November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

to be due to the asperities being similar in size causing them to be flattened during compression which then increases the surface contact area along with the friction factor. Analysis of Metal Flow Aluminum - Figure 9 shows the resulting workpiece shape for the aluminum pieces side pressed using high-temperature vegetable oil and a platen temperature of 149 °C (300 °F). As similar results were obtained for aluminum using both lubricants, only one data set is shown. Comparing the shape change of the workpiece as the workpiece orientation was changed from 0° to 90° shows that there is a noticeable change in the workpiece shape. This was found to occur at all the platen roughnesses considered in the study. Generally, as the orientation increases relative to the platen lay, the length of the workpiece decreased, and the width increased. This can be attributed to a few factors. One major factor is the orientation of the workpiece relative to the surface roughness of the platen. When the longitudinal axis of the workpiece is parallel to the surface roughness of the platen, metal flow in this direction is much less restricted, thus elongating the workpiece more than it would if it were flowing perpendicular to the grooves. Conversely, since the transverse axis is perpendicular to the surface lay, the metal flow is more restricted and does not move as freely in the width direction as the aluminum tends to fully conform to the platen surface and the asperities must undergo shear for sliding to occur. As the specimen angle approaches 90°, the metal flow becomes less resisted in the width direction and more restricted in the length direction which causes the workpiece to be shorter and wider compared to the 0° orientation. It is interesting to note that the flow at 90 degrees is more balanced and the outline remained essentially rectangular. In the workpieces oriented 45° relative to the surface lay, the samples have a parallelogram shape which can be attributed to the lay forcing the metal to shear and flowmore diagonally as seen in Figure 9. essentially rectangular. In the workpieces oriented 45° relative to the surface lay, the samples have a parallelogram shape which can be attributed to the lay forcing the metal to shear and flow more diagonally as seen in Figure 9.

Figure 7. Friction factors calculated at 0° (parallel) and 90° (perpendicular) relative to the lay for aluminum rings using vegetable oil lubricant at different platen temperatures.

Figure 8. Steel ring compression test results comparing average friction factor (m*) parallel to the lay (0°) and perpendicular to the lay (90°). Graphite was used as a lubricant at a platen temperature of 149 °C (300 °F). While friction factor generally increased with platen surface roughness, there are some points to consider. In both cases, a minima was observed. For aluminum this occurred at 1.02 µm while for steel the same minima occurred at 0.51 µm. In both cases this indicated that a change had occurred in the lubrication regime at the die-work interface. For aluminum, it was hypothesized that sliding occurred at the two lowest roughnesses and then sliding and shearing occurred at the three highest roughnesses in the direction parallel and perpendicular to the lay. At the remaining roughness, the minima at 1.02 mm was the result of mixed lubrication which provided the most separation between the platen and workpiece surfaces. In the case of steel, for platen roughness from Ra 0.25 µm (10 µin) to 0.51 µm (20 µin), the friction factor decreases (from 0.48 to 0.33 at a platen temperature of 149 °C) due to a decrease in real contact area as there are less asperities available to increase surface contact. As platen roughness was increased from Ra 0.51 µm (20 µin) to 1.52 µm (60 µin), the friction factor increases greatly (from 0.33 to 0.67 at a platen temperature of 149 °C) which is thought

(a) R a 0.25 µm (10 µin)

(b) R a 0.51 µm (20 µin)

(c) R a 1.02 µm (40 µin)

FIA MAGAZINE | NOVEMBER 2020 47 Comparing the shape change at each orientation, it can be deduced from the metal flow trends that maximum metal flow occurs in the path of least resistance. Because of this, the metal tends to flow more along the longitudinal axis when oriented parallel to the lay and more in the transverse direction when oriented perpendicular to the lay. Plots of true strain values also showed an interesting result. The most effect on both strains Figure 9. Deformed aluminum specimens after cigar testing at 149 °C (300 °F) platen temperature using high temperature vegetable oil lubricant. Specimens are arranged such that the angle between the longitudinal axis of the specimen and lay is in ascending order (i.e. 0°, 45°, and 90°). (f) R a 6.1 µm (240 µin) Figure 9. Deformed aluminum specimens after cigar testing at 149 °C (300 °F) platen temperature using high tem erature vegetable oil lubricant. Specimens are arranged such that t e angl between the longitudinal ax s of the specimen and lay is in ascending order (i.e. 0°, 45°, and 90°). (d) R a 1.52 µm (60 µin) (e) R a 3.3 µm (130 µin)