Process Parameters Optimisation

L-PBF has numerous process parameters that contribute to the quality of fabricated parts. The focus of this work, however, is on the critical process parameters that have a direct impact on the melt pool formation. These parameters will therefore have the major influence on the part density.

3.4.1.1. Ti-6Al-4V ELI

Process parameter optimisation was carried out using powder type 1 (T1). Layer thickness (LT), laser power (LP), scan speed (SS) and hatching distance (HD) were considered to be the most critical parameters because their effects on the delivered melt energy. In pulsed L-PBF systems, the laser does not fire continuously but rather in a discrete manner. In this case the scan speed is calculated according to point distance (PD, the distance between two consecutive points), as can be seen in Figure 3-10, exposure time (ET, the elapsed time for each laser beam firing), and jump speed (JS, the speed of galvanometer mirror when moving from point to point). Equation ( 3-5) was used to calculate the scan speed.

S𝑐𝑎𝑛 𝑆𝑝𝑒𝑒𝑑 (𝑆𝑆) = 𝑃𝐷 𝐸𝑇 +𝑃𝐷

𝐽𝑆 ( 3-5)

Figure ‎3-10: Point distance and hatching distance illustration for pulsed laser PBF systems

In this study the jump speed was kept constant at 5000mm/s for all builds as recommended by the equipment manufacturer, while the PD and ET were considered as variables in this study and were considered as optimisation parameters. The parameters and their selected levels are shown in Table 3-5. In pulsed L-PBF, however, it is not accurate to study the effect of scan speed as a single parameter on part quality. The scan speed can be obtained by different parameter combinations, but not all are suitable for use even when the combined values are identical. For instance, using a combination of a PD of 50µm and an ET of 50µs will lead to the same scan speed as a PD of 200µm and an ET of 200µs. Even though the value of scan speed is exactly the same, the latter combination may not be suitable for full density builds, as the size of the melt pool will not cover the distance between consecutive points (PD) even with the prolonged firing time (ET). Therefore, it is of paramount importance that each individual parameter is carefully selected. Thus, the scan speed value should only be used as a guideline, with further consideration given to the components which make up the scan speed value.

Table ‎3-5: Process parameters and their levels used in the experiments

Parameter Levels

1 2 3 4 5

Layer Thickness, LT - (µm) 20 30 50 70 100

Laser Power, LP - (W) 90 120 150 180 200

Point Distance, PD - (µm) 35 45 55 70 100 Exposure Time, ET - (µs) 50 70 100 150 200 Hatching Distance, HD - (µm) 50 60 70 85 100

The five process parameters (Table 3-5) were considered as control factors while the rest of process parameters were considered as to create a range of noise conditions.

Thus, for a five, five-level factors there are a total of 3125 (5⁵) different combinations that should be considered for a full factorial design. Using other fractional design such as Central Composite Design (CCD) and Box-Behnken Design (BBD) leads to 52-104 and 46-60 combinations (experiments) respectively. The range of the number of experiments of CCD and BBD depends on the number of factors that are considered as continuous or categorical factors. However, using the Taguchi approach for designing the experiments, the number of experiments required can be reduced drastically to save time and resources, and the approach still yields results in good agreement with other methods [138]. According to the Taguchi approach, the samples can be organised into only 25 groups. An orthogonal array of L25 was used for 5 parameters and 5 levels.

Each run was repeated four times. Minitab17 was used to build and analyse the experimental design.

The optimisation experiments (runs) and their process parameters were generated using the Taguchi methods (Table 3-6). Each run was repeated 4 times so that at the end of the experiment, there were 100 fabricated samples. For the same layer thickness, the machine is able to fabricate samples with different process parameters. As a result, the runs were grouped according to the LT. The parts were fabricated in five builds with varying layer thicknesses from 20µm to 100µm.

Table ‎3-6: Experimental runs generated by the Taguchi method for Ti-6Al-4V ELI and

For 316L-SS process parameters optimisation, the same process parameters were considered except the LP was kept fixed at the highest possible value. Using high laser power, however, widens the process window for other process parameters and provides

greater flexibility in investigating a wider range of process parameters on additively manufactured 316L-SS parts [50]. In addition, the result of optimising the process parameters for the Ti-6Al-4V ELI showed that using the LP with its maximum possible value improves the part density and can increase the system throughput. Therefore, the laser power in this study was used at its maximum value of 200W. The process parameters and their selected ranges are shown in Table 3-7.

Table ‎3-7: Range of the process parameters used in the experiments for 316L-SS

Parameter Range

min max

Layer Thickness, LT - (µm) 50 100 Point Distance, PD - (µm) 40 80 Exposure Time, ET - (µs) 50 150 Hatching Distance, HD - (µm) 50 120

The axial-points CCD of RSM was used to design the experiments for fabricating 316L-SS parts. For the four selected process parameters, the RSM suggested 31 runs in total (Table 3-8) which are classified as 16 cube points, 8 axial points and 7 centre points.

The levels of the factors (process parameters) were selected as axial points. The design was replicated 4 times. Minitab17 was used to design and analyse the experiments.

Table ‎3-8: Suggested runs by the RSM for 316L-SS