## 1 Introduction
In their work, the jet pressure, traverse feed rate, stand-off distance, and number of passes were varied for the post-treatment stage, whereas for the abrasive waterjet experiments, a single pass was carried out, with abrasive mass flow rate as an additional variable. Regarding the AWJ treatment, it was proven efficient for the removal of embedded particles, and although in most cases, roughness was reduced, in some cases, a considerable increase was observed both in the longitudinal and traverse direction. Similar conclusions were drawn by [PERSON] et al. [28], who underlined that although a pure WJ can smooth out machined surfaces, low energy or exposure time should be avoided as the surface features produced by the previous step cannot be removed. However, a very low feed rate should also be avoided as the roughness produced under these conditions is comparable to or higher than that of AWJ milling. [PERSON] et al. [29] conducted a comprehensive work on the effect of a plain WJ on the surface quality and integrity of the produced specimens. It was found that the grit embedment percentage becomes higher as the standoff distance increases and the jet pressure decreases, whereas the effect of the traverse feed rate was smaller. The surface roughness under pure WJ treatment was usually slightly higher due to the dislodgement of embedded particles, whereas the water droplets contributed to the smoothing of peaks to some extent. Moreover, the plain WJ treatment did not alter the residual stress distribution in most cases. [PERSON] et al. [30] conducted a thorough study on the effect of plain WJ cleaning on the removal of embedded grit and the surface quality of workpieces processed by an AWJ, indicating that water pressure is the most important parameter during cleaning and that this process can be very effective, leading up to 65% removal of embedded particles. Furthermore, one sustainable solution is the use of soluble abrasives in order to avoid the problem of particle embedment, which reduces fatigue life and poses challenges to the application of coatings, while they also offer a better material removal rate (MRR) than a plain WJ [31]. Finally, regarding the processing of implant materials, another option is the use of biocompatible abrasives, which were shown to produce surfaces with higher quality than those processed by a pure WJ, but are related to low MRR values [32]. Among the variants of pure WJ processing apart from a pure WJ, [PERSON], [PERSON], and [PERSON] recently proposed water droplet machining by means of a Rayleigh jet [33; 34] in order to avoid delamination when cutting composites, but at the expense of using low traverse rates. Another alternative type of WJ processing involves the combined use of water and air jets, based on the concept of a \"fuzzy jet\" [35] or injection principle (iAWJ) [36]. In general, the use of air, which is injected in the WJ, can be used as an additional factor that is regulated to achieve superior performance. Moreover, another alternative for improving fatigue life is to use ultrasonic pulsed WJ (PWJ) in order to enhance the effect of impact on the workpiece surface. Although the surface quality after the treatment deteriorated with Ra values up to almost 35 \(\upmu\)m for low traverse speeds, compressive stresses were developed, up to 345 MPa near the surface, thus improving the surface integrity [37]. In general, the use of a pulsating water jet can lead to alteration of the workpiece microstructure by modifying the geometry of the grains and increasing their misorientation to a large extent compared to the use of a continuous WJ. Furthermore, the surface quality deteriorates more rapidly with the use of a PWJ compared to a continuous WJ [38]. [PERSON] et al. [39] also proposed the use of a pulsating WJ as a more efficient way for material processing and tested this process under different pressures, nozzle diameters, and time durations.