November 2020 Volume 2

each orientation, there are some small differences between the workpieces. The workpieces oriented parallel (0°) to the platen lay tended to show more convex curvature at the ends parallel to the long transverse axis of the workpiece. For the workpieces whose orientation was perpendicular (90°) to the platen lay showed more convex curvature on the sides parallel to the longitudinal axis and almost none on ends parallel to the long transverse axis.

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Somewhat similar results were obtained for steel when width and longitudinal strains were plotted separately (Figure 13). It can be noted that only the width strains were affected at roughness values corresponding to typical die surfaces (i.e. 0.25 to 1.02 µm). In comparison, the longitudinal strains are the same at all roughness values that were used.

(c) R a 1.02 µm (40 µin)

(a) R a 0.25 µm (10 µin)

(b) R a 0.51 µm (20 µin)

(d) R a 1.52 µm (60 µin)

(e) R a 3.3 µm (130 µin)

(f) R a 6.1 µm (240 µin)

Figure 12. Deformed steel specimens after cigar testing at 149 °C (300 °) platen temperature using graphite lubricant. Specimens are arranged such that angle between the longitudinal axis of the specimen and lay orientation is in ascending order (i.e. 0°, 45°, and 90°). In all three orientations, the steel pieces tended to develop a similar shape. It is hypothesized that this is due to higher flow stress and friction factors compared to aluminum. Since steel has a higher flow stress, the asperities on the workpiece surface are better able to resist deformation by the platen surface such that metal flow is less affected by the platen lay. Due to the higher flow stress, the asperities on the steel surface have greater resistance and only partially fill the surface roughness grooves of the die, allowing for the steel pieces to slide over the platen asperities instead of having to fill the valleys between them and then undergoing shear as in the case of aluminum. As a result, this reduces the effect of lay directionality and makes the die surface essentially act as if it were more isotropic. In comparison for aluminum, the lower flow stress allows the workpiece surface to almost completely fill the grooves on the platen such that there is little flow resistance parallel to the lay while shearing must occur in the perpendicular direction. Since the flow stress of steel is much higher than that of aluminum, the effect of platen lay is not as evident. Figure 12 shows how, at the three different workpiece orientations, the shape of the steel workpiece varies very little, whereas with aluminum the shape changes significantly. Even though this is the case, the phenomenon regarding the corners or “ears” is still present and more notable due to the increased friction factor and decreased metal flow. It follows the same patter as aluminum where if the platen lay is parallel to the long transverse axis the friction factor is decreased in this direction thus causing the corner to rotate toward the longitudinal axis which can be seen in Figure 12 above. Inversely, if the platen lay is parallel to the longitudinal axis the friction factor is decreased in this direction, thus causing the corner to rotate toward the long transverse axis. 14 Figure 12. Deformed steel specimens after cigar testing at 149 °C (300 °) platen temperature using graphite lubricant. Specimens are arranged such that angle between the longitudinal axis of the specimen and lay orientatio is in asc nding order ( .e. 0°, 45°, and 90°).

Figure 13. Plot of longitudinal and width strains as a function of lay and surface roughness for AISI 1018 at a platen temperature of 149 °C. As noted previously, one factor that needs to be considered with respect to the metal flow and specimen shape during side pressing is the flow stress of aluminum and steel at the testing temperatures used. Aluminum has a flow stress between 50 - 125 MPa compared to low carbon steel, which is 600 - 800 MPa (approximately 6 – 8 times larger). As noted earlier, the shape of the aluminum specimens is much more influenced by the workpiece lay relative to the platen surface roughness. For aluminum, which has a lower flow stress, the metal tends to conform more to the surface roughness of the platen prior to initiating lateral flow. Since the metal conforms to the platen surface during compression, the platen topography inherently has more of an effect on how the metal moves in each direction. In the direction parallel to the lay, the metal is effectively channeled and easily slides, whereas the metal has to shear to move past the top of the grooves as depicted in Figure 14.

Figure 14. Shows how material is plowed and sheared more in the aluminum workpiece when compared to the steel workpiece where (a) full conformation to the platen surface occurs, (b) steel workpiece compressed showing minimal conformation and undergoes sliding and (c) aluminum workpiece compressed showing minimal conformation and undergoes sliding.

FIA MAGAZINE | NOVEMBER 2020 49