Metal Forming

A pipe being bent. Bending is one of several different metal forming methods.

Metal forming, a subset of fabrication, involves the reshaping of metals while still in the solid state. By taking advantage of the plasticity of certain metals, the forming process makes it possible to move a solid piece of metal from its current shape into the desired form. The metal forming process accomplishes this without melting the material, thereby avoiding any potential difficulties in the handling of molten metal or the integrity of molded products.

Hot Working vs. Cold Working

While hot working deforms metal above its recrystallization temperature, cold working is performed below it. Hot working is usually at temperatures higher than 60% of the metal’s melting point (on the Kelvin scale). Above the recrystallization temperature, metal grains, which are stressed from former processes applied to it, re-melt into new unstressed grains that are more ductile and less hard. This makes the metal easier to form.

Cold working is typically performed for most metals at room temperature or slightly higher. As long as the operating temperature is below the metal’s recrystallization temperature, it qualifies as cold working.

The temperatures involved in these processes are not static: they vary from metal to metal. In some cases, room temperature may mean hot working conditions.

Advantages and Disadvantages of Hot Working

Hot working is best for times when a large amount of deformation is needed. It takes a smaller amount of force to form hot metal, and hot forming allows the metal to stretch far more than in cold forming. Additionally, it doesn’t work harden metal, which allows the metal to keep its ductility. Hot working does have some disadvantages, however, including a rougher surface finish, less accurate dimensions, and a shorter tool life.

Advantages and Disadvantages of Cold Working

As opposed to hot working, cold working takes more energy but creates stronger products. Cold working provides closer tolerances, good surface finish, and requires minimum machining afterward. This method also creates more strength and hardness in the metal from strain hardening, and the way cold forming affects the metal’s grain flow can be useful for certain applications. Since cold forming also doesn’t require furnace or fuel, it allows the manufacturer to save on costs.

However, because the metal is cold worked, it takes more power to form it, and the amount of forming that can be accomplished is limited thanks to the strain hardening that happens. Additionally, cold worked metal can’t be deformed as easily as hot worked metal.

Metal Forming Manufacturing Processes

Most forming methods can be done at either hot or cold temperatures, depending on the effects needed. However, while some processes rely on bulk workpieces (like bars), others use sheet or plate to create a finished product.

Types of Bulk Metal Forming

Bulk metal forming involves materials with a low ratio of surface area to volume, like bars, tubes, or billets. Bulk forming encompasses techniques like roll forming, forging, extrusions, castings, and drawing.

• Rolling involves sending metal stock through a set of rollers, which deform the original stock and output it into the shape needed. Sheets, strips, rails and other similar shapes can be produced by rolling. This method produces tight, repeatable tolerances, high strength metal, and little or no scrap.
• Forging uses presses, hammers and other compressing devices to shape metal stock. Parts made by the cold forging process are often referred to as cold-headed parts. Often used for aerospace and automotive applications, forging’s main advantages come from the low amount of secondary services needed and the strength and hardness of the end product.
• Extrusion is a process whereby stock is forced through a die and emerges as a tube with a nearly identical cross-section to the die. The method of forming allows for hollow shapes without seam welding as well as complex cross sections.
• Casting is performed by pouring molten metal into a die or mold. Its advantages run toward complex parts and the wide range of alloys that can be used for it.
• Drawing resembles extrusion, except that the workpiece is pulled, not pushed through the die. While drawing is done on thicker pieces of metal, it should not be confused with deep drawing, which is a process applied to sheet metal.

Types of Sheet Metal Forming

Sheet metal forming uses metal forms with a high surface area such as plates and sheeting to form products. This can include deep drawing, bending, shearing, and stamping.

• Deep drawing involves drawing plate or sheet into the needed shape to the point that the final result has a height equal to or larger than its width. This method is good for complex geometry products, as well as fast production.
• Bending involves the reshaping of metal around a linear axis, and is usually accomplished through a press brake. Bending provides less residual stress than roll forming.
• Stamping uses a press to create indentations of the desired shape in metal. This method is good for higher volumes, including complex parts in short time periods. It can also produce consistent products without variation.

Metal Forming Tools

Thanks to the wide variety of metal forming processes, there is an equally large number of tools used for metal forming. Some of the most basic include:

• Presses, which create stampings. Presses shape metal by crushing it between a top and bottom ( the bottom is called the bed). They come in mechanical, hydraulic, and pneumatic types.
• Benders come in different varieties depending on whether the metal is a sheet or a bulk material. Press brakes create V- and U-shaped bends in sheet metal by pressing the sheet down into a die with a die block. There is also tube and bar bending equipment in manual and machine types.
• Roll forming machinery flattens and thins metal by running it between two rollers, and is often used to curl it into coils. Roll forming machinery can come in several different types depending on the need, including machines that can process multiple workpieces or that have multiple sets of rollers for a single piece.
• Dies are a form to shape raw metals. In addition to being used for stampings and bending, dies are also used for drawing and extrusion as metal is pulled or pushed through them to be shaped.
• Extruders force metal through a die to shape and strengthen it. Extruders can use direct, indirect, hydrostatic, lateral, or impact methods to extrude the metal.
• The opposite of extruders, Drawing machines pull metal through dies or draw plates to thin and strengthen it. Draw benches are also used for the drawing process.

Metal Forming Applications

Metal forming is used to create all manner of products, including tubes, pipes, metal sheets, fasteners, and wire. Many of the metal objects we encounter in everyday life, from thumbtacks to file drawers, were likely produced by one or more metal forming operations. Industrial metal forming is commonly used across multiple sectors, including the automotive industry, which uses metal forming for door frames and bumpers. It’s also used in aerospace, which forms metal for engine parts, blades, and structural parts. Metal is also formed for architectural purposes, such as decorative molding or roof parts.

Conclusion

Metal forming has several advantages as a process, including a quick production rate, lending the finished product better mechanical properties, and avoiding waste in the raw material. However, it is only one category of manufacturing processes.