# Pening Natural Aging of Aluminum Alloy by Ultra-High-Temperature and High-Pressure Cavitation
## 1 Introduction  
The mechanism of high-pressure water jet action on materials has been well understood for decades [1]. Shot peening [2, 3] and fine particle peening [4] are techniques for mechanically striking surfaces with particles to impart compressive residual stress. Another processing technology modifies surfaces with an abrasive water jet formed by adding an abrasive to water pressurized to about 300 MPa [5, 6]. When water jet cavitation (WJC) is irradiated with ultrasonic waves to increase the temperature and pressure, the cavitation has both mechanical and electrochemical affects. This is called multifunction cavitation [7, 8] (MFC, PCT Pub. No. W02016/136656, US Registered Patent [9]). WJC has a bubble radius of about 100 \(\upmu\)m, and the impact pressure at the time of bubble collapse is about 1000 MPa [10]. In contrast, in the ultrasonic cavitation (UC) used for cleaning, the bubble nuclei originally existing in the water with a radius of several micrometers are irradiated with ultrasonic waves, and the collapse pressure is small, but the temperature at the time of bubble shrinkage is several thousand kelvins [11, 12, 13]. MFC bubbles have a large radius of about 100 \(\upmu\)m, a collapse temperature of severalthousand kelvins, and a high collapse pressure of about 1000 MPa. Such bubbles are a feature of WJC and UC. The distribution and unsteady flow of WJC bubble clouds that are changed by sound pressure from incident ultrasonic waves have been clarified analytically and experimentally [14]. Previous studies have developed ultra-high-temperature and high-pressure cavitation (UTPC) obtained by attaching a swirl flow nozzle (SFN) used for water jet peening in air to further increase the bubble size, number of bubbles, temperature, and pressure of MFC [15]. In addition, a photon counting head was used to measure the photons (sonoluminescence) emitted from UTPC, MFC, UC, SFN-WJC, and WJC. The UTPC has the highest emission intensity, number of photons, and bubble interior temperature upon collapse. It was clarified that there is a correlation between the sonoluminescence results and the surface modification results of low-alloy steel [9]. Using these high-temperature and high-pressure bubbles, surface modifications of various metals and ceramics have been carried out. The foremost use of aluminium casting alloys is in the automotive industry (60% of the tonnage world-wide). These are mainly engine components elaborated from alloys with silicon, most of which also contain at least 3% copper. The main applications outside the automotive industry are in mechanical construction, electrical engineering, transport, household electrical appliances and ironmongery [16]. Studies on the processing and natural aging of aluminum alloys have been also reported. The influence of the stirring pin and pressing tool shoulder on the microstructural softening during friction-stir processing and subsequent natural aging behavior was investigated for a 6061-T6 aluminum alloy [17]. Severe plastic deformation (SPD) can be used to generate ultra-fine-grained microstructures and thus to increase the strength of many materials. Unfortunately, high-strength aluminum alloys are generally hard to deform, which imposes severe limits on the feasibility of conventional SPD methods. The low-temperature equal-channel angular pressing method was used to deform an AA7075 alloy, and the influence of natural aging of a solid solution heat treated AA7075 alloy (prior to SPD) on the microstructure and the mechanical properties after equal-channel angular pressing was studied [18]. In this study, the outermost surface of a material was solution-treated using UTPC processing to raise the temperature to at least 500 \({}^{\circ}\)C higher than the equilibrium phase diagram of the aluminum alloy. Then, the elements added to the aluminum alloy formed a supersaturated solid solution and became distributed uniformly.