# Evaluation of the Machinability of Ti-6 Al-4V Titanium Alloy by AWJM Using a Multipass Strategy
## 1 Introduction
The conclusions of the study referred to the influence of the process parameters on the paint removal level and surface roughness. Another research work considered the influence on the quality of the machined material with the use of different strategies based on single, double, and multi-cutting passes with different traverse speeds and direction strategies [25]. [PERSON] and [PERSON] [25] used a response surface method (RSM) for their analysis, and their experiments revealed significant conclusions, e.g., that the multipass AWJM process improves the surface quality in contrast to single-pass cutting. Changing the cutting direction in the multipass method allowed obtaining better results for surface quality compared to cutting in one direction, and the results also showed that the impact of posterior passes decreased. [PERSON] et al. [26] examined machining circular pockets using AWJM in titanium alloy Ti-6 Al-4V samples. The Box-Behnken design method was implemented, and the results of the experiment showed that well-selected process parameters can enhance the average milling depth by 47.8% and reduce the surface roughness by 38.1%. [PERSON], [PERSON], and [PERSON] [27] conducted an experiment in which hard-to-cut titanium aluminide TNM-B1 was machined using two kerfs, to realize the shape of a turbine blade. Two new quality parameters were introduced to describe the difference between the constant and variable kerf depths: a concave geometry factor fa, and a convex geometry factor fx. [PERSON] et al. [28] studied multipass AWJM efficiency when working on chromium molybdenum-vanadium (60 CrMoV18-5) steel alloy as a workpiece material. It was concluded that the effect of the multipass strategy was highly related to the process conditions and that the multipass cutting technique enables achieving a markedly higher efficiency. Another non-conventional study with the use of AWJM was introduced by [PERSON] et al. [29]. Experiments on polishing a stainless steel workpiece were conducted, where both single and multipass techniques were used. The results showed that the most appropriate process parameters for gaining the best quality of polished surface were a double-pass strategy, abrasive mesh 300, and a traverse rate of 39.96 mm/min. [PERSON] and [PERSON] [30] studied statistical possibilities for enhancing AWJM performance. The Taguchi method was used, which permitted focusing on the process parameters that had the most effect on the process. The conclusions showed that the stochastic effects resulting from the marble structure could be reduced, and the statistical methods used for the experiments allowed decreasing the number of cracks in the material after the machining process. The importance of adopting alternatives strategies to improve AWJM is even more significant in the case of the processing of hard-to-cut materials, such as hardened steel, titanium, and nickel alloys. In particular, Ti-6 Al-4V has a high strength-to-weight ratio, is more common than any other titanium alloys, represents more than 50% of the world's supply, and belongs to the \(\alpha+\beta\) alloy group [31; 32]. It is a common choice for applications such as compressor blades, airframe components, rocket engine cases, and rings used in jet engines. Approximately 155,000 kg of titanium will be consumed in 2020 by the -aviation sector, the largest consumer of titanium [33]. This alloy can be divided into two phases at usual atmospheric pressure and temperature: the \(\alpha\) phase with hexagonal close packed (HCP) crystallographic structure, and the \(\beta\) phase with body centered cubic (BCC) structure [34]. The Al, O, and N elements play a significant role in the stability of the \(\alpha\) phase, but the vanadium and iron elements stabilize the \(\beta\) phase [35]. Due to the superior characteristics of Ti-6 Al-4V, several authors have studied the machining of this alloy using AWJM. [PERSON], [PERSON], and [PERSON] [36] studied the effect of abrasive waterjet cleaning on the grit embedment and surface quality of this titanium alloy.