# Evaluation of the Machinability of Ti-6 Al-4V Titanium Alloy by AWJM Using a Multipass Strategy
## 1 Introduction  
Abrasive waterjet (AWJ) technology is a well-known and unconventional manufacturing method, in which a high-pressure fluid jet containing abrasive particles acts on the machined material, causing an erosion process and removing its fractions. This method has significant advantages, such as being environmentally-friendly, the possibility to machine all kinds of materials, especially hard-to-machine materials [1], and not causing a heat affected zone [2]. The method needs to be modernized in order to achieve a level of quality and efficiency that is in line with the requirements of new products in high end industries, such as the automotive and aerospace industries. Thus, research has been performed to enable the creation of more complex geometries in novel materials and composites through unconventional techniques such as AWJ. In order to control the process and find the best solutions, it is necessary to select appropriate process parameters and machining strategies [3; 4; 5]. The cutting capability of the AWJM process has been a major obstacle limiting its application to relatively thick materials, because of the depth-of-cut (DoC) or depth of penetration and kerf quality obtained [6; 7; 8]. Thus, an alternative multipass cutting strategy was introduced, in an attempt to increase the efficiency of the AWJM process and to improve the kerf quality. Multipass AWJM refers to the process in which the jet or cutting head travels over the same kerf many times during the cutting process. Multipass cutting has been found to be superior to single-pass cutting if the number of passes and the cutting parameters are properly selected [9]. By cutting the material at almost the same time with higher traverse speeds (vf) and lower mass flow rates (ma), multipass cutting offers bettercutting performance and lower energy consumption than single pass cutting. [PERSON] and [PERSON] [10] were among the first who took into consideration the possibility of improving technological performance using multipass strategies, as compared to single pass cutting. In their research, an 87% alumina ceramic sample was used and machined using the chosen process parameters. The jet traverse direction and jet traverse speed in each pass of a multipass operation were included in the design of the experiment. In the first and second parts of the tests, the first pass was able to cut through the sample, while in the third part more than one pass was needed to penetrate the material. It has been shown that multipass AWJM can be used to reduce the total actual cutting time and cost. [PERSON], [PERSON], and [PERSON] [11] investigated parameters in controlled-depth AWJM process, such as the jet impingement angle, traverse speed, waterjet pressure, and size of the garnet abrasive particles, to determine their influence on the roughness, waviness, and level of grit embedment of the titanium alloy sample. It was found that for achieving low waviness on the machined part, a high jet traverse speed, a small grit size, a low waterjet pressure, and a low jet impingement angle should be employed. The same research team completed experiments to determine the correlation between different abrasive properties and the quality of the surface in AWJ controlled-depth milling (AWJ-CDM) [12]. It was concluded that the material removal rate and surface roughness increase if the particle hardness is increased, that the shape factor and particle hardness have no significant effect on surface waviness, and the shape factor may have an effect on material removal and roughness. [PERSON] et al. [13] experimentally investigated the milling of a SiC ceramic plate, with a hardness level evaluated as 2500 HV under specific conditions. The influence of the jet feed rate, jet impingement angle, and the number of passes was investigated. It was concluded that real jet penetration does not follow a pure factorial rule (penetration depth \(\times\) number of passes). The research of [PERSON] et al.