PSI - Issue 54

Ivana Zetková et al. / Procedia Structural Integrity 54 (2024) 256–263 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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drilling value. This is the area of approximately 0.15-0.8 mm. The drilling took place from the face of the sample in the z-direction. The drilling method was used for a basic comparison of the parameter sets used due to its speed and relatively economic efficiency. However, this method is suitable for determining residual stresses in subsurface layers that are no longer affected by the influence of the surface topology. To track the profile of residual stresses in the surface area and its close proximity, the X-ray diffraction method was used to compare the SP and SP_120 sets, see Figure 5. The results are more detail described in the contribution of the collective of authors Trojan at al.

Fig. 5. Depth profile of macroscopic residual stresses.

From the macroscopic RS results, see Figure 5, it is clear that the depth profile varies significantly from sample to sample. Tensile RS still dominate in the near vicinity of the surface for both samples, however, for sample P120 (120°C platform preheating) a rapid decrease of RS can be observed with only compressive residual stresses being described with a maximum at a depth of about 120 μm. On the contrary, the RS for sample SP (defau lt platform preheating, i.e. 40 °C) show a tensile character up to a depth of 170 μm with a maximum of more than 300 MPa. This state of RS is unfavorable as high tensile values reduce the yield stress and promote crack initiation and their growth. However, at greater depths for the SP sample, compressive RS have also been described. The high tensile RS in the surface layers did not affect the strength, as the ultimate tensile strength is higher by about 50 MPa for the SP specimen, see Table 3. There could be a greater effect on fatigue life, but the surface tensile RS of specimen P120 probably explains one of the reasons why there was no increase in fatigue life between specimen P120 and SP (results will be the subject of following article). Therefore, further print optimization will be required to minimize surface RS (Trojan at al.) 3. Conclusion Based on the results of the drilling method, two sets of process parameters (T and SP_P120) were found to have the potential to reduce residual stresses in printed maraging steel C300. On the other hand, the results of this experiment cannot confirm the positive effect of the chessboard strategy, which is generally recommended for reducing residual stresses. Similar conclusion is presented by Lu et al. The curves lay in a similar scatter as for the SP set (-50 to -450 MPa) and the trend of the curves was also similar. Significant reduction of residual stresses (RS) was achieved by a combination of parameters labeled T, which involved a change of the complex VED parameter and the heat treatment regime. The VED value was reduced from 67 J/mm3 to 55 J/mm3, and the TZ change consisted of increasing the annealing temperature and dwell time from 820°C for 1 hour to 940°C for 2 hours. The residual stress values ranged in a nar row band (-120 to -180 MPa).