November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

Die Surface Texture and Metal Flow Effects By Joseph Domblesky*, Andrew Matcha, Kyle Wolf, and Ross Crowley, Department of Mechanical Engineering, Marquette University, Milwaukee, WI *Corresponding Author

Abstract Achieving optimal metal flow is vital to the North American forging industry. While part geometry has the most impact on die fill, it is well known that friction is also important.This is noteworthy in that, while part shape varies widely, friction is a common denominator in all forging processes. Researchers have hypothesized that friction can be selectively used to impede or promote metal flow on a tool surface, but to date no investigations have been conducted using forging-based conditions. The current article summarizes the results of an exploratory investigation that was conducted at Marquette University and was funded by FIERF to study the potential for using die surface topography to control metal flow in forging processes. While this will not eliminate the need for flash lands, it does offer an additional tool to reduce flash and/or forge sharper features. This technique also has some potential to promote longitudinal flow of prismatic parts over relatively long distances. Background and Key Ideas While friction is affected by a variety of factors in a forging process, surface topography is one parameter that can be controlled by the forge engineer and die room. In order to understand how surface topography can be used as a design tool, it is useful to review several key ideas related to forging tribology. At the microscopic level, all surfaces are analogous to mountainous terrain consisting of peaks (asperities) and valleys. Surface topography is normally characterized by surface roughness, waviness, and lay. Roughness represents the average peak (asperity)-valley distance along a given direction and is a function of the cutting tool geometry and machining operation that is used. Waviness differs from roughness by the broader spacing between the surface irregularities and is caused by tool deflection and vibration. Lay refers to the surface directionality and is mainly influenced by the tool path. With respect to die topography and metal flow, it must be considered that before bulk flow is initiated, the workpiece surface will completely fill or partially conform to the individual asperities on the harder tool surface and fill the surrounding valleys to varying degrees. As surface roughness increases, the difference in height between peaks and valleys gets larger and poses a greater obstacle to

workpiece metal movement due to interlocking of asperities on the die and workpiece surfaces. Similarly, the orientation, or lay, of the surface roughness will also influence the ease of metal flow. If the peaks/valleys are oriented in a particular direction, metal flowing parallel to the surface lay will still need to fill these valleys, but the lay acts like a riverbed channeling water flow as few obstacles remain to impede movement once bulk flow commences in that direction. Workpiece flow perpendicular to surface lay will generally be less than that of flow parallel to surface lay due to partial or full asperity interlocking. Finally, metal flow is also related to the mechanical entrapment of lubricants in the valleys on the surfaces. Entrapment is necessary because, as the valleys or troughs get deeper, they can retain more lubricant, thus allowing them to function as reservoirs under forging pressures.These reservoirs then act to supply lubricant and minimize die-workpiece interface contact while sliding occurs. However, it should be noted that insufficient or excessive surface roughness can result in peak-valleys that are too small or large to A purpose-built 22.6 metric ton (25 ton) hydraulic press that was built at Marquette University in a prior FIERF project was used to perform the study. A series of high-temperature compression tests were conducted to investigate the relationship between surface topography (roughness and lay) and metal flow. This included ring testing to determine how surface topography affected friction while side pressing (cigar) tests were used to study metal flow. Six platen sets were machined from AISI H-13 tool steel with surface roughnesses ranging from Ra 0.25 µm (10 µin) to 6.1 (240 µin) µm as listed in Table 1. Surface roughnesses ranging fromRa 0.25 – 1.52 µm were used to simulate finishes typically found on production dies while the two highest values (Ra 3.3 and 6.1 µm) were intended to simulate worn die surfaces. To ensure that each platen had a consistent finish and unidirectional lay, surface roughnesses between Ra 0.25 – 1.52 µm (10 – 60 µin) were made using a surface grinder. The higher roughness values of Ra 3.3 – 6.1 (130 and 240 µin) were made using a 12.7 mm (0.5 in) diameter ball endmill where parallel, linear grooves were made by varying the depth and spacing of each cut on a CNC mill. All platens were heat-treated after machining to allow effective lubrication action. Experimental Methodology

FIA MAGAZINE | NOVEMBER 2020 44