As in conventional PM, powder forging begins with custom-blended metal powders being fed into a die, then being compacted into a “green” shape, which is then ejected from the die. This compact, called a “preform,” is different from the shape the final part will acquire after being forged. Again as in the conventional PM process, the green compact is sintered (solid-state diffused) at a temperature below the melting point of the base metal in a controlled atmosphere furnace, creating metallurgical bonds between the powder particles and imparting mechanical strength to the preform. The heated preform is withdrawn from the furnace, coated with a high-temperature lubricant, and transferred to a forging press where it is close-die forged (hot worked). Forging causes plastic flow, thus reshaping the preform to its final configuration and densifying it, reducing its porosity to nearly zero. Powder forging produces parts that possess mechanical properties equal to wrought materials. Since they’re made using a net-shape technology, PF parts require only minor secondary machining and offer greater dimensional precision and less flash than conventional precision forgings. Parts fabricated through the PF process are subject to certain limitations. Tooling and the maximum press tonnage capabilities impose size and shape constraints on parts, just as in impression die hot forging. Annual production quantities in excess of 25,000 pieces are typically required to amortize the development costs of tool set-ups and maintenance. Finally, material systems are somewhat limited (all commercial PF products are steel). Typical Powder Forged Products—connecting rods, cams, bearing races, transmission components Typical Markets Using Powder Forged Parts— automotive, truck, off-road equipment, power tools
2. Powder Forging Powder forging (P/F) is used to produce components essentially free of internal porosity. The associated properties are equivalent to those developed in conventional precision forged products made from billets. The P/F process (Figure 10) is performed in three steps with the first two similar to normal PM processing. A preform is pressed as a conventional PM compact. The mass, density, and shape of the preform are controlled closely to ensure consistency in the characteristics of the final forged component. The preform is sintered with particular attention paid to the reduction of nonmetallic inclusions. The sintered preform is then reheated, placed in the forging die, and forged to full density. The tooling shape is designed to be close to that of the finished part with material flow controlled to fill the cavity completely.
Figure 10: Powder Forging Process-Schematic: Source "Powder Metallurgy Design Manual," Third Edition, MPIF.
