The following is a brief explanation of factors to consider when determining whether to use a die forging instead of a product made by other means.  It is by no means exhaustive, scientific, or conclusive.  It is merely a rough guideline to quickly determine the feasibility and economy of utilizing die forged products.  No guarantee of viability, accuracy or validity is intended or implied by the statements made herein.  Use of these rules of thumb by anyone for any reason is solely at the risk and judgment of the user.  Neither Mr. Silva, Ms. Winter, nor APP assumes any responsibility for the application of these rules of thumb by anyone.

It is assumed that the products under consideration are aluminum.  Therefore, the rules of thumb contained herein apply to aluminum die forgings only, although they may prove helpful when considering other forged materials.

David Silva, Vice President of Sales and Marketing, and Jacquelyn Winter, Chief Metallurgist, of Aluminum Precision Products, Inc. (APP), prepared this explanation.
As a preface to responding to this question, there are some overriding factors that demand the use of die forgings over parts made by other processes.  Due to certain important characteristics, such as grain orientation and enhanced properties resulting from high deformation and heat treat responsiveness, the die forging is the most suitable aluminum product for highly critical applications.  This will be reflected in the design parameters called out by airframe, automotive and other technical specifications.

There are various other factors that determine the benefit, feasibility and economy of choosing a forging over other methods of manufacture (e.g., casting, hogout,* extrusion, etc.).  These factors include strength, time, dimensional tolerances, return on or justification of non-recurring costs and manufacturability.  Some of these factors may be condensed into ‘general rules of thumb,’ which will be described below.

(*the term ‘hogout’ will be defined as a part that has been 100% machined from plate, forged block or extrusion)

We will assume for now that mechanical properties need not be addressed in our response to the question, ‘what makes a good die forging candidate?’  Castings, therefore, may be eliminated immediately, as it is well known that their mechanical properties are not in the same league as forged products.  However, it must also be stated that where the higher forged properties are not required, the economics of castings sometimes justify choosing the casting.

Parts machined from plate, forged block or extrusions, depending on the orientation of the grain, may have similar tensile properties as die forgings, although all would exhibit variations in performance when subjected to fatigue, stress corrosion and other cyclical applications, with die forgings usually outperforming the others.

For purposes of this exercise, we will assume that we are talking about aluminum die forgings vs. machined from plate, block or extruded product.

In order to eliminate the extrusion comparison from the onset, we can determine that products that can be oriented in such a way as to find the smallest cross-section, and therefore the smallest extrusion envelope, with the least amount of material removed by machining to achieve the final configuration, should be compared at once to the die forging with its added cost, if any, to machine to the final configuration.  A determination should be made whether the cost of the extrusion (including the machining required to achieve the final configuration) is lower than the cost of the final product made from the die forging.  Extrusion tooling is typically much cheaper than die forging tooling, and should therefore be included in the economic equation when comparing with the die forging.

Now that we have distilled the comparison to that between the die forging and parts machined from plate or block, we can focus on the major factors that determine a good die forging candidate.
I) Tooling:

If the number of pieces required for a given program is low (less than 50 pieces per year), the cost of die forging tooling likely will not justify choosing the die forging.  There are no (or seldom) tooling charges for plate or hand forged block, and the only non-recurring charges for the hogout are machine programming and fixtures; which, although some may be quite high, usually do not approach the higher costs of fabricating die forging tooling.  Additionally, if the forging concept requires significant machining, there would also be machine programming and fixtures required for the forged part, on top of the die forging tooling costs.

General rule of thumb #1:

  • If the investment in die forging tooling can be justified within 3-5 years or less, depending on the expected longevity of the program, the die forging should be considered.  Each company may have its own return on investment criteria.  Three to five years should be considered only as a rule of thumb.

II) Configuration:

This factor is perhaps the most elusive, yet the most important when considering whether to use a die forging vs a hogout.

When considering the smallest rectangle enveloping the entire part, if the height (or thickness) of that rectangle is low relative to the width and length, then the part might be less costly to machine from plate or block.  Particularly if the width and length provide for a large PVA (Plan View Area – the area upon which the forging pressure would be applied to fabricate the die forging), the tonnage required to forge such a part may require very large presses (generally, from 15 to 25 tons per square inch PVA are required to forge aluminum, depending on configuration and other factors).  The cost to operate large presses is high.

Additionally, if the dimensional tolerances are tighter than can be achieved by the forging process, the cost to machine such parts from plate or block, particularly if there are no complex contoured surfaces, may be only slightly higher than to machine the die forging.  The cost to machine such a die forging, added to the relatively high cost of forging may drive the total unit cost higher than the hogout cost.  Of course, when the cost of tooling is factored in, the overall cost to produce thin, high PVA parts is usually prohibitive.

General rule of thumb #2:

  • The more the die forging reduces the amount of metal removal to achieve the final configuration, the more likely the die forging will be economically feasible.

General rule of thumb #3:

  • parts with a height less than 2 inches should be reviewed as a hogout before choosing the die forging.

General rule of thumb #4:

  • parts with high PVA vs low height should be reviewed as a hogout before choosing the die forging.

Another configuration characteristic that often quickly determines the feasibility of the die forging over the hogout is the presence of complex contoured surfaces on the final configuration.  Machining such surfaces, particularly surfaces requiring kellering or other time-consuming measures, is very expensive.  When these surfaces are applied to the die forging tooling that will create such surfaces on the part during the forging process, the repeatability on the forgings is very good.  In cases where the forging tolerances are acceptable on such surfaces, therefore eliminating the complex, expensive machining, the economics of the comparison will most often be in favor of the die forging.

General rule of thumb #5:

  • parts with contoured surfaces (especially complex contoured surfaces) should always be studied as die forgings before choosing a hogout.

III) Tolerances:

The forging process cannot achieve the same tight tolerances as can the machining process.  Typical forging dimensional tolerances range from +/- 0.030” (for conventional forgings), to +/- .015” (for precision – near net forgings).  Also, contour tolerances, surface quality and other measurable features are usually more generous on the die forging than on the machined part.  In addition, the forging, depending on the configuration, usually requires additional material for releasing the part from the forging tooling during fabrication.  This material is known as ‘draft.’  Sharp corners are usually, if not always, replaced by corners and fillets to reduce or eliminate die wear.  These measures add material that often requires removal after the forging has been produced.  In many cases, the removal of this added material, as well as the tolerance and surface requirements may be localized on the part and may be machined on the die forging more economically than on the machined part.

General rule of thumb #6:

  • If forging tolerances are acceptable on most surfaces, the residual machining that may be required on the die forging is usually less costly than the machining required to make the part from plate or block.

Conclusion:

  • Due to certain important characteristics, such as grain orientation and enhanced properties due to deformation and heat treat responsiveness, the die forging is the most suitable aluminum product for highly critical applications.  This will be reflected in the design parameters called out by airframe, automotive and other technical specifications.
  • If the investment in die forging tooling can be justified within 3-5 years or less, depending on the expected longevity of the program, the die forging should be considered.  Each company may have its own return on investment criteria.  Three to five years should be considered only as a rule of thumb.
  • The more the die forging reduces the amount of metal removal to achieve the final configuration, the more likely the die forging will be economically feasible.
  • parts with a height less than 2 inches should be reviewed as a hogout before choosing the die forging.
  • parts with high PVA and low height should be reviewed as a hogout before choosing the die forging.
  • parts with contoured surfaces (especially complex contoured surfaces) should always be studied as die forgings before choosing a hogout.
  • If forging tolerances are acceptable on most surfaces, the residual machining, if any, that may be required on the die forging is usually less costly than the machining required to make the part from plate or block.