# Evaluation of the Machinability of Ti-6 Al-4V Titanium Alloy by AWJM Using a Multipass Strategy
## 3 Results and Discussion
### The Effect of Multipass Strategy on Material Removal Rate and Cutting Efficiency  
Apart from the kerf characteristics, which are related to the desired slot geometry and quality, it is important to estimate other crucial quantities of the AWJM process, such as the productivity, which is also essential from a different point of view. By altering process parameters or using a different number of passes, it is possible that the productivity, represented by material removal rate (MRR), can be regulated. From the observations of the slot geometry, it was deduced that during single pass milling, the amount of the removed material was decreased at higher traverse feed rates and increased at higher jet pressure values, as was anticipated due to the different energy values and contact time between the jet and the workpiece surface. Although this trend was still evident in the case of multiple passes, it was clear that the amount of removed material did not increase proportionally. Thus, as aforementioned, although the depth of penetration increased after each pass, the fact that the kerf width was almost unaffected and kerf taper was reduced, as well as the reduction of energy of the beam at higher depths, led to a less efficient material removal. Moreover, the previous interaction between the beam and material caused an increase in the residual compressive stress and difficulty with detaching the material during the erosion process. The time needed for removing the same quantity of material for every additional pass became longer. If the average MRR values are calculated, taking into consideration the different number of passes in each case, the results in Figure 9 clearly illustrate the previous observations about removed material and confirm the variation of MRR after multiple passes. It can be seen that the single-pass AWJM led to a greater MRR in all cases, with the MRR being reduced at higher traverse feed rates and increased at higher jet pressure values. In the case of multiple passes, at lower jet pressures, the effect of multiple passes on the MRR was less important, as the variation of MRR values was minimal. In some cases, the MRR increased under more passes and a faster feed rate, but this increase was rather small compared to the cases with a higher jet pressure. At higher jet pressures, a clear trend can be observed regarding MRR, as it was gradually decreasing, eventually being reduced to almost half the value of MRR for the single-pass milling. These results show that, although the process parameters and number of passes can alter kerf characteristics, it is essential to also consider the effect of these choices on the productivity and to avoid parameter combinations that render AWJM inefficient. It can be concluded that, in cases with a longer exposure time such as case 1, or higher kinetic energy, such as cases 4 and 5, the correlation between average MRR and number of passes was more evident, especially in the cases with higher kinetic energy, as the subsequent passes led to a lower overall MRR. On the contrary, when the exposure was shorter, the number of passes did not affect the MRR. As a rule of thumb, single pass cutting leads to higher MRR, but the final decision should take into account the values of kerf characteristics, in order to reach a compromise between productivity and slot quality. Finally, in order to quantify the cutting efficiency of AWJM, it is estimated as the ratio of the removed material per unit mass of abrasive removed [28]. The results in Figure 10 indicate generally similar trends to those for MRR, as the cutting efficiency is shown to decrease at higher traverse feed rates and increase at higher jet pressures for single-pass milling. The maximum efficiency was obtained for cases with a single pass. Moreover, for multipass milling at lower pressures, the variation of cutting efficiency was minimal, and in most cases, a higher traverse feed was slightly better than lower values.