Metal Forming: Principle, Process, Application, Advantages and Disadvantages.

What is Metal Forming?

Metal forming, also called material forming or forming, is a commonly employed manufacturing process that produces shaped metal workpieces by plastic deformation of the material without removing any material.

In essence, metal forming uses the plastic deformation of the material by applying stresses greater than the yield strength and lower than the ultimate tensile strength so that the metal takes on a permanent shape.

Metal forming is different from processing that removes material, such as machining (turning, milling, drilling), or processes that combine materials, such as welding.

Metal forming is a broad classification of processes, that encompasses many forming techniques, each suited for specific applications and specific micro-structure properties for the material being processed.

These processes are important in manufacturing a large product base which features products that can range in size from tiny electronic components to extremely large structures in the aerospace and automotive industries.

Fundamental Principles of Metal Forming

Metal forming fundamentally consists of the control of a force to plastically deform a material. The principle concepts involved in metal forming are:

• Yield Strength: The stress at which a material plastically deforms. Plastic deformation is permanent
• Tensile Strength: The maximum stress that a material can withstand before it “necks” and ultimately fractures. Any metal forming operation should remain below this stress to avoid failure.
• Ductility: The ability of a material to plastically deform without fracturing; highly ductile materials are generally the easiest to metal form.
• Work Hardening (straining hardening): A metal will change its internal structure when plastically deformed, causing a increase in the strength and hardness of the metal; this could be both beneficial (to strengthen the part) and an issue (it will take more force to deform the part again).
• Temperature: While processing metal at varying temperatures can greatly affect how the material is deformed, there are two key types of deformation to understand and be aware of:
  • Cold Forming: Typically performed at room temperature, cold forming will have decent dimensional control, good surface finish, and increase material strength due to work hardening. However, it also tends to require huge forces to be used due to less ductility of the material.
  • Hot Forming: Hot forming takes place above the recrystallization temperature, and will not only reduce necessary force, but can allow for large deformations due to the higher ductility of the material and reduced work hardening. On the downside, oxidation and a decreased surface finish, and dimensional control can happen when using this method.

Metal Forming Processes.

#1. Forging

Forging is considered to be one of the earliest forms of metalworking, and has been in practice for thousands of years. In forging, a compressive force is applied to runway from no deformation in the metal into the desired shape. This deformation style is primarily accomplished via hammering or pressing, and is generally done at a range of temperatures:

• Hot Forging: While hot forging can be performed at various temperatures, it is typically completed over the metal’s recrystallization temperature (while still below liquidus). Hot forging yields extremely large deformations with relatively small forces, increases ductility by refining the metal’s grain structure, as well as mechanical properties such as strength and toughness. Uses include, but are not limited to: components of engines, large bearings, and cutting tools.

• Cold Forging: Typically cold forging is completed at room temperature, and it provides greater artificial hardness and strength through work hardening in addition to providing great surface finishes and tolerances. However, because of increased forces to the given amount of deformation, the ductility of the given piece will be limited.

There are some common sub-processes of forging:

• Open (die) forging: the piece is not entirely enclosed by the dies that produce the forced deformation, providing a greater degree to form oversize pieces.
• Closed (die or impression-die) forging: the piece is completely enclosed by the dies making much simpler to produce complex geometries with relatively minimal waste of material.

#2. Rolling

Rolling is considered a metal forming process in which a metal billet, slab, or sheet is passed between a pair of rotating rolls. The rolls, turning in opposite directions, apply compressive forces to the material, reducing its thickness and increasing its length, thereby changing the shape of the material to a desired cross-section.

• Hot Rolling: Hot rolling occurs at hot temperatures, and it is intended to provide maximum thickness reduction from the form shown to its final cross-sectional shape. Hot rolling is the process for producing structural steel sections, railroad tracks, and the initial breakdown of ingots to slabs or billets. The heat produced allows the metal to become more ductile and deform significantly.
• Cold Rolling: Cold rolling occurs at room temperatures (at temperatures below the recrystallization temperature). Cold rolling provides the best surface finish, dimensional accuracy, and strength (through work hardening) than any other metal forming process. Cold rolling is often used to produce sheet metal, strips, and foils with tighter tolerances and with a wider range of surface finishes.

#3. Extrusion

Extrusion is characterized by the movement of a metal billet through an opening (die opening) where a cross-section is uniform and continuous. The extruded component may be a long component, such as rod, bar, tube, or a more complex profile, making this an ideal technique for process engineers. 

• Direct Extrusion: also know as forward extrusion – the ram and the billet (work material) travel in the same axial direction, pushing the metal through the stationary die.
• Indirect Extrusion: also known as backward extrusion – the die travels towards the stationary (non-moving) billet and the metal flows back around the ram. Indirect extrusion has the ability to produce longer lengths of extrusions and reduce friction.
• Hydrostatic Extrusion: the billet is immersed in a pressure fluid, which transmits the pressure to the billet and aids in pushing the billet through the die. This approach alleviates friction and enables brittle materials to be extruded.

Common applications include aluminum window frames and doors, piping, and other customized shapes.

#4. Drawing

Drawing is similar in concept to extrusion. The difference is that in drawing, the metal is pulled through a die instead of pushed through the die. Drawing is usually reserved for the processing of metals into wire, rods and tubes of reduced cross-section and with good surface finish.

• Wire Drawing: The reduced diameters of wires are usually produced by drawing, with the wire reduced through a series of dies.
• Rod Drawing: This operation is similar to wire drawing, but for larger diameter rods.
• Tube Drawing: Another version of drawing is tube drawing, which reduces the tube diameter and/or wall thickness.

Drawings offer some advantages over other processes in the manufacturing of parts i.e. an opportunity for extreme tight-tolerances, good mechanical properties.al properties (hardness and durability), and is commonly used for round shapes like pots and pans.

#5. Bending

Bending is a sheet metal forming procedure that exerts a force onto a sheet metal part to create a bend or angle. This is a primary operation for forming flanges, channels, and many different types of structural elements.

There are many methods of bending:

• V-bending: Uses a V-type die and punch to form angles, applicable for many types of bend angles.
• U-bending: Creates a U-shape within the sheet metal.
• Wiping Dies (Edge Bending): The sheet metal is held down on one side, with a punch being used to wipe/form the metal around the die edge creating the bend.
• Air Bending: The punch pushes the material into the die, but the fact that the metal/base does not touch the bottom of the die will afford the operator more control over the bend angle.
• Bottoming: The metal is pushed to the bottom of the die, this minimizes springback of the metal and affords the operator more accurate angles.
• Coining: Applying a lot of pressure very locally will create a well-defined bend with very low springback, especially useful when detailing small components.

#6. Deep Drawing

The deep drawing process is a direct sheet metal forming process in which a punch draws a flat sheet metal blank into a die cavity to produce a hollow, cup-shaped or box-shaped part. The depth of the formed part is generally more than the radius of the original blank.

Products made via the deep drawing process include beverage cans, sinks for the kitchen, automotive oil pans, and many domestic components. Advantages to the deep drawing process are many, including:

• High production speed for large runs.
• Added strength and durability due to work hardening.
• Cost-effectiveness in high-volume production no-labor & material waste.
• Complex geometry can be formed at once rather than with other operations and welding.
• Excellent finish with often little need for secondary processing.

#7. Stamping

Although stamping is a generic term describing sheet metal operations performed in presses with die, it is an extremely flexible and effective process that has gained popularity as a method for high-volume fabrication of parts with unique geometries from flat sheets or coils of metal.

Common Stamping Operations:

• Blanking: Cutting out a piece of whatever desired shape from a larger sheet metal blank. The cut-out piece is the desired part.
• Piercing (Punching): Putting holes into a sheet metal part. The removed material (slug) is scrap.
• Bending: Described above, forming angles and curves.
• Coining: Imprinting fine details or features into the surface of the metal using increased pressure.
• Progressive Die Stamping: A manufacturing method that is extremely high-volume, where a strip of metal is fed through a series of dies and each die performs operations to complete the part (cut, bend, form, etc.).

#8. Shearing

Shearing is a process in which chips are not formed, and the sheet metal is separated into geometries of the designer’s choosing. Shearing occurs with a pair of blades. One blade is stationary (die), the other blade (punch) is a moving part that will be cut up with the tool. Shearing operations include:

• Blanking: A blank is cut out of a larger sheet to make a complete part.
• Punching: Holes or cutouts are created within a sheet.
• Notching: Some smaller portion is removed from the edge of the sheet.
• Slitting: Longitudinal cuts made on coils of sheet metal that produces strips of a narrower width.
• Cutoff: A piece of metal is separated along a single line.

Shearing is performed before many other metal forming processes and is regarded as a clean operation.

#9. Hydroforming.

Hydroforming is a new and growing type of metal forming process that utilizes high-pressure fluid (typically water or oil) to form sheet metal to a die. The fluid acts like the male punch, forcing the metal to fill the female die cavity.

Some advantages of hydroforming are:

• Complex shapes: Will allow for intricate and difficult shapes to be formed with concavity that may not be possible with solid die stamping.
• Reduced seams/welds: Hydroforming eliminates the need of welding multiple parts together, creating a part that is unbroken which results in a stronger and more rigid and lighter part.
• Less structure stress: Hydroforming elongates stress concentrations and can have a better strength to weight ratio.
• Reduced tool costs: Hydroforming could require less tools compared to traditional stamping with some complex parts.

Hydroforming has become an amazing process to fabricate parts in the automotive industry for lighter and stronger unibody structures, while also being used in some aerospace and higher end industries where parts must have higher performance.

Advantages of Metal Forming

• Materials Savings: Instead of removing material like machining processes, metal forming does not result in material removal and therefore has less scrap with better use of the material.
• Enhanced Mechanical Properties: Work hardening can increase the strength and hardness of the formed part. Since forging establishes the flow line of the grain and processes the material’s cellular structure, the result is a more reliable grain structure, improved fatigue strength, and toughness.
• Rapid Production Rates: Many forming processes, most notable stamping and rolling processes, have many automated processes, which allow high production volumes with reduced labor costs. 
• Net Shape or near net shape: Many forming processes produce parts that require little to no additional machining, with reduced costs and manufacturing times.
• Surface Finish: Cold forming processes can also achieve very fine surface finishes.
• Cost Effectiveness for Mass Production: While some tooling costs can be very expensive, the per unit cost in mass production is often very low.

Disadvantages of Metal Forming

• High Tooling Costs: Tooling for dies and molds used for metal forming can be costly, which can be uneconomical for lower volume productions.
• Limited Geometric Complexity: While many geometries can be made by forming metal alone, some very complex geometries can be impossible to form and may use a combination of forming and machining.
• Spring back: For operations such as bending and perhaps others, after the forming force is removed, the part tends to recover elastically, which can cause the part to return in part to its original shape. Therefore, the part may require over-bending.
• Anisotropy: Plastic deformation can develop directional properties on the material that may not be desired for an application.
• Residual Stresses: A formed part can include residual stresses that require post-processing heat treatment to relieve.

Applications of Metal Forming

Metal forming can be found in many sectors of the economy where it is widely applied:

• Automobile Manufacturing: Metalworking contains many vehicle parts in the entire automobile industry, from small, complex engine parts, to heavy frame, to a simple body skin panel. It is the backbone of all automobile manufacturing as there is high volume production with confident tolerances (to meet safety standards) with strong lightweight parts.
• Aerospace: Production of low-weight, high-strength parts are essential in aerospace applications. Metalworking methods such as forging, extruding, and sheet metal, can support all parts in an aircraft including the fuselage, wings, engine components, and landing gear i.e. the structural parts.
• Construction and Architecture: The ability to form metal branches out from a few metal components, to make large bulks of metal including structural steel beams, columns, pipe, roofing, and even small decorative features out of flourished designs. Metalworking is widely acknowledged as an important material in modern buildings and infrastructure.
• Electronics: A variety of small delicate, and intricate metal components are often found in electronics including enclosures, connectors, heat sinks, and chassis, most created using highly accurate stamping and bending operations.
• Appliances: Washing machines, refrigerators, ovens, and other appliances use formed metal components to refine the internal structure, outer casing, and most components of function and assembly.
• Medical: The production of surgical instruments, implants, and many of the components of medical equipment often will require very tight tolerances and specific mechanical-properties of combinations of metals and features such as angles.
• Defense and Military: The need for military vehicles, heavy duty pieces and components such as items of ammunition, missile parts and infrequently, military vehicle parts rely on the “strength” and overall reliance of a forging manufacturing method, specifically.
• Energy: Also, metalworking continues to be utilized in the manufacture of items such as components for power generation equipment, pipelines, wind turbines and frames and fixtures for solar panels, all supported with metalworking techniques.

How to Choose the Right Metal Forming Technique?

The best metal forming process is determined by several considerations which include the following:

• Part Shape and Complexity: Simple forms may be produced by press braking, whereas shallow drawn, hollow parts, with complex features will require deep drawn methods. Intricate parts, which need to resist high loads and stress will utilize forging methods to achieve the right mechanical properties.  
• Material Properties: The ductility, strength, and work-hardening characteristics, of metal varies. For example, aluminum will deform differently than steel, copper, or titanium.
• Production Volume: When manufacturing in large quantities, stamping or rolling are preferred because of lower per-unit costs. Conversely, when manufacturing low quantities, lower tooling expense, or open-die forging may be used better criteria.
• Quality of Mechanical Properties: If mechanical properties of high strength, fatigue resistance, and closed die forging, or unhardened cold work, are required, most likely forging will be preferable to most or all other process.
• Tolerances and surface finish: Due to frictional effects, cold forming will have much higher mean dimensional tolerances, and surface finish better than hot forming.
• Cost and budget: There will be tooling costs, and costs of machine, in addition to labor costs.
• Lead Time: The length of time in developing and making tooling can also, help decide the manufacturing process.

Tips and Best Practices in Metal Forming

The following suggestions are meant to increase success and quality in metal forming:

• Material Characterization: You need to understand the properties of the material, including yield strength, tensile strength, % elongation, and work hardening.
• Die design and tooling: The main point here is that you must avoid tooling and dies made from rubbish. You will want quality dies that were manufactured to precision. Other areas to consider are how the metal will flow into the tooling, lubrication, and compensating for spring back.
• Lubrication: Applying ample lubrication is a critical component of metal forming, if you don’t want tool wear, excessive wear on the workpiece, and sticking and galling of the workpiece onto the dies, lubrication will decrease the friction between the two.
• Process Control: It is critical to control important parameters (force, speed of operation, temperature, lubrication, etc.) to help maintain the consistency of quality of parts made.
• Annealing (for cold working): If you are planning on doing any significant amount of cold working, you will require intermediate anneals to regain the ductility of the metal components and to decrease the chances of cracks.
• Simulation Software: Use computer aided engineering (CAD or CAE) software to simulate the processes (i.e. FEM analysis). The power to quickly simulate then calculate the flow of material, stress analysis and defects will have a significant impact on your costs (i.e. reduce the number of changes you will physically prototype if possible) while allowing you to optimize your die design and set your process parameters in advance.
• Quality Control: Quality control needs to be sustained through the procedure which can include dimensional checks, surface finish, and tensile tests.
• Tool maintenance: It is important to maintain dies and tooling. This will assist in the prolonging of a tools useful life and safe quality parts.
• Material handling: The handling of materials is critical. Adequate raw materials handling and formed parts handling is key in making sure you damage none of your components or parts and maintain quality.

Safety Considerations in Metal Forming

Metal forming operations use machinery that is powerful, and capable of a lot of force, which means safety needs to be prioritized and closely observed:

• Guarding Machines: All machines that have moving parts will have guarding to prevent a worker from becoming in contact with the moving part.
• Lockout/ Tagout Procedures: Lock out/device procedures are used to eliminate the possibility of machinery started accidentally while maintenance or repair is being preformed.
• Personal Protective Equipment (PPE): Workers will wear the appropriate PPE such as, safety glasses, hearing protection, safety shoes and gloves.
• Pinch Points: Be aware of pinch point areas for body parts in the moving part process.
• Material Handling Safety: Use proper lifting techniques and lifting/handling devices to move heavy work pieces.
• Emergency Stop buttons: Emergency “stop ” Buttons must be present and functional and accessible to the operator on all machines.
• Training: Operators must be trained to operate all machines, safety procedures, and to identify hazards.
• Fire Safety: Be aware of fire hazards that may occur during hot working operations, and be aware of fire suppression devices.
• Ventilation: Adequate ventilation must be provided to remove fumes or dust created by incremental processes.
• Noise Control: Control measures must be in place to control noise and provide hearing protection in loud environments.

Conclusion

Fundamentally, metal forming is the basis of modern manufacturing. Metal forming processes afford great advantages- in material usage, in mechanical properties and in high volume production.

There are numerous processes: ranging from traditional forging methods to highly sophisticated processes such as hydroforming; and the number of products produced by these processes is immense.

Metal products are necessary for much of our daily existence: from fabricating building structures to aluminum cans to aerospace applications and more!

It will be of utmost importance for engineers and manufacturers to understand metal forming principles and applications to reduce costs, increase productivity, and increase product performance.

References

• Groover, M. P. (2021). Fundamentals of Modern Manufacturing: Materials, Processes, and Systems. John Wiley & Sons.
• Kalpakjian, S., & Schmid, S. R. (2014). Manufacturing Engineering and Technology. Pearson.
• ASM International. (2006). ASM Handbook, Vol 14A: Metalworking: Forging and Extrusion. ASM International.
• Degarmo, E. P., Black, J. T., & Kohser, R. A. (2019). Materials and Processes in Manufacturing. John Wiley & Sons.
• TDH Manufacturing. (2023, August 15). The Benefits of Metal Forming. https://tdhmfg.com/the-benefits-of-metal-forming/
• ASM Handbook, Vol. 14B: Metalworking Processes. (2005). ASM International.
• Dieter, G. E., & Bacon, D. J. (1986). Mechanical Metallurgy (3rd ed.). McGraw-Hill.
• Schey, J. A. (1983). Tribology in Metalworking: Fricton, Lubrication and Wear. American Society for Metals.