The method was developed in 1970 in the Soviet shipbuilding industry but was later commercialised under the name Esonix UIT ^[31]. This method uses an electro-mechanical transducer that generates ultrasonic waves, which are then processed by a piezoelectric or magnetostrictive transducer, and a resonator is set in motion ^[31][32]. Ultrasonic impact peening is also called ultrasonic hammer peening or direct sonotrode-driven hammer peening. The tool used in this method is a steel pin driven in high-frequency oscillations by an ultrasonic generator. Deburring takes place through the interaction of the tool with the workpiece. The high-frequency impacts create indentations in the material, and thus the material is plastically deformed ^[33]. Figure 3 shows a comparison of the typical depth of influence between shot peening, laser shock peening, deep rolling, and hammer peening.

In 2000, a new pressing technique called ultrasonic nanocrystalline surface modification (UNSM) was patented by Pyoun due to the ease of creating a nanocrystalline structure ^[34]. UNSM is a method of material modification that involves peening the surface using movements of a tool that oscillates at a very high frequency. In a typical UNSM process, a tungsten carbide (WC) tip or Si₃N₄ ball is attached to an ultrasonic horn that strikes the surface up to 20,000 times per second at 1000 to 10,000 shots per square millimetres. To prevent the overheating of the shot peened workpiece, oil is used and sprayed on the workpiece ^[35][36].

The UNSM method has some advantages over other shot peening methods. The main advantage of this method is the possibility of extensive control of the process, i.e., applying a higher or lower static load, and changing the speed of oscillation by changing the amplitude of vibrations. Thus, this method allows high repeatability and the selection of ideal parameters for specific conditions. The method allows for grain fragmentation, introducing residual compressive stresses, reducing surface defects, and increasing hardness ^[37].

The UNSM method can be used to treat components after additive manufacturing, as demonstrated by Min Seob Kim et al. They investigated the behaviour of 316 L steel after direct energy deposition (DED) fabrication, after optimising the process, improving the roughness corrugation, and increasing the microhardness by 72%, 80%, and up to 71.2%, respectively ^[38]. Studies have also been conducted for titanium and its alloys ^[39], aluminium alloys after 3D printing ^[40], as well as 304 steel ^[41] or chromium-molybdenum steel ^[42].

The laser shock peening technology was developed at the Battelle Institute, dating back to the late 1960s. The laser shot peening (LSP) method uses pulses of laser light to generate shock waves that are expected to deform the material and leave residual compressive stress ^[28]. Laser light and a transparent layer are required, although a non-transparent layer is also often used. The temperature at the heating point reaches up to 10,000 °C and the pulse length is about 0.15 to 0.30 ns. LSP can be performed in two configurations, direct and limited ablation. A transparent layer (usually water) is used to reduce the plasma produced on the surface and increase the shock wave’s intensity. After the laser light beam passes through the transparent layer, it hits an opaque layer, usually made of aluminium, zinc, or copper, which is also called a sacrificial coating. The opaque layer is used to prevent direct laser ablation on the surface and melting ^[43]. An impulse of laser light reaches the absorption layer and then ionisation occurs in the heated zone, and a shock wave is generated, which plastically deforms the material ^[44]. The resulting compresive residual stresses (CRS) can reach up to 1.5 mm below the surface, depending on the method used ^[45]. Figure 4 shows different surface severe plastic deformation (SPD) treatments and the corresponding plastically deformed top surface of the target material.

In this method, balls are placed on two or more flaps which are fixed on a shaft. The tool is then made to rotate, which transfers energy to the balls, and the tool is brought close to the surface so that the balls begin to strike the material, thus deforming it, leaving numerous indentations ^[47].

Micro shot peening is carried out using minimal spherical particles called fine-particle or micro shot. The main difference from the SP is the sizes of shots that range from 0.03 to 0.15 mm in diameter. Generally, the micro shots are made of high-speed tool steel, cemented carbide, glassy alloy and ceramic, with high hardness ^[48].

It has been found that micro shot peening improves the scuffing resistance of 17CrNiM steel, and a combination of SP and micro shot peening gives even better results ^[49]. Other studies indicate better roughness, hardness, and CRS parameters compared to SP, and the combined treatment of SP and micro shot peening also improved all of these values compared to SP ^[50]. An improvement in fatigue strength for railway axle steels of 25% compared to the unpenned surface has also been observed ^[51]. Research is also being conducted on titanium alloys regarding the influence of shot size on material properties and corrosion resistance after the peening process ^[52].

In this method, the pressing medium is high-velocity water droplets hitting the surface. The droplets generate a high peak pressure which plastically deforms the surface. Water jet peening is a term used in many other peening methods, but it should be noted that it is different from cavitation peening. Therefore, in order to avoid misunderstandings, the classification used by H. Soyama was adopted. Water jet peening refers to methods using water jets, where water droplets are the pressing medium, and the mechanism is the collision of liquids. On the other hand, cavitation peening is a method using cavitation impacts, and the material deformation mechanism is due to shock waves. The problem of dividing these methods is probably due to the fact that, in classical cavitation peening, cavitation is induced by a jet of water injected at high speed into the water ^[53][54]. Results on spring steel, titanium, and nickel suggest that this may be a valuable method for applications where increased roughness and CRS are required ^[55].

The principle of this method is the same as for water jet peening, the difference being the medium, which is hydraulic oil, used to create the jet. With this method, it is possible to introduce CRS without significant changes in the surface topography. The maximum surface CRS value after oil jet peening can be 80–85% of the yield point, while for shot peening, it is 45–97%. For shot peening, the surface CRS is 30–80%, for water jet peening, it is 30–60%, and for cavitation water jet peening, it is about 89% of the yield strength ^[27][56][57]. By using oil jet peening, it is possible to improve CRS in materials such as low carbon steel or aluminium alloys.

This method uses powerful ultrasonic vibrations of a hard body with a high frequency, usually ultrasonic, which set spherical shots in motion. As a result, the intensity of pressing is very high. The main difference lies in the absence of the medium setting the shots in motion (in the form of air or steam). There are also shot collisions, so there is no uniform angle of incidence on the component as in the case of SP, where the angle is 90°, and there is no ball impact force ^[58]. Studies comparing ultrasonic shot peening (USP) and SP suggest two approaches that can be considered. Tekada found similar structural and mechanical properties for USP and SP, but concluded that under similar conditions, a larger volume of nanocrystallites was observed after SP. However, a different conclusion was reached by Dai and Shaw, who stated that, because the shot size used during USP can be larger than for SP, the unit impact is able to deliver more energy, despite the lower particle velocity ^[59]. According to Tekada, the coverage area in his condition was 170%/s and the particle velocity was >100 m/s for SP. For USP, the coverage area was 20%/s and the particle velocity < 20 m/s with a very large velocity distribution in contrast to USP. Despite many attempts, it has not been possible to provide a consistent model for multiple impacts ^[60]. Research in this area suggests that the type and composition of the peening balls, among other things, influence the intensity, but to a lesser extent than the length of peening time. USP gives good results for soft materials and reduces the probability of crack initiation and propagation. Positive aspects have also been found for the physical and mechanical properties of such treatment, such as improved corrosion resistance, increased contact, and fatigue strength ^[61].

The name cavitation—used in physics and technology—is derived from the Latin word ‘cavitas’ (emptiness, cavity). Cavitation is a rapid physical transition from a liquid to a gaseous state due to a decrease in pressure, which is associated with the appearance in the liquid of the so-called cavitation bubbles with huge implosion loads ^[68][69][70]. Bubbles filled with steam, dissolved gases or both steam and gases are formed in places with a pressure reduced to a critical value and implode in places with increased pressure. The time of collapse of the bubbles may be below 0.001 s.

The gas contained in the cavitation bubble causes the bubble to be reconstituted after implosion, and this process can repeat several times ^[71][72]. The determining factor for the formation of cavitation is the changing pressure field. There must be a region in the flow where the pressure drops to a value close to the saturated vapour pressure and then rises. The factors influencing the formation of cavitation are pressure, temperature and velocity of the fluid flow. Other factors that facilitate the cavitation phenomenon include the surface shape with which the liquid contacts the presence of impurities and gases dissolved in the liquid ^[73].

Osborn Raynolds first described the phenomenon of cavitation in 1894. It can be observed in liquids with a non-uniform velocity distribution. If the fluid rapidly increases its velocity, the static pressure of the fluid decreases. The boiling point of a fluid is closely related to the pressure surrounding the fluid. As the pressure decreases, the boiling point also decreases. If the fluid’s static pressure drops low enough, the fluid starts to boil and thus, cavitation bubbles form ^[74][75]. Figure 5 shows the collapse of a cavitation bubble, i.e., cyclic changes in bubble size with wave propagation in the liquid, followed by an implosion, i.e., a sudden reduction in bubble size leading to its disappearance. Implosion causes vapour to condense inside the bubble, and the implosion pressure ranges from several hundreds to several thousands of mega Pascals. Important for understanding the mechanism of material deformation during cavitation is the formation of microbubbles. If the collapsing bubble is in contact with the solid surface, the diameter of the microstructure can be approximately 10% of the bubble radius, and its velocity will be high enough to deform the solid surface ^[29]. If the asymmetric collapse of the bubble is at larger distances from the solid surface, there will be a significant reduction in its failure ^[76][77][78].

The formation of liquid microbubbles may also be due to the fact that the pulsating cavitation cloud interacts with spherically shaped bubbles close to the solid. These interactions cause the bubbles to oscillate, and their shape becomes unstable. If the amplitude of the bubble oscillation is large enough, microstructure formation begins to occur as the liquid surrounding the bubble begins to flow through the bubble towards the solid. The velocity of the micro-streams can range from 156 to 175 m/s, according to various sources ^[80][81]. Gibson has shown that jet formation, direction, and intensity are functions of the bubble–bubble interaction ^[82].

As a result of the interaction of bubbles in the cloud, the cloud collapse process experiences a slight deceleration in the primary phase and significant acceleration in the final phase of implosion. This results in the generation of a much higher final pulse pressure than is apparent from the analysis of the implosion of a single bubble ^[83][84]. The velocity of the micro stream produced by the interaction with pressure pulses resulting from the collapse of other bubbles and the propagation of the shock wave can be as high as 400 m/s ^[85].

Many types of cavitation can be considered, and successive authors have proposed their own divisions of this phenomenon. The most popular and, at the same time, the simplest division is the one proposed, among others, by Lauterborn ^[78]. Considering the causes of cavitation, researchers can distinguish the following types of this phenomenon ^[86]:

• optical cavitation;
• molecular cavitation;
• acoustic cavitation;
• hydrodynamic cavitation.