# Grain Size in Aluminum Alloy 6061 under Hot Ring Compression Test and after T6 Temper
## 4 Conclusions  
This study used a ring compression test with working temperature, upsetting speed, compression amount, and lubricant as process parameters. Each of the factors had three levels to form an orthogonal experiment table of L9 according to the Taguchi method to analyze the grain size after hot forging a 6061 aluminum alloy and after T6 temper. The surface layers of the specimen were peeled in order to eliminate the peripheral coarse grains from the previous processes used in preparing the billet.  
As a result, no peripheral coarse grains were found in any of the specimens, which were peeled and compressed. However, the relative significance of each factor still can be found by Taguchi analysis with ranking the process factors according to their influence on the mean grain size of each experiment. It is determined that the most significant factor is the lubricant, followed by the working temperature and then by the amount of compression, and the upsetting speed was negligible. In view of the two most significant factors, lubricant and working temperature, the usage of molybdenum disulfide can achieve smaller grains, while applying no lubricant can easily cause grain coarsening, and a higher working temperature of 500 \({}^{\circ}\)C can steadily conduct grain coarsening as well. Regarding the impact of each of these two factors to the grain size at each sampling position, the above conclusions are still applicable. The results from experiments in the ring compression test showed that the minimum mean grain size was 207 \(\upmu\)m and the maximum was 246 \(\upmu\)m, which could not be easily distinguished from that measured from the incoming alloy, 237 \(\upmu\)m. It is apparent that the effect of grain coarsening or grain refinement per hot forging is not recognizable. This can be attributed to the fact that the processing duration of forging operation is too short, so that the dynamic recrystallization and grain growth could not take place in the microstructure adequately.  
On the other hand, the compression tested aluminum alloys could have finer mean grain size after solid solution and artificial aging T6 temper. According to the Taguchi method and ANOVA, the only process factor in compression test that significantly influences the grain size after T6 temper is the compression amount. The greater the compression, the smaller the grain size achieved. A slight compression can cause grain sizes to be larger than both those of the incoming alloy and those just after compression test. The heating at elevated temperature during T6 temper can let the relatively strained alloy during compression test further undergo metadynamic or static recrystallization, so that the grain size is steadily smaller.  
In addition, this study also proved that only lubricants significantly affect the friction coefficient in a ring compression test of a 6061 aluminum alloy. The effect of working temperature on the friction coefficient is still relatively low.  
We hereby acknowledge that we received the grant of the Ministry of Science and Technology of the Republic of China for covering the costs to publish in open access.  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data; [PERSON] wrote the paper.  
The authors declare no conflict of interest.