# Pening Natural Aging of Aluminum Alloy by Ultra-High-Temperature and High-Pressure Cavitation
## 3 Results and Discussion
Figure 9a,b shows the shape image and corrosion potential image of the mirror-polished surface before processing, respectively, as measured by KFM. The shape image is flat due to mirror polishing, and the potential image has no bias and a uniform potential distribution. Figure 9c,d shows the KFM measurement results 72 days after UTPC processing of a specimen for 20 min; at this point, the hardness was starting to increase again. Although unevenness after processing is observed in the shape image, the potential image does not have a particularly biased distribution. In addition, the surface was leveled by raising the surface temperature to a level that generated solution treatment, and no remarkable peening marks were observed. Figure 9e,f provides the KFM measurement results after 104 days, when the hardness gradually increased after UTPC (W): 35 MPa, UC: 800 W, 60 min) processing. Although large irregularities are observed in the shape image, the potential image does not have a particularly biased distribution. In contrast, Figure 9g,h shows the KFM measurement results 7 days after LTPC processing of a specimen, which applies the highest compressive residual stress. A dent, which is considered to be a peening mark, is observed in the shape image. The surface potential at the peening mark is lower than that in the other regions. Further, the reason the peening marks were observed is that the surface did not reach the solution temperature and surface homogenization did not occur due to the lower processing temperature. Table 2 gives the surface potential results for the various specimens. KFM can measure the work function, which is a measure of the ease with which electrons are emitted from the surface. Because oxidation involves the transfer of electrons, the higher the surface potential, the higher the corrosion resistance. Among aluminum alloys, AC4 CH without processing has high corrosion resistance, and its surface potential after mirror polishing is as high as 1200 mV. The surface potential of special steel does not exceed 1000 mV, even after MFC processing [27]. Even if the aluminum alloy is mirror-polished, a passivation film is immediately formed and corrosion resistance is maintained. The surface potential of the specimen subjected to LPTC processing, which imparts the highest compressive residual stress, is almost the same as that of the as-received specimen after mirror polishing. UPTC processing, which causes peening natural aging, produced a decreased surface potential with 20 min of processing and increased potential with 30 min and 60 min of processing. After the outermost surface reaches the solution temperature and is subjected to the solution treatment by UTPC, precipitates such as GP zones are formed on the surface. The disparate surface potential results for the UTPC specimens are probably due to different formation states of these precipitates being created by different processing times (20 min vs. 30 min and 60 min). In the 60-min processed material, no change in surface potential was observed between 41 days and 104 days. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Processing Condition**} & \multicolumn{2}{c}{**Mirror**} & \multirow{2}{*}{**LTPC**} & \multirow{2}{*}{**UTPC (1)**} & \multirow{2}{*}{**UTPC (2)**} & \multirow{2}{*}{**UTPC (3)**} & \multirow{2}{*}{**UTPC (4)**} \\  & & & & & & \\ \hline \hline \multirow{2}{*}{Elapsed days after processing} & - & 7 & 72 & 70 & 41 & 104 \\ \hline \hline \multicolumn{1}{c}{Water jet pressure (MPa)} & - & 20 & 35 & 35 & 35 & 35 \\ \hline \hline \multicolumn{1}{c}{Ultrasonic output (W)} & - & 100 & 800 & 800 & 800 & 800 \\ \hline \hline \multicolumn{1}{c}{Processing time (min)} & - & 20 & 20 & 30 & 60 & 60 \\ \hline \hline \multicolumn{1}{c}{Surface potential (mV)} & 1319 & 1256 & 879 & 2871 & 1762 & 1700 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Surface potential of aluminum alloy (AC4 CH) by various processing.