ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 1 Copper alloy parts made by closed-die forging. Courtesy of Mueller Brass Company

Cylindrical slugs are sometimes partially flattened before forging to promote better flow and consequently better filling of

an impression. This can usually be done at room temperature between flat dies in a hammer or a press. A rectangular slug

is occasionally obtained by extruding rectangular-section bar stock and sawing slugs from it.

Upset forging is used less frequently for copper alloys than for steels, primarily because copper alloys are so easily

extruded. A part having a long shaftlike section and a larger-diameter head can often be made at less cost by extruding the

smaller cross section from a larger one than by starting with a small cross section and upsetting to obtain the head.

In the upsetting of copper alloys, the same rule applies for maximum unsupported length as is used for steels, that is, not

more than three times stock diameter. For the forging of brass, single-blow upsetting as severe as 3 to 1 (upset three times

starting diameter) is considered reasonable. In practice, however, upsets of this severity are rare. The degree of allowable

upset for other copper alloys is somewhat less than that for forging brass, generally in proportion to forgeability (Table 1).

Table 1 Relative forgeability ratings of commonly forged copper alloys

Ratings are in terms of the most forgeable alloy, forging brass (C37700).

Alloy Nominal composition

Relative

forgeability

(a)

C10200

99.95 min Cu

C10400

Cu-0.027 Ag

C11000

99.9 min Cu

C11300

Cu-0.027Ag + O

C14500

Cu-0.65Te-0.008P

C18200

Cu-0.10Fe-0. 90Cr-0.10

Si-0.05Pb

C37700

Cu-38Zn-2Pb

100

C46400

Cu-39.2Zn-0.8Sn

C48200

Cu-38Zn-0.8Sn-0.7Pb

C48500

Cu-37.5Zn-1.8Pb-0.7Sn

C62300

Cu-10Al-3Fe

C63000

Cu-10Al-5Ni-3Fe

C63200

Cu-9Al-5Ni-4Fe

C64200

Cu-7Al-1.8Si

C65500

Cu-3Si

C67500

Cu-39Zn-1.4Fe-1Si-0.1Mn

(a)

Takes into consideration such factors as pressure, die wear, and hot plasticity

In most designs, the amount of upset can be reduced by using slugs cut from specially shaped extrusions or by using one

or more blocking impressions in the forging sequence. Additional information on upset forging is available in the article

"Hot Upset Forging" in this Volume.

Ring rolling is sometimes used as a means of saving material when producing ring gears or similar ringlike parts. The

techniques are essentially the same as those used for steel and are described in detail in the article "Ring Rolling" in this

Volume. Temperatures are the same as those for forging the same alloy in closed dies.

Cost usually governs the minimum practical size for ring rolling. Most rings up to 305 mm (12 in.) in outside diameter are

more economically produced in closed dies. However, if the face width is less than about 25 mm (1 in.) it is often less

expensive to produce rings no larger than 203 mm (8 in.) in outside diameter by the rolling technique. The alloy being

forged is also a factor in selecting ring rolling or closed-die forging. For example, alloys such as beryllium copper that are

difficult to forge are better adapted to ring rolling. For these alloys, ring rolling is sometimes used for sizes smaller than

the minimum practical for the more easily forged alloys.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Forging Alloys

Copper C12200 and the copper alloys most commonly forged are listed in Table 1. They comprise at least 90% of all

commercially produced copper alloy forgings. Forging brass, the least difficult alloy to forge, has been assigned an

arbitrary forgeability rating of 100.

Table 2 Recommended die materials for the forging of copper alloys

Part configurations of varying severity are shown in Fig. 2.

Total quantity to be forged

100-10,000

10,000

Maximum severity

Die material

Hardness, HB

Die material

Hardness, HB

Hammer forging

Part 1 H11

6G, 6F2

405-433

341-375

H12

405-448

Part 2 6G, 6F2 341-375 6G, 6F2

H12

(a)

341-375

405-448

Part 3 6G, 6F2 269-293 6G, 6F2

302-331

Part 4 H11 405-433 H11

405-433

Part 5 6G, 6F2 302-331 6G, 6F2

(b)

302-331

Press forging

Part 1 H12

6G, 6F2

477-514

341-375

H12

477-514

Part 2 6G, 6F2 341-375 H12

477-514

Part 3

Part normally is not press forged from copper alloys

Part 4 H11 405-433 6G, 6F2

(c)

341-375

(a)

Recommended for long runs--for example, 50,000 pieces.

(b)

With either steel, use H12 insert at 405-448 HB.

(c)

With either steel, use H12 insert at 429-448 HB.

Some copper alloys cannot be forged to any significant degree, because they will crack. Leaded copper-zinc alloys, such

as architectural bronze, which may contain more than 2.5% Pb, are seldom recommended for hot forging. Although lead

content improves metal flow, it promotes cracking in those areas of a forging, particularly deep-extruded areas, that are

not completely supported by, or enclosed in, the dies. This does not mean that the lead-containing alloys cannot be forged,

but rather that the design of the forging may have to be modified to avoid cracking.

The solubility of lead in -brass at forging temperatures is about 2% maximum, but lead is insoluble in -brass at all

temperatures. Consequently, although a lead content of up to 2.5% is permissible in Cu-40Zn - brasses, lead in excess

of 0.10% in a Cu-30Zn -brass will contribute to catastrophic cracking.

Other copper alloys, such as the copper-nickels, can be forged only with greater difficulty and at higher cost. The copper-

nickels, primarily because of their higher forging temperatures, are sometimes heated in a controlled atmosphere, thus

complicating the process. The silicon bronzes, because of their high forging temperatures and their compositions, cause

more rapid die deterioration than the common forging alloys.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Machines

Most copper alloy forgings are produced in crank-type mechanical presses. With these presses, the production rate is

high, and less operator skill is needed and less draft is required than in forging copper alloys in hammers.

Press size is normally based on the projected (plan) area of the part, including flash. The rule of thumb is 0.5 kN of

capacity per square millimeter of projected area (40 tonf/in.

). Therefore, a forging with a projected area of 32.2 cm

in.

) will require a minimum of 1780 kN (200 tonf) capacity for forgings of up to medium severity. If the part is

complicated (for example, with deep, thin ribs), the capacity must be increased.

Speed of the press is not critical in forging copper alloys, but minimum duration of contact between the hot forging and

the die is desirable to increase die life. Detailed information on hammers and presses is available in the articles "Hammers

and Presses for Forging" and "Selection of Forging Equipment" in this Volume.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Dies

Dies designed for forging copper or copper alloys usually differ from those designed for forging the same shapes from

steel, as follows:

• The draft angle can be decreased for forging copper (3° max and often less than 3°)

• The die cavity is usually machined to dimensions that are 0.005 in./in. less than those for forging steels

• The die cavity is usually polished to a better surface finish for forging copper and copper alloys

Die materials and hardnesses selected for forging copper alloys depend on part configuration (forging severity) and

number of parts to be produced. Figure 2 illustrates the forging severities of parts listed in Table 2.

Fig. 2 Forged copper alloy parts of varying severity. See Table 2 for recommended die materials.

Whether the dies are made entirely from a hot-work steel such as H11 or H12 or whether or not inserts are used depends

largely on the size of the die. Common practice is to make the inserts from a hot-work steel and to press them into rings or

holders made from a low-alloy die block steel (Table 2) or L6 tool steel. Hardness of the ring or holder is seldom critical;

a range of 341 to 375 HB is typical. Details on the selection of die material and data on die wear and life are available in

the article "Dies and Die Materials for Hot Forging" in this Volume.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Preparation of Stock

The two methods most often used for cutting stock into slugs for forging are shearing and sawing.

Shearing is faster than other methods of cutting stock. In addition, no material is wasted in kerf. However, the ends of

sheared stock are rougher than those of sawed sections. Rough or torn ends usually cannot be permitted, because forging

defects are likely to nucleate from the rough ends. If shearing is used, best practice is to condition the sheared ends--for

example, with a radiusing machine.

Sawing with circular saws having carbide-tipped blades is widely used as a method of preparing stock because sawed

ends are usually in much better condition than sheared ends. The principal disadvantage of sawing is the loss of metal

because of the kerf. In addition, if the burrs left by sawing are not removed, defects are likely to develop in the forging.

Deburring of the saw sections by grinding, radiusing or barrel tumbling is always recommended.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Heating of Billets or Slugs

Optimal forging temperature ranges for ten alloys are given in Table 3. Atmosphere protection during billet heating is not

required for most alloys, especially when forging temperatures are below 705 °C (1300 °F). For temperatures toward the

top of the range in Table 3, a protective atmosphere is desirable and is sometimes required. An exothermic atmosphere is

usually the least costly, and it is satisfactory for heating copper alloys at temperatures above 705 °C (1300 °F).

Table 3 Recommended forging temperature ranges for copper alloys

Temperature range

Alloy

°C

°F

C12200

730-845

1350-1550

C18200

650-760

1200-1400

C37700

650-760

1200-1400

C46400

595-705

1100-1300

C62400

705-815

1300-1500

C64200

730-900

1350-1650

C67000

595-705

1100-1300

C67300

595-730

1100-1350

C67400

595-730

1100-1350

Gas-fired furnaces are almost always used, and furnace design is seldom critical. Open-fired conveyor chain or belt types

are those most commonly used.

Any type of pyrometric control that can maintain temperature within ±5 °C (± 10 °F) is suitable. As billets are discharged,

a periodic check with a prod-type pyrometer should be made. This permits a quick comparison of billet temperature with

furnace temperature.

Heating Time. The time at temperature is critical for all copper alloys, although to varying degrees among the different

alloys. For forging brass (alloy C37700), the time is least critical, but for aluminum bronze, naval brass, and copper, it is

most critical. Time in excess of that required to bring the billet uniformly to forging temperature is detrimental, because it

causes grain growth and increases the amount of scale.

Reheating Practice. When forging in hammers, all of the impressions are usually made in one pair of dies, and

reheating is rarely required. In press forging, particularly in high-production applications, blocking is often done

separately, followed by trimming before the forging is completed. The operations are likely to be performed in different

presses; therefore the partially completed forging is reheated to the temperature originally used.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Heating of Dies

Dies are always heated for forging copper and copper alloys, although because of the good forgeability of copper alloys,

die temperature is generally less critical than for forging aluminum. Dies are seldom preheated in ovens. Heating is

usually accomplished by ring burners. Optimal die temperatures vary from 150 to 315 °C (300 to 600 °F), depending on

the forging temperature of the specific alloy. For alloys having low forging temperatures, a die temperature of 150 °C

(300 °F) is sufficient. Die temperature is increased to as much as 315 °C (600 °F) for the alloys having the highest forging

temperatures shown in Table 3.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Lubricants

Dies should be lubricated before each forging operation. A spray of colloidal graphite and water is usually adequate.

Many installations include a spray that operates automatically, timed with the press stroke. However, the spray is often

inadequate for deep cavities and is supplemented by swabbing with a conventional forging oil.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Trimming

Brass forgings are nearly always trimmed at room temperature. Because the forces imposed on the trimming tools are less

than those for trimming steel forgings, the trimming of brass forgings seldom poses problems. Large forgings, especially

in small quantities, are commonly trimmed by sawing off the flash and punching or machining the web sections.

Trimming tools usually are used for trimming large quantities, especially of small forgings that are relatively intricate and

require several punchouts.

Materials for trimming dies vary considerably among different plants. In some plants, it is common practice for

normal trimming to make the punch from low-alloy die steel at a hardness of 46 to 50 HRC. One reason for using this

steel is economy; the punches are often made from pieces of worn or broken dies. Blades for normal trimming are

sometimes made by hardfacing low-carbon steels such as 1020.

In other plants both punches and blades are made from L6 steel and are heat treated to 52 to 56 HRC. Worn tools of this

material can be repaired by welding with an L6 rod, remachining, and heat treating; O1 tool steel heat treated to 58 to 60

HRC has also been used for punches and blades for cold trimming. When close trimming is required, blades and punches

fabricated from a high-alloy tool steel such as D2, hardened to 58 to 60 HRC, will give better results and longer life.

Hot trimming is sometimes used for one or both of the following reasons:

• For alloys such as aluminum bronzes that are brittle at room temperature

• When flash is heavy and sufficient power is not available for cold trimming

Hot trimming is usually done at 425 °C (800 °F).

Because of the lower forces involved, tools for hot trimming are simpler than those for cold trimming. Although the tool

materials discussed above can also be used for hot trimming, unhardened low-carbon steel will usually suffice as a punch

material. The same grade of steel with a hardfacing is commonly used as blade material.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Cleaning

Scale and excess lubricants are easily removed from copper and copper alloy forgings by chemical cleaning. Pickling in

dilute sulfuric acid is the most common method for cleaning brass and most other copper alloy forgings, although

hydrochloric acid can also be used. The compositions of sulfuric and hydrochloric acid solutions, the pickling procedures,

and the typical uses are given in Table 4.

Table 4 Cleaning solutions and conditions for copper and copper alloy forgings

Solution Composition Use

temperature,

°C (°F)

Uses

Sulfuric acid 4-15 vol% H

(1.83 specific gravity);

rem H

Room-60 (140)

Removal of black copper oxide scale from brass

forgings; removal of oxide from copper forgings

Hydrochloric

acid

40-90 vol% HCl (35% conc); rem H

O Room

Removal of scale and tarnish from brass forgings;

removal of oxide from copper forgings

"Scale" dip A 40% conc HNO

; 30% conc H

; 0.5%

conc HCl; rem H

Room

Used with pickle and "bright" dip to give a bright,

lustrous finish to copper and copper alloy forgings

"Scale" dip B 50% conc HNO

; rem H

O Room

Used with pickle and "bright" dip to give bright,

lustrous finish to copper and copper alloy forgings

"Bright" dip 25 vol% conc HNO

; 60 vol% conc

; 0.2% conc HCl; rem H

Room Used with pickle and "scale" dip to give bright,

lustrous finish to copper and copper alloy forgings

Aluminum bronzes form a tough, adherent aluminum oxide film during forging. An effective method of cleaning

aluminum bronze forgings is first to immerse them in a 10% solution (by weight) of sodium hydroxide in water at 75 °C

(170 °F) for 2 to 6 min. After rinsing in water, the forgings are pickled in acid solutions in the same way as brasses.

Alloys containing substantial amounts of silicon may form oxides of silicon removable only by hydrofluoric acid or a

proprietary fluorine-bearing compound. Alloys containing appreciable quantities of nickel are difficult to pickle in

solutions used for brasses, because nickel oxide has a limited solubility in these solutions. For these alloys, billets should

be heated in a controlled atmosphere, so that scale is kept to a minimum and can be removed by using the practice

outlined above and in Table 4 for brass.

Other methods of chemical cleaning can be used, depending largely on the desired finish. Additional information is

available in the article "Surface Engineering of Copper and Copper Alloys" in Surface Engineering, Volume 5 of the ASM

Handbook.

Appearance. When a bright, lustrous finish is desired, the metal can be pickled in the sulfuric or hydrochloric acid

pickles listed in Table 4 and then given two additional dips. Pickling removes surface oxides, and the second dip, a

"scale" dip, prepares the metal for the "bright" dip that follows. "Scale" dips and "bright" dips are mixtures of sulfuric and

nitric acids in proportions that vary widely from plant to plant. Generally, nitric acid accelerates the action of the dip,

while sulfuric acid slows it down. These solutions are used at room temperature. Parts are first dipped in the "scale" dip,

rinsed in water, dipped in the "bright" solution, rinsed in cold running water, and then rinsed in hot water and dried.

Compositions of "scale" and "bright" dips are listed in Table 4.

Surface Finish. In normal practice, the surface finish of cleaned forgings is expected to be 5 m (200 in.) or better.

By more precise control, a finish of 2.5 m (100 in.) or better can be obtained. Die finish is the major factor affecting

the surface finish of forgings. The type of alloy forged and the amount of draft have a minor influence on surface finish.

Forging of Copper and Copper Alloys

Robert A. Campbell, Mueller Brass Company

Minimum-Draft Forgings

Zero-draft forgings can be produced from copper alloys, but are usually impractical. However, the minimum-draft

concept is a practical approach for producing locating and clamping surfaces for machining operations, mating surfaces in

assemblies, or other functional shapes where dimensional tolerances on such surfaces are broad enough to include normal

forging tolerances but too close for normal draft angles.

Forging Design. The most obvious consideration is that any shape that has a negative draft angle would be impossible

to eject without damage to the die or workpiece. With zero draft, the smallest error of form or dimension can damage the

die and the workpiece. Therefore, a draft angle of ° should be considered the absolute minimum for production forging.

This very small amount of positive draft is sufficient to eliminate the possibility of negative draft while producing

forgings that have essentially zero draft.

Tolerances on closed-die forgings are normally ±0.25 mm (±0.010 in.) or better for small-to-medium forgings. It can be

seen from Table 5 that a small draft angle can easily be accommodated within these tolerance limits. For example, a draft

of ° would produce a taper of only 0.083 mm (0.00327 in.) on each side of a cavity 19 mm ( in.) deep. Because the

total taper of 0.166 mm (0.00654 in.) (both sides of the cavity) would be less than the usual 0.51 mm (0.020 in.) total

tolerance on the cavity diameter, the part would be within tolerance for a specification of parallel sides.

Table 5 Relation of draft angle to draft for minimum-draft forgings

Draft angle,

degrees

Draft, in./in.

Total taper on

diameter, in./in.

0.00219

0.00438

0.00436

0.00872

0.00873

0.01746

1 0.01745 0.03490

Die Design. Conventional forging practice calls for draft angles of 2° or more on press forgings and up to 5 to 7° for

hammer forgings. Draft angles of 1° or less increase cost. In general, as the draft angle is decreased, more force is

required to eject the forging from the die cavity or to withdraw the punch from a hole. Conventional forgings can usually

be ejected by a simple knockout pin. This method is not practical for minimum-draft forgings, because pin pressure would

be sufficient to damage the part.

Ejection of minimum-draft forgings is nearly always accomplished through the use of inserted dies built on die cushions

to provide a secondary action within the die. This provides a stripper action to the die so that ejection pressure is

distributed over an entire surface rather than concentrated on a pin. Such double-action dies are more expensive to build

and to maintain than solid dies, and their use slows the production rate.

Alloy Selection. Draft angles have no effect on the relative forgeability of copper-base alloys. Any alloy that can be

forged by conventional means can be forged to minimum draft angles.

Forging of Magnesium Alloys

Introduction

The forgeability of magnesium alloys depends on three factors: the solidus temperature of the alloy, the deformation rate,

and the grain size. Only forging-grade billet or bar stock should be used in order to ensure good workability. This type of

product has been conditioned and inspected to eliminate surface defects that could open during forging, and it has been

homogenized by the supplier to ensure good forgeability. Table 1 lists the compositions of magnesium alloys that are

commonly forged, along with their forging temperatures.

Table 1 Recommended forging temperature ranges for magnesium alloys

Recommended forging temperature

(a)

Workpiece

Forging dies

Alloy

°C °F °C

°F

Commercial alloys

ZK21A 300-370

575-700

260-315

500-600

AZ61A 315-370

600-700

290-345

550-650

AZ31B 290-345

550-650

260-315

500-600

High-strength alloys

ZK60A 290-385

550-725

205-290

400-550

AZ80A 290-400

550-750

205-290

400-550

Elevated-temperature alloys