The Significance of Pre-forming and Upsetting in Closed Die Forging Processes

Understanding Closed Die Forging and Its Core Techniques

Closed die forging is a precision metal-forming process that shapes heated metal billets under high pressure within a set of dies that contain the workpiece completely. Unlike open die forging, where the metal is not fully constrained, closed die forging allows manufacturers to produce complex, high-strength components with tight tolerances. The process is essential in industries such as aerospace, automotive, power generation, and heavy equipment manufacturing, where part reliability and performance are non-negotiable.

Two techniques that play a pivotal role in achieving these results are pre-forming and upsetting. While often discussed separately, these operations work in tandem to optimize material flow, reduce defects, and enhance the mechanical properties of the final part. Mastering these steps can mean the difference between a forged component that meets stringent specifications and one that fails prematurely.

The Role of Pre-Forming in Closed Die Forging

Pre-forming, also referred to as pre-shaping or blocking, is the initial deformation of the billet into an intermediate geometry that approximates the final part shape. This step is performed before the workpiece enters the finishing die, and its primary goal is to distribute metal volume appropriately so that the final forging stroke can fill the die cavities completely without defects.

Why Pre-Forming Matters

Without pre-forming, the raw billet would require excessive deformation in the finishing die. This can lead to several problems:

Incomplete die fill – material may not reach thin or deep sections of the die.
Lap and fold defects – metal folds over itself, creating weak points.
Excessive forging loads – requiring larger presses and higher energy consumption.
Premature die wear – the finish die experiences higher stresses and abrasion.

By pre-forming the billet, manufacturers can control how the metal flows during the final forging, ensuring that the part geometry is achieved with minimal effort. The pre-form die (often called a blocker die) typically has a simpler shape than the finishing die. It may include generous radii and draft angles to facilitate metal flow.

Types of Pre-Forming Operations

Depending on the part complexity and billet size, pre-forming can take several forms:

Fullering – reducing the cross-section of a portion of the billet to concentrate material in specific areas.
Edging – shaping the billet to a rough contour using edger rolls or a flat die.
Blocking – a more refined pre-shape that closely matches the finish die, often used for complex geometries.
Bending – for parts that require a curved axis, the billet is bent before final forging.

Each method is chosen based on material properties, final part design, and production volume. For high-volume runs, dedicated pre-form dies are designed to minimize finish die wear and maximize consistency.

Material Considerations in Pre-Forming

The pre-forming step must account for the material’s flow stress, temperature range, and workability. For example, materials like titanium and nickel-based superalloys have a narrow forging temperature window and require precise pre-forming to avoid cracking. Steels with high carbon content may need slower deformation rates to prevent strain hardening. Pre-forming helps reduce the number of reheating cycles, improving energy efficiency and reducing scale formation.

For applications where grain flow orientation is critical—such as aerospace turbine discs or automotive crankshafts—pre-forming is designed to align metal fibers in the direction of the primary stresses. This enhances fatigue resistance and overall part strength.

The Role of Upsetting in the Forging Process

Upsetting is a specific forging operation where the height of the billet is reduced while its diameter increases. It is one of the most fundamental forming processes and is frequently used in closed die forging to prepare billets for further shaping or to create a head on a blank.

How Upsetting Works

A billet is placed between two flat dies, and a compressive load is applied axially. As the height decreases, the material bulges outward. In controlled conditions—using lubricated dies and proper temperature—the bulge remains uniform, increasing the cross-section evenly. The upset ratio (height-to-diameter) must be kept within limits to avoid buckling. For typical steels, a ratio of 2:1 is safe; higher ratios require multiple upsetting steps or support tools.

Significance of Upsetting in Closed Die Forging

Upsetting serves several critical functions:

Grain flow refinement – the compression realigns the grain structure perpendicular to the axis of the original billet, improving strength in the radial direction.
Material concentration – upsetting gathers metal where needed, such as the head of a bolt or the flange of a gear blank.
Elimination of internal voids – the high compressive forces close porosity and weld internal cracks.
Reduction of billet length – preparing a shorter, fatter workpiece that can fit into the finishing die cavity.

Often, upsetting is performed as a pre-form step, especially for parts that require a large head relative to the shank. For example, in automotive connecting rods or heavy-duty fasteners, upsetting creates a preform that aligns with the subsequent blocker and finish dies.

Upsetting with Pre-Forming: A Combined Approach

In many closed-die forging sequences, upsetting is followed by pre-forming (blocking) before the final finish forging. This sequence allows precise control over material distribution. The upsetting step increases the cross-section and refines the grain, while the pre-forming die reshapes the upset billet into an intermediate geometry that approximates the final part. The finishing die then completes the shape with minimal deformation, reducing die wear and improving accuracy.

This combination is especially valuable for parts with significant variations in cross-section, such as turbine blades, connecting rods, and structural aerospace components. By designing the upsetting and pre-forming stages together, engineers can achieve near-net shapes that require little machining.

Comprehensive Benefits of Pre-Forming and Upsetting

When properly applied, these techniques deliver measurable advantages across the entire forging process. Below is an expanded exploration of the benefits mentioned in the original article.

Improved Material Flow and Reduced Defects

Metal flow during forging is influenced by friction, temperature, and die geometry. Pre-forming and upsetting create favorable flow patterns that guide the metal into the deepest and thinnest die cavities. This reduces the risk of cold shuts (when two metal streams meet without bonding) and folds (when surface metal gets trapped inside). A well-designed pre-form ensures that the material moves in a controlled, laminar manner rather than turbulent, which would cause defects.

Studies have shown that pre-forming can reduce the incidence of forging defects by up to 40% compared to direct forging from a cylindrical billet. This is critical for safety-critical components like landing gear parts or high-pressure valves.

Enhanced Dimensional Accuracy and Near-Net Shape

Closed-die forging already offers good dimensional tolerances, but incorporating pre-forming and upsetting pushes accuracy even further. Because the final deformation is smaller, the die experiences less elastic deflection, leading to more consistent part dimensions. For complex geometries, pre-forming reduces the need for subsequent machining operations, saving material and time. In some cases, parts can be forged to a near-net shape that requires only grinding or polishing.

Extended Tool Life and Reduced Downtime

The finishing die is the most expensive tool in the forging set. By using pre-form and upsetting stages, the finish die is only used for a relatively small amount of deformation, primarily improving surface detail and final dimensions. This reduces the mechanical and thermal stresses on the finish die, prolonging its life. Tool life can increase by 30-50% compared to forging without pre-forming. Additionally, the blocker dies used for pre-forming are simpler and more economical to replace, reducing overall tooling costs.

Superior Mechanical Properties Through Grain Flow

Upsetting and pre-forming align the grain structure of the metal to follow the part contours. For a forged connecting rod, for example, the grain flows along the shank and around the head, providing maximum strength where cyclic loads occur. This anisotropic property is a key advantage of forging over casting or machining from bar stock. Components with controlled grain flow exhibit higher fatigue strength, impact toughness, and resistance to stress corrosion cracking.

Standards such as ASTM A668 and AMS 6419 often require certain grain flow patterns, making pre-forming and upsetting essential for compliance.

Cost Efficiency and Process Optimization

Although adding pre-forming and upsetting steps increases the number of process stages, the overall cost per part often decreases due to:

Reduced material waste (less scrap from flash or machining)
Lower energy consumption (fewer reheating cycles)
Higher production rates (fewer rejected parts and die changes)
Extended tool life

A well-optimized forging sequence can yield overall cost savings of 15-25% compared to a process that skips pre-forming. For high-volume production, this translates to significant financial benefits.

Practical Applications Across Industries

Pre-forming and upsetting are not just theoretical concepts—they are applied daily in major manufacturing sectors.

Aerospace

Titanium and nickel alloy components such as fan discs, compressor blades, and structural bulkheads rely on precise pre-forming to achieve the required grain flow and mechanical properties. Upsetting is often used to create the boss or hub sections of these parts. Examples include the ASM International guidelines for forging aerospace alloys.

Automotive

Engine connecting rods, steering knuckles, and axle shafts are typically forged using upsetting to form the large ends and pre-forming to shape the intermediate geometry. High-strength steel forgings for automotive applications benefit from reduced weight and improved fatigue life.

Oil and Gas

Valve bodies, flanges, and drilling equipment often require upsetting to create thick-walled sections that can withstand high pressure. Pre-forming helps manage material flow for complex internal cavities.

Heavy Equipment and Mining

Large gears, spindles, and excavator components require massive billets that are first upset to refine the grain and then pre-formed to reduce the load on the finishing press. This is especially important for parts weighing several tons.

Key Process Parameters and Best Practices

To obtain the full benefits of pre-forming and upsetting, manufacturers must control several parameters:

Temperature uniformity – The billet must be heated uniformly to avoid thermal gradients that cause uneven flow or cracking. Pre-heating dies also helps maintain a consistent temperature.
Deformation rate – For most metals, a moderate strain rate (1-10 s⁻¹) is ideal. Extremely high rates can cause adiabatic heating and shear bands; low rates may allow excessive heat loss.
Lubrication – Proper lubricants (graphite, water-based, or oil-based) reduce friction and help control bulging during upsetting. If friction is too high, the material will barreled, leading to folds.
Die design – Pre-form dies should have generous radii and draft angles (3-5°) to facilitate flow. The upsetting dies must be flat and parallel to prevent uneven loading.

For reference, the Forging Industry Association (FIA) provides extensive resources on die design and process optimization. Manufacturers should also consult standards such as ISO 17022 for closed-die forging tolerances.

Common Defects Mitigated by Pre-Forming and Upsetting

Several forging defects are directly addressed by these techniques:

Defect	Cause	How Pre-Forming/Upsetting Helps
Laps	Metal folds back onto itself	Pre-forming creates smooth flow paths, reducing folding risk
Incomplete fill	Material not reaching cavity extremities	Upsetting concentrates metal where needed; pre-forming distributes volume
Internal cracks	Tensile stresses during forging	Compressive upsetting closes cracks; pre-forming reduces tensile strains
Scale pits	Oxide scale trapped in the surface	Pre-forming steps can be followed by descaling (e.g., with water jets) before final forging

By selecting appropriate upsetting ratios and pre-form shapes, manufacturers can virtually eliminate these defects in high-quality parts.

Conclusion: The Strategic Value of Pre-Forming and Upsetting

Pre-forming and upsetting are far more than optional steps in the closed-die forging sequence. They are essential techniques that enable the production of complex, high-performance parts with excellent material utilization and economical tool life. Whether forging a small aerospace fastener or a massive gear blank, the careful design of these intermediate stages directly impacts the final product’s quality and cost.

Engineers who invest time in upsetting and pre-forming process design—supported by simulation tools like finite element analysis (FEA)—can achieve near-net shapes with consistent grain flow and minimal defects. The result is components that meet the most demanding standards in aerospace, automotive, and heavy industries, while also improving sustainability by reducing scrap and energy consumption.

For further reading on closed-die forging techniques and best practices, the ASTM A668 standard for carbon and alloy steel forgings and resources from the Institution of Mechanical Engineers (IMechE) provide authoritative guidance. By mastering the interplay between upsetting and pre-forming, manufacturers can stay competitive in an industry that demands increasing precision and reliability.