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Open Die Forging Vs Closed Die Forging For Aluminum & Stainless Steel

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The decision to procure near-net material shapes has several advantages, primary among them cutting down on machining costs.  There are many choices to consider; casting, forging, sintered or additive manufacturing are just s few.  It turns out there is no simple solution that applies to all situations.  That is why today’s post will examine the differences between open die and closed die forgings, and how they apply to both aluminum and stainless steel.

One thing to keep in mind when choosing between techniques is what material will be specified.  Matching the right material with the right forging method for the required application is essential.  This task is complicated when there are so many different alloys and tempers to select from.  That’s why working with a trusted material supplier who has experience partnering with clients in a range of industries is so important.

What do we mean by forging?

Forging is one of the oldest methods of working with metal, dating back thousands of years.  It is defined as the process of forming and shaping metal using localized compressing forces.  It starts by heating a cast billet to the desired temperature (except in the case of cold forging), then applying high pressure force until the final form is achieved.

Forging brings with it a range of benefits, yielding finished pieces that are high quality.  It results in high strength parts that can be customized for both size and performance according to the specific needs of the application.  There are a variety of different types of forging methods, including open die, closed die, cold forging and seamless ring forgings.

What is open die forging?

A blacksmith is someone who shapes (or forges) metal by heating it in a furnace and then hammering it into shape against an anvil.  In the most simplistic terms, this is exactly what open die forging is: applying force to shape a piece of metal.

Traditional open die forging involves carefully placed hammer strikes that are used to shape the workpiece, which is held in place on a stationary anvil or other surface. The term die refers to the working tool that is in contact with the work piece, applying pressure to it. In the open die configuration, this means that these tools do not enclose the metal, so that the deformed material can flow freely after contact.  To achieve the desired shape, it is necessary to position and manipulate the workpiece accordingly.

The dies in open die forging may be flat or have a simple contour. The final shapes can be discs, hubs, blocks, shafts, flats, hexes, rounds, plate or other custom shapes, all testament to the versatility of open die forging.

The advantages of open die forging include a reduced chance of gaps or voids, increased fatigue resistance, improved microstructure, continuous grain flow, fine grain size, increased strength, high internal quality, good reliability, excellent impact resistance and greater ductility.  The finished pieces also respond well to subsequent thermal processing.

What is closed die forging?

Also known as impression-die forging, closed die forging involves a workpiece that is contained within a die that is attached to the anvil.  The hammer is also shaped, and as it drops on the metal, the workpiece is forced to flow into the die cavity and take on its contours.  Modern closed-die set ups can hammer the metal many times in a very short period.  The rapid pounding leads to excess metal being forced out of the die and trimmed away, leaving only enough metal to fill the die.

In high production operations, the workpiece may be moved through several dies to take it from its original form through subsequent shapes until it achieves its final shape.  This allows a manufacturer to quickly and efficiently create the desired form. This has also allowed for the closed die process to become more automated in recent years.  The ability to achieve uniformity is one of the major advantages of closed die forging.

Aluminum alloys suitable for forging

Forged aluminum is perfect for many applications, especially when safety and performance are needed from a lightweight metal.  Aluminum forged components are often found in parts that are located at the point of stress in a larger mechanism, such as gears or wheel spindles.

Industries where you are likely to find forged aluminum include aerospace, automotive, firearms and others that entail harsh environments and high standards for durability.  You will also find forged aluminum in hand tools such as pliers, hammers, wrenches, and surgical devices.

Some of the most commonly forged aluminum alloys include those used for commercial purposes, such as 6061 and 7075.  In the aerospace industry, you’re also likely to find alloys including 2024, 2219 and 7050.

Aluminum is suitable for both open die and closed die forging.  For open die, it’s possible to process extremely large pieces, weighing up to 200,000 pounds and 80 feet in length.  On the other hand, closed-die forged aluminum can produce an unlimited number of shapes, ranging in weight from a few ounces all the way to 25 tons.

Stainless steel alloys and forging

Stainless steel also offers several benefits when it comes to forging.  These include its elevated corrosion resistance.  Stainless steel is therefore a good choice for applications that are subject to stress cracking, crevice corrosion, pitting and deformation.  Forged stainless steel is also resistant to extreme heat, making it ideal for heat exchangers, furnaces and ovens.

Stainless steel alloys that are suitable for forging applications include those from the 300 series, such as 303, 304, 316 and 321.  The 400 series is also good for forging, including 410, 420 and 440.

Applications for forged stainless steel represent a wide variety of industries, everything from agriculture, construction, sports and recreation, transportation, petrochemical, aerospace, marine and medical.

To learn more about what stainless steel and aluminum alloys work best for open die and closed die forging applications, contact Clinton Aluminum today.

 

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