30079_34.pdf

CHAPTER 34

METAL FORMING, SHAPING,

AND CASTING

Magd E. Zohdi

Dennis B. Webster

Industrial Engineering Department

Louisiana State University

Baton Rouge, Louisiana

William E. Biles

Industrial Engineering Department

University of Louisville

Louisville, Kentucky

34.1 INTRODUCTION

11 01

34.4.3 Permanent-Mold Casting 1 123

34.4.4 Plaster-Mold Casting 11 25

34.4.5 Investment Casting 11 25

34.2 HOT- WORKING PROCESSES 1102

34.2.1 Classification of

Hot- Working Processes 1103

34.2.2 Rolling 11 03

34.2.3 Forging 11 05

34.2.4 Extrusion 11 07

34.2.5 Drawing 11 07

34.2.6 Spinning 11 10

34.2.7 Pipe Welding 11 11

34.2.8 Piercing

34.5 PLASTIC-MOLDING

PROCESSES 11 26

34.5.1 Injection Molding 11 26

34.5.2 Coinjection Molding 11 26

34.5.3 Rotomolding 11 26

34.5.4 Expandable-Bead Molding 1126

34.5.5 Extruding 11 26

34.5.6 Blow Molding 11 26

34.5.7 Thermoforming 11 27

34.5.8 Reinforced-Plastic

Molding 1127

34.5.9 Forged-Plastic Parts 11 27

11 11

34.3 COLD- WORKING PROCESSES 1112

34.3.1 Classification of

Cold- Working Operations 1112

34.3.2 Squeezing Processes 11 13

34.3.3 Bending

11 14

34.6 POWDER METALLURGY 11 27

34.6. 1 Properties of P/M

Products

34.3.4 Shearing

11 16

34.3.5 Drawing

11 18

1127

34.4 METAL CASTING AND

MOLDING PROCESSES 11 20

34.4.1 Sand Casting

34.7 SURFACE TREATMENT 11 28

34.7.1 Cleaning 11 28

34.7.2 Coatings 11 30

34.7.3 Chemical Conversions 1 132

11 20

34.4.2 Centrifugal Casting

1121

34.1 INTRODUCTION

Metal-forming processes use a remarkable property of metals—their ability to flow plastically in the

solid state without concurrent deterioration of properties. Moreover, by simply moving the metal to

the desired shape, there is little or no waste. Figure 34.1 shows some of the metal-forming processes.

Metal-forming processes are classified into two categories: hot-working processes and cold-working

processes.

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Rolling

Stretching

Forging

Straight bending

Extruding

Coining

Deep drawing

Spinning

Wire & tube

drawing

Piercing

and blanking

Fig. 34.1 Metal-forming processes.

34.2 HOT-WORKING PROCESSES

Hot working is defined as the plastic deformation of metals above their recrystallization temperature.

Here it is important to note that the crystallization temperature varies greatly with different materials.

Lead and tin are hot worked at room temperature, while steels require temperatures of 2000°F

(1100°C). Hot working does not necessarily imply high absolute temperatures.

Hot working can produce the following improvements:

1. Production of randomly oriented, spherical-shaped grain structure, which results in a net

increase not only in the strength but also in ductility and toughness.

2. The reorientation of inclusions or impurity material in metal. The impurity material often

distorts and flows along with the metal.

This material, however, does not recrystallize with the base metal and often produces a fiber structure.

Such a structure clearly has directional properties, being stronger in one direction than in another.

Moreover, an impurity originally oriented so as to aid crack movement through the metal is often

reoriented into a "crack-arrestor" configuration perpendicular to crack propagation.

34.2.1 Classification of Hot-Working Processes

The most obvious reason for the popularity of hot working is that it provides an attractive means of

forming a desired shape. Some of the hot-working processes that are of major importance in modern

manufacturing are

1. Rolling

2. Forging

3. Extrusion and upsetting

4. Drawing

5. Spinning

6. Pipe welding

7. Piercing

34.2.2 Rolling

Hot rolling (Fig. 34.2) consists of passing heated metal between two rolls that revolve in opposite

directions, the space between the rolls being somewhat less than the thickness of the entering metal.

Many finished parts, such as hot-rolled structural shapes, are completed entirely by hot rolling. More

often, however, hot-rolled products, such as sheets, plates, bars, and strips, serve as input material

for other processes, such as cold forming or machining.

In hot rolling, as in all hot working, it is very important that the metal be heated uniformly

throughout to the proper temperature, a procedure known as soaking. If the temperature is not uni-

form, the subsequent deformation will also be nonuniform, the hotter exterior flowing in preference

to the cooler and, therefore, stronger, interior. Cracking, tearing, and associated problems may result.

Fig. 34.2 Hot rolling.

Isothermal Rolling

The ordinary rolling of some high-strength metals, such as titanium and stainless steels, particularly

in thicknesses below about 0.150 in. (3.8 mm), is difficult because the heat in the sheet is transferred

rapidly to the cold and much more massive rolls. This has been overcome by isothermal rolling.

Localized heating is accomplished in the area of deformation by the passage of a large electrical

current between the rolls, through the sheet. Reductions up to 90% per roll have been achieved. The

process usually is restricted to widths below 2 in. (50 mm).

The rolling strip contact length is given by

L ~ VR(ho - h)

where R = roll radius

h0 = original strip thickness

h = reduced thickness

The roll-force F is calculated by

F = LwFavg

(34.1)

where w = width

Favg = average true stress

Figure 34.3 gives the true stress for different material at the true stress e. The true stress e is given

Fig. 34.3 True stress-true strain curves.

•"(*)

power/ro11=IS kw

(34-2)

where F = newtons

L = meters

N = rev per min

power -|=gg hp

(34.3)

where F = Ib

L = ft

34.2.3 Forging

Forging is the plastic working of metal by means of localized compressive forces exerted by manual

or power hammers, presses, or special forging machines.

Various types of forging have been developed to provide great flexibility, making it economically

possible to forge a single piece or to mass produce thousands of identical parts. The metal may be

1. Drawn out, increasing its length and decreasing its cross section

2. Upset, increasing the cross section and decreasing the length, or

3. Squeezed in closed impression dies to produce multidirectional flow

The state of stress in the work is primarily uniaxial or multiaxial compression.

The common forging processes are

1. Open-die hammer

2. Impression-die drop forging

3. Press forging

4. Upset forging

5. Roll forging

6. Swaging

Open-Die Hammer Forging

Open-die forging, (Fig. 34.4) does not confine the flow of metal, the hammer and anvil often being

completely flat. The desired shape is obtained by manipulating the workpiece between blows. Spe-

cially shaped tools or a slightly shaped die between the workpiece and the hammer or anvil are used

to aid in shaping sections (round, concave, or convex), making holes, or performing cutoff operations.

The force F required for an open-die forging operation on a solid cylindrical piece can be cal-

culated by

Fig. 34.4 Open-die hammer forging.

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