PSI - Issue 77

L.P. Borrego et al. / Procedia Structural Integrity 77 (2026) 681–687

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Fig. 5. SEM images of the cellular structure in transversal cross-section: (a) As-build condition and (b) heat-treated condition.

The low-temperature stress relieving heat treatment strategy (HT) did not introduce macroscopic alterations due to the moderately low thermal exposure, (Fig. 5(b). However, the high supersaturation of Si and Mg atoms in the Al lattice led to the development and coarsening of β -Mg2Si forerunner phases (red arrows) and Si precipitates inside the α -Al cells. The results of fatigue tests are presented in Fig. 6, in addiction with reference life proposed by the NP EN ISO 4210-8:2019 standard (2019).

0 20 000 40 000 60 000 80 000 100 000 120 000 140 000

Standarized target

100%

126%

78%

Number of cycles

As-built

Stress-relieved Conventional

Conditions

Fig. 6. Average fatigue results. The as-build cranksets failed in the region where the maximum load was achieved in the FEA (see Fig. 2), before reaching the target of 100,000 cycles, being 78,000 the average number of cycles achieved. Contrary, all the heat treated cranksets reached a medium value of 126,000 loading cycles without fracturing. Also, the conventional cranks failure also above the minimum number of cycles imposed by the standard, 100,000 cycles. Therefore, low temperature stress relief heat treatment presents high benefits in fatigue life by reducing the residual stresses produced during the Laser Powder Bed Fusion technic. Indeed, the heat treatment allowed the crank arm to reach 100,000 cycles, which can be attributed to the reduced sensitivity of the heat-treated condition to micronotches (defects), as already confirmed by Fernandes et al (2024a). This improvement is linked to the stabilized residual stress gradient, averaging 47 MPa, compared to the as-built condition, which exhibited an increasing linear gradient from 55 to 138 MPa.