Bulk Deformation Processes in Metalworking

Home , Draft (engineering), Ductility, Extrusion, Hot working, Screw press

Metal-Forming Processes Overview of Metal Forming

Rolling Performed as cold, warm, and hot working Forging Bulk Deformation Extrusion

Wire and bar drawing Metal Forming Mainly cold working Bending Sheet Shearing Metalworking Deep and cup drawing Bulk Deformation (Overview Cont’d)

rolling extrusion

Wire/bar drawing forging Sheet Metalworking (Overview Cont’d)

bending Deep/cup drawing

shearing Formability (workability) Formability of the material depends on: (1) Process variables - temperature……………… Desirable material properties in metal - strain……………… rate forming: – Low yield strength and high ductility - ………………stress

(2) Metallurgical changes (properties changes such as hardness)during deformation ,formation of voids, inclusions, precipitation, .... etc.

Ductility increases and yield strength decreases when work temperature is raised

 Any deformation operation can be accomplished with lower forces and power at elevated temperature Metal Forming Processes: Homologous Temp.  What is the parameter that determine working temperature???  Metal forming process temperature is measured by Homologus temperature  Homologous temperature expresses the temperature of a material as a fraction of its melting point temperature using the Kelvin scale • T: working temperature such Stainless steels have good strength and good resistance to corrosion and Process T/T oxidation at elevated temperatures m Cold working < 0.3 • Tm: melting point of metal (based on absolute temperature scale) Warm working 0.3 to 0.5 e.g. lead Hot working > 0.6

– Tm = 327 C – Formed at room temperature (20 C), …………………………………T/Tm = (20 +273)/(327 + .273) = 0.5 Warm working

 Most metals strain harden at room temperature But if heated to sufficiently high temperature and deformed, strain hardening does not occur Strain or Work Hardening

• Strain hardening (work hardening) is where a material becomes less ductile, harder and stronger with plastic deformation. • Encountered during cold working. • Percentage cold work can be expressed as:

Ao = original cross-sectional area

Ad = deformed cross-sectional area

Ductility ……decreases...…. with cold work Yield and tensile strength ……………increase Strain or Work Hardening

• Yield strength (sy) increases. • Tensile strength (UTS) increases. • Ductility (%EL or %AR) decreases. • Dislocation density increases with CW • Motion of dislocations is hindered as their density increases. • Stress required to cause further deformation is increased. • Strain hardening is used commercially to improve the yield and tensile properties. – cold-rolled low-carbon steel sheet – aluminum sheet • Strain hardening exponent n indicates the response to cold work (i.e. larger n means The influence of cold work on the greater strain hardening for a given stress–strain behavior for a low- amount of plastic strain). carbon steel. Cold Working • Performed at room temperature or slightly above. • Many cold forming processes are important mass production operations. • Minimum or no machining usually required (no oxidation). – These operations are near net shape or net shape processes.

Advantages of Cold Forming vs. Hot Working:  Better accuracy, closer tolerances.  Better surface finish.  Strain hardening increases strength and hardness.  Grain flow during deformation can cause desirable directional properties in product.  No heating of work required (less total energy) Cold Working Disadvantages of Cold Forming: • Equipment of higher forces and power required to shape material. • Surfaces of starting work-piece must be free of scale and dirt (to avoid surface defect during cold working). • Less ductility and high strain hardening limit the amount of forming that can be done. – In some operations, metal must be annealed to allow further deformation.

ANNEALING-A heat treatment to eliminate the effects of cold working.

Purposes of annealing: - …………relieve ..stress [residual stress] - ……………………increase ductility - …………………………produce a specific structure.. Annealing involves three steps Annealing

• Material in this condition (cold worked) is annealed, changes will begin to take place. These changes may be classified under three headings: 1. Stress relief 2. Recrystallization 3. Grain growth Effect of cold working on properties

The grain boundaries here is the disorder structure of high density dislocation which replace by the original fragmented grain boundaries Annealing Warm Working

• Performed at temperatures above room temperature but below recrystallization temperature.

Warm working: T/Tm from 0.3 to 0.5

Advantages of Warm Working: o Lower forces and power than in cold working. o More intricate work geometries possible. o Need for annealing may be reduced or eliminated. Hot Working

• Deformation at temperatures above recrystallization temperature: – In practice, hot working usually performed somewhat

above 0.5Tm – Metal continues to soften as temperature increases

above 0.5Tm, enhancing the advantage of hot working above this level [produce a specific structure] Hot Working

Why Hot Working? Capability for substantial plastic deformation of the metal - far more than possible with cold working or warm working.

Why? – Strength coefficient (K) is substantially less than at room temp. – Strain hardening exponent (n) is zero (theoretically). – Ductility is significantly increased. Advantages of Hot Working vs. Cold Working

• Work-part shape can be significantly altered. • Lower forces and power required (equipment). • Metals that usually fracture in cold working can be hot formed. • Strength properties of product are generally isotropic. • No strengthening of part occurs from work hardening.

Disadvantages of Hot Working:  Lower dimensional accuracy.  Higher total energy required. - Due to the thermal energy to heat the work-piece.  Work surface oxidation (scale), poor………… surface finish.  Shorter…..……… tool life Hot Work vs. Cold Work Hot Work Cold Work • Recrystallization takes place • NO Recrystallization • Less than <0.3 Tm • > 0.5 * Tm • Requires less force • Requires more force • Less residual stresses • Residual Stresses • Greater deformation • Strain Hardened possible • Better Surface Finish • Dimensional Variation • No oxides on the surface [Lower dimensional accuracy] after operation • Poor Surface Finish • lower costs for process • Oxidation of Surfaces and equipment • Expensive costs for process and equipment Friction in Metal Forming • In most metal forming processes, friction is undesirable: – Metal flow is retarded – Forces and power are increased – Wears tooling faster • Metalworking lubricants are applied to tool-work interface in many forming operations to reduce harmful effects of friction. • Benefits: – Reduced sticking, tool wear – Better surface finish – Removes heat from the tooling Material Behavior in Metal Forming

. Plastic region of stress-strain curve is primary interest because material is plastically deformed . In plastic region, metal's behavior is expressed by the flow curve: s  K n where K = strength coefficient; and n = strain hardening exponent . Flow curve based on true stress and true strain

. For most metals at room temperature, strength increases when deformed due to strain hardening . Flow stress = instantaneous value of stress required to continue deforming the material n Yf  K

where Yf = flow stress, that is, the yield strength as a function of strain

. Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest _ K n Y  f 1  n _ where Y f = average flow stress; and  = maximum strain during deformation process

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

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 and slabs and similar shapes . These processes stress metal sufficiently to cause plastic flow into the desired shape . Performed as cold, warm, and hot working operations

. 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

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

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

. Based on workpiece geometry . Flat rolling - used to reduce thickness of a rectangular cross section . 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

Sheet steel that undergoes acid pickling will oxidize (rust) when exposed to atmospheric conditions of moderately high humidity Shape Rolling For this reason, a thin film of oil or similar waterproof coating is applied to create a barrier to moisture in the air Rolling

• Starting material (blooms, billets, or slabs) are rolled into structural shape, rails, plate, sheet, strips, bars and rods by feeding material through successive pairs of rolls.

 Bloom - square or rectangular cross section with a thickness greater than 6” and a width no greater than 2x’s the thickness  Billets - square or circular cross section - smaller than a bloom  Slabs - rectangular in shape (width is greater than 2x’s the thickness), slabs are rolled into plate, sheet, and strips. Rolling

Plates: are generally regarded as having a thickness greater than 6mm, and are used for structural applications such as boilers, bridges, machine structure, girders, and ship hulls. Plates can be as much as 0.3 m thick for large boilers, and 100-125 mm thick for warships and tank armor. Sheets :are generally less than 6mm thick. They are used for automobile bodies, aircraft fuselages, office furniture and kitchen equipment's. Diagram of Flat Rolling

. Side view of flat rolling, indicating before and after thicknesses, work velocities, angle of contact with rolls, and other features.

. Hot-rolled plates are used in shipbuilding, bridges, boilers, welded structures for various heavy machines, tubes and pipes, and many other products. . Further flattening of hot-rolled plates and sheets is often accomplished by cold rolling, in order to prepare them for subsequent sheet metal operations. . The cold-rolled sheets, strips, and coils ideal for products ranging from automobiles to appliances and office furniture. Roll Flat Terminology

• ho = initial thickness of the strip, hf= final thickness, Vr= roll surfcae speed, Vf= the final speed of the strip (increses as the strip moves through the roll gap), L = contact length with the roll Roll Flat Terminology

 The basic flat rolling is shown in the figure.

 A strip of a thickness ho enters the roll gap and is reduced to a thickness

of hf by the powered rotating rolls at a surface speed Vr of the roll.  To keep the volume rate of metal flow constant, the velocity of the strip must increase as it moves through the roll gap. Constant material volume:

 At the exit of the roll gap, the ho wo Lo = hf wf Lf velocity of the strip is V . f  ho wo vo = hf wf vf (flow rate) Flat Rolling Terminology

. Draft = amount of thickness reduction

d t o tf

. Reduction = draft expressed as a fraction of starting stock thickness: d r  to

where d = draft; to = starting thickness; tf = final thickness, and r = reduction

Figure 13.6 Changes in the grain structure of cast or of large-grain wrought metals during hot rolling. Hot rolling is an effective way to reduce grain size in metals, for improved strength and ductility. Cast structures of ingots or continuous casting are converted to a wrought structure by hot working. Rolling

The rolling process (specifically, flat rolling) 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

Various configurations of rolling mills: (a) 2-high rolling mill.

44 Rolling Mill Configurations

 Two-high • Rolling mills are typically used for initial breakdown passes on the workpiece, with roll diameter ranging up to 1400 mm • In two high non reversing mills as two rolls which revolve continuously in opposite direction therefore smaller and less costly motive power can be used. Three-High Rolling Mill

Figure 19.5 Various configurations of rolling mills: (b) 3-high rolling mill.

46 Rolling Mill Configurations

 Three-high • Rolling mills are typically used for initial breakdown passes on the workpiece, with roll diameter ranging up to 1400 mm • It consists of a roll stand with three parallel rolls one above the other. Adjacent rolls rotates in opposite direction. So that the material is passed between the top and the middle roll in one direction and the bottom and middle rolls in opposite one so that thickness is reduced at each pass. • The rolls of a three high rolling mills may be either plain or grooved to produce plate or sections respectively. Four-High Rolling Mill

Various configurations of rolling mills: (c) four-high rolling mill.

48 Rolling: Rolling Mill Configurations

• Four-high • It has a roll stand with four parallel rolls one above the other. The top and the bottom rolls rotate in opposite direction as do the two middle rolls. The two middle are smaller in size than the top and bottom rolls which are called backup rolls for providing the necessary rigidity to the smaller rolls. • A four high rolling mill is used for the hot rolling of armor and other plates as well as cold rolling of plates, sheets and strips. Cluster Mill

Multiple backing rolls allow even smaller roll diameters

Various configurations of rolling mills: (d) cluster mill

51 Rolling: Rolling Mill Configurations  Cluster mill  It is a special type of four high rolling mill in which each of the two working rolls is backup by two or more of the larger backup rolls  Is suitable for cold rolling thin strips of high- strength metals  the rolled product obtained in cluster mill can be as wide as 5000mm (50 m) and as thin as 0.0025 mm.  The diameter of the smallest roll can be as small as 6 mm and is usually made of tungsten carbide for rigidity, strength and wear resistance  For rolling hard materials. Tandem Rolling Mill

A series of rolling stands in sequence

Various configurations of rolling mills: (e) tandem rolling mill.

54 Rolling Mill Configurations

Tandem rolling mill • It is a set of three rolls in parallel alignment so that a continuous pass may be made (thickness reduction) through each one successively.  Advantages • Reduced roll consumption • Tight tolerances for strip thickness. • The required strip thickness and flatness can be achieved more by tandem rolling mill ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Rolling: Shaped rolling or section rolling

• A special type of cold rolling in which flat slap is progressively bent into complex shapes by passing it through a series of driven rolls. • No appreciable change in the thickness of the metal during this process. • Suitable for producing moulded sections such as irregular shaped channels and trim. Shaped rolling or section rolling

A variety of sections can be produced by roll forming process using a series of Applications: forming rollers in a continuous method to - construction materials, roll the metal sheet to a specific shape. - partition beam - ceiling panel - roofing panels. - steel pipe - automotive parts - household appliances - metal furniture, - door and window frames - other metal products. Shape Rolling Shape Rolling products

L- beam

U- beam I- beam Thread Rolling

. (1) Start of cycle, and (2) end of cycle

Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies . Important process for mass producing bolts and screws . Performed by cold working in thread rolling machines . Advantages over thread cutting (machining): . Higher production rates . Better material utilization . Stronger threads and better fatigue resistance

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 of ring 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

Deformation process in which a thick-walled ring of smaller diameter is rolled into a thin-walled ring of larger diameter

Figure 19.7 Ring rolling used to reduce the wall thickness and increase the diameter of a ring: (1) start, and (2) completion of process.

65 Ring Rolling

• (1) start, and (2) completion of process Courtesy of www.fangyuanforging.cn Courtesy of www.offshore-technology.com

Courtesy of www.npa.nsk.com Courtesy of Courtesy of www.magictire.com www.southdevonrailway.org

Forging:

. Forging is a deformation process in which the work is compressed between two dies, using either impact or gradual pressure to form the part. . Today, forging is an important industrial process used to make a variety of high-strength components for automotive, aerospace, and other applications. 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 crankshafts, connecting rods, gears, aircraft structural components, jet engine turbine parts . Also, basic metals industries use forging to establish basic shape of large parts that are subsequently machined to final geometry and size

. Most forging operations are performed hot or warm, owing to the significant deformation demanded by the process and the need to reduce strength and increase ductility of the work metal. . Cold forging is also very common for certain products. . The advantage of cold forging is the increased strength that results from strain hardening of the component Forging:

. Either impact or gradual pressure is used in forging. . A forging machine that applies an impact load is called a forging hammer, while one that applies gradual pressure is called a forging press. 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

. Open-die forging - work is compressed between two flat dies, allowing metal to flow laterally with minimum constraint . Impression-die forging - die contains cavity or impression that is imparted to workpart . Metal flow is constrained so that flash is created . Flashless forging - workpart is completely constrained in die . No excess flash is created

. (a) Open-die forging, (b) impression-die forging, and (c) flashless forging

Compression of workpart between two flat dies . Similar to compression test when workpart 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

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 h   ln o h

where ho= starting height; and h = height at some point during compression

. At h = final value hf, true strain reaches maximum value

. (1) Start of process with workpiece at its original length and diameter, (2) partial compression, and (3) final size

. 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

Actual deformation of a cylindrical workpart in open-die forging, showing pronounced barreling: (1) start of process, (2) partial deformation, and (3) final shape

• Open-die forging is the simplest forging operation. • Part sizes may range from very small (the size of nails, pins, and bolts) to very large (up to 23 m, long shafts for ship propellers). • Open-die forging can be depicted by a solid workpiece placed between two flat dies and reduced in height by compressing it, this process is also called upsetting or flat die forging. Analysis of Open-Die Forging

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e . All of these factors cause the actual upsetting force to be greater than what is predicted, as an approximation, we can apply a shape factor as:

where F, Yf, and A have the same definitions as in the previous equation; and Kf is the forging shape factor

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e where μ = coefficient of friction; D = workpart diameter or other dimension representing contact length with die surface, mm (in); and h = workpart height, mm (in).

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

. (1) Just prior to initial contact with raw workpiece, (2) partial compression, and (3) final die closure, causing flash to form in gap between die plates

. In impression-die forging (sometimes called closed-die forging), the work-piece takes the shape of the die cavity while being forged between two shaped dies. . This process usually is carried out at elevated temperatures to lower the required forces and attain enhanced ductility in the workpiece. . During deformation, some of the material flows outward and forms a flash. Impression-die Forging: Impression-die Forging (closed die forging):

. The flash has an important role in impression-die forging: . The high pressure and the resulting high frictional resistance in the flash present a severe constraint on any outward flow of the material in the die. . Thus, the material flows into the die cavity, ultimately filling it completely. 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

. Advantages compared to machining from solid stock: . Higher production rates . Less waste of metal . Greater strength . Favorable grain orientation in the metal . Limitations: . Not capable of close tolerances . Machining is often required to achieve accuracies and features needed

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

. (1) Just before contact with workpiece, (2) partial compression, and (3) final punch and die closure

. Coining is a special application of true closed-die forging in which fine details in the die are impressed into the top and bottom surfaces of the workpart. . There is little flow of metal in coining, yet the pressures required to reproduce the surface details in the die cavity are high. . The process is also used to provide good surface finish and dimensional accuracy on workparts made by other operations. Flashless Forging: ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Forging Hammers

. Apply impact load against workpart: Two types: . Gravity drop hammers - impact energy from falling weight of a heavy ram . Power drop hammers - accelerate the ram by pressurized air or steam . Disadvantage: impact energy transmitted through anvil into floor of building . Commonly used for impression-die forging

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e . Drop forging hammer, fed by conveyor and heating units at the right of the scene (photo courtesy of Chambersburg Engineering Company).

. Diagram showing details of a drop hammer for impression-die forging

. Apply gradual pressure to accomplish compression operation . Types: . Mechanical press - converts rotation of drive motor into linear motion of ram . Hydraulic press - hydraulic piston actuates ram . Screw press - screw mechanism drives ram

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

. Upset forging to form a head on a bolt or similar hardware item: (1) wire stock is fed to stop, (2) gripping dies close on stock and stop retracts, (3) punch moves forward, (4) bottoms to form the head

. Examples of heading operations: (a) heading a nail using open dies, (b) round head formed by punch, (c) and (d) two common head styles for screws formed by die, (e) carriage bolt head formed by punch and die

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

. Swaging process to reduce solid rod stock; dies rotate as they hammer the work . In radial forging, workpiece rotates while dies remain in a fixed orientation as they hammer the work

Cutting operation to remove flash from workpart in impression-die forging . Usually done while work is still hot, so a separate trimming press is included at the forging station . Trimming can also be done by alternative methods, such as grinding or sawing

. Trimming operation (shearing process) to remove the flash after impression-die forging

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

. Also called forward extrusion . Starting billet cross section 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

. One of the problems in direct extrusion is the significant friction that exists between the work surface and the walls of the container. . A dummy block is often used between the ram and the work billet. The diameter of the dummy block is slightly smaller than the billet diameter, so that a narrowing of work metal (mostly the oxide layer) is left in the container, leaving the final product free of oxides. Direct Extrusion (forward extrusion) : Direct Extrusion:

. Hollow sections are possible in direct extrusion. . The starting billet is prepared with a hole parallel to its axis. This allows passage of a mandrel that is attached to the dummy block. . As the billet is compressed, the material is forced to flow through the clearance between the mandrel and the die opening. The resulting cross section is tubular. Hollow and Semi-Hollow Shapes

. (a) Direct extrusion to produce hollow or semi-hollow cross sections; (b) hollow and (c) semi-hollow cross sections

. As the ram penetrates into the work, the metal is forced to flow through the clearance in a direction opposite to the motion of the ram. . Since the billet is not forced to move relative to the container, there is no friction at the container walls, and the ram force is therefore lower than in direct extrusion. . Indirect extrusion can produce hollow (tubular) cross sections Indirect Extrusion

. Indirect extrusion to produce (a) a solid cross section and (b) a hollow cross section

. 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

. 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

. A wide variety of solid or hollow cross sections may be produced by extrusions. . Typical products made by extrusion are railings for sliding doors, window frames, tubing having various cross sections, aluminum ladder frames, and numerous structural and architectural shapes. Extrusion: 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

• Cold extrusion and warm extrusion are generally used to produce discrete parts, often in finished (or near finished) form. • The term impact extrusion is used to indicate high- speed cold extrusion. • Some important advantages of cold extrusion include increased strength due to strain hardening, close tolerances, improved surface finish, Impact Extrusion: Extrusion Die Features

. (a) Definition of die angle in direct extrusion; (b) effect of die angle on ram force

. 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

. 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 the die shape factor Kx

. Extruded cross section for a heat sink (courtesy of Aluminum Company of America)

. 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

Also called the reduction ratio, it is defined as

Ao rx  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

. The value of rx can be used to determine true strain in extrusion:

The pressure applied by the ram to compress the billet through the die opening depicted in our figure can be computed as follows:

where Yf = average flow stress during deformation, MPa

where ex = extrusion strain; and a and b are empirical constants for a given die angle. Typical values of these constants are: a = 0.8 and b = 1.2 to 1.5.

. Ram force in indirect or direct extrusion is simply pressure p from multiplied by billet area Ao:

. where P = power, J/s (in-lb/min); F = ram force, N (lb); and v = ram velocity, m/s (in/min)

. A billet 75mmlong and 25 mm in diameter is to be extruded in a direct extrusion operation with extrusion

ratio rx = 4.0. The work metal has a strength coefficient = 415 MPa, and strain-hardening exponent = 0.18. Use the Johnson formula with a = 0.8 and b = 1.5 to estimate extrusion strain. Determine the pressure applied to the end of the billet as the ram moves forward.

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

. Change in size of work is usually given by area reduction: A  A r  o f Ao

where r = area reduction in drawing; Ao = original area of work; and Ar = final work

. 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

. 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

. 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

. Hydraulically operated draw bench for drawing metal bars

. 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

. 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

. 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

. Wire is drawn through a draw die with entrance angle=15. Starting diameter is 2.5 mm and final diameter = 2.0 mm. The coefficient of friction at the work–die interface = 0.07. The metal has a strength coefficient K = 205 MPa and a strain-hardening exponent n = 0.20. Determine the draw stress and draw force in this operation.

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e SHEET METAL WORKING SHEET METAL WORKING

. Sheet metalworking includes cutting and forming operations performed on relatively thin sheets of metal. . Sheet-metal processing is usually performed at room temperature, The exceptions are when the stock is thick, the metal is brittle . . Most sheet-metal operations are performed on machine tools called presses. SHEET METAL WORKING

. Shearing is a sheet-metal cutting operation along a straight line between two cutting edges. . Shearing is typically used to cut large sheets into smaller sections for subsequent press-working operations. . It is performed on a machine called a power shears. Shearing, Blanking, and Punching: Shearing, Blanking, and Punching: • Blanking involves cutting of the sheet metal along a closed outline in a single step to separate the piece from the surrounding stock. The part that is cut out is the desired product in the operation and is called the blank.

• Punching is similar to blanking except that it produces a hole, and the separated piece is scrap, called the slug. The remaining stock is the desired part. Shearing, Blanking, and Punching: Bending Operations:

. Bending operations are performed using punch and die tooling. The two common bending methods and associated tooling are V-bending, and edge bending. . In V-bending, the sheet metal is bent between a V-shaped punch and die. Included angles ranging from very obtuse to very acute can be made with V-dies. . Edge bending involves cantilever loading of the sheet metal. Bending Operations:

Two common bending methods: (a) V-bending and (b) edge bending; (1) before and (2) after bending. Deep Drawing: Deep Drawing:

. Drawing is a sheet-metal-forming operation used to make cup-shaped, box-shaped, or other complex-curved and concave parts. . Common parts made by drawing include beverage cans, ammunition shells, sinks, cooking pots, and automobile body panels. Deep Drawing: Deep Drawing (cold, for sheetmetal) - Punch draws blank into die - Metal is supported on both sides to avoid wrinkling - Hold-down pressure (blank-holder force) is primary process variable if too high: tearing if too low: wrinkling www.endo-mfg.co.jp

Kalpakjian Spinning: Spinning:

. Spinning is a metal-forming process in which an axially symmetric part is gradually shaped over a mandrel or form by means of a rounded tool or roller. . Basic geometric shapes typically produced by spinning include cups, cones, hemispheres, and tubes. . Applications of conventional spinning include production of conical and curved shapes in low quantities. . The mandrel in spinning can be made of wood or other soft materials that are easy to shape. Spinning: Spinning Ideal for • Lower production volumes • Large parts • Inexpensive tooling