# Grain Size in Aluminum Alloy 6061 under Hot Ring Compression Test and after T6 Temper
## 3 Results
### Grain Size after T6 Temper
However, this setting could not achieve the largest grain size as described in Table 9. If temperature were changed to 500 \({}^{\circ}\)C and the lubricant to molybdenum disulfide as in the setting in experiment 8, or speed to 0.36 mm/min without lubricant as in experiment 1, or temperature to 450 \({}^{\circ}\)C and speed to 36 mm/min as in experiment 6, a larger grain could be achieved than by experiment 2. Among these experiments, the very same setting is the compression at one millimeter. This fact could also be found from the result not only in the rightmost column but also in the column of each individual position listed in Table 10. The most significant effect on the grain size is the compression, and its level at one millimeter has the lowest S/N ratio as well, while three millimeters has the highest S/N ratio (in 8 of all 9 positions). Furthermore, the second significant effect on the grain size is the temperature, in which 400 \({}^{\circ}\)C had a strong tendency (lowest in mean S/N ratio and all 9 positions) to have coarse grain, while 450 \({}^{\circ}\)C relatively easily (highest in mean S/N ration and only six positions) causes fine grain. Within the factors, lubricant and speed, their levels having larger or smaller grain, are very irregular among individual positions and therefore they had less effect on grain size. Resulting from the ANOVA, Table 11 shows that factors mentioned above with relative low difference of S/N ratio, such as temperature, lubricant, and speed, have also a low contribution (%C). The variation caused by them can then be regarded as random background and can be pooled to error, which is correct in view of F value, because their F values are relatively small when compared to the critical value 6.94. Thus, only compression is the remaining factor to grain sizes after T6 temper. It can be seen that T6 temper could further make grains finer and this refinement has an absolute relationship to the amount of compression. This can be attributed to the fact that at elevated temperature, severe ongoing plastic deformation beyond the critical strain can cause recrystallization, which is called dynamic recrystallization. If the plastic deformation is terminated, the larger retaining plastic strain will undergo further metadynamic recrystallization, while the smaller retaining plastic strain will undergo static recrystallization. These three recrystallizations will refine the grains. In the meantime, at such elevated temperatures, grain growth coarsens the grains to reduce their number and to release the grain surface energy as well as to lower the free energy [10]. Usually, the hot forging process takes place within a relatively short span of time, frequently less than one second. The dynamic recrystallization can be just initiated and take place in a short duration. Furthermore, some of the experiments had relative small amount of compression, which might restrict the initiation of dynamic recrystallization during the compression test. This caused the grain refinement to not be efficient. The subsequent heat treatments, such as solid solution (two hours) and aging (eight hours), had sufficient time for grains to have full recrystallization either by metadynamic recrystallization or by static recrystallization, so that finer grains could be obtained. Additionally, experiments 1, 6, and 8, which had small amounts of compression, had relatively coarse grains, which was due to their small plastic deformation amount. At the same time, grain growth still occurred and made grains coarser.