Forging

Forging

Forging is a manufacturing process in which metal is plastically deformed in the solid state under compressive forces to obtain parts with specific shapes, dimensions, and improved mechanical properties. Forging is typically performed using hammers (often power hammers) or dies.

Main Types of Forging

1.Hot Forging

Temperature range:For steel, typically 950–1260 °C (varies with different metals and alloys).

Main characteristics:

• Excellent plasticity and good formability; relatively low forming force required.
• Work hardening is offset by recrystallization, resulting in low internal stress.
• Oxide scale may form, reducing surface finish and dimensional accuracy.
• Suitable for mass production where high precision is not required.

2.Warm Forging

Temperature range:Below the recrystallization temperature but above the cold forging range. For steel, typically 650–950 °C.

Main characteristics:

• Very little or no oxide scale compared with hot forging; better surface finish.
• Dimensional accuracy is between hot forging and cold forging; forming force is lower than cold forging.
• Slightly reduced formability compared with hot forging; precise temperature control is required to prevent cracking.

3.Cold Forging

Temperature range:For steel, typically from room temperature up to about 150 °C. Aluminum and copper are usually cold forged at room temperature, while 200–400 °C is considered warm forging to improve ductility and reduce forming loads.

Main characteristics:

• No oxidation; excellent surface finish and high dimensional accuracy. 
• Significant work hardening increases yield strength and fatigue resistance.
• Requires high forming forces and robust equipment; limited to ductile metals such as aluminum, copper, and low-carbon steel.

4.Open-Die Forging (Free Forging)

Open-die forging refers to a process in which heated metal billets are subjected to impact or pressure between simple, general-purpose tools or between the upper and lower anvils of forging equipment. The material is compressed in the height direction and can freely extend and spread laterally, producing forgings with the required geometry and internal quality.

Open-die forging is generally carried out as hot forging and is suitable for small batch production, simple shapes, or large forgings. Common equipment includes forging hammers and hydraulic presses.

Basic operations include upsetting, drawing out, punching, cutting, bending, twisting, offsetting, and forge welding. It can be divided into manual forging and machine forging.

5.Die Forging

Die forging can be further classified into open-die forging and closed-die forging. In this process, metal billets are plastically deformed under pressure within die cavities of specific shapes to form forgings. Die forging also includes processes such as cold heading, roll forging, radial forging, and extrusion. Die forging is generally used for parts of relatively low weight produced in large quantities.

Open-Die Forging

Process principle:Uses dies that partially enclose the workpiece. Heated metal is formed by repeated hammering or pressing while freely flowing outward, requiring manual repositioning.

Advantages:

• Simple die structure and low tooling cost.
• Capable of producing large or irregular forgings.
• Allows controlled grain flow, improving strength.
• Suitable for small batches or customized production.

Disadvantages:

• Highly dependent on operator skill.
• Poor dimensional consistency and large machining allowances.
• Low material utilization.

Closed-Die Forging

Process principle:Upper and lower dies form cavities that match the part geometry. The preheated billet fills the cavity under pressure; excess material flows out as flash and is later trimmed. In flashless forging, the cavity is completely enclosed to eliminate flash.

Advantages:

• High dimensional accuracy and good repeatability.
• Suitable for mass production.
• Grain flow follows the part geometry, enhancing mechanical properties.

Disadvantages:

• Complex die design and high tooling cost.
• Flashless forging requires strict control of billet size and lubrication.
• High initial investment.

Advantages of Forging

Improved mechanical properties：
Forging enhances strength, hardness, toughness, and wear resistance due to microstructural and texture changes during deformation.

Reduced internal stress：
Plastic deformation during forging helps relieve internal stresses, reducing the risk of cracking or distortion in service.

Reduced processing time：
Compared with casting, rolling, and other metalworking processes, forging often requires fewer processing steps and less equipment, resulting in lower production costs.

Improved die life：
Uniform metal deformation leads to relatively lower die wear, helping extend die service life.

Greater design flexibility：
Since complex shapes can be formed directly by forging, designers have greater freedom to meet specific functional requirements.