ME 383 Exam 2 - CH 19: Bulk Deformation Process in Metal
MC 19.7 Which of the following operations are closely related to open-die forging (three best answers): (a) cogging, (b) flashless forging, (c) fullering, (d) impression-die forging, (e) Mannesmann process, (f) precision forging, (g) soaking, and (h) upsetting?
(a), (c), and (h).
MC 19.10 Which of the following are alternative names for indirect extrusion (two correct answers): (a) backward extrusion, (b) direct extrusion, (c) forward extrusion, (d) impact extrusion, and (e) reverse extrusion?
(a) and (e).
MC 19.9 Which of the following are classified as forging operations (four correct answers): (a) coining, (b) fullering, (c) impact extrusion, (d) roll piercing, (e) swaging, (f) thread rolling, (g) trimming, and (h) upsetting?
(a), (b), (e), and (h). Trimming, answer (g), although associated with forging, is a cutting operation.
MC 19.15 Which of the following bulk deformation processes are involved in the production of nails for lumber construction (three best answers): (a) bar and wire drawing, (b) extrusion, (c) flashless forging, (d) impression die forging, (e) rolling, and (f) upsetting?
(a), (c), and (d). Bar stock is rolled, and then drawn into wire stock. Upset forged is used to form the nail head.
MC 19.5 Production of pipes and tubes is associated with which of the following bulk deformation processes (three correct answers): (a) extrusion, (b) hobbing, (c) ring rolling, (d) roll forging, (e) roll piercing, (f) tube sinking, and (g) upsetting?
(a), (e), and (f)
MC 19.12 Which of the following stress or strength parameters is used in the computation of the force in an extrusion operation (one best answer): (a) average flow stress, (b) compression strength, (c) final flow stress, (d) tensile strength, or (e) yield strength?
(a).
MC 19.14 Theoretically, the maximum reduction possible in a wire drawing operation, under the assumptions of a perfectly plastic metal, no friction, and no redundant work, is which of the following (one answer): (a) zero, (b) 0.63, (c) 1.0, or (d) 2.72?
(b).
MC 19.8 Flash in impression die forging serves no useful purpose and is undesirable because it must be trimmed from the part after forming: (a) true or (b) false?
(b). Flash causes build-up of pressure inside the die, which forces the work metal to fill the die cavity.
MC 19.11 The production of tubing is possible in indirect extrusion but not in direct extrusion: (a) true or (b) false?
(b). Tube and pipe cross sections can be produced by either direct or indirect extrusion.
Although the workpiece in a wire drawing operation is obviously subjected to tensile stresses, how do compressive stresses also play a role in the process?
Compressive stresses are present in wire drawing because the starting metal is compressed as it is forced through the approach of the die opening.
What is draft in a rolling operation?
Draft is the difference between the starting thickness and the final thickness as the workpiece passes between the two opposing rolls.
What is extrusion?
Extrusion is a compression forming operation in which a workpiece is forced to flow through a die opening, thus taking the cross-sectional shape of the die opening.
What is a two-high rolling mill?
A two-high rolling mill consists of two opposing rolls between which the work is compressed.
Types of Forging Operations
A. Open‐die forging: work is compressed between two flat dies, allowing metal to flow laterally with minimum constraint B. Impression‐die forging: die contains cavity or impression that is imparted to work part ~ Metal flow is constrained so that flash is created C. Flashless forging: work part is completely constrained in die ~ No excess flash is created
2. Other Deformation Processes Related to Rolling
A. Thread rolling B. Ring rolling C. Roll piercing
List the accessory tools that can be used during open die forging according to the video on forging.
Accessory tools that can be used during open-die forging are (1) saddles, (2) blocks, (3) rings, (4) mandrels, and (5) punches.
In a wire drawing operation, why must the drawing stress never exceed the yield strength of the work metal?
Because if the drawing stress exceeded the yield strength, the metal on the exit side of the draw die would stretch rather than force metal to be pulled through the die opening.
Why is flash desirable in impression die forging?
Because its presence constrains the metal in the die to fill the details of the die cavity.
What is forging?
Forging is a deformation process in which the workpiece is compressed between two dies, using impact or gradual pressure to form the part.
Why is friction a factor in determining the ram force in direct extrusion but not a factor in indirect extrusion?
Friction is a factor in direct extrusion because the work billet is squeezed against the walls of the container so that friction resists the movement of the billet toward the die opening. In indirect extrusion, the billet does not move relative to the container walls, and thus there is no friction.
What are forging hammers commonly used for? (type of forging process)
Impression-die forging
Distinguish between direct and indirect extrusion.
In direct extrusion, also known as forward extrusion, a metal billet is loaded into a container, and a ram compresses the material, forcing it to flow through a die opening at the opposite end of the container. In indirect extrusion, also known as backward extrusion, the die is incorporated into the ram, and as the ram compresses into the metal billet, the metal is forced to flow through the die opening in a direction that is opposite (backwards) of the ram motion.
What is isothermal forging?
Isothermal forging is a hot forging operation in which the die surfaces are heated to reduce heat transfer from the work into the tooling.
What else is roll piercing called?
Mannesmann Process
what is the primary factor that makes the mechanical performance of forged parts better than cast parts in many situations?
The mechanical performance of forged or wrought parts is usually better because of the microstructure changes and the directional grain flow imparted during the forging process.
List the preforming operations discussed in the forming video.
The preforming operations discussed in the forming video are (1) edging - to increase the cross section of the work and (2) blocking - to refine the shape. Then, (3) finish forging is used to complete the shape.
One way to classify forging operations is by the degree to which the work is constrained in the die. By this classification, name the three basic types.
The three basic types are (1) open die forging, (2) impression die forging, and (3) flashless forging.
What are the two basic types of forging equipment?
The two types of forging machines are hammers, which impact the workpart, and presses, which apply a gradual pressure to the work.
What does the centerburst defect in extrusion have in common with the roll piercing process?
They are both examples of how compressive stresses applied to the outside surface of a solid cylindrical cross section can create high tensile stresses in the interior of the cylinder.
What is a trimming operation in the context of impression die forging?
Trimming is a shearing operation used to remove the flash on the workpiece after impression die forging.
What are common names for open-die forging?
Upsetting, upset forging
Identify some of the ways in which force in flat rolling can be reduced.
Ways to reduce force in flat rolling include (1) use hot rolling, (2) reduce draft in each pass, and (3) use smaller diameter rolls.
What is wire drawing and bar drawing?
Wire and bar drawing are bulk deformation processes in which the cross section of a wire or bar is reduced by pulling (drawing) it through a die opening.
MC 19.13 In which of the following extrusion operations is friction a factor in determining the extrusion force (one best answer): (a) direct extrusion or (b) indirect extrusion?
a
MC 19.3 Which of the following stress or strength parameters is used in the computation of rolling force (one best answer): (a) average flow stress, (b) compression strength, (c) final flow stress, (d) tensile strength, or (e) yield strength?
a
What is another name for direct extrusion?
forward extrusion
Forging Hammer Disadvantages
impact energy transmitted through anvil into floor of building
Name the four basic bulk deformation processes.
rolling, forging, extruding, wire and bar drawing
Bar Drawing
• Accomplished as a single‐draft operation: the stock is pulled through one die opening • Beginning stock has large diameter and is a straight cylinder • Requires a batch type operation
Swaging/ Radial Forging
• Accomplished by rotating dies that hammer a workpiece radially inward to taper it as the piece is fed into the dies • Used to reduce diameter of tube or solid rod stock • Mandrel sometimes required to control shape and size of internal diameter of tubular parts
Advantages of Impression-Die Forging
• Advantages compared to machining from solid stock: ~ Higher production rates ~ Less waste of metal ~ Greater strength ~ Favorable grain orientation in the metal
B. Indirect Extrusion
• Also called backward extrusion and reverse extrusion • Limitations of indirect extrusion are imposed by: ~ Lower rigidity of hollow ram ~ Difficulty in supporting extruded product as it exits die
A. Direct Extrusion
• Also called forward extrusion • Starting billet cross section is usually round • Final cross-sectional shape of extrudate is determined by die opening shape • As ram approaches die opening, a small portion of billet remains that cannot be forced through the die ~ This portion, called the butt, must be separated from the extrudate by cutting it off just beyond the die exit
Extrusion Ratio
• Also called the reduction ratio, it is defined as: rx = Ao/ Af ~ where rx = extrusion ratio; Ao = cross-sectional area of the starting billet; and Af = final cross-sectional area of the extruded section • Applies to both direct and indirect extrusion
Preparation of Work for Drawing
• Annealing: to increase ductility of stock • Cleaning: to prevent damage to work surface and draw die • Pointing: to reduce diameter of starting end to allow insertion through draw die
Forging Presses
• Apply gradual pressure to accomplish compression operation • Types: 1) Mechanical press: converts rotation of drive motor into linear motion of ram 2) Hydraulic press: hydraulic piston actuates ram 3) Screw press: screw mechanism drives ram
Forging Hammers
• Apply impact load against work part • Two Types: 1) Gravity drop hammers: impact energy from falling weight of a heavy ram 2) Power drop hammers: accelerate the ram by pressurized air or steam • Commonly used for impression-die forging
Types of Rolling
• Based on workpiece geometry: A. Flat rolling - used to reduce thickness of a rectangular cross section B. Shape rolling - square cross section is formed into a shape, such as an I-beam • Based on work temperature: ~ Hot Rolling- can achieve significant deformation ~ Cold rolling- produces sheet and plate stock
A. Thread Rolling
• Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies ~ Important for mass producing bolts and screws ~ Performed as cold working in thread rolling machines ~ Advantages over thread cutting (machining)
Area Reduction in Drawing
• Change in size of work is usually given by area reduction: r=(Ao-Af)/Ao ~ where r = area reduction in drawing; Ao = original area of work; and Ar = final work
Classification of Forging Operations
• Cold vs. hot forging: ~ Hot or warm forging - advantage: reduction in strength and increase in ductility of work metal ~ Cold forging-advantage: increased strength due to strain hardening • Impact vs. press forging: ~ Forge hammer- applies an impact force ~ Forge press- applies gradual force
What is a reversing mill in rolling?
A reversing mill is a two-high rolling mill in which the direction of rotation of the rolls can be reversed to allow the work to pass through from either side.
Hollow and Semi-Hollow Shapes
(a) Direct extrusion to produce hollow or semi‐hollow cross sections; (b) hollow and (c) semi‐hollow cross sections
C. Roll Piercing (Mannesmann Process)
(a) Formation of internal stresses and cavity by compression of cylindrical part (b) setup for producing seamless tubing
MC 19.2 The maximum possible draft in a rolling operation depends on which of the following parameters (two correct answers): (a) coefficient of friction between roll and work, (b) roll diameter, (c) roll velocity, (d) stock thickness, (e) strain, and (f) strength coefficient of the work metal?
(a) and (b)
MC 19.4 Which of the following rolling mill types are associated with relatively small diameter rolls in contact with the work (two correct answers): (a) cluster mill, (b) continuous rolling mill, (c) four-high mill, (d) reversing mill, and (e) three-high configuration?
(a) and (c)
Bulk Deformation Processes in Metalworking
1. Rolling 2. Other Deformation Processes Related to Rolling 3. Forging 4. Other Deformation Processes Related to Forging 5. Extrusion 6. Wire and Bar Drawing
Four Basic Bulk Deformation Processes
1. Rolling - slab or plate is squeezed between opposing rolls 2. Forging - work is squeezed and shaped between opposing dies 3. Extrusion - work is squeezed through a die opening, thereby taking the shape of the opening 4. Wire and bar drawing - diameter of wire or bar is reduced by pulling it through a die opening
In rolling of steel, what are the differences between a bloom, a slab, and a billet?
A bloom is a rolled steel workpiece with a square cross section of about 150 mm by 150 mm. The starting work unit for a bloom is an ingot heated in a soaking pit. A slab is rolled from an ingot or a bloom and has a rectangular cross section of about 250 mm by 40 mm. A billet is rolled from a bloom and has a square cross section of about 40 mm by 40 mm.
What is another name for flashless forging?
Precision forging process
Name some products that are produced by extrusion.
Products produced by continuous extrusion include structural shapes (window frames, shower stalls, channels), tubes and pipes, and rods of various cross sections. Products made by discrete extrusion include toothpaste tubes, aluminum beverage cans, and battery cases.
What are the reasons why the bulk deformation processes are important commercially and technologically?
Reasons why the bulk deformation processes are important include the following: (1) they are capable of significant shape change when hot working is used, (2) they have a positive effect on part strength when cold working is used, and (3) most of the processes produce little material waste; some are net shape processes.
List some of the products produced on a rolling mill.
Rolled products include flat sheet and plate stock, round bar and rod stock, rails, structural shapes such as I-beams and channels.
What is rolling in the context of the bulk deformation processes?
Rolling is a deformation process in which the thickness of the workpiece is reduced by compressive forces exerted by two opposing rolls. The rolls rotate, thus pulling and simultaneously squeezing the workpiece between them.
Besides flat rolling and shape rolling, identify some additional bulk forming processes that use rolls to effect the deformation.
Some other processes that use rolls are ring rolling, thread rolling, gear rolling, roll piercing, and roll forging.
What is sticking in a hot rolling operation?
Sticking is a condition in hot rolling in which the surface of the workpiece adheres to the rolls as the piece passes between the rolls, causing severe deformation of the metal below the surface in order to allow passage through the roll gap.
MC 19.16 Johnson's formula is associated with which one of the four bulk deformation processes: (a) bar and wire drawing, (b) extrusion, (c) forging, and (d) rolling?
b
MC 19.6 Which of the following stress or strength parameters is used in the computation of the maximum force in a forging operation (one best answer): (a) average flow stress, (b) compression strength, (c) final flow stress, (d) tensile strength, or (e) yield strength?
c
MC 19.1 The starting workpiece in steel hot rolling of plate and sheet stock is which of the following (one best answer): (a) bar stock, (b) billet, (c) bloom, (d) slab, or (e) wire stock?
d
5. Extrusion
• Compression forming process in which work metal is forced to flow through a die opening to produce a desired cross‐sectional shape • Process is similar to squeezing toothpaste out of a toothpaste tube • In general, extrusion is used to produce long parts of uniform cross sections • Two basic types: A. Direct extrusion B. Indirect extrusion
C. Flashless Forging
• Compression of work in punch and die tooling whose cavity does not allow for flash • Starting work volume must equal die cavity volume within very close tolerance • Process control more demanding than impression-die forging • Best suited to part geometries that are simple and symmetrical • Often classified as a precision forging process
A. Open‐Die Forging
• Compression of workpart between two flat dies ~ Similar to compression test when work part has cylindrical cross section and is compressed along its axis • Deformation operation reduces height and increases diameter of work • Common names include upsetting or upset forging
B. Impression‐Die Forging
• Compression of workpart by dies with inverse of desired part shape • Flash is formed by metal that flows beyond die cavity into small gap between die plates • Flash must be later trimmed, but it serves an important function during compression: ~ As flash forms, friction resists continued metal flow into gap, constraining metal to fill die cavity ~ In hot forging, metal flow is further restricted by cooling against die plates
Wire Drawing
• Continuous drawing machines consisting of multiple draw dies (typically 4 to 12) separated by accumulating drums • Each drum (capstan) provides proper force to draw wire stock through upstream die • Each die provides a small reduction, so desired total reduction is achieved by the series of dies • Annealing sometimes required between dies to relieve work hardening
5. Wire and Bar Drawing
• Cross section of a bar, rod, or wire is reduced by pulling it through a die opening ~ Similar to extrusion except work is pulled through die in drawing ~ (It is pushed through in extrusion!) • Although drawing applies tensile stress, compression also plays a significant role since metal is squeezed as it passes through die opening
Trimming
• Cuttingoperationtoremoveflashfromworkpartin impression‐die forging • Usually done while work is still hot, so a separate trimming press is included at the forging station
B. Ring Rolling
• Deformation process in which a thick‐walled ring of smaller diameter is rolled into a thin‐walled ring of larger diameter ~ As thick walled ring is compressed, deformed metal elongates, causing diameter to enlarge ~ Hot working process for large rings and cold working process for smaller rings ~ Products: ball and roller bearing races, steel tires for railroad wheels, and rings for pipes, pressure vessels, and rotating machinery
3. Forging
• Deformation process in which work is compressed between two dies ~ Oldest of the metal forming operations ~ Dates from about 5000 B C • Products: engine crank shafts, connecting rods, gears, jet engine turbine parts ~ Also, basic metals industries use forging to establish shape of large parts that are then machined to final geometry and size
1. Rolling
• Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls (shown below is flat rolling) • Rotating rolls perform two main functions: ~ Pull the work into the gap between them by friction between workpart and rolls ~ Simultaneously squeeze the work to reduce its cross section
Wire Drawing vs. Bar Drawing
• Difference between bar drawing and wire drawing is stock size ~ Bar drawing: large diameter bar and rod stock ~ Wire drawing: small diameter stock-wire sizes down to 0.03 mm (0.001 in.) are possible • Although the mechanics are the same, the methods, equipment, and even terminology are different
A. Flat Rolling Terminology
• Draft = amount of thickness reduction d = t o − tf • Reduction = draft expressed as a fraction of starting stock thickness: r = td where d = draft; to = starting thickness; tf = final thickness, and r = reduction
Drawing Practice and Products
• Drawing practice: ~ Usually performed as cold working ~ Most frequently used for round cross sections • Products: ~ Wire: electrical wire; wire stock for fences, coat hangers,and shopping carts ~ Rod stock for nails, screws, rivets, and springs ~ Bar stock: metal bars for machining, forging, and other processes
Extrusion Presses
• Either horizontal or vertical ~ Horizontal more common • Extrusion presses: usually hydraulically driven, which is especially suited to semi‐continuous direct extrusion of long sections • Mechanical drives: often used for cold extrusion of individual parts
Features of a Draw Die
• Entry region: funnels lubricant into the die to prevent scoring of work and die • Approach: cone‐shaped region where drawing occurs • Bearing surface: determines final stock size • Back relief: exit zone - provided with a back relief angle (half‐angle) of about 30° • Die materials: tool steels or cemented carbides
Rolling Mills
• Equipment is massive and expensive • Rolling mill configurations: ~ Two-high: two opposing rolls ~ Three-high: work passes through rolls in both directions ~ Four-high: backing rolls support smaller rolls ~ Cluster mill: multiple backing rolls on smaller rolls ~ Tandem rolling mill: sequence of two-high mills
Upsetting and Heading
• Forging process used to form heads on nails, bolts, and similar hardware products • More parts produced by upsetting than any other forging operation • Performed cold, warm, or hot on machines called headers or formers • Wire or bar stock is fed into machine, end is headed, then piece is cut to length • For bolts and screws, thread rolling is then used to form threads
Open-Die Forging with Friction
• Friction between work and die surfaces constrains lateral flow of work ~ This results in barreling effect • In hot open-die forging, effect is even more pronounced due to heat transfer at die surfaces ~ Which cools the metal and increases its resistance to deformation
Advantages of Thread Rolling over thread cutting (machining)
• Higher production rates • Better material utilization • Stronger threads and better fatigue resistance
Hot vs. Cold Extrusion
• Hot extrusion: prior heating of billet to above its recrystallization temperature • Reduces strength and increases ductility of the metal, permitting more size reductions and more complex shapes • Cold extrusion: generally used to produce discrete parts • The term impact extrusion is used to indicate high speed cold extrusion
Open‐Die Forging with No Friction
• If no friction occurs between work and die surfaces, then homogeneous deformation occurs, so that radial flow is uniform throughout workpart height and true strain is given by: ε = ln (h0/h) ~ where ho= starting height; and h = height at some point during compression • At h = final value hf, true strain reaches maximum value
Importance of Bulk Deformation
• In hot working, significant shape change can be accomplished •In cold working, strength is increased during shape change • Little or no waste - some operations are near net shape or net shape processes ~ The parts require little or no subsequent machining
Extrusion Die Features
• Low die angle: surface area is large, which increases friction at die‐billet interface ~ Higher friction results in larger ram force • Large die angle: more turbulence in metal flow during reduction ~ Turbulence increases ram force required • Optimum angle depends on work material, billet temperature, and lubrication
Bulk Deformation
• Metal forming operations which cause significant shape change by deforming metal parts whose initial form is bulk rather than sheet • Starting forms: ~ Cylindrical bars and billets ~ Rectangular billets, slabs, and similar shapes • These processes stress the metal sufficiently to cause plastic flow into the desired shape • Performed as cold, warm, and hotworking
Limitations of Impression-Die Forging
• Not capable of close tolerances • Machining is often required to achieve accuracies and features needed
Impression‐Die Forging Practice
• Several forming steps are often required ~ With separate die cavities for each step • Beginning steps redistribute metal for more uniform deformation and desired metallurgical structure in subsequent steps • Final steps bring the part to final geometry • Impression-die forging is often performed manually by skilled worker under adverse conditions
Shape of Extrusion Die Orifice
• Simplest cross-sectional shape is circular die orifice • Shape of die orifice affects ram pressure • As cross section becomes more complex, higher pressure and greater force are required • Effect of cross-sectional shape on pressure can be assessed by means of the die shape factor Kx
Advantages of Extrusion
• Variety of shapes possible, especially in hot extrusion ~ Limitation: part cross section must be uniform throughout length • Grain structure and strength enhanced in cold and warm extrusion • Close tolerances possible, especially in cold extrusion • In some operations, little or no waste of material
B. Shape Rolling
• Work is deformed into a contoured cross section rather than flat (rectangular) ~ Accomplished by passing work through rolls that have the reverse of desired shape • Products: ~ Construction shapes such as I-beams, L-beams, and U-channels ~ Rails for railroad tracks ~ Round and square bars and rods
