Drop Forging Explained: High Metal Strength

Drop forging is one of the most commonly used forging techniques to produce metal workpieces that require high strength.

But what exactly makes it different from other forging techniques? What are its different types? And what advantages does it provide?

Drop-forging is a metal-forming operation that involves using a power hammer to repeatedly impact a hot metal workpiece placed on a die. The workpiece is heated above its recrystallization temperature to enhance its malleability, followed by hammering to force it into the die cavity, enabling to forge complex shapes.

This article gives a detailed guide on drop forging by going through its process, types, and advantages.

In the end, the article provides a detailed differentiation between drop forging and press forging to help you understand which one is best suitable for your application.

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Drop Forging Explained: How it Works

Drop forging, also commonly referred to as hammer forging, is a metal-forming process in which the workpiece is shaped by using dies and pressure.

Although it works similarly to die punches, drop forging involves reshaping the workpiece, while punching is used to remove material from the workpiece.

In forging, the metal workpiece is heated above its recrystallization temperature to enhance its ductility, making it easy to alter its shape upon impact.

Generally, drop forging consists of two halves of a die, one mounted on the anvil and the other half mounted on the impacting surface of the ram or hammer.

The hot workpiece is placed in the die half mounted on the anvil, while the hammer is dropped on it, forcing the hot metal to mold according to the shape of the die.

This recurrent action compresses the metal until it completely fills the die cavity and forms the required geometry.

Rams or hammers are usually mechanically controlled by gears or a belt and chain mechanism, powered by a high-powered motor.

However, pneumatic or hydraulically controlled rams are preferred for forging larger workpieces of hard metals.

The excess materials that flow out from the sides of the dies is called flash. It requires post-processing to remove the flash and smoothen the edges.

Drop forging produces parts with higher load bearing capabilities when compared to machining.

Applications of Drop Forging

Industry	Application
Railway Industry	Used to manufacture railroads, gears, levers, connecting rods, etc.
Automobile Industry	Used to manufacture gears, levers, connecting rods, crankshafts, etc.
Marine Industry	Used to manufacture gears, levers, connecting rods, external sheets, etc.
Construction Industry	Used to manufacture Spanners, external metal sheets, etc.

Metal that undergoes drop-forging develops favorable material properties like high strength, ductility, and durability, making it suitable for various industries.

Drop-forged parts like gears, levers, external sheets, etc., are commonly used in the automobile, railway, marine, and construction industries. 

Apart from that, dro forging is preferable for forging of steel and other hard materials, due to its high impact load and ability to deform the workpiece.

Types of Drop Forging

The two main types of forging are open-die and closed-die/impression-die drop forging.

Open-Die Drop Forging

Open-die forging, also called Smith forging, is a typical drop forging method that allows the material to flow in all directions when impacted by the hammer or ram. 

This means that the material is not completely constrained and requires expert metalworking to manipulate the orientation of the workpiece and the striking angle to produce the required shape.

Depending on the user's requirement, the dies can be flat-shaped or have a specifically shaped cavity.

Cogging is an open-die drop forging process in which a flat or slightly curved hammer is used to roughly shape the workpiece to fit the die.

It is usually performed in the initial stages of all types of forging to roughly obtain the desired shape of the workpiece, after which closed-die forging is performed to attain the desired workpiece geometry.

Similarly, fullering is also an open-die drop forging process used to roughly shape the workpiece before performing closed-die forging.

Open die drop forging is primarily performed to obtain blanks and smooth-shaped objects.

Closed-Die Drop Forging

Closed-die drop forging, also known as impression-die forging, has a fundamental mechanism similar to open-die forging.

However, the dies used in this type of forging have a closed structure, restricting the flow of hot metal upon impact.

Although the flow of hot metal is restricted inside the dies, a minimal amount of flash is produced, requiring further machining to remove the flash from the workpiece.

The need for customized dies increases the cost of the forging process and is generally suitable for large-scale applications where identical parts are to be made in large volumes.

Advantages of Drop Forging

Forging provides various advantages over other metalworking processes, and these advantages can be classified as follows:

High Strength

Unlike other metalworking processes like casting, drop forging refines the properties of the metal.

Drop forging reorients the grain structure, resulting in greater strength and heat tolerance.

Due to this, drop forged objects can easily be subjected to high temperatures without damage.

Generally, hand tools are manufactured by implementing the drop forging technique.

High Ductility

Drop-forged workpieces are very ductile, meaning they can conveniently undergo hot and cold working operations.

On the other hand, casting produces brittle workpieces which are prone to breaking under compressive force.

Cost-efficient Process

The initial and running costs of drop forging are much lesser compared to other metalworking operations like casting and molding. 

Although purchasing customized dies for drop forging can increase the initial cost, the overall costs involved with drop forged products are reduced due to their low maintenance requirements.

Drop-forged objects are comparatively more durable, minimizing the need for frequent maintenance or replacement, and reducing the overall cost of the part. 

Quick Lead Time

Unlike other metalworking operations like rolling, stamping, or even casting, drop forging is a quick operation with a lesser lead time, thus making it ideal for bulk production in large-scale industries.

Although using a manual hammer to shape the metal workpiece can be a tedious task, implementing the modern approach drastically reduces the lead time.

A power hammer coupled with a closed die can shape the workpiece almost instantly.

Minimal Post-processing Requirements

Post-processing is generally required after almost every metalworking process.

However, the ability to produce workpieces with high dimensional accuracy reduces the need for the post-processing of drop-forged parts.

Minimal Material Wastage

The flash from the drop forging operation can be molten and reused in metal-forming processes such as casting or molding.

Thereby minimizing overall material wastage, which in turn saves costs and reduces environmental waste.

Drop Forging vs Press Forging

Drop Forging	Press Forging
Multiple hammer/ram impacts	Single hammer/ram impact
Higher pressure impacts	Lower pressure impacts
Intermittent impacts	Continuous impact
Suitable for small to medium-scale industries	Suitable for large-scale industries
Cheaper	More expensive
Workpiece can be hot or cold	Workpiece is cold
More significant deformation at the surface	Uniform deformation throughout the workpiece

In drop forging, the ram or hammer is dropped onto the workpiece multiple times with immense force, whereas, in press forging, the ram or hammer is gradually pressed onto the workpiece with continuous pressure.

Generally, press forging is suitable for large-scale industries due to its quick shaping of the workpiece, whereas drop forging is comparatively slower, making it suitable for small-scale projects.

Press forging does not necessarily involve heating the workpiece before forging. In contrast, drop forging requires the workpiece to be heated above the glass transition temperature.

Furthermore, drop forging is cheaper than press forging, both in terms of running and initial cost, thus making it more suitable for hobbyists.

Apart from that, drop forging generally involves reshaping the surface of the metal, whereas press forging produces a uniform deformation throughout the workpiece.

As a result, drop forging is suitable for applications where significant forming is not required, while press forging is ideal for completely transforming the shape of the workpiece.

Final Thoughts 

Drop forging is an excellent upgrade from traditional manual hammering to forge metal workpieces.

Open-die drop forging involves low initial costs, making it suitable for DIY applications. However, it requires practice to shape the workpiece without a constrained die.

On the other hand, closed die forging is best suited for large-scale applications where identical parts are to be made again and again.

Furthermore, the parts made by drop forging exhibit high strength, making them suitable for applications that require excellent wear resistance and durability, such as hand tools.

Frequently Asked Questions (FAQ)

What safety precautions are taken during drop forging?

The safety precautions an operator must implement during drop forging include wearing a face mask, safety goggles, safety shoes, gloves, and headgear. Due to the possibility of hot debris flying around, a face shield is preferable.

Can all metals undergo drop forging?

No, all metals cannot undergo drop forging. Some metals like tungsten, titanium, and cast iron do not effectively and easily undergo this process due to their material properties. Subjecting them to drop forging for extended periods can damage the workpieces.

What is a grain structure in metalworking?

A grain structure in metalworking is the orientation of all the atoms and molecules that make up a metal. A uniform grain structure leads to uniform properties throughout the metal workpiece and affects its mechanical properties such as ductility, brittleness, etc.