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A lesser-known cold-extrusion metal-forming technique, flowforming, makes seamless, near-net, complex, hollow metal parts. The process eliminates welds, which reduces the need for testing, and uses less energy than other manufacturing methods because it wastes less material.
Over the last 60 years, flowforming has been used in many industries. The process has gained favor in recent years because technical advancements now let it produce hollow, thin-wall, symmetrical shapes with close profile and wall-thickness tolerances. The technique works with many different materials including Inconel, Hastalloy, columbium, stainless steel (PH, ferretic, and austenitic), aluminums, and other high-strength alloys.
Flowforming is similar to metal spinning in that it forms metal against a hardened mandrel. The difference is that spinning uses a relatively thin starting material and a larger-diameter starting blank. Metal spinning, like deep drawing, forms the part to the mandrel or punch, but lacks control over the wall thickness. This can cause wide part variations.
In contrast, current flowforming techniques ensure a repeatable part because computer modeling and CNC equipment control wall thicknesses and profiles to a few thousandths of an inch. Optimal wall thicknesses for flowformed parts range from 0.010 to 0.375 in. Parts with a length-to-diameter ratio of 2.5:1 or greater typically produce the best results.
The design of the preform, the shape of the part before flowforming, is one of the most-critical starting points for successful parts. The first step is to reverse-engineer the final component. Additional computer modeling can then help cut development time and reduce wasted material. Preforms and the associated tools are unique to each different part. The preform is produced using deep drawing, machining, or forging.
Flowforming entails rotating and compressing the preform against the hardened mandrel. Metal is trapped between the mandrel and a set of two, three, or four forming wheels. The gap between the mandrel and wheels determines the finished-part thickness. This gap can change or remain constant anywhere along the length of the part. Flowforming is the only metal-forming process that can vary the wall thickness to produce thicker and thinner sections in any combination without removing material. Sizing, finishing, machining, and flange forming are typical secondary operations.
Because flowforming is conducted at ambient temperatures, many materials “cold work” due to strain introduced to the metal during the process. Cold working changes the metal’s mechanical properties. For example, cold working austenitic stainless steel typically doubles the mechanical strength. 304-grade stainless steel in an annealed state has a tensile strength in the mid-80-ksi range with an elongation of about 45%. After flowforming reduces the material’s thickness by 60%, its typical mechanical properties are 160-ksi ultimate tensile, with elongation reduced to approximately 15%.
Because the original specification may have been designed based on the starting material’s attributes, strain hardening is not necessarily an asset. Fortunately, flowformed parts can be heat treated (fully annealed) to restore the material’s original mechanical properties.