Metal forming summary
1. What main types of deformations do you know? Describe them.
a. Elastic deformation: reversible deformation, the bonds become longer but return to initial
shape. The volume is not constant during this deformation.
b. Plastic deformation: permanent deformation, the bonds become longer and planes are
sliding, then the planes remain in the new position. The volume is constant during this
deformation.
2. Engineering & true mechanical quantities
a. Stress:
i.
Explanation: Stress is the internal force within a material that resists deformation
when subjected to an external load. It is measured in force per unit area (e.g.,
Pascals, Pa).
ii.
Formula: Stress (σ) = Force (F) / Area (A)
iii.
Formula for Hooke's Law (in the elastic range): Stress (σ) = Young's Modulus (E)
× Strain (ε)
b. Strain:
i.
Explanation: Strain measures how much a material deforms in response to
applied stress. It is a dimensionless quantity, typically expressed as a fraction or
percentage.
ii.
Formula: Strain (ε) = (Change in Length (ΔL)) / Original Length (L)
c. Hooke's Law:
i.
Explanation: Hooke's Law describes the linear relationship between stress and
strain for materials within their elastic deformation range. It states that stress is
directly proportional to strain.
ii.
Formula: Stress (σ) = Young's Modulus (E) × Strain (ε)
d. Shear:
i.
Explanation: Shear stress is the force that acts parallel to the surface of a
material, causing deformation by displacing one layer of the material relative to
another.
ii.
Formula: Shear Stress (τ) = Force (F) / Area (A) perpendicular to the applied
force
iii.
Formula for Shear Strain: Shear Strain (γ) = Tangential displacement (Δx) /
Height (h)
iv.
In elastic state: Shear Stress (τ) = G × engineering shear strain (γ)
e. Torsion:
i.
Explanation: Torsion is the twisting of a structural component, like a rod or shaft,
due to a torque or twisting force. It is characterized by a shear stress acting in a
circular cross-section.
ii.
Formula for Torsional Shear Stress: Torsional Shear Stress (τ) = (Torque (T) ×
Distance from center (r)) / Polar Moment of Inertia (J)
iii.
Formula for Torsional Shear Strain: Torsional Shear Strain (θ) = Angle of twist (φ)
/ Length (L)
f. Yield Stress:
i.
Explanation: Yield stress is the stress at which a material begins to exhibit plastic
(permanent) deformation when subjected to an external load.
ii.
Formula: Yield Stress (σ_y) is typically determined from stress-strain data in a
tensile test and represents the stress at the point where plastic deformation
begins.
g. Tensile Strength:
i.
Explanation: Tensile strength is the maximum stress that a material can
withstand before it fractures or breaks under a tensile (pulling) load.
ii.
Formula: Tensile Strength (σ_t) is determined from the maximum stress reached
during a tensile test. It represents the material's ultimate strength.
h. Contraction:
i.
Explanation: Contraction, often expressed as a percentage, measures how much
a material contracts in a cross-sectional area during a tensile test.
ii.
Formula: Contraction (%) = [(Initial Area - Final Area) / Initial Area] × 100
i. Elongation:
i.
Explanation: Elongation, expressed as a percentage, measures how much a
material extends or stretches during a tensile test before it fractures.
ii.
Formula: Elongation (%) = [(Final Length - Initial Length) / Initial Length] × 100
3. Stress-strain curves
a. Linear region: elastic deformation
b. End linear region to ultimate strength: uniform plastic deformation
c. Ultimate strength to failure: necking, non-uniform/localized plastic deformation
4. Define the equivalent stress and strain.
a. Equivalent Stress: In metal forming, it is a measure that combines various stress
components (like normal and shear stresses) into a single value that represents the
material's response to deformation.
i.
Formula: σ_von Mises = √(σ₁² + σ₂² + σ₃² - σ₁σ₂ - σ₂σ₃ - σ₃σ₁)
b. Equivalent Strain: Similar to equivalent stress, it combines different strain components
into a single value representing the overall deformation in a material.
i.
Formula: ε_von Mises = √(ε₁² + ε₂² + ε₃² - ε₁ε₂ - ε₂ε₃ - ε₃ε₁)
5. Define the flow stress and draw a cold forming and a hot forming flow curve.
a. Flow Stress is the stress at which a material begins to plastically deform. It is a critical
parameter for understanding the material's behavior during metal forming.
b. Cold Forming Flow Curve: For cold forming, the flow stress typically increases rapidly
with strain due to strain hardening.
c. Hot Forming Flow Curve: For hot forming, the flow stress tends to decrease with strain as
the material softens at high temperatures.
6. Describe a method for measuring the flow curve (use figure).
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a. A typical method for measuring the flow curve is through tensile testing. A sample of the
material is pulled until it plastically deforms, and stress-strain data is collected and used
to determine the flow curve.
Describe the Coulomb friction model, and what are the limitations?
a. The Coulomb friction model describes friction as a force opposing motion proportional to
the normal force pressing the surfaces together. It assumes a constant coefficient of
friction.
b. Limitations of Coulomb Friction Model:
● It oversimplifies real-world friction, which can vary with speed, temperature, and
surface roughness.
● It doesn't account for the effects of lubrication, adhesion, or surface treatments.
Describe the Kudo (shear) friction model, and what are the limitations?
a. The Kudo (Shear) Friction Model considers the frictional force to be related to the shear
stress at the contact point between the tool and workpiece. It can better account for
complex frictional behavior.
b. Limitations of Kudo Friction Model:
● It still simplifies complex real-world friction behavior.
● Accurate determination of parameters can be challenging.
Describe a method for measuring the friction coefficient (use figure).
a. A common method involves conducting strip drawing tests or ring compression tests.
These tests provide data that can be used to calculate the coefficient of friction between
the tool and workpiece surfaces.
What are the roles of the lubricant? What requirements must be met?
a. Roles of Lubricants:
i.
Reducing Friction: Lubricants are used to minimize friction between the
workpiece and forming tools. Reduced friction helps in smoother deformation and
minimizes wear and tear on the tools.
ii.
Preventing Galling: Galling is a form of adhesive wear where metal from the
workpiece transfers to the tool surface. Lubricants act as a barrier to prevent this
transfer.
iii.
Improving Surface Finish: Lubricants can enhance the surface finish of the
formed part by reducing the likelihood of scratches or other surface defects.
iv.
Cooling: In some cases, lubricants also have cooling properties that can help
dissipate heat generated during the forming process.
b. Requirements for Lubricants:
i.
Adequate Lubrication: The lubricant must provide sufficient coverage to reduce
friction and prevent galling.
ii.
Compatibility: Lubricants should be compatible with both the material being
formed and the tools.
iii.
Ease of Application and Removal: They should be easy to apply to the workpiece
and should not leave residues that are difficult to remove.
iv.
Environmental Considerations: Lubricants should be safe for the environment
and for workers using them.
What factors limit the formability of the metals?
a. Yield Strength: Higher yield strength makes it more difficult to deform a metal without
causing plastic deformation.
b. Tensile Strength: The ultimate tensile strength determines the point at which the metal
will fail under tension.
c.
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Ductility: Low ductility restricts the amount of plastic deformation a metal can undergo
without fracturing.
d. Strain Hardening: Strain hardening can make it difficult to continue deforming the metal
after a certain point.
e. Temperature: Some metals become more formable at elevated temperatures (hot
forming) but may be less formable at low temperatures (cold forming).
What is plastic instability? Explain the process. What are the influencing factors?
a. Plastic instability refers to a phenomenon in metal forming where localized necking or
thinning occurs in a material. This is often a precursor to fracture or tearing. The process
involves:
i.
Necking: As a result of non-uniform deformation, some areas of the material
begin to narrow or neck down.
ii.
Thinning: The narrowing of the material is accompanied by a reduction in
thickness.
iii.
Strain Localization: The deformation becomes concentrated in these localized
areas, and strain increases significantly.
iv.
Fracture: Eventually, if not controlled, plastic instability leads to material fracture,
causing failure in the forming process.
b. Influencing Factors for Plastic Instability:
i.
Material Properties: The material's strength, ductility, and strain hardening
behavior can significantly influence the occurrence of plastic instability.
ii.
Forming Process Conditions: Factors such as the rate of deformation,
temperature, and lubrication can affect the likelihood of plastic instability.
iii.
Tool Geometry: The design of the forming tools and their interaction with the
workpiece can either promote or mitigate plastic instability.
iv.
Initial Material Thickness: Thinner materials are more susceptible to plastic
instability, especially if subjected to excessive deformation.
v.
Stress Concentrators: The presence of notches, holes, or other stress
concentrators can intensify plastic instability.
Draw a forming limit diagram (FLD) for sheet forming techniques.
Describe the factors used to characterize the anisotropy of sheet metals?
a. R-Value: R-value measures the anisotropy of a material by comparing the resistance to
deformation along the rolling direction (RD) and the transverse direction (TD). It's
expressed as (ε_RD / ε_TD), where ε represents strain.
b. Yield Locus: This graphical representation of a material's behavior under different loading
directions helps characterize its anisotropy.
c. Planar Anisotropy Index: This index quantifies how a material's properties vary in different
planes. It helps to understand how the material behaves in various directions.
d. Earling Ratio: The earling ratio is used to measure the earing tendency of a material,
which is the development of ears or ripples during deep drawing.
What is the meaning of the Lillet diagram?
a. The Lillet Diagram is a graphical representation of the relationship between the forming
limit diagram (FLD) and the mechanical properties of a material. It helps in predicting the
formability and failure modes of a material during sheet metal forming processes.
Compare the solid and liquid lubricants by their properties and field of application.
a. Solid Lubricants:
i.
Properties: Solid lubricants are typically in powder or solid form and are dry to the
touch.
ii.
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Applications: Commonly used in cold forming processes such as drawing,
extrusion, and stamping.
b. Liquid Lubricants:
i.
Properties: Liquid lubricants are in liquid or semi-liquid form and provide good
coverage of the workpiece.
ii.
Applications: Used in a wide range of metal forming processes, both hot and
cold, including rolling, forging, and deep drawing.
Describe shortly the steps of surface treatment of the workpiece prior to and after the forming.
a. Before Forming:
i.
Cleaning: Remove contaminants, such as oils, rust, and dirt.
ii.
Degreasing: Eliminate residual oils and greases.
iii.
Pickling: Remove oxides and scale.
iv.
Phosphating: Apply a phosphate coating to improve paint adhesion.
b. After Forming:
i.
Cleaning: Remove residues and contaminants.
ii.
Passivation: Prevent further oxidation and improve corrosion resistance.
iii.
Surface Coating: Apply paints, coatings, or other protective layers.
Describe the steps of the equilibrium calculation method.
a. Define the system and its components.
b. Determine the phase equilibrium conditions (pressure and temperature).
c. Calculate the chemical potential for each component.
d. Use the chemical potential to establish phase equilibrium.
What are the steps of the energy calculation method?
a. Determine the initial and final states of the system.
b. Calculate the work done on the system (e.g., deformation work) and the heat transfer.
c. Apply the first law of thermodynamics to calculate the change in internal energy.
d. Use the internal energy change to determine the change in temperature.
Describe the technique of extrusion by a schematic figure and name the main elements.
a. Extrusion is a metal forming process where a material (usually metal) is forced through a
die to create a product with a specific cross-sectional shape. Key elements include:
i.
Billet: The raw material.
ii.
Container: Holds the billet.
iii.
Ram: Exerts pressure on the billet.
iv.
Die: Shapes the material.
v.
Extruded Product: The final shape obtained after extrusion.
b. This process is often used to create long, continuous shapes like rods, tubes, and
profiles.
Draw sketches on the extrusion methods.
Calculate the force needed to extrude forward a cylinder with the given data.
Calculate the force needed to extrude backward a cup with the given data.
List the possible extrusion defects and give a solution for them.
a. Surface Cracking:
i.
Cause: Excessive friction or temperature, or improper die design.
ii.
Solution: Adjust the die and lubrication, control the material temperature, and use
a proper alloy.
b. Seam Formation:
i.
Cause: Poor billet quality, inadequate material flow, or excessive friction.
ii.
Solution: Ensure high-quality billets, optimize material flow, and reduce friction
through proper lubrication.
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l.
Piping or Centerline Cracking:
i.
Cause: Inclusions or voids in the billet material.
ii.
Solution: Use high-quality billet material with fewer inclusions, or remove defects
by crop cutting.
Surface Voids or Blisters:
i.
Cause: Gas entrapment during extrusion.
ii.
Solution: Ensure proper billet heating and degassing to minimize gas
entrapment.
Die Lines and Extrusion Lines:
i.
Cause: Die wear or improper die design.
ii.
Solution: Maintain the die properly, choose appropriate die materials, and
optimize the design.
Barrel Deflection or Swelling:
i.
Cause: Excessive pressure or temperature.
ii.
Solution: Control pressure and temperature within the recommended limits.
Extrusion Deflection:
i.
Cause: Inadequate die support or improper material flow.
ii.
Solution: Provide proper die support and optimize the billet and die design.
Surface Roughness:
i.
Cause: Poor die finish, inadequate lubrication, or excessive temperature.
ii.
Solution: Improve die surface finish, apply effective lubrication, and maintain
proper temperature control.
Size Variations:
i.
Cause: Variations in billet quality or process parameters.
ii.
Solution: Ensure consistent billet quality and tightly control process parameters.
Hollow Extrusions:
i.
Cause: Inadequate material flow to the center during extrusion.
ii.
Solution: Optimize die design and material flow to ensure proper filling of the
extrusion cavity.
Non-uniform Wall Thickness:
i.
Cause: Inconsistent material flow or die design.
ii.
Solution: Improve die design and optimize material flow for uniform wall
thickness.
Die Chipping or Breakage:
i.
Cause: Excessive die wear or high extrusion pressures.
ii.
Solution: Regularly maintain and replace dies as needed. Optimize the extrusion
process to reduce pressure.