Chapter 18: Extrusion Processes

The text distinguishes between the two fundamental classifications: direct extrusion, where the billet moves relative to the container creating significant friction, and indirect (inverted) extrusion, where the die moves into the stationary billet, thereby reducing friction forces and power requirements. A detailed examination of equipment covers the differences between horizontal and vertical hydraulic presses, alongside critical tooling components like die stacks, liners, and dummy blocks used to transmit pressure. Key process variables are analyzed, including the extrusion ratio, working temperature, and deformation speed, with specific focus on the trade-offs between flow stress and the risks of hot shortness or incipient melting at high speeds. The chapter explores the mechanics of deformation, presenting mathematical models for calculating extrusion pressure based on the ideal work of deformation, efficiency factors, and slab analysis, while noting that pressure generally correlates with the natural logarithm of the extrusion ratio. Significant attention is given to tribology and lubrication, particularly the Ugine-Sejournet process which utilizes molten glass to lubricate high-temperature steel extrusion. The discussion extends to metal flow patterns—homogeneous versus dead-metal zones—and common defects such as surface cracking (fir-tree cracking), center bursts (chevron cracking), and the extrusion defect (piping) caused by oxidized skin flowing into the product center. Advanced and specialized techniques are also covered, including cold extrusion for component production, hydrostatic extrusion which uses pressurized fluid to eliminate container friction, and impact extrusion for soft metals. Finally, the text details the production of seamless pipe and tubing through methods such as piercing with mandrels, the use of bridge and porthole dies for aluminum, and rotary piercing processes like the Mannesmann mill.