# Evaluation of the Machinability of Ti-6 Al-4V Titanium Alloy by AWJM Using a Multipass Strategy
## 3 Results and Discussion
### The Effect of the Multipass Strategy on the Depth of Penetration  
In this subsection, the research focused on checking the influence of the jet pressure, traverse feed rate, and the number of passes on the depth of penetration, with constant stand-off distance and abrasive mass flow rate values. Figure 3 shows the influence of machining one slot multiple times and other parameters on the depth. Grouping the input into five cases made it easier to separate the influence of particular parameters on the multipass method results. Different colors represent different numbers of passes, and \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**No. of Case**} & **Traverse Feed** & **Jet Pressure** & **Stand-Off** & **Abrasive Mass** \\  & **(mm/min)** & **(MPa)** & **Distance (mm)** & **Flow Rate (g/s)** \\ \hline
1 & 400 & 150 & & \\
2 & 500 & 150 & & \\
3 & 600 & 150 & 3 & 2 \\
4 & 500 & 225 & & \\
5 & 500 & 300 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental parameters. Figure 2: Indicative images of the produced slots: (**a**) 2D image of the produced slots under various process conditions, (**b**) 3D representation of the produced slot. for every case, 1 to 5, the pressure and traverse feed rate values were changed as shown in Table 2.  
In each case, the first three slots were machined using a 150 MPa jet pressure; and for the next two, the jet pressure was increased to 225 and 300 MPa, respectively. For the first slot, the traverse feed rate was 400 mm/min, for the third 600 mm/min, and the rest of the slots were machined using a 500 mm/min traverse feed rate. As we can see on the graph, all parameters were significant and influenced the results. The slots in the first case were deeper than in the second and third cases, due to the smaller value of the traverse feed rate. A slower movement of the nozzle above the workpiece material caused the removal of more material in the area per time unit and consequently deeper slots [6; 7]. In the fourth and fifth cases, the slots were deeper, because of the higher jet pressure [7]. Every additional pass through the slot generated deeper grooves [26]. This effect can be observed in each of the five cases. In the experiment, the mass flow rate was constant, the pressure increased, and the variation in the traverse feed rate values was not large. Despite the beam energy loss connected to the rising stand-off distance in every pass, the ratio of depths, calculated as the ratio of depths between each pass and the first one and featured in Figure 4, increased for each additional pass. The correlation between passes and depth was not thought to be directly linear. The values obtained by dividing the multipass results by the one pass result allowed us to approximate the increase in the depth of the slots after machining them repeatedly. Despite the change in parameters such as the jet pressure or nozzle feed rate, the changes for subsequent transitions between the indicated cases did not differ significantly. Using the multipass method we could predict that, after a second pass, the penetration depth will practically double; after a third, the depth of the slots will reach about three times the original value; and after four passes, it will reach values between four and five times higher than with a single cut. This can be probably attributed to the fact that, after  
Figure 3: Influence of multipass strategy on the depth of penetration. each pass, the geometry of the slot bottom surface becomes more irregular and non-flat compared to the first pass [13]. Thus, it is possible that in some cases, such as those with a lower traverse feed rate and lower pressure, the final pass can aggravate the variance of the slot geometry to a such a considerable extent that a larger depth of penetration is achieved.