3.1 Machining and tool wear
The samples were prepared by rough machining as described in Table 3. This machining involved tough intermittent machining and severe tool wear, e.g., chipping.
The tool wear propagation was analysed in the sub-surface after finishing showed an increasing flank- and crater wear on clearance and rake side respectively on tools used for the as-built PBF-EB. For the as-built PBF-LB material, the tool flank wear was lower on the clearance side and the coating was worn on the rake side, but no crater development. Both variants showed a clear notch at cutting depth, see Fig. 3.
Tool wear mechanisms in heat treated components was the same as in the as-built variants. The PBF-FBEB showed similar wear mechanism in both conditions while the PFB-LB had more flank wear in the heat treated condition but with less wear on the rake side, see Fig. 3.
The corresponding flank wear of the inserts has been summarized in Table 4. These results show a linear increase in flank wear with cutting length expect for the PBF-EB after 334.6 m which suffered from crated wear. Hence, this was replaced, and test restarted. It is further seen that for PBF-LB the heat treatment increased the tool wear with 49% but only 14% for the PBF-EB. Further, PBF-EB has induced an 87% higher tool wear compared to PBF-LB for the as-built condition and 43% higher for the heat treated condition.
Table 4
Measured maximum tool flank wear (VB_max) for the studied materials.
Cutting length [m]
|
PBF-LB
As-built [µm]
|
PBF-LB
Heat treated [µm]
|
PBF-EB
As-built [µm]
|
PBF-EB
Heat treated [µm]
|
334.6
|
52
|
78
|
77
|
217*
|
625.6
|
55
|
92
|
103
|
125
|
868.2
|
67
|
100
|
125
|
143
|
* Insert suffered from chipping failure as seen in Fig. 3.
3.2 Sample geometry and topography
To simplify the turning operation, rotational symmetric samples were produced for this cutting tests. However, due to the thermal heating and cooling in the printing process, stress relief operation of the samples was distorted prior to the machining operation. The geometry was measured using 3D-scanning and compared to the nominal CAD model as shown in Fig. 4. In general, the PBF-EB showed higher distortion for both conditions, the as-bult and heat treated, in comparison to the PBF-LB samples.
The topography, represented as 3D maps show significant differences between as-built PBF-LB and PBF-EB material, as seen in the Fig. 5. This difference is also seen in the Sa and S10z parameters which are mean values for three positions of each sample, and standard deviations. Results show a 6 times higher Sa for PBF-EB, 53.7 µm, compared to the PBF-LB 8.4 µm. The corresponding S10z-values were 203 µm for PBF-LB and 491 µm for PBF-EB. In principle, this means that to completely remove asperities from the printing process, at least 200 µm need to be removed from the PBF-LB surface and 500 µm from the PBF-EB surface. Additionally, the Sa and S10z values show a high variation between the three measurements, shown as high standard deviation. The two surfaces also show different surface features where the PBF-LB surface has unmelted particles while the PBF-EB surface instead show larger flake shaped features with a size of 0.5-1 mm. The topography after heat treatment, not included in Fig. 5, has similar topography values.
The topography was also evaluated as profiles from the cross sections with apparent differences as can be seen in Fig. 6. The two exemplified sections of the as-built material show an outer section consisting of protrusions and partially melted powder particles and an inner section which is completely melted. Comparing the cross-section profiles from the PBF-EB and PBF-LB samples shows an apparent difference in roughness. The PBF-EB profiles show a peak and valley structure that is not present in the PBF-LB profiles. It is also observed re-entrant features, as defined by Triantaphyllou et al. [42], linked to the peak and valley structure for the EB-PBF material, one example of which can be seen in Fig. 6A), whilst much smaller features are seen in the PBF-LB profiles, Fig. 6B). The protruding roughness features of the PBF-LB surface have directionality that can be described as waves in the direction of gravity within the build chamber. Such directionality is not observed in the PBF-EB profiles.
The 3D topography maps and calculated topography parameters, presented as the mean values from three measurements including the standard deviation, after finish machining are seen in Fig. 7. The results show apparent feed groves that are the main contribute to the surface roughness. The surface roughness represented by Sa is 0.3 µm for the PBF-LVB in both conditions while the PBF-EB has higher Sa values compared to PBF-LB, especially for the as-built condition. The higher Sa for the PBF-EB may be due to higher tool wear as the cutting edged becomes blunt, as observed in the tool wear results in Fig. 3. Similar trend is observed for the S10z value. The Sdr parameter show a slightly higher values for PBF-LB in heat treated condition and the PBF-EB in as-built condition, which implies more texture in the feed groves for these surfaces. Additionally, in the lower right of each condition the mean of ten profile is shown. This shows the difference in topography as well in terms of the depth and shape of the feed grooves. For PBF-EB the feed groves are deeper in as-built condition compared to the heat treated condition. The heat treated condition further show that the amplitude varies. The PBF-LB samples has a much lower feed groove amplitude, and the shape of the peaks are rounder, creating more of a sinusoidal shape compared to the PBF-EB peaks that is irregular. A long waviness is observed for all sample which most likely are connected to vibrations induced during machining.
3.3 Residual stresses
The surface residual stresses after the finish cut in the PBF-LB and EBP-EB samples in as-built and heat treated condition are shown in Fig. 8. The results are the mean values for three measurement positions along the building direction of the samples, and the error bars represent the standard deviation for the three measurements. The dashed lines show the bulk stresses measured at a depth 0.3 mm below the surface prior to the cutting test.
In general, the results show that turning has induced a significantly different stress condition in the samples related to the two directions, feed and cutting direction. For the as-built PBF-LB material a moderate compressive stress is introduced in the feed (build) direction and tensile stress in cutting direction, with similar magnitude as the bulk prior to machining. A significant change of stresses is observed for the heat treated material, which show tensile stresses in both directions ranging up to 900–1000 MPa in cutting direction and a moderate tensile stress of 300–400 MPa in feed direction. In relation to the different positions, a minor difference could be seen in Fig. 8 represented by the error bars.
The corresponding surface residual stresses after machining of the PBF-EB material is low compressive stress in feed direction and high tensile stress in cutting direction for the as-built material. The heat treated material show low tensile stresses in feed direction and high tensile stress in cutting direction. In relation to the different positions, a greater difference is observed for the PBF-EB material as shown by the error bars. For all sample, the variation around the samples were measured showing that the stresses could vary up to 140 MPa.
The residual stress profiles for the PBF-LB and PBF-EB after turning is shown in Fig. 9. The dashed lines show bulk values measured prior to the machining operation, in the as-build and heat treated conditions. The turning test of the PBF-LB as-built material shows great difference between the feed and cutting direction. The cutting direction shows a superficial tensile stress that drastically drops to a compressive stress below the surface. For both directions, a compressive zone is shown, which extends deeper for the cutting direction. At greater depth, both directions gradually become tensile, until the bulk stress condition is reached at 300 µm. Turning of PBF-LB in heat-treated condition induced high tensile stresses in the surface for both directions. The tensile stresses go into compressive stress at a depth of 10–30 µm followed by a deep compressive stress zone. The total impact depth, i.e., the depth where the bulk stresses are reached, is however lower compared to the as-built sample with an impact 150–200 µm deep.
The residual stress profiles after turning for the PBF-EB samples in as-built and heat treated condition are shown in Fig. 9. Generally, close to the surface it could be observed similar results but at a greater depths the heat treated condition has a deeper compressive stress compared to the as-built condition. It is further seen a greater variation in each measured position in the profile, seen in the error bars, which can be connected to large grains and texture in theses samples. For the as-built sample, both directions have tensile surface stresses that change into compressive ones below the surface at depth of 10–30 µm. The feed direction has a deep compressive zone and at a depth of 300 µm the bulk stresses are reached. However, there are a great variation in stresses at this depth for the two directions. The total affected depth is not completely covered for these measurements and are located deeper than 300 µm. Similarly, to the PBF-LB, machining of the heat treated samples show smoother shaped profiles characterized by tensile stress, rather high in tangential direction. Below the surface the stresses change into high compressive stresses with maximum at depth of 100–200 µm. It could further be observed a deeper impact with greater magnitudes compared to the as-printed profiles. The total impact depth not completely covered but in the range of 200–350 µm for feed direction and greater than 300 µm in cutting direction. It is shown that stresses differ greatly at depths greater than 150 µm.
The machining induced deformation could be evaluated from the diffraction peak broadening. In Fig. 10, the connected Full Width Half Maximum profiles are shown for the PBF-LB and PBF-EB samples in the two conditions. The results show a significant difference between the two material and also between the two conditions. The profiles for PBF-LB as smoother and show a low penetration depth of 25 µm while the heat treated condition is 75–100 µm deep. The surface deformation is also lower for the as-built condition. The PBF-EB show much greater variation in the profiles and a much smaller difference in the surface deformation. The penetration depth is difficult to precisely define as the profiles varies, but it is shown that the as-built material is typically around 75–100 µm while the heat treated condition show a penetration depth of 125–150 µm.
3.4 Microstructure and hardness
The as-built and heat treated microstructures were analysed in detail using Scanning electron microscopy (SEM). As shown in Fig. 11, the PBF-LB material have larger grains from the core of the specimen to the surface whereas the PBF-EB materials contained smaller grains, especially close to the surface.
The LB-PBF material in Fig. 11A) shows precipitates in the grain boundaries after printing, but no delta phase is evident. After heat treatment of the PBF-LB material, more precipitates can be observed along the grain boundaries as shown in Fig. 11B). In contrast to the PBF-LB samples, the PBF-EB has a clearly visible printed contour with a thickness of 300–600 µm. This is observed as a zone with comparably smaller and almost equiaxed grains compared to the much larger grains elongated in the build direction in the core, not included in Fig. 11. Strain in the grains (in the PBF-EB images) is evidenced by different shades of grey within the grains. Delta phase is present, evident as large white needle-like precipitates in the grain boundaries, especially in the as-built material in Fig. 11A). After heat treatment, the grain boundaries are increasingly more decorated with precipitates. Some strain has been relieved and delta phase is still present observed as high contracts within the grains in theses micrographs.
The polished cross sections of the different samples were also used for estimations the porosity near the surface. Table 5 shows the amount and type of porosity at different positions along the build direction for the studied materials showing higher from PBF-EB than for PBF-LB and that there are some differences at the different positions in the sample as well.
Table 5. Near surface porosity levels of the different samples at different positions.
Sample
|
Position
|
Spherical porosity [%]
|
Irregular porosity [%]
|
Total porosity [%]
|
|
Top
|
0.42
|
0.02
|
0.44
|
PBF-EB
|
Middle
|
0.42
|
0.26
|
0.63
|
|
Bottom
|
0.24
|
0.04
|
0.28
|
|
Top
|
0.1
|
0.03
|
0.13
|
PBF-LB
|
Middle
|
0.11
|
0.02
|
0.13
|
|
Bottom
|
0.005
|
0.005
|
0.01
|
The impact from the turning on the microstructure are shown in Fig. 12 for PBF-LB and PBF-EB. The deformation induced in the PBF-LB sample is low, no superficial deformed layer is shown in Fig. 12, and the precipitates decorating the grain boundaries are intact and still in place all the way to the surface. After finishing the PBF-EB material, where the printed contour is completely removed, the cut surface shows both strained grains, evidenced by shades of grey within grains, to a depth of about 40 µm and a superficial layer with grain refinement. The superficial layer is very distinct and about 2 µm thick when cutting in the as-printed material and less distinct, up to 10 µm thick when cutting in the heat-treated material.
The mean hardness profiles for the as-built and heat treated conditions from measurements at different positions along the sample, are shown in Fig. 13. The error bars represent the standard deviation for the three positions. The results indicate a significant difference between the PBF-LB and the PBF-EB in the as-printed condition, whereas after heat treatment they possess comparable hardness values. In the as-built condition, the PBF-EB material exhibit higher hardness compared to the PBF-LB.
Nanoindentation was used to measure the hardness of superficial regions with the results as seen in Fig. 14. As both PBF-EB materials showed transformed superficial regions a few microns thick, the first region to be measured (on all samples) was at depths between 1–3 µm. A heavily deformed region was found on the PBF-EB samples, so the second region measured on all samples was at depths between 8–30 µm to capture any such region. In addition to these two regions, also the hardness at a depth of 150 µm below the surface was measured. All measurements were made with a Berkovich tip and a target depth of 100 nm. All positions were confirmed and every depth below the surface was measured in microscope after the experiments. The values are different, but the order of the samples as measured in the deepest region corresponds to the order of the bulk Vickers hardness measurements shown in Fig. 16. It should be noted that the grain size is only slightly larger than the depth of these regions, which means the hardness levels may be influenced by differences in individual grains.
The measurements of the PBF-LB in as-built confirm that the cutting cause a significant increase in hardness of this material, both in the outermost surface and in the intermediate region below that. In contrast, the heat-treated PBF-LB material which has a higher starting hardness, has only a slight tendency to increase in hardness due to cutting. The two PBF-EB materials, as-built and heat treated, seem to be harder in the deep region rather than in the surface region and only the as-built material show a tendency to have an increase in the very outermost region.