Shaping Future Innovations with Radial Forging
Radial forging is a process in which high forces are transferred via surface contact to change the diameter of a workpiece by deformation. In radial forging machines, the workpiece rotation is accomplished in an optimized, direct-controlled manner by an oscillating rotating drive of the manipulator. The process is gaining popularity with the advent of electric vehicles.
As recently as the first half of the last century, a young engineer was not sure if he should pursue his passion for the emerging new aircraft engine industry or for the steel forming industry. He came to know the swaging process and realized how he can eliminate the pronounced weakness of limited-power transmission. After the turmoil of World War II, he founded a mechanical engineering company in Steyr, Austria, that offered this advanced machine type – the "radial forging machine.”
The term "rotary swaging" describes a somehow similar process. Swaging is an even older term from the early 19th century. The original meaning was "to shape or bend by use of a tool." In typical rotary swaging machines, pairs of opposing tool segments act repeatedly and sequentially, one after the other, onto a workpiece. Swaging machine types include internal, external and double rotating systems. For internal rotating systems, the dies rotate around the workpiece, and the dies only move radially for external rotating.
The principle behind radial forging can be subdivided into four parts: a drive by means of an eccentric shaft; the Scotch-Yoke principle (Fig. 1) for the transmission to a linear movement; and sinusoidal ram movement and a radial arrangement of these rams to suppress unnecessary material spreading.
Swaging and Radial Forging Contrasted
Despite the basic similarity of the processes, there are significant differences between swaging and GFM’s "Radialschmieden." Typically, rotary swaging machines perform only cold-forming processes in an open, oil-flushed system. The force is transmitted by convex contours. This limits the ability of swaging machines, which can transfer only limited forces and process only smaller part sizes (typically from wire size to bars under 4 inches in diameter).
The swaging process is less robust since the rotation of the workpiece is not actively controlled but only dragged along by the dies. This differs substantially to radial forging, during which high forces are transferred via surface contact and beneficial radial sinusoidal motion acts on the workpiece. In radial forging machines, the workpiece rotation is accomplished in an optimized, direct-controlled manner by an oscillating rotating drive of the manipulator, which leads to a more reliable and robust process.
More about Radial Forging
Austrian engineer Bruno Kralowetz first coined the term "radial forging." The company he founded was GFM, which is the German abbreviation for “company for manufacturing technique and machinery building” (Gesellschaft für Fertigungstechnik und Maschinenbau).
The first machines had three rams, and the material ran vertically through the machine. The operating principle (shown as horizontal material flow) and an image of the second radial forging machine built are shown in Figs. 2a and b, respectively.
Almost a century after the invention of radial forging, the most advanced radial forging machines rely on these basic principles: a drive by eccentric shaft, the scotch yoke principle to separate the rotary motion into a linear motion and the radial arrangement of the various number of rams. The method belongs under the collective term of "open-die forgings." From this perspective, it is a stroke- and energy-restricted incremental forging process.
On a normal crankshaft drive, the force is provided up to a certain angle, but this is not the case with the GFM radial forging machine. Large flywheels are used to reduce the necessary drive power and transform the machine from an angle-related force restriction into an energy-restricted machine.
From the incremental process, the benefits of reduced power requirements, versatile tools and a controllable process can be directly derived. Together with CNC-based accurate process control, the advantages of the precision forging process have been achieved for various types of cross-sectional shapes, such as round, square, rectangular, hexagonal or octagonal.
State-of-the-Art Radial Forging
Radial forging historically has offered exceptionally good surface quality and reduced machining allowances. The well-known SX machine types are still widely used worldwide. Over the decades, however, additional benefits could be achieved through different types of machines. The technology has evolved from simple shaping to deep penetrating forging. These characteristics have diverged and become more specific. The best-selling machine type for cold and semi-hot forging is the SKK machine with mechanical adjustment (Figs. 3a and 3b).
This reliable and robust process achieves the highest precision. This machine type was developed specifically for the requirements of the automotive industry. By use of a floating mandrel, unrivaled internal accuracy for hollow parts was economically possible. Drive shafts (Fig. 4a) and automatic-transmission gear shafts (Fig. 4b) are typical successful applications.
The machines can process different material qualities at different forming temperatures and thus offer unequaled flexibility. The most important properties of the different temperature ranges are summarized in Table 1.
A typical production cell for semi-hot forging is shown in Figure 5.
Type RF (radial forging) has been the state-of-the-art of radial forging for 20 years. In this system, the diameter adjustment of the tooling is done hydraulically by means of a cylinder. This type of machine provides sufficient power and the ability to penetrate deep into the centerline, allowing for a consolidated center while minimizing the overall reduction ratio. One advantage is the short contact times with the tools. This is of crucial importance for higher- and super-alloyed workpieces and is especially true for small cross sections and longer workpieces.
Extending technical boundaries was supported by the skip-stroke mode feature. With this type of machine, more material per stroke can be deformed while minimizing contact time to the tool and thus cooling of the workpiece. The successful consolidation and forging of fine grains is a combination of machine capabilities with tool and process know-how.
Modern simulation tools based on FEA allow such optimizations without lengthy, time-consuming and costly experiments. The sophisticated but easy-to-use FEA software tool is integrated into the control of modern RF machines. Intelligent routines allow fast simulation, although it is an incremental and therefore numerically complex calculation. The simulation tools provide indicators of porosity, development of the carbide network and banding.
For precision or geometric accuracy, only minor further improvements are expected. The limits that will and need to be further advanced are the formability of the various material grades.
The intrinsic formability is a material property itself. However, new radial forging machines expand the formability limits by using the advantageous states of stress during forging. The forming from all sides of the workpiece in combination with the optimized tool geometry and pass sequence enables the forging of materials susceptible to cracking. This is one of the most important advantages over alternative methods such as traditional open-die forming or rolling.
Low Operating Costs and Sustainability
Compared to open-die forging, the process requires significantly less power and less connection power (up to about 50%). This is partly due to a direct electric drive, which avoids the low energy efficiency of hydraulic drives, and partly due to the balancing effect of the flywheel. These advantages keep the operating expenses low and thus represent an environmentally friendly and sustainable technology.
Future of Radial Forging
The trends of radial forging are clearly defined. The rotor shafts for electric motors are now appearing on the automotive market. Their typical design seems to be based on the capabilities of radial forging technology. The best approach is a semi-hot forging process. Images of typical EV rotor shafts are shown in Fig. 6. As the number of electric vehicles increases, we will see radial forging as the main component in the manufacturing process.
Table 1. Typical characteristics and differences of hot, semi-hot and cold radial forging processes
Another topic is new and emerging steel grades and superalloys. The potential of steel and superalloys has not yet been fully exploited. Hundreds of material engineers are designing new grades to stay competitive against the advancing of composite materials. Lightweight design asks for higher mechanical and sometimes thermal load capability. The limited formability that is characteristic of the new grades is driving best-practice operations to radial forging.
On the machine side, modern technologies like servo controls, torque motors and sophisticated drive systems will dominate development. In order to increase the productivity and reliability of the process, limitations of the tooling must be addressed. Improved material grades, sophisticated coatings, environmentally friendly lubricants and well-designed internal cooling concepts are the real focus on the application side.
Radial forging found its niche in various complex-shaped, hollow parts and its capability to forge high-alloyed steel grades and superalloys. The process offers some interesting benefits that will become even more important and widespread due to emerging requirements on material, throughput, reliability and material properties.