BULK DEFORMATION PROCESSES IN METAL WORKING

BULK DEFORMATION PROCESSES IN METAL WORKING
Chapter 6 BULK DEFORMATION PROCESSES IN METAL WORKING

Overview of Metal Forming
Wire and bar drawing Extrusion Forging Rolling Performed as cold, warm, and hot working Mainly cold working Sheet Metalworking Bulk Deformation Metal Forming Large group of mfg processes in which plastic deformation is used to change the shape of metal workpieces Deep and cup drawing Shearing Bending

Bulk Deformation (Overview Cont’d)
rolling extrusion Wire/bar drawing forging

Sheet Metalworking (Overview Cont’d)
Deep/cup drawing bending shearing

Formability (workability)
Formability of the material depends on: Process variables ……………… temperature Desirable material properties in metal forming: Low yield strength and high ductility strain rate 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 oxidation at elevated temperatures Tm: melting point of metal (based on absolute temperature scale) Process T/Tm Cold working Warm working Hot working < 0.3 0.3 to 0.5 > 0.6 e.g. lead Tm = 327 C Formed at room temperature (20 C), …………………………………. T/Tm = ( )/( ) = 0.5 Warm working An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the maximum operating temperature (or peak operating temperature). Outside this range of safe operating temperatures the device may fail Most metals strain harden at room temperature according to the flow curve (n > 0)---- elastic + strain hardening But if heated to sufficiently high temperature and deformed, strain hardening does not occur Instead, new grains are formed that are free of strain The metal behaves as a perfectly plastic material; that is, n = ….

Perfectly plastic When the material is heated to sufficiently high temperature, and tension test is conducted the material will exhibit a perfectly plastic behavior Perfectly plastic: once the stress reaches yield stress, Y, it continues to undergo deformation at the same level. When the load is released, the material has undergone permeant deformation; there is no elastic recovery

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 ……...…. with cold work Yield and tensile strength …………… decreases 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 greater strain hardening for a given amount of plastic strain). The influence of cold work on the stress–strain behavior for a low-carbon steel.

Example: Cold Work Analysis
=15.2mm d =12.2mm Copper  • What is the tensile strength & ductility after cold working? % Cold Work 100 3 00 5 7 Cu 2 4 6 sy (MPa) % Cold Work UTS (MPa) 2 00 Cu 4 6 8 % Cold Work ductility (%EL) 2 4 6 Cu s y =300MPa TS=340MPa %EL =7%

Cold Working Advantages of Cold Forming vs. Hot 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) isotropic: Properties of a material are identical in all direction anisotropic: Properties of a material depend on the direction; for example, wood. In a piece of wood, you can see lines going in one direction; this direction is referred to as "with the grain". The wood is stronger with the grain than "against the grain". Strength is a property of the wood and this property depends on the direction; thus it is anisotropic. isotropic materials are useful since they are easier to shape, and their behavior is easier to predict. Anisotropic materials can be tailored to the forces an object is expected to experience. For example, the fibers in carbon fiber materials and rebars in reinforced concrete are oriented to withstand tension

Cold Working Disadvantages of Cold Forming:
Equipment of higher forces and power required to shape material. Surfaces of starting workpiece 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: - ………….. - …………………… - ………………………….. Annealing involves three steps relieve stress [residual stress] increase ductility produce a specific structure

Annealing Material in this condition (cold worked) is annealed, changes will begin to take place. These changes may be classified under three headings: Stress relief Recrystallization 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

Annealing Stress relief:
As the temperature of the material is raised so the vibrational energies of the individual atoms are increased and atomic movements can occur. Comparatively minor atomic movements result in the removal of the residual stresses associated with the locked-in elastic strains . This change, which occur at comparatively low temperature, has a negligible effect on the strength and hardness of the material, and the microstructure of the metal is unchanged in its appearance.

Annealing Recrystallization
When the temperature is raised further, the process of recrystallization begins. New unstressed crystals begin to form and grow from nuclei until the whole of a material has a structure of unstressed polygonal crystals. This change in structure is accompanied by a reduction in hardness, strength and brittleness to the original values prior to plastic deformation.

The driving force for the recrystallization process is the release of strain energy stored in the zones of high dislocation density.[ grain bondaries] The temperature at which recrystallization occurs is, for pure metal, within the range from one-third to one-half of melting temperature (k). Recrystallization temperature is not constant for all material. Why????

Recrystallization temperature is not constant for all material as its value is affected by: The a mount of plastic deformation prior to heating (its lower for very heavily cold worked metals than for samples of the same material which have received small amounts of plastic deformation). The composition (the presence of impurities or alloying elements will increase the recrystallization temperature of the material

Annealing Grain growth If the temperature is raised further, grain growth may occur following the completion of recrystallization, with some crystal grains growing in the size at the expense of others by a process of grain boundary migration or merge between small grain and large grain Small grains have larger GB area than large grains, and Since the dislocations are concentrated in these large GB area, these large GR becomes a high energy area. Consequently, these small grains with (large GB area), will have high energy GB areas. The High energy GB area wants to go to lower energy GB region (large grains). The driving force for grain growth is the release of grain boundary surface energy as the amount of total grain boundary surface is reduced, this will lead to the reduction of grain boundary area

Schematic representation of grain growth via atomic diffusion.
Growth of new grains will continue at high temperature. Grain growth occurs in both metals and ceramics at elevated temperature. Involves the migration of grain boundaries. Large grains grow at expense of small ones (small grains merge). Reduction of grain boundary area (driving force) for grains to grow. is the release of strain energy stored in the zones (grain boundaries) of high dislocation density. Reducing size Schematic representation of grain growth via atomic diffusion. Why do small grains merge with large grain? Small grains have larger GB area than large grains. Since dislocations are concentrated in the GB area, becomes a high energy area. So, small grains (large GB area), have high energy GB areas. High energy GB area wants to go to lower energy GB region (large grains).

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: Lower forces and power than in cold working. More intricate work geometries possible. Need for annealing may be reduced or eliminated.

Hot Working Why 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] 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
Workpart 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 workpiece. Work surface oxidation (scale), ………… surface finish. poor …..……… tool life Shorter

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, forces, power, tool wear Better surface finish Removes heat from the tooling Considerations in Choosing a Lubricant: Type of forming process (rolling, forging, sheet metal drawing, etc.) Hot working or cold working Work material Chemical reactivity with tool and work metals Ease of application Cost

The bulk deformation processes are important commercially and technologically (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

1. Rolling 2. Forging 3. Extrusion 4. Wire and bar drawing

Bulk Deformation Cylindrical billets
Metal forming operations which cause significant shape change by deforming metal parts whose initial form is bulk rather than sheet. Starting forms: Cylindrical billets Rectangular billets, 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

Importance of Bulk Deformation
In hot working, significant shape change can be accomplished at high temperature . 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

Hot Working of Metals Hot working is defined as the process of altering the shape or size of a metal by plastic deformation with the temperature above the recrystallization point. Being above the recrystallization temperature allows the material to complete grain growth during deformation :and to keep the ductility high and hardness and strength low. This is important because being above recrystallization keeps the materials from strain hardening, which ultimately keeps the yield strength and hardness low and ductility high.

Hot Working of Metals TR = recrystallization temperature

Cold Working Cold working is the process of altering the shape or size of a metal by plastic deformation with the temperature below the recrystallization point. Hardness and tensile strength are increased with the degree of cold work ( it becomes brittle depends to cold working percentage) whilst ductility and impact values are lowered. Processes include rolling, drawing, pressing, and extruding, it is carried out below the recrystallization point usually at room temperature. The cold rolling and cold drawing of steel significantly improves surface finish. (no oxides on the surface after operation)

Hot Work vs. Cold Work Hot Work Cold Work
Recrystallization takes place > 0.5 * Tm Requires less force Less residual stresses Greater deformation possible Dimensional Variation [Lower dimensional accuracy] Poor Surface Finish Oxidation of Surfaces Expensive costs for process and equipment Cold Work NO Recrystallization Less than <0.3 Tm Requires more force Residual Stresses Strain Hardened Better Surface Finish No oxides on the surface after operation lower costs for process and equipment

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{has fixed profile}, 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

Rolling

Rolling Rolling is the process of reducing the thickness or changing the cross section of a long workpiece by compressive forces applied through a set of rolls, thus the process is similar to rolling dough with a rolling pin to reduce its thickness. Rolling, which accounts for about 90% of all metals produced by metalworking processes, was first developed in the late of 1500s. The basic rolling operation is called flat rolling, or simple rolling, where rolled products are flat plate and flat sheet

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 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.

Rolling Traditionally, the initial form of material for rolling is an ingot; An ingot is a material, usually metal, that is cast into a shape suitable for further processing [materials prepared in bulk form] Rolling is first carried out at elevated temperature (hot rolling), wherein the coarse-grained, brittle, and porous cast structure of the ingot metal is broken down into a wrought structure, with finer grain size and improve properties An ingot is a material, usually metal, that is cast into a shape suitable for further processing.

Grain Structure During Hot Rolling
Figure 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.

The rolling process (specifically, flat rolling)
Deformation process in which work piece (slab or plate) thickness is reduced by compressive forces exerted by two opposing rolls. The 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 cross section. The rolling process (specifically, flat rolling)

Rolling One of the first primary processes to convert raw material into a finished product. Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls (shown below is flat rolling)

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.

Rolled Products Made of Steel

Rolled Products Made of Steel
Skelp (sometimes spelled scelp) is iron or steel rolled into narrow strips and ready to be made into pipe or tubing by being bent and welded

Rolling Rotating rolls perform two main functions:
Pull the work into the gap between them by friction between work part and rolls Simultaneously squeeze the work to reduce its cross section

Types of Rolling By geometry of work: By temperature of work:
Flat rolling - used to reduce thickness of a rectangular cross‑section Shape rolling - a square cross‑section is formed into a shape such as an I‑beam, structural shape, rails…. By temperature of work: Hot Rolling – most common due to the large amount of deformation required (petroleum and natural gas pipeline, For common structure of construction, bridges, ships and automobiles wheels). Cold rolling – produces finished sheet and plate.

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 Rolling of a ‘slab’ into a plate or sheet
Flat Rolling Hot Rolling of a ‘slab’ into a plate or sheet

Flat Rolling Terminology
Draft = amount of thickness reduction Reduction = draft expressed as a fraction of starting stock thickness: where d = draft; (ho or to) = starting thickness; (hf or tf )=final to of thickness, and r = reduction

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

Shape Rolling

Shape Rolling products
L- beam U- beam I- beam

Flat Rolling Shape Rolling
Sheet steel that undergoes acid pickling will oxidize (rust) when exposed to atmospheric conditions of moderately high humidity For this reason, a thin film of oil or similar waterproof coating is applied to create a barrier to moisture in the air Shape Rolling Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, rust or scale from ferrous metals, copper, and aluminum alloys.[1] A solution called pickle liquor, which contains strong acids, is used to remove the surface impurities. It is commonly used to descale or clean steel in various steelmaking processes

Rolling equipment's A variety of rolling equipment is available having several roll arrangements. Small-diameter rolls are preferable because the smaller the roll radius, the lower will be the roll force. On the other hand, small rolls deflect under roll forces and have to be supported by other rolls to maintain dimensional control

Rolling equipment's 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

Rolling Mill Configurations
( a) Two-high, (b) three-high, (c) four-high

(d) Cluster mill, (e) tandem rolling mill

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 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 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 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 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 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

Thread Rolling (1) start of cycle A deformation process used to form threads on cylindrical parts by rolling them between two dies. Important process Used for mass producing bolts and screws. Performed by cold working in thread rolling machines. Advantages over thread cutting (machining): Higher production rates. Stronger threads due to work hardening. Better fatigue resistance due to compressive stresses introduced by rolling. (2) end of cycle

Miscellaneous rolling operations
Thread Rolling Performed by cold working in thread rolling machines Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies with greater strength, because of the cold working involved. Important process for mass producing bolts and screws Advantages over thread cutting (machining): Higher production rates Stronger threads and better fatigue resistance Better fatigue resistance due to compressive stresses introduced by rolling

Thread Rolling (1) Start of cycle, and (2) end of cycle

Ring Rolling In this process, a small-diameter, thick ring is expanded into a large diameter, thinner ring by placing the ring between two rolls. Because of volume constancy, the reduction in thickness is accomplished for by an increase in the diameter of the ring. This process can be carried out at room temperature ( cold working) or at elevated temperature (hot working), depending on the size and strength of the product. Hot working process for large rings and cold working process for smaller rings.

Ring Rolling The advantages of ring rolling, compared with other processes for making the same part are : Shorter production runs Material saving Close dimensional tolerances Strengthening through cold working. Products: : typical parts made by ring rolling include large rings for rockets and turbines, steel tires for railroad, and rings for pipes

Ring Rolling (1) start, and (2) completion of process

Courtesy of www.fangyuanforging.cn
Courtesy of Courtesy of Courtesy of Courtesy of

Piercing Roll An important hot working process, this process is used to make long, thick-walled seamless tubing. Production of Seamless (not welded) thick-wall tubes. This process is based on the principle that when a round bar is subjected to a radial compressive forces Is carried out by an arrangement of rotating rolls. The axes of the rolls are skewed in order to pull the round bar through the rolls by longitudinal force of their rotary action. A mandrel assists the operation by expanding the hole and sizing the inside diameter of the tube. Because of the sever deformation that the metal undergoes in this process, it is important that the stock be of high quality and defect free.

Piercing Roll

Defects in Rolled parts
Successful rolling practice requires balancing many factors: Material properties Process variables Lubrications Material Parameters – ductility – coefficient of friction – strength, modulus Process Parameters – roller speed – power

There may be defects on the surfaces of the rolled plates and sheets, or there may be structural defects within the material. Structural defects are those that distort or affect the integrity of the rolled product. Some typical defects are Wavy edges are caused by bending of the rolls, whereby the edges of the strip become thinner than at the center, because the edges elongate more than the center [Due to the deflection of the rolls at the center]

Ziper cracks in the center of the strip and edge cracks: are usually caused by low material ductility and barreling edges.

Alligatoring: is a complex phenomena that results from inhomogeneous deformation of the material during rolling ( disturbing equilibrium of the residual stresses)

Surface defect may result from rust, dirty, impurities, and other causes related to prior treatment and working of the material. In hot rolling of bloom, billets, and slabs, the surface is usually preconditioned by various means, such as by scarfing (using a torch).

Wavy edge: are caused by bending of the rolls, whereby the edge of the strip become thinner than at center. Edge cracks Zipper crack in the center of strip Alligatoring: results from inhomogeneous deformation (residual stresses)

Other Characteristics
Residual Stresses can develop in rolled sheets and plates because of inhomogeneous plastic deformation of the material in the roll gap The deformation behavior of inclusion imbedded in the material matrix is one of the basic problem in the mechanics of inhomogeneous deformation Small-diameter rolls or small thickness reductions per pass tend to deform the metal plastically more at its surfaces than in the bulk ,this type of deformation generates compressive residual stresses on the surface and tensile stresses in the bulk. Large-diameter rolls or high reductions per pass tend to deform the bulk more than the surfaces, because of frictional constraint at the surface along the arc of contact. This situation generates residual stresses that are opposite to those of the previous case .

Residual Stresses in Rolling
Figure (a) Residual stresses developed in rolling with small rolls or at small reductions in thickness per pass. (b) Residual stresses developed in rolling with large rolls or at high reductions per pass. Note the reversal of the residual stress patterns.

In hot rolling, if the temperature of the workpiece is not uniform the flow of the material will occur more in the warmer parts and less in the cooler. If the temperature difference is great enough cracking and tearing can occur. Fins may be formed on the rolled bars if the metal forces itself into the clearance between the rolls When the metal is hot rolled, its surface will be not smooth. Cracks may form during cold rolling if the metal becomes too much work-hardened during the process.

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. At the exit of the roll gap, the velocity of the strip is Vf. Constant material volume: ho wo Lo = hf wf Lf  ho wo vo = hf wf vf (flow rate)

Roll Flat Terminology Since Vr is constant along the roll gap, but the strip velocity increases as it passes through the roll gap, sliding occurs between the roll and the strip. At one point along the arc of contact, the two velocity are the same, this point is called neutral point or No-slip point. To the left of this point, the roll moves faster than the workpiece, and to the right, the workpiece moves faster than the roll. Because in rolling, the frictional force on the left of the neutral point is greater than the frictional force on the right This difference results in a net frictional force to the right, making the rolling operation possible by pulling the strip into the roll gap

Flat Rolling --- Friction Forces
The rolling process is governed by the frictional force between the rollers and the workpiece. The frictional force at the entrance side is higher than that at the exit side. This allows the roller to pull the workpiece towards the exit end.

Friction P The figure illustrate the pressure distribution in the roll gap. The neutral point shifts toward the exist as the friction force decreases. The reason is that when friction approaches zero, which means that there is no friction between the roll and strip, so the rolls begin to slip ( no need to pressure to overcome the frictional forces) and the relative velocity between the roll and the strip is all in one direction

Flat Rolling – Terminology
Friction at the entrance controls the maximum possible draft. dmax = Maximum draft (mm) R = Roll radius (mm) m = Coefficient of friction However, μ depends on lubrication, work-piece and roller materials and temperature. When sticking occurs, m can be as high as 0.7

Rolling Analysis hf ho vo vf vr Assumptions: Infinite sheet
q L R = roller radius p = roll pressure L = contact length q = contact angle vr = roll speed ho = initial plate thickness hf = final plate thickness vo = plate entry speed Vf = plate exit speed Assumptions: Infinite sheet Uniform, perfectly rigid rollers Constant material volume: ho wo Lo = hf wf Lf where, Lo = initial plate length Lf = final plate length w = plate width  ho wo vo = hf wf vf (flow rate) The rolls contact the workpiece along an arc defined by θ A change in speed between the roller and workpiece occur along this arc

Friction vo<vl<vf
Maximum draft, which is the thickness reduction, is given as µ2R. Coefficient of friction typically: cold working 0.1 warm working 0.2 hot working 0.4 The reason is that the material at and near the die-specimen interfaces cools rapidly, whereas the rest of the specimen remains relatively hot. Since the strength of the material decreases with increasing temperature, the upper and lower portions of the specimen show a greater resistance to deformation than dose the center

Flat Rolling --- Friction Forces
Increasing friction  increasing forces and power requirements Max. draft is defined as the difference between the initial and final strip thickness or (h0-hf ) . It is a function of the coefficient of friction () and the roll radius (R) The higher the friction and the larger the roll radius, the higher the amount of draft.

Roll Force and Torque Roll force  a force perpendicular to the arc of contact The roll force in flat rolling: a= L/2 L = roll-strip contact length = 𝑅.∆ℎ w = the width of the strip (initial width) Yavg = the average true stress of the strip in the roll gap .

Rolling Analysis True strain Strain rate Apply average flow stress
R– (ho–hf)/2 L q Approximate roll force: F r where N.M per minute = Joule per minte ǂ Joule per second = watt angular velocity Torque estimated by T/roller = ~ 0.5 F L 𝑃𝑜𝑤𝑒𝑟=𝑇∗𝑤 𝑟𝑝𝑚 =0.5𝐿𝐹∗2π𝑁=π𝑁𝐹𝐿 (𝑓𝑜𝑟 𝑜𝑛𝑒 𝑟𝑜𝑙𝑙) Power = P = Tw = 2p N F L (for two rollers)

Power Requirement Power per roll (in S.I. units): _____in KW
F = roll force (N), L in meters Power per roll (in English units)__________ in hp: F = roll force (lb.) L = the roll-strip contact length (ft.) N = the rpm of the roll International System of Units

True strain and average flow stress
ϵ1 = Ln(h1/ho) The average flow stress: Ȳ = ( K . ϵ1n )/ (n + 1) Where ϵ1 : true strain in the roll gab, n: strain hardening exponent 1 The last equation can be used in both cases, i.e. when friction is significant or not.

Roll Force Influenced by: Roll radius Strip width Draft
Coefficient of friction The strength of the material

Example --- Calculation of Roll Force and Power
An annealed copper strip, 9 in. wide and 1.00 in. thick, is rolled to a thickness of 0.80 in. in one pass. The roll radius is 12 in., and the rolls rotate at 100 rpm. Calculate the roll force and the power required in this operation.

The roll strip contact length (L): Determination of average true stress (Yavg) The absolute value of the true strain:

True Stress-True Strain Curves
Figure 2.6 True stress-true strain curves in tension at room temperature for various metals. The curves start at a finite level of stress: The elastic regions have too steep a slope to be shown in this figure, and so each curve starts at the yield stress, Y, of the material. X 103 True Stress (psi X 103 ) Pounds per square inch psi

The value of the true strain: At a true strain of 0.223, the true stress is 40,000 psi Thus, the average true stress (Yavg) is about 26,000 psi At zero strain the true stress is 12,000 psi

The roll force is: The power for the two rolls is: 1 Feet is equal to 12 Inches

A (22.86 –cm) -wide 6061-O aluminum strip is rolled from a thickness of (2.54 cm) to (2.032 cm). for a roll radius of (30.48 cm) and roll rpm of 100, estimate the total power required for this operation?

Typical Values for K and n at Room Temperature

BULK DEFORMATION PROCESSES IN METAL WORKING

Similar presentations

Presentation on theme: "BULK DEFORMATION PROCESSES IN METAL WORKING"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

BULK DEFORMATION PROCESSES IN METAL WORKING

Similar presentations

Presentation on theme: "BULK DEFORMATION PROCESSES IN METAL WORKING"— Presentation transcript:

Similar presentations

About project

Feedback