Plastic Deformation Processes: Rolling, Forging and Extrusion

Materials and Processes

Processes Covered:

• Casting

• Pulverotechnology

• Polymers Processing (Molding, Injection, Inflation…)

• Forging

• Extrusion

• Rolling

• Bending

• Stamping

• Calendaring

• Machining (Turning, Punching, Drilling, Grinding)

• Thermal Cutting

• Welding (MIG, TIG, SER, Laser, SFL…)

• Brazing

• Adhesives

• Mechanical connections (Rivet / Bolts)

• Other processes (Coatings, Prototyping)

The 4 Types of Manufacturing Processes

Manufacturing processes can be categorized into four main types:

1. Molding Processes (Volume Creation):

   • Base material: Liquid

   • Process: Molten material is introduced into a mold cavity and acquires the mold's volume upon solidification.

   • Examples: Metal casting for automotive rims, plastic injection molding for keyboard keys.

2. Deformation Processes (Volume Geometry Alteration):

   • Base material: Solid

   • Process: Material is plastically deformed using compressive forces to achieve a new geometry.

   • Examples: Stamping to produce sinks, rolling to produce railway tracks.

3. Subtractive Processes (Volume Reduction):

   • Base material: Pre-form with a larger volume.

   • Process: Excess material is removed from a pre-form to obtain the desired volume.

   • Examples: Milling or cutting to produce coins.

4. Additive Processes (Volume Addition):

   • Process: Material is progressively added to build the final geometry.

   • Examples: Welding to produce a bicycle frame, 3D printing.

Classification of Mechanical Processes

Mechanical processes can be classified in several ways:

• Based on the final result:

  • Connection Processes (+)

  • Cutting Processes (-)

  • Deformation Processes (=)

• Based on physical principles:

  • Deformation

    • On the Plate

    • In the Volume

  • Cutting

    • Chip Removal

    • Punching

  • Fusion + Solidification

    • Welding

    • Casting

• Based on primary and secondary operations (M. F. Ashby):

  • Primary modeling processes

    • Casting (permanent, non-permanent molds, green sand, injected)

    • Moulding (polymer, glass)

    • Plastic Deformation (forging, extrusion, rolling, stamping, calendering)

    • Material Powdertechnology (applied to metallic and ceramic materials)

    • Special processes (CVD, electroforming, Hand Lay-up)

  • Secondary modelling processes

    • Cutting (machining, turning, punching grinding)

    • Treatments (Thermal) (quench, tempering, aging, mechanical work)

    • Connection (welding, brazing, bolted bonding, riveting, adhesives)

  • Finishing processes

    • Finishing (Polishing, Galvanizing, Anodizing, Painting)

• Classification in Processing and Connection Operations(M. P. Groover):

  • Modeling processes

    • Casting, Molding…

    • Powdertechnology

    • Plastic deformation

  • Improvement of the Mec. Props.

    • Heat treatments

  • Surface processing

    • Super cleaning and treatments.

    • Coatings and deposition

  • Cutting (material removal)

  • Permanent Connection

    • Welding

    • Brazing

    • Adhesive bonding

    • Mechanical connection

    • Bolted connection

    • Riveted connection

Plastic Deformation Processes

Plastic deformation processes include:

• Bending and Profiling

• Calendering

• Stamping

• Rolling

• Forging

• Extrusion (e Wire Drawing)

These processes can be further classified by:

• Temperature: Hot or Cold

• Matrix Type: Open or Closed

• Extrusion Type: Direct or Inverse, Conventional or Hydrostatic, By perfuration, By impact

• Part Geometry: Cylindrical or Conical, Cylindrical parts or Non-cylindrical parts

• Material State: On Volume or On the Plate

Rolling

Rolling Process Definition

Rolling is a plastic deformation process where material is forced between two rotating rollers (cylinders) moving in opposite directions with the same peripheral speed. The distance between the rollers is less than the material's thickness, causing deformation.

ε < 0

ε > 0

Products Obtained by Rolling

• Flat products: thin and thick sheets

• Non-flat products: structural bars, tubes, bars and profiles (U Beams, I Beams, Train Tracks, Thick steel sheet)

Importance of Rolling

Metallic materials in manufacturing undergo rolling operations during processing, making it a fundamental process.

Basic Principle

Frictional forces on the roller/material contact surfaces cause material propulsion. Thickness decreases (ε < 0), while length and potentially width increase (ε > 0).

Applications:

• Production of flat products (thin and thick plates)

• Production of non-flat products (Reins, tubes, bars and structural profiles)

Plate Enlargement During Rolling

Reducing thickness increases width if the sheet's width is comparable to its thickness. However, width increase is negligible when the sheet's width is much greater than its thickness.

Effect of Temperature

Hot Rolling:

• Temperature: T > 0.5 T_f (

• Advantages:

  • Temperature rise

  • Decreased mechanical strength, making plastic deformation easier

  • Increased ductility

  • Less cracking and complex part production

  • Structural modifications (grain size reduction for better mechanical behavior)

• Disadvantages:

  • High energy consumption

  • Poor dimensional control due to thermal cycling

  • Formation of surface oxide layers

Cold Rolling:

• Temperature: T < 0.3 T_f

• Advantages:

  • Production of parts with high mechanical resistance

  • Tight manufacturing tolerances

  • Excellent surface finish

  • Increased mechanical strength of the material

  • Allows good dimensional control

  • Improves the surface quality of products

• Disadvantages:

  • Hardening increases pressure, load, and power requirements

  • Reduced ductility increases cracking risk, especially in complex parts

  • Cold rolling of plates results in preferential grain orientation (anisotropy).

Difference between Hot and Cold Rolling

Typical Defects in Rolling

• Exaggerated bending of the rolling rolls due to separation force. Rollers can be manufactured with axial curvature to compensate (though costly).

• Excessive bending of the rollers

• Opening of the plate in the shape of an 'alligator's mouth'

• Ondulação

• Fendas devido às tensões residuais

• Fissuras provocadas pelas tensões residuais

Profile Rolling

Uses rollers with non-constant cross-sections to create varied part shapes

Ring Rolling

Reduces thickness and increases the diameter of ring-shaped parts through progressive radial compression using main drive and crazy rollers. Edge rollers control part thickness.

Thread Production by Rolling

Improves mechanical properties (strength and toughness) while increasing manufacturing productivity and lowering costs.

Forging

Forging Definition

Forging is a plastic deformation process where compressive forces, exerted by tools actuated by drop hammers or presses, change the shape of a material.

Applications of Forging

Forging enables manufacturing parts with diverse dimensions and shapes across various metal materials. Key industries include transport (automotive, aeronautics, rail, naval), military, industrial machinery, and energy production.

Examples of forged products:

• Axis

• Connecting rods

• MCI pistons

• Sprockets

• Gears

• Train wheels

• Hooks, forks and eyelets

• Lifting systems…

Characteristics of Forged Products:

• Good Quality Ratio - Production Costs

• Good ratio between mechanical strength and weight (i.e. good specific strength)

• Good fatigue resistance

• Good resistance to impact stresses

Forging - Classification

Open Die Forging

• Advantages:

  • Low cost of operation.

  • Suitable for small series manufacture.

  • Good suitability for varied dimensions, geometries and weights.

  • Excellent mechanical properties (good ductility, toughness and fatigue resistance).

• Disadvantages:

  • Limited to simple geometric shapes.

  • Does not allow for tight manufacturing tolerances (requires machining/grinding).

  • Low production rate.

  • Requires specialized workers with expertise.

Closed Die Forging

*Advantages:
* Reduces or eliminates secondary finishing and heat treatment operations
* Optimizes raw material consumption,
* Reduce energy consumption,
* Decrease or negate the costs associated with waste transformation.

• Disadvantages:

  • Complexity of the design of forgings and preforms

  • Increased forging forces,

  • Increased tool complexity.

Minting (Cold Forging)

• A process that is a substitute for Forging.

• The tools are illustrated and attention is drawn to the fine detail and surface finish.

Work Regime Temperature

Advantages of Cold Forging:

• Production of parts with high mechanical resistance;

• Tight manufacturing tolerances;

• Excellent surface finish.

Disadvantages of Cold Forging:

Hardening implies (for high levels of deformation):

• High pressure values developed in the tools;

• High load and power values required of machine tools.

• Reduced ductility decreases material formability and increases the risk of cracking (critical in parts with complex geometry)

Forgeability of Metals, in Decreasing Order

(See table in original document for specific temperature ranges for different metals and alloys)

Forging Defects

• Internal defects caused by an oversizing of the preform

• Forging cavities caused by buckling the web of the part being forged

Forging Forces

• What is the typical qualitative evolution of forging force in closed dies?

Equipment for making forgings:

• Hydraulic Presses

• Mechanical Presses

• Eccentric Mechanical Presses

• Friction Hammer of Fall

Criteria for choosing the presses:

Hydraulic presses

In the case of hydraulic presses, only the peak maximum load is of interest, since at any given moment the force applied by the press is given by

$F = Apiston x Pfluid$

Mechanical presses

In the case of mechanical presses, all the deformation energy is of interest, since in each stroke the press only has available the kinetic energy corresponding to the mass and velocity of its moving elements.

Costs per Part

Typical unit cost of forgings. Note the reduction in preparation costs depending on the number of pieces. From the figure it can be inferred that only for series > 1000 pieces the costs of tools and preparation already have a small incidence.

Cost comparison of a connecting rod manufactured with several processes – forging and casting.

Note that for large quantities forging is more economical. Sand casting is the most economical process for small quantities.

Relative Comparison (Technological Processes)

The same product can be obtained by means of different technological processes,resulting in different mechanical and geometric properties, associated costs, production times, etc.

• Pulverometallurgy

• Forging

• Cutting

Extrusion

Basic Principle

Extrusion is a plastic deformation process where material is forced through a die opening by applying high pressure via a punch, reducing and modifying its cross-section shape.

Applications of Extrusion

Extrusion is employed to manufacture components with diverse geometries across various industries, utilizing a wide range of metallic materials such as steels, aluminum, and copper alloys.

Products Obtained by Extrusion

While rods and tubes with constant cylindrical cross-sections are common, extrusion can produce diverse dimensions and shapes from various metal materials.

Examples:

• Aluminum Window and Door Extrusions

• Aluminium Profile for Windows and Doors

• Shower Room 6082 T6 Extrusions for Ocean Vessels

Cold and Hot Extrusion

Hot Extrusion

Tenviroment < T < Trecrystallization

• Níveis de deformação de material reduzidos;

• Ferramentas robustas;

• No aço (níveis de teor em carbono < 2%).

Cold

Trecrystallization< T < 0.75*Tfusion

• Higher levels of material deformation;

• Titanium alloys and steels with carbon content levels > 2%.

Classification of Extrusion Processes

Direct

The part is extruded in the opposite direction to the punch advance

Inverse

The part is extruded in the same direction as the punch action on the material

• Direct extrusion

• Hydrostatic extrusion

• Reverse Extrusion by Drilling

• Reverse Impact Extrusion

Direct Extrusion

Includes operations where material (1) is forced through the extrusion die (4) in the same direction as the applied load.

Hydrostatic Extrusion

Includes direct extrusion operations where material (1) is surrounded by hydraulic fluid (4) under high pressure inside the extrusion container (2).

Reverse extrusion by perforation

Integrates the set of extrusion operations in which the container is closed and the material is forced out of the die through the punch, that is, in the opposite direction to its advance.

Reverse impact extrusion

This group includes the set of extrusion operations in which the container is closed and the material is extruded through the space between the punch and the container in the opposite direction to its advance.

Lubrication

Is a very important factor in extrusion:

• Favors material flow

• Improves surface finish and process integrity

• Improves product quality

• Reduces extrusion forces

In some processes, glass (fiber or powder) is used, which, when melted, works as a lubricant (Séjournet Process). On metals with a high tendency to adhere to the matrix, a thin jacket of a soft metal (copper or mild steel) is used. In this process, the jacket also protects the surfaces from contamination or oxidation. This process is usually used in very reactive materials.

Direct Extrusion – Defects

The main defects that can occur during a direct extrusion operation are:

• Dart (or arrow) shaped fissures - They result from the existence of tensile stresses along the symmetry line of the region in plastic deformation.

• Sinks

• Surface Cracks - Results from low extrusion speeds and high strain rates

Energy to deform the "beata" (radial flow)

Direct Extrusion

• A - Energy required to accommodate the material to the geometry of the container

• B - Energy required to start extrusion of the material

• C - Energy required to plastically deform the volume of material

• D - Energy required to overcome the friction that develops at the contact interface between the material and the extrusion container.

Calculation of Extruded Parts

1. Determination of the extension to the outlet

   $ε = ln \frac{A<em>0}{A</em>1} = ln R$

2. Determination of the average value of the effective voltage

   $ε_{unif} = +1$

   $σ = K ε^n$

   Cold extrusion

   Q_p  a + bln R

3. Determination of extrusion pressure

   $p = Q<em>p σ</em>{unif}$

   Correction coefficient

   $R = \frac{A<em>0}{A</em>1} = \frac{r<em>0}{r</em>1}$

4. Determination of extrusion force and energy

   $F = p<em>e A</em>0$

Extrusion of Hollow Cross Section Products

The manufacture of extruded parts with a hollow cross-section can be carried out using two different techniques:

• Starting from raw material in the form of a rod where a hole was previously opened that will be maintained during extrusion through a mandrel fixed to the punch. (The hole can come directly from the casting, be made by machining, or by hot drilling).

• Using special extrusion dies, where the raw material is previously divided/segmented at the entrance of the die (in order to create the hollow region of the profile) and then connected by a pressure welding mechanism in welding chambers contiguous to the exit zone of the die. This technique is widely used in the hot extrusion of aluminum and lead alloys.

Wire Drawing

Wire Drawing Principle

In wire drawing, the raw material is forced through a spinneret (designation given to wire drawing dies) by applying a pulling force to the exit. As the raw material crosses the spinneret, it undergoes plastic deformation, giving rise to a product with a smaller cross-section and greater length, with good surface quality and excellent dimensional control.

Applications

Wire drawing is applied in the manufacture of parts that have axial symmetry of revolution, highlighting the production of wire and the reduction of the section of tubular components. Tube production is usually carried out by other technological hot processes, such as extrusion or rolling, with drawing being applied only to reduce diameters.

Zones

Wire drawing dies

are made of tool steel or tungsten carbide to ensure good durability and are made up of four distinct zones:

• Inlet zone – having a slightly larger angle than the drawing angle in order to facilitate the lubrication of the process.

• Drawing zone – having an angle that is usually between 5º and 15º.

• Cylindrical zone – included for manufacturing and maintenance reasons of the matrix, being essential to ensure good dimensional stability to the final product.

• Exit area – with an opening angle opposite to the entry and drawing angles in order to facilitate the exit of the final product.

Wire Drawing - Variants

• Wire drawing

• Tube drawing with Fixed chuck

• Tube drawing with Long chuck

• Tube drawing with floating mandrel