May 2019 Volume 1

FORGING RESEARCH

Where µ = coefficient of friction

friction law is generally preferred formeasuring friction in hot forging applications. The interface shear friction law (friction factor) isable tocharacterize friction insituations where the yield strength of the workpiece exceeds the shear stress at the die/workpiece junction through its comparison of shear yield stress, k, and interface shear strength, τ F . On the other hand, the coulombic friction law (coefficient of friction) is unable to determine friction under these conditions because it assumes that the frictional force isdirectlyproportional tothenormal force. While this is generally true in low pressure applications, in high pressure applications where the workpiece begins to yield, the frictional force is not proportional to the normal force, thus making the coefficient of friction inaccurate. The coefficient of friction is generally found by using the pin-on-disk test which is done under relatively low pressures due to the small size of the pin. The friction factor can be experimentally determined using the ring test. Because the friction factor is capable of accurately characterizing frictional forces under both sliding and sticking friction conditions, the ring test is well suited for the higher pressures and temperatures encounteredduring forgingprocesses. Details regarding the ring test andpin-on-disk test arewell established and can be found in the literature [1] [11] [16]. In most forging processes it can be considered that lubricated dies will tend to have lower friction between the die-workpiece interface compared to unlubricated dies. The idea that the use of a lubricant generally lowers the friction factor in hot forging has been well documented. Unlubricated dies generally inhibit sliding and demonstrate much less metal movement which is often due to asperity interlocking or sticking friction caused by contact welding of the metal pairs at the interface. Temperature can also modify frictional conditions as heating up the dies and reducing the workpiece flow stress enables the metal to flow more easily. For example, Kim [7] noted that an increased die temperature generally promoted the movement of the workpiece thus suggesting that friction factor decreased as the temperature increased. It should also be noted that Kim used a commercial cold forging lubricant, MEC HOMAT, to ensure that heated dies and workpieces wouldn’t affect the stability of the lubricant, as many lubricants break down at higher temperatures. 1.3 Die Surface Roughness and Lay Surface topography can be defined by several features which include surface roughness (height and width), waviness, and lay. Each of these features are illustrated schematically in Figure 1.3.1. Of these features, one of the most important is surface roughness. In the forging process, surface roughness has a direct effect on the 5 [2]

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

Since µ is determined by the normal force relative to the frictional force, it is generally assumed that µ is constant across the surface as the normal pressure is constant throughout. To simplify this the average of the values across the surface is taken and displayed as a singular value. Because interface shear cannot exceed shear yield stress in a real material the value of µ will always be greater than 0 and can vary from 0 to 0.577 for metals. At the limiting value of µ = 0.577, the interfacial shear stress may become larger than the yield strength of the asperity junction thus causing the weaker asperity, generally the workpiece, to shear off at the surface. It is also possible, at junctions welded together, for the material flow to develop at sub-surface layers. This is typically seen in sticking friction which is a result of metal to metal welding. Sticking friction can be defined as the workpiece surface remaining stuck to one or more of the die surfaces at the interface. With this being said this does not mean that the workpiece will not change in size at all as the metal is still moving at the subsurface level. This phenomenon is typically seen in hot forging when lubrication is not adequately applied. Since µ is a function of force and most upsetting is done at higher pressures m* is generally preferred due to its ease of calculation. The coulombic friction law can be calculated in most forging applications, however, due to the convenience of the ring test at higher temperatures and pressures the interface shear friction law is generally usedmore often [1] [11] [25]. At higher pressures, the interface shear friction law, utilizing the Von Mises yield criterion, is a better suited model for metal forming as it assumes that the frictional stress component is some fraction of the flow strength, σo, of theworkpiece:

= ∗ ,

= √3 or

∗ =

[2]

Where m* = the constant friction factor k = the shear yield stress τF = the interface shear strength

he constant friction factor e shear yield stress e interface shear strength

In cases where high normal stresses are present it is generally preferred to use m* over µ when defining frictional forces. The value for m* may vary from 0 to 1 wheream* valueof 0would represent an ideal frictionless interaction at the surface and a m* value of 1 would represent sticking friction [1] [11] [27]. Of the two laws mentioned above, the interface shear es where high normal stresses are present it is generally preferred to use m* defining frictional forces. The value for m* may vary from 0 to 1 wher a would represent an ideal frictionless interaction at the surface and a m* uld represent sticking friction [1] [11] [27]. two laws mentioned above, the interface shear friction law is generally

FIA MAGAZINE | MAY 2019 41