Bulk Metal Forming

Bulk Deformation

• Metal forming operations which cause significant shape change
• Starting forms: cylindrical bars and billets, rectangular billets and slabs, and similar shapes

Importance of Bulk Deformation

• In hot working, significant shape change can be accomplished
• In cold working, strength can be 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

Four Basic Bulk Deformation Processes

1. Rolling: slab or plate is squeesed 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

Rolling

• which work thickness is reduced by compressive forces exerted by tow opposing rolls

The Rolls

• The rotating rolls perform two main functions

1. Pull the work into the gap between them by friction between workpart and rolls
2. Simultaneously squeeze the work to reduce cross section

Types of Rolling

• By geometry of work
  • Flat rolling
    • Draft: amount of thickness reduction
      • d = t0 - tf
    • Reduction: draft expressed as a fraction of starting stock thickness
      • r = d / t0
    • d = draft
    • t0 = starting tthickness
    • tf = final thickness
  • Shape rolling
    • Work is deformed into a contoured cross-sectiion rather than flat
• By temperature of work
  • Hot rolling
  • Cold rolling

Rollling Mills

• Equipment is massive and expensive
• Two-high: two opposing large diameter rolls
• Three-high: work passes through both directions
• Four-high: backing rolls rupport smaller work rolls
• Cluster mill: multiple backing rolls on smaller rolls
• Tendem rolling mill: sequence of two-high mills

Thread Rolling

• Most important commercial process for mass producting bolts and screws
• Performed by cold working in thread rolling machines
• Advantages over thread cutting
  • Higher production rates
  • Better aterial utilization
  • Stronger threads due to work hardening
  • Better fatigue resistance due to compressive stresses introduced by rolling

Ring Rolling

• 언급만 잠깐 하고 넘어감

Forging

• Deformation process in which work is compressed between two dies
• Components: engine crangshafts, connecting rods, grears, aircraft structural components, jet engine turbine parts

Classification of Forging Operations

• Cold vs Hot forging
  • Hot or warm forging: most common, due to the significant deformation and the need to reduce strength and increase ductility of work metal
  • Cold forging: advantage is increased strength that results from strain hardening
• Impact vs press forging
  • Forge hammer - applies an impact load
  • Forge press - applies gradual pressure

Types of Forging Dies

• Open-die forging
  • work is compressed between two flat dies, allowing metal to flow laterally without constraint
  • Compression of workpart with cylindrical cross-section between two flat dies
• Impression-die forging
  • die surfaces contain a cavity or impression that is imparted to workpart, thus constraining metal flow - flash is created
  • 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 from part
• Flashless forging
  • workpart is completely constrained in die and no excess flash is produced
  • Compression of work in punch and die tooling whose cavity does allow for flash
  • Starting workpart volume must equal die cavity volume within very close tolerance
  • Process control more demanding than imporession-die forging
  • Best suited to part geometries that are simple and symmetrical
  • Often classified as a precision forging process

Impression-Die Forging Advantages and Limitations

• Advantages(compared to machining from solid storck)
  • Higher production rates
  • Conservation of metal(less waste)
  • Greater strength
  • Favorable grain orientation in the metal
• Limitations
  • Not capable of close tolerances
  • Machining often required to achieve accuracies and features needed, such as holes, threads, and mating surfaces that fit with other components

Forging Presses

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

Upsetting and Heading

• Forging process used to form heads on nails, bolts, and similar hardware products
• Cycle
  1. wire stock is fed to the stop
  2. gripping dies close on the stock and the stop is retracted
  3. punch moves forward
  4. bottoms to form the head

Swaging

• Accomplished by rotating dies that hammer a workpiece radially inward to taper it as the piece is fed into the dies

Trimming

• Cutting operation to remove flash from workpart in impression-die forging

Digital Inspection System

• 비접촉식 Digital 형상 측정 기술을 통하여 실물로부터 CAD Data를 생성하거나 CAD Data와 생산품(부품, PP, 양산품 등) 간의 2/3D 기하학적 차이를 비교검증하는 기술
• 측정기를 활용하는 두 가지 기술
• Reverse Engineering
  • 실물로부터 디지털화된 CAD Data 생성: 디지털 복제 기술
  • 제품은 있는데 CAD Model 없음
• Inspection
  • 기준 CAD Data와 실제품의 3차원 측정 데이터간의 신속 비교
  • 2D/3D Metrology, Comparison

Extrusion

• Compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape
• Two basic types of extrusion
  • Direct extrusion
  • Indirect extrusion

Direct Extrusion

• Also called forward extrusion
• This extra portion, called the butt, must be separated

Indirect Extrusion

• Also called backward extrusion and reverse extrusion
• Limitations of indirect extrusion are imposed by the lower rigidity of hollow ram and difficulty in supporting extruded product as it exits die

General Advantages of Extrusion

• Variety of shapes possible, expecially 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

Hot vs. Cold Extrusion

• Hot extrusion
  • prior heating of billet to above its recrystallizaion temperature
  • This 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

Extrusion Ratio

• Also called the reduction ratio
• rx = A0/Af(>1)
  • rx = extrusion ratio
  • A0 = cross-sectional area of the starting billet
  • Af = final cross-sectional area of the extruded section
• Applies to both direct and indirect extrusion

Die Angle

• Low
  • surface area is large, leading to increased friction at die-billet interface
  • Higher friction results in larger ram force
• High
  • more turbulence in metal flow during reduction
  • Turbulence increases ram force required
• Optimum angle depends on work material, billet temperature, and lubrication
• Maximum die angle = 90

Orifice Shape of Extrusion Die

• Simplest cross section shape = circular die orifice
• Shape of die orifice affects ram pressure
• As cross-section becomes more complex, higher pressure and greater force are required(ex: heat sink)

Extrusion Presses

• Either horizontal or vertical
  • Horizontal more common
• Extrusion presses
  • Usually hydraulically driven, which is expecially suited to semi-continuous direct extrusion of long sections
• Mechanical drives
  • Often used for cold extrusion of individual parts

Wire and Bar Drawing

• Cross-section of a bar, rod, or wire is reduced by pulling it through a die opening
• area reduction in drawing r
  • r = (A0-Af)/A0
  • A0 = orifinal area of work
  • Af = final work

Wire Drawing vs. Bar Drawing

• Difference is stock size
  • Bar: large diameter bar and rod stock
  • Wire: small diameter stock - wire sizes down to 0.03mm(0.001 in.)are possible

Drawing Practive and Products

• Drawing practive
  • Performed as cold working
  • Used for round cross-sections
• Products
  • Wire: electrical wire
  • Rod stock for nails, screws, rivets, springs
  • Bar stock: metal bars

Continuous Drawing

• Consisting of multiple draw dies separated by accumulating drums

Features of a Draw Die

• Entry region
• Approach
• Bearing surface
• Back relief
• Die materials

Preparation of the Work for Wire or Bar Drawing

• 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

Metal Forming

Metal Forming

• The tool, usually colled a die, applies stresses that exceed yield strength of metal

Stresses in Metal Forming

• Stresses to plastically deform the metal are usually compressive
• However, some forming processes
  • Stretch the metal(tensile stresses)
  • Other bend the metal(tensile and compressive)
  • Still others apply shear stresses

Material Properties in Metal Forming

• Desirable material properties
  • Low yield strength & high ductility
• These properties are affected by temperature
  • Ductility increases and yield strength decreases when work temperature is raised
• Other factors
  • Straint rate(변형속도) and friction

Bulk Deformation Processes

• Characterized by significant deformations and massive shape changes
• “Bulk” refers to workparts with relatively low surface area-to-volume ratios
• Starting work shapes include cylindrical billets and rectangular bars
• Basic bulk deformation processes: rolling, forging, extrusion, drawing

Sheet Metalworking

• High surface area-to-volume ratio of staring metal
• Basic sheet metalwokring operations: bending, drawing, shearing

Temperature in Metal Foraming

• Any deformation operation can be accomplished with lower forces and power at elevated temperature
• Three Temperature range in metal forming:
  • Cold
  • Warm
  • Hot working

Cold Working

• Performed at room temperature(보통 20도) or slightly above
• Important mass production operations
• Minimum or no machining usually required
  • These operations are near net shape or net shape processes

Advantages of Cold Forming vs. Hot Working

• Better Accuracy, closer toleratnces
• Better surface finish
• Strain hardening(변형 강화) increases strength and hardness
• Grain flow during deformation can cause desirable directional Properties in product
• No heating of work required

Disadvantages of Cold Forming

• Higher forces and power required
• Surfaces of staring workpiece must be free of scale and dirt
• Ductility and strainhardening limit the amount of forming that can be done

Warm Working

• Performed at temperatures above room temperature but below recrystallization temperature
• Dividing line between cold working and warm working often expressed in terms of melting point:
  • 0.3Tm, where Tm = melting point for metal(absolute temperature)

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

How Working

• Deformation at temperature above recrystallization temperature
• Recrystallization temperature = about one-half of melting point on absolute scale
  • In practive, hot working usually performed somewhat above 0.5Tm
  • Metal continues to soften as temperature increases above 0.5Tm, engancing advantage of hot working above this level

Why Hot Working?

• Capability for substaintial plastic deformation
• Why?
  • Strength coefficient is substantially less than at room temperature
  • Strain hardening exponent 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
• 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
  • Advantageous in cases when part is to be subsequently processed by cold forming

Disadvantages of Hot Working

• Lower dimensional accuracy
• Higher total energy required(due to the thermal energy to heat the workpiece)
• Work surface oxidation, poorer surface finish
• Shorter tool life

Friction in Metal Forming

• In most metal forming processes, friction is undesirable
  • Metal flow is retarted
  • Forces and power are increased
  • Wears tooling faster
• Friction and tool wear are more severe in hot woriking

Lubrication in Metal Forming

• Metalworking lubricants are applied to tool-work interface in many forming operations to reduce garmful effects of friction
• Benefits
  • Reduces sticking, forces, power, tool wear
  • Better surface finish
  • Removes heat from the tooling

Considerations in Choosing a Lubricant

• Type of forming processes(rolling, forging, sheet metal drawing, etc.)
• Hot working or cold working
• Work material
• Chemical reactivity with tool and work metals
• Ease of application
• Cost