Forging Process Types

The fundamental process of forging involves applying compressive forces to the workpiece using a variety of dies and tools. Hammering metal with stone tools is one of the most ancient and significant metalworking processes used to create jewellery, coinage, and other items. Large turbine rotors, gears, bolts and rivets, cutlery, hand tools, countless structural pieces for machinery, aeroplanes, and trains, as well as an assortment of other transportation equipment are examples of forged parts nowadays.

Blacksmiths have historically used a strong hammer and an anvil to accomplish simple forging operations. But most forgings need tools like a press or a powered forging hammer, along with a set of dies.

Depending on the homologous temperature, forging can be done at room temperature (cold forging) or at higher temperatures (warm or hot forging). Because the workpiece material is stronger, cold forging requires higher forces, and the material must be sufficiently ductile at room temperature to allow for the required deformation without cracking. Cold-forged parts have accurate dimensions and a fine surface quality. Although hot forging uses less force than cold forging, the components' surface finish and dimensional accuracy suffer as a result. Additional finishing processes, like heat treatment to change characteristics and machining to get precise final dimensions and a smooth surface, are typically applied to forgings. Precision forging, a key example of net-shape or near-net-shape, can reduce these finishing processes. When a material is forged, it is distorted by an impact load or a steady load. Forging is categorized as press forging or hammer forging depending on the kind of loading. Press forging includes progressive stresses, whereas hammer forging involves impact loads.

Open Die Forging

The workpiece is compressed in this method between two platens. Material flow in a lateral direction is unrestricted. Open die forging is a method that uses relatively simple-shaped dies to create products by gradually deforming them. The bottom die is fastened to the press bed or hammer anvil, and the top die is fastened to the ram. Heat is applied to the metal workpiece above recrystalline temperature, between 1900 and 24000 Celsius. The majority of open-die forging is done using flat dies.

Cogging, fullering, and edging are the three primary categories of open die forging. Cogging: Cogging, also known as drawing out, is a process wherein narrow dies are used to reduce an ingot's thickness to billets or blooms. Convex or concave-shaped dies are used in fullering and edging operations to lower the cross-section. As a result of material distribution, thickness decreases and material elongates. Upsetting is an open-die forging process.

Advantages of Die forging	Disadvantages of Die forging
Products are with greater strength	Less accuracy and tolerances
Fatigue resistance of the parts improved Reduce voids	Need to machine parts to get the desired accuracy and features.
Capable of producing very large parts which weigh about 136 metric tonnes.	Applications of Open die forging
Less material wastage
Products have fine grain size and continuous grain flow

Closed Die Forging

While impression die forging and closed die forging are very similar processes, closed die forging requires extremely precise control over the initial material removal to prevent flash formation. Otherwise, impression die forging is akin to the procedure. This method works well for large-scale production.

Advantages of Closed forging	Disadvantages of Closed forging
Reduce or no machining	No economic to small or short production runs due to the high cost of die
Less or no machining is required for its close tolerances	High setup cost for furnace dies and machines
Dimension with tight tolerances part can be made	Closed die forming is a dangerous process
Better surface finish and mechanical properties
Cost-effective for large production runs

Application of closed die forging

Railway, petrochemical, electrical, lifting and safety systems, industrial and agricultural machinery sectors.

Mining drilling bits, forestry wear parts

Impression Die Forging

The workpiece is pressed between the dies in impression die forging. The necessary shape is formed between the closing dies as the metal spreads to fill the cavities sunk in the dies. "Flash" is the term for a material that is forced out of the dies. As the top hits the anvil, the flash gives the dies some cushioning. The flash surrounding the workpiece is chopped off and thrown away as scrap. A good forging requires the material to fill the dies all the way to the top. It might take multiple hammerblows to accomplish this; one blow might not be enough.

Drop Forging

A closed impression die is used in drop forging to give the component the correct shape. The material inside the die cavity is repeatedly hammered to shape it. Drop hammers are the names of the tools used to deliver the blows.

A drop forging die is made up of two parts. The die's upper half is fixed to the ram, and the lower half is fixed to the machine's anvil. The lower die holds the heated stock. The metal is struck by the ram four or five times in rapid succession, causing the metal to spread out and fill the die cavity completely. The entire cavity is created when the two die halves close.

PRESS FORGING

Press forging dies and drop forging dies are comparable in terms of operation. Unlike drop forging, which shapes the metal through a succession of blows, press forging shapes it through a single, continuous squeezing action. It is achieved by using hydraulic presses to produce this squeezing. As a result of the hydraulic presses operating continuously, the material deforms uniformly throughout the depth. In drop forging, more hammer force is probably going to be transferred to the machine frame, whereas in press forging, the stock absorbs all of the force. Press forging yields a clean impression as opposed to jarred impressions, which are similar to drop-forged components.

Press forging has a smaller draft angle than drop forging. However, deforming requires a larger press capacity, so only smaller components are press forged in closed impression dies. 

The two halves of the die post are fastened to the bottom die to provide the required alignment. This allows the top die to slide only on the post and register the proper alignment. Better tolerance is ensured for press-forged components as a result.