May 2019 Volume 1

FORGING RESEARCH

1. INTRODUCTION 1.1 Background Information

by surface friction and roughness is a key aspect in understanding and controlling how a metal preform will deform, or flow, the way it does during the forging process. For a given die geometry, metal flow during a forging process is generally influenced by four factors which include: workpiece material, tool-workpiece interaction (i.e. friction), lubrication and deformation speed/ time [13]. A sub-factor within the tool- workpiece interaction that has been hypothesized is selective use of friction because an increase or decrease in friction has been known to impede or promote metal flow across the die surface. An example of this would be if a forging engineer wanted to promote or impede flow in certain areas of the die to control how it fills. Normally, metal is controlled within the die with appropriate die geometry and flash lands. It has been hypothesized that another way to control flow would be to increase the surface roughness of a certain area in the die cavity, which would allow for the metal to flow more readily in other areas where the surface roughness is lower. This would likely reduce the amount of excess billet material needed to fill the flash lands and/or enable “sharper” features to be forged. This technique has some potential to promote the forging of elongated parts that requiremetal to flow long distances. As such, utilizingselectivesurface roughness topromote die fill could lead to another die optimization tool as it will likely allow more complex parts without the need for overly complex die sets. It has long been known that the relative motion between two bodies in contact results in a resistance to movement which is defined as friction. The surface area of contact, in this case between the workpiece and dies, is a boundary of deformed metal. While the resistance at the boundary is often treated as a force, if the force is divided by the contact area, the resulting ratio then represents an interfacial shear stress in the material at the boundary. The two most common models that are used to represent the interfacial shear stress and define frictional resistance are the coulombic friction law (coefficient of friction) and the interface shear friction law (friction factor) [25]. At low pressures, the coulombic friction law provides a good representation of the frictional stress component as thiscomponent isdirectlyproportional to thepressure at the workpiece-die interface. Coulomb’s Law can be mathematically stated as: hypothesized that another way to control flow would be to increase the surface roug of a certain area in the die cavity, which would allow for the metal to flow more rea in other areas where th surface roughness is lower. This would likely r duce the a of excess billet material needed to fill the flash lands and/or enable “sharper” featur be forged. This technique has some potential to promote the forging of elongated pa that require metal to flow l ng distances. As such, utilizing selective surface roughn promote die fill could lead to an ther die optimization tool as it will likely allow mo complex parts without the need for overly complex die sets. It has long been known th t the relative motion bet ee two bodies in conta results in a resistance to movement which is defined as friction. The surface area of contact, in this case between the workpiece and dies, is a boundary of deformed met While the resistance at the boundary is often treated as a force, if the force is divide the contact area, he resulting ratio then represents an interfacial shear stress in the material at the boundary. Th two most common models that are used to represent t interfacial shear stress and define frictional resistance are the coulombic friction la (coefficient of friction) and the interface shear friction law (friction factor) [25]. At low pressures, the coulombic friction law provides a good representation frictional stress component as this component is directly proportional to the pressur t e workpiece-die interf ce. Coulomb’s Law can be mathematically stated as: = or = = Where µ = co fficient of friction [1]

Although worldwide consumption of metals has decreased in recent years, forging usage has continued to grow as a result of increasing demand for products that are stronger, tougher and more reliable. An indication of this growth can be seen in the global forging market which was valued at $66.05 million in 2016 and is predicted to growby 50%before 2025 [3]. Due to their superiormechanical properties and integrity, the use of forged components will continue to grow based on the need to increase operating temperatures, loads, and stresses in engineering applications, all while maintaining safety. Although competing metalworking processes such as casting, machining, and additive manufacturing are now able to produce higher performance parts, recent technological advances have enabled the forging industry to maintain superior reliability, improve product features, and use more efficient production methods ensuring that the forging process remains a viable manufacturing technique for the near future [28]. 1.2 Friction inHot Forging A key element of the forging process are the dies which enable the metal to be worked in the solid state and formed into complex shapes having a desirable grain flow pattern. As such, the way a die is designed can have a large impact on how a workpiece is formed. Consequentially, die design and optimization is an important topic and is of great practical interest to the forging industry. Two of the major challenges faced by forging engineers with respect to die design are how to promote efficient die fill and simultaneously maintain reasonable die life. While the forged part geometry is a primary influenceonhoweasily adiewill fill, friction is also an important element and is fundamental to all forging processes. During deformation processes, such as open and closed die forging, frictional resistance to sliding occurs at the interface between the workpiece and the dies. This frictional resistance isdue inpart to the surface asperities that arepresent onboth theworkpieceanddie. When the dies compress the workpiece, the asperities on both surfaces come into contact and act to impede the flow of the metal workpiece. High frictional forces at the interface between the die and the workpiece act to resist metal flow during forging, thus increasing the probability of incomplete die fill and producing a poor quality part [11]. While friction is affected by a variety of factors, a significant element is surface roughnesswhich also represents a fundamental parameter in all forging dies. Thus, understanding how metal flow is affected

N = the pressure normal to the interface F = the frictional force at the interface τ = the shearing stress at the interface

FIA MAGAZINE | MAY 2019 40