The effect of platform temperature combined with a T5 heat treatment on the fatigue life of additively manufactured aluminum alloy AlSi7Mg was characterized and understood. High-cycle fatigue tests were carried out on samples built with four platform temperatures (35 °C, 60 °C, 80 °C and 200 °C) and post-processing heat treatment strategies (F and T5). Microstructural and fractographic observations combined with microhardness measurements were performed. A log-normal statistical distribution regressed with 90% B-basis probabilities of survival revealed that specimens produced on a platform maintained at 80 °C and post-processed with a T5 heat treatment presented the highest fatigue life among the conditions tested. Precipitation of silicon within the aluminum cells during the T5 heat treatment is the proposed explanation for the improved fatigue life of the T5 samples. In the as-built condition, specimens produced at 200 °C were found to be less resistant to fatigue than the specimens built at lower temperatures. The coarser microstructure and lowest microhardness resulting from high-temperature manufacturing explain this reduced fatigue strength. All fatigue cracks initiated from manufacturing discontinuities. This led to a fatigue life prediction model based upon linear elastic fracture mechanics. The model was fitted to the experimental results of the F and T5 samples separately. With the exception of the 35 °C—T5 specimens, the predicted fatigue lives agree with the experimental results and literature.

The effects of surface roughness parameters and residual stresses induced by finish turning on fatigue life and crack propagation behavior of Inconel 718 super-alloy are investigated. A criterion serving as a good indicator of surface quality is also discovered. This is carried out for some specific turning conditions in order to control surface characteristics including surface roughness and the bi-axial state of surface residual stresses and to investigate the effects of these characteristics on fatigue behavior. Rotating bending fatigue (RBF) tests are performed on the machined specimens at room temperature. Characterization techniques including X-ray diffraction (XRD), laser confocal microscopy, and opto-digital microscopy are used to investigate surface residual stresses in axial (ARS) and hoop (HRS) directions, surface roughness, and secondary fatigue cracks, respectively. The huge dispersion in fatigue life of smooth turned Inconel 718 with very low arithmetic average roughness (Ra) values indicates that surface finishing is not the only reliable criterion for quality control of machined components. Multivariate statistical analyses show that axial surface residual stress is the most important surface characteristic affecting the fatigue life of machined Inconel 718 for the conditions tested. Changing ARS from compressive to tensile increases the number of secondary cracks and also decreases the fatigue life of turned specimens.

The effect of shielding gas on the mechanical and microstructural characteristics of ER410NiMo martensitic stainless steel weldments was investigated. Three weldments with various inclusion contents were manufactured using different shielding gas compositions and welding processes: gas metal arc welding (GMAW) with 100% argon (Ar), GMAW 85% Ar/15% carbon dioxide (CO2), and flux cored arc welding (FCAW) 75% Ar/25% CO2. The inclusions in each weldment were characterized by means of scanning electron microscope observations and energy-dispersive spectroscopy analysis. The weldments underwent postweld heat treatment, after which the chemical composition and reformed austenite proportion were measured to account for microstructural effects. Hardness measurements, tensile tests, and impact toughness tests using the Charpy method were performed. The results showed that the Charpy V-notch (CVN) absorbed energy decreases with increasing inclusion content. The highest CVN absorbed energy, 195 J, was obtained for the GMAW 100% Ar weld, which had the lowest inclusion content. GMAW 85% Ar/15% CO2, with four times more inclusions than the former, had a CVN absorbed energy of 63 J. The current manufacturing process, FCAW 75% Ar/25% CO2, was found to have an inclusion content three times higher than the GMAW 100% Ar weld but a CVN absorbed energy of 66 J, which is close to the GMAW 85% Ar/15% CO2 weld. The results showed that using GMAW 100% Ar as a replacement to FCAW 75% Ar/25 % CO2 would lead to a three-fold improvement in terms of absorbed impact energy. The effect of inclusions on tensile properties, which was not clearly identified as several factors, in addition to inclusion content, affects the weld strength and elongation. Overall, the yield and ultimate tensile strengths differed slightly: 724 and 918 MPa for GMAW 100% Ar, 746 and 927 MPa for GMAW 85% Ar/15% CO2, and 711 and 864 MPa for FCAW 75% Ar/25% CO2, respectively.

This study focuses on the fatigue resistance of aluminum friction-stir-welded (FSW) butt joints beyond the current design standards such as Eurocode 9 and CSA S6, which use generic S–N curves. A database of 1041 data points from 92 studies is analyzed using a statistical methodology by alloy, FSW variant, and stress ratio. The results show that fatigue strength more strongly depends on the alloy rather than on welding FSW variant. While existing codes give conservative estimation of the fatigue strength at a 50% probability of failure, they may be unconservative for lower probability of failure due to data scatter. This conclusion highlights the need for grade-specific S–N curves and additional guidance on scatter assessment.