ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 23

Limiting dome height curves for copper and copper alloys. LDH curves illustrate the overall ductility of

the coppers and copper alloys evaluated. See Table 8

for material designations, thicknesses, and tempers.

Source: Ref 5, 6.

Solving Forming Problems. In addition to displaying the relative formability of one material versus that of another,

forming limit and limiting dome height curves are valuable for identifying the cause of a sudden production problem that

might arise from changes in tooling, lubrication, or material suppliers. This permits the forming process to be modified to

maximize formability and productivity.

The most direct approach for determining if a sudden forming problem is materials related is to compare the LDH curve

for the material with that of the control lot of known good material. If only one region of the part is subject to critical

strains, it may be necessary only to test the blank width that will produce that critical value of minor strain. If the LDH

curve of the new material is the same as that of the control lot, then tooling or lubrication are suspect. If the LDH curve of

the new material is below that of the control lot, the material is the problem.

The best way to determine whether tooling or lubrication conditions have changed is to form a gridded sample under

current tool conditions from a control lot held in inventory. Strain distribution and critical grid strains measured on this

sample can be placed on the established forming limit curve and compared with those before the problem arose in order to

establish their relationship to known, safe strain levels. If changes are detected, they can often be remedied by adjusting

press conditions to change the magnitude of stretch or draw components.

This is illustrated in Fig. 24. Point A on this forming limit curve represents the strains in the critical region of a part when

the part was being formed satisfactorily. Point B represents the critical strains when forming became a problem because

the major and minor strains were too high. Draw beads, blank hold-down pressure, blank size, and/or lubrication can be

modified to change the amount of major and minor strains. The effects on critical strain can be compared on the forming

limit curve to ensure that the adjustments will indeed enable the part to be formed.

Fig. 24

Effect of tooling or lubrication on critical strain. Inadvertent changes in tooling or lubrication can shift

strain from Point A to Point B, causing parts that previously were readily formed to fail during forming.

Source:

Ref 5, 6.

A similar approach can be used to adjust the forming operation so that a less ductile material can be formed. Figure 25,

for example, shows forming limit curves for two materials (A and B) and the critical strain combination (point X)

measured on a formed part. Material A forms successfully, but Material B fractures during forming, as indicated by the

location of point X relative to the forming limit curve of each material.

Fig. 25

Effect of changes in forming operation on critical strain. Shifting strain from X to X' by increasing draw

or from X to X'' by increasing stretch permits Material B to be used in place of Material A despite its lower

ductility. Source: Ref 5, 6.

Because of the shape of the forming limit curves, it is possible to maintain approximately the same e

value for point X

but to fall in the safe region by changing e

, as indicated by points X' and X''. In this case, moving toward X' requires that

the draw component be increased during forming; moving toward X'' requires that the stretch component be increased.

Either can be accomplished by altering lubrication, tooling, and/or blank hold-down pressure, thus enabling the part to be

formed in Material B.

References cited in this section

Forming Limits Set for Copper Metals, Am. Mach., April 1983, p 99

"Forming Limit Analysis for Enhanced Fabrication," Report 310A, International Copper Research

Association, Dec 1981

Forming of Copper and Copper Alloys

Frank Mandigo and Jack Crane, Olin Corporation

Property Requirements for Various Formed Products (Ref 7)

Because the selection of an alloy begins with performance requirements even before formability is considered, this

section offers a brief discussion of alloy effects relative to the performance of copper alloys. When these factors are

matched with the best formability considerations, optimal material selection is attained.

Electrical Conductors. High conductivity is a primary requisite for many conductors, but not all conductors require

high conductivity. Strength and resistance to creep or softening can be traded for some loss of conductivity. Thermal

conductivity goes hand in hand with electrical conductivity and is usually required in all conductors operating in the high-

amperage range.

Terminals and connectors are used in both electronic and electrical applications. Electrical circuits require greater

current-carrying ability than electronic circuits, for which low-to-moderate conductivity will often suffice. Good

formability at the required strength level, well-defined load deflection characteristics, and resistance to stress relaxation

are usually required for both electrical and electronic connectors. Corrosion and stress-corrosion resistance and

solderability are also often demanded.

Other Electronic Applications. The primary electronic application for copper and copper alloys, other than

connectors and printed circuits, is as leadframes for semiconductor devices. Transistors, diodes, and integrated circuits

can be fabricated on a semiconductor chip often less than 5 mm (0.2 in.) square. The chip is bonded to a substrate or

leadframe, which serves both structurally and electrically to connect it to the outside world. Where copper alloys can be

used, the materials requirements are expressed in terms of cost, conductivity, strength, softening resistance, formability,

low-cycle fatigue resistance, and surface characteristics (such as plating, wire bonding, and plastic molding compound

adherence).

Hollow Ware, Flatware, and Decorative Applications. The production of hollow ware products requires

materials with good drawability. In addition, the material must have good solderability, corrosion resistance, sufficient

strength to resist denting during manufacture or use, and good buffing and plating characteristics; most hollow ware

products are silver or gold plated. The least expensive alloys that meet these criteria are the 10 to 30% Zn brasses. The

lower zinc levels are used for multiple-redrawing applications, and the higher zinc levels are for parts demanding higher

strength and/or deep-drawing capability. Copper and phosphorus-deoxidized copper are significantly softer and are

primarily used for decorative applications without plating, where the red color of the metal is considered appealing. They

have adequate drawability for parts that do not require sidewall ironing and a high work-hardening rate. Phosphorus-

deoxidized copper is required where brazing is needed.

Flatware items are generally produced by roll forming. In addition to good formability, flatwork alloys must have good

solderability, corrosion resistance, good buffing and plating characteristics, and low cost. Embossed items use annealed

tempers of materials with sufficiently low work-hardening rates to give faithful reproduction of detailed patterns. Copper-

zinc and copper-nickel-zinc alloys offer the required combination of properties; the zinc content is varied to suit the work

hardening needed. A copper-nickel-zinc alloy that has a silvery color is generally used for silver-plated flatware, and Cu-

30Zn is used for gold-plated flatware to minimize the color contrast between base metal and plate if damage to the plate

occurs.

Heat exchangers require good thermal conductivity, corrosion and stress-corrosion resistance, joinability, and strength

at modest cost. These requirements vary in importance for each application. Copper and copper alloys offer good

combinations of these properties. The two major heat exchanger applications are steam condenser tubing and automotive

radiators.

Condenser tubing must withstand potentially corrosive cooling water as well as the volatile components carried by the

steam, which condense on the tubing. Corrosion requirements are paramount, but strength at elevated temperature and

thermal conductivity are also required.

The copper alloys commonly used in power utility condensers include arsenic-, antimony-, or phosphorus-inhibited

brasses; aluminum bronzes; or copper-nickels, depending on corrosion and stress-corrosion requirements. For automotive

radiators, corrosion resistance, thermal conductivity, and fabricability are the primary requisites. Certain applications

require strength at elevated temperature. Fabricability demands the ability to solder and to braze. Resistance to both

atmospheric corrosion and corrosion by heat-transfer media and their decomposition products is required. Cooling fins are

made of pure copper or high-copper copper alloys.

Coinage. General requirements include low cost, attractive appearance and high density for high denominations (to give

the impression of intrinsic value), tarnish and corrosion resistance, modest strength, and ability to be coined easily.

Specific additional requirements for vending machine use include control of conductivity, density, magnetic permeability,

and eddy-current response. Apart from those few coins that are fabricated from stainless steel, most non-precious metal

coins are made of copper alloys, which can meet these requirements.

Ammunition. The cartridge case that contains the explosive powder and primer for ammunition is made by a cup and

draw process; a blanked disk is cupped, drawn to extend the sidewall, and redrawn for the same purpose. The drawing

process is repeated until the wall is sufficiently thinned and extended. The case can be annealed between a series of draws

to permit sufficient extension without cracking. The primary materials requirements are related to fabricability, but stress-

corrosion resistance and strength are also needed.

Cartridge brass (Cu-30Zn) is the most widely used copper alloy for shells and other ammunition; hence its name.

Cartridge brass offers the best materials compromise for such applications, having excellent deep drawability, moderate

redrawability, and sufficient strength. In press operations, its low coefficient of friction and absence of refractory oxides

contribute toward low press forces, low tool maintenance, and good surfaces on the parts being formed. It is also low in

cost. Formed cartridge cases must be stress-relief annealed to minimize the possibility of stress-corrosion cracking.

Reference cited in this section

M.B. Bever, Ed., Copper: Selection of Wrought Alloys, in

Encyclopedia of Materials Science and

Engineering, Vol 2, Pergamon Press and The MIT Press, 1986, p 866

Forming of Copper and Copper Alloys

Frank Mandigo and Jack Crane, Olin Corporation

References

1. Sheet Metal Industries--Yearbook, Fuel and Metallurgical Journals Ltd., 1972/1973

2. T.E. Bersett, Back to Basics: Properties of Copper Alloy Strip for Contacts and Terminals, in

Proceedings

of the Fourteenth Annual Connector Symposium, Electronic Connector Study Group, 1981

3. J.H. Mendenhall, Ed., Understanding Copper Alloys, Olin Corporation, 1977

"Advanced Sheet Metal Forming Course," Metals Engineering Institute Home Study and Extension Course,

American Society for Metals, 1979

5. Forming Limits Set for Copper Metals, Am. Mach., April 1983, p 99

"Forming Limit Analysis for Enhanced Fabrication," Report 310A, International Copper Research

Association, Dec 1981

7. M.B. Bever, Ed., Copper: Selection of Wrought Alloys, in Encyclopedia of

Materials Science and

Engineering, Vol 2, Pergamon Press and The MIT Press, 1986, p 866

Forming of Magnesium Alloys

Introduction

THE PRINCIPAL DIFFERENCE between forming magnesium alloys and forming steel, aluminum, or copper is

temperature. Although some forming of magnesium alloys can be done at room temperature, elevated temperatures are

used in most applications.

Cold Forming

Cold forming of magnesium alloys is restricted to mild deformation with a generous bend radius.

Bend Radii. Cylinders and cones can be formed from magnesium alloys at room temperature by using standard power

rolls. Simple flanges can be press formed at room temperature. Table 1 gives minimum radii for fast bending at room

temperature, as in a press brake. Slightly smaller bend radii than those given in Table 1 may be used when forming speeds

are slower, as in a hydraulic press or when proved by trial and error.

Table 1 Recommended minimum bend radii for the fast forming of magnesium alloys at room temperature

Alloy and temper

Minimum bend radius in terms of

workpiece thickness, t

Sheet 0.51-6.3 mm (0.020-0.249 in.) thick

(a)

AZ31B-O

5.5

AZ31B-H24

HK31A-O

HK31A-H24

HM21A-T8

HM21A-T81

LA141A-O

ZE10A-O

5.5

ZE10A-H24

Extruded flat strip 22.2 × 2.3 mm (0.875 × 0.090 in.) thick

AZ31C-F

2.4

AZ31B-F

2.4

AZ61A-F

1.9

AZ80A-F

2.4

AZ80A-T5

8.3

HM31A-T5

ZK21A-F

ZK60A-F

ZK60A-T5 12

(a)

Minimum bend radii are based on bending a 152 mm (6 in.) wide specimen through 90°

Surface Protection. In low-production cold forming, a common method of preventing surface damage is to apply tape

to critical areas of the work metal and, if feasible, to tool surfaces. It is especially important to keep the die clean and free

from particles of foreign metal that can become embedded in the surface of the workpiece and impair its corrosion

resistance.

Hard rubber inserts in dies are sometimes helpful in preventing damage to the work metal and in forming large radii.

However, this practice is not recommended for high production, because the inserts wear rapidly and cause nonuniform

workpieces.

Reworking of bends by straightening and rebending the same portion should not be done in cold forming, because of

the possibility of failure.

Springback can be as much as 30° for a 90° bend in cold forming of magnesium alloys.

Effect of Bending on Length. Unlike aluminum alloys and steel, which lengthen in bending, magnesium alloys

shorten, because the neutral axis moves slightly toward the tension side of the bend. For thin sheet, the extent of this

shortening is small, because the axis shifts only 5 to 10%. However, in thicker sheet when several bends are made, the

amount of shortening can be significant and must be allowed for in the development of the blank.

Stress Relieving. Magnesium-aluminum-zinc (AZ) alloys should be stress relieved after cold forming to prevent stress

corrosion. It may be desirable to stress relieve workpieces formed from the magnesium-thorium (HM, HK) alloys,

particularly if they require straightening in fixtures. Recommended temperatures and times for stress relieving the

magnesium alloys that are most commonly cold formed are given in Table 2.

Table 2 Stress relief treatments for the magnesium alloys most commonly cold formed

Temperature

Alloy and temper

°C °F

Time at

temperature, min

Sheet

AZ31B-O 260 500

AZ31B-H24 150 300

HK31A-H24 290 550

HM21A-T8 370 700

HM21A-T81

(a)

370 700

Extruded flat strip

AZ31B-F 260 500

AZ61A-F, AZ80A-F

260 500

AZ80A-T5 205 400

HM31A-T5 425 800 60

(a)

80 to 90% stress relief may be accomplished with a 30 min exposure at 400 °C (750 °F), but mechanical properties will be

reduced.

Forming of Magnesium Alloys

Hot Forming

Magnesium alloys, like other alloys with hexagonal crystal structures, are much more workable at elevated temperatures

than at room temperature. Consequently, they are usually formed at elevated temperatures. The methods and equipment

used in forming magnesium alloys are the same as those commonly employed in forming other metals, except for

differences in tooling and technique that are required when forming is done at elevated temperatures.

Working of metals at elevated temperatures has several advantages over cold working. Magnesium alloy parts usually are

drawn at elevated temperature in one operation without repeated annealing and redrawing, thus reducing the time

involved for making the part and also eliminating the necessity for additional die equipment for extra stages. Hardened

dies are unnecessary for most types of forming. Hot-formed parts can be made to closer dimensional tolerances than can

cold-formed parts because of less springback. Table 3 lists suggested maximum forming temperatures and times for

various wrought magnesium alloys. The times given indicate the maximum time the alloy can be held at temperature

without adversely affecting mechanical properties.

Table 3 Maximum forming temperatures and times for wrought magnesium alloys

Temperature

Alloy

°C °F

Time

Sheet

AZ31B-O 290 550

1 h

AZ31B-H24

165 325

1 h

345 650

15 min

370 700

5 min

HK31A-H24

400 750

3 min

Extrusions

AZ61A-F 290 550

1 h

AZ31B-F 290 550

1 h

M1A-F 370 700

1 h

AZ80A-F 290 550

30 min

AZ80A-T5 195 380

1 h

ZK60A-F 290 550

30 min

ZK60A-T5 205 400 30 min

Sheet and Plate. Rolled magnesium alloy products include flat sheet and plate, coiled sheet, circles, tooling plate, and

tread plate. These products are supplied in a variety of sizes.

The ability to use increased section thickness without weight penalty is of particular importance in designs that employ

magnesium sheet. Thick-sheet construction provides the rigidity necessary in a structure, without the need for costly

assembly of ribs and similar reinforcing members.

Rolled magnesium-alloy products can be worked by most conventional methods. For severe forming, sheet in the

annealed (O temper) condition is preferred. However, sheet in the partially annealed (H24 temper) condition can be

formed to a considerable extent. Because heat has significant effects on properties of hard-rolled magnesium, properties

of the metal after exposure to elevated temperature must be considered in forming. The design curves shown in Fig. 1

give minimum values suitable for design use. Although the curves are based primarily on tests of sheet 1.63 mm (0.064

in.) thick or less, check tests indicate reasonable applicability for thicknesses up to 6.35 mm (0.250 in.).

Fig. 1 Effect of exposure time at elevated temperature on the mechanical properties of AZ31B-

H24 sheet at

room temperature. (a) Tensile strength. (b) Tensile yield strength. Data are based on

tests of sheet 1.63 mm

(0.064 in.) thick, and have reasonable applicability for thicknesses up to 6.35 mm (0.250 in.).

Figure 1 shows how the properties of AZ31B-H24 change with exposure time at various temperatures. The curves have

been extrapolated above the typical property levels of AZ31B-H24 sheet. Thus, if the value selected from a curve exceeds

the actual property level of the material before exposure, the actual figure must be used. Also, it should be kept in mind

that the effects of multiple exposures at elevated temperature are cumulative.

AZ31B-H24 sheet is commonly hot formed at temperatures below 160 °C (325 °F) to avoid annealing it to room-

temperature property levels lower than the specified minimums. Annealing is a function of both time of exposure and

temperature; thus, temperatures higher than 160 °C can be tolerated if exposure is carefully controlled.

Thermal Expansion. Magnesium alloys have a very high rate of thermal expansion. At 260 °C (500 °F), for example,

the thermal expansion of magnesium is more than twice that of steel. Therefore, when parts made of magnesium alloys

are hot formed in tool steel or cast iron dies, the difference in the thermal expansion of the tool material and the work

metal must be considered.

Figure 2 shows the relation between the size of a magnesium part and the size of a steel die at 20 °C (70 °F) and at 205 °C

(400 °F). The dimensional factor need not be applied when the dies are made of zinc or aluminum alloys, because the

coefficients of expansion of these alloys are similar to those of magnesium alloys.

Fig. 2 Dimensional relations of a magnesium alloy workpie

ce and a steel die at room temperature and at a

forming temperature of 205 °C (400 °F). Room-

temperature dimensions of the steel die are determined by

multiplying the design dimensions of the magnesium part by 1.00270. Dimensions given in inches.

Precautions. Magnesium alloy stock to be hot formed must be clean. Protective coatings, oil, dirt, moisture, or other

foreign matter must be removed.

Dies, punches, and form blocks should be clean and free of scratches; tooling should be cleaned with solvent. Rust,

scratches, and minor imperfections can be removed by light polishing with fine-grit abrasive cloth. However, polishing

must not alter the dimensions of the tool.

In forming, the possibility of fire due to ignition of magnesium is remote. However, an ample supply of suitable fire-

extinguishing material, such as dry sand or commercially available powder, must be kept in the work area.

Forming of Magnesium Alloys

Heating of Workpieces

In most hot-forming methods, both the tools and the magnesium alloy work metal must be heated. Heating equipment

includes ovens, platen heaters, ring burners, electric heating elements, heat transfer liquids, induction heaters, and lamps

and other types of infrared heating. For small lots, tools and work may be heated by hand torches. Small dies that can be

handled rapidly can be heated in ovens adjacent to the forming equipment.

Heating Methods. Electric heating elements often are used for heating dies and other forming tools. Electric-resistance

heating can be used for heating some dies. Low-voltage high-amperage current is passed into the die through conducting

grips or clamps.