MIAE 311 - Manufacturing Processes - Lecture 9 - Fundamentals of Metal Forming
Introduction
Metal forming is the process of shaping metallic materials through plastic deformation, without removing or adding material. It is a fundamental manufacturing process, offering a wide range of techniques to create various shapes and sizes.
This process relies on the plasticity of engineering materials, allowing them to flow as a solid under applied forces without deteriorating their properties. Plastic deformation is permanent and does not recover when the force is removed, enabling metals to be formed into desired shapes.
Basic Concepts of Metal Forming
Stress-Strain Behavior: Metals deform when subjected to external forces. Initially, the deformation is elastic and reversible. However, when the applied stress exceeds the elastic limit, plastic deformation occurs, leading to a permanent change in shape.
Yield Strength (): Yield strength is the stress level at which permanent deformation begins. For metal forming to occur, the applied stress must exceed the yield strength of the material.
Ductility: Ductility is the ability of a material to undergo significant plastic deformation without fracturing. It is a critical property in metal forming, as it determines the extent to which a metal can be shaped without cracking or breaking.
Strain Hardening (Work Hardening): Strain hardening, also known as work hardening, is the phenomenon where the resistance to further deformation increases as the material undergoes plastic deformation. This effect is due to the increased dislocation density within the material's crystal structure.
Recrystallization: Recrystallization is a process where new, strain-free grains replace the deformed grains at elevated temperatures. This process is crucial in hot working, as it restores the material's ductility and prevents excessive strain hardening.
Grain Structure: Metals are crystalline materials composed of individual grains. The grain size, shape, and orientation influence the metal's properties and flow behavior during deformation. Understanding and controlling grain structure is essential in metal forming.
States of Stress
The state of stress applied during metal forming significantly influences the deformation behavior of the material.
Common classifications of stress states include:
Simple uniaxial tension: Involves pulling force in one direction.
Biaxial tension: Tensile forces applied in two directions.
Triaxial tension: Tensile forces applied in three directions.
Biaxial tension and compression: Combination of tensile and compressive forces in different directions.
Uniaxial compression: Compressive force applied in one direction.
Biaxial compression: Compressive forces applied in two directions.
Triaxial compression: Compressive forces applied in three directions.
Pure shear: Stress state where forces are parallel but in opposite directions.
Simple shear with triaxial compression: Combination of shear stress and compressive forces.
Biaxial shear with triaxial compression: Shear stress applied in two directions combined with compressive forces.
Forming Processes: Independent / Dependent Variables
Independent Variables: These are the aspects of the metal forming process that are directly controlled by the operator or engineer. They include:
Starting material: Type of metal, its composition, and initial properties.
Geometry: Shape and dimensions of the starting material.
Tool/die geometry: Design and dimensions of the tools and dies used in the process.
Lubrication: Type and amount of lubricant used to reduce friction.
Temperature: Temperature of the workpiece and tooling.
Speed: Rate at which the deformation is applied.
Amount of deformation: The extent to which the material is deformed.
Dependent Variables: These are the outcomes or results of the metal forming process, which are influenced by the independent variables. They include:
Force/power requirements: Amount of force and power needed to achieve the desired deformation.
Material properties: Changes in material properties, such as strength, hardness, and ductility.
Exit temperature: Temperature of the workpiece after deformation.
Surface finish: Quality and texture of the workpiece surface.
Precision: Accuracy and dimensional control of the formed part.
Controlling dependent variables requires careful selection and adjustment of appropriate independent variables.
Forming Operations
Metal forming encompasses a wide variety of operations, each designed to achieve specific shapes and properties. These operations include rolling, forging, extrusion, shear spinning, tube spinning, swaging/kneading, deep drawing, wire/tube drawing, stretching, straight bending, and contoured flanging.
Process Modeling
Process modeling, especially with finite element modeling (FEM), is a powerful tool for optimizing metal forming processes. FEM simulates the material's response under various conditions, allowing engineers to predict and improve the outcomes of the forming process.
FEM can simulate material behavior in processes like rolling, forging, extrusion, and casting, providing valuable insights into stress distributions, material flow, and temperature variations.
Additionally, FEM can simulate heat treatments and reduce the need for trial-and-error development, saving time and resources.
General Parameters
Successful metal forming requires thorough material characterization, including:
Strength/resistance to deformation: How much force is required to deform the material.
Conditions at different temperatures: How temperature affects the material's strength and ductility.
Formability limits: The extent to which the material can be deformed without fracturing.
Reaction to lubricants: How the material interacts with lubricants to reduce friction.
Effects of deformation speed: How the speed of deformation affects the material's behavior.
Friction and Lubrication
High forces and pressures are required to deform a material. Friction arises between the workpiece and the tooling, which can significantly affect the process.
In some processes, up to 50% of the energy is spent in overcoming friction. Reducing friction is essential for efficient metal forming.
Lubrication can alter material flow, create or eliminate defects, alter surface finish/dimensional precision, and modify product properties.
Friction Conditions
Metal forming friction differs from friction in mechanical devices due to the high pressures and large deformations involved.
For light, elastic loads, friction is proportional to the applied pressure ( is the coefficient of friction). However, this relationship changes at higher pressures.
At high pressures, friction is related to the strength of the weaker material. The lubricant's ability to reduce friction depends on its ability to maintain a separating film between the surfaces.
Temperature Concerns
Workpiece temperature significantly affects material properties. Temperature influences the material's strength, ductility, and strain hardening behavior.
Increasing temperature generally decreases strength, increases ductility, and decreases strain hardening rate. This is why many forming processes are conducted at elevated temperatures.
Hot working, cold working, and warm working are three main categories of metal forming based on temperature.
Hot Working
Hot working is plastic deformation performed above the recrystallization temperature. Above this temperature, the material continuously recrystallizes, preventing strain hardening.
Recrystallization removes strain hardening effects, allowing for large deformations without cracking.
Hot working may produce oxidation or scale formation on the surface of the workpiece, which needs to be removed.
Engineering properties can be improved through reorienting inclusions or impurities during hot working.
Temperature control is crucial in hot working; non-uniform temperatures can cause cracking due to differential thermal expansion and contraction.
Cold Working
Cold working is plastic deformation performed below the recrystallization temperature. Below this temperature, strain hardening occurs, increasing the material's strength and hardness.
Advantages of cold working include: no heating required, better surface finish/dimensional control/reproducibility, improved strength/fatigue/wear, minimized contamination.
Disadvantages of cold working include: higher forces required, less ductility, surfaces must be clean, intermediate anneals may be required to relieve strain hardening, undesirable residual stresses may develop.
Warm Forming
Warm forming is deformation at temperatures between cold and hot working. It offers a compromise between the two, balancing the benefits and drawbacks of each.
Advantages of warm forming include: reduced loads compared to cold working, increased ductility, fewer anneals required, less scaling/decarburization compared to hot working, better precision/surface finish than hot working.
Isothermal Forming
Isothermal forming is deformation under constant temperature. This is achieved by heating the dies and tooling to the same temperature as the workpiece.
Isothermal forming eliminates cracking from non-uniform surface temperatures and may use inert atmospheres to prevent oxidation.
Bulk Forming Processes
Bulk forming processes involve significant changes in the cross-sectional area of the workpiece. They are typically used to create parts with relatively simple shapes.
Primary processes reduce cast material into slabs, plates, and billets. These are intermediate shapes that can be further processed.
Secondary processes reduce shapes into finished/semi-finished products. These processes create the final shape and dimensions of the part.
Processes reduce thickness or cross-sections while sheet-forming maintains thickness and cross section.
Rolling
Rolling reduces thickness or changes the cross-section of a workpiece by passing it between rotating rolls. It is a widely used process for producing flat products.
Often the first process to convert cast material into a wrought product. Wrought products have improved mechanical properties compared to cast products.
Thick stock becomes blooms, billets, or slabs through rolling.
Hot Rolling and Cold Rolling
Hot rolling requires temperature control to ensure uniform grain size and prevent excessive oxidation.
Cold rolling produces smooth surfaces and accurate dimensions due to the absence of thermal distortion.
Ring Rolling
Ring rolling involves placing one roll through a thick-walled ring and pressing a second roll on the outside. This process expands the ring's diameter while reducing its thickness.
Produces seamless rings with circumferential grain orientation, which provides enhanced strength and fatigue resistance.
Forging
Forging is plastic deformation through localized compressive forces applied through dies. It is one of the oldest and most versatile metalworking processes.
Methods include drawing (elongating the workpiece), upsetting (increasing the diameter), and squeezing in closed impression dies (forming complex shapes).
Open-die Hammer Forging
Open-die hammer forging involves delivering an impact using a mechanical hammer. The workpiece is shaped by manipulating it between the hammer and anvil.
Computer-controlled hammers can provide varying blows, allowing for precise control of the forging process.
Impression-Die Hammer Forging
Impression-die hammer forging uses dies shaped to control metal flow. The workpiece is forced into the die cavity by the hammer blows.
Excess metal squeezes out as flash, which is later trimmed off.
Press Forging
Press forging is used for large or thick products and involves a slow squeezing action. This process provides more uniform deformation and flow compared to hammer forging.
Dies may be heated to improve metal flow and reduce forging forces.
Upset Forging
Upset forging increases the diameter of a workpiece by compressing its length. It is commonly used to form heads on bolts and fasteners.
Extrusion
Extrusion involves compressing metal and forcing it through a shaped die to produce products with a constant cross-section. It is a versatile process for creating complex shapes.
Common metals for extrusion include aluminum, magnesium, copper, and lead.
Wire, Rod, and Tube Drawing
Wire, rod, and tube drawing reduces the cross-section of a material by pulling it through a die. This process uses tensile force to deform the material.
Cold Forming, Cold Forging, and Impact Extrusion
Cold forming, cold forging, and impact extrusion involve squeezing material into shaped die cavities to produce precise parts. These processes are typically used for high-volume production.
Cold heading is a form of upset forging used to form heads on fasteners.
Piercing
Piercing is used to create thick-walled seamless tubing. The process involves deforming a heated billet into a rotating ellipse, which forms a hole in the center.
Other Squeezing Processes
Other squeezing processes include roll extrusion, sizing, riveting, staking, coining, and hubbing. These processes are used for various specialized applications.
Surface Improvement by Deformation Processing
Surface improvement can be achieved through deformation processing techniques such as peening, burnishing, and roller burnishing. These processes improve surface finish, hardness, and fatigue resistance.
Sheet-Forming Processes
Sheet-forming processes involve plane stress loadings and require lower forces than bulk forming. They are used to create parts from thin sheets of metal.
Main categories include shearing, bending, and drawing.
Shearing Operations
Shearing operations involve mechanical cutting without chips, burning, or melting. These processes are used to separate or trim sheet metal.
Includes blanking (cutting a shape out of a sheet), piercing (creating holes), notching (removing material from the edge), and trimming (removing excess material).
Types of Shearing
Types of shearing include simple shearing, slitting, piercing, blanking, lancing, perforating, notching, nibbling, shaving, cutoff, and dinking. Each process is designed for specific cutting operations.
Bending
Bending is plastic deformation about a linear axis with little surface area change. It is used to create curved shapes in sheet metal.
Forming involves multiple bends with a single die to create complex shapes.
Drawing/stretching involves non-linear deformation axes and is used to create deeper shapes.
Springback is the unbending that occurs after deformation due to elastic recovery.
Angle Bending
Angle bending is performed using bar folders and press brakes.
Roll Bending
Roll bending involves continuous three-point bending to create cylindrical or curved shapes.
Drawing and Stretching Processes
Drawing and stretching processes involve plastic flow over a curved axis, forming a 3D part. These processes are used to create complex shapes from sheet metal.
Spinning shapes sheet metal over a male form.
Deep Drawing and Shallow Drawing
Deep drawing and shallow drawing form solid-bottom cylindrical or rectangular containers from sheet metal.
Alternative Forming Operations
Forming with Rubber Tooling or Fluid Pressure replaces one of the dies with hydraulic pressure. This technique is used for forming complex shapes without the need for expensive dies.