PSI - Issue 38

Hugo Roirand et al. / Procedia Structural Integrity 38 (2022) 149–158 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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3.3. Tensile and micro-hardness behavior Hardness values are constant around 205 HV, except for SLM_Chess sample (Figure 5). The lower hardness for this specimen could be due to the high-elongated and large grains which composed its microstructure The yield strength is comparable for all samples except SLM_Chess specimen which has lower yield strength as Figure 5 shows. This could be due to the high defect density . For the other samples, the changes in meltpool morphology or grain shape do not seem to influence significantly the yield strength. The ultimate tensile strength (UTS) of SLM_Chess and Orlas specimens are respectively 20 % and 6 % lower than samples scanned with meander strategies. The defects study and fracture surface analysis suggest that defects, especially lack of fusion ones, are the cause of this UTS decrease. The main differences are on elongation. SLM_Chess sample has a very low rupture elongation compared to other samples, which is due to the high density of big lack of fusion. They act as a reduction in effective loading section and also they increase the stress concentrations which promote failure. The same effect can be observed for the Orlas specimen but with a lower intensity. It is interesting to notice that SLM_SR and SLM_Rot specimens have very similar tensile behaviour although the melt pool positions/morphology and grain morphology are significantly different. In consequence, the fine elongated grains observed on SLM_SR sample does not seem to play a key role concerning the tensile behaviour. Moreover, the more numerous gas pores observed for SLM_SR sample have no influence on monotonic tensile behaviour. It suggests that, it exists a threshold pore size under which it has no influence on the quasi-static properties.

Fig. 5. Microhardness results and representative tensile tests curves for the different specimens

3.4. Fatigue behavior The fatigue behaviour has been studied in order to point out the effect of the microstructure and defects generated by AM. Figure 6 clearly shows that the fatigue resistance of SLM_SR and SLM_Rot samples are much higher than for SLM_Chess and Orlas specimens. Fatigue behaviour of meander strategies specimens are very similar, so the conclusions done for monotonic behaviour could be extend to the fatigue loadings. Especially, the no-influence of the number of small gas pores is in accordance with the Kitagawa law which mentioned a minimum critical defect size for reducing fatigue lifetime (Liang et al., 2020; Morel et al., 2015). Under this critical size, cracks initiate from the microstructure like on persistent slip bands as Figure 7 illustrates it for SLM_SR and SLM_Rot specimens. On some of these specimens, cracks initiated from small defects close to the surface. However, points related to the samples initiated on microstructure and those for samples initiated on small pores are well aligned on the S-log(N) graph (Pineau et al., 2016; Schijve, 2009). This results suggest that pores in SLM_SR and SLM_Rot have a size very closed to the critical pore size defining the fatigue crack initiation mode (microstructure or defect crack initiation).