alloy, the forging process being used, the forging strain rate, the

type of forging being manufactured, lubrication conditions, and die temperature. The chemical compositions,

characteristics, and typical mechanical properties of all wrought titanium alloys referred to in this article are reviewed in

the article "Wrought Titanium and Titanium Alloys" in Properties and Selection: Nonferrous Alloys and Special-Purpose

Materials, Volume 2 of the ASM Handbook.

Forging of Titanium Alloys

G.W. Kuhlman, Aluminum Company of America

Titanium Alloy Classes

Because of the strong relationship among the required forging process parameters, deformation behavior, and mechanical

properties of the various titanium alloys, it is necessary to review the classes of titanium alloys that are forged and their

typical thermomechanical processing requirements, which exert a strong influence on forging part design and forging

process selection. Titanium and its alloys exist in two allotropic forms:

• The hexagonal close-packed (hcp) α phase

• The body-centered cubic (bcc) β phase

The more difficult to deform phase is usually present at low temperatures, while the more easily deformed β phase is

present at high temperatures. However, the addition of various alloying elements (including other metals and such gases

as oxygen, nitrogen, and hydrogen) stabilizes either the α or β phase. The temperature at which a given titanium alloy

transforms completely from α to β is termed the beta transus, β

, and is a critical temperature in titanium alloy forging

process criteria.

Titanium alloys are divided into three major classes, based on the predominant allotropic form(s) present at room

temperature:

• α/near-α alloys

• α-β alloys

• β/metastable β alloys

Each of these types of titanium alloys has unique forging process criteria and deformation behavior. Further, the forging

process parameters, often in combination with subsequent thermal treatments, are manipulated for each alloy type to

achieve the desired final forging microstructure and mechanical properties (heat treatment serves a different purpose in

titanium alloys from that in aluminum alloys or alloy steels, as discussed below). Table 1 lists most of the commonly

forged titanium alloys by alloy class, along with the major alloying elements constituting each alloy.

Table 1 Recommended forging temperature ranges for commonly forged titanium alloys

Forging temperature

(b)

Alloy

°C °F

Process

(a)

°C

°F

/near- alloys

Ti-C.P.

(c)

915 1675

C 815-900

1500-1650

Ti-5Al-2.5Sn

(c)

1050

1925

C 900-1010

1650-1850

Ti-5Al-6Sn-2Zr-1Mo-0.1Si 1010

1850

C 900-995

1650-1925

Ti-6Al-2Nb-1Ta-0.8Mo 1015

1860

940-1050

1040-1120

1725-1825

1900-2050

Ti-6Al-2Sn-4Zr-2Mo(+0.2Si)

(d)

990 1815

900-975

1010-1065

1650-1790

1850-1950

Ti-8Al-1Mo-1V 1040

1900

C 900-1020

1650-1870

IMI 685 (Ti-6Al-5Zr-0.5Mo-0.25Si)

(e)

1030

1885

C/B 980-1050

1795-1925