November 2020 Volume 2

matched that of the compressed steel workpieces. This supports the idea that aluminum does not conform and fill the surface grooves on the platen as well. Consequently, the aluminum asperities can effectively slide over the top of asperities as if the surface was isotropic.

FORGING RESEARCH AND TECHNOLOGY

As steel has a higher flow stress at the temperatures tested, optical microscopy confirmed that these surfaces showed much less conformation to the platen surface and reduced the effect of lay on the metal flow. Further confirmation of this was obtained by testing aluminum specimens at reduced temperatures to simulate the effect of warm working. These results are shown below in Figure 15. Aluminum cigar workpieces were tested at reduced working temperatures of 149 ºC (300 °F) and 20 ºC (68 °F) as a way to confirm that the trends observed were independent of the material and that aluminum and steel have similar behavior in warm and hot working. It should be noted that the die temperature was kept at 149 ºC (300 °F) for all three experiments.With a higher flow stress, it was hypothesized that aluminum would act like steel and retain a more rectangular shape with the geometry being less controlled by the orientation of the workpiece. This also meant the surface asperities would have significantly more resistance to conform to the platen surface and sliding would replace shearing. A comparison of steel at 1010 ºC (1850 °F), aluminum at 149 ºC (300 °F) and aluminum at 20 ºC (68 °F) can be seen in Figure 15. Comparing the geometries of steel at 1010 ºC (1850 °F) and aluminum at 149 ºC 300 °F) and 20 ºC (68 °F) one can see that the general shape of each aluminum workpiece is similar to those of the steel workpieces, especially in the workpieces compressed at 20 ºC (68 °F). It was found, that the lower that the aluminum workpiece temperature was, the more closely the final workpiece geometry matched that of the compressed steel workpieces.This supports the idea that aluminum does not conform and fill the surface grooves on the platen as well. Consequently, the aluminum asperities can effectively slide over the top of asperities as if the surface was isotropic.

(b) R a 1.02 µm (40 µin)

(c) R a 1.52 µm (60 µin)

(a) R a 0.51 µm (20 µin)

(f) R a 1.52 µm (60 µin)

(e) R a 1.02 µm (40 µin)

(d) R a 0.51 µm (20 µin)

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Figure 15. Comparison of steel cigar specimens at 1010 ºC (1850 °F) (a – c), aluminum cigar specimens at 149 ºC (300 °F) (d – f) and aluminum cigar specimens at 20 ºC (68 °F) (g – i). Steel specimens were compressed at platen temperatures of 149 ºC (300 °F) using a graphite lubricant and aluminum specimens were compressed at platen temperatures of 149 ºC (300 °F) using high temperature vegetable oil as a lubricant. (i) R a 1.52 µm (60 µin) Figure 15. Comparison of steel cigar specimens at 1010 ºC ( 850 °F) (a – c), aluminum cigar specimens at 149 ºC (300 °F) (d – f) and aluminum cigar specimens at 20 ºC (68 °F) (g – i). Steel specimens were compressed at platen temperatures of 149 ºC (300 °F) using a graphite lubricant and aluminum specimens were compressed at platen temperatures of 149 ºC (300 °F) using high temperature vegetable oil as a lubricant. The spread ratio was also calculated and plotted for steel and is shown below in Figure 16 for three different platen temperatures. (g) R a 0.51 µm (20 µin) (h) R a 1.02 µm (40 µin)

(a)

FIA MAGAZINE | NOVEMBER 2020 50

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