Bulk deformation process PDF

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This was a 120-page slide and a few pages in my textbook and i summarised it down in order to make it easier to read and remember....

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Bulk deformation process: Includes : ● Rolling ● Other deformation processes related to rolling ● Forging ● Other deformation processes related to forging ● Extrusion ● Wire / bar drawing This is a metal forming operation causing significant shape change by deformation in metal parts whose initial form is bulk rather than sheet. ● Starting forms - cylindrical bars & billets , rectangular billets & slabs & similar shapes. ● The process works by stressing metal sufficiently causing plastic flow into desired shape. ● Done as cold, warm , hot working operations. Importance: ● In hot working , sufficient shape change can be done. ● In cold working , strength can be increased during shape change ● Little / no waste (operations near net / net shape) Parts require little / no other machining. 4 basic bulk deformation processes: 1. Rolling - slab / plate squeezed between opposing rolls. 2. Forging - work squeezed & shaped between opposing rolls. 3. Extrusion - work squeezed through die opening , takes shape of the opening. 4. Wire / bar drawing - diameter of wire/bar reduced by pulling it through die opening.

1 Rolling: Thickness is reduced by compressive force of 2 opposing rolls. ● The rolls perform 2 main functions: ○ Pull work into gap between them by friction between workpart & rolls. ○ Squeeze work to reduce cross section. ● Types of rolling: ○ By geo of the part: ■ Flat rolling - reduces thickness of rectangular cross section. ■ Shape rolling - a square cross section is formed into a shape such as an I-beam. ○ By temp of work: ■ Hot rolling- most common due to large amount of deformation required. ■ Cold rolling- produces finished sheet/ plate stock.

Rolling types: ●

Flat rolling: Involves rolling of slabs, strips, sheets, plates, workparts of rec cross section where width is greater than thickness. Work squeezed between 2 rolls so the thickness is reduced by an amount called draft. Terminology: Draft - amount of thickness reduction.

Draft can be shown as fraction of starting stock thickness ,called reduction. Reduction - draft expressed as a fraction of starting stock thickness.

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Shape rolling: Work deformed into contoured cross section rather than flat. (rectangular) Done by passing work through rolls that have the reverse of desired shape. Products include: ● Construction hapes (I-beams, L-beams, U-channels) ● Rails for railroad tracks ● Round / square bars & rods. Shape rolling more complicated than flat rolling. The work starts as a square & requires gradual transformation through several rolls to achieve final cross section.

Rolling mills: Equipment is massive & expensive. Rolling mill configurations: ● Two-high (2 large opposing diameter rolls) ● Three- high ( work passes through both directions) ● four - high (backing rolls suport smaller work rolls) ● Cluster mill (many backing rolls on smaller rolls) ● Tandem rolling mill (sequence of 2 high mills)

3 high rolling mill: Theres 3 rolls in a vertical column & the direction of rotation of each roll remains unchanged. To achieve a series of reductions the work can be passed through from either side by raising or lowering the strrips after each pass. Need an elevator mechanism to raise & lower the work.

4 high rolling mill: Use 2 smaller diameter rolls to contact the work & 2 backing rolls behind them. Due to roll high forces, these smaller olls would deflect elasticity between their end bearing as the work passes through unless the larger backing rolls were used to support them.

Cluster mill: Multiple backing rolls allow even smaller roll diameters.

Tandem rolling mill: A series of rolling stands in sequence.

Thread rolling: Bulk D process used to form threads on cylindrical parts by rolling hem between 2 dies. ● Most important commercial process fro mass production of bolts/ screws ● Performed by cold working in thread rolling machines ● Advantages over thred cutting ( machining): ○ Higher production rates ○ Better material utilization ○ Stronger threads due to wrok hardening ○ Better fatigue resistance due to compressive stresses introduced by rolling.

4 Ring rolling: Deformation process where a thick walled ring of small diameter is rolled into thin walled rng of larger diameter. As thick walled ring is compressed, deformed metal elongates, causing diameter to enlarge. Hot working process for large rings & cold for smaller ones. Applications : ball & roller bearings racs, steel tires for railroad wheels, rings for pipes , pressure vessels & rotating mechanisms. Advantages: ● Material savings ● Ideal grain orientation ● Strengthening through cold working

2 Forging: Deformation process in which work is compressed between 2 dies. Oldest metal forming operation ( 5000BC) Components: ● Engine crankshafts ● Connecting rods ● Gears ● Aircraft structural components ● Jet engine turbine parts Basic metals industries use forging to establish basic form of large components that are subsequently machined to final shape & size. Classifications of foring operations: Cold vs hot forging: ● Hot / warm forging: most common , due to significant deformation & need to reduce strength & increase ductility of work metal. ● Cold forging : advantage is increased strength that results from strain hardening of part. Impact vs press forging: ● Forge hammer - applies an impact load ● Forge press - applies gradual pressure Types of forging dies: Open die forging - work compressed between 2 flat dies , allowing metal to flow laterally without constraint. Impression die forging- die surfaces contain a cavity / impression that is imparted to workpart, thus constraining metal flow - flash is created. Flashless forging - workpart completely constrained in die & no excess flash is produced.

Open die forging: Compression of workpart with cylindrical cross section between 2 flat dies. ● Similar to compression test ● Deformation operation reduces height & increases diameter of work ● Common names - upsetting / upset forging Open die forging with no friction: If no friction occurs between work & die , then homogeneous deformation occurs, so that radial flow is uniform throughout workpart height & true strain is given by:

Open die forging with friction: Friction between work & die constraints lateral flow of work , causing barreling effect. I hot open die forging the effect is een more pronounced due to heat transfer at & nea die surface which cools the metal & increases resistance to deformation.

Impression die forging ( closed die forging): Compression of part by die with inverse of desired shape. ● Flash is formed by metal that flows beyond die cavity into small gap between ie plates. ● Flash must be trimmed but serves an important function during compression: ○ As flash forms , friction resists continued metal flow into gap, constraining material to fill die cavity ○ In hot forging , metal flow is further restricted by cooling against die plates.

Impression die forging practice: Several forming steps required with seperate die cavities: ● Beginning steps redistribute metal for more uniform deformation & desired metallurgical structure in next steps ● Final steps bring part to final geo ● Impression die forging is often performed manually by skilled operator under adverse conditions. Advantages: ● High production rates ● Conservation of metal ● Greater strength ● Fav grain orientation Limitations: ● Not capable of close tolerances ● Machining often required to get accuracies & features needed (holes, threads & mating surfaces that fit with other components) Flash forging: Compression of work in punch & die tooling whose cavity does not allot for flash. ● Starting work part volume must equal die cavity volume within very close tolerance. ● Process control more demanding than impression die forging. ● Best suite to part geo that are simple / symmetrical. ● Classified as precision forging process.

Forging dies, hammers & presses: Equipment used are hammers & presses & forging dies, they are special tooling use in these machines. Auxiliary equipment ike furnaces, mechanical devices to load & unload work & trimming stations are required. ● Forging hammers ( drop hammers): Apply an impact load against workpart - 2 types: ● Gravity drop hammer - impact energy from falling weight of a heavy ram ● Power drop hammer - accelerate the ram by pressurized air / steam Disadvantages: Impact energy transmitted through anvil into floor of building. Most commonly used for impression die forging. How hammers work: Upper portion of forging die is attached to ram & lower portion to anvil. In operation the work is placed on lower die & ram is lifted then dropped. When upper die hits work , the impact energy causes part to take form of die cavity. Several blows facilitates desired change in shape.

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Forging presses: Apply gradual pressure rather than sudden impact to get compression operation. Types: ● Mechanical presses: converts rotation of drive motor into linear motion of ram (cranks, knuckles , joints & eccentrics) ● Hydraulic presses : hydraulic piston accelerates ram. ● Screw presses: screw mechanism drives ram.

Other deformation processes: 1. Upsetting & heading: Forging process used to form heads on nails , bolts & similar products. More parts produced by upsetting than other forging methods. Performed cold, warm or hot on machines called headers or formers. Wire or bar stock is fed to machine , end is headed then cut into length of piece. Bolts & screws the thread rolling is then used to from threads.

Swaging: Done by rotating dies that hammer workpart radially inward to taper it as the piece is fed into die. Used to reduce the diameter of tube or solid rod stock. Mandrel may be needed to control shape / size of int diameter of tbe.

Trimming: cutting operation to remove flash. Done while work is hot so separate trimming press is included at forging station. Trimming can also be done by alt methods ( grinding / sawing)

Extrusion: Compression forging process in which work metal is forced to flow through a die opening to produce desired cross section shape. Process similar to squeezing toothpaste out. Extrusion used to produce long parts with uniform cross section. Types: ● Direct ● Indirect

Direct extrusion: Called forward extrusion. As Ram approaches die opening , a small portion of billet remains that cannot be forced through die opening. This extra portion (called butt) must be separated from extruded product by cutting it just beyond die exit. Starting billet cross section usually round but final shape determined by die opeeing.

Indirect extrusion: Called backward / reverse extrusion. Workpart is in chamber that is closed on 1 side. Metal extrusion die located on ram which exerts force on work part. The extruded product flows in opp direction to that of ram. The ram is hollow to accommodate the part. Limits of indirect extrusion are imposed by lower rigidity of hollow ram & difficult in supporting extruded product as it exits die.

Advantages of extrusion: ● Variety shapes (limits - part cross section must be uniform throughout length) ● Grain structure & strength enhanced in cold & warm extrusion ● Close tolerance possible ( especially in cold extrusion) ● In some operations little / no waste of material. Hot vs Cold extrusion: ● Hot - prior heating of billet to above recrystallization temp. This reduces strength & increases ductility of metal , permitting more size reductions & more complex shapes. ● Cold - generally used to produce discrete parts. The term impact extrusion is used to indicate high speed cold extrusion.

Extrusion ratio: Also called reduction ratio , defined as :

Comments on die angle: Low die angle - surface area large , leading to increased 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 needed) Optimum angle depends on work material , billet temp, and lubrication. Orifice shape of extrusion die: Simplest cross section shape - circular die orifice. Shape of die orifice affects ram pressure. As cross section becomes complex, higher pressure & greater force needed. Extrusion presses: Either horizontal / vertical (horizontal most common) Extrusion presses - normally hydraulic , which is especially suited to semi continuous direct extrusion of long sections. Mechanical drives - often used for cold extrusion of individual parts. WIre & bar drawing: Cross section of bar , rod , wire is reduced by pulling it through a die opening. Similar to extrusion except work pulled through die in drawing ( pushed through extrusion) Although drawing applies tensile stress, comparison also plays a role since metal is squeezed as it passes through the die opening.

Area reduction in drawing : Change in size of work is usually given by area reduction:

Wire drawing vs bar drawing: Diff between them is stock size: ● Bar drawing: large diameter bar & rod stock ● Wire drawing: smaller diameter stock - wire sized 0.03mm are possible Although the mechanics are the same, the methods, equip & even terminology are diff. Drawing practice & products: Drawing practice: Usually done as cold working. Most frequently used for round cross section. Products; ● Wire - elec wire , wire stock for fences , coat hangers and shopping carts. ● Rod stock for nails, screws, rivets and springs. ● Bar stock: metal bars for machining , foring and other processes. Bar drawing: Dine as a single draft operation - stock pulled though one die opening. Beginning stock has large diameter & is straight cylinder. This necessitates a batch type operation.

Wire drawing: Continuous drawing mechanism has multiple draw dies separated by accumulating drums( 412 dies) Each drum provides force to draw wore stock through upstream die. Each die provides small reduction , so desired total reduction is achieved by the series. Annealing ( increase ductility) sometimes required between dies.

Features of draw die: Entry region - funnels lubricant into die to prevent scoring of work & die. Approach - cone shaped region where drawing iccures. Bearing surface - determines final stock size. Back relief - exit zone - provided with a back relief angle ( half angle ) - 30 degrees. Die materials - tool steels / cemented carbides.

Prep of work for wire or bar drawing: ● Annealing - increase ductility ● Cleaning - prevent damage to work surface & draw die ● Pointing - reduce diameter of starting end to allow insertion through draw die.

Sheet metal working: ● Bending ● Deep / cup drawing ● Shearing process & miscellaneous process 1 Bending : In sheet metal work is defined as the straining of the metal around straight axis. During bending operation , metal on the inside of neutral plane is compressed, while metal on outside is stretched. Metal is plastically deformed so the bend takes a permanent set upon removal of the stress that caused it. Bening produces little/ no change in thickness of sheet metal.

V - bending & edge bending: Bending operations are done using punch & die tooling. V bending: Performed with V die The sheet metal is bent between a V shaped punch and die. Used for low production operations.

2 Edge bending: Performed with wiping die Involved cantilever loading of sheet metal. Pressure pad used to apply force Fh to hold base of part against die, while punch force part to yield bend over the edge of the die. Limited to bends of 90 degrees of less. Wiping dies are more complicated & costly that V dies. Used for high production work.

Bend allowance: Metal stretches is the bend radius is small relative to stock thickness. Estimation of stretch is necessary. Determine length of neutral axis before bending to account for stretching of final bend section. This length is called bend allowance. Formula :

Spring back: When bending pressure is removed at the end of the deformation operation, elastic energy remains in the bent part, causing it to recover partially toward its original shape. The elastic recovery called spring back defined as the increase in angle of the bent part relative to the included angle of the forming tool after the tool is removed & is expressed as :

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