Forging is a manufacturing process that many industries rely on. In fact, hundreds of forged parts can be found in aircraft, vehicles, agricultural equipment, trains, mining machinery, and more. Forgings are stronger and more reliable than other processes such as casting and welding. In addition, because the metal forging process can efficiently produce complex geometries, it can be a more cost-effective manufacturing process.
A common misconception is that forging is a universal method; in fact, forging manufacturers can use a variety of methods. In general, forging can be divided into two categories: by tool classification and by temperature classification.
In this article, we will introduce each forging method and highlight the main types of equipment used in forging.
Forging Method
Tooling Forging
The words "mold" and "tool" are often used interchangeably in the forging industry. They refer to equipment used to form (usually preheated) billets, ingots, preformed parts, or bars. When comparing forging by the tool used, there are three main methods: impression die, open die, and rolled ring.
Impression Die Forging
Through impression die forging, the metal is extruded/compressed between two custom dies, each with a pre-cut outline/cavity (or impression) of the final part shape.
Various types of equipment, such as mechanical presses or drop hammers, are then used to compress the billet or preform while it undergoes plastic deformation. In each impression, the workpiece is forced into the cavity as flash (excess metal) flows into the mold. As the flash cools, pressure builds up in the impression, which helps ensure a more even composition. Then, the flash is removed manually or with trimming dies.
Impression die forging is mainly used to produce complex symmetrical and asymmetrical forgings. The method can also achieve closer tolerances than open die forging.
Open Die Forging
In open die forging, the workpiece (ingot, bar, billet, or preformed part) is deformed between the upper and lower flat dies, and the movement of the workpiece itself contributes to the formation of the final part to a large extent.
In addition to flat dies and hammers (or presses), other tools are available for open die forging, including saddles, blocks, rings, mandrels, and punches.
Parts manufactured by open die forging are usually simple in shape (long and rectangular, flat and round, etc.) and can be further processed after the forging process.
Open die forging is used to produce parts of various sizes; however, when complexity is involved, impression die forging is usually the preferred method over open die forging.
Rolled Ring Forging
The last processing method is rolled ring forging. Unlike impression die and open die, the roll forging method does not use dies, and it is used to make a very special shape: the ring.
This forging process can be used to manufacture seamless rings of all types and sizes. Roll forging is more popular than machining, cutting, and casting because it can produce parts with tight tolerances. In addition, unlike casting, forging eliminates internal porosity, which reduces creep, fatigue, and other problems.
In roll forging, the blank is usually preformed. The preforming technique used in roll forging is called punching. As the name suggests, a hole is punched in the stock to create a donut shape. The preformed part is then passed through the rolling mill until the final part is produced.
All types of components can be manufactured by roll forging, including engine bearings, wheel bearings, flanges, and gears.
Forging Process Divided by Temperature
Metal forging processes can also be classified according to the actual temperature of the workpiece. Different temperatures affect the plasticity of the metal, the final quality of the part, and may determine the final mechanical properties of many components, including strength, ductility, and impact or fatigue resistance.
When forging is classified by temperature, there are three main methods: hot forging, warm forging, and cold forging.
Hot Forging
In hot forging, the dies/tooling are usually preheated to a specific temperature, and then the workpiece itself is heated to a predetermined temperature. The temperature used in hot forging varies according to the raw materials used, the parts being produced, and any specific application requirements.
In hot forging, heat makes the metal more malleable, and the constant pressure of a die that squeezes the hot metal produces a more refined grain structure, resulting in stronger, more malleable parts.
Warm Forging
In warm forging, the workpiece is heated within a specific range: above the work hardening temperature but below the scaling temperature.
Warm forging covers a range of forged products that do not require the expense and strict tolerances of cold forging nor the higher strength yield of hot forging. Warm metal forging is often used on parts that are already close to their final shape, such as shafts and gears.
Cold Forging
As the name suggests, the cold forging process is carried out at lower temperatures (anything between room temperature and a few hundred degrees).
Cold forging also utilizes other processes, including bending, stamping, rolling, and extrusion, depending on the type of part being produced. The metals used in this process are also typically annealed or softened during the forging process, and continuous lubrication is required during this process to prevent frictional heat.
Although cold forging is extremely precise and often requires little finishing, hot forging is still the preferred method for complex custom forgings. In addition, cold-forged parts are more likely to contain residual stress, which can eventually lead to cracking, warping, and other problems.
Equipment for Forging
If you've ever seen or heard terms like press forging or hammer forging, this refers to the type of equipment used in a particular forging process.
Presses
During open die forging or impression die forging, the manufacturer can use a press, which allows the deformation of the workpiece to be controlled by high pressure. There are several types of presses: screw presses, hydraulic presses, and mechanical presses.
A screw press drives the power ram on the screw shaft up or down to press-form the metal. Hydraulic presses use cylinders under hydraulic oil pressure, and mechanical presses are actuated by gears, levers, or other mechanical operators connected to motors and crankshafts.
Drop Hammers
Next, a drop hammer, as the name suggests, uses the force of a heavy metal hammer to strike the workpiece when dropped from a height (gravity-driven) or power-assisted. Powerful blows release kinetic energy to deform the metal, and typically about three blows at each stage are sufficient to form the molten metal, although this number can vary.
Counterblow Hammers
For very large metal parts, the counterblow hammer transfers the high impact energy of the tonnage hammer to the die. With each strike, the hammer moves in the opposite direction, forming large forgings more precisely. Counterblow hammers can be operated by hydraulic or pneumatic drive systems.
Ring Rollers/Rolling Mills
Ring rollers, rolling mills, or reduction mills are primarily used for seamless rolled ring forging but can also be used to preform billets during open die forging or impression die forging. In a rolling mill, the pre-formed part is passed through the rolls of the machine and then the final part is shaped (additional processing may be required after the part has passed through the mill). Some mills even include a manipulator for punching billets.
Is One Forging Method Better Than Another?
No particular forging method is inherently "better" than another. Each has unique advantages depending on the size, shape, and number of parts, the complexity of forged parts, or any required strength or mechanical properties.
Certain part geometries are also better suited to certain processes and equipment. In our previous article, we highlighted the many advantages of each forging process. Click here to learn about the benefits of each process.
Conclusion
Forging is an important process that provides unmatched strength, reliability, and cost-effectiveness for manufacturing a wide range of components in a variety of industries. Understanding the different forging methods, whether by tooling or temperature, enables manufacturers to choose the best process for each specific application. From the precision of cold forging to the versatility of hot forging, each technique has its own advantages. By utilizing the right forging methods and equipment, manufacturers can produce high-quality parts that meet demanding standards and perform reliably in their respective applications. As technology advances, the forging industry continues to innovate, ensuring that forged parts remain a cornerstone of modern manufacturing.
