Bulk-Deformation Processes

TABLE 6.1 General characteristics of bulk deformation processes.

PROCESS GENERAL CHARACTERISTICS Forging Production of discrete parts with a set of dies; some finishing operations usually

necessary; similar parts can be made by casting and powder-metallurgy techniques; usually performed at elevated temperatures; dies and equipment costs are high; moderate to high labor costs; moderate to high operator skill.

Rolling Flat Production of flat plate, sheet, and foil at high speeds, and with good surface finish,

especially in cold rolling; requires very high capital investment; low to moderate labor cost.

Shape Production of various structural shapes, such as I-beams and rails, at high speeds; includes thread and ring rolling; requires shaped rolls and expensive equipment; low to moderate labor cost; moderate operator skill.

Extrusion Production of long lengths of solid or hollow products with constant cross-sections, usually performed at elevated temperatures; product is then cut to desired lengths; can be competitive with roll forming; cold extrusion has similarities to forging and is used to make discrete products; moderate to high die and equipment cost; low to moderate labor cost; low to moderate operator skill.

Drawing Swaging

Production of long rod, wire, and tubing, with round or various cross-sections; smaller cross-sections than extrusions; good surface finish; low to moderate die, equipment and labor costs; low to moderate operator skill. Radial forging of discrete or long parts with various internal and external shapes; generally carried out at room temperature; low to moderate operator skill.

Impression-Die Forging

FIGURE 6.14 Schematic illustration of stages in impression-die forging. Note the formation of flash, or excess material that is subsequently trimmed off.

Flat-And-Shape-Rolling Processes

FIGURE 6.29 Schematic outline of various flat-and-shape-rolling processes. Source: American Iron and Steel Institute.

Classification

Name Characters Cost Skill

Forging Production of discrete parts with dies High High skill

Rolling (Flat) Flat plate, sheet, foil in long length High Equipment cost Low skill

Rolling (shape) Various structural shapes, I-beam Expensive Equipment Moderate

ExtrusionLong length of solid or hollow products with constant cross-section.

Moderate to high die and equipment cost Moderate

Drawing Production of long rod and wire Moderate cost Low skill

Swaging

Radial forging of discrete or long parts with various internal and external shapes; generally carried out at room temperature;

Moderate cost

low to moderate operator skill.

Bulk-Deformation Processes

Grain Flow Lines

FIGURE 6.2 Grain flow lines in upsetting a solid steel cylinder at elevated temperatures. Note the highly inhomogenous deformation and barreling. The different shape of the bottom, section of the specimen (as compared with the top) results from the hot specimen resting on the lower, cool die before deformation proceeded. The bottom surface was chilled; thus it exhibits greater strength and hence deforms less than the top surface. Source: J. A. Schey et al., IIT Research Institute.

Finite Element Simulation

FIGURE 6.11 Plastic deformation in forging as predicted by the finite-element method of analysis. Source: Courtesy of Scientific Forming, Inc.

Plastic Deformation in Plane StrainFIGURE 6.13 Examples of plastic deformation processes in plane strain, showing the h/L ratio. (a) Indenting with flat dies. This operation is similar to cogging, shown in Fig. 6.19. (b) Drawing or extrusion of strip with a wedge-shaped die, described in Sections 6.4 and 6.5. (c) Ironing; see also Fig. 7.54. (d) Rolling, described in Section 6.3. As shown in Fig. 6.12, the larger the h/L ratio, the higher the die pressure becomes. In actual processing, however, the smaller this ratio, the greater is the effect of friction at the die-workpiece interfaces. The reason is that contact area, and hence friction, increases with a decreasing h/L ratio.

Impression-Die Forging

FIGURE 6.14 Schematic illustration of stages in impression-die forging. Note the formation of flash, or excess material that is subsequently trimmed off.

AnalysisSimple shapes, without flashSimple shapes, with flashComplex shapes, with flash

3-55-88-12

F = (Kp)(Yf)(A)

TABLE 6.2 Range of Kp values in Eq. (6.21) for impression-die forging.

Load-Stroke Curve in Closed-Die Forging

FIGURE 6.15 Typical load-stroke curve for closed-die forging. Note the sharp increase in load after the flash begins to form. In hot-forging operations, the flash requires high levels of stress, because it is thin-that is, it has a small h-and cooler than the bulk of the forging. Source: After T. Altan.

Heading

FIGURE 6.17 Forging heads on fasteners such as bolts and rivets. These processes are called heading.

Piercing Operations

FIGURE 6.18 Examples of piercing operations.

Cogging Operation

FIGURE 6.19 Schematic illustration of a cogging operation on a rectangular bar. With simple tools, the thickness and cross-section of a bar can be reduced by multiple cogging operations. Note the barreling after cogging. Blacksmiths use a similar procedure to reduce the thickness of parts in small increments by heating the workpiece and hammering it numerous times.

Roll Forging Operation

FIGURE 6.20 Schematic illustration of a roll forging (cross-rolling) operation. Tapered leaf springs and knives can be made by this process with specially designed rolls. Source: After J. Holub.

Manufacture of Spherical BlanksFIGURE 6.21 Production of steel balls for bearings by the skew-rolling process. Balls for bearings can also be made by the forging process shown in Fig. 6.22.

FIGURE 6.22 Production of steel balls by upsetting of a cylindrical blank. Note the formation of flash. The balls are subsequently ground and polished for use as ball bearings and in other mechanical components.

Internal Defects In Forging

FIGURE 6.24 Internal defects produced in a forging because of an oversized billet. The die cavities are filled prematurely, and the material at the center of the part flows past the filled regions as deformation continues.

FIGURE 6.23 Laps formed by buckling of the web during forging.

Defect Formation In Forging

FIGURE 6.25 Effect of fillet radius on defect formation in forging. Small fillets (right side of drawings) cause the defects. Source: Aluminum Company of America.

Forging A Connecting Rod

FIGURE 6.26 Stages in forging a connecting rod for an internal combustion engine. Note the amount of flash that is necessary to fill the die cavities properly.

Features Of A Forging Die

FIGURE 6.27 Standard terminology for various features of a typical forging die.

Hot-Forging Temperature RangesMetal C F Metal C FAluminum alloysCopper alloysNickel alloys

400-450625-950870-1230

750-8501150-17501600-2250

Alloy steelsTitanium alloysRefractory alloys

925-1260750-795975-1650

1700-23001400-18001800-3000

TABLE 6.3 Hot-forging temperature ranges for various metals.

Presses Used In Metalworking

FIGURE 6.28 Schematic illustration of various types of presses used in metalworking. The choice of the press is an important factor in the overall operation.

Stages in Shape Rolling of an H-section part

Shape Rolling- Angles

Special Rolling

Special Products (Shapes)

Special Methods

Non-Uniform Rolling (Deformation)

Thread Rolling

Bulk deformation process used to form threads on cylindricalparts by rolling them between two diesMost important commercial process for mass producing boltsand screws|Performed by cold working in thread rolling machines|Advantages over thread cutting (machining):

Higher production ratesBetter material utilizationStronger threads due to work hardeningBetter fatigue resistance due to compressive stressesintroduced by rolling

Thread rolling with flat dies:(1) start of cycle, and (2) end of cycle

•The thread is formed by the axial flow of material in the work piece. • The grain structure of the material is not cut, but is distorted to follow the thread form.• Rolled threads are produced in a single pass at speeds far in excess of those used to cut threads. • The resultant thread is very much stronger than a cut thread. It has a greater resistance to mechanical stress and an increase in fatigue strength. Also the surface is burnished and work hardened.

Ring Rolling

Ring rolling used to reduce the wallthickness and increase the diameter of a ring:(1) start, and (2) completion of process

(a) Schematic illustration of Ring-rolling operation. Thickness reduction results in an increase in the part diameter.

(b) Examples of cross-sections that can be formed by ring-rolling

Production of Seamless Pipe & Tubing

Assel Method

Transverse Rolling

•Using circular wedge rolls.• Heated bar is cropped to length and fed in transversely between rolls.• Rolls are revolved in one direction.

Powder Rolling

Metal powder is introduced between the rolls and compacted into a ‘green strip’, which is subsequently sintered and subjected to further hot- working and/ or cold working and annealing cycles.

Advantage :- Cut down the initial hot- ingot breakdown step (reduced capital investment).- Economical - metal powder is cheaply produced during the extraction process.- Minimise contamination in hot- rolling.- Provide fine grain size with a minimum of preferred orientation.

Powder Rolling

Continuous casting and hot rolling

Continuous rolling

Example for hot strip mill process

Hot and Cold Flat Rolling

Changes in grain structure during hot-rolling

Other CharacteristicsOther Characteristics

Residual stresses – produces: Compressive residual stresses on the surfacesTensile stresses in the middle

TolerancesCold-rolled sheets: (+/- ) 0.1mm – 0.35mmTolerances much greater for hot-rolled plates

Surface roughness Cold rolling can produce a very fine finishHot rolling & sand have the same range of surface finish

Gauge numbers – the thickness of a sheet is identified by a gauge number

Schematic Illustration of various roll arrangements : (a) two-high; (b) three-high; (c) four-high; (d) cluster mill

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Drawing

FIGURE 6.62 Variables in drawing round rod or wire.

FIGURE 6.63 Variation in strain and flow stress in the deformation zone in drawing. Note that the strain increases rapidly toward the exit. The reason is that when the exit diameter is zero, the true strain reaches infinity. The point Ywire represents the yield stress of the wire.

Tube Drawing

FIGURE 6.67 Various methods of tube drawing.

SwagingFIGURE 6.71 Schematic illustration of the swaging process: (a) side view and (b) front view. (c) Schematic illustration of roller arrangement, curvature on the four radial hammers (that give motion to the dies), and the radial movement of a hammer as it rotates over the rolls.

FIGURE 6.72 Reduction of outer and inner diameters of tubes by swaging. (a) Free sinking without a mandrel. The ends of solid bars and wire are tapered (pointing) by this process in order to feed the material into the conical die. (b) Sinking on a mandrel. Coaxial tubes of different materials can also be swaged in one operation.

Cross-Sections Produced By Swaging

FIGURE 6.73 (a) Typical cross-sections produced by swaging tube blanks with a constant wall thickness on shaped mandrels. Rifling of small gun barrels can also be made by swaging, using a specially shaped mandrel. The formed tube is then removed by slipping it out of the mandrel. (b) These parts can also be made by swaging.