# Evaluation of the Machinability of Ti-6 Al-4V Titanium Alloy by AWJM Using a Multipass Strategy
## 3 Results and Discussion
### The Effect of a Multipass Strategy on the Kerf Taper Angle  
One of the important quality parameters for cutting a workpiece material and also for creating more complex geometries is the kerf taper angle. It would be highly desirable to produce machine elements, parts, and geometries in a way that would enable the processing of their edges such that they were perfectly perpendicular to the face of the element. It can therefore be concluded that the challenge for engineering is to reduce the kerf taper angle. As previous studies showed, this parameter is strongly affected by all variable process parameters [9,13]. In Figure 7, for cases 1-3, where lower pressure values were selected, one pass of the nozzle generated explicitly higher values of the kerf taper angle, whereas subsequent passes reduced this value, something that was also observed in the relevant literature [10,15,16,21]. Among the input variables that can reduce the angle when their values are increased, we can indicate the jet pressure and the number of passes, whereas the jet traverse rate, in most cases, increases the kerf taper angle. In one case out of the five, it can be seen that the fourth pass increased the angle value, but this might have been an exception related to measurement error or the impact of the other process parameters. Choosing adequate process parameters could reduce the taper angle, without the need to use a multipass strategy [13]. For example similar or better results could be achieved after machining twice using a 150 MPa jet pressure and 500 or 600 mm/min traverse feed rate, instead of machining using once using a 300 MPa jet pressure and 500 mm/min traverse feed rate. The reduction in kerf angle is probably partially related to the increasing distance between the nozzle and the workpiece. In general, the water beam enters the already created slot, deepening it and also removing more material on the sides of the slot, which makes the angle sharper. There is also an undeniable relationship between the groove depth and diminishing kerf taper angle. For a better comparison, the ratio between one and multiple passes was generated and is featured in Figure 8. The measurement results indicated that the ratio between one and several passes was less than one, thus indicating a decreasing trend of kerf taper angle with a higher number of passes. It can be deduced that there was a clear trend that caused the reduction of kerf taper angle after conducting additional passes [28]. In most cases, the successive passes had less and less impact on the result. Additionally, we can conclude that the other parameters, such as jet pressure and traverse feed rate, also visibly affected  
Figure 6: Ratio of top kerf width obtained with four passes, with respect to the width of the first pass under various experimental conditions. variations in the angle values. In the graphs, the differences between cases changed, so this means the values of the ratio of kerf taper angle differed in the studied cases. The smallest difference existed in the first case, between 3 and 4 passes, and then, after changing the pressure to 225 MPa, the difference increased. After using a jet pressure value of 300 MPa, the previously obtained effect of reducing the angle when increasing the number of passes no longer occurred; thus, as can be seen in the last case, the kerf taper angle increased when increasing the number of passes from three to four. Figure 8: Ratio of kerf taper angle obtained with 2, 3, or 4 passes, with respect to the taper angle of the first pass under various experimental conditions. Figure 7: Variation of the kerf taper angle for an increasing number of passes and different process conditions. From the aforementioned discussion, it can be concluded that there was a direct effect of multiple passes on the kerf taper angle, with the increase of passes leading directly to a smaller kerf taper angle and a sharper shape of the incision profile of the slots.