# Pening Natural Aging of Aluminum Alloy by Ultra-High-Temperature and High-Pressure Cavitation
## 3 Results and Discussion  
Figure 3 shows photographs of the LTPC processing surface jetted at a fixed point. When fixed-point injection is performed with using the SFN, an erosion region is formed at the injection center, and a peening region is formed around the transition region of the erosion region. In particular, the distribution of the peening region becomes pronounced after processing of 5 min or more. Figure 4 shows the photographs of UTPC processing surface jetted at a fixed point. Compared to LTPC, the peening area is expanded in the peripheral direction. Although the area of the perforated disc-shaped peening region of LTPC is small, the compressive residual stress is higher than that of UTPC, which has a large peening region, due to the peening artificial aging. Figure 5 shows the changes of surface roughness with processing time in the peening region after UTPC and LTPC processing. Arithmetic mean roughness Ra increases with processing time, but there is no significant difference in surface roughness between UTPC and LTPC up to 20 min. However, from the KFM observation results, the number of  
Figure 4: Surface photographs after UPTC processing (alloy: AC4 CH, jet: 35 MPa, UC: 800 W). (\(\Circle\): roughness and hardness measurement area). Figure 3: Surface photographs after LPTC processing (alloy: AC4 CH, jet: 20 MPa, UC: 100 W). peening marks is larger in LTPC than in UTPC, and the surface of LTPC is relatively smooth compared to that of UTPC, but the UTPC surface has many fine irregularities. The Vickers microhardness of the untreated material was 85.3 HV. Figure 6 shows the time course of hardness after UTPC processing at various times. No increase in hardness was observed after 2 min or 5 min of processing, but an increase in hardness was observed after processing for 10 min or more. The hardness of the material treated for 20 min increased the most. After 20 min of UTPC processing, the hardness reached its maximum value 21 days after processing, and then the hardness decreased and increased again. The reason the hardness increased after 10 min or more of UTPC processing is as follows. The outermost surface is solution treated by UTPC processing, as shown in Figure 2, and the supersaturated solid solution elements Mg and Si are perfectly mixed in aluminum at the solution temperature. After that, when cavitation stops and the temperature decreases to room temperature, the surface becomes a supersaturated solid  
Figure 5: Change of surface roughness with processing time of LPTC and UTPC (LTPC: jet 20 MPa, UC 100 W; UTPC: jet 35 MPa, UC 800 W) (Evaluation length 5 mm)  
Figure 6: Relationship between hardness and elapsed time after UPTC processing (WJ: 35 MPa, UC: 800 W) (Average of standard deviation: 2 min 5.9 HV, 5 min 7.5 HV, 10 min 9.9 HV, 20 min 9.5 HV, 30 min 11.2 HV, 60 min 6.9 HV). solution, and Mg and Si gradually precipitate as GP zones. The surface is plastically deformed by the microjet when cavitation bubbles collapse, and compressive residual stress is imparted. This compressive stress causes dislocations on the surface, and the dislocations are trapped in the precipitates and hardened by the Orowan mechanism. The reason the hardness of material processed for 20 min and 30 min decreases at first and then increases again is thought to be a result of natural aging as the structure changes from GP I phase to GP II phase and \(\theta^{\prime}\) phase. It is considered that the compressive residual stress generated causes new dislocations that are related to the pinning effect on the precipitate. In contrast, it can be recognized that after 60 min of processing, unlike the material processed for 20 min, hardening does not occur immediately after processing. The hardness immediately after processing was about 160 HV, which was about the same as that for the LTPC 20-min processing and LTPC 30-min processing shown in Figure 7. However, after 60 days, the 60-min processed surface was hardened to 183 HV. Al-0.62% Mg-0.39% Si alloy was quenched in water after solution treatment and naturally aged.