Mti Unit III

8/8/2019 Mti Unit III

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Unit III Bulk deformation

Processes


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Hot working

In materials science, hot working refers to

processes where metals are plastically

deformed above their recrystallization

temperature. Being above the recrystallizationtemperature allows the material to recrystallize

during deformation. This is important because

recrystallization keeps the materials from strain

hardening, which ultimately keeps the yieldstrength and hardness low and ductility high.


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Advantages of Hot working

Decrease in yield strength, therefore it is easier to workand uses less energy or force

Increase in ductility

Elevated temperatures increase diffusion which canremove or reduce chemical in homogeneities

Pores may reduce in size or close completely duringdeformation

In steel, the weak, ductile, face-centered-cubic austenite

microstructure is deformed instead of the strong body-centered-cubic ferrite microstructure found at lowertemperatures


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Disadvantages of HW

Undesirable reactions between the metal andthe surrounding atmosphere (scaling or rapidoxidation of the workpiece)

Less precise tolerances due to thermalcontraction and warping from uneven cooling

Grain structure may vary throughout the metalfor various reasons

Requires a heating unit of some kind such as agas or diesel furnace or an induction heater,which can be very expensive


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Cold working

Cold working is a metalworking process that employswork hardening because the work piece is shaped at atemperature below its recrystallization temperature,usually at the ambient temperature.[1] Cold forming

techniques are usually classified into four major groups:squeezing, bending, drawing, and shearing.

Such deformation increases the concentration ofdislocations which may subsequently form low-anglegrain boundaries surrounding sub-grains. Cold workinggenerally results in a higheryield strength as a result ofthe increased number of dislocations and the Hall-Petcheffect of the sub-grains, and a decrease in ductility. Theeffects of cold working may be reversed by annealing thematerial at high temperatures where recovery and

recrystallization reduce the dislocation density.


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Advantages of CW

No heating required

Better surface finish

Superior dimensional control

Better reproducibility and interchangeability

Directional properties can be imparted into the

metal

Contamination problems are minimized


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Disadvantages of CW

Greater forces are required

Heavier and more powerful equipment andstronger tooling are required

Metal is less ductile Metal surfaces must be clean and scale-free

Intermediate anneals may be required tocompensate for loss of ductility thataccompanies strain hardening

The imparted directional properties may bedetrimental

Undesirable residual stress may be produced


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Forging

Forging is the term for shaping metal by usinglocalized compressive forces. Cold forging isdone at room temperature or near roomtemperature. Hot forging is done at a high

temperature, which makes metal easier to shapeand less likely to fracture. Warm forging is doneat intermediate temperature between roomtemperature and hot forging temperatures.Forged parts can range in weight from less thana kilogram to 170 metric tons. Forged partsusually require further processing to achieve afinished part.


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Hot and Cold forging

Hot forging is defined as working a metal aboveits recrystallization temperature. The mainadvantage of hot forging is that as the metal is

deformed the strain-hardening effects arenegated by the recrystallization process.

Cold forging is defined as working a metal below

its recrystallization temperature, but usuallyaround room temperature. If the temperature isabove 0.3 times the melting temperature (on anabsolute scale) then it qualifies as warm forging.


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Open-die drop-hammer forging

Open-die forging is also known as smith forgingIn open-die forging ahammer comes down and deforms the workpiece, which is placedon a stationary anvil. Open-die forging gets its name from the factthat the dies (the working surfaces of the forge that contact the

workpiece) do not enclose the workpiece, allowing it to flow exceptwhere contacted by the dies. Therefore the operator needs to orientand position the workpiece to get the desired shape. The dies areusually flat in shape, but some have a specially shaped surface forspecialized operations. For instance, the die may have a round,concave, or convex surface or be a tool to form holes or be a cut-offtool.

Open-die forging lends itself to short runs and is appropriate for artsmithing and custom work. Other times open-die forging is used torough shape ingots to prepare them for further operations. This canalso orient the grains to increase strength in the required direction.


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Impression-die forging

Impression-die forging is also called closed-die forging.In impression-die work metal is placed in a dieresembling a mold, which is attached to the anvil.Usually the hammer die is shaped as well. The hammeris then dropped on the workpiece, causing the metal toflow and fill the die cavities. The hammer is generally incontact with the workpiece on the scale of milliseconds.Depending on the size and complexity of the part thehammer may be dropped multiple times in quicksuccession. Excess metal is squeezed out of the die

cavities, forming what is referred to as flash. The flashcools more rapidly than the rest of the material; this coolmetal is stronger than the metal in the die so it helpsprevent more flash from forming. This also forces themetal to completely fill the die cavity. After forging theflash is removed.


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Closed-die forging

In this type of forging the die cavities are completely closed, whichkeeps the workpiece from forming flash. The major advantage to thisprocess is that less metal is lost to flash. Flash can account for 20 to45% of the starting material. The disadvantages of this processinclude additional cost due to a more complex die design and the

need for better lubrication and workpiece placement.

Closed-die forging has a high initial cost due to the creation of dies and

required design work to make working die cavities. However, it has low

recurring costs for each part, thus forgings become more economical with

more volume. This is one of the major reasons closed-die forgings are

often used in the automotive and tool industry. Another reason forgingsare common in these industrial sectors is because forgings generally have

about a 20% higher strength to weight ratio compared to cast or machined

parts of the same material.


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Press forging

Press forging is variation of drop-hammer forging. Unlike drop-

hammer forging, press forges work slowly by applying continuous

pressure or force. The amount of time the dies are in contact with

the workpiece is measured in seconds (as compared to the

milliseconds of drop-hammer forges). The press forging operationcan be done either cold or hot.

Press forging can be used to perform all types of forging, including

open-die and impression-die forging. Impression-die press forging

usually requires less draft than drop forging and has betterdimensional accuracy. Also, press forgings can often be done in one

closing of the dies, allowing for easy automation.


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Forging


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Cold Forging


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Roll forging

Roll forging is a process where round or flat bar stock is reduced inthickness and increased in length. Roll forging is performed usingtwo cylindrical or semi-cylindrical rolls, each containing one or moreshaped grooves.

A heated bar is inserted into the rolls and when it hits a stop the rollsrotate and the bar is progressively shaped as it is rolled out of themachine. The work piece is then transferred to the next set ofgrooves or turned around and reinserted into the same grooves.This continues until the desired shape and size is achieved. Theadvantage of this process is there is no flash and it imparts afavorable grain structure into the workpiece.

Examples of products produced using this method include axles,tapered levers and leaf springs.


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Roll Forging


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Drop Forging


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Press Forging


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Roll forming

Roll forming is a continuous bending

operation in which a long strip ofmetal

(typically coiled steel) is passed through

consecutive sets of rolls, orstands, each

performing only an incremental part of the

bend, until the desired cross-section

profile is obtained. Roll forming is ideal forproducing parts with long lengths or in

large quantities.


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A variety of cross-section profiles can be produced, but each profilerequires a carefully crafted set of roll tools. Design of the rolls startswith a flowerpattern, which is the sequence of profile cross-sections, one for each stand of rolls. The roll contours are thenderived from the profile contours. Because of the high cost of the rollsets, simulation is often used to validate the designed rolls andoptimize the forming process to minimize the number of stands andmaterial stresses in the final product.

Roll formed sections have an advantage over extrusions of a similarshapes. Roll formed parts are generally much lighter and stronger,having been work hardened in a cold state. Another advantage is

that the part can be made having a finish or already painted. Laboris greatly reduced since volume is a major consideration forchoosing the roll forming process.


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The process of roll forming is one of the simplermanufacturing processes. It begins with a large spool ofmetal strips, usually between 1 in. and 20in. in width,and 0.004 in. and 0.125 in. thick. This is held by a devicecalled a dispenser. The metal strip is then unrolled andfed into a machine starting with the stock feeder which isconnected to the cutoff attachment. After the cutoffattachment, the metal strip is fed into the forming rolls.These mating die-set rolls are constructed to form thedesired shape in stages sequentially by means of

various shaped rolls. The layout of these rolls can beflower shaped as mentioned previously, progressiveupper/lower rolls, side rolls, or as overhung spindle rolls(known as cluster roll configurations). These different rollconfigurations are used according to the job that needsto be done.


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Rolled sections


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Flat strip rolling


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Rolling Mills


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Rolling mills


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Cluster mills


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Tube piercing


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Shape rolling


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Hot rolling

Hot rolling is a hot working metalworkingprocess where large pieces ofmetal, such asslabs or billets, are heated above theirrecrystallization temperature and then deformed

between rollers to form thinner cross sections.Hot rolling produces thinner cross sections thancold rolling processes with the same number ofstages. Hot rolling, due to recrystallization, willreduce the average grain size of a metal while

maintaining an equiaxed microstructure whereas cold rolling will produce a hardenedmicrostructure


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Cold rolling

Cold rolling is a metalworking process in which

metal is deformed by passing it through rollers at

a temperature below its recrystallization

temperature. Cold rolling increases the yieldstrength and hardness of a metal by introducing

defects into the metal's crystal structure. These

defects prevent further slip and can reduce the

grain size of the metal. Cold rolling is most oftenused to decrease the thickness of plate and

sheet metal.


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Drawing

Drawing is a metalworking process which uses tensileforces to stretch metal. It is broken up into two types:sheet metal drawing and wire, bar, and tube drawing.The specific definition for sheet metal drawing is that it

involves plastic deformation over a curved axis. For wire,bar, and, tube drawing the starting stock is drawnthrough a die to reduce its diameter and increase itslength. Drawing is usually done at room temperature,thus classified a cold working process, however it may

be performed at elevated temperatures to hot work largewires, rods or hollow sections in order to reduce forces.


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Rod drawing

Bars or rods that are drawn cannot be coiled thereforestraight-pull draw benches are used. Chain drives areused to draw workpieces up to 30 m (98 ft). Hydrauliccylinders are used for shorter length workpieces.

The reduction in area is usually restricted to 20 to 50%,because greater reductions would exceed the tensilestrength of the material, depending on its ductility. Toachieved a certain size or shape multiple passes through

progressively smaller dies or intermediate anneals maybe required.


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Wire drawing

Wire drawing is a metalworking process used to reduce thediameter of a wire by pulling the wire through a single, or series of,drawing die(s).

There are many applications for wire drawing, including electrical

wiring, cables, tension-loaded structural components, springs, paperclips, spokes for wheels, and stringed musical instruments. Althoughsimilar in process, drawing is different from extrusion, because indrawing the wire is pulled, rather than pushed, through the die.

Drawing is usually performed at room temperature, thus classified acold working process, but it may be performed at elevatedtemperatures for large wires to reduce forces. Wires can also bedrawn into different shapes, although this is much more difficult thandiameter reductions. More recently drawing has been used withmolten glass to produce high quality.


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Rod and wire drawing


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Drawing operation


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Tube drawing

Tube drawing is a metalworking process to size tube byshrinking a large diameter tube into a smaller one, bydrawing the tube through a die. This process produceshigh quality tubing with precise dimensions, good

surface finish, and the added strength ofcold working.Because it is so versatile, tube drawing is suitable forboth large and small scale production.

There are five types of tube drawing: tube sinking,

mandrel drawing, stationary mandrel, moving mandrel,and floating mandrel. A mandrel is used in many of thetypes to prevent buckling or wrinkling in the work piece.


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Tube drawing


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Extrusion

Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of thedesired cross-section. The two main advantages of this processover other manufacturing processes is its ability to create verycomplex cross-sections and work materials that are brittle, becausethe material only encounters compressive and shearstresses. It

also forms finished parts with an excellent surface finish.

Extrusion may be continuous (theoretically producing indefinitelylong material) or semi-continuous (producing many pieces). Theextrusion process can be done with the material hot or cold.

Commonly extruded materials include metals, polymers, ceramics,concrete and foodstuffs.


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Hot extrusion

Hot extrusion is done at an elevatedtemperature to keep the material from workhardening and to make it easier to push thematerial through the die. Most hot extrusions are

done on horizontal hydraulic presses that rangefrom 250 to 12,000 tons. Pressures range from30 to 700 MPa (4,400 to 102,000 psi), thereforelubrication is required, which can be oil orgraphite for lower temperature extrusions, or

glass powder for higher temperature extrusions.The biggest disadvantage of this process is itscost for machinery and its upkeep.


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Cold extrusion

Cold extrusion is done at room temperature or near roomtemperature. The advantages of this over hot extrusionare the lack of oxidation, higher strength due to coldworking, closer tolerances, good surface finish, and fast

extrusion speeds if the material is subject to . Materials that are commonly cold extruded include: lead,

tin, aluminum, copper, zirconium, titanium, molybdenum,beryllium, vanadium, niobium, and steel.

Examples of products produced by this process are:

collapsible tubes, fire extinguishercases, shockabsorbercylinders, automotive pistons, and gear blanks.


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Extrusion


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Extrusion


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Extrusion equipment

There are many different variations of extrusion equipment. They vary by four majorcharacteristics:

Movement of the extrusion with relation to the ram. If the die is held stationary andthe ram moves towards it then its called "direct extrusion". If the ram is heldstationary and the die moves towards the ram its called "indirect extrusion".

The position of the press, either vertical or horizontal.

The type of drive, either hydraulic or mechanical.

The type of load applied, either conventional (variable) orhydrostatic.

A single or twin screw auger, powered by an electric motor, or a ram, driven byhydraulic pressure (often used for steel and titanium alloys), oil pressure (foraluminum), or in other specialized processes such as rollers inside a perforated drumfor the production of many simultaneous streams of material.


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Direct extrusion

Direct extrusion, also known as forward extrusion, is themost common extrusion process. It works by placing thebillet in a heavy walled container. The billet is pushedthrough the die by a ram or screw. There is a reusabledummy block between the ram and the billet to keep

them separated. The major disadvantage of this processis that the force required to extrude the billet is greaterthan that need in the indirect extrusion process becauseof the frictional forces introduced by the need for thebillet to travel the entire length of the container. Becauseof this the greatest force required is at the beginning ofprocess and slowly decreases as the billet is used up. Atthe end of the billet the force greatly increases becausethe billet is thin and the material must flow radially to exitthe die. The end of the billet, called the butt end, is notused for this reason.


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Indirect extrusion

In indirect extrusion, also known as backwards extrusion, the billet and containermove together while the die is stationary. The die is held in place by a "stem" whichhas to be longer than the container length. The maximum length of the extrusion isultimately dictated by the column strength of the stem. Because the billet moves withthe container the frictional forces are eliminated. This leads to the followingadvantages:

A 25 to 30% reduction of friction, which allows for extruding larger billets, increasing

speed, and an increased ability to extrude smaller cross-sections There is less of a tendency for extrusions to crack because there is no heat formed

from friction

The container liner will last longer due to less wear

The billet is used more uniformly so extrusion defects and coarse grained peripheralszones are less likely.

The disadvantages are:

Impurities and defects on the surface of the billet affect the surface of the extrusion.These defects ruin the piece if it needs to be anodized or the aesthetics areimportant. In order to get around this the billets may be wire brushed, machined orchemically cleaned before being used.

This process isn't as versatile as direct extrusions because the cross-sectional area islimited by the maximum size of the


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Hydrostatic extrusion

In the hydrostatic extrusion process the billet is completelysurrounded by a pressurized liquid, except where the billet contactsthe die. This process can be done hot, warm, or cold, however thetemperature is limited by the stability of the fluid used. The processmust be carried out in a sealed cylinder to contain the hydrostaticmedium. The fluid can be pressurized two ways:

Constant-rate extrusion: A ram or plunger is used to pressurize thefluid inside the container.

Constant-pressure extrusion: A pump is used, possibly with apressure intensifier, to pressurize the fluid, which is then pumped tothe container.


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The advantages of this process include

No friction between the container and the billet reduces force requirements.This ultimately allows for faster speeds, higher reduction ratios, and lowerbillet temperatures.

Usually the ductility of the material increases when high pressures are

applied. An even flow of material.

Large billets and large cross-sections can be extruded.

No billet residue is left on the container walls.

The disadvantages are:

The billets must be prepared by tapering one end to match the die entryangle. This is needed to form a seal at the beginning of the cycle. Usuallythe entire billet needs to be machined to remove any surface defects.

Containing the fluid under high pressures can be difficult

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Mti Unit III