## 1 Introduction  
Non-conventional techniques, usually employing a high energy beam, have revolutionized the manufacturing domain, as they have essentially enabled the rendering of various challenging features with higher efficiency and lower cost than conventional techniques [1]. Especially, in the case of hard-to-cut materials such as hardened steels, nickel superalloys, and titanium alloys, the use of non-conventional techniques can be beneficial as expensive machining setups, use of special cutting fluids, and assisted technologies can be avoided and the feasibility of the process is definitely increased. Moreover, as in the case of material removal processes, a high surface quality is a major requirement, and several non-conventional processes that have multiple capabilities in order to offer a considerably improved surface quality can be employed as an alternative to the traditional finishing processes. Among the non-conventional machining processes, AWJ machining has attracted a lot of attention, especially regarding roughing of workpieces as it can achieve considerably high productivity, even for hard-to-cut materials such as hardened steels, titanium alloys, and nickel-based superalloys [2]. Moreover, it can be regarded as an eco-friendly process given that no harmful liquids are employed and no harmful gases are emitted during this process [3]. However, although AWJ machining is an inherently eco-friendly process, there is still the possibility for improvement in terms of sustainability indicators, something that can be achieved by not only maintaining high standards regarding environmental criteria but also considering the economic and social dimensions of sustainability. Actually, the sustainability of this process has been less frequently studied than the sustainability of other processes and the use of recycled, eco-friendly abrasives such as glass beads has been less reported in the relevant literature compared to conventional abrasives such as garnet, alumina, and silicon carbide. Various types of features can be created by AWJ technology such as slots, contours, through cuts, holes, and pockets, among others [2]. Especially in the case of pockets it is important to choose appropriate process parameter values in order to obtain high dimensional accuracy and also acceptable levels of roughness and waviness. However, due to the intense nature of contact between the abrasive waterjet and the workpiece surface, the quality of roughed pockets is rather low and post-processing steps are required. Thus, pure WJ is preferred to be used in finishing operations, in order to avoid many shortcomings of the AWJ process, such as particle embedment [4], something that will be discussed afterwards [5]. Regarding post-processing, a pure WJ can be employed for polishing and other related purposes, such as deburring or cleaning. For example, [PERSON] et al. [6] employed a pure WJ for coating stripping and various researchers indicated that a pure WJ can be rather effective for paint removal, although an AWJ can more easily remove material, as the latter is associated with deterioration of the surface quality [7; 8; 9]. Several authors have employed a pure waterjet for other purposes as well, such as modification of the surface integrity. For example, various authors have studied the effect of a pure WJ treatment on the fatigue life of the processed surfaces. [PERSON] et al. [10] determined the effect of multiple WJ passes (up to six) on the surface quality, hardness, and fatigue life. It was shown that the surface roughness increased considerably with an increased number of passes and also the surface hardness had a notable increase. Moreover, although the compressive residual stresses increased in respect to the number of passes, the fatigue life was decreased. The same scientific group, in another study [11], also investigated the effect of WJ peening on stainless steel specimens by different strategies.