May 2019 Volume 1

FORGING RESEARCH

final workpiece geometry as it influences how the metal flows, the forging loads required, and the effectiveness of the lubricant. First, surface roughness can be thought of as a series of peaks and valleys, or troughs and crests, that the deformed metal, or workpiece, will partially or completelyfill beforeslidingbegins tooccur. Thesepeaks and valleys are formed by asperities of varying heights and widths. Asperities formon the die surface during the diemanufacturingprocessandare influencedby the type of tools and surfacing process used. Surface roughness is a result of the fact that die cavities are machined from blocks of tool/die steel and the machining process leaves the dies with a particular surface roughness. Surface roughness can have a detrimental effect on the outcome of the workpiece flow if uncontrolled as the metal may flow in a pattern that leaves an undesirable finished workpiece geometry. Secondly, waviness of the surface can be distinguished from roughness by the broader spacing between the surface irregularities as seen in Figure 1.3.1. These large irregularities form when uncontrollable effects such as tool deflection and vibration occur during the machining process. Third, the lay of the workpiece is defined as the direction of the predominant surface pattern of the surface roughness. Surface lay results from controlling the direction of the tool used in the machining process which could include grinding, milling, and turning, just to name a few. Grinding is used because a surface grinder is able to produce a unidirectional laydueto itsperpendicular rotationrelative to the surface it’s grinding. An end mill can produce an arced or unidirectional lay depending on how the tool is used. If thework piece is facedoffusing thebottomof the endmill it will produce an arced lay pattern. Alternatively, if the side of the end mill is used it will produce a near unidirectional lay. Lastly if a lathe is used to turn the face of a workpiece the resulting surface lay will be in a spiral pattern [1] [11].

Measuring the surface roughness can be difficult as the peak-valley height is not consistent from asperity to asperity. For this reason, surface roughness is usually represented as an average value. This calculation determines the average roughness value across the surface which is then defined by a single Ra value (µm). Figure 1.3.2 below shows a visual representation of how R a is defined.

Figure 1.3.2 - Example of surface cross-section and Ra Measurement [26]. It is generally assumed that friction increases with surface roughness, but this has been proven to not be strictly true in recent studies [11] [19]. If one were to analyze a complete plot of the friction factor over a broad rangeof surface roughness, onewouldbeable tosee that the highest frictional values are seen as surfaces with very low and high surface roughness. For metal pairs, the friction tends to be high on very smooth surfaces (low R a values) because of the real area of contact which becomes much larger due to the increased number of asperities in contact. With very rough surfaces (high Ra values) the friction is also high, but it is due to the need for the asperities to lift over each other or shear in order to continue movement. At intermediate surface roughness values, the friction is reduced and is almost independent of roughness. These intermediate surface roughnesses are typically seen in forging applications. Currently surface roughness iscontrolled inmost forging applications in a way to promote easy metal flow and die fill. Most forging dies are typically machined to have a surface roughness around R a 1.52 µm (60 µin) [11]. For a metal to flow, the compressed metal must initially fill or partially fill each valley between individual asperities before it can start to stick or move over the surface. As surface roughness increases, the difference in height between these peaks and valleys gets larger and poses a greater obstacle to workpiece metal movement. The orientation, or lay, of the surface roughness also

Figure 1.3.1 – Features of surface topography [1].

FIA MAGAZINE | MAY 2019 42