3.1 – Cutting forces
Table 2 shows the machining tests results. This table is essentially divided by the feed rate since this was the imposed variable to differentiate cutting conditions. The results presented are ordered in ascending order of resulting machining cutting force F.
For each machining test, the dynamometer outputs three values: the maximum force value and the maximum and minimum force values for each axis. The average of these two last values was used to determine the composition of the correspondent force.
Also, in this table are presented the contributions in the percentage of each force component in the resulting cutting force.
To compare the cutting forces measured in each test sample was calculated the resulting machining force F from the average values of the measured forces Fc, Ff and Fp using the following equation:
Table 2 – Machining forces results
Material
|
f
[mm/rev.]
|
Fp Avg
[N]
|
% Fp
|
Ff Avg
[N]
|
% Ff
|
Fc Avg
[N]
|
% Fc
|
F
[N]
|
EBM - T03
|
0.1
|
49,4
|
45%
|
19,5
|
18%
|
40,3
|
37%
|
66,7
|
Ti-6Al-4V
|
66,9
|
49%
|
24,3
|
18%
|
45,2
|
33%
|
84,3
|
EBM - T01
|
0.2
|
65,0
|
42%
|
23,1
|
15%
|
65,1
|
42%
|
94,8
|
EBM - Full
|
66,9
|
42%
|
23,6
|
15%
|
67,3
|
43%
|
97,8
|
EBM -T02
|
67,9
|
41%
|
25,4
|
15%
|
71,3
|
43%
|
101,7
|
Ti-6Al-4V
|
84,0
|
41%
|
37,8
|
19%
|
80,5
|
40%
|
122,3
|
Table 2 shows the feed rate effect in the cutting force values. There is an increase of approximately 50% in the cutting force for the same EBM test samples geometry when the feed is doubled and about 45% in the wrought test samples for the same feed rate variation. As the feed rates increases, the material volume with which the tool is in contact also increases. This condition increases the cutting force in the area of the tool in contact with the material which is cutting.
In the case of turning a cone, in which the tool moves in two axes (X and Z) simultaneously, the force ratio is different from that normally observed in cylindrical turning. In this case study, it was found that the passive force assumed a predominance over the other machining force components. From the results obtained, it is possible to observe that for 0.1mm/rev. feed rate, the passive force is higher compared to the cutting force, and that in the case of 0.2mm/rev. feed rate there is a proximity of the values of these two forces.
Comparing the forces resulting from the tests carried out with feed rates of 0.1 and 0.2mm/rev, respectively (Figure 3), it is possible to notice the difference in behaviour between the two types of materials used and the consistency of behaviour for each type of material. In the wrought alloy test sample, there is a gradual increase in the resulting machining force while in the EBM test samples the machining force is stable.
In figure 3, it is possible to see that, for the two cutting conditions, the EBM test samples always present lower values of cutting forces compared to the wrought ones throughout the entire machining test. It is also possible to observe that the shell effect of the EBM test samples does not show a significant impact between them in terms of differences in cutting forces.
3.2 – Surface roughness
Figure 4 presents the machined surface of a wrought and EBM test samples at f=0.2mm/rev. Comparing the two images, it is possible to see that in the EBM test sample, the tool marks are more identifiable. As demonstrated before, EBM test samples require lower cutting forces, therefore, have better machinability.
Another determining factor has to do with the fact that Young's modulus of EBM is higher than that of the wrought. This means that EBM has a lower rigidity, thence more subject to vibrations during the machining process.
As a result of the aforementioned, EBM offers less resistance to be machined and is less responsive to tool contact. Therefore, as a consequence of this, the tool can imprint more vividly on the EBM test sample surface its path. In wrought test samples, where hardness is superior, this results in a lesser irregulate surface or lower roughness.
The values of the Ra, RzD and Rt roughness, measured in each test sample with a feed rate of 0.1mm/rev are presented in figure 5. From what can be observed in all measurements, the Ti-6Al- 4V wrought test sample has the lowest roughness values.
Figure 6 shows the values of Ra, RzD and Rt, for a feed rate of 0.2mm/rev. For this feed rate value, the wrought test sample still presents a lower value.
Also, the EBM-Full test sample generally presents the lowest roughness values of the EBM set test samples. As it is more compact than the other EBM test samples, since it is filled inside, it has greater robustness than the shell-type test pieces and therefore has higher deformation resistance. This implies that the component structure influences roughness values.
Regarding the measured roughness values, comparing the results of the tests with different types of feed, it is clear that for higher feed values there is an expected increase in the roughness value. This leads to the conclusion that the structure of a 3d printed part influences its behaviour when subjected to the action of a cutting tool, and that the more fragile this structure, the lower its resistance to machining. In this particular case, the result was a higher surface roughness.
Observing Fig.7 and Fig 8, it is clear to see the variation of the roughness parameters as the feed rate increases. For the Ti-6Al-4V test sample, there was an increase of 218% for Ra and, for the EBM test sample an increase of 180% for the same parameter.
The results obtained are consistent with the work carried out by other researchers 22,23 in which wrought test samples presented better roughness results than the EBM ones or AM in general.
3.3 – Micro-CT
From the Micro-CT analysis is possible to see the differences in consistency and density of each test sample (Fig. 7). As can be seen in the wrought test sample designated as Ti-6Al-4V in the figure, there is a uniformity in its density that is not possible to verify in the remaining test samples obtained by EBM.
From the images obtained it is also possible to identify and distinguish the surfaces that weren’t machined. These have a surface irregularity characteristic of parts obtained by 3D metal printing. This surface irregularity or discontinuity has a direct effect on the mechanical properties of the material, being directly related to a decrease in the elastic modulus and yield strength of AM parts. The different mechanical properties of the materials under analysis will influence the results obtained. Therefore, it is expected for an EBM component to have lower resistance to deformation compared to a part obtained from a wrought bulk.
This can be confirmed by observing table 2, where for EBM test samples the cutting forces are always lower than those registered for the wrought test sample. The surface discontinuity of EBM test samples offers lower resistance to the cutting tool, which, on its displacement along the surface, will find material gaps and thus be relieved contact with the material. On the contrary, on wrought test samples, the contact of the tool with the material is continuously, leading to permanent tool stress on the material.
In addition to the factors previously referred, the hardness of the material is itself a variable to be taken into account when interpreting the results obtained. The hardness of the tested materials, approximately 350 HV for Ti-6Al-4V and 300 to 330 HV for EBM test samples, is also the reason for the different cutting forces. A harder material is more difficult to cut, this means that a higher reaction force from the material is applied against the cutting tool during its displacement along the machined surface.
On the side of the cutting tool, there was no built-up edge formation, tool wear or chatter. Since the cutting operation was a single pass with ap = 0.15mm which corresponds to a finishing operation in an AM part, the stress impressed over the cutting tool was minimal so that it could somehow cause any major damage to the cutting tool.
3.3 – Chip formation
In machining, the chip formed during cutting depends on several factors, such as cutting parameters, use and type of coolant and tool geometry, among others24.
After the completion of each test, the formed chips were collected for later classification and analysis. In figure 9 it is possible to observe the chips obtained for EBM and wrought test samples for the two feed rates used in the machining tests.
From figure 8 can be observed that EBM chips have a regularly tubular and helical shape with short and long sizes, and wrought titanium chips have a snarled shape.
According to ISO 3685-1977 (E), EBM chips can be classified as long washer-type helical chips for 0.1 mm/rev feed rate, and as long tubular chips for 0.2 mm/rev. Wrought chips can be classified as snarled ribbon chips for 0.1 mm/rev feed rate and as snarled tubular chips for 0.2 mm/rev. The chip geometry variation in both materials reveals that also in this case there is a direct influence of the feed rate variation.
Analysing the collected chips from the machining point of view of interest, the best chip formation was achieved with EBM and feedrate of 0.2mm/rev. Regular and short chips allow more efficient removal of the cutting zone, with that the coolant can be reached that zone and decrease the cutting temperature, with the benefits already deeply studied and known for the reduction of cutting forces, an increase of tool life and a better surface roughness.