4. PROCESS PARAMETER DETERMINATION
4.1. Process Parameters Optimisation for L-PBF of Ti-6Al-4V ELI
The experimental runs that were generated by the DOE (Chapter 3.4.1.1) were conducted to investigate and identify the optimum combination of process parameters which give the highest part density. The Taguchi method was used to optimise the process parameters of the Ti-6Al-4V ELI. The experimental results are illustrated in Table 4-1 and in Figure 4-1, where it can be seen that the maximum average relative density (RD) recorded is 99.97%, which was for run number 6. The average RD measurement is the result of three separate readings for each of the four cubes built in each run. The accuracy of measurements of the selected method depends on the surface roughness of all sides of the sample and the surface tension of the liquid (distilled water). When the surface is smooth, the variation of the measurements is small and vice versa.
Table 4-1: Average relative density (RD) resulted from the experimental runs of
Figure 4-1: Relative density of the Ti-6Al-4V ELI cubes for each run, indicating measurement accuracy
The porosity was visualised by using X-ray computed tomography (CT) using CTH225 LC model (X-TEK SYSTEM LTD, UK). Figure 4-2 shows samples of CT scanned cubes. The selected cubes represent the maximum and minimum relative density, cube 21 from run 6 and cube 61 from run 16 respectively (Figure 4-2 (a) and (c)). The other scanned cubes (b and d) were selected according to the average high and average low
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RD. The majority of porosity distribution was located in the centre of the samples. In cube number 27, however, the porosity was more concentrated to the right-hand side of the cube. This could be caused by the machine architecture along with the process parameters (i.e. the gas flow direction is from the right to the left). The calculated RD value via the CT scan was slightly higher than that calculated by the Archimedes’
principle. The calculated RD of the CT scanned cubes 21, 27, 61 and 95 was 99.99%, 99.90%, 93.24% and 98.94% respectively while the calculated RD using Archimedes’
principle was 99.96%, 99.46%, 88.87% and 98.92% respectively. This variation is believed to be due to the incapability of the CT scan to detect small pores, due to resolution limitations of the machine.
Figure 4-2: CT scanning of the internal porosity accumulation of samples 21, 27, 61 and 95 from Run number 6, 7, 16 and 24 respectively identified by colour according to pore size For the most effective process parameters, the Signal-to-Noise ratio S/N ratio was analysed and summarised in Table 4-2. The Delta value represents the difference between maximum and minimum S/N ratio for the levels of each factor. The value of
Delta represents the significance of a factor, with a higher Delta value indicating a greater significance of that factor.
Table 4-2: Response Table for Signal to Noise Ratios (Larger is better) for resultant density
S/N for factor:
Level LT LP PD ET HD
1 12.87 12.64 12.76 12.71 12.75
2 12.87 12.72 12.76 12.7 12.85
3 12.67 12.74 12.87 12.81 12.69
4 12.64 12.79 12.61 12.7 12.8
5 12.66 12.81 12.71 12.79 12.61
Delta 0.23 0.17 0.26 0.11 0.24
Rank 3 4 1 5 2
The highest values for the S/N ratio of the factors identify the ideal level in terms of the control factor settings, which minimizes the effects of the noise factors. The factors are ranked according to their effectiveness. As shown in Table 4-2 and Figure 4-3, point distance (PD) and hatch distance (HD) are the most significant factors (Rank 1 and Rank 2 respectively). Layer thickness (LT) is the third most influential factor on the process, followed by laser power (LP) and finally, exposure time (ET).
Figure 4-3: Main effects plot for S/N ratios (larger is better) for the resultant density
The optimal parameter combination from the selected experimental design identified is:
layer thickness of 30µm, laser power of 200W (maximum power gives greater flexibility in choosing other parameter ranges [50]), point distance of 55µm, exposure time of 100µs, and hatching distance of 60µm.
Validation experiments were carried out using the same materials and equipment as previously described. The levels of factors were selected around the best combination of parameters that were established by the Taguchi method. Layer thickness and laser power were fixed at 30µm and 200W respectively. The experiments were based on a full factorial experiment (Table 4-3).
Table 4-3: Validation process parameters levels
# Parameter Levels
1 Layer Thickness - (µm) 30
2 Laser Power - (W) 200
3 Point Distance - (µm) 50 55 60 65
4 Exposure Time - (µs) 50 100
5 Hatching Distance - (µm) 55 60 65
The results from the validation experiments show that there are additional combinations of point distance and hatching distance that produce higher density parts (Figure 4-4).
Two other combinations of process parameters were obtained: the first where the PD is 50µm, ET is 50µs, and HD is 65µm and the second where the PD, ET, and HD are 65µm, 50µs, and 65µm respectively.
Figure 4-4: Results of validation experiments comparing the relative density with respect to the point distance and hatching distance at exposure time of 50µs and 100µs.
From Table 4-4, the density of the Ti-6Al-4V ELI parts fabricated using the new parameters combinations, identified via the validation experiments, was higher in comparison to the density of the parts obtained by either the Taguchi method or the manufacturer’s recommended profile. Using the exposure time suggested by the Taguchi method (ET of 100µs) preserves the robustness of the process and ensures the system is less sensitive to any slight changes in other parameters such as the PD and the HD. The relative density percentage given in Table 4-4 is in comparison to the theoretical density of titanium alloy (4.43g/cm3 [154]).
Table 4-4: Comparison of results of the process parameter combinations found by Taguchiandvalidationexperimentsagainstthemanufacturer’sprofile for Ti-6Al-4V ELI
Parameter Taguchi Valid.
Process 1
Valid.
Process 2
Manufacturer’s profile
Layer Thickness - (µm) 30 30 30 30
Laser Power - (W) 200 200 200 200
Point Distance - (µm) 55 50 65 75
Exposure Time - (µs) 100 50 50 50
Hatching Distance - (µm) 60 65 65 65
Relative Density – (%) 99.62 99.83 99.72 99.53
Figure 4-5 shows the relationship between volumetric energy density (VED) and relative density for the different layer thicknesses. The results indicate that it is possible
to fabricate parts with a larger layer thickness and still obtain high density parts. The effect of changing the layer thickness is comprehensively investigated in the next chapter, in addition to changing the particle size distribution of the primary powder.
Figure 4-5: The relative density of Ti-6Al-4V ELI parts versus VED for each layer
thickness obtained from optimisation phase
Using a layer thickness of 30µm gives the highest relative density. It is, however, possible to use thicker layers with an adjusted VEDs in the optimal parameter window, to fabricate high density parts, as with all layer thicknesses investigated it was possible to achieve a relative density of approximately 99%.
The noticeable variation in density for a single layer thickness might be caused by a combination of parameters, not solely by the overall value of VED. For the second (2nd) and third (3rd) points of the 70µm layer thickness in Figure 4-5, it can be noted that the combination of process parameters led to similar VED values, however, the part densities varied significantly. The main effect in this case was the point distance (PD).
The third point had almost double the point distance of the second point, 100 vs 55µm respectively and the density of the third point is 97% while the second point has a density of 99%. It is clear from above that the value for the VED alone is not appropriate to achieve a successful build, and that it is of paramount importance to understand the individual components which in combination deliver a specific value of VED. Therefore, the values of process parameters should be carefully selected to
establish certain values of scan speed or VED. Figure 4-6 shows that there is a linear relationship between LT and RD, and also LP and RD but the other three factors exhibit a non-linear relationship with respect to RD. Using a small PD, high ET or small HD results in a high energy density which contributed and resulted in evaporation and the development of keyhole porosity. In contrast, high PD, small ET or high HD causes a lack of fusion due to insufficient energy density being applied to fully melt the powder.
Therefore the relation of these three factors to the relative density was non-linear.
Moreover, it is expected that the LP also has a non-linear relation with RD which would be if an excessive LP was used, however, due to limitations of the current system used in this study, the maximum LP achievable was 200W, up to which the relation appeared linear.
Figure 4-6: Main effects plot for RD of Ti-6Al-4V ELI parts and its relationship with each parameter
Table 4-5 shows the optimum process parameters that were used in the investigation of particle size distribution and layer thickness in the next chapter.
Table 4-5: Optimum process parameters of L-PBF for Ti-6Al-4V ELI Parameter Value