May 2019 Volume 1

FORGING RESEARCH

influences the ease of metal flow. If the peaks/valleys are oriented in a particular direction, and the metal is flowing parallel to the surface lay, it may still need to fill these valleys, but few obstacles remain to impede movement thus making overall flow easier in that direction. Workpieceflowperpendicular tosurface laywill generally be less than that of flowparallel tosurface roughnessdue to partial or full asperity interlocking. Controlling surface lay thus has the potential to improve workpiece flow, reduce lubrication consumption and reduce the amount of flash needed in the forging process. In forging, lubrication is often used topromoteworkpiece sliding. There are generally four types of lubrication mechanisms that can exist: boundary, full-film, hydrodynamic and mixed-layer lubrication. The most common lubrication method found in forging is mixed layer as it is a mixture of hydrodynamic and boundary lubrication. In hydrodynamic lubrication a thin layer of lubricant separates the die and the workpiece surface. It is difficult to maintain full hydrodynamic conditions because of the low sliding velocity and high pressures that are present during forging. Therefore, much of the lubricant is pushed into the valleys and a high percentage of the peaks are exposed to boundary conditions. At the peaks the surface treatment of the die acts as the lubricant at the die/workpiece interface. This type of mechanism is referred to as mixed-layer lubrication. The effects of each lubrication method on the coefficient of friction can be seen below in Figure 1.3.3.

The effect of surface roughness and lay are also related to the use of lubrication as surface roughness has been known to help the mechanical entrapment of lubricants. The quantity entrapped depends on the volume of free space available in the troughs so that as the roughness increases, additional lubricant can become entrapped [23]. Entrapment can be a powerful mechanism for both solid and liquid lubricants because as the troughs get larger, they are able to retain more lubricant thus allowing for them to hold up better under higher normal pressures. It should be noted that excessively large surface roughnesses will eventually lead to peak valleys that are too large for lubrication entrapment to have a positive effect on metal flow. Within the deformation zone, the entrapped pressurized lubricant and roughness gives rise to micro plasto-hydrodynamic (PHD) lubrication. This effect becomes most important when process configuration and local sliding conditions would wipe off the lubricant from a very smooth surface. It should also be noted that a rougher finish is more tolerant of variations in production conditions as it can accommodate wear particles. So as a die begins to “load up” withworkpiece particles, it won’t have asmuch effect on the final geometry of the part as the troughs can act as repositories and hold some of these wear particles. The above effects are also influenced by the lay of surface roughness. Surfaces with a lay parallel to the sliding direction allow for the lubricant to escape more easily than if the lay is perpendicular to sliding. Therefore, lubricant intake, lubricant retention, and the micro- PHD lubrication mechanism are all potentially more effective if the surface roughness is oriented perpendicular to the sliding direction [22]. Based on these ideas, controlling surface roughness direction, or lay, could be influential in a forging application as the ability to change the workpiece shape can be directly related to how the metal is flowing relative to the surface lay. Optimizing the surface lay would in turn reduce the number of asperities in contact at the die-workpiece interface allowing for metal to flowmore readily at the surface. 1.4 Summary of Problem In many forging applications die design and flash lands are generally used asmeans of controllingmetal flowand die fill. It is also known that friction plays a role in the flow of themetal within the die cavity. What is less understood is what effect surface roughness and lay have on metal flow throughout this process. During a metal forming process, on the die faces, there are local frictional forces acting opposite to the direction in which the metal is moving. These forces are in large part due to surface roughness and lay which thus leads to the belief that both factors play a particularly important role in respect to optimizing metal flow on the die-workpiece interface.

Figure 1.3.3 - Stribeck curve showing onset of various lubricationmechanisms [1].

FIA MAGAZINE | MAY 2019 43