Drop Forging

Chapter 1: What is Drop Forging?

Drop forging is a metal working process that shapes and forms a metal bar or billet placed between two dies. The metal piece is compressed, under great force, into the form and shape of the dies to produce a product or component. The mechanism used for the drop forging process involves the use of a drop hammer that is lifted by air, a belt, or chain and forced down by air, gravity, or hydraulics. The two portions of the die include a lower half, fixed position die that sits on the anvil of the forge and an upper fixed die that is attached to the ram, which is forced down onto the lower half of the die..

Drop forging, in its simplest form, is hammering a piece of metal multiple times until it takes a specified shape or form. The blows of the hammer are continuous until the metal piece or billet fills the die cavity. Part of the design of forging dies is to force excess metal out of the impression, which is referred to as flash. Although the forming of a component or part takes place when the hammer is pounding a metal piece into the die, the excess metal, or flash, is removed during a secondary or finishing process using various methods and techniques.

Two forms of drop forging are open die and closed die, which are differentiated by the type and form of the die. In both cases, the process of drop forging is the same but the type of die is different with closed dies encapsulating the workpiece while open dies are open and allow the metal to flow out of the die.

Chapter 2: Types of Drop Forging

Of the many methods used to shape and form metal products, drop forging is one of the most aggressive due to the amount of force that is used to transform a slug of metal into a viable product. The two types of drop forging are open die and closed die that differ in how the workpiece is shaped by the die. In basic terms, closed die drop forging is a process where the die completely covers the workpiece and encloses it during the shaping process. With open die drop forging, which is referred to as smith forging, the die does not enclose the workpiece as it is formed by the multiple blows of the hammer.

Open Die Drop Forging

Although open and closed die forging use the same process to deform a billet and force it into a specified shape, they differ in the form of the die plates. In the case of open forging, also known as free forging or smith forging, simple dies are surfaces that come in contact with the workpiece are flat, semi-round, concave, convex, or V-shaped and never encase the metal billet. Repetitive blows from the hammer alter the shape and grain of the metal such that it takes on the profile of the die plates.

Open die forging consists of the application of pressure to a billet at its base and both sides. It is a hot forming process that uses standard dies to form a limitless array of components, parts, and products that can range from a few pounds up to several hundred tons. The workpiece is heated to improve its plasticity and lower the amount of force required to reshape the workpiece.

The process of open die forging allows for rough and finishing of metal products and allows the workpiece to move freely. Very large workpieces of several tons can be shaped using open die drop forging. The types of products produced include discs, hubs, blocks, shafts, step shafts, step shafts with flanges, sleeves, cylinders, flats, hexes, rounds, and plates. Aside from standard shapes and forms, open die forging is capable of producing custom shapes for a specific purpose.

Other common uses of drop forging are short runs, art smithing, rough shaping of ingots, and the reorientation of the grain structure of metals to increase their strength and reduce porosity. It is very common to use open die drop forging to improve the machinability of metals and their mechanical properties such as resistance to wear.

Open Die Forging Dies

Dies for open die forging consist of a top and bottom that are capable, in their simplest form, of creating a single impression. The pairs of dies are such that each portion has part of the impression to be formed or created. In some shaping processes, a workpiece may pass through a series of dies to achieve the desired shape. The majority of open drop forging dies are flat with round swaging dies, V-dies, mandrels, pins, and loose tools being the exception, which are used depending on the configuration and size of a part.

Examples of Open Die Processes

• Notching – With notching, a tool that is similar in shape to a chisel is used. The operator of the forge rotates the workpiece such that an even impression is created around the circumference of the workpiece. The workpiece is rotated multiple times to achieve the proper placement and shape of the notches.

  

  
  

• Shaping – Multiple shapes are produced from open die forging using the same method as that which is used for notching. The basic difference in the processes is the use of machine jaws to hold the workpiece in place and move it through the process, rotating it prior to each blow from the hammer.

  

  
  
• Cogging – Cogging is used to change the thickness of a workpiece using a flat or curved die, which increases the length of the workpiece. As with other open die forgings, the workpiece is struck several times until the desired shape is achieved. In many cases, the process of cogging is performed in a series and is normally a roughing procedure to prepare a workpiece for further processing.

Closed Die Forging

Closed die forging, also known as impression forging, has a die that is similar to a mold that sets on the anvil and is the lower half of the shape to be formed. Depending on the shape to be created, the ram or hammer may also have a die shaped like a mold to create the other half of the impression. The workpiece is normally heated to make it more ductile and is placed on the lower die on the anvil. The ram, with the other half of the die, drops, with great force, onto the die and anvil. The multiple blows from the ram forces the metal of the workpiece into the upper and lower molds.

As with open die forging, the ram may strike the anvil several times in order to achieve the desired shape and fill the contour of the die. Any metal or material that is forced out of the die, referred to as flash, cools quickly and blocks flow between the dies. Unlike the dies for open die forging, the dies for closed die forging are carefully designed, precision crafted, and accurately machined, a factor that increases the cost of closed die forging. The accuracy and craftsmanship in the preparation of closed forging dies make it a net shape or near net shape process.

During the plastic deformation of the workpiece, in closed die forging, the grain structure of the workpiece is compressed and aligned to the shape of the product, which increases its directional strength while reducing stress on corners and fillets. The result of the closed die drop forging deformation is products that are stronger and more durable with characteristics that are similar to that of parts machined from solid pieces of metal.

One of the concerns regarding closed die forging is the durability of the dies, which have to endure a high amount of stress during the process, a factor that can reduce the useful life of a part. Typically, closed dies are able to endure 5000 up to 7000 cycles before having to be replaced. Although this is a general rule, more complex and intricate dies that are used with high strength alloys may only last a little over 100 cycles. In addition, dies with sharp edges and tight tolerances may have an even shorter life cycle.

The key to the strength of closed die forging dies is their design, which has to account for partition lines, draft angles, ribs, radii, and fillets. Most designers are able to marginalize any concern for draft angles using various design techniques. Radii are necessary for the flow of the metal and to help reduce tool wear.

Cold Drop Forging

Drop forging that is performed at room temperature is referred to as cold forging, where the workpiece is not heated prior to the forging process. Exceptional dimensional accuracy and surface smoothness is achieved with cold forging. During the process, blanks are deformed and work hardened as dies experience heavy loads. Dies are high strength and have a hard film applied during their machining to help with wear and to avoid adhesion.

The cold drop forging method can be used with open and closed dies that have received the proper treatment. It is the preferred process for forging soft metals, such as aluminum, and is less expensive. Aside from the film that is placed on the dies for cold forging, workpieces are normally coated with a lubricant to keep the workpiece from sticking to the die, which can reach temperatures of 250°C up to 450°C (482°F up to 842°F) due to the friction generated by the process.

As with hot drop forging, cold forging may include a series of forging processes completed in succession to achieve the appropriate shape. Cold forging is flexible and efficient with many advantages such as producing products with improved mechanical properties, dimensional accuracy, eco-friendly, and the ability to produce complex and intricate parts.

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Chapter 3: Metals Used for Drop Forging

The choice of metal to be used to produce a drop forging product is the main factor in the drop forging process. The characteristics and properties of the metal plays an important part in the final decision. Although there are several metals that can be used for drop forging, the three most common metals are various types of steel, copper, and aluminum. An additional factor in regard to the choice of metal is the metal’s reaction to being heated.

Another deciding factor in regard to the choice of metals is the manufacturer since certain manufacturers specialize in specific metals or have equipment that specializes in a group of metals. Manufacturers work with their clients in the choice of metal based on the client’s CAD rendering and the services the manufacturer can provide.

Steel

Of the many different metals that can be used for drop forging, steel is the most used, has the highest demand, and the lowest cost. As an alloy of iron and carbon, the addition of different amounts of carbon and other alloys adjusts and changes the characteristics and properties of steel, creating different types of steel for different applications.

At low temperatures, the microstructure of steel is in the ferrite and iron carbide field. At high temperatures, the microstructure moves to the austenite phase. Each phase exhibits a different crystalline structure and different properties. With drop forging, austenite steel is mainly used due to its strength and ability to endure the process.

Combinations of different alloys are used to create the different types of steels. This aspect of steel makes it possible to choose an alloy that ideally fits a process or application and makes it possible to select a type of steel that can be processed by drop forging. Although there are over 3000 different grades of steel, only a few of them have the resilience to be used in the drop forging process.

• Alloy Steel – Alloy steel is an overarching term that refers to a variety of steels that have had their properties changed, altered, and enhanced by the addition of other metals, such as silicon, manganese, molybdenum, or nickel. The purpose for adding these metals is to add characteristics like corrosion resistance, malleability, conductivity, wear resistance, and strength. During the selection process, manufacturers work with their clients to choose a steel that fits the requirements of drop forging and the properties of the client’s product.
• Microalloy Steel – Microalloy steel contains small amounts of niobium, vanadium, titanium, molybdenum, zirconium, or boron. The grain microstructure of the metal is refined to make precipitation hardening easier. Microalloyed steels can be cold and hot forged with cold working making the steel as strong as carbon steel with a uniform hardness and a ferrite pearlite microstructure.
• Carbon Steel – The high carbon content of carbon steel gives it higher strength to endure the drop forging process. As the carbon content of carbon steel increases, the metal becomes harder and stronger due to heat treatment but less ductile.
• Stainless Steel – Stainless steel is one of the more popular forms of steel due to its resistance to corrosion and heat as well as its durability and strength. There are many grades of stainless steel with a few common ones being the most widely used. For drop forging, grades 304, 304L, 316, and 316L are austenitic stainless steels that are commonly used. Stainless steels require a higher temperature for the drop forging process compared to other steel options.

Aluminum

Aluminum is a soft metal that is easy to shape and can easily be deformed under pressure. As with steel, there is an endless number of aluminum alloys, each of which has different properties, characteristics, and features. Although it is generally assumed that aluminum is resistant to corrosion and rust, the level of those properties varies across the grades of aluminum. The different alloys of aluminum are divided into ones for casting and ones for forging. Aluminum alloys for forging include 6061, 6063, 6082, and 7075.

• Alloy 6061: Alloy 6061 is known for its high strength, excellent machinability, and corrosion resistance.
• Alloy 6063: Alloy 6063 has good thermal conductivity and is used in heat sinks and applications where thermal conductivity is needed.
• Alloy 6082: Alloy 6082 is a high-strength alloy that is used for structural applications. It is weldable and has good corrosion resistance.
• Alloy 7075: Alloy 7075 is another high strength alloy with fatigue strength and high resistance to corrosion. It is the strongest of the four alloys but is difficult to weld.

Copper

The characteristics of copper that make it a choice for drop forging is its thermal and electric conductivity, its ductility, and corrosion resistance. As with the other metals that are used for drop forging, copper is combined with an assortment of alloys to form other types of copper and provide a selection of coppers for various applications. The characteristic that differentiates the types of copper is their copper content, which can range from 70% up to 99%. The copper alloys that are used for forging include brass, high copper, leaded copper, copper-nickel, and nickel-silver.

• Brass – Brass is the most commonly chosen copper alloy that is used for drop forging due to its resistance to corrosion and its ability to be forged into intricate and complex shapes. Aside from its malleability, brass is highly durable, readily available, and a cost-effective alloy. It is a copper-zinc alloy that has a yellowish color and is forged into highly durable fittings for piping and valves.
• High Copper Alloys – High copper alloys have a copper content of at least 94% and small amounts of silver and tin. It is a high strength alloy that is durable and corrosion resistant. The strength of high copper is increased with the addition of zinc, nickel, iron, or aluminum. Regardless of what metals are alloyed with copper, it still maintains its electrical conductivity.
• Leaded Copper – Leaded copper contains copper and lead that are combined with zinc, aluminum, tin, or nickel to achieve particular properties and characteristics. The addition of lead to copper increases its workability and its strength. The workability of leaded copper makes it ideal for forging products with intricate shapes with high tolerances. The restrictions on the use of leaded copper are due to its lead content, which is toxic and has to be handled with care.
• Copper-Nickel – Copper-nickel is widely used for marine applications due to its resistance to the effects of saltwater corrosion. Although copper-nickel forgings are strong and durable, they are lightweight, making them ideal for use in boats and other marine applications. A unique characteristic of copper-nickel is its ability to retain its strength at temperatures up to 400°C (750°F).
• Nickel-Silver – Nickel-silver, also known as German silver, is a copper, nickel, and zinc alloy that has been used for many years in the production of coins and jewelry due to its distinctive color. The use of nickel-silver for decorative items is due to its resistance to corrosion and tarnishing, which is higher than other copper alloys. Nickel-silver is an easy metal to forge due to its malleability when heated.

Titanium

Although steel, aluminum, and copper are commonly used for the production of drop forged parts, titanium, a strong, low-density metal, is also used. Titanium has the strength and durability of carbon steel and stainless steel but is 40% lighter. Forgings made of titanium can endure environments that reach 537.78°C (1000°F). The factor that restricts the use of titanium for drop forging is its cost, which can be as high as $50 per kilogram while stainless steel can cost as much as $1.50 per kilogram.

Chapter 4: Types of Drop Forging Hammers

The drop forging process is completed using different forms of equipment with each form following the basic principles of drop forging, which is to repeatedly strike a workpiece until it takes on the desired shape and form. The key component in drop forging is the impression hammer that can take several different shapes and designs powered by air or hydraulics.

Hammers are classified as single effect, double effect, and counterblow, depending on the drive for the ram. The purpose of the hammer on a drop forge is to provide impact strain to deform a workpiece. The hammer thrusts the ram at the workpiece multiple times and is described by the energy of the blows or strokes it delivers.

Air Powered Drop Hammer

Air powered drop hammers are the most powerful drop forging machine. Although they were introduced during the first industrial revolution, air powered drop hammers have remained in general use with hammers that can reach up to 350 inches per second. The basic construction of an air powered hammer is that of all forms of drop forges and includes an anvil base that is supported by two columns that guide the ram.

A significant upgrade to air powered drop hammers is the addition of programmable air hammers that allow operators to set the number of blows and the force of each blow. The programmability makes cycle times faster and running clearances are lowered due to the ram guiding method. The configurations of the forging have a closer match to provide tighter tolerances with cycle rates at 80 to 120 strokes per minute.

Board Drop Hammer

With the board drop hammer, maple boards are attached to the ram and are used to raise the ram using rollers that are pressed against the board. At a preset height, the pressure on the rolls is released and the ram drops by gravity or an operator. Unlike pneumatic and hydraulic drop forges, there is no force applied to the ram, which falls by gravity with the force of the blows to the workpiece dependent on the weight of the ram, which can range between 453.59 kg up to 2267.96 kg (1000 lbs. to 5000 lbs.).

Board drop hammers are an old design that are used in situations where power is expensive or difficult to get.

Air Lift Hammers

Air lift drop hammers use compressed air to lift the ram, a process that is similar to board drop hammers. They are a gravity type of drop forge that allows the length of the stroke and the velocity of the ram to be controlled to vary the amount of energy that is delivered to the workpiece. A steel rod connected to a piston lifts the ram using the force of the compressed air. Once the ram is raised, it is held in position by an air circuit that controls a clamp to hold the ram. The force of the ram is program controlled, which also controls the number of blows.

Air lift hammers are capable of delivering 50 to 75 strokes per minute. The popularity of air lift hammers is due to their lower maintenance cost and ease of operation.

Counterblow Hammers

With counterblow drop hammers, an upper ram is driven downward or sideways using steam or air as a lower ram moves upward or sideways to meet the other ram. Since there is no anvil, the workpiece is placed between the two rams and is shaped by their upward and downward action or sideways action for horizontal counterblow forges. The design of the counterblow hammer is to overcome the shortcomings of the traditional hammer and anvil method. Hammer and anvil drop hammer forges have to be positioned vertically such that the excess energy generated by each blow of the hammer can be dissipated into the structure of the forge or the surface below the forge.

Counterblow forges can be positioned horizontally since any excess energy created by the shaping of the workpiece becomes recoil. The two rams of a counterblow hammer strike one another, which reduces the force that is absorbed by the frame of the forge. Only one of the hammers is powered and moves the otherram by a coupling that is hydraulically powered. The mechanism of the counterblow hammer produces a unique flow pattern on the workpiece. As with traditional hammer and anvil drop forges, counterblow hammers can use open and closed dies.

The idea for counterblow forging is based on the idea that two bodies of equal mass forced toward each other will concentrate the absorption of energy. When a workpiece is placed between them, it is deformed by the impact of the energy generated by the two masses. The nature of the impact energy is preset and regulated for precise time controls to provide consistent energy for metal deformation.

The speeds and cycles for counterblow hammers is the same as double acting hammers with lower productivity due to the inability to perform multiple impressions. Tolerances have a broader span and guiding clearances are twice as much as those for traditional hammer and anvil forges. Also, loading, handling, and unloading are far easier with traditional hammer and anvil drop forges. Counterblow hammers are widely used in Europe with a few located in the United States. The largest counterblow hammers can deliver 900,000 foot pounds of energy and are used for the manufacture of very large workpieces.

Chapter 5: Advantages of Drop Forging

Drop forging is used to manufacture a wide assortment of high tolerance products and produces components and parts for industries that require high strength and durable products. The factor that differentiates drop forging from any other manufacturing technique is the grain structure of the finished products.

The alignment of the grains in a metal determines the characteristics and properties of the products produced from a metal. Various methods are used to shape products and help maintain the original grain structure. Drop forging is one of the primary shaping methods that helps maintain and improve the grain structure of products to give the products strength and a lasting enduring quality.

Strength

Drop forging improves the mechanical properties of metals, which results in products having exceptional strength, toughness, and durability as well as resistance to fatigue and impact. The deformation process aligns the grain structure, reducing weak points and improving structural integrity.

Structural Integrity

One of the major reasons to use drop forging is the compressive force that is applied to a workpiece, which eliminates internal voids, porosity, and defects in the metal. As a result, workpieces have improved structural integrity and strength. It is essential that any defects, deformities, or voids in the metal be removed, the removal of which enables components to endure heavy loads, vibrations, and stressful operating conditions.

Dimensional Accuracy

As every designer and engineer knows, the dimensional accuracy of a part is necessary in order to achieve tight tolerances and consistent shapes. With drop forging, dies and controlled deformation ensure that products meet design specifications and fit into the assemblies for which they are designed. The accuracy and consistency of drop forging removes the need for secondary machining and adjustments.

Cost

Components for applications can be produced using a variety of manufacturing processes. One of the factors that influences the decision as to what method will be used to produce a component is the cost of the process, which, in some cases, can be too high and impractical. Drop forging is a high-volume process capable of producing thousands of parts in a short time. It reduces material waste due to its accuracy and produces high strength to weight ratio parts.

Versatility

Although there are specific metals that perform best when drop forged, the number of metals that can be used in the process are endless and range from soft easily formable metals to high hardness ones that require more aggressive deforming. This factor allows designers, engineers, and manufacturers to choose metals that best fit their project based on the desired properties of an application.

Design Flexibility

The versatility in the choice of metals allows designers and engineers to create components with intricate details, complex configurations, and unique and unusual geometries, which would be difficult and expensive to produce using any other manufacturing process. The shape, size, and form of a part can be produced in any customized shape such that it exactly fits design requirements and needs.

Drop Forging Grain Structure

Changes to grain flow is important in regard to the manufacturing of components that have to withstand high stress and impact. The adjustment of grain flow during the drop forging process improves the impact and fatigue resistance of parts, which makes it less likely that they will fail by fracturing or cracking during use. In addition, drop forging increases the ductility of metals, a factor that allows them to undergo changes in shape without breaking.

The grain flow created by drop forging directly affects the mechanical properties of a component or part. It determines the push or pull of grains when placed under stress or fatigue. Since cracks are more likely to occur parallel to the grain structure, stress applied to a component should be perpendicular to the grain structure. Drop forging produces components that have their grain structure aligned in the direction of maximum strength

The durability and strength of forged parts is why they are used in power generating industries, the defense industry, and aerospace. In essence, the use of finely structured grain formation is essential for industries that cannot afford unreliable components.

Chapter 6: Industries that Rely on Drop Forging

One of the fascinating facts about drop forging is its origins, which dates back hundreds of years to China where bronze and iron were hammered into weapons, tools, and household items. The original version of drop forging did not involve impact machinery that applied tons of force to a metal piece but was performed by a strong man with a hammer. It is amazing that this ancient manufacturing art has progressed through the ages to become an essential part of so many industries.

Automotive Parts

As any mechanic or engineer can tell you, components for automobiles have to be exceptionally strong and durable. It is for this reason that critical parts of cars are produced using drop forging.

• Crankshafts: Crankshafts provide rotational motion and transfer power from the engine to the transmission.
• Connecting Rods: Connecting rods are the other part of the transfer of power. They are connected to the piston that drive the crankshaft.
• Suspension Systems: Suspension systems are not the most exciting part of an automobile but are complex and include several parts produced by drop forging, such as control arms, ball joints, and tie rods, provide stability, control, and supports.
• Steering Components: Steering components include pitman and idler arms that ensure accurate steering control.

Hand Tools

Drop forged parts for automobiles are extremely critical and require the highest amount of strength and durability. Hand tools are on a more practical level and require the same characteristics. In most cases, they have a simple design and are normally forged from a single billet. From DIY users up to professionals, hand tools have to be made of materials that won’t fail and are dependable.

• Hammers: The term hammer refers to a wide array of tools that come in different shapes and sizes and are designed of high strength metals to pound in nails, remove and bend materials, and perform other high strength activities.
• Wrenches: As with hammers, wrenches come in a wide assortment of sizes and require high strength to tighten or loosen bolts and nuts. An essential part of wrenches is their torque capacity, which makes them ideal for mechanical and plumbing applications.
• Pliers: Pliers are like all other tools in that they come in a wide variety of shapes and sizes and require durability and strength in order to endure stress to which they are exposed.

Construction

Of all the various industries, the construction industry depends on tools and components that are capable of withstanding the pressure and stress of constant use. Although it is common to find hand tools as part of construction, there are more critical tools that require greater strength and endurance due to the importance of the application for which they are used.

• Shackles and Hooks: Shackles and hooks are used in riggings and lifting applications. They are forged to form a secure connection and strong enough to withstand the weight of a load.
• Sprockets and Gears: Sprockets and gears are made of hardened metals that are capable of reliable power transmission for machinery and industrial equipment. The key properties of sprockets and gears is their ability to withstand the stress placed on them by heavy duty machinery.
• Flanges and Fittings: Flanges and fittings are an essential part of piping systems. They are designed to provide solid and leak proof connections.

Aerospace Components

The guiding principle for all aeronautics components is reliability and safety. Unlike the parts for other industries, failure of parts in the aeronautics industry can mean the loss of lives. The critical nature of aerospace necessitates the use of drop forging to produce the highest quality and most reliable components.

• Jet Engine Parts: Turbine blades, compressor discs, and shafts used in jet engines, where high strength and resistance to extreme temperatures are essential, have been produced by drop forging for decades.
• Landing Gear Components: Landing gear parts, such as struts and brackets, provide required stability and support during landings and takeoffs.
• Wing Fittings: Wing fittings, that are simple in structure, are an essential part of airplane construction since they ensure that wings are properly positioned and secure.

Conclusion

Drop forging is a manufacturing process that applies multiple blows of several tons to a workpiece to shape and form it into a viable component, part, or product.

The dies of drop forging can be open or closed and are factors that differentiate drop forging processes.

The wide use of drop forging is due to the reliability and dependability of the parts that are manufactured by the process. During the compression of a workpiece, the grain structure of the metal is changed to create different and more durable properties.

Drop forging is an ancient manufacturing process that began hundreds of years ago with a man with a hammer and progressed over the centuries to become a highly efficient and reliable form of manufacturing.

Industries that rely on tight tolerances and dimensionally accurate parts depend on drop forging for the most critical parts of their assemblies.

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