To design a component that requires the ultimate metallurgical advantage of forging, a design engineer must understand how to generate a design to maximize forging advantages. Forging is the initial, critical process to assure product quality. All other steps are included to generate a finished, integrated product to achieve the optimum result.
While specialized forging processes can produce nearly complete components, forging is not generally considered a process to provide net shapes. Materials that flow more easily in a solid state at lower temperatures can be forged net or near net. In alloys requiring higher force and temperature to achieve metal flow, the ability to create fine detail or net shape is restricted.
Limitation is the strength of the die materials.
Even when heated to high temperatures, forged metals are as strong as the dies used to form the material. Die life will be short, while repair and replacements costs will be high. Therefore, the initial design considerations will be based on the type of material, the quantity to be produced, and the process steps required after forging to achieve the finished component.
First “rule of thumb” is that parts required in low volume and higher temperature material will argue for open die forging processes. This creates simple shapes, requiring significant secondary operations. Some low volume items may require impression die forging. In those cases, several approaches can be used to hold down the initial cost of tooling. As tooling becomes more complex, tool costs will rise.
High volume, lower temperature materials will argue for impression die forging.
Volumes and configurations may allow specialized tooling and forging processes dedicated to an end use component or family of similar design. Processes such as cold and warm forging might be considered where net shaped forgings are commonly produced. In many cases, the tooling costs are very high. The costs are only justified by large, ongoing volume demand for parts.
What about the moderate middle, where the greatest volume of forgings is produced? How are forgings designed when quantities range from hundreds to several tens of thousands per year? The job becomes more difficult for the designer. Understanding some basic rules of impression die forging is essential to making a cost-effective decision.
It is useful to think of impression dies as tools working in two dimensions to create an object in three dimensions. Typical forge dies consist of two blocks of hot work tool steel with matching impressions cut into the face of each block. When the blocks are brought together with force, the material trapped between the blocks is forced to flow into the impressions.
The special skill of the die designer is not just to create a single cavity, but a series of forming cavities. These forming cavities shape material from an original simple shape into the desired forged configuration.
These steps must cause material flow that prevents the creation of any laps or seams.
The progressive forming must not overstress the material being forged risking defects or risk breaking the dies. The deformation energy must drive a thermomechanical process that guarantees refined metallurgical structure. Although these concerns are for forging die design engineers, the effects of the forming requirements, dictated by the features of the desired end product, impact the final forged configuration.
Every impression die forging must be designed with dimensional features inherent to the process. Forging designs must include draft angles that open to the parting line – where the two dies come together – which allow the part to release from the die cavity.
Areas of reverse draft result in the production of a single forging that requires destruction of the die to remove the part form the cavity. Zero draft angles can be designated on side walls. This can be somewhat successful in production of forgeable materials, and when ejectors are built into dies. A common drawback is that polishing and die wear from repeated part production will cause the cavity to open. It is always better to plan for some draft, even if no draft is the goal.
Forge tooling must avoid sharp corner radius and fillets.
Material flowing over sharp edges will break down the outside radius. Material forced into sharp internal corners will tend to cause the die to crack. Wedge shaped cavities, thin or thick forging sections, and complex configurations over a large plan area will increase forging difficulty and reduce die life.
In more difficult to forge materials, draft must increase. Therefore, the radii and fillets must become more generous to preserve die life and assure consistent productivity.
The benefit of net or near net shaping requirements that will negatively impact the costs of die repair and reduced productivity must be recognized. It must be determined if features may be more economically achieved in secondary machining operations, especially if machining operations are already anticipated to achieve the finished configuration.
Many designers are immersed in programming complex tool paths with expensive multi axis mills to whittle components from relatively inexpensive blocks of raw material. Many others been attracted to the potential of AM to produce a net shaped item, requiring no further operations to finish the part. The hope of such processes is that basic raw material can be put into a high-tech machine and the required item will magically appear, quickly and cheaply, a fully functional, finished part at the end of the process.
The truth is that achieving such a result is not so simple.
There is no guarantee of a serviceable component. True wrought properties are only achieved though thermomechanical processing. Often the most cost-effective methods are achieved by using combinations of forging, heat treating and machining to yield a superior product, competitive in the marketplace.
Additional design forging information, and guideline tolerances can be found here.