Stage I – Several demonstration components were SLM fabricated using the EOS system with Magics supports in order to assess the feasibility of the process to produce particular
CHAPTER 4: PROCESS OPTIMISATION: INFLUENCE OF THE SLM PARAMETERS ON THE MICROSTRUCTURE &
4.4 IN625 Parametric Studies
The parametric investigation of IN625 was carried out in a very similar way to the previous CMSX486 study. Literature (see Chapter 2) suggests that IN625 is a good candidate for laser processing as it does not show the same susceptibility to weld-induced cracking.
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This parametric study again aimed to rapidly assess the process parameters for IN625 however in this case the central composite design method was used for only three parameters:
laser power, scan speed and scan spacing. The island size did not show any significance in the previous study and so was eliminated from this DOE. Also based on an initial examination of the samples, there was no cracking observed in the as-fabricated sate so the results were obtained for void area fraction only and utilised optical (rather than SEM) microscopy to aid the rapid nature of the investigation.
i. Powder Size Analysis Results & Discussion
As with the previous studies, the powder size distribution was measured. The argon gas-atomised IN625 powder was supplied by LPW Technology Ltd. in the size fraction +15-53 m. Figure 4.45 shows the verified size distribution.
Figure 4.45: Particle size distribution for argon gas atomised IN625 powder as; black dashed lines indicate the ideal size distribution boundaries. Key sizing data is provided in the inset table.
The powder size distribution lies clearly within the ideal boundaries showing a maximum at almost exactly 53 m and a minimum particle size slightly greater than 16 m. The particle
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sizes appear to show a good normal distribution with little skew as indicated by the similar mean and D50 (median) values.
ii. SEM Powder Examination: Results & Discussion
Examination of the ground and polished powder was carried out in a similar way to that of CM247LC and CMSX486. Figure 4.46 shows a typical image from the set taken for porosity measurement; it shows similar morphologies to the previous powders examined. There appear to be some poorly shaped particles caused by the attachment of two individual particles and evidence of liquid droplets colliding with solid particles causing misshaping and the characteristic crescent voids as seen previously.
Figure 4.47 shows a BSE SEM micrograph of an individual particle. It appears to show a finely dendritic morphology; however there does not appear to be any significant elemental segregation across the diameter of the particle as seen in the EDX linescans presented in Figure 4.48.
The overall porosity quantification is presented on Table 4.10. In general the IN625 powder appears to show much lower porosity in terms of average pore size and overall porosity area fraction than the previous two powders.
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Figure 4.46: BSE SEM micrograph showing a typical sample of ground and polished IN625 as used for the porosity quantification. Note the misshapen particles due to particle joining during solidification and the
‘crescent’ shaped pores formed by particle collision during solidification.
Figure 4.47: BSE SEM micrograph of an individual ground and polished particle of IN625 argon gas atomised powder.
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Figure 4.48: EDS linescans across two individual IN625 powder particles (Yellow line on micrograph denoting linescan profile). EDS results show no significant segregation across the particle. Note: The increase in C and decrease in Ni towards either end of the scan is due to the onset of the particle edge and
the effect of the mounting Bakelite.
Table 4.10: Porosity quantification results for In625 powder.
Total number of particles analysed 447 Minimum pore size to be measured
(to eliminate noise) 0.30 m2 Total number of pores analysed 203 Mean Pore equivalent diameter 1.19 m Maximum Pore equivalent diameter 2.97 m
% Porosity of the analysed powder 0.14%
iii. SLM Parametric Study: Void Quantification Results
The raw results for the parametric study carried out for IN625 are presented in Table 4.11. the study investigated laser power (100 – 200 W); scan speed (1000 - 3000 mm/s) and scan
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spacing (a1; 0.2-0.8). the raw data has been plotted against energy density (J/mm2, Equation 4-1) Figure 4.49.
Table 4.11: Parametric DOE study results for the SLM processing of IN625 investigating 3 parameters measuring porosity only.
Figure 4.49: Plot showing porosity (%) against energy density (J/mm2) for SLM of IN625.
The Design-Expert predicted an R2 value of 83% when fitting these results to a linear model.
In this case only the individual parameters laser power, scan speed and scan spacing were
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examined for statistical significance. The ANOVA table is presented in Table 4.12 and shows that all of these parameters have a P-value of <0.05 and so are all significant.
Table 4.12: ANOVA for void area fraction of IN625. Blue rows indicate significant terms and red row indicates overall model significance.
The linear model produced for these results is given in Equation 4-5.
Equation 4-5: Empirically determined relationship between the process parameters and the void fraction for IN625.
The comparison of the model against the actual void fraction values is shown in Figure 4.50.
Figure 4.50: Plot showing comparison of predicted values against actual results for the void fraction of SLM-fabricated IN625.
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Figure 4.51, Figure 4.52 and Figure 4.53 show that the void fraction increases with increasing scan speed and scan spacing, but decreases with increasing laser power.
Figure 4.51: Plot showing predicted influence of laser power on void fraction for IN625. Dashed lines indicate the 95% confidence limits of the model.
Figure 4.52: Plot showing predicted influence of scan speed on void fraction for IN625. Dashed lines indicate the 95% confidence limits of the model.