ME310

Manufacturing Processes

Rahul Panat

School of Mechanical and Materials Engineering

Washington State University

1

Deformation Processes - II

Week 6

2

OUTLINE

•Rolling Processes

•Different Rolling Operations

•Mechanics of Rolling

•Slab Analysis for Rolling

•Pressure Distribution in Rolling

•Defects in Rolling

ROLLING

• The process of reducing thickness or changing cross-section of a long work-piece by compressive forces applied through a set of rolls.

• Often the first process that is used to convert material into a finished wrought product

• Thick stock can be rolled into blooms, billets, or slabs– Blooms have a square or rectangular cross section (>36 sq. in.)

– Billets are usually smaller than a bloom and can have a square or circular cross sections.

– Can be further rolled into structural shapes

– Slabs are a rectangular solid with a width greater than twice the thickness.

– Can be used to produce plates, sheets, or strips

• Rolling accounts for 90% of all metalworking!!!

DIFFERENT ROLLING OPERATIONS

• Flat rolling

• Shape rolling

http://www.youtube.com/watch?v=6xnKmt_gsLs

Videos:

https://www.youtube.com/watch?v=AuuP8L-WppI -0-1.5min

http://www.youtube.com/watch?annotation_id=annotation_280168&feature=iv&src_vid=AuuP8L-WppI&v=k6iODHla6qY 0-1min

BASIC ROLLING PROCESS

• Metal is passed between two rolls that rotate in opposite directions

• Friction acts to propel the material forward

• Metal is squeezed and elongates to compensate for the decrease in cross-sectional area

MECHANICS OF ROLLING

• Friction is important

• Neutral point– Vr=Vworkpiece (no slip

point)

– Direction of friction is different across the neutral point

– Desired to be close to exit

Forward slip:

Frictional forcesDraft, h0 – hf = m2R

R

http://www.youtube.com/watch?v=Kg1FIIaEwEE

SLAB ANALYSIS OF ROLLING

Stresses acting on an element in rolling: (a) entry zone and (b) exit zone

Rolling pressure increases with – Strength of material– Coefficient of friction– R/hf ratio– Thickness reduction

L: Roll strip contact lengthw = Width of the stripYavg = Average true stress of the strip in the roll gap

N: Revolutions per minute of the rolls

PRESSURE DISTRIBUTION IN ROLLING

Pressure distribution in the roll gap as a function of the coefficient of friction. Note that as friction increases, the neutral point shifts toward the entry. Without friction, the rolls will slip, and the neutral point shifts completely to the exit.

Pressure distribution in theroll gap as a function ofreduction in thickness. Notethe increase in the areaunder the curves withincreasing reduction, thusincreasing the roll force.

Pressure distribution as a function of front and back tension in rolling. Note the shifting of the neutral point and the reduction in the area under the curves (hence reduction in the roll force) as tensions increase.

CONTROL OF ROLLING THICKNESS

DEFECTS IN ROLLING

Wavy edges

Zipper cracks in the center of strip

Edge cracks

Alligatoring

Residual stress

The effect of roll radius on the typeof residual stresses developed inflat rolling: (a) small rolls and/orsmall reduction in thickness; and(b) large rolls and/or largereduction in thickness.

ROLLING PROCESSES

Roll Forging

Skew Rolling

Thread Rolling

QUESTIONS?

REVIEW QUESTIONS

• Rolling is a bulk deformation process in which the thickness of the work is reduced by compressive forces exerted by two opposing rolls: – (a) true

– (b) false

REVIEW QUESTIONS

• The maximum possible draft in a rolling operation depends on which of the following parameters (two correct answers): – (a) coefficient of friction between roll and work,

– (b) roll diameter,

– (c) roll velocity,

– (d) stock thickness,

– (e) strength coefficient of the work metal

Draft, h0 – hf = m2R

EXTRUSION

• Metal is compressed and forced to flow through a shaped die to form a product with a constant cross section

• May be performed hot or cold

• A ram advances from one end of the die and causes the metal to flow plastically through the die

• Commonly extruded metals: aluminum, magnesium, copper, and lead

RAM

TYPES OF EXTRUSION

Direct Extrusion Indirect Extrusion

Hydrostatic extrusion Impact extrusion

ADVANTAGES OF EXTRUSION

• Many shapes can be produced that are not possible with rolling

• Amount of reduction in a single step is only limited by the equipment, not the material or the design

• Dies are relatively inexpensive

• Small quantities of a desired shape can be produced economically

Video Time!Direct Extrusion

http://www.youtube.com/watch?v=9MqzKihSrFIImpact Extrusio

http://www.youtube.com/watch?v=zMhVPId-pRA

EXTRUSION OF HOLLOW SHAPES

Mandrels may be used to produce hollow shapes or shapes with multiple longitudinal cavities

(a) the mandrel and ram have independent motions; (b) they move as a single unit.

http://www.youtube.com/watch?v=MfMXwhs0gTk 2-4 min

EXTRUDED PRODUCTS

(a)-(c) Examples of extrusions and products made by sectioning them.Source: Kaiser Aluminum. (d) Examples of extruded cross-sections. Source:(d) Courtesy of Plymouth Extruded Shapes.

FORCES IN EXTRUSION PRESSURE

Lubrication is important to reduce friction and act as a heat barrier

Metal flow in extrusion◦ Flow can be complex

◦ Surface cracks, interior cracks and flow-related cracks need to be monitored

◦ Process control is importantDiagram of the ram force versus ramposition for both direct and indirectextrusion of the same product. The areaunder the curve corresponds to the amountof work (force x distance) performed. Thedifference between the two curves isattributed to billet-chamber friction.

5.12.1 ,8.0

)ln( 0

ba

A

AbaYp

f

ROD OR WIRE DRAWING

A process in which the cross-sectional area of a bar or tube is reduced or changed in shape by pulling it through a converging die.

Tube drawing animation

http://www.youtube.com/watch?v=QKAg1yMZIpY

Tube cold drawing process

http://www.youtube.com/watch?v=zlueIHudt4k

]3

2)ln()1[( 0

m

f

dA

AY

TUBE DRAWING OPERATION

Examples of tube-drawing operations, with and without an internal mandrel.Note that a variety of diameters and wall thicknesses can be produced fromthe same tube stock (that has been produced by other processes, such asextrusion

ROTARY SWAGING

A process in which a rod or tube is reduced in diameter by the reciprocating radial movement of two or four dies (external hammers)

– Reduce diameter

– Produce taper or add point

Usually limited to a diameter of 2 in.

(a) Schematic of rotary-swaging process. (b) Forming internal profiles ina tubular workpiece by swaging.

SWAGING OPERATIONS

Reduction of outer and inner diameters of tubes by swaging. (a) Free sinkingwithout a mandrel. The ends of solid bars and wire are tapered (pointing) bythis process in order to feed the material into the conical die. (b) Sinking on amandrel. Coaxial tubes of different materials can also be swaged in oneoperation.

Swaging process animationo http://www.youtube.com/watch?v=ilL8ViUDUKc

SWAGED PRODUCTS

(a) Typical cross-sections produced by swaging tubular blanks with aconstant wall thickness on shaped mandrels. Rifling of small gunbarrels also can be made by swaging. (b) Typical parts made byswaging. Source: Courtesy of J. Richard Industries.

SUMMARY

• There are a variety of bulk deformation processes

• The main processes are – Rolling, forging, extrusion, and drawing

• Each process has its limits and advantages as to capabilities

• The correct process depends on the desired shape, surface finish, quantity, etc.

• Therefore, its important to understand each process’- Operation principles

- Mechanics fundamentals

- Limits and advantages