Fatigue crack growth behavior of a selective laser melted AlSi10Mg

A critical review on the effects of process-induced porosity on the mechanical properties of alloys fabricated by laser powder bed fusion

“…ageing treatment resulted in better fatigue life than as-built Crack initiation occurred in sub-surface pores for as-built samples and in subsurface coarsened Si or intermetallic precipitates in HIP ? aged samples While coarsened precipitates caused crack initiation, they also impeded crack propagation [224] AlSi10Mg R = -1 HIP can cause metal above sub-surface defects to deform inward, resulting in surface defects Additional surface machining after HIP is required to further improve fatigue life [120] AlSi10Mg R = 0.1 Machining improves fatigue life, but pores exposed by machining will become responsible for fatigue failure [121] AlSi10Mg R = 0.1 Improvements to fatigue life is limited by machining due to exposure of previously sub-surface/internal pores [28] AlSi10Mg R = -1 Shot-peening can reduce sub-surface porosity and also causes the spheroidization of sub-surface LoF pores Shot-peening improved fatigue life [225] AlSi10Mg R = -1 Improvement to fatigue life by shot-peening was attributed to compressive surface stresses rather than any influence on surface/sub-surface porosity [226,227] AlSi10Mg R = 0.1 Friction stir welding as a surface treatment was found to improve fatigue life by closing surface pores and globularizing the Si phase [142] AlSi10Mg R = 0.1 Heat treatment that coarsens the Si phase improves fatigue life porosity closure across the entire cross-section using HIP becomes far more effective since internal porosity becomes more critical. Similarly, for LPBF IN718, Ardi et al found that despite porosity reduction through HIP (0.39% to 0.08%), room temperature high-cycle fatigue performance worsened due to microstructural changes while shot-peening improved fatigue life [128].…”

“…Similarly, the HIP and ageing study on LPBF AlSi10Mg by Schneller et al [141] noted that while coarsened Si particles can act as crack initiation sites if competing pores are small enough, they can also impede crack propagation. On the other hand, in the as-built condition, because the fine dendritic Si phase confers high strength but easily facilitates crack propagation, the loss of this microstructure through a conventional heat treatment (where the Si phase is dissolved through solution treatment and subsequent ageing causes it to reprecipitate as coarse discrete particles) may improve fatigue life if the improved resistance to crack propagation more than compensates for the corresponding reduction in load bearing capacity due to reduced strength [142]. This dichotomy explains why ageing treatment was found to be beneficial in [142] but detrimental in [86,143].…”

“…On the other hand, in the as-built condition, because the fine dendritic Si phase confers high strength but easily facilitates crack propagation, the loss of this microstructure through a conventional heat treatment (where the Si phase is dissolved through solution treatment and subsequent ageing causes it to reprecipitate as coarse discrete particles) may improve fatigue life if the improved resistance to crack propagation more than compensates for the corresponding reduction in load bearing capacity due to reduced strength [142]. This dichotomy explains why ageing treatment was found to be beneficial in [142] but detrimental in [86,143]. An interesting study by Bao et al shows that the coarsening of the Si phase during a fatigue test at 400 °C can actually result in void formation that leads to fatigue failure, while fatigue life at lower temperatures was driven by existing porosity, as illustrated in Fig.…”

Residual stress in laser-based directed energy deposition of aluminum alloy 2024: simulation and validation

“…Therefore, the hole-drilling method would hardly be applicable in this case. The approach of X-ray diffraction has already been applied to additively manufactured parts [18], including DED [16,19,20]. Unfortunately, as a drawback, assessing the residual stress over the surface of the deposited metal is more difficult with this method: Through-thickness stress gradients within the probed volume complicate the analysis and interpretation of results, and the roughness of the as-deposited material also increases the measurement uncertainty.…”

Heat Treatments for Stress Relieving AlSi9Cu3 Alloy Produced by Laser Powder Bed Fusion

“…This treatment aims at removing residual stresses, which derive from volumetric changes of the material upon cooling and are exacerbated by the layer-by-layer nature of LPBF [ 25 , 26 ]. Indeed, residual stresses are known to negatively affect the behaviour of LPBFed aluminium alloys under several perspectives, including fatigue life [ 27 ] and corrosion resistance [ 28 ]. Their reduction or removal is usually obtained by means of annealing treatments, which are performed in a temperature range, which lies between the one used for ageing treatment (usually from 140 °C to 180 °C) and the one able to induce a proper solubilization of the material (above 480 °C).…”